Comparison of the In Vitro and In Vivo Electrochemical Performance of Bionic Electrodes

Nonlinear effects at the electrode-tissue interface of deep brain stimulation electrodes

Objective.The electrode-tissue interface provides the critical path for charge transfer in neurostimulation therapies and exhibits well-established nonlinear properties at high applied currents or voltages. These nonlinear properties may influence the efficacy and safety of applied stimulation but are typically neglected in computational models. In this study, nonlinear behavior of the electrode-tissue interface impedance was incorporated in a computational model of deep brain stimulation (DBS) to simulate the impact on neural activation and safety considerations.Approach.Nonlinear electrode-tissue interface properties were incorporated in a finite element model of DBS electrodesin vitroandin vivo,in the rat subthalamic nucleus, using an iterative approach. The transition point from linear to nonlinear behavior was determined for voltage and current-controlled stimulation. Predicted levels of neural activation during DBS were examined and the region of linear operation of the electrode was compared with the Shannon safety limit.Main results.A clear transition of the electrode-tissue interface impedance to nonlinear behavior was observed for both current and voltage-controlled stimulation. The transition occurred at lower values of activation overpotential for simulatedin vivothanin vitroconditions (91 mV and 165 mV respectively for current-controlled stimulation; 110 mV and 275 mV for voltage-controlled stimulation), corresponding to an applied current of 30μA and 45μA, or voltage of 330 mV at 1 kHz. The onset of nonlinearity occurred at lower values of the overpotential as frequency was increased. Incorporation of nonlinear properties resulted in activation of a higher proportion of neurons under voltage-controlled stimulation. Under current-controlled stimulation, the predicted transition to nonlinear behavior and Faradaic charge transfer at stimulation amplitudes of 30μA, corresponds to a charge density of 2.29μC cm-2and charge of 1.8 nC, well-below the Shannon safety limit.Significance.The results indicate that DBS electrodes may operate within the nonlinear region at clinically relevant stimulation amplitudes. This affects the extent of neural activation under voltage-controlled stimulation and the transition to Faradaic charge transfer for both voltage- and current-controlled stimulation with important implications for targeting of neural populations and the design of safe stimulation protocols.

Limitations in the electrochemical analysis of voltage transients

Objective. Chronopotentiometric voltage transients (VTs) are used to assess the performance of bionic electrodes. The data obtained from VTs are used to define the safe operating conditions of clinical devices. Various approaches to analysing VTs have been reported, and a number of limitations in the accuracy of the measurements in relation to electrode size have been noted previously.Approach. The impact of electronic hardware and electrode configuration on VTs is discussed.Main results. The slew rate, rise time, sample time, minimum pulse length and waveform averaging characteristics of the electronic hardware, and electrode configuration will impact on VT measurement accuracy. Subsequently, activation and polarisation voltage measurements, and the definition of safe stimulation levels can be affected by the electronic hardware and electrode configuration.Significance. This article has identified some limitations in the previous literature related to the measurement and reporting of VTs and subsequent analysis of access and polarisation voltages. Furthermore, the commonly used Shannon plot used to define safe stimulation protocols does not correct for uncompensated resistance, account for electrode roughness or changes in electrode configuration. The creation of a safe stimulation plot which has been corrected for uncompensated resistance would generate more widely applicable stimulation guidelines for clinical devices used in different anatomical locations such as endovascular neural interfaces.

Functional Electrochemistry: On-Nerve Assessment of Electrode Materials for Electrochemistry and Functional Neurostimulation

Emerging therapies in bioelectronic medicine highlight the need for deeper understanding of electrode material performance in the context of tissue stimulation. Electrochemical properties are characterized on the benchtop, facilitating standardization across experiments. On-nerve electrochemistry differs from benchtop characterization and the relationship between electrochemical performance and nerve activation thresholds are not commonly established. This relationship is important in understanding differences between electrical stimulation requirements and electrode performance. We report functional electrochemistry as a follow-up to benchtop testing, describing a novel experimental approach for evaluating on-nerve electrochemical performance in the context of nerve activation. An ex-vivo rat sciatic nerve preparation was developed to quantify activation thresholds of fiber subtypes and electrode material charge injection limits for platinum iridium, iridium oxide, titanium nitride and PEDOT. Finally, we address experimental complexities arising in these studies, and demonstrate statistical solutions that support rigorous material performance comparisons for decision making in neural interface development.

A Comparative Study on the Effect of Substrate Structure on Electrochemical Performance and Stability of Electrodeposited Platinum and Iridium Oxide Coatings for Neural Electrodes

Implantable electrodes are crucial for stimulation safety and recording quality of neuronal activity. To enhance their electrochemical performance, electrodeposited nanostructured platinum (nanoPt) and iridium oxide (IrOx) have been proposed due to their advantages of in situ deposition and ease of processing. However, their unstable adhesion has been a challenge in practical applications. This study investigated the electrochemical performance and stability of nanoPt and IrOx coatings on hierarchical platinum-iridium (Pt-Ir) substrates prepared by femtosecond laser, compared with the coatings on smooth Pt-Ir substrates. Ultrasonic testing, agarose gel testing, and cyclic voltammetry (CV) testing were used to evaluate the coatings' stability. Results showed that the hierarchical Pt-Ir substrate significantly enhanced the charge-storage capacity of electrodes with both coatings to more than 330 mC/cm2, which was over 75 times that of the smooth Pt-Ir electrode. The hierarchical substrate could also reduce the cracking of nanoPt coatings after ultrasonic, agarose gel and CV testing. Although some shedding was observed in the IrOx coating on the hierarchical substrate after one hour of sonication, it showed good stability in the agarose gel and CV tests. Stable nanoPt and IrOx coatings may not only improve the electrochemical performance but also benefit the function of neurobiochemical detection.

