# Polydimethylsiloxane (PDMS)-Based Flexible Resistive Strain Sensors for Wearable Applications
## 2 Materials and Fabrication Methods  
Flexibility and extensibility of materials are key to fabricating wearable resistive strain sensors. The substrate functions as the flexible support structure of the wearable resistive strain sensors; the properties of the substrate materials directly determine the sensor's elastic performance. Many commercialized polymers can be used to construct flexible substrates for wearable resistive strain sensors; these include polyethylene terephthalate (PET) [48], polyimide (PI) [49], polyethylene (PE) [50], polyurethane (PU) [51], Ecoflex [52], and PDMS [53]. Among them, PDMS is the most widely used material for flexible substrate fabrication because of its excellent comprehensive performance. Another important consideration for flexible sensors is the appropriate active material as the sensing element. The function of the sensing element is to convert external mechanical stimuli into electrical signals. To date, PDMS-based flexible resistive strain sensors have been fabricated by using conducting materials (e.g., EGaIn, Ti/Au, silver nanowires (AgNWs), and Ag nanoparticles (Ag NPs)) [54; 55; 56; 53], carbon materials (e.g., graphene, graphene oxide (rGO), carbon nanotubes (CNTs), and carbon black (CB)) [57; 58; 4; 59], and their hybrid micro/nano-composite [60]. As the material science and microelectromechanical (MEMS) technology has developed, many methods have become available for fabricating flexible sensors. In this section, the typical fabrication methods of PDMS-based flexible resistive strain sensors are summarized as follows. The filtration method [61; 62] is a powerful tool for obtaining uniform composite films with filter membranes; the films can be used as a sensing element converting the mechanical deformation into electrical signals. [PERSON] et al. demonstrated a multidimensional strain sensor composed of prestrained AgNW percolation network lays, which was obtained by the filtration of prestrained AgNW solution and transferred to a PDMS substrate [54]. [PERSON] et al. reported highly stretchable strain nanopapers by mixing crumpled graphene with nanocellulose through vacuum filtration to obtain uniform composite films, and then embedding in a PDMS matrix (Figure 1a) [63]. Printing technology [64; 65] involves printing the composite through a deposition nozzle with directly pattern electrodes onto a PDMS substrate as the resistive sensing element. A stretchable electronic was fabricated by [PERSON] et al. using a direct writing approach where the liquid metal EGaIn was injected into a PDMS substrate through a blunt syringe needle [66]. [PERSON] et al. applied a coaxial printing method to embed EGaIn into the PDMS and developed a stretchable and conductive cable for monitoring the frequency and amplitude of a curved object (Figure 1b) [67]. [PERSON] et al. reported a new type of flexible strain sensor that can detect both tensile and compressive strains, using single-step direct transfer patterning to transfer Ag NPs-ink from the donor substrate to the microstructured PDMS [47]. The micromolding method [68; 45; 58; 69; 70] employs micromolds to fabricate electrodes or flexible substrates that are especially suitable for irregular microstructures. [PERSON] et al. demonstrated highly sensitive skin-mountable strain gauges packaged in the substrate of PDMS with carbon black-doped PDMS as straight resistors, and joined by serpentine-shaped interconnects made with carbon-nanotube-doped PDMS through a repeated photoresist pattern and rinse [58]. A piezoresistive sensor-integrated PDMS cantilever was proposed by [PERSON] et al. The micropatterned PDMS cantilever was prepared by patterning photoresist film on Si wafer; the sensing element (\(\Pi\)/Au layers) was integrated onto the PDMS cantilever through photolithography and metal etching processes (Figure 1c) [53]. Coating techniques [71; 72; 73; 74] use deposition or evaporation methods to make sensing material uniformly grow on the surface of the polymer layers. [PERSON] et al.