Ion Releasing Abilities of Phosphate Invert Glasses Containing MgO, CaO or SrO in Tris Buffer Solution

CaO-TiO2-P2O5-Na2O invert glasses developed in our group showed bioactivity and good sinterability. In the present work, phosphate invert glasses, in which CaO was substituted with MgO or SrO (PIG-MO, M = Mg, Ca, Sr), were prepared and their ion-releasing behaviors were evaluated. Glass transition temperature (Tg) and crystallization temperature (Tc) were estimated from differential thermal analysis. (Tc – Tg)/Tg value, which indicates glassification of the glass, of PIG-MgO was higher in comparison with those of the other glasses. Laser Raman spectroscopic analysis indicated that peaks due to P2O7 4– group in the spectrum of PIG-MgO were smaller than those in the spectra of PIG-CaO and PIG-SrO. The structure of PO4 3–-M2+PO4 3– was expected to form easily, resulting in the good glassification of PIG-MgO. The ion amounts dissolved from PIG-MgO containing a large amount of PO4 3– group were 2 ∼ 3 times larger than those of the other glasses.


Introduction
CaO-TiO 2 -P 2 O 5 -Na 2 O invert glasses (Phosphate Invert Glasses, denoted by PIGs), which contain ortho-and pyrophosphate groups, were successfully prepared by a conventional melting method in our group [5]. Powders of the glasses have good sinterability when heated at 850°C [4]. Hydroxyapatite deposits on the glass and glass-ceramic surfaces after soaking in simulated body fluid (SBF) [4,6]. The glass-ceramics was successfully coated on a Ti-29Nb-13Ta-4.6Zr alloy [7,10] and then expected to be applicable to coating for artificial implant materials.
Trace amounts of Si 4+ , Sr 2+ , Mg 2+ , and Zn 2+ ions were reported to enhance bone formation [3,8,11,17]. We have investigated in PIGs which release these ions. Mg 2+ and Sr 2+ ions can be easily substituted with Ca 2+ ions in PIG. Mg 2+ ion concentration in body has been reported to influence bone strength [11]. Sr 2+ ion has been reported to inhibit bone resorption by osteoclasts and enhance bone formation by osteoblasts [8]. Wu et al. developed calcium-magnesium phosphate cement. The calciummagnesium phosphate cement induced higher proliferation of human osteosarcoma MG63 cells in comparison with calcium phosphate cement [16]. Qie et al. prepared calcium polyphosphate substituted with strontium. The proliferation and alkaline phosphatase (ALP) expression of osteoblastlike cell line (ROS17/2.8) were higher on the calcium polyphosphate substituted with < 20 mol% strontium than on an unsubstituted one [13].
The present work was a fundamental study for developing new types of glasses with Mg 2+ , Ca 2+ and Sr 2+ ion releasability. PIG, in which CaO was substituted with MgO or SrO, was prepared by a conventional meltquenching method and evaluated in their solubilities in Tris buffer solution (pH 7.40), assuming to the body system. The glass structures and their glassification were discussed.

