Crack-Templated Patterns in Thin Films: Fabrication Techniques, Characterization, and Emerging Applications

Abstract

Crack-templated thin films, inspired by naturally occurring patterns such as leaf venation, spider webs, and the networked structure of dried egg white, represent a paradigm shift in the design of functional materials. Traditionally, cracks in coatings are seen as defects to be avoided due to their potential to compromise mechanical integrity and performance. However, in this context, cracks are deliberately induced and meticulously controlled to serve as templates for versatile applications. This review explores the latest advances in preparation techniques, including solvent evaporation and thermal stress induction, with a focus on the interplay between material properties (e.g., polymers and ceramics) and process parameters (e.g., drying rates and temperature, layer thickness, substrate interactions) that govern crack behavior. The resulting crack patterns offer tunable features, such as density, width, shape, and orientation, which can be harnessed for applications in semitransparent electrodes, flexible sensors, and wearable and energy storage devices. Our study aims to navigate the advancements in crack engineering in the last 10 years and underscores its importance as a purposeful and versatile strategy for next-generation thin-film technologies, offering a novel and affordable approach to transforming perceived defects into assets for cutting-edge thin-film technologies.

1. Introduction

Crack-templated thin films, inspired by the intricate and interconnected patterns found in nature—such as drying mud (Figure 1c), leaf venation (Figure 1d), spider webs, reptile skin, and the networked structure of dried egg whites [1]—have emerged as a transformative approach in the design of functional materials [2]. Cracks are not only found in nature but also in human artifacts, such as paintings, or biological materials, like dried blood. These human-made occurrences offer unique opportunities for practical applications; for instance, crack patterns in artwork (Figure 1a,b) provide insight into art conservation and authentication [3,4], while the patterns formed in dried blood hold potential forensic value [5].

These naturally occurring patterns reveal an inherent ability to balance complexity and functionality, providing a powerful tool for engineering advanced materials with non-uniform pathways. Traditionally, cracks in coatings and films have been viewed as undesirable defects that compromise mechanical strength and overall performance [6].

The concept of crack templates has evolved significantly in fracture mechanics research. Initially, Padovan and Guo introduced a moving template analysis for crack growth, allowing accurate prediction of crack propagation in fatigue environments [7]. This approach was extended to multiple crack problems by the same group, demonstrating its effectiveness in modeling complex mixed-mode behaviors [8].

However, in this innovative context, cracks are deliberately induced and precisely tuned to serve as functional templates for a wide range of applications, by a process commonly known as “crack lithography” [2,9].

This review delves into the forefront of crack engineering, emphasizing the deliberate control of crack formation through different preparation techniques. These include solvent evaporation [10], thermal stress induction [11], and mechanical deformation [12], all of which are influenced by material properties (e.g., polymers [13], ceramics [14], and hybrid composites [15]) and processing conditions (e.g., drying rates [16], thermal gradients [17], film thickness [18], and substrate chemistry [19]). Mathematical relationships have been established between some of these parameters—such as film thickness, material adhesion, drying temperature, and stress distribution—allowing researchers to dig deeper and better predict crack behaviors using appropriate computational modeling [20,21].

By tailoring these parameters, crack patterns can be prepared with customizable features such as crack density, width, shape, and orientation, unlocking opportunities for unique material functionalities.

The resulting crack networks are versatile and scalable, enabling applications in emerging technologies such as semitransparent electrodes [22], flexible sensors [23], wearable electronics [24], and optoelectronic components [25]. The crack width can range from ~0.5 to ~200 μm for acrylic resin and SiO₂ nanoparticle dispersions, respectively [23,26], extending up to A4 sheet size [22], with a thickness varying between 100 and 1000 nm [27], as demonstrated using acrylic resin as a model system.

This approach offers significant advantages in terms of cost-effectiveness and material sustainability, allowing cracks to be understood in their genealogy and transformed from defects into assets with purposeful functionality. By bridging the fields of materials science, physics, and engineering, this review will highlight the relevant papers from the last 10 years (2014–2024) that address the fabrication, optimization, and applications of crack templates, presenting crack engineering as a bold and innovative strategy for advancing next-generation thin-film technologies.

2. Crack Templating

2.1. Concept and Mechanism of Crack Formation

Cracking occurs when the tensile stress applied to a material exceeds its ability to resist fracture, a property quantified as fracture toughness or the critical stress intensity factor (Kc). This phenomenon involves the breakdown of atomic bonds without complete separation and is influenced by the stress intensity factor (K), which measures the local stress amplification near a crack tip. The magnitude of K depends on material properties, crack geometry, and the magnitude and rate of applied loads. There are three primary fracture modes: mode I (opening or tensile), mode II (in-plane shear), and mode III (out-of-plane shear) [28]. While real-world cracking often involves a combination of these modes, thin-film and substrate systems typically experience pure mode-I fractures, making the mode-I fracture toughness (KIc) and stress intensity factor (KI) the critical determinants of crack propagation. Cracks grow when KI ≥ KIc and remain stable when KI < KIc, allowing for potential predictability and control through precise material and process engineering [29].

Crack formation in thin films is predominantly driven by the interplay of stresses that arise during and after film formation (

Table 1. Overview of the main factors influencing crack formation, including the types of stresses (intrinsic and residual), contributing material defects (e.g., grain boundaries, voids, surface irregularities, and adhesion), and crack orientations (random, unidirectional, bi/multidirectional) as discussed in Section 2.1 and Section 2.2.

Aspect of Crack Formation	Details
Stresses	Intrinsic: Stresses originating from the material’s internal structure (e.g., due to phase changes, thermal expansion, or material defects).
	Residual: Stresses remaining in a material after processing or deformation (e.g., due to cooling, phase transformations, or manufacturing processes).
Contributors/defects	Grain boundaries: Cracks can form or propagate along grain boundaries, which are weak points in crystalline materials.
	Voids: Internal voids or pores within the material that can lead to crack formation under stress.
	Surface irregularities: Surface roughness or defects, including scratches or inclusions, that can act as crack initiation sites.
	Adhesion to the substrate: Poor adhesion between layers or to the substrate can lead to delamination or crack initiation.
Orientation	Random: Cracks propagate in random directions based on stress distribution and material properties.
	Unidirectional: Cracks propagate in a single direction, typically in response to directional stresses (e.g., along the length of a material).
	Bi/Multidirectional: Cracks propagate in multiple directions, often influenced by complex loading or multi-axial stress conditions.

). A key contributor is intrinsic stress, which develops in situ during the film’s deposition or transformation processes, originating from interactions such as particle–particle forces or particle–substrate adhesion. This stress contrasts with residual stress, which persists in the film after it has cooled to room temperature and is a result of structural or thermal changes occurring during processing. When these stresses exceed the material’s fracture strength, cracks are initiated, influencing the film’s structural integrity. These stresses can originate from various sources, including thermal expansion mismatch, shrinkage during drying or curing, and mechanical deformation. For instance, thermal stresses often occur when a film cools down after deposition or processing, leading to contraction that may differ between the film and its substrate due to mismatched coefficients of thermal expansion. An example of this phenomenon occurring in sol–gel deposition of ceramic thin films has been described in detail by H. Kozuka [30]. Similarly, solvent evaporation in liquid-based coatings can cause volumetric shrinkage, generating tensile stress within the film.

The study of Kumar et al. [31] investigates the evolution of cracks in desiccating TiO₂ colloidal films as model films, emphasizing the hierarchical and temporal development of crack patterns. Crack geometry was analyzed using image analysis and a recursive algorithm, classifying cracks into primary (generally, about 46% of the formed cracks in films), secondary (~43%), and tertiary (~13%) generations based on their intersection angles, widths, and temporal progression. Primary cracks appear first, followed by secondary and tertiary cracks that increase connectivity. As drying progresses, cracks widen and propagate due to accumulating stress, with the film’s thickness influencing crack width, area, and spacing (Figure 2). By analyzing crack widths in fully dried films, the study reconstructed the temporal sequence of crack formation, and the recursive algorithm efficiently identifies crack hierarchies, aligning with experimental observation. This model enhances understanding of crack stress profiles, hierarchical development, and crack generation statistics. It also provides insights for analyzing historical paintings, correlating crack morphology with drying processes to estimate age and material properties.

Other contributors include the accumulation of stress at defects or imperfections, such as voids, grain boundaries, or surface irregularities, which act as stress concentrators and thus as crack nucleation sites [32]. Additionally, the film’s adhesion to the substrate plays a critical role; weak adhesion can lead to delamination or detachment, whereas strong adhesion may intensify internal stresses, increasing the likelihood of cracking [33]. Factors such as loading types, substrate confinement, and competing stress–relaxation processes like interface debonding and buckling instability further influence crack pattern formation and propagation [34].

The interplay of these factors, along with the material’s mechanical properties—such as elasticity, toughness, and thickness—determines the onset and propagation of cracks in thin films. Understanding these mechanisms is crucial for improving thin-film reliability and developing controlled crack pattern fabrication techniques.

2.2. Orientation and Morphology

As mentioned in Table 1, the orientation of the so-obtained crack pattern can be random, unidirectional, or multidirectional (Figure 3) [29].

Randomly oriented crack patterns commonly arise during solvent evaporation and chemical vapor deposition (CVD) processes due to tensile stresses induced by drying or thermal mismatches. In drying-mediated systems, stresses evolve non-uniformly, as seen in colloidal films of fine coffee powder or TiO₂ and SiO₂ nanoparticles [27], where initial defects nucleate cracks that propagate linearly before branching or merging into fractal-like patterns. The crack density and width are influenced by factors such as film thickness, particle size, and material properties, with drying-induced stresses sometimes creating complex structures like spiral cracks due to humidity gradients and twisting stress fields. In CVD processes, residual stresses caused by thermal expansion mismatches generate random cracks in TiCN/WC-Co and TiCN/α-Al₂O₃ bilayers [35]. Random crack propagation is influenced by the crystallographic orientation of the substrate, leading to preferential directions unless dampened by elastic interlayers or isolated by engineered trenches. Such strategies can also prevent cracks from penetrating or spreading, demonstrating the role of material properties, substrate structure, and stress control in guiding crack formation.

Unidirectional and bidirectional/multidirectional crack patterns can be generated through precise control of applied stresses, with various methods available for each desired condition.

Unidirectional cracks can be created by introducing tensile stresses, such as through temperature gradients or mechanical strain. Temperature gradients generate stress fields through differential heating and cooling, with the resulting crack patterns varying from straight to nonlinear depending on the thermal stress intensity [36]. Mechanical strain, when applied to elastic substrates like polydimethylsiloxane (PDMS), induces cracks in a brittle film layer aligned perpendicular to the tensile direction [37], with parameters like strain magnitude and rate offering fine control over crack position and alignment. Additionally, unidirectional cracks can be formed using techniques like CVD, which creates residual tensile stress in brittle films such as Si₃N₄, with stress-concentrating features like sharp notches facilitating crack initiation and homogeneous propagation [38]. Periodic propagation of unidirectional cracks has been studied by Mondal and colleagues, specifically using a film thickness gradient formed under the gravity flow of a colloidal dispersion (acrylic resin nanoparticles) on an inclined substrate [39]. In this case, the direction of the drying front dictates the direction of parallel cracks, and experimental parameters can be optimized to control periodicity, crack width, and perpendicular interconnects.

To achieve bi- or multidirectional crack pathways, various methods are employed, including mechanical strain, CVD, conventional solvent evaporation, and photolithography. For bidirectional cracks, the work of Li et al. introduces a method to guide cracks along desired bidirectional pathways in thin films coated on substrates using generalized biaxial stress on a polyimide film [40]. By adjusting biaxial stress ratios, cracks can be guided along specific bidirectional paths, influenced by the initial angle of prefabricated cracks and the applied stress configuration. Experimental results validate the theoretical model, demonstrating accurate control of crack propagation along predetermined pathways. In contrast, CVD-based methods for multidirectional cracks involve creating residual tensile stress in films like SiO₂ or Si₃N₄, where prefabricated structures such as trenches or multilevel micropatterns modulate crack initiation, propagation, and termination. The lower residual stress of SiO₂ compared to Si₃N₄ allows for localized cracks without deep penetration into the substrates. In particular, Guo et al. explored the kinetics of crack formation in graphene films on liquid copper substrates via CVD [38]. Interestingly, cracks in graphene tend to propagate along a zigzag direction. This selective zigzag cracking is formed by the stress concentrations at the V-shaped defects, together with the tensile stress driven by gas applied on graphene films.

Another study by Ma and colleagues reveals that distinct polygonal crack patterns in cornstarch–water suspensions result from two shrinkage mechanisms [41]. Initial evaporation induces capillary stress at the interface, causing primary cracks. Secondary cracks emerge due to particle deswelling as the system continues to dry. These mechanisms highlight the potential for scalable multiscale crack patterning, applicable to large surfaces such as wet mud.

