Introduction: Cell behaviour is regulated by numerous environmental parameters related to the composition, surface topography and nano-mechanical properties of the extracellular matrix[1]. Cells grow preferentially on specialised microstructures that imitate their original extracellular matrix, the chemical and mechanical properties of which been tailored to mimic the cells’ original environment. This study aims to relate differences in microstructure of polyurethane scaffolds, prepared by means of three different processing techniques, to changes in mechanical properties measured on the micron and nano length scales.
Materials and Methods: The following forms of medical grade polyurethane (b9 ‘A’ series thermoplastic poly-ether urethanes, Biomer Technology Ltd., Runcorn, UK) were compared: melt processed polymer, solvent-cast films and sub-micron fibres fabricated by electrospinning. Comparison of the thermal and mechanical properties was made by differential scanning calorimetry (DSC) and nanoindentation, whereas the fibre morphology and surface roughness were quantified by scanning electron (SEM) and atomic force microscopy (AFM), respectively.
Results and Discussion: Thermal analysis of each of the polymer forms revealed differences in polymer microstructure (Fig. 1). Whereas two distinct endotherms of approximately equal size are evident in the thermograms of melt-processed samples, suggesting an equal distribution of type I and type II hard segment crystallites, solvent-cast films demonstrate a larger type I endotherm, indicating increased hard segment phase mixing. In contrast, the electrospun fibrous forms exhibit a larger type II endotherm, suggesting a greater degree of phase separation in those samples, which may be attributed to the manner in which the fibres are formed.
Figure 1. Thermal analysis (DSC) of the same polyurethane processed by each method.
A two-fold increase in stiffness, as measured by nanoindentation, was observed, with electrospun forms demonstrating an average modulus of 12.3 ± 5.8 GPa compared to solvent-cast films (5.88 ± 0.91 GPa). This increase may be attributed in part to differences in microstructure revealed by DSC.
Figure 2. Comparison of electrospun and cast forms by means of AFM (top row), SEM (bottom left) and Nanoindentation (bottom right).
Conclusion: Thermal analysis of polyurethane films cast from solvent, melt processed pellets and microfibrous scaffolds fabricated by means of electrospinning, reveal differences in polymer microstructure, which we attribute to the method of processing in each case. Further studies are warranted in order to determine the extent to which the degree of phase separation has on protein and cell-surface interactions. The findings of these studies may have a bearing on biomaterials selection and scaffold design.
The authors acknowledge the support of the Scottish Funding Council and the UK Engineering & Physical Sciences Research Council (EPSRC) Doctoral Training Centre in Medical Devices, University of Strathclyde (EPSRC Grant Ref. EP/F50036X/1) for the studentships awarded to MC and DE, respectively
References:
[1] Trappmann B, Gautrot JE, Connelly JT, Strange DGT, Li Y, Oyen ML, Cohen Stuart MA, Boehm H, Li B, Vogel V, Spatz JP, Watt FM, Huck WTS, Nature Materials (2012); 11: 642 - 649