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Shape memory alloys (SMAs) provide new insights in biomedical engineering with the unique properties they exhibit, in applications such as cardiovascular stents, guide wires and organ frame retractors.
SMAs are metallic alloys that ‘remember’ the thermomechanical treatments they have been subjected to and have two unique material properties:
- The heat induced recovery of the material to a ‘remembered’ shape (shape memory effect)
- The ability to recover the original shape after large deformations induced by a mechanical load (superelasticity)
The alloys most commonly used in biomedicine are nickel-titanium alloys.
Shape Memory Effect
The material can ‘remember’ two distinct shapes; a high-temperature shape, and a low-temperature shape. This phenomenon results from a crystalline phase change in the material, when the alloy is heated through its transformation temperatures it will change from a martensitic to an austenitic crystalline structure.
Two-way shape memory effect is the reversible shape change of the material with thermal cycling, achieved after specific, repeat thermomechanical treatments known as training procedures.
Superelasticity is the recovery of an SMA from abnormally large strains. Similar to the shape memory effect, a phase transformation occurs between the martensite and austenite phases. This time, it is due to a mechanical stress, when an SMA is loaded in the austenite phase the alloy will transform to the martensitic crystalline structure. As it has formed above the normal austenitic transformation temperature, the martensite returns immediately to an austenite phase, removing the deformation and creating a rubber-like elasticity.
What is a Biomaterial?
A biomaterial is defined as a nonviable material used in a medical device, intended to interact with biological systems. The biocompatibility of a material is the ability of a material to perform with an appropriate host response, examples of appropriate host responses include resistance to blood clotting and resistance to bacterial colonization. The corrosion resistance of an implant material influences its functional performance and durability, and it is a primary factor governing biocompatibility.
In most areas of minimally invasive surgery, the instruments are miniaturized to reduced unnecessary trauma to the patient. The increasing use of diagnostic MRI and MRI guidance techniques requires specific materials with sufficient visualization. The following requirements are defined for medical devices and instruments:
- Excellent mechanical properties
- Excellent biocompatibility
- Excellent biomechanical compatibility (elastic pseudo-biological behavior)
- Excellent kink and break resistance
- MR compatibility
Nitinol as a Biomaterial
Nitinol is a shape memory alloy, a metal alloy of nickel and titanium with two elements present in approximately equal atomic percentages. When properly treated through electropolishing Nitinol exhibits excellent biocompatibility compared to that of Titanium and other alloys used for bone implants.
There is a usually a dualism between mechanical characteristics, biocompatibility, and radiological applicability. Generally, the higher the mechanical stability of a material the lower the biocompatibility and radiopacity. Ceramics such as AL203 Sapphire or calcium hydroxylapatite yield excellent tissue integration but low radiopacity and elasticity. Conversely, stainless steel has a lower biocompatibility but more favorable elastic modulus and ductility.
Titanium is harder to machine but is more resistant to corrosion in a biological environment, and it is considered as the best biocompatible metal. Nitinol is an exception to this as it has an elasticity ten times higher than stainless steel along with biocompatibility close to titanium. It has low toxicity and high corrosion resistance as well as its unique mechanical properties.
Applications of Nitinol
Organ Frame Retractor
One established medical use of Nitinol exploits its deformability to create a large frame that collapses to allow insertion through small access ports. The frame retractor shown below creates a roughly spherical shape 100 mm in diameter which can be used in the abdomen to separate and support organs. It is withdrawn into a tube of 5mm diameter for insertion and extraction through the abdomen wall in a small opening, something which could not be achieved with conventional materials.
A coronary stent is a mesh tube that is used to treat narrow or weak arteries, a stent is placed in an artery to restore blood flow through narrow or blocked arteries by supporting the artery. Self-expanding stents are manufactured with a diameter larger than the target vessel, and a transformation temperature set at around 30 °C.
The stent is plastically deformed in the low-temperature face and stored in a sheath to avoid premature enlargement. When at body temperature inside the patient it will expand when the sheath is removed. The stent will then resist forces on it with an opposing radial force.
