Showing papers by "Nigel H. Lovell published in 2018"

Computationally Efficient Adaptive Error-State Kalman Filter for Attitude Estimation

Abstract: A geometrically intuitive quaternion-based complementary attitude and heading reference system (CAHRS) proposed in our previous work estimated the attitude of a magnetic and inertial measurement unit (MIMU). The method used two correction factors, $\mu _{_{a}}$ that determined the rate at which the accelerometer corrected the inclination angle, and $\mu _{_{m}}$ that governed the rate at which the magnetometer corrected the yaw angle. Improvements to the filter have been made by embedding each correction factor within an error-state Kalman filter (KF), enabling the correction rates to behave adaptively. The revised filter only estimates the error in two variables, thus remaining computationally efficient (65 addition, 88 subtraction, and 214 multiplication operations) compared with established algorithms in the literature for attitude estimation that utilize a KF or extended KF. The accuracy of the attitude estimated (i.e., the pitch, roll, and yaw angle errors $\theta _{\text {RMSE}_{_{}}}^{\circ }$ , $\phi _{\text {RMSE}}^{\circ }$ , and $\psi _{\text {RMSE}}^{\circ }$ ) by the adaptive error-state KF was compared with the CAHRS algorithm and a cascaded KF that is representative of state-of-the-art methods. Each algorithm was assessed using a publicly available data set in which the attitude of a foot-worn magnetic and inertial measurement units was recorded by a motion capture system while participants walked and ran around a room for one or three minutes ( $\phi _{\text {RMSE}}^{\circ } = 2.08^\circ $ , $\theta _{\text {RMSE}}^{\circ } = 1.98^\circ $ , and $\psi _{\text {RMSE}}^{\circ }= 5.25^\circ $ ).

A Low-Power Fall Detector Balancing Sensitivity and False Alarm Rate

Abstract: Falls in older people are a major challenge to public health. A wearable fall detector can detect falls automatically based on kinematic information of the human body, allowing help to arrive sooner. To date, most studies have focused on the accuracy of an offline algorithm to distinguish real-world or simulated falls from activities of daily living, while neglecting the false alarm rate and battery life of a real device. To address these two important metrics, which significantly influence user compliance, this paper proposes a low-power fall detector using triaxial accelerometry and barometric pressure sensing. This fall detector minimizes power consumption using both hardware- and firmware-based techniques. Additionally, the fall detection algorithm used in this device is optimized to achieve a balance between sensitivity and false alarm rate, while minimizing the power consumption due to algorithm execution. The fall detector achieved a high sensitivity (91%) with a low false alarm rate (0.1149 alarms per hour), and a commercially-viable battery life (1125 days).

Closed-Loop Efficient Searching of Optimal Electrical Stimulation Parameters for Preferential Excitation of Retinal Ganglion Cells.

Abstract: The ability for visual prostheses to preferentially activate functionally-distinct retinal ganglion cells (RGCs) is important for improving visual perception. This study investigates the use of high frequency stimulation (HFS) to elicit RGC activation, using a closed-loop algorithm to search for optimal stimulation parameters for preferential ON and OFF RGC activation, resembling natural physiological neural encoding in response to visual stimuli. We evaluated the performance of a wide range of electrical stimulation amplitudes and frequencies on RGC responses in vitro using murine retinal preparations. It was possible to preferentially excite either ON or OFF RGCs by adjusting amplitudes and frequencies in HFS. ON RGCs can be preferentially activated at relatively higher stimulation amplitudes (>150 μA) and frequencies (2-6.25 kHz) while OFF RGCs are activated by lower stimulation amplitudes (40-90 μA) across all tested frequencies (1-6.25 kHz). These stimuli also showed great promise in eliciting RGC responses that parallel natural RGC encoding: ON RGCs exhibited an increase in spiking activity during electrical stimulation while OFF RGCs exhibited decreased spiking activity, given the same stimulation amplitude. In conjunction with the in vitro studies, in silico simulations indicated that optimal HFS parameters could be rapidly identified in practice, whilst sampling spiking activity of relevant neuronal subtypes. This closed-loop approach represents a step forward in modulating stimulation parameters to achieve appropriate neural encoding in retinal prostheses, advancing control over RGC subtypes activated by electrical stimulation.

