Biomaterials Tutorial

Metals

Maxi Boeckl
University of Washington Engineered Biomaterials

Elements in the periodic table that have good electrical and thermal conductivity are classified as metals.  In addition, these elements have high strength and stiffness and good ductility.  Some of the metals are magnetic, such as iron, cobalt and nickel.   Some become superconductors at extremely low temperatures or high pressure, such as copper, barium, ruthenium and tungsten complexes.  

Common metals used in implants—either as pure elements or as alloys (mixture of metals)—include titanium, platinum, iridium, gold, iron (as stainless steel alloy), vanadium, cobalt, aluminum (ceramic) and zirconium (ceramic).  The advantage of alloys over metals is that the properties of the material can be fine-tuned to its application needs. Changing the elements and their concentrations present in the alloy leads to stronger, more flexible materials.

Titanium

Titanium is a mineral deposit found in mines as ilmenite (FeTiO3) and rutile (TiO2). Pure metallic titanium is extracted from rutile.  Four grades of commercially pure titanium exist which vary greatly in strength, mostly due to minor differences in the amount of oxygen they contain (0.18 - 0.4 wt % oxygen).

Due to its good properties, titanium is used in total joint replacement, dental implants, plates and screws. It is a relatively corrosion-resistant implant material. Nevertheless, metallic debris is often seen in tissue surrounding titanium implants.

Gold

Gold is a relatively inert metal, and one of the first materials used as an implantable material (as a dental tooth implant) since the 18th century.  Besides being used as an implant, gold is often coated onto substrates (glass, mica, or silicon wafers) in order to form self-assembled monolayers (SAMs). SAM technology is often used in nanotechnology, microchips, and micro contact printing. 

Platinum

Platinum by itself is a soft metal, which dissolutes and forms complexes in saline, a sign of corrosion.  Thus, it is often used in the form of an alloy (mixture of metals). Platinum is often used in the manufacture of stimulatory electrodes, such as cardiac pacemakers.  More commonly, iridium and iridium oxide are used for electrodes.

References:

1. Biomaterials science: An introduction to materials in medicine, 2nd edition. Ratner BD, Hoffman AS, Schoen FJ, Lemons JE. San Francisco: Elsevier Academic Press, 2004.
2. Beeves CJ, Robinson JL. Some observations on the influence of oxygen content on the fatigue behavior of α-titanium. J. Less-Common Metals 1969; 17: 345-352.
3. Conrad H, Doner M, de Meester B. Critical review: Deformation and fracture. In: Jaffee RI, Burte HM, editors. Titanium, science and technology, vol. 2. Warrendale, PA: TMS, 1973.