Responsive materials and mechanisms as thermal safety systems for skin-interfaced electronic devices

Abstract

Soft, wireless physiological sensors that gently adhere to the skin are capable of continuous clinical-grade health monitoring in hospital and/or home settings, of particular value to critically ill infants and other vulnerable patients, but they present risks for injury upon thermal failure. This paper introduces an active materials approach that automatically minimizes such risks, to complement traditional schemes that rely on integrated sensors and electronic control circuits. The strategy exploits thin, flexible bladders that contain small volumes of liquid with boiling points a few degrees above body temperature. When the heat exceeds the safe range, vaporization rapidly forms highly effective, thermally insulating structures and delaminates the device from the skin, thereby eliminating any danger to the skin. Experimental and computational thermomechanical studies and demonstrations in a skin-interfaced mechano-acoustic sensor illustrate the effectiveness of this simple thermal safety system and suggest its applicability to nearly any class of skin-integrated device technology.

Multifunctional Materials Strategies for Enhanced Safety of Wireless, Skin-Interfaced Bioelectronic Devices

“…This temperature represents the approximate value at which pain/discomfort is expected on human skin. [6, 30-32] The International Electrotechnical Commission (IEC) defines 48°C as the maximum temperature that healthy adult human skin can be in contact with a medical device part, for contact times between 1-10 min. [33] Various mass fractions of the leuco dye can be incorporated to yield desired properties (Figure 2E).…”

“…Experiments to examine the relevance in the context of safety hazards associated with an overheating battery involve SH PDMS and SH PDMS + HGMs structures that fully encapsulate a LiPo battery housing that contains a resistive heater designed to simulate thermal runaway. [6] Passing 250 mA through the heater causes a significant temperature increase (Figure 4F) in the battery structure. At 180 s after supply of current, the temperature of the heater of the 2.5 wt% and 5.0 wt% HGMs composites are ∼10.2°C and ∼14.5°C higher, respectively, than that of SH PDMS, indicating that the thermally insulating nature of the HGMs helps to contain and isolate the heat to the battery structure (Figure 4G).…”

“…In practical applications, the thermal insulating aspects of the HGMs can be combined with other thermal protective strategies that exploit materials, layouts, and circuit designs. [6, 52-54]…”

“…The process for simulating an overheating battery (thermal runaway) appears elsewhere. [6] Briefly, 250 mA of current was applied to a SH PDMS or SH PDMS + HGM composite-encapsulated battery enclosure with heater (BwH) device, consisting of a lithium polymer (LiPo) battery enclosure (DNK201515, DNK Power) with a copper resistive heating element. The resulting temperature increase inside the BwH was monitored via measurement of the voltage drop across the BwH.…”

“…While many safeguards, spanning careful quality control to voltage/current protection circuits, are important, they do not address all possible safety hazards. [3, 6-7] For example, mechanical failures in the encapsulation structure that follow from prolonged cycles of use can expose the user to mechanical and electrical hazards associated with the electronic components. [8-12] Polymer chemistries that offer self-healing capabilities and strain-limiting features have advantages over simple silicones as encapsulants.…”