Conductive Hydrogel Electrodes for Delivery of Long-Term High Frequency Pulses.

Abstract: Nerve block waveforms require the passage of large amounts of electrical energy at the neural interface for extended periods of time. It is desirable that such waveforms be applied chronically, consistent with the treatment of protracted immune conditions, however current metal electrode technologies are limited in their capacity to safely deliver ongoing stable blocking waveforms. Conductive hydrogel (CH) electrode coatings have been shown to improve the performance of conventional bionic devices, which use considerably lower amounts of energy than conventional metal electrodes to replace or augment sensory neuron function. In this study the application of CH materials was explored, using both a commercially available platinum iridium (PtIr) cuff electrode array and a novel low-cost stainless steel (SS) electrode array. The CH was able to significantly increase the electrochemical performance of both array types. The SS electrode coated with the CH was shown to be stable under continuous delivery of 2 mA square pulse waveforms at 40,000 Hz for 42 days. CH coatings have been shown as a beneficial electrode material compatible with long-term delivery of high current, high energy waveforms.

A biopotential optrode array: operation principles and simulations

Abstract: We propose an optical electrode ’optrode’ sensor array for biopotential measurements. The transduction mechanism is based on deformed helix ferroelectric liquid crystals which realign, altering the optrode’s light reflectance properties, relative to applied potential fields of biological cells and tissue. A computational model of extracellular potential recording by the optrode including the electro-optical transduction mechanism is presented, using a combination of time-domain and frequency-domain finite element analysis. Simulations indicate that the device has appropriate temporal response to faithfully transduce neuronal spikes, and spatial resolution to capture impulse propagation along a single neuron. These simulations contribute to the development of multi-channel optrode arrays for spatio-temporal mapping of electric events in excitable biological tissue.

A Multiphysics Biventricular Cardiac Model: Simulations With a Left-Ventricular Assist Device

Abstract: Computational models have become essential in predicting medical device efficacy prior to clinical studies. To investigate the performance of a left-ventricular assist device (LVAD), a fully-coupled cardiac fluid-electromechanics finite element model was developed, incorporating electrical activation, passive and active myocardial mechanics, as well as blood hemodynamics solved simultaneously in an idealized biventricular geometry. Electrical activation was initiated using a simplified Purkinje network with one-way coupling to the surrounding myocardium. Phenomenological action potential and excitation-contraction equations were adapted to trigger myocardial contraction. Action potential propagation was formulated within a material frame to emulate gap junction-controlled propagation, such that the activation sequence was independent of myocardial deformation. Passive cardiac mechanics were governed by a transverse isotropic hyperelastic constitutive formulation. Blood velocity and pressure were determined by the incompressible Navier-Stokes formulations with a closed-loop Windkessel circuit governing the circulatory load. To investigate heart-LVAD interaction, we reduced the left ventricular (LV) contraction stress to mimic a failing heart, and inserted a LVAD cannula at the LV apex with continuous flow governing the outflow rate. A proportional controller was implemented to determine the pump motor voltage whilst maintaining pump motor speed. Following LVAD insertion, the model revealed a change in the LV pressure-volume loop shape from rectangular to triangular. At higher pump speeds, aortic ejection ceased and the LV decompressed to smaller end diastolic volumes. After multiple cycles, the LV cavity gradually collapsed along with a drop in pump motor current. The model was therefore able to predict ventricular collapse, indicating its utility for future development of control algorithms and pre-clinical testing of LVADs to avoid LV collapse in recipients.

Evaluation of an mHealth-Based Adjunct to Outpatient Cardiac Rehabilitation

Abstract: A pilot study was conducted to determine if a smartphone-based adjunct to standard care could increase the completion rate of a cardiac rehabilitation program (CRP). Based on historical completion rates, 66 participants who were about to commence a hospital-based CRP were randomized so that half received three devices embedded with near-field communication, namely, a smartphone [pre-installed with an application (app) designed specifically for cardiac rehabilitation], portable blood pressure monitor, and weight scale while completing the CRP. The completion rate among participants who were randomized to the intervention group was 88%, compared to 67% in the control group ( ${p}$ = 0.038). This combined with the week-to-week frequency with which participants in the intervention group measured their blood pressure ( $\times$ 5/week) demonstrated the ability of the intervention to increase the proportion of patients who completed the CRP. No significant differences were found between the treatment groups for the measurements taken at baseline and prior to discharge from the CRP. A statistically significant correlation ( ${r}$ = 0.472; ${p}$ = 0.013) was found between the average time participants walked each day (as estimated via the smartphone app) and participants’ six minute walking distance (6MWD) before they were discharged from the CRP (a clinically validated measurement).

