Wrinkled Parylene Coating for Implantable Strain Sensor Encapsulation

Implantable strain sensors integrated on organ surfaces can monitor organ deformations, such as the filling and emptying of the bladder and the motility of the stomach, thereby providing important information about their functional states. A major challenge lies in achieving a large strain range while ensuring long-term stability inside the body fluid environment. Commonly used materials to achieve large stretchability, such as soft elastomers, have relatively high water permeability, which can lead to degradation of sensing performance and pose safety risks. The biocompatibility requirements present additional material challenges to stretchable encapsulation for long-term use. In this work, we present a wrinkled parylene coating for the encapsulation of stretchable strain sensors, which are made of silicone elastomer and spray-coated carbon nanotubes in a parallel-plate capacitor configuration. Conformal deposition of a parylene thin film on a pre-stretched, elastomer-based strain sensor followed by the release of the prestrain creates wrinkles in the parylene coating. This wrinkled parylene coating provides over 50% mechanical stretchability and biocompatible encapsulation. The coating can also increase the gauge factor of the strain sensors from 1 to 2 by suppressing the Poisson’s effect of the elastomer-based sensor. We investigate the influences of the coating thickness on the sensor performance, and demonstrate the long-term stability of these encapsulated strain sensors through cyclic stretching with thermally accelerated aging inside saline. The fabrication simplicity and biocompatibility of this wrinkled parylene encapsulation make it potentially useful for other stretchable, implantable devices.