An Implantable Fully-Packaged Current-Controlled Wireless Near-Adiabatic Neural Stimulator Achieving 71.7% Peak Efficiency and 13.5% Efficiency Variation Across Supported Stimulation Current Range

Recent advances in implanted peripheral nerve stimulators (IPNSs) have shown promise in regulating physiological functions and treating various disorders [1]–[4]. Wireless-powered IPNSs with miniaturized volumes can enable direct placement at the stimulation site rather than connecting the stimulators to leads at distances, which are proven effective in reducing risks and increasing longevity [3], [4]. However, miniaturization has brought two critical issues: 1) available power received wirelessly by the IPNS reduces drastically, especially for deeply implanted devices; 2) tissue heating is exacerbated. Minimizing power dissipation, which is dominated by energy consumption during stimulation events, can be an impactful way to accommodate the issues above. Therefore, improving the energy efficiency of such stimulators is the key to yield a more pragmatic adoption. The current-controlled-stimulators (CCSs) are often in favor compared to voltage-controlled-stimulators (VCSs) and switch-capacitor stimulators (SCSs) due to their safety, area efficiency, and established methods of charge balancing [5]–[13]. However, they are generally facing efficiency challenges, especially when a broad range of stimulation currents <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathrm{I}_{\text{STIM}})$</tex> is needed: conventional CCSs [5], [6] with constant supply dissipate >50% of their output power across current sources. Thus, techniques for generating dynamic supplies are needed: inductive DC-DC converters are efficient, though requiring bulky off-chip inductors [7] or coils [8], the form factor is larger than desired; multi-ratio switch-capacitor-converter (SCC)-based solutions [9], [10] are area-efficient but lossy when creating smooth ramps; rectifier-based solutions [11], [12] can achieve high peak efficiencies, but the efficiencies vary >50% over the supported stimulation current range due to loading effects in the rectifiers. In addition, the stimulation voltage is limited to the rectified voltage, without cascading boost-converters or charge-pumps (CPs), the stimulation voltage <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathrm{V}_{\text{STIM}})$</tex> can be too low to accommodate the wide ranges of electrode-tissue interface impedance, especially for deeply implanted devices. Therefore, to date, a practical miniaturized IPNS featuring efficient broad-range current-controlled stimulation has yet to be demonstrated.