Emma Brunton

Characteristics of electrode impedance and stimulation efficacy of a chronic cortical implant using novel annulus electrodes in rat motor cortex

OBJECTIVE Cortical neural prostheses with implanted electrode arrays have been used to restore compromised brain functions but concerns remain regarding their long-term stability and functional performance. APPROACH Here we report changes in electrode impedance and stimulation thresholds for a custom-designed electrode array implanted in rat motor cortex for up to three months. MAIN RESULTS The array comprises four 2000 µm long electrodes with a large annular stimulating surface (7860-15700 µm(2)) displaced from the penetrating insulated tip. Compared to pre-implantation in vitro values there were three phases of impedance change: (1) an immediate large increase of impedance by an average of two-fold on implantation; (2) a period of continued impedance increase, albeit with considerable variability, which reached a peak at approximately four weeks post-implantation and remained high over the next two weeks; (3) finally, a period of 5-6 weeks when impedance stabilized at levels close to those seen immediately post-implantation. Impedance could often be temporarily decreased by applying brief trains of current stimulation, used to evoke motor output. The stimulation threshold to induce observable motor behaviour was generally between 75-100 µA, with charge density varying from 48-128 µC cm(-2), consistent with the lower current density generated by electrodes with larger stimulating surface area. No systematic change in thresholds occurred over time, suggesting that device functionality was not compromised by the factors that caused changes in electrode impedance. SIGNIFICANCE The present results provide support for the use of annulus electrodes in future applications in cortical neural prostheses.

In vivo comparison of the charge densities required to evoke motor responses using novel annular penetrating microelectrodes

[This corrects the article on p. 265 in vol. 9, PMID: 26029097.].

A comparison of microelectrodes for a visual cortical prosthesis using finite element analysis.

Altering the geometry of microelectrodes for use in a cortical neural prosthesis modifies the electric field generated in tissue, thereby affecting electrode efficacy and tissue damage. Commonly, electrodes with an active region located at the tip (“conical” electrodes) are used for stimulation of cortex but there is argument to believe this geometry may not be the best. Here we use finite element analysis to compare the electric fields generated by three types of electrodes, a conical electrode with exposed active tip, an annular electrode with active area located up away from the tip, and a striped annular electrode where the active annular region has bands of insulation interrupting the full active region. The results indicate that the current density on the surface of the conical electrodes can be up to 10 times greater than the current density on the annular electrodes of the same height, which may increase the propensity for tissue damage. However choosing the most efficient electrode geometry in order to reduce power consumption is dependent on the distance of the electrode to the target neurons. If neurons are located within 10 μm of the electrode, then a small conical electrode would be more power efficient. On the other hand if the target neuron is greater than 500 μm away—as happens normally when insertion of an array of electrodes into cortex results in a “kill zone” around each electrode due to insertion damage and inflammatory responses—then a large annular electrode would be more efficient.

Restoration of vision using wireless cortical implants: The Monash Vision Group project

Monash Vision Group is developing a bionic vision system based on implanting several small tiles in the V1 region of the visual cortex. This cortical approach could benefit a greater proportion of people with total blindness than other approaches, as it bypasses the eyes and optic nerve. Each tile has 43 active electrodes on its base, and a wirelessly powered electronic system to decode control signals and drive the electrodes with biphasic pulses. The tiles are fed with power and data using a common transmitting coil at the back of the patients head. Sophisticated image processing, described in a companion paper, ensures that the user experiences maximum benefit from the small number of electrodes. This paper describes key features of this system.

Optimising electrode surface area to minimize power consumption in a cortical penetrating prosthesis

In the design of cortical stimulating prostheses for applications such as vision perception or motor control it is preferable to use wireless power and data transfer to maintain a biological barrier to infection. This puts a constraint on the amount of power that can be supplied to the implant, in turn limiting the power that can be used for stimulation. Design of electrodes for such prostheses have considered factors such as efficiency of stimulation and penetrating capacity; here we consider the design of electrodes from the power consumption perspective. We use the simple electrode geometry of a sphere to determine if the surface area of the electrode can be chosen in order to minimize the power consumed during stimulation. As is known to happen when an electrode is inserted to penetrate into brain tissue, we have assumed that, due to mechanical damage from electrode insertion and the response of brain tissue to the foreign body of an electrode, there will be a kill zone around the electrode where no viable neurons are present. Using realistic thicknesses for the kill zone from the published literature and our own unpublished work, we demonstrate that when the width of the kill zone is known, an electrodes surface area can be chosen so that the power consumed during stimulation is minimized.

