Christian Henle

Brain–computer interfaces: an overview of the hardware to record neural signals from the cortex

Brain-computer interfaces (BCIs) record neural signals from cortical origin with the objective to control a user interface for communication purposes, a robotic artifact or artificial limb as actuator. One of the key components of such a neuroprosthetic system is the neuro-technical interface itself, the electrode array. In this chapter, different designs and manufacturing techniques will be compared and assessed with respect to scaling and assembling limitations. The overview includes electroencephalogram (EEG) electrodes and epicortical brain-machine interfaces to record local field potentials (LFPs) from the surface of the cortex as well as intracortical needle electrodes that are intended to record single-unit activity. Two exemplary complementary technologies for micromachining of polyimide-based arrays and laser manufacturing of silicone rubber are presented and discussed with respect to spatial resolution, scaling limitations, and system properties. Advanced silicon micromachining technologies have led to highly sophisticated intracortical electrode arrays for fundamental neuroscientific applications. In this chapter, major approaches from the USA and Europe will be introduced and compared concerning complexity, modularity, and reliability. An assessment of the different technological solutions comparable to a strength weaknesses opportunities, and threats (SWOT) analysis might serve as guidance to select the adequate electrode array configuration for each control paradigm and strategy to realize robust, fast, and reliable BCIs.

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.

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).

Mapping of sheep sensory cortex with a novel microelectrocorticography grid.

Microelectrocorticography (µECoG) provides insights into the cortical organization with high temporal and spatial resolution desirable for better understanding of neural information processing. Here we evaluated the use of µECoG for detailed cortical recording of somatosensory evoked potentials (SEPs) in an ovine model. The approach to the cortex was planned using an MRI‐based 3D model of the sheeps brain. We describe a minimally extended surgical procedure allowing placement of two different µECoG grids on the somatosensory cortex. With this small craniotomy, the frontal sinus was kept intact, thus keeping the surgical site sterile and making this approach suitable for chronic implantations. We evaluated the procedure for chronic implantation of an encapsulated µECoG recording system. During acute and chronic recordings, significant SEP responses in the triangle between the ansate, diagonal, and coronal sulcus were identified in all animals. Stimulation of the nose, upper lip, lower lip, and chin caused a somatotopic lateral‐to‐medial, ipsilateral response pattern. With repetitive recordings of SEPs, this somatotopic pattern was reliably recorded for up to 16 weeks. The findings of this study confirm the previously postulated ipsilateral, somatotopic organization of the sheeps sensory cortex. High gamma band activity was spatially most specific in the comparison of different frequency components of the somatosensory evoked response. This study provides a basis for further acute and chronic investigations of the sheeps sensory cortex by characterizing its exact position, its functional properties, and the surgical approach with respect to macroanatomical landmarks. J. Comp. Neurol. 522:3590–3608, 2014.

Scaling limitations of laser-fabricated nerve electrode arrays

A laser technology for manufacturing implantable electrode arrays, using spin-on polydimethylsiloxane (PDMS) and platinum foil as materials, was investigated in respect to its scaling limitations. The following aspects were analyzed: Minimal width and centre-to-centre distance of platinum tracks, the ability of spin-on PDMS to flow between platinum tracks with very narrow gaps and the electrical insulation properties of thin spin-coated PDMS.

Mechanical characterization of neural electrodes based on PDMS-parylene C-PDMS sandwiched system

Manufacturing of neural electrodes based on metal foil and silicone rubber using a laser is a simple and promising method. A handicap of such electrode arrays is the mechanical robustness of the thin metal tracks that connect the electrode sites with the interconnection pads. Embedding of structured parylene C foil in silicone rubber turned out to be an interesting method to increase the robustness. Test samples with 12.5 μm thick platinum tracks and a 15 μm thick embedded and RIE-structured parylene C foil showed more than 800 % higher ultimate strength until breakage of the tracks. Different structured parylene C foil showed increasing robustness with increasing hole-spacing.

Evaluation of μECoG electrode arrays in the minipig: experimental procedure and neurosurgical approach.

Emerging research on brain-machine interfaces (BMIs) requires the development of animal models for testing implantable BMI electrodes. New models are necessary in order to characterize and test newly constructed electrodes in an acute environment, and their properties and performance need to be evaluated in long-term, chronic implantations. Owing to their availability, small size and neuroanatomical similarity to the human brain, minipigs are frequently used for neurological studies. Despite this fact, there are still no standardized experimental and neurosurgical procedures available for recording of cortical potentials using implantable BMI electrodes in minipigs, and, until now, it was unclear whether these animals could also be used for long-term subdural electrode implantations. We have therefore evaluated the potential use of minipigs for acute and chronic implantation of micro-electrocorticogram (μECoG) electrodes we newly developed for BMI applications and we present a standardized neurosurgical approach to the minipigs cerebral cortex. A neurophysiological setup is described which is suitable to perform recordings of somatosensory evoked potentials (SEPs) with high spatial resolution - down to approx. 1-mm inter-electrode distance. Perioperative management, anesthesia and anatomical landmarks for electrode placement are discussed and common surgical pitfalls are described. While, due to their specific cranial anatomy, minipigs appear not optimally suited for chronic subdural implantations, the findings of the present study indicate that μECoG recording from the minipig cortex is a valuable new approach for acute in vivo characterization of subdural BMI electrode function.

Electrical Characterization of Platinum, Stainless Steel and Platinum/Iridium as Electrode Materials for a New Neural Interface

This paper describes the design and fabrication of a 48-channel microelectrode, based on laser-patterned silicone rubber (polydimethylsiloxane, PDMS) and different metal foils. Samples with platinum, platinum/iridium and stainless steel as electrode materials were fabricated and electrochemically characterized. The electrochemical performance was investigated by electrical impedance spectroscopy. Supplementary, pulse tests were carried out, in order to calculate the charge injection capacities. All samples have the same contacts sizes, what allows a comparison between the three different metal foils.

Miniaturized neural interfaces and implants

Neural prostheses are technical systems that interface nerves to treat the symptoms of neurological diseases and to restore sensory of motor functions of the body. Success stories have been written with the cochlear implant to restore hearing, with spinal cord stimulators to treat chronic pain as well as urge incontinence, and with deep brain stimulators in patients suffering from Parkinsons disease. Highly complex neural implants for novel medical applications can be miniaturized either by means of precision mechanics technologies using known and established materials for electrodes, cables, and hermetic packages or by applying microsystems technologies. Examples for both approaches will be introduced and discussed. Electrode arrays for recording of electrocorticograms during presurgical epilepsy diagnosis have been manufactured using approved materials and a marking laser to achieve an integration density that is adequate in the context of brain machine interfaces, e.g. on the motor cortex. Microtechnologies have to be used for further miniaturization to develop polymer-based flexible and light weighted electrode arrays to interface the peripheral and central nervous system. Polyimide as substrate and insulation material will be discussed as well as several application examples for nerve interfaces like cuffs, filament like electrodes and large arrays for subdural implantation.