Membrane isolation of repeated-use sweat stimulants for mitigating both direct dermal contact and sweat dilution

Abstract

With the device integration of sweat stimulation, sweat becomes a stronger candidate for non-invasive continuous biochemical sensing. However, sweat stimulants are cholinergenic agents and non-selective to just the sweat glands, and so, direct placement of sweat stimulants poses additional challenges in the possibility for uncontrollable transport of the stimulant into the body and challenges in contamination of the sweat sample. Reported here is membrane isolation of repeated-use sweat stimulants for mitigating direct dermal contact, dilution of the sweat stimulant, and contamination of the sweat sample. The membrane dramatically reduces passive diffusion of the sweat stimulant carbachol by roughly two orders of magnitude, while still allowing repeated sweat stimulation by iontophoretic delivery of the carbachol through the membrane and into the skin. Both in-vivo and in-vitro validation reveal feasibility for reliable integration of sweat stimulants within a wearable device for use periods of 24 h or more. In addition, advanced topics and confounding issues such as stimulant gel design, osmotic pressure, and ionic impurities are speculatively and theoretically discussed.

A Wearable Nutrition Tracker

“…[11,12] A rapid rise in wearable sweat analytical devices in recent years brings promising new applications for sweat in health care. [13][14][15][16][17][18][19][20][21][22] For instance, sweat ethanol level is correlated with blood ethanol, and uric acid in sweat can reflect plasma uric acid level in gout patients. [23,24] These discoveries guide the possibility of sweat analysis in wider applications such as in nutritional assessment.…”

Skin-Interfaced Wearable Sweat Sensors for Precision Medicine

“…In an equine model, the microneedle patch produces a much higher sweating volume per unit area and unit dose compared to the pilocarpine hydrogel (Figure e,f). While the previous studies focused on local sweating, the first use of carbachol for sweat extraction was integrated into a band-aid-shaped system with an external iontophoresis device (Figure g). ,, With the custom-made carbachol gels compared with pilocarpine gels, the group also studied the sweating duration at high sweat rates (Figure h) and at low sweat rates (Figure i) over a long time frame (over 10 h). More recently, a flexible laser-engraved iontophoresis patch with a much smaller form factor was developed with small carbachol gels cast onto laser-engraved graphene electrodes, and on-demand sweat induction was achieved by integrated FPCB (Figure j) .…”

Wearable Potentiometric Sensors for Medical Applications

“…Indeed, other wearable sensors have employed commercial iontophoresis units for sweat collection that may be further incorporated with potentiometric sensors [70]. As an example, ActiveDose II (ActivaTek, Gilroy, CA, USA) and the Wescor Nanoduct system merit mentioning [71]. Furthermore, one recent work presented by the group of Heikenfeld investigates in detail the sweat rate provided by a carbachol-based iontophoresis system using an impedance electrode similar to that reported by the group of Javey [39].…”