Nano-SiC reinforced Zn biocomposites prepared via laser melting: Microstructure, mechanical properties and biodegradability

1. Introduction

Recently, biodegradable metals have been extensively investigated and developed as potential bone biomaterials. Among them, Zn and Zn-based materials have drawn increasing interest on account of their desirable degradation behavior [[1], [2], [3]]. Some studies have proved that pure Zn possessed relatively suitable degradation rate compared with Mg and Fe, as well as good biocompatibility in vivo [[4], [5], [6], [7]]. As is well-known, Zn is an essential micronutrient in the human body and plays a vital role in nucleic acid metabolism, gene expression, cell signaling and apoptosis [8]. More importantly, Zn can stimulate osteoblast differentiation and mineralized tissue formation [9]. It is also involved in inhibiting osteoclastic bone resorption and preserving the bone mass in the human body [10]. Thus, biodegradable Zn and Zn-based materials are excellent candidates for bone repair. However, the compressive strength of pure Zn (20-50 MPa) is poor compared with the human cortical bone (110-180 MPa) [11,12], which has become the main obstacle in the application of bone repair.

Metal matrix nanocomposites are promising approach to tailor the performance of matrix. Karimzadeh et al. [13] fabricated Zn-Al₂O₃ nanocomposite and found that Al₂O₃ nanoparticles enhanced the hardness and the wear resistance of Zn. However, the Al element in Al₂O₃ might cause health issues during long-term implantation such as senile dementia [14]. Additionally, Yang et al. [15] prepared Zn-hydroxyapatite (HA) nanocomposites using spark plasma sintering and found that HA could adjust the degradation rate and improve the biocompatibility of the nanocomposites, but immolated the compressive strength. Thus, it is necessary to seek an appropriate reinforcer for Zn with both excellent mechanical properties and favorable biocompatibility.

Nano-silicon carbide (SiC) has been widely used in many fields of engineering as reinforcer due to its excellent mechanical performance, including high strength, hardness and modulus, as well as high thermal stability and low thermal expansion coefficient [16,17]. What’s more, SiC is a kind of bioceramics with good biocompatibility and Si is also a crucial element for bone growth and development [18]. Many studies have reported that SiC was comparable to HA in regard to osteogenic properties [19,20]. Reddy et al. [21] incorporated nano-SiC in Al matrix and significantly enhanced the compressive strength, tensile strength and hardness by 80%, 51% and 128%, respectively. Zhang et al. [22] illustrated the enhancement of mechanical properties in AZ91D Mg alloy due to the grain refinement and uniform distribution of SiC nanoparticles. Thus, nano-SiC might be an eligible mechanical reinforcer of Zn for orthopedic implant applications.

In this work, nano-SiC reinforced Zn biocomposites were prepared via laser melting, aiming to improve the mechanical properties. The effects of SiC contents (0, 0.5, 1, 2 and 3 wt%) on the microstructure and mechanical properties of Zn-xSiC biocomposites were systematically investigated. The corresponding reinforcing mechanisms were discussed in detail. Furthermore, the degradability and biocompatibility of the biocomposites were also studied to evaluate their feasibility as orthopedic implants (Table 1).

2. Experimental procedures

2.1. Material preparation

In this study, commercial Zn powder with a size range from 15 to 20 μm, and nano-SiC powder had an average size of 50 nm were used as original materials. Both powders have a purity of 99.9% and were supplied by Shanghai Naiou Nanotechnology Co. Ltd., China. In order to prepare nano-SiC to reinforce Zn biocomposites, different amounts of nano-SiC powder (0, 0.5, 1, 2 and 3 wt%) were mixed with pure Zn powder by using mechanical milling in a DECO-PBM-V-0.4 L planetary ball mill (DECODK Co. Ltd., China) for 2 h at 250 rpm to get a homogeneous particle distribution.

Subsequently, nano-SiC reinforced Zn biocomposites (denoted as Zn-xSiC, where x = 0, 0.5, 1, 2 and 3 wt%, respectively) were fabricated on a homemade laser melting system. The processing parameters were shown as follows: laser power was 80 W, scanning speed was 12 mm/s, laser spot diameter was 0.15 mm and hatch spacing was 0.12 mm. The samples were fabricated layer-by-layer under argon gas protection to avoid oxidation during the laser melting process.