Differential electrical responses in retinal ganglion cell subtypes: effects of synaptic blockade and stimulating electrode location.

Abstract: OBJECTIVE Visual prostheses have shown promising results in restoring visual perception to blind patients. The ability to differentially activate retinal ganglion cell (RGC) subtypes could further improve the efficacy of these medical devices. APPROACH Using whole-cell patch clamp, we investigated membrane potential differences between ON and OFF RGCs in the mouse retina when their synaptic inputs were blocked by synaptic blockers, and examined the differences in stimulation thresholds under such conditions. By injecting intracellular current, we further confirmed the relationship between RGC stimulation thresholds and resting membrane potentials (RMPs). In addition, we investigated the effects of stimulating electrode location on the differences in stimulation thresholds between ON and OFF RGCs. MAIN RESULTS With synaptic blockade, ON RGCs became significantly more hyperpolarized (from -61.8 ± 1.4 mV to -70.8 ± 1.6 mV), while OFF RGCs depolarized slightly (from -60.5 ± 0.7 mV to -58.6 ± 0.9 mV). RGC stimulation thresholds were negatively correlated with their RMPs (Pearson r value: -0.5154; p-value: 0.0042). Thus, depriving ON RGCs of synaptic inputs significantly increased their thresholds (from 14.7 ± 1.3 µA to 22.3 ± 2.1 µA) over those of OFF RGCs (from 13.2 ± 0.7 µA to 13.1 ± 1.1 µA). However, with control solution, ON and OFF RGC stimulation thresholds were not significantly different. Finally, placement of the stimulating electrode away from the axon enhanced differences in stimulation thresholds between ON and OFF RGCs, facilitating preferential activation of OFF RGCs. SIGNIFICANCE Since ON and OFF RGCs have antagonistic responses to natural light, achieving differential RGC activation could convey more natural visual information, leading to better visual prosthesis outcomes.

Adaptive template matching of photoplethysmogram pulses to detect motion artefact.

Abstract: OBJECTIVE The photoplethysmography (PPG) signal, commonly used in the healthcare settings, is easily affected by movement artefact leading to errors in the extracted heart rate and SpO2 estimates. This study aims to develop an online artefact detection system based on adaptive (dynamic) template matching, suitable for continuous PPG monitoring during daily living activities or in the intensive care units (ICUs). APPROACH Several master templates are initially generated by applying principal component analysis to data obtained from the PhysioNet MIMIC II database. The master template is then updated with each incoming clean PPG pulse. The correlation coefficient is used to classify the PPG pulse into either good or bad quality categories. The performance of our algorithm was evaluated using data obtained from two different sources: (i) our own data collected from 19 healthy subjects using the wearable Sotera Visi Mobile system (Sotera Wireless Inc.) as they performed various movement types; and (ii) ICU data provided by the PhysioNet MIMIC II database. The developed algorithm was evaluated against a manually annotated 'gold standard' (GS). MAIN RESULTS Our algorithm achieved an overall accuracy of 91.5% ± 2.9%, with a sensitivity of 94.1% ± 2.7% and a specificity of 89.7% ± 5.1%, when tested on our own data. When applying the algorithm to data from the PhysioNet MIMIC II database, it achieved an accuracy of 98.0%, with a sensitivity and specificity of 99.0% and 96.1%, respectively. SIGNIFICANCE The proposed method is simple and robust against individual variations in the PPG characteristics, thus making it suitable for a diverse range of datasets. Integration of the proposed artefact detection technique into remote monitoring devices could enhance reliability of the PPG-derived physiological parameters.

Comparing perilymph proteomes across species.

