A voltage-controlled current source with regulated electrode bias-voltage for safe neural stimulation

Maximizing Charge Injection Limits of Iridium Oxide Electrodes with a Programmable Anodic Bias Circuit

Efficacious stimulation of neural tissues requires high charge injection capacity while minimizing electrode polarization. Applying anodic bias on certain electrode materials is a way to enhance charge injection both in vitro and in vivo. We developed an embedded neurostimulator system that enabled a digital control of user-defined bias levels, without requiring a potentiometer or external voltage source. Comparison of charge injection with and without anodic-bias, as well as at different bias potentials were conducted in phosphate-buffered saline with Blackrock iridium oxide microelectrodes. Results showed that a nine-fold increase in current intensity and charge injection capacity, was achieved with a 0.7 V anodic bias and within electrochemically safe limits.

A microfluidic device for noninvasive cell electrical stimulation and extracellular field potential analysis

We developed a device that can quickly apply versatile electrical stimulation (ES) signals to cells suspended in microfluidic channels and measure extracellular field potential simultaneously. The device could trap cells onto the surface of measurement electrodes for ES and push them to the downstream channel after ES by increasing pressure for continuous measurement. Cardiomyocytes, major functional cells in heart, together with human fibroblast cells and human umbilical vein endothelial cells, were tested with the device. Extracellular field potential signals generated from the cells were recorded. We found that under electrical stimulation, cardiomyocytes were triggered to alter their field potential, while non-excitable cells were not triggered. Hence this device can noninvasively distinguish electrically excitable cells from electrically non-excitable cells. Results have also shown that increased cardiomyocyte cell number led to increased magnitude and occurrence of the cell responses. This relationship could be used to detect the viable cells in a cardiac tissue. Application of variable ES signals on different cardiomyocyte clusters has shown that the application of ES clearly boosted cardiomyocytes electrical activities according to the stimulation frequency. In addition, we confirmed that the device can apply ES onto and detect the electrical responses from a mixed cell cluster; the responses from the mixed cluster is dependent on the ratio of cardiomyocytes. These results demonstrated that our device could be used as a tool to optimize ES conditions to facilitate the functional engineered cardiac tissue development.

Human Papillomavirus E6 biosensing: Current progression on early detection strategies for cervical Cancer

Prognosis of early cancer detection becomes one of the tremendous issues in the medical health system. Medical debates among specialist doctor and researcher in therapeutic approaches became a hot concern for cervix cancer deficiencies early screening, risk factors cross-reaction, portability device, rapid and free labeling system. The electrical biosensing based system showed credibility in higher specificity and selectivity due to hybridization of DNA duplex between analyte target and DNA probes. Electrical DNA sensor for cervix cancer has attracted too many attentions to researcher notification based on high performance, easy to handle, rapid system and possible to miniaturize. This review explores the current progression and future insignificant for HPV E6 genobiosensing for early Detection Strategies of Cervical Cancer.

Mechanical deformation of thin film platinum under electrical stimulation

Thin-film-based electrodes used to interact with nervous tissue often fail quickly if used for electrical stimulation, impairing their translation into long-term clinical applications. We initiated investigations about the mechanical load on thin-film electrodes caused by the fact of electrical stimulation. Platinum electrodes of Ø 300μm on a polyimide carrier were subjected to approximately 50 000 asymmetrical, biphasic stimulation pulses in vitro. The electrode's surface was investigated optically by means of white-light interferometry. The structural expansion for the metallic surface subjected to stimulation was measured to reach roughly 30%. The study points towards a failure mechanism of thin-films being of mechanical nature, inherent to the unavoidable electrochemical processes involved (change in lattice constants) during electrical stimulation at the electrode's surface. Based on further scientific facts, we set 3 hypotheses for the exact mechanisms involved in the failure of thin-films used for electrical stimulation, opening a new door for research and improvement of novel neuroprosthetic devices.

Note: Temperature effects in the modified Howland current source for electrical bioimpedance spectroscopy

The aim of this study is to show how the modified Howland current source (MHCS) is affected by temperature changes. The source has been tested in a temperature range from 20 to 70 °C and frequency range from 100 Hz to 1 MHz. Parameters like output current, output impedance, total harmonic distortion, and oscillation have been measured. The measurements were made inside a temperature controlled environment. It was showed that the MHCS is stable at temperatures below 70 °C. Operational amplifiers with a low temperature drift and matching resistor should be carefully considered in order to prevent oscillations at high temperatures.

