Electrodes implanted into neural systems are known to degrade due to encapsulation by surrounding tissues. The mechanisms of electrode-tissue interactions and prediction of the behavior of electrode are yet to be achieved. This research will aim at establishing the fundamental knowledge of interfacial interactions between the host biological environment and an implanted electrode. We will identify the dynamic mechanisms of such interfacial interactions. Quantitative analysis of the electrical properties of interface will be conducted using Electrochemical Impedance Spectroscopy (EIS). Results will be used to develop a general model to interpret electrical circuitry of the interface. This is expected to expand our understanding in the effects of interfacial interactions to the charge transport. The interfacial interactions of an implanted electrode with neural system will be studied in two types of electrodes: silver and graphene coated. The interfacial impedance of both samples will be studied using EIS. The development of the cellular interaction will be investigated using histological procedure. X-ray photoemission spectroscopy (XPS) will be employed to study the chemical effects on the silver electrodes. Atomic force microscopy and Raman spectroscopy will be used for material characterization of graphene-coated electrodes. In the study of silver electrode, two mechanisms affecting the interfacial impedance are proposed. First is the formation of silver oxide. The other is the immuno-response of tissue encapsulation. Histological results suggest that higher cell density cause higher impedance magnitude at the interface. It is also found that the cellular encapsulation dominates the increase in impedance for longer implanted time. In the study of graphene-coated electrode, it is found that the graphene can strongly prevent the metal substrate from being oxidized. It not only provides good electrical conductivity for signal transport, but also reduces the speed of the accumulation of tissue around the electrode. Such characteristics of graphene have great potential in the application of neural implant. Finally, the dynamic mechanisms of biological interaction are proposed. A model is also developed to represent the general circuitry of the interface between an implanted electrode and the neural system. The model has three major components, which are interfacial double layer, cellular encapsulation, and the substrate. The model presented in this study can compensate for selection and prediction of materials and their behaviors.

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Mechanical trauma to the brain, both big and small, and the method to protect the brain in its presence is a crucial field of research given the large population exposed to neuronal trauma daily and the benefit available through better understanding and injury prevention. A population of particular interest and risk are youth athletes in contact sports due to large accelerations they expose themselves to and their developing brains. To better monitor the risk these athletes are exposed to, their accumulation of head acceleration events (HAEs), a measure correlated with harmful neurological changes, was tracked over sport seasons. It was observed that few significant differences in HAEs accumulated existed between players of ages from middle school to high school, but there did exist a difference between sports with girls' soccer players accumulating fewer HAEs than football players. This highlights to risk youth athletes are exposed to and the importance of improved technique and individual player size. To better monitor HAEs for each individual, a novel head segmentation program was developed that extracts player specific geometries from a single T1 MRI scan that can improve the accuracy of HAE monitoring. Acceleration measures processed with individualized head model versus those using a standardized head model typically displayed higher accelerations, highlighting the need for individualized measure for accurate monitoring of HAEs and risk of neurological changes. In addition to the large accelerations present in contact sports, the small but constant strains produced by neural implants embedded in the brain is also an important field of neuro-mechanical research as the physical properties of neural implants have been found to contribute to the chronic immune response, a major factor preventing the widespread implementation of neural implants. To reduce the severity of the immune response and provide improved chronic functionality, researchers have varied neural implant design and materials, finding general trends but not precise relationships between the design factors and how they contribute the mechanical strain in the brain. Performing a large series of mechanical simulations and Cotter's sensitivity analyses, the relationships between neural design factors and the stain they produce in the brain was examined. It was found that the direction which neural implants are loaded contributes the most to the strain produced in the brain followed by the degree of bonding between the brain and the electrode. Directly related to the design of electrodes themselves, it was found that in most cases reducing the cross-sectional area of the probe resulted in a larger decrease of mechanical strain compared to softening the implant. Finally, a study was performed quantifying the resting micromotion of the brain utilizing a novel method of soft tissue micromotion measurement via microCT, applicable within the skull and the throughout the rest of the body.

