Wireless networks of injectable microelectronic stimulators based on rectification of volume conducted high frequency currents

Abstract

ObjectiveTo develop and in vivo demonstrate threadlike wireless implantable neuromuscular microstimulators that are digitally addressable.ApproachThese devices perform, through its two electrodes, electronic rectification of innocuous high frequency current bursts delivered by volume conduction via epidermal textile electrodes. By avoiding the need of large components to obtain electrical energy, this approach allows the development of thin devices that can be intramuscularly implanted by minimally invasive procedures such as injection. For compliance with electrical safety standards, this approach requires a minimum distance, in the order of millimeters or a very few centimeters, between the implant electrodes. Additionally, the devices must cause minimal mechanical damage to tissues, avoid dislocation and be adequate for long-term implantation. Considering these requirements, the implants were conceived as tubular and flexible devices with two electrodes at opposite ends and, at the middle section, a hermetic metallic capsule housing the electronics.Main resultsThe developed implants have a submillimetric diameter (0.97 mm diameter, 35 mm length) and consist of a microcircuit, which contains a single custom-developed integrated circuit, housed within a titanium capsule (0.7 mm diameter, 6.5 mm length), and two platinum-iridium coils that form two electrodes (3 mm length) located at opposite ends of a silicone body. These neuromuscular stimulators are addressable, allowing to establish a network of microstimulators that can be controlled independently. Their operation was demonstrated by injecting a few of them in the hind limb of anesthetized rabbits and inducing controlled and independent contractions.SignificanceThese results show the feasibility of manufacturing threadlike wireless addressable neuromuscular stimulators by using fabrication techniques and materials well established for chronic electronic implants. This paves the way to the clinical development of advanced motor neuroprostheses formed by dense networks of such wireless devices.

First-in-human demonstration of floating EMG sensors and stimulators wirelessly powered and operated by volume conduction

“…The demonstrated miniature electronic circuit will be integrated in an ASIC that will be included in a tubular implant with electrodes at opposite ends. The implantation procedure of this tubular conformation has been demonstrated to be successful in several assays, including those reported in (22). This first-in-human demonstration opens the possibility of using coupling by volume conduction in networked neuroprosthetic systems.…”

“…The muscle contractions obtained during electrical stimulation were weak compared to those obtained in previous electrical stimulation studies using this WPT approach (17,22). Because of the limitations explained above related to the frequent breakage of the thin-film electrodes when implanting the floating device, during the procedure it was prioritized the alignment with respect to the applied electric field and the settling in a single tissue over electrical stimulation (i.e., powering and bidirectional communications were deemed more important in the study).…”

“…They are not the final proposed implantable devices to be chronically implanted. The electronics are meant to be integrated in the future into an ASIC to be included in a flexible threadlike implant with two electrodes at opposite ends as that shown in (22). In other words, the limitations faced with the thin-film electrodes will not be faced with a conformation based on a tubular body mostly made of silicone.…”

“…This would allow to obtain fully implantable flexible threadlike EMG sensors and stimulators that can be easily implanted by injection, that can be employed in chronic studies and developed for future commercial use. We recently demonstrated in the hindlimbs of two anesthetized rabbits that fully injectable microstimulators (overall diameter of 0.97 mm) are able to rectify these volume conducted HF bursts to cause local low frequency currents capable of stimulation (22). The microstimulators include a hermetic capsule that houses an ASIC, two dc-blocking capacitors and a capacitance for voltage stabilization.…”

“…We have recently reported the development and evaluation of semi-implantable devices based on WPT by volume conduction that are capable both of performing stimulation and EMG sensing (17). These are proof-ofconcept prototypes of the electronic architecture that will be integrated into an ASIC to obtain flexible threadlike devices as those demonstrated in vivo in (22), with stimulation, EMG sensing and bidirectional communication capabilities. The external system and a bidirectional communications protocol that allows wireless communication between the external system and the semi-implantable devices are also described in (17).…”