Multifunctional Conductive Hydrogels for Next Generation Bioelectronics

The mismatch between electronic devices and biological tissue is based on two fundamental differences in their mechanical and functional properties causing a separation between the two worlds. While intensive research has been conducted to reduce the mechanical mismatch, reducing the functional disparity has been largely neglected. In biological systems, electrical and biomolecular signals are closely interconnected. Electronic devices on the other hand rely purely on electronic signal transmission. A technological recapitulation of the biological principle in one soft materials system could bridge both the mechanical and the functional gap between electronics and living matter and could enable seamless integration of electronic devices into biological tissues.<br/>Targeting that challenge, we developed a new class of conductive metamaterials which mimic but also transcend the natural extracellular matrix. We synthesized a semi-interpenetrating network of the electroconductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) within sulfated/sulfonated polymer hydrogels (SSPH). Due to the modular character of the hydrogel system, the PEDOT:SSPH materials are tunable in their electrical properties such as impedance and charge storage capacity. Charge storage capacities of more than 1100 mC/ml could be reached enabling highly efficient electrical stimulation using these materials as electrodes or electrode coatings. Furthermore, by tuning the integral and local anionic charge density of the doping SSPH matrix, the specific affinity of the hydrogel to differently charged biomolecules could be varied. In combination with PEDOT, which has an electrically tunable redox state, the PEDOT:SSPH materials are capable of delivering differentially charged small molecules and proteins in electronically controlled ways. Depending on the polarity of the applied potential differently charged molecules could be released. Furthermore, by tuning the amplitude of the applied potential, the amount of the released molecules could be precisely controlled. By fabricating organic electrochemical transistors (OECTs) we demonstrated the feasibility of creating hydrogel sensor units from the PEDOT:SSPH materials. We showcased the potential of the sensors to act in direct combination with the biomolecule release system. Using the hydrogel OECTs low oxygen levels could be measured in physiological solutions. Subsequently an active release of Vascular Endothelial Growth Factor (VEGF) was induced. This caused the formation of blood vessel like structures in a 3D HUVEC cell culture embedded in the biomimetic hydrogel.<br/>With this, our newly developed biomimetic metamaterials combine and link tunable electrical conductivity and specific biomolecular affinity. Applying these materials to design multimodal tissue interfaces that are able to stimulate and sense biological signals, we aim to further push the boundaries between living matter and electronic devices.