Capacitive Feedthroughs for Medical Implants

Important technological advances in the last decades paved the road to a great success story for electrically stimulating medical implants, including cochlear implants or implants for deep brain stimulation. However, there are still many challenges in reducing side effects and improving functionality and comfort for the patient. Two of the main challenges are the wish for smaller implants on one hand, and the demand for more stimulation channels on the other hand. But these two aims lead to a conflict of interests. This paper presents a novel design for an electrical feedthrough, the so called capacitive feedthrough, which allows both reducing the size, and increasing the number of included channels. Capacitive feedthroughs combine the functionality of a coupling capacitor and an electrical feedthrough within one and the same structure. The paper also discusses the progress and the challenges of the first produced demonstrators. The concept bears a high potential in improving current feedthrough technology, and could be applied on all kinds of electrical medical implants, even if its implementation might be challenging.

Cuff electrodes for very small diameter nerves -- prototyping and first recordings in vivo

A fabrication method for cuff electrodes to interface small nerves was developed. Medical grade silicone rubber conforms the body of the cuff and insulation of the wires, platinum was used as metal for the embedded wiring and contacts. Planar electrode arrays where fabricated using a picosecond laser and then positioned into a carrying tube to provide the third dimension with the desired inner diameter (Ø 0.3-0.5 mm). The post preparation of the cuffs after structuring allows the fabrication of a stable self-closing flap that insulates the opening slit of the cuff without the need of extra sutures. Basic for the success of the cuff is the laser-based local thinning of both the silicone rubber and the metal at defined sections. This is critical to permit the PDMS' body to dominate the mechanical properties. Finite element modeling was applied to optimize the displacement ability of the cuff, leading to design capable of withstanding multiple implantation procedures without wire damage. Furthermore, the contact's surface was roughened by laser patterning to increase the charge injection capacity of Pt to 285 μC/cm(2) measured by voltage transient detection during pulse testing. The cuff electrodes were placed on a small sympathetic nerve of an adult female Sprague-Dawley rat for recording of spontaneous and evoked neural activity in vivo.

Improved polyimide thin-film electrodes for neural implants

Thin-film electrode arrays for neural implants are necessary when large integration densities of stimulating or recording channels are required. However, delamination of the metallic layers from the polymer substrate leads to early failure of the device. Based on new adhesion studies of polyimide to SiC and diamond-like carbon (DLC) the authors successfully fabricated a 232-channel electrode array for retinal stimulation with improved adhesion. Layers of SiC and DLC were integrated into the fabrication procedure of polyimide-platinum (Pt) arrays to create fully coated metal wires, which adhere to the polyimide substrate even after 1 year of accelerated aging in saline solution. Studies on the inter-diffusion of Pt and SiC were conducted to establish an optimal thickness for a gold core of the platinum tracks, which is used for reducing the electrical track resistance. Furthermore, the electrochemical behaviour of the stimulating contacts coated with IrOx were studied in a long-term pulse tests over millions of pulses showing no deterioration of the coating.

Feedback control of electrode offset voltage during functional electrical stimulation

Control of the electrode offset voltage is an important issue related to the processes of functional electrical stimulation because excess charge accumulation over time damages both the tissue and the electrodes. This paper proposes a new feedback control scheme to regulate the electrode offset voltage to a predetermined reference value. The electrode offset voltage was continuously monitored using a sample-and-hold (S/H) circuit during stimulation and non-stimulation periods. The stimulation current was subsequently adjusted using a proportional-integral (PI) controller to minimise the error between the reference value and the electrode offset voltage. During the stimulation period, the electrode offset voltage was maintained through the S/H circuit, and the PI controller did not affect the amplitude of the stimulation current. In contrast, during the non-stimulation period, the electrode offset voltage was sampled through the S/H circuit and rapidly regulated through the PI controller. The experimental results obtained using a nerve cuff electrode showed that the electrode offset voltage was successfully controlled in terms of the performance specifications, such as the steady- and transient-state responses and the constraint of the controller output. Therefore, the proposed control scheme can potentially be used in various nerve stimulation devices and applications requiring control of the electrode offset voltage.

High-porous platinum electrodes for functional electrical stimulation

This paper reports on the preparation and characterization of highly porous platinum electrodes for functional electrical stimulation. Thin-film platinum electrodes were roughened by electrochemical deposition of platinum-copper alloys and subsequent removal of copper using cyclic voltammetry (CV). Prepared samples were characterized by electrochemical impedance spectroscopies (EIS), CVs and long-term pulse testing. The electrodes exhibited an increased porosity by a factor of ~238 when compared to sputtered platinum electrodes. EIS measurements showed a decreased impedance of about 85% and pulse tests suggested a stable coating over at least 250 million pulses.

