Of the five senses, touch is the most widely distributed throughout the body, and perhaps the most fundamental. A single fingertip has over 2500 touch receptors, which sense and transmit enough information to allow us to discriminate spatial distances as small as 40 micrometers (Tee, et al, 2013). Receptors distributed throughout our hands can sense extremely gentle pressures of around 100 pascals (equivalent to the feeling of a penny resting on a fingertip) as well as pressures of greater than 100 kilopascals (the feeling of gripping an object very tightly). Having such a wide range of sensitivities allows us to perform extremely delicate tasks, such as flipping the pages of a textbook, or pipetting primers into tiny PCR tubes.
From a materials engineering point of view, human skin is nature’s version of an elastomer-based pressure sensor, or, in layman’s terms, an extremely flexible material that is able to feel when it’s being pushed on, pulled, flexed or grazed. For many years, recreating the features of human skin by artificial means eluded researchers. Recently, however, several labs have made enormous strides in recreating the properties of human skin electronically. Over the past decade, Prof. Ali Javey and colleagues at Lawrence Berkeley National Laboratory developed a number of plastic- and rubber-based electronic skin prototypes. Meanwhile, in the laboratory of Prof. Zhenan Bao at Stanford University, graduate student Benjamin Tee helped develop two types of flexible electronic skin, including one that is able to repair itself after being damaged.
The first prototype developed in the Bao laboratory relies on capacitors to detect changes in pressure and flexible organic transistors to amplify and relay signals emitted by the capacitors in a system known as a capacitive pressure sensor. This system is likely familiar to you, because a form of capacitive sensing is used in touch screens for smartphones and tablet computers. The basic building block of capacitive sensing systems are capacitors, devices that are used to temporarily store electrical energy. Though there are several types of capacitors, all contain at least two electrical conductors—usually metal plates—separated by a non-conductive insulator known as a dielectric. When current flows through a circuit and hits the capacitor, a difference in electrical potential is created between the capacitor’s plates, causing positive charge to collect on one plate and negative charge on the other. This in turn causes an electrical field to develop across the dielectric. The electrical field that is created can remain in the capacitor even if the current ceases to flow through the circuit. In this way, energy can be stored by the capacitor in the form of a static voltage spanning the conductors. When stored energy is released, the resulting current can be picked up by devices called transistors and amplified into a readable output current that is much stronger than the input current.
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