(31) Implantable Flexible GHz Waveguide for High Data Throughput Neural Recording Interfaces

Introduction: Advanced implantable neural interfaces continue to increase in channel, or electrode, count for the purpose of accessing neural activity at cellular scales. Devices having thousands of electrodes on a single penetrating shank have been demonstrated [1]. Increasing electrode counts concomitantly increases trace counts, typically requiring tradeoffs to minimize shank dimension and trace resistance. Although time-domain multiplexing allows a reduction in the number of traces required, neural data is sacrificed [2] and the method does not easily extend to passive polymer-based neural interfaces.
To address these competing requirements, we present a novel concept that simultaneously supports the data bandwidth for 100’s of electrodes using only a few conductors.

Methods: A novel flexible, penetrating shank paradigm is introduced in which a twin-lead transmission line terminated in a micro coil wirelessly communicates with an integrated circuit for multi-channel neural recording. To demonstrate this principle, electromagnetic simulations in ANSYS HFSS were performed on the transmission line to characterize microwave frequency (300 MHz-300 GHz) signal in the surrounding lossy, biological medium for typical shank lengths. Modelling was used to produce a coil geometry with maximal Q-factor and coupling coefficient. Prototypes were fabricated via femtosecond laser micromachining from a polyimide substrate and combined with a 75-micron diameter gold wire.

Results: With a 1 GHz carrier signal, we demonstrated a data rate capable of supporting 300 electrodes at a 20 kHz sampling rate with 8-bit digitization through transient simulations. The simulated transmission line had a characteristic impedance of 48.0 Ω and an insertion loss of -16 dB for a 5 mm long shank. The coil (2 turns, 1.5 × 0.5 mm area) had an inductance of 20 nH and a Q-factor of 23. To an inset, concentric coil of similar geometry, a coupling coefficient of 0.71 and a maximum power transfer efficiency of 68% (load = 50 Ω) with resonance at 1 GHz was simulated.

Conclusion: This work demonstrates the feasibility of using transmission lines for microwave frequency communication in penetrating shanks for neural recording having hundreds of electrodes. At microwave frequencies, bandwidths supporting thousands of electrodes simultaneously are possible. Additional work is underway to further reduce insertion loss for longer shank lengths and demonstrate this concept in fabricated shank prototypes.