Mechanical strength of microspheres produced by drying of acoustically levitated suspension droplets

Spray drying is widely used in pharmaceutical manufacturing to produce microspheres from solutions or suspensions. The mechanical properties of the microspheres are reflected by the morphology formed in the drying process. In suspension drying, solids dissolved in the carrier liquid may form bridges between the suspended primary particles, producing a microsphere structure which is resistant against mechanical loads. Experiments with individual, acoustically levitated droplets were performed to simulate the drying of suspension droplets in a spray drying process. The suspensions studied consisted of a binary liquid mixture as the carrier liquid, and primary particles of suspended lactose material which is partially soluble in the liquid. The solubility of lactose was varied by the composition of the liquid mixture. The experiments revealed longer first and second drying stages for higher lactose solubility. Electron micrographs revealed the morphology of individual microspheres produced by drying in the levitator. Microspheres with only primary particles and no visible crust were obtained for low lactose solubility, whereas higher contents of dissolved lactose resulted in a more densely packed microsphere with crust formation. To quantify the hardness of individual microspheres, the maximum breaking force upon mechanical loading was measured for a range of varying suspension compositions. These measurements confirmed that densely packed structures with a thick crust reveal high mechanical strength. It was shown that, for primary lactose particles to be conserved in spray drying, the dissolved lactose mass loading Xd must be below 5.2%.