Nanotechnology Strategies for tissue engineering

an example of a tissue engineering concept that involves seeding cells within porous biomaterial scaffolds. a, Cells are isolated from the patient and may be cultivated (b) in vitro on two-dimensional surfaces for efficient expansion. c, Next, the cells are seeded in porous scaffolds together with growth factors, small molecules, and micro- and/or nanoparticles. The scaffolds serve as a mechanical support and a shape-determining material, and their porous nature provides high mass transfer and waste removal. d, The cell constructs are further cultivated in bioreactors to provide optimal conditions for organization into a functioning tissue. e, Once a functioning tissue has been successfully engineered, the construct is transplanted on the defect to restore function.

Nature Nanotechnology – Nanotechnological strategies for engineering complex tissues

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Tissue engineering aims at developing functional substitutes for damaged tissues and organs. Before transplantation, cells are generally seeded on biomaterial scaffolds that recapitulate the extracellular matrix and provide cells with information that is important for tissue development. Here we review the nanocomposite nature of the extracellular matrix, describe the design considerations for different tissues and discuss the impact of nanostructures on the properties of scaffolds and their uses in monitoring the behaviour of engineered tissues. We also examine the different nanodevices used to trigger certain processes for tissue development, and offer our view on the principal challenges and prospects of applying nanotechnology in tissue engineering.

Nanotechnologies clearly have had an impact on tissue engineering and still have great potential to advance therapeutic methods based on tissue engineering. The synthesis of new nanostructures and their incorporation into existing macro- and microtechnologies have led to improvements in the ability to provide a true biomimetic microenvironment to the developing tissue. But challenges still need to be addressed. It is believed that engineered nanomaterials such as carbon nanotubes, nanowires and other inorganic materials will be increasingly used in tissue engineering. In recent years, several reports have indicated possible negative effects of carbon nanotubes on cells and their potential to provoke oxidative stress, inflammation, genetic damage and long-term pathological effects. Therefore the biocompatibility and biodegradation of inorganic nanomaterials need to be thoroughly investigated before they can be safely applied in clinical trials. (For a comprehensive review on the toxicology of nanomaterials, see Kunzmann et al.105.) Another challenge that must be addressed is the complexity of creating 3D porous scaffolds with nanotopographies. The interaction of cells with various nanoscale topographies has proved to be important for creating monolayers of functioning tissues but ‘scaled-up’ versions of these nanopatterning technologies remain to be achieved. Moreover, it is crucial to discover the key factors promoting the assemblies of different tissues and to create specific microenvironments. Developing nanotechnological tools for controlling and guiding cells to desired locations in 3D matrices will be useful for engineering complex multicellular constructs such as epithelial and vascularized tissues.

the information provided to cells by the extracellular matrix (ecM). a, ECM fibres provide cells with topographical features that trigger morphogenesis. Adhesion proteins such as fibronectin and laminin located on the fibres interact with the cells through their transmembrane integrin receptors to initiate intracellular signalling cascades, which affect most aspects of cell behaviour. Polysaccharides such as hyaluronic acid and heparan sulphate act as a compression buffer against the stress, or serve as a growth factor depot. b–d, Illustrations of the heart, liver and bone at the level of organ (left) and tissue and cell/matrix interaction (centre), followed by scanning electron micrographs of engineered scaffolds (right). The ECMs of various tissues have different composition and spatial organization of molecules to maintain specific tissue morphologies. For example (b), the ECM of muscle tissues, such as the heart, forces the heart cells (cardiomyocytes) to couple mechanically to each other and to form elongated and aligned cell bundles that create an anisotropic syncytium. Nanogrooved surfaces (SEM image) are suitable matrices for cardiac tissue engineering because they force cardiomyocytes to align. c, Cells composing epithelial tissues are polarized and contact three types of surfaces for efficient mass transfer: the ECM, other cells and a lumen. Nanofibres modified with surface molecules can promote cell adhesion and tissue polarity (SEM images). d, Bone is a nanocomposite material consisting primarily of a collagen-rich organic matrix and inorganic hydroxyapatite nanocrystallites, which serve as a chelating agent for mineralization of osteoblasts. The scaffold structure (SEM image), stiffness and hydroxyapatite nanopatterning on the surface (inset) can enhance osteoblast spreading and bone tissue formation. SEM images reproduced with permission from: b, ref. 56, © 2010 NAS; c, ref. 59, © 2009 Elsevier; d, ref. 65, © 2010 Elsevier

nanodevices in tissue engineering. a, Three-dimensional, free-standing nanowire transistor probe for electrical recording. The probe is composed of a kinked nanowire (yellow arrow) and a flexible substrate material. The device is used to penetrate the membrane of living cells (inset) and measure intracellular signals (lower panel). b, Biosensors based on carbon nanotubes are used for the detection of genotoxic analytes, including chemotherapeutic drugs and reactive oxygen species. Upper figure shows a schematic of a sensor made from a DNA and a single-walled carbon nanotube complex bound to a glass surface through a biotin-BSA (orange) and neutravidin (blue) linkage. Lower figures reveal the spectral changes arising from the interaction of the nanotube sensor with (from left to right): a chemotherapeutic agent, hydrogen peroxide, singlet oxygen and hydroxyl radicals (blue curve, before addition of analytes; green curve, after addition of analytes). Figures reproduced with permission from: a, ref. 99, © 2010 AAAS; b, ref. 101, © 2010 NPG.

We believe that future strategies could involve incorporating intelligent nanoscale biosensors inside scaffolds to follow the development of engineered tissues after transplantation, and according to biochemical composition or tissue behaviour, trigger the release of compensating cues. For example, when oxygen levels are low, the system would release angiogenic factors for fast new blood vessel formation. We also believe that incorporating nanomaterials, such as nanotubes or wires on the outer surfaces of 3D scaffolds, could serve to reduce inflammatory responses to transplanted engineered tissues by modulating macrophage adhesion and viability. For example, zinc oxide nanorods (50 nm in diameter and around 500 nm long) sputtered on polyethylene terephthalate discs inhibited the formation of the typical foreign body capsule occurring after subcutaneous transplantation in mice

Finally, we envisage the use of smart nanoparticulate systems that will recruit stem cells to desired sites in the body and instruct the formation of tissues in vivo, on demand, by triggering the release of chemical attractants. Smart controllable nanorobots could potentially circulate inside the body, find diseased tissues and repair them by destroying defective cells and molecules, or encourage cells to regain function. These robots could be powered by biologically inspired nanomotors based on conversion of chemical energy (usually stored as adenosine triphosphate) into mechanical energy. We foresee nanowire-based brain-machine interfaces that could assist paralysed patients by re-routing movement-related signals around injured parts of the nervous system.

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