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Article

Development of an Electrowetting-on-Dielectric Cellulose-Based Conductive Sensor Using Direct Inkjet Printed Silver Nanoparticles

by
Oriol Caro-Pérez
1,
Maria Blanca Roncero
1,* and
Jasmina Casals-Terré
2
1
CELBIOTECH Paper Engineering Research Group, Department of Chemical Engineering, Universitat Politècnica de Catalunya BarcelonaTech (UPC), Colom Street 11, 08222 Terrassa, Spain
2
MicroTech Lab, Department of Mechanical Engineering, Universitat Politècnica de Catalunya BarcelonaTech (UPC), Colom Street 11, 08222 Terrassa, Spain
*
Author to whom correspondence should be addressed.
Polysaccharides 2024, 5(4), 761-782; https://doi.org/10.3390/polysaccharides5040048
Submission received: 9 October 2024 / Revised: 27 November 2024 / Accepted: 29 November 2024 / Published: 2 December 2024
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Abstract

:
In the quest for sustainable and efficient solutions for modern electronics, flexible electronic devices have garnered global attention due to their potential to revolutionize various technological applications. The manufacturing of these devices poses significant challenges, particularly regarding environmental sustainability and ease of production. A novel method employing direct inkjet printing of silver nanoparticle (npAg) ink onto cellulose nanocrystal (CNC) substrates is presented, offering a promising alternative to conventional methods. This study demonstrates the ability of CNCs to serve as a flexible and biodegradable substrate that does not require complex post-printing treatments to achieve adequate electrical performance. This method was implemented in the fabrication of an electrowetting-on-dielectric (EWOD) device, achieving circuit patterns with high resolutions and reduced resistances. The findings not only validate the use of CNCs in flexible electronic applications but also underscore the potential of advanced printing techniques to develop flexible electronics that are environmentally sustainable and technically feasible.

1. Introduction

The development of flexible electronic devices has transitioned from a promising technology to a daily reality due to advancements in various polymers [1]. These devices offer advantages such as surface adaptability, lightness, durability under mechanical stress, and low cost. The most commonly used material is polyethylene terephthalate (PET) and its derivatives [2,3,4,5]. However, with increasing environmental awareness, particularly regarding materials that generate microplastics, current research focuses on using biodegradable materials. Cellulose and its derivatives have garnered significant interest for their potential in sustainable electronics [6,7,8,9]. Several techniques have been employed to create conductive patterns on cellulose substrates, with screen printing and nanoparticle-based inkjet printing being among the most prominent [10]. Initial research on patterning conductive materials onto cellulose explored a range of techniques, including chemical deposition [11,12,13,14,15], airbrushing [16,17], inkjet printing [6,12,18], and screen printing [19,20,21]. Each method has been employed to deposit conductive materials onto cellulose substrates, enabling the formation of integrated circuits. However, cellulose’s inherently high surface roughness poses challenges as a substrate for electrical circuits. This roughness impairs print resolution, hindering the formation of precise circuit patterns and increasing resistance due to difficulties in achieving a homogeneous conductive ink layer on the surface [22].
Recent investigations have further advanced the field of cellulose-based electronic devices. Ahmad et al. [23] explored the electrohydrodynamic (EHD) jet printing of silver nanoparticle electrodes on cellulose substrates for moisture sensor applications. In their study, meander-shaped electrodes were fabricated with line resolutions below 150 µm. The resistance of these electrodes varied significantly, with values ranging from 50 kΩ to 8 MΩ, depending on the number of printed layers and post-printing treatment processes applied. Seeking to enhance resolution and decrease resistance, Jaiswal et al. [24] designed a cellulose-based electrocardiogram (ECG) device. They incorporated the hydrophobic agent dimethyl ketene dimer (AKD) into a substrate of cellulose nanofibril (CNF) and hydroxyethyl cellulose (HEC), using sorbitol as a plasticizer, which improved mechanical properties by increasing elongation and tensile strength. Using screen printing and nanoparticle silver (npAg) ink, they achieved resistances as low as 24.1 mΩ per square, with line sizes down to 100 µm. Corletto et al. [25] investigated the printing of conductive patterns using functionalized carbon nanotubes (CNTs) on CNC substrates through advanced discontinuous dewetting topography (TDD) and liquid bridge transfer (LBT) printing techniques. The CNTs were functionalized via acid oxidation to enhance their dispersion in aqueous solutions, allowing for precise printing. Subsequently, a 32% hydrochloric acid treatment was applied to the printed patterns to increase conductivity, achieving up to 41.7 S/m. This study successfully printed patterns with lateral resolutions as fine as 925 nm, demonstrating the viability of CNC substrates for flexible and sustainable electronics applications, though the potential rigidity due to the absence of a plasticizer remains unaddressed. Finally, Wibowo et al. [26] developed conductive and elastic cellulose/PEDOT hybrid films for flexible electronic applications, utilizing a combination of nonionic surfactants and surface modifiers such as 11-aminoundecanoic acid (11-AA) to enhance conductivity and elasticity. Selected for its biodegradability and skin compatibility, cellulose was paired with PEDOT doped with ethylene glycol to reduce electrical resistance, achieving values as low as 156 Ω/sq. These films exhibited only a 1.21-fold change in resistance after 100 stretch–relaxation cycles at 30% strain, notable for their low hysteresis and mechanical stability. The surface of the film must be functionalized to ensure conductivity, as outlined in this study.
Previous studies in cellulose-based electronics often require complex post-printing treatments and functionalized substrates to achieve adequate electrical and mechanical performance. Gabriel et al. [27] used a highly porous, microporous polytetrafluoroethylene (PTFE) membrane as a substrate within a liver-on-a-chip system, sealing its porosity with a biocompatible SU-8 layer before printing conductive inks. Sowade et al. [28] utilized corrugated cardboard as a substrate for UHF antennas and enhanced the surface by applying a UV-curable ink primer. This primer effectively reduced the surface roughness and absorptiveness of the cardboard, which was essential prior to the deposition of the conductive silver ink. The application of the primer was a necessary step to ensure the formation of a functional conductive layer, thereby streamlining the substrate preparation process while maintaining the requisite properties for electrical functionality. Gong and Sinton [29] explored paper-based microfluidics for diagnostic applications, employing selective hydrophilization of cellulose substrates using wax printing or hydrophobic polymer treatments to guide liquid flow. Additional enhancements included the incorporation of metallic particles, such as gold or silver, through chemical or physical deposition methods, and coatings with graphene oxide (GO) followed by reduction processes to improve electrical conductivity. These processes increase manufacturing complexity and may limit the environmental benefits of biodegradable substrates. This work focuses on developing a flexible substrate composed of CNC and xylitol, allowing for the creation of conductive electrical patterns through inkjet printing. The aim is to provide a sustainable and biodegradable alternative to conventional materials, demonstrating that CNC-based substrates can be effectively used in EWOD devices.
Building upon these goals, this study addresses the limitations of current solutions by introducing a novel approach that leverages the unique properties of CNC combined with xylitol as a substrate. This innovation enables the direct inkjet printing of silver nanoparticles without requiring complex post-printing treatments or surface functionalization. Compared to existing solutions, the proposed sensor simplifies fabrication, reduces resistance, and enhances pattern resolution. Furthermore, it offers a biodegradable and sustainable alternative for flexible electronics, aligning with the growing demand for environmentally friendly manufacturing methods.
EWOD is a microfluidic technology that uses electric fields to control droplet movement. The device consists of a conductive layer, a dielectric layer that prevents short-circuiting, and a hydrophobic layer that reduces friction, facilitating droplet manipulation. Applying voltage alters the droplet’s contact angle, causing it to move toward the activated electrode. The process is reversible, as the contact angle is restored upon voltage removal, but the droplet remains in its new position.

