Multi-Parametric Exploration of a Selection of Piezoceramic Materials for Bone Graft Substitute Applications

Abstract

This work was devoted to the first multi-parametric unitary comparative analysis of a selection of sintered piezoceramic materials synthesised by solid-state reactions, aiming to delineate the most promising biocompatible piezoelectric material, to be further implemented into macro-porous ceramic scaffolds fabricated by 3D printing technologies. The piezoceramics under scrutiny were: KNbO₃, LiNbO₃, LiTaO₃, BaTiO₃, Zr-doped BaTiO₃, and the (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃ solid solution (BCTZ). The XRD analysis revealed the high crystallinity of all sintered ceramics, while the best densification was achieved for the BaTiO₃-based materials via conventional sintering. Conjunctively, BCTZ yielded the best combination of functional properties-piezoelectric response (in terms of longitudinal piezoelectric constant and planar electromechanical coupling factor) and mechanical and in vitro osteoblast cell compatibility. The selected piezoceramic was further used as a base material for the robocasting fabrication of 3D macro-porous scaffolds (porosity of ~50%), which yielded a promising compressive strength of ~20 MPa (higher than that of trabecular bone), excellent cell colonization capability, and noteworthy cytocompatibility in osteoblast cell cultures, analogous to the biological control. Thereby, good prospects for the possible development of a new generation of synthetic bone graft substitutes endowed with the piezoelectric effect as a stimulus for the enhancement of osteogenic capacity were settled.

Keywords: bone graft substitutes; in vitro testing; macro-porous scaffolds; physico-chemical characterization; piezoceramics; robocasting.

Figure 1

XRD diagrams of sintered ceramics: (a) BT; (b) Zr:BT; (c) BCTZ50; (d) LNO; (e,f) KNO; and (g,h) LTO, sintered conventionally (conv.) (a–e,g) or by (f,h) SPS. The ICDD reference diffraction lines of each targeted phase are presented (in orange colour) alongside the corresponding sample XRD pattern.

Figure 2

FWHM values of the most intense diffraction peak of each ceramic sample, sintered either conventionally (conv.) (solid symbols) or by SPS (hollow symbols).

Figure 3

(a) Density values of the sintered piezoceramic materials. FE-SEM morphology of the conventionally sintered (b) BT; (c) Zr:BT; (d) BCTZ50; (e) LNO; and (f) LTO and (g) spark plasma sintered LTO ceramic disks. Insets: FE-SEM images collected at: lower magnification in the case of (b) BT, to enable the visualization of the general morphology of the sintered disks, and at higher magnification for the finer grained materials—(d) BCTZ50; and both (f) conventionally and (g) spark plasma sintered LTO, to disclose the morphology and size of the constituting grains.

Figure 4

(a,b) Dielectric constant of the: (a) BT, Zr:BT, BCTZ50 and (b) LNO and LTO sintered ceramics. (c) Dielectric losses (dissipation factors) of the BT, Zr:BT, BCTZ50, LNO and LTO ceramics.

Figure 5

The (a) total and (b) remnant polarization vs. electric field of BT, Zr:BT and BCTZ50. (c) Total polarization vs. electric field in the case of LNO.

Figure 6

(a) Longitudinal piezoelectric constant d₃₃; (b) planar coupling factor k_p; and (c) mechanical quality factor Q_mp of the BT, Zr:BT-Zr and BCTZ50 sintered ceramics.

Figure 7

(a) Hardness; (b) modulus of elasticity; and (c) H³/E² ratio (“plastic index”) determined for the BT, Zr:BT and BCTZ50 sintered ceramics based on indentation tests.

Figure 8

pH value of the DMEM/F12-FBS cell culture medium after 36 h of incubation in the presence of BT, Zr:BT, BCTZ50, LNO and LTO sintered ceramics.

Figure 9

(a) hFOB 1.19 cell proliferation of the sintered ceramics, as assessed by an MTS assay performed after 36 h of culturing; (b) cytotoxicity of the investigated piezoceramics, as inferred by an LDH test after 36 h of culturing; (c–g) morphology of hFOB 1.19 cells after 36 h of culturing on the surface of the (c) BT; (d) Zr:BT; (e) BCTZ50; (f) LNO; and (g) LTO disks, as evidenced by epifluorescence microscopy. The actin cytoskeleton is stained with red (Alexa Fluor™ 546 phalloidin), whilst the cell nuclei are counterstained with blue (DAPI).

Figure 10

(a) Batch of sintered BCTZ50 scaffolds printed by robocasting. (b) General view of the filament arrangement and size, as evidenced by a low magnification FE-SEM image of a BCTZ50 scaffold fractured along its height. (c) Micro-structure of the sintered BCTZ50 filaments, as revealed by high-magnification FE-SEM analysis.

Figure 11

(a–c) hFOB 1.19 cell proliferation values recorded for the BCTZ50 scaffolds, determined by (a) MTS, (b) LDH and (c) Acridine orange (AO) tests after 14 days of culturing. (d) Cytotoxicity of the BCTZ50 scaffolds evaluated by an LDH test after 14 days of cell culturing. (e–j) Effective colonization (after 14 days) of the BCTZ50 scaffolds with hFOB 1.19 cells evidenced by (e–g) epi-fluorescence microscopy images focused on the first three printed layers and by (h) FE-SEM analysis. (e–g) The actin cytoskeleton is stained with red (Alexa Fluor™ 546 phalloidin, Thermo Fisher Scientific, Waltham, MA, USA), whilst the cell nuclei are counterstained with blue (DAPI). (h) Several cell bundles are indicated on the FE-SEM image with red arrows. (i,j) Morphology of hFOB 1.19 cells grown for 14 days on the surface of the filaments constituting the BCTZ50 macro-porous scaffolds, revealed by FE-SEM investigations.