Some animals, such as the chameleon and cephalopod, have the remarkable capability to change their skin colour. This unique characteristic has long inspired scientists to develop materials and devices to mimic such a function. However, it requires the complex integration of stretchability, colour-changing and tactile sensing. Stanford University researchers have developed an all-solution processed chameleon-inspired stretchable electronic skin (e-skin), in which the e-skin colour can easily be controlled through varying the applied pressure along with the applied pressure duration. As such, the e-skin's colour change can also be in turn utilized to distinguish the pressure applied. The integration of the stretchable, highly tunable resistive pressure sensor and the fully stretchable organic electrochromic device enables the demonstration of a stretchable electrochromically active e-skin with tactile-sensing control. This system will have wide range applications such as interactive wearable devices, artificial prosthetics, health monitoring and smart robots.
Human skin provides a remarkable network of sensors with highly sensitive pressure, temperature and vibration sensing. Skin can transduce environmental stimuli into physiological signals, which are then interpreted by brain. Electronic skin (e-skin) is an artificial skin that mimics the properties of skin using electronic devices. Inspired by human skin, e-skin has many potential applications.
Unlike human skin, both animal and insect skin exhibit additional functions, for example, the chameleon's skin has colour-changing abilities. A chameleon shifts its skin colour through controlling the skin pigment cell for purposes in camouflage, temperature maintenance and communication. Since chameleons cannot generate any body heat, the colour of their skin can in turn be used to regulate their body temperature. Mimicking the colour-changing ability of chameleons can also be achieved using approaches such as mechanical or electrical control.
The e-skin developed by the Stanford team, led by Ho-Hsiu Chou, consists of two main components: a stretchable microstructured polymer that can modify its voltage upon an applied pressure, and a stretchable electrochromic polymer that can be either red or blue, depending on the applied voltage. They demonstrated an e-skin with both interactive colour-changing and tactile-sensing properties by attaching the pressure-sensitive polymer to a commercially purchased teddy bear's paw, and connected it to the electrochromic polymer which they mounted on the bear's abdomen. Upon applying a weak handshake (~50 kPa), the colour of the electrochromic polymer turned from dark red to blue grey. Releasing the handshake reverts the colour to dark red, whereas applying a strong handshake (~200 kPa) changes the colour again to pale blue.
Notably, for the user-interactive devices, the toxicity and carcinogenicity of carbon nanotubes have raised concerns as they have rather similar shapes as asbestos. Previous reports have demonstrated that the longer and thicker carbon nanotubes (lengths >5 μm and diameter >20 nm) will induce significantly more DNA damage and inflammation compared with the lower aspect ratio63, 64. Here the researchers use the much shorter and smaller diameter SWNT (bundle lengths range from 0.5 to 1.5 μm, along with an average bundled diameters of 4-5 nm), which should greatly reduce the potentially adverse effect. Furthermore, the team also considers that proper encapsulation of this system is needed. A number of elastic substrates, such as silicone, polyurethane or fluoroelastomers, are biocompatible and highly stretchable. They are also easily processed. Therefore, the researchers believe that encapsulation with such elastomers is a potentially compliant method for further e-skin applications.
In our future studies, the researchers aim to introduce various colours of electrochromic polymers and array designs to enable a wider and more dynamic colour range for high contrast and high resolution. Such systems should be promising for applications in military applications, artificial prosthetics, smart robots, smart phones, smart watches, and any other kind of wearable devices.
Source and images: Nature Communications