2. Materials and Methods

2.1. Materials

Crystalline nanocellulose (1) was obtained from the University of Maine (Orono, ME, USA). Xylitol was sourced from Acros Organics BVBA (Geel, Belgium). Sicrys Silver nanoparticle ink (npAg) in diethylene glycol monomethyl ether from PVnanocell (Migdal HaEmek, Israel) with a concentration of 20% w/t was utilized for electrode printing. Epoxy-based negative photoresist (SU-8) GM1050 from GERSTELTEC SARL (Pully, Switzerland) was used for the dielectric layer. A PDMS mixture (Dow Inc., Torrance, CA, USA) was then prepared using a Sylgard 184 Silicone Elastomer Kit and its curing agent was used in a 10:1 ratio as a hydrophobic layer.

2.2. Formation of CNC Films with Xylitol

CNC films with varying proportions of xylitol were prepared using a technique previously developed in our group to enhance flexibility and optimize mechanical properties [30]. Xylitol was selected for its ability to improve elongation and tensile strength in CNC-based films, providing flexibility without significantly compromising rigidity. The xylitol content ranged from 0% to 20% relative to the dry weight of CNC. Concentrations above 20% were avoided as they resulted in excessively soft films prone to deformation.
CNC was dispersed in deionized water to form a 2% consistency suspension. Xylitol was added in the desired proportion, and the mixture was stirred using a magnetic stirrer at room temperature for 2 h to promote uniformity. Although longer stirring times were not tested, the selected duration was sufficient to prepare a visually stable dispersion. The resulting solutions were poured into polystyrene Petri dishes and allowed to evaporate under controlled conditions (23 °C, 50% relative humidity) for 5 days. These conditions were chosen to minimize film cracking and ensure reproducibility, as variations in temperature and humidity are known to influence film morphology and mechanical properties [31,32]. The dried films had a thickness of approximately 250–300 µm.
The preparation process was repeated across multiple batches, producing films with consistent mechanical and electrical properties. A representative illustration of the preparation process is included in Figure 1 to visualize the key stages of the procedure.

2.3. Preliminary Conductivity Testing on CNC Films

To evaluate the conductivity of npAg and optimize curing conditions, preliminary tests were conducted. The recommended curing temperature for npAg ink is 130 °C to achieve conductivity. However, higher temperatures degrade CNC properties. Therefore, CNC and xylitol (CNC+Xyl) films were tested with manually traced npAg lines. Curing temperatures of 100, 110, 120, and 130 °C were used, with durations of 10, 20, and 30 min. The objective was to identify the conditions that ensure conductivity while preserving the physical properties of the CNC films. The results indicated that the minimum temperature and time required to achieve conductivity while maintaining material integrity was 120 °C for 30 min.

2.4. Mechanical Property Evaluation

The mechanical properties of the CNC films were evaluated by analyzing tensile stiffness and elongation using a Metrotec T5K quality control instrument equipped with a 500 N load cell. The testing speed was set to 10 mm/min. CNC+Xyl films with various combinations, ranging from 100% CNC to 80% CNC (as described in the section Formation of CNC Films with Xylitol), were tested, including both unexposed films and films exposed to the ink curing temperature (120 °C for 30 min). For the films exposed to the curing temperature, they were left for a minimum of 24 h in a controlled environment at 23 °C and 50% RH. Both types of films were stored under the same controlled conditions. This assessment provided insights into the tensile stiffness and elongation characteristics of the films under different xylitol concentrations, with the objective of selecting the CNC+Xyl proportion that exhibits the best physical properties after undergoing the npAg ink curing process.

2.5. Preliminary Electrode Printing Tests

A Dimatix printer (Model DMP-2831, FUJIFILM Dimatix, Inc., Santa Clara, CA, USA) equipped with Fujifilm Dimatix 2.5pl SAMBA cartridges using npAg ink was used. First, the waveform, power, and temperature of the SAMBA cartridge were defined to ensure the ejection of perfectly round droplets, ensuring high-quality printing. Straight lines of npAg ink, consisting of single droplets, were printed on CNC+Xyl films with different spacing between droplet centers (10, 15, 20, and 25 µm). Using white light interferometry (Filmetrics Profilm3D Profilometer, Dimatix optics), the printed lines were observed to determine the optimal droplet center spacing for continuous and well-defined lines.
Subsequently, straight lines with a thickness of 1 mm and lengths of 5, 10, 15, 20, and 25 mm were printed with the previously defined optimal droplet center spacing of 15 µm. Different films with 1 to 5 layers were printed. These lines were evaluated using impedance measurements with a Keysight B2962A to assess the electrical properties based on the number of layers.

