Tekla S. Perry
IEEE Spectrum
The Biostamp can replace today's clunky biomedical sensors
I turn the key to start the little Ford SUV I’ve rented for my visit to the University of Illinois at Urbana-Champaign, and a message flashes briefly on the dash: “Tire pressure low.” I ignore it. My own car is 12 years old; I’m not accustomed to a car that monitors its own health. Turns out, though, that the little Ford wasn’t kidding. The next morning I find the car has a flat tire.
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| The Ultimate Wearable: John Rogers and his research team at the University of Illinois have developed a way of building circuits that act like skin, collect power wirelessly, and can be worn just about anywhere on the body. |
Modern cars are laden with sensors that constantly monitor the vehicle’s vitals and indicate, for example, when a filter needs replacing or whether the air bag is working. Electronics diagnose failures after they happen and even predict problems that are imminent. Wouldn’t it be great if we could monitor our bodies in much the same way.
That very idea, ironically enough, is what has brought me to Illinois. I’m here to see John Rogers, a materials science professor at the University of Illinois with a bold vision: Someday, he believes, we will all have sensors on our bodies that send information to a mobile phone, similar to the way a car’s sensors feed the vehicle’s computer.
HOW IT WORKS
Inside the Biostamp
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| While all biostamps have a few common characteristics – they stretch like skin, include flexible circuits, and can be powered wirelessly – different functions require different sensors. The butterfly sensor on the left is designed to monitor exposure to the sun’s ultraviolet (UV) rays, the center sensor uses sensitive dyes to detect chemicals in sweat, and the sensor at the right uses electronic circuitry to measure blood pressure. |
We’ve already taken the first steps in this direction. Many of us now wear fitness bands that track our activity and heart rate and assume we’re sleeping if we don’t move for a while. But most of these bands aren’t exactly chic or unobtrusive, so even the more die-hard among us take them off sometimes. And the information they dispense is interesting but hardly vital: They can’t detect signs that you’re getting sick or tell your doctors anything they need to know, much less replace an office visit.
But there’s no reason why they can’t, says Rogers. Think about your last medical exam. Your doctor checked your pulse, your temperature, your blood pressure, and maybe your blood oxygen. If any anomalies showed up, you may have been sent for further tests – perhaps an electrocardiogram for your heart, a blood test to check for diabetes, electromyography if you were having muscle weakness, or possibly even a polysomnogram at a sleep lab to check for apnea. All of these tests require specialized and costly equipment, trained medical technicians, or invasive pokes.
These tests – and more – could be accomplished by means of sensors so light, durable, and comfortable that you could wear them on your body for weeks at a time. It’s no distant dream: At press time, several sensors developed by members of Rogers’s research team had entered or were about to enter clinical trials in the United States and Europe, and the first commercial versions were expected to become available by the end of this year.
Rogers says these sensors are so much like skin that you don’t notice you’re wearing them – and I didn’t have to take his word for it. I wore one on my inner forearm for more than a week. This version was a test unit that simply transmitted a greeting when triggered by an Android smartphone; units with biosensors haven’t yet been made available to journalists.
Simple though it was, my sensor delighted me. It clung unobtrusively and tenaciously to my arm as I went about my life – showering, sleeping, and exercising. It also got me thinking about how future versions of these sensors are going to make our lives better – not a decade from now but within a couple of years, Rogers promises.
The Illinois team isn’t the only one trying to make skinlike electronics. Takao Someya is leading a group at the University of Tokyo that’s working to develop electronic skin made of organic semiconductors and carbon nanotubes. Zhenan Bao at Stanford is also working with organic semiconductors to develop an electronic film that would be as sensitive as human skin and could be applied over robotic limbs. And researchers at the University of California, San Diego, are developing inks that would allow scientists to draw sensors directly onto the skin.
But Rogers’s skinlike sensors are poised to be the first to get out of the lab and onto our bodies. In 2008, Rogers teamed with Roozbeh Ghaffari to start a company called MC10, in Cambridge, Mass., to turn his group’s research on stretchable electronics into commercial health care products. MC10 today has about 60 full-time employees, US $60 million in venture capital and corporate investment, and one product on the market: the Checklight, a skullcap for precisely measuring accelerations during athletes’ head impacts. It’s not a skinlike sensor patch, but it does bend to conform to the shape of a body part. (Rogers serves on the board of MC10 and helps plan the company’s research and technology efforts with Ghaffari, who is now MC10’s chief technology officer.)
