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Structured Gels: Mechanics, Responsivity and Applications
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Structured Gels: Mechanics, Responsivity and Applications

A special issue of Gels (ISSN 2310-2861). This special issue belongs to the section "Gel Analysis and Characterization".

Deadline for manuscript submissions: closed (30 April 2024) | Viewed by 22766

Special Issue Editors

Dr. Mutian Hua

E-Mail Website
Guest Editor
Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, USA
Interests: structural hydrogels; soft robotics; 3D printing; dynamic covalent bond
Special Issues, Collections and Topics in MDPI journals
Prof. Dr. Shuwang Wu

E-Mail Website
Co-Guest Editor
Department of Material Science and Engineering, Suzhou University, Suzhou 215003, China
Interests: hydrogel
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Gel-related research has seen tremendous growth in the past several decades. Higher demands on gel properties and measures for their control arise as gel-based materials are exposed to new interdisciplinary studies and applications. For instance, high fatigue resistance is desired for gels that experience cyclic loadings when used as structural components in soft robots; low tortuosity and high ion diffusivity are desired for gels used as solid electrolytes and electrodes in supercapacitors and batteries; dynamic crosslinking and bonding is desirable for gels used as tissue scaffolds or for self-healing studies. Structures at various length scales have proven to significantly impact the physical and chemical properties of gel materials both in nature and in practice. Therefore, the design of gels with specific molecular functions, network complexities, microstructures and macro-architectures could yield hydrogels with advanced properties in terms of mechanics, diffusivity, sensitivity, adhesion, biocompatibility, and recyclability. Recent developments have shown that by harnessing chemical synthesis and engineering controls, gels could exhibit a multitude of controllable structures that lead to significant enhancement of properties and dynamics that were infeasible with conventional hydrogel designs. This Special Issue is dedicated to the surging studies of structure-property relationships of novel gel materials, with a focus on molecular design, network engineering, and structural controls. It is hoped that the topics will stimulate new research and discoveries in the field of structured gels.

Dr. Mutian Hua
Prof. Dr. Shuwang Wu
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Gels is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2100 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • structure-property relationship
  • mechanical properties
  • responsivity
  • diffusivity
  • sensitivity
  • adhesion energy
  • network design

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Related Special Issue

Published Papers (10 papers)

