Power/Performance Bits: Sept. 11

Transistor thread; MEMS microphone actuators.


Thread transistor
Researchers at Tufts University developed a thread-based transistor that can be fashioned into simple, all-thread based logic circuits and integrated circuits which could be woven into fabric or worn on the skin, or even surgically implanted.

The thread-based transistor (TBT) is made of a linen thread coated with carbon nanotubes, creating a semiconductor surface. Two thin gold nanowires are connected as source and drain. Surrounding the thread is an electrolyte-infused gel comprised of silica nanoparticles that self-assemble into a network structure and an ionic liquid which is connected to a third gate wire. The electrolyte gel (or ionogel) can be deposited onto the thread by dip coating or rapid swabbing and is resilient under stretching and flexing.

Manufacture of 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 (G), f) cross-sectional view of TBT. Electrolytes – EMI: 1-ethyl-3methylimidazolium TFSI: bis(trifluoromethylsulfonyl)imide (Source: Nano Lab, Tufts University)

Previously, the team 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 developed in this study allow the creation of logic circuits that control the behavior and response of those components. The researchers created a multiplexer (MUX) and connected it to a thread-based sensor array capable of detecting sodium and ammonium ions, which are 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. “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.”

“The development of the TBTs was an important step in making completely flexible electronics, so that now we can turn our attention toward improving design and performance of these devices for possible applications,” added Sameer Sonkusale, professor of electrical and computer engineering at Tufts. “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.”

MEMS microphone actuators
Engineers at Binghamton University devised a more reliable way to control MEMS microphones by combining two methods for electrostatic actuation. Using both parallel-plate and levitation actuators led to a predictable linearity that neither of those systems offered on its own.

This would allow MEMS microphones to be boosted high enough to make background noise from electronics a non-issue, said Ronald N. Miles, distinguished professor of mechanical engineering at Binghamton. “The electronic noise is really hard to get rid of. You hear this hiss in the background. When you make really small microphones – which is what we want to do – the noise is a bigger and bigger issue. It’s more and more of a challenge. This is one path toward avoiding that and getting the noise down.”

Shahrzad (Sherry) Towfighian, associate professor of mechanical engineering at Binghamton, explains that typically, MEMS actuators are two plates with a gap between them, which close when it receives a certain voltage. While those are difficult to fine tune, adding two electrodes on the sides of the plates creates a levitation effect that simultaneously pushes them apart and allows better control over the device. “Combining the two systems, we can get rid of nonlinearity,” she said. “If you give it some voltage, it stands at some distance and maintains that over a large range of motion.”

“In a sensor, life is much easier if it moves one unit and the output voltage increases in one unit, or something in proportion as you go,” added Miles. “In an actuator, you’re trying to push things, so if you’re giving it twice as much voltage, you want it to go twice as far and not four times as far.”

The two actuators work so well together because the nonlinearities inherent in each cancel out, said Miles. “They tend to be in opposite directions. We’re able to show that over a significant range, they’re linear. By having both of these electrode configurations, it gives you more knobs to turn and more adjustments you can make with applying voltages to different electrodes. With a simple parallel plate, you have one voltage across them and you don’t have much design freedom. With this, there are more electrodes and you get much more control over the design.”

Beyond microphones, the team sees possibilities for the new actuator design in gyroscopes, accelerometers, pressure sensors and other kinds of switches.

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