Tufts University engineers are making transistors from a material you’re more likely to see in a fabric store than in the field of electronics.

The semiconductor, produced entirely from linen thread, enables the Tufts team to create electronic devices that can ultimately be woven into clothing, worn on the skin, or someday even implanted surgically for diagnostic monitoring.

In a study published in ACS Applied Materials and Interfaces, the authors describe engineering the first thread-based transistors (TBTs), which can be fashioned into simple, all-thread based logic circuits and integrated circuits.

The circuits replace the last remaining rigid component of many current flexible devices.

The Tufts-developed transistors conform to different shapes and allow free movement without compromising function – a valuable biomedical application, according to researcher Sameer Sonkusale.

“There are many medical applications in which real-time measurement of biomarkers can be important for treating disease and monitoring the health of patients. The ability to fully integrate a soft and pliable diagnostic monitoring device that the patient hardly notices could be quite powerful,” said Sonkuale, a professor of electrical and computer engineering at Tufts University.

How to Make a Thread-Based Transistor

The first step of making a thread-based transistor is to coat a linen thread with nanotubes. The coating creates a semiconductor surface through which electrons can travel.

SEM images of linen thread coated with carbon nanotube (CNTs) for Tufts University thread-based electronics
SEM images of (a) bare linen thread and (b) linen thread after coating with CNTs. These images show the relative roughness of the linen thread substrate and that no apparent thread morphology changes occur upon addition of CNT solution. (Image Credit: Tufts)

Attached to the thread are two thin gold wires – a “source” of electrons and a “drain” where the electrons flow in and out.

A third wire, called the gate, is attached to material surrounding the thread. Small changes in voltage through the gate wire allow a large current to flow through the thread between the source and drain – the basic principle of a transistor.

Perhaps the most innovative ingredient in the production process is the electrolyte-infused gel material surrounding the thread and connecting to the gate wire. The ionogel, made up of silica nanoparticles that self-assemble into a network structure, can be easily deposited onto the thread by dip coating or rapid swabbing. In contrast to the solid-state oxides or polymers used as gate material in classical transistors, the electrolyte is resilient under stretching or flexing.

a diagram of how to make a thread-based transistor, which includes nanotube coated linen threads, two thin gold wires, and a gate
The steps of producing thread-based transistors (TBTs): a) linen thread, b) attachment of source (S) and drain (D) thin gold wires, c) drop casting of carbon nanotubes on the surface of the thread, d) application of electrolyte infused gel (ionogel) gate material, e) attachment of the gate wire. f) shows a cross-sectional view of TBT. Electrolytes EMI: 1-ethyl-3methylimidazolium TFSI: bis(trifluoromethylsulfonyl)imide. (Image Credit: Tufts)

The Tufts researchers created a simple small-scale integrated circuit called a multiplexer (MUX) and connected it to a thread-based sensor array capable of detecting sodium and ammonium ions – important biomarkers for cardiovascular health, 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.”

In an interview with Tech Briefs, Owyeung reveals the big possibilities that begin with such a tiny thread.

Tech Briefs: What inspired you to make this thread-based transistor?

Rachel Owyeung: Our group has previously explored threads for electrochemical sensors, microfluidics, optical sensors, and even drug delivery on threads, but we still relied on conventional silicon-based electronics. For many biomedical applications that require diverse or large quantities of sensors, or the sensing area is deep within the tissue, there is a need to integrate electronics with the sensors for multiplexed readout and amplification. This is easier when the entire platform, including the electronics, is extremely thin, soft, and pliable for intimate integration of the device with the biological tissue without impairing function. Thus, we needed to develop transistors that fit this description, as they are essential building blocks of any advanced sensor or electronic circuit.

Tech Briefs: What are the advantages of a completely thread-based transistor, compared to a traditional transistor?

Rachel Owyeung: Threads have many interesting properties compared to silicon or other substrates for transistors, such as chemical tunability through material diversity, mechanical flexibility, and they can provide a natural interface to three-dimensional tissues and organs.

Additionally, centuries of process and material refinement in the textile industry have uniquely positioned threads as the ideal substrate for wearable technology, since threads are extensively engineered to be placed in intimate contact with the body (e.g., clothes or wound dressings) and are already manufactured at scale.

Tech Briefs: What applications are most exciting to you?

Rachel Owyeung: Thread-based transistors are particularly ideal for wearable or biomedical applications where intimate interfacing of soft and compliant devices would improve the user experience. We’re particularly excited about exploring this technology for diagnostic monitoring devices, where the flexibility of the electronics not only leads to greater patient/user comfort, but also provides a conformal contact for more accurate readings of the monitored biomarkers.

Tech Briefs: What was most challenging part of the design process for you?

Rachel Owyeung: Threads have a few major challenges when it comes to electronics. A thread’s surface is highly uneven and rough from the many fibers that coalesce to form the thread. Threads also have an unconventional aspect ratio and geometry compared to typical planar silicon chips. These aspects make fabrication of the transistors challenging, as typical methods cannot be used as is. That’s where the thoughtful material selection comes in. We chose our semiconductors and gate materials (our ionogel) because they could be easily deposited by dip coating or swabbing and are more resilient to flexing or stretching.

Tech Briefs: What will you work on next?

Rachel Owyeung: We are looking into ways to improve the scalability of the devices. Ultimately, a high-throughput fabrication process would be great for demonstrating more complex circuits with these thread-based transistors.

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