Temperature as an exteroceptive sense: Challenges remain in thermal modeling of skin–object interactions

The sense of temperature has two aspects: an enteroceptive aspect that provides inferences about the physiological state of the body and is important for homeostasis and thermoregulatory behavior, and an exteroceptive aspect that provides inferences about the outside world and is important for object recognition and environmental exploration. Among the papers published in this journal, the recent study by Rykaczewski [1] is one of the few devoted to the exteroceptive aspect of temperature sense. Specifically, it focuses on estimation of thermal contact resistance, which is an important but rarely explored issue in thermal modeling of skin–object interactions. In response to this paper, I would like to briefly discuss the remaining challenges in predicting thermal interactions between the skin and an object during contact.

When the hand (or other body parts) is brought into contact with a cooler object, heat is conducted out of the skin. The corresponding changes in skin temperature determine thermal sensations associated with contact. Modeling skin temperature responses during skin–object interactions is important for engineering applications aiming to create a more realistic experience of contact in virtual environments (for a review, see ref [2].) and for safety guidelines that intend to prevent injuries during contact cold exposure (for a review, see ref [3].). However, the heat transfer process at the skin–object interface is complicated, depending on the object’s material composition and other biological and physical factors related to the skin, the object, and the skin–object interface. In thermal modeling of skin–object interactions, the challenges lie in how to incorporate these factors into a model so that model simulations can be considered predictive for the intended contact scenarios.

A common way to model skin temperature responses during contact cold exposure is to fit empirical data obtained in a certain contact scenario (e.g., a bare hand touching a metallic surface in a cold environment) to a modified Newtonian model [4]. Although this empirical modeling approach is straightforward, its limitations include that the modified Newtonian model is only valid up to the point of the start of cold-induced vasodilation (CIVD) and that numerous measurements have to be completed in order to predict the outcome of contact cold exposure in different contact scenarios. Challenges remain before a model can take into account influences from the thermoregulatory control mechanisms, such as CIVD, and incorporate various contact materials and environmental conditions in different cold exposure scenarios.

For thermal display applications, theoretical, rather than empirical, thermal modeling is the major methodology used for predicting thermal interactions between the skin and an object. These thermal models typically use a transient heat conduction equation to describe the heat transfer processes within the skin and the object, and the thermal contact resistance is incorporated into the boundary conditions at the skin-object interface [5]. The existing thermal models often assume brief contact so that the skin and object can be given a simplified representation as two inanimate materials with semi-infinite dimensions. Although models with the semi-infinite body assumption are simple and in general give good predictions for brief contacts, there remains room for improvement. First, the inanimate material assumption of the skin neglects biological factors such as blood perfusion and metabolic heat generation, which could have significant effects on the skin temperature responses even during brief contacts, especially when the contacted object is thin and/or has low thermal conductivity. Second, the semi-infinite body assumption neglects any influence from the actual geometry of the object, which can have an impact on thermal sensations even during a brief contact. Just consider the temperature difference felt when touching an aluminum block and a piece of aluminum foil. Besides the issues with the semi-infinite body assumption, as discussed by Rykaczewski [1], the estimation of thermal contact resistance at the skin–object interface also requires further refinement. Currently, the models available for estimating thermal contact resistance are for metal-metal or gel-metal surfaces. Consequently, the accuracy of the thermal contact resistance estimation would deteriorate for contact materials that have lower thermal conductivity. In sum, challenges must be overcome before a model can take into account the influences of object geometry and biological factors and incorporate various contact materials in a thermal contact resistance estimation.

Over the past two decades, numerous advances have been made in modeling of thermal responses of the skin to contact for both engineering and safety purposes. Because of the difference in application focus, the models for these applications have been developed independently. Each of them has its own approach and limitations. Regarding directions for future research, communications between these related fields (e.g., between researchers interested in thermal perception, thermal regulation, and thermal displays) should be encouraged. Such an interdisciplinary knowledge exchange would especially advance thermal modeling for skin–object interactions as well as have a general impact on research on the dual nature (enteroceptive and exteroceptive) of the sense of temperature.