Shape memory biomaterials

Biomaterials with shape memory and superelasticity

Did you know that 420,000 hip replacements and 210,000 knee replacements are implanted in Europe every year? Is there a material that can instantly heal fractures and promote the formation of bone tissue? Will there ever be intelligent prostheses that can adapt their response to the needs of the locomotor system at any given moment?

Bioengineering: a multidisciplinary science

The increase in life expectancy has brought with it a growing demand for healthcare derivatives and the emergence and development of new medical technologies. This has led to the emergence of a discipline with an enormous future, Bioengineering, which seeks to combine medical developments with the development of new materials and engineering designs.

Biomechanics, for example, studies the mechanical behaviour of the human body, identifying the behaviour of tissues and organs, as well as the interaction of the body with different types of prostheses, implants and other artificial elements.

Biomaterials

Biomaterials are one of the most important advances in medicine today: they improve the quality of life of patients and reduce the healing and convalescence time for diseases and trauma.

They can be a metal, a ceramic or a polymer. The only condition it must meet is that it must be biocompatible, i.e. it must not be rejected by the human immune system. Moreover, its mechanical properties must be compatible with the organ in which it is incorporated. In addition, it must be easy to sterilise, handle and insert.

In addition to their scientific importance, these alloys have numerous applications, ranging from space technology to medicine and all kinds of industrial applications.

Current research is focusing on metal-metal or ceramic-ceramic combinations with low coefficients of friction, low-nickel stainless steels, shape-memory materials, polyethylenes irradiated with low-energy electrons, biodegradable materials with controllable mechanical strength, and so on.

A multidisciplinary example: the development of prostheses for the locomotor system

The average ageing of the population in Western countries, coupled with the increasing rate of traffic accidents, has led to an increase in the number of fracture operations, many of which require prostheses or fixations. Finding the best prosthesis designs is a challenge, as is studying their long-term influence on the surrounding tissues. This is done by means of programmes that simulate the behaviour of the prosthesis-organ assembly, making it possible to reproduce its evolution over time and even consider possible future situations: the appearance of bone loss, loosening between implant and bone, lack of fracture stabilisation…

In this sense, the “great couple” of joint prostheses is metal-polyethylene, where the metal is a type of stainless steel or cobalt-chromium-molybdenum alloys. Polyethylene has the advantage of high mechanical performance: fatigue resistance, low friction, toughness and self-lubrication.

Shape memory materials

A shape-memory material is able to remember a previously established shape, even after severe deformation, when subjected to a rise in temperature. This makes them very useful when designing biomaterials that, for example, need to occupy a certain space inside the body and cannot be inserted inside the body at that size.

The first shape memory material was discovered at the U.S. Naval Ordnance Laboratory in 1963: the equiatomic nickel-titanium alloy (NiTi). Subsequent research has identified this property in other alloys, such as copper alloys with aluminium, nickel or zinc, or nickel-aluminium alloys. These materials have a large number of industrial applications.

Physics of shape memory materials

Shape-memory materials have two different phases. At low temperature, the phase is called martensite and at high temperature, austenite. In the martensite phase the material is more malleable and easier to work than in the austenite phase.

By heating the material to a temperature where all of it is austenite, we can give it the shape we want it to resemble later. It is then cooled until all the material has been transformed into martensite. This transformation occurs without any change in the shape of the material, but as this phase is very malleable, the shape can easily be changed. This new shape is maintained as long as the material is not reheated.

Upon reheating and transformation back to austenite, the material returns to its original shape. This behaviour is known as one-way shape memory materials and is the simplest.

Obviously, the temperatures at which these transformations occur vary depending on the type of alloy and this behaviour can be modified by appropriate thermal and mechanical treatments of the material.

How does this transformation take place?

From the point of view of the atomic structure, the transformation from austenite to martensite takes place in two stages:

1.- A displacement of the atoms appears which, in principle, could lead one to think that the external shape of the piece is changing. This does not occur because the martensite adapts to the initial shape of the austenite by a self-matching mechanism, known as twinning.

