1. Introduction
It has been reported previously that the hydrated calcium phosphate structure known as hydroxyapatite (HA) is considered to be biocompatible with human hard tissues and displays Osseo conductive characteristics [1] [2]. Accordingly, HA can be assumed as superior material for clinical and medical applications. However, HA is well known to have reduced mechanical strength in comparison with natural bone. This drawback is assumed to be the most serious obstacle for some specific claims, particularly for load-bearing implants [3] [4]. For this reason, bioactive glass-ceramics are considered as alternatives to the pure HA to be used as fillers and bone graft [5]. This may due to their enhanced hardness and mechanical strength which lead to their good ability to form strong bonds with living bone [6]. Besides, the apatite of such bioglass-ceramics has to be characterized by its good crystalline structure. The enhanced crystallinity leads to mechanical strength and biocompatibility stronger than that of the pure HA. Consequently, load bearing applications appear to need a perfect mix of bioactivity and preferred mechanical characteristics of crystalline apatite phases to be used.
Recently, developing new biomaterials was focused on silica free glasses and glass-ceramics [7] [8] [9] resulting from the ease crystallization of the apatite phases through stimulating suitable environments including doping with activating agents such as CeO2, Nd2O3, CrF2, etc. [10] [11] [12]. Besides, the precipitation of crystalline apatite phases within the as-prepared glass network was expected to be enhanced [10]. It was reported that amorphous glasses were considered to have a limited bioactivity in comparison to that of natural bones. But the crystallized glass-ceramics can simply possess the biocompatibility and tight bonding to the bone subsequent in the growth of healthy tissues to their surfaces [11].
Mechanical properties were observed to be enhanced using different amalgamations among apatite and various glass phases. Amongst them, apatite Ca5(PO4)3, wollastonite (CaSiO3) or CaTeO3 have been suggested to be used as clinically bone-repairing materials [13] [14] [15]. Besides, flour—apatite (Ca5(PO4)3 F—is conveyed as a customary type which is can be considered as a promising material in biodental applications including dental roots, and joint prostheses.
Recent researches have been focused on silicate free glasses as bioactive materials. Studies on borate and mixed former borophosphates, to our knowledge, are growing field of studies reported by several authors [7] [8] [9]. Therefore, trials will be done in this work to explore the bioactive properties and structure of borate glass-ceramics as a new biomaterial. Moreover, borate glasses containing CeO2 (as an agent for crystallization) will be studied. The presence of CeO2 in the network of borate network will enhance the bioactivity of the material since CeO2 is reported to be used as a nucleating and crystallizing agent of apatite and wollastonite phases.
2. Experimental Methods
Glasses of basic composition xCeO2∙(45 − x)B2O3∙24.5Na2O∙24.5CaO∙6P2O5 were synthesized using ordinary melt annealing technique. Pure analytical grade chemical of ceric oxide supplied by Sigma Aldrich Co., orthoboric acid, ammonium dihydrogen phosphate, sodium and calcium were used in their carbonate form supplied by ElNasr pharmaceuticals. Glass nomination and composition was listed in Table 1.
X-ray diffractogram of the studied samples is recorded using PAN analytical X-Pert PRO machine operated with 30 kV utilizing copper target with λkα = 1.540 Å within Braggs angles extended from 4˚ to 70˚. Surface and bulky morphologies of studied samples were recorded using (JEOL, JSM-5400) scanning and transmission electron microscopy (SEM. TEM) supported with EDAX and ED units, respectively. Studied samples were coated with a thin layer of gold. The Vickers microhardness numbers (HV) was recorded as an average of 15 indentation in a triplicate sample measured in a polished surface using FM-7 micro hardness tester at 25-g force load to ensure homogeneity and reproducibility of measurements.
31P MAS NMR experiments were also conducted at resonance frequency (202.4 MHz) using a 3.2 mm diameter rotor spinning at 15 kHz. Solid NH4H2PO4 was used as a secondary reference compound and the signal from this set to 0.8 ppm. A pulse length of 2.5 μs and a recycle delay of 5 s was applied.
3. Results Discussion
Figure 1 and Figure 2 show the XRD spectra of the studied glasses as a function of CeO2 concentration. The XRD spectra of glasses which contain low CeO2 concentration (0, 0.5 and 0.8) Figure 1 possess a wide hump that supports the amorphous structure of these glasses. On the other hand, glasses of higher CeO2 concentrations (Figure 2) exhibit intense sharp diffraction peaks superimposed on a widespread diffraction hump. The sharpness of several peaks may is due to the presence of different phases enriched with fine crystals in the investigated samples.
Positions of indicated sharp diffraction peaks on XRD diffractogram shown in Figure 2 is compatible with that reported for cerium sodium borate and calcium
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Table 1. Glass abbreviation and composition.
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Figure 1. XRD spectra of glasses containing 0, 0.5 and 0.8 mol% CeO2.
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Figure 2. X-ray diffraction (XRD) patterns of borate glasses containing different CeO2 concentrations.
phosphate phases [16]. These phases were classified as Ca2CeB2O7 and Na4CeB2O7, in combination with various calcium phosphate phases as, Ca2(P2O7) and CaCe(PO3). These phases were considered to be amongst the most bioactive and biocompatible phases in the matrix of the borophosphate glass-ceramics network [17].
