This Wearable Sweat Sensor Keeps Tabs on Your Ovulation by Monitoring Estradiol Levels

Researchers from the California Institute of Technology (Caltech) have developed a sweat sensor designed to monitor the level of estradiol — an estrogen-class hormone — in a woman's sweat, as a non-invasive approach to monitoring everything from fertility to the progress of hormone replacement therapy (HRT).

"People often ask me if I could make the same kind of sweat sensor for female hormones," explains Wei Gao, assistant professor of medical engineering and whose lab has built sweat-based sensors for cortisol, COVID-19, and more, "because we know how much those hormones impact women's health."

Estradiol levels can be used for accurate fertility monitoring, helping those who are attempting to conceive — whether naturally or through in-vitro fertilization (IVF). Traditional non-invasive approaches for this, typically focused around monitoring body temperature, aren't accurate enough to provide good results — but as estradiol levels increase before ovulation, the team's sweat sensor could provide earlier and more accurate ovulation alerts.

Gao also claims that the sensor could be useful for those undergoing hormone replacement therapy (HRT) for a lack of estradiol, replacing invasive blood-draw monitoring of estradiol levels with continuous readings from a simple adhesive patch.

The patch in question is based on the earlier sensors from Gao's lab, and uses a flexible plastic membrane with microfluidic passages to wick sweat away from the body and into a chamber with inkjet-printed gold nanoparticles and titanium carbide MXene films and containing a single-strand DNA aptamer which binds to estradiol. It's considerably more sensitive than previous devices, though: estradiol is only present at low volumes in the blood, and there's 50 times less in sweat.

In testing, the sensor — which the team says could be miniaturized into a compact ring-like wearable and placed on the patient's finger — proved able to track estradiol levels in sweat over the course of its wearer's reproductive cycle, while also monitoring sweat pH, salt levels, and skin temperature, used to calibrate the sensor to each individual subject.

The team's work has been published in the journal Nature Nanotechnology under closed-access terms.