In neonatal intensive care units, eliminating cumbersome wires from patient monitoring has driven the development of compact, skin-mounted devices. These wireless systems integrate batteries, sensors, analog electronics, and radio modules into soft, flexible packages. While freeing patients from tethered equipment, they introduce a new challenge: preventing skin injury from overheating due to battery discharge or component failure. Conventional safeguards rely on electronic temperature sensors and shutdown circuits, but these can themselves fail. A materials-based, self-powered approach offers an alternative.
The design centers on a thin, flexible bladder embedded between the device’s battery and the skin within the elastomeric enclosure. This bladder contains Novec 71DE, a non-flammable, low-toxicity liquid with a boiling point of 41 °C at atmospheric pressure. Under normal operation, device and skin temperatures remain below this threshold, keeping the bladder flat at about 125 µm thick. If overheating pushes temperatures above the boiling point, the liquid vaporizes rapidly, expanding the bladder to nearly 5 mm at 46.6 °C. This expansion both delaminates the device from the skin and creates an insulating vapor layer.
The bladder’s construction uses ethylene vinyl alcohol (EVOH) film, heat-sealed to encapsulate roughly 10 µL of liquid. The EVOH provides high impermeability, retaining more than 99.8% of the liquid over 100 days at room temperature, and withstands strains up to 340% without rupture. Even after 1,000 bending cycles, the seal remains intact. The total added weight is only about 40 mg, with negligible impact on flexibility.
Thermal testing in an oven showed expansion begins within 10 s of reaching the boiling point, achieving 50% of maximum volume in 30 s and full expansion in about a minute. At full vaporization, the internal pressure reaches ~138 kPa, matching the liquid–vapor saturation pressure, and the thermal resistance between device and skin increases sharply. This barrier maintains skin-contact temperatures below medical safety limits of 48 °C for 1–10 min exposure.
To simulate battery thermal runaway, researchers embedded a resistive heater inside a battery pouch and mounted it on a skin phantom. A bare device under these conditions drove skin temperatures to nearly 60 °C. Adding static air pockets of 1 mm and 5 mm thickness reduced skin temperatures to ~47 °C and ~41 °C, respectively, but at the cost of increased device thickness. The liquid–vapor bladder, by contrast, remained unobtrusive during normal operation and only created a thermal gap when triggered. Upon heating, the bladder began expanding around 15 s, absorbing heat through vaporization and slowing the skin temperature rise. At full expansion, the device lifted several millimeters from the skin, reducing contact area to as little as 1% of the original. This dynamic isolation kept the skin below 45 °C even under heat generation approaching 1 W for 10 min.
Finite element analysis confirmed the bladder’s performance, showing heat transfer ratios to the skin dropping from 29.3% at 30 s to just 5.4% at 180 s after activation. Without delamination, ratios remained comparable to those of a 5 mm air pocket. The expanded bladder maintained its insulating state indefinitely for as long as temperatures stayed above the boiling point.
Integration into a wireless mechano-acoustic health monitor demonstrated practical viability. The device, equipped with a high-bandwidth 3-axis accelerometer, measured respiratory motion, cardiac vibrations, and speech patterns without any degradation in signal quality due to the bladder. The added thickness was only 0.125 mm, and flexibility was unchanged since the bladder sat beneath the rigid battery.
In a live battery short-circuit test, the bladder expanded in response to the generated heat, lifting the device from a skin phantom and keeping the contact temperature under 48 °C, despite the device surface exceeding 65 °C. Thermal simulations indicated no visible skin damage under these conditions. This passive, materials-driven mechanism thus offers a robust, fail-safe layer of protection for skin-mounted electronics, activating only when needed and requiring no external power to operate.