Tissue-engineering

Tissue engineering is a recent discipline that is closely related to cell therapy, but combines knowledge of molecular and cell biology with traditional concepts from biomaterials engineering, bioreactors, biomechanics, and controlled drug release, aiming at the development of new tissues. 

The first tissue engineering product to be approved targeted the treatment of burns and consisted of keratinocytes cultivated in vitro, which form a tissue that is later transplanted to the patient. Beyond this product, others are already being commercialized for a wide range of applications. Examples are Carticel (marketed by Genzyme), which consists of chondrocytes employed in the treatment of cartilaginous defects caused by acute or repetitive traumas Apligraf (Novartis), used in the treatment of venous ulcer and DACS SC (Dendreon), utilized for reconstitution of the immune system after chemotherapy. 

Tissue engineering is a new field that has developed with the progress of science (47). Tissue engineering involves concepts and techniques from various fields of science such as fife science, engineering, medical science and the like. 

Tissue engineering aims to understand the relationship between the structure and function of a body tissue and producing a substitute for a damaged body tissue or organ for transplantation purposes so as to maintain, improve or restore the function of a human body. 

Biomaterials for tissue engineering applications and the very methods of use have been detailed (48). One typical tissue engineering technique comprises the following steps (47)  

After the transplantation is achieved, oxygen and nutrients are provided to the transplanted cells in biodegradable porous pol5mier due to the diffusion of bodily fluids until a blood vessel is new ly formed. 

When the blood vessel is formed, the cells are cultivated and divided in order to form a new tissue and organ. EHuing the formation of new tissue and organ, the pol5nner scaffolds become degraded and eventually disappear (47). 

Tissue engineering, and specifically synthetic tissue biology, is a newly emerging discipline which seeks to engineer tissues and form them into complex biological assemblages (Ben-Nissan, 2(X)4). One approach in this method is to reverse-engineer 

In search of scaffolding materials, we have so far identified candidate biomatrices in nature, with varied chemical homologies and structural analogies to human extracellular matrices and whole tissues. They include nacre marine shell, marine sponge skeletons, echinoderm skeletal elements, and coral skeletons. The utility of selected species of these marine animals has been applied to the regeneration of human bone and cartilage. However, the full utility in these tissues and other tissues has yet to be harnessed and exploited. 

Hydroxyapatite (HAp) [Caj (PO )g(OH2)] is the main inorganic mineral constituent close to human bone chemistry, and is also an outstanding synthetic bone substitute because of its osteoconductive properties. HAp ceramics can be manufactured synthetically from its constituents via a range of production methods. In addition, they have been manufactured by demineralizing bovine or human hard tissues. 

Calcium phosphates are also prepared from natural marine structures such as corals (Roy and Linnehan, 1974 Vago et al., 2002 Papacharalambous and Anastasoff, 1993), mussel (Macha et al., 2013), sea shells (Bahar et al., 2003), sea urchin (Vecchio et al, 2007 Samur et al., 2013), land snail shells (Kel et al., 2012), cuttlefish bone (Rocha et al., 2006), and pearl (Shen et al., 2006) to name just a few. HAp powders have commonly been prepared using a variety of techniques such as wet chemical synthesis, hydrothermal conversion, solid-state reaction, and calcination of bone. 

Based on observed tissue response, synthetic bone-graft substitutes can be classified into inert (e.g., alumina, zirconia), bioactive (e.g., hydroxyapatite, bioactive glass), and resorbable substitutes (e.g., tricalcium phosphate, calcium sulfate). Of these, resorbable bone-graft substitutes are preferred for bone defect filling because they can be replaced by new natural bone after implantation, p-tricalcium phosphate (Ca3(PO )2, p-TCP) is one of the most widely used bone substitute material, due to its faster dissolution characteristics. Preparation of magnesium-substituted tricalcium phosphate ((Ca, Mg)3(PO )2, p-TCMP) has been reported by precipitation or hydrolysis method in solution. These results indicate that the presence of Mg stabilizes the p-TCP structure (LeGeros et al., 2004). The incorporation of Mg also increases the transition temperature from p-TCP to a-TCP and decreases the solubility of p-TCP (Elliott, 1994 Ando, 1958). 

