Thermal Transport Characteristics of Human Skin Measured In Vivo Using Ultrathin Conformal Arrays of Thermal Sensors and Actuators

Abstract

Measurements of the thermal transport properties of the skin can reveal changes in physical and chemical states of relevance to dermatological health, skin structure and activity, thermoregulation and other aspects of human physiology. Existing methods for in vivo evaluations demand complex systems for laser heating and infrared thermography, or they require rigid, invasive probes; neither can apply to arbitrary regions of the body, offers modes for rapid spatial mapping, or enables continuous monitoring outside of laboratory settings. Here we describe human clinical studies using mechanically soft arrays of thermal actuators and sensors that laminate onto the skin to provide rapid, quantitative in vivo determination of both the thermal conductivity and thermal diffusivity, in a completely non-invasive manner. Comprehensive analysis of measurements on six different body locations of each of twenty-five human subjects reveal systematic variations and directional anisotropies in the characteristics, with correlations to the thicknesses of the epidermis (EP) and stratum corneum (SC) determined by optical coherence tomography, and to the water content assessed by electrical impedance based measurements. Multivariate statistical analysis establishes four distinct locations across the body that exhibit different physical properties: heel, cheek, palm, and wrist/volar forearm/dorsal forearm. The data also demonstrate that thermal transport correlates negatively with SC and EP thickness and positively with water content, with a strength of correlation that varies from region to region, e.g., stronger in the palmar than in the follicular regions.

Intrinsically Stretchable Biphasic (Solid–Liquid) Thin Metal Films

“…The soft epidermal thermal actuators provided a well-controlled heat source on the skin (Figure 4c), opening opportunities in noninvasive or minimally invasive biomedical mapping. [16] We also fabricated an electromechanical actuator with a dielectric elastomer sandwiched between soft biphasic electrodes (Figure 4d and Figure S16 and Movie S3, Supporting Information). The tip of the cantilever actuator traced the predicted displacement of voltage-controlled actuation.…”

Multimodal epidermal devices for hydration monitoring

“…D(τ), computed for a relevant range of τ, and a representative experimental data set with its corresponding fit appear in Supplementary Figure S5. For ease of processing large quantities of data such as those obtained in clinical trials described subsequently, a simplified analytical expression 15 can be useful,…”

“…Here we describe recent advances in multimodal sensors of skin hydration, via measurements of intrinsic thermal and electrical properties, in which advanced materials, mechanics and concepts render devices that have soft, 'skin-like', or 'epidermal', formats [11][12][13][14][15][16][17][18][19][20] . The thin geometries and compliant mechanics lead to a mode of integration with the skin that does not require application of pressure, and instead relies on van der Waals adhesion forces alone.…”

“…Consequently, devices that offer, in a single platform, capabilities for measuring both the thermal and the electrical characteristics of the skin provide improved insights into the state of hydration. In general, thermal transport techniques probe 15,24 the flow of heat deep into the epidermis. Here, the response changes with hydration simply because the thermal properties of water are different than those of the tissue itself.…”

Toward Self‐Powered Wearable Adhesive Skin Patch with Bendable Microneedle Array for Transdermal Drug Delivery

“…Wearable flexible electronic medical devices have received major attention recently owing to their considerable practicability for several applications,1, 2, 3, 4, 5, 6, 7, 8, 9, 10 including health monitoring and drug delivery for disease treatment 11, 12, 13. The traditional drug delivery method using hypodermic needles may be an unpleasant process to many patients.…”