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Research

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16 pages, 3026 KiB  
Article
Impact of the Preparation Method on the Formulation Properties of Allantoin Hydrogels: Evaluation Using Semi-Solid Control Diagram (SSCD) Principles
by Robert-Alexandru Vlad, Teodora-Cătălina Dudici (Vlăgea), Muhammad Ali Syed, Paula Antonoaea, Emöke Margit Rédai, Nicoleta Todoran, Cornelia-Titiana Cotoi, Magdalena Bîrsan and Adriana Ciurba
Gels 2024, 10(1), 58; https://doi.org/10.3390/gels10010058 - 12 Jan 2024
Cited by 1 | Viewed by 1690
Abstract
Allantoin possesses numerous beneficial properties for the skin, like anti-irritant effects, wound healing, skin hydration, and epithelization. In this paper, we investigated a suitable preparation method for an allantoin hydrogel using the Semi-Solid Control Diagram (SSCD) method and characterized its rheological and consistency [...] Read more.
Allantoin possesses numerous beneficial properties for the skin, like anti-irritant effects, wound healing, skin hydration, and epithelization. In this paper, we investigated a suitable preparation method for an allantoin hydrogel using the Semi-Solid Control Diagram (SSCD) method and characterized its rheological and consistency behavior. To accomplish this, xanthan gum (XG) was selected as a model gelling agent. Briefly, four hydrogels were prepared, two without allantoin (coded M01 and M02) and two with allantoin (M1 and M2). Similarly, the formulations were either prepared through magnetic stirring (M01 and M1) or homogenization in a mortar (M02 and M2). The prepared hydrogels were evaluated using the SSCD for specific parameters and indexes. The Good Quality Index (GQI) shows a higher value for the formulation, M1 = 6.27, compared to M2 = 5.45. This result is also underlined by the value of M01 = 6.45, which is higher than M02 = 6.38. Considering the consistency, the formulation M01 possessed the highest spreadability, followed by M02 and then the allantoin hydrogels M1 and M2. The rheological behavior had a thixotropic pseudoplastic flow for all the formulations. The use of SSCD pictographs outlined the rheological properties that need improvement, the method that is suitable to prepare the allantoin hydrogels, and the influence of the allantoin suspended in the XG hydrogel. Full article
(This article belongs to the Special Issue Structured Gels: Mechanics, Responsivity and Applications)
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12 pages, 4243 KiB  
Article
Soft and Conductive Polyethylene Glycol Hydrogel Electrodes for Electrocardiogram Monitoring
by Dongik Lee, Jihyang Song, Jungwoo Kim, Jaebeom Lee, Donghee Son and Mikyung Shin
Gels 2023, 9(12), 957; https://doi.org/10.3390/gels9120957 - 6 Dec 2023
Cited by 1 | Viewed by 2337
Abstract
The measurement of biosignals in the clinical and healthcare fields is fundamental; however, conventional electrodes pose challenges such as incomplete skin contact and skin-related issues, hindering accurate biosignal measurement. To address these challenges, conductive hydrogels, which are valuable owing to their biocompatibility and [...] Read more.
The measurement of biosignals in the clinical and healthcare fields is fundamental; however, conventional electrodes pose challenges such as incomplete skin contact and skin-related issues, hindering accurate biosignal measurement. To address these challenges, conductive hydrogels, which are valuable owing to their biocompatibility and flexibility, have been widely developed and explored for electrode applications. In this study, we fabricated a conductive hydrogel by mixing polyethylene glycol diacrylate (PEGDA) with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) polymers dissolved in deionized water, followed by light-triggered crosslinking. Notably, this study pioneered the use of a PEGDA−PEDOT:PSS hydrogel for electrocardiogram (ECG) monitoring- a type of biosignal. The resulting PEGDA−PEDOT:PSS hydrogel demonstrated remarkable conductivity while closely approximating the modulus of skin elasticity. Additionally, it demonstrated biocompatibility and a high signal-to-noise ratio in the waveforms. This study confirmed the exceptional suitability of the PEGDA−PEDOT:PSS hydrogel for accurate biosignal measurements with potential applications in various wearable devices designed for biosignal monitoring. Full article
(This article belongs to the Special Issue Structured Gels: Mechanics, Responsivity and Applications)
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17 pages, 3102 KiB  
Article
Polymeric Hydrogels for Intervertebral Disc Replacement/Integration: Playing with the Chemical Composition for Tuning Shear Behavior and Hydrophilicity
by Devid Maniglio, Elia Bissoli, Emanuela Callone, Sandra Dirè and Antonella Motta
Gels 2023, 9(11), 912; https://doi.org/10.3390/gels9110912 - 17 Nov 2023
Viewed by 1371
Abstract
Damages to the intervertebral disc (IVD) due to improper loading or degeneration result in back pain, which is a common disease affecting an increasing number of patients. Different strategies for IVD remediation have been developed, from surgical treatment to disc replacement, by using [...] Read more.
Damages to the intervertebral disc (IVD) due to improper loading or degeneration result in back pain, which is a common disease affecting an increasing number of patients. Different strategies for IVD remediation have been developed, from surgical treatment to disc replacement, by using both metallic and non-metallic materials. Hydrogels are very attractive materials due to their ability to simulate the properties of many soft tissues; moreover, their chemical composition can be varied in order to assure performances similar to the natural disc. In particular, for the replacement of the IVD outer ring, namely, the anulus fibrosus, the shear properties are of paramount importance. In this work, we produced hydrogels through the photo-induced crosslinking of different mixtures composed of two hydrophilic monofunctional and difunctional polymers, namely, poly(ethyleneglycol) methyl ether methacrylate (PEGMEMA) and poly(ethyleneglycol) dimethacrylate (PEGDMA), together with a hydrophobic molecule, i.e., tert-butyl acrylate (tBA). By changing the ratio among the precursors, we demonstrated the tunability of both the shear properties and hydrophilicity. The structural properties of hydrogels were studied by solid-state nuclear magnetic resonance (NMR). These experiments provided insights on both the structure and molecular dynamics of polymeric networks and, together with information obtained by differential scanning calorimetry (DSC), allowed for correlating the physical properties of the hydrogels with their chemical composition. Full article
(This article belongs to the Special Issue Structured Gels: Mechanics, Responsivity and Applications)
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16 pages, 1646 KiB  
Article
A Preliminary Study on the Release of Bioactive Compounds from Rice Starch Hydrogels Produced by High-Pressure Processing (HPP)
by Anna D’Aniello, Katerina Koshenaj and Giovanna Ferrari
Gels 2023, 9(7), 521; https://doi.org/10.3390/gels9070521 - 27 Jun 2023
Cited by 5 | Viewed by 2313
Abstract
This work aimed to carry out a preliminary study on the release of bioactive compounds loaded into starch-based hydrogels produced by high-pressure processing (HPP). As a study case, the experiments were carried out on rice starch HPP hydrogels. Rice starch (20% w/ [...] Read more.
