Design, characterization and testing of a thin-film microelectrode array and signal conditioning microchip for high spatial resolution surface laplacian measurement.

Cardiac mapping has become an important area of research for understanding the mechanisms responsible for cardiac arrhythmias and the associated diseases. Current technologies for measuring electrical potentials on the surface of the heart are limited due to poor spatial resolution, localization issues, signal distortion due to noise, tissue damage, etc. Therefore, the purpose of this study is to design, develop, characterize and investigate a custom-made microfabricated, polyimide-based, flexible Thin-Film MicroElectrode Array (TFMEA) that is directly interfaced to an integrated Signal Conditioning Microchip (SCM) to record cardiac surface potentials on the cellular level to obtain high spatial resolution Surface Laplacian (SL) measurement. TFMEAs consisting of five fingers (Cover area = 4 mm2 and 16 mm2), which contained five individual microelectrodes placed in orthogonal directions (25-µm in diameter, 75-µm interelectrode spacing) to one another, were fabricated within a flexible polyimide substrate and capable of recording electrical activities of the heart on the order of individual cardiomyocytes. A custom designed SCM consisting of 25 channels of preamplification stages and second order band-pass filters was interfaced directly with the TFMEA in order to improve the signal-to-noise ratio (SNR) characteristics of the high spatial resolution recording data. Metrology characterization using surface profilometry and high resolution Scanning Electron Microscope (SEM) indicated the geometry of fabricated TFMEAs closely matched the design parameters \u3c 0.4%). The DC resistances of the 25 individual micro electrodes were consistent (1.050 ± 0.026 kO). The simulation and testing results of the SCM verified the pre-amplification and filter stages met the designed gain and frequency parameters within 2.96%. The functionality of the TFMEA-SCM system was further characterized on a TX 151 conductive gel. The characterization results revealed that the system functionality was sufficient for high spatial cardiac mapping. In vivo testing results clearly demonstrated feasibility of using the TFMEA-SCM system to obtain cellular level SL measurements with significantly improved the SNRs during normal sinus rhythm and Ventricular Fibrillation (VF). Local activation times were detected via evaluating the zero crossing of the SL electro grams, which coincided with the gold standard (dV/dt)min of unipolar electro grams within ± 1%. The in vivo transmembrane current densities calculated from the high spatial resolution SLs were found to be significantly higher than the transmembrane current densities computed using electrodes with higher interelectrode spacings. In conclusion, the custom-made TFMEASCM systems demonstrated feasibility as a tool for measuring cardiac potentials and to perform high resolution cardiac mapping experiments

Design and Optimisation of Extracellular Microelectrodes

The work described in this thesis concerns the development and application of design methods for the optimisation of thin film metal microelectrodes, to be used for recording the electrical signals generated by neurons in culture

Analysis of Factors Affecting the Performance of Retinal Prostheses Using Finite Element Modelling of Electric Field Distribution in the Retina

