Many of us see health trackers, such as Apple watches and Fitbits, on a near-daily basis, but what if they could be drawn directly onto your skin instead? Researchers at the University of Houston have created a new pen-and-ink method to create health sensors on your skin using just stencils and conductive ink.
A draw-on-skin system, or DoS, would allow for better skin adhesion
Wearable bioelectronics (specifically containing heart sensors) include fitness tracking wristbands, chest straps, and patches, and rely on two main processes. Chest straps and patches use electrocardiography (ECG) to detect electrical activity, whereas fitness trackers can use either ECG or photoplethysmography (PPG). PPG emits hundreds of flashes of green light per second, which interact with the blood flowing through the skin under the device. Since blood is red, it reflects red light and absorbs green light. With each heartbeat, the blood flow to your wrist increases, and the more blood there is, the more light is absorbed. The sensors in the tracker can measure this change in absorption to give pulse readings, typically shown in beats per minute. ECGs use electrodes to track the electrical impulses generated by the heart. These impulses cause the muscle to contract and pump blood around the body.
Currently, there are problems with each available option. PPG is much less reliable than ECG since tattoos, lighting, and skin colour can reduce the accuracy of light-based readings. Wristwatches using ECG typically require you to hold your finger down on a sensor to get a reading, and wearable ECGs have up to two leads, compared to the standard 12 on ECG machines used in hospitals. Patches, which use wires embedded in adhesive silicone to make a circuit, are thin and flexible, but can be very expensive and sensitive to motion because they do not follow the movement of skin perfectly, leading to an inconsistent interface between the electronics and skin. In fact, the biggest problem with wearable sensors are motion artifacts which reduce the quality of data.
A draw-on-skin system, or DoS, would allow for better skin adhesion. Faheem Ershad and his team used silver flakes to make a conductive ink that can be used on human skin – with the help of stencils and modified ballpoint pens – to craft devices such as electrophysiological (EP) and skin hydration sensors. An electrical lead is attached to the device to transmit the data to a computer and supply power. By creating two more inks (a semiconductor and type of insulator called a dielectric) temperature and strain sensors can also be developed.
The researchers tested the durability of the ink by drawing on artificial skin and stretching, poking, and twisting to observe any damage, and concluded there were almost no visible effects except minimal cracking when stretched. They then confirmed its compatibility with skin using mice models and measuring inflammation. This means the ink is both suitable and ultra-conformal, so the device sticks to and follows the contours of the skin extremely well. Immunity to motion artifacts, alongside being unaffected by the presence of sweat, is an advantage over current technologies such as gel or mesh electrodes, which are all affected by stretching and compression.
Using stress tests, a DoS EP sensor was able to monitor and detect changes in heart rate and give clear ECG traces, suggesting that DoS EP systems are suitable for cardiac diagnostics both during normal activity and clinical examinations.
The DoS system presents a significant leap in bioelectronic technologies. It is more customisable (both in shape due to the use of ink and also in its ability to draw active electronics), simple enough that it requires no dedicated equipment, and is therefore much cheaper than current options. The combination of its simplicity and durability also means that imperfections can be fixed very quickly, instead of the whole device needing to be replaced as with mesh electrodes, for example. As research progresses, making the DoS system completely wireless could revolutionise health monitoring, providing an affordable, accessible, and personalised way to keep track of health indicators.