Validation of the iButton and Flex sensors for measuring skin temperature

Purpose: Skin temperature is a critical component of athletes’ heat strain during exercise in the heat. This study evaluated the innovative Flex sensor (BodyCap, Caen, France), allowing real-time measurements, against a data logger (iButton hygrochron DS1923, Analog Devices, Wilmington, USA), commonly used in sports science. Methods: In a first study, the accuracies of iButton faces as well as the Flex sensor were tested on a constant-temperature Peltier module affixed to a brass plate. In a second study, concurrent skin temperature measurements were conducted with both sensors on eight participants cycling in hot conditions. Results: Both faces of the iButton underestimated plate temperature, with the base (ME = mean error: 0.59 ± 0.25 °C at 30 °C, 1.62 ± 0.48 °C at 40 °C) being significantly more accurate than the top (ME: 1.14 ± 0.26 °C at 30 °C, 3.02 ± 0.71 °C at 40 °C, p < 0.001). The rate of temperature increase of the base was also higher than the top at 30 °C (1.97 ± 0.16 °C/min vs 1.49 ± 0.15 °C/min) and 40 °C (4.41 ± 0.46 °C/min vs 3.14 ± 0.16 °C/min, both p < 0.001). The Flex sensor underestimated plate temperature (ME: 1.31 ± 0.32 °C at 30 °C, 3.00 ± 0.84 °C at 40 °C), showing larger errors than the iButton base (p < 0.001). During exercise, the Flex sensor (36.91 ± 1.19 °C) measured higher skin temperature compared to the iButton (36.50 ± 1.15 °C), with an ME of -0.41 ± 0.55 °C. Conclusion: In the absence of a gold standard, for scenarios where real-time monitoring is not required, using the iButton with its base applied to the skin is therefore recommended. To enhance the Flex sensor’s accuracy, implementing a calibration protocol before use is strongly suggested, enabling its full potential for real-time monitoring.