2.2. Microstructure characterization

The morphologies of the Zn powder, SiC powder and representative Zn-2SiC mixed powder were observed using scanning electron microscopy (SEM, Phenomprox, Phenom-World BV, Netherlands) and energy dispersion spectrometry (EDS). The particle size of Zn powder was determined via a Malvern instrument (Mastersizer 3000, Malvern Instruments Ltd., UK). The optical microstructures of the Zn-xSiC nanocomposites were examined through light microscopy (DM2500, Leica, Germany). For this test, the samples were grinded using abrasive papers (P800-P4000), polished by using diamond pastes with 1.5 and 0.5 μm particles and etched in a 1 ml HNO₃ + 49 ml H₂O solution. The microstructures and the elemental compositions of the Zn-xSiC nanocomposites were examined using SEM and EDS. The fracture surfaces were also visualized through SEM to analyze the fracture behavior. The phase identification of the original powder, the mixed powders and the Zn-xSiC nanocomposites were carried out by using X-ray diffraction (XRD, Bruker AXS Inc., German) over a 2θ range of 20°-90° at a scan rate of 2°/min.

2.3. Mechanical properties

The mechanical properties of the Zn-xSiC nanocomposites were investigated using hardness tests and compression tests. Vickers micro-hardness tests were conducted by using a hardness tester (Shanghai Taiming Optical Instrument Co. Ltd., China) under a 10 N load for a time of 15 s. Five indentations were made at each sample randomly. In accordance with ASTM-E9-09, compression tests were conducted at room temperature on a universal testing machine (ZLC-50 M, Jinan Zhongluchang Instruments Co. Ltd., China) at a deformation rate of 0.2 mm/min. Afterwards, the compressive yield strength (CYS) was calculated based on the stress-strain curves. And the standard error was calculated from ten measurements per group.

2.4. Electrochemical tests

The electrochemical tests were carried out in simulated body fluid (SBF 8.036 g/L NaCl, 0.354 g/L NaHCO₃, 0.311 g/L MgCl₂·6H₂O, 0.292 g/L CaCl₂, 0.231 g/L K₂HPO₄·3H₂O and 0.225 g/L KCl, pH 7.4 [23]) with a Multi Autolab M204 workstation (Metrohm, Switzerland). In the tests, a three-electrode cell was used with the Zn-xSiC samples as the working electrode, a platinum counter electrode and a saturated calomel electrode as reference electrodes. The open circuit potentials (OCP) of the samples were monitored for 5400 s in SBF to obtain stable. The potentiodynamic polarization experiments were conducted at a scan rate of 1 mV/s and the polarization curves were recorded and fitted. Subsequently, the corrosion potential (E_corr) and corrosion current density (i_corr) were analyzed by means of linear fit according to Tafel extrapolation, and corrosion rates were calculated according to Ref. [24].

2.5. Immersion tests

The long-term corrosion behavior of Zn-xSiC samples were evaluated using immersion tests including corroded surface morphology and weight loss. According to the procedures outlined in ASTM G31-72, the Zn-xSiC samples were polished and then immersed in SBF with the ratio of volume-to-surface area of 30 mL/cm² at 37 °C [25]. After 21 d immersion, the samples for corroded morphology observation were taken out from SBF, gently washed, dried and then characterized by SEM coupled with EDS. The weight loss tests were also conducted to the further evaluation of the corrosion rate. Another group samples were removed from SBF after 50 days’ immersion. The corrosion products on the sample surface were removed using a solution containing 200 g/l CrO₃ and dried. The corrosion rate by weight loss is determined as follows:

C=KW/(Dat) (1)

where C represents the corrosion rate (mm/year), K represents a conversion coefficient 8.76 × 10⁴, W represents the weight loss (g), D represents the density of the composite (g/cm³), A represents the initial sample surface area (cm²) and t represents the immersing time (h).