Drawbacks of Nitinol and Conclusions
Currently, the main limitations of nitinol are due to the relative infancy of the material, resulting in a lack of reliable information. Robertson et al carried out one of the first fracture-mechanics-based approaches to fatigue in Nitinol, focused on thin walled (~400 μm thick) tubing with an austenite finish temperature of Af = 25-30 °C, identical to self-expanding Nitinol Stents.
Fatigue crack growth behavior measure over a large range of growth rates and positive load ratios revealed a fatigue threshold significantly higher than previously thought. Conversely, the fracture toughness of the tested Nitinol was significantly lower than previously reported. This shows there may be a lack of reliable information on Nitinol and unexpected failures in the future if the failure modes are not understood properly.
The list of medical applications shown for Nitinol is not extensive, there are many orthopedic and soft tissue reconstructive surgeries that could take advantage of the shape memory effect and superelasticity. The novel attributes of shape memory alloys make Nitinol superior to traditional alloys in a wide variety of medical implants, devices, and implements.
Shape memory polymer technology is still in its relative infancy, but in areas such as cardiovascular applications, the lower molecular weight could make them more suitable than metal alloys such as nitinol.
References and Further Reading
A. Pelton, A. M., 2000. Superelastic shape-memory technology of Nitinol in medicine. Min lnvas Ther & Allied Technol, 6(9), pp. 59-60.
Buddy D. Ratner, A. S. H. F. J. S. J. E. L., 2013. Biomaterials Science: An Introduction to Materials in Medicine. Oxford: Society For Biomaterials.
Bundy, K. J., 1994. Corrosion and other electrochemical aspects of biomaterials. Critical Reviews in Biomedical Engineering, 22(2-3), pp. 139 - 251 .
D Stoeckel, A. P. T. D., 2003. Self‐Expanding Nitinol Stents ‐ Material and Design Considerations, Fremont, California: NDC.
Horizon AAA Repair Brochure
Lexcellent, C., 2013. Shape-Memory Alloys Handbook, New Jersey: John Wiley & Sons.
Lorenza Petrini, F. M., 2010. Biomedical Applications of Shape Memory Alloys. Volume 2011.
M H Aras, O. M. C. B. M. K. E. O. a. A. H., 2010. Comparison of the sensitivity for detecting foreign bodies among conventional plain radiography, computed tomography and ultrasonography. Dentomaxillofac Radiol, 39(2), pp. 72-78.
Mantovani, D., 2000. Shape Memory Alloys: Properties and Biomedical Applications. Smart Materials Overview, 52(10), pp. 36-44.
How Does Nitinol Work? All About Nitinol Shape Memory and Superelasticity.
R. Lahoz, J. P., 2004. Training and two-way shape memory in NiTi alloys: influence on thermal parameters. Journal of Alloys and Compounds, 381(1 - 2), pp. 130-136.
Zeus SX Self-Expanding Nitinol Stent System
Scott W. Robertson, R. O. R., 2007. n vitro fatigue–crack growth and fracture toughness behavior of thin-walled superelastic Nitinol tube for endovascular stents: A basis for defining the effect of crack-like defects. Biomaterials, February, 28(4), pp. 700-709.
Stedman's, 7th Revised edition (2011). Medical Dictionary for the Health Professions and Nursing. Philadelphia: Lippincott Williams and Wilkins.
Stoeckel, D., 2000. Nitinol medical devices and implants. Minimally Invasive Therapy & Allied Technologies, 9(2), pp. 81-88.
T. G. Frank, W. X. &. A. C., 2000. Instruments based on shape-memory alloy properties for minimal access surgery, interventional radiology and flexible endoscopy. Minimally Invasive Therapy & Allied Technologies, 9(2), pp. 89-98.
What Is a Stent?
Wayman, K. O. a. C. M., 1998. Shape Memory Materials. Cambridge: Cambridge University Press.