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High frequency, stretchable circuits could yield new era of wearable electronics

The unique structure has been inspired by twisted-pair telephone cables - essentially, they contain two ultra-tiny intertwining power transmission lines in repeating S-curves

27 May 2016 | Editor

Researchers at the University of Wisconsin—Madison - led by Zhenqiang "Jack" Ma, the Lynn H. Matthias Professor in Engineering and Vilas Distinguished Achievement Professor in electrical and computer engineering, have created what the researchers believe to be the world's fastest stretchable, wearable integrated circuits, an advance that could be a key driver for the Internet of Things and a play an important role in the growing connected, high-speed wireless world.

According to the researchers 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.

The say what makes the new, stretchable integrated circuits powerful is their unique structure, inspired by twisted-pair telephone cables. Essentially, they contain 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.

University of Wisconsin - Fabricated in interlocking segments like a 3-D puzzle, the new integrated circuits could be used in wearable electronics that adhere to the skin like temporary tattoos

Figure: University of Wisconsin - Fabricated in interlocking segments like a 3-D puzzle, the new integrated circuits could be used in wearable electronics that adhere to the skin like temporary tattoos

Further, 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. At these dimensions they are tiny enough to be highly effective in epidermal electronic systems, among many other applications.

The advance could allow health care staff to monitor patients remotely and wirelessly. 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.

Currently, the researchers' stretchable integrated circuits can operate at radio frequency levels up to 40 GHz. 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. With wavelength sizes between a mm and a meter, microwave radio frequencies are electromagnetic waves that use frequencies in the .3GHz to 300GHz range - these falls directly in the 5G range.

Jack Ma’s group has been developing 'transistor active devices' for the past decade. This latest advance marries the researchers' expertise in both high-frequency and flexible electronics.

The work was supported by the Air Force Office of Scientific Research.

Zhenqiang 'Jack' Ma, the Lynn H. Matthias Professor in Engineering and Vilas Distinguished Achievement Professor in electrical and computer engineering at UW–Madison, said, "We’ve found a way to integrate high-frequency active transistors into a useful circuit that can be wireless. " Jack added "This is a platform. This opens the door to lots of new capabilities."

Stretchable Twisted-Pair Transmission Lines for Microwave Frequency Wearable Electronics

Yei Hwan Jung | Juhwan Lee | Yijie Qiu | Namki Cho | Sang June Cho | Huilong Zhang | Subin Lee | Tong June Kim | Shaoqin Gong | Zhenqiang Ma

First published: May 2016 | DOI: 10.1002/adfm.201600856

Abstract

Stretchable electrical interconnects based on serpentines combined with elastic materials are utilized in various classes of wearable electronics. However, such interconnects are primarily for direct current or low-frequency signals and incompatible with microwave electronics that enable wireless communication. In this paper, design and fabrication procedures are described for stretchable transmission line capable of delivering microwave signals. The stretchable transmission line has twisted-pair design integrated into thin-film serpentine microstructure to minimize electromagnetic interference, such that the line's performance is minimally affected by the environment in close proximity, allowing its use in thin-film bioelectronics, such as the epidermal electronic system. Detailed analysis, simulations, and experimental results show that the stretchable transmission line has negligible changes in performance when stretched and is operable on skin through suppressed radiated emission achieved with the twisted-pair geometry. Furthermore, stretchable microwave low-pass filter and band-stop filter are demonstrated using the twisted-pair structure to show the feasibility of the transmission lines as stretchable passive components. These concepts form the basic elements used in the design of stretchable microwave components, circuits, and subsystems performing important radio frequency functionalities, which can apply to many types of stretchable bioelectronics for radio transmitters and receivers.

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