Thermal transport characteristics of human skin measured in vivo using ultrathin conformal arrays of thermal sensors and actuators.

R Chad Webb,Rafal M Pielak,Jonas Kurniawan,Guive Balooch,Philippe Bastien,Juha Niittynen,John A Rogers,Nam Heon Cho,Athena Lin,Joshua Ayers,Megan Manco,Viktor Malyrchuk,Victor M Ugaz

doi:10.1371/journal.pone.0118131

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.

Highlights

Skin is the largest organ of human body and it provides one of the most diverse sets of functions
Existing in vivo approaches couple the use of laser heating or induced changes in the temperature of the ambient with infrared thermography [4,5,6], or they exploit rigid probes that press against the skin [7,8]
An infrared image collected during the heating sequence (Fig. 2a) shows results of local, rapid heating generated by a single element

Summary

Introduction

Skin is the largest organ of human body and it provides one of the most diverse sets of functions. Existing in vivo approaches couple the use of laser heating or induced changes in the temperature of the ambient with infrared thermography [4,5,6], or they exploit rigid probes that press against the skin [7,8]. These and other previously reported methods only apply to certain regions of the skin; they do not readily allow thermal mapping measurement or determination of anisotropic properties and they operate effectively only in controlled, laboratory settings. Assessments of the skin at six different body locations in twenty-five human subjects illuminate systematic variations in both the thermal conductivity and thermal diffusivity, for which measurements by optical coherence tomography (OCT), and electrical impedance yield additional insights into the underlying physiology

Methods

Results

Conclusion