Long-term stability of self-assembled monolayers on electropolished L605 cobalt chromium alloy for stent applications

Commercially available drug-eluting stents have the potential to induce inflammatory and hypersensitive adverse reactions due to their polymer coating. The use of self assembled monolayers (SAMs) as an alternate drug delivery platform for stents has recently been demonstrated. In this study, the formation and stability of phosphonic acid SAMs were investigated using the material and surface preparation commonly used to make ultra-thin stent struts-electropolished L605 Cobalt Chromium (CoCr) alloy. Methyl (⁻CH₃) and carboxylic acid (⁻OOH) terminated phosphonic acid SAMs were coated on electropolished CoCr alloy using a combination of solution immersion and dip-evaporation cycle deposition methods. SAMs-coated CoCr alloy specimens were thoroughly characterized using contact angle goniometry, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). These characterizations suggested that uniform and well-ordered monolayers were coated on the electropolished CoCr alloy. The long-term physiological stability of monolayers was investigated in tris-buffered saline (TBS) at 37°C for up to 28 days. Contact angles, FTIR, XPS, and AFM suggested that both ⁻CH₃ and ⁻COOH terminated phosphonic acid SAMs desorb from electropolished CoCr alloy surfaces in a biphasic manner. A significant desorption of ⁻CH₃ and ⁻COOH terminated SAMs occurs within 1-3 days followed by a slower desorption for up to 28 days. Thus, there is a need to develop techniques that can improve the long-term stability of SAMs on electropolished CoCr alloy for stent and other biomedical applications.