Tissue engineering can perhaps be best defined as the use of a combination of cells, engineering materials, and suitable biochemical factors to improve or replace biological functions in an effort to effect the advancement of medicine. MacArthur and Oreffo (as cited in "References") defined tissue engineering as "understanding the principles of tissue growth, and applying this to produce functional replacement tissue for clinical use." A typical tissue engineering solution consists of a number of parts as alluded to above. This article will discuss each part in turn, along with its implications.
Tissue engineering solves problems by using living cells as engineering materials. These could be artifical skin that includes living fibroblasts, cartilage repaired with living chondrocytes, or other types of cells used in other ways.
Cells became available as engineering materials when scientists at Geron Corp. discovered how to extend telomeres in 1998. Before this, laboratory cultures of healthy, noncancerous mammalian cells would only divide a fixed number of times, up to the Hayflick limit.
The cells are often categorized by their source. "Autologous" cells come from the same body as that to which they will be reimplanted. "Allogenic" cells come from another body. "Xenogenic" cells come from another species.
Autologous cells have the fewest problems with rejection and pathogen transmission—however in genetic disease, suitable autologous cells are not available. In severe burns, autologous cells will not be available in sufficient quantities. Autologous cells also must be cultured from samples before they can be used. This takes time, so autologous solutions are not very quick.
For more information upon the sources of cells see this article on cell selection for tissue engineering (http://tissue.medicalengineer.co.uk/Sources+of+Cells.php).
Cells as found above are generally implanted, or 'seeded' into a scaffold material which serves at least one of the following purposes:
- Enhances structural properties
- Delivers biochemical factors
- Delivers or allows delivery of vital cell nutrients
- Exerts certain mechanical and biological influences to modify the behaviour of the cell phase
Many different materials (natural and synthetic, biodegradable and permanent) have been investigated.
A commonly used material is PLA - poly lactic acid. This is a material which degrades within the human body to form lactic acid, a naturally occurring chemical which is easily removed from the body. This means that during the time that cells are fabricating their own natural matrix structure around themselves, the PLA is able to provide structural integrity within the body, and eventually it will break down, leaving the neotissue (newly formed tissue which will take over the mechanical load).
- Biomedical engineering is a closely related (and often regarded as parental) field.
- MacArthur, B. D. & Oreffo, R. O. C. (6 January 2005). Bridging the gap. In Nature, 433, 19.