“Fashion is not about utility.” The Devil Wears Prada, a film for fashionistas, may have been confident declaring that accessories are chosen to “express individual identity,” but wearable sensors suggest your outfit may soon communicate more than just your sense of style.
As the name suggests, the mission of sensors is to sense physical phenomena. What is arguably more interesting, however, is their ability to turn those phenomena into useful data. For example, cars have sensors to detect how much gas (or battery charge) is left in the tank and how hot the engine is running. Smartphones can recognize a fingerprint or orient the screen in (hopefully) the right direction when turned sideways.
Healthcare provides a plethora of opportunities for deploying sensors to improve diagnostics and disease management. Those same sensors in a smartphone, for example, may be used to detect when a senior citizen falls and alert emergency care. And chemical biosensors can be designed to quantify blood chemistry and transduce that information into digital form to help doctors monitor worrisome changes in disease progression. How will these sensors work, and how will they be integrated into our lives?
One approach is to integrate into something ubiquitous: clothing. In a recent publication in the journal Lab on a Chip , mechanical engineers Xiyuan Liu and Peter B. Lillihoj of Michigan State University describe the creation of the first electrochemical sensor embroidered into a piece of clothing. In the ongoing pursuit for less invasive diagnostics, this clever design could enable seamless and pervasive monitoring for patients with chronic diseases.
Diabetes is one such ailment overdue for an upgrade. Many people suffering from diabetes regularly prick their fingers to check blood glucose levels and manage their health. In a world of Apple Watches and Fitbits, bloodletting is a prickly nuisance that offers embarrassingly sparse data. Glucose, a glycemic indicator for diabetes, happens to exist in bodily fluids other than just blood. Google’s healthcare subsidiary, Verily, prototyped a contact lens that quantifies glucose levels from tears and transmits the data to a smartphone every second. Unfortunately, the dubious link between tear glucose levels and blood glucose levels may render their efforts to create this unobtrusive and ever-present sensor fruitless. But glucose is also present in sweat. Clothing made from cotton can easily absorb and transport sweat to embedded sensors. Glucose and other markers of physiological health go along for the ride.
In order to fabricate these embroidered sensors, the Michigan State researchers needed threading that could both conduct electricity and target glucose. Polyester threads were first coated with either silver/silver-chloride or carbon ink; then baked at 120 ºC. These inks rendered the threads electrically conductive, even after repetitive bending and twisting. The carbon threads were then immersed in a solution of glucose-oxidase (GOx), dried overnight, and stitched into a small patch of cotton.
GOx, an enzyme that catalyzes the oxidation of glucose, was critical for finding and exclusively reacting with free-floating glucose. This chemical reaction causes electrons to transfer to the conductive threads. To measure this rush of electrons, a small voltage was applied to the threads. The rate of change in current was directly proportional to the concentration of glucose.
Other researchers have integrated sensors into textiles before. A group from the University of California, San Diego equipped a pair of knickers with screen-printed electrodes . Why underwear? The elastic waistband, the authors argued, remains in close, direct contact with the skin and thus easily absorbs sweat. But screen-printed electrodes are fragile and integrate poorly with other textile-based electronic components. The embroidered sensors constructed using modified polyester are more robust, working while bent and improving the ability to create fully integrated systems.
Some groups want to go even further, adding insulin delivery to the system. In a collaboration between the University of Texas at Austin and several Korean institutions, Hyunjae Lee and colleagues developed a closed-loop device to measure glucose and administer insulin . Their sophisticated sticker was embedded with microelectronics and purportedly painless microneedles that measured glucose and regulated insulin delivery. These types of closed-loop devices aim for better safety by managing insulin spikes in real-time and eliminating human mismanagement.
Sensors hidden in our everyday routines suggest the possibility of a personal, biometric dashboard updating you on your vital health signs at all times. Multiple sensors could provide constant feedback on things such as heart rate, stress levels, blood pressure, and dehydration. Liu predicts small antennas could be woven into the fabric to wirelessly send data to a smart-device. This will not be trivial. The power and bandwidth demands of electronics capable of collecting and transmitting the massive datasets expected from wearable sensors remain an obstacle. But as the anticipated Google Jacket (equipped with its own conductive fibers for sensing touch) shows, textile-based sensors may become increasingly trendy. Fashion designers may have to rethink their definition of an accessory.
 Liu, X. and P.B. Lillehoj. Embroidered electrochemical sensors for biomolecular detection. Lab Chip 16(11), 2093-2098 (2016).
 Yang, Y.-Li. et al. Thick-film textile-based amperometric sensors and biosensors. Analyst 135(6), 1230-1234 (2010).
 Lee, H. et al. A graphene-based electrochemical device with thermoresponsive microneedles for diabetes monitoring and therapy. Nat Nanotech 11(6), 566–572 (2016).