Stretchable Encapsulation for Implantable Strain Sensors

Implantable strain sensors integrated on organ surfaces can monitor organ deformations, such as bladder filling and stomach motility, thereby providing important information about their functional states. A major challenge lies in achieving large strain ranges while ensuring biocompatibility and long-term stability inside physiological fluid environments. Commonly used stretchable materials have relatively high water permeability, which can lead to degradation of sensing performance. This work presents a method to provide highly stretchable, biocompatible, compliant, and stable encapsulation for implantable capacitive strain sensors. Conformal deposition of parylene, a widely used encapsulation material with limited stretchability, followed by controlled mechanical buckling, creates microscale wrinkles in the parylene coating. A thermal annealing step reduces Young's modulus of parylene, which converts globally buckled thick (>5 μm) parylene coating into microscale wrinkles. This simple annealing step effectively enhances the stretchability and barrier properties of the parylene coating. The resulting biocompatible wrinkled parylene encapsulation provides over 60% mechanical stretchability and a normalized water vapor transmission rate of 0.07 g mm/m²/day, offering one of the best combinations of barrier properties and stretchability among different encapsulation materials. In addition, the uniaxially microwrinkled encapsulation results in a more than doubled gauge factor for capacitive strain sensing by suppressing the Poisson effect. Thermally accelerated dynamic testing of encapsulated strain sensors validates their long-term stability. Additionally, strain sensing using encapsulated sensors sutured on a bladder phantom and ex vivo porcine bladders demonstrates their potential for real-time organ deformation sensing. The versatility of this encapsulation method makes it promising for a wide variety of stretchable implantable devices, supporting continuous organ monitoring and targeted therapy.