Tissue engineering
307065
219642049
2008-06-16T05:38:52Z
Oleg Alexandrov
153314
no need for ...
{{Portal|Engineering}}
'''<nowiki></nowiki>Tissue engineering''' is the use of a combination of [[Cell (biology)|cell]]s, [[engineering]] and [[Materials science|materials]] methods, and suitable [[biochemistry|biochemical]] and physio-chemical factors to improve or replace [[biology|biological]] functions. While most definitions of tissue engineering cover a broad range of applications, in practice the term is closely associated with applications that repair or replace portions of or whole tissues (i.e., [[bone]], [[cartilage]], [[blood vessels]], [[Urinary bladder|bladder]], etc.). Often, the tissues involved require certain mechanical and structural properties for proper function. The term has also been applied to efforts to perform specific biochemical functions using [[cell (biology)|cell]]s within an artificially-created support system (e.g. an [[artificial pancreas]], or a [[Bioartificial liver device|bioartificial liver]]). The term '''[[Stem cell treatments|regenerative medicine]]''' is often used synonymously with tissue engineering, although those involved in regenerative medicine place more emphasis on the use of [[stem cell]]s to produce tissues.
In 2003, the [[National Science Foundation|NSF]] published a report entitled "The Emergence of Tissue Engineering as a Research Field" [http://www.nsf.gov/pubs/2004/nsf0450/start.htm], which gives a thorough description of the history of this field.
[[Image:Alcian stain micromass.jpg|right|thumb|300px|Micromass cultures of C3H-10T1/2 cells at varied oxygen tensions stained with [[Alcian blue]].]]
A commonly applied definition of tissue engineering, as stated by [[Robert Langer|Langer]] and Vacanti, is "an [[interdisciplinary]] field that applies the principles of engineering and life sciences toward the development of biological substitutes that restore, maintain, or improve [[Biological tissue|tissue]] function or a whole organ".<ref>Langer, R & Vacanti JP, Tissue engineering. Science 260, 920-6; 1993.</ref> Tissue engineering has also been defined as "understanding the principles of tissue growth, and applying this to produce functional replacement tissue for clinical use."<ref>MacArthur, B. D. & Oreffo, R.O.C. (2005). "Bridging the gap". Nature 433, 19.</ref> A further description goes on to say that an "underlying supposition of tissue engineering is that the employment of natural biology of the system will allow for greater success in developing therapeutic strategies aimed at the replacement, repair, maintenance, and/or enhancement of tissue function."{{Fact|date=March 2007}}
Powerful developments in the multidisciplinary field of tissue engineering have yielded a novel set of tissue replacement parts and implementation strategies. Scientific advances in biomaterials, stem cells, growth and differentiation factors, and biomimetic environments have created unique opportunities to fabricate tissues in the laboratory from combinations of engineered extracellular matrices ("scaffolds"), cells, and biologically active molecules. Among the major challenges now facing tissue engineering is the need for more complex functionality, as well as both functional and biomechanical stability in laboratory-grown tissues destined for transplantation. The continued success of tissue engineering, and the eventual development of true human replacement parts, will grow from the convergence of engineering and basic research advances in tissue, matrix, growth factor, stem cell, and developmental biology, as well as materials science and bioinformatics.
==Cells==
[[Image:Epithelial-cells.jpg|right|thumb|100px|Stained cells in culture]]
Tissue engineering utilizes living cells as engineering materials. Examples include using living [[fibroblast]]s in [[skin]] replacement or repair, [[cartilage]] repaired with living [[chondrocyte]]s, or other types of cells used in other ways.
Cells became available as engineering materials when scientists at Geron Corp. discovered how to extend [[telomere]]s in 1998, producing [[Biological immortality#Cell lines|immortalized cell lines]]. Before this, laboratory cultures of healthy, noncancerous mammalian cells would only divide a fixed number of times, up to the [[Hayflick limit]].
===Extraction===
From fluid tissues such as [[blood]], cells are extracted by bulk methods, usually [[centrifugation]] or [[apheresis]]. From solid tissues, extraction is more difficult. Usually the tissue is minced, and then digested with the [[enzyme]]s [[trypsin]] or [[collagenase]] to remove the [[extracellular matrix]] that holds the cells. After that, the cells are free floating, and extracted using centrifugation or apheresis.<br>
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.
