According to ‘Langer’ and ‘Vacanti’- “An interdisciplinary field that applies the principlesof engineering and life sciences toward the development ofbiological substitutes that restore, maintain, or improvetissue function or a whole organ.”It is also defined as- “Understanding the principles of tissue growth, andapplying this to produce functional replacement tissue forclinical use."
In vitro meat: Edible artificial animal muscle tissue cultured in vitro. Artificial pancreas: Research involves using islet cells to produce and regulate insulin, particularly in cases of diabetes. Artificial skin constructed from human skin cells embedded in collagen. Artificial bone marrow Artificial bone
Tissue engineering utilizes living cells as engineering materials. Examples: -Fibroblasts in skin replacement or repair. -Cartilage repaired with 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, producing immortalized cell lines.
These cells are extracted from fluid tissues such as Blood, usually by centrifugation or apheresis. From solid tissues, extraction is more difficult. Usually the tissue is minced, and then digested with the enzyme Trypsin or collagenase to remove the extra-cellular matrix that holds the cell. After that, the cells become free floating, and extracted using centrifugation or apheresis. Digestion with ‘Trypsin’ is very dependent on temperature and ‘Collagenase’ is less temperature dependent.
These are obtained from the same individualto which they will be re-implanted. Autologous cells havethe fewest problems with rejection and pathogentransmission. Autologous cells also must be culturedfrom samples before they can be used, this takes time, soautologous solutions may not be very quick.
Allogenic Cells come from the body of a donor of thesame species. While there are some ethical constraints tothe use of human cells for in vitro studies, the employmentof dermal fibroblasts from human foreskin has beendemonstrated to be immunologically safe and thus a viablechoice for tissue engineering of skin.
These are isolated from individuals of another species.In particular animal cells have been used quite extensivelyin experiments aimed at the construction of cardiovascularimplants. (Mouse embryonic stem cells)
These are isolated from genetically identicalorganisms, such as twins, clones, or highly inbred researchanimal models. Primary cells come from an organism and Secondarycells are from cell bank.
Cells are often implanted or seeded into an artificial structure capable of supporting three-dimensional tissue formation. These structures, typically called scaffolds.Scaffolds usually serve at least one of the following purposes:-Allow cell attachment and migration-Deliver and retain cells and biochemical factors.-Enable diffusion of vital cell nutrients and expressed products.-Exert 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.Most of these materials have been known in the medicalfield before the advent of tissue engineering as a researchtopic, being already employed as bio-resorbable sutures. Examples of these materials are collagen andsome polyesters (PLA, PGA, PCL )
A number of different methods has been described in literature for preparing porous structures to be employed as tissue engineering scaffolds.1. Nanofiber Self-Assembly: Molecular self-assembly is one of the few methods to create biomaterials with properties similar in scale and chemistry to that of the natural in vivo extracellular matrix (ECM). Moreover, these hydrogel scaffolds have shown superior in vivo toxicology and biocompatibility compared with traditional macro-scaffolds and animal-derived materials
2. Textile Technologies: These techniques include all the approaches that have been successfully employed for the preparation of non- woven meshes of different polymers. In particular non-woven polyglycolide structures have been tested for tissue engineering applications: such fibrous structures have been found useful to grow different types of cells. The principal drawbacks are related to the difficulties of obtaining high porosity and regular pore size.
3. Solvent Casting & Particulate Leaching (SCPL): This approach allows the preparation of porous structures with regular porosity, but with a limited thickness. First the polymer is dissolved into a suitable organic solvent then the solution is cast into a mold filled with porogen particles. Such porogen can be an inorganic salt like sodium chloride, crystals of saccharose, gelatin spheres or paraffin spheres. Once the porogen has been fully dissolved a porous structure is obtained.
4. Gas Foaming: To overcome the necessity to use organic solvents and solid porogens a technique using gas as a porogen has been developed. First disc shaped structures made of the desired polymer are prepared by means of compression molding using a heated mold. The discs are then placed in a chamber where are exposed to high pressure CO2 for several days. During this procedure the pores are formed by the carbon dioxide molecules that abandon the polymer, resulting in a sponge like structure.
5. Emulsification/Freeze-drying: This technique does not require the use of a solid porogen. First a synthetic polymer is dissolved into a suitable solvent (e.g. polylactic acid in dichloromethane) then water is added to the polymeric solution and the two liquids are mixed in order to obtain an emulsion. Before the two phases can separate, the emulsion is cast into a mold and quickly frozen by means of immersion into liquid nitrogen. The frozen emulsion is subsequently freeze-dried to remove the dispersed water and the solvent, thus leaving a solidified, porous polymeric structure.
6. Thermally Induced Phase Separation (TIPS): Similar to the previous technique, this phase separation procedure requires the use of a solvent with a low melting point that is easy to sublime. Then phase separation is induced through the addition of a small quantity of water: a polymer-rich and a polymer-poor phase are formed. Following cooling below the solvent melting point and some days of vacuum-drying to sublime the solvent a porous scaffold is obtained.
7. Electrospinning: A highly versatile technique that can be used to produce continuous fibers from submicron to nanometer diameters. In a typical electro spinning set-up, a solution is fed through a spinneret and a high voltage is applied to the tip. The buildup of electrostatic repulsion within the charged solution, causes it to eject a thin fibrous stream. A mounted collector plate or rod with opposite or grounded charge draws in the continuous fibers, which arrive to form a highly porous network.
8. CAD/CAM Technologies: Since most of the above described approaches are limited when it comes to the control of porosity and pore size, computer assisted design and manufacturing techniques have been introduced to tissue engineering. First a three-dimensional structure is designed using CAD software, then the scaffold is realized by using ink-jet printing of polymer powders or through Fused Deposition Modeling of a polymer melt.
One of the continuing, persistent problems with tissueengineering is mass transport limitations. Engineeredtissues generally lack an initial blood supply, thus makingit difficult for any implanted cells to obtain sufficientoxygen and nutrients to survive, and/or function properly. Self-assembly may play an important role here, bothfrom the perspective of encapsulating cells and proteins, aswell as creating scaffolds on the right physical scale forengineered tissue constructs and cellular in growth. A recent innovative method of construction uses anink-jet mechanism to print precise layers of cells in amatrix of thermo-reversible gel.
In many cases, creation of functional tissues andbiological structures in vitro requires extensive culturing topromote survival, growth and inducement of functionality. In general, the basic requirements of cells must bemaintained in culture, which include O2, pH, humidity,temperature, nutrients and osmotic pressure maintenance. Tissue engineered cultures also present additionalproblems in maintaining culture conditions. In standardcell culture, diffusion is often the sole means of nutrientand metabolite transport. However, as a culture becomeslarger and more complex, such as the case with engineeredorgans and whole tissues, other mechanisms must beemployed to maintain the culture.
In many cases, bioreactors are employed tomaintain specific culture conditions. The devices arediverse, with many purpose-built for specificapplications.
National Institutes of Health National Science Foundation National Research Council of Canada