Skin-Interfaced Flexible Sensors for Continuous Monitoring of Thermal, Mechanical, Electrical, and Respiratory Signals

Abstract

Skin-interfaced flexible sensors enable physiological monitoring during daily activities, sleep, and rehabilitation, but a sensitive device or a clear waveform does not by itself establish the validity of the physiological information obtained. Many existing reviews emphasize sensor materials, fabrication, device design, and sensing performance. This review instead examines how physiological information is preserved or altered from its origin to its eventual interpretation. We focus on flexible physical sensors for skin temperature, mechanical activity, surface biopotentials, electrodermal activity, and respiration-related monitoring. The scope is restricted to selected physical and skin surface electrical sensing pathways, while optical, biochemical, and implantable technologies are not reviewed in detail. The analysis links the physical quantity available at the skin to its transmission across the skin–device interface, sensor response, signal fidelity, and the physiological output that the measurement can reasonably support. This approach distinguishes direct measurements from derived or clinically interpreted quantities and shows where additional reference-based validation becomes necessary. Across the reviewed modalities, major limitations arise when interface conditions or environmental and mechanical interference alter the relationship between the physiological source and the recorded signal. Reliable monitoring therefore depends on preserving this relationship under intended wearing conditions and demonstrating agreement with references appropriate to the claimed output. By focusing on this measurement process, the review complements materials- and device-oriented surveys and provides a basis for judging flexible sensors according to the physiological information that their measurements can support.