Development of a battery-free ultrasonically powered functional electrical stimulator for movement restoration after paralyzing spinal cord injury

Abstract

Background Functional electrical stimulation (FES) is used to restore movements in paretic limbs after severe paralyses resulting from neurological injuries such as spinal cord injury (SCI). Most chronic FES systems utilize an implantable electrical stimulator to deliver a small electric current to the targeted muscle or nerve to stimulate muscle contractions. These implanted stimulators are generally bulky, mainly due to the size of the batteries. Furthermore, these battery-powered stimulators are required to be explanted every few years for battery replacement which may result in surgical failures or infections. Hence, a wireless power transfer technique is desirable to power these implantable stimulators. Methods Conventional wireless power transduction faces significant challenges for safe and efficient energy transfer through the skin and deep into the body. Inductive and electromagnetic power transduction is generally used for very short distances and may also interfere with other medical measurements such as X-ray and MRI. To address these issues, we have developed a wireless, ultrasonically powered, implantable piezoelectric stimulator. The stimulator is encapsulated with biocompatible materials. Results The stimulator is capable of harvesting a maximum of 5.95 mW electric power at an 8-mm depth under the skin from an ultrasound beam with about 380 mW/cm 2 of acoustic intensity. The stimulator was implanted in several paraplegic rats with SCI. Our implanted stimulator successfully induced several hindlimb muscle contractions and restored leg movement. Conclusions A battery-free miniature (10 mm diameter × 4 mm thickness) implantable stimulator, developed in the current study is capable of directly stimulating paretic muscles through external ultrasound signals. The required cost to develop the stimulator is relatively low as all the components are off the shelf. Electronic supplementary material The online version of this article (10.1186/s12984-019-0501-4) contains supplementary material, which is available to authorized users.

Ultrasound-driven piezoelectric current activates spinal cord neurocircuits and restores locomotion in rats with spinal cord injury

“…Detailed developmental procedure of a piezoelectric stimulator utilized in this study is described elsewhere (Alam et al 2019). In brief, a piezoelectric ceramic Barium Titanate (BaTiO 3 ) with around 1 MHz resonance frequency and a signal conditioning circuit (Villard voltage doubler) were used to convert ultrasound signals into piezoelectric stimulation pulses.…”

“…All piezo-ceramics were first examined for their center frequency for generating maximum piezoelectric voltage (see Supplementary document). After prototyping, the entire stimulator was encapsulated with a biocompatible silicone coating as described previously (Alam et al 2019). A pair of Teflon-coated stimulation wires (AS-632, Cooner Wire, United States) were connected to the piezostimulator for the delivery of stimulation current.…”

“…Furthermore, battery-powered stimulators require a secondary surgery as battery life is projected to be 5-10 years, depending on its discharge and recharge cycles (Kane et al 2011;Loeb et al 2006). In contrast, our recently developed piezoelectric stimulator (Alam et al 2019) does not require any implanted power source; rather the power is delivered via ultrasound beam from an external ultrasound probe. This allows us to design a distributed stimulation system that can deliver piezoelectric currents to the targeted organ.…”

Ultrasound powered piezoelectric neurostimulation devices: a commentary

“…1 Working principle of the ultrasound-driven piezoelectric stimulator interfacing the spinal cord for the restoration of involuntary locomotion. The ultrasound energy provided by the probe is converted to electrical energy for neural stimulation by the implanted device height of 4 mm (Alam et al 2019) yielding a volume of 314 mm 3 . Compared to some commercial button cells (5.8 mm in diameter and 1.6 mm), the size of the piezoelectric stimulator is still relatively large and could benefit from further shrinking so that there will be more options to implant the device into anatomical pockets of small animals.…”

MiniVStimA: A miniaturized easy to use implantable electrical stimulator for small laboratory animals

“…Other approaches exist like tiny implants with rechargeable batteries [ 29 ] requiring frequent charging cycles or ultrasonically powered devices [ 19 , 30 ] that relay on an ultrasonic transducer during stimulating. All of them need additional animal handling or efforts during the experimental time, or equipment installed to or near the animal housing.…”