Implantable medical devices (IMD) with stimulation system-on-chip (SoC) have been essential techniques for disease treatments and rehabilitations. As neuromuscular stimulation injects a large amount of stimulus energy into the body, its energy efficiency and safety should be carefully considered, which otherwise damages cellular tissues. Conventional current stimulation suffers from large power losses across current sources. Even adopting the adaptive supply voltage, the stimulator efficiency is still limited below 60% . The switched capacitor stimulation (SCS) system charges the capacitor and transfer its charges to the tissue, achieving stimulator efficiency up to 84% -. However, previous SCS systems only operate with AC input voltages directly from wireless power, which can be interrupted in loosely-coupled inductive links. To take advantages of using a rechargeable battery or a supercapacitor for reliable IMD operation, the SCS system that can efficiently operate with both DC and AC inputs is required. Also, more aggressive techniques to further improve stimulator efficiency and efficacy are highly needed.
|Title of host publication||2022 IEEE Custom Integrated Circuits Conference, CICC 2022 - Proceedings|
|Publisher||Institute of Electrical and Electronics Engineers Inc.|
|Publication status||Published - 2022|
|Event||43rd Annual IEEE Custom Integrated Circuits Conference, CICC 2022 - Newport Beach, United States|
Duration: 2022 Apr 24 → 2022 Apr 27
|Name||Proceedings of the Custom Integrated Circuits Conference|
|Conference||43rd Annual IEEE Custom Integrated Circuits Conference, CICC 2022|
|Period||22/4/24 → 22/4/27|
Bibliographical noteFunding Information:
C1-4 are 1μF/each. Fig. 3 shows the measured waveforms of the switching charger when charging VCAP from 0V (left) or 1.5V (right) to the target voltage of 3V. While the duty ratio increases at 2.2MHz, the capacitor can be charged from 0V to 3V within 50μs. When the VRES is 1.5V, the capacitor starts charging once VSL reaches 1.5V with the optimal duty ratio that can prevent large fluctuation of the inductor current, IIND. Thus, IIND can start charging immediately from its optimal current level, ensuring higher charging efficiency with minimized I2R losses. The measured capacitor charging efficiencies depending on initial VCAP levels, i.e., VRES, are described in Fig. 4 (left). Higher target voltages, VTG, leads to higher charging efficiencies, achieving up to 90% when charging the 1μF capacitor from 0V to 3V. The proposed iSCS with DC input has 13% higher efficiency than conventional AC-input-only SCS in . The iSCS still achieves 9% higher efficiency even when it utilizes AC input with an additional AC-DC converter . When charging starts from higher VRES of 1.5V to 3V, higher charging efficiency up to 92.7% was measured with shorter charging time of 28μs. Fig. 4 (right) shows algorithm, implementation, and measured waveforms of the proposed offset-control charge balancing, which adaptively adds offset to capacitor voltages by adjusting VTG, leading to active charge balancing between cathodic and anodic stimuli. In vivo animal experiments with iSCS were conducted for rectus muscle stimulation with rabbits to modulate eye movements, which can be used for paralytic strabismus treatments. Fig. 5 (left) shows the conceptual illustration and measured eye movements by injected stimulus energy. The iSCS system resulted in higher efficacy with 1.2x longer eye movement of 6mm and 13% smaller stimulus energy of 29.7μJ than conventional rectangular current stimulation, thanks to its energy-efficient decaying exponential waveforms. Fig. 5 (right) shows the measured in vivo waveforms for multi-switched-capacitor stimulation. When the iSCS charges VCAP1 to 3V, VCAP2 is discharged to stimulate ocular muscle of a rabbit, creating decaying exponential current stimuli, ISTIM, which can be more effective in activating tissues than conventional rectangular stimuli. By changing stimulus direction between selected electrodes, biphasic stimulation can be applied with cathodic and anodic stimuli. The offset-control charge balancing sequentially minimizes the residual charges with intended offsets. Fig. 6 compares the proposed iSCS system with state-of-the-art stimulators. The iSCS system achieved higher capacitor charging efficiency up to 92.7% and shorter charging time of 28μs for 1μF capacitor charging from 1.5V to 3V. The charge transfer efficiency from capacitors to electrode/tissue models (1kΩ+1μF) was 99.2%, resulting in overall stimulator efficiency up to 92%. The proposed system improves both stimulator efficiency with iSCS and stimulus efficacy with its decaying exponential waveforms. Acknowledgement: This work was supported by the Technology Innovation Program (20010712) funded By Ministry of Trade, Industry & Energy, Korea. Chip fabrication was supported by IC Design Education Center, Korea. References:  Z. Luo et al., “A Digitally Dynamic Power Supply Technique for 16-Channel 12 V-Tolerant Stimulator Realized in a 0.18-µm 1.8-V/3.3-V Low-Voltage CMOS Process,” IEEE TBioCAS, 2017.  H.-M. Lee et al., “A Power-Efficient Switched-Capacitor Stimulating System for Electrical/Optical Deep-Brain Stimulation,” IEEE ISSCC, 2014.  Y. Jia et al., "A Trimodal Wireless Implantable Neural Interface System-on-Chip," IEEE ISSCC, 2020.  W.-Y. Hsu et al., “Compact, Energy-Efficient High-Frequency Switched Capacitor Neural Stimulator with Active Charge Balancing,” IEEE TBioCAS, 2017.  G. Namgoong et al., “A 6.78 MHz, 95.0% Peak Efficiency Monolithic Two-Dimensional Calibrated Active Rectifier for Wirelessly Powered Implantable Biomedical Devices,” IEEE TBioCAS, 2021.  A. Urso et al., “An Ultra High-Frequency 8-Channel Neurostimulator Circuit With 68% Peak Power Efficiency,” IEEE TBioCAS, 2019.
This work was supported by the Technology Innovation Program (20010712) funded By Ministry of Trade, Industry and Energy, Korea. Chip fabrication was supported by IC Design Education Center, Korea
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ASJC Scopus subject areas
- Electrical and Electronic Engineering