*3.3.1 Wearable strain sensor*

Wearable strain sensor converts strain into electrical signal. Many applications, such as human health monitoring, require enough stretchability range from tiny deformation (small than 1%) to large deformations (as large as 100%) and high sensitivity. There are two main strategies to enhance the sensitivity. One is choosing proper sensing materials. Various kinds of nanomaterials are tested, as seen in **Table 1**. For example, by coating graphene on woven fabric structure, a maximum elongation of 57% and a GF of 416 and 3667 at lower and higher strains are achieved. Combining graphene and nanocellulose into nanocomposite, it shows ultrahigh sensitivity with GF of 502 at 1% strain and 2427 at 6% strain.

The second strategy is structure engineering. As discussed in above section, cracks can greatly enhance the change of resistance. Network cracks formed in multilayer CNT films on PDMS composite result in both high gauge factor (maximum value of 87) and a wide sensing range (up to 100%) of the strain sensor, which allows the detection of strain as low as 0.007% with excellent stability (1500 cycles) [27].

To improve stretchability, many strategies have been developed. One strategy is using intrinsically flexible materials and the relative stiff components bridged with highly flexible interconnects [48]. When the intrinsic stretchability of flexible material is not enough, structural engineering can be used to further enhance their stretchability. The fragmented structure with connected islands can form a lot of cracks, which can relieve most of the applied strain through opening and enlargement


#### **Table 1.**

*Performance of wearable electromechanical sensor fabricated with typical nanomaterials.*

of cracks. Deformable structures are widely used. For instance, the horseshoe and filamentary serpentine have been patterned with nanomaterials, which can accommodate large strain [49, 50]. Porous structures such as sponge and foam are also employed to improve the stretchability [51]. Wrinkled structure based on CNT film is produced and integrated on an Ecoflex substrate, allowing conductivity up to 750% elongation, an approximate 60 times increase versus nonwrinkled films [52].

Significant progress has been achieved on the sensitivity and stretchability, but there are some challenges still existing. Most resistive wearable strain sensors suffer from at least one of these problems, which are nonlinear response, large hysteresis, and irreversibility. The irreversibility mainly origins from partial slides back of sensing materials and irreversibly recover of cracks. Hysteresis is mainly caused by the viscoelasticity of polymers and the friction between the sensing materials and the polymer matrix. The rearrangement of sensing materials and opening of cracks are also responsible for time delay between electrical output and mechanical input. Nonlinear response mainly results from crack propagation and tunneling effect, which is always exponential as discussed above. Therefore, the performance of resistive wearable strain sensor should be evaluated from more aspects in further research.

**85**

**Figure 3.**

*Wearable Electromechanical Sensors and Its Applications DOI: http://dx.doi.org/10.5772/intechopen.85098*

strain sensors remain constant in the entire strain range.

*3.3.2 Wearable pressure sensor*

ing the tunneling resistance [54].

no fracture one [55].

sensitivity of 0.8 kPa<sup>−</sup><sup>1</sup>

Compared with resistive wearable strain sensor, capacitive strain sensors possess good linearity with low hysteresis, fast response, and are less susceptible to overshoot and creep. Nanomaterial-based stretchable conductors are usually used as the electrodes for capacitive strain sensors. Highly stretchable silicone, such as PDMS, Dragon Skin, and Ecoflex are commonly used as the dielectric layer sandwiched between two electrodes. For example, a capacitive strain sensor is fabricated with stretchable AgNW/PDMS conductors as the top and bottom electrodes and Ecoflex as the dielectric material [53]. The GF of this sensor reaches 0.7 and its stretchability is up to 50%. Moreover, it also has a good linearity. While capacitive strain sensors exhibit smaller GFs than the resistive strain sensors, they are ideal for applications where the strain is relatively large. In addition, the GFs of capacitive

Wearable pressure sensor converts pressure into electrical signal. Pressure sensor can be fabricated with interlocked structures, percolative networks of nanomaterials, microfabricated structures (e.g., micropyramids, micropillars), porous structures (e.g., sponges, foams, porous rubbers), and so forth. For example, **Figure 3a** presents a pressure sensor fabricated with interlocked microdome array. The contact between microdome increases when pressure is applied, thus decreas-

To improve the sensitivity of piezoresistive pressure sensor, structural surface

modification of the electrodes is an effective strategy. Incorporation of nano/ microscaled structures can provide large changes in contact resistance, allowing for detections of smaller pressures. For example, through coating polyurethane sponge with graphene to form fracture structure, a two-order of magnitude increase in sensitivity within the 0–2 kPa regime is demonstrated compared with

For the capacitive pressure sensor, the separation between two electrodes decreases with the pressure, resulting in an increase in capacitance. The property of dielectric materials almost determines the pressure sensitivity. Lower elastic modulus means a larger strain ε under a given pressure. The dielectric constant increased with pressure and low Poisson's ratio would all benefit the performance. High

foam as the dielectric material [56]. There are several methods been demonstrated to fabricate highly deformable dielectric materials, including using commercial

*(a) Schematic of the fabrication procedure and mechanism of pressure with interlocked microdome arrays. (b) Response characteristics of the flexible capacitive pressure sensor based on the PDMS microarray dielectric layer.*

has been reported by using a GO-based low elastic modulus

*Wearable Electromechanical Sensors and Its Applications DOI: http://dx.doi.org/10.5772/intechopen.85098*

Compared with resistive wearable strain sensor, capacitive strain sensors possess good linearity with low hysteresis, fast response, and are less susceptible to overshoot and creep. Nanomaterial-based stretchable conductors are usually used as the electrodes for capacitive strain sensors. Highly stretchable silicone, such as PDMS, Dragon Skin, and Ecoflex are commonly used as the dielectric layer sandwiched between two electrodes. For example, a capacitive strain sensor is fabricated with stretchable AgNW/PDMS conductors as the top and bottom electrodes and Ecoflex as the dielectric material [53]. The GF of this sensor reaches 0.7 and its stretchability is up to 50%. Moreover, it also has a good linearity. While capacitive strain sensors exhibit smaller GFs than the resistive strain sensors, they are ideal for applications where the strain is relatively large. In addition, the GFs of capacitive strain sensors remain constant in the entire strain range.