This dissertation proposes a computational framework targeted at improving the design of currently employed retinal prostheses. The framework was used for analysing factors impacting the performance of prostheses in terms of electrical stimulation for retinal neurons, which might lead to a perception of pixelated vision. Despite their demonstrated effectiveness, the chronic and safe usage of these retinal prostheses in human and animal trials is jeopardised due to high stimulation thresholds. This is related to the distance between the stimulating electrodes and the retinal neurons resulting from the implantation procedure. The major goal of this dissertation was to evaluate the stimulation efficacy in current implantable planar microelectrode-based retinal prostheses and consequently demonstrate their weakness, thereby providing scope for the development of future implants. The effect of geometrical factors i.e., electrode-retina distance and electrode size on stimulation applied to the retina by retinal prostheses was studied. To this end, a finite element method based simulation framework to compute electric field distribution in the retina was constructed. An electrical model of the retina was an integral part of the framework, essentially represented by a resistivity profile of the multi-layered retina. The elements of a retinal prosthesis were modelled by incorporating realistic electrode sizes, an anatomical and electrical model of the retina, a precise positioning of stimulation and return electrodes and the location of the implant with respect to the retina representing the epiretinal and subretinal stimulation schemes. The simulations were carried out both in quasi-static and direct current (DC) modes. It was observed that electrode-electrolyte interface and tissue capacitance could be safely neglected in our model based on the magnitude of the applied voltage stimulus and frequencies under consideration. Therefore, all simulations were conducted in DC mode. Thresholds and lateral extents of the stimulation were computed for electrode sizes corresponding to existing and self-fabricated implants. The values and trends obtained were in agreement with experiments from literature and our collaborators at the les Hôpitaux Universitaires de Genève (HUG). In the subretinal stimulation scheme, the computed variation of impedance with electrode-retina distance correlated well with time varying in vivo impedance measurements in rats conducted in collaboration with the Institut de la Vision, INSERM, Paris. Finally, it was also reiterated that the currently employed retinal prostheses are not very efficient due to a significant distance between the stimulation electrode and the retinal cells. In addition, I present a new experimental technique for measuring the absolute and local resistivity profile in high-resolution along the retinal depth, based on impedance spectroscopy using a bipolar microprobe. This experiment was devised to extract the resistivity profile of an embryonic chick retina to construct an electrical model for the simulation framework to simulate in vitro retinal stimulation experiments conducted by HUG collaborators. We validated the capability of the technique in rat and embryonic chick retinas. In conclusion, the computational framework presented in this dissertation is more realistic than those found in literature, but represents only a preliminary step towards an accurate model of a real implantation scenario in vivo. The simulation results are in agreement with results from clinical trials in humans for epiretinal configuration (literature) and with in vitro results for epiretinal and subretinal stimulation applied to chick retinas (HUG). The developed simulation framework computes quantities that can form a reference for quality control during surgery while inserting implants in the eye and functionality checks by electrophysiologists. Furthermore, this framework is useful in deciding the specifications of stimulation electrodes such as optimal size, shape, material, array density, and the position of the reference electrode to name a few. The work presented here offers to aid in optimising retinal prostheses and implantation procedures for patients and eventually contributes towards improving their quality of life

High Resolution Multi-parametric Diagnostics and Therapy of Atrial Fibrillation: Chasing Arrhythmia Vulnerabilities in the Spatial Domain

After a century of research, atrial fibrillation (AF) remains a challenging disease to study and exceptionally resilient to treatment. Unfortunately, AF is becoming a massive burden on the health care system with an increasing population of susceptible elderly patients and expensive unreliable treatment options. Pharmacological therapies continue to be disappointingly ineffective or are hampered by side effects due to the ubiquitous nature of ion channel targets throughout the body. Ablative therapy for atrial tachyarrhythmias is growing in acceptance. However, ablation procedures can be complex, leading to varying levels of recurrence, and have a number of serious risks. The high recurrence rate could be due to the difficulty of accurately predicting where to draw the ablation lines in order to target the pathophysiology that initiates and maintains the arrhythmia or an inability to distinguish sub-populations of patients who would respond well to such treatments. There are electrical cardioversion options but there is not a practical implanted deployment of this strategy. Under the current bioelectric therapy paradigm there is a trade-off between efficacy and the pain and risk of myocardial damage, all of which are positively correlated with shock strength. Contrary to ventricular fibrillation, pain becomes a significant concern for electrical defibrillation of AF due to the fact that a patient is conscious when experiencing the arrhythmia. Limiting the risk of myocardial injury is key for both forms of fibrillation. In this project we aim to address the limitations of current electrotherapy by diverging from traditional single shock protocols. We seek to further clarify the dynamics of arrhythmia drivers in space and to target therapy in both the temporal and spatial domain; ultimately culminating in the design of physiologically guided applied energy protocols. In an effort to provide further characterization of the organization of AF, we used transillumination optical mapping to evaluate the presence of three-dimensional electrical substrate variations within the transmural wall during acutely induced episodes of AF. The results of this study suggest that transmural propagation may play a role in AF maintenance mechanisms, with a demonstrated range of discordance between the epicardial and endocardial dynamic propagation patterns. After confirming the presence of epi-endo dyssynchrony in multiple animal models, we further investigated the anatomical structure to look for regional trends in transmural fiber orientation that could help explain the spectrum of observed patterns. Simultaneously, we designed and optimized a multi-stage, multi-path defibrillation paradigm that can be tailored to individual AF frequency content in the spatial and temporal domain. These studies continue to drive down the defibrillation threshold of electrotherapies in an attempt to achieve a pain-free AF defibrillation solution. Finally, we designed and characterized a novel platform of stretchable electronics that provide instrumented membranes across the epicardial surface or implanted within the transmural wall to provide physiological feedback during electrotherapy beyond just the electrical state of the tissue. By combining a spatial analysis of the arrhythmia drivers, the energy delivered and the resulting damage, we hope to enhance the biophysical understanding of AF electrical cardioversion and xiii design an ideal targeted energy delivery protocol to improve upon all limitations of current electrotherapy