2.6. Electrode Printing on CNC+Xyl Films

The electrodes were printed using a Dimatix printer with SAMBA 2.5 pL cartridges. For the printing configuration, a single inkjet nozzle was used, operating at a voltage of 25 V. The cartridge temperature was set to 28 °C, while the printing tray temperature was maintained at 40 °C, optimizing the conditions for the ink deposition process. The printing process was executed in two stages. Initially, four layers of npAg ink were applied, focusing on high-resolution printing of the device’s action electrodes. This step ensured a very small gap between electrodes without electrical communication between them. In the second stage, four layers of npAg ink were printed to form the connection pads linking the electrodes and the power source adapter.
Pre-pattern line printing was configured to guarantee optimal ink charge in the nozzle’s chamber, ensuring effective ink delivery during the printing process. The electrodes were treated in a two-stage curing process: they were initially dried at 50 °C for 15 min and then cured in an oven at 120 °C for 30 min, as per the ink manufacturer’s specifications. For this configuration, a printing resolution was achieved that resulted in an electrode gap of 80 µm.

2.7. Implementation of Dielectric and Hydrophobic Layers

For the dielectric layer, an epoxy-based negative photoresist (SU-8) (GM1050 from GERSTELTEC SARL, Pully, Switzerland) was used. The CNC+Xyl films with printed electrodes were pre-treated with oxygen plasma for 60 s at 60 W to enhance the adhesion of the SU-8. Spin-coating was applied at 4200 rpm for 40 s. Pre-curing was conducted at 80 °C for 20 min. A 15 s UV exposure followed, with post-curing at 80 °C for 20 min.
A PDMS mixture (Dow Inc., Torrance, CA, USA) was prepared using a Sylgard 184 Silicone Elastomer Kit and its curing agent in a 10:1 ratio and used as the hydrophobic layer. The device was treated with oxygen plasma for 60 s at 60 W to enhance adhesion. The coating was applied by spin-coating at 5000 rpm for 1 h to achieve a layer thickness of approximately 5 μm. The device was thermally cured at 80 °C for 2 h following a 5 min rest period.

2.8. Optical Evaluation of Printed Layers

The initial characterization of the printed layers was performed using the integrated optics of the Dimatix inkjet printer and a conventional microscope. This optical evaluation focused on the quality of the prints, defined by the sharpness of the printed edges, the absence of dispersed ink droplets, and the uniform coverage of ink on the printed features. The evaluation was conducted across printed layers ranging from 1 to 5 layers to assess both resolution and print quality as the number of layers increased. Additionally, the spacing between printed lines was measured to determine the maximum resolution achievable with the selected printing configuration.

2.9. Thickness and Roughness Measurements

The thickness of the printed layers was characterized using white light interferometry with a Filmetrics Profilm3D Profilometer (San Diego, CA, USA). This technique was applied to measure the thickness of the npAg electrode layer. The dielectric and hydrophobic layers’ thickness and surface roughness were also characterized using the same Filmetrics Profilm3D Profilometer.

2.10. Statistical Analysis

To analyze the differences between groups, the normality of the data distribution was first assessed using the Shapiro–Wilk test [33]. Based on the results of this test, an analysis of variance (ANOVA) [34] was performed for variables with a normal distribution. For variables that did not meet the normality assumption, a non-parametric Kruskal–Wallis test was applied [35]. Pairwise comparisons were conducted using Tukey’s test following ANOVA [36], or Dunn’s test following the Kruskal–Wallis test (35,36). All data were analyzed using R statistical software (R version 4.3.1 build 2023-06-16).

