shape memory alloys - biotinet workshop belgium... · •(niti-) shape memory alloys are well...
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Shape Memory Alloys
Vormgeheugenlegeringen
Alliages à mémoire de forme
Formgedächtnislegierungen
prof. dr. ir. Jan Van Humbeeck
MTM-KULeuven BioTiNet Leuven March 6, 2012
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Requirements
• A (thermoelastic) martensitic transformation
Shape deformation by reorientation of martensite variants (Martensite phase).
Shape deformation by strain-induced martensite (Beta phase)
Transformation temperatures below 150°C
Small Hysteresis
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Transformation Temperatures
martensite
martensite
beta
beta
As Af
Ms Mf
T
heating
cooling 10 K
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Reorientation
Shape recovery after reorientation
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Alloy systems exhibiting SME
• NiTi alloys and its derivatives
• Cu-based alloys (Cu-Zn-Al, Cu-Al-Ni, Cu-Al-Mn, Cu-Al-Be)
• Fe-based alloys with ε-martensite (Fe-Mn-Si, Fe-Ni-Co-Ti)
• CuZr-based alloys
• β-Ti alloys
• Magnetic Field Driven SMA (Ni-Mn-Ga)
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Transformation temperatures as function on Ni-content. Lines denoted with 400°C and 500°c give the TT after annealing at those temperatures.
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Functional Properties
• One Way Shape Memory Effect
• Two Way Shape Memory Effect
• Actuator
• Constrained Recovery Force
• Superelasticity
• High Damping Capacity
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Biomedical Applications
• Orthodontic devices
• Guidewires
• Non-invasive surgical instruments
• Orthopaedic implants
• Stents
• Filters
• Exo-prosthesis
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Securing the Human Body
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Requirements for Medical Implants (biomaterials)
• The reliability of the mechanical functions and functional properties
• The chemical reliability (the resistance to deterioration of their properties in a biological medium, the resistance to expansion, dissolution, corrosion)
• Biological reliability-biological compatibility, lack of toxicity and carcenogenicity, resistance to the
formation of thrombus and antigens.
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Biocompatibility of NiTi
• NiTi is bio-inert (osteopermissive)
– intermetallic compound
– Ti20 surface layer
• Deviation from stoichiometric composition (with Ni contents exceeding or depletion on ~ 2at% and more) leads to a raise of electrochemical activity and to a lowering of the corrosion resistance
• Alloying with elements of the Pt-group (Ru, Rh, Os, Ir, Pd, Pt) or Mo increases corrosion resistance. TiNi-Mo has a higher passivity
• Alloying with Cu, Fe, Mn, Al results in a slight drop of the corrosion resistance
• Ionic beam treatment, TiN and TiCN, increases the corrosion resistance and suppresses the dissolution of Ni from NiTi alloys
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Corrosion resistance
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Orthodontic Applications
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•Steerable biliary guidewire (Director)
Guidewires: -high flexibility and high kink resistance
Superelastic NiTi endodontic files for root canal surgery
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A Simon filter, shown in longitudinal and transverse views of the deployed state
A 1-mm-diam urological grasper (Bacher, Tuttlingen, Germany) demonstrates kink resistance. The shaft comprises a nitinol wire concentrically placed in a nitinol tube; the distal end is a hinged, stainless-steel grasper, which opens to an approximately 90° included angle.
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Biorthex porous NiTi for bone reconstruction
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Stents
• Self-expanding stents based on the superelastic properties
• Balloon-expandable stents
• Expansion by the shape memory effect
• Drug delivery stents
• Temporary Stents (removable)
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Stents produced by laser-cutting from tubes
Wire-stent
Self-expandable stent (Angiomed)
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“Stents”
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Conclusions
• (NiTi-) Shape memory alloys are well established functional materials.
• The main application sector is medical.
• Since the value of the functional properties is very much dependent on the thermomechanical history, it is difficult to provide correct design data on the material.
• Creative design might open new application possibilities