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Smart materials have been one of the fastest growing materials needed for medical device manufacturing. Smart materials, according to the McGraw-Hill Dictionary of Scientific & Technical Terms, are defined as 'Materials that can significantly change their mechanical properties (such as shape, stiffness, and viscosity), or their thermal, optical, or electromagnetic properties, in a predictable or controllable manner in response to their environment'. It is this property of changing according to its material that makes smart materials very valuable in manufacturing today. Perhaps one of the most useful smart materials comes in the form of memory shape alloys, specifically nitinol. Memory shape alloys have many applications in medical devices used today. They are highly prized for their exceptional superelasticity, their shape memory, their good resistance to fatigue and wear, and their relatively good biocompatibility. This makes them the perfect candidate for many in-vivo medical devices.
The shape-memory effect was first observed in copper-zinc and copper-tin alloys by Greninger and Mooradian in 1938, but it was only in the early 1960s that Buehler and his colleagues discovered and patented nitinol, a nickel-titanium alloy created in the Naval Ordnance Laboratory (NOL). This lab was formerly located in White Oak, Maryland and was the site of considerable work that has had useful impact upon world technology. The White Oak site of NOL has now been taken over by the Food and Drug Administration but has still left its legacy in the name nitinol (nickel + titanium + NOL- the initials of the Naval Ordinance Laboratory) (Gautam, et al. 2008). Their smart metal alloy, however, is 55% nickel by weight and may thus have allergic, toxic, or carcinogenic effects. For short-term use, in-vitro and clinical data strongly support nitinol as a safe biomaterial which is at least as good as stainless steel or titanium alloys also available to designers.
Medical Applications of Shape Memory Alloys
Muscles are the power of the body, used to turn energy into movement and motion. Shape memory alloys can be used to in their solid-state phase to make devices from muscle wires.
Applications of shape memory alloys in the medical field are numerous. Their flexibility at one temperature and one way shape memory effect when heated to their transformation temperature make these alloys key materials for various medical methods. The inability of shape memory materials to combine to other metals requires some adaptation to be developed. A common material for this is nickel-titanium. Nickel-titanium has an excellent torque transfer characteristic which is just one of the many reasons this material is used for fabricating medical equipment (Yoshida, et al. 2010). A few notable applications are catheters, medical guide wires, bone plates and stents. Bone plates comprised of shape memory alloys, assist in repairing broken bones by making use of the body's natural temperature to contract and maintain pressure for proper healing. (Georgia Inst. Of Tech, 2007)
Catheters are used in a number of procedures such as therapeutics, diagnostics, and ablative procedures. Used in the medical field for administration of fluids, drainage, and provide a method to insert surgical instruments, catheters are tubes that can be placed in a body cavity, vessel, or duct. In the case of blood vessels, the catheter must move around the bends and angles to reach the desired destination. Stiff materials would not be flexible enough for this procedure and may cause a rupture in the vessel. Due to heat restrictions and risk of damage, only specific shape memory alloys can be used for many of these delicate processes. A solution for this problem is provided by the R-phase transformation, which is a specific type of martensite transformation that occurs in certain nickel-rich Ni-Ti alloys (Langelaar, et al. 2010). Travelling through the vessels is a difficult task, so a steering mechanism is implemented into a catheter to maneuver throughout the body.
Currently catheters are equipped with integrated micro-actuators that allow controlled bending, which yields enhanced maneuverability compared to conventional catheters. Actuators consist of guide wires that bend when energy runs through them such as an electric current that heat the shape memory material. The simplistic designs of the actuator allows for high strains and stresses needed for a process. There are few actuating mechanisms which produce more useful work per unit volume than nitinol (Williams, et al. 1999). Guiding wires also known as pull wires or shaping wires, are located along the tube to allow for motion in many directions.
Figure 1: Actuation energy density diagram.
Above: This demonstrates that shape memory alloys are more effective in actuators than many of the current materials on the market. Guide wires provide flexibility, shape memory, and pseudoelasticity. When a greater stiffness is required, the thickness of the wire may be increased to meet performance standards. Shape memory alloys allow for the catheter to return to its original geometry when the tension in the wire is removed. One adaptation formed due to the lack of metallurgical joining is a stainless steel sleeve, known as a crimp sleeve, to hold the wires to the catheter (Stoeckel, 2010). The sleeve brings up the problem of increasing the diameter of the catheter. To prevent breakage in a material, more flexibility and ductility is ideal. In medical applications, nitinol has higher ductility allowing more plastic deformation without fracturing due to the temperature of the human body.
