THE PRESENCE OF MgO, MnO, SrO AND ZnO IMPROVING

SURFACE TRANSFORMATIONS ON A STANDARD BIOGLASS

Gilderman S. L., Silmara C. S., Euler A. dos S.Laboratório de Biomateriais, P²CEM/Universidade Federal de sergipe, São Cristovão (SE), Brasil

E-mail: euler@ufs.br

Abstract. In this study, the synthesis of bioactive glass-based system (SiO2.CaO.Na2O.P2O5) and containing four different modifiers (SiO2.CaO.Na2O.P2O5.ZnO.MnO.MgO.SrO) were prepared using the method sol-gel. Tablets were produced from the powder obtained by uniaxial pressing and sintering at 700° C. the bioactivity was measured by immersion in McCoy's 5A modified medium under sterile conditions at 37° C for 1, 4 and 7 days. TGA / DTA, XRD, DRIFT, and SEM-EDS were used to characterize the changes and the bioglass surface after immersion. As expected, the introduction of modifiers induced a decrease in crystallization temperature in comparison with those measured for the standard bioglass. The XRD analysis confirmed the presence of an amorphous structure as crystalline peaks were not observed. Spectra showed bands DRIFT large vibration asymmetric and symmetric stretching of Si-O-Si (1090 and 800 cm -1, respectively). After soaking, globular precipitates appeared gradually coating the entire surface. EDS spectrum also showed a gradual increase of Ca and P on the surface and a lower Si, suggesting formation of a CaP-rich layer. This behavior was remarkably pronounced on the bioglass containing modifiers. Spectra showed absorption bands DRIFT additional around 609 cm-1 (v4 O-P-O) and 972 cm-1 (v1 P-O), indicating the formation of a layer of phosphate after immersion. The XRD, using low angle of incidence have shown no crystalline phase, which would suggest the formation of an amorphous calcium phosphate or nano crystals. Probably, the breaking of the network caused by the insertion of glass modifiers facilitated its dissolution and, consequently, the reprecipitation process.

Kenwoods: bioglass, apatite precipitation, sol-gel.

1. INTRODUCTION

The bioglass biocompatibility has been accessed by its ability in forming an apatite layer after immersion in biological fluids [KOKUBO and TAKADAMA, 2006]. However, it is known that not only apatite formation can remarkably affect the cellular metabolism but also the ionic dissolution products by changing the intracellular ionic concentrations [HOPPE, GÜLDAL and BOCCACCINI, 2011]. One way of controlling the dissolution and reprecipitation process is to insert elements called network modifiers into bioglass systems. In general, these modifiers have a strong ionic character and act disrupting the covalent network formed by SiO4 and PO4 tetrahedral units, increasing the reactivity of bioglass [ARCOS and VALLET-REGÍ, 2010]. It is known that some network modifiers are completely involved in osteogenesis and angiogenesis [HOPPE, GÜLDAL and BOCCACCINI, 2011]. In this sense, our objective in this work was to evaluate the effect of the insertion of ZnO, MnO, MgO and SrO in a standard bioglass (SiO2.CaO.Na2O.P2O5) on the dissolution/reprecipitation process after immersion in a biological fluid.

2. MATERIALS AND METHODS

A standard bioglass (SiO2.CaO.Na2O.P2O5) and a bioglass containing four different modifiers (SiO2.CaO.Na2O.P2O5.ZnO.MnO.MgO.SrO), called here of BV and BV4M, respectively, were prepared using the sol-gel method [SBOORI, RABIEE, et al., 2009]. In this sense, tetraethyl orthosilicate (TEOS) was hydrolyzed in nitric acid followed by the addition of triethylphosphate (TEP), under agitation. The other reagents were added consecutively, under continuous agitation, in amounts shown in table 1. The gel was dried for 10 days and then grounded and sieved. Tablets were produced from the obtained powder by uni-axial pressing and calcination at 700°C. The samples were immersed in McCoy's 5A modified medium under sterile conditions at 37°C for 1, 4 and 7 days. TGA/DTA, XRD, DRIFT and SEM-EDS were used to characterize the bioglass and the surface transformations after immersion.

Table 1 - Nominal composition of bioglass pattern synthesized by sol-gel.

PRECURSORSAmount in grams per each 150 mL

of HNO3 0,1 mol L-1

BV BV4M

(*)TEOS 66,646 66,646(**)TEP 4,549 4,549

Ca(NO3)2.4H2O 30,700 29,77

NaNO3 2,125 2,125

Zn(NO3)2 - 1,60 x10-3

Mn(NO3)2.xH2O - 7,85 x 10-6

Mg(NO3).6H2O - 0,911

Sr(NO3)2 - 1,71 x 10-2

3. RESULTS AND DISCUSSION

From TG/DTA curves was possible to observe three distinct regions of weight loss (Fig. 1 and 2). The first lost (stage 1) occurred around 60°C at 120oC and was followed by the appearance of endothermic peaks commonly attributed to the loss of residual water. The second one (stage 2) was observed around 270°C and corresponded to the chemical desorption of water, which was accompanied by exothermic peaks. The last one (stage 3) occurred around 520°C with a large endothermic peak generated by both volatilization of NO2, CO2 and reactions between remaining alkoxide groups. A small exothermic peak at 924°C and 889°C is observed for BV and BV4M, respectively, corresponding to the beginning of crystallization [MA, CHEN, et al., 2011; MA, CHEN, et al., 2010].