This work aimed to carry out a preliminary study on the release of bioactive compounds loaded into starch-based hydrogels produced by high-pressure processing (HPP). As a study case, the experiments were carried out on rice starch HPP hydrogels. Rice starch (20% w/w) and green tea extract (2% w/w), suspended in distilled water, were treated by HPP at processing conditions enabling starch gelatinisation, namely 600 MPa for 15 min at room temperature. Additional experiments were carried out on samples that were further loaded with glycerol (5% w/w). Gel formation was assessed by analysing the gelatinisation extent, structuring level, and swelling power of the samples. At the processing conditions utilised, stable hydrogels were obtained even in the presence of the extract and/or the glycerol in the starch suspension. As expected, the colour of the hydrogels formed was affected by the addition of green tea extract in the starch solution. HPP starch hydrogels were characterised by Fourier transform infrared spectroscopy (FT-IR) to determine the interactions between the different compounds utilised in the formulation. Moreover, the release kinetics of bioactive compounds from HPP rice starch hydrogels was evaluated using a vertical Franz diffusion cells system, simulating a transdermal pattern. The diffusion of bioactive compounds was measured spectrophotometrically and via HPLC analysis. A controlled release of bioactive compounds from the hydrogel structure was detected, suggesting that small molecules, such as polyphenols, positively interacted with the rice starch HPP hydrogel network that allowed a smooth and constant release of these bioactive compounds over time. Full article
(This article belongs to the Special Issue Structured Gels: Mechanics, Responsivity and Applications)
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17 pages, 4591 KiB  
Article
pH-Sensitive Poly(acrylic acid)-g-poly(L-lysine) Charge-Driven Self-Assembling Hydrogels with 3D-Printability and Self-Healing Properties
by Maria-Eleni Kargaki, Foteini Arfara, Hermis Iatrou and Constantinos Tsitsilianis
Gels 2023, 9(7), 512; https://doi.org/10.3390/gels9070512 - 25 Jun 2023
Viewed by 1607
Abstract
We report the rheological behavior of aqueous solutions of a graft copolymer polyampholyte, constituted of polyacrylic acid (PAA) backbone grafted by Poly(L-lysine) (PAA-b-PLL). The graft copolymer self-assembles in aqueous media, forming a three-dimensional (3D) network through polyelectrolyte complexation of the oppositely [...] Read more.
We report the rheological behavior of aqueous solutions of a graft copolymer polyampholyte, constituted of polyacrylic acid (PAA) backbone grafted by Poly(L-lysine) (PAA-b-PLL). The graft copolymer self-assembles in aqueous media, forming a three-dimensional (3D) network through polyelectrolyte complexation of the oppositely charged PAA and PLL segments. Rheological investigations showed that the hydrogel exhibits interesting properties, namely, relatively low critical gel concentration, elastic response with slow dynamics, remarkable extended critical strain to flow, shear responsiveness, injectability, 3D printability and self-healing. Due to the weak nature of the involved polyelectrolyte segments, the hydrogel properties display pH-dependency, and they are affected by the presence of salt. Especially upon varying pH, the PLL secondary structure changes from random coil to α-helix, affecting the crosslinking structural mode and, in turn, the overall network structure as reflected in the rheological properties. Thanks to the biocompatibility of the copolymer constituents and the biodegradability of PLL, the designed gelator seems to exhibit potential for bioapplications. Full article
(This article belongs to the Special Issue Structured Gels: Mechanics, Responsivity and Applications)
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12 pages, 3007 KiB  
Article
Thermo-Responsive Injectable Hydrogels Formed by Self-Assembly of Alginate-Based Heterograft Copolymers
by Konstantinos Safakas, Sofia-Falia Saravanou, Zacharoula Iatridi and Constantinos Tsitsilianis
Gels 2023, 9(3), 236; https://doi.org/10.3390/gels9030236 - 17 Mar 2023
Cited by 10 | Viewed by 2088
Abstract
Polysaccharide-based graft copolymers bearing thermo-responsive grafting chains, exhibiting LCST, have been designed to afford thermo-responsive injectable hydrogels. The good performance of the hydrogel requires control of the critical gelation temperature, Tgel. In the present article, we wish to show an alternative [...] Read more.
Polysaccharide-based graft copolymers bearing thermo-responsive grafting chains, exhibiting LCST, have been designed to afford thermo-responsive injectable hydrogels. The good performance of the hydrogel requires control of the critical gelation temperature, Tgel. In the present article, we wish to show an alternative method to tune Tgel using an alginate-based thermo-responsive gelator bearing two kinds of grafting chains (heterograft copolymer topology) of P(NIPAM86-co-NtBAM14) random copolymers and pure PNIPAM, differing in their lower critical solution temperature (LCST) about 10 °C. Interestingly, the Tgel of the heterograft copolymer is controlled from the overall hydrophobic content, NtBAM, of both grafts, implying the formation of blended side chains in the crosslinked nanodomains of the formed network. Rheological investigation of the hydrogel showed excellent responsiveness to temperature and shear. Thus, a combination of shear-thinning and thermo-thickening effects provides the hydrogel with injectability and self-healing properties, making it a good candidate for biomedical applications. Full article
(This article belongs to the Special Issue Structured Gels: Mechanics, Responsivity and Applications)
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22 pages, 5228 KiB  
Article
The Influence of the Hydroxyl Type on Crosslinking Process in Cyclodextrin Based Polyurethane Networks
by Cristian Peptu, Alexandra-Diana Diaconu, Maricel Danu, Catalina A. Peptu, Mariana Cristea and Valeria Harabagiu
Gels 2022, 8(6), 348; https://doi.org/10.3390/gels8060348 - 2 Jun 2022
Cited by 9 | Viewed by 3483
Abstract
The influence of the hydroxyl groups (OH) type on the polyaddition processes of isocyanates represents a critical approach for the design of multicomponent polyurethane systems. Herein, to prove the effect of hydroxyl nature on both the isocyanate-OH polyaddition reactions and the structure/properties of [...] Read more.
The influence of the hydroxyl groups (OH) type on the polyaddition processes of isocyanates represents a critical approach for the design of multicomponent polyurethane systems. Herein, to prove the effect of hydroxyl nature on both the isocyanate-OH polyaddition reactions and the structure/properties of the resulting networks, two structurally different cyclodextrins in terms of the primary and secondary groups’ ratio were analyzed, namely native β-cyclodextrin (CD) and its derivative esterified to the primary hydroxyl groups with oligolactide chains (CDLA). Thus, polyurethane hydrogels were prepared via the polyaddition of CD or CDLA to isophorone diisocyanate polyethylene glycol-based prepolymers (PEG-(NCO)2). The degradable character of the materials was induced by intercalating oligolactide short sequences into the polymer chains composing the polymer network. In order to establish the influence of the OH type, the synthesis of polyurethane hydrogels was analyzed by a rheological investigation of the overall system reactivity. Materials properties such as swelling behavior, thermal properties and hydrolytic degradation were influenced by the reaction feed. Specifically, the presence of primary OH groups leads to more compact networks with similar water uptake, disregarding the CD content, while the predominance of secondary OH groups together with the presence of oligolactide spacers leads to the fine tuning of the water swelling properties. Full article
(This article belongs to the Special Issue Structured Gels: Mechanics, Responsivity and Applications)
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Review