3. Results and Discussion

3.1. Mechanical Properties of the Films

The first layer of the EWOD device serves as the substrate for the subsequent layers. Therefore, it must be flat and exhibit sufficient stiffness to resist deformation under minimal force, while also maintaining enough flexibility to prevent breakage when subjected to stress. Previous studies from our group have demonstrated that adding certain polyols to CNC improves flexibility by reducing the inherent stiffness of CNC films, with xylitol being particularly effective [30]. To optimize both flexibility and resistance to deformation, CNC films with varying xylitol contents (5%, 10%, 15%, 20%) were tested. Pure CNC films were excluded from further analysis due to significant deformation during formation, making them impossible to manipulate and shape into the desired device form. The mechanical properties were compared between films exposed to the curing temperature (120 °C for 30 min) and those that had not been exposed to these conditions. This allowed for a direct comparison between films that underwent the same treatment as the finalized sensor and untreated films.
Films that were not cured consistently exhibited higher tensile strength compared to their cured counterparts, particularly at lower xylitol concentrations. For instance, at 5% xylitol content, the not-cured films reached a tensile strength of 10.8 MPa, compared to 4.93 MPa in the cured films. However, as the concentration of xylitol increased to 10%, this difference reduced significantly, with the tensile strength of not-cured films measuring 9.21 MPa and that of the cured films at 4.61 MPa. At 15% and 20% xylitol content, the tensile strength followed a similar pattern, but the values began to converge.
The ANOVA results indicated a statistically significant effect of xylitol concentration on tensile strength for the cured group (p = 0.0325). This suggests that variations in xylitol content have a more pronounced impact on the mechanical properties of the films when they are cured. The post hoc Tukey test revealed significant differences between certain xylitol concentrations, particularly between 10% and 15% xylitol in the cured films (p = 0.0471), further supporting that 15% xylitol might represent an optimal concentration for maximizing tensile strength after curing. Interestingly, the tensile strength differences at higher concentrations (15% and 20% xylitol) were not as marked for the no-cured group (8.24 MPa and 5.67 MPa, respectively). In contrast, the cured group showed notable differences, confirming that the curing process, in combination with xylitol content, significantly impacts tensile strength, with 15% xylitol providing an optimal balance.
The effectiveness of xylitol as a plasticizer can be attributed to its structural similarity to cellulose, as it possesses free hydroxyl groups that enhance interactions with the CNC matrix. These interactions, mediated through hydrogen bonding, improve compatibility and strengthen the matrix, enhancing flexibility and tensile strength. The unique crystalline structure of CNC, with its high density of hydroxyl groups, further promotes uniform plasticizer distribution, contributing to improved mechanical properties [37,38,39].
However, increasing the concentration of plasticizers typically reduces tensile strength due to the intercalation of plasticizer molecules between CNC chains, which increases spacing and weakens intermolecular forces. At higher concentrations, xylitol can disrupt the structural integrity of the CNC matrix, leading to a less cohesive and structurally weaker network. This phenomenon aligns with the observed data, where tensile strength decreases as xylitol concentration exceeds 5%. Similar behavior has been reported in cellulose-based systems with plasticizers, where increasing plasticizer content beyond optimal levels results in reduced stiffness and tensile strength due to weakened intermolecular interactions [40]. Balancing xylitol concentration is thus crucial for improving flexibility without excessively compromising mechanical properties [30].
Interestingly, the observed higher tensile strength in films that were not continuously cured compared to cured films can be attributed to the thermal effects on hydrogen bonding and the relaxation of internal stresses within the cellulose nanocrystal (CNC) matrix. Studies have shown that thermal treatment can disrupt the hydrogen bonding network, reducing the intermolecular interactions that contribute to mechanical strength [41]. Additionally, the curing process may induce thermal relaxation of internal stresses, which can further decrease the material’s resistance to tensile forces. These mechanisms provide a plausible explanation for the differences in tensile strength observed between cured and no-cured films, highlighting the complex interplay between thermal treatment, plasticizer distribution, and the CNC matrix.
As shown in Figure 2B, an increasing trend in elongation is observed with higher xylitol content in films not exposed to curing temperature. CNC films with 5% xylitol exhibited an elongation of 1.85%, compared to 2.4% in films with 20% xylitol. This trend suggests that xylitol addition enhances film flexibility, potentially preventing cracking or breakage under stress. This clearly indicates that the addition of xylitol improves the flexibility of CNC films. However, within the range of 5% to 20% xylitol content, no statistically significant differences were observed for no-cured films (ANOVA p-value = 0.21), suggesting that while xylitol enhances flexibility, further increases in xylitol content do not yield significant improvements without exposure to curing temperatures. In contrast, curing at 120 °C resulted in significant differences in elongation between the films (ANOVA p-value = 0.02), with the CNC film containing 15% xylitol showing the best performance after curing, with an elongation of 1.65%. This highlights that, despite the loss in elongation due to the curing process, 15% xylitol significantly enhances flexibility, making it an optimal candidate for the EWOD device substrate. Interestingly, no significant differences were observed between the cured and no-cured films at 15% xylitol (p-value = 0.134), suggesting that this concentration maintains a balance in mechanical properties, regardless of the curing process. This highlights that, despite the loss in elongation due to the curing process, 15% xylitol significantly enhances flexibility, making it an optimal candidate for the EWOD device substrate.
As illustrated in Figure 2C, the tensile stiffness of CNC films varies with both xylitol content and exposure to curing temperatures. For no-cured films, tensile stiffness decreases as xylitol content increases. Films with 5% xylitol exhibit the highest stiffness with 6.72 GPa, while films with 20% xylitol show the lowest stiffness of 3.84 GPa. This suggests that adding more xylitol softens the CNC matrix, reducing overall stiffness, likely due to the plasticizing effect of xylitol disrupting the crystalline structure. The ANOVA for no-cured films showed a significant difference between groups (p-value < 0.001).
In contrast, for cured films, tensile stiffness decreases at lower xylitol concentrations (5% and 10%) compared to no-cured films, indicating that curing significantly impacts mechanical integrity. A slight increase in stiffness is observed in films with 15% xylitol after curing, suggesting an optimal balance where xylitol provides flexibility without overly compromising tensile stiffness. However, the lack of significant differences in stiffness among cured films (ANOVA p-value = 0.448) suggests that xylitol content does not drastically alter stiffness post-curing, allowing for flexibility in formulation depending on the desired properties.
The incorporation of polyol plasticizers improves the performance characteristics of cellulose-based films [30,42]. Polyol molecules integrate into the CNC matrix, disrupting the strong hydrogen bonds between cellulose chains. This disruption reduces the inherent stiffness of the CNC films, allowing the cellulose fibers to slide past one another more easily when subjected to stress [43]. Consequently, the films become more ductile, exhibiting increased elongation before rupture. However, a significant loss in the mechanical benefits of using xylitol is observed at concentrations below 15%, as the exposure to curing temperature has a considerable impact on reducing these mechanical properties [44,45]. This modification not only decreases the rigidity of the films but also enables them to better absorb mechanical stress without breaking, making them more suitable for applications that demand flexible and durable materials. However, the exposure to the curing temperatures of npAg ink leads to a partial loss of these properties. Despite this, the substrate continues to provide sufficient flexibility and tensile strength. This phenomenon can be related to the thermal degradation behaviors observed in natural fibers, where cellulose degradation requires high activation energies, thus playing a crucial role in maintaining the structural integrity of the fiber under thermal stress. Such thermal resilience ensures that, despite some loss of mechanical properties, the modified CNC films retain essential functional characteristics for demanding applications [46].
Regarding the addition of xylitol to CNC, surface roughness measurements revealed a value of 167 nm (Sa) using white light interferometry. This suggests that the use of plasticizers not only enhances mechanical flexibility but also improves surface homogeneity, which is beneficial for the application of conductive inks. This level of roughness is relatively low and comparable to values found in other CNC substrates, where reduced surface roughness minimizes material absorption into surface pores, enhancing performance [12]. Consistent with previous studies, plasticized CNC films exhibit smoother surfaces compared to pure CNC films [47]. Furthermore, recent research indicates that the low surface roughness of nanocellulose substrates may be sufficient to support conductive layers created by inkjet-printed nanoparticles, highlighting their potential in forming reliable conductive patterns [48].