Safety of multi-channel stimulation implants: a single blocking capacitor per channel is not sufficient after single-fault failure

One reason given for placing capacitors in series with stimulation electrodes is that they prevent direct current flow and therefore tissue damage under fault conditions. We show that this is not true for multiplexed multi-channel stimulators with one capacitor per channel. A test bench of two stimulation channels, two stimulation tripoles and a saline bath was used to measure the direct current flowing through the electrodes under two different single fault conditions. The electrodes were passively discharged between stimulation pulses. For the particular condition used (16 mA, 1 ms stimulation pulse at 20 Hz with electrodes placed 5 cm apart), the current ranged from 38 to 326 μA depending on the type of fault. The variation of the fault current with time, stimulation amplitude, stimulation frequency and distance between the electrodes is given. Possible additional methods to improve safety are discussed.

In vivo validation of custom-designed silicon-based microelectrode arrays for long-term neural recording and stimulation

We developed and validated silicon-based neural probes for neural stimulating and recording in long-term implantation in the brain. The probes combine the deep reactive ion etching process and mechanical shaping of their tip region, yielding a mechanically sturdy shank with a sharpened tip to reduce insertion force into the brain and spinal cord, particularly, with multiple shanks in the same array. The arrays' insertion forces have been quantified in vitro. Five consecutive chronically-implanted devices were fully functional from 3 to 18 months. The microelectrode sites were electroplated with iridium oxide, and the charge injection capacity measurements were performed both in vitro and after implantation in the adult feline brain. The functionality of the chronic array was validated by stimulating in the cochlear nucleus and recording the evoked neuronal activity in the central nucleus of the inferior colliculus. The arrays' recording quality has also been quantified in vivo with neuronal spike activity recorded up to 566 days after implantation. Histopathology evaluation of neurons and astrocytes using immunohistochemical stains indicated minimal alterations of tissue architecture after chronic implantation.

On the stability of poly-ethylenedioxythiopene as coating material for active neural implants

This article deals with the stability of poly-ethylenedioxythiopene (PEDOT) coatings under high loads of current pulses. Test parameters were chosen to match many peripheral nervous system applications in regard of charge injection, pulse width, and repetition frequency. PEDOT coatings were characterized with electrochemical impedance spectroscopes and pulse tests. After 60-100 million pulses, impedance increased and a more capacitive behavior was observed. A mean-time-to-failure of 127 million pulses could be calculated, suggesting a stable coating for at least this time frame with superior properties in regard of conventional platinum electrodes.

An active approach for charge balancing in functional electrical stimulation

Charge balancing is a major concern in functional electrical stimulation, since any excess charge accumulation over time leads to electrolysis with electrode dissolution and tissue destruction. This paper presents a new active approach for charge balancing using long-term offset regulation. Therefore, the electrode voltage is briefly monitored after each stimulation cycle and checked if it remains within a predefined voltage range. If not, an offset current is adjusted in order to track the biphasic current mismatch in upcoming stimulations. This technique is compared to a previously introduced active charge balancer as well as commonly used passive balancing techniques. Subsequently, the techniques are verified through experiments on a platinum black electrode in 0.9% saline solution.

Comparison of three current sources for single-electrode capacitance measurement

The capacitance of a single electrode is usually measured by injecting a current to the electrode and measuring the resultant voltage on the electrode. In this case, a voltage-controlled current source with a high bandwidth is needed because the impedance is inversely proportional to the excitation frequency. In this design note, three different current sources are discussed: (1) the Howland current source, (2) a modified Howland current source, and (3) a dual op-amp current source. The principle and dynamic performances are presented and compared. Simulation and experimental results show that although the Howland current source has the lowest (i.e., worst) output impedance, its output is the most stable among the three current sources when the frequency changes. Therefore, it is suitable for single-electrode capacitance measurement. Initial tests have proven the feasibility of single-electrode capacitance sensor with the Howland current source.

Recent advances in charge balancing for functional electrical stimulation

Charge balancing is a major concern in functional electrical stimulation, since any excess charge accumulation over time leads to electrolysis with electrode dissolution and tissue destruction. Its major function is to ensure that the mean value of electrode voltage is kept within a safe level. However, it serves as a failure protection as well. This paper presents an overview on recent advances in this field, both passive and active (closed-loop) charge balancing techniques.