Endogenous Biomaterials for Implantable Energy Storage
Authors: Milroy Craig 
Published: February 2020

The United States National Academy of Engineering has included the need to “Advance health informatics” among its 14 Grand Challenges for Engineering in the 21st Century. Bioelectronics, in concert with the Internet of Things, which seeks to enable the facile integration of implantable devices with “big data”-driven health monitoring, promises to empower personalized healthcare. For example, advances in bioelectronics have produced implantable devices for in vivo biosensing, monitoring, and therapeutics that are fabricated with biocompatible materials and seamlessly interface with biological systems and living tissue, and could therefore have enormous utility for “precision medicine”. In contrast, the field of implantable energy storage is relatively unexplored. Batteries for implantable devices currently require bulky casing because they contain toxic electrolytes and non-endogenous active materials that elicit immune responses which negatively impact device performance and long-term functionality. Therefore, development of new materials is paramount for safety and miniaturization.
Ideally, implantable devices could achieve maximum biocompatibility if they exclusively comprised endogenous materials. In this context, dopamine is an electroactive, essential neurotransmitter that exhibits catechol/quinone redox functionality, and is thus an excellent candidate for implantable energy-storage. However, electronics and batteries for implantable medical devices must also resist nonspecific protein adsorption, as biofouling is the primary contributor to decreased sensitivity of implantable devices such as glucose sensors. Biological hydrogels have proven useful for masking implantable devices since they are soft, elastic, and highly permeable to low molecular weight molecules; they also effectively resist the non-specific protein adsorption that leads to cell adhesion and biofouling.
To combine the electroactive properties of dopamine and the non-adhesive biocompatibility of hydrogels, we chemically conjugated hyaluronic acid (HA) and dopamine (DA) via one-step carbodiimide conjugation to form an electroactive biocomposite. Hyaluronic acid is an endogenous polysaccharide found throughout human endothelial and neural tissues, and is a primary constituent of the extracellular matrix. It has well-characterized properties, is generally non-adhesive to cells, and inhibits glial cell response. The dopamine-hyaluronic acid (DAHA) composite can be electrodeposited onto electronically conducting substrates to form an electroactive polydopamine-hyaluronic acid (p(DAHA)) biopolymer for bioelectronic energy storage. The pseudocapacitive p(DAHA) exhibits catechol-quinone interconversion, stable, long-term electroactivity for 400 cycles, and high pseudocapacitance (up to ~ 900 F g-1) and discharge capacity (~ 130 mAh g-1 at ~ 10 A g-1) in physiological phosphate-buffered saline solution. These characteristics predispose it for bioelectronic energy storage, i.e., as a supercapacitor or, when coupled with an implantable Ag/AgCl electrode, a biobattery with an operating voltage ~ 0.85 V.