© IEEE 2020. Contact Burn Injuries Part II: The influence of object shape, size, contact resistance, and applied heat flux May Yen, Ph.D. Exponent Inc. Natick, MA USA myen@exponent.com Francesco Colella, Ph.D. Exponent Inc. Natick, MA USA fcolella@exponent.com Harri Kytomaa, Ph.D. Exponent Inc. Natick, MA USA hkytomaa@exponent.com Boyd Allin Facebook Inc. Seattle, WA USA boydallin@fb.com Alex Ockfen Facebook Inc. Redmond, WA USA alex.ockfen@fb.com Abstract—Increasing use of consumer electronics such as wearables brings new concerns associated with long duration, low temperature skin burn risk. Contact with these devices of low thermal mass results in the temperature of the device changing as energy is transferred from the device to the skin during contact. Current regulatory standards concerned with contact burn injury thresholds are designed assuming that the thermal energy in the hot contacting device is infinite and that the temperature of the object does not change significantly during contact. Furthermore, geometrical aspects of the contacting objects (i.e. contact shape, object size) and operational aspects (i.e. presence or absence of heat source associated with active components) are not accounted for in the standards. This paper is the second of a two-part series that discusses a numerical methodology that relies on the concept of cumulative equivalent exposure to evaluate contact burn injury thresholds. Part I described a burn injury model which numerically solves the transient heat transfer equation in living tissues and presents the burn injury threshold conditions associated with finite thermal mass objects. In Part I, the effect of a finite thermal mass is analyzed for an infinite plate of several finite thicknesses. In Part II, the sensitivities to object shape, size, thickness, contact resistance and applied heat flux are considered. Keywords— burn injury, modeling, cumulative equivalent exposure, sensitivity I. INTRODUCTION Part I of this paper series discussed the general aspects of the regulatory guidance for burn threshold surface temperature and contact duration limits [1,2,3] Part 1 also outlined a number of important aspects associated with the regulatory framework. Specifically, the ISO 13732 standard assumes that the surface temperature of the object remains constant after contact with the tissue. The ASTM standard recognizes that there exists a difference between the object surface temperature, the object- skin interface temperature, and skin contact temperature, which is defined as the temperature at the epidermis-dermis interface. All the standards assume the surface temperature of the touched object remains constant and neglect the surface temperature reduction associated with the transfer of energy from the object to the tissues. Furthermore, only a limited number of contact parameters are considered in the standard. They include the thermal resistance between the heat source and surface of the device and the influence of the surface finish and material. Part I outlined the limitation of the regulatory framework associated with long contact times where, according to the standards, a burn injury is always predicted regardless of material, finish or other factors such as the size of object [4]. This “infinite” contact time limit is demonstrably not valid for cases where the contacting object (and its surface temperature) cools due to the heat transfer to the skin. This is particularly true for low thermal mass objects and long duration exposures. Part II of the paper series addresses some of the additional limitations of the regulatory standards with regards to the impact on the time-temperature contact burn threshold of the object size and shape (i.e. large, circular, elongated), contact resistance with the skin and presence of an applied heat flux. The influence of object shapes and applied heat flux is of particular interest for the consumer electronics and wearable devices industry. The methodology followed in this study is largely similar to that discussed in Part I of this series [4]. The thermal damage assessment is based on the tissue temperature and the duration of the thermal exposure and is estimated using the concept of cumulative equivalent minutes at 43°C (CEM43°C) [5]. This model allows time-temperature history to be converted to an equivalent duration exposure at 43°C as: 43°C = ∫  43−()  , Eqn. 1 where CEM43°C is the cumulative equivalent minutes at 43°C, t is the duration of the thermal exposure, R is a constant (R (T<39°C)=0, R(T<43°C)=0.25, R(T>43°C)=0.5) and T is the temperature at the tissue. Large tissue-specific databases are available in the literature that summarize the relation between CEM43°C values and the observed damages to the tissues. In the case of the skin, most of the CEM43°C threshold values are based on the work of Henriquez and Moritz [6]. In this study, a 600 min CEM43°C for thermal damage threshold has been used as defined by the scientific literature [6]. II. MODEL In order to understand the influence of the object contact conditions on the propensity to cause a skin burn, a 2D heat transfer model was developed. As described in Part I of this study, the model solves for the conduction of heat from a hot contacting object into human tissue layers. The Pennes bioheat equation [7], shown in Eqn. 2 is numerically solved to simulate the evolution of the temperature distribution through the skin. The Pennes bioheat equation accounts for blood perfusion, in which blood flow through the skin carries heat away from the contact area, and metabolic heat generation effects in the dermal and hypodermal layers of the skin. The computational model integrates for CEM43°C as indicated in Eqn. 1. The model developed for this study was used to simulate the three geometry configurations shown in Fig. 1. The first