Effect of nitinol wire surface properties on albumin adsorption

The superelastic, shape memory alloy nitinol (∼50% nickel and ∼50% titanium) is an important medical device material used for stent applications. However, the role specific surfaces properties have in protein adsorption remain controversial. In this study the effects of nitinol wire surface roughness, hydrophobicity and elemental composition upon albumin adsorption are investigated. In particular, we demonstrate that the technique of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry in the so-called surface mode can be used for the direct detection of albumin on the wire surfaces. In addition, albumin adsorbing to the wires was determined by using 125I-labelled albumin. Albumin was detected on all wire samples. Surface roughness and hydrophobicity appeared to have no effect on albumin adsorption. There was however a clear correlation between the surface nickel and oxygen concentration and the amount of albumin adsorbed. Samples with higher levels of nickel and less oxygen in the surface oxide layer of the wires showed increased albumin adsorption, which could lead to improved biocompatibility. However, nickel is a toxic substance and can cause many adverse effects on humans, and thus nitinol with a slightly enriched surface nickel concentration that does not exhibit nickel release may have potential as a medical device material.