Composite bone tissue engineering scaffolds produced by coaxial electrospinning

Electrospinning of polylactic acid (PLA)/calcium phosphates (CaPs) has been widely investigated for bone tissue engineering, however the significant reduction in mechanical properties and the rapid loss of the structural integrity of the scaffolds upon inclusion of high filler content is still challenging. Coaxial electrospinning has gained attention for tissue engineering applications due to the enhanced quality and the functionality of the resulting fibres compared to the basic electrospinning process. In this study, core and shell polycarpolactone (PCL)- PLA/micro-HA fibrous scaffolds were produced via coaxial electrospinning. To optimise the shell component, PLA solutions of concentrations ranging from 5 to 25 wt%, and containing 10-40 vol% of either spray dried HA (HA1), sintered HA (HA2) or beta tricalcium phosphate (β-TCP) were electrospun using single-needle electrospinning. However, only 15 and 20 wt% PLA solution with 10 or 20 vol% CaPs produced electrospun scaffolds. Inclusion of all these fillers significantly reduced the mechanical properties of the scaffolds compared to non-filled PLA while increasing fibre diameter and non-homogeneity. TCP-containing scaffolds showed reduced mechanical properties compared to HA1- and HA2-filled scaffolds and increased TCP agglomerations along the fibres. Introduction of HA1 and HA2 into PLA scaffolds decreased the degradation rate of the scaffolds while increasing the bioactivity. However, apatite formation on the fibre surfaces was lower than previously reported due to the lower surface area of micro-HA particles compared to nano-HA in addition to the lack of sufficient HA particles on fibres surface. The higher surface area of HA1 did not significantly affect the rate of bioactivity, however it increased the thermal stability of scaffolds compared to HA2-filled scaffolds and led to further reduction in mechanical properties in vitro than HA2. Scaffolds with either HA lost their mechanical integrity within 28 days of SBF immersion. As for the core component, changing the solvent system was found to affect the stability of the Taylor cone during electrospinning, and subsequently the morphology of the resultant fibre. Introduction of PCL as the core component in coaxial scaffolds increased both the tensile strength and strain at failure. The mechanical properties were influenced by the flow rate ratio between the core and shell components. Coaxial scaffolds with and without HA exhibited gradual release of BMP 2 with only 12.8-13.6% released over 96 hours. They also supported cell attachment and spreading over 21 days of culture. However, control scaffolds had improved cell spreading compared to HA-containing scaffolds due to increased fibre uniformity and decreased fibre diameter. Tubular scaffolds made of core and shell structured fibres were also produced using rotating needle collector with G16 and G21 needle producing the internal diameter. Coaxial electrospinning with rotating needle collector produced fibres with improved circumferential alignment compared to stationary collector and increased fibre non-uniformity in HA-containing scaffolds. Bioactivity of tubular coaxial scaffolds was also significantly increased due to partial encapsulation of HA particles, and large areas of coaxial scaffolds were covered with apatite layer after 12 weeks of immersion in SBF. On the other hand, coaxial scaffolds with no HA showed no bioactivity even after 12 weeks of SBF immersion. Tubular control and HAcontaining coaxial scaffolds had significantly higher mechanical stability in vitro and showed gradual reduction in their mechanical properties over 12 weeks of immersion in either PBS or SBF. The results obtained suggest that coaxial electrospinning is a promising technique to produce bone tissue scaffolds with high content of CaPs while preserving the structural and mechanical integrity of the scaffolds. Bioactivity of scaffolds can be significantly increased by incorporating the CaP in the shell layer while mechanical properties of the coaxial scaffolds can be tailored by changing the core composition and diameter. However, further studies should be carried on to enhance the uniformity and alignment of the coaxial fibres in order to improve the mechanical properties of scaffolds