Figure 2: Stress-strain curve of common shape memory alloy, nickel-titanium
At body temperature (310K), nitinol will have a high percentage of strain at low stress meaning more ductility.
One of the largest medical uses for shape memory alloys is in stents. A stent is a tube that is inserted into an artery to hold it open. Stents are needed when the walls of the artery are not strong enough to remain open and need support to ensure that blood is able to flow. The stent is put in place during a procedure called an angioplasty (Stent Facts, 2010). In order to get the stent into the artery, it needs to be collapsed and inserted into a catheter. Shape memory alloys allow doctors to collapse the stent to a much smaller diameter, and have it return to its original shape after leaving the catheter inside the artery. The original use of shape memory alloys in stents was in the form of a simple coil. The coil was tightly wound in the catheter and then expanded once it was inserted into the artery and warmed. The expanded size of the coil is chosen to be slightly larger than the inner diameter of the target vessel, which means the coil will not be able to fully expand inside the artery. The shape memory alloy, in its warmed state, will continue to attempt to expand, which will put a continuous outward pressure on the walls of the artery. This will ensure that the artery remains open. In more recent times, simple coil stents are used more for non-vascular applications such as preventing bladder obstruction. The simple coil stents that are still in use today are used in vascular cases where easy retrieval is required. The shape memory alloy allows the stent to hold its form in the body, but still be easy to deform back to a straight wire for removal (Sutou, et al. 2006).
More modern shape memory alloy stents are made in forms other than a coil. The shape memory alloy can be formed into a braided or knitted coil. The downside of this is that the points where the wires cross form thicker walls, which are undesirable in a stent. Although the braided and knitted shape memory alloy stents were a step up in functionality from the simple coils, the thicker walls made them undesirable for many cases. The next level of shape memory alloy stents occurred once scientists determined how to make the alloys in flat sheets rather than just wire. Laser cutting a pattern into a flat sheet of the alloy, then rolling and welding it at various points creates a stent with no overlapping wires at the walls. Sheet style stents are thin, but also structurally supportive when heated to body temperature. This gives them more flexibility than the simple coil models and is a better use of the shape memory alloys characteristics (Sutou, et al. 2006).
An older style coil stent in both its compressed and expanded forms
Examples of sheet style stents: Top- Jostent SelfX (made by Jomed), Bottom- Dynalink (made by Guidant)
Examples of braided style stents: Left- ZA Stent (made by Cook), Right- Symphony Stent (made by Boston Scientific)
General hazards of inhaling Nitinol include irritation, coughing, and shortness of breath. If ingested gastrointestinal disorders are possible. Skin contact and eye contact include irritation with possible redness and pain. None of these side effects are chronic. (SMDS 2008)
Complications of Nickel-Titanium in Medical Applications
Of the wide range of alloys that contain the properties of shape memory alloys, nickel-titanium and copper-based alloys hold the most value commercially. Nickel-titanium, also known as nitinol, is an equi-atomic mixture of the two metals. Concerns have risen over this alloy for the fear of nickel being released into the body (Williams, et al. 1999). It is important in medical equipment for the materials to be biocompatible, or the ability of the material to perform with a necessary response. In most medical procedures no response is typically desired. To determine if nitinol meets these criteria, the properties of titanium, nickel, and the combination of the two can be looked at.
Titanium is a metal with a high resistance to corrosion. It is not particularly reactive and therefore is effective for medical uses where the device needs to be in the human body for an extended period of time (Lagoudas, 2010). It contains no characteristics of toxicity. Titanium is also a very strong material, however it is rarer and more difficult to manufacture than other materials. This makes titanium expensive compared to other alternatives.
Nickel is a metal which is extremely reactive. Nickel is toxic to the human body and may cause massive inflammation and interaction with proteins. These properties raise questions on whether nitinol alloy is safe for medical uses. The benefits of using nickel in medical devices is that nickel increases flexibility and lowers the expense when alloyed with more expensive materials such as titanium (Langelaar, et. al. 2010 ).
