Around the world, people are living longer with more active lifestyles thanks to continuing advancements in medical technology. This evolution in technology stems from the development of sophisticated metals and alloys that are finding new uses in internal and external medical applications. From improvements in diagnostic guide wires to new alloys for permanent implants in the body, metals continue to find new uses.

A sampling of pacemaker-related components made of a titanium alloy. (Credit: Ulbrich Stainless Steels & Special Metals)

Titanium has been a consistent performer for a number of years, tantalum is emerging, and copper is re-emerging, while advancements in technology are creating a worldwide demand for a variety of other metals along with new versions of long-established standards. Niobium and nitinol are ideal for use in both internal and external medical applications.

The metals industry has a long history of innovation, development, and processing metals and alloys, in step with medical devices development — from tiny screws for the smallest implants to complex surgical tools operated robotically. A large segment of the medical-metals market is the production of materials for hypodermic needles. Hypodermic-needle manufacturers use various production processes, but most needles are fabricated from flat strip or flat foil product, which are roll formed, welded into a tube, and redrawn to different needle diameters.

Wide Variety of Metals and Uses

Titanium is the acknowledged workhorse metal used in medical applications, particularly in internal applications. It resists corrosion and connects to human bone when properly treated, with fewer negative reactions than other metals. Osseointegration is a unique phenomenon where the body's natural bone and tissue bond to the titanium implant, which firmly anchors the implant in place. Titanium is also a staple in the medical field for uses such as shields for implanted devices that control heart function; products that dispense medicine and perform various neurostimulation; and orthopedic rods, pins, and plates.

An example of catheter wire. (Credit: Ulbrich Stainless Steels & Special Metals)

Pure titanium has low density, high strength, and high corrosion resistance. Titanium is a good candidate material when processing or forming into subassemblies or finished components. It is considered to be physiologically inert. Titanium alloys Ti-6Al-4V and Ti-6Al-4V ELI are especially nonreactive with fluids in the human body, and are commonly used in medical devices due to the low risk of rejection.1

The human body accepts titanium much more readily than other metals, including stainless steel. Some of the most common uses for titanium are in orthopedic surgery (specifically back surgery), and as hip, knee, and shoulder and elbow-joint replacements. Some forms of replacement heart valves use titanium housings or material support rings. Titanium pegs are used to attach prosthetic eyes and ears as a result of recent medical advances. For example, the P-KTM Sleeved Peg System is used to couple the Bio-eye hydroxyapatite (HA) orbital implant (a registered trademark of Integrated Orbital Implants, San Diego, CA) to the artificial eye to create a fully integrated motility prosthesis. Titanium is a standard shield material in such implanted medical devices as pacemaker cases and centrifuges due to its resistance to attack by body fluids, high strength, and low modulus.2

In addition to its use inside the body, titanium is an ideal choice for surgical instruments, such as drills, forceps, retractors, scissors, needle holders, and Lasik eye-surgery equipment. The metal does not interfere with medical tests requiring MRIs or CT scans.

Niobium. There is growing interest in niobium and its alloys for use in medical devices. It is frequently found in devices such as pacemakers because the metal is physiologically inert. Niobium treated with sodium hydroxide forms a porous layer that aids osseointegration, which also makes it an attractive alternative for internal medical applications.

Tantalum is another increasingly popular metal that is highly corrosion resistant and has been used in medical devices as simple as diagnostic marker bands for more than 50 years. Tantalum is especially useful in shaped-wire applications, and its corrosion resistance makes it attractive for implants.

Pure tantalum is ideal for use in permanent bone implants and other uses including vascular clips, flexible stents to prevent arterial collapse, and in repairing bone fractures. It is not ferromagnetic, and, therefore, is MRI compatible.

Nitinol is a nickel-titanium (about 51 percent Ni) shape memory alloy with superelastic properties — a reversible response to an applied stress. Shape memory refers to nitinol's ability to undergo deformation at one temperature, and then recover its original shape upon heating above its transformation temperature. Nitinol's extraordinary ability to accommodate large strains, coupled with its physiological and chemical compatibility with the human body have made it an often sought after material in medical device engineering and design.

Spools of various alloys. (Credit: Ulbrich Stainless Steels & Special Metals)

At higher temperatures, nitinol assumes a cubic crystal structure referred to as austenite (also known as the parent phase). At lower temperatures, it spontaneously transforms to a more complicated monoclinic crystal structure known as martensite. The temperature at which austenite transforms to martensite is generally referred to as the transformation temperature — more specifically, martensite begins to form at the so-called Ms temperature, and the temperature at which it is complete is called the Mf temperature. Those two facets of its structure — shape memory and superelastic properties — allow nitinol to exhibit a reversible response to an applied stress, which itself is caused by a phase transformation between the austenitic and martensitic phases of a crystal.

Crucial to nitinol's properties are two key aspects of this phase transformation. First is that the transformation is reversible, meaning that heating above the transformation temperature will revert the crystal structure to the simpler austenite phase. The second key point is that the transformation in both directions is instantaneous.

Martensite's crystal structure has the unique ability to undergo limited deformation substantially without breaking atomic bonds. This type of deformation is known as twinning, which consists of the rearrangement of atomic planes without causing permanent deformation. It is able to undergo about 6-8 percent strain in this manner.3

When martensite is reverted to austenite by heating, the original austenitic structure is returned, regardless of whether the martensite phase was deformed. Thus, the name shape memory refers to the fact that the shape of the high-temperature austenite phase is remembered, even though the alloy is severely deformed at a lower temperature.

However, heat treating nitinol is delicate — and critical in fine tuning the transformation temperature. Aging time and temperature control the precipitation of various nickel-rich phases, and thus control how much nickel resides on the nickel — and titanium-lattice; by depleting the matrix of nickel, aging increases the transformation temperature. The combination of heat treatment and cold working is essential in controlling the properties of nitinol.

Nitinol devices — such as stents — can be fabricated at one temperature, deformed or folded smaller at another temperature, then inserted into an artery where the body heats the material above its transformation temperature where it returns to its original size. Phase transformation also allows a device to fully recover after it has been bent to a high rate of strain (up to 7 percent).4

This superelastic effect allows use of nitinol devices that have been bent or shaped to allow introduction or use inside the body. Tools such as small grasping and biopsy devices can extend from a tube and expand to a much larger area than devices made from standard alloys.