Applications of Shape Memory Alloys in the medical field

Shape Memory Alloys (SMA) are a success story in the medical applications market with an enormous growth in usage. Huge advances from the surgical point of view mean a great opportunity for new commercial applications.

The alloys with shape memory have the unique property of “shape effect” (simple and double effect of the shape memory), super elasticity and biocompatibility. The usefulness of a material from this point of view can be established on the base of three selection criteria chemical, biological and mechanical), corrosion resistance, are very ductile and can be easily deformed, great capability of absorption of vibration (due to the easiness of the displacement of the internal interfaces of these alloys), the capacity to convert the heat energy in mechanical energy, and so on.

Smart materials have been given a lot of attention mainly for their innovative use in practical applications. One example of such materials is also the family of shape memory alloys (SMA) which are arguably the first well known and used smart material.

The body is a complicated electrochemical system that constitutes an aggressive corrosion environment for implants which are surrounded by bodily fluids of an aerated solution containing 0.9% NaCl, with minor amounts of other salts and organic compounds, serum ions, proteins and cells which all may modify the local corrosion effect.

High acidity of certain bodily fluids is especially hostile for metallic implants. It is important to understand the direct effects of an individual component of the alloy since it can dissolve in the body due to corrosion and it may cause local and systemic toxicity, carcinogenic effects and immune response. The cytotoxicity of elementary nickel and titanium has been widely researched, especially in the case of nickel, which is a toxic agent and allergen. Nickel is known to have toxic effects on soft tissue structures at high concentrations and also appears to be harmful to bone structures, but substantially less than cobalt or vanadium, which are also routinely used in implant alloys. Experiments with toxic metal salts in cell cultures have shown decreasing toxicity in the following order: Co > V > Ni > Cr > Ti > Fe.

On the other hand, titanium is recognized to be one of the most biocompatible materials due to the ability to form a stable titanium oxide layer on its surface. In an optimal situation, it is capable of excellent osteointegration with the bone and it is able to form a calcium phosphate-rich layer on its surface, Figure 1, very similar to hidroxyapatite which also prevents corrosion. Another advantageous property is that in case of damaging the protective layer the titanium oxides and calcium phosphate layer regenerate.

Figure 1: Formation of hydroxyapatite layer on titanium oxide film

The properties and biocompatibility of nitinol have their own characteristics which are different from those of nickel or titanium alone. In vitro NiTi biocompatibility studies on the effects of cellular tolerance and its cytotoxicity have been performed on various cell culture models. Human monocytes and microvascular endothelial cells were exposed to pure nickel, pure titanium, stainless steel and nitinol. Nitinol has been shown to release higher concentrations of Ni²⁺ ions in human fibroblast and osteoblast cultures, which did not affect cell growth.

Nitinol, a group of nearly equiatomic, nickel titanium (NiTi) alloys is widely recognized and accepted for medical use. Nitinol’s shape memory, super elasticity, and high wear resistance have allowed novel instrumentation and implants to be designed in surgery fields as versatile as orthopedics to vascular interventions.

Nitinol’s excellent malleability and ductility allow it to be manufactured in the form of wires, ribbons, tubes, sheets, or bars, thereby providing a wide spectrum of opportunities for medical applications. For example, the biomaterial has been shown to be suitable not only for minimally invasive procedures, which are often performed in outpatient clinics, but also on patients whose health and age status will not allow for major “open” surgeries. Additionally, Nitinol is known to be appropriate for treatment of younger populations, especially children with congenital defects.

Move than thirty years ago, when the first study on Nitinol implantation was completed, Castleman (1976) had written that while the development road for Nitinol as a biomaterial may be long, its future looked bright. We are operating now in that future. From its discovery in 1962 at the U.S. Naval Ordnance Laboratory, the Nitinol road has not always been smooth or paved. Being American in origin, Nitinol quickly established roots in other parts of the world, where researchers from both West and East contributed to the maturation of Nitinol as a biomaterial: new shape memory features were discovered, new multicomponents and porous shape memory alloys were developed, and new operation techniques utilizing Nitinol’s potential were elaborated.

In vitro and in vivo studies in animal and human models unanimously concluded that Nitinol is biocompatible and efficacious as a biomaterial. As mentioned above, inappropriately treated Nitinol will always present a problem with Ni release. In order to avoid troubling issues with Ni release and stent metal failures, the surface and bulk of the alloy need to be scrutinized.

Nitinol chemical heterogeneity needs to be eventually recognized with regards to medical applications, the specifics of particulate size and distribution need to be better understood aiming at better corrosion resistance and fatigue life. An approach: the thicker the surface oxide layer, the better protection against corrosion and Ni release is not applicable to super elastic Nitinol. Surface coatings and modifications with various energy sources do not seem to improve Nitinol biocompatibility. The NiTi matrix alloy itself is not a reservoir for Ni release, but Ni-enriched interfaces formed due to heat treatments (and especially Ni in a nonoxidized, elemental state) are such a source.

The immunostimulatory effect of NiTi, which may be deduced from in vitro and in vivo studies, merits further in-depth investigation. A new aspect of bare Nitinol surfaces, related to the possibility of manipulating Nitinol thrombogenicity, is also of great interest for both the applications requiring nonthrombogenic surfaces, like stents, and for high thrombogenic surfaces required for defect closures and orthopedic, and osseogenic implants.

