IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS 1 MagNI: A Magnetoelectrically Powered and Controlled Wireless Neurostimulating Implant Zhanghao Yu*, Student Member, IEEE, Joshua C. Chen*, Fatima T. Alrashdan, Student Member, IEEE, Benjamin W. Avants, Yan He, Student Member, IEEE, Amanda Singer, Jacob T. Robinson, Senior Member, IEEE, and Kaiyuan Yang, Member, IEEE Abstract—This paper presents the first wireless and pro- grammable neural stimulator leveraging magnetoelectric (ME) effects for power and data transfer. Thanks to low tissue absorption, low misalignment sensitivity and high power transfer efficiency, the ME effect enables safe delivery of high power Implant levels (a few milliwatts) at low resonant frequencies (∼250 kHz) to mm-sized implants deep inside the body (30-mm depth). The presented MagNI (Magnetoelectric Neural Implant) con- Coil sists of a 1.5-mm2 180-nm CMOS chip, an in-house built Battery 4 × 2 mm ME film, an energy storage capacitor, and on-board Magnetic electrodes on a flexible polyimide substrate with a total volume Microcontroller Driver of 8.2 mm3. The chip with a power consumption of 23.7 µW includes robust system control and data recovery mechanisms under source amplitude variations (1-V variation tolerance). The Fig. 1. Conceptual diagram showing a wearable spinal cord neurostimulation system delivers fully-programmable bi-phasic current-controlled system for pain relief, the implant is remotely powered via magnetic fields. stimulation with patterns covering 0.05-to-1.5-mA amplitude, 64- to-512-µs pulse width and 0-to-200-Hz repetition frequency for RF NF US OP MT ME Better neurostimulation. Body Absorption Index Terms—Wireless neurostimulator, implantable device, Implant Size Peneatration bioelectronics, magnetoelectric effect, wireless power transfer Functional Flexibility Subject Mobility I. INTRODUCTION Operating Reliability Worse EUROSTIMULATION holds significant promise as a Fig. 2. Comparison of wireless power transfer modalities for bioelectronic N tool to modulate nerves for both neuroscience research implants. (RF: radio-frequency field. NF: near-field inductive coupling. US: and clinical therapies. Peripheral nerve stimulation (PNS) is ultrasound. OP: optoelectronics. MT: magnetothermal nanoparticles. ME: a common approach to treat neuropathic pain. For example, magnetoelectric effects.) devices can be implanted to deliver electrical pulses to the spinal cord, which can help prevent pain signals from reaching would be programmable so that they can be reconfigured to to the brain [1], [2]. suit user needs. Fig. 1 illustrates the concept of the proposed A fundamental challenge in developing miniature neural im- spinal cord stimulation system. The wireless implant receives plants is delivering power to devices inside the body. The use power and data from the portable battery powered transmitter of a wired power supply causes failures for neural implants, (TX) via magnetic field; the microcontroller, magnetic field as lead wires increase the risks of infections, restrict device driver, and the battery are assembled in a wearable belt. arXiv:2107.02995v1 [q-bio.NC] 7 Jul 2021 deployment and affect subject mobility [3], [4]. Batteries add While various wireless neural implants exploiting radio- considerable weight and increases device footprint [5]. They frequency (RF) electromagnetic (EM) [6], [7], inductive cou- are also required to be replaced or recharged frequently, which pling [8]–[16], ultrasonic [17]–[21], and optical [22], [23] limits their long-term clinical applications. Compared to wired power transfer have been reported, achieving safe and reliable or battery powered implants, wirelessly powered battery-free wireless power transfer with the size and power constraints neural stimulators have the potential to provide less invasive, to neural implants is still challenging. Existing technologies longer lasting interfaces to nerves. Ideally, these implants cannot simultaneously satisfy all the desired properties as This work was supported in part by the National Institute of Biomedical summarized in Fig. 2. Radio-frequency EM waves are capable Imaging and Bioengineering of the National Institutes of Health under of delivering power to implants deep in the tissue [6], [7]. award U18EB029353 and the National Science Foundation under award However, they wrestle with size limitations of the receiver’s ECCS-2023849. (Corresponding Authors: Kaiyuan Yang, [email protected]; and Jacob T. Robinson, [email protected]) antenna since efficient power delivery with electromagnetic All authors are with Rice University, Houston, TX 77005, USA. J. T. waves requires antenna sizes comparable to the wavelength. Robinson is also with Baylor College of Medicine, Houston, TX 77030, USA. Higher frequency