A 24-Channel Neurostimulator IC With Channel-Specific Energy-Efficient Hybrid Preventive-Detective Dynamic-Precision Charge Balancing

Abstract

This paper presents the design, development, and experimental characterization of a 24-channel programmable charge-balanced current-mode neurostimulator IC. Each channel is equipped with a quad-threshold voltage-based charge imbalance detection and a dedicated hybrid preventive-detective charge balancing circuit. The interplay of the preventive and detective control loops utilized for charge balancing has resulted in minimizing the power and timing overhead of the proposed strategy for maintaining a charge-neutral electrode-tissue interface, while avoiding the risk of unintended stimulation. The design offers dynamic programmability for the safe and unsafe charge imbalance thresholds, as well as for the balancing speed and precision. The IC is fabricated in a standard $0.18~\mu m$ CMOS technology with an overall active area of $2.27mm^{2}$ . Experimental characterization results of different circuit blocks are presented and discussed. Additionally, the IC’s efficacy in conducting charge-balanced stimulation is experimentally validated under various scenarios and for the full range of stimulation current magnitude, showing the balancing accuracy, latency, and active time. Experiments are conducted both with a simplified electrical model of the interface impedance as well as in vitro. Compared to the state-of-the-art stimulators with a closed-loop charge balancer, the presented work offers the most energy-efficient charge balancing technique, the shortest required inter-pulse interval (i.e., neutralization time), and the highest balancing precision.

Keywords

• Charge balancing
• neurostimulator
• implantable IC
• neural interface
• energy-efficient design
• closed-loop control