Aggregation and dispersion behavior of nanometer and submicrometer scale TiO 2 particles in aqueous suspension were investigated using three kinds of mechanical dispersion methods: ultrasonic irradiation, milling with 5-mm-diameter balls, and milling with 50 lm beads. Polyacrylic acids with molecular weights ranging from 1200 to 30 000 g/mol were used as a dispersant, and the molecular weight for each dispersion condition was optimized. Viscosities and aggregate sizes of the submicrometer powder suspensions were not appreciably changed in the ultrasonic irradiation and 5-mm-ball milling trials. In contrast, in the trials in which nanoparticle suspension was used, ultrasonic irradiation produced better results than 5-mm-ball milling. Use of ultrasonication enabled dispersion of aggregates to primary particle sizes, which was determined based on the specific surface area of the starting TiO 2 powders, even for relatively high solid content suspensions of up to 15 vol%. Fiftymicrometer-bead milling was also able to disperse aggregates to the same sizes as the ultrasonic irradiation method, but 50-lmbead milling can be used only in relatively low solid content suspensions. It was concluded that the ultrasonic dispersion method was a useful way to prepare concentrated and highly dispersed nanoparticle suspensions.

J Dent Res 59(2)b137, February 1980 Stern et at. (J Dent Res 43:873, 1964) indicated that when the enamel surfaces of extracted human teeth were exposed to the ruby laser, a degree ot alteration was imparted to the surface that reduced subsurface demineralization. For the clinical application of laser to caries prevention, the following attributes would be desirable: a) minimum energy density to avoid the damages of the soft tissues, especially the dental pulp, and b) an ability to easily lead the laser beam to the restricted area of the oral cavity by means of a flexible beanm guide. Stern et al. (.J Dent Res 51:455, 1972) ani d Yamamnoto et al. (J Oral Patli 3:7. 1974) have studied the enamel exposed to the C)2 laser and the Pockels Q-switched Nd:YAG-, laser, respectively. Because of the wavelength in the infrared zone and the high peak power, these laser beams can be guided only by a manipulator which consists of rigid arms and mirrors. The manipulator is not equal to optical fiber for precise aiming, control and transmission loss. Recently, we found that remarkable acid resistance was imparted to the enamel by irradiation with an acousto-optically Q-switched Nd:YAG laser at a low energy density and that this laser beam could be guided easily by a single flexible optical fiber wvithout any loss of laser energy.In this experiment, 60 freshly extracted noncaries human permanent incisors were used. The labial surfaces of each tooth were cleaned, driedl in air, coated with laser absorption material, and exposed to the laser. The laser beam was guided with a step index cylindrical quart? fiber of 300,um in diameter. The irradition conditions xvere peak power of IOOkW, pulse width of lOOnsec, repetition rate of 1kHz, average output of lOW, spot size of about 3.5mm, and irradiation time of 0.6 to 1 .0 sec. After irradiation, the laser absorptioni material was removed, and the samples were placed in a demineralizing solution vhich consisted of hydroxyethyl cellulose and O.1M lactate buffer of pH 4.5, for a period of four days at 370C to create in vitro incipient carious lesion. After demineralization, ground sections were prepared for microradiography. The temperature rise of the dental pulp during laser irradiation wvas measured with a thermocouple.Macroscopically, enamel transparency vas slightly increased in the lased area. The microradiograph of the unlased enamel area showed subsurface radiolucency. On the other hand, that of the corresponding lased enamel area showed no subsurface demineralization as shown in the ligure (remarkable acid resistance in 44 (73%1c) and moderate acid resistance in 16 (27%/k.)). [his observation clearly demonstrates that the enamiiel exposed to the laser effectively inihibited formation of the caries-like lesion. The temperature rise measured at the dental pulp was about 200C. This temperature rise above body tenmperature (37(C) to 57'tC vill produce pulp destruction for exposures longer than ten seconds (Goldman and Rockwell: Laser in Medicine, New York, LoIndoi, Paris: (,ordoii...

Toward a practical light source for two-photon bioimaging, we have generated kilowatt peak power of 0.77 mum wavelength and 5 ps optical pulse via second-harmonic generation of the amplified output from a gain-switched 1.55 mum semiconductor laser. This compact scheme and stable optical-pulse-source has been successfully used for the two-photon fluorescence bioimaging of actin filaments in PtK2 cells.

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