Synthetic Fibres

The mechanical and physical properties of any biomaterial for tissue specific applications depends on the reinforcement material, i.e. fibres or tubes. The scaffold material must demonstrate high porosity with interconnected pores and a high surface area. However, a lack of good mechanical properties impedes their use for many load bearing tissue engineering applications. Natural origin scaffold materials (e.g. chitosan, collagen and hyaluronic acid), synthetic polymers such as poly(lactic acid) (PLA), poly(glycolic acid) (PGA), poly(lactic acid-co-glycolic acid) (PLGA) have been used for many tissue engineering applications. [124, 125] developed chi- tosan fibres reinforced with porous collagen in an effort to improve the mechanical properties of porous collagen scaffold. They studied the relationship between both volume and length of the fibre (which was greater than or equal to critical length) and the compressive properties of the scaffold. [125] proposed a modified empirical formula for (critical length) fibre reinforced porous materials by introducing a modifying factor (F1), which represented the elasticity of the fibres as shown in the equation below:

Where lcr is the critical length of the fibres, dr the diameter of the fibres, cf the compressive strength of the fibres, Tm the shear strength of the matrix material.

Also, for the relationship between volume content of the fibres and the compressive strength of the porous scaffold, they modified formula to calculate compressive strength of the scaffold as shown in the equation below:

Where oc, Vf, Ef, Em, cm, F0, O represents the compressive strength, volume content of the fibres, compressive modulus of the fibres, compressive modulus of the matrix material, compressive strength of matrix material, modifying factor and porosity of the intensified material respectively [125]. Carbon microfibre and collagen nanofibre reinforcement in collagen Type I matrices showed that the interaction between collagen nanofibres with matrix was greater than that between carbon microfibres and matrix. This study indicated the importance of material choice and size of the fibre, which directly influenced the interaction between fibres and matrix and also the scaffold mechanical properties [126].

Incorporation of fibres not only improved the mechanical properties of the scaffold but also maintained the size and shape of the medical device or implant while maintaining the bioactivity. It is well known that collagen based gel contract while cellular remodelling and reduce in the dimensions. To overcome this problem, collagen I has been reinforced with poly(ester terephthalate) microfibres. Collagen Type I fibres were extruded into polymerising buffer and incorporated into Type I collagen gels as fibre bundles. The addition of fibres maintained the gel dimensions [127-129].

Pore size and surface area of composites play important roles with respect to fibre-reinforced scaffolds. The diameter of the fibres is inversely proportional to the surface area. Also, the fibre length and volume decreases the porosity of the scaffold due to intertwined and low solubility in pore forming reagents. Therefore, to reduce the porosity and pore size, the reinforcement material should be added in a limited way [124]. The mechanical integrity of fibres and matrix increases with chemical crosslinking when compared to conventional mechanical mixing. Chitin fibres have been integrated into a PLA matrix successfully when crosslinked and the crosslinking treatment did not invoke a cytotoxic effect or reduce the impact of the environment for efficient cell growth [130].

As discussed previously, the mechanical properties of reinforcing material depends on biocompatibility and biodegradability of the scaffold. The mechanical properties of the composite affect the adhesion of the cells. The softer the substrate then the better for bone marrow- and adipose tissue-derived mesenchymal stem cells attachment when compare to a stiffer substrate [131]. Biodegradable PLA decreases the pH rapidly over time due to degraded products, which affects the microenvironment. [132] reinforced PLA with chitin fibres (30 vol.%) and showed improved pH variation, which was maintained between 7.0 and 7.2 over 16 weeks in vitro by neutralising the acidic degradation by-products of PLA with chitin degradation by-products. Furthermore, the chitin reinforced PLA demonstrated better cell attachment and proliferation of human osteoblast (HOBt) like cells [132]. An alginate was reinforced with single-walled carbon nanotubes (SWCNT) via a freeform fabrication technique. The resultant composite exhibited improved biocompatibility and a higher strength (436 vs. 542 kPa) and secondary modulus (7.5 vs. 426 kPa) when compared with pure alginate [133].

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