Abstract: Objectives/Hypothesis Biological components of perilymph affect the electrical performance of cochlear implants. Understanding the perilymph composition of common animal models will improve the understanding of this impact and improve the interpretation of results from animal studies and how it relates to humans. Study Design Analysis and comparison of the proteomes of human, guinea pig, and cat perilymph. Methods Multiple perilymph samples from both guinea pigs and cats were analysed via liquid chromatography with tandem mass spectrometry. Proteins were identified using the Mascot database. Human data were obtained from a published dataset. Proteins identified were refined to form a proteome for each species. Results Over 200 different proteins were found per species. There were 81, 39, and 64 proteins in the final human, guinea pig, and cat proteomes, respectively. Twenty-one proteins were common to all three species. Fifty-two percent of the cat proteome was found in the human proteome, and 31% of the guinea pig was common to human. The cat proteome had similar complexity to the human proteome in three protein classes, whereas the guinea pig had a similar complexity in two. The presence of albumin was significantly higher in human perilymph than in the other two species. Immunoglobulins were more abundant in the human than in the cat proteome. Conclusions Perilymph proteomes were compared across three species. The degree of crossover of proteins of both guinea pig and cat with human indicate that these animals suitable models for the human cochlea, albeit the cat perilymph is a closer match. Level of Evidence NA Laryngoscope, 2017

Long-term anesthetic protocol in rats: feasibility in electrophysiology studies in visual prosthesis

Abstract: Electrical stimulation of excitable cells provides therapeutic benefits for a variety of medical conditions, including restoration of partial vision to those blinded via some types of retinal degeneration. To improve visual percepts elicited by the current technology, researchers are conducting acute electrophysiology experiments, mainly in cats. However, the rat can provide a model of a range of retinal diseases and possesses a sufficiently large eye to be used in this field. This article presents a long-term anesthetic protocol to enable electrophysiology experiments to further the development of visual prostheses. Six Long-Evans rats (aged between 14 and 16 weeks) were included in this study. Surgical anesthesia was maintained for more than 15 h by combining constant intravenous infusion of ketamine (24.0-34.5 mg/kg/h), xylazine (0.9-1.2 mg/kg/h), and inhaled isoflurane in oxygen (<0.5%). Overall heart rate, respiratory rate, and body temperature remained between 187-233 beats/min, 45-58 breaths/min, and 36-38 °C, respectively. Neural responses to 200-ms light pulses were recorded from the superior colliculus using a 32-channel neural probe at the beginning and before termination of the experiment. Robust responses were recorded from distinct functional types of retinal pathways. In addition, a platinum electrode was implanted in the retrobulbar space. The retina was electrically stimulated, and the activation threshold was determined to be 5.24 ± 0.24 μC/cm2 . This protocol may be used not only in the field of visual prosthesis research, but in other research areas requiring longer term acute experiments.

Detection of Atrial Fibrillation from RR Intervals and PQRST Morphology using a Neural Network Ensemble

Abstract: Early detection and discrimination of cardiac arrhythmia, atrial fibrillation (AF) in particular, is essential for timely intervention to improve patient outcomes. In this work, an algorithm was developed to classify ECG records as normal, AF, other arrhythmia, or too noisy to classify. This algorithm, which was an entry for the PhysioNet Computing in Cardiology Challenge 2017 (the Challenge), is described. Artifact masking and QRS detection were applied to lead-I equivalent ECG records and 17 features were extracted which captured the irregularity of the RR intervals, the PQRST morphology, and artifact/noise. An ensemble of ten neural networks (NN) was trained on the features from a training set of 5,970 records. A final classification was taken by majority vote over the 10 classifiers. The trained NN models were validated on a further 2,558 ECG records and then tested on a blind out-of-sample test set of 3,658 records. A mean $F_{1}$ score across the four classes of 0.78 for the training/validation sets and 0.80 for the testing set was achieved. A higher $F_{1}$ score for the testing set indicates that overtraining did not occur, unlike most entries to the Challenge (winner mean $F_{1}$ score of 0.89 for training/validation set, and 0.83 for testing set). Performance of the Challenge winner was not ideal and there is evidence of overtraining, indicating the difficulty of classifying AF from single-lead ECG. The features and method described here performed comparably and overtraining did not occur (high likelihood of generalization) indicating a good starting point for future work.

