Fibre Reinforced Hydrogels
Hydrogels are primarily composed of synthetic or natural hydrophilic polymers. The 3D structure of hydrogels resembles that of natural ECM, absorbing large amounts of water or biological fluids, demonstrating a highly porous network that allows cell and nutrients migration and cell proliferation [5, 37-39]. Hydrogel composition must meet certain parameters in order to be successful as a tissue engineered scaffold with respect to its mechanical (e.g. Young’s modulus), physical (e.g. mass transport), and biological (cell/matrix interaction) properties [40, 41]. The mechanical properties of the hydrogel largely depends on the cross-link density  and the nature of network (e.g. chemical or physical). Physical networks are reversible and can be synthesised using polymers such as collagen , chitosan  and aminated hyaluronic acid . Whereas, chemical networks are achieved by covalent bonding and are non-reversible, e.g. chitosan/hyaluronic acid , PEG/fibrin- ogen , and alginate  based hydrogels. However, such hydrogels are weak isotropic materials and fail to provide any load-bearing capabilities in vivo. They also demonstrate low stability and survival following engraftment [49, 50].
To improve the mechanical integrity of hydrogels, different methods have been developed to allow fibre reinforcement. The most commonly used methods for fibre fabrication are electrospinning and electrospraying . Researchers have developed highly injectable composite based hydrogels using small fibres (1 qm-1 mm) [20, 21, 28, 29, 32, 52, 53]. Fibre reinforced hydrogels can be tailored to demonstrate anisotropic mechanical properties [30, 33, 54, 55]. Moreover, fibre reinforced hydrogel systems containing electrospun fibres have been shown to exhibit good mechanical properties, high levels of cell growth and differentiation [53, 56-58].