Biocompatibility considerations at stimulating electrode interfaces

Advances in Soft and Dry Electrodes for Wearable Health Monitoring Devices

Electrophysiology signals are crucial health status indicators as they are related to all human activities. Current demands for mobile healthcare have driven considerable interest in developing skin-mounted electrodes for health monitoring. Silver-Silver chloride-based (Ag-/AgCl) wet electrodes, commonly used in conventional clinical practice, provide excellent signal quality, but cannot monitor long-term signals due to gel evaporation and skin irritation. Therefore, the focus has shifted to developing dry electrodes that can operate without gels and extra adhesives. Compared to conventional wet electrodes, dry ones offer various advantages in terms of ease of use, long-term stability, and biocompatibility. This review outlines a systematic summary of the latest research on high-performance soft and dry electrodes. In addition, we summarize recent developments in soft materials, biocompatible materials, manufacturing methods, strategies to promote physical adhesion, methods for higher breathability, and their applications in wearable biomedical devices. Finally, we discuss the developmental challenges and advantages of various dry electrodes, while suggesting research directions for future studies.

Fabrication of High Aspect Ratio Millimeter-Tall Free-Standing Carbon Nanotube-Based Microelectrode Arrays

Microelectrode arrays of carbon nanotube (CNT)/carbon composite posts with high aspect ratio and millimeter-length were fabricated using carbon-nanotube-templated microfabrication with a sacrificial "hedge". The high aspect ratio, mechanical robustness, and electrical conductivity of these electrodes make them a potential candidate for next-generation neural interfacing. Electrochemical measurements were also demonstrated using an individual CNT post microelectrode with a diameter of 25 μm and a length of 1 mm to perform cyclic voltammetry on both methyl viologen and dopamine in a phosphate-buffered saline solution. In addition to detection of the characteristic peaks, the CNT post microelectrodes show a fast electrochemical response, which may be enabling for in vivo and/or in vitro measurements. The CNT post electrode fabrication process was also integrated with other microfabrication techniques, resulting in individually addressable electrodes.

Vision rehabilitation in the case of blindness

This article examines the various vision rehabilitation procedures that are available for early and late blindness. Depending on the pathology involved, several vision rehabilitation procedures exist, or are in development. Visual aids are available for low vision individuals, as are sensory aids for blind persons. Most noninvasive sensory substitution prostheses as well as implanted visual prostheses in development are reviewed. Issues dealing with vision rehabilitation are also discussed, such as problems of biocompatibility, electrical safety, psychosocial aspects, and ethics. Basic studies devoted to vision rehabilitation such as simulation in mathematical models and simulation of artificial vision are also presented. Finally, the importance of accurate rehabilitation assessment is addressed, and tentative market figures are given.

Protein adsorption and peroxidation of rat retinas under stimulation of a neural probe coated with polyaniline

For the purpose of investigating the potential use of conducting polymers, i.e. polyaniline (PANi), as electrode coating material for improving the function of neural probes, a PANi-coated platinum (Pt) electrode was prepared by the in situ polymerization method. Protein adsorption was observed by atomic force microscopy/scanning electron microscopy and sodium dodecyl sulfate polyacrylamide gel electrophoresis, as well as quantification. Peroxidation of rat retinas was evaluated by determination of conjugated dienes and PLOOH, which were quantified by UV-visible spectrophotometer and high-performance liquid chromatography. The stability of PANi coating for 6 months was also estimated with an in vitro electrical stimulation system. This revealed that: (1) PANi with regular and compact nanoparticles 20-40 nm in diameter was successfully polymerized on the uncoated platinum electrode surface; (2) the PANi-coated Pt electrode adsorbed fewer retinal fragments and induced less peroxidation than the uncoated platinum electrode; (3) in contrast to the uncoated platinum electrode, the PANi-coated Pt electrode surface tended to aggregate retinal fragments rather than spread them, which may help to reduce inflammation and scar formation in long-term implantation; (4) the PANi coating exhibited excellent properties in terms of the intactness and the stable nanoparticle morphology after 6 months' electrical stimulation, while corrosion occurred on the uncoated platinum electrode after 1 month.

