Biomechanical energy as a power source for ingestible devices

Ingestible devices have become useful tools for non-invasive monitoring of the gastrointestinal (GI) tract in real-time for physiological changes and potentially for intestinal sampling, drug delivery and physical- and chemical sensing. Currently, most of the ingestible electronic devices rely on batteries to power the functional electronic units, however, the presence of batteries makes these devices bulky while posing a risk in terms of biocompatibility. In this review, inherent biomechanical movements within the GI tract are presented as a power source for ingestible devices as an alternative to batteries by comprehensively discussing the mechanical movements occurring along the GI tract in terms of their frequency and associated forces. This is followed by discussions on design and performance of various miniature ingestible and implantable mechanical energy harvesting devices that can be applied in the GI tract with a focus on device development, electromechanical output and in vivo performance. Further, discussions are presented on flexible electronics that are interfaced with the energy harvesting devices for power management. Finally, the current state and the challenges of electromechanical characterization of mechanical energy harvesting devices for the GI tract are discussed along with a perspective on standardized testing of such devices.