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WIREs Nanomed Nanobiotechnol
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Injectable foams for regenerative medicine

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The design of injectable biomaterials has attracted considerable attention in recent years. Many injectable biomaterials, such as hydrogels and calcium phosphate cements (CPCs), have nanoscale pores that limit the rate of cellular migration and proliferation. While introduction of macroporosity has been suggested to increase cellular infiltration and tissue healing, many conventional methods for generating macropores often require harsh processing conditions that preclude their use in injectable foams. In recent years, processes such as porogen leaching, gas foaming, and emulsion‐templating have been adapted to generate macroporosity in injectable CPCs, hydrogels, and hydrophobic polymers. While some of the more mature injectable foam technologies have been evaluated in clinical trials, there are challenges remaining to be addressed, such as the biocompatibility and ultimate fate of the sacrificial phase used to generate pores within the foam after it sets in situ. Furthermore, while implantable scaffolds can be washed extensively to remove undesirable impurities, all of the components required to synthesize injectable foams must be injected into the defect. Thus, every compound in the foam must be biocompatible and noncytotoxic at the concentrations utilized. As future research addresses these critical challenges, injectable macroporous foams are anticipated to have an increasingly significant impact on improving patient outcomes for a number of clinical procedures. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Implantable Materials and Surgical Technologies > Nanotechnology in Tissue Repair and Replacement

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Literature progression in the areas of minimally invasive surgery and injectable biomaterials. The plotted values correspond to the number of papers published each year related to these topics. The numbers were obtained by conducting a search in PubMed and including papers published up to 2012.
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Drug delivery from injectable PUR scaffolds. (a) Fluorescent images obtained using a DAPI filter depicting the distribution of naphthalene compounds in an LDI‐glycerol PUR foam. Compounds were uniformly distributed throughout the foams, as evidenced by the blue color throughout the material. (Reprinted with permission from Ref 88. Copyright 2010 IOS Press). (b) SEM image of an LTI‐polyester foam. (c) Confocal images of LTI‐polyester foams augmented with fluorescein isothiocyanate (FITC)‐labeled bovine serum albumin (BSA) as a labile powder. In contrast to the LDI‐glycerol + napthol foams, discrete BSA‐FITC particles can be observed uniformly distributed throughout the material.
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Schematic summarizing the synthesis of injectable polyurethane (PUR) foams from viscous liquid precursors. Component 1 comprises a polyisocyanate or isocyanate (NCO)‐terminated prepolymer. Component 2 includes all compounds with reactive hydrogen groups that can react with the prepolymer or control the properties of the foam, including the polyol, water, catalyst, matrix particles, and biologics. As evidenced by the rheological properties, at the G′‐G″ crossover point (identified as the working time) the material transitions from a viscoelastic liquid to an elastic solid. Water reacts with the prepolymer to form carbon dioxide gas bubbles, which results in the formation of a macroporous foam (SEM image). In contrast to aromatic polyisocyanates, foams prepared from lysine‐derived (LTI, LDI) or aliphatic (hexamethylene diisocyanate trimer (HDIt)) show a mild (<20°C) exotherm. (Reprinted with permission from Ref 95. Copyright 2009 Elsevier).
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Thermoresponsive injectable polyHIPEs. (a) Diagram illustrating the formation of crosslinked oil‐in‐water thermoresponsive polyHIPEs using the temperature‐triggered lower critical solution temperature (LCST) phase transition of polyNIPAAm chain segments as a means of non‐covalently crosslinking injectable HIPEs. (b) SEM image of a thermoresponsive dextran‐bpolyNIPAAm polyHIPE with 20% (w/v) polymer concentration. (Reprinted with permission from Ref 57. Copyright 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim).
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Injectable propylene fumarate dimethacrylate (PFDMA) polyHIPE. (Reprinted with permission from Ref 19. Copyright 2011 American Chemical Society).
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Schematic of the emulsion templating process to obtain a polyHIPE.
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SEM images of biphasic calcium phosphate cements. (a) A biphasic CPC fabricated by mixing α‐tricalcium phosphate (α‐TCP, 40%), calcium sulfate hemihydroate (CSH), and an aqueous solution containing 2.5 wt% Na2HPO4. The cement initially contained 20 wt% α‐TCP (×400). Both small (approximately 1 µm) crystals of CSD (labeled D in panel a and viewed at higher magnification in the inset image) and larger residual CSH (labeled H) crystals were observed in the cement. Voids formed by dissolution of the CSH crystals (labeled SH) are also evident in the SEM image. (Reprinted with permission from Ref 32. Copyright 2002 Wiley Periodicals, Inc). (b) Microstructure of cements with 8.7 mM of SDS after 5 days of setting (L/P = 0.32 mL g−1) shows macropores (approximately 20 µm) stabilized by the SDS. (Reprinted with permission from Ref 11. Copyright 2003 Willey Periodicals, Inc).
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Images of macroporous and nonporous PPF based scaffolds. Scanning electron micrography (top) and magnetic reasonance microscopy (bottom) images of either nonporous (a, d) or macroporous (b,c,e,f) PPF scaffolds. (Reprinted with permission from Ref 72. Copyright 2008 Elsevier).
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Shape memory cryogels as injectable materials. (a) Depiction of cryogel synthesis; (b) proof of injectability by loading a square shaped cryogel into a syringe and displaying the shape after injection; (c) rhodamine‐labeled gels with varying size maintain their shape after injection; (d) variable geometries maintain their shape after injection; (e) stress versus strain curves for conventional and cryogels showing recoverable deformation; and (f) SEM image of cell attachment to cryogels. (Reprinted with permission from Ref 5. Copyright 2012 National Academy of Sciences).
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Immunostaining of cells remaining in macroporous fibrin scaffolds. Cells stained for myogenin, an early marker of muscle tissue differentiation after 8 days (a–c); cells stained for MYH after 16 days (d–f); and cells stained for ACTN and ACTA1 after 16 days of culture showing multicell merging indicating muscle fiber differentiation. (Reprinted with permission from Ref. 21. Copyright 2013 Elsevier).
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Therapeutic Approaches and Drug Discovery > Emerging Technologies
Implantable Materials and Surgical Technologies > Nanotechnology in Tissue Repair and Replacement

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