Flexible sensors that can be work on the body have been around for decades, but they usually start to degrade after repeated stress from mechanical deformation. Skin can stretch and bend along with muscle movements that most sensor materials can’t keep up with before starting to tear, rendering those sensors ineffective. Scientists have undergone extensive research to measure the strains for health monitoring systems using some materials that can detect high-level (40-100%) strains, such as movement of fingers and limb joints, others that detect mid-level (10-40%) strains, including swallowing and facial movements and still others sensitive to low-level manipulations (<1%-10%) that are capable of detecting pulses and vocal cord movement.
To retain those sensitivity levels without material degradation, scientists from the Terasaki Institute have developed a new flexible sensor that retains a high level of conductivity and sensitivity with minimal wear and tear. To achieve their goal, the scientist used a PEDOT: PSS, or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate top layer, which provides that aforementioned conductivity and sensitivity but rates at an insufficient level for flexibility. So the scientists turned to nature for answers and found that snakes can stretch to several times their size when devouring prey. Their scales allow them to do so as they overlap one another, and during feeding time, they slide past each other with skin interspersed among them.
That concept enabled the scientists to design their sensor, which takes that PEDOT: PSS top layer and affixes it to a bottom elastomer substrate. They then stretched the material to a 50% strain level, resulting in cracks and microscale pieces of material with interspersed PEDOT: PSS islands. They then added another layer of PEDOT: PSS material on top of the sensor and stretched it to a 100% strain level, which naturally aligned to the first layer, resulting in a highly conductive flexible sensor that helps mitigate tearing. The scientists state their new sensor could be used to monitor circulation and heart functions for users who have difficulty vocalizing or swallowing.