This process involves the establishment within the martensite of regions separated by boundaries, called twin boundaries, where the arrangement of atoms in one region is the mirror image of the adjacent region. If the martensite is then deformed by the application of a force or a load, the boundaries are displaced, with some regions growing at the expense of others: this is called de-matting. This process results in a change in the external shape of the part.

2.- Subsequent heating of this martensite deformed by total or partial demagnetisation only has the possibility of changing it to austenite, which implies, and this is the fundamental thing, also returning to the original external shape. In short, heating the martensite with any degree of demagnetisation and with the deformation that has occurred, the same austenitic phase will always be obtained, with the original external shape.

Superelasticity

Another method of obtaining martensite without cooling the austenite is to subject the material in the austenite phase to tensile stress. Above a certain critical value, what is called stress-induced martensite (SIM) is generated. This process takes place in a certain temperature range.

If the temperature is too high, the material is irreversibly deformed plastically before it transforms into martensite.

If the temperature is very low, the martensite is thermally stable and therefore when we stop applying force, the material does not change shape because it remains martensite.

In the appropriate intermediate temperature range the phase of the material is austenite, with the final shape. If stress is applied, the transformation to martensite takes place. Given the properties of this phase, the stress produces a large deformation, which is fully recoverable when the stress is released because the transformation back to austenite takes place as martensite is not thermodynamically stable at this temperature. At the end the material has the same shape as at the beginning. This behaviour is known as superelasticity.

Application of these materials in medicine

Of the shape memory materials, only the alloy based on nickel-titanium in equiatomic proportions is biocompatible, competing with other biomaterials such as stainless steels and titanium-chromium alloys. If we add to this its shape memory properties, superelasticity and high mechanical vibration damping capacity, its use in traumatology, stomatology, radiology and orthopaedics is not surprising.

Traumatology

Staples are manufactured, mainly with one-way shape memory. They are inserted deformed (martensite) and, on acquiring the temperature of the human body, regain their previous shape (austenite), which forces the fractured bones to be repositioned, holding them together during healing. Before implantation, the staples must remain at low temperatures. Experimentally, correctors for spinal deformities (e.g. scoliosis) based on superelasticity and intervertebral disc replacement elements with cushioning capacity have been developed.

Stomatology

The applications focus on orthodontic archwires. In this case, the material is previously deformed so that, once placed in the patient’s mouth, it begins to exert a constant force until the necessary correction has been achieved (superelasticity).

Liquid crystals in nature

Interventional radiology

This was one of the first medical applications. A filter was devised for the vena cava to retain possible blood clots. This element is introduced deformed into the bloodstream and in the vein it recovers its open umbrella shape.

Stents

These stents are used to “open” the bile, oesophageal, tracheal or vascular ducts of the human body. They are inserted deformed with reduced diameters and, by elasticity, they recover their size and open the obstruction. Superelasticity behaviour has also been used in some precision systems such as catheterisation guidewires and vascular stenting devices.

What do we do at INMA?

We develop applications of NiTi shape memory alloy in medicine.

In collaboration with Interventional Radiology Departments and Minimally Invasive Surgery Centres, we are developing stents for colonic and oesophageal applications.

Among the different shape memory alloys, we use the nickel-titanium alloy for its characteristics both from the point of view of the austenite-martensite transformation and the thermo-mechanical properties derived from it: one-way shape memory, two-way shape memory, superelasticity and anelasticity. The combination of these properties together with its biocompatibility has allowed its application in the fields of industry and medicine.

We are also investigating the modelling of the material by means of constitutive equations that account for thermo-mechanical phenomena, as a first step in the design of devices. The applications we are working on are colon stents, joint splints and bone anchors.

We are studying how to improve polymeric materials for joint prostheses

Polyethylene ultra-high molecular weight polyethylene (PEUAPM) is the weak material of the metal-polymer or ceramic-polymer pair in current hip and knee prostheses. Our research is aimed at developing methods to reduce the wear of the material and to extend the in vivo life of the prosthesis. In our laboratories we are trying to study the ageing mechanisms that are occurring in this material. This research is carried out in collaboration with research groups from the Miguel Servet Hospital.

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More information

Instituto de Nanociencia y Materiales de Aragón