Have you ever debated about what to eat for dinner? Probably most of us have. But have you ever thought about what you should/shouldn’t eat for dinner? When it comes to food restriction, it is usually associated with either allergy or metabolic conditions. As food passes through our digestive system, nutrients become absorbed and circulate in blood, and some get further metabolized into metabolites. The circulating metabolites and nutrients can reveal important health conditions, such as glucose in blood. For diabetic patients, sugar intake was controlled to maintain a safe circulating glucose level. For people with gout, they know that they should restrict purine intake, as purine consumption increases the uric acid (UA) level in blood, which could trigger an acute and painful gout attack. However, most of the food labels, either processed or not, don’t provide purine levels. It is hard for gout patients to know if the food is safe or not for them until they reflect on what they eat after the attack happens.

One potential way is to monitor the UA levels after a certain food consumption and take UA-lowering medicine immediately before the attack happens. It is possible to do finger pricking many times a day and measure the blood UA concentration, but is there an easier and less invasive way for this monitoring process?

A potential solution we proposed is a wearable sensor platform that can measure sweat metabolite levels in real time. Using a facile fabrication method based on CO₂ laser scribing, we created a patch that integrates a metabolite chemical sensor, a sweat sampling microchannels (microfluidics), and vital sign sensors for respiration rate, heart rate and temperature sensing. By engraving patterns on polyimide and tuning designs with different laser parameters, we obtained sensitive graphene/carbon-based chemical sensor and vital sign sensors. Similarly, the microfluidics were created by laser scribing on medical adhesives, assembling layers of channels/inlets/outs, and attaching to the sensors. The use of temperature sensor calibrates the chemical sensing result based on real-time temperature condition;  the use of microfluidics prevents contaminants from skin and improves temporal response of the sweat chemical sensor. For real-time sensor readout, a flexible printed circuit board was designed and developed for sensor data acquisition, processing and wireless data transmission to a mobile phone.

With this lab-on-skin patch, we could monitor the sweat UA and tyrosine (Tyr) in real time on human body. Tyrosine is a conditionally essential amino acid that is associated with metabolic disorders and eating disorders. Compatible with previous literature, our lab-on-skin patch measured a lower sweat Tyr in trained athletes than in untrained individuals. Interestingly, the sweat UA levels in gout patients are higher than those in healthy subjects, and the sweat UA level after medication for a patient is much lower than before medication. The pilot study of sweat-serum UA correlation shows the potential of using sweat UA as a biomarker for gout monitoring and potentially therapeutic evaluation.

With the lab-on-skin patch, we are investigating more circulating nutrients and metabolites that are indicative of human health. Maybe in future, when people tap on their electronic watch, they can see all of their health-relevant parameters measured from the sensors, and the data analysis will tell you to take meds immediately, or to eat less fish or potatoes for the day!

These results were recently published in Nature Biotechnology: “A Laser-Engraved Wearable Sensor for Sensitive Detection of Uric Acid and Tyrosine in Sweat”, Y. Yang, Y. Song, X. Bo, J. Min, O. S. Pak, L. Zhu, M. Wang, J. Tu, A. Kogan, H. Zhang, T. K. Hsiai, Z. Li, W. Gao, Nature Biotechnology, 2019, doi:10.1038/s41587-019-0321-x.

The full paper can be found here: https://www.nature.com/articles/s41587-019-0321-x.