Figure 1 - TGA curves of the bioglass BV and BV4M after being dried at 120°C, heating rate of 5o min-1.

Figure 2 – DTA curves of the bioglass BV and BV4M after being dried at 120°C, heating rate of 5o min-1. Temperature of crystallization (*).

Figure 3 - XRD profiles of the bioglasses after sintering in four different temperatures.

Figure 4 - SEM images obtained from bioglasses surface after immersion in culture medium.

The decrease of approximately 35°C seen for the crystallization temperature is probably related to the presence of the network modifiers in the modified bioglass (BV4M)

[CHIANG, BIRNIEIII and KINGERY, 1997]. According to these results, a sintering temperature below 800° C would be able to produce a bioglass free of residual synthesis products and of crystalline phases. Indeed, XRD patterns obtained from samples sintered from 600 to 900° C (fig.3) have confirmed the absence of crystalline phases for the samples sintered above 800°C.

The SEM images for the bioglasses before and after immersion in McCoy’s 5A modified medium revealed a gradual deposition of spherical particles on the surfaces along time (Fig. 4). The precipitated features suggest the formation of apatite layer on the surfaces [NAYAK, KUMAR and BERA, 2010; AGATHOPOULOS, TULYAGANOVA, et al., 2006]. These precipitates entirely covered the surfaces after 7 days of immersion. This behavior was markably pronounced on the modified bioglass BV4M, fig.4.

The standard elements from the standard bioglass (Si, Ca, P and Na) were detected by EDS analysis (Fig.5 and 6). However, the network modifiers were not detected by EDS, except the Mg (Fig. 6). Probably, the very low quantities added in substitution to the Ca avoided the detection. The intensities of the Ca and P peaks (Fig. 5 and 6) increased as a function of immersion time, suggesting the formation of a CaP-rich layer onto bioglasses surface. At the same time, it was possible to observed a decrease of the Si peak, which lead us to confirm this gradual CaP precipitation on the surfaces. If we compare the Si peak decrease in both BV and BV4M along time, it is evident that the precipitation onto BV4M was faster than onto the BV. It is also observed that Mg signal become more pronounced along time of immersion indicated that CaP layer can easily incorporate this element from McCoy’s medium. DRIFT spectra are shown in Fig.7 and 8. The vibrational modes for Si-O-Si can be observed as a broad absorption band around 500 cm-1 and 1035cm-1 [SBOORI, RABIEE, et al., 2009; LUCAS-GIROT, MEZAHI, et al., 2011] for all spectra.

Figure 5 –EDS spectra obtained from BV surface before and after immersion in the McCoy’s 5A modified medium.

Figure 6 – EDS spectra obtained from BV4M surface before and after immersion in the McCoy’s 5A modified medium.

Figure 7 – DRIFT spectra of BV4M before (control) and after immersion in the McCoy’s 5A modified medium.

Figure 8 – DRIFT spectra of BV before (control) and after immersion in the McCoy’s 5A modified medium.

The main P-O vibration mode rises in the same region of the Si-O-Si one, which can strongly difficult the identification of the phosphate precipitation onto bioglasses. However, other phosphate vibration modes around 609 cm-1 (v4 O-P-O) and 972 cm-1 (v1 P-O) can be observed on the spectra after immersion [LUCAS-GIROT, MEZAHI, et al., 2011; SBOORI, RABIEE, et al., 2009], confirming the presence of this functional group in the precipitates. A slight band at 870 cm-1 (related to the C-O vibration mode) indicates the presence of carbonate in the deposited layer. . This leads us to assume that a carbonated apatite layer was probably formed on the surfaces during immersion in the culture medium.

4. CONCLUSIONS

Both standard and modified bioglasses produced in this work have exhibited surface bioactivity. However, the insertion of the network modifiers in the BV4M seemed to remarkably improve its surface reactivity relatively to the BV. Probably, the disruption of the glass network caused by the insertion of the modifiers facilitated its dissolution and, consequently, the reprecipitation process.

AGRADECIMENTOS

Authors would like to thank the FAPITEC/SE, FAPERJ, CAPES and CNPq for providing financial support to this project.

REFERENCES

AGATHOPOULOS, S. et al. Formation of hydroxyapatite onto glasses of the CaO–MgO–SiO2 system with B2O3, Na2O, CaF2 and P2O5 additives. Biomaterials, 2006. 1832 - 1840.

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