Shape memory alloys (SMA) on the basis of nickel-titanium (NiTi; Nitinol ) find use in clinical medicine due to their special mechanical properties: First, the shape memory effect in which the material ªremembersº its initial shape after deformation in the cold, followed by subsequent heating (one-way shape memory effect), and second, a very high elasticity (termed superelasticity) making it exceptionally flexible. These unique properties of NiTi-SMA have inspired different biomedical applications such as cardiovascular devices, orthopaedic implants, surgical instruments or orthodontic devices. [1,2] The shape memory effect is clinically used in staples for foot surgery. [1,3,4] Intravascular stents and orthodontic wires make use of the superelasticity. An adverse reaction of the body due to a release of nickel was not confirmed. [5±7] In general, NiTi has a good resistance towards in vivo surface corrosion and also to nickel ion release. An extensive in vivo study showed that it compared well to a titanium implant in a 12-month sheep implantation experiment. [8] Microstructurally, shape memory behaviour is based on a diffusionless and reversible phase transformation between the low-temperature martensite and the high-temperature austenite phases. The transformation is characterized by the phase transformation temperatures (PTTs) with a start, a peak and a finish temperature (abbreviations M s , M p and M f for the transformation to martensite, A s , A p and A f for the reverse transformation to austenite). Under certain conditions, the transformation is more complex with the intermediate R-phase (multi-step transformation). For technical applications, two kinds of shape memory effects are preferentially used, as mentioned above. First, the ªone-way shape memory effectº: When a specimen is cooled down to its martensitic state, martensite variants are formed by shearing in different directions from the austenite structure. The different variants can be easily directed by mechanical loading. Upon subsequent heating, the material ªremembersº its initial shape by forming the austenite phase again. Second, the ªpseudoelasticº or ªsuperelasticº behaviour is based on the formation of stressinduced martensite (SIM) in stress direction. This occurs in a temperature range where the austenite structure is thermodynamically stable without an external load, but transforms into martensite (SIM) under mechanical load. After unloading the material, the martensite variants transform back into austenite and the material recovers its previous shape. [9,10] Typically, shape memory implants are prepared by casting and formed by rolling, drawing and forging to semifinished parts. Shape setting is accomplished by sawing, milling, drilling or more recently by spark plasma eroding and laser cutting. [11] These processes require many steps which may be avoided by a high-throughput process from powder metallurgy, the so-called metal injection moulding (MIM). In this method, a mixture of metal powder and organic binder is injected at about...
