Fiber-Templated 3D Calcium-Phosphate Scaffolds for Biomedical Applications: The Role of the Thermal Treatment Ambient on Physico-Chemical Properties

Abstract

A successful bone-graft-controlled healing entails the development of novel products with tunable compositional and architectural features and mechanical performances and is, thereby, able to accommodate fast bone in-growth and remodeling. To this effect, graphene nanoplatelets and Luffa-fibers were chosen as mechanical reinforcement phase and sacrificial template, respectively, and incorporated into a hydroxyapatite and brushite matrix derived by marble conversion with the help of a reproducible technology. The bio-products, framed by a one-stage-addition polymer-free fabrication route, were thoroughly physico-chemically investigated (by XRD, FTIR spectroscopy, SEM, and nano-computed tomography analysis, as well as surface energy measurements and mechanical performance assessments) after sintering in air or nitrogen ambient. The experiments exposed that the coupling of a nitrogen ambient with the graphene admixing triggers, in both compact and porous samples, important structural (i.e., decomposition of β-Ca₃(PO₄)₂ into α-Ca₃(PO₄)₂ and α-Ca₂P₂O₇) and morphological modifications. Certain restrictions and benefits were outlined with respect to the spatial porosity and global mechanical features of the derived bone scaffolds. Specifically, in nitrogen ambient, the graphene amount should be set to a maximum 0.25 wt.% in the case of compact products, while for the porous ones, significantly augmented compressive strengths were revealed at all graphene amounts. The sintering ambient or the graphene addition did not interfere with the Luffa ability to generate 3D-channels-arrays at high temperatures. It can be concluded that both Luffa and graphene agents act as adjuvants under nitrogen ambient, and that their incorporation-ratio can be modulated to favorably fit certain foreseeable biomedical applications.

Keywords: Luffa; applications; biomedical; graphene; marble; reinforced bio-products; sintering ambient.

Figure 1

The comparative XRD patterns of the (a,c) compact and (b,d) porous samples thermally-treated in (a,b) air and (c,d) nitrogen ambient at 1200 °C/8 h. On the bottom of each graph column are presented the ICDD reference diffraction files of the main constitutive crystalline phases (i.e., β-TCP, α-TCP, and α-Ca₂P₂O₇).

Figure 2

The XRD diagrams of the (a) compact and (b) porous specimens thermally-treated in nitrogen ambient at 1200 °C/8 h, zoomed in the angular region 2 θ = 28.8–31.4° (highlighted in yellow in Figure 1) to emphasize the β-TCP phase transformation.

Figure 3

The comparative FTIR-ATR spectra of the (a,c) compact and (b,d) porous samples thermally-treated in (a,b) air and (c,d) nitrogen ambient at 1200 °C/8 h.

Figure 4

The FTIR-ATR spectra of the control samples: pure β-TCP (Sigma-Aldrich), β-TCP + α-TCP blend (obtained by heat-treating the Sigma-Aldrich β-TCP powder at 1400 °C/4 h in air) and α-Ca₂P₂O₇ (synthesized by co-precipitation, see ref. [16]).

Figure 5

The morpho-compositional evolution of the compact and porous samples thermally-treated in air and nitrogen ambient at 1200 °C/8 h.

Figure 6

The comparative mass loss and total shrinkage of the (a) compact and (b) porous samples thermally-treated in air and nitrogen ambient at 1200 °C/8 h.

Figure 7

Nano-CT 3D reconstruction of the porous samples thermally-treated in air and nitrogen ambient at 1200 °C/8 h. The porosity variation of all samples is provided as percentage bars on the sides of the figure, along with the calculated values (n = 5, mean ± SD).

Figure 8

Contact angle measurements—performed with two wetting agents: water (W) and ethylene glycol (EG) and surface free energy (SFE) results for the compact samples thermally-treated in air and nitrogen ambient at 1200 °C/8 h.

Figure 9

Comparative compressive strength results of the compact and porous samples thermally-treated in air and nitrogen ambient at 1200 °C/8 h.