Precision thermometry of the skin can, together with other measurements, provide clinically relevant information about cardiovascular health, cognitive state, malignancy and many other important aspects of human physiology. Here, we introduce an ultrathin, compliant skin-like sensor/actuator technology that can pliably laminate onto the epidermis to provide continuous, accurate thermal characterizations that are unavailable with other methods. Examples include non-invasive spatial mapping of skin temperature with millikelvin precision, and simultaneous quantitative assessment of tissue thermal conductivity. Such devices can also be implemented in ways that reveal the time-dynamic influence of blood flow and perfusion on these properties. Experimental and theoretical studies establish the underlying principles of operation, and define engineering guidelines for device design. Evaluation of subtle variations in skin temperature associated with mental activity, physical stimulation and vasoconstriction/dilation along with accurate determination of skin hydration through measurements of thermal conductivity represent some important operational examples.

Ultrathin conformal devices for precise and continuous thermal characterization of human skin.

This paper is one of several in this Special Issue of the International Journal of Hyperthermia that discusses the current state of knowledge about the human health risks of hyperthermia This special issue emanated from a workshop sponsored by the World Health Organization in the Spring of 2002 on this topic It is anticipated that these papers will help to establish guidelines for human exposure to conditions leading to hyperthermia This comprehensive review of the literature makes it clear that much more work needs to be done to clarify what the thresholds for thermal damage are in humans This review summarizes the basic principles that govern the relationships between thermal exposure (temperature and time of exposure) and thermal damage, with an emphasis on normal tissue effects Methods for converting one time-temperature combination to a time at a standardized temperature are provided as well as a detailed discussion about the underlying assumptions that go into these calculations There are few in vivo papers examining the type and extent of damage that occurs in the lower temperature range for hyperthermic exposures (eg 39-42 degrees C) Therefore, it is clear that estimation of thermal dose to effect at these thermal exposures is less precise in that temperature range In addition, there are virtually no data that directly relate to the thermal sensitivity of human tissues Thus, establishment of guidelines for human exposure based on the data provided must be done with significant caution There is detailed review and presentation of thermal thresholds for tissue damage (based on what is detectable in vivo) The data are normalized using thermal dosimetric concepts Tables are included in an Appendix Database which compile published data for thresholds of thermal damage in a variety of tissues and species This database is available by request (contact MWD or PJH), but not included in this manuscript for brevity All of the studies reported are for single acute thermal exposures Except for brain function and physiology (as detailed in this issue by Sharma et al) one notes the critical lack of publications examining effects of chronic thermal exposures as might be encountered in occupational hazards This review also does not include information on the embryo, which is covered in detail elsewhere in this volume (see article by Edwards et al) as well as in a recent review on this subject, which focuses on thermal dose

Basic principles of thermal dosimetry and thermal thresholds for tissue damage from hyperthermia

A dynamic model predicting human thermal responses in cold, cool, neutral, warm, and hot environments is presented in a two-part study. This, the first paper, is concerned with aspects of the passi...

https://www.physiology.org/doi/pdf/10.1152/jappl.1999.87.5.1957

A computer model of human thermoregulation for a wide range of environmental conditions: the passive system.

The role of porous media in modeling flow and heat transfer in biological tissues

The UTCI-Fiala mathematical model of human temperature regulation forms the basis of the new Universal Thermal Climate Index (UTC). Following extensive validation tests, adaptations and extensions, such as the inclusion of an adaptive clothing model, the model was used to predict human temperature and regulatory responses for combinations of the prevailing outdoor climate conditions. This paper provides an overview of the underlying algorithms and methods that constitute the multi-node dynamic UTCI-Fiala model of human thermal physiology and comfort. Treated topics include modelling heat and mass transfer within the body, numerical techniques, modelling environmental heat exchanges, thermoregulatory reactions of the central nervous system, and perceptual responses. Other contributions of this special issue describe the validation of the UTCI-Fiala model against measured data and the development of the adaptive clothing model for outdoor climates.

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UTCI-Fiala multi-node model of human heat transfer and temperature regulation.

A new simplified three-dimensional bioheat equation is derived to describe the effect of blood flow on blood-tissue heat transfer. In two recent theoretical and experimental studies [1, 2] the authors have demonstrated that the so-called isotropic blood perfusion term in the existing bioheat equation is negligible because of the microvascular organization, and that the primary mechanism for blood-tissue energy exchange is incomplete countercurrent exchange in the thermally significant microvessels. The new theory to describe this basic mechanism shows that the vascularization of tissue causes it to behave as an anisotropic heat transfer medium. A remarkably simple expression is derived for the tensor conductivity of the tissue as a function of the local vascular geometry and flow velocity in the thermally significant countercurrent vessels. It is also shown that directed as opposed to isotropic blood perfusion between the countercurrent vessels can have a significant influence on heat transfer in regions where the countercurrent vessels are under 70-micron diameter. The new bioheat equation also describes this mechanism.

A New Simplified Bioheat Equation for the Effect of Blood Flow on Local Average Tissue Temperature

A new theoretical model supported by ultrastructural studies and high-spatial resolution temperature measurements is presented for surface tissue heat transfer in a two-part study. In this first paper, vascular casts of the rabbit thigh prepared by the tissue clearance method were serially sectioned parallel to the skin surface to determine the detailed variation of the vascular geometry as a function of tissue depth. Simple quantitative models of the basic vascular structures observed were then analyzed in terms of their characteristic thermal relaxation lengths and a new three-layer conceptual model proposed for surface tissue heat transfer. Fine wire temperature measurements with an 80-micron average diameter thermocouple junction and spatial increments of 20 micrometers between measurement sites have shown for the first time the detailed temperature fluctuations in the microvasculature and have confirmed the fundamental assumptions of the proposed three-layer model for the deep tissue, skeletal muscle and cutaneous layers.

Theory and experiment for the effect of vascular microstructure on surface tissue heat transfer--Part I: Anatomical foundation and model conceptualization.

Heat Conduction With Melting or Freezing in a Corner

In this paper the conceptual three-layer representation of surface tissue heat transfer proposed in Weinbaum, Jiji and Lemons [1], is developed into a detailed quantitative model. This model takes into consideration the variation of the number density, size and flow velocity of the countercurrent arterio-venous vessels as a function of depth from the skin surface, the directionality of blood perfusion in the transverse vessel layer and the superficial shunting of blood to the cutaneous layer. A closed form analytic solution for the boundary value problem coupling the three layers is obtained. This solution is in terms of numerically evaluated integrals describing the detailed vascular geometry, a capillary bleed-off distribution function and parameters describing the shunting of blood to the cutaneous layer. Representative heat transfer results for typical physiological conditions are presented.

Theory and Experiment for the Effect of Vascular Microstructure on Surface Tissue Heat Transfer—Part II: Model Formulation and Solution

Heat transfer between a circular free impinging jet and a solid surface with non-uniform wall temperature or wall heat flux—1. Solution for the stagnation region

Latif M. Jiji

Papers

A New Simplified Bioheat Equation for the Effect of Blood Flow on Local Average Tissue Temperature

Theory and experiment for the effect of vascular microstructure on surface tissue heat transfer--Part I: Anatomical foundation and model conceptualization.

Heat Conduction With Melting or Freezing in a Corner

Theory and Experiment for the Effect of Vascular Microstructure on Surface Tissue Heat Transfer—Part II: Model Formulation and Solution

Heat transfer between a circular free impinging jet and a solid surface with non-uniform wall temperature or wall heat flux—1. Solution for the stagnation region