SKIN HYDRATION SENSOR APPLICATOR

Through advancements in the fabrication of thin copper traces, newer versions of the device have been able to increase the proportion of heat applied to the skin by mitigating thermal conduction via the circuit. This, in turn, increases the signal-to-noise ratio for thermal measurement data, increasing device precision.

However, pressure effects have hindered the functionality of the device for a long time. When skin - especially dry skin - is pressed, water from the lower layers (dermis) is forced into the epidermis. This is a bit like jumping on a dry trampoline positioned above a puddle. Sure, before you jump on it, the trampoline is dry, but when once you dip into that puddle, it springs back up having absorbed all of that moisture. A more accurate analogy might be pressing on a kitchen sponge that is half-wet. Once you push down, you have irreversibly forced water into the upper portion of the sponge.

So, what does this mean for the functioning of the skin hydrations sensor? My research and engineering team at Wearifi found variations in the application techniques used to manually adhere the device to the skin (force, angle, location of grip) will significantly affect measured hydration due to this effect. Conventional skin hydration sensors (corneometers) commonly use spring mechanisms and/or pressure sensors to control pressure effects, but these turn out to be bulky, rigid, and cumbersome. In 2021, I made it my summer research project to come up with a better solution for this problem.

At Wearifi, in association with the Querrey Simpson Institute of Bioelectronics (QSIB), I proposed, designed, fabricated, and validated a compliant mechanism that partially decouples the force from the user’s hand from the sensing region of the device, mitigating pressure effects. I hypothesized that I could create such a mechanism using stereolithography (SLA).

Creating the applicator was fundamentally a materials selection and geometry optimization problem. Using Formlabs’ clear photopolymer resin, I developed a series of initial designs with differing compliant geometries and tested their ability to control pressure effects on dry skin.

These embodiments had varying success, but the clear photopolymer’s high elastic modulus and low yield strength limited their effectiveness and durability.

While some of these prototypes managed to increase the precision of measurements compared to lamination (the conventional method of applying the device), they failed to control for pressure effects.

Then I had a breakthrough. I had been going about my design the wrong way, instead of trying to create bendable structures by thinning out a brittle material, I realized that I could solve all of these problems by designing an applicator comprised of a flexible photopolymer (Formlabs Flex 80A). This version, which I termed the Flex 2 Applicator, successfully improved both device precision and accuracy. I presented my work at the 2021 QSIB Summer Symposium, winning the 1st-place presentation award out of a pool of 43 undergraduates.

I made further improvements to the Flex 2, eventually settling on a design called the Flex 4 which improved upon aesthetics, user-friendliness, and printability. Finite Element Analysis was later performed on its geometry to further quantify and validate its ability to decouple forces from the user from forces on the sensor. My work was later published in the journal Advanced Medical Materials in early November 2022, on a paper detailing new improvements to the skin hydration sensor, linked here.