RF is necessary for mm-scale implants, (*Zhanghao Yu and Joshua C. Chen contributed equally to this work.) ©2020 IEEE. Personal use is permitted, but republication/redistribution but suffers from higher tissue absorption [24], limiting the requires IEEE permission. amount of power that can be safely delivered. Near-field IEEE TRANSACTIONS ON BIOMEDICAL CIRCUITS AND SYSTEMS 2 Capacitor ME Transducer ulators [11], [27], this proof-of-concept lacks robust control of SoC the stimulation patterns and thus is highly sensitive to changes - + of coupling between the TX and the implants. For clinical ap- plications, the stimulation timing and amplitude must be well Flex Board controlled and programmable by the user to ensure the safety and reliability of the stimulator. Furthermore, due to the lack Volume: 8.2 mm3 of energy storage and charge balancing techniques, this work Weight: 28 mg has difficulties in providing large-power stimulating pulses and eliminating residual charge. Therefore, there is a critical need to create wireless neural stimulators that simultaneously achieve clinical safety, miniaturization, operation reliability mm 5 and flexibility, and programmable stimulation parameters. Electrodes To meet all the desired properties and circumvent problems 9.1mm mentioned above, we present MagNI (Magnetoelectric Neural Implant), the first untethered and programmable neural stim- Fig. 3. Illustrations of the proposed neurostimulation implant. ulator that exploits ME effects for power and data transfer, which integartes a 1.5 mm2 180-nm CMOS system-on-chip (SoC), an in-house built 4 mm × 2 mm x 0.12 mm ME inductively coupling has been well developed in wireless transducer, a single energy storage capacitor, and 1-mm2 on- power transfer [8]–[16]. It has less tissue absorption than board electrodes on a flexible polyimide substrate, as shown in RF because of the lower operating frequency. However, its Fig. 3. The proposed device features: (1) a miniature physical power delivery is sensitive to perturbations in the distance dimension of 8.2 mm3 and 28 mg, (2) adaptive system control and angle, especially when the coil is small. Ultrasound is and data transfer mechanisms robust under source amplitude another promising method to further reduce implant size and variations, (3) a 90% chip efficiency due to its low static body absorption [18]–[21]. Compared to inductively coupled power down to 23.7 µW, and (4) the capability to perform coils, its efficiency is more robust to the source-receiver fully programmable bi-phasic current stimulation covering misalignment [25]. However, it must overcome significant 0.05 to 1.5 mA amplitude, 64 to 512 µs pulse width, and 0 path loss caused by reflections at the boundaries between air, to 200 Hz frequency ranges, making it appropriate for spinal bone and tissue, which have different densities and acoustic cord stimulation to treat chronic pain. properties. To alleviate the reflection between the air and the This paper is an extended version of [30], with more body, ultrasound gel is typically required for the transmitter. comprehensive analysis, discussions, and measurements on the The need for frequent replacements of the ultrasound gel safety, robustness, and power efficiency of the proposed ME and the need for the transmitter to be in contact with the power transfer mechanism. The rest of the paper is organized skin can be inconvenient and unreliable for long-term clinical as follows: Section II presents qualitative and quantitative treatments. Further, the gel cannot mitigate the in-body path analysis of safety, misalignment sensitivity and efficiency −1 −1 loss, which is 22-dB cm MHz in the skull [26]. Optical for the ME power transfer for miniaturized neural implants; power delivery is also advantageous in the miniaturization of Section III describes the detailed design and implementation neural implants [22], [23], but it suffers energy loss due to of the proposed SoC and neural stimulator; Section IV gives scattering and may have difficulties in supporting the neural experimental results, including stimulation variability, charge stimulation with higher power requirements. imbalance, impedance of on-board electrodes, and power Among the various modalities of wireless power transfer, transfer efficiency; Section V concludes this paper. low-frequency magnetic field is believed to be one of the best mechanisms to safely deliver power deep inside body, II. MAGNETOELECTRIC POWER AND DATA TRANSFER FOR because of its low absorption and strong penetration. Re- NEURAL IMPLANTS cently, magneothermal deep brain stimulation using magnetic nanoparticles has been demonstrated by [27]. However, the A. Magnetoelectric Transducers system has limited capabilities because of difficulties in the A magnetoelectric