Battery-powered implantable nerve stimulator for chronic activation of two skeletal muscles using multichannel techniques

Choosing the right electrode representation for modeling real bioelectronic interfaces: a comprehensive guide

Objective.Producing realistic numerical models of neurostimulation electrodes in contact with the electrolyte and tissue, for use in time-domain finite element method simulations while maintaining a reasonable computational burden remains a challenge. We aim to provide a straightforward experimental-theoretical hybrid approach for common electrode materials (Ti, TiN, ITO, Au, Pt, IrOx) that are relevant to the research field of bioelectronics, along with all the information necessary to replicate our approach in arbitrary geometry for real-life experimental applications.Approach.We used electrochemical impedance spectroscopy (EIS) to extract the electrode parameters in the AC regime under different DC biases. The pulsed electrode response was obtained by fast amperometry (FA) to optimize and verify the previously obtained electrode parameters in a COMSOL Multiphysics model. For optimization of the electrode parameters a constant phase element (CPE) needed to be implemented in time-domain.Main results.We find that the parameters obtained by EIS can be used to accurately simulate pulsed response only close to the electrode open circuit potential, while at other potentials we give corrections to the obtained parameters, based on FA measurements. We also find that for many electrodes (Au, TiN, Pt, and IrOx), it is important to implement a distributed CPE rather than an ideal capacitor for estimating the electrode double-layer capacitance. We outline and provide examples for the novel time-domain implementation of the CPE for finite element method simulations in COMSOL Multiphysics.Significance.An overview of electrode parameters for some common electrode materials can be a valuable and useful tool in numerical bioelectronics models. A provided FEM implementation model can be readily adapted to arbitrary electrode geometries and used for various applications. Finally, the presented methodology for parametrization of electrode materials can be used for any materials of interest which were not covered by this work.

Micropyramid structured photo capacitive interfaces

Optically driven electronic neuromodulation devices are a novel tool in basic research and offer new prospects in medical therapeutic applications. Optimal operation of such devices requires efficient light capture and charge generation, effective electrical communication across the device's bioelectronic interface, conformal adhesion to the target tissue, and mechanical stability of the device during the lifetime of the implant-all of which can be tuned by spatial structuring of the device. We demonstrate a 3D structured opto-bioelectronic device-an organic electrolytic photocapacitor spatially designed by depositing the active device layers on an inverted micropyramid-shaped substrate. Ultrathin, transparent, and flexible micropyramid-shaped foil was fabricated by chemical vapour deposition of parylene C on silicon moulds containing arrays of inverted micropyramids, followed by a peel-off procedure. The capacitive current delivered by the devices showed a strong dependency on the underlying spatial structure. The device performance was evaluated by numerical modelling. We propose that the developed numerical model can be used as a basis for the design of future functional 3D design of opto-bioelectronic devices and electrodes.

Strategies for generating prolonged functional standing using intramuscular stimulation or intraspinal microstimulation

Spinal cord injury (SCI) often results in the loss of the ability to stand. The goal of this study was to implement a functional electrical stimulation (FES) system for restoring prolonged periods of standing after SCI. For this purpose, we tested two control strategies: open-loop and closed-loop control, and two stimulation paradigms: non-interleaved intramuscular stimulation (IM-S) and interleaved intraspinal microstimulation (ISMS). The experiments were conducted in anesthetized cats. Stimulation was applied to the muscles through IM-S electrodes implanted in the main knee and ankle extensor muscles, or to the spinal cord through ultra-fine ISMS wires implanted within the ventral horn of the lumbosacral enlargement. The cats were partially supported over parallel force plates and accelerometers were secured to the hindlimbs above and below the ankle joint. Ground reaction forces and knee and ankle joint angles were measured by the force plates and accelerometers, respectively. The closed-loop controller used these feedback signals to modulate the amplitude of stimulation applied to the extensor muscles. The open-loop controller applied constant levels of stimulation which were determined before the onset of each trial. The duration of standing achieved using closed-loop control of IM-S was significantly longer than that achieved with open-loop control (approximately 2 times longer). The increase in the duration of standing corresponded with a decrease in the rate of force decay and a lower average injected current during closed-loop control. Standing was further improved with the use of ISMS. Closed-loop control of interleaved ISMS resulted in a period of standing > 3 times longer than the best trial generated using non-interleaved IM-S. There was also a significant improvement in the balance of force between the two hindlimbs. The results suggest that a system which uses closed-loop control in conjunction with interleaved ISMS could achieve prolonged FES standing in people with SCI.