2.6. In vitro cytocompatibility

Osteoblast-like MG-63 cells (American Type Culture Collection, Manassas, USA) were cultured in a humidified atmosphere (5% CO₂ and 37 °C) in Dulbecco’s modified eagle medium (DMEM, Gibco BRL, USA), which also contained 10% fetal bovine serum, 100 U/mL penicillin and 100 mg/mL streptomycin. The cytocompatibility of Zn-xSiC nanocomposites was evaluated by means of indirect contact method. All samples were firstly sterilised under high temperature and high pressure, and then immersed in DMEM for 72 h (sample surface area/solution volume = 1.25 cm²/mL) based on ISO 10993-5:1999 [26]. Afterwards, the soaking solution were withdrawn, centrifuged and and then stored at 4 °C.

For fluorescent staining assay, osteoblast-like MG-63 cells were cultured in a 24-well tissue culture plate with DMEM for 4 h at 1 × 10⁵ cells/mL. Afterwards, the DMEM were removed and replaced by 100% extracts of each sample. After being cultured for 24 h and 72 h, the cells were washed twice using phosphate buffered solution and incubated with Calcein-AM and Ethidium homodimer-1 reagents. Then, the cells were observed using a BX60 fluorescence microscopy (Olympus, Japan).

For CCK-8 tests, osteoblast-like MG-63 cells were cultured in a 96-well culture plate for 24 h (2000 cells/well). Thereafter, the DMEM was replaced by the 100% and 50% extracts of each sample, respectively. After 24 and 72 h, 10 μl CCK-8 solution (Dojindo, Japan) was added and continued culturing for 4 h. Then, the spectrophotometric absorbance was determined at a wavelength of 450 nm by using an Infinite M200Pro microreader (Tecan, Austria). The number of living cells calculated by the obtained optical density (O.D.) values, and a DMEM group without extracts was used as control.

2.7. Statistical analysis

All of the experiments data were presented in terms of the mean ± standard deviations (SD). Statistical differences between groups were analyzed by Student's t-test and the results were considered statistically significant when p < 0.05.

3. Results and discussion

3.1. Microstructure

The powder morphologies of pure Zn, nano-SiC and Zn-2 wt% SiC mixed powders were observed using SEM and are shown in Fig. 1. It could be observed that nano-SiC powder (Fig. 1(a)) showed a fluffy morphology with a large surface-to-volume ratio, which could provide more contact area with matrix. The EDS spectrum of nano-SiC powder (Fig. 1(b)) indicated strong peaks of C and Si. The XRD pattern (Fig. 1(c)) demonstrated that the characteristic peaks of the nano-SiC powers were in accord with the standard peaks of SiC. In comparison, Zn particles showed a regular spherical shape with a smooth surface (Fig. 1(d)). The particle size of Zn powder was analyzed using Malvern laser particle size meter and the average size was 15 μm (Fig. 1(e)), which was similar to the size obtained from the SEM appearance. Fig. 1(f) exhibits the XRD pattern of Zn powder that only presented the high intensity peaks corresponding to Zn without other phases. The XRD results confirmed the high purity of both Zn and nano-SiC powders. The typical morphology of Zn-2SiC mixed powders is shown in Fig. 1(g), in which the Zn particles did not show obvious deformation and nano-SiC particles distributed homogeneously on the surface of Zn particles. This was further demonstrated by EDS analysis (Fig. 1(h)). The XRD pattern of mixed Zn-2SiC powder was shown in Fig. 1(i). The results showed only the strong peaks for Zn while the peaks of SiC were absence in the mixed powder, which might be due to the low amount of nano-SiC that could not be captured by XRD.

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Fig. 1.   (a) SEM micrograph, (b) EDS analysis and (c) XRD pattern of nano-SiC powder; (d) SEM micrograph, (e) particle size distribution and (f) XRD pattern of Zn powder; (g) typical micrograph of Zn-2SiC mixed powder after 2 h mechanical milling; (h) corresponding EDS map of Si element distribution (red), (i) XRD pattern of the mixed powder.

