Tissue engineering: the biophysical background

Tissue engineering is the construction, repair or replacement of damaged or missing tissue in humans and other animals. This engineering may take plac e within the animal body or as tissue constructs to be made in a bioreactor for later grafting into the animal. The minimal set of materials for this are the appropriate types of cell. Usually, however, non-living substrata a re used as well. These substrata may be nothing more than materials that bu lk up any voids in the damaged tissue and provide the mechanical strength t hat has been lost when the tissue is damaged or removed. They may serve a s imilar pair of functions in the bioreactor. They can do much more in terms of pattern formation. The orientations and morphology of the cells, the arr angement of intercellular material as it is laid down and the relationships between different cell types in the repairing or construct tissue are all of importance, for these should resemble the correct normal tissue as close ly as possible. Most of these requirements are ones involving pattern forma tion. This review discusses the various ways in which tissue pattern can be engin eered chiefly from a biophysical standpoint. Unpatterned cells are effectiv ely not tissue. This engineering includes the use of topography on the subs trata, chemical patterning of adhesive and other cues for the cells, mechan ical force application to cause cell orientation and appropriate synthetic responses and electrical fields. The review also discusses the methods used to impart the appropriate cues to and through the materials which are ofte n biodegradable polymers. The article gives particular attention to regions of research and practice where the involvement of the physicist or biophys icist is of importance.