Fabrication and Characterization
The copolymer PLA-PCL blend contained a 70/30 PLLA/PCL ratio, and was termed PLC for abbreviation. First a colloidal solution was prepared by mixing PLC with acetone. BNNTs were also mixed separately with acetone and the solution was added to PLC colloid, which was appropriately mixed and finally cast to form films. For comparison purposes pure PLC films were also prepared using a similar process. The thickness of the resulting films was 500 ^m, 250 ^m and 200 ^m for the PLC, 2 wt% BNNT/PLC and 5 wt% BNNT/PLC, respectively .
Figure 4.5a, b reveals a top view of the microstructure of the pure PLC and 5 wt% BNNT/PLC composite. It is seen that the addition of BNNTs decreased the porosity and also resulted in a smoother surface. The polymer coated BNNTs are shown Fig. 4.5c which is a cross-sectional view of a composite film.
Performing uniaxial tensile tests indicated that the addition of BNNTs increased the strength of the PLC. The samples were loaded to a maximum strain of 2.4%, and upon unloading they all resumed their initial shape, i.e. neither plastic deformation nor fracture was observed. The stress values recorded at this maximum strain were 2.67 MPa, 4.98 MPa and 5.59 MPa for the PCL, 2 wt% BNNT/PLC and 5 wt% BNNT/PLC, respectively . The ability of composites to retain the flexibility of pure PLC, despite the addition of BNNTs, was due to the high flexibility of BNNTs  and the ability of acetone to allow for a good incorporation of this second phase into the polymer matrix. The higher strength of the composite materials was due to short fiber strengthening which resulted from the addition of stiffer fibers, of an appropriate aspect ratio, in a matrix.
Fig. 4.5 Scanning electron microscopy images of the cross-section of (a) PLC and (b) PLC- 5BNNT composite showing the difference in porosity. Inset shows high magnification SEM images. (c) Scanning electron microscopy image of the fracture surface of PLC-5%BNNT composite, illustrating polymer coated BNNTs of varying diameters (Reproduced with permission from Ref. )
In order to test the applicability of these scaffolds for bone regeneration human osteoblasts (ATCC CRL-11372) were seeded in culture medium (at 34 °C) and after 60 h they were stained with a solution containing fluorescein di-acetate (FDA) for observing live-dead cells with a fluorescent microscope. Such fluorescent microscopy indicated that the viability of osteoblasts significantly increased for the BNNT/ PLC composites, as it was 60% for PLC, while for both BNNT composites it was ~90% . This suggests that the cell viability did not differ between the 2 wt% and 5 wt% BNNT content composites, but the addition of BNNTs increased the biocompatibility of such scaffolds. This could be partly due to the fact that rough surfaces have been shown to obstruct the proliferation of osteoblasts  and since the BNNT composites had smoother surfaces they could promote osteoblast viability.