The current generation of smartphones is one we constantly carry around in our hand. The next generation of smartphones may well be on our hand, like an artificial skin that is packed with electrical circuits, sensors and actuators. Thanks to new materials, we are moving towards skin electronics.
We have become familiar with a wide range of wearables, like smart glasses and watches, fitness trackers and VR headsets for computer games. We currently wear all of these mobile gadgets as accessories on our clothes. And that is directly related to the materials from which electronic devices are made and the energy that all these wearables consume.
In fifteen years’ time, however, we will be living in a new age, beyond the smartphone. At least according to Zhenan Bao, Professor of Chemical Engineering at Stanford University and brand-new honorary doctor at KU Leuven. “Electronics are currently built with hard and breakable materials. Future technology will be built into our clothes, but will also be wearable on our bodies, or even implanted. To this end, we are currently designing new organic electronic materials: they are flexible, and they can repair themselves, just like our body. Some are even biodegradable. And they contain sensors, electric circuits and displays, so that they can constantly measure and communicate with their environment.”
In other words, it is a search for a second skin, a synthetic and enhanced version of human skin. Bao calls it “skin electronics”. Just like our own skin registers touch, pressure, heat or pain with millions of nerve endings, artificial skin would use sensors to measure our temperature, blood pressure, heart rate and blood sugar level. And just like all our skin sensations are translated into signals to the brain, skin electronics can send wireless signals to a smartphone or computer, and in the future, even directly to our brains. And if it breaks, the electronics will be able to repair themselves.
The development of these skin electronics is possible thanks to new, organic materials. These materials are made of the same elements as living beings: primarily carbon combined with hydrogen, nitrogen, oxygen and sulphur. This makes the organic materials light-weight, flexible and biocompatible. The best-known organic electronics example is OLED – organic light-emitting diode – a material that is used for the screens of televisions and smartphones. And experiments are also being conducted with ultrathin, rollable solar panels that are printed with organic materials.
Electronic skin is actually a thin layer of stretchable plastic, and this plastic is made primarily of polymers. These are large molecules constructed of a chain of small – and in this case organic – building blocks. “This is a new generation of plastic materials that is biocompatible and functional. Biocompatible because the materials are made of organic molecules that are safe to use on or in the human body. And functional because these polymers can conduct electricity. You can arrange the atoms in the polymers in such a way that they become conductors, like copper, or semiconductors, like silicon.” And there is a range of interesting applications for these skin electronics.
Just like our own skin senses heat using millions of nerve endings, artificial skin might be able to measure our temperature using sensors.
One example of a near-term application is electronic skin as a sticking plaster that monitors your health: for example, band-aid electronics can monitor and transmit information about your blood pressure. This is what the start-up Pyrames, which Bao co-founded, is working on: “For babies in intensive care units, it is very important to monitor their condition via their blood pressure. This is currently done using a needle, which is very painful and difficult for the tiny and fragile babies.” The start-up is therefore developing a wearable non-invasive blood pressure monitor with very sensitive pressure sensors on the wrist: paper-thin and the size of a stamp, and it is literally applied under a plaster.
One step further is electronic skin as an implant. For example, to place a small sensor on the artery to monitor one’s blood flow. “Or, if we can place a piece of electronic skin around a nerve, the e-skin can stimulate that nerve: this might be a solution for paralyzed people.” On the other hand, biodegradable skin electronics offer other possibilities. After an operation, the surgeon would be able to implant a piece of electronic skin, which would temporarily measure if everything is developing well and would then biodegrade and disappear. Such implants will take a long time to develop because implants require long-term safety testing.
Electronic skin can not only supplement our human skin but can also genuinely replace it: “Imagine wrapping an electronic skin full of sensors around a prosthetic hand. This would imitate the sense of touch. People who have lost a limb would thus be able to feel again.” The challenge of such smart prosthetics is not to build in one sensor, but millions – like the millions of nerve endings in our fingertips – in addition to the necessary circuits to process all the signals that allow the patient to feel and decide on how to control the prosthetic. Bao is thinking even further ahead: “Over time, we will be able to connect our sensorial electronics to the human nervous system.” The prosthetic will thus truly be connected to the human brain.
