(T1130-01-01) Acoustic Levitation: A Novel Experimental Tool to Study Drying Kinetics and Particle Formation of Pharmaceutical Excipients

Purpose: It is significantly important, in terms of modeling and prediction of parameters in spray drying to have a thorough understanding of the drying process. In Spray drying the generation of small droplets is followed by solvent evaporation and the creation of a final powder product. The exact knowledge of the drying process, i.e. evaporation and transport processes inside the droplets, solid component distribution, and shell/skin formation, are of great interest for modeling and predicting product characteristics for given operating conditions. Acoustic Levitation as a novel method for single particle/droplet levitation technique enables a contactless method for holding droplets in the air and can successfully simulate the real spray process parameters such as temperature, humidity, and gas velocity.

Methods: Excipients chosen for this study included D-mannitol and Dextrin. An aqueous solution with a different initial concentration of the excipients was prepared. The ultrasonic levitator consists mainly of an ultrasonic wave emitter (piezo crystal and transducer flange) that generates waves at 58kHz (see Fig. 1). The acoustic radiation pressure exerted on the droplet generates an acoustic force that counteracts the gravity force; thus, the droplet can be levitated.

Results: The experimental results on drying of dextrin aqueous solution of 10% and 30% initial solids load were carried out at different air-drying temperatures, i.e. 30-90°C. The evaporation of the water leads to a higher concentration of solids. As the solvent evaporates, the local density and viscosity near the surface layers take on higher values. The droplet starts to solidify and a skin forms on the surface. The results show that higher temperature initiates skin formation at an earlier stage, for instance at 60°C air drying temperature, skin formation begins after aprox. 250 s while it takes almost 450 s at 30°C. The evaporation flux of the solvent decreases significantly -almost 3-5 times less- once the falling rate period starts, the solvent here faces extra resistance due to skin/shell formation. The experimental results in terms of the drying curves were compared with a theoretical model (REA) and show a good agreement.

Conclusion: Single droplet drying was carried out using the acoustic levitation method which is a novel tool to model drying kinetics occurring in pilot or industrial spry dryers. This technique is allowing additional information on drying kinetics and mechanisms of particle formation, where various process parameters and material properties i.e. viscosity can be investigated to study its influence on capsule/full particle formation. In the present study, the drying behavior of dextrin droplets/particle has been investigated using a single droplet drying device (Acoustic Levitator). This experimental technique enables to mimic the spray drying process occurred in industrial applications in terms of process parameters and various feed formulations. The results show that higher temperature initiate skin formation at earlier stage. The RAE model was applied to model the drying curves of dextrin solution. Comparison of the experimental data with the REA approach shows good agreement. The drying kinetics obtained for a single droplet of chitosan in the presented drying method could be very beneficial for large-scale modeling of the spray drying process and can be used as a sub-model in computational fluid dynamics (CFD) modeling.

References: Al Zaitone, B. and A. Al-Zahrani (2020). "Modeling Drying Behavior of an Aqueous Chitosan Single Droplet Using the Reaction Engineering Approach." AAPS PharmSciTech 21(8): 315.
Al Zaitone, B. and A. Al-Zahrani (2021). "Spray Drying of Cellulose Nanofibers: Drying Kinetics Modeling of a Single Droplet and Particle Formation." Chemical Engineering & Technology 44(7): 1270-1277.
Al Zaitone, B., A. Al-Zahrani, O. Ahmed, U. Saeed and A. A. Taimoor (2022). "Spray Drying of PEG6000 Suspension: Reaction Engineering Approach (REA) Modeling of Single Droplet Drying Kinetics." Processes 10(7).
Al Zaitone, B., A. Al-Zahrani, S. Al-Shahrani and A. Lamprecht (2020). "Drying of a single droplet of dextrin: Drying kinetics modeling and particle formation." International Journal of Pharmaceutics 574: 118888.
Jiang, W.-Z., Y. Cai and H.-Y. Li (2017). "Chitosan-based spray-dried mucoadhesive microspheres for sustained oromucosal drug delivery." Powder Technology 312: 124-132.

Acknowledgements: This work was supported by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant No. (4-135-36-RG). The author, therefore, gratefully acknowledge the DSR technical and financial support. The travel support by the College of Engineering, United Arab Emirates University is appreciated.

Figure 1: Schematic diagram of the experimental set-up: a) acoustic levitator, CMOS
camera and macro lens. b) LED light source, heating and cooling chambers.

Figure 2: Experimental drying curve evolution, dry mass content (X) versus time and the corresponding REA model (solid lines). a) Cs0=0.1, b) CS0 =0.3, air drying temperatures were 30°C, 60°C and 90°C.