The development of a washable and durable smart textile to measure electrodermal activity for early stress recognition

Abstract

This paper presents the results of the development of a new sock garment with integrated electrodes for monitoring physiological signals for stress detection in people with intellectual disabilities or dementia. Misunderstood stress-induced behaviours reduce the quality of life of these individuals and complicate caregiver support and treatment, as the correct interpretation of these behaviours. One of the physiological parameters most related to stress is electrodermal activity (EDA). It shows a direct response to the sympathetic nervous system activation ('fight or flight' response) in the form of a change in skin electrical properties such as skin conductance (SC) or skin impedance (SI). The phasic component of EDA is associated with short-term events and occurs in the presence of stimuli that control sweat gland activity. Therefore, analysis of this signal can be used as an indicator of emotional arousal or stress.To continuously measure EDA on an individual, a comfortable, durable, and easy-to-use carrier is essential. Current medical electrode patches (carriers) have limited user-friendliness because of their large shape and risk of skin irritation during extended use. Besides, the daily disposal of electrode patches would pose a major supply chain challenge and generate large amounts of medical waste. Furthermore, depending on the target group, classic wrist sensors may not be accepted by patients due to their discomfort and removed during recording. Considering the above limitations, a garment sock with integrated electrodes was proven to be the most efficient location in terms of signal quality, comfort, and an optimal alternative to standard medical electrodes. This allows the electrodes to be applied in one handling while maintaining permanent spacing and positioning of the electrodes on the skin. This garment can also be reused several times after regular washing cycles. Screen printing was chosen as a method for incorporating conductive electrodes onto garments. Conductive inks can be printed onto the garment directly or onto a thermoplastic polyurethane (TPU) film, which has been proven to be a suitable material for this type of integration. Screen printing onto these films offers both high flexibility and stretchability. The printing process allows the use of complex designs, such as stacking layers and printing dielectric insulating layers on top of the conductive layers. Different types of connectors were studied and designed to convert this stretchable film into a fixed connector tail with strain relief. Finally, test prints were made in a lab to validate each material and ink combination of silver, carbon, and dielectric inks. This aim was to achieve the desired robustness, and flexibility and to optimise the position of the sensors to achieve a good balance between patient comfort and good EDA signal output.The work showed that the use of advanced screen-printing technologies in the smart sock was the best solution to ensure high wear comfort while maintaining good signal quality even after repeated use and washing while maintaining low costs and high flexibility during production. In addition, the sheet-to-sheet production method proved to be cost-effective and enabled rapid changes in the material stack and sock design.