3.2. Results of npAg Ink Printing

After optimizing the Dimatix printer to eject perfectly spherical droplets, the next step was to define the spacing between droplet centers. The optimization of drop spacing (DS) in the inkjet printing of npAg ink is crucial for achieving a continuous and conductive printed layer for the proper functioning of the EWOD device. Additionally, this DS must provide sufficient resolution to maintain a minimal gap between electrodes, which is a requirement for the optimal performance of the EWOD system [49,50]. A series of dot lines with different drop spacing (DS) values of 10 µm, 15 µm, 20 µm, and 25 µm were printed on a glass substrate. As shown in Figure 3, the line printed with a DS of 25 µm resulted in discontinuities, highlighting the need for an appropriate DS to ensure a continuous layer. In contrast, DS values between 10 µm and 20 µm provided complete continuity, with line thickness increasing as DS decreased. This is expected, as smaller DS values deposit more ink in the same area, increasing line thickness. This increase, from 115 µm at 10 µm DS to 40 µm at 25 µm DS, underscores the need to balance continuity and resolution. A smaller DS improves both, but it also risks excessive ink deposition, potentially reducing print resolution. Similar findings were reported by Ohira et al. [51], who investigated the dynamics of inkjet-printed conductive polymer droplets on cellulose nanopapers. They demonstrated that optimal drop spacing is critical for achieving continuous conductive lines, as larger spacing leads to discontinuities, in line with our observations.
The use of white light interferometry to measure layer thickness revealed a maximum of approximately 0.5 µm in all DS configurations. The results suggest that the choice of DS is not only for ensuring a continuous printed layer but also for achieving the desired line thickness, which is essential for the electrical performance of the printed electrodes.
In Figure 4, the technical print of scaled concentric squares, whether increasing or decreasing in size, allows for the determination of the smallest possible spacing between two lines without contact. This information is useful for designing the spacing between electrodes in the EWOD device, ensuring that the lines do not touch, thus preventing short circuits and maintaining proper device function. The minimum spacing between lines defined for a DS of 15 µm ranges between 50 and 60 µm.
Five lines with a thickness of 1 mm and varying lengths between 5 mm and 25 mm were printed, with an average thickness of 0.5 µm at a DS of 15 µm, using one to five layers. This method facilitates the assessment of both npAg surface coverage and its corresponding impact on electrical conductivity. As shown in Figure 5, with three or more printed layers, surface coverage becomes uniform, achieving full coverage of the CNC surface with the fourth layer. However, starting from the fifth layer, a noticeable decrease in the resolution of the printed line edges is observed. This loss of resolution may be attributed to possible nozzle blockages or partial obstructions as the number of printed layers increases, affecting the deposition of npAg. Nozzle blockages are a common issue in the inkjet printing of nanoparticles, which can further compromise the quality of the printed layers [42]. The thickness of the printed lines was measured using white light interferometry microscopy, obtaining a maximum thickness of 0.5 µm to 1 µm.
Table 1 summarizes the results for resistance (Ω) and sheet resistance (μΩ/sq) of npAg-printed lines at different layer counts. In Figure 6, the resistance of printed npAg lines is shown as a function of the number of layers and line length. A decrease in resistance was observed with an increasing number of layers across all tested lengths (5 mm, 10 mm, 15 mm, 20 mm, and 25 mm). Specifically, for a 5 mm line, resistance decreased from 9.04 Ω at two layers to 1.26 Ω at four layers. Similar trends were observed for longer lines, with resistance values ranging from 13.5 Ω to 2.51 Ω at 10 mm, and from 31.5 Ω to 7.03 Ω at 25 mm as the number of layers increased.
The resistivity of printed npAg lines was evaluated as a function of the number of layers and the line length, as shown in Figure 7. The data show a clear decrease in resistivity with an increasing number of layers for all tested lengths (5 mm, 10 mm, 15 mm, 20 mm, and 25 mm). For a 5 mm line, resistivity dropped from 1807.5 µΩ·sq with two layers to 281.3 µΩ·sq with four layers and further to 184.1 µΩ·sq with five layers. Similar reductions were observed for longer lines, with resistivity decreasing from 1350.3 µΩ·sq to 221.0 µΩ·sq as the number of layers increased from two to five for a 10 mm line. It is noteworthy that the differences in resistivity between different lengths for the same number of layers decrease as the number of layers increases. In studies using the same method, employing a Dimatix printer and npAg ink but utilizing acetylated cellulose nanofiber as a substrate, sheet resistances of 11 mΩ/sq and 7 mΩ/sq have been reported [52].
The ANOVA results by the number of layers yielded a p-value < 0.001, indicating significant differences across the groups. Tukey’s post hoc test, as highlighted in Figure 7, shows statistically significant differences between nearly all pairs of layers (p adj < 0.001), with the exception of the comparison between four and five layers, where the differences were not significant (p adj = 0.378).
The decrease in resistivity observed with the increasing number of layers can be attributed to the improvement in both the continuity and density of the conductive pathways. As additional layers of npAg are deposited, the likelihood of forming a continuous conductive network across the printed surface increases, reducing the overall resistivity. As observed in the microscopic images of the printed layers in Figure 5, each additional layer results in improved uniformity and coverage of the surface. Additionally, statistically significant differences between two, three, and four layers, confirmed by Tukey’s post hoc test, highlight the importance of layer number for optimal electrical performance. This enhancement in conductivity results from a more uniform coverage of the nanoparticles, which improves the electrical connectivity between them. However, it is important to note that the rate of resistivity reduction diminishes after a certain number of layers, indicating a saturation point. Beyond this point, further layers do not significantly contribute to the reduction in resistivity, possibly due to issues like nozzle blockages or uneven deposition, which may limit the effectiveness of additional layers. This is confirmed by the lack of statistically significant differences between the examples with four and five layers, as indicated by Tukey’s post hoc test (p-value > 0.05), further emphasizing the saturation point in resistivity reduction.
Moreover, the relationship between line length and resistivity demonstrates that while longer lines exhibit higher resistivity initially, the differences in resistivity between varying lengths diminish as the number of layers increases. This suggests that at higher layer counts, the conductive network becomes more homogenous across different line lengths, ensuring more consistent electrical performance. This finding highlights the importance of optimizing both the number of layers and the line length to achieve the desired balance between conductivity and pattern resolution. Another strategy to reduce the resistivity of the printed lines could involve decreasing the drop spacing (DS) below 15 µm during the inkjet printing process. However, this would likely compromise the print resolution, as smaller DS values may lead to excessive ink deposition, causing line broadening and reducing precision. As a result, the gap between electrodes in the EWOD device would need to be increased to prevent electrical communication or short-circuiting, which could affect the overall performance of the device. Therefore, while reducing DS could improve conductivity, it must be balanced with maintaining the necessary resolution for the electrode gap.
Additionally, the low surface roughness of CNC is a key advantage for this type of device [53]. A smoother surface reduces the likelihood of imperfections that could trap droplets or disrupt their movement across the surface in EWOD devices. This ensures more uniform wetting and electrowetting behavior, which is crucial for controlling droplet manipulation [54,55]. Furthermore, reduced roughness minimizes areas where conductive ink could pool unevenly, improving the overall precision and performance of printed circuits [56]. This smooth surface aids in the creation of a consistent dielectric layer, preventing voltage leakage or breakdown [57]. The method used to obtain CNC through acid hydrolysis may have introduced sulfate ester groups onto the CNC surface due to the use of sulfuric acid in the process. These groups could enhance the interaction between the ink and the cellulose, improving ink adhesion and potentially influencing overall print quality [58,59].