Another PLL-based hydrogel for tissue engineering was recently prepared. A new branched polymer, poloxamine-poly(L-lysine) acrylate, was synthesized and photo-crosslinked in aqueous medium to yield hydrogels 

Motivated by the development of cardiac tissue engineering based on electrically active electrospun nanofibers, Fernandes and co-workers reported on the preparation of electrospun hyperbranched PLL nanofibers containing polyaniline in the form of nanotubes.Both electroactivity and biocompatibility demonstrated by the composite nanofibers opens the possibility of using this material as a scaffold in cardiac tissue engineering. 

Einally, linear amphiphilic PEG-Z)-PLLA-Z)-PLL triblock copolymers were synthesized and blended with PLLA for film formation. Investigation of the film surface revealed an enrichment of PLL blocks on the surface of the PLLA film. Human osteoblast tests performed on different PLLA films showed that the triblock copolymers were much more effective in promoting cell adhesion and proliferation compared to the PEG-6-PLLA diblock-modified and virgin PLLA films. The self-segregation of the PEG-6-PLLA-A-PLL triblock copolymers on the film surface demonstrated a potential application in the preparation of functional scaffolds for tissue engineering. 

Cell transplantation has been proposed as a strategy to achieve organ replacement or tissue repair for a variety of therapeutic needs, including diseases of the liver, pancreas, and other tissues. In cell transplantation, donor tissue is dissociated into individual cells or small groups of cells, which may then be attached to or encapsulated in a support matrix, and transplanted to the patient to restore lost tissue function. A number of approaches to cell transplantation have been explored. 

One approach to achieve this goal is based on the following observations (i) Every tissue undergoes constant remodeling because of attrition and renewal of consituent cells, (ii) Isolated cells will tend toward forming 

Several criteria define the ideal material for a cell transplantation matrix, (i) The material should be biocompatible, in the sense that it does not provoke a connective tissue response which will impair the function of the new tissue (ii) it should be resorbable, to leave a completely natural tissue replacement (this is important, because it could avoid some of the problems that occur in long-lasting polymers such as those used in breast implants) (iii) it should be processable into a variety of shapes and structures which retain their shape once implanted and (iv) the surface should interact with transplanted cells in a way which allows retention of differentiated cell function and which promotes cell growth if such growth is desired. 

From the perspective of biocompatibility, degradability, and process-ability, synthetic polymers have many advantages over complex natural polymers such as collagen. One class of polymers in particular, polyesters in the family of polylactic acid (PLA), polyglycolic acid (PGA), and copolymers of lactic and glycolic acids (PLGAs), most closely meets the listed criteria. These polymers have been approved by the FDA for in vivo 

The observation that cells adhered to the substrates equally as well as to the controls in a cell concentration range suitable for seeding transplant devices prompted a study of cell growth and retention of differentiated function on the polymer films. Culture was carried out for 5 days. Of the polymer substrates tested, only the blend of PLLA and PLGA 85 15 was suitable for maintenance of cells in culture the number of cells attached 

much of the research on biomaterials can be classified into three major fields tissue engineering, development of replacement parts, and creation of blood substitutes. Although there is some overlap among these fields, they serve well as organizing themes for recent developments in the science of biomaterials development. 

Two other definitions provided in a2002reportby the International Technology Research Institute are  

The important theme in all of these definitions is the desire of the drafters to move away from the use of donor or artificial organs or tissues as replacement for damaged body parts and explore mechanisms hy which the body can he encouraged to heal itself. This theme is reflected in two terms sometimes used as synonyms for tissue engineering regenerative medicine and reparative biology. 

The treatment of damaged skin has been a problem for medical workers for centuries. In some cases, skin damage results from a surgical procedure, such as during the amputation of some body part. Far more often, however, skin damage occurs as the result of a burn. 

Burns are classified into one of three categories, depending on the severity and symptoms presented. The most serious type of burn, a third-degree burn, may actually he the least painful because nerves in the skin are destroyed, and the burn victim loses all sense of feeling in the affected area. But third-degree burns are also the most serious, since all three layers of skin, epidermis, dermis, and subcutis, are destroyed. Historically, third-degree burns over a large part of a person s body led almost inevitably to death. 

