UC San Diego

Internet of Things (IoT) is becoming pervasive in our daily lives. Wearable technologies will expand the connectivity of IoT and will increase the interaction between technology and human body. Micro Electro-Mechanical Systems (MEMS) microfabrication techniques that involve bulk Si micromachining and thin film processing have allowed us to develop electronic systems that are based on Si and other advanced materials that are flexible, wearable, and implantable. Wearable and implantable electronics equipped with sensors enable us to perform real-time health monitoring from above and below the skin, respectively, and can replace conventional bulky electrophysiological monitoring devices and systems.

Research efforts in wearables and implantables have intensified in the last decade tackling several aspects of the sensor technology, embedded signal processing and conditioning, energy harvesting, connectorization, functionality, longevity and reliability. However, there are still technical challenges that impose restrictions for their widespread adoption. On top of these challenges is the power source for the wearable or implantable device. Energy harvesting is expected to replace conventional battery systems that power wearables and implantables. In this dissertation, we focus on solar energy as an energy source for self-powered electronics.

In Chapter 1, the motivation of the dissertation together with a brief survey of state of the art in flexible and wearable electronics with energy harvesting system and implantable medical devices are discussed.

In Chapter 2, we disclose our parametric studies on solar cells with different microwire surface and array morphologies to understand the effect of surface passivation, surface crystal orientation on surface recombination and carrier collection on SiMW solar cells with radial p-n junctions as well as their emitter series resistances with an overall goal of maximizing their power conversion efficiencies.

In Chapter 3, we present an approach for self-powered wearable electronics by means of the monolithic integration of SiMW solar cells with Si MOSFETs on a Silicon on Insulator (SOI) wafer that is subsequently transferred to flexible substrates. The fabrication details and its application to a voltage-controlled oscillator and electrophysiological monitoring are discussed.

In Chapter 4, we discuss the details of the novel fabrication processes for the development of a stylet guided depth/laminar probe and of a surface electrocorticography (ECoG) grid that is fabricated with bio-compatible polymers (Polyimide and Parylene C) including their electrochemical characterization and their use in vivo for electrophysiological recordings in rats.