Intelligent Nanomaterials for Wearable, Stretchable Strain Sensor Applications: Comparison
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Please note this is a comparison between Version 2 by Amina Yu and Version 1 by Sundarrajan subramanian.

nNanostructured materials such as nanowires (NWs) [1,2,3,4], nanoparticles (NPs), nanofibers (NFs), and nanomaterials such as carbon nanotubes (CNTs), carbon fibers (CFs), graphite, and graphene (GE) have earned a wide focus in research due to their eminent physical, mechanical, chemical, and electrical properties. In addition, the ease in cost, synthesis of these nanostructures in different morphologies, and further fabrication of devices for an extensive range of applications have made them essential in the field of device fabrication.

  • wearable and stretchable strain sensor
  • intelligent nanomaterials

1. Different Types of Stretchable Strain Sensor

1.1. Resistive-Type Strain Sensors

Resistive-type strain sensors are typically composed of active sensing materials combined with flexible and stretchable supporting substrates [138,139][1][2]. The sensitive materials can be in the form of conductive micro-/nanomaterials-based polymer composites, thin films, or conductive yarns/fabrics. The conductive network of sensitive materials serves as a resistor under the application of a potential bias (voltage). When stretched, the electrical resistance of the sensitive network changes as a function of the applied mechanical strain. The resistance variations upon stretching originate from the geometrical changes, i.e., length and cross-sectional area [65,66,139][2][3][4]. The sensitive materials recover back to their initial state after experiencing a tensile/compressive strain, which is measured and makes it the principle behind resistive-type sensors.
Resistive-type WSSs have been fabricated using different synthesis techniques, namely, electrospinning, spray coating, inkjet printing, chemical route synthesis, sputter-coating, printing, liquid phase blending, and filtration.
A composite film made of graphene NPs and PDMS with high conductivity has been reported for WSS [83][5]. Initially, these graphene NPs were transferred onto the PDMS substrates using medical tape and pressed mechanically to obtain high conductivity [83][5]. A spray-coating technique was used for CNTs on elastic rubber film, producing highly SSS [84][6]. In another study, CNTs/carbon black hybrid network spin coated on PDMS substrate for synergistic conductive strain sensors [144][7] was reported. A conductive fiber made from Ag NWs and polyurethane composites, fabricated wearable sensing devices and the bonding between Ag and polyurethane were also investigated [145][8].
This novel structure minimized in-plane stretching disturbance, leading to customizable SSS. A chemical vapor deposition was used to grow vertically aligned CNT bundles followed by rolling and transfer onto the Si substrate, which thereby fabricated a stretchable strain sensor [142][9]. Aerosol jet printing of polyimide (PI)/Ag NWs enables multi-functional strain sensors with tunable resistance; initially, PI and Ag NWs were deposited on a glass slide with spin-coated PMMA (see Figure 3e) [148] [10]. Strain sensors based on multiple-layered structures demonstrate capabilities such as strong adhesion and conformal lamination on different surfaces without the use of conventional fixtures and/or tapes. Chen et al. synthesized a functionalized organic nanoparticle embedded in a hydrophobic breathable coating on textiles. Subsequent inkjet printing of continuous conductive electrical path onto the pretreat coating reduced the sheet resistance of graphene-based printed e-textiles by three orders of magnitude from 1.09 × 106 Ω/sq. to 2.14 × 103 Ω/sq [146,149][11][12].

