Kanezaki Akio, Hirata Akimasa, Watanabe Soichi, Shirai Hiroshi
National Institute of Information and Communications Technology, EMC Group, Tokyo, Japan.
Biomed Eng Online. 2009 Sep 23;8:20. doi: 10.1186/1475-925X-8-20.
Because the possibility of millimeter wave (MMW) exposure has increased, public concern about the health issues due to electromagnetic radiation has also increased. While many studies have been conducted for MMW exposure, the effect of dielectric permittivities on skin heating in multilayer/heterogeneous human-body models have not been adequately investigated. This is partly due to the fact that a detailed investigation of skin heating in a multilayer model by computational methods is difficult since many parameters are involved. In the present study, therefore, theoretical analyses were conducted to investigate the relationship between dielectric permittivities and MMW-induced skin heating in a one-dimensional three-layer model (skin, fat, and muscle).
Approximate expressions were derived for the temperature elevation and temperature difference in the skin due to MMW exposure from analytical solutions for the temperature distribution. First, the power absorption distribution was approximated from the analytical solution for a one-layer model (skin only). Then, the analytical expression of the temperature in the three-layer model was simplified on the basis of the proposal in our previous study. By examining the approximate expressions, the dominant term influencing skin heating was clarified to identify the effects of the dielectric permittivities. Finally, the effects of dielectric permittivities were clarified by applying partial differentiation to the derived dominant term.
Skin heating can be characterized by the parameters associated with the dielectric permittivities, independently of morphological and thermal parameters. With the derived expressions, it was first clarified that skin heating correlates with the total power absorbed in the skin rather than the specific absorption rate (SAR) at the skin surface or the incident power density. Using Debye-type expression we next investigated the effect of frequency dispersion on the complex relative permittivity of tissue. The parametric study on the total power absorbed in the skin showed that skin heating increases as the static permittivity and static conductivity decrease. In addition, the maximum temperature elevation on the body surface was approximately 1.6 times that of the minimum case. This difference is smaller than the difference caused by the thermal and morphological parameters reported in our previous study.
This paper analytically clarified the effects of dielectric permittivities on the thermally steady state temperature elevation and the temperature difference in the skin of a one-dimensional three-layer model due to MMW exposure.
由于毫米波（MMW）暴露的可能性增加，公众对电磁辐射引起的健康问题的关注度也随之提高。虽然已经针对毫米波暴露开展了许多研究，但在多层/异质人体模型中，介电常数对皮肤发热的影响尚未得到充分研究。部分原因在于，由于涉及众多参数，通过计算方法对多层模型中的皮肤发热进行详细研究存在困难。因此，在本研究中，我们进行了理论分析，以研究一维三层模型（皮肤、脂肪和肌肉）中介电常数与毫米波引起的皮肤发热之间的关系。
根据温度分布的解析解，推导了毫米波暴露导致皮肤温度升高和温差的近似表达式。首先，从单层模型（仅皮肤）的解析解中近似得出功率吸收分布。然后，根据我们先前研究中的提议，简化了三层模型中温度的解析表达式。通过研究这些近似表达式，明确了影响皮肤发热的主导项，以确定介电常数的影响。最后，通过对导出的主导项进行偏微分，明确了介电常数的影响。
皮肤发热可以由与介电常数相关的参数来表征，而与形态学和热学参数无关。利用推导得出的表达式，首先明确了皮肤发热与皮肤吸收的总功率相关，而非与皮肤表面的比吸收率（SAR）或入射功率密度相关。接下来，我们使用德拜型表达式研究了频率色散对组织复相对介电常数的影响。对皮肤吸收的总功率进行的参数研究表明，随着静态介电常数和静态电导率的降低，皮肤发热增加。此外，体表的最大温度升高约为最小情况的1.6倍。这种差异小于我们先前研究中报道的由热学和形态学参数引起的差异。
本文通过分析明确了介电常数对一维三层模型中毫米波暴露导致的皮肤热稳态温度升高和温差的影响。