===Types of cells===
Cells are often categorized by their source:
* '''Autologous''' cells are obtained from the same individual to which they will be reimplanted. Autologous cells have the fewest problems with rejection and pathogen transmission, however in some cases might not be available. For example in [[Genetic disorders|genetic disease]] suitable autologous cells are not available. Also very ill or elderly persons, as well as patients suffering from severe burns, may not have sufficient quantities of autologous cells to establish useful cell lines. Moreover since this category of cells needs to be harvested from the patient, there are also some concerns related to the necessity of performing such surgical operations that might lead to donor site infection or chronic pain. Autologous cells also must be cultured from samples before they can be used: this takes time, so autologous solutions may not be very quick. Recently there has been a trend towards the use of [[mesenchymal stem cell]]s from [[bone marrow]] and [[fat]]. These cells can differentiate into a variety of tissue types, including [[bone]], [[cartilage]], [[fat]], and [[nerve]]. A large number of cells can be easily and quickly isolated from fat, thus opening the potential for large numbers of cells to be quickly and easily obtained. Several companies have been founded to capitalize on this technology, the most successful at this time being [http://www.cytoritx.com Cytori Therapeutics].
[[Image:Mouse embryonic stem cells.jpg|right|thumb|200px|[[Mus musculus|Mouse]] [[Mammalian embryogenesis|embryo]]nic stem cells. [http://www.news.wisc.edu/packages/stemcells/labphotos.html '''More lab photos'''] ]]
* '''Allogenic''' cells come from the body of a donor of the same species. While there are some ethical constraints to the use of human cells for ''in vitro'' studies, the employment of dermal [[fibroblast]]s from human foreskin has been demonstrated to be immunologically safe and thus a viable choice for tissue engineering of skin.
* '''Xenogenic''' cells are those isolated from individuals of another species. In particular animal cells have been used quite extensively in experiments aimed at the construction of cardiovascular implants.
* '''Syngeneic''' or '''isogenic''' cells are isolated from genetically identical organisms, such as twins, clones, or highly inbred research animal models.
* '''Primary''' cells are from an organism.
* '''Secondary''' cells are from a cell bank.
* '''Stem cells''' (''see main article: [[stem cell]]'') are undifferentiated cells with the ability to divide in culture and give rise to different forms of specialized cells. According to their source stem cells are divided into "adult" and "embryonic" stem cells, the first class being [[multipotency|multipotent]] and the latter mostly [[pluripotency|pluripotent]]; some cells are [[totipotency|totipotent]], in the earliest stages of the embryo. While there is still a large ethical debate related with the use of embryonic stem cells, it is thought that stem cells may be useful for the repair of diseased or damaged tissues, or may be used to grow new organs.
==Engineering materials==
Cells are often implanted or 'seeded' into an artificial structure capable of supporting [[dimension|three-dimensional]] tissue formation. These structures, typically called [[scaffold]]s, are often critical, both ''[[ex vivo]]'' as well as ''[[in vivo]]'', to recapitulating the ''in vivo'' milieu and allowing cells to influence their own microenvironments. 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
[[Image:Kohlenstoffnanoroehre_Animation.gif|thumb|left|330px|This animation of a rotating [[Carbon nanotube]] shows its 3D structure. Carbon nanotubes are among the numerous candidates for tissue engineering scaffolds since they are [[biocompatible]], resistant to [[biodegrade|biodegradation]] and can be functionalized with [[biomolecule]]s. However, the possibility of toxicity with non-biodegradable nano-materials is not fully understood.]]
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. [[Biodegradation|Biodegradability]] is often an essential factor since scaffolds should preferably 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. Injectability is also important for clinical uses.
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 [[suture]]s. Examples of these materials are [[collagen]] or some linear [[aliphatic]] [[polyester]]s.
New biomaterials have been engineered to have ideal properties and functional customization: injectability, synthetic manufacture, [[biocompatibility]], non-immunogenicity, transparency, nano-scale fibers, low concentration, resorption rates, etc. PuraMatrix, originating from the MIT labs of Zhang, Rich, Grodzinsky and Langer is one of these new biomimetic scaffold families which has now been commercialized and is impacting clinical tissue engineering.
A commonly used synthetic material is [[polylactide|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 [[polyglycolide|polyglycolic acid]] (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.
Scaffolds may also be constructed from natural materials: in particular different derivatives of the [[extracellular matrix]] have been studied to evaluate their ability to support cell growth. Proteic materials, such as collagen or [[fibrin]], and polysaccharidic materials, like [[chitosan]] or [[glycosaminoglycan]]s (GAGs), have all proved suitable in terms of cell compatibility, but some issues with potential immunogenicity still remains. Among GAGs [[hyaluronan|hyaluronic acid]], possibly in combination with cross linking agents (e.g. [[glutaraldehyde]], [[carbodiimide#EDC|water soluble carbodiimide]], etc...), is one of the possible choices as scaffold material. Functionalized groups of scaffolds may be useful in the delivery of small molecules (drugs) to specific tissues.
===Synthesis of tissue engineering scaffolds===
A number of different methods has been described in literature for preparing porous structures to be employed as tissue engineering scaffolds. Each of these techniques presents its own advantages, but none is devoid of drawbacks.