3.3. Layer Device Characterization

Heart-shaped electrodes were printed, as shown in Figure 8A. According to the consulted literature, this geometry is the most optimal for droplet movement on the surface of an EWOD device [60]. Four layers were printed with a drop spacing (DS) of 20 µm. The radius of each electrode is 2.5 mm, and the gap between electrodes was reduced to 120 µm without electrical communication between them, as illustrated in Figure 8C. Although a printing resolution of 60 µm was achieved, the gap size was increased to 120 µm due to the high operating voltage of 120 V. This adjustment was made to prevent dielectric layer breakdown under high voltage.
Heart-shaped electrodes were printed, as shown in Figure 8A,B. According to the consulted literature, this geometry is the most optimal for droplet movement on the surface of an EWOD device [61]. Four layers were printed with a drop spacing (DS) of 20 µm. The diameter of each electrode is 3,5 mm, and the gap between electrodes was reduced to 140 µm without electrical communication between them, as illustrated in Figure 8C.
The SU-8 layer, serving as the dielectric layer, was characterized with a measured thickness of approximately 1 µm (±150 nm). The PDMS layer, acting as the hydrophobic layer, exhibited a thickness of approximately 5 µm (±150 nm). The contact angle of the PDMS layer with a pure water droplet was 115° (± 2.5°), indicating its hydrophobic nature.

3.4. Device Actuation

A study of droplet movement was conducted using four different voltages: 100 V, 120 V, 150 V, and 180 V. At 150 V and 180 V, the hydrophobic layer detached from the surface without causing a short circuit. This indicates that the SU-8 coating can withstand these voltages; however, the PDMS layer cannot sustain voltages higher than 120 V.
The performance of the EWOD device was assessed by measuring the contact angle change under different applied voltages. As shown in Figure 9A,B, droplet movement was observed at 100 V using a top-view camera. A reduction in the contact angle by an average of 20° was noted (Figure 10A,B). However, clear displacement of the droplet on the surface was not achieved at this voltage. In contrast, at 120 V, as depicted in Figure 9C,D, significant droplet movement was observed, with a contact angle reduction of up to 30° (Figure 10C,D). It is important to consider that reducing the thickness of the dielectric and hydrophobic layers could potentially lower the activation voltage required for droplet manipulation. This performance can be compared to similar EWOD systems utilizing SU-8 as the dielectric layer and PDMS as the hydrophobic layer. Naseri et al. [62] reported a contact angle reduction of 20° at 100 V and up to 30° at 120 V. (2022), which closely aligns with the results obtained in this study. However, their device required a higher voltage (150–180 V) for effective droplet displacement, partly due to limitations in the hydrophobic properties of the PDMS layer. Additionally, their device used indium tin oxide (ITO) as the conductive material, which may influence the electric field distribution and droplet manipulation. It is important to note that Naseri did not provide details on the thickness of the hydrophobic layer, which limits a direct comparison of layer performance. These results underscore the importance of optimizing both the dielectric and hydrophobic layers to achieve efficient droplet manipulation at lower voltages.
Thinner layers can enhance the electric field intensity across the droplet interface, thus allowing lower voltages to achieve similar or improved fluid manipulation capabilities. However, a dielectric layer that is too thin could lead to breakdown under high voltage. For this reason, materials with high dielectric constants were sought. Several dielectric materials have been explored for their potential in EWOD devices [63]. Inorganic materials such as SiO2 and Si3N4 offer high dielectric constants and low driving voltages [64]. However, their deposition methods are complex and prone to defects. Integrating SiO2 or TiO2 with an organic substrate like CNC+Xyl would require addressing challenges such as adhesion, mechanical stability, and compatibility with the organic matrix. These materials are typically deposited using techniques like chemical vapor deposition (CVD), which may not be directly applicable to the CNC+Xyl system. Future work could explore hybrid approaches combining these inorganic materials with SU-8 to leverage their high dielectric constants while ensuring compatibility with the current substrate.
ZnO, with its high polarity, can further enhance wettability but may reduce dielectric efficiency [65]. Fluoropolymer materials like Teflon AF and Cytop™ are widely used for their hydrophobic properties and breakdown resistance, though they come at a higher cost and exhibit porous structures [66]. Other polymeric materials such as Parylene-C and polydimethylsiloxane (PDMS) are appreciated for their flexibility and ease of processing but often come with trade-offs in terms of lower dielectric strength or reduced breakdown resistance [67]. Polyimide offers high thermal stability and mechanical strength, though at a higher cost. The use of the organic polymer SU-8 offers several advantages, including ease of application, low cost, and a balanced dielectric constant.
The analysis of displacement revealed a lack of uniformity in the droplet’s movement speed, as can be observed in the video provided in the Supplementary Materials Video S1. Difficulties in droplet movement are attributed to the inherent hydrophobic properties of PDMS [68]. Although PDMS provides a hydrophobic surface, issues with liquid adhesion may occur, leading to irregular droplet movement. Fluorinated materials like Teflon, commonly used in EWOD devices, do not exhibit these limitations due to their superior liquid-repellent properties. However, the use of Teflon contradicts the goal of developing devices based on renewable materials like CNC. The choice of PDMS, despite its challenges, is justified by its lower environmental impact [69,70], aligning with the objective of creating more sustainable devices. Although a higher voltage may be required for effective operation, it has been demonstrated that PDMS can be utilized at voltages as low as 120 V, enhancing the feasibility of its use in EWOD systems without significantly compromising energy efficiency [71].
Nonetheless, this device demonstrates the feasibility of inkjet printing with npAg directly onto nanocellulose without the need to modify or functionalize the cellulose surface or alter the conductive ink formulation. This may be attributed to two characteristics of CNC: the tight matrix formed by CNC films and the acidic hydrolysis process used to produce CNC from cellulose, which could potentially introduce sulfur groups. These groups may enhance the adhesion of the ink to the surface more effectively than other cellulose derivatives [59].

