This paper provides a review on some of the significant research work done on wearable flexible sensors (WFSs). Sensors fabricated with the flexible materials have been attached to a person along with the embedded system to monitor a parameter and transfer the significant data to the monitoring unit for the further analyses. The use of wearable sensors has played a quite important role to monitor the physiological parameters of a person to minimize any malfunctioning happening in the body. This paper categorizes the work according to the materials used for designing the system, the network protocols, and different types of activities that were being monitored. The challenges faced by the current sensing systems and future opportunities for the WFSs regarding its market values are also briefly explained in this paper.

The outstanding properties of graphene have initiated myriads of research and development; yet, its economic impact is hampered by the difficulties encountered in production and practical application. Recently discovered laser-induced graphene is generated by a simple printing process on flexible and lightweight polyimide films. Exploiting the electrical features and mechanical pliability of LIG on polyimide, we developed wearable resistive bending sensors that pave the way for many cost-effective measurement systems. The versatile sensors we describe can be utilized in a wide range of configurations, including measurement of force, deflection, and curvature. The deflection induced by different forces and speeds is effectively sensed through a resistance measurement, exploiting the piezoresistance of the printed graphene electrodes. The LIG sensors possess an outstanding range for strain measurements reaching >10% A double-sided electrode concept was developed by printing the same electrodes on both sides of the film and employing difference measurements. This provided a large bidirectional bending response combined with temperature compensation. Versatility in geometry and a simple fabrication process enable the detection of a wide range of flow speeds, forces, and deflections. The sensor response can be easily tuned by geometrical parameters of the bending sensors and the LIG electrodes. As a wearable device, LIG bending sensors were used for tracking body movements. For underwater operation, PDMS-coated LIG bending sensors were integrated with ultra-low power aquatic tags and utilized in underwater animal speed monitoring applications, and a recording of the surface current velocity on a coral reef in the Red Sea.

Physical sensors form the fundamental building blocks of a multitude of advanced applications that detect and monitor the surroundings and communicate the acquired physical data. The everlasting need for more compliant, low-cost, and energy-efficient sensor solutions has led to considerable interest in enhancing their features and operation limits even further. While graphene has emerged as a promising candidate material, due to its outstanding electrical and mechanical properties, it is still not available in large volumes for practical applications. Meanwhile, Laser-Induced Graphene has opened new perspectives for a versatile, durable, printed physical sensing platform capable of detecting various physical parameters across a range of conditions and subjects. In this review, LIG physical sensors were categorized into four broad types based on their transduction mechanism: mechanical, thermal, magnetic, and electromagnetic. We summaries various design strategies established for preparing reliable physical sensors without the involvement of chemical treatments, synthesis, and multi-step fabrication processes. The review considers the effects of laser choice, lasing environment, and parameters on graphene properties. We also discuss a broad spectrum of applications of LIG physical sensors in fields ranging from healthcare, tactile sensing, environmental monitoring, energy harvesting, and soft robotics to desalination and THz modulation.