A Wafer-Scale Etching Technique for High Aspect Ratio Implantable MEMS Structures

Microsystem technology is well suited to batch fabricate microelectrode arrays, such as the Utah electrode array (UEA), intended for recording and stimulating neural tissue. Fabrication of the UEA is primarily based on the use of dicing and wet etching to achieve high aspect ratio (15:1) penetrating electrodes. An important step in the array fabrication is the etching of electrodes to produce needle-shape electrodes with sharp tips. Traditional etching processes are performed on a single array, and the etching conditions are not optimized. As a result, the process leads to variable geometries of electrodes within an array. Furthermore, the process is not only time consuming but also labor-intensive. This report presents a wafer-scale etching method for the UEA. The method offers several advantages, such as substantial reduction in the processing time, higher throughput and lower cost. More importantly, the method increases the geometrical uniformity from electrode to electrode within an array (1.5 ± 0.5 % non-uniformity), and from array to array within a wafer (2 ± 0.3 % non-uniformity). Also, the etching rate of silicon columns, produced by dicing, are studied as a function of temperature, etching time and stirring rate in a nitric acid rich HF-HNO(3) solution. These parameters were found to be related to the etching rates over the ranges studied and more-importantly affect the uniformity of the etched silicon columns. An optimum etching condition was established to achieve uniform shape electrode arrays on wafer-scale.

A Novel Method of Fabricating Convoluted Shaped Electrode Arrays for Neural and Retinal Prostheses

A novel fabrication technique has been developed for creating high density (6.25 electrodes/mm(2)), out of plane, high aspect ratio silicon-based convoluted microelectrode arrays for neural and retinal prostheses. The convoluted shape of the surface defined by the tips of the electrodes could compliment the curved surfaces of peripheral nerves and the cortex, and in the case of retina, its spherical geometry. The geometry of these electrode arrays has the potential to facilitate implantation in the nerve fascicles and to physically stabilize it against displacement after insertion. This report presents a unique combination of variable depth dicing and wet isotropic etching for the fabrication of a variety of convoluted neural array geometries. Also, a method of deinsulating the electrode tips using photoresist as a mask and the limitations of this technique on uniformity are discussed.

Stable biopassive insulation synthesized by initiated chemical vapor deposition of poly(1,3,5-trivinyltrimethylcyclotrisiloxane)

The permanent implantation of electronic probes capable of recording neural activity patterns requires long-term electrical insulation of these devices by biopassive coatings. In this work, the material properties and neural cell compatibility of a novel polymeric material, poly(trivinyltrimethylcyclotrisiloxane) (poly(V3D3)), are demonstrated to be suitable for application as permanently bioimplanted electrically insulating films. The poly(V3D3) polymeric films are synthesized by initiated chemical vapor deposition (iCVD), allowing for conformal and flexible encapsulation of fine wires. The poly(V3D3) also exhibits high adhesive strength to silicon substrates, a common material of manufacture for neural probes. The poly(V3D3) films were found to be insoluble in both polar and nonpolar solvents, consistent with their highly cross-linked structure. The films are pinhole-free and extremely smooth, having a root-mean-square (rms) roughness of 0.4 nm. The material possesses a bulk resistivity of 4 x 1015 Ohm-cm exceeding that of Parylene-C, the material currently used to insulate neurally implanted devices. The iCVD poly(V3D3) films are hydrolytically stable and are demonstrated to maintain their electrical properties under physiological soak conditions, and constant electrical bias, for more than 2 years. In addition, biocompatibility studies with PC12 neurons demonstrate that this material is noncytotoxic and does not influence cell proliferation.

