Soft Materials and Devices for Brain-Electronics Interface

Abstract

Recent advances in materials sciences, electronics and bioengineering have paved the way for new forms of communication and interaction between human technology and living systems, such as wearable devices and neuroprosthetics. However, a fundamental challenge remains to enable such applications: living systems are soft, wet and conduct with ions, while electronics is stiff, dry and conduct with electrons. To bridge this gap, soft materials have been used to realize electronic, pneumatic, thermal, or mechanical functionalities, but it is still unclear if they can be used as encapsulation. Encapsulation is a ubiquitous component of all real-world devices, which ensures reliability and longevity by limiting exchanges with the outside. At “small scale”, from sub-millimeter-thickness gas barriers in wearable devices to sub-micron-thickness dielectric encapsulations for seamless brain-electronics interface, encapsulation plays a critical role. This work focuses on encapsulation strategies for soft devices and brain-electronics interface. The thesis first describes the fundamental trade-off between mechanical stiffness and permeability of materials to show that soft and low-permeability materials do not exist in nature, which limits the longevity of soft devices (chapter one). Then, this paradigm is illustrated with the example of elastomer-coated hydrogel fibers (chapter two). A solution is provided to increase their longevity based on rational material choices, geometry, and thermodynamic considerations. In chapter three, a generic solution inspired from mechanical instabilities – wrinkles – is described to breakdown the stiffness – permeability trade-off. Chapter four to seven focus on soft and stretchable devices for brain-electronics interface. The electrochemical impedance instability in hydrocarbon-based dielectric elastomer thin films, caused by a diffusion-induced ionic conductivity when immersed in biofluids, prevents their use as encapsulation for brain-electronics interface (chapter 4). Two directions are then explored to realize soft brain-electronics interfaces. First, stretchable mesh nanoelectronics, where the electrodes are encapsulated by a stiff and low-permeability dielectric polymer, is designed to integrate with cardiac and brain organoids and captures electrophysiological signals at the single-cell resolution (chapters 5 and 6). Then, we realize ultra-soft and ultra-high density brain probes with a photo-patternable perfluoropolyether elastomer of stable dielectric impedance in biofluids, breaking down the trade-off between sensors integration and tissue-level flexibility (chapter 7).