Numerical modelling of electrical stimulation for cartilage tissue engineering

In this thesis, the design and validity of numerical models of electrical stimulation for cartilage tissue engineering are critically assessed at different scales. In sum, the results of this thesis pave the way for experimentally validated numerical models of electrical stimulation devices for cartilage tissue engineering. Furthermore, models of tissue samples can be developed down to the cellular scale and will contribute to the development of patient-specific stimulation approaches

Developing piezoelectric biosensing methods

Biosensors are often used to detect biochemical species either in the body or from collected samples with high sensitivity and specificity. Those based on piezoelectric sensing methods employ mechanically induced changes to generate an electrical response. Reliable collection and processing of these signals is an important aspect in the design of these systems. To generate the electrical response, specific recognition layers are arranged on piezoelectric substrates in such a way that they interact with target species and so change the properties of the device surface (e.g. the mass or mechanical strain). These changes generate a change in the electrical signal output allowing the device to be used as a biosensor. The characteristics of piezoelectric biosensors are that they are competitively priced, inherently rugged, very sensitive, and intrinsically reliable. In this study, a compound label-free biosensor was developed. This sensor consists of two elements: a Love wave sensor and an electrochemical impedance sensor. The novelty of this device is that it can work in both dry and wet measurement conditions. Whilst the Love wave sensor aspect of the device is sensitive to the mass of adsorbed analytes under both dry and wet conditions with high sensitivity, the sensitivity coefficients in these two conditions may be different due to the different (mechanical) strengths of interaction between the adsorbed analyte and the substrate. The impedance sensor element of the device however is less sensitive to the mechanical strength of the bond between the analyte and the sensing surface and so can be used for in-situ calibration of the number of molecules bound to the sensing surface (with either a strong or weak link): conventional Love wave sensors are not sensitive to material loosely bound to the surface. Thus, a combination of results from these two sensors can provide more information about the analyte and the accuracy of the Love wave sensor measurements in a liquid environment. The device functions with label-free molecules and so special reagents are not needed when carrying out measurements. In addition, the fabrication of the device is not too complicated and it is easy to miniaturise. This may make the system suitable for point-of-care diagnostics and bio-material detection. The substrate used in these sensors is 64°Y–X lithium niobate (LiNbO3) which is a kind of piezoelectric material. On the substrate, there is a pair of interdigital transducers (IDTs) which are composed of 100 Ti/Au split-finger pairs with a periodicity (λ) of 40μm. The acoustic path length, between both IDTs, is 200λ and the aperture between the IDTs is 100λ. On top of the substrate and IDTs, there is a PMMA guiding layer with an optimised thickness ranging from 1000 nm to 1300 nm. In addition, a gold layer with thickness 100 nm is deposited on the guiding layer to act as the electrodes for the electrochemical impedance sensor. The biosensor in this study has been used to measure Protein A, IgG, and GABA molecules. Protein A is often coupled to other molecules such as a fluorescent dye, enzymes, biotin, and colloidal gold or radioactive iodine without affecting the antibody binding site. In addition, the capacity of Protein A to bind antibodies with such high affinity is the driving motivation for its industrial scale use in biologic pharmaceuticals. Therefore, measuring Protein A binding is a useful method with which to verify the function of the biosensor. IgG is the most abundant antibody isotype found in the circulation. By binding many kinds of pathogens including viruses, bacteria, and fungi, IgG protects the body from infection. Also, IgG can bind with Protein A well so the biosensor here could also measure IgG after a Protein A layer is immobilised on the sensing area. GABA is the main inhibitory neurotransmitter in the mammalian central nervous system. It plays an important role in regulating neuronal excitability throughout the nervous system. The conventional method to measure concentrations of GABA under the extracellular conditions is by using liquid chromatography. However, the disadvantages of chromatographic methods are baseline drift and additions of solvent and internal standards. Therefore, it is necessary to develop a simple, rapid and reliable method for direct measurement of GABA, and the sensor here is an attractive choice. When the Love wave sensor works in the liquid media, it can only be used to measure the mass of analytes but does not provide information about the conditions of molecules bound with the sensing surface. In contrast, electrochemical impedance sensing based on the diffusion of redox species to the underlying metal electrode can provide real-time monitoring of the surface coverage of bound macromolecular analytes regardless of the mechanical strength of the analyte-substrate bond: the electrochemical impedance measurement is sensitive to the size and extent of the diffusion pathways around the adsorbed macromolecules used by the redox species probe i.e. it is sensitive to the physical area of the surface covered by the macromolecular analyte and not to the mass of material that is sensed through a mechanical coupling effect (as in a Love wave device). Although electrochemical impedance measurements under the dry state are quite common when studying batteries and their redox/discharge properties, these are quite different sorts of systems to the device in this study. Therefore, integrating these two sensors (Love wave sensor and electrochemical impedance sensor) in a single device is a novel concept and should lead to better analytical performance than when each is used on their own. The new type of biosensor developed here therefore has the potential to measure analytes with greater accuracy, higher sensitivity and a lower limit of detection than found when using either a single Love wave sensor or electrochemical impedance sensor alone

