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Accelerated tests for neural implants
Keywords: reliability, implants, accelerated tests
Neural implants are used for stimulating different neural pathways of interest as well as for sensing electrical and electrochemical function of the neurons. They play a critical role in the emerging efforts to develop therapeutics in bioelectronic medicine for a variety of disorders including neurological disorders and dysfunctions in mental health. However, implants in the brain and in the peripheral nervous system fail rather unpredictably in long-term conditions lasting several months. The most common and the debilitating modes of failure are at the interface between the implant (often a conductive electrode) and the surrounding tissue that include a complex interplay between biotic and abiotic factors. Biotic factors such as acute implantation injury, and chronic foreign body response result in changes in the extracellular matrix and the cellular milieu that can exacerbate typical abiotic factors such as corrosion and delamination leading to further injection of metal, metal oxide ions in the extracellular space. Accelerated bench-top tests that have been very successfully used in electronic industry to predict lifetimes are less accurate for predicting life-times for implantable devices since the biotic factors involved in the failures are not accurately understood and accelerated. The overall goal is to develop accelerated tests that will be more predictive of lifetimes under chronic in vivo conditions. We propose tests that will involve the use of a soft siloxane scaffold to minimize both acute injury and chronic foreign body responses in the brain tissue to an implant. We test the following hypotheses (a) soft siloxane scaffolds minimize biological perturbations in the surrounding tissue that typically accompany an implant and (b) accelerated bench top tests involving such scaffolds will therefore be more accurate in predicting lifetimes of implants in vivo experiments. We performed in vivo and bench top experiments to test the above two hypotheses under 3 different conditions for implants (condition #1) for monitoring electrical activity of neurons, (condition #2) for monitoring electrochemical activity of neurons and finally (condition #3) for neurostimulation. Measurements included electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), amperometry and neuronal recordings in bench top and in vivo rodent experiments. In bench top experiments, 10-20 mM H2O2 and 40 mg/ml albumin were used to model the reactive oxidation species (ROS) and blood-brain breakage respectively. In in vivo experiments lasting over one year, the broad-band electrochemical impedance spectra were significantly more stable for implants (both tungsten and Pt/Ir) with a soft scaffold than those without. Preliminary histological and immunochemical analysis demonstrate a diminished biological response in the surrounding tissue when the implants are encapsulated in a soft siloxane scaffold. Electrochemical sensors developed for tyrosine were more stable (in 3 different biological mediums) when encapsulated with a soft scaffold compared to sensors that did not have one. Preliminary statistical analyses show that the use of a soft scaffold improves the consistency of the experimental measurements in long- term experiments under all 3 experimental conditions (condition #1, #2 and #3). Current work is focused on lifetime predictions based on statistical models of the above experimental data.
Jit Muthuswamy, Associate Professor, Biomedical Engineering
Arizona State University
Tempe, AZ

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