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Abstract
Stretchable electronics have been widely studied over the past decade because they offer excellent mechanical properties. This allows them to stay in stable integration with soft, deformable surfaces such as biological tissues and soft robotics. Intrinsically stretchable materials, such as polymer conductors, semiconductors and nanomaterial assemblies, are better options than their rigid counterparts for building electronics with higher stretchability and lower stiffness, which further improve conformability and stability. Although significant progress has been made in developing intrinsically stretchable devices, relatively little effort has focused on reducing their Young’s modulus to match that of soft tissues. Most stretchable electronic materials still have moduli that are several orders of magnitude higher than biological tissues. Achieving a tissue-like modulus is crucial for maintaining a stable, conformal interface between the device and surrounding tissue. Additionally, while much work has aimed at preserving the structural integrity of stretchable devices during deformation, it is also important to ensure that they provide reliable measurements and consistent performance even when stretched or bent. This is especially crucial for applications such as closed-loop control in soft robotic systems, which depend on consistent and precise sensing for effective operation. In my Ph.D. research, I focused on the development of intrinsically stretchable electronic devices for better interfacing with biological tissues with three major objectives: building fully stretchable circuits for wearable applications; imparting tissue level softness to the intrinsically stretchable electronics; maintaining deformation-unperturbed sensing performance for stretchable pressure sensors. First, I developed a fully stretchable power management circuit for a wearable energy-harvesting system based on a stretchable triboelectric nanogenerator. This stretchable circuit includes a four-transistor-based rectifier and a stretchable supercapacitor. It maintains stable electrical performance and keeps working when stretched by up to 100% strain. Second, to match the softness of biological tissues, I innovated a generalizable strategy of soft interlayer design for stretchable electronics. Based on the strategy, I successfully fabricated an ultrasoft transistor array on a hydrogel substrate, which has an effective Young’s modulus of 5.2 kPa, and can be stretched up to 100%. Benefits from the highly matched mechanical properties, the ultrasoft stretchable systems have demonstrated excellent biocompatibility through an implanted immune study, as well as in-vitro study based on a living isolated mice heart. Third, to solve the problem of unstable performance of stretchable sensors during deformation, I created a fully stretchable and highly sensitive pressure sensor that keeps stable sensing performance even when stretched. This was done by controlling strain distribution in a micro-pyramid array. Besides, I also developed a second-generation stretchable pressure sensor using electrode made of ultrasoft, highly conductive composite combined with ionically conductive microbeads as the sensing element. This sensor keeps its pressure sensing accuracy even when bent or stretched, which shows promising functionality on soft robotic systems with deforming surfaces. In the final chapter, I give the summary and future perspectives to the development of intrinsically stretchable electronics for wearable, implantable and soft robotic applications.