# Improved Smart Pillow for Remote Health Care System
## 1 Introduction
### Evolution of Measurement of Core Body Temperature (MCBT)  
The human body can regulate heat production and heat loss in varying circumstances to maintain constant temperature, such that the balance between heat produced and lost can be represented by the means of body temperature. Core Body Temperature (CBT) [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12] is the temperature of the interior of the body, which is opposed to the peripheral temperature such as that of the skin. The traditional and relatively accurate method to measure CBT is by use of a rectal thermometer, which is invasive and inconvenient. As CBT is a vital parameter to determine the patient's health status, researchers work continuously to find non-invasive methods to measure CBT effectively and accurately. Zero-Heat-Flux (ZHF) and Dual-Heat-Flux (DHF) are two of the main methods that are extensively tested and productized. Zero-Heat-Flux (ZHF) is the method of measuring the deep body temperature. This is where an area on the skin without heat flux from the core to the surface is constructed,such that when thermal equilibrium reaches the peripheral skin temperature is the same as CBT theoretically. The method by [PERSON] et al. in 1973 [1] was incrementally improved or adapted by various researchers and manufacturers [2; 3; 4; 5; 6; 7]. The diagram of the original ZHF probe is shown in Figure 1, with the size of the probe about 6 cm \(\times\) 6 cm \(\times\) 0.6 cm. The upper side of the diagram will be the side closest to the skin. The probe is comprised of silicone rubber, with one temperature sensor \(T_{1}\) installed in the middle of the upper surface, the other temperature sensor \(T_{2}\) and a heater is embedded inside the rubber. They are all connected to hardware the control the heater output to ensure that the reading of \(T_{1}\) is equivalent to \(T_{2}\) over time. Silicone rubber is selected because it is easy to be shaped and its thermal conductivity is relatively low (approximately 0.1 W/mK) to avoid heat diffusion, so it provides a thermal insulation at the skin's surface to limit heat lost to the ambient environment. To measure core temperature accurately and reliably, the device requires a long time to reach thermal equilibrium between the various thermal compartments and the ambient environment. The conclusions reached by ZHF are slightly different from the evaluation of several researchers. [PERSON] et al. [1] said that the probe gives a temperature closely comparable with other methods for CBT in the resting subject, while [PERSON] et al. [2] believed that probe based on ZHF is not accurate enough for performing a single individual MCBT, yet it seems to be a valid, non-invasive alternative for monitoring circadian rhythm profiles. [PERSON] et al. [3] tested the SpotOn Prototype ZHF cutaneous thermometer by 3M [4] and agreed that CBT can be noninvasively measured using the ZHF method. [PERSON] [5] employed the CTM-205 by [PERSON] (Tokyo, Japan) and found that the application of the equipment is limited by its high cost and relatively slow response rate. Two temperature sensors are employed to make it a double-sensor thermometer. [PERSON] et al. [6] evaluated the accuracy of Tcore from Draeger [7] under heat stress, they found that the temperatures from Tcore were within \(\pm\)0.3 \({}^{\circ}\)C of the rectal measurement. Dependency on a heater element is the evident weakness of ZHF, so while it is non-invasive, its inflexibility makes it not economical. Based on heat transfer, an improved method named Dual-Heat-Flux (DHF) was proposed to overcome the issue brought by the heater of ZHF. As implied by the name, DHF means two heat flux channels are employed in comparison to one heat flux channel in ZHF. To construct two channels, four sensors are employed, and the height of channels must be different to form two discrete but comparable channels. The two channels could be integrated together or separated. An integrated probe is shown in Figure 2; with its shape resembling a hat.