**2.5.2 Materials and methods**


An extraoral device was used to adjust the flow rate of a 0.1-mm-thick film of artificial saliva over an agarose disk to determine the clearance half-time in the same manner as that performed intraorally (Dawes et al, 1989). Then, from the relationship between the intraoral and extraoral half-times, the salivary film velocities of the UAB and UPB sites were estimated. The half-time at UAB and UPB were evaluated by the method of study 2.1.

Fig. 14 shows the equipment used. The diameter of the well in the lower part of the device was 6 mm, the same as the width of the 0.1-mm-deep slot in the upper part. Thus, the fluid was directed over the surface of the gel. The well was 4 mm from the end of the device.

The well in the lower part of the device was filled with 1 mol/L KCl in 1 % agarose, as described for study 2.1 (Fig.3) and the upper and lower parts of the device were held together with three spring clamps.

Fig. 14. An extraoral device for salivary flow rate study.

The device was maintained at 37 °C, and de-ionized water at the same temperature was infused with an infusion pump (Model 2000 IW, Harvard Apparatus Co., USA), the flow rate of which was adjustable over a wide range. The pump activated a 5-mL syringe connected to the device via polyethylene tubing.

For an experiment to be initiated, the flow rate of the pump was set to 1.07 ml/min. As soon as the water filled the tubing and completely covered the gel, a stop watch was started, and the flow rate of the pump was set to the desired value. After a pre-determined time, the pump was stopped, the two halves of the acrylic device were separated, and the agarose gel was removed with a needle and transferred to an appropriate volume of 100-ppm NaCl. The potassium concentration was determined by atomic absorption spectrophotometry.

For each flow rate, the experiment was repeated using five different gels for different durations to enable up to 70% of the KCl to be cleared from the gel. For each flow rate, a control gel that had not been exposed to water was used to determine the initial potassium concentration. The experiment was repeated three times for each flow rate. A least-squares straight line was fitted by computer to the potassium concentration plotted against the square root of time, and the half-time was calculated.