Stimulation of peripheral nerves using conductive hydrogel electrodes

Abstract: Nerve block via electrical stimulation of nerves requires a device capable of transferring large amounts of charge across the neural interface on chronic time scales. Current metal electrode designs are limited in their ability to safely and effectively deliver this charge in a stable manner. Conductive hydrogel (CH) coatings are a promising alternative to metal electrodes for neural interfacing devices. This study assessed the performance of CH electrodes compared to platinum-iridium (PtIr) electrodes in commercial nerve cuff devices in both the in vitro and acute in vivo environments. CH electrodes were found to have higher charge storage capacities and lower impedances compared to bare PtIr electrodes. Application of CH coatings also resulted in a three-fold increase in in vivo charge injection limit. These significant improvements in electrochemical properties will allow for the design of smaller and safer stimulating devices for nerve block applications.

Multi-optrode arrays: a new path towards brain/machine interface

Abstract: Brain/machine interfaces will play a significant role in the coming decades. As a means to enhance human abilities they are set to fundamentally change the course of humanity. Today, there is no clear winner in the race towards a practical in-vivo interface. We present here the latest work completed at UNSW using multi-optrode arrays, a technology based on ferroelectric liquid crystals that has the potential to deliver 1 million neuronal connections: a stated goal of the DARPA program.

Computational Models And Tools For Developing Sophisticated Stimulation Strategies For Retinal Neuroprostheses

Abstract: Improvements to the efficacy of retinal neuroprostheses can be achieved by developing more sophisticated neural stimulation strategies to enable selective or preferential activation of specific retinal ganglion cells (RGCs). Computational models are particularly well suited for these investigations. The electric field can be accurately described by mathematical formalisms, and the population-based neural responses to the electrical stimulation can be investigated at resolutions well beyond those achievable by current state-of-the-art biological techniques. In this study, we used a biophysically-and morphologically-detailed RGC model to explore the ability of high frequency electrical stimulation (HFS) to preferentially activate ON and OFF RGC subtypes, the two major information pathways of the retina. The performance of a wide range of electrical stimulation amplitudes (0 - $100~\mu \mathbf {A}$) and frequencies (1 – 10 kHz) on functionally-distinct RGC responses were evaluated. We found that ON RGCs could be preferentially activated at relatively higher stimulation amplitudes $( > 50 {\mu } \mathrm {A})$ and frequencies $( >2$ kHz) while OFF RGCs were activated by lower stimulation amplitudes (10 to $50 {\mu } \mathrm {A})$ across all tested frequencies. These stimuli also show great promise in eliciting RGC responses that parallel RGC encoding: one RGC type exhibited an increase in spiking activity during electrical stimulation whilst another exhibited decreased spiking activity, given the same stimulation parameters.

A Multi-Domain Continuum Model of Electrical Stimulation of Healthy and Degenerate Retina

Abstract: A continuum multi-domain model of electrical stimulation of the retina is presented and validated against retinal ganglion cell (RGC) excitation thresholds reported in a recently published in vitro experimental study. We applied our model to investigate the response of the RGC layer to electrical stimulation during mid-to-late stage retinal degeneration for both epiretinal and suprachoroidal configurations. Interestingly, our model predicted that suprachoroidal stimulation of the degenerate retina required increased current thresholds, mainly because of the presence of the glial scar layer. In contrast, epiretinal stimulation thresholds were almost similar for both healthy and degenerate models. The latter finding implies that there is no influence of the glial scar layer on epiretinal stimulation current thresholds.

Computational Simulation of Array-based Electroporation in the Cochlea

Abstract: We present a computational model for predicting electric field distributions following array-based closed-loop electroporation in the cochlea. The model geometry was reconstructed from magnetic resonance images of the guinea pig cochlea and an eight-channel electrode array embedded within this geometry. The model’s electrode voltage output waveform was obtained from electric potential mapping conducted in physiological solution following constant-current stimulation using the electrode array. Our simulations predict that a tandem electrode configuration with four ganged cathodes and four ganged anodes produces a larger area in target tissue where the electric field is within the range for successful gene transfer compared to an alternate paired anode-cathode electrode configuration. These findings corroborate published in vivo evidence comparing the two configurations and support the utility of the developed model as a tool to optimize the efficacy of electroporation electrodes.