We investigated the fabrication of highly porous scaffolds made of three different materials [poly(propylene fumarate) (PPF) polymer, an ultra-short single-walled carbon nanotube (US-tube) nanocomposite, and a dodecylated US-tube (F-US-tube) nanocomposite] in order to evaluate the effects of material composition and porosity on scaffold pore structure, mechanical properties, and marrow stromal cell culture. All scaffolds were produced by a thermal-crosslinking particulate-leaching technique at specific porogen contents of 75, 80, 85, and 90 vol%. Scanning electron microcopy, microcomputed tomography, and mercury intrusion porosimetry were used to analyze the pore structures of scaffolds. The porogen content was found to dictate the porosity of scaffolds. There was no significant difference in porosity, pore size, and interconnectivity among the different materials for the same porogen fraction. Nearly 100% of the pore volume was interconnected through 20microm or larger connections for all scaffolds. While interconnectivity through larger connections improved with higher porosity, compressive mechanical properties of scaffolds declined at the same time. However, the compressive modulus, offset yield strength, and compressive strength of F-US-tube nanocomposites were higher than or similar to the corresponding properties for the PPF polymer and US-tube nanocomposites for all the porosities examined. As for in vitro osteoconductivity, marrow stromal cells demonstrated equally good cell attachment and proliferation on all scaffolds made of different materials at each porosity. These results indicate that functionalized ultra-short single-walled carbon nanotube nanocomposite scaffolds with tunable porosity and mechanical properties hold great promise for bone tissue engineering applications.

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We have investigated the dispersion of single-walled carbon nanotubes (SWNTs) and functionalized SWNTs (F-SWNTs) in the unsaturated, biodegradable polymer poly(propylene fumarate) (PPF) and examined the rheological properties of un-cross-linked nanocomposite formulations as well as the electrical and mechanical properties of cross-linked nanocomposites. F-SWNTs were produced from individual SWNTs by a diazonium-based method and dispersed better than unmodified SWNTs in both un-cross-linked and cross-linked PPF matrix. Cross-linked nanocomposites with F-SWNTs were superior to those with unmodified SWNTs in terms of their mechanical properties. Specifically, nanocomposites with 0.1 wt % F-SWNTs loading resulted in a 3-fold increase in both compressive modulus and flexural modulus and a 2-fold increase in both compressive offset yield strength and flexural strength when compared to pure PPF networks, whereas the use of 0.1 wt % SWNTs gained less than 37% mechanical reinforcement. These extraordinary mechanical enhancements considered together with Raman scattering and sol fraction measurements indicate strong SWNT-PPF interactions and increased cross-linking densities resulting in effective load transfer. With enhanced mechanical properties and capabilities of in situ injection and cross-linking, these SWNT/polymer nanocomposites hold significant implications for the fabrication of bone tissue engineering scaffolds.

This work investigated the effects of the use of a surfactant or the functionalization of single-walled carbon nanotubes (SWNTs) on their dispersion in uncrosslinked poly(propylene fumarate) (PPF) and the mechanical reinforcement of crosslinked composites as a function of the SWNT concentration. Rheological measurements showed good dispersion of SWNTs in uncrosslinked PPF at low concentrations of 0.05 wt% and SWNT aggregation for higher concentrations for all formulations examined. Mechanical testing demonstrated significant reinforcement in the compressive and flexural mechanical properties of crosslinked nanocomposites which peaked for low SWNT concentrations of the order of 0.05 wt%. For example, a 74% increase was recorded for the compressive modulus and a 69% increase for the flexural modulus of nanocomposites with functionalized SWNTs at a 0.05 wt% loading. Nevertheless, this reinforcement was not related to the use of a surfactant or the functionalization of the SWNTs tested. Scanning electron microscopy examinations of fractured nanocomposite surfaces revealed the formation of SWNT aggregates at higher concentrations corroborating the rheological and mechanical data. These results suggest that the dispersion of individual SWNTs in a uncrosslinked formulation is pivotal to the development of injectable nanocomposites for bone tissue engineering applications.