The optical microstructures of Zn-xSiC nanocomposites prepared via laser melting are demonstrated in Fig. 2. The grain shape of pure Zn (Fig. 2(a)) was irregular columnar crystal, and the average grain size was approximately 250 ± 9.7 μm. In contrast to pure Zn, the grain size of Zn-xSiC nanocomposites significantly decreased to 90 ± 2.5 μm, 50 ± 3.8 μm, and 15 ± 1.7 μm on average for the Zn-0.5SiC, Zn-1SiC, Zn-2SiC nanocomposites, respectively (Fig. 2(b)-(d)). It was worth noting that Zn-2SiC nanocomposite exhibited almost equiaxed grain structure, which was conducive to the mechanical properties [27]. With further increasing nano-SiC content to 3 wt% (Fig. 2(e)), the grain size distribution became nonuniform, which might be due to inhomogeneous distribution or agglomeration of excessive nano-SiC. On the one hand, during metal solidification, the undissolved nano-SiC particles promoted heterogeneous nucleation and hindered the rapid grain growth of Zn, thus showing a refinement effect in Zn-xSiC nanocomposites [28] On the other hand, the rapid solidification of laser melting may inhibit the grain growth and the separation of components, which favors the formation of small grain [29].

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Fig. 2.   Optical microstructures of (a) Zn, (b) Zn-0.5SiC, (c) Zn-1SiC, (d) Zn-2SiC, (e) Zn-3SiC nanocomposites and (f) computed values of average grain size.

The microsurface morphology of the nanocomposites are showed in Fig. 3. Compared with pure Zn (Fig. 3(a)), the Zn-xSiC nanocomposites showed some black spots which were marked by red circles (Fig. 3(b) and (c)). EDS analysis were carried out on point A and point B in the Zn-2SiC nanocomposite (Fig. 3(d)) to determine the element composition. According to the results of EDS, the main elements at point A (Fig. 3(h)) were Zn and that on point B (Fig. 3(i)) were Si and C, indicating the black spots as nano-SiC. In addition, the elemental map analysis of Si (Fig. 3(g)) revealed that nano-SiC particles homogeneously distributed within the Zn-2SiC nanocomposites. With the increasing content of nano-SiC, it was observed that small agglomerations were formed in Zn-3SiC nanocomposite as designated in Fig. 3(e). An EDS scan was conducted at line C, which revealed that the content of Zn element gradually decreased while that of Si increased along the scanning-path of line C, confirming that the aggregation of nano-SiC.

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Fig. 3.   SEM micrographs of Zn-xSiC nanocomposites fabricated via laser melting for (a) pure Zn, (b) Zn-0.5SiC, (c) Zn-1SiC, (d) Zn-2SiC, (e) Zn-3SiC; corresponding EDS analysis of (h) point A, (i) point B, and (f) line C; (g) EDS mapping of Si element distribution in Zn-2SiC (red).

The phase compositions of Zn-xSiC nanocomposites were identified by XRD and present in Fig. 4. The diffraction peaks of pure Zn were obviously visible with hexagonal close packed crystal structure. After the incorporation of nano-SiC, the relative intensities for peaks corresponding to the (101), (110), (004) and (201) crystal planes of Zn slightly changed, which could be the reason for the difference in grain shape in the optical microstructures. However, it was noted that no SiC peaks were detected in the XRD spectra of Zn-2SiC nanocomposites due to the low amount of nano-SiC. In addition, the results showed no presence of new phases, indicated no reactions between Zn and nano-SiC reinforcer. Similar results were also found in SiC reinforced Al-based composites [30].

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Fig. 4.   XRD spectra of laser melting processed pure Zn and Zn-2SiC nanocomposite.

3.2. Mechanical properties

Mechanical performance is an important factor for bone implants, because they need to provide structural support at the implant site [[31], [32], [33]]. The compressive properties of laser melting processed Zn-xSiC nanocomposites were studied at room temperature, as showed in Fig. 5. The CYS increased with increasing nano-SiC content from 22.4 ± 3.1 MPa for pure Zn to 121.8 ± 5.8 MPa for Zn-2SiC nanocomposite (Fig. 5(a)). A similar trend was also found in microhardness (Fig. 5(b)), in which pure Zn showed 41 ± 1.2 HV while that of Zn-2SiC nanocomposites reached 74.1 ± 3.0 HV. It was noted that the CYS was reduced with nano-SiC content up to 3 wt%. This might be due to the weakened strengthening effect by the aggregation of nano-SiC reinforce, because the distribution characteristics of reinforcer play a vital role in the mechanical properties of nanocomposites [34,35]. The results above indicated considerable strengthening and hardening effects of nano-SiC in Zn matrix. These improvements were mainly attributed to the grain refinement and the uniform distribution of nano-SiC reinforcer in the matrix. On the one hand, grain refinement enhanced the grain boundary density and thus promoted the resistance to dislocation movement under load, leading to mechanical enhancement [36,37]. On the other hand, the nano-SiC particles distributed along Zn grain boundaries could be served as hard second phase, which allowed the effective stress transfer from the matrix to nano-SiC, thereby improving the strength and hardness. In addition, the vastly different thermal expansion coefficients of nano-SiC and Zn could form internal thermal stress and further strengthen the Zn matrix.

