Versatile sweat bioanalysis on demand with hydrogel-programmed wearables (2023)

Bioanalysis with blood has been one of the most significant diagnostic tools for modern medicine. However, owing to its invasive nature, it is not accessible to most people until annual physical examination or development of an obvious disease symptom. Application of this powerful tool to early diagnostics and treatment of chronic disease (e.g., diabetes (Vargas et al., 2019)) is highly restricted (Bariya et al., 2018). As a consequence, quite a few diseases are not properly diagnosed until they advance to the middle or late stage, and their treatment is much more challenging than in the early stage. Therefore, people are increasingly interested in non-invasive bioanalysis with externally secreted body fluids (e.g., sweat) for continuous health monitoring and early diagnostics of diseases, leading to development of various wearables (Gao et al., 2016; Keene et al., 2019; Koh et al., 2016; Moreddu et al., 2020).

Skin is the largest human organ characterized by a lot of external secretory glands such as eccrine, apocrine and sebaceous glands. For the eccrine gland, there are 2-4 million of them distributed all over the skin. One of the crucial functions of the eccrine gland is sweating for thermoregulation (Baker, 2019; Quinton, 1983). For this purpose, sweat is continuously generated on the skin even without any external stimulation. The normal sweat flow rate ranges from 10 to 250nLmin⁻¹ cm⁻² (Emaminejad et al., 2017; James et al., 2012; Lin et al., 2020), and sweat contains many types of biochemical species, which originate mainly from interstitial fluid so that their levels are closely correlated to physiological state. Therefore, real-time sweat bioanalysis can provide a sensitive means for non-invasive diagnostics and health management.

Although the collection of sweat is non-invasive, it is not user-friendly in most current practice. Without stimulation, secretion of sweat is of a very low rate, which is hard to observe or collect due to evaporation (Baker, 2019; Quinton, 1983). Consequently, stimulated sweat induced by iontophoresis (Emaminejad et al., 2017; Simmers et al., 2018), exercise (Tai et al., 2018; Yang et al., 2020), or heat (Zhang et al., 2019) for the sampling of substantial volume is often involved in current sweat analysis. For elderly people, patients and people with limited mobility, stimulated sweat by exercise or heat is problematic. Sweat collection by iontophoresis can be painful and may cause skin irritation. Frequent stimulation using drugs may lead to tolerance and decreased sweat rate. Therefore, the application of the stimulated sweat collection is highly restricted (Baker, 2019; Kim et al., 2019; Lin et al., 2020).

Furthermore, current interpretation of sweat test results relies on an established blood-sweat correlation so that the sweat target level can be converted to its corresponding blood level. A time lag is usually reported for target concentrations in blood and sweat (Baker, 2019; Emaminejad et al., 2017; James et al., 2012; Sonner et al., 2015). However, the levels of biochemical species in sweat are dramatically affected by a range of factors that can hardly be predicted or related to health, including reabsorption, evaporation, sweat rate, substances on the epidermis and even metabolism of eccrine glands (Baker, 2019; Kim et al., 2019; Quinton, 1983). The stimulation for sweat can remarkably distort the blood-sweat correlation and undermine the clinical significance of the sweat test results.

Therefore, the collection of representative samples plays a pivotal role in clinical application of sweat bioanalysis, which has drawn considerable research attention (Baker, 2019; Lin et al., 2020; Quinton, 1983). To avoid the problems of stimulated sweat, collection of unstimulated sweat for bioanalysis has been reported (Baker, 2019; Emaminejad et al., 2017; Kim et al., 2019). For example, Bariya and coworkers proposed a glove-based method to collect naturally secreted sweat for bioanalysis (Bariya et al., 2020). Nyein and coworkers reported a wearable microfluidic device for sampling and monitoring pH, Cl⁻ and levodopa in unstimulated sweat (Nyein et al., 2021). Nagamine and coworkers used agarose gel to extract lactic acid in sweat for continuous potentiometric detection (Nagamine et al., 2019). Likewise, Shuyu and coworkers reported the use of agarose hydrogel for analysis of lactic acid and caffeine in sweat (Lin et al., 2020). However, most of these methods are limited to simple one-step detection, and multi-step bioanalysis for a wide range of valuable targets is still challenging.

Here we report a versatile method for on-demand sweat bioanalysis based on hydrogel-programmed wearables. Unlike previously reported hydrogel-based sensors, which depend on sweat diffusion in the hydrogel to the electrode surface (Moon et al., 2022; Saha et al. 2021, 2022; Sempionatto et al., 2021), the sensor we propose utilizes a thermoresponsive hydrogel that can not only passively collect the sweat but also actively release the sweat into a microfluidic channel. Our design facilitates the formation of a stable electrode-solution interface and sequential reactions. The hydrogel involved is a thermoresponsive poly(n-isopropyl acrylamide) (pNIPAM) polymer that is featured by a lower critical solution temperature (LCST). Under its LCST (i.e., 42°C), the hydrogel gradually absorbs sweat that is slowly secreted from the skin. Above the LCST, the hydrogel undergoes a phase transition and releases the absorbed sweat or preloaded reagents into a microfluidic channel for bioanalysis. The fluidic manipulation process is programmed by electric heating to accomplish not only one-step detection of glucose but also multi-step immunoassay of cortisol on the wearables. Using our method, on-demand bioanalysis can be performed within 1h without disturbing daily life from stimulated sweat collection. We also evaluate the clinical applicability of our method by comparing our test results with those obtained in blood samples or conventional stimulated sweat samples.