*3.3.2 Wearable pressure sensor*

*Wearable Devices - The Big Wave of Innovation*

Strain 9.6

Mxene Strain 64.6

Mxene Pressure 4.05 kPa<sup>−</sup><sup>1</sup>

(0–250%) 37.5 (250–500%)

(0–30%) 772.6 (30–70%)

(0–1.0 kPa) 22.56 kPa<sup>−</sup><sup>1</sup> (1–3.5 kPa)

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

AuNW (gold nanowire)

Carbon nanofiber

Carbon nanofiber

Carbon nanotube

Carbon nanotube

**Table 1.**

Carbonized silk

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].

**Material Type Sensitivity Stretchability Linearity Durability** 

AgNW Strain 150,000 60% 0.989 200 [33] AgNW Pressure 1.54 kPa<sup>−</sup><sup>1</sup> 0.6 Pa-115 kPa linear 5000 [34]

AuNW Pressure 1.14kPa<sup>−</sup><sup>1</sup> 13 Pa-5 kPa Linear 5000 [36] Carbon black Strain 647 20% Nonlinear 200 [37] Carbon black Pressure 4.2 kPa<sup>−</sup><sup>1</sup> 0–30 kPa 0.996 30,000 [38]

Strain 70 250% Nonlinear 500 [35]

Strain 72 300% Nonlinear 8000 [39]

Pressure 4.2 kPa<sup>−</sup><sup>1</sup> 1.0 Pa-2 kPa Nonlinear 10,000 [40]

Strain 80 100% Nonlinear 1500 [42]

Pressure 0.209 kPa<sup>−</sup><sup>1</sup> 5.0 Pa-50 kPa Nonlinear 5000 [43]

Graphene Pressure 1.2 kPa<sup>−</sup><sup>1</sup> 0–25 kPa Linear 1000 [44] Graphene Strain 1054 26% Nonlinear 500 [45]

500% Nonlinear 10,000 [41]

130% Nonlinear 5000 [46]

0–3.5 kPa Nonlinear 10,000 [47]

**(cycles)**

**Refs**

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.

**84**

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 decreasing the tunneling resistance [54].

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 no fracture one [55].

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 sensitivity of 0.8 kPa<sup>−</sup><sup>1</sup> has been reported by using a GO-based low elastic modulus foam as the dielectric material [56]. There are several methods been demonstrated to fabricate highly deformable dielectric materials, including using commercial

#### **Figure 3.**

*(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.*

porous tapes, using special molds (e.g., the surface of matte glass, a micromachined Si mold, or the surface of lotus leaf) to create microstructures in elastomers, using sugar cubes as the template to create porous elastomers and fabricating buckled structures through prestretching and releasing. As the dielectric constant of air is smaller than that of the dielectric material used for the sensor, the effective dielectric constant is increased under pressure when the air gap is compressed. For example, **Figure 3b** shows a flexible pressure sensor with high sensitivity been built, which is a typical sandwich structure by combining a microarrayed PDMS dielectric layer with PDMS substrates. The top/bottom electrode material is PDMS substrate coated with AgNWs, and the dielectric layer is a PDMS with microarray structure, which is used to improve the pressure sensitivity. The results show that it possesses high sensitivity (2.04 kPa<sup>−</sup><sup>1</sup> ) in low-pressure ranges (0–2000 Pa), low detection limits (<7 Pa), and fast response times (<100 ms). Meanwhile, it also has excellent bending and cycling stability [57].

Progress has also been made on wearable piezoelectric and triboelectric pressure sensors. For example, it has been reported that a novel piezoelectric pressure sensor was fabricated through sandwiching freestanding electrospun polyvinyledenedifluoride-trifluoroethylene (PVDF-TrFE) nanofiber arrays [58] or electrospun PVDF-TrFE nanofiber between two electrodes. It can detect very tiny pressures as low as 0.1 Pa and has high sensitivity up to 1.1 V kPa<sup>−</sup><sup>1</sup> for pressure range from 0.4–2 kPa. In a representative work, a pressure-responsive triboelectric nanogenerator is used to gate the graphene transistors. Such graphene tribotronics showed a pressure sensitivity of ≈2% kPa<sup>−</sup><sup>1</sup> at a pressure of 10 kPa.
