Sweat-Based Diagnostics: Potential and Challenges

The field of personalized and remote healthcare has been growing at an unprecedented pace, and wearable devices sit at the forefront of this revolution. Sweat, an underexplored bio-fluid, offers an intriguing avenue for continuous and non-invasive health monitoring due to its richness in physicochemical biomarkers. Nevertheless, several technical obstacles hinder the advancement of sweat-based diagnostics, such as power requirements, device miniaturization, and integration.

Energy Requirements of Wearable Biosensors: A Barrier to Overcome

The high power demand of wearable sensors presents a significant challenge. Most of these devices rely on bulky batteries, which require frequent recharging. This dependency on non-renewable power sources limits the devices' sustainability and user convenience, highlighting the urgent need for alternative energy harvesting strategies.

Ambient Light as an Energy Source: A Viable Solution?

Ambient light emerged as a promising energy source for wearable devices due to its abundant availability during daily activities. While silicon-based photovoltaics technology currently dominates the commercial market, it falls short in addressing the power needs of wearable devices due to its bulkiness, fragility, and insufficient power conversion efficiency (PCE) under low or indoor illumination.

Harnessing the Power of Perovskite Solar Cells

Perovskite solar cells (PSCs) piqued our interest due to their favorable properties, including high absorption coefficients, high structural defect tolerance, and tunable bandgap. After extensive research and optimization, we designed a flexible perovskite solar cell (FPSC) capable of efficient energy harvesting under diverse lighting conditions. The module exhibited record-breaking PCE under indoor illumination, demonstrating its potential as a sustainable power source for wearable biosensors.

From Concept to Reality: The LumiStat Wearable Biosensor

The culmination of our research led to the development of LumiStat – a solar-powered, multifunctional wearable device that represents a leap forward in sweat-based diagnostics (Figure 1). LumiStat's capabilities extend to autonomous sweat induction, dynamic microfluidic sweat sampling, multiplexed monitoring of sweat biomarkers, and wireless data transmission. The device demonstrated remarkable performance, operating continuously for over 12 hours under various light conditions, signifying its practical applicability.

Figure 1. Battery-free wearable device powered by quasi-2D flexible perovskite solar cell for multiplexed wireless biomolecular analysis in varied illumination.

A Leap Forward: LumiStat, the Wearable Biosensor

Our quest culminated in the development of LumiStat – a solar-powered, multifunctional wearable device, marking a significant leap forward in sweat-based diagnostics. LumiStat's functionalities span autonomous sweat extraction, dynamic microfluidic sweat sampling, multiplexed monitoring of sweat biomarkers, and wireless data transmission. Operating continuously for over 12 hours under various light conditions, LumiStat's performance attests to its practical applicability.

From Lab to Limelight: On-Body Validation Trials

The next phase of our journey involved subjecting LumiStat to rigorous on-body evaluation trials. Our wearable device demonstrated commendable resilience and adaptability when worn under different outdoor and indoor illumination conditions. We successfully validated the accuracy of the device’s sweat rate sensor and observed efficient and prolonged sweat extraction.

In monitoring key biomarkers, LumiStat's proficiency was evident both during sedentary and exercise trials. We achieved a high correlation between blood glucose levels and sweat glucose levels in sedentary oral glucose intake studies across multiple subjects, attesting to the device's potential for non-invasive glucose monitoring. Our efforts also highlighted the importance of personalized metabolic monitoring, taking into account the distinct pH levels in sweat induced by different approaches. Furthermore, we mitigated potential noise due to motion artifacts by employing hardware filters integrated onboard and smoothing algorithms in our custom app.

During a day-long, cross-activity multimodal monitoring of sweat biomarkers, LumiStat demonstrated its ability to perform consistently across various lighting environments and physical activities (Figure 2). The device's power consumption was intelligently adapted based on the available illumination, ensuring continuous sensor measurements. This extended, real-life trial further underlined LumiStat's potential for 12-hour operation, opening up possibilities for continuous health monitoring.

Figure 2. Flexible perovskite solar cell-powered wearable device enables multiplexed wireless biomolecular analysis across wide activity ranges and illumination environments over 12 hours.

A Glimpse into the Future

Despite the promising results, we realize that our work with LumiStat is far from complete. Future developments will focus on enhancing the long-term stability of the sensor patch and exploring correlations between sweat and blood biomarker levels through large-scale human trials.

The modular design of LumiStat opens up possibilities of incorporating different biosensors, broadening the horizon for identifying a myriad of target biomarkers. We envision LumiStat playing an integral role in fields such as sports science, daily health tracking, and care for various health conditions.

Conclusion

The development of LumiStat illuminates the path towards sustainable, personalized healthcare, merging daily life with health monitoring. Our work on this solar-powered wearable biosensor is an ongoing journey, filled with challenges to overcome and opportunities to explore. This innovative approach brings us closer to the essence of preventive medicine, highlighting the role of wearable healthcare technologies in shaping the future of personalized care.