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1 / 8 Case Study: Non-Invasive Core Body Temperature Measurements

greenTEG AG

Technoparkstrasse 1 8005 Zürich, Switzerland

T: +41 44 632 04 20 F: +41 44 632 14 62

[email protected] greenTEG.com

Case study: Non-invasive core body temperature measurements

during sleep and daily life with greenTEG gSKIN®BodyTemp KIT.

Lukas Durrer and Michele Zahner

greenTEG AG, 8005 Zürich, Switzerland, Contact: [email protected]

1. Abstract

This case study presents body temperature measurements obtained with the gSKIN®BodyTemp KIT. The measurements were

taken on the forehead, on the chest and on the upper arm during sleep, daily life and physical activity.

The gSKIN® BodyTemp KIT is the world’s first device that enables:

Easy and accurate non-invasive and continuous core body temperature monitoring under free-living conditions

Detection of important thermoregulatory events

The gSKIN® BodyTemp technology presented in this study can be easily integrated into any wearable device and will enable:

Mobile sleep analysis

Early diagnosis of health disorders affecting body temperature

Heat stroke risk detection during sports and heavy work

Ovulation tracking

Investigation of thermoregulation and other physiological events

Patient monitoring

The current system works well on proximal measurement sites (chest, upper arm, forehead) and under varying environmental

conditions. The measurement accuracy drops during physical activity and on distal measurement sites (wrist).

greenTEG is currently working on an improved version of the sensor hardware and algorithm that includes additional sensors,

enabling accurate body temperature measurement at the wrist and during sports in the near future.

2. Introduction – relevance of core body temperature measurements

Early diagnosis of diseases is a powerful tool to increase quality of life and at the same time a promising way to stop or at least slow down the cost explosion in the health systems in developed countries. Wearable devices are seen as key technology for early diagnosis because they allow for continuously monitoring of key vital parameters of humans. Combining continuous monitoring of vital signs with state-of-the-art data analysis (big data or machine learning for example), early detection of health issues becomes possible. Besides heart rate, oxygen saturation, blood pressure and respiration rate, core body temperature (CBT) is one of the five most important vital parameters of a human being. While heart rate, oxygen saturation and respiration rate are commonly measured in the state-of-the-art wearable devices, non-invasive core body temperature and blood pressure sensors are not yet available. Several scientific studies have concluded that major health disorders such as chronic stress [Oka2001], insomnia [Lack 2008], Alzheimer’s disease [Musiek 2015] and Parkinson’s disease [Zhoung 2013] have repercussions on one’s core body temperature. CBT is also an important indicator for healthy people when they travel (jetlag [Kojima 2013]), for athletes willing to maximize their performance during competition [Rosa 2016], for detecting ovulation [Baker2001] or for preventing heat stroke. According to scientific literature, a key parameter for non-invasive and continuous core body temperature measurement is the heat flux from the human body to the environment [Gunga 2008] and [Niedermann 2014].

mailto:[email protected]


greenTEG AG


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greenTEG produces highly sensitive and robust heat flux sensors that are small enough to be easily integrated into any wearable

device and smart textile. For demonstration purposes, greenTEG has developed a demonstration system, the gSKIN®BodyTemp

KIT, for measuring core body temperature at the forehead and the chest.

3. Measurement equipment – The gSKIN® BodyTemp KIT

The gSKIN® BodyTemp KIT consists of a standalone three-channel data logger, up to three gSKIN® BodyTemp sensors and a

readout software for downloading recorded measurement data via a USB linked to a PC (Figure 1). The BodyTemp sensors are

provided with single-use dermal foam patches for a reliable attachment to the skin. This allows integrating the measurements

into the daily routine of a person.