Lastly, crack photolithography represents an innovative nanofabrication approach that exploits controlled cracking in photoresist materials to generate multidirectional and mixed-scale patterns with exceptional resolution and scalability. This technique integrates seamlessly with standard photolithography processes, ensuring cost-effectiveness and broad accessibility. By enabling precise manipulation of crack geometry, crack photolithography facilitates the creation of complex, arbitrarily shaped nanopatterns across large surface areas (e.g., on SU-8, photoresist) [42]. This versatile method has been effectively utilized to fabricate master molds for nanofluidic devices, which are subsequently replicated via soft lithography, underscoring its potential in advanced micro/nanomanufacturing applications.

Figure 3. Visual representation of unidirectional ((a) acrylic resin) and multi/bidirectional cracks ((b,c) polyimide and graphene, respectively). (a) Reprinted from ref. [39], Copyright © 2018, with permission from Elsevier. (b) Reprinted from ref. [40], Copyright © 2022, with permission from Elsevier. (c) Reprinted from ref. [38], Copyright © 2022, with permission from Elsevier.

Figure 3. Visual representation of unidirectional ((a) acrylic resin) and multi/bidirectional cracks ((b,c) polyimide and graphene, respectively). (a) Reprinted from ref. [39], Copyright © 2018, with permission from Elsevier. (b) Reprinted from ref. [40], Copyright © 2022, with permission from Elsevier. (c) Reprinted from ref. [38], Copyright © 2022, with permission from Elsevier.

3. Materials for Crack Template Fabrication

3.1. Bio-Based Materials

Egg white, a bio-based material predominantly composed of proteins such as ovalbumin, conalbumin, and lysozyme, has garnered increasing attention in various scientific fields due to its unique properties and sustainability. As a natural polymer with excellent film-forming capabilities, egg white has found application in diverse areas, including food, pharmaceuticals, and materials science [43]. The use of egg white in this context offers several advantages, including its renewable nature, low environmental impact, and biocompatibility [44]. In non-cracked coatings, Chandrabose et al. utilized egg white foam to synthesize highly porous layered double hydroxides and mixed metal oxides, resulting in enhanced specific surface area and porosity [45]. Also, Li et al. employed egg white as a template and dispersing agent to produce ZnO nanosheets, with improved ethanol sensing and antibacterial properties [46].

Egg white has been widely investigated as a model for studying the drying behavior of protein solutions due to its well-defined composition and typical biological properties. The work of Gao and colleagues investigates the formation of crack patterns in drying droplets of protein solutions [1]. The study explores how varying concentrations of egg white solution influence the resulting crack patterns and their mechanisms. Droplets with different protein concentrations were deposited on a silicon wafer, and their drying processes were monitored. The experiments revealed that as the droplets dried, distinct crack patterns formed, which were categorized into two main types: “daisy” and “wavy-ring” patterns. These patterns were influenced by the concentration of the protein solution, with higher concentrations leading to radial cracks originating from the perimeter, while lower concentrations resulted in periodic, wavy cracks.

Recently, egg white, thanks to the above-mentioned properties has emerged as an effective bio-based material in the fabrication of crack-patterned surfaces [47]. When applied as a thin film, egg white undergoes a drying process that induces spontaneous crack formation due to inherent stress and structural changes as the material transitions from liquid to solid [48]. Voronin and colleagues developed a cracked template using egg white to create transparent EMI shielding films with high performance [49,50], as well as transparent heaters [51]. Egg white has previously been exploited by Peng et al. to fabricate TCEs [52]. These studies demonstrate the versatility of egg white as a bio-template in materials science, and parameters such as concentration, drying conditions, and substrate type can be adjusted, allowing for precise control over crack patterns and their periodicities.

3.2. Ceramic-Based Materials

Materials like titanium dioxide (TiO₂), zinc oxide (ZnO), and silicon dioxide (SiO₂) are frequently used in the preparation of thin films; in particular, sol–gel methods have been utilized for the development of crack-patterned networks for functional coatings. TiO₂ thin films have found applications in environmental remediation, hydrogen production via water photo-splitting, odor control, self-cleaning surfaces, and degradation of dye pollutants. The photocatalytic performance of TiO₂ thin films depends on their textural properties, including thickness, surface roughness, grain size, pore size distribution, and porosity. These characteristics are closely influenced by sol properties such as viscosity, the water-to-alkoxide ratio, precursor concentration, and the presence of complexing agents or surfactants [53].

A TiO₂-based approach was employed to create crackle network templates for functional coatings [14]. TiO₂ nanoparticle powder was dispersed in ethanol and ultrasonicated to form a suspension, with ethyl acetate added to enhance crack formation while maintaining reduced film thickness. This suspension was drop-coated onto substrates such as glass, quartz, and PET, with the volume adjusted to substrate size (e.g., 30 μL per cm²). Crackle patterns formed spontaneously upon drying in air. Subsequently, Au metal was deposited via physical vapor deposition, and the TiO₂ template was removed through water washing with mild sonication, leaving behind the desired metal network. In the work of Muzzillo et al. [54], TiO₂ suspensions, prepared at 30% wt/vol in deionized water and sonicated to reduce agglomeration, were used to create crack templates via blade coating (Figure 4a). The thickness of the TiO₂ templates was controlled by adjusting the blade height (50–350 μm), with the suspensions drying within seconds due to rapid solvent evaporation. After template formation, Cu was deposited onto the TiO₂ using e-beam evaporation. The TiO₂ templates were successfully lifted off by immersion in water followed by mild sonication. In the same work, different templates were compared to TiO₂, specifically polystyrene (PS) and poly(methyl methacrylate) (PMMA) nanoparticle suspensions. The lift-off procedures varied: water immersion was sufficient for TiO₂, whereas tetrahydrofuran was required for PS and PMMA. Additionally, Cu deposition methods differed, with e-beam evaporation used for TiO₂ and PMMA and DC sputtering for PS to improve pattern fidelity.

ZnO, a luminescent semiconductor with a wide direct band gap of approximately 3.3 eV, is widely used in the optoelectronic field. ZnO films are notable for their transparency across the visible spectrum, making them suitable for diverse applications. ZnO-based nanocomposites have garnered significant interest due to their high surface-to-volume ratio, which allows interactions between ZnO nanoparticles and surrounding materials to modify emission spectra, thereby enhancing optoelectronic and photocatalytic properties [55]. In the work of Melnychenko et al. [56], ZnO nanoparticles, purchased as a 40 wt% ethanol dispersion with particle sizes < 130 nm, are added to the PVP solution due to their compatibility in ethanol. The mixture is homogenized at 55 °C to ensure uniform dispersion and suspension stability, lasting up to three days. Next, the dispersion is spin-coated onto a transparent substrate and the sample is heated to 80 °C to induce cracks, driven primarily by ethanol evaporation. The cracking process has thus been studied by varying ZnO-to-PVP ratios, maintaining the polymer matrix constant while incrementally increasing ZnO content, on different substrates (Figure 4b).

SiO₂ nanoparticles have gained considerable attention in materials science due to their unique properties, making them valuable for various technological applications such as photocatalysis, sensors, solar cells, and memory devices. Their remarkable physical and chemical characteristics have driven extensive research, particularly in the preparation and characterization of SiO₂ thin films. These films are noted for their large energy gap, excellent visible and near-IR transmittance, high refractive index, and dielectric constant [57]. The study of Gupta et al. presents a scalable spray-coating method to fabricate crack templates on both flat and curved surfaces (Figure 4c), which serve as sacrificial templates for creating metal wire networks through physical metal deposition [58]. Two crackle precursors were used, one of those incorporating SiO₂ particles. The crackle precursor was prepared by diluting SiO₂ particles with a commercial diluter and ethyl acetate in a 3:1 volume ratio to achieve a 0.2 g/mL concentration. After resting overnight, the solution was ultrasonicated and then spray-coated onto surfaces using an airbrush system under controlled conditions (25 °C, 45% RH). This process enabled the formation of crackle networks with controlled widths. In another work, a commercial SiO₂ nanoparticle-based crackle paint was prepared by ultrasonicating the dispersion in a diluter and allowing it to rest overnight. This suspension was applied to PET substrates via drop, spin, or rod coating [22]. Upon air drying, a spontaneous crackle network pattern formed, aided by film-forming agents that ensured smooth and uniform coating over large areas, for subsequent metal deposition in the cracked spaces. Solvents such as ethyl acetate and pentyl acetate enhanced the spreading and leveling of the dispersion, respectively.

Figure 4. (a) Coating and subsequent drying of TiO₂ colloidal suspension. Reprinted with permission from ref. [54]. Copyright © 2020, American Chemical Society. (b) Cracked ZnO template on (1) quartz, (2) sapphire, and (3) glass substrates. Reprinted from ref. [56]. Copyright © 2022 the authors. Published by American Chemical Society. (c) Photograph (1) and optical microscope image (2) of SiO₂ cracked template on a round-bottom flask. Adapted with permission from ref. [58]. Copyright 2014 © American Chemical Society.

Figure 4. (a) Coating and subsequent drying of TiO₂ colloidal suspension. Reprinted with permission from ref. [54]. Copyright © 2020, American Chemical Society. (b) Cracked ZnO template on (1) quartz, (2) sapphire, and (3) glass substrates. Reprinted from ref. [56]. Copyright © 2022 the authors. Published by American Chemical Society. (c) Photograph (1) and optical microscope image (2) of SiO₂ cracked template on a round-bottom flask. Adapted with permission from ref. [58]. Copyright 2014 © American Chemical Society.

3.3. Polymeric, Hybrid, and Composite Materials

Strategies used to prevent or promote cracking include incorporating polymers like polyvinylpyrrolidone (PVP) in films made of inorganic nanoparticles of different sizes. It was demonstrated that PVP can increase the critical thickness of sol–gel-derived ceramic thin films containing titanium alkoxide, suppressing crack formation. This could be due to the presence of C=O groups that could work as “capping agents” for the OH groups of the metalloxane polymers, suppressing the condensation reaction and promoting structural relaxation [30]. However, the presence of PVP does not necessarily suppress crack formation: in another study, Koga and colleagues demonstrated that PVP showed a stronger adhesion to particles in silica suspension. Compared to polyvinyl alcohol, particularly PVA-1 (degree of saponification: 78–82 mol%) and PVA-2 (>98 mol%), cracks that formed in the presence of PVP showed a constant length; the lowest was registered for PVA-1, whereas PVA-2 led to a decrease in cracks only at high concentrations (Figure 5). In addition, PVA-1 led to periodic crack pattern, while PVA-2 and PVP preferably led to randomly crackled patterns. It was thus concluded that not only adsorption of polymers on particles (in this case, higher for PVA-1 rather than PVA-2, with good adhesion properties also shown for PVP) but also their miscibility (good for PVA-1 but low for PVA-2 and PVP) are fundamental issues for controlling the formation of cracks during drying of colloid–polymer suspensions [59].

Another experimental study using titanium dioxide (TiO₂) particles of different sizes (90 nm and 150 nm) in suspensions with water-soluble polymers (e.g., PVA and PEG) revealed that the critical cracking thickness (hCCTh) increases nonlinearly with polymer adsorption, following a master curve independent of particle size [13]. Polymers with higher molecular weights (in this study, 98–99 mol% hydrolyzed PVA with a degree of polymerization of n = 1700) enhance elasticity and toughness, suppressing cracks more effectively, while low-adsorption polymers (in this case, PEG) exhibit minimal impact on hCCTh. In general, polymer adsorption reduces capillary stress and introduces polymer-specific stresses; entangled polymers form networks that increase elasticity and toughness, suppressing crack propagation. These findings underscore the potential of optimizing polymer adsorption and suspension properties to design crack-free particulate coatings, when needed.

Hybrid and composite materials have emerged as innovative solutions for enhancing the performance and functionality of thin films in a variety of applications, including optoelectronics, sensors, catalysis, and energy storage. These materials combine distinct components—organic, inorganic, or both—resulting in films with properties superior to those of their individual constituents, and the formation of cracks and fractures has been investigated in films based on organosilica composites, woven carbon/epoxy (C8) composites, laminated composites, or carbon nanotubes (CNTs) as nanofillers in carbon fiber-reinforced polymer (CFRP) composites

It is well known that, in sol–gel-derived films, rapid drying induces mechanical stresses that may cause fracturing, with silica films being more prone to cracking than organosilica films [15]. Kappert et al. demonstrated that silica films exhibit a critical thickness of 300 nm, above which they fracture due to drying-induced stresses. In contrast, organosilica films, synthesized using various precursors, show significantly higher critical thicknesses. Moreover, organosilica films exhibit saturation in crack spacing for thicker layers due to partial delamination, which reduces stress and halts further cracking.

Arasan and colleagues investigated the effects of crack length and crack location on the fracture of glass and carbon fiber-reinforced composites, underscoring the importance of crack configuration, layer arrangement, and material composition in optimizing the fracture resistance [60].

Recently, another study [61] investigated the fracture behavior of a hybrid composite laminate composed of a metal foil (Al, Ti, or NiTi) layer at the center of sixteen layers of plain woven carbon fabric with an epoxy matrix. The study highlighted the successful integration of in situ fiber optics measurements with computational models, enabling a more comprehensive understanding of crack growth in hybrid interfaces. Future research could benefit from the use of this model to investigate the effects of surface preparation on interfacial adhesion strength in hybrid metal–polymer composite systems.

