You might think that temporary tattoos look cool, but what if they could also collect and transmit information about your heart rate, temperature, muscle contractions or brain waves?

A new flexible electronic circuit promises to do just that, by moving with the skin and staying in place without any adhesive. The research used existing semiconductor technology to imprint integrated circuits onto a thin, flexible silicon film that can be applied directly on the skin. The device is described in a new paper published online August 11 in Science.

"The goal is really to blur the distinction between electronics and biological tissues," John Rogers, a professor of materials science and engineering at the University of Illinois at Urbana-Champaign (U.I.C.U.) and co-author of the new study, said in a podcast interview.

The new technology might soon allow monitoring to become "simpler, more reliable and uninterrupted," Zhenqiang Ma, a professor at the University of Wisconsin–Madison's Department of Electrical and Computer Engineering, wrote in an essay in the same issue of Science.

Rogers, who is working with Cambridge, Mass.–based start-up company mc10, Inc., to commercialize the device, and his team have already demonstrated the patch can be used to measure vitals—and they suggest that it could one day be used to help stimulate muscles, speed wound healing, improve prosthetics and even communicate with video games.

Slim fit
The most powerful force of this new patch is its thinness. "At an intuitive level, it's really pretty simple," Rogers said in the podcast. "If you make anything thin enough it becomes flexible." So, instead of a more typical one-half-millimeter-thick silicon wafer, they used a 50-nanometer-thick silicon membrane.

Rogers calls the membrane itself "kind of disgusting," noting in a Wednesday press briefing that "it looks a little bit like it's been ripped off—or excreted from—the body." But that is, in some sense, the point: to arrive at a material in which "the distinction between the mechanics and the tissue is blurred," he said Wednesday. And to get around the problem of application, the group took a cue from the temporary tattoo industry and used a plastic backing that is peeled off after application. (Rogers and his colleagues are fond of demonstrating the sensor's application on a temporary pirate tattoo.)

The ultra-flexible patches were made via transfer printing (or "inking and printing"), in which the chips are assembled on two silicon layers and then transferred onto the elastomer polymer base material, which is designed to conform to the skin.

Although the materials and components are not particularly new, they are "configured in geometries that are unusual," Rogers said. The circuits are formed into an open mesh shape, "almost like a spider web of electronics that we embedded in a very thin elastomer skin," he explained. That means, "we have not had to go back and reinvent semiconductor materials or reinvent transistor design," he said. And just by slimming everything down and assembling it in the right configurations, his team hopes they are leading the way to new "opportunities in biointegration."

Xiaolin Zheng, an assistant professor of mechanical engineering at Stanford University, has been working on a similar device and is excited about the new report. Her team's device, described in a recent Nano Letters paper, relies on a different manufacturing process that involves a layer between the circuitry and silicon that helps to prevent stress on larger circuits in the application process. As Ma pointed out in his essay, however, the fact that the polyester layer and the sensor layer are the same thickness in the new device means that they "develop opposite strains that cancel, so the middle circuit layer experiences little stress no matter which direction the device is bent."

Although Zheng's group used straight metal rather than the more deformable shapes in the new patch, the Stanford team's version is substantially slimmer, measuring in at about 0.8 micron. "Our device is even more flexible and can easily achieve conformal coating onto curved surfaces," she says.

Easy on
Much of today's monitoring equipment requires bulky hardware, such as heart monitors that cardiac patients often have to drag or tote around. But this new device would be "almost mechanically invisible to the wearer," Rogers noted in the podcast. Being slim and sleek would allow it to go where other devices can be awkward or invasive. One possible application could be for premature babies, who, because of their small size "just aren't compatible" with bulkier, hardwired sensors, Rogers said. It could also be a more appealing monitoring method for sleep studies, nixing the need for cumbersome, disruptive equipment and wires.

The flexible circuit also enables a large number of electrodes to be in contact with the skin over a small space—an improvement over current electrodes that typically are individually applied and wired.

And instead of requiring potentially irritating adhesive gel or tape, it takes advantage of nonspecific, generalized adhesion forces, known as van der Waals forces—the same ones that allow geckos to walk up walls. "We're exploiting mechanical processes and geometries to optimize the strength of that adhesion force," Rogers explained in the podcast. "You can put it on like a temporary tattoo, and it will stay there." It stays in place even in the shower, he reports.

In current tests, the patches have stayed on for about a week. Adding some sort of adhesive would probably help seal the deal in the real world, Rogers says. But he likes that the current version shows that external adhesives are "not absolutely required."

Going wireless
Getting power to such a small, flexible patch is a key issue in making it usable. Currently, the team has been able to integrate inductive coils that can pick up power when a nearby primary source operates at high enough levels to power LEDs and other components.

But what happens if someone needs to wear this thing all around town? The next step will be to integrate capacitors or batteries that can store power for later. And borrowing from the already-existing flexible photovoltaic technology, the team is looking toward solar as a possible partner for a built-in battery.

To be a major breakthrough, though, the patch will ultimately need to be entirely wireless. The developers currently rely on small wires that connect to slim ribbon cables to download information, but is already at work integrating wireless radio capabilities.

Like a temporary tattoo, these patches do not cling forever. Rogers pointed out that with the skin's constant renewal, a cycle that takes about two weeks, the patch would need to be replaced. As Zheng notes, the silicon in the new patch is itself part of the device. So "the wafer used for fabrication is not reusable," she says. With her group's mid-layer, the silicon wafer can be used again and again.

Body language
Research at Johns Hopkins University has shown that "these devices are not just used to monitor the body but to stimulate it," Rogers said. "The electrical interface works in the other direction as well, so you can stimulate muscle contractions." And being slim and streamlined means that the device "doesn't restrain the motion of the muscle" in the process.

The technology could also some day aid in wound healing, by providing heat or electrical stimulation around a wound. With light integration, the circuits could also be used to assess tissue via spectroscopy.

The device has been attached to subjects' throats as they speak, and using a computer program to recognize patterns, the signals can navigate simple video games with verbal controls. Such communication methods could also be useful for those who suffer from diseases that affect the larynx, as well as possibly controlling prosthetics, Rogers noted.

Rogers hopes that one day they would be able to change the chip's language from that of voltage to one that is more readily understandable to the body, such as fluids and ions. And that would enable interaction "with the tissues at a very deep level," Rogers noted.

In the meantime, mc10 is already working to bring a version of the patch to market. And they have reportedly struck a deal with Reebok to start integrating these sorts of sensors into sportsware.

Although her group could be competing with Rogers's, Zheng says that she is "happy to see that we are working toward the same goal [but] with different approaches."