Figure 1: gSKIN® BodyTemp data logger and sensor


greenTEG AG


T: +41 44 632 04 20 F: +41 44 632 14 62


4. Experimental

greenTEG has performed several measurement campaigns for validating the gSKIN®BodyTemp KIT. This case study reports three

measurements performed under different measurement conditions (see Table 1 for details):

Test 1: Controlled environmental conditions (night in a sleep lab)

Test 2: Free-living conditions

Test 3: Physical activity

The aim of these tests was to compare the core body temperatures calculated by the gSKIN® BodyTemp KIT placed at three

body locations (forehead, chest, upper arm) with an invasive reference measurement. An ingestible radio pill was selected for

this purpose (VitalSense Capsule by Respironics Inc., USA, Equivital™ LifeMonitor, Hidalgo Ltd, UK)

Measurement Procedure

Participants ingested the radio pill between 6 to 8 h before starting the test to ensure the pill to be located in the intestine

during the recording period. This measure minimizes temperature artifacts due to cold beverage or food intake. 15 minutes

before starting the test the three available sensors were placed on the skin at their respective measurement position using a

single-use foam patch provided by greenTEG. Once the gSKIN® BodyTemp KIT was set, the participant started the data

recording. An initial resting period of ten minutes was needed for the autocalibration of the sensors to apply (forehead and

chest). For distal measurement locations (i.e. upper arm), an inner body temperature value of 35°C was assumed as starting

point. The gSKIN®BodyTemp KIT continuously recorded every second.

The Equivital™ LifeMonitor (Hidalgo Ltd, UK) was used to record the data of the radio pill temperature. The participant started

the data collection simultaneously with the gSKIN®BodyTemp KIT, taking a measurement every 15 seconds.

Data analysis

The agreement of the reference signal (radio pill) and the BodyTemp signals was quantified by an analysis of the Bland-Altman type. The mean of the difference signal (bias) and the standard deviation of the difference signal (SD) were both calculated. The limit of agreement was defined as ±1 SD.


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Table 1. Test protocol and body locations selected for the gSKIN® BodyTemp KIT in the three experiments reported (sleep, free-living and activity).

Test 1: Controlled environmental conditions The first measurement was recorded during the night, under controlled environmental conditions in a sleep lab. The gSKIN®BodyTemp sensor was placed on the chest. The recording lasted 8 hours, starting from 11 PM. The participant spent the night in the sleep lab where he slept until 7 PM.

Test 2: Free-living conditions In the second experiment, another participant was wearing a gSKIN® BodyTemp sensor on the forehead and one on the middle of chest (sternum). The recording lasted 14 hours, starting from 5 PM. At the starting time the participant was still working in his office. The gSKIN® BodyTemp KIT allowed the participant to finish his work (5 PM to 6.30 PM), to go outside and travel by train (6.30 PM to 7 PM), to arrive home and perform his daily domestic routine as usual before he went to bed at 11 PM and woke up at 7 PM.

Test 3: Activity In the third experiment, another participant was wearing a gSKIN® BodyTemp sensor on the forehead, middle of chest and upper arm. The recording consisted of 30 minutes of sitting on a desk followed by 15 minutes of continuous stepping (step bench) exercise. Finally, the participant cooled down for 30 more minutes at the desk.


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5. Results and Discussion

Test 1: Controlled environmental conditions (sleep lab)

Figure 2 shows the raw output signals of the gSKIN® BodyTemp KIT (yellow line), the reference (radio pill, orange line) and the

skin temperature (blue line) measured at the chest. The gSKIN® BodyTemp sensors were positioned and the measurement

started just before going to bed. The core body reading catches up with the radio pill reference only at 11 PM because the

sensor takes 5 to 10 minutes to thermally equilibrate (on a covered chest sometimes up to 20min). After this equilibration

period the non-invasively measured gSKIN® BodyTemp signal follows the core pill signal with a bias of –0.16 °C and limits of

agreement of ± 0.25 °C.

Figure 2: Core body temperature measurements recorded with the gSKIN® BodyTemp KIT at the chest under controlled environmental conditions. Orange: Radio pill measurement (reference); Yellow: non-invasive core body temperature measurement with the gSKIN® BodyTemp KIT; Blue: skin temperature at the chest.


greenTEG AG


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Test 2: Free-living conditions

Figure 3 shows how the gSKIN® BodyTemp KIT (green line) performed under real free-living conditions in comparison to the

reference (radio pill, orange line) and the skin temperature (blue line) measured at the chest. The measurement started at 5 PM

in the office. At 6.30 PM, the person left the office and went home, where he spent the evening with his family. At 11 PM, he

went to bed. The non-invasive measurement has a bias of –0.13 °C and limits of agreement of ±0.28 °C.