4. Conclusions

In this study, an electrowetting-on-dielectric (EWOD) sensor was developed using direct inkjet printing of silver nanoparticles onto a cellulose nanocrystal (CNC) substrate. The results demonstrated that CNC substrates provide a viable and biodegradable platform for flexible electronic device fabrication, enabling the printing of high-resolution electrodes with reduced electrical resistance. Notably, this was achieved without the need for surface functionalization of the substrate or alterations to the conductive ink formulation.
The combination of CNC with xylitol as a substrate simplifies the manufacturing process by reducing post-treatment requirements while contributing to the environmental sustainability of the final product. The successful printing of circuit patterns with high resolution and low resistance highlights the potential of these substrates for flexible electronic applications, addressing the growing demand for biodegradable materials and eco-friendly manufacturing methods.
The EWOD devices fabricated in this study demonstrated reliable performance, validating the optimized printing technique for practical applications. These findings pave the way for future research into biodegradable materials and their integration into flexible electronic devices, aligning with global efforts to minimize the environmental footprint of electronic manufacturing.
While the current device integrates CNC+Xyl as a sustainable substrate, its manufacturing process also reduces environmental impact by eliminating the need for surface functionalization or ink modifications. However, the SU-8 dielectric layer and the PDMS hydrophobic layer, although used in minimal thicknesses and with relatively low environmental impact, are materials that persist in the environment. Future research should explore alternative materials with similar properties—such as high dielectric constants, hydrophobicity, and compatibility with flexible electronics—that could enable the development of a fully biodegradable device. Such efforts would further align with global sustainability goals and enhance the eco-friendliness of EWOD systems.
Future improvements to the EWOD device could focus on optimizing the thickness and uniformity of the dielectric and hydrophobic layers. Reducing the SU-8 layer’s thickness while maintaining its breakdown resistance could significantly lower the activation voltage required for droplet manipulation. Exploring dielectric materials with higher dielectric constants, such as SiO2 and TiO2, may enhance device performance, though their compatibility with the CNC+Xyl substrate requires further study. Additionally, alternative hydrophobic materials could be investigated to improve droplet mobility and reduce adhesion issues. While fluorinated polymers are widely used due to their excellent hydrophobic properties, their environmental impact makes them less suitable for sustainable designs. Developing non-fluorinated, environmentally friendly hydrophobic materials would align better with the sustainability goals of this device and future advancements in flexible electronics.
Aligning with global sustainability goals, such as the United Nations Sustainable Development Goals (SDGs), particularly SDG 12 (Responsible Consumption and Production) and SDG 13 (Climate Action), optimizing the device for lower voltage thresholds and identifying biodegradable or recyclable hydrophobic materials would further strengthen its contribution to eco-friendly electronic systems.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/polysaccharides5040048/s1, Video S1: EWOD Device Actuation with CNC+Xyl Substrate.

Author Contributions

Conceptualization, O.C.-P., M.B.R. and J.C.-T.; methodology, O.C.-P., M.B.R. and J.C.-T.; investigation, O.C.-P.; writing—original draft preparation, O.C.-P.; writing—review and editing, M.B.R. and J.C.-T.; supervision, M.B.R.; funding acquisition, M.B.R. and J.C.-T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Spain’s Ministry of Science and Innovation and Spain’s State Research Agency under grant numbers PID2020-114070RB-I00 (CELLECOPROD), CPP2021-009021, and CTQ2017-84966-C2-1-R, supported by the State Research Agency of the Ministry of Economy, Industry and Competitiveness.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data are contained within the article or Supplementary Material.

Conflicts of Interest

The authors declare no conflict of interest.

