Biodegradation of a photocuring hydrogel layer in vitro to form an interface between biological tissue and a neurostimulator electrode

Introduction

This work investigates third-generation bioelectronics combining soft electronic materials with multifunctional adaptive human–machine interfaces responsive to mechanical and environmental stimuli. Emphasis is placed on a biodegradable material for the tissue–electrode interface of a neurostimulator, as implantation triggers a foreign body response that causes a sharp impedance increase upon encapsulation.

Methods

Photocuring employed an HTTP MARK MOPA laser (600 mW, 100 ns, 30 kHz, 1070 nm). The hydrogel comprised MWCNTs (0.027 mg/mL), reduced graphene oxide (0.013 mg/mL), eosin Y (0.3 mg/mL), magnetite nanoparticles (0.04 mg/mL), albumin (40 mg/mL), chitosan (24 mg/mL), and collagen (26 mg/mL).

Conductivity was measured with an ST2258C station and an ST2571A-F01 four-point probe on a 9×9 mm layer; thickness was determined micrometrically. Biodegradation was assessed via passive hydrolysis in phosphate-buffered saline and enzymatic hydrolysis in proteinase K/lysozyme (1/1) over one week, with activities of 0.8–42 nkat/mL and solution refreshed every two days. Biocompatibility was evaluated on Neuro 2A cells over three days.

Results

The material exhibited a conductivity of 76.8 mS/cm at a thickness of 350 μm. The hydrogel swelled to 385%, with dry and wet masses of 25 mg and 53 mg. Under passive hydrolysis, mass loss reached 83% (day 4) and 90% (day 8), with swelling of 203% and 59%. Enzymatic hydrolysis at 8 nkat/mL yielded the highest degradation rate: 88% mass loss (day 4) and 94% (day 8), with swelling decreasing to 79% and 6%. Cell proliferation increased by 20% versus the control without hydrogel.

Conclusion

Implanted electrodes become progressively encapsulated by glial cells, increasing impedance and degrading signal quality, ultimately rendering the neurostimulator ineffective. We propose a temporary, gradually resorbable layer on the electrode surface, leaving it in a conditioned environment.

Funding: The work was supported by the Ministry of Education and Science of the Russian Federation (project FSMR-2024-0003).