Stretchable electronics (elastronics) is an emerging class of electronics. It is used for building electronic circuits by depositing stretchable electronic devices and circuits onto stretchable substrates or by embedding them completely in stretchable materials, such as silicones or polyurethanes.
Stretchable devices have a wide range of potential uses in healthcare, energy and the military. Examples include cyber skin for robotic devices, imparting a network of sensors on a fully conformable, stretchable cyber skin. Other examples include: in vivo implantable sponge-like electronics, flesh-like devices with embedded electronic nervous systems and next-generation wearable devices.
Wearable elastronics
Non-invasive wearable robots can interact seamlessly with the human body, enabling the detection of diseases. Soft robots can be used to implement minimally invasive surgeries, such as of the brain. Where precision is important, such robots may have more scope than a human surgeon.
By integrating multiple stretchable components such as temperature, pressure and electrochemical sensors, it is possible to create a material resembling human skin. It uses signals from sweat, tears or saliva for real-time, non-invasive healthcare monitoring. Also, it can be used for smart prosthetics or robots with enhanced sense capabilities. However, at present, fabrication of artificial skin remains time-consuming and complex.
Elastronics research
Researchers from Seoul National University and MC10 (a flexible-electronics company that originated at MIT based in Lexington, Massachusetts) have developed a patch that is able to detect glucose levels in sweat. It can deliver the medicine needed on demand (insulin or metformin). The patch consists of graphene riddled with gold particles and contains sensors that detect temperature, pH level, glucose and humidity.
Strategies for developing stretchable electronics
Many different stretchable electronic components are being developed. Currently, there are two main strategies for manufacturing stretchable electronics.
In the first strategy, they can be made by using the same components used for conventional rigid printed circuit boards, the substrate and the interconnections being made stretchable rather than flexible or rigid.
Intrinsically stretchable materials, such as rubber, can endure large deformations. However, these materials have limitations, such as high electrical resistance. Also, when rigid components are deposited onto stretchable substrates, the interconnections will be subjected to high mechanical strain whenever the substrate is flexed.
The second method is to make non-flexible materials stretchable using innovative design. For example, brittle semiconductor materials such as silicon can be grown on a pre-stretched surface. It is then allowed to compress, creating buckling waves.
Another strategy involves linking ‘islands’ of rigid conductive materials together using flexible interconnections, such as soft or liquid metals. Origami-inspired folding techniques can be used to make foldable electronic devices. In the future, stretchable electronics may be enhanced with new capabilities, such as wireless communication, self-charging or even self-healing.
The cost of stretchable electronics
Low-cost stretchable conductors and electrodes are being made from silver nanowires and graphene. Graphene is a thin, two-dimensional layer of carbon atoms arranged in a hexagonal lattice. It is the basic building-block of graphite.
Graphene has a multitude of unique mechanical, thermal, electrical and optical properties. Defects in graphene can make graphene weaker and with quite different, yet no less interesting properties.
Graphene has extremely high electrical current density (a million times that of copper) and intrinsic mobility (100 times that of silicon). It has lower resistivity than any other known material at room temperature, including silver. There are also some methods to turn it into a superconductor because it can carry electricity with 100% efficiency.
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The technical issues
An urgent technical problem is the need for stretchable energy conversion and storage devices, such as batteries. Zinc-based batteries are promising candidates: however, more work is required to make them commercially viable.
Nanomaterials and fullerenes
Nanomaterials describe, in principle, materials where a single unit is sized (in at least one dimension) between 1 to 1000 nm but usually is 1 to 100 nm. Nanomaterials are slowly becoming commercialised and are beginning to emerge as commodities.
Fullerenes are conceptually graphene sheets rolled into tubes or spheres. These include the carbon nanotubes (or silicon nanotubes) which are of interest both because of their mechanical strength and their electrical properties.
Where are stretchable electronics used?
The most commonly used stretchable energy storage device is based on active materials for double-layer supercapacitors–carbon-based materials such as single-walled carbon nanotubes (SWCNTs)– due to their excellent electrical conductivity and high surface areas.
References
Wei Wu. Wuhan University, China. weiwu@whu.edu.cn, Paper link: https://doi.org/10.1080/14686996.2018.1549460
National Institute for Materials Science. The future of stretchable electronics. https://phys.org/news/2019-03-future-stretchable-electronics.html
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