Design, fabrication, and characterization of biocompatible soft strain sensors for implantable applications

Cardiovascular diseases remain a major global health challenge, with effective post-operative monitoring critical for improving patient outcomes and reducing hospital readmissions. This study presents the design, fabrication, and characterization of biocompatible soft strain sensors for implantable applications, focusing on continuous and precise cardiac strain monitoring. Two sensor types were developed: resistive and capacitive. While the resistive sensors provided initial insights, they faced durability limitations under cyclic loading, leading to the design of capacitive sensors with interdigitated configurations. The capacitive sensors were constructed with polydimethylsiloxane encapsulation and 25 µm copper conductive elements. Theoretically, this design yielded a baseline capacitance of 0.228 pF and demonstrated sensitivity increments of 10, 20, and 30 fF under 5%, 10%, and 15% strain, respectively. Durability tests included continuous cyclic loading at 10% strain for over 12 hours, where the sensors exhibited minimal baseline drift and consistent signal integrity. Additionally, the capacitive sensors maintained stability under large strain amplitudes of 25% and demonstrated pressure insensitivity, ensuring operational reliability in dynamic physiological environments. Characterization under simulated cardiac conditions using a ViVitro Superpump confirmed the sensor's ability to detect minimal volumetric changes and to accurately trace distinct phases of a heartbeat. These findings underscore the potential of the proposed sensors for real-time telemetric cardiac monitoring, offering high sensitivity, durability, and biocompatibility. This work lays a critical foundation for next-generation implantable devices aimed at enhancing cardiovascular care.