Organ printing refers to the placement of various cell types into a soft scaffold. This is fabricated by a computer-aided design template (12). The simultaneous printing of cells and biomaterials has been elucidated, which allows the precise placement of cells and proteins within three-dimensional hydrogel structures. Both the scaffold structure, and also the type of tissue that can be grown within the scaffold, can be controlled. 

In particular, three-dimensional tissue constructs with human cells have opened a new avenue for tissue engineering for pharmaceutical and pathophysiological applications (8). These technologies have a great potential to estimate the dynamic pharmacological effects of drug candidates, metastasis processes of cancer cells, and toxicity expression of nanomaterials. 

Quantitative and time-lapse analysis in environments on a microscopic scale for drug diffusion, drug toxicity, and the release of molecules from drug delivery carriers are useful to explore physiological processes. 

Hydrogels are three-dimensional hydrophilic networks with the ability to absorb and retain large amounts of water (15). Hydrogels can be from natural or synthetic polymers and are attractive materials for tissue engineering applications because of their excellent biocompatibility, biodegradabUity, elasticity and compositional similarities to the extracellular matrix. 

Inkjet printing of cells and polymers has been done (16). Microtissue arrays have been arranged by inlqet printing of single cells (17). 

The fibrillar structure of collagen is important for cell attachment, proliferation and differentiation function, and mimicking its structure may lead to engineered tissue which more closely resembles native tissues. Polymer nanofibres are an important class of nanomaterials which have been focused for the past ten years on the field of tissue engineering. Nanostructured materials are extremly small in size, falling 

In general terms, biodegradable polyesters such as PLA, and poly (s-caprolactone) (PCL) are suitable choices for constructing nanofibrous scaffolds, by their good processability and mechanical properties. In fact, electrospun fibres of these polymers could replicate the physical dimensions and morphology of the major components in native ECM. On the other 

Mo et al. prepared P(EEA-CE)-electrospun fibres where cells were well proliferated as monolayer cultures. Khatri et produced PCE-PEEA tubular nanofibres by electrospinning using different PCE-PEEA ratios to obtain good mechanical stability combined with faster biodegradability. They concluded that these materials are viable in tissue engineering applications, due to their biocompatibility, and can be considered as good substrates for in vivo or in vitro research. 

The 3D structure of the electrospun scaffolds should allow the cells to be fully differentiated and should permit them to migrate freely. The influence of 3D PEEA nanofibre scaffolds on in vivo bone formation was studied by Schofer et besides the effect of the incorporation of bone morphogenetic protein 2 (BMP-2) to enhance efficiency. BMP-2 is involved in the development of bone and cartilage tissues and induces osteoblast 

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A further committee. The Committee for Advance Therapies, is to be established under regulation (EC) No. 1394/2007 to deal with issues raised by emerging drug products based on gene therapy, somatic cell therapy and tissue engineering. 

Potential Usage of TMC-Based Copolymers in Tissue Engineering.232... 

Wang and coworkers first reported the use of these monomers as a novel elastomeric material for potential application in soft tissue engineering in 2002. The molar ratio of glycerol to sebacic acid they used was 1 1. The equimolar amounts of the two monomers were synthesized by polycondensation at 120°C for three days. The reaction scheme is shown in Scheme 8.1. To obtain the elastomers, they first synthesized a prepolymer and then poured an anhydrous 1,3-dioxolane solution of the prepolymer into a mold for curing and shaping under a high vacuum. 

PEG/PBT copolymers are also very good matrix materials for the release of growth factors in tissue engineering. Proteins have been delivered from PEG/PBT microspheres with preservation of protein delivery of complete activity. In the case of protein delivery from PLGA and poly(ortho ester) microspheres, the protein activity was significantly reduced. " ... 

Potential Usage oe TMC-Based Copolymers in Tissue Engineering 8.5.4.1 TMC-DLLA Copolymers in Heart Tissue Engineering... 