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Fig. 5.   Mechanical properties of Zn-xSiC nanocomposites: (a) CYS; (b) microhardness (*p < 0.05, **p < 0.01).

The SEM morphology of fracture surfaces of Zn-xSiC nanocomposites are illustrated in Fig. 6. It can be seen that pure Zn (Fig. 6(a)) showed many cleavage cracks and steps, which were taken as a typical pattern of brittle fracture. These cleavage cracks intersected with twist grain boundary led to the formation of “rivers”, which indicated the local propagation direction [38]. In comparison with pure Zn, the fracture surfaces of Zn-0.5SiC (Fig. 6(b)) and Zn-1SiC (Fig. 6(c)) nanocomposites appeared some shallow dimples and finer fracture morphology. This might be related to the grain refinement owing to the evenly distributed nano-SiC particles. Meanwhile, the tearing ridges were clearly visible, revealing that the fracture was gradually transformed from cleavage fracture into quasi-cleavage fracture [39]. For Zn-2SiC nanocomposite, many rough and small dimples can be observed on the fracture surface (Fig. 6(d)), suggesting a more ductile fracture. However, Zn-3SiC nanocomposite presented different fracture morphology with many flat brittle facets (Fig. 6(e)). EDS analysis at point A showed the presence of Zn, C, and Si elements (Fig. 6(f)), suggesting that nano-SiC particles presented in the crack walls. This might resulted in serve stress concentration and further the premature fracture of nano-SiC agglomerates [40].

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Fig. 6.   SEM morphologies of fracture surfaces of (a) pure Zn, (b) Zn-0.5SiC, (c) Zn-1SiC, (d) Zn-2SiC, (e) Zn-3SiC and (f) EDS analysis at point A. The typical fracture characteristics were indicated by white arrows.

3.3. Electrochemical characteristics

Potentiodynamic polarization measurement of Zn-xSiC nanocomposites was performed in SBF solution to investigate the microgalvanic efficiency, and the potentiodynamic polarization curves are illustrated in Fig. 7(a). None passivation behavior could be seen in the polarization curves for both samples with and without nano-SiC, indicating no formation of passive film on the sample surfaces. The I_corr and E_corr were determined from the polarization curves and are shown in Fig. 7(b). Compared with pure Zn (‒1.005 ± 0.020 V), the E_corr of Zn-SiC nanocomposites shifted negatively and ranged from ‒1.059 ± 0.025 V (Zn-0.5SiC) to ‒1.170 ± 0.018 V (Zn-3SiC). It can be observed that Zn-3SiC nanocomposite showed the highest I_corr of 9.706 ± 0.107 μA/cm² compared with pure Zn (7.641 ± 0.124 μA/cm²), Zn-0.5SiC (8.612 ± 0.089 μA/cm², Zn-1SiC (9.706 ± 0.107 μA/cm²) and Zn-2SiC (10.380 ± 0.114 μA/cm²). As shown in Fig. 7(c), the calculated corrosion rates were ranged from 0.114 ± 0.038 mm/year (pure Zn) to 0.230 ± 0.027 mm/year (Zn-3SiC) with increasing amount of nano-SiC. These results revealed the decreased corrosion resistance of Zn-xSiC nanocomposites with the increasing amount of nano-SiC.

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Fig. 7.   Potentiodynamic polarization curves of pure Zn and Zn-xSiC nanocomposites in SBF solution (*p < 0.05).