Figure 3: Core body temperature measurements during day and night under free-living conditions recorded with the gSKIN®BodyTemp KIT placed at the chest and a radio pill.

Overall, more than 10 overnight measurements on forehead and chest under free-living conditions were made. On average, the

measurements had limits of agreement of ±0.25°C.

“This accuracy is perfectly adequate for clinical use” [Sessler 2008 citation of figure 2].


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Test 3: Compensation of environmental fluctuations

The main advantage of using heat flux sensors for CBT determination is the ability to compensate environmental influences very

efficiently (more than 80% compared to skin temperature measurements). This property can be easily seen in Figure Figure 3

(blue highlighted area). As the person went to bed and covered the sensor area (chest) with a blanket, the skin temperature

started to steadily increase from 34.5°C to 36°C. However, the BodyTemp KIT was able to identify this skin temperature increase

as resulting from a covering artifact and correctly predicted a slight decrease in body temperature, in agreement with the signal

of the radio pill.

The blue highlighted area in Figure 4 illustrates the behavior of the BodyTemp signal in case of faster and more pronounced

changes in environmental conditions. In this particular measurement, the participant exited a well-heated building (23°C) and

stepped outside (8°C), resulting in a rapid drop of the forehead skin temperature by 5.5°C (blue curve). greenTEG’s BodyTemp

sensor (Error! Reference source not found.a orange curve) was able to almost fully compensate the rapid temperature drop and

registered only a slight decrease of the (uncovered) head temperature. greenTEG is working on an algorithm for exposed

sensors that will tolerate rapid environmental changes even better. If the sensor is covered by clothing as the one on the chest

shown in Figure 3 (same person), the current algorithm is already capable of compensating the less pronounced fluctuations in

skin temperature.

Figure 4: Same measurements as shown in Figure 3 measured at the forehead.

Test 3: Activity

We would like to use the third test to illustrate the limitations of the gSKIN® BodyTemp KIT. As mentioned before, the algorithm

is optimized for low activity conditions, like sleep or working on a desk, and if the sensors are positioned on the forehead and on

the chest.


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Figure 5 presents the gSKIN® BodyTemp KIT readings at the forehead (red), chest (green) and upper arm (yellow) in comparison

to the radio reference pill reading for a well-defined activity test protocol (see figure 5 below) . 30 minutes after start time, all

four curves highly correlated during working on the desk. As soon as the physical activity started, the core body temperature

measured by the radio pill started to rise steadily, whereas the core body temperature obtained from the forehead and the

upper arm showed a slight initial decrease (probably convective cooling due to the body movement). However, the most

prominent artifact is the high overshoot of all three gSKIN®BodyTemp temperature signals to 41°C and 42°C, while the core

body temperature maximum observed by the radio pill did not exceed 38.5°C.

Figure 5. Activity test: Thermophysiological measurement with the gSKIN®BodyTemp KIT. Body temperature recorded during stepping activity (step bench) at the forehead (red), chest (green) and upper arm (yellow) in comparison to the reference ingestible thermometer enclosed in the radio pill (blue).

The thermoregulatory reactions of the body during physical activity are very complex. In order to detect and compensate for all

thermoregulatory events during activity, additional physiological parameters have to be integrated into the BodyTemp system.

These parameters can be measured using additional non-invasive sensors available commercially. These sensors signals are not

yet part of greenTEGs gSKIN®BodyTemp KIT.

6. Conclusions

The gSKIN®BodyTemp KIT from greenTEG is able to monitor core body temperature accurately, non-invasively and continuously

during sleep and the daily routine if sensors are placed on the forehead and on the chest. Although promising at this stage, the

use of the gSKIN®BodyTemp KIT for thermoregulatory research i.e. circadian rhythm and REM sleep phase detection requires

further verification.

Accurate core body temperature detection during increased physical activity and at distal body positions (upper arm and wrist)

requires a more elaborated algorithm in combination with additional sensors.

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