(Nanowerk Spotlight) Imagine a future where a simple patch worn on the skin could keep tabs on your health as you go about your day, flagging concerning changes before symptoms even appear. This tantalizing vision has driven intense research into wearable biosensors that can non-invasively monitor diverse physiological parameters. While consumer devices like smartwatches can already track basics like heart rate and activity, the next frontier is tapping into the trove of molecular biomarkers carried in bodily fluids like sweat.
The sweat produced by our eccrine glands, which are the most numerous type of sweat gland found all over the body, contains dozens of health-relevant indicators including electrolytes like sodium and potassium, metabolites like glucose and lactate, hormones like cortisol, and even drugs or their byproducts. This biochemical richness, combined with the fact that eccrine sweat is readily accessible on the skin surface, has made it an attractive sampling fluid for non-invasive wearable biosensing compared to blood, the gold-standard biological matrix that requires painful needle sticks to access.
Sweat’s accessibility and richness have made it an attractive target for non-invasive sensing compared to blood, the gold-standard matrix that requires painful needle sticks. However, reliably and selectively measuring minute concentrations of sweat biomarkers, while maintaining sensor performance through repeated mechanical stresses, has proven incredibly difficult. This is especially true for multiplexed sensors that track multiple markers in parallel – a key capability since individual measures often only tell part of the story.
For example, in diabetes management, blood glucose is a central metric. But alcohol consumption can also cause blood sugar to spike, so tracking glucose alone could paint an incomplete or misleading picture. Contextual indicators like blood alcohol content are critical to making sense of anomalous glucose readings. But packing sensors for glucose, alcohol and other markers onto a flexible patch is an enormous engineering challenge. Each sensing modality has unique requirements that conventional microfabrication techniques struggle to satisfy simultaneously.
In a significant step towards practical multiplexed sweat sensing, researchers at The Hong Kong University of Science and Technology, have now unveiled a new wearable patch that leverages custom printable inks to enable the seamless integration of glucose, alcohol, pH and temperature sensors. By carefully optimizing the inks’ rheological properties and employing a precise droplet-based printing process, the team achieved unparalleled control over the sensors’ architecture and interfaces.
The fully printed integrated system for multiplexed epidermal sweat analysis. a,b) Schematic diagrams and c) photo of the fully printed sensor array for simultaneous multiplexed biosensing monitoring, including pH, glucose, alcohol, temperature, and reference sensing electrodes integrated on a PET substrate. Scale bar, 5 mm. d) Configurations of the sensor electrodes. (Adapted from DOI: 10.1002/adma.202311106, CC BY) (click on image to enlarge)
The printed films form interpenetrating networks that maximize contact area, conductivity and stability between layers – feats difficult to replicate with standard methods like spin-coating or physical vapor deposition. At the same time, the mesh-like structure remains porous enough for sweat to permeate through to the buried enzyme-doped layers. This unique morphology, combined with mediating nanomaterials and optimized mass loading, grants the biosensors remarkable sensitivity and range.
In artificial sweat solutions, the glucose and alcohol sensors respectively demonstrated sensitivities of 313 and 0.87 microamps per millimolar per square centimeter – over 5 times better than prior printed sweat biosensors. The pH sensor achieved a slope of 71 millivolts per pH unit. Just as vitally, the sensors exhibited excellent stability, drifting less than 0.1 microamps or millivolts per hour during 30-hour continuous operation.
To validate real-world applicability, the researchers integrated the printed sensor array with a flexible wireless circuit and used it to monitor sweat dynamics as volunteers consumed sugary and alcoholic beverages. The expected spikes in glucose and alcohol were detected with high fidelity, closely matching reference measurements by benchtop instruments. The pH and temperature readings provided valuable context to refine the enzymatic sensor responses.
Printing the full stack using tailored functional inks imbues the fabrication process with impressive scalability and design freedom compared to traditional methods confined to cleanroom settings. The approach also facilitates the dense integration of multiple sensing modalities to paint a more comprehensive picture of the wearer’s physiological state. A glucose sensor on its own has limited diagnostic utility; a glucose sensor cross-referenced against alcohol, pH, temperature and other markers is far more revealing.
Looking ahead, this highly integrated yet non-invasive sensing paradigm could be transformative for personalized and predictive medicine. A single wearable patch that continuously logs numerous health indicators would offer doctors an unprecedented window into a patient’s day-to-day physiological fluctuations. Such rich multimodal datasets could elucidate subtle patterns to catch declining health earlier or optimize treatment regimens. For patients themselves, real-time feedback on diet choices, alcohol intake and metabolic markers may boost engagement and compliance.
Major hurdles remain in refining the printed biosensors’ selectivity, longevity and robustness to meet the stringent demands of medical use. Rigorous clinical trials are necessary to validate sweat-based measurements against gold-standard blood draws across diverse populations. And ensuring sensor stability through repeated mechanical stresses will be critical for durable long-term use. But with diligent optimization work, this sophisticated approach to non-invasive health tracking could become a powerful tool for timelier diagnoses and interventions.
By cleverly melding state-of-the-art printed bioelectronics, enzymes, nanomaterials and wireless circuitry, the team devised a versatile platform for continuous and imperceptible on-body sensing. The multiplexed sweat analysis patch represents an impressive feat of cross-disciplinary engineering, spanning materials science, electrochemistry, biochemistry and electrical design. Though still in early stages, this ambitious effort lights the way towards next-generation wearables that will reshape point-of-care testing, telemedicine and early disease detection.
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