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Review Article

# Overview of Human Thermal Responses to Warm Surfaces of Electronic DevicesOPEN ACCESS

[+] Author and Article Information
Han Zhang, Alan Hedge

Department of Design and
Environmental Analysis,
Cornell University,
Ithaca, NY 14850

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received August 25, 2016; final manuscript received June 17, 2017; published online xx xx, xxxx. Assoc. Editor: Satish Chaparala.

J. Electron. Packag 139(3), 030802 (Jul 13, 2017) (9 pages) Paper No: EP-16-1100; doi: 10.1115/1.4037146 History: Received August 25, 2016; Revised June 17, 2017

## Abstract

Literature was reviewed and summarized on a few topics including: existing standards about the limits of devices' surface temperature, recent studies on the devices that caused discomfort and skin damage, human thermal sensation thresholds, the factors that affect thermal sensation, and the subjective ratings in the studies of thermal sensation. At the end, recent research on human subjective and objective testing was also summarized. The purpose of the review is to give an overview of cutaneous human thermal sensation and comfort, and how they are affected by the surface temperature of electronic devices.

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## Introduction

As technologies develop, electronic computing devices have become ubiquitous to the general population, such as laptops, tablets, other types of mobile computers, and wearable computers. Globally, over 2 billion people use tablet computers and laptop computers every day [1]. By October 2015, 45% of adult Americans owned a tablet computer [2]. Wearable computers are projected to be growing rapidly in the upcoming decade [3].

However, as the form factor of computers becomes smaller in size, heat dissipation has become a major limiting factor to both higher performance computing power, and users' thermal comfort [46]. Cases have been reported that users may feel uncomfortable, and even experience “toasted skin syndrome” when devices with heat are in contact with users' skin for hours [79]. Therefore, it is critical to understand user thermal comfort and efficient ways to dissipate heat.

This paper is an overview of the literature of thermal sensation and comfort and its relation to the surface temperature of electronic devices. It includes three major sections, current standards, and design guidelines about devices' surface temperatures, recent research on device-related discomfort and skin damage, existing literature on human thermal sensation thresholds, the related factors, and the subjective ratings in the studies related to thermal sensation.

## Current Standards on the Limit of Surface Temperatures

A few standards are widely used as references to prevent the risks of the burns of the skin. The upper limit of surface temperature is defined to be 45.0 °C for prolonged contact in most standards [1012]. Similarly, when in contact with a surface, 44 °C (temperature at the human skin) was also considered to be safe not to cause any reversible skin injury for a short period of 6 h or less [13]. In addition, the standards also provide information about the temperature limits with different materials, including coated and uncoated metal, wood, plastic, and ceramic materials. It is shown that for a shorter contact of 1 min, the surface temperature burn thresholds are higher for wood and plastic surface at 60 °C, while the thresholds were lower for uncoated and coated metal surfaces at 51 °C [11]. However, when the contact period increases to 10 min, the burn thresholds for surfaces of all the materials are 48 °C [11]. The threshold value decreases to 43 °C for 8 h and longer contact. Besides the burn thresholds, ISO 13732-1 also provides guidelines to assess burn risks, task analysis, and protective measures [12].

Information related to human sensation in response to surfaces with moderate temperatures is also provided in the standard of ISO TS 13732-2 [14]. ISO 13732-2 specifies thermal sensation information when hands are in contact with hand rail or door knobs. For an initial contact with a surface of 40–50 °C, people feel the wood and plastic to be less warm than steel and aluminum. However, no information is provided about thermal responses to prolonged contact with warm surfaces in the standard. In ISO 13732-2, a few factors that might affect thermal sensation were introduced, such as body part thermal state, skin temperature, environmental temperature, type of object, heat source, contact coefficient and thermal diffusivity, contact duration, and contact pressure. ASTM C1055-2014 also includes information on the surface temperature ranges and related thermal sensation.

