System Bits: April 11

Tiny transistors made from self-assembled carbon nanotubes
While carbon nanotubes can be used to make very small electronic devices, they are difficult to handle. Now, researchers from the University of Groningen, the University of Wuppertal, and IBM Zurich, have developed a method to select semiconducting nanotubes from a solution, and make them self-assemble on a circuit of gold electrodes.

The team acknowledges the results look deceptively simple: a self-assembled transistor with nearly 100 percent purity and very high electron mobility — but it took ten years to get there. University of Groningen Professor of Photophysics and Optoelectronics Maria Antonietta Loi designed polymers which wrap themselves around specific carbon nanotubes in a solution of mixed tubes. Thiol side chains on the polymer bind the tubes to the gold electrodes, creating the resultant transistor.

Artists impression of carbon nanotubes wrapped by polymers with thiol side chains (yellow spheres), assembled on gold electrodes. (Source: University of Groningen)

In previous work, the researchers said they learned a lot about how polymers attach to specific carbon nanotubes. Specifically, these nanotubes can be depicted as a rolled sheet of graphene, the two-dimensional form of carbon. Depending on the way the sheets are rolled up, they have properties ranging from semiconductor to semi-metallic to metallic. Only the semiconductor tubes can be used to fabricate transistors, but the production process always results in a mixture.

They had the idea to use polymers with thiol side chains some time ago, the idea being that as sulphur binds to metals, it will direct polymer-wrapped nanotubes towards gold electrodes. So while the researchers were working on the problem, IBM patented the concept, but there was a big problem in the IBM work in that the polymers with thiols also attached to metallic nanotubes and included them in the transistors, which ruined them.

The solution was to reduce the thiol content of the polymers, with the assistance of polymer chemists from the University of Wuppertal. As a result, they’ve now shown that this concept of bottom-up assembly works, and that by using polymers with a low concentration of thiols, they can selectively bring semiconducting nanotubes from a solution onto a circuit.

Spray-on memory for bendable digital storage
Duke University researchers reminded that USB flash drives are already common accessories in offices and college campuses but thanks to the rise in printable electronics, digital storage devices like these may soon be everywhere – including on groceries, pill bottles and even clothing. To this end, the team believes it has brought closer a future of low-cost, flexible electronics by creating a new “spray-on” digital memory device using only an aerosol jet printer and nanoparticle inks.

The device is analogous to a 4-bit flash drive, and is the first fully-printed digital memory that would be suitable for practical use in simple electronics such as environmental sensors or RFID tags, the team said. It is jet-printed at relatively low temperatures, and therefore could be used to build programmable electronic devices on bendable materials like paper, plastic or fabric.

Duke University researchers have developed a new “spray-on” digital memory (upper left) that could be used to build programmable electronics on flexible materials like paper, plastic or fabric. They used LEDS to demonstrate a simple application. (Source: Duke University)

At the core of the new device, which is about the size of a postage stamp, is a new copper-nanowire-based printable material that is capable of storing digital information.

Whereas most flash drives encode information in series of silicon transistors, which can exist in a charged state, corresponding to a “one,” and an uncharged state, corresponding to a “zero,” the new material, made of silica-coated copper nanowires encased in a polymer matrix, encodes information not in states of charge but instead in states of resistance. By applying a small voltage, it can be switched between a state of high resistance, which stops electric current, and a state of low resistance, which allows current to flow.

And, unlike silicon, the nanowires and the polymer can be dissolved in methanol, creating a liquid that can be sprayed through the nozzle of a printer.’

Sensor-laden shoes for seniors
Using what exists currently as a tangle of wires, sensors, circuits and motors, Rice University bioengineering students are building a device that can help people who have impaired sensation in their feet stay upright and avoid falls.

In what could someday be a simple-powered insole that can go into any shoe to provide additional tactile sensation to improve the wearer’s motor skills, that sensory feedback could prevent a tumble. And for many elderly and patients with diabetes who might have lost some ability to feel their extremities, that can be a lifesaver.

The team of Megan Kehoe, Yuqi Tang, Suzanne Wen, Daniel Zhang and Allen Hu, senior bioengineering majors working with faculty adviser Eric Richardson, accepted the challenge posed by Dr. Mehdi Razavi, director of electrophysiology clinical research at the Texas Heart Institute. Razavi asked students working on their required capstone projects at Rice’s Oshman Engineering Design Kitchen to find a way to help his patients maintain their balance.

The students call themselves “All the Feels,” solved the problem by dividing the foot into four zones. Under each they placed a sensor that measures the pressure on the foot, which is used to determine how much tactile sensation the user should feel. Above each sensor they placed a vibrating motor, not unlike that found inside a cellphone, to provide additional sensation to the wearer. They hope wearers will eventually learn to process the feedback unconsciously and adjust their strides automatically to navigate stairs or uneven terrain.

See the team’s web page here.

Four strategically placed sensors were built into the sole of a prototype shoe created by Rice University engineering students. The sensory feedback device is intended to prevent falls in people with impaired sensation in their feet. (Source: Rice University)