Chapter 1. Introduction
1.2 Working mechanisms of various electronic skins
1.2.1. Transduction modes for mechanical stimuli-senitive e-skins
Piezoresistivity: Piezoresistivity is the change in the resistance of materials caused by mechaniscal stimuli. As one of the most widely used transduction modes, piezoresistive e-skins can detect various mechanical stimuli by transducing them into changes in resistance. There are several types of piezoresistive e-skins based on polymer composites, where piezoresistivity is derived from a change in the band structure of semicronductors,12 modification of tunneling resistance between conductive fillers,13 and break up and reforming of percolating pathways.14 Figure 1.3a shows the conventional piezoresistive e-skins based on pressure-sensitive rubber (PSR), composed of elastomeric polymer matrix and conductive carbon materials, such as graphite, carbon nanotube (CNT), and graphene. When the mechanical stress is applied to the piezoresistive e-skins, deformation of PSR modulates conductive networks between conductive fillers and decreases bulk film resistance (Rb), enabling piezoresistive sensing. In this mechanism, piezoresistive e-skins show the highest sensing performances near the perforlation threshold of the conductive fillers in the elastomeric polymer matrix.
Recently, introduction of microstructures in the PSRs have attracted much attentions because of much enhanced sensor performances, including sensitivity, response time, durability, etc.7 In general, contact resistance (Rc) between microstructured piezoresistive polymer composite and electrode is several orders higher than Rb owing to small contact area. Therefore, overall sensor resistance highly depends on the variation in Rc rather than that in Rb, although both Rc and Rb decreases with applied mechanical stress. Since the microstructured e-skins can be designed to have high variation in contact area between microstructured piezoresistive polymer and electrode, the Rc of them can be decreases from almost insulating state to conductive state near Rb, resulting in much enhanced sensor performances than PSR- based e-skins.15 To further maximize variation in Rc, various nano/microstructures (such as interlocked, porous, hierarchical, and crack structures), which emulate biostructures, have been introduced for the high performance piezoresistive e-skins.7
29
Capacitance: Capacitive e-skins have beend widely researched owing to the adventages of high sensitivity, simple device structure, compatibility with static force detection, and low power consumption. They transduce various mechanical stimuli into a change in the capacitance of the device.
The capacitance (C) of a parallel plate capacitor is definced as C = εA/d, where ε is the dielectric dielectric constant, A is the area of electrode, and d is the distance between electrodes. The pressure sensing mechanism of typical capacitive e-skins is shown in Figure 1.3b. In the early stage, the working mechanism of capacitive e-skins depend on the variation in A or d in response to applied mechanical stimuli, which results in a change in capacitance. Recently, various researches about pressure-dependent dielectric constant have intensively studied because of their high sensitivity to mechanical stimuli.16 One of the effective methods to assign the pressure-dependent dielectric constant in capacitive e-skins is introduction of nano/microstructures in dielectric layer, which show high variation in effective dielectric constant in response to mechanical stimuli.
Piezoelectricity: Piezoelectricity refers to the ability of piezoelectric materials to generate electrical potential in reseponse to mechanical stimuli, because of non-centrosymmetric crystal structures of them. Therefore, piezoelectric e-skins can produce electric potential in response to mechanical stress based on inorganic materials such as ZnO, lead zirconium titanate (PZT), and BaTiO3, and organic materials such as PVDF, PVDF-based copolymers, poly-l-lactide, MoS2, and non-synthetic biocompatible protein (Figure 1.3c).17 Owing to their high flexibility, the polymer-based materials (PVDF, PVDF-based copolymers, and PVDF-based composite) have mainly studied for the flexible piezleoectric sensors. Because the piezoelectric e-skins can generate and quantify stress-responsive electric potential, they have been utilized as energy harvesting devices, as well as self-powered e-skins.
In addition, owing to instantaneous generation of electric signals, piezoelectric e-skins can detect dynamic forces with high frequency, such as vibrations, sound, and slip. Because piezoelectricity can be generated by changing the polarization state of piezoelectric materials, the piezoelectricity highly depends on the crystalline phase of them, where higher polar phase induces higher polarization state.18-
19 The classical way to transform the phase of piezoelectric materials from nonpolar to polar phase is applying high electric fields through the film, which is known as electric poling method. However, it requires too high applying voltage and their high piezoelectric performances continuously decrease over time, addressing need of new stratege for high-performance piezoelectric materials.20-21 Recently, introduction of nanomaterials with oxygen-containing functional groups to piezoelectric polymers can possess high piezoelectricity without electric poling process.20,22-23 In those piezoelectric polymer composites, the steric effect between nanomaterials and piezoelectric polymers through hydrogen bond restrict and order the PVDF change segments, transforming phase of piezoelectric polymers from nonpolar σ-phase to polar β-phase. The other method to further improve the piezoelectric performance is introduction of surface micropatterns (e.g., dome, pyramid, and pillar) and bioinspired structures (e.g.,
30
interlocking, hierarchical, porous), which can enable a large localized stress in the nano/microstructures and large surface area, resulting in significant piezoelectricity compared to planar e-skins22,24
Triboelectricity: Triboelectric e-skins convert mechanical energy to electrical energy through a conjugation of triboelectrification and electrostatic induction, caused by various contact motions such as vertical touch, shear friction from sliding motion, and torsional stress (Figure 1.3d).25 Since the triboelectric output critically relies on differences in triboelectric polarity between two contacting materials and contacting surface areas, the most important factor for high-performane triboelectric e- skin is a rational design of materials in reference to triboelectric series.26 Although two contacting materials have similar position in triboelectric series, chemically modified surface charge by self- assembled monolayers using an end-fluorine terminated group, thiol with a different head group, and atomic-level halogens and amines influences the surface potential and surface-charge density, enhancing triboelectric outputs.17 Because the triboelectrification occurs at the surfaces of two different materials, modification of surace morphologies can highly enhance the triboelectric perforamnces by increasing surface area and amplifying stress concentration at the contacting nano/microstructures.25,27 For a structural approach, uniquely designed micro/nanoscale structures such as surface micropatterns, micropore, nanoscale assembly, and hierarchical nanoporous and interlocked micro-ridge structures have been suggested, which enables the increases in surface area and stress-induced deformability, thereby producing enhanced power up to a few tens of milliwatts.7,28
Figure 1.3. Transduction modes for mechanical stimuli-senitive e-skins. (a) Piezoresistive e-skins based on PSR. (b) Capacitive e-skins of a parallel-plate capacitor. (c) Piezoelectricity generated in single film-based e-skins. (d) Triboelectricity generated in e-skins during contact/separation cycles.
Current
Current
Pressure (a) Piezoresistivity
+
− +
− +
−
+
− +
− +
− +
− +
− +
− +
−
(b) Capacitance
Pressure
P
+ + + + + + +
− − − − − − − (c) Piezoelectricity
Pressure − − − − − − −+ + + + + + + (d) Triboelectricity
+ + + + + + +
− − − − − − −
− − − − − − − + + + + + + +
Contact
Separation
31
1.2.2 Transduction modes for temperature-senitive e-skinstemperature-dependent behaviors