Repeated in vivo electrochemical activation and the biological effects of microelectromechanical systems drug delivery device

The repeated activation of a microelectromechanical systems (MEMS) drug delivery device was studied in vivo in rats to examine the effect of implantation on the device operation and the effect of electrochemical activation on the inflammatory and wound-healing response. The MEMS devices were fabricated from a silicon wafer into which reservoirs were etched and covered with gold membranes. The membranes were electrochemically removed when an anodic voltage was applied. Devices were implanted subcutaneously both with and without stainless steel mesh cages for 4, 7, 14, 21, or 28 days before activation. Devices were activated every other day for five activations. Leukocyte concentrations indicated that both the application of voltage and the gold corrosion products elevated the inflammatory response which was resolved within 48 h after each activation. The efficiency of gold membrane removal was not impaired throughout the implantation, although a bimodal distribution of background current densities was observed after long implantation times. The thickness of the fibrous capsule surrounding the MEMS devices was similar between activated and control devices explanted at each time point. It was concluded that the repeated activation of MEMS drug delivery devices was successful and the activation produced an acceptable biological response that resolved promptly.

Methods for reducing biosensor membrane biofouling

The deleterious effect that biofouling has on sensor stability is a serious impediment to the development of long term implanted biosensors. This paper reviews the surface modification strategies currently employed to minimize membrane biofouling of in vivo sensors. Nine sensor modifications are discussed herein: hydrogels, phospholipid-based biomimicry, flow-based systems, Nafion, surfactants, naturally derived materials, covalent attachments, diamond-like carbons, and topology.

Bioactive surface coatings for nanoscale instruments: effects on CNS neurons

A method is described for depositing onto medical instruments highly biocompatible and bioactive surface coatings that can promote and stabilize cell attachment. The coatings were made by first depositing thin films of materials, such as diamond-like carbon, or metals, including tantalum, tungsten, platinum, gold, iridium, palladium, and brass. These surfaces were further altered to either promote or inhibit cell growth and spreading by an additional overcoat of biological materials, including the extracellular matrix proteins, laminin, fibronectin, and collagen IV. The deposition technique used a metal or carbon plasma, and the important properties of film adhesion, hardness, density, and smoothness are tailored by control of the ion bombardment energy. The films are translucent enough to permit high resolution light microscopy for rapid and detailed examination of tissue response. These bioactive substrates have been tested on primary central nervous system neurons, and the growth response is excellent. Equally successful have been our attempts to anchor neurons, without associated proliferation of non-neuronal cells, using coatings of poly-d-lysine. The method and the materials could have important ramifications in a number of areas of research and biotechnology, for example for chronic implantation of microelectrode arrays in the cerebral cortex for neuroprosthetic and neural monitoring application and for research on the human central nervous system. Possible application in nonneuronal fields, such as for coronary artery stents and pacemaker electrodes, also are discussed.

Continuous in vivo monitoring in diabetes: the subcutaneous glucose concentration

The most advanced glucose sensors are measuring amperometrically the hydrogen peroxide generated in a stoichiometric relation to the prevailing glucose concentration during glucose oxidase-mediated glucose oxidation. They proved useful in commercially available glucose analysers and in experimental subcutaneous (sc) monitoring. Here it is shown (a) that under steady state conditions the s.c glucose concentration is nearly identical to that in blood, (b) that sc. inserted glucose electrodes do mirror the intracorporal glucose concentration both under hypo-, normo-, and hyperglycaemic conditions with a clinically relevant accuracy, (c) that automated feedback control of intracorporal glucose concentration is possible applying the output of sc. glucose sensor as an input to the computer that controls the insulin pump, and (d) that stable function of sc. sensor may be accomplished over intervals up to one day; in some cases applications over up to ten days have been reported. The underlying problem consists in an insufficient functional biostability which is a function of biocompatibility and size of the sensor, of its sterility, and of the permanent skin penetration. The latter is still required to get the device in place, to keep it in function, and to make use of the data under any condition. At this time, sc. glucose electrodes to be employed as hypoglycaemia-warning systems over one day are considered clinically important and technically achievable.