Fabrication and Characterization
Hydroxy lated BNNTs (BNNT-OH) chitosan scaffolds were prepared by mixing 2 mL of chitosan with 1 mL BNNT-OH suspension in H2O as described in . In order to examine the effect of the BNNTs a control group of pure chitosan scaffolds was also prepared.
Scanning electron microscopy images, such as those depicted in Fig. 4.3a, b, illustrated a porous microstructure for both chitosan and chitosan/BNNT-OH scaffolds. However, the addition of BNNT-OH resulted in an increased pore size, which is preferred for tissue ingrowth. This was due to the hydrophobicity of BNNTs. The average pore sizes were determined from the randomly chosen pores on the SEM images and as seen, the pore sizes of the chitosan/BNNT-OH scaffold were larger than the pure chitosan scaffold. Although the BNNTs became more hydrophilic with the introduction of hydroxyl groups into their structures, they still exhibited some hydrophobicity. Hence, when they were added into the hydrophobic chitosan solution, the water present formed larger droplets than in pure chitosan in order to avoid the BNNTs. As a result when H2O sublimated larger pores formed in the nanocomposite scaffolds .
In addition to increasing the pore size, the hydrophobicity of BNNTs also resulted in a lower swelling ratio as it repelled water molecules and reduced the amount that could enter the composites. Particularly, the swelling ratio was found to be 60 and 100 for the chitosan/BNNT-OH and pure chitosan scaffolds, respectively.
Finally, as expected, the BNNT-OH reinforced composites also exhibited an increased strength, despite their larger pore size. Performing compression tests indicated that for pure chitosan 35 kPa were necessary to induce a 70% strain, while the chitosan/BNNT-OH required a 55 kPa stress . A drawback here is that up to a ~35% strain the stress-strain curves for both types of materials coincided; hence a higher content of BNNT should most likely be considered in future studies, to attain better mechanical properties at lower strains.