In all types of PHAs, P4HB is of the most interest because it was used in the degradable scaffold that resulted in the first successful demonstration of a tissue-engineered tri-leaflet heart valve in a sheep animal model. Its copolymers with PHB and polyhydroxyoctanoate (PHO) are also promising in tissue engineering because of their nontoxic degradation products, stability in tissue culmre media, and the potential to tailor the mechanical and degradation properties to match soft tissue. 

The growing interests in finding tissue-engineering solutions to the devastating worldwide problem of cardiovascular disease has prompted the attractions of PHAs in the heart valves and vessel patches tissue engineering. ... 

As a typically flexible material, P4HB has been widely researched in cardiovascular, wound healing, orthopedic, drug delivery, and tissue-engineering fields. ... 

In order to develop a tissue-engineered heart valve, a group at Children s Hospital in Boston evaluated several synthetic absorbable polyesters as potential scaffolding materials for heart valves. Unfoitu-nately, the most synthetic polyesters proved to be too stiff to be function as flexible leaflets inside a tri-leaflet valve. " In the late 1990s, a much more flexible PHAs called poly-3-hydroxyoctanoate-co-3-hydroxyhexanoate (PHO) was used as the scaffold material for the valve leaflet, and then the entire heart valve. ... 

Stock et al. used P4HB scaffolds and tissue engineered the patch with a porosity of 95% and pore sizes in the range of 180-240 p,m by salt-leaching and solvent evaporation. The sheep autologous cells (endothelial, smooth muscle, and fibroblast cells) were seeded on the scaffold before implantation. Results confirmed that the cell-seeded implants induced progressive tissue regeneration with no thrombus formation, stenosis, or dilatation. 

Another example was done by Opitz et al. They utilized P4HB scaffolds to produce viable ovine blood vessels, and then implanted the blood vessels in the systemic circulation of sheep. Enzymatically derived vascular smooth muscle cells (vSMC) were seeded on the scaffolds both under pulsatile flow and static conditions. Mechanical properties of bioreactor-cultured blood vessels which were obtained from tissue engineering approached those of native aorta. 

They also seeded autologous vSMC and ECs obtained from ovine carotid arteries to study autologous tissue-engineering blood vessels in the descending aorta of juvenile sheep. They found that after three months implantation, grafts were fully patent, without dilatation, occlusion, or intimal thickening. A continuous luminal EC layer was formed. However, after six months ... 

Yang J, Webb A, and Ameer G. Novel citric acid-based biodegradable elastomers for tissue engineering. Adv Mater, 2004, 16, 511-516. 

Danger R. Biomaterials in drug dehvery and tissue engineering One laboratory s experience. Acc Chem Res, 2000, 30, 94-101. 

Deschamps AA, Clause MB, Sleijster WJ, Bruijn JD, Grijpma DW, and Feijen J. Design of segmented poly(ether ester) materials and structures for the tissue engineering of bone. J Control Rel, 2002, 78, 175-186. 

Webb A, Yang J, and Ameer G. Biodegradable polyester elastomers for tissue engineering. Exp Opin Biol Ther, 2004, 4(6), 801-812. 

Matsumura G, Miyagawa-Tomita S, Shin Oka T, Ikada Y, and Kurosawa H. First evidence that bone marrow cells contribute to the construction of tissue-engineered vascular autografts in vivo. Circulation, 2003, 108, 1729-1734. 

Matsumura G, Hibino N, Ikada Y, Kurosawa H, and Shin oka T. Successful application of tissue engineered vascular autografts Clinical experience. Biomaterials, 2003, 24, 2303-2308. 

Watanabe M, Shin oka T, Tohyama S, et al. Tissue-engineered vascular autograft Inferior vena cava replacement in a dog model. Tissue Eng, 2001, 7, 429 39. 

Pego AP, Siebum SB, Luyn MJAV, et al. Preparation of degradable porous structures based on 1,3-trimethylene carbonate and D,L-lactide(co)polymers for heart tissue engineering. Tissue Eng, 2003, 9, 981 994. 

Sodian R, Hoerstrup SP, Sperling JS, Daebritz S, Martin DP, Moran AM, Kim BS, Schoen FJ, Vacanti JP, and Mayer JE. Early In. vivo experience with tissue-engineered trileaflet heart valves. Circulation, 2000, 102(111), 22-29. 

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