Juan S. Ordonez

Fabrication of multi-layer, high-density micro-electrode arrays for neural stimulation and bio-signal recording

The electrode-tissue interface is of principal importance in neuroprosthesis. Indeed the successes of the cochlear implant and other therapeutic devices are directly attributable to the design and fabrication techniques of their interfaces with neural tissue, that is, the electrode or electrode array. Traditional fabrication techniques are often labor-intensive and do not lend themselves to automation thereby increasing the cost of the electrode, and owing to fabrication variability, potentially compromising the reliability of the devices incorporating them. Exacerbating the difficulties in electrode fabrication further is the fact that only a handful of materials have been demonstrated to be biologically inert. These same materials are often among the most difficult to utilize in the fabrication of neural electrodes. In the present paper, a new methodology for automated fabrication of high-density electrode arrays is presented. Using exclusively biologically-inert raw materials, laser machining techniques combined with multiple layer structuring is shown to achieve feature sizes of the order of 25 mum. As an illustrative example, a 98 electrode array for interfacing with surviving retinal tissue through a visual prosthesis for the blind is presented. Overall dimensions of the array are of the order of 8.7 times 9.4 mm, consistent with approximately 25 degrees of visual field.

Cytotoxicity of implantable microelectrode arrays produced by laser micromachining

Implantable high-density microelectrode arrays have been successfully fabricated using laser micromachining of conventional implant materials, polydimethylsiloxane (PDMS) and platinum (Pt) foil. This study investigates the impact of modifying PDMS and Pt with high power laser beams and the possible toxicity of by-products that may remain on the implantable device. Materials were characterised both chemically and biologically through x-ray photoelectron spectroscopy (XPS), cell growth inhibition assays and a direct contact cell proliferation assay. It was found that laser micromachining produces oxides of silicon and platinum on the PDMS and Pt respectively. While the chemical properties of materials were altered, there was negligible change in the biological response to either extracts or cell growth directly on the composite electrode array.

Hermetic electronic packaging of an implantable brain-machine-interface with transcutaneous optical data communication

Future brain-computer-interfaces (BCIs) for severely impaired patients are implanted to electrically contact the brain tissue. Avoiding percutaneous cables requires amplifier and telemetry electronics to be implanted too. We developed a hermetic package that protects the electronic circuitry of a BCI from body moisture while permitting infrared communication through the package wall made from alumina ceramic. The ceramic package is casted in medical grade silicone adhesive, for which we identified MED2-4013 as a promising candidate.

Interconnection technologies for laser-patterned electrode arrays

Standard interconnection technologies are reviewed in respect to their applicability to electrically and mechanically connect laser-patterned nerve electrodes made from silicone rubber and platinum foil to wires and screen printed alumina substrates. Laser welding, gap welding, microflex ball bonding, and soldering are evaluated. Corresponding processes were established and evaluated in respect to their mechanical strength. Best results were obtained by soldering. If soldering cannot be used because of regulatory reasons, parallel gap welding and microflex are recommended. Laser welding provides weaker interconnects with only moderate reproducibility.

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.

Fabrication and test of a hermetic miniature implant package with 360 electrical feedthroughs

A fabrication technology for small hermetic implant packages with large numbers of electrical feedthroughs is presented. First prototypes were fabricated on a ceramic substrate of 25·25 mm area, having a metal cup of 5 mm height soldered to it. These prototypes provide 360 feedthroughs. The electrical properties of the feedthroughs are characterized and the leakage rate of the packages is determined using helium fine leak tests. The amount of water sealed inside the packages is investigated. Based on maximum allowable water vapour concentrations inside hermetic packages reported in literature and applying a commonly accepted mathematical model, we predicted a minimum lifetime to water-induced failure of a few hundred years.

Stretchable tracks for laser-machined neural electrode arrays

An easy and fast method for fabrication of neural electrode arrays is the patterning of platinum foil and spin-on silicone rubber using a laser. However, the mechanical flexibility of such electrode arrays is limited by the integrated tracks that connect the actual electrode sites and the contacts to which wires are welded. Changing the design from straight lines to meanders, the tracks can be stretched to a certain extend defined by the shape of the meanders. Horse-shoe-like designs described by an opening angle θ = 3D 60° and ratio between curvature radius r and track width w of r/w = 3D 3.6 permitted stretching of 14.4% before track breakage. For r/w = 3D 11.7 a maximum elongation at break of 19.7% was measured. Larger opening angles θ provided even better flexibility but with increasing θ, the tensile strength and the electrical conductance of a single track is compromised and the maximum integration density (tracks per area) decreases.

Patterning of Silicone Rubber for Micro-Electrode Array Fabrication

Five methods of micro-patterning silicone rubber, (polydimethylsiloxane, PDMS) are reviewed. Three of the methods are experimentally evaluated in order to find the most suitable process parameters to expose electrode sites and contact pads of PDMS-embedded platinum structures for fabricating micro-electrode arrays. Rated by applicability, type of PDMS involved (implantable-grade required), achievable aspect ratio (AR) and minimum feature size (FS), the feasibility of two methods was demonstrated: laser-ablation, using a Nd:YAG laser (AR = 5.3, FS > 100 mum) and dry etching using a reactive ion etcher (AR = 10, FS > 10 mum).

Let There Be Light—Optoprobes for Neural Implants

Over the past decades, optical technologies have entered neural implant technologies. Applications such as optogenetics, near-infrared spectroscopy (NIRS), and direct-near-infrared stimulation (NIS) request technical devices that combine electrical and optical recording as well as stimulation capabilities using light sources and/or optical sensors. Optoprobes are the technical devices that meet these requirements. This paper provides basic insights into optogenetic mechanisms, the background of NIRS and NIS, and focuses on fundamental requirements of technical systems from a biological background. The state of the art of optoprobes is reviewed and attention is drawn on the potential long-term stability of these technical devices for chronic neural implants. Further, material selection for stiff and flexible devices, applicable light sources, waveguide and coupling concepts, packaging paradigms as well as system assembly and integration aspects are discussed in view of biocompatible and biostable devices. This paper also considers the physical background of light scattering and heat generation when light sources are implanted into biological tissue.

Silicone rubber and thin-film polyimide for hybrid neural interfaces — A MEMS-based adhesion promotion technique

Strong permanent adhesion between thin-film polyimide (BPDA-PPD) and silicone rubber (MED-1000) was achieved through deposition of a chemically-transitive intermediate adhesion promoting layer. Plasma-enhanced chemical vapor deposition (PECVD) of SiC and SiO2 was used to grow a thin 50 nm layer directly on a 5 μm thin polyimide substrate. The deposition at low pressures permitted the fabrication of an adaptive covalent bond transition from sp2-hybridized carbon (in polyimide) towards the sp3 bonding in SiC, continuing to SiO2 which provides a good bonding partner for one-component poly-dimethyl siloxane (PDMS). The fabricated laminates together with reference probes containing no adhesion promoting layer were subjected to intense accelerated aging at 125°C and 130 kPa (pressure cooker) over 96 hrs in phosphate buffered saline solution. While the reference polyimide-PDMS laminates failed just after 30 min in the pressure cooker, no failure was detected on samples using the proposed adhesion promoter technique. Mechanical loading of the samples resulted in cohesive crack formation at the polyimide, propagating across the bulk with no evidence of adhesive failure between any of the materials. The strong permanent adhesion brings the fabrication of hybrid neural interfaces one step forward, achieving the combination of thin-film manufacturing and PDMS.