## Heat as a Cause of Discomfort and as a Way of Interaction

###### Device-Caused Discomfort and Skin Damages.

To understand how prevalent thermal discomfort is among laptop users, Zhang and Hedge [9] surveyed over 100 normally working laptop computers in the field, and correlated the surface temperature with users' thermal discomfort ratings. It was found that participants' thermal discomfort had a positive relationship with the maximum temperature of the bottom surface. Overall, about 20% surveyed users reported discomfort in the thigh area. The bottom surface temperature was measured with a calibrated infrared (IR) camera and a surface thermometer. Maximum temperature spots were found from the IR image analysis. The maximum bottom surface temperature of a laptop ranged from 22 °C to 45.4 °C. Finally, a few factors that were tested to be significant in the statistical model that are related to the maximum surface temperatures of a laptop include the years of manufacturing, screen size, the use of a power cord, and the software program that was running. In addition to the survey results, a few potential factors such as the thermal solutions and the design of laptops can also affect surface temperature.

Besides thermal discomfort, multiple cases of skin damage because of the prolonged use of laptops have been reported in medical studies [1520]. The skin damages ranged from a deep skin burn to a more common condition known as “erythema ab igne,” characterized as reticular, pigmented skin lesions [7]. Further, prolonged use of a hot laptop can increase the thigh temperature, leading to an increase in scrotal temperature up to 2.8 °C [21].

Erythema ab igne, or toasted skin syndrome, is caused by repetitive exposure to mild heat that is either cutaneous or radiation [7,22]. Heat ranging from 43 to 47 °C usually can cause this condition [7,22]. Historically, erythema ab igne has been found among elderly patients and people in cold climates being too close to hot devices such as a fireplace, heater, radiator, heating blanket, and stove [17]. However, in recent years, erythema ab igne has been reported with mobile devices, such as laptops [7] and cell phones [23]. Between 2004 and 2012, 15 reports on laptop-induced erythema ab igne were found in different languages (English, French, German, Portuguese, and Swedish journals) [7].

The typical cases reported for laptop-induced erythema ab igne are mostly young adults with an age of 26 or younger. However, only a minority are middle school students [7]. A possible reason is that middle school adolescents are more likely to use a desktop at school or home instead of a laptop [7]. It is commonly observed that erythema ab igne or skin burn appears after the user's use of a laptop on the thighs for 2–3 h per day for a period of time. The patients tend to be college students or similarly aged information technology workers [7].

More serious skin damages caused by laptop computers have been reported, such as second- and third-degree skin burns [16,19,2426]. For example, a patient had a deep second-degree skin burn on the right thigh after he left a laptop on his thighs for 6 h [16]. Under optimal ventilation, on a hard surface desk, the laptop bottom's average maximum surface temperature was measured to be about 37.2 °C, which is lower than the superficial skin burn threshold [22]. However, when ventilation is compromised, the surface temperature of the bottom of a laptop can increase rapidly. For example, in the case reported in Ref. [16], the patient's thigh blocked the hot air flow of the ventilation exhaust and thus it led to a much higher surface temperature [16]. Similarly, three more cases were reported where patients had full thickness skin burns that required surgery [19,24,25]. Tsang et al. reported that a patient fell asleep and left the computer on his lap and woke up 3 h later with a right thigh burn [24]. Paprottka et al. reported that a wheelchaired patient with paraplegia had second- and third-degree burns on both of her feet after 1 h using a laptop on her lap [25]. A laptop power adaptor was reported to lead to a young man's full-thickness burn on his lower leg [19].

###### The Application of Thermal Feedback in Human–Computer Interaction.

Temperature changes from electronic devices have been used as thermal feedback for human–computer interaction. Thermal feedback has the potential to express emotional messages and to be used as a notification [27]. In a virtual reality environment, thermal feedback was used for users to recognize different materials such as copper, aluminum, brass, and bronze [28]. The thenar eminence is optimal for thermal feedback, and the rates of temperature change of 1 °C/s and 3 °C/s of are necessary for participants to perceive the signals [29]. For the cutaneous sense, when the skin temperature is below 30 °C, people perceive constant coldness; and when it is above 36 °C, people sense warmth [30]. Thermal feedback can also help with emotional expression, and the haptic modality increased closeness [27,31]. More specifically, warm and cold thermal feedback can indicate positive and negative meaning [27].

