Hydrogels and conductive hydrogels for implantable bioelectronics

Abstract

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

Lifetime engineering of bioelectronic implants with mechanically reliable thin film encapsulations

“…Hydrogel materials have also been implemented in a variety of ways in recent years [84,136,[145][146][147][148]. Composed of a loosely crosslinked polymer matrix interspersed with water, hydrogels are notable for their superior mechanical compatibility with nervous tissues, and can easily be designed to exhibit elastic modulus values of <100 kPa [149]. Due to their low modulus and viscoelastic properties, hydrogel-based devices can achieve excellent conformability to convoluted surfaces, making them great substrate candidates for cortical electrode arrays.…”

“…Due to their low modulus and viscoelastic properties, hydrogel-based devices can achieve excellent conformability to convoluted surfaces, making them great substrate candidates for cortical electrode arrays. Common materials for hydrogels include polyvinyl alcohol (figure 1(l)) and sodium alginate (figure 1(M)), as well as collagen, gelatin, fibrin, hyaluronic acid, poly(ethylene glycol), polyacrylamide, and poly(hydroxyethyl methacrylate) [149]. Hydrogels can be combined with conductive materials to create soft conducting layers as alternatives to metallized tracks [150], a promising avenue for the creation of highly stretchable interconnects.…”

An oriented MXene/silk fibroin nanofiber hydrogel with high strength and strain response ability

“…Nowadays, hydrogels have attracted much attention due to their unique swelling ability, flexibility, biocompatibility, porosity and other properties. [8][9][10] They are widely used in many fields such as bioengineering, smart sensing, and environmental pollution treatment, and can achieve excellent electrical conductivity and strain response by doping with ions, [11][12][13] doped carbon nanomaterials, [14][15][16] or doped metal nanomaterials. 17,18 However, the relatively poor mechanical properties and fragility characteristics limit the application of hydrogels in flexible sensor components.…”

A 10-micrometer-thick nanomesh-reinforced gas-permeable hydrogel skin sensor for long-term electrophysiological monitoring

“…Skin bioelectronics capable of long-term, continuous health monitoring offers a powerful route for timely disease prevention, screening, diagnosis, and treatment in personalized health care ( 1 – 3 ). Hydrogels, as a group of cross-linked polymers with high water content, have gained considerable attention in skin bioelectronics by virtue of their similarities to biological tissues and versatility in electrical, mechanical, and biofunctional engineering ( 4 – 6 ). Hydrogel-enabled skin bioelectronic devices have seen tremendous health care applications ( 7 , 8 ), such as biometric signals detection ( 9 ), human-machine interfaces ( 10 ), wound healing ( 11 ), and precision therapy ( 12 ).…”