Hydrogels and conductive hydrogels for implantable bioelectronics

Hydrogels are a class of soft materials, which display unique biomimetic properties to biological tissues. Their mechanical properties, high water content, and porosity resemble that of extracellular matrix so that cell growth and proliferation can be reliably supported. In vitro studies report that mechanosensitive cells found in the central nervous system, such as astrocytes and glia, display reduced activation, thus promoting lower foreign body reaction, when cultured on hydrogel substrates of <1-kPa modulus. This observation provides an opportunity to explore whether soft hydrogels should be integrated in or form implantable neural interfaces and offer long-term biointegrated neurotechnologies. This article highlights recent progress in hydrogel materials and associated technologies for the design of implantable bioelectronics. Essential structural, mechanical, and electronical properties of hydrogels and composite hydrogels are briefly reviewed. Manufacturing methods suitable for these multiscale and multifunctional materials are presented. The final section presents hydrogel-based implantable bioelectronics for the brain and outlines current challenges and future opportunities. Graphical abstract