1.2. Capacitive-Type Strain Sensors

Wearable capacitive-based strain sensors are often fabricated by sandwiching an insulating film known as a dielectric layer between two stretchable electrodes [66][4]. Under an applied voltage, the accumulated opposite charges on each electrode cannot flow through the dielectric layer, yielding a parallel-plate capacitor with an initial capacitance of C0, expressed as
C0 = ε0 εr Ac/d
                   
 
where Ac denotes the overlapped area of electrodes, d is the thickness of the dielectric layer, εr represents the dielectric constant of the dielectric material, and ε0 is the permittivity of vacuum. The capacitance of strain sensors depends on the resistance value of the electrodes, which increases under the stretching due to the changes in the capacitive area [66,150][4][13].
Resistive-type strain sensors offer high sensitivities and outstanding gauge factors, but a practical application requires characteristics such as linear strain response, high stretchability, and low hysteresis, which are the major limitations of resistive-type sensors, which can be overcome by parallel-plate capacitor structures. Nur et al. fabricated a wrinkled-shape PDMS dielectric layer, thermally evaporated gold on perylene electrodes, and further transferred it onto the PDMS to construct a capacitive type of strain sensor.

1.3. Optical Strain Sensors (OSS)

Wearable sensors based on the optical strain principle typically comprise a stretchable waveguide flanked by light emitters and photodetectors. Since the time of the first fabrication techniques in electronics, such as soft lithography and 3D printing, flexible polymeric waveguides have been investigated for wearable strain-sensing applications [68,69,155][14][15][16]. The principal sensing mechanisms behind the OSS are the difference between the incident (initial illumination applied) and the reflected light (light received at the photodetector) upon deformation [68][14]. The optical types of strain sensors are more promising to overcome challenges (less effective in environmental disturbances) faced during the other two resistive-type and capacitive-type sensors. An optical-type stretchable strain sensor based on the change in optical transmittance of the CNT-embedded Eco-flex film is presented [156][17]. MWCNTs were spray-coated and embedded into the Eco-flex substrate, and the microcrack propagation in this MWCNT film led to optical transmittance change. The sensor responses were observed to be independent of the intensity of the light source and the strain rate. The sensor was utilized to detect the bending of the finger and wrist for the control of the robot arm. Furthermore, the applications of this sensor to the real-time monitoring of neck posture, carotid pulse, and facial expression were demonstrated [156][17]. Stretchable OSS have been fabricated from optically transparent polymeric materials such as hydrogels and elastomers [68,69,119,156,157,158][14][15][17][18][19][20]. A graphene/PDMS fiber with high tensile and good transmittance has been demonstrated to detect tensile strain up to 150%. 

1.4. Thermoelectric Strain Sensors

In the case Fofr TE technology, conventional systems have been considered as a power source for integrated sensors of pressure, corrosion, heat flow, vibration, heart rate, and other stimuli. The utilization of conjugated polymers as the active component/sensitive layer of a TE device is a relatively recent concept, but it has the potential to enable a flexible wearable sensor that can operate at low power and could be fabricated at a low cost. Other desirable attributes, such as low toxicity, ubiquity, and abundance of constitutive elements, as well as the ease in processing by various established coating or printing techniques, are added advantages of thermoelectric strain sensors [46,47][21][22]. A highly stretchable and wearable self-powered temperature sensor was fabricated using TE inks such as Ag NPs, graphene, and PEDOT: PSS [159][23].

1.5. Piezoelectric-Based Strain Sensors

Piezoelectricity is a mechanism in which electrical voltage is directly generated under external deformation due to the electrical dipole moments in PE materials (PEM) [161][24]. Strain sensors based on sensitive materials with a high PE coefficient can induce electric current in an external circuit, leading to the detection of mechanical deformations with high sensitivity and fast response. Therefore, PE materials can usually be used for strain sensors and as energy-harvesters. An active, self-driven wearable PE sensor can be used to monitor respiration rates and rhythm at multiple body peripherals during various physical movements. It is anticipated that self-powered energy harvesters would be highly desirable for next-generation wireless and wearable electronics. The most widely used PE materials, such as lead zirconate titanate (PZT) and lead magnesium niobate-lead titanate (PMN-PT), possess the qualities of hardness, stiffness, and brittle ceramics. However, these PE ceramics are too brittle to integrate with flexible electronics. and hence ferroelectric polymers such as poly(vinylidene fluoride) (PVDF) and its copolymer poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) are composited with PE materials as these polymers provide sufficient mechanical flexibility and small polarization, while PE materials offer a sufficient PE response. Hence, excellent and flexible PE materials made from composites [162[25][26],163], thin PE films [164][27], and NFs [165,166,167,168,169][28][29][30][31][32] have been intensively investigated for their improved PE performance, cost-effectiveness, and mechanical flexibility [162,163][25][26].

