An integrated self-healable electronic skin system. (Courtesy of Zhenan Bao, Donghee Son, Jiheong Kang, and Orestis Vardoulis).Electronic skin that heals itself after damage just like human skin could now be possible, according to new research [Son et al., Nature Nanotechnology (2018), https://doi.org/10.1038/s41565- 018-0244-6]. Wearable devices, which monitor heart rate for example, or life-like robotics and prosthetics need flexible, conformable skin-like electronic systems. But such systems also need to be robust enough to endure the wear and tear of human movement. Now a team, led by Zhenan Bao at Stanford University, Korea Institute of Science and Technology, Kyung Hee University, Asahi Kasei Corporation, and Samsung Advanced Institute of Technology, has developed a self-healing electronic skin system that can repair itself, unaided, after any damage.
“Electronic skin is a soft and stretchable electronic device inspired by human skin that is capable of sensing various external stimuli such as temperature, touch, and humidity,” explains one of the first authors of the study, Donghee Son, along with Jiheong Kang and Orestis Vardoulis.
The system devised by the team relies on a combination of a conducting nanostructured network embedded in a self-healing polymer matrix. The researchers used a tough, self-healing polymer, which incorporates strong and weak bonding units on a poly(dimethylsiloxane) backbone. The polymer can accommodate strains of up to 1600% and has extremely high fracture toughness. Embedded into the top surface of this conformable yet robust polymer matrix is a conductive network of either carbon nanotubes (CNTs) or silver nanowires (AgNWs).
When the composite is damaged – with a small cut, for example – the polymer matrix gradually repairs itself. Over the course of the few hours, the conductive nanostructured network follows suit, rearranging itself until conductive pathways are rebuilt. After about 24 h at room temperature, the researchers found that electrical resistance, which reaches infinitely high levels immediately after damage to the material, returns to pre-damage levels. Likewise the resistance-strain behavior also returns to normal after about 12 h. Moreover, the physical damage to the electronic skin system was almost indiscernible to the eye after a couple of days. Incredibly, even if the material is cut through entirely, the severed surfaces can still self-heal and reconstruct if brought back into contact with each other with only a small increase in resistance.
“[This is] the first report of the molecular-level movement of polymer chains translating into macroscopic rearrangement of a conductive network,” says Son. “We have observed a new phenomenon in the reconstruction of a nanostructured conductive network.”
The team used either CNT or AgNWs polymer composites to create self-healing active components such as interconnects and electrodes for functional devices. A tough, self-healing polymer is used as the device substrate and to encapsulate the functional layers, which include self-healable ECG and strain/pressure sensors, protecting them from damage. A light emitting capacitor (LEC) array is also incorporated into the electronic skin system.
“Our integrated electron skin system can detect physiological signals and wirelessly transmit the recorded data to LEC arrays to display the user’s health condition in real-time,” points out Son.
The new electronic skin system is biocompatible and water-insensitive, as well as suitable for large-scale synthesis, making it ideal for wearable healthcare and prosthetic skin applications.
“Our work represents a new milestone in self-healing electronics,” says Son. “A combination of high toughness and autonomous self-healability would potentially validate future unbreakable wearable electronics.”
Marek W. Urban of Clemson University believes that Bao and her team have demonstrated one of the most elegant interplays of science and engineering in designing multi-functional self-healable electronic skin concepts. which may have many future applications.
“What is particularly impressive is the integration of inorganic and organic components each bringing unique properties, with potential applications in soft robotics, prosthetic skin, and flexible electronics,” he comments. “Self-reconstruction of conducting nanostructures in contact with a dynamically cross-linked polymer networks is particularly impressive.”
The only significant hurdle to overcome is the high power consumption of the LEC display, which will need to be reduced to extend device lifetime.
This article was originally published in Nano Today 22 (2018) 5–6.