Introduction: Adequate strength, tailored stiffness, and an interconnected porous architecture are crucial requirements for functional tissue engineered (TE) bone scaffolds, however the high porosity reduces the mechanical properties[1]. A novel approach to the existing design challenge is the deposition of a stiff and strong polymer-nanocomposite coating using Layer-by-Layer (LbL) assembly. LbL assembly involves the adsorption of oppositely charged polyelectrolytes onto a substrate resulting in multilayer films with controlled physical and structural properties that can be tailored by changing process parameters[2]. In this work, a polymer-nanocomposite coating was deposited onto foam template under systematically altered conditions, and the physical, structural and mechanical properties were characterized.
Materials and Method: LbL assembly was conducted using aqueous solutions of polyethyleneimine (PEI), polyacrylic acid (PAA) (Sigma Aldrich), and Na+-Montmorillonite clay (MTM) (donated by BYK), prepared as previously described[3]. Open-cell polyurethane (PU) foams (30 PPI, The Foam Shop Ltd.) were alternately subjected to cationic (PEI), and anionic (PAA and MTM) solutions (Figure 1). Deposition of a single PEI/PAA/PEI/MTM multilayer was repeated in intervals of five multilayers to obtain 60 multilayers coated specimens.
Figure 1 LbL assembly onto porous foam template.
Specimens were dried and the total coating mass was determined before deposition of subsequent multilayers. Process parameters’ limits (Figure 2) were selected following literature review, and the experimental investigation was conducted using two level, ½ fractional factorial design (Design-Expert Version 5.0.8, State-Ease).
Figure 2: Upper and lower limits of LbL assembly parameters.
The mechanical properties of were determined by quasi-static mechanical testing in compression and following ASTM D1621-10. MG63 osteoblast-like cells were cultured onto 10 multilayer-coated planar substrates for 120 hours followed by nucleus (DAPI) and F-actin (Phalloidin) staining. Cell viability was determined by LDH assay.
Results and Discussion: All coated foams showed a consistent increase in mass and compressive modulus as a function of number of multilayer deposited. Varying the process parameters (Figure 2) resulted in fabricating 60 multilayer-coated specimens with diverse physical and mechanical properties covering a range of material-property-space (Figure 3). Coated specimens exhibited improved stiffness spanning over two orders of magnitude compared to uncoated (from 0.082 up to 16.64 MPa). Preliminary cell studies indicated no cytotoxicity when PAA was deposited as the last layer of the material system.
Figure 3 Compressive modulus versus density for foams coated with 60 multilayers under varying conditions. Insets of exemplary SEM images with 60 multilayer coated foam.
Conclusion: LbL assembly was implemented to fabricate polymer-nanocomposite coated foams. The results suggested that varying the process parameters allows for fabrication of coated foams with tailored physical and mechanical properties. This approach offers a pioneering strategy for fabricating materials with tailored structure and mechanical properties spanning several orders of magnitude and that are suitable for TE bone scaffold applications.
References:
[1] Hutmacher, D. (et al.), J Tissue Eng Regen M, 1: 245-60, 2007
[2] Dubas, S. (et al.), Macromolecules, 32: 8153-60, 1999
[3] Ziminska, M. (et al.), Proceedings of the 26th ESB Conference, Liverpool, UK, 2014