*3.2.2.3 Polymer*

*Wearable Devices - The Big Wave of Innovation*

CNT are allotropes of carbon with a cylindrical nanostructure, which possesses excellent electrical conductivity and mechanical properties. It has been demonstrated that a single CNT shows strong structural effect and has a GF higher than 1000. However, wearable electromechanical sensor fabricated with single CNT is difficult and hard to realize mass production. Thus, CNT is usually intermingled into polymer substrates and its excellent conductivity plays an important role in electromechanical sensor construction. Wearable capacitive and piezoresistive electromechanical sensors have all been demonstrated by depositing CNT onto substrate or forming composite with polymers. For the piezoresistive composite sensor, the resistance change is mainly due to the strain-varied intertube tunneling resistance. The maximum GF can be achieved when the concentration of CNT is near the percolation threshold (PH). When the CNT loading is much lower than PH, the distance between adjacent CNTs is larger than their cut-off distance and there is almost no tunneling resistance. On the contrary, when CNT loading is much higher than PH, the CNTs can form dense 3D network and most of CNTs would connect with each other, resulting in a small intertube resistance. In this case, the contact resistance dominates the behavior, which will significantly decrease the GF. For piezoresistive film sensor, the variation of resistance gains almost a tenfold increment compared with nanocomposite type, but its cycle durability is not favorable enough because of unexpected cracks and desquamations. CNTs are also used to form wrinkle structure on a soft substrate via heating of the film or a prestrained substrate and are utilized to fabricate high-performance wearable strain sensor. Due to outstanding electroconductibility, excellent mechanical properties, great thermal characteristic and optical transmittance, graphene becomes the most promising sensing material for the development of wearable electromechanical sensor [28]. Graphene has been developed as electrode material for capacitive sensor and filler for piezoresistive sensor. A variety of graphene electromechanical sensors with different forms have been demonstrated, including porous foams, flakes, ripples, woven fabrics, and films. For example, the GWF film, which can be fabricated either by CVD or dip coating, consisted of many overlapping microribbons and features a good trade-off between sensitivity and stretchability, making it suitable for wearable strain sensors. It shows fascinating stretchability (a tolerable strain up to 57%) and sensitivity (GF = 416 for 0 < ε < 40%, and GF = 3667 for 48 < ε < 57%) by encapsulating the obtained GWFs in natural rubber latex [29].

Metal possesses excellent electrical conductivity and has been widely used in wearable electromechanical sensors. There are four forms of metal developed, which are nanowires, nanoparticles, stretchable configurations, and liquid state at room temperature. Nanowires (NWs) and nanoparticles (NPs) are usually used to prepare piezoresistive composites or conductive ink. For example, silver nanowire (AgNW) can be embedded into PDMS to build resistive-type strain sensor. Because the adhesion between AgNWs and polymers is not as strong as carbon nanomaterials, AgNW interconnection is easy to be broken. The resistance will irreversibly increase after buckling and wrinkling if the AgNW film is just simply coated on the surface of polymer. In addition, AgNWs are easy to be oxidized. Therefore, AgNW layer is often sandwiched between two polymer layers, ensuring AgNWs to move back along their determined paths and be free from oxidation [23]. The stretchable configurations of metal are on the basis of the strategy "structures that are flexible and stretch." Coiled buckled, serpentine and woven structures have been utilized to endow flexibility and stretchability to metals. The liquid metal, like Ga and its alloys, maintains the liquid state at room temperature. With the help of microfluidic

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*3.2.2.2 Metal materials*

Conductive polymers possess favorable electroproperties and can participate in building sensing materials. An attractive feature of conductive polymer is the mechanical similarity between them and many insulated substrate polymers. PEDOT-based polymers are the most common sensing materials for their thermal stability, high transparency, and tunable conductivity. Among them, poly(3,4 ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) is one of the promising conductive polymers due to its excellent solubility in water. However, the dried PEDOT:PSS film is easy to form hard particles inside, which may induce fissure and then decrease electrical conductivity. It is not suitable for continuous bending and stretching. To solve this problem, porous substrates have been developed for printing and permeating PEDOT:PSS ink, such as fabrics and cellulose paper, which can greatly promote their adhesion. This strategy greatly improves the stability of wearable electromechanical sensor fabricated with PEDOT:PSS ink [30]. The polyvinyledenedifluoride (PVDF) is another appealing sensing material with many attractive properties, such as piezoelectric property, especially appropriate for piezoelectric wearable electromechanical sensors. Moreover, other conductive polymers such as PPy, poly(3-hexylthiophene-2,5-diyl) (P3HT) and PANI have also been utilized to fabricate wearable sensors [31]. More recently, ionic liquid (IL), a kind of salt that keep liquid state at room temperature, has attracted extensive attention [32]. Similar to liquid metals, IL can also be embedded in PDMS-based microchannels to fabricate wearable electromechanical sensor.
