Principles of long-term fluids handling in paper-based wearables with capillary–evaporative transport

Abstract

We construct and investigate paper-based microfluidic devices, which model long-term fluid harvesting, transport, sensing, and analysis in new wearables for sweat analysis. Such devices can continuously wick fluid mimicking sweat and dispose of it on evaporation pads. We characterize and analyze how the action of capillarity and evaporation can cooperatively be used to transport and process sweat mimics containing dissolved salts and model analytes. The results point out that non-invasive osmotic extraction combined with paper microfluidics and evaporative disposal can enable sweat collection and monitoring for durations longer than 10 days. We model the fluid flow in the new capillary–evaporative devices and identify the parameters enabling their long-term operation. We show that the transport rates are sufficiently large to handle natural sweat rates, while we envision that such handling can be interfaced with osmotic harvesting of sweat, a concept that we demonstrated recently. Finally, we illustrate that the salt film deposited at the evaporation pad would eventually lead to cessation of the process but at the same time will preserve a record of analytes that may be used for long-term biomarker monitoring in sweat. These principles can be implemented in future platforms for wearable skin-interfacing assays or electronic biomarker monitors.

Wearable Osmotic-Capillary Patch for Prolonged Sweat Harvesting and Sensing

“…Due to the existence of continuous fluid evaporation, the pad acts as a repository of any solute and biomarkers dissolved in the sweat . The evaporation of water from the pad maintains the capillary pressure in the paper strip to promote continuous long-term fluid flow toward the pad.…”

“…The use of paper microfluidic channels can be an extremely simple and efficient approach for long-term sweat harvesting and management. Not only is paper is a readily available and inexpensive material but its porous nature and the ability to control flow rates via capillary wicking and evaporation make it very suitable as a medium to transport and manage sweat. , Evaporation-assisted passive pumping has been used effectively by researchers to develop several microfluidic devices. − Yet, to the best of our knowledge, the combined actions of passive osmotic microfluidic pumping and evaporation-assisted fluid management via paper have not been yet deployed in an operational wearable device for biomarker analysis in sweat.…”

State of Sweat: Emerging Wearable Systems for Real-Time, Noninvasive Sweat Sensing and Analytics

“…Additionally, these methods are not amenable to applications that require frequent, repeated stimulation events (as comparable to blood glucose measurements). Recent efforts to support daily health assessments demonstrate the potential for collection of sweat at a consistent flow rate generated either during showering (Figure C) or by natural perspiration processes − (Figure D). By virtue of the passive nature and circumvention of resource and exertion requirements, these alternative stimulation methods may significantly expand the breadth of potential applications for sweat analytics.…”

Lab-on-Paper Devices for Diagnosis of Human Diseases Using Urine Samples—A Review

“…For example, fast technical analysis (FTA) cards have now been successfully commercialized for the collection and storage of blood samples, urine samples, plasmids, cells, and viruses [41][42][43][44]. In addition, many lab-on-paper platforms have also been developed for the collection and storage of blood, urine, saliva, sweat and tear samples [45][46][47][48][49]. For instance, Shay et al developed a wearable lab-on-paper platform with a capillary-evaporative transport function for the collection of sweat samples for long-term measurement and analysis [49].…”

“…In addition, many lab-on-paper platforms have also been developed for the collection and storage of blood, urine, saliva, sweat and tear samples [45][46][47][48][49]. For instance, Shay et al developed a wearable lab-on-paper platform with a capillary-evaporative transport function for the collection of sweat samples for long-term measurement and analysis [49]. As shown in Figure 1a, typical paper-based sample collection and storage devices comprise three parts, namely a region for storing the sample fluid, a closed paper channel for transporting the sample fluid, and a paper pad with a large surface area for driving the sample fluid continuously through the device under the effects of evaporation.…”