Touch Sensors for Flexible Wearable Devices: Comparison
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Please note this is a comparison between Version 2 by Lindsay Dong and Version 1 by Changyoon Jeong.

The traditional touch sensors are bulky, complicated, inflexible, and difficult-to-wear devices made of stiff materials. The touch screen is gaining further importance with the trend of current IoT technology flexibly and comfortably used on the skin or clothing to affect different aspects of human life. Flexible sensors have advantages over conventional rigid devices. These sensors are nontoxic and lightweight and can be worn comfortably on the body

  • flexible sensor
  • wearable devices
  • touch sensor

1. Touch-Sensor Working Principles

There are three types of sensing actions: touch, movement, and pressure. The distance over which actions are performed differentiates fingertip touch and movement, and fingertip pressure level provides a third sensing action. Regular touch occurs when the finger is at a zero distance from the sensors and no force is applied. With increasing pressure, the amount of force increases (pressure sensing). Sensors that detect movement can use a combination of multiple sensing points or proximity sensors (the current distance exceeds zero).

1.1. Single Mechanism

1.1.1. Capacitive Touch Sensors

Capacitive sensor technologies can be broadly divided into two main categories: mutual-capacitive and self-capacitive technologies [16][1]. For a self-capacitive system, capacitive changes are measured relative to earth ground. It operates on the basis of the parallel-layer model, where the electrode and bottom (or user’s finger) act as the two layers of the capacitor. As capacitance is added with each “touch”, the capacitance on the electrode of the self-capacitive system increases [26][2]. Contrarily, any intended or unintended capacitance between two “charge-holding objects” can be termed mutual capacitance [27][3]. A mutual capacitance between intersecting elements of columns and rows can be intentionally created via projected capacitance touch sensors. With the help of such system electronics, a single scan can detect multiple touches by measuring each node (intersection) individually. The surface capacitance measurement is used by some touch devices to determine the human body (finger). The sensor is contacted by a finger, forming a capacitor dynamically [28][4].

1.1.2. Resistive Touch Sensors

Resistive touch sensors respond to pressure [29][5]. These sensors are composed of several layers, with the most critical layer formed by two electrodes separated by another layer with poor conduction. Herein, a light pressure resulting from contact can predominantly change the resistance. Apart from a finger, other stimuli, such as a stylus, can also be sensed by these sensors. Resistive touch sensors were the earliest used sensors, with the advantages of low cost, fast response, a linear output, and high durability. However, such sensors require power, making them unsuitable for wearable systems (i.e., low-power systems) [12][6].

1.1.3. Piezoelectric and Triboelectric Touch Sensors

Piezoelectric materials translate deformation into electrical energy, enabling the development of a piezoelectric sensor [22][7]. Applying slight mechanical pressure (light touch) to a so-called piezoelectric material results in the separation of the electric charges because of electrical dipole moments, generating electrical voltage. When the external force is withdrawn, the polarization phenomenon disappears. By detecting the change of the electrical signal, it can be used to realize pressure sensing. Pressure sensing can be achieved by detecting the changing electrical signal. Some of the commonly used piezoelectric materials are BaTiO3, PZT, ZnO, PVDF, PbTiO3, and polypropylene (PP) [31][8]. Piezoelectric sensor performance may hinder as a result of cracking due to the pressure applied to traditional piezoelectric materials to overcome the problem of cracks and hole defects.

1.1.4. Triboelectric Touch Sensors

Triboelectric sensors (or contact-electrification) is employed by a triboelectric nanogenerator (TENG) [21[9][10],34], where a physical contact (a touch) generates a potential electrical signal without reliance on an external power supply. Triboelectric nanogenerators (TENGs) form the basis of triboelectric sensors. The inductive charges are generated on the friction layers of TENGs under external forces owing to the electron affinity differences. Upon the removal of the external force, current starts to flow between electrodes as a result of the formation of internal potential. Moreover, the current signals are analyzed for the sensing function. Sensors using electrostatic induction for the detection of human motions have been available since the introduction of triboelectric nanogenerators in 2012 [35][11]. The triboelectric nanogenerators may transform the received signals of body movements into electrical impulses, allowing the sensor to function without the need for external power. Commonly used as friction layer materials, they can easily lose or gain electrons, such as PTFE, PDMS, PI, nylon [36][12] copper, and silver.

2. Hybrid Multisensor Mechanisms

2.1. Two-Principle Integration Mechanism

A wearable sensor based on a single-principle mechanism is unable to detect multiple features and could not meet the requirements for multifunctional detection. Therefore, it is necessary to develop hybrid mechanisms to detect multiple properties and enable the fabrication of multifunctional sensors. Two-principle mechanisms consist of various integrated mechanisms, such as triboelectric–piezoelectric mechanisms or capacitive–piezoelectric mechanisms. These two-principle mechanisms are used extensively in wearable devices. Tang et al. demonstrated a working mechanism based on simultaneous triboelectric and piezoelectric mechanisms in self-generated powered sensors [40][13]. The two-principle mechanisms operate in different directions because the triboelectric components consist of four working electrodes that operate in the xy plane, whereas the piezoelectric element functions in the z plane. This integration has shown extensive applicability in the human–machine interaction and other wearable electronic devices [40][13].