Lastly, the study of Ayyagari et al. [62] explored the role of CNTs as nanofillers in CFRP composites to improve their fracture properties. CNTs were grown using a low-temperature technique in specific patterns on the surface of carbon fibers to create a controlled interface between the fiber and the epoxy matrix. The impact of CNT growth on the composite’s fracture toughness was assessed using a DCB test and digital image correlation to monitor crack propagation. Additionally, a finite element simulation model based on the cohesive zone method was employed to model the fracture behaviors of different composite configurations. The results showed that a coarser CNT growth pattern enhanced resistance to crack propagation, improving interlaminar fracture toughness. The study concluded that an optimal CNT growth topology is essential to enhance the composite’s resistance to delamination and improve its fracture toughness.

4. Preparation Techniques for Crack-Templated Thin Films

As previously stated, crack-templated thin films are typically prepared by inducing controlled cracking through various methods, which exploit material responses to external stimuli. A few of the most common techniques are solvent evaporation, thermal stress induction, mechanical stretching, and UV curing, each offering distinct advantages and characteristics.

Solvent evaporation relies on the stress generated during the drying of colloidal suspensions or polymer solutions, as uneven shrinkage leads to the formation of hierarchical crack networks [10,63]. Thermal stress is employed by exposing thin films to temperature gradients or heating cycles, causing differential expansion or contraction within the film or substrate, thereby initiating cracks [64]. Mechanical strain involves the deliberate application of tensile or compressive forces to thin films, creating fractures normally aligned with the applied stress direction [12]. Lastly, UV curing uses photopolymerization to solidify a liquid precursor (e.g., photoresist), with the subsequent contraction during curing inducing crack patterns [65].

Each method enables fine control over the crack morphology, including the width, length, and spatial distribution, making these techniques versatile tools in the development of structured thin films for diverse scientific and industrial applications. Before delving into the preparation methods of cracks, it is important to briefly discuss the deposition techniques used to create the initial thin films that serve as templates for crack formation. These coating methods play a crucial role in determining the uniformity, thickness, and stress distribution of the film, which ultimately influence the morphology of the resulting cracks.

4.1. Deposition Techniques

Crack templates are deposited using several established techniques, each offering specific advantages in controlling film thickness, uniformity, and drying dynamics. Key parameters can be adjusted for each method to influence the resulting crack patterns, and some of the main deposition techniques used include the following:

• Spin coating: a high-speed technique where a liquid solution is evenly distributed over the substrate through centrifugal force [26,56]. Tunable parameters: solution viscosity, spin speed, spin time, and substrate surface energy. These factors influence film thickness, uniformity, and drying rate, thereby affecting crack density and morphology.
• Drop coating: a straightforward approach where droplets of the solution are deposited onto the substrate and allowed to spread and dry, forming crack networks [14]. Tunable parameters: droplet volume, deposition position, solution concentration, drying environment (temperature and humidity), and substrate inclination. These parameters dictate the spreading dynamics and final crack distribution.
• Blade coating: a method in which a solution is spread across the substrate using a blade or knife, enabling controlled film thickness over large areas [54]. Tunable parameters: blade speed, solution viscosity, blade height, substrate temperature, and drying atmosphere. These settings determine the uniformity, thickness, and drying kinetics, which directly affect crack formation.
• Dip coating: in this technique, a substrate is immersed into a solution and then withdrawn at a controlled speed. As the solvent evaporates, a thin film forms on the substrate surface, which develops cracks due to stresses induced during drying or shrinkage. Additionally, biphasic dip coating can also be exploited to induce the self-ordering of the cracks, by using an immiscible second solvent [66]. Tunable parameters: withdrawal speed, solution viscosity, immersion time, drying conditions (temperature, humidity, and airflow), and solution composition.
• Spray coating: a versatile deposition method used to create crack templates by atomizing a solution into fine droplets and spraying them onto a substrate. This method is particularly advantageous for coating large areas, irregular surfaces, or substrates with complex geometries [58]. Tunable parameters: spray nozzle specifications, spray pressure, solution flow rate, substrate distance from the nozzle, spray angle, drying conditions (temperature, humidity, airflow), solution composition (solute-to-solvent ratio, solvent volatility), and substrate properties.
• Electron beam evaporation: a physical vapor deposition technique where a high-energy electron beam is used to evaporate a source material, typically in a vacuum chamber. The vaporized material condenses on the substrate, forming a thin film. Cracks can form due to thermal stresses induced by differences in the thermal expansion coefficients of the substrate and deposited film [11]. Tunable parameters: electron beam energy, deposition rate, vacuum pressure, substrate temperature, material properties of the source and substrate, and post-deposition annealing conditions.

4.2. Solvent Evaporation

Solvent evaporation plays a crucial role in thin film formation and morphology. A minimal model for solvent evaporation and absorption in thin films has been developed, which can predict drying dynamics and film formation across various experimental conditions [67]. This model also explains the formation of a solute-rich “skin” layer at the evaporating surface and captures the propagation of a saturation front during solvent absorption, demonstrating its applicability to diverse practical contexts.

Tomar and colleagues investigated the formation of cracks in drying films of trimethylsiloxysilicate polymer solution, emphasizing the critical role of solvent evaporation [10,63]. During drying, the evaporation of solvent from polymer solutions induces shrinkage stresses, which ultimately results in cracking. Thin films exhibited rapid and uniform drying, leading to negligible or isolated cracking due to faster stress buildup and higher strain rates. Conversely, thicker films experienced slower evaporation rates due to the formation of a surface “skin”, causing stress accumulation at later drying stages. This delayed stress buildup was associated with isolated cracks, which became extensive in very thick films, accompanied by delamination. The hierarchical drying process, also influenced by film thickness and type of substrate (parameters later discussed in this review), highlights solvent evaporation as a key factor dictating crack behavior, stress evolution, and the final morphology of polymer films.

Another study highlights the pivotal role of solvent evaporation in the formation of crack patterns in colloidal deposits [68]. As water evaporates from a drop of colloidal suspension, a solid particle deposit forms at the pinned contact line and expands inward. The evaporation-driven shrinkage generates stresses within the deposit, leading to the formation of radial and orthoradial cracks (Figure 6). Radial cracks emerge first and propagate inward, guided by pressure gradients caused by water flow through the porous deposit. Orthoradial cracks develop later between radial cracks, requiring higher tensile stresses due to partial stress relaxation in preexisting fractures. The dynamics of water transport within the deposit, controlled by evaporation and pore pressure gradients, govern the spatial and temporal evolution of these crack patterns, emphasizing the intricate interplay between drying-induced stresses and solvent removal. The pressure gradient is described by Darcy’s law for pore pressure gradients (Equation (1)):

$${\displaystyle \frac{\partial P}{\partial r}}=-{\displaystyle \frac{\mu u}{k}}$$

(1)

where P(r,t) is pore pressure, μ is water viscosity, u is radial water velocity, and k is permeability. This pressure drives radial water flow toward evaporating regions, accelerating the drying process and propagating solidification fronts. The flow velocity within the deposit peaks at the boundary of the liquid region and drops to zero at the orthoradial crack front (u(r_or) = 0) since water is unable to pass through orthoradial cracks. Consequently, the pressure decreases along r, reaching its most negative value at the orthoradial crack front.

4.3. Thermal Stress Induction

Thermal stress is another key factor contributing to crack formation and delamination in thin films. These defects result from differences in thermal expansion coefficients and elastic properties between the film and substrate, causing localized stress concentrations at the interface and subsequent delamination [69]. In multilayer configurations, such as Si-SiO₂-Al structures, pulsed heating of interconnects can induce thermal shocks, which promote crack formation in thin silicon oxide layers [70]. Though cracks often arise from the mismatch in thermal expansion coefficients between the film and substrate, temperature also influences solvent evaporation rates, where rapid evaporation at higher temperatures induces stress gradients, promoting localized mechanical failure and crack formation.

Li et al. utilized heating during the CVD process to induce cracking in a tungsten interlayer, which was then used to deposit a diamond film. The cracks in the interlayer are replicated in the diamond film, improving the wear resistance of the underlying stainless steel [71].

Lama et al. investigated the role of temperature during the drying of colloidal suspensions, particularly in the context of the “coffee ring effect”, a crackled, ring-like deposit around the periphery of droplets when the solvent evaporates. Overall, it is important to note that the coffee ring effect contributes to localized mechanical stresses and inhomogeneities in material deposition, which can act as a precursor to crack formation in films, coatings, or other materials that undergo evaporation or drying processes [64]. Experimental observations show that at lower substrate temperatures (e.g., 25 °C), cracks in the deposit tend to be irregular and branched. In contrast, at higher substrate temperatures (e.g., 45 °C and above), the cracks become more regular and aligned, with a noticeable increase in their ordering and periodicity. The key mechanism behind this transition is the thermal gradient induced by the increased substrate temperature, which drives Marangoni flow, a flow of fluid caused by surface tension differences due to temperature variations [72]. At temperatures below 45 °C (e.g., 25 to 34 °C), the drying process is primarily driven by capillary flow, which pushes the solvent outward, resulting in irregular crack patterns due to steep gradients in the deposit height. As the temperature increases between 34 °C and 50 °C, a combination of capillary and Marangoni flows occurs. The latter, induced by thermal gradients, causes inward fluid movement that forms a “coffee-eye” pattern, where a central stain appears. At higher temperatures (50 °C to 70 °C), the Marangoni flow becomes more dominant, directing the particle deposition toward the center and reducing the number of crack branches. This shift leads to more uniform crack spacing and a clearer transition from disordered to ordered crack morphology. Additionally, the annular region of the deposit extends further inward, and the central stain grows wider with increasing temperature. These findings indicate that temperature influences both the crack formation and the overall distribution of the particulate deposit.

In another work, the use of shock drying has been exploited in order to obtain crack formation on an egg white sacrificial template [49]. In the process of peeling the perimeter of cracked cells (a strategy used to increase the thickness of the metal sputtered on the cracked template), two critical parameters influence the outcome, the moistening time and the shock drying conditions, alternated in the cracking phase of the egg white. The moistening time, using saturated water vapor, was experimentally optimized, with values of 3, 5, and 7 s tested. The optimal moistening time was found to be 3 s, as longer times led to over-moistening, causing the merging of cracks and disrupting the fracture network. The second important parameter is the shock drying temperature, in the temperature range of 100–140 °C, eventually leading to the choice of 120 °C for 5 s. The temperature needed to be sufficiently high to induce maximum mechanical stresses in each individual cell of the cracked template, but not so high as to harden the egg white layer, as this would have complicated its removal (in the washing steps) during the process of obtaining the mesh transparent conductor. The shock drying process was, in this case, crucial for creating the right conditions for crack formation and template peeling.

Lastly, the fabrication of a Pd-based H₂ gas sensor has exploited a self-cracked WO₃ thin film as a template, where thermal stress induced during deposition creates nanogaps at the crack edges (Figure 7) [11]. WO₃ is deposited onto a flexible PDMS substrate through e-beam evaporation, and the cracks form due to the thermal stress generated by the differing thermal expansion coefficients (TECs) and elastic moduli of PDMS and WO₃. The thermal stress is calculated using Equation (2):

$$\mathsf{\Sigma }=\alpha \times \mathrm{E}\times \mathsf{\Delta }\mathrm{T}$$

(2)

where α is the TEC, E is the elastic modulus, and ΔT is the temperature change. During deposition, the WO₃ film experiences tensile stress up to 1400 times greater than that of PDMS, which, upon cooling, leads to compressive stress and crack formation. Comparative studies with Al₂O₃ and SiO₂ showed that WO₃ generates the highest thermal stress, with Al₂O₃ cracking but with a lower crack density. Because the stress was applied uniformly across the entire deposited area, the formation of gaps was highly consistent. This means that a uniform self-cracked template can be created regardless of the sample size, making the process particularly beneficial for large-scale and mass production.

4.4. Mechanical Stretching

Controlled tensile or compressive forces applied to thin films create stress concentrations that may result in crack formation. Investigating how cracks propagate in thin metal films under applied strain is important, particularly for adjusting the conductivity in flexible thermoelectric sensors [73] or in flexible gold electrodes [37]. Film thickness and loading ratio also significantly influence crack patterns and fracture strain, with thicker films exhibiting different behaviors under equibiaxial and uniaxial strain for nickel thin films [18].

Periodic crack patterns can be obtained through bending processes. The study of Lim and colleagues presents a novel approach for creating well-aligned gold nanoparticle (Au-NP) arrays on liquid PDMS substrates, cracked by oxygen plasma and subsequent bending (Figure 8a) [12]. This technique involves applying stepwise stress with an orientation angle, as well as point stress, and controlling the liquid PDMS thickness. Various patterns, including periodically aligned, mesh-type, concentric, and wide/narrow Au-NP arrays, were fabricated without the need for additional reducing or stabilizing agents (Figure 8b). The method relies on oxygen plasma treatment to create high-reducing-capacity regions on PDMS and induce cracks, which serve as templates for Au-NP formation.

