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Polymers aid transparent, highly conductive ultrathin film; fast and stretchy circuits for wearable electronics

Polymers aid transparent, highly conductive ultrathin film; fast and stretchy circuits for wearable electronics

29-Jun-16

The consumer are seeing a plethora of options of lively assortment of smart wearable electronics that wirelessly monitor vital boy signs, fitness or sun exposure to play music, charge other electronics or even purify the air. A team of University of Wisconsin-Madison engineers has created the world’s fastest stretchable, wearable integrated circuits, an advance that could drive the Internet of Things and a much more connected, high-speed wireless world. Led by Zhenqiang “Jack” Ma, the Lynn H. Matthias Professor in Engineering and Vilas Distinguished Achievement Professor in electrical and computer engineering at UW-Madison, the researchers published details of these powerful, highly efficient integrated circuits in the journal Advanced Functional Materials. The advance is a platform for manufacturers seeking to expand the capabilities and applications of wearable electronics- including those with biomedical applications - particularly as they strive to develop devices that take advantage of a new generation of wireless broadband technologies referred to as 5G. With wavelength sizes between a millimeter and a meter, microwave radio frequencies are electromagnetic waves that use frequencies in the .3 gigahertz to 300 gigahertz range. That falls directly in the 5G range. In mobile communications, the wide microwave radio frequencies of 5G networks will accommodate a growing number of cellphone users and notable increases in data speeds and coverage areas. In an intensive care unit, epidermal electronic systems (electronics that adhere to the skin like temporary tattoos) could allow health care staff to monitor patients remotely and wirelessly, increasing patient comfort by decreasing the customary tangle of cables and wires.
What makes the new, stretchable integrated circuits so powerful is their unique structure, inspired by twisted-pair telephone cables. They contain, essentially, two ultra-tiny intertwining power transmission lines in repeating S-curves. This serpentine shape - formed in two layers with segmented metal blocks, like a 3-D puzzle - gives the transmission lines the ability to stretch without affecting their performance. It also helps shield the lines from outside interference and, at the same time, confine the electromagnetic waves flowing through them, almost completely eliminating current loss. Currently, the researchers’ stretchable integrated circuits can operate at radio frequency levels up to 40 gigahertz. The advance could allow health care staff to monitor patients remotely and wirelessly, increasing patient comfort by decreasing the customary tangle of cables and wires. And, unlike other stretchable transmission lines, whose widths can approach 640 micrometers (or .64 millimeters), the researchers’ new stretchable integrated circuits are just 25 micrometers (or .025 millimeters) thick. That’s tiny enough to be highly effective in epidermal electronic systems, among many other applications. Ma’s group has been developing what are known as transistor active devices for the past decade. This latest advance marries the researchers’ expertise in both high-frequency and flexible electronics.
“We’ve found a way to integrate high-frequency active transistors into a useful circuit that can be wireless,” says Ma, whose work was supported by the Air Force Office of Scientific Research. “This is a platform. This opens the door to lots of new capabilities.” Other authors on the paper include Yei Hwan Jung, Juhwan Lee, Namki Cho, Sang June Cho, Huilong Zhang, Subin Lee, Tong June Kim and Shaoqin Gong of UW-Madison and Yijie Qiu of the University of Electronic Science and Technology of China.
A new self-healing material could solve many wearable woes. The experimental electronics material works in high humidity and even after being cut in half - once it heals itself. The physical limitations of existing materials are one of main problems when it comes to flexible electronics, be it wearables, medical or sports tech. If a flexible material breaks, it either stays broken, or if it has some self-healing properties it may continue to work, but not so well. However, a team from Penn State have creating a self-healing, flexible material that could be used inside electronics even after multiple breaks. The main challenge facing researchers led by Professor Qing Wang, was ensuring that self-healing electronics could restore "a suite of functions". The example used explains how a component may still retain electrical resistance, but lose the ability to conduct heat, risking overheating in a hypothetical wearable, which is never good. The nano-composite material they came up with was mechanically strong, resistant against electronic surges, thermal conductivity and whilst packing insulating properties. Despite being cut it in half, reconnecting the two parts together and healing at a higher temperature almost completely heals where the cut was made. The thin strip of material could also hold up to 200 grams of weight after recovering.
Unlike other healable materials, the boron-nitrate nanosheets the Penn State team used are unaffected by moisture, meaning it could also be used in high humidity environments like the shower. "This is the first time that a self-healable material has been created that can restore multiple properties over multiple breaks, and we see this being useful across many applications," said Qing Wang. "We need conducting elements in circuits but we also need insulation and protection for microelectronics."

An ultrathin film that is both transparent and highly conductive to electric current has been produced by a cheap and simple method devised by an international team of nanomaterials researchers from the University of Illinois at Chicago and Korea University. The film - actually a mat of tangled nanofiber, electroplated to form a "self-junctioned copper nano-chicken wire" - is also bendable and stretchable, offering potential applications in roll-up touchscreen displays, wearable electronics, flexible solar cells and electronic skin. The finding is reported in Advanced Materials. "It's important, but difficult, to make materials that are both transparent and conductive," says Alexander Yarin, UIC Distinguished Professor of Mechanical Engineering, one of two corresponding authors on the publication. The new film establishes a "world-record combination of high transparency and low electrical resistance," the latter at least 10-fold greater than the previous existing record, said Sam Yoon, who is also a corresponding author and a professor of mechanical engineering at Korea University. The film also retains its properties after repeated cycles of severe stretching or bending, Yarin said - an important property for touchscreens or wearables.
Manufacture begins by electrospinning a nanofiber mat of polyacrylonitrile, or PAN, whose fibers are about one-hundredth the diameter of a human hair. The fiber shoots out like a rapidly coiling noodle, which when deposited onto a surface intersects itself a million times, Yarin said. "The nanofiber spins out in a spiral cone, but forms fractal loops in flight," Yarin said. "The loops have loops, so it gets very long and very thin." The naked PAN polymer doesn't conduct, so it must first be spatter-coated with a metal to attract metal ions. The fiber is then electroplated with copper - or silver, nickel or gold. The electrospinning and electroplating are both relatively high-throughput, commercially viable processes that take only a few seconds each, according to the researchers. "We can then take the metal-plated fibers and transfer to any surface - the skin of the hand, a leaf, or glass," Yarin said. An additional application may be as a nano-textured surface that dramatically increases cooling efficiency. Yoon said the "self-fusion" by electroplating at the fiber junctions "dramatically reduced the contact resistance." Yarin noted that the metal-plated junctions facilitated percolation of the electric current - and also account for the nanomaterial's physical resiliency. "But most of it is holes," he said, which makes it 92% transparent. "You don't see it.

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