Development of a stretchable strain sensor for detection of facial expressions

Abstract

Recently, flexible and wearable electronic devices have been getting a lot of attention from the world due to their ability to interact with the human body. These devices can be easily mounted on clothing or directly adhered onto the skin of any part of the human body. Conventional sensors such as electrocardiogram and smart watches usually use fabrication methods that are complicated and expensive. Therefore, this project presents a low-cost fabrication technique called screen printing to fabricate a simple resistive type strain sensor based on silver (Ag) ink and Tegaderm film. These sensors fit with current stretchable sensor requirements such as being elastic, curvilinear, conforms to the skin conformity, and is biocompatible. In this work, stretchable strain sensors with a straight-line shape were developed and fabricated for facial expression detection. The design of the sensor was optimized and has dimensions of 30mm x 1mm x 0.0047mm. This sensor has been tested and was found to have good stretchability and sensitivity. Tegaderm film was used as the substrate as it can conform and adhere well to the skin. It has a very low Young’s Modulus of 4MPa, and superior stretchability of up to 300%, which is well above that of human skin of 30%. The electrode was fabricated using stretchable silver ink due to its cost-effectiveness, versatility, and high conductivity. A total of 30 healthy subjects of ages ranging from 20 to 45 years old were involved in the real-time experiment. The developed strain sensor was able to detect small strains induced by different emotional expressions which are Neutral, Smile, Sad, and Disgust when they were attached to the forehead, upper lip, lower lip, and left cheek. Sensors that were placed at the upper lip area showed the highest change in resistance and were very sensitive in the detecting the different human emotions. This work shows that stretchable strain sensors can be effectively used as a low-cost, easily portable method to detect facial expressions. When coupled with rehabilitative devices, these sensors can be used to determine whether the patients are having any pain or discomfort when exercising.