MC10 started making the first skin patches – the company calls them Biostamps – in late 2012. Most of these early units were used for internal development or codevelopment efforts with partners. MC10 began developing a new generation of the technology in late 2014; most of these Biostamps are now going to medical researchers for use in clinical trials. Consumer-wellness Biostamps are also being developed for companies targeting their own special niches. A cosmetics company may package a sun-monitoring Biostamp with sunscreen, for example, or a pharmaceutical company could include motion- and temperature-monitoring Biostamps with a package of medication.
The basic Biostamp is a thin sticker about the size of a British ten pence or an American quarter. It looks like a temporary tattoo a child might get at a birthday party, but because it has been designed to be mechanically similar to skin, it can’t really be felt by the wearer once it’s applied. A Biostamp can contain hundreds of thousands of transistors, as well as resistors, LEDs, and a radio-frequency antenna. It’s waterproof and breathable, and it costs just tens of cents when manufactured in quantity. It can be worn for a week or so, before the normal shedding of skin cells begins to force the thin substrate to peel from the skin, like an early-season sunburn.
A Biostamp is built out of stretchable circuits supported by an extremely thin sheet of rubber. To make these circuits, Rogers and his colleagues in Illinois start by fabricating their transistors, diodes, capacitors, and other electronic devices on wafers of any common semiconductor material. They typically use silicon but could also use gallium arsenide or gallium nitride. These are not ordinary semiconductor wafers; they’re kind of like the Oreo cookie of semiconductor wafers. They have a thin top layer of semiconductor material, a thicker bottom layer of the same material that acts as a rigid support during manufacture, and a sacrificial layer of a different material in between. In the case of a silicon wafer, this sacrificial layer is silicon dioxide. After the device manufacture is complete, a chemical bath eats away that central layer and frees the thin top layer.
Then a stamp made of soft silicone presses onto the wafer. Raised areas on the stamp lift away selected electronic devices in the same way a rubber stamp picks up ink from a stamp pad. After picking up the devices, the silicone stamp deposits them onto a temporary substrate, usually a plastic-coated glass plate. This plate then goes through a standard photolithography process that connects the devices with copper conductors in the form of serpentine coils, which make the connections stretchable.
The next step is to transfer the interconnected devices from the plastic-coated glass onto what will go to the consumer – a thin sheet of rubber already attached to a plastic backing sheet, with a layer of adhesive in between. To do this, a machine pushes the rubber against the array of devices and coils that are still clinging to the plastic-coated glass. A final chemical bath dissolves the plastic between the electronic circuits and the glass, leaving the circuits attached to the rubber. And the last step happens when the Biostamp gets into the hands of the user – who exposes the adhesive and sticks the rubber-backed electronics onto the skin.
In many Biostamps, all the electronics are created using this process. In some cases, however, a Biostamp design incorporates a microprocessor without packaging, which the researchers thin down to 5 to 10 micrometers. A few more micrometers of flexible resin cover the circuitry to protect it from water. For now, though, most Biostamps don’t have full-blown microprocessors on board. Most of those now being tested simply gather data and transmit it; analysis happens elsewhere, generally on a smartphone or tablet.
A Biostamp powers itself by harvesting energy from near-field communication (NFC) radio signals, typically from the wearer’s cellphone. It communicates with the phone the same way. NFC, which sends data at 13.56 megahertz, is a feature of almost all current-model smartphones, which use it for wireless-payment schemes. At the moment the stamps work only with Android phones, but the hardware is compatible with the type of NFC technology on the newer iPhones.
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| Biostamp Brigade: These stamps include a variety of ultrathin unpackaged electronics, flexible circuits, and sensitive dyes. They harvest energy and communicate wirelessly and can support a variety of sensors to allow the monitoring of different bodily functions. |
The stamp converts the RF energy picked up by an antenna to electrical energy by means of an inductive coil. A Biostamp can generate tens of milliwatts of power when within a meter or so of a phone transmitting an NFC signal. For longer-distance power gathering, a Biostamp can be built to receive radio signals at frequencies between 1 and 2.5 gigahertz from a transmitter up to several meters away.
The current tattoo-like versions of the technology don’t store energy, although Rogers’s group and MC10 have already built and tested stretchable batteries and supercapacitors. But in a hospital room, say, with an NFC transmitter under the bed or a longer-distance RF transmitter in a corner, Biostamps can operate continuously and indefinitely.