* '''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 macroscaffolds and animal-derived materials.
* '''Textile technologies''': these techniques include all the approaches that have been successfully employed for the preparation of [[Non-woven textiles|non-woven meshes]] of different [[polymer]]s. 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.
* '''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 (e.g. polylactic acid could be dissolved into [[dichloromethane]]), 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. The size of the porogen particles will affect the size of the scaffold pores, while the polymer to porogen ratio is directly correlated to the amount of porosity of the final structure. After the polymer solution has been cast the solvent is allowed to fully evaporate, then the composite structure in the mold is immersed in a bath of a liquid suitable for dissolving the porogen: water in case of sodium chloride, saccharose and gelatin or an aliphatic solvent like [[hexane]] for paraffin. Once the porogen has been fully dissolved a porous structure is obtained. Other than the small thickness range that can be obtained, another drawback of SCPL lies in its use of organic solvents which must be fully removed to avoid any possible damage to the cells seeded on the scaffold.
* '''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 [[carbon dioxide|CO<sub>2</sub>]] for several days. The pressure inside the chamber is gradually restored to atmospheric levels. During this procedure the pores are formed by the carbon dioxide molecules that abandon the polymer, resulting in a sponge like structure. The main problems related to such a technique are caused by the excessive heat used during compression molding (which prohibits the incorporation of any temperature labile material into the polymer matrix) and by the fact that the pores do not form an interconnected structure.
* '''Emulsification/Freeze-drying''': this technique does not require the use of a solid porogen like SCPL. 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 drying|freeze-dried]] to remove the dispersed water and the solvent, thus leaving a solidified, porous polymeric structure. While emulsification and freeze-drying allows a faster preparation if compared to SCPL, since it does not require a time consuming leaching step, it still requires the use of solvents, moreover pore size is relatively small and porosity is often irregular. Freeze-drying by itself is also a commonly employed technique for the fabrication of scaffolds. In particular it is used to prepare collagen sponges: collagen is dissolved into acidic solutions of [[acetic acid]] or [[hydrochloric acid]] that are cast into a mold, frozen with liquid nitrogen then lyophilized.
* '''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. For example [[dioxane]] could be used to dissolve polylactic acid, 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. Liquid-liquid phase separation presents the same drawbacks of emulsification/freeze-drying.
* '''CAD/CAM Technologies''': since most of the above described approaches are limited when it comes to the control of porosity and pore size, [[Computer-aided design|computer assisted design]] and [[Computer-aided manufacturing|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.
==Assembly methods==
One of the continuing, persistent problems with tissue engineering is mass transport limitations. Engineered tissues generally lack an initial blood supply, thus making it difficult for any implanted cells to obtain sufficient oxygen and nutrients to survive, and/or function properly.
Self-assembly may play an important role here, both from the perspective of encapsulating cells and proteins, as well as creating scaffolds on the right physical scale for engineered tissue constructs and cellular ingrowth.
It might be possible to print organs, or possibly entire organisms. A recent innovative method of construction uses an ink-jet 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.<ref>Mironov, V., Boland, T., Trusk, T., Forgacs, G. & Markwald, R.R., Organ printing: computer-aided jet-based 3D tissue engineering. Trends in Biotechnology 21, 157-61; 2003.</ref>
==Tissue culture==
In many cases, creation of functional tissues and biological structures ''in vitro'' requires extensive [[cell culture|culturing]] to promote survival, growth and inducement of functionality. In general, the basic requirements of cells must be maintained in culture, which include [[oxygen]], [[pH]], [[humidity]], [[temperature]], [[nutrient]]s and [[osmotic pressure]] maintenance.
Tissue engineered cultures also present additional problems in maintaining culture conditions. In standard cell culture, [[diffusion]] is often the sole means of nutrient and metabolite transport. However, as a culture becomes larger and more complex, such as the case with engineered organs and whole tissues, other mechanisms must be employed to maintain the culture.
Another issue with tissue culture is introducing the proper factors or stimuli required to induce functionality. In many cases, simple maintenance culture is not sufficient. [[Growth factor]]s, [[hormone]]s, specific metabolites or nutrients, chemical and physical stimuli are sometimes required. For example, certain cells respond to changes in oxygen tension as part of their normal development, such as [[chondrocyte]]s, which must adapt to low oxygen conditions or [[hypoxia]] during skeletal development. Others, such as endothelial cells, respond to [[shear stress]] from fluid flow, which is encountered in [[blood vessels]].
===Bioreactors===
{{main|Bioreactor}}
In many cases, [[bioreactor]]s are employed to maintain specific culture conditions. The devices are diverse, with many purpose-built for specific applications. Bioreactors allow for precise and continuous control of culture conditions and also allow for introduction of different stimuli to tissue cultures.
