Soft and Stretchable Conductive Hydrogel-based Implants for Neuromodulation

Introduction: Implantable electronic devices have broad applications for various disease diagnoses and treatments. Existing commercial electronic implants rely on conductive materials with high Young’s Modulus, such as gold and platinum. Mechanical disparity across rigid electronics-tissue interfaces has been a significant drawback affecting the biocompatibility of electronic implants. Soft materials with Young’s modulus matching that of biological tissues can significantly reduce adverse immune responses and are highly desired for next-generation bioelectronics.

Materials and

Methods: We introduced a nontoxic additive, for example, a nutritive sweetener D-sorbitol, to poly(3,4- ethylenedioxythiophene):poly(styrene sulfonate) aqueous solution to yield conductive hydrogel. The micro-molding approach is employed to fabricate conductive hydrogel (CH)-based soft electronic implants. We characterized the mechanical, electrochemical, and electrical properties of CH and validated the performance of CH based soft implants for sciatic nerve stimulation and simultaneous muscle action, potential recording in rats. Also, in vitro and in vivo biocompatibility are analyzed through live and dead cell assay, histology, and immunofluorescence. We also analyzed rat gait behavior over chronic implantation.

Results, Conclusions, and Discussions: In this work, we realized soft CH with electronic and ionic dual conductivity for high-performance electronic implants. The CH exhibits a low
Young’s modulus of ~3 Mpa is much lower than commonly used metals in electronic implants, such as platinum (~170 Gpa). The CH shows an excellent electrical conductivity of 56 S/cm. At the sweeping rate of 100 mV s, the CH-based electrode possesses a charge storage capacity (CSC ) of 102 mC/cm, much higher than the platinum electrode with a CSC of 2 mC/cm. The CH-based electrode offers a charge injection capacity of 46.91 mC cm higher than the platinum electrode (0.56 mC/cm ). In vivo experiments show that CH-based soft electrodes can activate a leg movement at a low voltage of 0.1V at 100Hz, while the platinum electrode requires a minimum of 0.25V. The histological and immunohistochemistry analysis of electrode and nerve tissue interface and rat gait behavior studies confirmed the superior biocompatibility of CH electrodes to platinum ones.

CH-based soft electronic implants reduced the modulus discrepancies between the soft biological tissues and the implants, thus lessening the adverse immune response. The soft CH electrodes demonstrated more efficient neuromodulation than platinum electrodes. The rats with CH implanted showed no sign of inîammation in the chronic biocompatibility investigation. The biocompatible, high-performance CH can be employed to realize various implantable neuromodulation devices, such as deep brain stimulators and vagus nerve stimulators.

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