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48 pages, 9421 KiB  
Review
Diversity of Bioinspired Hydrogels: From Structure to Applications
by Alexandra Lupu, Luiza Madalina Gradinaru, Vasile Robert Gradinaru and Maria Bercea
Gels 2023, 9(5), 376; https://doi.org/10.3390/gels9050376 - 2 May 2023
Cited by 14 | Viewed by 3322
Abstract
Hydrogels are three-dimensional networks with a variety of structures and functions that have a remarkable ability to absorb huge amounts of water or biological fluids. They can incorporate active compounds and release them in a controlled manner. Hydrogels can also be designed to [...] Read more.
Hydrogels are three-dimensional networks with a variety of structures and functions that have a remarkable ability to absorb huge amounts of water or biological fluids. They can incorporate active compounds and release them in a controlled manner. Hydrogels can also be designed to be sensitive to external stimuli: temperature, pH, ionic strength, electrical or magnetic stimuli, specific molecules, etc. Alternative methods for the development of various hydrogels have been outlined in the literature over time. Some hydrogels are toxic and therefore are avoided when obtaining biomaterials, pharmaceuticals, or therapeutic products. Nature is a permanent source of inspiration for new structures and new functionalities of more and more competitive materials. Natural compounds present a series of physico-chemical and biological characteristics suitable for biomaterials, such as biocompatibility, antimicrobial properties, biodegradability, and nontoxicity. Thus, they can generate microenvironments comparable to the intracellular or extracellular matrices in the human body. This paper discusses the main advantages of the presence of biomolecules (polysaccharides, proteins, and polypeptides) in hydrogels. Structural aspects induced by natural compounds and their specific properties are emphasized. The most suitable applications will be highlighted, including drug delivery, self-healing materials for regenerative medicine, cell culture, wound dressings, 3D bioprinting, foods, etc. Full article
(This article belongs to the Special Issue Structured Gels: Mechanics, Responsivity and Applications)
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Other

