Feeling Through the Abyss 

Bendable silicon sensors are a feat of nano-construction, but the sensory communications they allow verge on the fantastic 

In 2004, James Gregory, a surgeon specializing in malignancies of the liver and bile ducts, was frustrated with what he could not feel.

Gregory, then a clinical professor at the University of Illinois at Urbana-Champaign, was tired of using robotic devices and cameras to diagnose diseased tissue. Surely, he thought, there must be a way to increase the feeling through a latex surgical glove so that, instead of probing at the body with mechanical devices, a doctor could interact with the nuance of a fingertip.

He found his solution less than a mile away from the university’s medical center, in the lab of a friend, a nanoengineer named John Rogers.  

Rogers specializes in nanoscale engineering, a field that adapts electronic circuits so they can flex, stretch and bend in decidedly non-mechanistic ways. The emerging area is an interdisciplinary abnormality – Rogers holds joint appointments at UIUC in the departments of Chemistry, Bioengineering, Mechanical Science, Electrical Engineering and Computer Engineering – and the work produced by the Rogers Research Group reads like a smut novel from a futurist fantasy.  In the last year the lab has designed an electrocardiogram reader that sticks to the left valve of the heart – monitoring heartbeats wirelessly to an external device – and a tissue-thin circuit that acts like a compression bandage for diseased tissue, dissolving automatically into the blood stream when the body repairs.

The lab’s latest invention is an initial answer to Gregory’s question: a smart finger sheath, which wires tactile information through a layer of latex.

Enhancing touch requires a feat of deception. The sheath uses a set of exterior sensors to register the size, shape and texture of an object by pressure; then, it wires the information to tiny, interior pads, which mimic the feeling of touch by shooting small electrical currents into the nerve receptors of the finger – a feat of electrotactile technology, which “tricks the skin into feeling an object that’s not there,” says Rogers.

The prototype is detailed in the August issue of Nanotechnology.

Electrotactile technology, which uses mechanics to replicate the senses, has been around since the mid-80s. It was lauded as a groundbreaking innovation that could recreate sight, sound and touch in people with sensory damage. It largely hasn’t lived up to the hype.

Aside from “a few tentative probes into the marketplace,” the technology “kind of fizzled out,” said Kurt Kaczmarek, a neuroscientist at the University of Wisconsin who is considered one of the fathers of electrotactile technology.

“For a number of years the main issue of electronic recreation of sensation has been controlling the feel and perception in a way that feels natural,” explains Kaczmarek. The mechanics of the fingertip – in particular its small sensitivity range, shape, and sweatiness – make it a particularly erratic vessel for electrical currents. So difficult, in fact that Kaczmarek’s groundbreaking device, BrainPort, an electrotactile camera that transmits visual information to the blind through touch, bypasses the fingertip problem in favor of the tongue.

Thus the challenge of designing Rogers’ device was one of materials, not conception.

Circuits are based on interlocking semi-conducted wafers, which are largely flat and brittle. “They have the principles of glass: if you bend them you break them, if you drop them they shatter,” says Rogers. “They’re very linear, but the human body is not very linear.” 

Semi-conducted wafers have successfully been engineered into a slight curve, to mirror the horizons of the brain or heart.

 But to work with the finger, things have to move – a lot.

“If you think about a finger cuff or the skin itself, it bends and wrinkles to be sure, but it also stretches like a rubber band,” says Rogers. “The sensors can’t just “bend, they have to flex and stretch.” 

To work around the problem, Rogers’s team initially tried to recreate touch physically by rigging up a device with tiny inflatable balloons that would stimulate the finger nerves manually.

It didn’t work. “The problem is: How do you do 100 balloons? And how do you have 100 airlines to inflate each one? But you can make a million electrical stimulations,” he said.

So instead, they decided to rethink the circuit.

First they shaved down the base of the semiconductor to a tiny sliver that resembles a bendable silicon Frisbee.  Then the team changed the shape of the conductors – from straight lines to a stretched out slinky of interlocking circles that Rogers calls “narrow filaments in a serpentine geometry.” 

The result: a row of semi-conductors that fit “intimately” with the skin.

From an engineering perspective, the flexible circuit is “very impressive,” says Darren Lipomi a nanoengineer at the University of California, San Diego. More importantly the flexible circuit allows room for further innovation.

“If you want to put electronics inside the body for medical purposes, if you want to make a device which is resistant to mechanical failure for fieldwork – all these materials need to respond to strain in a way that can’t destroy them,” says Lipomi.

The glove-prototype is already under development with MC10, a Cambridge start-up, co-founded by Rogers, which specializes in small, stretchable electronic devices. (The company won a technology innovation award from the Wall Street Journal week for another Rogers project – a translucent patch, which can monitor the blood and heart beat continuously through a square-inch of skin.)

Still, the surgical device is a long way from the operating room, says Elyse Kabinoff, marketing director for MC10. Translating an academic study to a product “can take a year or two, or a decade or more,” she wrote in an email, and “medical products, particularly devices, take much longer – based in part on regulatory approvals.”

In the distant future, Rogers can imagine a myriad of uses for the technology behind the glove: attach an electrode at the tip and arrhythmia can be measured with a finger; link the glove to the Internet and suddenly Amazon and eBay have a tactile reality for virtual textile shopping. Or, straight out of science fiction: Think of what could happen with a whole suit…

“You string several of these circuits together and you really have incredible options,” says Rogers.

Even without a flight of imagination, Kaczmarek believes that Rogers’ flexible circuits may allow for a wider range of electrotactile innovation.

“It’s a huge step in the right direction,” explains Kaczmarek.