We describe smart engineered shape memory polymer (SMP) substrates, which have been proposed for use in biomedical devices extensively over the past decade. Specifically, the paradigm of softening bioelectronics medicines enables devices such as neural interfaces to be implanted while mechanically rigid and subsequently soften in physiological conditions. Harris et al. have demonstrated softening intracortical electrodes based on the significant swelling of thermally and water sensitive polymer substrates. Building upon this work, we have further demonstrated the fabrication, characterization and demonstration of softening neural interfaces with 5 micron minimum feature sizes patterned using full-photolithography reaching temperature up to 85°C on softening substrates with minimal swelling. SMP substrates are thiol-ene/acrylate copolymers designed to position the glass transition temperature (Tg) to near 55°C, such that after plasticization in fluid, the Tg shifts 20°C triggering softening. This paradigm allows surgeons adequate time for implantation, and maintains sub 3% swelling of the substrate to minimize abiotic device failure and delamination of the patterned Parylene-C barrier coating. We balance mechanical buckling forces, created by modulus mismatches between the device modulus at insertion and that of both agarose gel (in vitro experimental model) and the cortex of a laboratory rat. Other studies have shown how higher modulus materials, such as silicon, tungsten, Parylene-C and polyimides maintain sufficient stiffness to allow implantation into tissue. Our devices match these supra 1 GPa insertion properties, but chronically behave mechanically more similarly to polydimethylsiloxane.

Softening polymer substrates for chronically soft neural interface

249th National Meeting (2015)

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