Research paper Effect of the spraying conditions and nozzle design on the shape and size distribution of particles obtained with supercritical fluid drying Andre ´anne Bouchard a, * , Natas ˇa Jovanovic ´ b , Anne H. de Boer c ,A ´ ngel Martı ´n d , Wim Jiskoot b,e , Daan J.A. Crommelin b , Gerard W. Hofland a , Geert-Jan Witkamp a a Process Equipment, Delft University of Technology, Delft, The Netherlands b Department of Pharmaceutics, Utrecht University, Utrecht, The Netherlands c Department of Pharmaceutical Technology and Biopharmacy, University of Groningen, Groningen, The Netherlands d Departamento de Ingenierı ´a Quı ´mica y Tecnologı ´a del Medio Ambiente, Universidad de Valladolid, Valladolid, Spain e Division of Drug Delivery Technology, Leiden University, Leiden, The Netherlands Received 20 August 2007; accepted in revised form 31 March 2008 Available online 11 April 2008 Abstract In the perspective of production of dry therapeutic protein formulations, spray drying of lysozyme (as a model protein) into super- critical carbon dioxide was studied. The effects of the nozzle (i.e., co-current coaxial converging and converging–diverging, and T-mixer impinging) and process conditions (i.e., flow rates, pressure) on the drying of the lysozyme prepared in aqueous solution dried with super- critical carbon dioxide enriched with ethanol were investigated. The particle size distribution, width of particle size distribution and mor- phology were used to determine the effect of the various parameters assessed. Particles with a median size of 1.5, 5 or 25 lm were produced depending of the nozzle selected. A basic comparative study of the nozzle was done by computational fluid dynamics, but the differences in particle size could not be depicted by these computations. The proportional increase of the flow rates (up to fivefold) caused a decrease in particle size (7- to 12-fold), and doubling the pressure caused a moderate decrease of the size (5–20%). The individual effect of the supercritical carbon dioxide, ethanol and solution streams was explained with a mass transfer model. Changing the ratio between flow rates slightly affected the particle size in various ways because of the swelling and shrinking stages of the drying droplet in super- critical carbon dioxide enriched with ethanol. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Atomisation; Lysozyme; Particle size distribution; Mass transfer; Computational fluid dynamics 1. Introduction The necessity for appropriate protein stabilisation meth- ods increases as the number of labile protein based thera- peutics being approved and introduced to the market is greater than ever [1]. Proteins are often unstable in liquid formulations because of chemical and physical degradation reactions [1], and traditional drying processes such as freeze- or spray-drying potentially cause harmful stresses on them [2]. The use of supercritical fluid (SCF) as drying medium for the precipitation of proteins had been previ- ously suggested because of the mild process conditions [3]. Despite the poor solubility of water in supercritical car- bon dioxide (SC-CO 2 ), the production of protein powders from aqueous solution is favoured over drying from organic solutions as organic solvents can negatively affect protein stability [4] and are often poor at dissolving pro- teins. Drying of aqueous protein solutions by SCF has been reported by several authors [5]. The influence of process flow rates on the particle morphology was recently demon- strated using lysozyme as solute [6]. In that study, three 0939-6411/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ejpb.2008.03.020 * Corresponding author. Process Equipment, Delft University of Tech- nology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands. Tel.: +31 15 278 5561; fax: +31 15 278 6975. E-mail address: a.m.j.l.bouchard@tudelft.nl (A. Bouchard). www.elsevier.com/locate/ejpb Available online at www.sciencedirect.com European Journal of Pharmaceutics and Biopharmaceutics 70 (2008) 389–401