Localized cracks on vertically aligned golden nanowires (V-AuNWs) embedded in PDMS films can be generated after applying a repeated strain within 10 cycles using a uniaxial moving stage [24,74]. This approach allows for the creation of programmable cracks on flexible, integrated patterns, which can be used to develop wearable sensors potentially sensing physiological signals, such as pressure, strain, glucose, and lactate levels, with potential applications expanding to soft robotics and digital healthcare in the future.

4.5. UV Curing

UV-curable resins are widely used in additive manufacturing, particularly stereolithography (SLA) [75]. In ceramic SLA applications, resin composition plays a crucial role in defect formation during the debinding process. Adding non-reactive components to the photocurable resin can reduce polymerization shrinkage and alter thermal decomposition, leading to fewer delamination and intra-laminar cracks, if not deliberately needed [76].

The study of Yoon and colleagues explores the role of UV curing in the formation of cracks within a polymer layer inside anodic aluminum oxide (AAO) pores, which can be used to create smaller nanopores [65]. The template was exposed to UV light at a wavelength of 365 nm for 90 s. After curing, the front surface showed that most pores were blocked by the cured polymer, while many pores on the rear surface remained empty. The curing process also generated cracks along the inner walls of the pores, indicating that the adhesion between the polymer and AAO surface was insufficient to withstand the volume shrinkage during curing. Additionally, the crack geometry varied: without plasma treatment, cracks formed along the perimeter of the pores, but with improved adhesion (after plasma treatment), cracks appeared more arbitrarily within the pore. Future research should therefore focus on these parameters to better control the final outcomes.

Another study examines the tensile fracture behavior and properties of epoxy composite films reinforced with hollow and solid polymer microspheres (HMs and SMs, respectively), focusing on the role of UV curing in enhancing interfacial adhesion [77]. It was found that strong interfacial adhesion between the polymer microspheres and the epoxy matrix is achieved through chemical bonding between the epoxy groups on the microspheres and the epoxy matrix via cationic UV-induced photopolymerization. The results show that HMs provide better tensile properties than SMs due to stronger interfacial bonding, highlighting the potential of HMs/epoxy syntactic foams for applications in electrical and optical areas.

5. Process Parameters Governing Tunable Features of Crack Patterns

Crack formation in thin films and coatings, depending on the materials investigated (e.g., bio-based, ceramic, hybrid), is influenced by a range of process parameters that enable precise control over their density, width, shape, and orientation. Key factors described below include the drying rate and temperature, thickness, wetting–drying cycles, solvents, and interactions between the substrate and the film. Additionally, particle shape anisotropy and nematic orientation in liquid crystal-based systems offer advanced strategies for aligning and structuring cracks. It is evident that isolating the contribution of a single parameter is not feasible, as these parameters invariably interact and exert their influence in combination. The impact of the parameters discussed on crack formation across various materials is summarized in

Table 2. Summarized table of the influence of discussed parameters on crack formation.

Parameter	Materials	Effect on Crack Formation	References
Drying Temperature	- SiO2 films - TiO2 films	Higher temperatures: ordered, aligned cracks due to Marangoni flow. Lower temperatures: irregular, finer cracks due to capillary-driven flow.	[14,64]
Drying Rate	- SiO2 nanospheres	Faster drying: smaller fragments, near-crystalline structures. Slower drying: larger fragments, amorphous structures, reduced crack propagation.	[16]
Wetting–drying cycles	- SiO2 nanospring film	Non-connected crack networks evolve and stabilize over cycles, exhibiting a shielding effect. Growth through existing cracks and nucleation of new cracks. Crack length increases in a staircase pattern over cycles.	[78]
Layer thickness	- Metallic trilayer systems	Thicker middle layers and higher compressive stress in top layers: higher critical strain for crack initiation, lower crack density.	[79]
	- Polymeric films (PMMA, PC)	Thinner layers (~30 nm): improved stiffness and strength during thermal aging.	[80]
	- Piezoelectric thin films	Thinner films exhibit higher strength and Weibull modulus.	[81]
	- Nickel thin films	Thinner films (20–100 nm): higher strength in equibiaxial strain tests (EBST) vs. uniaxial tests (UST). Thicker films (600 nm): similar strength in EBST and UST. Straighter cracks in thinner films; sinuous cracks in thicker films.	[18]
	- Acrylic resin nanoparticles	Crack width and spacing grow linearly with layer thickness, critical threshold at ~700 nm for interconnected patterns.	[27]
	- Chromium	Thinner films (100 nm): early, widespread, and branched cracks even at low strain. Thicker films (200 nm): delayed crack initiation, fewer but deeper and more linear cracks.	[82]
Solvents	- SiO2, TiO2 ethanolic suspensions	Addition of ethyl acetate to ethanolic suspensions promotes uniform crack patterns.	[14,22]
	- Acrylic resin	Water + IPA mixtures enhance crack uniformity.	[83]
	- Epoxy resin	Addition of DMSO to prepolymer enhances crack generation via volume shrinkage during curing.	[65]
Substrate Wettability	- Si - SiO2/Si	Hydrophobic substrates show slower crack propagation and higher stress resistance compared to hydrophilic substrates.	[19]
Particle Size	- Kaolinite clay	Larger particles reduce crack density but increase crack width and length.	[84]
Surface Charge of Colloids	- Colloidal SiO2	Positively charged colloids: wider spacing and faster growth. Negatively charged colloids: narrower spacing, slower growth.	[85]
Substrate Hardness	- Silicon resin	Softer substrates: lower critical thickness, more cracking is produced. Harder substrates: films remain crack-free.	[10]
Particle Shape Anisotropy	- Hematite ellipsoids	Isotropic particles: radial cracks. Anisotropic particles: azimuthal (circular) cracks due to alignment of ellipsoids’ major axis with contact line.	[86]

.

5.1. Drying Rate and Temperature

As previously discussed, temperature significantly influences crack patterns during the drying phase. However, this effect is evidently dependent on the type of substrate selected.

For example, in the case of aqueous dispersions of silica, Lama et al. highlighted how, at lower temperatures, cracks are irregular and branched due to capillary-driven flow, while higher temperatures (above 45 °C) enhance Marangoni flow, resulting in more ordered, aligned cracks with uniform spacing as thermal gradients drive particle deposition inward [64].

Piroird and colleagues also investigated the mechanisms of crack formation in silica nanosphere thin films, highlighting the impact of drying rate on this process [16]. The drying rate (modulated by regulating the relative humidity) influences the fragmentation and nanostructure arrangement of the nanospheres: in particular, fast drying (RH = 10%) produces numerous small fragments and a near-crystalline nanoscale structure, while slow drying (RH = 95%) results in fewer, larger fragments and an amorphous structure. Introducing a long holding period during drying favors particle agglomeration, altering the nanostructure to an amorphous form and yielding larger, more fracture-resistant macroscopic crack patterns compared to continuous rapid drying.

The cracking behavior at lower drying rates has also been an object of interest for TiO₂ films [14]. Interestingly, in this case, the width of crackles and the areas of polygons in the network decrease with lower drying temperatures due to slower solvent evaporation, which affects stress release in the film. At lower temperatures, the network features finer crackles (<1 μm), with maximum crackle width reducing linearly from 37 μm at 328 K to 18 μm at 223 K (Figure 9).

The impact of substrate temperature on crack morphology and particle arrangement during drying exhibits distinct patterns, which are also affected by particle shape—a topic that will be explored later in this review. For shape-anisotropic hematite particles (alpha-Fe₂O₃ ellipsoids with an aspect ratio > 2.0), lower substrate temperatures (25–34 °C) produce periodic circular cracks, with particles near the cracks aligning their major axes parallel to the crack direction. However, at higher temperatures (40–50 °C), the cracks become fragmented and disordered, with surface particle alignment diminishing, though bulk ordering along crack surfaces remains. This transition is attributed to increased thermal energy, which enhances randomness, and reduced drying time. Conversely, for shape-isotropic hematite particles (spheres with aspect ratios ~1.0–1.3), low temperatures yield disordered, interconnected crack networks with dense crack formation near droplet edges. In contrast, higher temperatures result in highly ordered radial cracks, with improved periodicity despite a loss of particle ordering. Additionally, applying a magnetic field during drying counteracts thermal randomization, restoring particle alignment and periodic crack morphology even at elevated temperatures [17].

5.2. Wetting–Drying Cycles

Research on wetting–drying cycles reveals significant impacts on crack patterns in thin films and soils, leading to different outcomes depending on the considered material (in most cases, colloidal suspensions of acrylic resin particles or silica nanospheres). For example, in helical nanostructured coatings, which are films made of an assembly of silica nanosprings, non-connected crack networks evolve and stabilize over repeated cycles, exhibiting a shielding effect [78]. In nanospring films, cracks progressively extend with each cycle, primarily through the growth of existing cracks and the nucleation of new cracks at sufficient distances from the existing network. Crack growth tends to stabilize once local energy is insufficient to drive further propagation. The total crack length increases in a staircase-like pattern over successive cycles, while the network topology evolves with an increasing number of dead-end cracks and low junction connectivity. After approximately ten cycles, the network stabilizes as crack growth ceases. This behavior contrasts with brittle films, where cracks rapidly invade the film and lead to partial delamination. Nanospring films avoid delamination due to their compliant nature, resulting in localized crack propagation and dead-end formation. This phenomenon is more prevalent in materials with low elastic energy, such as thin films of nanosilica or nanoclay, where cracks form open networks with dead ends. Crack propagation is energetically unfavorable in such compliant systems, leading to distinct patterns and stabilization over repeated stresses.

Repeated wetting and drying cycles play a critical role in enhancing the electrical transport properties of Cu networks by promoting the rearrangement of acrylic resin particles and improving crack formation [26]. The process is shown in Figure 10. Quantitative analysis of the current-effective backbone within these networks reveals that, prior to drying cycles, the backbone region, arising from interconnected and non-interrupting cracks, accounts for only 63% of the network. However, after 20 drying cycles, this value increases to 82%, accompanied by a 150% increase in total backbone length. Initially, the backbone is highly non-uniform, with a sparse distribution of conductive pathways. Over successive wetting and drying cycles, the network undergoes a significant rearrangement of particles, leading to a more uniform distribution of conductive wires. This improved wire density enhances the current distribution and reduces the sheet resistance, which is crucial for the performance of TCE networks.

5.3. Layer Thickness

The impact of layer thickness on crack behavior in thin films is significant across various materials and applications. To measure the thickness of crack templates, common techniques include optical profilometry (non-contact, high-resolution light interference) [58], stylus profilometry (mechanical surface scanning) [87], atomic force microscopy (nanometer-scale probing) [49], and ellipsometry (polarization-based thickness analysis) [15]. Similarly, microscopy is commonly used to measure crack width, providing precise dimensional analysis. To illustrate the influence of layer thickness, metallic trilayer systems composed of Mo, Cu, and Cr were analyzed. Thicker middle layers, combined with higher compressive stress in the top layer, were found to increase the critical strain required for crack initiation and reduce the maximum crack density [79]. For polymeric films based on PMMA and polycarbonate (PC), decreasing the layer thickness to ~30 nm improves stiffness and strength stability during thermal aging [80]. In piezoelectric stacks containing Pb(Zr_0.52Ti_0.48)O₃ (PZT) of varying thicknesses grown on Si wafers (coated with thin LaNiO₃/SiO₂ layers), thinner films exhibit higher characteristic strength and Weibull modulus, with fracture initiating in the piezoelectric layer before propagating through other layers (Figure 11) [81].

Godard and colleagues [18] analyzed the stress–strain behavior in metal (in this case, nickel) thin films of different thicknesses (20, 100, and 600 nm) under uniaxial (UST) and equibiaxial (EBST) strain tests. The results showed that thinner films (20 and 100 nm) exhibited higher strength in EBST compared to UST, while the thicker 600 nm film displayed similar strength in both tests. Crack propagation was observed to occur at higher strain levels for thinner films, with the fracture strain in the thinnest films (20 nm) being higher in EBST than in UST. Cracking behavior in thinner films led to straighter cracks, while thicker films exhibited more sinuous cracks, particularly in UST. The crack spacing was found to be approximately proportional to the film thickness, with a mean fragment length of 125 times the sample thickness. This relationship held for both parts of the stress–strain curve and across different strain tests, suggesting that crack density is closely related to film thickness.

Focusing on hybrid materials, the cracking process of PVP-ZnO layers was examined by Melnychenko et al., by varying the mix ratio of PVP to ZnO while keeping the PVP amount constant. The amount of added ZnO was gradually increased in 100 μL increments. For a 100 μL addition of ZnO, no cracking was observed. At 200 μL, some domain formation occurred, but the layer remained intact. Cracking began to appear with the 300 μL addition, though the spaces between domains were still not visible. A satisfactory cracking effect was achieved at 400 μL, where distinct spaces between domains allowed for the entry of metallic material during evaporation. Based on these results, the optimal PVP to ZnO ratio was determined to be 5:1 [56].

