*3.1.1 Sensitivity and linearity*

*Wearable Devices - The Big Wave of Innovation*

*C* = κ\_\_

*A*

in which κ, A, and d represent the permittivity of the medium between two plates, the overlap area, and the distance between two plates, respectively. When any of them is changed by the mechanical stimulus, the capacitance would be changed. For capacitive strain sensor, when the strain ε is applied, the length of capacitor along the strain direction would be increased, which is expressed as (1 + ε)l0, while the width and thickness of dielectric layer would be decreased, which is expressed as (1 − νelectrode)w0 and (1 − νdielectric)d0, respectively. The νelectrode and νdielectric are used to represent the Poisson's ratios of flexible electrodes and dielectric layer, respectively. If both flexible electrodes and dielectric layer have same Poisson's

*C* = (1 + ε)*C*<sup>0</sup> (4)

The equation indicates that the capacitance of capacitive strain sensor is linear with the applied strain. However, the linear relationship is only suitable for limited strain range. When the applied strain is higher than certain value, the relationship

For capacitive pressure sensor, the sensitivity (S) of capacitance to pressure is

*S* = δ(Δ*C*/*C*0)/*P* (5)

where ΔC is the variation of capacitance (C–C0) and P presents applied pressure. The most popular structure for the wearable pressure sensor is interlock

As **Figure 1c** shows, iontronic sensor is based on the iontronic interface sensing mechanism. The iontronic interface usually exists at the nanoscale interface between the electrode and the electrolyte. The electrode forms ionic-electronic contact with ionic gel. The electrons on the electrode and the counter ions from the iontronic film accumulate and attract to each other at a nanoscopic distance, leading to an ultrahigh unit-area capacitance. Compared to traditional parallel plate capacitive sensors, iontronic sensor has a higher surface area and its electrical capacitance is at last 1000 times larger. This excellent property is suitable for wearable electromechanical sensors. In addition, this special mechanism enables iontronic sensor immunity to environmental or body capacitive noises. So far, ion gels and ionic

As **Figure 1d** shows, the sensing mechanism of piezoelectric sensor is piezoelectric effect. Piezoelectric means that electric change accumulates in piezoelectric materials when mechanical stress is applied. Many materials have piezoelectric property, such as crystals, certain ceramics, and even biological matter. When strain or pressure is applied, there is a change in electrical polarization inside the material, resulting in a change in surface charge (voltage) at the surface of the piezoelectric material. In general, the electrical signal of piezoelectric sensor is voltage, which

ratio, then the capacitance upon stretching could be calculated as:

between different axes cannot be obtained simply by the Poisson's ratio.

structure, which is hard to make accurate analysis.

liquids are the most popular materials for iontronic sensor.

can be collected by measuring two different surfaces.

*<sup>d</sup>* (3)

**78**

given by:

**2.3 Iontronic sensors**

**2.4 Piezoelectric sensors**

Sensitivity is the magnitude of electrical response to measured mechanical stimulus, which is an important parameter. For strain sensor, sensitivity is called gauge factor (GF), which is defined as GF = ΔR/R0 for resistive type and GF = ΔC/C0 for capacitive type. For pressure sensor, pressure sensitivity (PS) is defined as PS = (ΔR/R0)/P. Sensitivity can be affected by functional material, sensing mechanism, and structural configuration. The materials with large piezoresistive or piezoelectric coefficient are desired. Tunneling effect and crack/ gap structures in piezoresistive sensors have been proven to be effective in promoting sensitivity. However, most highly sensitive sensors always show limited stretchability.

Linearity characterizes degree of deviation from linear relationship between electrical signals and mechanical stimulus. High linearity is convenient for the calibration and data processing process. However, there is always a contradiction between sensitivity and linearity because crack propagation and tunnelingeffect-induced resistance change are usually exponential. For instance, piezoresistive strain sensors often exhibit varied sensitivity in different strain ranges, which is induced by the nonlinear heterogeneous deformation. In addition, capacitive sensors with microstructured dielectric also suffer the similar problem.

## *3.1.2 Hysteresis and response time*

Hysteresis and response time are another two important parameters in evaluating dynamical performance of electromechanical sensor. Hysteresis means the dependence of the performance on its history, which should be reduced or avoided. In general, capacitive sensors show immediate responding to the variation of overlapped area, featuring a lower hysteresis. Meanwhile, piezoresistive sensors have slower response due to the interactive motion between sensing material and polymer substrate. The interfacial binding between sensing material and substrate greatly affects the optimization of hysteresis. The full recovery of sensing material position is hindered by the interfacial slide, leading to a high hysteresis behavior. Meanwhile, to avoid the friction-induced buckling and facture in sensing materials, a weak adhesion is needed. It is reported that using low viscoelastic polymer substrate and improved configuration can partially eliminate hysteresis. However, it is still a large challenge to optimize hysteresis by novel material and structural engineering. Response time illustrates the speed to achieve steady response to applied mechanical stimulus, and response delay exists in nearly all composite-based sensors because of the viscoelastic property of polymers. Relatively, piezoresistive device has a larger response time than others because it needs more time to reestablish percolation network in resistive composites. In addition, lower modulus materials are popular for wearable electromechanical sensor, which can further decrease the response speed of resistive sensors. Moreover, based on structural design, the newly developed crack-based piezoresistive sensors show an appealing response time (about 20 ms) because cracks can reversibly connect and disconnect with loading and unloading of mechanical stimuli [15].

