Recent advances in noninvasive flexible and wearable wireless biosensors

Flexible and wearable biosensors have great potential to interface with skin and noninvasively extract biofluids for real time and continuous monitoring of physiological status. Soft electronics is the prime factor in these wearables, regardless of sensing mechanism and fabrication technology. Wireless connectivity would be a valuable addition to enhance wearable biosensor's scope for remote and resource limited settings. These skin-adaptable, user-friendly, battery-free (although not all), and noninvasive devices continuously and simultaneously monitor wearer well-being and transfer data wirelessly. Thus, they have a great potential to improve quality of life with timely diagnostics and hence early treatments. However, they remain in the early stages, with relatively conventional sensing modalities, battery requirements, soft electronics fabrication limitations, and practicable size restrictions to retain skin compatibility. We classify wearable biosensors by sensing functionality, such as skin temperature, pH, heart rate, sweat glucose, uric acid, sweat electrolyte, cerebrospinal shunt flow, and toxic chemicals, and discuss challenges and prospects for these biosensors.