The behavior of crack formation in organic polymers was investigated by Rao and colleagues, optimizing various parameters in order to obtain different thicknesses of both cracks and subsequently formed metal wires [27]. As the layer thickness of the acrylic resin (crack precursor, CP) increases, both the crack width and the spacing (cell size) grow linearly, with a critical thickness of 700 nm, beyond which well-defined cracks appear and interconnectivity becomes present. For thicknesses up to 700 nm, no significant cracks are observed, but above this threshold, crack patterns with increasing widths and spacings are formed. The work also details the characteristics of various TCEs fabricated using crack templating on different substrates, emphasizing the variation in metal wire thickness and their associated properties. The wire thickness ranges from 90 nm in AgCP on quartz to 720 nm in Cu-PdCP on PET. Notably, thinner wires (such as those made from AgCP on glass) exhibit lower sheet resistance and higher specular transmittance compared to thicker wires, like those from Cu-PdCP on PET, which have higher resistance and lower transmittance.

A simple, scalable method to generate parallel crack patterns, again in a drying acrylic resin film, is presented, which exploits a thickness gradient formed through the gravity-driven flow of a colloidal dispersion on an inclined substrate [39]. The gravity flow creates the thickness gradient, while the drying front governs the crack orientations. Experimental investigation reveals that the dried film thickness decreases as the inclination angle increases, due to the increased dragging force of gravity. Additionally, both crack width and periodicity are observed to increase linearly with the film thickness, a behavior consistent with desiccating cracks, where stress decreases with increasing film thickness. The inclination angle is shown to control the film thickness, offering a way to achieve desired crack patterns.

Lastly, the study of Alkhazali et al. examines the mechanical degradation of chromium (Cr) thin films on polyethylene terephthalate (PET) substrates for flexible electronic applications. Cracking is induced by tensile strain applied at a constant speed of 1 mm/s, focusing on film thickness (100 nm vs. 200 nm) and strain levels (3.75%, 5%, and 6.25%). In this case, thinner films (100 nm) develop early, widespread, and branched cracks even at low strain (3.75%), which intensify with increased strain, leading to rapid mechanical and electrical failure. In contrast, thicker films (200 nm) delay crack initiation, forming fewer but deeper and more linear cracks, which become more severe at higher strains. A combined study of multiple parameters provides a better understanding of the cracking phenomenon, helping to optimize the design of electronic components [82].

5.4. Solvents

The impact of the solvent is critical in the cracking process, as it predominantly governs the evaporation dynamics, which directly influence crack formation and propagation. Solvent evaporation plays a significant role, particularly in techniques involving solution-based deposition methods, such as those utilizing colloidal suspensions of polymeric or ceramic materials.

In the case of SiO₂ and TiO₂ colloidal suspensions, as reported by Kiruthika and colleagues, the addition of ethyl acetate improves both spreading and, consequently, uniform crack pattern formation with reduced film thickness due to its higher evaporation rate and lower viscosity and surface tension compared to ethanol [14,22].

Solvent mixtures, such as water and isopropyl alcohol (IPA) near azeotropic composition, can lead to narrower cracks in colloidal films due to unique evaporation and wetting properties [83]. One study explores the critical role of solvent composition in forming crack templates using an acrylic resin-based colloidal dispersion. Solvents like water and IPA alone exhibit limited crack formation, with water leading to radial cracks and IPA forming unconnected patterns. However, mixtures of water and IPA, particularly at the azeotropic ratio of 15:85, demonstrate superior performance (Figure 12). This composition promotes high wettability, efficient evaporation, and optimal adhesion, resulting in a uniform distribution of fine, interconnected cracks with minimum crack width (~400 nm) and spacing (~15 μm), the lowest values reported for desiccating cracks. The exceptional properties of the 15:85 mixture arise from its low boiling point, fast evaporation, and reduced surface tension, which enhance wetting and stress-induced crack density. In contrast, IPA-rich compositions (>90%) suffer from particle agglomeration and poor dispersion stability, while pure solvents lack the combined benefits of the azeotropic mixture. The optimized templates, when used in TCE devices, demonstrate reduced wire width and spacing compared to templates formed with commercial diluters, highlighting the practical benefits of using carefully tuned solvent mixtures.

The study of Yoon and co-workers investigates the addition of DMSO into a UV-curable epoxy resin to enhance crack generation by increasing volume shrinkage during curing [65]. DMSO is selected for its excellent wettability on anodic aluminum oxide (AAO) surfaces. Solutions with varying resin-to-solvent ratios (70%, 50%, and 30% resin) were prepared, with DMSO’s properties closely matching those of the resin, ensuring minimal variability in final results. The close match in molecular weights and other properties between DMSO and the resin allows for flexible solvent usage without significantly affecting the uniformity of the resulting cavities. This approach provides a robust method for achieving controlled and uniform crack templates through solvent-induced volume shrinkage.

In another example of epoxy coatings on a rigid glass substrate, this time in a different solvent, residual solvent initially reduces crack susceptibility through plasticization but can ultimately contribute to increased internal stress and crack probability as it evaporates during curing [88]. This study investigates crack formation induced by internal stress in a solvent-based epoxy/para amino di-cyclohexyl methane (PACM) coating, focusing on crack susceptibility and morphology. The residual solvent plays a double role, initially reducing crack susceptibility due to plasticization but later promoting high reactant conversion. Upon solvent evaporation, significant internal stress develops, increasing the likelihood of crack initiation and growth. Moreover, elastic modulus trends reveal that higher solvent content delays stress-induced defect formation, with samples containing 25% solvent showing a dominant plasticizing effect and lower final modulus compared to those with 15% solvent. These findings highlight that solvent introduction can modulate coating properties to reduce stress and cracking, but long-term evaluations are still needed.

5.5. Substrate Interactions and Surface Properties

The role of substrate and surface interactions in determining crack formation in colloidal films has been extensively examined. For example, substrate wettability is a critical factor to be considered [19].

A colloidal nanosuspension of polystyrene (PS) nanoparticles has been studied on glass substrate treated differently, in order to adjust hydrophobicity: plasma-treated glass was considered hydrophilic, simply cleaned glass was considered intermediate, and PDMS-coated glass was considered hydrophobic (contact angles of 96°, 36°, and 5°, respectively). The higher the hydrophobicity, the lower the evaporation rate, due to the lower contact between the surface and the suspension. The crack propagation velocity is reduced from 300 μm/s for the hydrophilic surface to 25 μm/s for the hydrophobic surface. Moreover, the critical intensity factor, which indicates the stress-bearing capacity of the film, is significantly higher for hydrophobic substrates, enabling these films to resist crack formation under greater stress. Thus, hydrophobic substrates lead to greater stress thresholds and more robust film stability, while hydrophilic substrates reduce stress tolerance.

The nature of substrate–particle interactions further influences the level of order in the cracked pattern: in the case of sessile drops of DNA-coated gold nanoparticles (Figure 13), hydrophilic Si substrates are associated with the development of ordered stripe patterns, whereas hydrophobic SiO₂/Si substrates tend to produce coffee-ring-like, radial cracks, which supports the findings of Ghosh and colleagues [19,89].

The particle size also plays a pivotal role: larger particles reduce crack density while increasing crack width and length in clay layer desiccation [84]. Additionally, the surface charge of colloids affects cracking behavior, with positively charged colloids displaying wider crack spacing and accelerated crack growth rate compared to negatively charged colloids, with the disposition of particles strongly influenced by the particle–particle and substrate–particle interaction [85].

The drying dynamics of a silicon resin on different substrates has also been evaluated by Tomar and co-workers, particularly on glass, acrylic substrates, LDPE, and PDMS. Interestingly, softer substrates led to a decrease in critical thickness, as the ability to absorb stress is reduced, while harder substrates allow thicker films to remain crack-free [10].

Another key factor to keep in mind is particle shape anisotropy, which typically allows crack formation along one specific direction. When sessile drops with isotropic particles evaporate, they typically form radially oriented cracks. However, when the colloidal particles are ellipsoidal, the cracks tend to align azimuthally (forming circular cracks). This is because ellipsoids align their major axis parallel to the three-phase contact line of the pinned sessile drop [86].

It has been demonstrated that the process of droplet evaporation and the formation of ringlike particulate deposits is similar for both isotropic and anisotropic particles at various substrate temperatures. However, at temperatures above 40 °C, the crack patterns for these particles differ significantly. For shape-isotropic particles, there is an increased order in crack formation, while for shape-anisotropic particles, there is a progressive disorder, resulting in smaller, fragmented cracks as the temperature rises [17]. Programmed crack patterns for liquid crystal polymer networks have already been exploited to fabricate anisotropic, transparent conductors, with cracks filled with AgNP ink [90].

6. Applications of Crack-Templated Patterns

Crack-templated patterns have garnered significant attention over the years due to their versatility and unique structural properties. The ability to fill cracks with materials such as metals (or other substances) to create functional meshes further extends the utility of these patterns, allowing for the development of optoelectronic devices, where they serve as templates for fabricating conductive networks, touch screens, and EMI-shielding devices (whose main features are summarized in

Table 3. Summary of crack template and mesh optical/electrical properties for electronic device applications.

Crack Template Material	Mesh Material	Thicknesses	Transmittance	Resistance	Applications	Ref.
Acrylic emulsion	Cu	1.7–3.5 μm (Cu mesh)	85.8% (550 nm)	0.18–0.53 Ω/sq	EMI shielding and EM-compatible optoelectronic devices	[91]
Crackle precursor	Au–Cu	5 nm (Au) 650 nm (Cu) ~10 μm (crack width)	~88%	~5 Ω/sq	Microfluidic devices, capillary cooling systems, optoelectronics	[92]
Egg white	Ag	~130 μm (cracks) 3–10 μm (Ag network) 15 μm (Ag-Re micro-nanosheets)	~88.1%	~9 Ω/sq	Flexible display technologies, lighting, sensing	[93]
Acrylic emulsion (CA-600)	Ag	~1.8 μm (Ag) <5 μm widths, 5 μm depths (cracks)	91%–93%	0.54–1.4 Ω/sq	Transparent electrodes and EMI shielding	[94]
Silicate synthetic clay (Laponite), pH values: 7.8–12.7	Graphite	Few μm	Peak: ~80% (540 nm);	10–50 kΩ	Solar cells, touch screens, transparent heaters	[95]
Low-cost crackle precursor	Al_SnO2_WO3	530 nm (WO3)	Filters at 400 nm (pH 8.7, 12.2, 12.5); transparent all λ at other pH values: ~88% (decreasing with applied voltage)	5 Ω/sq	Smart windows	[96]
PMMA (30% wt) on CdTe/CuGaOx	Au	50 nm (Au) 3 nm (CuGaOx) 2 μm (wire width)	~77%	8.3 Ω/sq	Photovoltaics	[97]
Egg white	Ag	2.5 μm (Ag) 4 nm (PMMA spacer in sandwich structure)	~89%	0.28–1.59 Ω/sq	EMI shielding	[49]
ZnO + PVP on quartz, sapphire or glass	Ag	200 nm (Ag) 6 μm (crack layer)	~83% (550 nm) (quartz > sapphire and glass)	Quartz = 13.55 Ω/sq; sapphire = 10.45 Ω/sq; glass = 13.65 Ω/sq	Encryption, solar cells, touch panels, light-emitting diodes, transparent heaters	[56]
Acrylic resin	Al-SnO2	<30 μm	85% (Al-SnO2) 92% (Al)	5 Ω/sq	Smart windows	[98]
Egg white	Cu-Ag, Ni-Ag	1.92 μm (Cu) 0.95 μm (Ni) 200 nm (Ag)	82%–88% (Cu-Ag) 78%–87% (Ni-Ag) (550 nm)	0.06–1.52 Ω/sq (Cu-Ag); 0.7–9.3 Ω/sq (Ni-Ag)	EMI shielding	[50]
TiO2	WO3/Ag	30 nm (WO3) 3–10 μm (crack width)	81%	1.36 Ω/sq	Smart windows	[25]
Crackle precursor	Al-SnO2	~400 nm (Al) ~200 nm (SnO2)	~83% (550 nm)	5.5 Ω/sq	Optoelectronics and photovoltaics	[99]
TiO2, PMMA, or PS	Cu	3–45 μm (PS and PMMA); 25–140 μm (TiO2); Cu = 1/3 of template thickness	80%–90% (TiO2) >90% (PS and PMMA) (450–890 nm)	≥100 Ω/sq	Photovoltaics	[54]
Egg white	Ag	200–600 nm (Ag)	84%–91% (550 nm)	1.6–21 Ω/sq	Transparent heaters (e.g., anti-fogging and anti-icing coatings)	[51]
Acrylic emulsion	Cu	1 μm (Cu)	~85%	0.83 Ω/sq	EMI shielding, Joule heater	[100]
Nail polish + polyimide (PI)	Ag	100–400 nm (Ag)	~85%	1.5 Ω/sq	Photovoltaics	[101]
Sputtered Au under tensile stretching	Au	120 nm (Au)	n. a.	~205 Ω (V-AuNWs/PDMS thin film) ~7100–7300 Ω (V-AuNWs-based sensor)	Wearable electronic tattoos (e.g., glucose and lactate sensors)	[24]
Acrylic resin	Cu	80 nm (Cu) ~400 nm (crack width) ~15 μm (crack spacing)	~87.5%	10 Ω/sq	Optoelectronic applications (e.g., solar cells, touch screens, transparent heaters)	[83]
Nail polish	Au	~500 nm (Au)	>90%	100 Ω/sq	Cosmetically adaptable devices, wearable strain sensors	[23]
Crackle finish paint	Au	n.a.	~60%	~4 Ω/sq	Enzymeless glucose sensors	[102]
Nail polish/egg white	Ag	100 nm (Ag electroless deposition) 2.5 μm (Ag electroplated)	80%–82% (pre- and post-electroplating)	0.008–0.01 Ω/sq	Transparent conductor for, e.g., OLEDs, thin film solar cells, and EMI shielding	[103]
Strain application onto Pt layer on polyurethane (PU) membrane	Pt	~10 nm (Pt) 100 μm (PU membrane)	~50% (400–800 nm)	Under 50% strain: R1 (islands) = 4.94 kΩ R2 (bridges of two adjacent islands) = 2.96 kΩ/%	Strain sensors, whole-body motion monitoring	[104]
Acrylic resin	Cu	~1.7 μm (film) ~700 nm (crack width)	88.2%	31 Ω/sq	Touch screens, solar cells	[26]
Egg white	Ag, Cu	60–6000 nm (after electroplating)	60%–95%	0.03–3 Ω/sq	LED lighting, solar cells	[52]
High-stress thin silicon nitride (Si3N4)	Ag	40 nm (AgNW diameter) 1.65–2.05 μm (Si3N4 film)	85%	92 Ω/sq (highest crack density)	Touch screens	[105]
Acrylic resin (CP1), SiO2 (CP2)	Ag	80 nm (Ag) 0.5–30 μm (CP1) 24–150 μm (CP2)	86% 70%	6 Ω/sq (CP1) 2 Ω/sq (CP2)	Transparent heaters, curved surfaces	[58]
Acrylic emulsion (CP2), SiO2 (CP1) + Pd/Au	Cu	300 nm (Cu) 50–100 μm (CP1 width) 4–8 μm (CP2 width)	67%–75%	<5 Ω/sq	Large-area applications (e.g., large panel displays), touch screens and solar cells	[22]
TiO2	Au	~100 nm (Au) ~10 μm (TiO2)	~82%	3–6 Ω/sq	Optoelectronics	[14]
Acrylic resin	Au, Cu, Ag, Pd, Al, Zn	90–700 nm (metal wire) 1–4 μm (film)	78%–88%	1.6–52 Ω/sq	Transparent, flexible heaters	[27]

