Highly Conformable Chip-in-Foil Implants for Neural Applications

Abstract

Demands on neural interfaces in terms of functionality, high spatial resolution, and longevity have recently increased. These requirements can be met with sophisticated silicon-based integrated circuits. Embedding miniaturized dice in flexible polymer substrates significantly improves the adaptation to the mechanical environment in the body and thus the systems’ structural biocompatibility as well as the ability to cover larger areas of the brain. This work addresses main challenges in developing a hybrid chip-in-foil neural implant. Assessments were related to: first, the mechanical compliance to the recipient tissue that allows a long-term application, and second, the suitable design that allows the implant’s scaling and modular adaptation of chip arrangement. Finite element model studies were performed to identify design rules regarding die geometry, interconnect routing, and positions for contact pads on dice. Providing edge fillets in the die base shape was an effective measure to improve die-substrate integrity and increase the area available for contact pads. Furthermore, the routing of interconnects in the immediate vicinity of die corners should be avoided, as the substrate in these areas is prone to mechanical stress concentration. Contact pads on dice should be placed with a clearance from the die rim to avoid delamination when the implant is conformed to a curvilinear body. A microfabrication process was developed to transfer, align and electrically interconnect multiple dice into conformable polyimide-based substrates. The process enabled arbitrary die shape and size and independent target positions on the conformable substrate from the die position on the fabrication wafer.