3.4. Degradation behavior

The degradation behavior of the Zn-xSiC nanocomposites was studied using immersion tests in SBF. The corrosion rates were estimated from the weight loss of samples and are shown in Fig. 8. The weight loss of pure Zn reached about 0.0167 mg/(cm² d), corresponding to a corrosion rate of 0.085 mm/year, closing to the value previously reported by Li et al. [41]. After the incorporation of nano-SiC, the corrosion rates of the biocomposites were increased to 0.096 mm/year for Zn-0.5SiC, 0.108 mm/year for Zn-1SiC, 0.115 mm/year for Zn-2SiC, and 0.134 mm/year for Zn-3SiC, respectively. The accelerate degradation was consistent with the electrochemical tests. This could be ascribed to the microstructure refinement which formed high boundary density between the Zn matrix and nano-SiC particles, providing more corrosion sites for corrosive fluid invasion into the Zn-xSiC nanocomposites. In addition, SiC, as a biocompatible semiconductor, is widely applied in biosensors which are characterized by high conductivity and low chemical activity under physiological condition [42]. Therefore, the nano-SiC located at the grain boundary might promote the charge transfer in Zn-xSiC nanocomposites, thereby accelerating corrosion. It is known that an ideal bone implant should progressively degrade at a suitable rate (approximately 0.2-0.5 mm year^-1) to match the bone healing process [43]. Although pure Zn possessed relatively suitable degradation rate compared with Mg and Fe, the degradation behavior needs to be further optimized with tunable degradation rates to match the bone healing rate [15,44,45]. In this study, the degradation rate of pure Zn was increased from 0.085 mm/year to 0.13 mm/year after the addition of SiC, which was more comparable to the bone healing rate. As reported in many studies [46,47], the corrosion rate in terms of mg/cm²/day represents the average daily weight loss and can be used to compare with the daily allowance to demonstrate the biocompatibility of biomaterials. In this study, the highest corrosion rate of Zn-xSiC nanocomposites corresponded to a weight loss of 0.0250 mg/(cm² d), i.e. the average daily weight loss during 50 d degradation, which was far below the daily allowance of Zn (15 mg/d) [48].

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Fig. 8.   Corrosion rates of Zn-xSiC nanocomposites calculated from weight loss after 50 d immersion in SBF (*p < 0.05).

For corrosive surface morphology, pure Zn and Zn-2SiC nanocomposite were selected as representative due to the optimal combination of strength and hardness of Zn-2SiC nanocomposite. The corrosive surface morphologies after immersed for 21 d are presented in Fig. 9. Some scratches were still visible on the corrosive surface of pure Zn (Fig. 9(a)), indicating that the corrosion layer was relatively thin. With the addition of nano-SiC, the corrosion products became denser with big spherical precipitates (Fig. 9(b)), which further confirmed the increased corrosion rates of Zn-xSiC nanocomposites. According to the EDS map analysis, the main elements of degradation products were Zn, O, C, Si, Ca and P. It thus could be speculated that the products mainly consisted of zinc hydroxide, calcium phosphates and calcium carbonates [49]. It is known that calcium phosphates are beneficial to the interface bonding between bone and biodegradable implant [50,51].

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Fig. 9.   Surface morphologies and EDS map analysis of laser melting processed Zn-xSiC nanocomposites after immersion for 21 d: (a) pure Zn; (b) Zn-2SiC nanocomposite.

3.5. Cytocompatibility

As degradable implant biomaterials, the safety and biocompatibility of Zn-xSiC nanocomposites should be considered. In the present study, osteoblast-like MG-63 cells were used to evaluate the cytocompatibility of Zn-xSiC nanocomposites by using fluorescence microscopy and CCK-8 assay.

Fluorescence images of osteoblast-like MG-63 cells cultured in 100% extracts for 1 d and 3 d are demonstrated in Fig. 10(a). It can be observed that there was no significant difference of cell morphology among Zn-xSiC nanocomposites for 1 d. After 3 d, the filopodia extending from the cells were clearly visible in the Zn-1SiC and Zn-2SiC nanocomposites compared with pure Zn. This was a typical characteristic of cellular growth and division, indicating normal cellular activity of osteoblast-like MG-63 cells [11,52]. Therefore, nano-SiC might have a positive influence in the interaction between osteoblast-like MG-63 cells and Zn.