Materials and methods
60MO·30P 2 O 5 ·7Na 2 O·3TiO 2 (mol%, M = Mg, Ca, Sr, nominal composition, denoted by PIG-MO) were prepared. Glass batches were prepared using MgO (99.5%), CaCO 3 (99.5%), SrCO 3 (98%), H 3 PO 4 (85% liquid), Na 2 CO 3 (99.5%), and TiO 2 (99.5%). All regeagents were purchased from Kishida Chemical Co. The reagents were mixed with distilled water to make a slurry, and then dried at 140°C overnight. The resulting sample was melted at 1400°C for 30 min and then quenched on a stainless steel, resulting in the formation of the glass. Glass transition temperature (Tg) and crystallization temperature (Tc) of the resulting glasses were estimated from differential thermal analysis (DTA, Rigaku Thermo plus, TG8120), which was performed until 1000°C at a heating rate of 5°C/min. Glass structure was examined by laser Raman spectroscopy (LRS, JASCO, NRS-2000, 514.5 nm). The glasses were pulverized into 125 ∼ 300 µm in size. Tris buffer solution was prepared using 6.118 g of Trishydroxymethylaminomethene (NH 2 C(CH 2 OH) 3 , Kishida Chemical Co.) and 1 liter of distilled water and its pH was adjusted to 7.40 at 37°C using hydrochloric acid. 15 mg of the glass powders were soaked in 15 mL of Tris buffer solution at 37°C for 1 week. The amounts of the ions dissolved from the glasses were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Shimadzu, ICPS-7000). Figure 1 shows Tg, Tc, and (Tc -Tg)/Tg, which indicates a degree of glassification [12]. The (Tc -Tg)/Tg value of PIG-MgO is twice larger than that of PIG-CaO, and the value of PIG-SrO is half of that of PIG-CaO. PIG-MgO is suggested to show good glassification due to its larger (Tc -Tg)/Tg value than those of the others. Figure 2 shows laser Raman spectra of the glasses. The peaks matching to phosphate groups are as follows; 1040 cm -1 : P-O stretching due to non-bridging oxygen Q 1 species, 950 cm -1 : P-O stretching due to non-bridging oxygen Q 0 species and 740 cm -1 : P-O-P stretching due to bridging oxygen Q 1 species [1]. Q 0 and Q 1 indicate orthophosphate (PO 4 3-) and pyrophosphate (P 2 O 7 4-) groups, respectively. The peaks matching to Ti-O polyhedra are as follows; 880 cm -1 : TiO 4 tetrahedra and 650 cm -1 : TiO 6 octahedra [15]. The peaks due to Q 0 , Q 1 and TiO 6 appeared in the all of the spectra. The peaks due to TiO 4 appeared in PIG-CaO and PIG-SrO. The pyrophosphate (Q 1 ) peaks of PIG-MgO was smaller than those of the other glasses. All peaks red-shifted with increasing atomic number of M (Mg, Ca, Sr) in the glasses. Dietzel proposed F is field strength, Z is cation valance and a is distance between cation and anion. The field strength of Mg 2+ , Ca 2+ , and Sr 2+ ions are 0.53 (4-fold coordination) or 0.46 (6fold coordination), 0.33, and 0.28 valance/Å 2 , respectively [2]. Nelson et al. reported that Raman peaks of phosphate groups in alkaline earth metal containing phosphate glasses red-shifted with increasing the atomic number of the metals [9]. As the atomic numbers of the metals increase, their field strengths decrease. This causes attributes to the increase in the angle of P-O-P bonds in PO 4 tetrahedra. The Raman peaks red-shifted with the increase in O-P-O angle, due to decrease in its bonding energy [14]. Figure 3(a) shows the ion concentration in Tris buffer solution after soaking the glass powders for 1 week, and Figure 3(b) shows the ratios of the dissolved ion-amount to the total one in the glasses. The ion amounts dissolved from PIG-MgO were about 2 ∼ 3 times larger than those from the other glasses.

Results and discussion
The field strength of Mg 2+ ion is larger than those of Ca 2+  PIG-MgO is high, while the glass contains a larger amount of Q 0 than the others. In the case of PIG-CaO and PIG-SrO, pyrophosphate groups (Q 1 ) formed predominantly, since the field strengths of Ca 2+ and Sr 2+ are smaller than that of Mg 2+ .
The bonding strength of P 2 O 7 4--M 2+ may be larger than that of PO 4 3--Mg 2+ . The dissolution behavior of PIG-MgO is suggested to be attributed to a large amount of orthophosphate group in the glass.

Conclusions
MO-TiO 2 -P 2 O 5 -Na 2 O invert glasses (M = Mg, Ca, Sr) were prepared and evaluated in their glass structures and ion-releasing abilities in Tris buffer solution. Although PIG-MgO contained a larger amount of orthophosphate group than the other glasses, it showed good glassification due to the high field strength of Mg 2+ . On the other hand, PIG-MgO showed lower chemical durability due to a large amount of orthophosphate group. M 2+ ions in the phosphate invert glasses play an important role in their glassification and ion-releasing.