Dynamic Mechanical Properties in Conducting Hydrogels for Bioencapsulating Electronics

Dynamic bonding in hydrogels leads to valuable biomedical materials properties. Shear-thinning and self-healing are used for (1) injectable materials that fill a tissue void (such as for traumatic injuries), (2) injectable cell therapeutics, (3) 3D extrusion printing of biomaterials and bioprinting of cells, and (4) as matrices for tissue engineering which can be dynamically remodeled by cells. These properties have been achieved to some degree in electronic materials, but exploiting these properties for cell- and tissue biointerfacing specifically has been very limitedly explored. Here, we aimed to achieve dynamic mechanical properties in conducting polymer hydrogels by fabricating poly(3,4-ethylene-dioxythiophene):polystyrene sulfonate (PEDOT:PSS) into a granular material. A granular material is comprised of tightly packed solid particles surrounded by a liquid or gas. In this jammed state, the presence of interparticle interactions keeps the particles together and the aggregate of particles behaves as a solid material. These interactions, however, are weak enough that when a sufficient force is applied, the interactions are disrupted and particles displace relative to each other. Disruption (shear-thinning) and reformation (self-healing) can occur repeatably, thus giving rise to the material’s dynamic mechanical properties. As the first step, we developed methods for fabricating PEDOT:PSS hydrogel microparticles using water-in-oil emulsion methods. We next demonstrated these particles can successfully generate granular materials after packing by vacuum filtration and possess the appropriate shear-thinning and self-healing properties. We have also shown this material can be used as an ink for 3D extrusion printing as a material by itself or as a bioink when cells are safely encapsulated into the material, as quantified by high cell viability when residing in the microporous structure among the particles. We envision that our success of achieving this unique material class in conducting hydrogels will open doors for new uses, including cell-laden or “biohybrid” electronics as well as tissue-injectable electrodes.