Imagine if you were to wrap electronic skin full of sensors around a prosthetic hand, you would be able to imitate the sense of touch.
And whatever is possible on a prosthetic hand is also possible on a robotic hand. Skin electronics can give the robot the necessary dexterity, Bao says. “Current robots are constructed from hard materials, without sensors for touch or handling objects. This leads to accidents when humans and robots interact.” An operation robot that assists a surgeon, for example, needs to know how deep to cut. The Bao Lab at Stanford developed an electronic glove for a robotic hand, with various layers of sensors that measure the intensity and direction of pressure. “We can already programme a robotic hand to touch a raspberry without crushing it. But it is not yet a robot that can feel and identify a raspberry and then carefully pick it up.”
New materials for the wearables of today or the skin electronics of tomorrow will not solve all our problems. There is still the major issue that all these high-tech gadgets require power, such as batteries. While microelectronics have developed at lightning speed, battery technology has fallen behind. Just think, for example, of the lithium-ion battery in a smartphone, the capacity of which diminishes over time.
The challenge is thus to make small, rechargeable batteries with larger energy density. New materials are also being explored in this area, such as lithium metal batteries. But this new generation of batteries does have certain downsides: the material in the battery expands while it is charging and shrinks as the battery discharges during use. Over time, the material breaks down, reducing the battery’s power – which is precisely the opposite of the desired result. Even worse, such batteries can have electrical shorting or even catch fire.
At Stanford they not only work on making lithium metal batteries last longer, but also on elastic batteries. Bao’s team has developed a new coating for lithium metal batteries: these polymers help lithium ions to move through, but they are solid and thus do not leak. At the same time, the material can stretch to up to almost twice its original length without losing energy. “We have prototype batteries with this new technology. But we still have a lot of work to do because you have to find a solution that works for all the components of the battery.” And yet there is already a vision for the future: an electronic skin with its own, built-in elastic battery.
Bao’s research group at Stanford is not the only centre that conducts research into skin electronics or new batteries. But Zhenan Bao is a cut above the rest because she draws a straight line from the chemistry of new materials to the development of applications. “I have learned to start from today’s problems to develop new fundamental science.” This sounds almost obvious, but it is not, and it makes her work unique and extraordinary. As a world authority in the field of organic electronics, Bao will receive an honorary doctorate at KU Leuven’s Patron Saint’s Day.
Professor Francisco Molina-Lopez at the Department of Materials Engineering spent a few years working at the Bao Lab in Stanford as a postdoctoral researcher. “Bao is an inspiration and role model to many researchers because she combines fundamental and applied research in a multidisciplinary way. She also leads her research group in a way that prioritises curiosity and collaboration.” Molina-Lopez is the nominator of the honorary doctorate, along with his colleague Marc Heyns, who is also affiliated with the research centre Imec.
Bao’s work is actually a very unconventional type of electronics, Heyns adds. Today’s electronic appliances are built with inorganic components: silicon, germanium, metals… “These materials are rigid and consistent. The technology is optimal but has reached its limits. To make more technological progress, we need new materials.” And that is precisely where Bao is making a difference: “She not only demonstrates in her many papers in Nature and Science that there are possibilities. She also always thinks a few steps ahead, about how to manufacture and apply the materials.”
A vision of the future emerges: an electronic skin with its own, built-in elastic battery.
To achieve this with organic materials is very difficult, Molina-Lopez continues. “Organic materials behave more chaotically by their nature and perform less well. But they have many advantages: they conduct electricity and are, moreover, cheap and easy to produce. And because they are elastic and non-toxic, you can use them on or in the body.”
Molina-Lopez is now developing his own research group in Leuven. The goal is to study how electricity can be produced from residual heat (heat that is released during all kinds of industrial processes but which is then usually wasted). “We are studying how we can use semi-conducting polymers that can extract thermo-electricity and how we can manufacture these new materials with 3D printing. But the primary goal is to improve the performance of these polymers by aligning the molecules with one another; just like Bao, I will fiddle around with polymers on a molecular level.”