Bioelectronic devices are being increasingly used to study, diagnose, and treat various physiological processes and conditions. However, a mechanical mismatch between rigid and dry materials of conventional electronics and soft and hydrated biological components limits interfacial surface area between the device and biological components and can even cause a foreign body response to implantable devices. This has motivated the development of conducting polymer hydrogels to create more tissue-mimetic bioelectronic interfaces. These water-swollen polymer networks are not only soft and hydrated like the body but also inherently electronically conducting, allowing them to act as an interfacial bridge between electronic materials and biological components. However, many conducting hydrogels are optimized for a specific mode of biointerfacing, such as injection, conformal surface contact, or encapsulation, rather than as versatile platforms capable of adaptation. As a result, the application and impact of these materials is limited by the need to redesign them for each new bioelectronic interface.

Our project proposes the use of a conducting granular hydrogel as an adaptable conducting hydrogel bioelectronic interface. We use a highly scalable and inexpensive water-in-oil emulsion methodology to fabricate micrometer-scale hydrogel particles (i.e. microparticles) from the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). When densely packed, these microparticles form an electronically conducting granular hydrogel with dynamic mechanical properties. The conducting polymer PEDOT:PSS imparts inherent electronic conductivity to the individual microparticles, while contact between adjacent microparticles facilitates electronic conductivity throughout the granular hydrogel. Frictional forces at microparticle-microparticle junctions are the most influential forces within the material and give rise to shear-thinning and self-healing dynamic mechanical properties. Under no or low forces, the material behaves as a paste-like solid. Application of sufficient force disrupts the microparticle-microparticle contact points and frictional forces, causing the material to flow like a liquid. Upon removal of the applied force, microparticle-microparticle contact points and frictional forces are recovered, and the material begins to behave as a paste-like solid again. These dynamic mechanical properties enable extrusion of the conducting granular hydrogel via 3D printing and maintenance of 3D printed shapes, conformation to topographically diverse tissue surfaces, and encapsulation of tissue structures, making it highly adaptable to a variety of interfaces.

To demonstrate the unique capabilities afforded by the dynamic mechanical properties and electronic conductivity of the material, we formed bioencapsulating electrodes with the conducting granular hydrogel and validated their performance via electroantennography. Electroantennography measures changes in potential across insect antennae in response to odors to better understand insect olfaction and guide the development of biohybrid sensors for complex, dynamic environments. Conventional rigid electronic materials can only achieve limited contact with the small, cylindrical antennas. However, the combination of dynamic mechanical properties and electronic conductivity facilitated the formation of bioencapsulating granular hydrogel electrodes around the antennae ends and successful recording of potential changes in response to odor applications. With further development, we envision the conducting granular hydrogel as a highly adaptable bioelectronic platform capable of forming 3D-printed, conformal, and bioencapsulating interfaces that reduce the need for specialized materials while enhancing the impact of hydrogel-based bioelectronics.