Fluid interfaces for selective bioparticle recovery

Biological products are often produced as or included in particles, such as inclusion bodies, crystals, cell organelles, viruses and virus-like particles. Selective harvesting of these specific particles is crucial for efficient product recovery. It requires separation techniques that have the ability to separate the target particles from the liquid phase and in general also from other particles, such as cells, cell debris, by-products or immobilised catalysts. In many cases the target particles contain the product already at high purity, making selective particle recovery advantageous. Conventional separation methods such as centrifugation and filtration are inefficient in many particle-particle separations due to the small density differences and overlapping particle size distributions of the particles that require separation. New physical separation methods are therefore desirable. Particles can adsorb to fluid interfaces. Under specific conditions this process is selective for one type of particle in a particle mixture, which makes the phenomenon useful for particle-particle separation. Selectivity in the adsorption process can be created in several ways. This chapter deals with the use of fluid interfaces for particle separation both from an experimental and theoretical point of view. It shows that interfacial separation is very valuable for separation of biotechnological particle mixtures. Two principles for creating the fluid interfaces are discussed: 1) injection of the dispersed phase, and 2) emulsification. Selectivity in these processes can be created by selective particle-interface collision, selective particle attachment, selective particle detachment, or a combination of these sub-processes. Experimental examples from literature are given for all of these subprocesses. For example, ampicillin and phenylglycine crystals were separated by making use of solvent-water interfaces (Jauregi et al., 2001; 2002; Hoeben et al., 2005). In this case the crystals showed competition for the interface and under certain conditions partitioning of the phenylglycine crystals occurred from one liquid phase to the other. A second example is the separation of polyhydroxyalkanoate inclusion bodies from ruptured fermentation broth making use of the air-water interface (Van Hee et al., 2006b). The selectivity was created by selective aggregation of the inclusion bodies, which enhanced their transport to the interface (more favourable hydrodynamics) relative to that of the other particles in the mixture. Finally, the flotation behaviour of synthetic micrometer-sized bioparticles was investigated by determining the relation between the properties of the surface groups of the particles and their adsorption behaviour to the airwater interface (Van Hee et al., 2006c). Particle aggregation again showed to be essential for these small particles. In addition, the floatability of the particles was dominated by the adsorption kinetics of their surface groups. Besides these examples, many other examples from literature are used to illustrate the mechanisms that can be applied for interfacial bioparticle separation.