Non-Invasive Body Temperature Measurement with Skin Sensor

This article covers body temperature measurements attained with the gSKIN® BodyTemp KIT. The measurements were taken on the chest, on the forehead, and on the upper arm during sleep, daily life, and physical activity.

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

  • Easy and accurate non-invasive and continuous core body temperature monitoring under free-living conditions
  • Detection of vital thermoregulatory events

The gSKIN® BodyTemp technology discussed in this article can be easily combined into any wearable device and will enable:

  • Mobile sleep analysis
  • Ovulation tracking
  • Patient monitoring
  • Early diagnosis of health disorders affecting body temperature
  • Heat stroke risk detection during sports and heavy work
  • Investigation of thermoregulation and other physiological events

The present system functions well on proximal measurement sites (upper arm, chest, forehead) and under variable environmental conditions. The measurement accuracy declines during physical activity and on distal measurement sites (wrist).

greenTEG is presently working on a better version of the sensor hardware and algorithm that includes extra sensors, allowing accurate body temperature measurement at the wrist and during sports in the near future.

Introduction – Relevance of Core Body Temperature Measurements

Early diagnosis of diseases is a robust tool to boost the quality of life and at the same time an encouraging way to stop or at least decelerate the cost explosion in the health systems in developed countries. Wearable devices are seen as a core technology for early diagnosis as they enable continuous monitoring of main vital parameters of humans. Integrating continuous monitoring of vital signs with advanced data analysis (big data or machine learning for instance), early detection of health problems becomes possible.

Besides blood pressure, respiration rate, heart rate and oxygen saturation, core body temperature (CBT) is one of the five most critical vital parameters of a human being. While oxygen saturation, heart rate, and respiration rate are normally measured in the advanced wearable devices, non-invasive core body temperature and blood pressure sensors are still not available.

A number of scientific studies have determined that key health disorders such as insomnia [Lack 2008], Alzheimer’s disease [Musiek 2015], chronic stress [Oka2001], and Parkinson’s disease [Zhoung 2013] have consequences on one’s core body temperature. CBT is also a crucial indicator for athletes willing to make the most of their performance during competition [Rosa 2016], for healthy people when they travel (jetlag [Kojima 2013]), for detecting ovulation [Baker2001] or for preventing heat stroke.

According to scientific literature, the main 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]. greenTEG creates very sensitive and powerful heat flux sensors that are sufficiently small to be easily combined into any wearable device and smart textile. For demonstration reasons, greenTEG has created a demonstration system, the gSKIN® BodyTemp KIT, for measuring CBT at the forehead and the chest.

Measurement Equipment – The gSKIN® BodyTemp KIT

The gSKIN® BodyTemp KIT has 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 built with single-use dermal foam patches for a reliable attachment to the skin. This allows combining the measurements into the everyday routine of a person.

gSKIN® BodyTemp data logger and sensor.

Figure 1. gSKIN® BodyTemp data logger and sensor.

Experimental

greenTEG has conducted a number of measurement campaigns for confirming 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 positioned at three body locations (chest, forehead, upper arm) with an invasive reference measurement. An ingestible radio pill was chosen 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 hours prior to starting the test to ensure the pill is located in the intestine during the recording period. This measure reduces temperature artifacts due to cold beverage or food intake. Fifteen minutes prior to starting the test, the three available sensors were positioned on the skin at their respective measurement position using a single-use foam patch provided by greenTEG. Once the gSKIN® BodyTemp KIT was fixed, the participant started the data recording. An early resting period of 10 minutes was required for the auto-calibration 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 taken as starting point. The gSKIN® BodyTemp KIT uninterruptedly recorded every second.

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

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.
Controlled environmental conditions
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.
Free-living conditions
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.
Activity

 

Results and Discussion

Test 1: Controlled Environmental Conditions (Sleep Lab)

Figure 2 illustrates 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 began just before going to bed. The core body reading catches up with the radio pill reference only at 11 PM as the sensor takes 5 to 10 minutes to thermally equilibrate (on a covered chest at times up to 20 minutes). 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.

Core body temperature measurements

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.

Test 2: Free-Living Conditions

Figure 3 displays how the (green line) performed under real free-living conditions in comparison to the reference (gSKIN® BodyTemp KIT radio pill, orange line) and the skin temperature (blue line) measured at the chest. The measurement began at 5 PM in the office. At 6.30 PM, the person went home from the office. 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.

Core body temperature measurements

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.

Generally, more than 10 overnight measurements on forehead and chest under free-living conditions were done. 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]

Test 3: Compensation of Environmental Fluctuations

The key advantage of using heat flux sensors for CBT determination is the ability to compensate environmental impacts quite efficiently (over 80% compared to skin temperature measurements). This property can be easily noticed in Figure 3 (blue highlighted area). As the person went to bed and covered the sensor area (chest) with a blanket, the skin temperature began 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 appropriately predicted a slight decrease in body temperature, in keeping with the signal of the radio pill.

The blue highlighted area in Figure 4 shows the behavior of the BodyTemp signal in case of faster and more distinct variations in environmental conditions. In this specific measurement, the participant exited a well-heated building (23 °C) and stepped outside (8 °C), resulting in a fast drop of the forehead skin temperature by 5.5 °C (blue curve). greenTEG’s BodyTemp sensor was able to nearly fully compensate the rapid temperature drop and registered only a mild decrease of the (uncovered) head temperature. greenTEG is working on an algorithm for exposed sensors that will tolerate fast environmental changes even better. If the sensor is covered by clothing, as the one on the chest displayed in Figure 3 (same person), the current algorithm is already capable of compensating the less pronounced fluctuations in skin temperature.

Core body temperature measurements

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

Test 3: Activity

The third test is used to illustrate the limitations of the gSKIN® BodyTemp KIT. As stated before, the algorithm is enhanced for low-activity conditions, like sleep or working at a desk, and if the sensors are situated on the forehead and on the chest. Figure 5 signifies the gSKIN® BodyTemp KIT readings at the chest (green), forehead (red), and upper arm (yellow) in comparison to the radio reference pill reading for a precise activity test protocol (refer Figure 5 below). Thirty minutes after start time, all four curves highly correlated during working at a desk. As soon as the physical activity began, the core body temperature measured by the radio pill began to rise steadily, while the core body temperature attained from the forehead and the upper arm revealed a slight initial decrease (perhaps convective cooling because of 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 go beyond 38.5 °C.

Thermophysiological measurement with the gSKIN®BodyTemp KIT.

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 multifaceted. So as to detect and compensate for all thermoregulatory events during activity, extra physiological parameters have to be combined into the BodyTemp system. These parameters can be measured using additional commercially available non-invasive sensors. These sensors signals are still not part of greenTEGs gSKIN® BodyTemp KIT.

Conclusions

The gSKIN® BodyTemp KIT from greenTEG is able to monitor core body temperature accurately, non-invasively, and uninterruptedly during sleep and everyday routines, if sensors are positioned on the chest and on the forehead. Although encouraging at this stage, the use of the gSKIN® BodyTemp KIT for thermoregulatory research, i.e. circadian rhythm and REM sleep phase detection requires more verification.

Accurate CBT detection during increased physical activity and at distal body positions (upper arm and wrist) requires a more expanded algorithm in combination with extra sensors.

greenTEG AG.

This information has been sourced, reviewed and adapted from materials provided by greenTEG AG.

For more information on this source, please visit greenTEG AG.

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