Floating EMG sensors and stimulators wirelessly powered and operated by volume conduction for networked neuroprosthetics

Abstract

Background Implantable neuroprostheses consisting of a central electronic unit wired to electrodes benefit thousands of patients worldwide. However, they present limitations that restrict their use. Those limitations, which are more adverse in motor neuroprostheses, mostly arise from their bulkiness and the need to perform complex surgical implantation procedures. Alternatively, it has been proposed the development of distributed networks of intramuscular wireless microsensors and microstimulators that communicate with external systems for analyzing neuromuscular activity and performing stimulation or controlling external devices. This paradigm requires the development of miniaturized implants that can be wirelessly powered and operated by an external system. To accomplish this, we propose a wireless power transfer (WPT) and communications approach based on volume conduction of innocuous high frequency (HF) current bursts. The currents are applied through external textile electrodes and are collected by the wireless devices through two electrodes for powering and bidirectional digital communications. As these devices do not require bulky components for obtaining power, they may have a flexible threadlike conformation, facilitating deep implantation by injection. Methods We report the design and evaluation of advanced prototypes based on the above approach. The system consists of an external unit, floating semi-implantable devices for sensing and stimulation, and a bidirectional communications protocol. The devices are intended for their future use in acute human trials to demonstrate the distributed paradigm. The technology is assayed in vitro using an agar phantom, and in vivo in hindlimbs of anesthetized rabbits. Results The semi-implantable devices were able to power and bidirectionally communicate with the external unit. Using 13 commands modulated in innocuous 3 MHz HF current bursts, the external unit configured the sensing and stimulation parameters, and controlled their execution. Raw EMG was successfully acquired by the wireless devices at 1 ksps. Conclusions The demonstrated approach overcomes key limitations of existing neuroprostheses, paving the way to the development of distributed flexible threadlike sensors and stimulators. To the best of our knowledge, these devices are the first based on WPT by volume conduction that can work as EMG sensors and as electrical stimulators in a network of wireless devices.

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

“…The electronic system consists of 1) an external system that delivers the HF current bursts for WPT and bidirectional communications, and 2) wireless devices for EMG sensing and electrical stimulation. This electronic system was extensively described in (17). Yet some aspects are highlighted here to facilitate the reading of this first- in-human demonstration.…”

“…This is also done with the two proximal contacts located in the top and bottom layers of the filament, creating a proximal electrode of 3.8 mm 2 (Electrode ‘B’). The impedance magnitude of the electrodes, as measured across two electrode pairs in 0.9% NaCl, is flat beyond 10 kHz, and lower than 190 Ω at 1 MHz (17). The distance between the electrodes’ centers is 30 mm.…”

“…The analog front-end (AFE) designed is explained in detail in (17). It has been configured to have a gain of 54 dB, a signal-to-noise ratio (SNR) of 46 dB, and a common mode rejection ratio (CMRR) of 88 dB.…”

“…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-of- concept 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.…”

“…These are proof-of- concept 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). The devices were evaluated using an agar phantom and in hindlimbs of anesthetized rabbits.…”