148 Defining the operating space for spray drying of red blood cells

Abstract

The ability to store red blood cells (RBCs) in the dry state at room temperature would be extremely beneficial, particularly in areas where reliable access to electricity is unavailable. In this study, we explore the potential for using spray drying for preservation of RBCs in the dried state. Spray drying involves atomization of a liquid feed into small droplets and drying of the droplets using a heated gas stream, resulting in a powder composed of the solid components of the liquid feed. Previously, we demonstrated the ability to atomize a suspension of RBCs using a two-fluid nozzle with humidified nitrogen gas without significant shear-induced hemolysis [M. McLean, X.Y. Han, A.Z. Higgins, Spray Drying for Preservation of Erythrocytes: Effect of Atomization on Hemolysis, Biopreservation and Biobanking 11 (2013) 122–123]. Here, we explore the potential spray drying process space using bench-scale tests in order to understand the balance between producing sufficiently dry RBC powder and maintaining the integrity of thermally labile RBCs. We investigated the temperature sensitivity of RBCs in order to determine a maximum tolerated drying gas temperature, recognizing that evaporative cooling of drying droplets results in droplet temperatures at wet-bulb. Suspensions of RBCs were exposed to temperatures up to 65 °C for a period of 10 s. At 50 °C and below the RBCs remained intact, exhibiting hemolysis levels of less than 10%. However, exposure to higher temperatures resulted in substantial increases in hemolysis. A wet bulb temperature of 50 °C is consistent with a drying gas inlet temperature of 115 °C at 5% relative humidity, indicating that a process inlet temperature of 115 °C would result in less than 10% hemolysis due to process temperatures. We also investigated the equilibrium relationship between relative humidity and RBC moisture content by drying RBC suspensions in the presence of various saturated salt solutions. The results of these experiments indicate that an equilibrium moisture content of 5% corresponds with a relative humidity between 15% and 20%. Assuming equilibrium at the drying chamber outlet, these results bracket the liquid feed to drying gas feed ratio necessary to achieve a target moisture content of 5%. In addition, our current results together with our previous measurements of droplet size after atomization allow estimation of the droplet drying time. The predicted drying time for a 70 μ diameter aqueous droplet in isolation with a drying gas at 115 °C is on the order of 1 s. Although in actual practice increased mass transfer resistance would be expected as a result of droplet–droplet interactions, previous experimental observations of industrial spray drying confirm the drying time estimate to within an order of magnitude. This short drying time underscores a major advantage of spray drying as a method for achieving the dry state. Source of funding: None declared. Conflict of interest: None declared. adam.higgins@oregonstate.edu