Identification of sensory information in mixed nerves using multi-channel cuff electrodes for closed loop neural prostheses

The addition of sensory feedback is expected to greatly enhance the performance of motor neuroprostheses. In the case of stroke or spinal cord injured patients, sensory information can be obtained from electroneurographic (ENG) signals recorded from intact nerves in the non-functioning limb. Here, we aimed to identify sensory information recorded from mixed nerves using a multi-channel cuff electrode. ENG afferent signals were recorded in response to mechanical stimulation of the foot corresponding to three different functional types of sensory stimuli, namely: nociception, proprioception and touch. Offine digital signal processing was used to extract features for use as inputs for classification. A quadratic support vector machine was used to classify the data and the five fold cross validation error was measured. The results show that classification of nociceptive and proprioceptive stimuli is feasible, with cross validation errors of less than 10%. However, further work is needed to determine whether the touch information can be extracted more reliably from these recordings.

Separability of neural responses to standardised mechanical stimulation of limbs

Considerable scientific and technological efforts are currently being made towards the development of neural prostheses. Understanding how the peripheral nervous system responds to electro-mechanical stimulation of the limb, will help to inform the design of prostheses that can restore function or accelerate recovery from injury to the sensory motor system. However, due to differences in experimental protocols, it is difficult, if not impossible, to make meaningful comparisons between different peripheral nerve interfaces. Therefore, we developed a low-cost electronic system to standardise the mechanical stimulation of a rat’s hindpaw. Three types of mechanical stimulations, namely, proprioception, touch and nociception were delivered to the limb and the electroneurogram signals were recorded simultaneously from the sciatic nerve with a 16-contact cuff electrode. For the first time, results indicate separability of neural responses according to stimulus type as well as intensity. Statistical analysis reveal that cuff contacts placed circumferentially, rather than longitudinally, are more likely to lead to higher classification rates. This flexible setup may be readily adapted for systematic comparison of various electrodes and mechanical stimuli in rodents. Hence, we have made its electro-mechanical design and computer programme available online

Monash Vision Group’s Gennaris Cortical Implant for Vision Restoration

The Gennaris bionic vision system is a wireless device that has been designed to directly stimulate the primary visual cortex to restore useful vision to people with bilateral, irreversible blindness. Here, we describe the end-to-end system and the design of each component. The rationale for design decisions is provided, including the benefits of cortical stimulation, the need for wireless power and data transmission and the format of the autonomous implant tiles and penetrating micro-electrode arrays. We discuss the broad population of people for which this device may provide benefit, with reference to specific indications of blindness.

Multichannel cuff electrodes for peripheral nerve stimulation and recording

In the development of neuroprostheses to restore sensory and motor function to disabled patients the choice of the electrodes to be used remains an important consideration. The optimal electrode design should be minimally invasive and be capable of recording or stimulating selectively a large number of nerve fibers. Additionally, the electrodes should be capable of delivering stimulation within electrochemically safe limits. Here we report on the use of a multi-contact cuff electrode for stimulation and recording from peripheral nerves. Nerve cuffs with 16 electrodes, comprising 4 rings of 4 electrodes, were implanted around the sciatic nerve of two rats. The electromyogram signal (EMG) was recorded in response to electrical stimulation delivered by the electrodes, and the electroneurogram signal (ENG) was recorded in response to sensory stimulation applied to the ipsilateral foot. Visually detectable muscle movements were elicited with charge injections ranging from 4.6 to 8.2 nC. ENG recordings in response to sensory stimulus allowed for the onset and culmination of sensory stimulation to be detected using mean absolute value of the signal. Initial results indicate that flexion and extension of the ankle joint can be differentiated by combining information recorded from pairs of electrodes. The results of this study indicate that multi-contact cuffs can be used for decoding neural signals; however, more data needs to be collected for classification of sensory movements to be tested.

Chronic thresholds for evoking perceptual responses in the rat sensory cortex

Stimulation of neural tissue for the remediation of brain and sensory deficits requires that stimulation paradigms are selected carefully to deliver the most stable, efficient and safest stimulation to evoke the desired therapeutic response. Here we asked two questions of a penetrating cortical prosthesis use to evoke sensory-guided behavior: 1) does the threshold charge required to evoke a behavioral response change over time? 2) what effect does changing the frequency of stimulation have on stimulus threshold? To answer these questions we implanted a 4-electrode array into the somatosensory (tactile) cortex of a Sprague Dawley rat. The threshold charge to evoke a behavioral response was measured weekly over a 9-week period. Stimulation frequencies of 50 or 200 Hz were used, while all other stimulus parameters were kept constant. Within a maximum current limit of 100 μA and with a pulse width of 200 μs, we reliably elicited a behavioral response on 2 electrodes. Over the 9 week implantation period there was an initial increase in threshold current at 4 weeks, followed by a decrease at week 5 post-implantation; by week 8 post-implantation, thresholds appeared to have stabilized. Although we could reliably evoke a response at both 50 and 200 Hz, the stimulus frequency of 50 Hz required on average a lower threshold charge to evoke a response.