2.2. Three-Principle Integration

Hybridization involving three integration mechanisms will play a tremendous role in future wearable devices for use in wide applications, such as the human–machine interface, smart robots, and wearable electronics. To further improve the performance of sensing materials, it is important to introduce different mechanistic parameters into sensing mechanisms to expand their applicability in wearable and other devices. Various polymeric materials, such as PVDF, PDMS, PET, polyurethane (PU), polypropylene (PP), and other nanomaterials, are required to improve the cracking performance of materials subjected to great pressure or temperature changes, thereby requiring the integration of more than one mechanism for enhancing the sensing ability of the developed materials. Zhao et al. [49][14], for example, developed high-performance sensors by combining three effects: triboelectricity, piezoresistivity, and piezoelectricity. They fabricated a nanofilm of carbonized polyacrylonitrile/barium titanate (PAN-C/BTO) using the electrospinning method. The resultant nanofilms could independently and simultaneously detect pressure and curvature with high and enhanced sensitivity. These sensors could be used multifariously in detecting human motion, such as finger tapping and swallowing.

3. Nanocomposite Material for Flexible Touch Sensors

Flexible sensors should be made of a lightweight material that is biocompatible, comfortable, and does not cause irritation. Nanocomposite materials mostly include metallic thin films [52[15][16],53], carbon nanotubes (CNTs) [54[17][18],55], metal nanowires (NWs) [56][19], metal nanoparticles (NPs) [57][20], and conductive polymers [58,59][21][22]. Good electrochemical activity, high electrical conductivity, and a large active area make the NWs, CNTs, and conductive polymers preferred choices for sensors. In addition, NW/CNT composites can be coated/printed directly onto a substrate [60][23] to produce a sensor with high sensitivity, stretchability, and durability. Conductive polymers, such as poly(3,4-ethylene dioxythiophene) (PEDOT), and especially its complex with poly(styrene sulfonate) (PEDOT:PSS), can be synthesized by chemical or electrochemical deposition [61][24] and exhibit high conductivity, good transmission of light, good processability in water, and high flexibility. For the mass production of sensors, printable conductive materials are particularly advantageous because they enable nearly all sensor parts to be printed. Several metal conductive inks have been shown to be promising because of their tendency to disperse in solvents and their compatibility with different printing technologies. Ag nanoparticle-based inks and nanowires have been extensively studied as flexible electrodes or conductors for polyethylene (PE) [61][24]. However, Cu nanoparticle-based ink, which is inexpensive and highly conductive, has attracted particular attention [62][25]. Inks based on carbon nanomaterials (e.g., CNTs and graphene) have also been shown to be printable and stretchable for flexible sensors [63,64][26][27]. The integration of Ag nanowires (AgNWs) [65][28], ITO [66][29], graphene [67][30], PDMS [68][31], CNTs [68][31], and PEDOT:PSS [69][32] has led to transparent stretchable electrodes suitable for certain touch displays and photovoltaic applications. The fabrication of transparent electrodes/conductors faces a major challenge in the trade-off relationship among flexibility, transparency, and conductivity, which depends on the conductive filler concentration [70][33]. This challenge is particularly relevant to touch sensors such as touch screens and fingerprint sensors. Liquid metals and liquid ionic materials are intrinsically flexible. Ionic additives can improve the conductivity and stretchability of PEDOT:PSS, leading to conductivity as high as 4100 S cm−1 at 100% strain, as demonstrated by Wang et al. [58][21], and enabling the fabrication of soft sensors that can detect both positive and negative pressures from −60 to 20 kPa [71][34]. Liquid metals—specifically, eutectic gallium–indium alloys—can be used to fabricate flexible circuits via the integration of room-temperature liquid metals (RTLMs) and water-soluble poly(vinyl alcohol) (PVA) because of their high conductivity, intrinsic stretchability, and low piezoresistivity. Thus, RTLMs and PVA can be integrated into flexible circuits [72][35], asymmetric force sensors can be prepared from hydrophilic polymer networks [73][36], and soft sensors can be produced by a 3D-printed rigid micro-bump [74,75][37][38]. In dynamic applications, liquid components introduce reliability problems. An excellent solution would be to embed the liquid metals into elastomers, such as PVA. Furthermore, a polymer matrix combined with a wide range of nanofillers can produce various polymer nanocomposites. Nanofillers can have either two- or three-dimensional structures, similar to CNTs or graphene. The properties of CNTs, such as their high mechanical strength, high aspect ratio, and outstanding electrical properties, make them a particularly promising material. CNTs can be embedded in various polymer matrices to produce materials with diverse properties. Unfortunately, the high Van der Waals forces of CNTs cause the particles to tend to cluster or agglomerations. Thus, the distribution of CNTs in polymers strongly affects the performance of nanocomposites. There have been many efforts to achieve a homogeneously dispersed CNTs within the polymer matrix. This is required to fabrication sensors and establish the foundation for high repeatability. Numerous approaches have been developed to tailor and improve the dispersion of CNTs/polymer composites via solution mixing, including direct mixing, in situ polymerization and melt processing.

