Mechanical Biocompatibility, Osteogenic Activity, and Antibacterial Efficacy of Calcium Silicate-Zirconia Biocomposites

Abstract

Zirconia ceramics with high mechanical properties have been used as a load-bearing implant in the dental and orthopedic surgery. However, poor bone bonding properties and high elastic modulus remain a challenge. Calcium silicate (CaSi)-based ceramic can foster osteoblast adhesion, growth, and differentiation and facilitate bone ingrowth. This study was to prepare CaSi-ZrO₂ composites and evaluate their mechanical properties, long-term stability, in vitro osteogenic activity, and antibacterial ability. The Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) bacteria and human mesenchymal stem cells (hMSCs) were used to evaluate the antibacterial and osteogenic activities of implants in vitro, respectively. Results indicated that the three-point bending strength of ZrO₂ was 486 MPa and Young's modulus was 128 GPa, which were much higher than those of the cortical bone. In contrast, the bending strength and modulus of 20% (201 MPa and 48 GPa, respectively) and 30% CaSi (126 MPa and 20 GPa, respectively) composites were close to the reported strength and modulus of the cortical bone. As expected, higher CaSi content implants significantly enhanced cell growth, differentiation, and mineralization of hMSCs. It is interesting to note the induction ability of CaSi in osteogenic differentiation of hMSCs even when cultured in the absence of an osteogenic differentiation medium. The composite with the higher CaSi contents exhibited the greater bacteriostatic effect against E. coli and S. aureus. In conclusion, the addition of 20 wt % CaSi can effectively improve the mechanical biocompatibility, osteogenesis, and antibacterial activity of ZrO₂ ceramics, which may be a potential choice for load-bearing applications.

Figure 1

SEM micrographs of (a) ZrO₂ and (b) CaSi raw powders used in this study.

Figure 2

XRD patterns of the CaSi powder and various ZrO₂-CaSi composites. Notably, ZCS30 had the lower peak intensity than the other samples (triangle down solid: ZrO₂; box solid: β-Ca₂SiO₄; and circle solid: CaZrO₃ or Ca₃ZrSi₂O₉).

Figure 3

Fatigue curves of various samples under applied three-point bending stress versus the number of cycles to failure.

Figure 4

XRD patterns of various ZrO₂ samples containing (a) 0, (b) 10, (c) 20, and (d) 30 wt % CaSi before and after soaking in an SBF solution with pH 5 for predetermined time durations.

Figure 5

Surface SEM images of various ZrO₂-based materials before and after soaking in an SBF solution with pH 5 for 1, 3, and 6 months. The scale bars are 300 nm.

Figure 6

EDS profiles of various ZrO₂-based materials before and after soaking in SBF (pH 5.0) for 6 months. The changes in the (Ca + Si)/(Ca + Si + Zr) ratio by weight of various samples after soaking in SBF.

Figure 7

(a) Biaxial flexural strength and (b) hardness of various ZrO₂ samples containing different CaSi contents before and after soaking in an SBF solution with pH 5.0 for predetermined time durations. Asterisk represents a statistically significant difference (p < 0.05) from the corresponding as-prepared samples.

Figure 8

(a) Porosity and (b) weight loss of various ZrO₂ samples containing different CaSi contents after soaking in an SBF solution with pH 5.0 for predetermined time durations.

Figure 9

Cell growth of hMSCs cultured on the surfaces of various samples after various time points. Asterisk represents a statistically significant difference (p < 0.05) from the ZCS0 control.

Figure 10

ALP activity of hMSCs on the surfaces of various samples after 7 and 14 days when cells were cultured in the growth medium (a) with and (b) without the differentiation induction agents. Asterisk represents a statistically significant difference (p < 0.05) from the ZCS0 control.

Figure 11

Photographs of hMSCs and the quantitative assay of the calcium deposit after staining with alizarin red S on the surfaces of various samples when cultured in the growth medium (a) with and (b) without the differentiation induction agents for 14 and 21 days. Asterisk represents a statistically significant difference (p < 0.05) from the ZCS0 control.

Figure 12

Viability of (a) E. coli and (b) S. aureus bacterial species cultured on the various sample surfaces for different time points. Viability is normalized to the control without materials. Asterisk represents a statistically significant difference (p < 0.05) from the ZCS0 control.

Figure 13

SEM micrographs of E. coli bacterial adhesion on the surfaces of (a) ZCS0 control, (b) ZCS10, (c) ZCS20, and (d) ZCS30 after 24 h of culture.

Figure 14

SEM micrographs of S. aureus bacterial adhesion on the surfaces of (a) ZCS0 control, (b) ZCS10, (c) ZCS20, and (d) ZCS30 after 24 h of culture.

Figure 15

Schematic diagram showing the CaSi effect on characteristics of ZrO₂.