Human skin has a fascinating variety of diverse physical properties. It is versatile enough to discern small changes in the feel of various materials and at the same time capable of shielding the body from environmental injury and hostile biological entities. As our primary interface with the environment, the skin offers complex tactile sensing capabilities that allow many of our critical activities to take place. For instance, force measurements help promote gripping strength and temperature sensing abilities sharpen environmental perception and help in dodging harmful temperatures.
Electronic devices are being designed to imitate the properties of human skin and are referred to as electronic skin (e-skin). Such devices have found valuable application in prosthetics, advanced robotics and health tracking technologies. For measuring temperature and tactile signals from the skin, methods have rapidly advanced because of inventions in processing techniques and materials. This article will review how e-skin systems are exceeding the abilities of natural human skin.
The electronic devices with biological skin properties mimic their temperature and tactile sensing abilities. Tactile sensing abilities include measurements of shear forces, strain, pressure and vibrations. For gripping objects, shear forces and pressure measurements are vital, and for tracking body movements, strain measurements are important. Moreover, for gauging an object's texture, vibration information is crucial. Popular methods that are used to transform these physical stimuli into electrical signals require changes in capacitance or resistance and voltage generation by piezoelectric sensors.
In piezoresistive tactile sensing, strain and pressure sensors convert the strain changes into resistance changes by altering the resistance of the contact present between the conducting parts or resistivity of a conducting part. Elastomers with ingrained conducting fillers are popular since they are readily available and inexpensive. Traditionally, carbon black was used as conductive filler by pressure-based rubbers. But such materials generally have large hysteresis, low sensitivity and largely depend upon temperature. Therefore, low-density materials like carbon nanotube aerogels with small elastic moduli can increase sensitivity. Additionally, modern conductive fillers with sharp characteristics, including micro-structured nickel particles, boost tunneling current dependence on the material's strain state by focusing the electrical field at sharp tips, thus enhancing sensitivity.
For capacitive tactile sensing, strain, shear forces and normal forces are measured according to the changes in the gap between the plates of a dielectric and its area. Dielectric materials also have higher sensitivity because of their property of high compressibility. As a result, air gap systems have been widely used. Moreover, capacitive devices are also not inherently sensitive to temperature and have high sensitivity. Even so, they are prone to intrusion from the environment that can alter their fringe fields. Solid and thin dielectric materials show sluggish time responses because of the viscous properties of the materials. On the other side, air gap systems need wide air gaps, which result in small capacitance values, and reduced sensitivities. Therefore, the dielectrics are often micro-structured into shapes of a pyramid to dodge the bulk elastomers' viscous properties even with thin dielectrics. This results in fast time response. The extremely small thickness of dielectrics along with high capacitance allowed the manufacturing of transistors that are extremely sensitive to transistors.
For piezoelectric tactile sensing, the sensors use piezoelectric transduction techniques. Piezoelectricity refers to the voltage generated when a strain is applied to a material. Piezoelectric-based sensors efficiently detect small-timescale force changes and vibrations but exhibit imprecise characteristics when it comes to pressure sensing. Moreover, piezoelectric materials also react to changes in temperature, reducing the tactile signal's selectivity. "Piezo-electret" devices also use the same methods, but the permanent charges found in the pores of materials having low-density form their dipoles. Therefore, piezo-electrets are beneficial over piezoelectric devices because of their smaller dependence on temperature and higher electromechanical transduction coefficients.
Temperature sensing commonly depends upon the resistivity of metals that is sensitive to temperature (i.e., resistance's temperature coefficient). Nevertheless, finding out the slight resistance variations with temperature needs complex circuits. Experts are also using semiconductor P-N junctions for sensing temperature as they offer benefits of more efficient multiplexing and higher sensitivity. Conductive polymer composites are also used for such purposes. They show huge variations in conductivity when the temperature reaches the melting point of the polymer matrix. Nonetheless, a conductor and a polymer matrix, or any mixture of two materials, do not show reproducibility when made to undergo cycling. However, composites of three materials such as a conductive filler, a polymer with a melting point in the desired temperature range and a polymer with a high melting point have shown decent reproducibility and outstandingly high sensitivity. The produced electric signal can also be read wirelessly because of such composites.
Rapid innovations in deployment methods, processing techniques and materials have allowed significant development in the abilities of electronic devices that can imitate human skin. The sensing mechanisms have gained from the use of micro- and nano-technologies. However, given a variety of e-skin devices that transcend the properties of human skin, advances in all aspects are still possible, for example, improving the response time, reducing cost and deploying several functions.