Recyclable ‘Smart Skin’ Monitors Biological Signals on Demand

Skin can serve as a crucial indicator of health, for instance, dry skin might signal a need for hydration. Both clinicians and patients could benefit immensely if skin could be even more informative, capable of actively monitoring and communicating vital health data like glucose levels in sweat or heart rate. While there have been notable advancements in wearable health monitors, a multifunctional, skin-interfaced electronic device that adheres intrinsically to a singular, cost-effective material platform hasn’t been developed until now. Researchers have now introduced an adhesive sensing device that can be directly applied to human skin to continuously track the wearer's health indicators.

Researchers at Penn State College of Engineering (University Park, PA, USA) have engineered a so-called "smart skin" that can be reprogrammed to detect a variety of health metrics and is even recyclable. Traditional methods for creating flexible electronics are typically complex and expensive, and despite the flexibility of the base materials, the sensors themselves often lack this flexibility. This rigidity can restrict the overall flexibility of the device. This research team had previously worked with biomarker sensors made from laser-induced graphene (LIG), which is crafted by laser-patterning 3D networks onto a porous, flexible substrate, turning the material into conductive graphene. However, these LIG-based sensors on flexible substrates do not naturally stretch, limiting their ability to mold to the human skin, which constantly changes in shape, temperature, and moisture—particularly important during physical activities when monitoring things like heart rate, nerve activity, or sweat glucose levels is crucial. Although it is possible to transfer LIG onto stretchable materials, this often diminishes the quality of the graphene.

Consequently, programming these sensors to monitor specific biological or electrophysiological signals can be challenging, and the quality of sensing is frequently compromised. A preferable approach involves embedding porous 3D LIG directly onto a stretchable substrate. The researchers achieved this by creating an adhesive composite that incorporates polyimide powders—which enhance strength and heat resistance—and amine-based ethoxylated polyethylenimine dispersed in a silicone elastomer. This stretchable composite not only supports the direct creation of 3D LIG but also allows the device to adhere seamlessly to the varying contours of human skin. The device has been proven experimentally to monitor various biomarkers like pH, glucose, and lactate in sweat and can perform as effectively as traditional finger prick tests.

Moreover, the device can be easily reprogrammed to measure heart rate, nerve activity, and glucose levels in real time. Reprogramming involves simply applying clear tape to the LIG networks and peeling it off, allowing the substrate to be re-lasered up to four times before it becomes excessively thin. Beyond this point, the entire device is recyclable. Importantly, the device maintains its adhesiveness and functionality even on moist or sweaty skin. It currently operates on battery power or via near-field communication nodes, akin to a wireless charger, but there is potential for it to eventually harvest energy and transmit data over radio frequencies. This would transform it into a self-sufficient, stretchable, adhesive platform for continuous health monitoring. The team is planning further developments in collaboration with medical professionals to eventually utilize this technology for managing diseases like diabetes and monitoring conditions such as infections or wounds.

“We would like to create the next generation of smart skin with integrated sensors for health monitoring — along with evaluating how various treatments impact health — and drug delivery modules for in-time treatment,” said Huanyu “Larry” Cheng, the James L. Henderson, Jr. Memorial Associate Professor of Engineering Science and Mechanics in the Penn State College of Engineering.

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