Synthesis and characterization of all-polymer electrically conductive hydrogels
Date
2024
Authors
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Publisher
University of Delaware
Abstract
The bio-electronic interface is a critical area of research focused on integrating electronic devices with biological systems. One major challenge in this field is the mechanical mismatch between conventional inorganic electronics and the soft, flexible nature of biological tissues. This mismatch can lead to discomfort, damage, and unreliable performance when interfacing with biological systems. To address this issue, researchers are developing all-polymer electrically conductive hydrogels. These materials are soft, biocompatible, and capable of adopting multiple properties, making them ideal for bio-electronic applications. Conductive hydrogels can conform to biological tissues, providing a more comfortable and reliable interface. Additionally, their electrical conductivity enables effective signal transmission between electronic devices and biological systems. By combining mechanical flexibility with electrical functionality, conductive polymer hydrogels represent a promising solution for advancing the integration of electronics with the human body for applications in medical diagnostics, neural interfaces, and wearable devices. This thesis will describe new methods we have developed towards synthesizing all-polymer conductive hydrogels with a wide range of stiffness (kPa – MPa), adhesion, and capability to photo-crosslink for advanced manufacturing. ☐ Firstly, a simple method to transform a range of different hydrogels into electronically conductive hydrogels without affecting the mechanical properties of the parent hydrogel is reported by in-situ polymerization of a water-soluble and neutral conducting polymer precursor: 3,4–ethylenedioxythiophene diethylene glycol (EDOT DEG). The resulting conductive hydrogels are homogenous, have conductivities around 0.3 S m−1, low impedance, and maintain an elastic modulus of 5–15 kPa, which is similar to the preformed hydrogel. ☐ Secondly, we introduce a novel approach to synthesize conductive hydrogels by photo-printing in one step. Our approach combines the simultaneous photo-crosslinking of a polymeric scaffold, poly(styrene sulfonate-co-coumarin acrylate) (P(SS-co-CoumAc)), and the polymerization of 3,4-ethylene dioxythiophene (EDOT), without additional photocatalysts. The photo-mediated conductive hydrogels exhibit an elastic modulus of 4–11 MPa, a strain at break up to 16%, coupled with an electronic conductivity of 9.2 S m–1 suitable for wearable electronics. Furthermore, the conductive hydrogels can be photo-patterned to achieve micron-sized structures with high resolution. The photo-crosslinked hydrogels are used as electrodes to record stable and reliable surface electromyography (sEMG) signals. ☐ Thirdly, building upon the method introduced in Chapter 2, a versatile approach is reported to address the challenges associated with conductive hydrogels in adhesive bioelectronic interfaces. The goal is to achieve optimal mechanical and electrical properties, as well as adhesion for enhanced integration into devices and biological interfaces. This method incorporates a conductive PEDOT-DEG polymer into zwitterion-containing adhesive preformed hydrogels. The resulting adhesive and conductive hydrogels exhibit electrical conductivity around 0.3 S m–1, low impedance, and tunable elastic moduli of 10–20 kPa. These conductive hydrogels demonstrate conformal adhesion to various substrates, including human skin, plastics, and metals. ☐ Lastly, we describe our preliminary efforts towards conductive materials that can modulate their mechanical properties between ‘hard’ and ‘soft’ under a low-voltage electrical stimulus. The approach involves synthesizing conductive polymer hydrogels coordinated with Fe(III) ligand end-groups, where Fe(III) acts as a reversible crosslinker that weakens when reduced to Fe(II), allowing the material to become 'softer' with a negative voltage and 'harder' with a positive voltage. Furthermore, incorporating conductive polymers into metal-crosslinked polymeric networks could facilitate rapid switching between 'hard' and 'soft' materials under low applied voltages. These materials would offer lightweight and conformable options for potential applications in wearable actuators, soft motors for seamless human–machine interfaces, and fluidic control gates for drug delivery or microfluidics.
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Keywords
Inorganic electronics, Advanced manufacturing, Hydrogels, Surface electromyography, Polymer