n. a. Not available.

). Furthermore, crackled patterns can find applications in biotechnology, enabling the development of microfluidic devices, wearable medical devices, and cell scaffolds. Additionally, their role in advanced material design, such as hierarchical porous structures for energy storage, gas sensing, and filtration membranes, highlights the transformative potential of crack-templated structures in addressing technological and industrial challenges.

6.1. Sustainability

The use of crack templates to produce transparent conductive electrodes (TCEs) offers several advantages over traditional methods, particularly in terms of sustainability and resource efficiency. Crack templating, being itself a spontaneous process, involves the formation of cracks in thin films during the drying or solidification of deposited materials, contrasting with conventional electrode fabrication processes that often require energy-intensive steps, complex equipment, and the use of large quantities of solvents or chemicals [106]. This reduces the energy consumption and operational costs associated with more complex fabrication techniques, while at the same time lowering the environmental impact and simplifying waste disposal [93,107].

Furthermore, the materials utilized in crack templating are typically more abundant, cost-effective, and less toxic compared to those required in conventional methods. Common materials such as polymers and metals, which are both sustainable and easier to source, can be employed. The process also operates at lower temperatures, further reducing energy demand and associated costs [106].

The sustainability of the crack template process is increasingly vital in addressing environmental challenges, especially those posed by the production of TCEs. Traditional methods often generate metallic micro-nanosheets (MMNSs)—2D materials with a sheet-like morphology and nanometer-scale thicknesses that can extend to micron-scale dimensions—as waste. Some of these methods render MMNSs unrecyclable, contributing to substantial environmental pollution. Yan et al. demonstrated an innovative approach by recycling Ag micro-nanosheets (Ag–Re MMNSs) to create a conductive Ag paste [93]. This material offers advantages such as adjustable sheet width, higher surface area ratio, and compatibility with solution processing, setting a new standard for eco-friendly practices in the production of stretchable conductive pastes and TCEs. Additionally, the study by Lim et al. [12] supports sustainability efforts by introducing a simple crack template reduction lithography method to fabricate gold nanoparticle arrays without relying on additional reducing agents or conventional MEMS processes.

6.2. Transparent Conductive Electrodes (TCEs)

By incorporating metals or other materials into the crack patterns formed in drying colloidal films, researchers can produce highly conductive pathways while maintaining transparency (Figure 14). This approach leverages the natural crack formation to create intricate, conductive networks without compromising the optical clarity of the electrode. Such electrodes could have significant applications, among others, in flexible electronics and optoelectronic devices.

In 2014, Kiruthika and co-workers exploited crack templates to form metal wire networks as new-generation electrodes. In one work, crack templates were prepared by depositing a dispersion of SiO₂ nanoparticles in acrylic resin on PET flexible substrates. Then, electroplating and electroless deposition were employed to fill the cracks with Cu at the desired thickness, with Pd or Au nanoparticles serving as seeds. Two length scales of Cu wire networks were produced for different applications: broader networks for large panel displays and finer networks for touch screens and solar cells. The resulting Cu TCEs exhibit high transmittance (67%–75%) and low sheet resistance (<5 Ω/sq), with up to 90% transmittance achievable at 141 Ω/sq. These TCEs are large-area (A4 size), highly interconnected, and stable against oxidation and temperature, and remain flexible without significant changes in resistance after multiple bending cycles. Additionally, the wettability of the TCEs can be altered from hydrophobic to hydrophilic using UV–ozone treatment without affecting resistance. This method holds significant potential for the optoelectronics industry, with future research focusing on metal-free solution processing [22]. In another work, Au metal was deposited by a physical vapor deposition system on a crackled TiO₂ template to a thickness of ~100 nm. The Au wire network-based TCEs on glass, quartz, and PET exhibit a transmittance of approximately 82% across a wide spectral range, extending up to 1500 nm for PET and glass and 3000 nm for quartz, thanks to a metal fill factor of about 7.5%. The TCEs show sheet resistance values between 3 and 6 Ω/sq, comparable to or better than ITO films and other nanowire or nanotube-based TCEs. PET substrates used for flexibility tests showed less than 1% temporary change in resistance when subjected to adhesion and flexibility tests, including bending, crumbling, rolling, and twisting, indicating robust performance: the Au wire networks maintained stability and high flexibility under aggressive handling [14].

The combination of electroless deposition and subsequent electroplating was later used to enhance the thickness of Ag mesh [103]. In this case, electroplating both increases conductivity and reduces sample roughness.

More recently, Yan and co-workers employed a “kill-two-birds-with-one-stone” strategy using cracking lithography to create two types of flexible conductors. First, the morphology of flexible nano-textured conductors (f-NTCs) is controlled through a wetting–drying cycling process, achieving 88.1% transmittance and 8 Ω/sq sheet resistance after Ag deposition via magnetron sputtering technology. A flexible electroluminescent device using f-NTCs later demonstrated a luminescence intensity of 819 lx. Additionally, residual Ag micro-nanosheets removed with the template are collected, cleaned via ultrasonication treatment, and used to develop a stretchable conductive Ag paste with excellent mechanical performance and a low percolation threshold (~13%). This Ag paste is designed to be used to fabricate a human motion sensor that aligns well with joint movements, with an innovative strategy that helps promote environmental sustainability [93].

The study of Pujar and colleagues highlights the benefits of low wire spacing in networks (ranging in multiple applications), which reduces charge carrier recombination and improves charge collection efficiency. In this context, reducing wire width from 8 μm to 400 nm increases transmittance from ~92% to ~95% despite a 40-fold higher wire density. This enhances the resolution of touch screens and reduces response time in micro-transparent heaters. For applications like solar cells, a bi-modal width distribution of metal meshes is crucial, since the narrower mesh is used for efficient charge carrier collection, while the wider one is important to achieve low sheet resistance [83].

6.3. EMI-Shielding Devices

The advent of radio-frequency electromagnetic radiation has significantly transformed communication networks. As our dependence on electronic devices and systems increases, there is a growing demand for efficient electromagnetic interference (EMI)-shielding materials that can preferably maintain transparency. In this context, the combination of crack lithography and metal meshes has become crucial.

The study of Zarei and colleagues evaluates the performance of metal mesh structures for EMI shielding across 8–18 GHz. The fabricated meshes achieved average shielding effectiveness (SE) values of 37.4 to 42.5 dB, indicating consistent and effective EMI shielding. Reflection is the primary shielding mechanism, with reflection coefficients around 0.95, while absorption plays a secondary role with coefficients around 0.05. The study highlights the importance of small width and substantial thickness in metal meshes for enhancing EMI shielding efficiency. This method achieved wire widths under 5 μm, demonstrating superior performance compared to previous similar approaches [94].

In another work, a Cu mesh film was fabricated and tested (Figure 15). The Cu mesh achieved an average EMI-SE of 40.7 dB in the X-band (8–12 GHz) through both reflection and absorption mechanisms. The mesh maintains high optical transmittance (over 85%), outperforming other materials such as MXene, cellulose composites, and graphene. These results show that increasing the thickness of the Cu mesh enhances its EMI SE, albeit with a slight reduction in transmittance. The optimal configuration, with an 18.6% coverage ratio and 800 μm² cell size, achieves the highest average EMI SE value of 40.4 dB. Additionally, the Cu mesh exhibits excellent mechanical durability, with minimal changes in SE after bending and stretching tests, proving to be a promising alternative for next-generation applications in both wearable electronics and wireless communication devices [91].

Voronin and co-workers fabricated Cu-Ag and Ni-Ag meshes using galvanic deposition of Cu and Ni (300 s) on a Ag layer. The galvanic deposition of metal on a Ag seed mesh significantly enhances the SE against electromagnetic waves. Both Cu and Ni depositions increase SE, with longer deposition times yielding stronger SE. For example, Cu-Ag mesh at 10 GHz shows SE improvements from 31.9 dB to 42.3 dB, with deposition times ranging from 15 to 300 s. Ni-Ag meshes show lower SE values compared to Cu-Ag but still improve with increased deposition time. The SE behavior is consistent across the X-band (8–12 GHz) and K-band (18–26.5 GHz), with SE values being slightly lower at higher frequencies. Comparing mesh samples with similar sheet resistances or optical transmittance reveals that Cu-Ag meshes generally achieve higher SE due to copper’s superior conductivity [50]. A two-layer sandwich structure was later proposed by the same group, with two Ag layers separated by a thin PMMA layer (4 nm), in order to enhance shielding and maintain transparency (>80%). The crack template was, in this case, characterized by localized peeling at the cell perimeters, obtained by adjusting exposure to moist and hot air, together with surface energy. It has been demonstrated that EMI shielding performance inversely correlated with sheet resistance values, reaching >70 dB. Even after 1000 bending cycles (5 mm radius), no change was observed in sheet resistance, both at 20 and 85 °C, confirming the mechanical durability of the Ag mesh [49].

Lastly, Walia and co-workers used crack lithography to create a flexible Cu mesh starting with an acrylic emulsion on a PET substrate, ultimately featuring high interconnectivity, low sheet resistance (0.83 Ω/sq), and a network-like structure, allowing it to trap microwave radiation effectively. The Cu mesh film achieved an EMI SE of approximately 49 dB at 18 GHz, maintaining 85% transparency. This high SE was achieved by increasing Cu mesh thickness and decreasing wire spacing without significantly affecting transparency. The film was also laminated for improved scratch resistance. Additionally, the multifunctional aspect was demonstrated with a flexible Joule heater, reaching about 80 °C at 5 V. The study also noted a significant attenuation of Wi-Fi signals and doorbell signals, indicating effective EMI shielding capabilities. Cu demonstrated superior SE capacities over Al and Sn and similar effectiveness to Ag, guaranteeing lower costs [100].

6.4. Smart Windows

For smart windows, these crack-templated patterns can serve as an effective means of integrating transparent heating elements or EMI shielding. By coupling these structures with advanced materials, such as thermotropic and thermochromic materials, smart windows can switch between clear and opaque states, reducing energy consumption and enhancing comfort. The conductive networks can maintain the clarity and aesthetics of the windows while providing additional functionalities such as defogging, temperature control, and protection from harmful electromagnetic radiation, making smart windows a valuable addition to modern architectural designs.