### *3.1.3 Durability*

Durability is the ability to remain its performance, without requiring excessive maintenance or repair, when it is normally used. It is usually measured by cyclic stability for wearable electromechanical sensor. Cyclic stability is sensor endurance to periodic loading and unloading cycles. The sensing material film on polymer substrate is easy to form buckling, facture, and even stripping after enough cycles, which results in cyclic instable problem. For example, the sensitivity of graphene woven fabric (GWF) strain sensor decreases 24% after about 1000 cycles from 0 to 2% [16].

Endowing sensor with self-healing is a novel way to promoting durability. Several works have been reported on wearable electromechanical sensor. **Figure 2a** shows a stretchable self-healing piezoresistive strain sensor using single wall carbon nanotube (SWCNT) in self-healing hydrogel (SWCNT/hydrogel) as the conductive sensing channel [17]. The cutting groove is partially healed after 30 s and totally restored to normal after 60 s at room temperature without any external assistance. It also shows the repetitive cutting-healing processes with five cycles at the same location. The average efficiencies are 98 ± 0.8% for the five self-healing cycles within about 3.2 s, indicating that the SWCNT/hydrogel possesses significant and repeatable electrical restoration performance. **Figure 2b** shows that a self-healing sensor with tunable positive/negative piezoresistivity is designed by the construction of hierarchical structure connected through supramolecular metal-ligand coordination bonds [18]. The electrical resistance of the repaired samples only slightly increases after multiple cutting/healing cycles. However, the increase of electrical resistance is neglectable, which is lower than one order of magnitude, indicating its excellent electrical selfhealing ability. The high-healing efficiency is estimated to be 88.6% after the third healing process, and the healed wearable strain sensor still show good flexibility, high sensitivity, and accurate detection capability, even after bending over 10,000 cycles.

#### *3.1.4 Biocompatibility*

Wearable electromechanical sensors are usually directly used on human skins, so biocompatibility is also important. The main danger comes from sensing materials, which is usually nanomaterial other than substrate materials, which is a polymer. For example, it has been reported that injecting large quantities of CNTs into mice lungs could cause asbestos-like pathogenicity because of the small size and needle-like morphology of CNT [19]. To improve the biocompatibility, organic active materials, such as polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene)

#### **Figure 2.**

*(a) Self-healing properties of SWCNT/hydrogel-based strain sensor. (b) Electrical self-healing properties of supramolecular-elastomer-based strain sensor.*

**81**

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

**3.2 Materials for wearable electromechanical sensor**

cotton, are also highly desirable substrate materials [41].

fabricated with pencil-on-paper shows high GF up to 536.61 [27].

*3.2.2 Materials for active elements*

*3.2.2.1 Carbon nanomaterials*

*3.1.5 Self-power*

*3.2.1 Materials for substrate*

(PEDOT), have generally been used. The carbonized cotton or silk also presents

Power is the basic element for wearable system. Wearable devices with self-power ability attract more and more attentions, which can greatly extend their application scenarios and is particularly suitable for long-lasting wearables. Self-power wearable electromechanical sensor has been demonstrated so far using triboelectric [21], photovoltaic [22], piezoelectric [23], radiofrequency, thermoelectric (TE) systems [24], and others [25]. Among them, TE technology is rather attractive because of the utilization of conjugated polymers as the active component, which is also flexible, enabling a new generation of novel, low-cost, low-powered wearable electromechanical sensors [26].

Substrate is mainly responsible for flexibility and stretchability, and directly determines the comfort level and long-term reliability. Polydimethylsiloxane (PDMS), a commercial silicone elastomer with intrinsic high stretchability (up to 1000%), nontoxic, nonflammability, hydrophobicity, and good processability, has been frequently used. Though cannot be stretched for its relatively high modulus (about 2~4 GPa), polyethylene terephthalate (PET) features good transparency (>85%), high creep resistance, and excellent printability. Silicone elastomers including Ecoflex, Sylgard, Dragon Skin, and Silbione are biocompatible and their maximum stretchability is up to 900%. They are suitable flexible substrate because of their strong adhesion onto target surfaces. Ecoflex® rubber is a newly developed, highly stretchable and skin safe silicone with better stretchability and lower modulus, which has been used in the sensors requiring more severe flexibility and stretchability. Polyimide (PI) is another frequently used substrate because it can maintain flexibility, creep resistance and tensile strength under the condition of high temperature (up to 360°C) and acids/alkalis. Thus, PI is compatible with micromanufacturing process and many types of wearable electromechanical sensor are possible to be designed and implemented on it. Natural materials are also explored and developed to produce flexible substrate because they are easily biodegraded, such as cellulose paper. Moreover, the natural textiles, like silk and

Carbon nanomaterials including graphite, CNT and graphene, have been widely used in fabricating wearable electromechanical sensors. Graphite is a conductor and attracts more and more attentions with development of pencil-on-paper electronics [54]. Graphite flakes in pencil lead is easy to be deposited on paper surface by the physical friction between lead tip and porous cellulose paper. Moreover, structural edges in graphite flakes results in a strain-induced resistance variation of pencil traces, making them suitable for strain sensor. The contact area between graphite flakes increases by compressing the trace and decreases when the tension strain is applied, leading to the decrease or increase of resistance. The wearable strain sensor

great potentials in constructing biocompatible wearable sensors [20].

(PEDOT), have generally been used. The carbonized cotton or silk also presents great potentials in constructing biocompatible wearable sensors [20].
