Nanotechnological strategies for engineering complex tissues

Abstract

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.

Figure 1. 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.

Figure 2. 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. , © 2010 NAS; c, ref. , © 2009 Elsevier; d, ref. , © 2010 Elsevier.

Figure 3. Recreating ECM components using nanoscale tools

a, ECM nanofibres produced by electrospinning polymeric fibres contain nanoparticles that release epidermal growth factor (green) and bovine serum albumin (red) in parallel. b, Self-assembled peptide amphiphile nanofibres. c,d, Alginate scaffolds containing short motifs of ECM adhesion proteins such as RGD encouraged mesenchymal stem cells to spread and attach to the matrix (c), whereas on unmodified scaffolds (d) only cell–cell interactions were seen (collagen fibres, green; nuclei, red). e, Epithelial cells respond to nanopatterning by alignment and elongation along the grating axis. f, On smooth substrates, cells are mostly rounded. Figures reproduced with permission from: a, ref. , © 2009 Wiley; b, ref. , © 2009 AAAS; c and d, ref. , © 2009 Elsevier; e and f, ref. , © 2003 Company of Biologists.

Figure 4. 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. , © 2010 AAAS; b, ref. , © 2010 NPG.