Long-term stability of self-assembled monolayers on 316L stainless steel and L605 cobalt chromium alloy for biomedical applications

Kaufmann, Christopher
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Surface properties of biomaterial implants play a critical role in determining biocompatibility since the body immediately interacts with the implant surface. Surface modifications using a self-assembled monolayer (SAM) is one method which can provide a wide range of functionalities to control a biomaterial's surface chemistry. This method not only enables manipulation of surface chemistry but also opens up numerous possibilities to tether any biological molecule or therapeutic drugs to a metal biomaterial surface.

A SAM is a single 1-10 nm thick layer of organic molecules self assembled on metal or metal oxide surfaces by adsorption from a solution. The SAM molecule contains (a) a chemical head group which has a strong affinity towards the surface, (b) a long hydrocarbon chain, and (c) a terminal functional group. A variety of biomolecules involving proteins, peptides, DNA, carbohydrates, antibodies, and therapeutics have been attached to SAMs for biomedical applications. Of particular interest is the use of SAMs as a platform for drug delivery from coronary stents, the medical device implanted in over a million patients per year.

Although the use of SAMs in biomedical applications is promising, the long term stability of the monolayers under physiological conditions remains unclear. In addition, SAM coatings have primarily been studied on a variety of mechanically polished metal surfaces. However, most commercially available metal implants such as coronary stents are primarily finished by electrochemical polishing. The goal of this study was to investigate the formation and stability of methyl- and carboxyl-terminated phosphonic acid SAMs on mechanically polished 316L stainless steel (MPSS), electropolished 316L stainless steel (EPSS), and electropolished L605 cobalt chromium alloy (EPCC).

The effect of various annealing conditions on the stability of monolayers was thoroughly investigated. The results showed that when a SAM system is heated above its two-dimensional melting point for 6 hours or more, the SAM quality and ordering begins to decrease. Thus, a mild annealing (below the system two-dimensional melting temperature) may be preferred for increasing SAM stability. Based on the findings in this study, it is recommended to allow SAMs to sit at room temperature for 1 to 18 hours after deposition to provide time for a gradual SAM structural reordering to occur.

Using dodecylphosphonic acid and 11-phosphoundecanoic acid, both methyl- and carboxyl-terminated SAMs were successfully coated and characterized on MPSS and for the first time on EPSS and EPCC. The monolayer stability was investigated in Tris buffered saline (TBS) at 37°C for a period of 28 days using contact angle goniometry, fourier transformed infrared spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. SAMs on SS were partially stable under physiological conditions for up to 28 days, with only a slight decrease in monolayer integrity after immersion in TBS for 1-3 days. Carboxyl-terminated SAMs were more stable than methyl-terminated SAMs on SS. Also, both the carboxyl- and methyl-terminated SAMs were more stable on MPSS than on EPSS. In addition, steam autoclave sterilization did not adversely affect the SAM integrity on EPSS. On EPCC, the integrity of SAMs was severely compromised over 28 days in TBS. The significant instability is most likely due to extensive phosphoric acid contamination from the electropolishing process or inherent differences in the metal oxides.

Overall, this thesis has demonstrated the long-term stability of phosphonic acid SAMs on MPSS or EPSS for potential use in biomedical applications. Although the long-term stability of monolayers on EPCC is inferior to EPSS, a period of SAM stability over 1-3 days may be sufficient to control the initial biological cascade that occurs at the biomaterial surface upon implantation. Hence, phosphonic acid SAMs on EPCC may have potential applications in controlling the initial biological responses to an implant material.

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Cobalt Chromium, Electropolish, Phosphonic Acid, Self Assembled Monolayer, Stability, Stainless Steel
Biomedical Engineering