Tissue engineering aims to create medical devices that, once implanted, will replace or enhance tissue function that has been impaired by disease, injury, or age.
These devices are typically created by seeding a biomaterial scaffold (a sculpted porous sponge) with cells. It is preferable that the cells be harvested from the patient - a device made from cells that are a genetic match are less likely to be rejected by the immune system. If the patient's own cells are not an option, cells from another human donor can be used. The type of cell depends on the tissue - engineered liver tissue would require hepatocytes to restore liver function, while tissue engineered cartilage would make use of chondrocytes.
Cells attach to the scaffolds and reorganize them to form functional tissue by proliferating, synthesizing extracellular matrix, and migrating along the scaffold. This reorganization can begin to occur outside of the body in a bioreactor and then continue after implantation into a patient. The scaffolds are often made of polymers designed to degrade slowly and safely in the body, disappearing as the cells make their renovations. They can be extremely complex chemically: a good scaffold must facilitate cell attachment without provoking an immune response, permit the diffusion of nutrients from the blood, and, at least initially, mimic the mechanical properties of the tissue. Additionally, the scaffold can be constructed with or designed to release factors that can beneficially manipulate the behavior of the cells in the device.
A tissue engineer first has to consider the function of the tissue - must it be strong? Elastic? Should it release certain proteins, like insulin from the pancreas, or filter toxins, like the kidneys? Then a scaffold must be designed that, when combined with cells, will eventually duplicate those functions, and continue to function properly in the body for years to come. Depending on the type of tissue, the design process can involve a variety of disciplines including mechanical engineering, molecular biology, physiology, medicine, polymer chemistry, and nanotechnology.
There are already several types of tissue engineered skin in clinical use, and a great deal of research going into tissue engineered liver, bone, blood vessel, nerve grafts, and pancreas.
Regenerative medicine is a term that is often used synonymously with tissue engineering, though the former often neglects the scaffold and focuses on tissue regeneration via application of stem cells. Since tissue engineers also may make use of stem cells, there is some overlap between the two fields.