## The Thresholds of Cutaneous Thermal Sensation and Related Factors

###### The Thresholds for Non-Noxious Warmth Sensation and Noxious Heat Sensation.

Depending on the body sites and testing methods, the thresholds for a person to start to have warm sensation are between 33 °C and 35 °C [3237]. When the local skin temperature reaches 42–45 °C, people start to have pain sensations. The threshold for thermal discomfort is 43 °C when human skin is in contact with copper or aluminum [37,38]. In general, the threshold for heat pain is consistent between 44 °C and 45 °C [12,36,39,40]. However, body sites can affect the pain threshold. For example, Defrin et al. found that the average pain threshold was 42 °C for chest skin, but was 44.5 °C for foot skin. In some studies, the heat toleration threshold is defined as tolerance threshold, meaning the participants are exposed to stimuli until they could not tolerate [4143].

Skin burn can happen at temperatures close to or overlapping with the pain threshold for the human, ranging from 42 to 47 °C [4447]. With a prolonged contact with local warm surfaces, a skin burn can happen with temperatures above 43 °C [46]. However, for a short-period contact that is less than 10 s, the skin burn threshold may vary widely according to different contact surface materials. For example, in ISO 13732-1, it is shown that a skin burn will not occur at a temperature of 55 °C, when the skin is in contact with bare uncoated metals for 10 s. However, the threshold can be increased to over 65 °C when skin is in contact with smooth ceramic, glass, and stone for 10 s because of the lower thermal conductivity of these materials.

###### The Factors That Affect the Thresholds of Warm Sensation or Heat Pain.

The testing results for thermal sensation vary significantly among different laboratory studies. For example, the threshold for warm sensation on the chest can range from 34.6 to 36.2, depending on the testing methods [36]. The pain threshold can vary from 37.5 °C for the maxillary facial skin to 45 °C on forearm [48,49]. Multiple studies have found that the temperature thresholds for warm or heat pain can be lower for proximal than distal body parts [32,50]. However, Hagander et al. (2000) did not find significant difference between proximal and distal skin locations [34].

Multiple reasons can contribute to the inconsistency in the results, including types of stimuli, testing methods, experimental conditions, individual differences, skin area, age, gender, and body sites, type of device, rate of temperature change, probe size, psychophysical algorithm, race, body mass index, smoking/alcohol consumption, and local skin temperature [34,36,5153]. Sections 4.2.14.2.14 will provide an overview of the effect of each factor on thermal sensation.

###### Testing Procedure.

Some researchers have used the “methods of limits” (MLI) to test local thermal sensation thresholds [3234,43,48]. Others have used the “methods of levels” (MLE) for thermal sensation testing [3436]. In the MLI, participants are in contact with the stimulus while the stimulus's temperature increases from skin temperature until participants begin to have thermal sensations. In the MLE, participants are tested with multiple levels of stimuli, to approach a level where participants start to have a certain thermal sensation. In general, MLI is a faster test method but it may provide higher absolute thresholds; MLE takes more time but may be more accurate. In clinical applications, both of the two methods are used [54].

###### Rate of Temperature Change.

The rate of temperature change can affect the thermal sensation threshold, but the effect varies among studies [32,5558]. In general, a faster rate of change such as 1 °C/s and 3 °C/s may lead to an overestimated threshold for thermal sensation or heat pain [32,56]. However, the effect was minimized if MLI was used for testing instead of MLE [57]. A slow rate of temperature change such as 0.02 °C/s may lead to a more comfortable perception of heat than a faster rate such as 0.15 °C/s [58].

###### Stimulus Duration.

In the range of 2.5–10 s, the increase of the duration of a stimulus was shown to decrease the threshold of heat pain [32,57]. However, in a recent study [58], thermal sensation did not change significantly with the increase of contact duration.

###### Stimulus Size.

The heat pain threshold tends to decrease as the area of heat stimulation increases [34,35,48,59]. Meh and Denišlič showed that with a probe surface of 25 × 50 mm, the heat pain threshold at the thenar eminence was 37.7 °C for female participants and was 40.2 °C for males [48]. While the heat pain threshold was at 43.4 °C with a probe surface of 30 × 30 mm [34] and was 45.6 °C with a probe surface of 25 × 25 mm [35]. Similarly, at the lower medial calf, a larger thermode (12.5 cm2) produced a significantly lower threshold of the mean warm threshold of 35.5 °C than a smaller thermode (3.75 cm2) of 36.5 °C [59]. A larger thermode was also shown to produce lower thresholds in large-scale studies [32,59,60].

###### Local Skin Temperature.

Local skin temperature does not significantly affect the thermal sensation threshold [34,48,57], or the heat pain threshold [61,62]. Local skin temperature ranging from 27 °C to 37 °C have no significant effect on the warm thermal threshold and only shows a minor effect on the cool thermal threshold [34].