An implantable bi-directional wireless transmission system for transcutaneous biological signal recording

This study presents an implantable microcontroller-based bi-directional transmission system with an inductive link designed for biological signal sensing. The system comprises an external module and an implantable module. The external module incorporates a high-efficiency class-E transceiver with amplitude modulation scheme and a data recovery reader. The transceiver sends both power and commands to the implanted module, while the reader recovers the recorded biological signal data and transmits the data to a personal computer (PC) for further data processing. To reduce the effects of interference induced by the 2 MHz carrier signal, the implanted module uses two separate coils to perform the necessary two-way data transmission. The outward backward telemetry circuitry of the implanted module was based on the loadshift keying (LSK) technique. The transmitted sensed signal had a 10-bit resolution and a read-out rate of 115 kbps. The implanted module, measuring 4.5 x 3 x 1.2 cm3, was successfully verified in animal experiment in which the electroneurogram (ENG) signal was recorded from the sciatic nerve of New Zealand rabbits in response to nociceptive stimulation of foot. The reliable operating distance of the system was within about 3.5 cm with an efficiency of around 25%. Our present study confirms that the proposed biological signal sensing device is suitable for various implanted applications following an appropriate biocompatible packaging procedure.

Functional and biological test of a 20 channel implantable stimulator in sheep in view of functional electrical stimulation walking for spinal cord injured persons

A newly developed implantable stimulator with 20 output channels, mainly intended for the stimulation of lower extremities in paraplegics, was implanted in 6 sheep over a time period of 26 weeks. Five epineural electrodes each were used to contact various nerves at different locations to elicit hip and knee extension and flexion and to make carrousel and selective stimulation possible. Different electrode application strategies in view of paraplegic standing and walking were investigated. Additional implanted electrodes allowed M-wave monitoring for selectivity investigations in 3 sheep. Stimulator, electrode leads, and electrodes proved to be reliable. Selective stimulation with electrodes placed on the trunk of the sciatic nerve could be demonstrated but with bad reproducibility. Histological investigation of the tissues surrounding electrodes and leads showed the expected stable foreign body response. Strong hip and knee extension could be gained in all cases while only weak flexion forces could be elicited in most cases. Muscle biopsies showed that daily stimulation for 8 h at threshold level caused an increase in muscle Type I fibers and a decrease in Type IIc fibers. Implants and electrodes fulfill the most important functional and biological criteria for their clinical application for paraplegic walking. The intention to provide selective flexion functions via epineural stimulation could not be demonstrated sufficiently in this animal model.

Basic design and construction of the Vienna FES implants: existing solutions and prospects for new generations of implants

We can distinguish 3 generations of FES implants for activation of neural structures: 1. RF-powered implants with antenna displacement dependent stimulation amplitude; 2. RF-powered implants with stabilised stimulation amplitude; and 3. battery powered implants. In Vienna an 8-channel version of the second generation type has been applied clinically to mobilisation of paraplegics and phrenic pacing. A 20-channel implant of the second generation type for mobilisation of paraplegics and an 8-channel implant of the third generation type for cardiac assist have been tested in animal studies. A device of completely new design for direct stimulation of denervated muscles is being tested in animal studies. There is a limited choice of technologically suitable biocompatible and bioresistant materials for implants. The physical design has to be anatomically shaped without corners or edges. Electrical conductors carrying direct current (D.C.) have to be placed inside a hermetic metal case. The established sealing materials, silicone rubber and epoxy resin, do not provide hermeticity and should only embed DC-free components. For electrical connections outside the hermetic metal case welding is preferable to soldering; conductive adhesives should be avoided. It is advisable to use a hydrophobic oxide ceramic core for telemetry antenna coils embedded in sealing polymer. Cleaning of all components before sealing in resin is of the utmost importance as well as avoidance of rapid temperature changes during the curing process.