(J-409) Development of a Multi-Walled Carbon Nanotube-Based Drug-Eluting Coating for Metallic Implant Surfaces

Introduction:: Multi-walled carbon nanotubes (MWCNTs) are a promising substrate for drug immobilization and release. While a lot of work regarding the use of MWCNTs as individual drug carriers has been performed, the idea of employing them as a drug-loaded interface on metallic implant surfaces is still less popular. The unique property pool and tuneability of MWCNTs make them an attractive candidate for a coating medium to enhance the blood and biocompatibility of metallic surfaces. Pristine MWCNTs are hydrophobic and agglomerate easily when dispersed in host fluid. The two most widely employed approaches for tuning the chemical affinity and surface properties of MWCNTs are wet chemical and non-thermal plasma functionalization. Compared to most wet chemical processes, plasma processing of materials has the advantage of shorter reaction times and is not accompanied by unwanted process waste and sample contamination, which is crucial in the preparation of nanomaterials for biomedical applications. Additionally, plasma processes offer a wide range of MWCNT functionalizations depending on the gas composition, power, treatment time, and pressure used. Plasma treatments can improve the reactivity of MWCNTs by introducing functional groups to individual tube walls. The introduction of polar groups converts the hydrophobic nature of pristine MWCNTs into a strong hydrophilicity required for in vivo applications. The goal of this study is to use the plasma-modified MWCNTs as a high-surface area coating material for the release of pharmaceutical agents from the surface of a medical implant.


Materials and Methods:: MWCNTs were synthesized by chemical vapor deposition, then underwent functionalization using a low-pressure capacitively-coupled radiofrequency (RF) argon/oxygen/ethane plasma to produce negatively charged MWCNTs. The functionalized MWCNTs were sonicated in reverse osmosis (RO) water to create a stable suspension. Subsequently, electrophoretic deposition (EPD) of MWCNTs onto a 316L stainless steel (SS) plate was carried out by applying a constant DC voltage to the MWCNT-RO water suspension. The cathode was a graphite plate. EPD voltages and times were varied from 0 to 40 V and 1 to 60 min, respectively. Characterization of the MWCNT coating was carried out by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and profilometry.



The coating was pre-treated with an argon plasma, and subsequently with a continuous ammonia plasma to graft primary amine (-NH₂) to individual MWCNTs. To determine the experimental parameters which lead to the highest degree of -NH₂ grafting, different RF plasma power levels (5-100 W) and treatment times (1-60 min) were investigated. The pressure was kept constant at 1 Torr. The chemical composition of the pristine, argon plasma pretreated and ammonia plasma functionalized MWCNTs was assessed by XPS. For the present investigation, the aim was to bind -NH₂ groups to MWCNTs since they can be covalently coupled to carboxyl groups (-COOH) of various drug compounds via cross-linking agents. The presence of -NH₂ was confirmed by thermogravimetric analysis coupled with infrared spectroscopy (TGA-IR) and 4-trifluoromethyl benzaldehyde (TFBA) vapour chemical derivatization.


Results, Conclusions, and Discussions:: After argon/oxygen/ethane plasma functionalization, the at% of MWCNT surface oxygen increased from zero to 12%. The production of a stable MWCNT-RO water suspension was achieved without the use of an external surfactant. During EPD, negatively charged MWCNTs were drawn towards the positively charged SS plate. Since the electrolyte was water-based, hydrolysis of water took place. The oxygen formation at the SS anode did not interfere with the deposition of a smooth coating on the anode due to the inherent porosity of the MWCNT coatings. While the applied EPD was kept constant, the effective current decreased over time as more charged species were removed from the electrolyte and a growing MWCNT coating accumulated on the surface of the SS anode. The deposited MWCNT coating had a smooth and uniform black appearance. The MWCNT coating had a composition of 94 at% carbon and 6 at% oxygen. A MWCNT coating thickness of ~15 μm was produced for the highest EPD voltage (40 V) and time (60 min).



The argon plasma pre-treatment preceding the ammonia plasma functionalization rendered the MWCNTs more defective by ionic bombardment, which enhanced the effect of the ammonia plasma treatment. The highest at% nitrogen results of 15 % were observed for continuous ammonia plasmas at 5 W, 1 Torr, a flow rate of 10 sccm, and a processing time of 45 min. The TGA-IR spectra revealed the presence of -NH₂ solely. Chemical derivatization showed an average percentage of 1.5% -NH₂ on the MWCNT surface for the most successful ammonia plasma functionalization protocol. As such, roughly 10% of the total at% nitrogen on the MWCNT surface were part of -NH₂ groups.



In this study, we have successfully deposited a -NH₂ functionalized MWCNT coating on a pristine 316L SS plate. In the next phase of the study, we will further characterize the coating, as well as investigate the drug loading and elution properties of the coating.




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