Conducting Polymer Granular Hydrogel Bioinks for 3D Printed In Vitro Bioelectronic Devices

Conjugated polymers have recently been used in their hydrogel form for the creation of soft bioelectronics with improved cell interfacing and the potential for enhanced measurement and stimulation of cellular activities. Cell-encapsulating hydrogels offer a truly 3D culture environment and when designed appropriately can be used as bioinks for 3D printing which grants spatially controlled deposition of cells and materials simultaneously. We developed a bioink based on the conducting polymer poly(3,4-ethylene-dioxythiophene):polystyrene sulfonate (PEDOT:PSS) by creating a granular hydrogel form. Emulsion methodology was developed to fabricate PEDOT:PSS hydrogel microparticles (microgels) with high circularity and monodispersity. Filtration techniques were used to size select microgels with diameters between 10 and 60 µm. When centrifuged to remove significant water, the conducting polymer microgels achieved dense packing characteristic of a granular material state. Increasing centrifugal force decreased the void fraction of the material which increased conductivity. Rheological investigations confirmed shear-thinning and self-healing properties, both ideal for 3D bioprinting for extrusion and ability to keep shape, respectively. Granular PEDOT:PSS hydrogels were in fact 3D printable via pneumatic extrusion and formed various 3D configurations. Evaluation of the material’s printability in the granular state also revealed that increased centrifugal force facilitated better printability as evidenced by increased maximum filament length and filament diameter better matching the chosen nozzle diameter. Structural stability of the material within an aqueous environment for up to three months was achieved using a collagen hydrogel overlay. Human dermal fibroblasts encapsulated and cultured within the granular hydrogel showed high cell viability over fourteen days demonstrating cytocompatibility. This developed conducting granular hydrogel bioink exhibits balanced properties of printability, conductivity, and cellular responses for the additive manufacturing of future in vitro bioelectronics.