Waterproof, electronics-enabled, epidermal microfluidic devices for sweat collection, biomarker analysis, and thermography in aquatic settings

Abstract

Noninvasive, in situ biochemical monitoring of physiological status, via the use of sweat, could enable new forms of health care diagnostics and personalized hydration strategies. Recent advances in sweat collection and sensing technologies offer powerful capabilities, but they are not effective for use in extreme situations such as aquatic or arid environments, because of unique challenges in eliminating interference/contamination from surrounding water, maintaining robust adhesion in the presence of viscous drag forces and/or vigorous motion, and preventing evaporation of collected sweat. This paper introduces materials and designs for waterproof, epidermal, microfluidic and electronic systems that adhere to the skin to enable capture, storage, and analysis of sweat, even while fully underwater. Field trials demonstrate the ability of these devices to collect quantitative in situ measurements of local sweat chloride concentration, local sweat loss (and sweat rate), and skin temperature during vigorous physical activity in controlled, indoor conditions and in open-ocean swimming.

Fig. 1. Waterproof, skin-like microfluidic/electronic device.

(A) Exploded view schematic illustration of the key layers of a representative device. (B) Microfluidic channel geometry. (C) Optical micrograph that shows the microfluidic inlet and outlet ports and the colorimetric reagent. (D) A dye composed of blue and red water-soluble particles that dissolve at different rates results in a flow-driven change in color. Measuring the number of turns of filled channels yields the total volume of collected sweat (1 turn = 1.5 μl). (E) Near-field communication (NFC) coil for wireless measurements of skin temperature. (F) Sweat collection in aquatic environments without contamination is enabled by the use of small outlet geometries (r = 0.25 mm) and constituent polymer materials (SIS) that are hydrophobic and largely impermeable to water and water vapor. Dip coating an encapsulation of this same material enables underwater operation of the electronics, including the NFC coil, integrated circuit chip, and indicator light-emitting diode (LED). Photo credit: P. Gutruf, Northwestern University.

Fig. 2. Fabrication and characterization of soft microfluidic systems constructed with SIS.

(A) SIS solvent cast on a bas-relief wafer forms a uniform coating. (B) Evaporation of the solvent leaves behind a thin layer of SIS conformal to the bas-relief. (C) The channel layer, bottom layer, and adhesive laminate together and bond via application of light pressure after demolding. (D) Cross-sectional micrograph of a microfluidic channel showing the contoured geometry of the top surface. (E) The high strain to failure (>2000%) and low elastic modulus (0.83 MPa) enable demolding of thin, delicate microfluidic structures. (F) Geometry-dependent flow rate in SIS microfluidic channels as a function of filling length for a fixed pressure of 2 kPa. (G and H) The high-barrier properties and low water uptake of SIS enable stable collection and storage of sweat in both aquatic and arid environments. Photo credit: J. Choi, Northwestern University.

Fig. 3. Mechanics of conformal epifluidic systems.

(A) Optical micrographs of devices with tapered edges on the skin before and after pinching to form wrinkles. (B) Delamination from the skin of a nontapered device due to wrinkling. (C) Stretched. (D) Compressed. (E) Twisted. (F) Pulled. (G) Before and after stretching to ~400%. Simulation results and experimental observations of mechanical deformations including (H) stretching (15%), (I) bending (r = 3 cm), and (J) twisting (67.5°). The maximum interfacial stress is <5 kPa for all cases, which is much lower than the threshold of skin sensitivity (20 kPa). Photo credit: J. Choi, Northwestern University.

Fig. 4. Sweat collection from aquatic and dryland athletes.

(A) A subject wearing an epifluidic device during a swimming study. (B) A subject wearing an epifluidic device during a biking study. (C) The sweat collection area, defined by the geometry of the adhesive layer, is a circle with r = 3 mm. Vents, corresponding to regions without the adhesive, extend radially from the center to reduce the number of occluded sweat glands and, thereby, to minimize the effects of compensatory sweating. (D) Experimental and theoretical (ideal gas law) data on backfilling of a device after submerging at various depths. (E) A good correlation exists between the sweat volume per area collected via the epifluidic devices and that obtained with an absorbent pad as a result of biking and swimming exercises. (F) A good correlation between the sweat volume per area collected via the epifluidic devices and the percentage of body weight loss as a result of biking exercise. (G) Representative images of epifluidic devices on three IRONMAN triathletes after exercising. Photo credit: (A) J. T. Reeder and (B) J. Choi, Northwestern University, and (G) K. Barnes, Gatorade Sports Science Institute.

Fig. 5. Digital thermography and chloride sensing.

(A) The NFC electronics consist of a magnetic loop antenna, NFC chip with an on-board temperature sensor, LED, and passive components. (B) Skin temperature measurements during 20-min sessions of swimming and biking elucidate effects of the environment, perfusion, and sweat generation. (C) The a* and b* color values associated with reference chloride solutions after reacting with silver chloranilate. Inset: Photographs of SIS devices with a silver chloranilate reagent after reacting with reference chloride solutions. (D) Images of devices used for measuring chloride concentrations in sweat generated by swimming and biking. (E) LAB color values from the colorimetric reagent for chloride measurements compared to results obtained with a chloridometer.

Fig. 6. Sweat collection in unconventional scenarios.

Thin, conformal mechanics associated with devices introduced here enable sweat collection under high levels of deformation and from unusual locations including the (A) neck, (B) armpit, and (C) forehead. (D) Soft, thin device construction enables conformation to delicate skin, such as that of a baby during a warm bath. (E) High-barrier properties of the SIS enable reliable collection of sweat at low rates (<4 μl/hour) over 8 hours from the lower back of a sedentary subject. Photo credit: (A to C) J. T. Reeder and (D) S. Xu, Northwestern University.