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Fig. 10.   (a) Fluorescence images of osteoblast-like MG-63 cells cultured in 100% extracts, and (b) CCK-8 results in 100% and 50% extracts of Zn-xSiC nanocomposites for different period with the data normalized to the control group.

The viability of osteoblast-like MG-63 cells cultured in the extracts of Zn-xSiC nanocomposites for 1 d and 3 d were accessed by CCK-8 tests, as shown in Fig. 10(b). After 1 d incubation, there was little different in the number of cells in the 100% extracts, which ranged from 69% to 73%. As reported, a suitable dilution of extract is applicable for biodegradable metals to better simulate the vivo conditions [53]. Therefore, the extracts in this study were diluted to 50% and used for CCK-8 assay. The results showed that the cell activity of Zn-xSiC nanocomposites in 50% extracts were more than 75%, indicating good biocompatibility to osteoblast-like MG-63 cells. With the progress of incubation time, the nanocomposites exhibited favorable cell viability for both the 50% and 100% extracts. Moreover, there were no significant differences among Zn-xSiC nanocomposites. This might be ascribed to the biphasic roles of nano-SiC in the biocompatibility of Zn-xSiC nanocomposites: On the one hand, many previous studies had reported that SiC was a promising biomaterial with good biocompatibility and hydroxyapatite-like osseointegration [[54], [55], [56]]. Moreover, it was capable of promoting cell proliferation and osteogenic differentiation [57]. On the other hand, the addition of nano-SiC accelerated the degradation of Zn-xSiC nanocomposites, which might be harmful to cell proliferation due to the large amount of Zn ions release [58]. As a consequence, nano-SiC did not show significant influences on the viability of osteoblast-like MG-63 cells.

Overall, the in vitro tests in this study indicated the acceptable biocompatibility of Zn-xSiC nanocomposites. It should also be noted that SiC can hardly be degraded in the human body. Previous in vivo and in vitro studies have demonstrated that SiC can be absorbed through phagocytosis by macrophages, depending on the particle size. Nevertheless, the metabolic pathway of SiC is still a key and unsolved factor for the application of Zn-xSiC nanocomposites in bone repair. Therefore, further in vivo studies should be carried out to reveal both the metabolic pathways of nano-SiC and the degradation behavior of Zn-xSiC nanocomposites. Moreover, although the uniform distribution of nano-SiC in the nanocomposites was achieved after the laser melting process, the underlying mechanisms was not involved in this study. And the relationships between the laser melting parameters and the distribution of nano-SiC in the nanocomposites should be studied in the future work.

4. Conclusion

In this study, Zn-xSiC nanocomposites processed using laser melting were investigated for use of biodegradable metallic bone implants. The results showed that the incorporation of nano-SiC could greatly improve the mechanical properties of pure Zn. The CYS and hardness of Zn-2SiC nanocomposite were increased to 121.8 MPa and 74.1 HV, respectively. The enhancement was ascribed to grain refinement, Orowan strengthening and dispersion strengthening. Meanwhile, Zn-xSiC nanocomposites possessed higher degradation rates (ranging from 0.096 mm/year to 0.134 mm/year) in comparison with pure Zn (0.085 mm/year). Cell culture tests also showed good cytocompatibility of the Zn-xSiC nanocomposites as characterized by the normal morphology and viability of MG-63 cells. This study suggested that the nano-SiC reinforced Zn biocomposites may be promising candidates for bone implant applications.

Acknowledgements

This work was supported financially by the National Natural Science Foundation of China (Nos. 51705540, 81871494 and 81871498), the Hunan Provincial Natural Science Foundation of China (Nos. 2018JJ3671 and 2019JJ50588), the Guangdong Province Higher Vocational Colleges & Schools Pearl River Scholar Funded Scheme (2018), the Open Sharing Fund for the Large-scale Instruments and Equipments of Central South University, the Project of Hunan Provincial Science and Technology Plan (No. 2017RS3008), the Shenzhen Science and Technology Plan Project (No. JCYJ20170817112445033), the National Postdoctoral Program for Innovative Talents (No. BX201700291), the Hunan Science and Technology Innovation Plan (Nos. 2018SK2105 and kq1606001) and the China Postdoctoral Science Foundation (No. 2018M632983).