9 research outputs found

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

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    <div><p>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 <i>in vivo</i> 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 <i>in vivo</i> 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.</p></div

    Ultrathin, conformal device for evaluating thermal transport characteristics and validation on human skin.

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    <p>(<b>a</b>) Photograph of a device laminated onto a subject’s cheek. (<b>b</b>) Magnified view showing the location of the heater (red), a sensing element 3.5 mm away from the heater (blue), 4.7 mm away (black), and 5.8 mm away (green). (<b>c</b>) Magnified view during deformation. (<b>d</b>) Optical coherence tomography image of a region of a human palm before and (<b>e</b>) after mounting the array (blue).</p

    Clinical data correlation analysis.

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    <p>(<b>a</b>) Scatterplot matrix representation for the entire data set (all 6 body locations: cheek, volar and dorsal forearm, wrist, palm, and heel on 25 total subjects). Pairwise correlation analyses include the thermal characteristics (<i>k</i>, W m<sup>-1</sup>°C<sup>-1</sup>; <i>ρc</i><sub><i>p</i></sub>, J cm<sup>-3</sup>°C<sup>-1</sup>; <i>α</i>, mm<sup>2</sup> s<sup>-1</sup>) and stratum corneum thickness (SC-thick, μm), epidermal thickness (EP-thick, μm), and stratum corneum hydration (SC-h, arbitrary units). Data for different body areas are represented by different colors. The red line represents the pairwise linear regression slope. The pink shaded clouds represent the 95% bivariate normal density ellipse. Assuming the variables are bivariate normally distributed, this ellipse encloses approximately 95% of the points. (<b>b</b>) The bivariate correlations for the entire data set are represented using a color coding (HeatMap) scheme associated with a clustering of the descriptors. Dark red is associated with Pearson Correlation Coefficient, R, equal to 1 and dark blue is associated to R = -1. The Pearson correlation coefficients are given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0118131#pone.0118131.s010" target="_blank">S1 Table</a>.</p

    Spatial mapping of thermal transport associated with low level heating on the surface of the skin.

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    <p>(<b>a</b>) Spatial map of the changes in temperature at each sensor (i.e. element) in the array. The data processing uses an adjacent-average filter (window size = 8 s) and normalization to Element 16. The red highlight and colored boxes represent the elements boxed in the same colors in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0118131#pone.0118131.g001" target="_blank">Fig. 1b</a>. (<b>b</b>) Change in temperature at elements 3.5 mm away (blue), 4.7 mm away (black) and 5.8 mm away (red) from element responsible for thermal actuation. The solid and dashed lines represent experimental data and best fit calculations, with <i>k</i> ~ 0.35–0.43 W m<sup>-1</sup> K<sup>-1</sup> and <i>α</i> ~ 0.12–0.15 mm<sup>2</sup> s<sup>-1</sup>. (<b>c</b>) Results of finite element modelling of an array on a cheek, in the same arrangement as <b>b</b>.</p

    Thermal flow associated with low level transient heating on the surface of the skin.

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    <p>(<b>a</b>) Infrared image during heating at a single thermal actuator in an array device on the skin. (<b>b</b>) Finite element modelling results for the distribution of temperature during rapid, low level heating at an isolated actuator on the skin, after 1.2 s of heating at a power of 3.7 mW mm<sup>-2</sup>. (<b>c</b>) Spatial map of the rise in temperature due to transient heating sequentially in each element in the array. The solid black lines are experimental data; the red dashed lines are best fit calculations. The strong rise shown in upper leftmost element results from local delamination of the device from the skin. (<b>d</b>) Experimental data (solid lines) and best fit calculations (dashed lines) for the cheek (black) and heel (blue), along with extracted thermal transport properties.</p

    Clinical data correlation analysis for regions with significant stratum corneum thickness.

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    <p>The same correlation analysis as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0118131#pone.0118131.g006" target="_blank">Fig. 6</a> for the (<b>a</b>) palm and (<b>b</b>) heel.</p

    Clinical data distributions.

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    <p>Boxplot representation of the data (open circles). The mean is represented by a black diamond shape. The top and the bottom line of the box are the first and third quartiles, and the middle line of the box is the second quartile—the median. The lower (upper) whisker represents the minimum (maximum) observation above (below) the 1.5 Inter Quartile Range (IQR) below (above) the lower (upper) quartile. Data distributions for the (<b>a</b>) stratum corneum thickness (SC-thick), (<b>b</b>) stratum corneum hydration (SC-h), (<b>c</b>) epidermis thickness (EP-thick), (<b>d</b>) thermal conductivity (<i>k</i>), (<b>e</b>) volumetric heat capacity (<i>ρc</i><sub><i>p</i></sub>), and (<b>f</b>) thermal diffusivity (<i>α</i>).</p

    Anisotropic convective effects associated with near surface blood flow.

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    <p>(<b>a</b>) Spatial map of changes in temperature at each element for a device located at the volar aspect of the wrist. The position of the thermal actuator coincides with a large vein. (<b>b</b>) Difference in temperature between element 11 (E11) and element 3 (E3). The results show effects of anisotropic heat flow in the wrist, compared to isotropic distributions typically observed on a region of the body such as the cheek. The vertical red dashed lines correspond to initiation and termination of heating, respectively.</p
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