Systematic Study On Shape Memory Alloy Endo-Stent

Development of TiNi interventional endo-stents have evolved into a key area of applied researches for TiNi shape memory alloy thanks to the fact that its biocompatibility has come to gain widespread acceptance over the last twenty years. A systematic study of the TiNi stents has been carried out including stent material and surface examination, structural analysis and animal experimentation with the following main results: 1. A thin-wire tension installation is first designed specific to the woven structure of the stents, which is then employed to study the mechanical performance of the stent wires and its deterioration as a result of stress corrosion in a simulated body fluid. It is shown that stress corrosion in the acid environment comparable to human gastric juice leads to a remarkable drop in the stress plateau and elongation of TiNi wires (c = 0.1). This is mainly attributable to breaking of the oxide film formed from wire heat-treatment and the micro-cracks introduced into the wire surface as disclosed by microscopic examinations. 2. A mathematical model of the stent hoop strength is established through linear elastic mechanics. Computation based on this model identifies the main factors determining stent hoop strength as wire diameter, stent size, half-angle, number of ends and stent length for the criss-cross mesh stent and wire diameter, stent size, number of ends and end-angle for the Z-stent. 3. A hydraulic test installation is first designed to characterize stent hoop strength and measurements with stents of different parameters using this installation have established the precision of the mathematical model. 4. Surface study of the alloy shows that its oxidation at 500℃ of memory heat-treatment follows a logarithmic rule of weight growth and exposure at this temperature normally results in an oxide film below 300μm in thickness with primarily titanium oxide. The formation of oxide film decreases the passivation current during electro-chemical corrosion, but it also causes a diminish of the passivation region. TiNi alloy with an oxide film shows higher nickel release rate than mechanically polished samples in an acid environment. 5. Animal experimentation with TiNi stents for treatment of canine abdominal aortic aneurysm has testified to the good biocompatibility of TiNi alloy with intact internal elastic membranes and no inflammatory cells infiltration found around the nitinol wire. TiNi mesh stent is favorable with its integrity, easy delivery and high recovery rate while its function to exclude aortic aneurysms is dominated by the lacerated length of the aneurysmal sac.