The properties when nickel and titanium are alloyed together usually take on those of titanium. During the manufacturing process an outer layer of titanium oxide forms. Although some nickel will still exist on the exterior, the toxicity is greatly reduced. When choosing a material for medical instruments, a risk/benefit analysis controls which alloy will be used. Nitinol is chosen because it holds great benefits and is very safe to use. Extensive testing of this material has been done and is still occurring to limit complications (Yoshida, et al. 2010).
Figure 3: Break-down potential as a function of oxide thickness on Nitinol; insert: results of potentiostatic corrosion tests of Nitinol samples
Safety During Medical Application
When considering the use of shape memory alloys (such as nitinol), in medical applications, it becomes necessary to evaluate the safety of the materials for use in the human body. Biocompatibility and corrosion are two factors that come into play when considering placement into humans. Properly treated nitinol implants are corrosion resistant and compatible in humans. These implants form a surface oxide layer that protects the base material from most corrosion. There are some concerns of the nickel content dissolving from the Nitinol and causing adverse affects. However, other alloys containing high levels of nickel, such as MP35N or 300 series stainless steel, have been used in orthodontics, orthopedics, and cardiovascular applications, all the while displaying good biocompatibility. (Stoeckel, et al. 2003)
Studies have shown that in vitro dissolution of nitinol dental archwires in saliva released an average of 13.05 mg/day nickel. This number is significantly lower than the average dietary intake of 200-300 mg/day. There was no increase in the nickel blood level throughout the study. A comparative in vitro cell culture study was performed to measure nickel release from nitinol and 316L stainless steel in fibroblast and osteoblast cell culture media. The nickel content was higher in the nitinol group for the first day, but rapidly decreased over time to achieve similar levels as the stainless steel. The nickel content never reached toxic levels in the nitinol and did not interfere with the cell growth. It was found that samples prepared by mechanical polishing released higher amounts of Ni-ions than those prepared by electropolishing. In order to evaluate the effect of polishing on nickel release, mechanically polished and electropolished samples of nitinol, MP35N, and 316L stainless steel were immersed in solution for a period of over 1000 hours. Samples prepared by electropolishing released smaller amounts of Ni-ions than those with mechanical polishing. The electropolishing process removes excess nickel from the surface and forms an enriched layer of titanium. (Stoechel, et al. 2003)
A study on blood compatibility was conducted on nitinol and stainless steel stents using an ex vivo, AV-shunt porcine model. It was concluded that nitinol is significantly less thrombogenic than stainless steel, meaning that when used in the human body it has a much lower chance of causing blood clots. It is thought that the titanium-oxide rich surface layer on the nitinol prevents denaturation of fibrinogen and minimizes platelet-rich thrombus formation within the stent after implantation. (Thierry, et al. 2000)
Figure 4: Ni ion release from Nitinol, MP35N, and stainless steel
Table 1: Ratio of Ni to Ti in the surface of mechanically, electropolished or passivated samples of Nitinol, MP35N and stainless steel
Comparison of Shape Memory Alloy Nickel-Titanium to Stainless Steel
The ability of shape memory alloys to return to their original position after large strains are induced is similar to that of rubber. However, unlike rubber, shape memory alloys are strong and noncorrosive much like stainless steel. Both nickel-titanium and stainless steel have long fatigue life. Many stainless steels contain nickel to maintain an austenitic structure. Higher nickel content guarantees superior resistance to corrosive cracking. Stainless steel has a relatively lower cost compared to nitinol mainly due to larger production numbers. Only about two hundred tons were produced in 1998 compared to a few hundred thousand tons of stainless steel (Lagoudas, 2010). Alloying a metal raises the production expenditure but changes the tensile and shear strength of the initial metals. The properties of shape memory alloys are better than those of stainless steel and therefore are the chosen material for certain applications.
Figure 6: Phases of Alloys
Figure 5: Stress-strain-temperature diagram exhibiting the shape memory effect of nitinol
Above: Shape memory alloys have two phases, each with a different crystal structure and
properties. One is the high temperature phase, called austenite, and the other is the low temperature phase, martensite. Each martensitic crystal formed can have a different orientation direction, called a variant. The assembly of martensitic variants can exist in two forms. Twinned martensite, which is formed by a combination of self-accommodated martensitic variants and detwinned or reoriented martensite in which a specific variant is dominant (Lagoudas, 2010).