1.6. Triboelectric Strain Sensors

As mentioned in the past sections, a triboelectric sensor also converts mechanical energy into an electrical response [173,174,175][33][34][35]. When two thin materials with opposite tribo-polarity come into contact with each other, a charge transfer at their interface results in the creation of an output potential [161][24]. The intensity of the generated potential is directly proportional to the interaction with the external load/deformation regarding time and area. Despite some advantages of PE and triboelectric strain sensors, they usually operate under fast mechanical deformations, and it is difficult to measure the strain history over large stretching cycles because of the fast charge transfer [161,173,174,175][24][33][34][35].
Microstructured PDMS films/PET spacer layers were constructed using a triboelectric nanogenerator that could fully contact human skin and be used for harvesting mechanical energy and detecting human motion. Initially, a 200 nm Al was sputtered on the front of the micro-pyramid-structure PDMS film as both the friction layer and the electrode, and the 200 nm Al was sputtered on the back of another micro-pyramid-structure PDMS film as another electrode. Then, a spacer layer (PET) was placed between the two PDMS films. Further, the conductive fabric was placed on the Al film to form an external contact. Finally, the device was covered by the pure PDMS film as a protective coating. Due to its wearable and flexible nature, the well-designed triboelectric generator provides a new way to achieve a power supply for wearable electronic devices and act as an active sensor to monitor human gesture movements [176][36]. The energy generated by human motion was collected by multi-layered electrodes in triboelectric nanogenerators (TENGs). An Al foil was used as both electrodes and as a friction layer, a fluorinated ethylene propylene (FEP) film served as the friction layer, and copper deposited on it served as the second electrode. The TENG in shoe insoles could be driven to generate a voltage of about 700 V output and a short circuit transfer charge of 2.2 mC. This provided a new kind of TENG that could work in different conditions. Ouyang et al. fabricated a self-powered pulse sensor (SUPS) based on a triboelectric active sensor [119][18]. The SUPS was composed of four parts: friction layers, electrodes, a spacer, and encapsulation layer. Nanostructured Kapton (n-Kapton) film served as one triboelectric layer, and the Cu layer deposited on its backside acted as one electrode. The nanostructured Cu (n-Cu) film served as both a triboelectric layer and an electrode. The fabricated SUPS was ultrasensitive and low in cost, which is important for any wearable sensors [176][36].

1.7. Field-Effect Type Strain Sensors

To utilize the stretchability of a polymer or plastic, metal electrodes were used on rubber substrates. The embedded active components, such as transistors and diodes in rubber sheets, are integrated with wavy metal wires [178][37]. As an essential element of various intelligent electronic devices such as organic thin-film transistors, they possess the integrated functionality of signal transduction and amplification. In principle, the combination of the above properties is ideal for e-skin and health monitoring sensing applications [45,58,59,179][38][39][40][41].

