Recent developments in Nitinol surfaces for implantation

Nitinol as an implant material conquers steadily new areas of medical application as versatile as cardiovascular and orthopaedics. Because of the need to keep a low profile of miniature implant devices and considering the absence of compatibility between superelasticity of Nitinol and mechanical properties of various coatings, bare NiTi surfaces are of particular interest for implantation. Versatile applications require different in essence surface properties. In this paper, bare Nitinol surfaces were analyzed. Together dense and porous Nitinol offer the whole spectrum of topographies, chemistry, oxide thicknesses, and corrosion resistances. Native highly corrosion resistant oxides differ in structure from amorphous to crystalline and exhibit either conducting or semiconducting properties important for blood contacting applications. Various topographies can be designed both on nano and micro scales. These native surfaces free of stresses associated with coatings and interfaces are suitable for better cell attachment and migration, and thereby for faster implant healing and patient recovery. The disadvantages of mechanically polished finish surfaces are outlined. Surface chemistry in regard with lasting nickel release from Nitinol into biological solutions is analyzed and illustrated with examples. Various surface treatment and pre-treatment protocols contributing to lasting nickel release are discussed. Surface oxide thicknesses critical for the performances of superelastic implant devices are also evaluated.