Costs of Shape Memory Alloys such as Nickel-Titanium
Alloys such as nitinol have poor formability in the manufacturing process which increases the production costs of such materials. The complex behavior of the material makes the development of shape memory alloys adaptive structures a challenging task. In this case, it is generally accepted that systematic, model-based design approaches and design optimization techniques can be of great assistance (Langelaar et al. 2010). However, as more applications for these materials are needed, the price will decrease.
Currently, shape memory alloys are commercially available from a limited number of producers. When more production of these alloys begins, production costs will reduce. World production is small in contrast to other metal commodities. Competition drives prices lower in a market. Newer technology in manufacturing will also make the production more effective. Prices for shape memory alloys were over one dollar per gram of material in the 1990s. Today, the costs are roughly ninety percent lower.
Whatever the cost may be, shape memory alloys such as nickel-titanium are one of the only materials capable of such miniscule instrumentation with the desired properties. Shape memory alloys are effective for their cost due to reliability and multiple functions (Stoeckel, 2010). Many applications of shape memory alloys only require a small amount of material. With prices around that of similar steels, shape memory alloys are gaining more attention in a variety of applications.
Figure 7: Cost ratio-output volumetric work chart for both one-shot and one hundred-shot actuators.
Above: The best material lies towards the upper left corner as it corresponds to low material cost
for the same output work (Lagoudas, 2010). It indicates that CuZnAl is the best, while Ni-Ti is the least. However, it may be more advantageous to use Ni-Ti because of reduced voltage requirements due to much higher resistivity, which results in cheaper equipment in cyclic applications. Copper based alloys are less stable and more brittle than Ni-Ti. Although less expensive, copper based alloys have found little approval for applications.
Current studies at the University of OULU have been conducted in order to demonstrate that bone modeling can be controlled by using a functional implant such as a NiTi nail which can be used to bend a normal shaft of the long bone. The method could also be applied inversely, such as straightening a deformed bone. Fractures and especially frequent fractures lead to angular deformity and bowing of long bones. Operative treatment has usually consisted of cortical osteotomies with cast, internal fixation, or external fixation (Kujala, 2003). However, these are relatively large operations with much postoperative pain and a risk for complications. Implantation of a bending rod would be a much smaller operation for the patient with reduced postoperative recovery. It might even be possible to insert the nails using minimally invasive techniques which would require a minute incision. Thus, the functional nail presented might provide an easier, quicker, cheaper, and less painful way to correct such bone deformities in the future.
In addition, Prototype piping in nuclear reactors has been wound with pre-stretched Ni-Ti wire, which leaves very high compressive stresses in the pipe. Tennis racket strings have been tested in China and the USA with both countries claiming performance superior to existing string materials (Deurig, 1995). Furthermore, a variety of damping applications are being examined including such motivated projects as railroad wheel tires and damping mechanisms for suspension bridges.
Moreover, the maximum Ms temperature achieved in Ni-Ti binary alloys is 100 degrees Celsius and for several years scientists have searched extensively for ways to increase this. Ms temperature or Martensite start temperature is the temperature at which the transformation from austenite to martensite begins on cooling. Until just two years ago the only alloys showing hope were extremely expensive alloys such as Ti- Pd-Ni and Ti-Pt-Ni. Recently, however two new alloys are showing a great deal of promise, Ni-Ti-Hf and Ni-Ti-Zr31. These alloys prove that transformation temperatures of over 300 degrees C are possible (Deurig, 1995). However, it is too early to know what the cost of the alloys will be and if other properties will be as good as the original alloys. Luckily, these first indications seem positive. One advantage if such an Ms temperature is possible would include the use of nitinol in circuit breakers and in automotive applications.
Shape memory alloys are quickly becoming a common material used in medical applications today. The adverse uses of alloys, such as nitinol, allow for improved stents, catheters, bone plates, medical procedures, and more. These advanced materials are helping to shape medical technology for the future. Through their durability and unusual prowess for changing shape they have become the future of medical material.