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13 pages, 932 KiB  
Systematic Review
The Influence of Fluoride Gels on the Physicochemical Properties of Tooth Tissues and Dental Materials—A Systematic Review
by Paweł J. Piszko, Aleksandra Piszko, Jan Kiryk, Adam Lubojański, Wojciech Dobrzyński, Rafal J. Wiglusz, Jacek Matys and Maciej Dobrzyński
Gels 2024, 10(2), 98; https://doi.org/10.3390/gels10020098 - 26 Jan 2024
Cited by 6 | Viewed by 1937
Abstract
The aim of the presented systematic review is to update the state of knowledge and relate the properties and composition of fluoride gels to their potential application. This article aims to explore the effect of fluoride gel application on changes in the properties [...] Read more.
The aim of the presented systematic review is to update the state of knowledge and relate the properties and composition of fluoride gels to their potential application. This article aims to explore the effect of fluoride gel application on changes in the properties of dental biomaterials and tooth tissues. The review includes articles assessing studies on the effects of fluoride gel on dental tissues and materials. Employing the PRISMA protocol, a meticulous search was conducted across the PubMed, Scopus, and Web of Science databases, utilizing keywords such as fluoride, gel, and properties. The publications were selected without limitation by the year of publication, and then Cohen’s κ test was used to assess the agreement of the respondents. Exclusion criteria included non-English studies, opinion pieces, editorial papers, letters to the editor, review articles and meta-analyses, clinical reports, studies lacking full-text accessibility, and duplicates. The quality of the chosen papers was assessed by two independent reviewers. A total of 2385 were located in databases, of which only 17 met the inclusion criteria. All publications showed increased surface mineralization, and seven studies showed the effect of fluoride gel on the surface of dental tissues. Three articles stated a negative effect of fluoride gels on titanium and stainless steel alloys and glass ionomer fillings. The effects on shear bond strength and plaque deposition require further investigation because the study results are contradictory. Full article
(This article belongs to the Special Issue Structured Gels: Mechanics, Responsivity and Applications)
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11 pages, 1509 KiB  
Technical Note
Crosslinking of Bovine Gelatin Gels by Genipin Revisited Using Ferrule-Top Micro-Indentation
by Vincent Ball
Gels 2023, 9(2), 149; https://doi.org/10.3390/gels9020149 - 10 Feb 2023
Cited by 2 | Viewed by 1673
Abstract
(1) Background: Gelatin is widely used in food science, bioengineering, and as a sealant. However, for most of those applications, the mechanical properties of gelatin gels need to be improved by means of physical or chemical crosslinking. Among the used chemical agents, genipin [...] Read more.
(1) Background: Gelatin is widely used in food science, bioengineering, and as a sealant. However, for most of those applications, the mechanical properties of gelatin gels need to be improved by means of physical or chemical crosslinking. Among the used chemical agents, genipin allows low cytotoxicity in addition to improved Young’s modulus. However, the mechanical properties of gelatin–genipin gels have only been investigated at the macroscale, and there is no knowledge of the influence of the genipin concentration on the surface homogeneity of Young’s modulus. (2) Methods: To this aim, the influence of genipin concentration on Young’s modulus of gelatin gels was investigated by means of ferrule-top micro-indentation. The data were compared with storage moduli obtained by shear rheology data. (3) Results: Ferrule-top indentation measurements allowed us to show that Young’s moduli of gelatin–genipin gels increase up to a plateau value after approximately 12 mg/mL in genipin and 4 h of crosslinking. Young’s moduli distribute with high homogeneity over 80 µm × 80 µm surface areas and are consistent with the storage moduli obtained by shear rheology. (4) Conclusions: It has been shown that ferrule-top indentation data fitted with the Hertz model yield Young’s moduli of gelatin–genipin gels which are consistent with the storage moduli obtained by characterization at the macroscale using shear rheometry. In addition, Young’s moduli are homogenously distributed (with some irregularities at the highest genipin concentrations) and can be increased by two orders of magnitude with respect to the uncrosslinked gel. Full article
(This article belongs to the Special Issue Structured Gels: Mechanics, Responsivity and Applications)
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