In order to improve the immediate response to implantation and also longterm implant performance, it is necessary to understand specifics of plasma proteins and platelet interactions with Nitinol surfaces. A deeper insight is needed into Nitinol surfaces:
1) the electronic structure of Nitinol surface oxides, their conductivity and reactivity, nanostructure and defects, surface charge, and oxide stoichiometry;
2) their fracture mechanics, microstructure, compositions;
3) the studies of explants.

Although Nitinol has been in use for more than 40 years, important features continue to be discovered as scientists try to grasp its complexity.

Metal ion release study also revealed very low concentrations of nickel and titanium that were released from nitinol. Researchers therefore concluded that nitinol is not genotoxic. For in vivo biocompatibility studies of nitinol effect, different experiments have been done on animals. Several in vivo nitinol biocompatibility studies which were done in the last decade disclosed no allergic reactions, no traces of alloy constituents in the surrounding tissue and no corrosion of implants. Studies of rat tibiae response to NiTi, compared with Ti-6Al-4V and AISI 316L stainless steel, showed that the number and area of bone contacts was low around NiTi implants, but the thickness of contact was equal to that of other implants. Normal new bone formation was seen in rats after 26 weeks after implantation. Good biocompatibility results of NiTi are attributed to the fact that implants are covered by a titanium oxide layer, where only small traces of nickel are being exposed.

Corrosion resistance of SMA has also been studied in vivo on animals. Plates and stents have been implanted in dogs and sheeps for several months. Corrosion has been examined under microscope and pitting was established as predominant after the implants were removed. Thus surface treatments and coatings were introduced. The improvement of corrosion resistance was considerable, since pitting decreased in some cases from 100 μm to only 10 μm in diameter.

The trends in modern medicine are to use less invasive surgery methods which are performed through small, leak tight portals into the body called trocars. Medical devices made from SMAs use a different physical approach and can pull together, dilate, constrict, push apart and have made difficult or problematic tasks in surgery quite feasible. Therefore unique properties of SMAs are utilized in a wide range of medical applications. Some of the devices used in various medical applications are listed below (Fig 2).

Stents are most rapidly growing cardiovascular SMA cylindrical mesh tubes which are inserted into blood vessels to maintain the inner diameter of a blood vessel. The product has been developed in response to limitations of balloon angioplasty, which resulted in repeated blockages of the vessel in the same area. Ni-Ti alloys have also become the material of choice for superelastic self-expanding (SE) stents which are used for a treatment of the superficial femoral artery disease.

The Simon Inferior Vena Cava (IVC) filter was the first SMA cardiovascular device. It is used for blood vessel interruption for preventing pulmonary embolism via placement in the vena cava. The Simon filter is filtering clots that travel inside bloodstream. The device is made of SMA wire curved similarly to an umbrella which traps the clots which are better dissolved in time by the bloodstream.

The Septal Occlusion System is indicated for use in patients with complex ventricular septal defects (VSD) of significant size to warrant closures that are considered to be at high risk for standard transatrial or transarterial surgical closure based on anatomical conditions and/or based on overall medical condition. The system consists of two primary components; a permanent implant, which is constructed of an SMA wire framework to which polyester fabric is attached, and a coaxial polyurethane catheter designed specifically to facilitate attachment, loading, delivery and deployment to the defect.

The most representative instruments such as guide wires, dilatators and retrieval baskets exploit good kink resistance of SMAs. Open heart stabilizers are instruments similar to a steerable joint endoscopic camera. In order to perform bypass operations on the open heart stabilizers are used to prevent regional heart movements while performing surgery. Another employment of the unique properties of SMAs such as constant force and superelasticity in heart surgery is a tissue spreader used to spread fatty tissue of the heart.

In general, conventional orthopedic implants by far exceed any other SMA implant in weight or volume. They are used as fracture fixation devices, which may or may not be removed and as joint replacement devices. Bone and nitinol have similar stress-strain characteristics, which makes nitinol a perfect material for production of bone fixation plates, nails and other trauma implants.

Figure 2: Examples of nitinol medical devices

Shape memory fixators are one step forward applying a necessary constant force to faster fracture healing. The SMA embracing fixator consists of a body and sawtooth arms. New SMA inter-locking intramedullary nails have many advantages compared to traditional ones. For example, when cooled SMA inter-locking nails are inserted into a cavity, guiding nails are extracted and body heat causes bending of nails into a preset shape applying constant pressure in the axial direction of the fractured bone. The SMA effect is also used in surgical fixators made from wire.

The SMA Patellar Concentrator was designed to treat patellar fractures. The device exerts continuous compression for the fixation of patella fracture.

Dentists are using devices made from SMA for different purposes (Figure3). NiTi based SMA material performs exceptionally at high strains in strain-controlled environments, such as exemplified with dental drills for root canal procedures. The advantage of these drills is that they can be bent to rather large strains and still accommodate the high cyclic rotations. Superelastic SMA wires have found wide use as orthodontic wires as well. Lately a special fixator for mounting bridgework has been developed.

Figure 3: Dental applications of nitinol