The determination of skin surface pH via the skin volatile emission using wearable colorimetric sensors

Biodiagnostic sensors in the form of wearables have become of widespread interest in both the scientific and clinical communities in recent years due to their potential for monitoring human health, well-being and physical performance. However, collecting biofluids from the skin to enable biochemical analysis using wearables has proved challenging to date. This research seeks to overcome the need for fluid collection by designing a wearable sensing platform capable of monitoring the volatile emission from skin with the aim of collecting biodiagnostic information from the body. We investigate the use of a simple wearable colorimetric sensing platform incorporating sensor spots comprising encapsulated bromocresol green pH indicator dye in an enclosed headspace above the skin. The sensor spots undergo a colour change in response to basic volatile nitrogen compounds such as ammonia and amines being emitted from skin. By deploying this wearable in a healthy participant study, a strong correlation between sensor colour response and skin surface pH was demonstrated, despite a significant inter-individual variability being noted. Sensor response was observed to be highly dependent on gender as well as body site, and attributed to factors such as gland and microbial composition differences. Finally, the wearable’s ability to detect changes in skin surface pH in response to topical skin treatments was demonstrated. Overall, this work demonstrates a novel and simple approach to wearable biodiagnostics that exploits the skin volatile emission to monitor skin physiology without the need for microneedles or the requirement to harvest f luid from the skin