Soft strain sensors for implantable applications

The design of materials that can integrate useful devices with soft, hydrated, and curvilinear substrates such as organs has been a grand challenge in the field of medical devices and soft robotics in recent years. Materials currently used in medical devices (e.g. copper, polyethylene) are rigid and lack the compliance necessary to mitigate scar tissue formation and are thereby incompatible with chronic implantation. More specifically, the Young's modulus of device materials is of particular importance for seamless integration with organs, but materials in current medical devices exhibit moduli 3-6 orders of magnitude higher than those of internal organs. In addition, conventional materials are often unable to meet the high strain requirements of dynamic organs. This dissertation aims to provide a set of engineering solutions to address the need for soft and implantable devices, with a focus on biocompatibility. The materials studied in this dissertation are applied as implantable strain sensors with moduli and maximum strains compatible for chronic integration with internal organs. Finally, the implantable capabilities of the materials and devices presented are demonstrated in vivo in porcine models to monitor cardiac output.

The first part of this dissertation aims to apply adhesive hydrogels as strain sensors. Hydrogels are a class of materials that can seamlessly interact with many types of excitable tissue owing to their tunable low Young's moduli and large equilibrium swelling in water. By measuring their change in resistance in response to strain, they may be applied as biocompatible and soft strain sensors. However, due to the presence of electrochemical reactions from the applied voltage, enhanced conductivity is often a prerequisite, resulting in the preclusion of biocompatibility. We demonstrate that biocompatible hydrogels can be used as highly sensitive and stable strain sensors without conductive fillers by measuring their impedance in a high frequency regime. The second part of this dissertation explores the possibility of applying polydimethylsiloxane (PDMS) as a biocompatible and soft strain sensor via photonics. PDMS is a widely applied elastomer in the field of medical devices and soft robotics with demonstrated biocompatibility, and its use may facilitate faster clinical translation due to its established history in FDA-approved medical devices. Optical strain sensing has advantages over its electronic counterpart owing to its immunity to electromagnetic interference, broader range of applicable materials, and compatibility with state- of-the-art chemo-optical analytical methods. By measuring attenuation changes through the soft optical fiber during applied strain and tuning its modulus to match those of tissue, PDMS is demonstrated as a soft and implantable optical strain sensor with a form factor relevant for internal organs. In the last part of this dissertation, the remaining challenges and opportunities for strain sensing materials towards chronically implantable devices are summarized and discussed.