Making transistors out of thread for flexible electronic devices

Tufts University develops thread-based transistors
In the thread-based transistors developed by Tufts University, carbon nanotube coated threads carry electric current between source (S) and drain (D) wires, in response to voltage sensed at gate wire (G). (Tufts University)

In recent years, researchers have been busy trying to fabricate electronics on flexible substrates. One of the remaining hurdles has been the transistor, but that may be changing. A team of Tufts University engineers have developed a transistor made from linen thread that could expedite the development of fully flexible electronic devices that conform to different shapes and allow free movement without compromising function.

In a study published in ACS Applied Materials and Interfaces, the Tufts University researchers describe engineering the thread-based transistors (TBTs), which can be designed into simple, all-thread based logic circuits and integrated circuits. When combined with thread-based sensors, these transistors enable the creation of completely flexible, multiplexed devices.

Up to now, most flexible electronics devices have been manufactured by patterning metals and semiconductors into bendable “wavy” structures, or by using intrinsically flexible materials such as conducting polymers. Many of these circuits have been designed into medical devices that conform and stretch with the biological tissue in which they are embedded.

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The Tufts University researchers say, however, that thread-based electronics have superior flexibility, material diversity, and allow manufacturing without the need for costly cleanrooms. The thread-based electronics can include diagnostic devices that are extremely thin, soft and flexible enough to integrate seamlessly with the biological tissues that they are measuring.

Previously, the Tufts researchers developed a suite of thread-based temperature, glucose, strain and optical sensors, as well as microfluidic threads that can draw in samples from, or dispense drugs to, the surrounding tissue. The thread-based transistors now developed allow the creation of logic circuits that control the behavior and response of the sensors. The authors created a small-scale integrated circuit called a multiplexer (MUX), and connected it to a thread-based sensor array able to detect sodium and ammonium ions, considered key biomarkers for cardiovascular health and liver and kidney function.

“In laboratory experiments, we were able to show how our device could monitor changes in sodium and ammonium concentrations at multiple locations,” said Rachel Owyeung, a graduate student at Tufts University School of Engineering and first author of the study. “Theoretically, we could scale up the integrated circuit we made from the TBTs to attach a large array of sensors tracking many biomarkers, at many different locations using one device.”

Making a thread-based transistor involves coating a linen thread with carbon nanotubes, which creates  a semiconductor surface through which electrons can travel. There are two gold wires attached to the thread. One functions as a “source” of electrons, and the other as a “drain” where the electrons flow out. A third wire attached to material surrounding the thread, called the gate, responds to voltage changes to regulate the flow of current between the source and drain, in effect functioning like a transistor.

According to the researchers, one key element is the use of an electrolyte-infused gel as the material surrounding the thread and connected to the gate wire. In the study, the gel is made up of silica nanoparticles that self-assemble into a network structure. The electrolyte gel (iongel) can be easily deposited onto the thread by dip coating or rapid swabbing. The ionogel remains resilient under stretching or flexing.

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