Selected Papers from the 1st International Electronic Conference on Biosensors (IECB 2020)

The scope of this Special Issue is to collect some of the contributions to the First International Electronic Conference on Biosensors, which was held to bring together well-known experts currently working in biosensor technologies from around the globe, and to provide an online forum for presenting and discussing new results. The world of biosensors is definitively a versatile and universally applicable one, as demonstrated by the wide range of topics which were addressed at the Conference, such as: bioengineered and biomimetic receptors; microfluidics for biosensing; biosensors for emergency situations; nanotechnologies and nanomaterials for biosensors; intra- and extracellular biosensing; and advanced applications in clinical, environmental, food safety, and cultural heritage fields

Translational pipelines for closed-loop neuromodulation

Closed-loop neuromodulation systems have shown significant potential for addressing unmet needs in the treatment of disorders of the central nervous system, yet progress towards clinical adoption has been slow. Advanced technological developments often stall in the preclinical stage by failing to account for the constraints of implantable medical devices, and due to the lack of research platforms with a translational focus. This thesis presents the development of three clinically relevant research systems focusing on refinements of deep brain stimulation therapies. First, we introduce a system for synchronising implanted and external stimulation devices, allowing for research into multi-site stimulation paradigms, cross-region neural plasticity, and questions of phase coupling. The proposed design aims to sidestep the limited communication capabilities of existing commercial implant systems in providing a stimulation state readout without reliance on telemetry, creating a cross-platform research tool. Next, we present work on the Picostim-DyNeuMo adaptive neuromodulation platform, focusing on expanding device capabilities from activity and circadian adaptation to bioelectric marker--based responsive stimulation. Here, we introduce a computationally optimised implementation of a popular band power--estimation algorithm suitable for deployment in the DyNeuMo system. The new algorithmic capability was externally validated to establish neural state classification performance in two widely-researched use cases: Parkinsonian beta bursts and seizures. For in vivo validation, a pilot experiment is presented demonstrating responsive neurostimulation to cortical alpha-band activity in a non-human primate model for the modulation of attention state. Finally, we turn our focus to the validation of a recently developed method to provide computationally efficient real-time phase estimation. Following theoretical analysis, the method is integrated into the commonly used Intan electrophysiological recording platform, creating a novel closed-loop optogenetics research platform. The performance of the research system is characterised through a pilot experiment, targeting the modulation of cortical theta-band activity in a transgenic mouse model