Titanium Phosphate Glass-Ceramics with Silver Ion Exchangeability

Journal
J. Am. Ceram. Soc., 82 [3] 765–67 (1999)
Titanium Phosphate Glass-Ceramics with Silver Ion Exchangeability
Toshihiro Kasuga,* Masayuki Nogami,* and Yoshihiro Abe*
Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
Na2O–CaO–TiO2–P2O5 glass-ceramics containing Nasicontype crystals were found to have silver-ion exchangeability. The Nasicon-type phase contains Ca2+ and Na+ ions which are located in two different sites of the conduction channels: the Na+ ions with high mobility can be exchanged easily with Ag+ ions in aqueous solution. The Ag+-ionexchanged glass-ceramics showed excellent bacteriostatic activity and a relatively high strength of 160 MPa in bending. I. Introduction
T IS known that some silver-bearing materials have bacteriostatic activity, that is, effectiveness in inhibiting the growth of microorganisms. Possible mechanisms for inhibition by silver ions such as interference of electron transport, binding to DNA, or interaction with cell membranes have been suggested,1 but they are unconfirmed. Bacteriostatic materials such as zeolites, calcium phosphates, silica gels, or clay minerals in which Ag+ ions are borne have recently been introduced commercially.2 These materials have a tendency to release Ag+ ions.3 Extreme release of the ions may be toxic in the human body and shorten the life of the materials. A new type of porous ceramic with the surface phase consisting predominantly of AgTi2(PO4)3 crystal and the interior phase of LiTi2(PO4)3 crystal has been prepared by exchange of Ag+ ions for Li+ ions; it shows excellent bacteriostatic activity4 and the Ag+ ions are chemically stable in water and in the presence of Na+ ions.5 If high-strength ceramics containing Nasicon-type crystals with silver-ion exchangeability could be prepared, they would have the great advantage of bacteriostatic activity given by introducing Ag+ ions into their surface layers. Such materials could play an important role in the development of dental applications. For example, they could be applied to castable dental crowns utilizing a lost-wax technique.6,7 Since bacteriostatic activity can be produced locally by the ion exchange method in portions of the crown, intrusion of bacteria from the gums would be expected to be suppressed effectively. For castable dental crowns, various properties such as high mechanical strength, hardness similar to that of natural dental enamel, good compatibility with the body, and color resemblance are also required. In the present work, Na2O–CaO– TiO2–P2O5 glass-ceramics containing Nasicon-type crystals were prepared; they were found to show relatively high strength and have Ag+ ion exchangeability.
I
II.
Experimental Procedure
Nominal compositions of the base glasses were (100 − x)NaTi2(PO4)3⅐xCa3(PO4)2 (in mol%). The batch mixture was prepared by using raw materials such as reagent-grade
Na2CO3, CaCO3, TiO2, and H3PO4 (85% liquid). The mixture was put into a Teflon beaker with a small amount of water and stirred well to make a slurry. After the slurry was dried, the resultant product was melted in a platinum crucible at 1350°C for 1 h in air. The melt was poured onto a carbon plate to form the base glass; a glass with a composition of x ‫ ס‬52 could be obtained by casting the melt and subsequent cooling to room temperature in air. Glasses with compositions other than x ‫ס‬ 52 were prepared by using a splatting method due to an extreme devitrification tendency. In the present work, the glass with a composition of x ‫ ס‬52 was selected as the mother glass. The temperature for heat treatment was determined by differential thermal analysis (DTA). The glass was heated from room temperature to 650°C and held at this temperature for 20 h for nucleation, and subsequently it was heated to 700°C for 12 h for crystal growth. Crystalline phases of the crystallized glass were identified by X-ray (CuK␣) diffraction (XRD). The microstructure of the specimen was examined by scanning electron microscopy (SEM) incorporating X-ray microanalysis using energy dispersive spectrometry (EDS). To examine silver-ion exchangeability, the glass-ceramic was soaked in 50 mL of 1 mM AgNO3(aq) at 37°C. A 0.5-g sample of the glass-ceramic grains obtained by pulverization into Յ150 ␮m particles was used. After soaking for various periods, the amounts of ions such as Ag+, Na+, Ca2+, Ti4+, and P5+ in the solution were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The three-point bending strength of the glass-ceramic was measured at a loading rate of 0.1 mm/min. A span length of 12 mm and a rectangular-prism-shaped specimen, 3 mm × 3 mm × 20 mm with a tensile surface polished by 0.5-␮m diamond powders, were used. Vickers hardness was measured by indentation at a load of 9.8 N. The critical stress intensity factor, KIC, was estimated from the indentation crack length by using the median crack equation introduced by Niihara et al.8 Evaluation of bacteriostatic effectiveness of the glassceramics against Escherichia coli (IFO 3301) was followed by the so-called dropping method.9 In this test, a sintered alumina plate was used as a negative control material. A phosphate buffer solution containing Na+ ions (PBS) was prepared by dissolving 0.7 g of NaH2PO4, 2 g of Na2HPO4, and 5 g of NaCl into 1 L of distilled water (DW). A bacteria cell suspension was prepared by adding 105 bacterial cells/mL to the PBS; 0.2 mL of the suspension was put onto a glass-ceramic plate with dimensions of 10 mm × 10 mm × 2 mm and was subsequently incubated for 18 h at 36°C. After the incubation, 1.8 mL of the PBS was added and mixed well. The diluted suspension was added to a soybean-casein-digest (SCD) culture medium. After the suspension was incubated for 24 h at 36°C, the number of living bacterial cells was counted. III. Results and Discussion
L. F. Francis—contributing editor
Manuscript No. 189979. Received August 3, 1998; approved December 18, 1998. *Member, American Ceramic Society.
In preliminary investigations, attempts were made to prepare glass-ceramics in the system LiTi2(PO4)3–Ca3(PO4)2 by heat treatment of the base glasses. No dense materials, however, could be obtained because of the formation of numerous cracks during the cooling process. In the present work, fine-textured, dense glass-ceramics could be prepared by controlled crystal765