2. Sensing Mechanisms

Sensing mechanisms depend on the type of sensor (for instance, the capacitive type and the optical type), the type of sensing materials, their surface interaction with supporting materials, and the fabrication process. For instance, the resistance of resistive-type sensors can be modified with respect to their geometry (length, cross-section) upon external stretching. The geometric effect is more dominant in liquid–metal-based stretchable strain sensors [186,187,188][42][43][44]. For instance, elastic hollow fibers composed of triblock copolymer resin were fabricated into strain sensors, and a liquid metal alloy (eutectic gallium indium) was injected into the cavity for better conductivity [189][45]. In another studyone, PDMS fibers were injected with low toxicity liquid metal that enabled them to detect strain variation in geometrical changes [190][46].
Geometric effects play an important role in capacitive and optical strain sensors, changes in the capacitive area, and the thickness of the dielectric layer, leading to a shift in the capacitance as a function of either external stretching or compression. Whereas the optical type of strain sensors relies on the attenuation in the light transmission on the stretching of optical waveguide [68[14][18][47],119,191], the majority of sensing mechanisms are based on the intrinsic resistive response of the sensing materials. This could be defined by the change in the electrical resistance of the materials in response to external deformations. The intrinsic resistance of a sensor dramatically increases upon changes in the bandgap, and carbon nanotubes and oxide-based nanowires endure a very high resistive response, which leads to the development of highly sensitive strain sensors incorporated into highly stretchable substrates. However, large mechanical mismatch and weak interfacial adhesion between micro-/nanoscale materials and stretchable supporting materials dramatically lower the contribution of their intrinsic resistive response to the overall sensing performance of stretchable strain sensors. The sensing behavior of resistive-type strain sensors can be attributed to the changes in the conduction network upon deformations. As per the percolation behavior, a minimal number of nanomaterials is required to establish conducting pathways within a film or composite. Once the adjacent nanomaterials are connected, electrons can pass through established conducting networks. Upon stretching, nanomaterials-based resistive-type strain sensors lose their overlapping area and electrical connection, which leads to increased overall electrical resistance. The disconnection/mismatch is because of large stiffness between nanomaterials and stretchable supporting materials, leading to slippage and debonding of nanomaterials in the context of major stretching. Nanowires and nanoflakes take greater advantage of the disconnection mechanism. The resistance shift in CNT-based thin films has been reported to be due to changes in interconnecting pathways during the stretching and destretching [84][6]. A conductive network of Au nanosheets has also produced similar conductive networks [192][48]. Upon stretching, the nanosheets slip toward the direction of the external load, decreasing the contact area between them and thus increasing the overall resistance of the strain sensor. In addition to disconnections, cracks also appear on top surfaces of the soft polymers or natural fibers upon stretching. Cracks propagate in stress-concentrated areas. Although the cracks are undesirable, microcracks have been utilized in conductive thin films to develop highly sensitive strain sensors. Microcracks have been observed in CNT-based sensors [102[49][50],193], graphene-based strain sensors [98,194,195][51][52][53] and graphene derivatives [92[54][55],196], metal nanowires, and nanoparticle-based [197,198][56][57] strain sensors. The rapid separation of nanomaterials at the microcrack edges dramatically limits the electrical conduction paths within the thin films, leading to a significant increase in the electrical resistance of strain sensors under the applied tensile strain [66][4]. Recently, controlled cracks have been utilized as an effective mechanism to promote the sensitivity of strain sensors [113,198][57][58]. Au thin-film-based strain sensors have been fabricated on PDMS substrate to focustudy on the effect of cracks on the sensitivity and electrochemical response [194][52]. In one study, graphene-based ultra-sensitive and stretchable strain sensors fabricated with reversible parallel microcracks [199][59]. The length and density of microcracks were increased with applied strain, and microcrack edges were reconnected upon releasing the strain, ensuring the recovery of the electrical resistance after the complete strain release. Strain sensors are based on natural fibers, and the fiber breakage in the direction of the applied strain leads to crack propagation and subsequent increase in electrical resistance [116][60]. Natural-fiber-based