Engineered Polymer Interfaces for Flexible (Bio)electronics, 3D Printing and the Internet of Things

Walter Voit
University of Texas at Dallas

Abstract: Polymer engineering, specifically tuning monomer chemistries, polymerization kinetics and thin film interfaces, seeks to address grand challenges in neuroscience, semiconductor processing, the internet of things and additive manufacturing.

We demonstrate peripheral and cortical neural stimulators and recording devices using engineered low cure stress softening polymer substrates. Polymers can be implanted at moduli of more than 1 GPa and soften toward the modulus of tissue. Encapsulation materials include combinations of zwitterions, hydrophobic thiol-based polymers, parylene-C, silicon carbide, silicon nitride, aluminum oxide and ultrananocrystalline diamond. Neural interfaces allow for delivery of a large amount of information transfer, but current microstimulators or microrecorders fail chronically or are poorly suited for interfacing with small biological structures, such as sensory peripheral nerves. We discuss chronic device failure through design of both materials and devices to overcome various failure mechanisms. We demonstrate the effects of self-coiling vagus nerve stimulators and self-wrapping cochlear implants. We demonstrate spinal stimulators that reduce inflammation and tissue response and behave in a manner similar to ball electrodes for use in understanding long-term muscle plasticity. We demonstrate softening peripheral neural interfaces for modulating sensory and motor input toward closed-loop feedback for prosthetics.

We demonstrate large area flexible substrates compatible with full photolithographic process with 2 micron minimum features sizes and microfabrication processes at temperatures up to 300°C. Substrates compatible with high mobility semiconductors such as zinc oxide and indium-gallium zinc oxide are highlighted. Devices are thermally cycled through thermal transitions and compared to state-of-the-art flexible substrates including biaxially oriented poly (ethylene naphthalate), polydimethylsiloxane, polyimides and others. Furthermore, we use these processes to show a host of sensor and energy harvesting applications powering the emerging internet of things. Pressure sensors on one of the world’s softest fully elastic materials are demonstrated with matrix-addressed pixel accuracy. Engineered temperature sensors show changes in electrical performance of more than 6 orders of magnitude over less than 5 °C. Antenna technologies are discussed that allow wireless powering of flexible devices. We explore failure limits in strain tolerant thin film conductors on flex.

We demonstrate methods to 3D print materials that are isotropic, namely deformation perpendicular to the print grain does not lead to poorer thermomechanical properties than deformation in the XY plane along the print grain. This is accomplished using thiol-click chemistries and recently developed stereolithography printing techniques including our patent pending Z-Cup® paradigm. Tough materials across modulus ranges are printed into complex shapes and compared to state-of-the-art 3D printed materials. We take advantage of partial polymerization of oxygen-insensitive resins that can subsequently chemically crosslink into successive layers leading to strong tough materials.

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