4. Manufacturing Technologies

In current studies, spinning, coating, printing, and transferring are indispensable methods that strongly influence the electromechanical and flexible properties of touch sensors because they govern the material and the manufacturing process used. Yarns or surfaces can be coated with conductive coatings. In this context, graphene nanoplatelets and carbon nanofibers painted along both sides of a rubber piece result in flexible and stretchable capacitive touch sensors with a low sheet resistance (~10 Ω sq−1). Chen et al. [8][39] developed a flexible touch sensor using electron-induced perpendicular graphene sheets implanted on porous carbon films (Figure 5a). These sheets displayed high sensitivity (0.13 kPa−1 at pressures less than 0.1 kPa and 4.41 MPa−1 at pressures greater than 10 kPa) as well as a fast response time of 66 ms when the substrate was ~0.5 mm Si. Conductive printing techniques enable the fabrication of flexible touch sensors using printing technology. A transparent capacitive touch sensor fabricated using the inkjet printing technique has been proposed for use in freestanding nanoparticle arrays made of ultrafine polydopamine with a controlled line-to-line separation (similar to pitch) 5b [86][40]. A self-powered touch sensor with a polyethylene terephthalate substrate with an area of 10 × 10 mm2 and a thickness of 380 µm was found to be capable of powering diodes, supplying power to a device, or charging a capacitor. It is a paper-based harvester screen-printed using a mesh [87][41]. Recently, researchers have focused extensively on 3D-printing technology because of its high commercialization potential, ease of integration, and large-scale manufacturing capability.

5. Applications of the Touch Types

5.1. E-Skins-Based Approach

An electronic skin is an electronic device that mimics human skin properties by being flexible, stretchable, and self-healing. E-skins have multiple applications, including prosthetics, robotics, and skin-attachable devices. E-skins made of ultrathin materials, such as PET or poly(ethylene naphthalate), are a good choice for a small, sustainable deformations. In addition, elastomer substrates, such as polyurethane (PU), latex, and poly(polydimethylsiloxane) (PDMS) may be used in applications that require stretchability [92][42]. Asghar et al. [93][43] used magnetically generated microstructures (MPs/PDMSs) to show a piezo-capacitive flexible pressure sensor. In the device, pressure can be sensed over a wide range (0 to 145 kPa) with a fast response (50 ms), as well as high cyclic stability (>9000 cycles).

5.2. E-Textiles-Based Approach

As one of many wearable sensors, e-textiles in stretchy fabrics have drawn attention from both the academic and industrial worlds because of their supreme wearability in allowing seamless fitting across different body sizes and shapes [97][44]. Traditionally, e-textiles have been created by knitting, weaving, or embroidering functional fibers into fabrics in a twisted or coaxial structure, or embedding functional nanoparticles directly into fabrics. Through adding electronic elements, e-textiles, such as conductive fibers or fabrics, can be used in wearable devices, the human–machine interface, or for controlling/monitoring applications [98][45].

5.3. E-Healthcare-Based Approach

For healthcare implementation devices must be low energy consumers and biocompatible to prevent skin irritation. In e-healthcare, wearable bio-chemical/chemical sensors are featured as examples of a flexible touch-sensing. Their greatest issue is maintaining precision in a variable working environment with multiple impacts, such as temperature and humidity [104][46]. Normally, these biochemical or chemical sensors are often connected to human skin or are woven into garments and fabrics in order to detect external toxins or monitor the level of specific (blood) compounds 7/7 [105][47]. With the addition of an electronic unit, e-textiles, such as conductive nanofibers or nanofabrics, can be used in wearable devices, data collection, control, and for monitoring applications. A temperature sensor e-textile made up of rGO flakes that have been applied to a bleached cotton yarn used batch dyeing in a high-throughput manner (1000 kg/h) [99][48]. In addition to the overcoat, cotton yarns were knitted into scaffold shapes via an automatic knitting machine to ensure high mechanical stability against a cyclic analysis at 25–55 °C. The great sensitivity and rapid response time of carbon-based nanomaterials, such as CNTs, have also made them an attractive alternative to rGO flakes for temperature sensors [100][49].

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