PROGRAM | Materials Science and Engineering

By: Liangqi Ouyang Chair: David Martin

ABSTRACT

This thesis describes methods for improving the reliability of Poly(3,4-ethylenedioxythiophene) (PEDOT) and derivatives as direct interfaces between neural tissue and engineered biomedical devices. We investigated methods to improve the performance of these materials by depositing them directly in living neural tissue, and by tailoring their mechanical strength and adhesion by the design, synthesis, and characterization of appropriately functionalized thiophene monomers.
In the first part of this thesis, to establish conducting pathways between the neurons and glial-sheath-encapsulated electrodes, localized electrochemical polymerization of PEDOT in living tissue was investigated. For this purpose, EDOT monomer solution was infused through a microcannula and electrochemically polymerized into PEDOT under the current delivered via electrodes adjacent to the cannula. The deposition was typically a cloud formed near the tip of the working electrode, extending out into the gel or tissue matrix. After 2 mins of deposition at 2 V in agarose gel, ~200 μm of deposition was found. The PEDOT deposition decreased the system impedance magnitude at 1 kHz to 20% of its value before any deposition. There seemed to be a window time for the polymerization. Polymerized at different scarring stage post initial device implantation, it was found that the impedance increase rate for 2 weeks was slowest when performed between 3 to 4 weeks post surgery. Immunohistology showed that the in vivo deposited PEDOT was still associated with secondary scarring. A hippocampus dependent behavior task, delayed alternation (DA), proved that the in vivo polymerization did not cause significant deficit in the local neural functions.
In the second part, to improve the mechanical strength of PEDOT coatings, a crosslinking strategy was examined. A 3-armed branched EDOT derivative, 1,3,5-tri[2-(3,4-ethylenedioxythienyl)]-benzene (EPh), was copolymerized with EDOT at different monomer feed ratios to create highly branched or crosslinked PEDOT. At 0.1% to 0.5% EPh feed ratio, the impedance of the copolymer films was comparable to PEDOT homopolymer. Further increasing EPh feed ratio decreased the conductivity of the copolymer. UV-Vis-NIR spectra indicated that the increased EPh feed caused a decrease in the effective conjugation length. It was also found that the incorporation of EPh significantly improved the mechanical properties of the copolymer films. PeakForce QNM AFM tests showed that the surface Young’s modulus for PEDOT was 1.35 ± 0.48 GPa. At 0.5% EPh, it was increased to 4.9 ± 2.1 GPa.
In part 3 of this thesis, an amine functionalized EDOT derivative, EDOT-methylamine (EDOT-NH2), was electrografted onto platinum or iridium tin oxide (ITO) substrates as an adhesion promoting layer. PEDOT was consecutively deposited onto the EDOT-NH2 film modified substrates, forming covalently bonded PEDOT coating. After 1 hour of ultrasonication test, the PEDOT / EDOT-NH2 film showed minimal materials loss while normal PEDOT was cleared away from the substrate after only 5 seconds of sonication.
In the last part of this thesis, approaches for locating EDOT and PEDOT in the brain for the in vivo polymerization as well as in-situ imaging of cell-materials interfaces were investigated. Fluorescenct tags were introduced onto the EDOT molecule in order to trace the diffusion of EDOT. It was found that the bulky side group prevented the subsequent polymerization of EDOT. Only at very low concentrations (less than 1% of tagged EDOT in EDOT) could any PEDOT be deposited. Finally, a focused ion beam scanning electron microscope (FIB-SEM) method and a tissue clearing method (CLARITY) for optical imaging of the tissue-materials interface were investigated.

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