==Examples of tissue engineering technologies==
*[[Bioartificial liver device]] - several research efforts have produced hepatic assist devices utilizing living [[hepatocyte]]s.
*[[Artificial pancreas]] - research involves using [[islet cell]]s to produce and regulate [[insulin]], particularly in cases of [[diabetes]].
*Artificial [[Urinary bladder|bladder]]s - [[Anthony Atala]] ([[Wake Forest University]]) has successfully implanted artificially grown bladders into seven out of approximately 20 human test subjects as part of a long-term experiment.<ref name="cnngrow">[http://www.cnn.com/2006/HEALTH/conditions/04/03/engineered.organs/index.html Doctors grow organs from patients' own cells], ''[[CNN]]'', April 3, 2006</ref>
*[[Cartilage]] - lab-grown tissue was successfully used to repair knee cartilage.<ref name="knee">[http://www.newscientistspace.com/article.ns?id=dn9483 Lab-grown cartilage fixes damaged knees - health - 05 July 2006 - New Scientist Space<!-- Bot generated title -->]</ref>
* [[Doris Taylor]]'s heart in a jar.
==See also==
*[[Biomedical engineering]] is a close (and often regarded as parental) field.
*[[Biological engineering]] is a broader field that generally encompasses tissue engineering and related fields (e.g. [[biomaterials]]).
*[[Molecular self-assembly]]
===Agencies that support tissue engineering research===
*[[National Institutes of Health]]
*[[National Science Foundation]]
*[[National Research Council of Canada]]
==References==
<div class="references-small">
<references/>
:* Davis, M.E., et al., Injectable Self-Assembling Peptide Nanofibers Create Intramyocardial Microenvironments for Endothelial Cells. Circulation 111:442-450 (2005).
:* Ma, Peter X.: Scaffolds for tissue fabrication - ''Materials Today'', May 2004, 30-40.
:* Holmes, T.C., et al., Extensive neurite outgrowth and active synapse formation on self-assembling peptide scaffolds. PNAS USA 97 :6728 (2000).
:* Semino, C.E., et al., Entrapment of migrating hippocampal neural cells in 3D peptide nanofiber scaffold. Tissue Engineering 10:643 (2004).
:*{{cite paper
| author = Mikos, A. G. & Temenoff J. S.
| title = Formation of highly porous biodegradable scaffolds for tissue engineering
| date = 2000
| publisher = Electronic Journal of Biotechnology 3, 114-119
| url = http://www.scielo.cl/pdf/ejb/v3n2/art03.pdf
}} URL accessed on April 28, 2006.
:*{{cite paper
| author = National Science Foundation (U.S.A.)
| title = The Emergence of Tissue Engineering as a Research Field
| date = 2004
| url=http://www.nsf.gov/pubs/2004/nsf0450/start.htm
}} URL accessed on April 28, 2006.
:*Nerem, R.M., in Principles of Tissue Engineering. Lanza, Langer and J Vacanti (eds), 2000.
</div>
*[http://publishing.royalsociety.org/tissue-engineering Dedicated issue of ''Philosophical Transactions B'' on Tissue Engineering the Heart. Some articles are freely available.]
==External links==
'''Organizations'''
*[http://www.ctecohio.org Clinical Tissue Engineering Center] State of Ohio Initiative for Tissue Engineering (National Center for Regenerative Medicine)
*[http://www.ptei.org Pittsburgh Tissue Engineering Initiative]
*[http://www.tesma.org Tissue Engineering Society of Malaysia Pages]
*[http://www.termis.org Tissue Engineering and Regenerative Medicine International Society]
'''Directories and Repositories'''
*[http://www.hukm.ukm.my/tissueengineering/ Malaysia Tissue Engineering Laboratory Pages]
*[http://tissue.medicalengineer.co.uk Tissue engineering articles and information]
*[http://www.tissue-engineering.net Tissue Engineering Pages]
'''Research Initiatives'''
*[http://icpet-itpce.nrc-cnrc.gc.ca/success_te.html Institute for Chemical Process and Environmental Technology] Tissue engineered (TE) corneas
*[http://organprint.missouri.edu/index.php Organ Printing] Multi-site NSF-funded initiative
*[http://www.team-research.org Team Research] Tissue Engineering and Medical Research Programmes
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[[Category:Chemical engineering]]
[[Category:Cell biology]]
[[Category:Cloning]]
[[Category:Bioengineering]]
[[ar:هندسة النسج]]
[[de:Tissue Engineering]]
[[es:Ingeniería de tejidos]]
[[eo:Hista inĝenierarto]]
[[it:Ingegneria tissutale]]
[[nl:Weefselkweek (dier)]]
[[ja:再生医学]]
[[pt:Engenharia de tecidos]]