Recently, Prof. Yude Su’s group at SIAR, University of Science and Technology of China, published a perspective paper titled "Wearable Microbial Fuel Cells for Sustainable Self-Powered Electronic Skins” in ACS Applied Materials & Interfaces. This work is a collaboration with Prof. Ming Zhou at Northeast Normal University and Prof. Lu Lu at Harbin Institute of Technology.
Electronic skin (e-skin) is a fast-growing technology that has broad applications in real-time health diagnostics and monitoring. The emergence of power-consuming e-skins also evokes the development of wearable energy suppliers. Existing e-skin platforms are predominately powered by bulky batteries or other energy storage devices which can suffer frequent recharges and undesirable replacements. Energy autonomous system, where the power to drive the wearable devices is collected from ambient surroundings or human body, paves the way to achieve continuous and sustainable operation of e-skins. Potential autonomous energy resources for e-skin include sunlight, human motion, sweat biofuel, body moisture, and body heat, among which the biofuel in human sweat is a most natural and accessible candidate. Recently, people have developed skin-worn enzymatic biofuel cells, where the substances in human sweat serve as fuels to autonomously produce electrical power. Such wearable enzymatic biofuel cell has demonstrated promising power-delivery efficiency when integrated into e-skin platforms. However, enzyme-based fuel cells typically suffer poor stability and short lifetime (several days), limiting their practical applications. In contrast to enzyme-based fuel cell, microbial fuel cell (MFC) utilizes metabolism of microbes (can self-repair and self-replicate), and can significantly improve the robustness of the system. Capable of delivering power for up to several years, wearable MFC represents a promising candidate for sustainable self-powered e-skin. Additionally, in contrast to the costly preparation of pure enzyme, wearable MFC takes advantage of the abundant and easy-access electrogenic microbes in particular those inhabiting on human skin.
The standardized architecture of wearable MFCs has not been fully established, but two types of configurations, the super thin patch MFC (Fig. 1) and textile MFC (Fig. 2), have been proposed.
The former one with a sandwich electrode assembly can be attached directly on the skin and can easily access the sweat biofuels (Fig. 1). Its flat structure allows incorporation of a microfluidic module for sweat collection and holding of excess sweat in a defined reservoir. Its improvement should lie in integrating more subunits within one super thin device, which allows enhancement of power output by connecting subunits in series or parallel.
Fig. 1 Schematic illustration of an on-skin super thin patch MFC and proposed device configurations
The latter one was constructed based on a nonconductive yarn, where the anode and cathode were fabricated by coating conductive material and catalyst horizontally along the yarn, and a pristine region of the yarn between anode and cathode serves as an ion exchange channel (Fig. 2). Its 1D structure can be extended to 2 and 3D woven fabrics to further integrate more subunits to enhance overall power output, but the structure cannot help collect and store sweat.
Fig. 2 Schematic illustration of a textile MFC device.
Despite of the great promise of wearable MFCs, the cytotoxicity and pathogenicity of microbes need to be addressed. One feasible packaging approach is to confine these microbes with a kind of semipermeable membrane. In addition, advanced encapsulation strategy can be employed to immobilize the microbes and prohibit any potential leakage.
Albeit conceptually viable, the development of wearable MFC is still in its infancy. In addition to the biosafety concern, several other issues need to be addressed to improve the reliability of wearable MFCs. These issues include the instable power output of MFC, the appropriate selection of on-body position to place wearable MFC, realization of on-demand power supplying from wearable MFCs, over-growth of waste biomass in wearable MFCs. Resolving these limitations will enable wearable MFC as a reliable autonomous “living” electricity generator to sustainably power the e-skin.
Paper link: https://pubs.acs.org/doi/10.1021/acsami.2c00313
