Jozefina Katić

Modification of a Nitinol Surface by Phosphonate Self-Assembled Monolayers

Nitinol, as a shape memory alloy, is attractive material for medical implants and devices. Contrary to titanium, corrosion by releasing Ni2+ ions occurs during a long-term contact of Nitinol with (physiological) solutions containing Cl- ions. In order to develop chemically/electrochemically stable surfaces and interfaces, Nitinol was modified by self-assembled monolayers of dodecylphosphonate (-OH and –CH3 terminated) films, which were characterized by XPS, PM-IRRAS and contact angle measurements. Strongly bounded well-ordered films of high homogeneity and resistance were synthesized. An innovative method that allows in situ study of influence of thermal annealing, following SAM formation, on their protecting properties in simulated (physiological) solutions is presented. Changes of structural sensitive impedance parameters were correlated with the changes in the interfacial layer. Effective thermal annealing greatly enhances the quality of the self-assembled alkyl-phosphonate films, which behave as non-ideal dielectrics, i.e., the solid/liquid interfaces formed represent the blocking contact preventing charge-transfer reactions.

Kinetics of passivity of NiTi in an acidic solution and the spectroscopic characterization of passive films

Anodic polarization of nitinol in acetic acid under galvanostatic conditions produces oxide films composed mainly of TiO2. An exponential current-field relation is valid during ionic conduction through the growing oxide, in which the field coefficient is related to the jump distance. Transport processes in anodic films have been discussed in terms of a cooperative mechanism developed for amorphous oxide films on valve metals, in which both metal and oxygen ions were involved in ionic conduction. For more crystalline oxide structure of passive films on nitinol, formed during a prolonged potentiostatic conditions, the charge transfer takes place only through the oxygen vacancies as mobile species via a high-field-assisted mechanism. Based on the results of the Mott–Schottky analysis, these films behave as n-type semiconductors indicating that oxygen vacancies formed during the film formation and growth act as electron donors. The barrier/protecting and electronic/semiconducting properties of the passive films as well as their chemical composition were studied using electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy.

Synthesis and characterization of calcium phosphate coatings on Nitinol

In recent years, coating of metal orthopedic implants with bioactive layers to promote fixation with bones has become increasingly common. Calcium phosphate coatings on the Nitinol surface were formed using two low-temperature methods: sol–gel and electrochemically assisted deposition. The coatings formed were characterized using: X-ray diffraction analysis, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy. Cyclic voltammetry studies were carried out in the deposition solution to determine parameters for electrodeposition and to understand electrochemistry of deposition. The barrier properties and corrosion resistance of coatings were tested in the physiological Hanks’ solution using electrochemical impedance spectroscopy. The sol–gel deposited coating consisted of two phases, hydroxyapatite and tricalcium phosphate (TCP). Apatite coatings containing TCP offered the opportunity to create a grafting material with high bioactivity and bioresorbility. The electrodeposited coating consisted of Ca-deficient HAp which resembles to biological HAp.

Surface Modification of Nitinol by Biocompatible Passive Films

Nitinol, near-equiatomic nickel-titanium alloy exhibits unique properties, shape memory effect and superelasticity, and is therefore of huge importance in biochemical engineering for medical applications. Metal implants, because of their irreplaceable mechanical properties, are built in different part of human body (as bone replacements, stents). These materials corrode in contact with aggressive body fluids. Knowledge and prediction of electrochemical processes is necessary for designing and optimizing biocompatible coatings on Nitinol, which is the main goal of this investigation. Electrochemical methods will be used not only for in vitro investigation of Nitinol reactivity, but also for the functionalization of its surface. Nitinol surface will be modified with biocompatible coatings (anodically formed passive films) and surface characterization of these films will be done by highly-sophisticated surface-analytical methods.