From Swell to Signal: Hydrogel Polymer Design in Biosensor Performance

In the field of biomedical engineering, a significant challenge persists in developing hydrogel-based electrodes that maintain high conductivity, water retention, and long-term functionality while ensuring biocompatibility and biodegradability for clinical applications, such as wearable biosensors and ECG monitoring.

Traditional synthetic hydrogels, petroleum derived polymers such as polyacrylamide or polyacrylic acid, often excel in mechanical tunability and electrical performance but fall short in environmental sustainability and biological integration, leading to issues like skin irritation from prolonged contact, limited reusability, and non-degradable waste that hinders their translation into patient-friendly devices.

This problem is exacerbated by the reliance on synthetic polymers, which may introduce toxicity or reduce long-term stability due to water evaporation and degradation under physiological conditions. A promising alternative involves bio-based polymers such as chitosan, a cationic polysaccharide derived from crustacean shells, blended with anionic polymers like alginate to form polyelectrolyte complex (PEC) hydrogels.

By systematically manipulating the chitosan to alginate ratio, researchers can tune ionic crosslinking density, network porosity, swelling capacity, and water retention- key factors that directly influence electrode performance. However, the effects of specific polymer composition ratios in long term hydration stability and functionality in relevant media remain unexplored.

Evidence from recent studies highlights these limitations of synthetic systems and the potential of natural polymers (Sriramakrishnan et al. 2025). The underutilization of bio-based polymers like chitosan, derived from natural sources such as crustacean shells, which offer inherent biocompatibility, antibacterial properties, and degradability but have not been systematically optimized for electrode performance.

The purpose of this study was to evaluate the functionality of a hydrogel-based electrode. A quantitative experimental design was used to examine how changes in polymer composition affected conductivity, responsiveness, and water retention—key performance indicators identified in biomaterials research (Zhou et al., 2020). Previous studies indicated that polymer chemistry and crosslinking density strongly influenced electrical behavior and hydration stability, supporting the selection of these variables (Cheng et al., 2024; Li et al., 2019).

Hydrogel samples with varying polymer ratios served as the subjects, eliminating ethical concerns. Conductivity was measured using a conductivity meter, responsiveness with an EMG sensor, and water retention through mass loss experiments in a controlled environment. All samples were synthesized under identical conditions, cast to uniform dimensions, and tested using standardized procedures to ensure replicability. Data were analyzed using comparative statistics, including means, ranges, and percentage changes.

The independent variable was polymer composition, specifically the chitosan-to-alginate ratio (e.g., 1:1, 2:1, 1:2, 1:4, 4:1). Dependent variables included responsiveness, swelling capacity, and water retention, measured by percentage mass change over time. These variables were selected because they reflect critical performance indicators: responsiveness indicates signal detection ability, while water retention and swelling capacity relate to hydration stability and adhesion.

A quantitative approach was appropriate as it enabled precise comparison across formulations (Zhao et al., 2026). Chitosan and alginate solutions were prepared separately and combined in predetermined ratios, with total polymer mass held constant. Mixtures were stirred, molded, gelled by freezing, rinsed, and crosslinked using calcium chloride. Hydrogels were tested for swelling in various media and monitored for mass changes. Water retention was assessed after swelling by measuring mass loss over 18–24 hours.

The mass of each sample was recorded at 15 minutes intervals after gently blotting excess solvent with adsorbent tissue paper. The swelling capacity was measured and recorded over a 24 hour period. Swelling capacity rose over time in Phosphate Buffered Saline (PBS) for all formulations. The 2,1 Chitosan: Alginate ratio exhibited the highest swelling, peaking near 95%, followed closely by the 1,2 ratio at approximately 90-92%. The 4,1 and 1,4 formulations showed moderate and similar swelling capacities, while the 1,1 ratio displayed the lowest swelling. All curves showed rapid initial swelling and subsequent gradual decline or stabilization up to 1200 minutes.

Almost all formulations exhibited low and relatively stable swelling for most chitosan: alginate formulations in Distilled Water (DW). The 1,2 ratio showed the highest swelling. The 4, ratio starts negative, remained low then shows a small recovery. The 1,4 ratio exhibits moderate swelling. In contrast, the 2,1 and 1,1 ratio displayed very low and nearly flat swelling profiles close to zero throughout the experiment, the 2,1 ratio performed poorly.

All formulations exhibited negative swelling from the very beginning in Ethanol. The 2,1 ratio showed the least negative value, followed closely by the 4,1 and 1,1 ratios. In contrast, the alginate rich, 1,2 and 1,4 ratios displayed the most negative swelling.

Mass retention (resistance to drying) was evaluated by monitoring mass loss over time under controlled environmental conditions (25°C, 50% relative humidity) to simulate skin adhesion scenarios. Pre-swollen hydrogel samples of uniform size were placed on a non-absorbent surface, and their mass was measured at regular intervals (every 30 minutes for the first 4 hours, then hourly) until significant dehydration occurred.

