In vitro and skin patch sensitivity testing of a carbon nanotube-based wearable fabric sodium sensor

Carbon nanotube (CNT)-based materials are currently being developed for use in human health monitoring due to their high electrical conductivity and sensitivity. However, the skin irritation potential and overall safety of CNTs have not been fully characterized when used in wearable sensor applications. In this study, in vitro and human patch testing were used to determine the skin irritation potential of a wearable CNT-based sweat sodium sensor which was created for commercial applications. Human 3T3 fibroblast cells were exposed to the constituent components of the sensor and cell viability was evaluated using a regular toxicity (MTT) assay. The obtained results showed no statistical difference between the relative cell viabilities of the control cells and the cells exposed to the sensor components using a one-way ANOVA (p > 0.05). A modified leaching test was performed and determined that a very low concentration of MWCNTs of 0.0867 µg/mL, or 0.05% of the initial MWCNT concentration, was leached from the sodium sensor under wet conditions that mimic sweating. The concentration was found to be below previously reported toxic levels for MWCNTs. Pilot-scale patch testing trials were performed on thirteen subjects at rest and on eleven subjects during exercise to further support the in vitro skin irritation test findings. During the patch and exercise testing trials, the participants experienced either no irritation or minimal irritation (p > 0.05). Combining the in vitro testing, modified dynamic leaching test, human patch testing, and exercise skin irritation testing results, it can be concluded that the sweat sodium sensor was not a skin irritant in this use case. Further testing will involve reconstructed human epidermis or a 3D in vitro skin model and subsequent human patch and exercise testing on a wider demographic of participants are needed to validate these findings.