Functional Enhancement and Characterization of an Electrophysiological Mapping Electrode Probe with Carbonic, Directional Macrocontacts

Electrophysiological mapping (EM) using acute electrode probes is a common procedure performed during functional neurosurgery. Due to their constructive specificities, the EM probes are lagging in innovative enhancements. This work addressed complementing a clinically employed EM probe with carbonic and circumferentially segmented macrocontacts that are operable both for neurophysiological sensing ("recording") of local field potentials (LFP) and for test stimulation. This paper illustrates in-depth the development that is based on the direct writing of functional materials. The unconventional fabrication processes were optimized on planar geometry and then transferred to the cylindrically thin probe body. We report and discuss the constructive concept and architecture of the probe, characteristics of the electrochemical interface deduced from voltammetry and chronopotentiometry, and the results of in vitro and in vivo recording and pulse stimulation tests. Two- and three-directional macrocontacts were added on probes having shanks of 550 and 770 μm diameters and 10-23 cm lengths. The graphitic material presents a ~2.7 V wide, almost symmetric water electrolysis window, and an ultra-capacitive charge transfer. When tested with clinically relevant 150 μs biphasic current pulses, the interfacial polarization stayed safely away from the water window for pulse amplitudes up to 9 mA (135 μC/cm2). The in vivo experiments on adult rat models confirmed the high-quality sensing of LFPs. Additionally, the in vivo-prevailing increase in the electrode impedance and overpotential are discussed and modeled by an ionic mobility-reducing spongiform structure; this restricted diffusion model gives new applicative insight into the in vivo-uprisen stimulation overpotential.

A Flexible Conductive Electrode Using Boronic-Acid Modified Carbon Dots

Flexible electrodes are becoming a topic of interest for a range of applications including implantation. They can be used for neural signal recording and for electrical stimulation of atrophying muscles. Unlike the traditionally used metal electrodes that are harsh to the body's tissues, flexible electrodes conduct electricity while preserving the delicate tissues. Polydimethylsiloxane (PDMS), a non-conductive synthetic polymer characterized by its flexibility, low cost, biocompatibility, and durability during implantation, has been explored as a matrix for flexible electrodes. This study reports the synthesis of composite boronic acid-modified carbon dots (BA-CDs)/PDMS electrode materials. The performance of the composite electrode is evaluated electrochemically (for its conductivity and charge storage capacity) and mechanically (Young's modulus). Furthermore, the effect of increasing the PDMS crosslinking density on the electrode's performance is studied based on the hypothesis that a higher crosslinking will bring the BA-CDs closer together, thereby facilitating the movement of electrons. Results of this study showed that incorporating 10% BA-CDs dispersed with 16% glycerol in 74% PDMS with a higher crosslinking density resulted in a bulk impedance of 47.7 Ω and a conductivity of 2.68×10-3 S/cm, both of which surpassed that of the same composition with lower crosslinking. The synthesized flexible electrode material was capable of charge storage although the charge storage capacity (0.00365 mC/cm2) was lower than the safe limit for some tissue activation. Furthermore, the electrode maintained a modulus of elasticity (0.2322 MPa) that is compatible with biological soft tissues.Clinical Relevance- This study reports a conductive electrode that has a flexibility compatible with that of biological tissues for future purposes such as neural signal recording and tissue electrical stimulation (e.g. atrophying muscles). The reported BA-CD/PDMS electrode overcomes the limitations of the harsh metals previously used as implantable electrodes that harm the biological tissues due to their high rigidity.

Electrochemistry in a Two- or Three-Electrode Configuration to Understand Monopolar or Bipolar Configurations of Platinum Bionic Implants

Electrodes are used in vivo for chemical sensing, electrophysiological recording, and stimulation of tissue. The electrode configuration used in vivo is often optimised for a specific anatomy and biological or clinical outcomes, not electrochemical performance. Electrode materials and geometries are constrained by biostability and biocompatibility issues and may be required to function clinically for decades. We performed benchtop electrochemistry, with changes in reference electrode, smaller counter-electrode sizes, and three- or two-electrode configurations. We detail the effects different electrode configurations have on typical electroanalytical techniques used on implanted electrodes. Changes in reference electrode required correction by application of an offset potential. In a two-electrode configuration with similar working and reference/counter-electrode sizes, the electrochemical response was dictated by the rate-limiting charge transfer step at either electrode. This could invalidate calibration curves, standard analytical methods, and equations, and prevent use of commercial simulation software. We provide methods for determining if an electrode configuration is affecting the in vivo electrochemical response. We recommend sufficient details be provided in experimental sections on electronics, electrode configuration, and their calibration to justify results and discussion. In conclusion, the experimental limitations of performing in vivo electrochemistry may dictate what types of measurements and analyses are possible, such as obtaining relative rather than absolute measurements.

Self-doped conducting polymers in biomedical engineering: Synthesis, characterization, current applications and perspectives

Recent studies willingly agree that conducting polymers (CPs) are attractive materials for biomedical engineering purposes, mainly because of their unique physicochemical characteristics combining electrical conductivity and high biocompatibility. Nevertheless, the applicability of CPs is restricted by their limited stability under physiological conditions, associated with a decrease in electrical conductivity upon dedoping. Accordingly, modifying chemical structure of CPs to exhibit a self-doping effect seems to be an appealing approach aimed to enhance their functionality. The aim of this review is to provide a current state-of-the-art in the research concerning self-doped CPs, particularly those with potential biomedical applications. After presenting a library of available structure modifications, we describe their physicochemical characteristics, focusing on achievable conductivities, electrochemical, optical and mechanical behaviour, as well as biological properties. To highlight high applicability of self-doped CPs in biomedical engineering, we elaborate on biomedical areas benefiting most from using this type of conducting materials.