In 2020, Li and colleagues used commercial TiO₂ as a colloidal pattern material (Figure 14). It was dispersed in ethyl alcohol and ethyl acetate and then dropped on a pre-cleaned PET substrate, forming uniform crackle patterns after drying. Laser beams were used to cut wider mesoscale lines in the dried patterns, obtaining a leaf vein-like structure. A WO₃/Ag conductive network was then created by magnetron sputtering, depositing a layer of tungsten trioxide (WO₃) followed by a layer of silver (Ag). The TiO₂ pattern was later removed by ultrasonic vibration, resulting in hierarchical metal grid electrodes. The resulting flexible TCEs demonstrated a sheet resistance of 1.36 Ω/sq with 81% transmittance. These electrodes were used to fabricate high-stability flexible electrochromic devices (ECDs) for smart windows: the insertion and extraction of Li+ ions into and out of WO₃ result in reversible changes between colored and bleached states (Figure 16), and the cycling performance shows that this process has good reversibility, stability, and durability. The switching times for coloring and bleaching are 6.8 and 14.7 s, respectively, with corresponding voltages of −1.4 V for coloration and 0.2 V for bleaching. Overall, these ECDs showed good electrochemical performance, stable optical contrast, and high cycling stability, comparable to previously reported works [25].

Mondal and co-workers fabricated large-area SnO₂-coated Al mesh (Al-SnO₂) hybrid electrodes with the same aim. Au and Al meshes were fabricated using sacrificial crackle templates: Au was deposited via thermal evaporation over a spin-coated acrylic resin template and subsequently removed using chloroform, yielding interconnected Au micro-wire meshes. For Al, a large-area spray-coating method was optimized to form uniform crackle templates, followed by thermal evaporation and water-based template removal. These metal meshes were further functionalized with SnO₂ coatings: the Au mesh was coated with SnO₂ via multiple spin-coating steps and annealing, while the Al mesh utilized spray deposition of a SnCl₂ precursor at high temperatures. Al was preferred over Au for a notable reduction in device cost. The hybrid electrode demonstrated 85% transparency and a sheet resistance of approximately 5 Ω/sq. It was used to fabricate polymer-dispersed liquid crystal (PDLC) devices, enabling large-area window fabrication. The mixture for the PDLC film was sandwiched between two 10 × 10 cm² Al-SnO₂ electrodes and cured under UV light, resulting in a nearly opaque white film. Applying 30 V made the device highly transparent, with response and recovery times of 13 ms and 161 ms, respectively [98]. Subsequently, a WO₃-coated (deposited via reactive ion sputtering) Al-SnO₂ hybrid electrode was exploited as a low-cost dual-functioning electrochromic/charge storage device, for future applications in “zero-energy buildings”. The WO₃ film transitions between transparent (bleached) and opaque (dark) states are due to lithium ion intercalation/deintercalation, with a fast response recovery times of 11 and 5 s, respectively (twice as fast as ITO-based devices) [96].

6.5. Transparent Heaters

Another potential application of crack-templated patterns is the fabrication of transparent heaters, with the aim of providing efficient thermal management solutions while integrating seamlessly into transparent and flexible technologies. An analytical model has been developed to provide insights into key design parameters for these transparent heaters, in terms of geometrical parameters and material properties [108].

Gupta and co-workers were among the first employing crack-templated patterns for transparent heater fabrication on both flat and curved surfaces. Two types of crackle precursors (CPs), acrylic resin (CP1) and SiO₂ particles (CP2), were used to create crack templates with widths ranging from 2 to 250 μm and fill factors between 15% and 42%. These templates were applied to various curved and flat substrates, including tubes, lenses, and PET sheets. The resulting metal wire networks, mimicking the crack patterns, showed excellent optoelectronic properties: low sheet resistance and high transmittance (6 Ω/sq at 86% for AgCP1 and 2 Ω/sq at 70% for AgCP2). Transparent heaters were fabricated using these networks, achieving uniform heating on different substrates with low response times (<20 s), low input voltages (<5 V), and high thermal resistance (515 °C cm²/W). For example, a lens reached 60 °C under subzero conditions with a 6 V application, demonstrating exceptional defrosting performance. These heaters also have potential applications in cell culturing, biomonitoring, and chemical reactions, requiring uniform and constant heating [58].

In another study, a transparent flexible heater was created by connecting a AgCP/PET-derived TCE to an external DC power supply (Figure 17). The heater showed a gradual increase in temperature with applied voltage, reaching up to 110 °C uniformly across the TCE area. The performance of the heater was characterized by a high thermal resistance of 420 °C cm²/W, significantly higher than that of carbon nanotube-based heaters. IR images demonstrated consistent temperature maintenance even when the heater was bent. The resistance of the TCE changed minimally (<0.5 Ω) during bending tests conducted thousands of times, indicating excellent durability. The heater also exhibited high performance in adhesion, sonication, and chemical inertness tests, highlighting its robustness and potential for practical applications [27].

Voronin and colleagues successfully fabricated a transparent heater using a Ag mesh deposited on a cracked egg white template. Two concentrations of egg white were tested, specifically 1 mL/L (Template A) and 3 mL/L (Template B). Ag was sputtered onto both templates at thicknesses of 200 nm and 600 nm. Both thicknesses remained stable up to 350 °C, with only minor differences in thermal resistance values, and exhibited fragmentation at 500 °C. Template B, characterized by a higher fill factor, was slightly less transparent but offered greater uniformity, enhancing heat distribution and minimizing the occurrence of hot spots and local degradation risks. The best efficiency was observed on polyethylene/PET substrates rather than glass, due to PET’s lower specific heat capacity, which resulted in faster heating and cooling times (<25 s). Furthermore, the PET-based heaters maintained stability over 100 cycles, demonstrating their potential for practical applications [51].

6.6. Solar Cells

Crack-templated patterns have emerged as a promising approach for fabricating transparent conductive metal grids in solar cells, offering a low-cost alternative to traditional methods. This technique is particularly advantageous for organic solar cells, which can be designed to be transparent and colorful, enabling their integration into architectural elements or interior design features [109,110,111]. By combining functionality with aesthetic versatility, crack-templated patterns open new possibilities for innovative energy-harvesting solutions in both residential and commercial spaces.

The study of Muzzillo and colleagues focuses on optimizing the transparent conduction performance of solar cells using metal grids (Figure 18) onto substrates of soda–lime glass (SLG), SLG coated with Al:ZnO, and F:SnO₂. Traditional figures of merit are inadequate for these non-uniform grids, necessitating more detailed calculations. In this study, linear regression correlated crack spacing with transmittance, while grid sheet resistance and transmittance were correlated for each material. The primary method for controlling performance trade-offs was adjusting the crack template thickness: thinner templates resulted in narrower cracks and higher transmittance but also higher grid sheet resistance (TiO₂ and PMMA were used as cracking substrates). The metal thickness needed to be less than one-third of the template thickness for successful lift-off. Calculations showed that the crack-templated grids could reduce power loss for narrow solar cells (0.5–2 cm wide) and resistive semiconductors (≥100 Ω/sq), making them suitable for thin-film photovoltaic modules [54]. Subsequently, the application in bifacial photovoltaics (solar cells that can capture sunlight from both the front and rear sides of the panels) was investigated, particularly by combining a CuGaOx buffer with crack-patterned metal grids. CuGaOx, a recently developed transparent hole contact layer, provides passivation and low contact resistance. Inserting CuGaOx between CdTe and Au layers improves front illumination time-resolved photoluminescence (TRPL) lifetimes. TRPL lifetimes refer to the analysis of the decay dynamics of photoluminescence (light emission) from a material when it is illuminated from the same side where the photoluminescence is detected. Longer lifetimes typically correlate with higher-performing solar cells, leading to higher current output and indicating better material quality. Interestingly, replacing blanket Au with crack-patterned Au grids (on a PMMA layer acting as a cracked sacrificial template) further enhances TRPL lifetimes, demonstrating significant performance improvements in the solar cell structure. This technique enabled improvements in the fill factor and further protected the sample, avoiding contaminant-associated open-circuit voltage (Voc) losses, obtaining a 25% increase in bifacial power density compared to CdTe records [97].

Another study highlights the potential of a high-adhesion metallic network for improving the performance and durability of flexible photovoltaic modules, mimicking the bamboo-root ground-gripping structure. Fabricated through a low-temperature, solution-based process without high vacuum processing, the process involves spin coating, plasma etching, electroplating, and lamination. The final network on a flexible PET substrate shows significant haze, which, while unsuitable for display applications, is beneficial for flexible light sources and photovoltaic applications. The dendritic nanowire structure enhances haze (~20%), making it particularly suitable for flexible solar cells [101].

6.7. Medical Devices

Crack lithography has emerged as a promising technique for creating nanoscale patterns in biomedical devices. Researchers have developed various crack lithography techniques, ranging from random to multidirectional patterns, enabling the creation, among others, of bioassay platforms and biomimetic sensors.

Jeon and colleagues developed an omni-purpose stretchable strain (OPSS) sensor (Figure 19d) utilizing a highly dense nano-cracking structure (crack density ≈ 10⁷/m), demonstrating high sensitivity and a wide sensing range. The OPSS sensor features a straightforward design, comprising a stretchable polyurethane (PU) membrane coated with a platinum (Pt) layer deposited via magnetron sputtering, where nano-cracks develop upon the application of strain. By controlling the grain size and thickness of the sputtered Pt layer (~10 and ~3 nm, respectively), the sensor achieved high sensitivity (Gauge factor ≈ 30), a wide working range (strain up to 150%), great linearity (R² = 0.9814), and fast response time (<30 ms). The study also improved sensor performance by enhancing the adhesion between the Pt layer and the PU membrane with silane vapor treatment, effective particularly in higher-Pt-thickness regimes (>10 nm). Joint-level and skin-level monitoring tests subsequently confirmed that the sensor could monitor whole-body motions, including abduction, adduction, rotation, flexion, and extension motions. It was successfully adapted for real-time Morse code communication via fingertip and eye blinking motions, aiding paralyzed patients (which could find application in pathologies like amyotrophic lateral sclerosis). Additionally, a glove-type hand motion detector was developed, capable of measuring individual finger movements [104].

In another study, a 20 nm thick Pt layer was deposited onto a 10 μm thick cured polyurethane acrylate (PUA) layer to create a nano-cracked mechanosensor with self-healing properties. Briefly, PUA was applied to a PET film, followed by UV curing for 10 h. The Pt layer was then deposited on the PUA surface and cracks were generated by bending the film along a curvature with a 1 mm radius. Lastly, a 40 μm thick self-healing polymer (SHP) was coated onto the sensor using an automatic film applicator with a constant blading velocity of 2 mm/s at 70 °C. The SHP facilitated self-healing through low-temperature polymerization driven by hydrogen bonding between amino and carbonyl groups. The self-healing process was significantly accelerated by applying IR radiation, achieving a rate 20 times faster at 75 °C compared to room temperature. The study demonstrated that heating at 50 °C for 10 min fully restored both the cracks and the sensor’s sensitivity. Furthermore, this sensor exhibited remarkable durability, maintaining functionality for up to 1 million cycles [112].

A Au microwire network semi-embedded into a PDMS substrate was fabricated by Gupta and co-workers in order to obtain a wearable strain sensor with high sensitivity for small movements of human organs (e.g., hand gestures, eye blinking, and chewing). A crackle template is formed by spin coating a mixture of chloroform and crackle precursor (acrylic resin) onto a silicon substrate. Next, an electroless gold deposition process is performed by dipping the substrate into a gold solution, allowing Au to deposit in the interconnected crackle pattern. After cleaning the substrate to remove the sacrificial layer, a transfer process to PDMS is carried out. Uncured PDMS is poured onto the gold-deposited substrate, allowed to settle, and then heated to form a PDMS layer embedded with a gold wire network, which is later peeled off to obtain the sensor material. The developed sensor exhibits high optical transmittance (85%) and an effective strain range of 0.02%–4.5%, with a gauge factor over 10⁸. Transparency is crucial for allowing physicians to monitor underlying skin or body parts, and the Au network also recovered completely after strain release, highlighting its potential for use in medical devices [23].

Glucose sensors can significantly benefit from the potential of crack-patterned designs. In this work, nickel (Ni)-based transparent enzymeless glucose sensors were developed by combining Ni with various alkanes (C4-C16) and incorporating a Au mesh on a crackle paint substrate. For the Au mesh electrodes, the crackle paint was mixed with IPA and water, spin-coated on glass substrates, and dried to form a crack network. Au was then deposited via physical vapor deposition, and the crack template was removed with chloroform, leaving behind the Au network. The unique butyl chains in nickel butylthiolate exhibit defective conformations, leading to highly exposed Ni^II redox centers, which enhance redox activity. Ni-C4SH ink displayed a lower potential (0.55 V) and higher current density (1.06 mA), making it suitable for enhanced electrocatalytic applications. The transparent Ni-C4SH/Au mesh sensor exhibited a broad detection range for glucose concentrations (0.5–2 mM in the lower region and 2–11 mM in the higher region), outperforming the Ni-C4SH/FTO electrode used as control. The synergistic effects of the Au mesh and Ni-C4SH, due to the larger projection area, enabled the sensor to exhibit a wide linear range and a low limit of detection (LOD). These electrodes can be conveniently used as single-use chips for detecting glucose in blood serum and bodily fluids like urine and tears, in which glucose is present in low concentrations [102].

