Thermoplastic polyurethane as a base material for soft, high-density neural interfaces

Polymers used as substrate and encapsulation for implants are typically deposited layer-by-layer, leaving polymer–polymer interfaces that adhere poorly and become a critical failure point. Thermoplastic polyurethane (TPU) elastomers combine four properties not found together in any other candidate: decades of clinical use in long-term implants and availability in medical grade, a stiffness well below that of polyimide and parylene C, compatibility with microfabrication, and thermoplastic self-fusion, which removes the polymer–polymer interface entirely. This work introduces TPU as a base material for soft, high-density neural interfaces and demon-strates one route to realizing them. Gold is patterned on a sacrificial copper sheet by maskless lithography and transferred into TPU by vacuum lamination, giving conformally coated tracks down to 15 µm at 30 µm pitch within a single, merged polymer body. An additional electroplating step produces a nanostructured “pinecone” gold surface that increases the electrode electrochemically active area, approximately doubling the charge injection capacity. Daisy-chain silicon chips were also embedded within the soft polymer body. The films transmit up to 88% at 470 nm, a wavelength commonly used for optogenetics. TPU showed no cytotoxicity towards differentiated SH-SY5Y cells and only a mild tissue response after 9.5 months of subcutaneous rat implantation.