By Vince Sullivan, PhD and Anjana Bhuta-Wills, BD Technologies, Research Triangle Park, NC; and Shoreh Hajiloo, ICEM CFD Engineering, Livonia, MI

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An accurate CFD model is a potentially powerful development tool for evaluating and predicting the performance of medical devices. In studies conducted at BD Technologies, FIDAP was used to evaluate the operation of the BD AccuSprayT Nasal Delivery System. The model developed in this study accurately simulated fluid flow through the device at appropriate device actuation velocities.

The BD AccuSpray Delivery System is designed to reach acceptable pressure for actuation with minimal variability

The BD AccuSpray device is a nasal delivery system based on BD's HypakT prefillable syringe technology. It creates a spray by forcing liquid through a pressure swirl atomizer when the user depresses the plunger on the device. A thin intact sheet of liquid is formed in the shape of a cone at the exit orifice, and breaks up into droplets of an appropriate size for delivery of drugs to the nasal mucosa. The AccuSpray device has been used to deliver a new live attenuated influenza vaccine, which has recently completed Phase III clinical trials, and is currently under review at the FDA. Other intranasal vaccines are in the preclinical testing stage.

Fluid is forced into the swirl chamber through a valve, resulting in rotational flow prior to release

A model was developed using four design stages. First, internal single-phase flow characteristics were modeled. Second, a 2D model of the two-phase atomization flow at the device nozzle was simulated. Third, empirical equations were obtained from the literature, relating particle size to model output parameters such as cone angle and intact sheet thickness at the nozzle; these were used to validate the model. Fourth, a 3D, two-phase flow of the atomization process was simulated using FIDAP's volume of fluid (VOF) model. These results were then compared to average spray particle sizes produced by AccuSpray, as determined experimentally by a Sympatec© laser diffractometer.

FIDAP was used to illustrate the path of the fluid as it moves past the valve, into the swirl chamber and out the nozzle. In addition, the VOF models were able to simulate the formation of a hollow cone, and the initial jet breakup of the fluid as it leaves the AccuSpray nozzle.

Average particle size (Sauter mean diameter) as a function of plunger velocity for two correlations and experimental measurements; agreement is best when velocities in excess of 80 m/sec are used

Predictions of average particle size were obtained using semi-empirical correlations based on the fluid's physical properties and the FIDAP results. There was a close correlation between the model and experimental results, indicating that the model accurately simulates the AccuSpray operation. The experimental particle size data showed that reproducible particle size in the appropriate range for nasal deposition occurs when plunger velocities exceed 80 mm/sec. While the experimental results were closely matched by model results at higher velocities, the model underestimated particle size at lower velocities. The reason for the discrepancy at lower actuation rates is believed to be due to the fact that the model assumes fully turbulent flow. This assumption appears to be correct only at higher velocities.

Using CFD to model liquid atomization is complex because the random physical instabilities at the air-liquid interface that cause atomization can be difficult for CFD to capture. Even so, the AccuSpray study has given BD confidence that their new design does allow the user to reliably reach an actuation velocity high enough to achieve full spray atomization. The results of this study indicate the power of CFD modeling, when combined with rigorous analytical testing, to accurately predict device performance.

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