Polylactide Layer for Surface Protection in Ni-Ti Shape Memory Alloy

The Ni-Ti shape memory alloys have well known from their practical application in medicine. Apart from numerous medical applications there still exists discussion over their biocompatibility and corrosion resistance. Especially, nickel is known from its carcinogenesis when appears in the free state.  In state of the alloy – nickel solubility in biological environment is low. In order to eliminate a fear of its diffusion to a human body surface of the shape memory alloys is covered by protective layer. It was reported that, for Ni-Ti alloys, coating with titanium nitrides, oxides or diamond-like seems to be an attractive way to create a barrier against ion of nickel and sufficiently reduces corrosion. Additionally, surface smoothing reduces adhesion of bacteria as well as a biofilm formation.  However, too thick and/or stiff layer can limit or completely block shape memory effect. That’s, why biocompatible polymers became at the point of interest when covering of Ni-Ti alloys is considered.

In presented work polylactide was used as a protective layer deposited on the surface of Ni-Ti shape memory alloy. Structure of obtained layer as well as its influence on the shape memory effect in covered alloy was studied.  The thin strip with nominal chemical composition Ni₅₀Ti₅₀ was produced using twin roll casting technique. Surface of the samples was modified using L-lactide/DL-lactide copolymer 80/20 Purasorb PLDL 8038 (PURAC Biochem, Netherland). Homogonous L-lactide/DL-lactide copolymer layer 2-3µm thick was deposited on surface of Ni-Ti shape memory alloys. Deposited polylactide layer follows elongation, caused by shape memory effect, up to 5% keeping continuous surface without any cracks. Covering NiTi shape memory alloy with use of the polylactide can be useful way for surface protection, when it is applied in medicine for short-time implants.