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Polysaccharides 05 00048 g001
Figure 1. Schematic representation of the preparation process for CNC–xylitol films. (1) Cellulose nanocrystals (CNCs) and xylitol are mixed to form a 2% consistency suspension. (2) The mixture is stirred magnetically for 2 h at room temperature to ensure uniformity and then poured into Petri dishes. The solutions are dried under controlled conditions (23 °C, 50% RH) for 5 days to produce the final films.
Figure 1. Schematic representation of the preparation process for CNC–xylitol films. (1) Cellulose nanocrystals (CNCs) and xylitol are mixed to form a 2% consistency suspension. (2) The mixture is stirred magnetically for 2 h at room temperature to ensure uniformity and then poured into Petri dishes. The solutions are dried under controlled conditions (23 °C, 50% RH) for 5 days to produce the final films.
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Figure 2. Mechanical properties of CNC films with varying xylitol content, comparing cured and no-cured films. (A) Tensile strength results, showing variations with different xylitol concentrations and the effect of curing. (B) Elongation results, illustrating the differences in film flexibility based on xylitol concentration and curing. (C) Tensile stiffness results, highlighting the changes in stiffness with varying xylitol concentrations and curing. Error bars indicate 95% confidence intervals. Each group and condition includes 20 samples.
Figure 2. Mechanical properties of CNC films with varying xylitol content, comparing cured and no-cured films. (A) Tensile strength results, showing variations with different xylitol concentrations and the effect of curing. (B) Elongation results, illustrating the differences in film flexibility based on xylitol concentration and curing. (C) Tensile stiffness results, highlighting the changes in stiffness with varying xylitol concentrations and curing. Error bars indicate 95% confidence intervals. Each group and condition includes 20 samples.
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Figure 3. White light interferometry images of inkjet-printed npAg ink lines at different drop spacings (DS): 10 µm, 15 µm, 20 µm, and 25 µm. The images show the top view of the printed lines, revealing variations in line thickness and continuity across different DS values. The color scale indicates the height profile in micrometers (µm), with red representing higher areas and blue representing lower areas.
Figure 3. White light interferometry images of inkjet-printed npAg ink lines at different drop spacings (DS): 10 µm, 15 µm, 20 µm, and 25 µm. The images show the top view of the printed lines, revealing variations in line thickness and continuity across different DS values. The color scale indicates the height profile in micrometers (µm), with red representing higher areas and blue representing lower areas.
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Figure 4. (A) Global view of the 1-layer printed concentric squares pattern, used to evaluate the minimum spacing between printed lines. (B) Close-up images captured using the Dimatix printer’s integrated optics, showing the measured spacing between the printed lines.
Figure 4. (A) Global view of the 1-layer printed concentric squares pattern, used to evaluate the minimum spacing between printed lines. (B) Close-up images captured using the Dimatix printer’s integrated optics, showing the measured spacing between the printed lines.
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Figure 5. Microscope images showing the results of npAg inkjet printing on CNC+Xyl films. The left column images are at 20× magnification, and the right column images are at 80× magnification. (A) shows 1 layer of npAg, (B) shows 2 layers, (C) shows 3 layers, (D) shows 4 layers, and (E) shows 5 layers.
Figure 5. Microscope images showing the results of npAg inkjet printing on CNC+Xyl films. The left column images are at 20× magnification, and the right column images are at 80× magnification. (A) shows 1 layer of npAg, (B) shows 2 layers, (C) shows 3 layers, (D) shows 4 layers, and (E) shows 5 layers.
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Figure 6. Resistance of printed npAg lines as a function of the number of layers and line length.
Figure 6. Resistance of printed npAg lines as a function of the number of layers and line length.
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Figure 7. (A) Resistivity of printed npAg lines as a function of the number of layers and line length. (B) Resistivity of printed npAg lines as a function of the number of layers.
Figure 7. (A) Resistivity of printed npAg lines as a function of the number of layers and line length. (B) Resistivity of printed npAg lines as a function of the number of layers.
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Figure 8. (A) The printed EWOD device showing heart-shaped electrodes. (B) The original printed pattern is depicted. (C) A close-up image of the electrode gap captured using the Dimatix printer’s optics. The gap between electrodes is clearly visible, demonstrating the achieved 80 µm separation without electrical communication. The image contrast was enhanced to differentiate the CNC+Xyl substrate from the npAg ink.
Figure 8. (A) The printed EWOD device showing heart-shaped electrodes. (B) The original printed pattern is depicted. (C) A close-up image of the electrode gap captured using the Dimatix printer’s optics. The gap between electrodes is clearly visible, demonstrating the achieved 80 µm separation without electrical communication. The image contrast was enhanced to differentiate the CNC+Xyl substrate from the npAg ink.
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Figure 9. (A) Initial position of the droplet on the device surface before activation. (B) Upon applying a voltage of 100 V, movement of the droplet is observed on the device surface, though no clear displacement occurs. (C) Initial position of the droplet on the surface before activation. (D) When the device is activated with 120 V, both movement and displacement of the droplet are observed on the surface of the EWOD device.
Figure 9. (A) Initial position of the droplet on the device surface before activation. (B) Upon applying a voltage of 100 V, movement of the droplet is observed on the device surface, though no clear displacement occurs. (C) Initial position of the droplet on the surface before activation. (D) When the device is activated with 120 V, both movement and displacement of the droplet are observed on the surface of the EWOD device.
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Figure 10. Contact angle (CA) measurements of a droplet on the device surface under different voltage conditions. (A,C) show the initial contact angles at 0 V, with CAs of 106.7°/103.0° and 100.0°/102.0°, respectively. (B) At 100 V, the contact angles decrease to 86.6°/82.8°, indicating partial wetting of the surface. (D) At 120 V, the contact angles further decrease to 71.8°/75.7°.
Figure 10. Contact angle (CA) measurements of a droplet on the device surface under different voltage conditions. (A,C) show the initial contact angles at 0 V, with CAs of 106.7°/103.0° and 100.0°/102.0°, respectively. (B) At 100 V, the contact angles decrease to 86.6°/82.8°, indicating partial wetting of the surface. (D) At 120 V, the contact angles further decrease to 71.8°/75.7°.
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Table 1. Resistance (Ω) and sheet resistance (Ω/□) of npAg-printed lines with different lengths, ranging from 5 mm to 25 mm. The thickness of the printed lines is 1 mm, with a line thickness of 1 µm. Twenty readings were taken for each of the five printed lines.
Table 1. Resistance (Ω) and sheet resistance (Ω/□) of npAg-printed lines with different lengths, ranging from 5 mm to 25 mm. The thickness of the printed lines is 1 mm, with a line thickness of 1 µm. Twenty readings were taken for each of the five printed lines.
Layer Numbers Resistance (Ω)Sheet Resistance (µΩ/sq)
MeanSDMeanSD
219.77.971400213
39.314.6361421.9
43.82.0225020.4
53.141.1722537.9
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MDPI and ACS Style

Caro-Pérez, O.; Roncero, M.B.; Casals-Terré, J. Development of an Electrowetting-on-Dielectric Cellulose-Based Conductive Sensor Using Direct Inkjet Printed Silver Nanoparticles. Polysaccharides 2024, 5, 761-782. https://doi.org/10.3390/polysaccharides5040048

AMA Style

Caro-Pérez O, Roncero MB, Casals-Terré J. Development of an Electrowetting-on-Dielectric Cellulose-Based Conductive Sensor Using Direct Inkjet Printed Silver Nanoparticles. Polysaccharides. 2024; 5(4):761-782. https://doi.org/10.3390/polysaccharides5040048

Chicago/Turabian Style

Caro-Pérez, Oriol, Maria Blanca Roncero, and Jasmina Casals-Terré. 2024. "Development of an Electrowetting-on-Dielectric Cellulose-Based Conductive Sensor Using Direct Inkjet Printed Silver Nanoparticles" Polysaccharides 5, no. 4: 761-782. https://doi.org/10.3390/polysaccharides5040048

APA Style

Caro-Pérez, O., Roncero, M. B., & Casals-Terré, J. (2024). Development of an Electrowetting-on-Dielectric Cellulose-Based Conductive Sensor Using Direct Inkjet Printed Silver Nanoparticles. Polysaccharides, 5(4), 761-782. https://doi.org/10.3390/polysaccharides5040048

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