###### Skin Type.

Glabrous skin, in general, have a higher heat pain threshold than hairy skin [49,57,63,64]. More specifically, glabrous skin areas such as the thenar eminence and pedis plantum have approximately 3 °C higher heat pain threshold than hairy areas, such as the dorsal lateral forearm and lateral calf at the leg.

###### Gender.

Studies have shown inconsistent results of thermal thresholds on the effect of gender [33,35,52,65,66]. A few studies showed that gender did not affect the pain thresholds [3537,52,59]. However, Harju et al. found a gender difference in heat pain threshold on the upper arm. Other studies also found that females showed greater thermal pain sensitivity than males [48,67,68].

###### Age.

Thermal sensitivity, defined as the ability to differentiate temperatures, decreases with the increase of age [50,69]. Stevens and Choo tested participants' thermal sensations by using a temperature stimulator, with a baseline temperature of 33 °C in a range of ± 10 °C [50]. An increase in age was associated with a decrease in the acuity of differentiating thermal change, especially in the feet and the belly. The whole body thermal sensitivity was tested, including finger, thenar, forearm, upper arm, cheek, lips, lower back, belly, thigh, calf sole, and toes. In general, peripheral body parts tend to have a worse deterioration in thermal acuity than central parts, and the researchers suggested slowing of peripheral circulation because of aging. The cutaneous thermal sensitivity was also shown to linearly decrease with aging, from years of 21 to 92 [69].

###### Height.

Participant's height does not have an effect on thermal sensation thresholds except a weak correlation with heat pain threshold on the hands (Pearson correlation r = 0.312, p = 0.037) [35].

###### Body Sites.

In general, face is the most heat-sensitive body part, followed by trunk, hands, and arms, and the lower extremities have similar or lower sensitivities than upper extremities [32,34,49,50,54,56].

More specifically, for the general population, the heat pain threshold is the lowest at the chest with an average temperature of 42 °C, and highest at the foot with a mean temperature of 44.5 °C [36]. The warm sensation threshold is higher at the lateral upper chest with an average of 34.8 °C, and at the feet ranges from 34.6 °C to 35.7 °C [32,35,36]. The warm sensation threshold wrist and hand area ranges from 32.7 °C to 33.9 °C [36,66]. The above thresholds were mostly derived from methods of levels. In most studies, face and trunk tend to be more sensitive to thermal stimuli and have lower thresholds than the arm and hands, while the legs and feet might have similar or lower sensitivities [50,56].

###### Neurological Conditions.

Neurological conditions can affect the thresholds of thermal sensation, such as stroke, diabetes, white fingers, and spinal cord injuries [7075]. For most of the cases, patients with conditions will have higher heat paint thresholds. For example, patients with vibration-induced white finger have a higher threshold of heat pain at 42.1 °C versus 39.9 °C for the control group [72]. Patients with small fiber neuropathy have a higher warm sensation threshold of 38.1 °C versus 35.14 °C for the control group in hand area [74].

###### Mechanical Contact.

The mechanical contact with a thermal stimulus affects how people perceive the intensity and quality of the heat. When participants held and released warm stimulus ranging from 38 °C to 43 °C in hands, they reported less intense than the condition when they held the stimulus constantly [76].

On the other hand, the tactile information such as roughness and vibrotactile sensitivity are also moderated by skin temperatures [7683]. Commonly, the tactile information such as roughness will be degraded with the decreases of skin temperature, and upgraded with the increase of skin temperature [77,82].

###### Materials.

Materials, including the contact surface of a stimulus and the clothing materials between a stimulus and the skin, can affect thermal sensation and comfort [41,46,8486]. A few key factors such as materials' thermal conductivity, specific heat, and specific gravity can significantly change thermal sensation [86]. A few surface materials of stimuli, such as wood, aluminum, nylon, and steel have been tested when people were touching handrails [46,84]. Aluminum was perceived to be warmer than wood or nylon surfaces when surface temperatures were above 30 °C. Webb tested astronauts about conductive heating pain and burns with metal surfaces [46]. A variety of body areas were tested, including hand, kneecap, fingertip, hand palm, forearm, and upper arm. Clothing was also tested, ranging from bare skin to different suits. It was shown that the tolerance temperature varied greatly depending on the types of clothing [41]. Elbow (with suits) and knees (bare skin) can even get second-degree skin burns without the sensation of heat pain [46]. Halvey et al. also found that clothing (cotton and nylon) can reduce a stimulus's intensity [85].