Neural implants have been developed to aid with different neurological disorders. Although there are neural implants that are used to treat patients today, there is still room for improvement in the field. The material used in neuroprosthetics is of particular importance and could lead to problems, if the material stiffness does not match the one of the tissue. Hence, soft and flexible implants are important to decrease the discrepancy between the tissue and the implants. Different materials have been used as substrates to make the implants soft, some materials are even biodegradable. Further, the design of the implants is also of importance to make the devices flexible. In this thesis, optimisation of the substrate layer for a photovoltaic cortical implant was performed. The degradation of PLGA as a potential material used as a substrate layer, was investigated. Moreover, for improvement of the design Parylene C was used. Mechanical and electrical tests were done to investigate how the manipulation of the devices affects their performance. The results showed that the degradation of PLGA started after 600 h. Further, the best shape and the width of the bridges, that was the Parylene C strips connecting the photovoltaic cells were chosen, as well as the thickness of Parylene C. The mechanical and electrical results indicated that the number of cycles does not affect the material performance as much, where the highest number. of cycles was 10000 cycles. As compared to the effect of the manual handling of the devices. / Neurala implantat har utvecklats för att hjälpa till vid olika neurologiska sjukdomar. Även om neurala implantat används för att behandla patienter i dag finns det fortfarande utrymme för förbättringar på detta område. Materialet som används i neuroproteser är särskilt viktigt och kan leda till problem om materialets styvhet inte motsvarar vävnadens styvhet. Därför är mjuka och flexibla implantat viktiga för att minska klyftan mellan vävnaden och implantaten. Olika material har använts som substrat för att göra mjuka implantat, vissa material är till och med biologiskt nedbrytbara. Dessutom, är implantatens design också viktig för att göra enheterna flexibla. I den här rapporten optimerades substratskiktet för ett solcells kortikalimplantat. Nedbrytningen av PLGA som ett potentiellt material som kan användas som substratskikt undersöktes. Dessutom användes Parylen C för optimering av konstruktionen. Mekaniska och elektriska tester utfördes för att undersöka hur manipuleringen av enheterna påverkar deras prestanda. Resultaten visade att nedbrytningen av PLGA började efter 600 h. Vidare valdes den bästa formen och bredden på broarna, dvs. de Parylen C-remsor som förbinder solcellerna, samt tjockleken på Parylen C. De mekaniska och elektriska resultaten visade att antalet cykler inte påverkar materialets prestanda särskilt mycket, med det högsta antalet cykler på 10 000 cykler. Jämfört med effekten av den manuella hanteringen av enheterna.

Stable single-unit recordings from the nervous system using microelectrode arrays can have significant implications for the treatment of a wide variety of sensory and movement disorders. However, the long-term performance of the implanted neural electrodes is compromised by the formation of glial scar around these devices, which is a typical consequence of the inflammatory tissue reaction to implantation-induced injury in the CNS. The glial scar is inhibitory to neurons and forms a barrier between the electrode and neurons in the surrounding brain tissue. Therefore, to maintain long-term recording stability, reactive gliosis and other inflammatory processes around the electrode need to be minimized. This work has succeeded in the development of neural electrode coatings that are capable of sustained release of anti-inflammatory agents while not adversely affecting the electrical performance of the electrodes. The effects of coating methods, initial drug loadings on release kinetics were investigated to optimize the coatings. The physical properties of the coatings and the bioactivity of released anti-inflammatory agents were characterized. The effect of the coatings on the electrical property of the electrodes was tested. Two candidate anti-inflammatory agents were screened by evaluating their anti-inflammatory potency in vitro. Finally, neural electrodes coated with the anti-inflammatory coatings were implanted into rat brains to assess the anti-inflammatory potential of the coatings in vivo. This work represents a promising approach to attenuate astroglial scar around the implanted silicon neural electrodes, and may provide a promising strategy to improve the long-term recording stability of silicon neural electrodes.