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WIREs Nanomed Nanobiotechnol
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In vivo injectable gels for tissue repair

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Abstract The desire to reduce healthcare costs while improving outcomes drives minimally invasive methods to replacing traditional surgical procedures. Various treatments that would previously have needed open‐type surgeries can be carried out using endoscopes, catheters, and needles. These advantages have become especially obvious for tissue engineering and regenerative medicine with in vivo gel injectable nanomaterials. In this review, the state of the art in this rapidly developing field is given. This is done by contrasting functional evaluation in vitro with in vivo followed by describing (1) synthetic materials, (2) the body's own polymers, (3) polymers in nature, (4) self‐assembled peptides, and (5) new innovations and combinations. With increased understanding of the relationship between material characteristics and the outcome in vivo more rational design criteria are emerging . WIREs Nanomed Nanobiotechnol 2011 3 589–606 DOI: 10.1002/wnan.91 This article is categorized under: Implantable Materials and Surgical Technologies > Nanotechnology in Tissue Repair and Replacement

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Synthesis of multiarm polyethyleneglycol–peptide conjugates, n‐PEGMMPLys, through a Michael‐type addition reaction. Incorporation of an MMP‐sensitive peptide substrate makes gels susceptible to the action of enzymes. Gel degradation releases covalently bound VEGF from the networks to stimulate angiogenesis (Reprinted with permission from Ref 23. Copyright 2007 American Chemical Society)

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The change of regenerated bone (mm) in clinical trials. A statistically significant increase (*P < 0.01.) at 3, 6, 12, and 24 months compared to preoperative bone height. Bar: standard deviation (Reprinted with permission Ref 115. Copyright 2008 Mary Ann Liebert, Inc.)

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Peptide RADA16‐I. (a) Amino acid sequence and molecular model of RADA16‐I, the dimensions are 5 nm long, 1.3 nm wide, and 0.8 nm thick; (b) AFM images of RADA16‐I nanofiber scaffold, 8 µm × 8 µm; (c) 2 µm × 2 µm; and (d) 0.5 µm × 0.5 µm. Note the different heights of the nanofiber, 1.3 nm, in (d) suggesting a double layer structure; photographs of RADA16‐I hydrogel at various condition; (e) 0.5 wt% (pH 7.5); (f) 0.1 wt% (pH 7.5, Tris–HCl); (g) 0.1 wt% (pH 7.5, phosphate buffered saline) before sonication (h) reassembled RADA16‐I hydrogel after four times of sonication, respectively (Reprinted with permission from Ref 105. Copyright 2009 Elsevier)

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Schematic representations of a bifunctional chain‐extended ureido‐pyrimidinone (UPy)‐modified polymers. (a–c) The first pictures show a schematic drawing of the polymers, the second pictures are enlargements that indicate the dimerization of the UPy‐moieties and the third figure shows the chemical structure of two dimerized UPy‐units. (Reprinted with permission from Ref 97. Copyright 2006 Elsevier)

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Deacetylation of chitin exposes the amine functionality to give chitosan with cationic characteristics. Chitosan triggers coagulation of blood and is being employed as wound dressing. In preclinical trials, it has demonstrated to assist in bone formation

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Polypeptides with glycine in almost every third residue are formed, modified, and assembled inside the cell into a triple helical procollagen (1). The procollagen is secreted outside the cell (2), where the registration peptides are cleaved to give tropocollagen (3). Tropocollagen assembles into fibrils (4), that are stabilized by covalent crosslinking, followed by further structuring into collagen fibers (5). Collagen is catabolized enzymatically for its turnover in the body by collagenases (6) that cleaves peptide linkages

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Fibrin plays an important role in hemostasis, or blood clotting. This protein is formed when monomers of fibrinogen assemble into fibers via enzyme catalysis, forming a three‐dimensional network. As it is highly biocompatible, injectable to form a gel in vivo and commercially available in high purity, it has been employed for almost all conceivable applications in tissue engineering and is clinically applied for wound healing, cartilage, and skin engineering

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The sequential injection of hyaluronic acid–tyramine conjugates and enzymes forms biodegradable hydrogels in vivo by enzyme‐induced oxidative coupling, offering high potential as a promising biomaterial for drug delivery and tissue engineering (Reprinted with permission from Ref 59. Copyright 2005 Royal Society of Chemistry)

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Hyaluronan chemical structure: polymeric repeat of D‐glucuronic acid and N‐acetylglucosamine. The asterisk (*) represents potential sites of chemical conjugation, i.e., hydroxyl and carboxyl groups. Crosslinked hyaluronan has been used as tissue fillers for more than a decade. In vivo crosslinkable gels have been demonstrated clinically useful for bone and cartilage regeneration

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