###### Ambient Temperature.

Ambient temperature has been proved to be a significant factor that affects cutaneous thermal sensation and comfort [37,8789]. In general, cool environment (13–15 °C) can reduce the intensity of the hot stimuli, while warm environment (33–35 °C) can increase the intensity and unpleasantness [37,87]. A significant interaction effect was found for thermal sensation in tested hand areas between indoor temperature and surface temperature [37].

###### Subjective Ratings to Measure Warmth Sensation and Heat Pain.

In psychophysics and psychological studies, subjective rating scales are commonly used to understand participants' feelings in contact with thermal stimuli, at both innocuous and noxious temperatures. Among pain scales, the visual analog scale (VAS) and numeric pain rating scale have been widely used for both patients' pain due to disease and experiment induced pain. For both acute and chronic pain, these two scales are equally sensitive and either can be used [75].

Subjective ratings can present richer information than thermal thresholds. For example, although gender did not affect the pain thresholds, females tend to rate higher pain for all body sites than males [35]. Pain ratings were consistent among body sites, although there was greater variability for noxious than innocuous temperature thresholds. Similarly, Harju found no age or gender differences for warmth and heat pain thresholds for the thenar, foot, and knee, except there was a difference for the upper arm. In contrast, subjectively perceived intensity was varied by age, gender, and specific body areas.

Strigo et al. used a VAS to measure participants' responses of perceived intensity and unpleasantness for thermal stimuli, with 0 as no thermal sensation or no pain, and 100 as extremely unpleasant or extreme pain [87]. Green had used a seven-level general labeled magnitude scale to determine the intensity and quality of thermal sensations induced by innocuous stimuli, ranging from “no sensation” to “strongest imaginable sensation” [76]. Ray used a five-point thermal comfort scale to record participants' ratings to thermal stimuli [90]. Zhang et al. used a mixed VAS and labeled scale to derive ratings of thermal sensation and thermal comfort from participants [37].

## Recent Research on Device-Related Thermal Comfort and Future Research Opportunities

###### Human Subject Testing and Mathematical Modeling on Local Sensation and Local Comfort.

Local thermal comfort and local thermal sensation are affected by the ambient environment a person is in, and by the stimuli a person that is in contact. Multiple studies have integrated human subject tests to the mathematical prediction models of thermal comfort and thermal sensation [9193]. For example, a series of studies [91,92] have shown that the local thermal sensation acts a function of whole body temperature, local skin temperature, the rate of temperature change, and the body temperature over time, as expressed in formula (1). Researchers demonstrated that the above factors are positively correlated with local sensation [91]. In the formula, $Tskin,i$ means skin temperature, means the skin temperature at certain body part I, and $Tcore$ means core temperature [91]. Local sensation was measured in the ASHRAE 7-point scale, with “very hot” and “very cold” as two additional descriptor points Display Formula

(1)

Similarly, local thermal comfort has been modeled and validated in a follow-up study [92], as shown in formula (2). Local sensation is an independent variable for the logistic model for local comfort. In the formula, offset means local sensation at which maximum comfort occurs, and maxcomfort means the maximum comfort when local sensation equals to the offset [92]. Local comfort was also measured in the ASHRAE 7-point scale, with “very uncomfortable” and “very comfortable” as two additional descriptors Display Formula

(2)$Local comfort=Logistic function (Local sensation+offset)+maxcomfort$

Researchers have also explained thermal sensation from a neurophysiological perspective [93]. Kingma et al. [93] have utilized the predicted mean vote model and the conceptual model from neuropathway, such as neurons' discharge rates and skin temperature to predict thermal sensation rate. The thermal sensation model is a validated regression model with multiple coefficients Display Formula

(3)$S=β0+β1Hwarm+β2Pwarm$

Similar to the model (1), thermal sensation was also correlated with core temperature and skin temperature. In the model, $S$, defined as thermal sensation, was measured in the ASHRAE 7-point scale. $Hwarm$ was defined as the neuron discharge rate for core temperature. $Pwarm$ was defined as the average discharge rate of warm thermoreceptors at skin. The model and the regression coefficients were tested to be statistically significant. The coefficients are $β0=−44.0±32.8$, $β1=−14.3±9.0$, and $β3=−2.3±0.9$. The model's $r2=0.89$, with a $p−value<2.5×10−8$.

