Introduction: Electrospinning has become an increasingly favoured method in biomaterials and is often used for tissue engineering scaffolds, in particular for soft tissues. Electrospun nanofibres have a very similar structure to the nanofibrous collagen found in the extracellular matrix of many tissues, which contribute significantly to the mechanical properties. Mimicking the morphology of natural networks in artificial scaffolds will allow similar mechanical properties to the natural healthy tissue resulting in a successful replacement. This study considers the mechanical properties of the electrospun meshes in relation to the polymer solutions used and the morphology of the mesh.
Materials and Methods: Eight different solutions of gelatin in acetic acid were electrospun repeatedly. Four different porcine gelatins with bloom strengths[1] ranging between 100 g and 300 g were used to create solutions at concentrations of 10 and 12 wt %, in a solvent of 90 v/v % aqueous acetic acid. The resulting nanofibrous meshes were analysed by scanning electron microscopy (SEM) to determine average fibre diameter and fibre directionality. Multiple unidirectional tensile tests were also conducted on each mesh produced using an Instron (5544 Universal Testing Machine). Some meshes were further analysed by focused ion beam (FIB) SEM in order to visualise their 3D structure.
Results and Discussion: As the concentration and bloom strength of the gelatin increases, the average diameter of the fibres was found to increase from 75 nm to 254 nm, as well as an increasing variability in the diameter, demonstrated in Figure 1.
However, the concentration and bloom strength were shown to have little effect on the mechanical properties of the meshes. The viscosity and the conductivity of the solutions also had little effect on mechanical properties, although viscosity does strongly influence the fibre diameter. Further analysis showed that the mechanical properties displayed little dependence on fibre diameter, shown in Figure 2. This result is unexpected and contradictory to many other author’s findings[2],[3].
The modulus of a porous material is strongly affected by the bulk volume fraction. Here, the same volume of polymer is used to create each of the meshes, of similar total volume, hence the fibre volume fraction must be similar despite varying fibre diameter. FIB SEM images have supported this theory, showing that the porosity of the meshes vary in addition to the fibre diameter. This implies that tensile properties are more dependant on porosity of the mesh than the average fibre diameter.
Conclusion: The concentration and bloom strength of electrospun gelatin are shown to not affect tensile properties of the meshes despite affecting the fibre diameter. It is shown that the porosity of the electrospun meshes has a greater effect on mechanical properties than fibre diameter, and that in future porosity of meshes should be reported as the controlling variable.
This work was funded by the Engineering and Physical Sciences Research Council (1354760)
References:
[1] Chua, W. K., & Oyen, M. L. (2009). Viscoelastic Properties of Membranes Measured by Spherical Indentation. Cellular and Molecular Bioengineering, 2(1), 49–56.
[2] Wong, S.-C., Baji, A., & Leng, S. (2008). Effect of fiber diameter on tensile properties of electrospun poly(l-caprolactone). Polymer, 49(21), 4713–4722.
[3] Huang, Z.-M., Zhang, Y., Ramakrishna, S., & Lim, C. (2004). Electrospinning and mechanical characterization of gelatin nanofibers. Polymer, 45(15), 5361–5368.