Every chitosan to alginate ratio (1,1; 2,1; 1,2; 4,1; 1,4) exhibited negative mass retention values in Ethanol over 30 hours. The 1,2 ratio retained the least negative water content, while the 2,1 ratio showed the most negative values. The other ratios fell between these extremes by the end of the experiment.

The swelling and water retention behaviors of chitosan–alginate hydrogels were strongly influenced by both the immersion medium and the chitosan-to-alginate ratio, reflecting interactions among ionic crosslinking, electrostatic repulsion, and osmotic forces. In phosphate-buffered saline (PBS, pH 7.4), all formulations exhibited increasing positive swelling over time. The 2:1 ratio showed the highest swelling, followed by 1:2, while the 1:1 formulation demonstrated the lowest swelling. This was attributed to dense polyelectrolyte complex formation at near-stoichiometric ratios, which increased crosslinking density and restricted expansion. Moderately imbalanced ratios, particularly 2:1, appeared to optimize porosity and hydrophilicity while maintaining structural integrity.

In distilled water, swelling remained minimal and stable for most formulations, except the 1:2 ratio, which reached approximately 150% swelling before declining. This behavior resulted from unscreened electrostatic repulsion and the Donnan effect, where excess negative charges increased osmotic pressure and water influx. The 2:1 ratio showed negligible swelling, highlighting the importance of ionic strength.

In 95% ethanol, all samples displayed negative swelling, indicating dehydration. Alginate-rich formulations showed the greatest mass loss, while balanced and chitosan-rich samples were less affected due to slightly stronger network stability.

Water retention results indicated gradual mass loss after swelling in all media. The 1:2 ratio showed the highest retention, while the 2:1 ratio exhibited poor long-term stability despite high initial swelling. Extreme ratios showed reduced mechanical stability, especially in PBS. Overall, hydrogel performance depended on balancing crosslink density and charge.

The findings from this study highlighted the tunable and environment-responsive nature of chitosan–alginate hydrogels. Swelling and water retention were strongly influenced by polymer ratio and surrounding medium. PBS promoted stable swelling, distilled water amplified electrostatic repulsion through the Donnan effect, and ethanol caused consistent dehydration. These results demonstrated their potential as stimuli-responsive biomaterials. It was also observed that high initial swelling did not guarantee long-term water retention, emphasizing the need to balance crosslinking density with mechanical stability.

Future research should focus on refining polymer ratios to optimize hydration, conductivity, and strength. Exploring intermediate ratios could have identified ideal compositions. Incorporating dual crosslinking methods may have improved structural stability while preserving responsiveness. Additional analyses, including SEM, rheology, and FTIR, would have provided deeper insight. Expanding testing to include pH variation, degradation, and complex biological environments would have further supported the development of advanced hydrogel-based biomedical applications.

Cheng, S., Zhu, R., & Xu, X. (2024). Hydrogels for next generation neural interfaces.

Ho, T.-C., Chang, C.-C., Chan, H.-P., Chung, T.-W., Shu, C.-W., Chuang, K.-P., … Tyan, Y.-C. (2022). Hydrogels: Properties and applications in Biomedicine.

Kaliampakou, C., Lagopati, N., Pavlatou, E. A., & Charitidis, C. A. (2023). Alginate–gelatin hydrogel scaffolds; an optimization of post-printing treatment for enhanced degradation and swelling behavior.

Li, H., Cao, J., Wan, R., Feig, VR., Tringides, CM., Xu, J., Yuk, H., Lu, B., (2024). Pedots-based conductive hydrogels: Design, fabrications, and applications.

Pan, X., Wang, Q., He, P., Liu, K., Ni, Y., Chen, L., Ouyang, X., Wang, H., Xu, S. (2020). A bionic tactile plastic hydrogel-based electronic skin constructed by a nerve-like nanonetwork combining stretchable, compliant, and self-healing properties.

Sriramakrishnan, J., BS, A., Thakur, G., Kumar, P. (2025). Sustainable hydrogels as conductive platforms for neural applications

Taisescu, O., Dinescu, V. C., Rotaru-Zavaleanu, A. D., Gresita, A., & Hadjiargyrou, M. (2025). Hydrogels for peripheral nerve repair: Emerging materials and therapeutic applications.

Tessier, A., Zhuo, S., & Kabiri Ameri, S. (2024). Ultrasoft long-lasting reusable hydrogel-based sensor patch for Biosignal recording.

Wu, S., Wu, S., Zhang, X., Feng, T., & Wu, L. (2023). Chitosan-based hydrogels for bioelectronic sensing: Recent advances and applications in biomedicine and Food Safety.

Zhao, W., Tu, H., Chen, J., Wang, J., Liu, H., Zhang, F., & Li, J. (2026). Functionalized Hydrogels in neural injury repairing.

Zhou, L., Ramezani, H., Sun, M., Xie, M., Nie, J., Lv, S., He, Y. (2020). 3D printing of high-strength chitosan hydrogel scaffolds without any organic solvents.