More recent human subject studies [37,58,94] have demonstrated more specifically how electronic devices can affect local thermal sensation and thermal comfort, which also supports the results from the mathematical predictions from the previous studies [9193]. Figure 1 shows a human subject test with a simulated tablet in an environmentally controlled chamber [37]. More specifically, researchers have found an interaction effect of surface temperature of a tablet computer and the ambient temperature on local thermal sensation and comfort on hands, as shown in Figs. 2 and 3 [37]. Both the surface temperature of the tablet and the ambient temperature have a significant effect on thermal sensation and thermal comfort on the fingers. In the range of 34–44 °C, as the surface temperature increases, the average thermal sensation on fingers increases from neutral to hot. The trend is similar across the tested environmental temperature, although a hotter stimulus was reported to be significantly less hot in a lower ambient temperature of 13 °C than in a higher ambient temperature of 33 °C [37].

Similarly, as the surface temperature increases, the average thermal discomfort score on fingers increases from neutral to uncomfortable. However, the neutral threshold is different among the ambient temperature. The neutral rating is about 42 °C at 13 °C, about 40 °C at 23 °C, and about 38 °C at 33 °C. The highest discomfort ratings were also lower at a lower ambient temperature.

The scores in Figs. 1 and 2 were measured in 0–100 visual analog scale, different from the s-point ASHRAE scale. However, the verbal descriptors are the same as the models (1) and (2) from previous studies [91,92].

As shown in the functions (1) and (2), the rate of temperature change ($dTskin/dt$) can also affect thermal sensation, and thus affect thermal comfort. This was also demonstrated by a recent study on the effect of the rate of temperature change tested with a table computer [94]. In the study, participants were required to hold a simulated tablet computer that is heated in the peripheral areas as shown in Fig. 4. Four conditions with different rate of temperature change were tested, as shown in Fig. 5. The results showed that a slower rate of temperature change of 0.02 °C/s led to a higher thermal comfort, as shown in Fig. 6. The findings suggest a potential method to improve thermal comfort by dissipating tablet computer heat at a lower temperature change rate.

###### Quantitative Modeling and Experiment on Skin Temperatures.

Research has rarely been conducted on quantitative modeling on device-related thermal comfort. One of the first known studies to characterize finger's heat transfer with a mobile phone's surface is by Sripada et al. [95]. The researchers developed a gel finger and a multilayer heating surface to simulate the process of human hand in touch with a mobile phone, as illustrated in Figs. 7 and 8. Multiple surface materials such as aluminum and polycarbonate were tested to measure the change of temperatures on the device surface and in the gel finger over time.

Simulation results were curve fitted with experimental data for verification. Figure 9 is an example of the temperature measurement in the three holes of the gel finger with in contact with aluminum. The experimental data and simulation results matched well. The gel finger was proved to be able to quantify the temperature change and required power for the heated simulated phone surface. In addition, the results showed that the power levels were different between materials for the gel finger to reach to 52 °C. An aluminum surface needs 3.9 W, while a polycarbonate surface needs 1.8 W, as shown in Fig. 9.

The advantages of the measurements from such a simulated finger are repeatable, objective, and efficient, compared to the human subjective testing. However, more works can be done to correlate the temperature change with human perception directly.

###### Future Research Opportunities.

The packaging of electronics is becoming smaller and thinner, while the processors are increasing in power. Heat dissipation has become a major limitation for computing in small form factors. The limitation is likely to continue to be challenging in the near future. To solve such challenges, research opportunities exist in a few of the following areas: correlating thermal sensation and comfort between quantitative modeling and objective testing devices; how thermal comfort can be affected by smaller packaging form factors; and how to dissipate heat more efficiently for higher performance computing technologies.

###### Human Subjective Testing and Objective Testing Device.

One of the possibilities is to correlate the existing research findings from human subject tests with simulation devices. Traditionally, in the fields of ergonomics, psychophysics, physiology, and biological science, thermal sensation and comfort have been studied in depth from the perspectives of human physiology, animals, and human's interaction with electronic devices [1126]. On the other hand, in the field of electronic packaging, research is more focused on packaging technologies [6]. Only recently, research has emerged from the fields of ergonomics and electronic packaging to study the effect of the surface temperature on user thermal comfort in the range of moderate heat [9496].

