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.
Neural recording and modulation has evolved rapidly in recent years. Closed-loop neuromodulation systems have been successfully demonstrated for the treatment of Parkinson's disease and epilepsy. Chronically implanted medical devices, requiring compliance to rigorous safety regulations, employ numerous safety measures to protect the patient. One such measure to prevent direct current from being applied to the tissue in case of a system failure is typically the usage of external blocking capacitors between the electrodes and the neuromodulator. These capacitors can cause significant magnetic resonance imaging (MRI) magnetic susceptibility artifacts that appear as a shading effect. This paper presents some of the challenges which arise when evolving neuromodulation hardware from benchtop prototypes to biomedical systems for chronic implantation in humans. We propose a novel safety measure to mitigate the MRI shading issue while still complying to the relevant safety regulations. Furthermore, we propose several additional features that improve the flexibility and usability of a 32-channel neuromodulation platform. The neuromodulator was fabricated in a 180 nm HV CMOS process and realizes a fully digital-to-neural interface.
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