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Biomedical shape memory alloys

Production and testing of biomedical shape memory alloys, development of suitable training methods and devices to induce the 2-way shape memory effect, and investigation of its long-term stability and testing of alloy biocompatibility.

Biomedical shape memory alloys (B-SMA) are commonly used in bioengineering applications as they combine important qualities such as resistance to corrosion, biocompatibility, fatigue resistance, MR compatibility, and kink resistance, with two unique thermo-mechanical behaviours: the shape memory effect and the pseudoelastic effect. They allow B-SMA devices to undergo large mechanically induced deformations and then to recover the original shape by thermal loading or simply by mechanical unloading. Shape memory behaviour represents a material's ability to recover from large deformations (of approximately 10%) with a thermally induced phase transformation. The shape memory material deforms at room temperature, causing a permanent deformation similar to the deformation of copper when wound around the finger. Unlike copper, however, when heated above a critical temperature (typically around 100 degrees Celsius), shape memory materials immediately spring back to their original memorised shape. The binary alloy of nickel (Ni) and titanium (Ti) is one of the most widely used B-SMA materials. (B-SMA) amongst others can sense thermal stimuli and convert them directly into mechanical work. Ti-Ni shape memory alloys have been used successfully as biomedical materials owing to their superior shape memory property and superelasticity. However, the Ni-hypersensitivity and toxicity of Ni has been pointed out in Ti-Ni alloys. Although the Ti-Ni alloys have been successfully applied for many medical products, the developments of Ni-free shape memory alloys is also strongly required in order to pursue absolute safety. The beta-type Ti alloys reveal a martensitic transformation from beta (disordered BCC) to two metastable martensite structures, either hexagonal martensite (alpha') or orthorhombic martensite (alpha'') by quenching. The martensite structure changes from alpha' to alpha'' above a critical alloying content. Transformation strains from the b to alpha'' is accommodated primarily by internal twinning. The reversion of alpha'' to b is related to the shape memory effect in beta-type Ti alloys. The martensitic transformation start (Ms) temperature can be controlled by the addition of alloying elements. However, for the biomedical application, alloy designs with combinations of non-toxic elements are required. Our group will systematically investigate firstly new Ti-base shape memory alloys, in order to develop Ni-free biomedical shape memory alloys and secondly Cu-based B-SMA which are significantly cheaper. Furthermore, Cu-based B-SMA are the option, if high transformation temperatures and reasonable resistance against the functional fatigue are required (also the martensite-finish-temperature can be adjusted far above 100 degrees Celsius through selection of the Al-content). The most important disadvantage of polycrystalline Cu-based B-SMA is, due to high elastic anisotropy, the small size of the shape memory effect (SME): up to 4% at one-way SME and only up to 1.5% at two-way SME. To manufacture small components, thin bands (ribbons) or wire are necessary. By employing melt spinning, a thin ribbon is produced directly from the melt and wire from several steps of drawing. Furthermore, rapid solidification allows for high supersaturations, and through this it is possible to adjust the temperatures of transformation in a wider range. While there is not so much knowledge available on Ni-Ti alloys, there is however quite a lot of knowledge available on bulk Cu-based B-SMA alloys. The objective of this project is to inform about recent achievements and future prospects of all aspects of biomedical shape memory alloys, ranging from fundamentals to applications. B-shape memory alloys have attracted attention due to their fascinating properties for applications as well as their fundamental aspects of deformation and transformation behaviour. The Ti–Ni and Ti–Ni–X alloys have been successfully used for many applications in the fields of household appliances, machinery, transportation technology, construction technology, medical applications and so on. Recently, intensive research into developing new shape memory alloys, such as thin film shape memory alloys, ferromagnetic shape memory alloys, high temperature shape memory alloys and Ni-free biomedical shape memory alloys, has been carried out in order to break through the limitation of existing shape memory alloys. The B-SMA alloy is also considered to be one of key materials for constructing smart structures and systems. Since the medical field has tried to integrate B-SMA into new implantology, our team will focus on integrating advanced industrial materials for use in human implantology also. A critical concern will be the use of B-SMA to improve physical and mechanical characteristics of implants while assuring that the devices improve the quality of life as unobtrusively as possible. Keywords: shape memory alloys, bio-medical, 2-way SME (Shape Memory Effect).
Acronym: 
BIO-SMA
Project ID: 
3 971
Start date: 
01-01-2007
Project Duration: 
36months
Project costs: 
1 170 000.00€
Technological Area: 
Medical technology
Market Area: 
Bioinformatics

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