The data from recent emerging research in the users' responses of the heat from tablet devices [37,94] can be correlated with the gel finger temperature change over time [95].

###### Thermal Comfort With Smaller Packaging Form Factors.

Some wearable computers have shown overheating issues when videos were played for a prolonged time [8]. Wearable computers such as head-mounted or wrist-worn require the form factors to be smaller or thinner than mobile phones, thus posing a more challenging problem for heat dissipation. It can be anticipated that more issues and needs would appear for computers in smaller sizes than currently popular computers such as tablet and mobile.

###### Innovative Heat Dissipation Technologies.

There have been many well-established ways to improve energy efficiency and to dissipate heat effectively for handheld devices. A common thermal solution to control the peak mobile computers' skin temperature and to improve thermal comfort is to use thermal interface materials, such as copper foil for passive cooling [97]. Different combinations of materials and dimensions of heat spreaders, case materials, air gap thickness and, etc., have been used for varied applications [98]. Frequency control algorithms have also been developed [4,5] to control the duty cycling to optimize power consumption. There have also been efforts to apply new technologies such as phase-changing materials to help with heat dissipation [99101].

Recent ergonomics studies have shown alternative possibilities to improve local thermal comfort from human factors perspectives. For example, using material's textures with coarse patterns and a slower rate of temperature change may improve thermal comfort [94,96]. In the future, innovative solutions with the new technologies to achieve heat control and dissipation to accommodate the preference of human comfort as suggested by previous studies will help to dissipate heat more efficiently while improving user thermal comfort. For example, technologies such as phase-changing materials [99101] and frequency control algorithms [4,5] can be used to control temperature change rate to achieve the improvement of heat dissipation designs from the users' perspective.

## Conclusions

A comprehensive literature review was conducted on standards about surface temperature limits, studies about thermal comfort and skin damages, and different aspects that may affect thermal sensation, such as age, body sites, and stimuli size. Average thresholds for thermal sensation and heat pain were also discussed on the body parts that might be frequently in contact with electronic devices, including hand areas and leg areas. Subjective ratings were introduced to measure warm sensation and heat pain. At last, recent research on human subject testing and human-related thermal measurement was summarized, and future research opportunities were discussed.

## Funding Data

• Intel Corporation (Grant No. 69282/A001).

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Topics: Heat , Temperature , Skin , Testing
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## Figures

Fig. 1

A participant holding the heating surface (left) and the IR image (right) [37]

Fig. 2

Fingers thermal sensation scores with surface temperature [37]. X-axis shows the back-surface temperature of the simulated tablet computer ranging from 34 °C to 44 °C and Y-axis shows the participants rated scores on fingers' thermal sensation. The lines in different patterns show the responses in different ambient temperatures of 13 °C, 23 °C, and 33 °C. Error bar is 1 standard error from the mean.

Fig. 3

Fingers thermal comfort scores with surface temperature [37]. X-axis shows the back-surface temperature of the simulated tablet computer ranging from 34 °C to 44 °C and Y-axis shows the participants rated scores on fingers' thermal comfort. The lines in different patterns show the responses in different ambient temperatures of 13 °C, 23 °C, and 33 °C. Error bar is 1 standard error from the mean.

Fig. 4

Heated areas for different heating conditions [94]: (a) the constant heated tablet back, and the two shades at both the left and right sides show the heated areas and (b) the heated areas that switched between left and right sides with shades

Fig. 5

Thermal sensation and comfort rating change over time and the designed surface temperature change [94]. Each graph shows the trend of the surface temperature as constant a temperature (A), 0.02 °C/s change rate (B), at 0.15 °C/s change rate (C), and at 0.15 °C/s change rate at two sides (D).

Fig. 6

Overall rating on thermal sensation scores and thermal discomfort scores across all four conditions [94]. Error bars indicate standard errors.

Fig. 7

The structure of Gel finger and the setup of the test on a simulated smartphone [95]

Fig. 8

Basic layers of the test phone (a) and a gel finger with temperature measurement at three holes (b) [95]

Fig. 9

An example of the comparison of simulation results and experimental data in the gel finger [95]

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