Multifunctional Conducting Polymer Coatings for Magnesium Implants

Magnesium’s potential to degrade completely in vivo with safe corrosion byproducts make it a promising biomaterial for use in a wide array of implants from orthopedic fixation devices that do not require a removal surgery to peripheral nerve guides that degrade away after regeneration. However, a few issues need to be addressed before widespread clinical use can be realized. Firstly, Mg often degrades too rapidly in vivo. Rapid degradation can result in the mechanical instability and gas bubbles that may separate the implant from the tissue. Secondly, Mg implants lack the ability to combat potential infection and the inflammatory foreign body response. Lastly, Mg alone lacks a versatile functionalization method to incorporate tissue specific cues that better guide tissue growth and regeneration. To overcome these issues, conducting polymer based coatings with a combination of functionalities in corrosion control, drug release, and biofunctionalization are investigated in this thesis. A composite coating of conducting polymer poly 3,4-ethylene dioxythiophene (PEDOT) doped with Graphene Oxide (GO) has been developed for improving Mg implant performance for applications such as orthopedic fixation, stents, and peripheral nerve regeneration. The PEDOT/GO coating decreased Mg corrosion throughout 22days of immersion in a phosphate buffered solution. Corrosion protection is attributed to an initial passive barrier followed by electrochemical coupling of the coating with the Mg to from a more protective Mg phosphate layer. Additionally, anti-inflammatory drug Dexamethasone was incorporated into the PEDOT/GO film and it was shown that Mg corrosion current could drive drug release. Lastly, the carboxylic acid groups of the GO sheets exposed at the surface of the PEDOT/GO coating were used to immobilize multiple bioactive molecules. Specifically, immobilized poly ethylene glycol (PEG) prevented both bacterial and fibroblast attachment, while nerve growth factor (NGF) attachment increased neurite sprouting from PC12 cells. These results suggest that the PEDOT/GO coating has the potential to be a versatile coating that can provide corrosion protection and add biologically relevant cues to the Mg implan

In Vivo Electrochemical Analysis of a PEDOT/MWCNT Neural Electrode Coating

Neural electrodes hold tremendous potential for improving understanding of brain function and restoring lost neurological functions. Multi-walled carbon nanotube (MWCNT) and dexamethasone (Dex)-doped poly(3,4-ethylenedioxythiophene) (PEDOT) coatings have shown promise to improve chronic neural electrode performance. Here, we employ electrochemical techniques to characterize the coating in vivo. Coated and uncoated electrode arrays were implanted into rat visual cortex and subjected to daily cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for 11 days. Coated electrodes experienced a significant decrease in 1 kHz impedance within the first two days of implantation followed by an increase between days 4 and 7. Equivalent circuit analysis showed that the impedance increase is the result of surface capacitance reduction, likely due to protein and cellular processes encapsulating the porous coating. Coating’s charge storage capacity remained consistently higher than uncoated electrodes, demonstrating its in vivo electrochemical stability. To decouple the PEDOT/MWCNT material property changes from the tissue response, in vitro characterization was conducted by soaking the coated electrodes in PBS for 11 days. Some coated electrodes exhibited steady impedance while others exhibiting large increases associated with large decreases in charge storage capacity suggesting delamination in PBS. This was not observed in vivo, as scanning electron microscopy of explants verified the integrity of the coating with no sign of delamination or cracking. Despite the impedance increase, coated electrodes successfully recorded neural activity throughout the implantation period

Mechanical failure modes of chronically implanted planar silicon-based neural probes for laminar recording

Penetrating intracortical electrode arrays that record brain activity longitudinally are powerful tools for basic neuroscience research and emerging clinical applications. However, regardless of the technology used, signals recorded by these electrodes degrade over time. The failure mechanisms of these electrodes are understood to be a complex combination of the biological reactive tissue response and material failure of the device over time. While mechanical mismatch between the brain tissue and implanted neural electrodes have been studied as a source of chronic inflammation and performance degradation, the electrode failure caused by mechanical mismatch between different material properties and different structural components within a device have remained poorly characterized. Using Finite Element Model (FEM) we simulate the mechanical strain on a planar silicon electrode. The results presented here demonstrate that mechanical mismatch between iridium and silicon leads to concentrated strain along the border of the two materials. This strain is further focused on small protrusions such as the electrical traces in planar silicon electrodes. These findings are confirmed with chronic in vivo data (133–189 days) in mice by correlating a combination of single-unit electrophysiology, evoked multi-unit recordings, electrochemical impedance spectroscopy, and scanning electron microscopy from traces and electrode sites with our modeling data. Several modes of mechanical failure of chronically implanted planar silicon electrodes are found that result in degradation and/or loss of recording. These findings highlight the importance of strains and material properties of various subcomponents within an electrode array