strain sensors with high sensitivity and stretchability and with controlled crack propagation and fiber breakage have been fabricated [117,119][18][61]. Electron tunneling occurs when electrons pass through a gap between two conductive nanomaterials with a short distance between them [66][4]. Conductive nanocomposites made of functional nanomaterials and polymer matrices have not only direct electrical paths through connected nanomaterials but also tunneling conduction between adjacent nanomaterials [65,67][3][62]. The minimum distance (nonconductive barrier) through which electrons pass through is to create a quantum tunneling junction is called the cut-off distance. The cut-off distance depends on several factors, including the type of insulating material, conductive fillers, and processing parameters. The tunneling resistance originated from quantum electron junctions. In CNT-based nanocomposite strain sensors, the strain response arises from changes in the tunneling resistance [200,201][63][64]. In nanocomposite strain sensors, CNTs are often entangled and self-folded within polymeric matrices. When stretched, entangled CNTs are more susceptible to unfolding rather than sliding, leading to changes in the tunneling resistance among neighboring CNTs. It is important to point out that the tunneling effect differs from the disconnection mechanism where many connected networks are separated due to the slippage of nanomaterials within polymer matrices.
In addition to the affecting factors, the gauge factor is a quantifier for a sensitivity of a strain sensor, which is defined by the ratio between the relative change in the output signal to the applied strain [65][3]. The value of a gauge factor of a strain sensor depends on various factors such as sensing microstructures, elements, fabrication process, and sensing mechanism [138,202][1][65]. For example, the gauge factor of a resistive-type strain sensor is defined as a gauge factor = (ΔR/R0)/ε, where ΔR/R0 is the relative change of resistance and ε is the applied strain. Zhou et al. fabricated a CNT-PDMS-composite-based resistive type strain sensors showed sensitivity 107 at 50% strain [203][66]. High sensitivity was attributed to the carbon nanotube networks in the cracked area. Highly stretchable and ultrasensitive (gauge factor of 88,443 at 350% strain) strain sensors made of 3D printed graphene and PDMS nanocomposite followed by plasma treatment, and polyethyleneimine coating, and finally, the deposition and reduction of GO particles was reported [204][67]. The gauge factor originates from the disconnection of the rGO at low strain range, whereas the open mesh structure can enhance the sensitivity at high strain levels. Different astudiepects have shown a wide range of gauge factor values starting from 1.2 to 102,351 [111,113,116,117][58][60][61][68]. It has been noticed that the gauge factor of the strain sensors can be controlled by geometric engineering [111][68]. Serpentine-shaped active materials exhibit lower sensitivity than straight ones, and a strain sensor based on the fragmented conductive cotton fabric-Eco-flex composite showed ultra-high sensitivity up to 102,351 within the strain range of 342–400% [113][58].
The maximum theoretically achievable gauge factor value for capacitive-type strain sensors is about one (gauge factor = (ΔC)/C0ε  =  ((1  +  ε) C0 − C0)/C0ε  =  1), which means that most of the stretchable capacitive-type sensors have gauge factor values less than one (gauge factor 1). Carbon-black-filled elastomer composite and silicone elastomer-based strain sensors showed gauge factor value of about 0.98 at 500% stretchability [113][58]. Capacitive-type stretchable sensors with hollow elastomeric fibers filled with liquid metal networks reported gauge values of up to 0.82 [205][69]. Nevertheless, this theoretical limitation of capacitive-type sensors has demonstrated the possible enhancement of sensitivity by more than one (gauge factor 1) due to geometrical alterations and novel material formulations [151,206][70][71]. A capacitive-type strain sensor using Au-film electrodes achieved a gauge factor of about 3 under 140% strain [206][71]. In another studyone, highly sensitive capacitive-type sensors have been reported using ionic hydrogels, and Ag-nanofiber-based nanocomposites have achieved a very high gauge factor of 165 under an ultrahigh strain of 1000% [151][70]. The improvement in sensitivity is because of the incorporation of Ag nanofibers leading the hydrogel–metal interface. In optical-type strain sensors, the sensitivity is quantified by an output power loss of a stretchable waveguides under mechanical deformations [68,207][14][72]. In a study, tThe sensitivities of stretchable strain sensors made of thermoplastic elastomer and dye-doped PDMS optical fibers were 10 and 3.62 dB ε−1, respectively [69,119][15][18]. CNTs-embedded Eco-flex film-based optical-type stretchable strain sensors achieved high sensitivity with gauge factor ≈ 30 [208][73].

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