Christina Hassler

Characterization of parylene C as an encapsulation material for implanted neural prostheses

The applicability of parylene C as an encapsulation material for implanted neural prostheses was characterized and optimized. The adhesion of parylene C was tested on different substrate materials, which were commonly used in neural prostheses and the efficiency of different adhesion promotion methods was investigated. On Si(3)N(4), platinum, and on a first film of parylene C, a satisfactory adhesion was achieved with Silane A-174, which even withstood standard steam sterilization. The adhesion to gold and polyimide could not be improved sufficiently with the tested methods. Furthermore, tensile tests and measurements of the degree of crystallinity were performed on untreated, on steam sterilized, and on annealed parylene C layers to investigate the influence of thermal treatment. This led to more brittle and stiffer films due to an increase in the crystalline portion in the parylene layers. Finally, an electrochemical impedance spectroscopy was used to test if a parylene C layer was able to protect a metallic structure against corrosion on a Si(3)N(4) substrate. The results indicated that this could be only possible by treating the substrate with Silane A-174. To receive parylene C layers with a good encapsulation performance, it is important to consider the materials, which are used in the neural prosthesis, to find the best suited process parameters.

In vivo monitoring of glial scar proliferation on chronically implanted neural electrodes by fiber optical coherence tomography.

In neural prosthetics and stereotactic neurosurgery, intracortical electrodes are often utilized for delivering therapeutic electrical pulses, and recording neural electrophysiological signals. Unfortunately, neuroinflammation impairs the neuron-electrode-interface by developing a compact glial encapsulation around the implants in long term. At present, analyzing this immune reaction is only feasible with post-mortem histology; currently no means for specific in vivo monitoring exist and most applicable imaging modalities can not provide information in deep brain regions. Optical coherence tomography (OCT) is a well established imaging modality for in vivo studies, providing cellular resolution and up to 1.2 mm imaging depth in brain tissue. A fiber based spectral domain OCT was shown to be capable of minimally invasive brain imaging. In the present study, we propose to use a fiber based spectral domain OCT to monitor the progression of the tissues immune response through scar encapsulation progress in a rat animal model. A fine fiber catheter was implanted in rat brain together with a flexible polyimide microelectrode in sight both of which acts as a foreign body and induces the brain tissue immune reaction. OCT signals were collected from animals up to 12 weeks after implantation and thus gliotic scarring in vivo monitored for that time. Preliminary data showed a significant enhancement of the OCT backscattering signal during the first 3 weeks after implantation, and increased attenuation factor of the sampled tissue due to the glial scar formation.

Chronic intracortical implantation of saccharose-coated flexible shaft electrodes into the cortex of rats

Within this study, polyimide based shaft electrodes were fabricated and dip-coated in molten saccharose to stiffen them for insertion into the brain tissue. These electrodes were then implanted successfully into the cortex of whistar rats and the insertion force during implantation was recorded. Electrochemical impedance spectroscopy was performed immediately after implantation and in regular time intervals up to 201 days after implantation to monitor the tissue response to the implanted electrodes. Depending on the measured electrode pairs and the rats, the impedance spectra behaved different over time. Either they showed a constant decrease in impedance at 1 kHz, or they showed an initial decrease to increase again later. Furthermore, physiological signal recording was performed by stimulating the rats with acoustic signals and simultaneously recording the response on the different electrode sites. Multi-unit activity was detected until 37 days after implantation with an averaged signal-to-noise ratio of 2 to 4.

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.

Deposition Parameters Determining Insulation Resistance and Crystallinity of Parylene C in Neural Implant Encapsulation

Miniaturized neural implants often use Parylene C as an FDA approved material for system encapsulation. The quality of the layer with respect to water absorption and salt intrusion limits the lifetime of these implants. Within this study we measured the resistance of Parylene C (poly-chloropara-xylylene) thin films in saline solution as a function of film thickness, pressure while deposition and substrate metal. The measurement of the resistance was done with an electrochemical setup. In addition, the crystallinity of the deposited films was investigated with an x-ray diffractometer (XRD) and film thickness was measured with a white-light interferometer.

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.

In situ monitoring of brain tissue reaction of chronically implanted electrodes with an optical coherence tomography fiber system

Neural microelectrodes are well established tools for delivering therapeutic electrical pulses, and recording neural electrophysiological signals. However, long term implanted neural probes often become functionally impaired by tissue encapsulation. At present, analyzing this immune reaction is only feasible with post-mortem histology; currently no means for specific in vivo monitoring exist and most applicable imaging modalities provide no sufficient resolution for a cellular measurement in deep brain regions. Optical coherence tomography (OCT) is a well developed imaging modality, providing cellular resolution and up to 1.2 mm imaging depth in brain tissue. Further more, a fiber based spectral domain OCT was shown to be capable of minimally invasive brain intervention. In the present study, we propose to use a fiber based spectral domain OCT to monitor the the progression of the tissues immune response and scar encapsulation of microprobes in a rat animal model. We developed an integrated OCT fiber catheter consisting of an implantable ferrule based fiber cannula and a fiber patch cable. The fiber cannula was 18.5 mm long, including a 10.5 mm ceramic ferrule and a 8.0 mm long, 125 μm single mode fiber. A mating sleeve was used to fix and connect the fiber cannula to the OCT fiber cable. Light attenuation between the OCT fiber cable and the fiber cannula through the mating sleeve was measured and minimized. The fiber cannula was implanted in rat brain together with a microelectrode in sight used as a foreign body to induce the brain tissue immune reaction. Preliminary data showed a significant enhancement of the OCT backscattering signal during the brain tissue scarring process, while the OCT signal of the flexible microelectrode was getting weaker consequentially.