Carbon and Inorganic Nanostructure Reinforced Polymeric Nanocomposites for Bone Tissue Engineering

Introduction: Poly(propylene fumarate) (PPF) is a biocompatible, biodegradable polymer widely investigated for bone tissue engineering applications. Various nanomaterials such as fullerenes, single- and multi- walled carbon nanotubes (SWCNTs, MWCNTs) and alumoxane nanoparticles have been used as reinforcing agents to improve the mechanical properties of PPF nanocomposites [1, 2, 3]. In this study, we have investigated the mechanical properties of PPF nanocomposites fabricated using various 1D and 2D carbon and inorganic nanomaterials such as single- and multi- walled carbon nanotubes (SWCNT, MWCNT), single- and multi- walled graphene oxide nanoribbons (SWGONR, MWGONR), graphene oxide nanoplatelets (GONP), tungsten di-sulfite nanotubes (WS₂) and molybdenum di-sulfite nanoplatelets (MSNP) as reinforcing agents [4]. The objectives of this study were to investigate (1) the effects of nanostructure architecture (1D nanostructure (e.g. nanotube) vs. 2D nanostructure (e.g. graphene)), and (2) effects of chemical composition (inorganic vs. carbon nanoparticles) of reinforcing agents on the mechanical properties of PPF.

Materials and Methods: PPF was synthesized using a two-step condensation reaction of diethyl fumarate and propylene glycol as described previously and characterized by H¹-NMR (structure) and GPC (molecular weight). Longitudinal unzipping method was used to synthesize SWGONRs and MWGONRs form SWCNTs and MWCNTs, respectively. Modified Hummer’s method was used to synthesize GONPs. Inorganic nanomaterials (WS₂ nanotubes and MSNPs) were synthesized using literature methods. Nanomaterials were characterized using Raman spectroscopy, AFM and HRTEM. Specimens were thermally crosslinked (60°C) into sample sizes according to ASTM standards for compression and three-point bend testing. Sol fraction analysis was performed to assess the crosslinking density of PPF nanocomposites after thermal crosslinking. 

Results and Discussion: PPF nanocomposites with 2D nanostructures showed significant improvements in the mechanical properties compared to positive controls (1D nanostructure reinforced PPF composites) and baseline controls (PPF composites without nanomaterials). For example, 0.2 wt% loading of MSNPs leads to ~ 100% increase in the Young’s modulus compared to PPF composites (baseline control). Additionally, inorganic nanostructures showed better or equivalent mechanical reinforcement compared to carbon nanostructures at all loading concentrations. The results indicate that the extent of mechanical reinforcement is closely dependent on the nanostructure morphology and follows the trend: nanoplatelets > nanoribbons > nanotubes. In general inorganic nanoparticles show equivalent or better mechanical reinforcement compared to carbon nanoparticles. TEM analysis of crosslinked nanocomposites indicates good dispersion of nanomaterials in the polymer matrix. Sol-fraction analysis showed increase in the crosslinking density of PPF nanocomposites at all concentrations of MSNP and WS₂, and higher concentrations of GONP and MWGONR (0.1-0.2 wt%). These results suggest that the increase in the mechanical properties of 2D carbon and inorganic nanocomposites is due to increases in the crosslinking density of PPF (maybe due to formation of PPF-nanomaterial crosslinking). Additionally, presence of structural defects and functional groups on the nanostructures can lead to increased polymer-nanomaterial interaction, thereby increasing the mechanical properties.

Conclusion: Carbon (SWCNT, MWCNT, MWGONR, SWGONR and GONP) and inorganic- nanoparticles (WS₂ nanotubes and MSNPs) as reinforcing agents significantly improve the mechanical properties of PPF. In general, 2D and inorganic nanostructures show equivalent or better mechanical reinforcement than 1D and carbon nanostructures, respectively.

Figure Caption: Compressive Modulus of PPF Nanocomposites

References:

1. B. Sitharaman et al., J Biomater Sci Polym Ed 18, 655, 2007.

2. A. S. Mistry et al., J Biomed Mater Res A 89, 68, 2009.

3. G. Lalwani et al., Acta Biomaterialia 9 (9), 8365-8373.

4. G. Lalwani et al., Biomacromolecules 14 (3), 900-909, 2013.