In 2003, the NSF published a report titled [http://www.nsf.gov/pubs/2004/nsf0450/start.htm "The Emergence of Tissue Engineering as a Research Field"], which gives a thorough description of the history of this field.

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.

Cells
Tissue engineering solves problems by using living cells as engineering materials. These could be artificial 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. "Primary" cells are from an organism. "Secondary" cells are from a cell bank.

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.

From fluid tissues such as blood, cells are extracted by bulk methods, usually centrifugation or aspheresis.

From solid tissues, extraction is more difficult. Usually the tissue is minced, and then digested with the enzymes trypsin or collagenase to remove the cellular matrix that holds the cells. After that, the cells are free floating, and extracted using centrifugation or aspheresis.

Digestion with trypsin is very dependent on temperature. Higher temperatures digest the matrix faster, but create more damage. Collagenase is less temperature dependent, and damages fewer cells, but takes longer and is a more expensive reagent.

Engineering materials
Cells as found above are generally implanted or 'seeded' into an artificial structure capable of supporting three-dimensional tissue formation. Such devices, usually referred to as scaffolds, serve at least one of the following purposes: Enhance structural properties Deliver biochemical factors Deliver or allow delivery of vital cell nutrients Exert certain mechanical and biological influences to modify the behaviour of the cell phase

To achieve the goal of tissue reconstruction, scaffolds must meet some specific requirements. A high porosity and an adequate pore size are necessary to facilitate cell seeding and diffusion throughout the whole structure of both cells and nutrients. Biodegradability is essential since scaffolds need to be absorbed by the surrounding tissues without the necessity of a surgical removal. The rate at which degradation occurs has to coincide as much as possible with the rate of tissue formation: this means that while cells are fabricating their own natural matrix structure around themselves, the scaffold 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.

Many different materials (natural and synthetic, biodegradable and permanent) have been investigated. Most of these materials have been known in the medical field before the advent of tissue engineering as a research topic, being already employed as bioresorbable sutures. Examples of these materials are collagen or some linear aliphatic polyesters. A commonly used synthetic material is PLA - polylactic acid. This is a polyester which degrades within the human body to form lactic acid, a naturally occurring chemical which is easily removed from the body. Similar materials are polyglycolic acic (PGA) and polycaprolactone (PCL): their degradation mechanism is similar to that of PLA, but they exhibit respectively a faster and a slower rate of degradation compared to PLA.

Assembly methods

One of the continuing, persistent problems with tissue engineering is to grow or construct a system of blood vessels to feed an organ. Each layer of cells must be no more than two cell-thicknesses from a source of oxygen. Use of growth factors has been ineffective.

It might be possible to print organs, or possibly entire organisms. A recent innovative method of construction uses an inkjet mechanism to print precise layers of cells in a matrix of thermoreversable gel. Endothelial cells, the cells that line blood vessels, have been printed in a set of stacked rings. When incubated, these fused into a tube.