The crystalline phases presented by CeO2 rich borate glasses can precipitate apatite units with Ca/P ratio near to unity [18] [19]. This leads that adequate degree of crystallinity is enhanced by the effect of more CeO2 addition as can be seen from Figure 3.
Results based on TEM and EDP, (Figure 4) agree well with that obtained from XRD. Both would confirm the amorphous nature of glasses containing low concentrations of CeO2. Figure 4(a) is introduced as an example (0.8 mol% CeO2). There are no resolved electron diffraction patterns due to the amorphous nature of glassy samples while no ordered structure can be detected. Then it can be concluded that the ceramics with CeO2 < 1 mol% show a common microstructure that contains nonuniform distributed species. On the other hand, TEM micrographs of samples contain higher CeO2 concentration indicates that spheroids
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Figure 3. Crystallinity changes as a function of cerium content.
morphology as the most dominant morphology. Such spheroid crystals can be observed to cover utmost of the studied area of the sample under investigation. The growing of spherical crystals (Figures 4(b)-(d)) is the main feature of the cerium ions which can be distributed in any glass composition. Simply, on increasing CeO2 contents the microstructure is appeared to contain clearer distributed species with less grain boundaries and less porosities (Figure 4(b)). However, larger amounts of CeO2 result in the complexation of microstructure. For the ceramics with CeO2 = 8, 15 and 20 mol%, the microstructure becomes crowded with some anisometric layers containing some polycrystalline species. The existence of these interconnected layers, can enhance the mechanical strength, since the hardness-number Figure 5 is found to increase with increasing CeO2 concentrations. This is attributed to the increase of CeO4 concentration which leads to enhancing the phase formation caused by higher content of CeO4. The presence of the polycrystalline phases enriched with CeO4 enhances the hardness number of the glass and decreases the crack length due to indentation processes (Figure 5) and reduces the grain growth.
It can be shown from TEM micrographs that the capacity of precipitated crystal increases with increasing CeO2 concentration. For instance, accumulation of the precipitated crystals is increased and distributed in multilayers which causes a darkness of the bulk structure of the investigated sample. The high capacity of the crystalline species results in the interconnections between the spheres distributed in several layers. The EDP of cerium rich phase, Figure 4, clearly reveals the sharpest diffraction rings confirmed the highest crystallinity of composition contains 20 mol%.
The surface morphology of two selected samples is represented by scanning electron micrographs (SEM) Figure 6. It can be shown that there is a great difference between the morphologies presented for sample of low CeO2 (0.8 mol%) and of the highest CeO2 concentration (20 mol%). The scanned surface of sample of 20 mol% CeO2 contains a crystalline species of larger size than that presented for sample of 0.8 mol% CeO2. The capacity of the accumulated phases is considered as the main reason for appearance of EDS peak with higher intensity than that of 0.8 glass. In addition, EDEX spectra a composition of 20 mol%
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Figure 4. Transmission electron micrographs supported with electron diffraction (TEM/ED) of some selected samples containing variable amounts of cerium oxide. (a) 0.8 mol% CeO2; (b) 2.0 mol% CeO2; (c) 8.0 mol% CeO2; (d) 15 mol% CeO2; (e) 20 mol% CeO2.
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Figure 5. Vicker’s hardness (HV) number as a function of cerium content.
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Figure 6. SEM/EDAX micrographs of selected samples containing 0.8 and 20 mol% CeO2.
CeO2 contains duple peak for Ce2+ ions which confirm more accumulation of crystalline borate species involving Ce ions.
31P MAS NMR experiments offer direct information about the Qn units in the phosphate network of the investigated glasses. (Q) is P ions and (n) is the number of bridging bonds. The Q0 species have been found in the main glass network. It is shown in Figure 7 that the chemical shift value of glasses containing 0 and 2 mol% CeO2 is in the order of Q0 species (9 ppm) which is the dominant structural species. The frequency peaks of glasses of 0 and 2 mol% CeO2 compositions are considered to corresponding to orthophosphate Na3PO4 or Ca3(PO4)2 or mixing between them. Adding of more CeO2 has some effect on the 31P-NMR spectra, since the value of chemical shift is relatively changed in composition of 12 and 20 mol% CeO2 glasses (around 2 ppm for composition 20 mol% CeO2), Figure 7. These changes may be considered due to a mixture of cerium sodium
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Figure 7. 31P NMR spectra of borate glass Ce free and of glasses containing different CeO2 concentrations.
or calcium phosphate crystalline phases. The decrease of chemical shift from 9 to 2 ppm means that some of cerium cations are coordinated with PO4 groups forming cerium phosphate phases [20] [21].
There is a second remark on 31P NMR spectra of glass containing 12 and 20 mol CeO2. The spectra are relatively broader than that of glasses containing 0 and 2 mol%. This may be considered due to formation of some ordered apatite phases containing Ce ions as a charge compensator, since Ce-O-P bond is longer than that of P-ONa+.
4. Conclusion
From all above spectra, it can be concluded that the small change in NMR chemical shift of phosphate network (9 to 2 ppm) upon CeO2 addition should be due to some modification of phosphate network by CeO2. This means that few of CeO2 are consumed to modify the phosphate network. But most of CeO2 concentrations are highly consumed to modify B2O3 network resulting in creation high reduction in the fraction of boron tetrahedral units.