Electrodes or microelectrode arrays used for stimulation of nerve tissue and sensing or recording of neural electrical activity are the basis of emerging devices and treatments for various cardiac, neurological, retinal, and hearing disorders. Fabricated to have very specific sizes, geometries, and profiles, as well as electrical, electrochemical, and mechanical properties to match the biological requirements of their intended applications, their amount and diversity are vast.
There has been a growing need for electrode miniaturization to reduce patient trauma and induced scar tissue, in addition to improvements in device performance, longevity, battery life, effectiveness, selectivity, and patient compatibility. Studies center on the development of high-density microelectrode arrays to stimulate or record neural activity in various neurostimulation devices, cochlear implants, retinal prostheses, and multi-electrode electrophysiology catheters. A higher density array of electrodes allows for a greater number of discrete neurons or neuron groups to be activated, resulting in increased localization and control of the desired biological response. Research groups, startup companies, and contract and medical device manufacturers are developing neurostimulation, cardiac, electro-physiology, and cochlear devices as well as retinal prostheses, with a focus on the construction of high-density micro-electrode arrays or single electrodes with enhanced electrical and electrochemical properties at their interface with the biological environment.
High-charge storage/injection capacity, low impedance, and high-capacitance electrodes and microelectrode arrays are of great interest to most device manufacturers. Most sensing, recording, and stimulating applications — including deep brain stimulation and ultra-high-density heart mapping catheters used in electrophysiology devices — aim for the best signal-to-noise ratio with the highest selectivity. The need to use implantable microelectrode arrays with multiple sensing, recording, and stimulation sites close to the tissue is critical. Fabrication of electrodes small enough for communication with single neurons is feasible considering the overall dimensions of the implant. However, size reduction of the actual conducting site is inevitably accompanied by an increase in the impedance of the electrode. Smaller electrodes suffer from low signal-to-noise ratios and reduced charge transfer capacity. The size of an electrode for clinical use is determined by a trade-off between high selectivity (obtained by small size) and optimized electrochemical characteristics (obtained by electro-chemically available surface area.) Larger electrodes inject more charge before exceeding electrochemically safe limits but consume more space. As a result, the spatial selectivity or resolution of a device produced from such electrodes is limited.
A greater number of electrodes results in more geometric surface area, increasing charge injection capacity and capacitance, and enabling the delivery of a higher resolution signal. This is expected to improve device performance. However, due to the space limitations within organs such as the brain, spinal cord, cochlea, and eye, it is not possible to increase the number of electrodes without reducing their size. This significantly lessens the deliverable charge, again impacting device performance.
An alternative approach increases the electrode electrochemical surface area (ESA), producing a larger number of electrodes for improved selectivity and resolution but with a smaller geometric surface area (GSA). A greater number of electrodes can be introduced into the device or in the construction of the microelectrode array, in turn leading to increased performance and selectivity, lower power consumption, and improved fidelity.
Several classes of materials and technologies are effective in increasing ESA and improving electrochemical performance of the electrodes. For example, iridium oxide (IrO2), titanium nitride (TiN), and black or porous platinum coatings, conductive polymers, electrochemical surface roughening, nanostructured scaffolds, two-dimensional materials, and carbon nanotubes have been shown to enhance the charge injection capacity and overall properties of electrodes and microelectrode arrays. There are major differences between these materials, technologies, and their respective manufacturing techniques in terms of performance, durability, scalability, throughput, and capital investment. There are also material and precious metal requirements for the manufacturing process since most electrodes are made from platinum group metals and their alloys.
Many of these techniques pose technological challenges in a manufacturing environment. Difficulties include an inability to employ serial or in-line processing approaches in production, a need for costly time-consuming vacuum/batch processes, and the selective masking and coating of areas of interest on the electrode surface. Despite the favorable electrochemical properties some of these materials and technologies offer, more important challenges include poor adhesion of coatings and additive layers to the underlying substrate or electrode surface, affecting electrode function, as reported in platinum black, IrO2 or conducting polymers.
Surface morphology of materials is a key factor in governing various surface properties such as optical, mechanical, wetting, chemical, biological, and electrochemical characteristics of solid surfaces. Ultrashort pulse and femtosecond laser technology have emerged as novel and versatile technologies for producing a variety of micro- and nanostructured surfaces suitable for a wide range of applications in photonics, plasmonics, optoelectronics, biochemical sensing, micro- and nanofluidics, optofluidics, and biomedicine, among others.
Pulse Technologies Inc. has developed a patented technology using ultrashort pulse lasers for hierarchical surface restructuring (see Figure 1) of electrode materials for implantable medical device applications (see Figure 2). Ultrashort pulse lasers offer the unique advantage of athermal material ablation with no induced damage such as heat affected zone, micro cracking, surface debris, and recast layer. This technology can engineer and tune surface texture and morphology (see Figure 3) to increase surface roughness and available surface area to enhance electrochemical performance of the electrodes by several orders of magnitude.