Enzymatic glucose and lactate sensors can be fabricated using crack templates, leveraging elastomer-embedded vertically aligned gold nanowires (v-AuNWs) as electronic-skin tattoos. The V-AuNWs/PDMS thin film was fabricated by spin coating PMMA and photoresist on a silicon wafer, followed by photolithography to form electrode patterns. V-AuNWs were synthesized on the patterned photoresist, and a PDMS layer was applied to transfer the AuNWs into the PDMS film. Programmable cracks were induced by sputtering gold or silver thin films and applying strain (20%) to the V-AuNWs/PDMS electrodes. For glucose and lactate sensors, carbon nanotubes were coated on the electrodes, followed by electrodeposition of Prussian blue and drop-casting of glucose or lactate oxidase. The sensors demonstrated excellent performance, with areal sensitivities of 41.3 μA mM⁻¹ cm² for glucose and 3.4 μA mM⁻¹ cm⁻² for lactate. However, the sensing performance diminished under strain up to 30% due to the potential formation of cracks and delamination in the enzymatic layer [24].

Gong and colleagues further explored V-AuNWs to develop a novel sensor for detecting acoustic stimuli that is potentially wearable and suitable for cochlear implants. In this case, cracks were generated by applying a repeated strain of for 10 cycles using a uniaxial moving stage The sensor demonstrated exceptional stability, maintaining functionality at 1% strain for up to 5000 cycles and at 50% strain for up to 1000 cycles. It was capable of detecting frequencies up to 3000 Hz, encompassing musical notes and tones. An array of sensor strips was utilized to create an artificial basilar membrane prototype. This sensor exhibited high sensitivity (ranging from 0.48 to 4.26 Pa⁻¹) and impressive frequency selectivity (319–1950 Hz), covering the critical range for human communication, which typically spans from 300 to 3500 Hz [74].

Cracked patterns could help in measuring the contractility of cultured cardiomyocytes (CMs). The sensor consists of a crack sensor that bridges Ag islands with carbon nanotubes (CNTs) embedded in PDMS, with a stable period of over 2 million cycles. The fabrication begins by preparing a glass slide with microgrooves using lithography and photoresist. The PDMS layer is then spin-coated and cured, followed by the deposition of a Ag film. CNT-PDMS strips are subsequently screen-printed onto the PDMS surface, connected by Ag electrodes, and prestretched to form even cracks. The sensor is then integrated onto a PDMS support and sealed with a glass tube. The disconnection and reconnection of Ag cracks (propagating in a zigzag direction, as previously discussed for graphene [38]) result in a high sensitivity with a gauge factor of ~108,000. The addition of CNTs significantly reduced noise and enhanced the signal-to-noise ratio. The Ag/CNT-PDMS crack sensor was integrated onto a thin microgroove PDMS film for culturing cardiomyocytes, mimicking in vivo myocardial structures (Figure 19a–c): the microgrooves enabled anisotropic growth and consistent contraction of CMs, leading to improved contractility measurements. Measurements showed that the sensor could reliably monitor CMs’ contractility, beating rate, and waveform during a 14-day culture period. Additionally, the sensor was used to measure contraction signals in vivo from a rat’s beating heart, showing excellent conformal contact and stability. The study also evaluated cardiac drug effects using verapamil and isoproterenol, commonly used cardiac drugs, demonstrating the sensor’s effectiveness in detecting dynamic changes in CMs’ contractility induced by different drug doses. Overall, the Ag/CNT-PDMS crack sensor offers a promising tool for continuous, high-sensitivity monitoring of cardiomyocyte activity and drug effects [113].

Researchers are actively working to replicate synaptic networks that emulate the complex functionalities of neurons and synapses in the human brain. The final biomedical application highlighted in this review involves the fabrication of neuromorphic devices achieved through the self-organization of hierarchical aluminum and silver micro-nanostructures. This study introduces an in-plane device architecture comprising aluminum (Al) islands, tens of micrometers wide, separated by a network of cracks (12–20 μm) filled with deposited silver (Ag) agglomerates and nanoparticles (~27 nm in diameter). An acrylic resin-based crack precursor was used to create an interconnected crack pattern on Al-coated glass substrates. The precursor was mixed with a water–IPA solution and spin-coated at 1000 rpm for 60 s, forming a uniform crack pattern. This crack template served as a mask to etch the underlying Al film using an etchant solution. After etching, the crack template was removed with chloroform, leaving behind an Al island structure with interconnected microgaps (c-Al). The final substrate was cleaned with water and IPA and dried with nitrogen. This architecture facilitates scalable fabrication of neuromorphic devices while maintaining optimal operational parameters. The device emulates biological neural networks, where the Ag agglomerates function as “neurons” and the microgaps serve as “synapses”. Under an applied electric field, conductive filaments form across the microgaps, analogous to synaptic connections. The energy consumption per synapse in the most promising configuration is approximately 1.3 fJ, comparable to biological synapses. The large Al islands in the architecture are advantageous for integrating external signals into neuromorphic circuitry and support scalability, allowing for large-area fabrication [114].

Figure 19. Effect of microgrooves on cardiomyocyte (CM) growth: (a,b) Microscopic and immunofluorescence images show CMs grown on microgrooves and flat PDMS films after 7 days (scale bars: optical images, 80 μm; immunofluorescence, 20 μm; blue dye: DAPI, green dye: α-actin). (c) Sarcomere orientation angles, showing differences between CMs on flat and microgrooved surfaces. Adapted with permission from ref. [113]. Copyright © 2022, American Chemical Society. (d) Schematic representation of omni-purpose stretchable strain sensor (Pt layer on PU membrane). Adapted with permission from ref. [104]. Copyright © 2017, American Chemical Society.

Figure 19. Effect of microgrooves on cardiomyocyte (CM) growth: (a,b) Microscopic and immunofluorescence images show CMs grown on microgrooves and flat PDMS films after 7 days (scale bars: optical images, 80 μm; immunofluorescence, 20 μm; blue dye: DAPI, green dye: α-actin). (c) Sarcomere orientation angles, showing differences between CMs on flat and microgrooved surfaces. Adapted with permission from ref. [113]. Copyright © 2022, American Chemical Society. (d) Schematic representation of omni-purpose stretchable strain sensor (Pt layer on PU membrane). Adapted with permission from ref. [104]. Copyright © 2017, American Chemical Society.

6.8. Other Applications

Other applications of crack-templated patterns not already mentioned above include touch screens, fluidic devices, filtration membranes, gas sensors, cellular patterning platforms, and encryption.

Suh and colleagues investigated the use of cracked patterns for touch-screen applications. The developed flexible and transparent conductor, produced on a wafer scale with high reproducibility, demonstrates excellent mechanical robustness, enduring repeated bending and scratching, typical of touch-screen panel usage. Additionally, its random nanowire bundle patterns prevent the Moiré pattern issue, which is typically associated with repetitive/periodic patterns, resulting in a wavy or rippled visual artifact that can distort the image [105].

The transparent conductive surfaces fabricated and tested by Haque and colleagues exhibited varied crack network characteristics depending on the pH during their fabrication. All samples showed continuous, interconnected cracks, with the Euler Characteristic (χ) quantifying the network’s fragmentation. A more branched crack network, associated with more negative χ values, could enhance touch-screen sensitivity by improving signal distribution, though it increases resistance. At pH 12.5, where χ is near zero, the network achieves minimal resistance, ideal for touch-screen functionality. In contrast, at pH 8.7, the more negative value of χ (closer to −25) correlates with a significant resistance of 40 kΩ, highlighting the importance of optimizing pH to ensure efficient electrical pathways for touch-screen applications [95]. The crucial role of wire density had already been highlighted by Kumar and co-workers: after 20 drying cycles of the cracking template, the resulting wire density in the TCE network increases and becomes uniform, improving current distribution significantly, reducing sheet resistance and ensuring consistent performance for achieving high-resolution touch screens [26].

Microfluidic devices, recently optimized for various biomedical and optoelectronic applications, can benefit from the cost-effective crack template method. A Cu mesh, prepared by electroless deposition for 7 min onto the Au seed layer (which was itself deposited onto cracked pattern), ensured controlled microchannel formation and will be the object of future studies for real-time applications such as cooling systems [92].

Recent research has explored various methods to prepare filtration membranes with enhanced properties. In this context, Yoon and colleagues emphasized the role of the crack template method to form membranes with precise control over key parameters, such as pore size and plasma treatment of the AAO template. This approach allows the UV-curable resin to form cracks on the polymer layer, creating cavities smaller than the template’s original pores. Such membranes are expected to enable filtration with finer screening dimensions than currently available options while minimizing hydraulic resistance, since cavities are confined to the surface. Future work will focus on evaluating filtration performance and hydraulic properties [65].

The study of Evrova and co-workers highlights the critical role of crack formation in modifying and enhancing the properties of hybrid scaffolds composed of PLGA and PEO for biomedical applications. The addition of the hydrophilic polymer PEO to the hydrophobic PLGA induced morphological changes, such as circular pores and longitudinal cracks in the fibers, which were pivotal to scaffold performance. These cracks, forming within days and varying with PEO concentration, accelerated fiber disintegration, altered fiber morphology, and increased scaffold porosity, creating unique fibrous microenvironments. These crack-induced modifications significantly influenced cellular responses, enhancing myoblast attachment, proliferation, differentiation, and self-alignment. The newly acquired flexibility allowed myoblasts to interact dynamically with the evolving microenvironment, promoting superior myotube formation compared to traditional electrospun scaffolds. This approach underscores the significance of cell–material interactions in modulating cellular patterning and suggests that such a method could be adapted for other polymeric scaffolds in tissue engineering and cellular studies [115].

Kim and colleagues presented an efficient approach to manufacturing hydrogen sensors with high recovery rates, using Pd and a self-cracked template. Hydrogen gas sensors are typically made using metal oxide semiconductors (e.g., In₂O₃, NiO, WO₃), which detect gas through adsorption/desorption on the sensor surface. While oxide-based sensors are highly sensitive, they require a heating process (around 150 °C) and suffer from poor selectivity. In contrast, Pd selectively reacts with hydrogen at room temperature, causing a phase change that expands the lattice and increases conductivity. These properties are combined in these Pd-based sensors with intentionally created nanogaps, which increase conductivity when exposed to hydrogen, allowing for efficient high-concentration hydrogen detection. In this study, the self-cracking of the WO₃ thin film occurs due to the thermal stress difference between the substrate PDMS and the WO₃, which is caused by the heat produced during the deposition process. The H₂ sensor proved to be capable of detecting hydrogen concentrations of 1%–5% with a gap range of 0.5–2 mm, and exhibits a very fast recovery time of approximately 1 s [11].

One last application of crack-templated patterns could be found in encryption and encoding data. Melnychenko and colleagues’ example involves determining intersection points between cracks where a horizontal line is cut off, and this could be read optically or electrically (Figure 20). However, challenges include data recovery and the types of information that can be encrypted. Future applications would require optimization of the template, such as enhancing electrode durability with protective layers or matching substrate size to equipment specifications [56].

7. Conclusions and Future Directions

In conclusion, the development of crack-templated patterns has significantly advanced the field of transparent conductive electrodes, providing a cost-effective and scalable method for creating highly conductive, optically transparent, and flexible materials. Key parameters include the thickness of the crack template, which directly affects crack footprint, width, and spacing. Thinner templates generally produce narrower cracks, resulting in higher transmittance, but also increase the sheet resistance of the obtained metallic grid. The choice of materials for the sacrificial crack layer, such as acrylic resins, ceramics, or egg white, determines the uniformity and morphology of the crack patterns. The solvent of the colloidal suspension also plays a pivotal role in the generation of hierarchical cracks. Additionally, the reaction time and solution used in particular processes, such as electroplating, can control the length and thickness of the resulting metallic nanowires. These tunable features allow for precise adjustment of the network’s properties, making crack-templated patterns highly adaptable for various applications, including flexible electronics, transparent heaters, and photovoltaic devices.

Future research should explore optimizing the crack-templating process to improve the uniformity and reproducibility of these patterns across larger areas. Additionally, investigating the integration of crack-templated patterns with emerging materials, such as perovskites or organic semiconductors, could lead to the development of next-generation solar cells and light-emitting devices. Potential applications extend beyond current uses in flexible electronics and photovoltaics. For instance, the unique properties of these crack-templated patterns could be harnessed in the development of advanced sensors, wearable devices, and smart textiles. Moreover, their adaptability to curved and irregular surfaces makes them ideal for automotive and aerospace applications, where conformability and durability are critical, and possibly for cellular patterning, to control the spatial arrangement of cultured cell lines, allowing researchers to study their behavior in more physiologically relevant environments.