Rare earth metals see new medical uses

While the importance of rare earth metals is widely acknowledged in manufacturing sophisticated devices for use in space, defense, clean, green and consumer technologies, the increasingly innovative and effective roles REE are playing in the development of new medicines and medical technologies get little public attention.

New developments in medical technology are expected to increase the use of surgical lasers, magnetic resonance imaging, and positron emission tomography scintillation detectors. Medical applications using REE appear to be on the rise as researchers find more and more ways to capitalize on the unique properties of these minerals, whether used singly, in combination with one or more, or with other metals.

A variety of minerals are used to create drugs and medical therapies and devices that assist health care workers in treating disease and maintaining human health, according to researchers at the National Institutes of Health. They range from common ones like quartz used to make medical instruments to exotic minerals such as hubnerite, a source of the metal tungsten, which is used in radiation shielding.

Tantalum, for example, is a very porous refractory metal that encourages bone to grow and attach to it, making tantalum useful for implants when used in the presence of bone. The material is also used for diagnostic marker bands. However, tantalum is more often used as a composite due to the element's cost and rarity.

Rare earth elements consist of yttrium and the 15 lanthanide elements (lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium). Scandium is also found in most rare earth element deposits and is sometimes classified among them.

Though called "rare" because they are seldom found in substantial quantities in nature, all of the REE, except promethium, are actually more abundant than silver or mercury.

REE in medicines

Some 55% of REE are used in manufacturing catalysts such as batteries, while 45% are key components of compounds, ceramics and alloys, many of which are used in making lifesaving medicines and high-tech medical devices.

Lanthanum, for example, is known to affect human metabolism, lowering cholesterol levels, blood pressure, appetite, and risk of blood coagulation.

Lanthanum carbonate is used to lower phosphate levels in patients with end-stage kidney disease. High levels of phosphate can make it hard for the body to absorb calcium, which can cause serious medical problems.

Yttrium is used to help treat cancer in at least two ways, by directly targeting cancer cells or poisoning the blood flowing to the cancer tissue. A form of yttrium, known as yttrium-90, can be used as part of radiation therapy by attaching the element to antibodies that can specifically bind to a tumor's cells or by injecting millions of Y-90 radioactive beads into arteries carrying blood to a tumor. The advantage of either form of yttrium therapy is that it is highly targeted to the tumor's location. This ends up hurting tumor cells while minimizing any toxic effects to healthy parts of the body.

Samarium is another REE that is used in the palliative treatment of bone pain associated with osteoblastic metastatic bone lesions. It also is administered intravenously.

Medical implants

Ceramic technology is a growing sector in medical manufacturing, as demonstrated with the development of electronic devices as implants. Some companies and manufacturers are developing neurostimulators-used to pulse nerves and treat medical symptoms-that are dependent on ceramic composition.

Zirconia ceramic is as an extremely high-strength material and is generally stronger than alumina. Because of such resilience, it has been applied to devices that typically generate sufficient wear, such as femoral balls in hip replacements. While this material is known for its strength, it is regularly combined with the metal yttrium to prevent the breakdown of the product.

The most common metals and alloys used in medical implants include stainless steel, cobalt-chrome alloy, titanium, and nickel-titanium alloy (nitinol).

Other metals, such as gold, platinum, silver, iridium, and tungsten, are also common in many medical devices.

X-rays, lasers and more

In medical device and equipment technology, REE are especially prized for properties that make them excellent contrast agents and the most powerful magnets.

Terbium, for example, has atomic number 65 and is used especially in alloys for electronic equipment and as a green phosphor in electronic displays.

Not isolated in its form until the advent of ion exchange techniques, terbium is used to dope calcium fluoride, calcium tungstate and strontium molybdate, which are materials used in solid-state devices and as a crystal stabilizer of fuel cells, which operate at elevated temperatures.

It has been used to improve the safety of medical x-rays by allowing the same quality image to be produced with a much shorter exposure time.

By doping tiny amounts of chromium, neodymium, erbium, thulium, ytterbium and a few other metals into transparent crystals, ceramics or glasses, manufacturers are producing active medium for solid-state lasers.

Doping is the intentional introduction of impurities into a pure semiconductor for the purpose of modulating its electrical, optical and structural properties.

Erbium, with atomic number 68, is another REE that is used especially in lasers, infrared light filters, and optical fibers. It is used in manufacturing lasers for use in aesthetic and dental implant surgery.

Erbium lasers also have excellent bactericidal properties because its energy ruptures the cell membranes of bacteria when absorbed into intracellular water.

Erbium lasers are considered versatile because they can be used in good hard-tissue applications, although their soft-tissue applications are limited because of poor hemostasis, or ability to prevent or stop bleeding.

Another REE, holmium, is a relatively soft and malleable silvery-white metal. It is too reactive to be found uncombined in nature, but when isolated, it is relatively stable in dry air at room temperature. However, it reacts with water and corrodes readily and also burns in air when heated.

Holmium is found in the minerals monazite and gadolinite and is usually commercially extracted from monazite using ion-exchange techniques.

Holmium has the highest magnetic strength of any element, and therefore is used to create the strongest artificially generated magnetic fields, when placed within high-strength magnets as a magnetic pole piece or magnetic flux concentrator.

Applications include holmium-doped yttrium iron garnet (YIG) and yttrium lithium fluoride (YLF) in solid-state lasers, and in optical isolators and in microwave equipment.

Holmium lasers emit at 2.1 micrometers, and are used in medical, dental, and fiber-optical applications.

Thulium, the rarest of REE, is a soft, silver-like metal that is used in portable X-rays machines and in treating health problems with a thulium laser. Because the metal is not economical to mine, it is very expensive and thus impractical for some uses.

Promethium, the last of the REE to be discovered, was identified in 1945. It has the longest half-life, 17.7 years, of all elements in the lanthanide series.

While promethium is viewed as an ideal metal for portable X-ray devices, no successful application has been properly developed yet. Instead, the element's beta radiation has been used primarily in industry to measure the thickness of materials, and the isotope promethium 147 has been used in nuclear batteries.

Magnets and MRI

The strong magnetic field properties of magnets have led to their use in many fields, including medicine and dentistry. Early research indicates that magnets made of neodymium appear to hold great promise for both diagnostic and therapeutic purposes.

Neodymium magnets are incorporated in some medical devices, including magnetic resonance imaging devices used to diagnose and treat chronic pain syndrome, arthritis, wound healing, insomnia, headache, and several other diseases due to their ability to generate a static magnetic field.

"An increase in their (neodymium magnets) usage has been observed over the past decade. These magnets are thought to have a curing effect and are therefore sometimes called "magic magnets," scientists reported in the journal Northern Clinics of Istanbul.

Another REE, gadolinium, has no large-scale applications but is seeing a variety of specialized medical uses.

Because the mineral has a high neutron cross-section, it is used to target tumors in neutron therapy.

Gadolinium also possesses unusual metallurgic properties, with as little as 1% of gadolinium improving the workability and resistance of other metals, such as iron, chromium, and related alloys to high temperatures and oxidation. Its paramagnetic characteristics make the metal useful for magnetic resonance imaging.

Solutions of organic gadolinium complexes and gadolinium compounds are used as an intravenous MRI contrast agent to enhance images in medical magnetic resonance imaging and magnetic resonance angiography procedures. Once injected, gadolinium-based contrast agents accumulate in abnormal tissues of the brain and body, which provides a greater image contrast between normal and abnormal tissues, facilitating the location of abnormal cell growths and tumors.