The hierarchical surface structures induced are comprised of varying length scales ranging from micro- to nanostructures (see Figures 4 and 5). This surface hierarchy greatly increases available electrochemical surface area (ESA), in turn significantly enhancing electrochemical performance of the electrode.
Some performance benefits — including an increase in charge storage capacity and reduction in impedance derived from hierarchical surface restructuring of electrodes — can be measured using electrochemical techniques. Cyclic voltammetry (CV) was used to measure charge storage capacity, and electrochemical impedance spectroscopy (EIS) was used to measure impedance and specific capacitance. Both tests were performed in a three-electrode cell (see Figure 6) comprising a Ag/AgCl reference electrode, a coiled Pt counter-electrode and identically sized electrodes using commercially available phosphate-buffered saline (PBS) solution.
All potentials were recorded with respect to Ag/AgCl. All CV tests were measured at a 50 mV/s sweep rate between potential limits of –0.6 and 0.8 V, beginning at open-circuit potential and sweeping in the positive direction first. Total charge storage capacity (CSCtotal) was calculated by integrating the area under the cyclic voltammagrams for a bare Pt10Ir electrode and a series of electrodes restructured under varying laser restructuring conditions (see Figure 7). The voltammagrams compare electrodes restructured under various pulsing conditions by adjusting laser pulse energies to tune surface morphology and hierarchy (see Figure 7a). The CV behavior of the highest-performing hierarchically restructured electrode exhibiting the largest voltammagram is compared against a smooth Pt10Ir electrode and a 4-μm-thick TiN coating (see Figure 7b).
The Pt10Ir electrode exhibits distinct oxidation and reduction peaks similar to Pt electrodes. TiN voltammogram has the approximately rectangular shape expected for an electrode exhibiting only double-layer capacitance. The laser restructured Pt10Ir electrodes, on the other hand, exhibit substantially larger voltammagrams that are both semirectangular, indicating double-layer capacitance similar to TiN, and also contain an oxidation peak at 0.8 V and a small reduction peak near 0.1 V inherent to Pt10Ir as shown in the inset CV voltammagram of Pt10Ir. Impedance was measured using EIS over a 0.1–105 Hz frequency range using a 10 mV root-mean-square (rms) sinusoidal excitation voltage about a fixed potential between –0.6 and 0.8 V. All measurements were made with Gamry potentiostats and vendor-supplied software. All data reported for EIS are an average of three samples per restructuring condition, tested three times, i.e., a total of nine measurements. Most notably, at frequencies below 1000 Hz, EIS tests and impedance measurements (see Figure 8) exhibit approximately up to two orders of magnitude reduction in impedance for hierarchically restructured electrodes compared to pristine Pt10Ir electrodes. At higher frequencies, all electrodes exhibit resistive behavior dominated by electrolyte conductivity. Specific capacitance was calculated using EIS data and common Randles model.
One of the advantages of hierarchically restructured surfaces compared to TiN coatings is the ability to tune the surface topography and porosity and thus engineer its ESA. TiN exhibits large CSC at slow sweep rates, but at higher sweep rates, access to all the available charge is limited by pore resistance of TiN and the tightly packed nature of the TiN pillars contrary to hierarchically restructured electrodes (see Figure 9).
Use of ultrashort pulse lasers enables restructuring with various surface topographies, pore size, depth, and intercolumnar spacing to reduce pore resistance in order to increase CSC and specific capacitance as shown in Figures 7 and 10. CV tests and CSCtotal measurements demonstrate more than 80-fold increase in total charge storage capacity (CSCtotal) and over 400-fold increase in specific capacitance of Pt10Ir electrodes via hierarchical laser restructuring (see Figure 10). The results also demonstrate that the charge storage capacity and specific capacitance of hierarchically restructured electrodes exceed that of TiN coatings.
Every electrode or microelectrode array has very specific electrochemical performance requirements for an intended application. The tunability and flexibility of this technology for the design of optimal surface topographies make ultrashort pulse laser technology commercially viable, cost-effective, and capable of revolutionizing the electrode, microelectrode array, and long-term implantable device markets.
Despite the favorable electrochemical properties of various coating technologies, there are challenges associated with the majority of these techniques, such as poor adhesion of the coatings to the underlying substrate or electrode surface, low structural and chemical stability, and poor long-term durability. In most vacuum coating technologies, undesired thermal stresses are also introduced into the coating structure, which in turn leads to durability and performance issues.
Hierarchical laser restructuring technology offers numerous promises from material, manufacturing, and cost perspectives, and also from an improved performance viewpoint. The technology is an ideal candidate for next-generation sensing, recording, and stimulating electrode and microelectrode array applications.
This article was written by Dr. Shahram Amini, Director of Research and Development, Pulse Technologies, Quakertown, PA. For more information, visit here .