A Miniaturized, Battery-free Implant for Spinal Stimulation

Introduction: Electrical stimulation of the spine has several therapeutic applications such as treating injured nerves and managing chronic pain. Traditional spinal cord stimulators are typically composed of an electrode to deliver stimulation to the posterior columns of the spinal cord. The electrode is connected to a pulse generator via a wire to a subcutaneously implanted housing that contains the electronics (pulse generator and battery). The main drawback of this approach is the increased risk of infections and complications due to wire movement and failure. Furthermore, the stimulator is usually programmed to offer a limited set of stimulation modalities, and it lacks the ability to adjust the stimulation parameters (frequency, pulse width, and number of pulses) to offer any stimulation protocol.
Wireless, battery-free implants for spinal cord stimulation offer the advantage of reduced complications due to wire movement failures. They are typically composed of a wireless reception element (antenna or coil) and a rectifier circuit to convert the wireless signal into DC voltage to be used to power the stimulator and deliver electrical stimulation. We propose a 0.1 mm-thick, wireless, and battery-free implant for spinal stimulation that can be programmed via an external antenna to generate any required stimulation protocol that is composed of bursts of monophasic voltage pulses. The in vivo test results show that the implant can trigger limb response when used to stimulate a rodent’s spinal cord. The results demonstrate the implant’s utility for future chronic animal studies as a tool for treating spinal injury and controlling pain.

Materials and

Methods: The implant is fabricated on a 0.1 mm-thick, 8×8 mm2 parylene/PDMS substrate. PDMS is coated on a 4-inch silicon wafer with a 10:1 curing agent ratio. Parylene (8 µm) is used to improve the adhesion of RF-sputtered gold to PDMS. Photolithography is performed to obtain the gold tracks. The electronics are assembled using conductive silver epoxy. A parylene encapsulation layer (2 µm) protects the implant
The battery-free implant circuit is composed of a dipole antenna operating at 2.4 GHz and a rectifier that converts the wireless signal in a 30 µF capacitor. It delivers monophasic voltage pulses in response to an on-off keying modulated wireless signal to a wire electrode (120 µm, exposed tip = 1 mm).
The acute animal test was performed on a 330 g rat, under an approved protocol. The rat was anesthetized by 2% isoflurane, and a 4 cm incision was made over the lumbar spine. A segment of a 22-gauge needle was slid over the wire to act as a guide cannula. The needle was placed onto the spine between L3 and L4, then slowly inserted 1 cm into the spinal canal. The needle was then withdrawn while holding the electrode wire in place. The ground wire was tucked under the skin near the tail. After inserting the electrode into the spinal cord, test stimulation pulses were applied using the RF power/control source. The wire was slowly withdrawn in 1 mm increments until reaching a position where strong limb movement was evoked.

Results, Conclusions, and Discussions: Results
The implant benchtop testing confirms that the device can delivers monophasic voltage pulses in response to on-off keying modulation of the wireless signal at 2.4 GHz, and can generate a medically relevant stimulation protocol composed of 10 pulses at 100 Hz with a pulse width of 200 µs delivered at a period of 1 sec. The maximum output voltage was 11.9 V at 25 mm to the external antenna and a maximum transmit power of 30 dBm. The device can still generate monophasic voltage pulses with a magnitude of 5 V at a transmit power of 17 dBm and distance of 25 mm.
The in vivo test showed that the device can trigger motor response by stimulating motor neurons, and a limb deflection around 3.5 mm was measured from the resting position using a video analysis software. The external antenna was placed 25 mm away from the implant, and the transmitted power was set to 27 dBm.
Conclusions
We designed, fabricated, and tested a wireless, battery-free implant for spinal stimulation with miniaturized dimensions to enable its application in rodents. The benchtop and in vivo testing confirmed the implant’s ability to generate an externally configurable stimulation protocol and trigger motor response in a rodent. The animal work was performed under Arizona State University IACUC (Institutional Animal Care and Use Committee) approved protocol 24-2049R in compliance with all university guidelines and regulations.
Discussions
The work presented herein demonstrated the ability to perform spinal stimulation using a miniaturized wireless, battery-free implant. The future research plan is to perform chronic experiments on moving animal models such as controlling pain and improving recovery from spinal injury. In addition, we are aiming to further reduce the implant’s dimensions using highly integrated CMOS fabrication to enable multichannel operation for improved targeting of specific spinal regions and optimize the electrode design to precisely target specific spinal neurons.

Acknowledgements (Optional): Research reported in this publication was supported by HHS-NIH: National Institute of Biomedical Imaging and Bioengineering (NIBIB) of the National Institutes of Health under award number R21 EB028396 and 2022 Mayo Clinic and Arizona State University Alliance for Health Care Collaborative Research Seed Grant Program. A.Q.-H. acknowledges the support from the William J. and Charles H. Mayo Professorship, the Mayo Clinic Clinician Investigator Award, the Florida Department of Health Cancer Research Chair Fund, and the Monica Flynn Jacoby Endowed Chair.