# Polydimethylsiloxane (PDMS)-Based Flexible Resistive Strain Sensors for Wearable Applications
## 1 Introduction  
Wearable electronic technology integrates electronic devices into clothing, accessories, human skin, and is even implanted in vivo and realizes measurements of body sensing, data storage, and mobile computing [1; 2; 3; 4; 5; 6; 7; 8; 9]. The front-end component of wearable systems is a wide variety of wearable sensors, which can be used to measure all kinds of physiological parameters, such as body temperature [10; 11], myoelectricity [12], heart rate [13], and blood glucose [14]. They are also used for human motion detection, such as acceleration [15], muscle ductility [6; 16], and foot pressure [17; 18]; as well as environment monitoring, for example, position coordinates [19], temperature [20; 21], humidity [22; 23], and atmospheric pressure [24]. Different from other functional wearable sensors, strain sensors are attached to the joint or even directly mounted on the muscle for human motion inspection [25; 26; 27; 28; 29]. They must be capable of not only bending to conform to the topography of the skin but also stretching during natural body activities. Recently, some wearable strain sensors have been proposed to monitor the bending/unbending state of a joint [30] and to collect physiological signals of a volunteer in relaxation and after exercise [31; 32].  
Wearable strain sensors typically consist of a sensitive element and a supporting mechanism, and their flexibility is deeply influenced by the sensing mechanism and the substrate material. Compared to other transduction techniques, including capacitive [33], optoelectric [34], and piezoelectric [35], the resistive transduction technique is utilized as the primary choice of sensing mechanism owing to its high spatial resolution, relatively simple input/output control system, and low noise susceptibility [36; 37].  
Polydimethylsiloxane (PDMS) has been a common material for microfabrication for decades. Thanks to its stable chemical properties, biological compatibility, transparency, and good thermal stability, and especially its adhesion and non-adhesion areas that are clearly visible under UV light and can be easily adhered to the surface of electronic materials [38; 39; 40], PDMS is widely used for microfluidic chips [41], micropumps [42], and electronic skin [43; 44]. Particularly, PDMS as a flexible substrate for wearable resistive strain sensors has been developed because of its broad applications in entertainment technology, human-machine interfaces, personal healthcare, and sports performance monitoring, etc. [45; 46; 47].  
In this review, we primarily focus on microfabrication materials and methods, strain sensing performances, and the applications of PDMS-based flexible resistive strain sensors. The article is organized as follows: first, various component materials and the fabrication methods are described in detail. Second, we compare and summarize the performance parameters between different PDMS-based flexible resistive strain sensors in terms of stretchability, gauge factor, linearity, and durability. Third, we emphasize the applications of PDMS-based flexible resistive strain sensors in some related fields. The last section provides conclusions and the challenges in future research.