Comparison of different encapsulation strategies for living cells and mechanical characterization of microspheres by scanning acoustic microscopy

Microencapsulation is a novel strategy for immobilization and immunoprotection of cells where microcapsules are used for long term production of recombinant proteins, hormones, and growth factors in culture or in vivo. The focus of this study was the comparison of three different strategies for the production of calcium cross-linked alginate beads of a small size (< 350 mm) to immobilized and immunoprotected mammalian cells. The following strategies for bead production have been used, a) the AirJet technology (coaxial gas flow extrusion), b) the vibrating nozzle technology, and c) the JetCutter technology and for the successful immunoprotection of the cells were used the established alginate/poly-L-lysine/alginate complexation as the polymeric system. It was shown that all three methods may be used for production of homogeneous beads with a diameter < 350 mm. While the vibrating nozzle technique was limited to an alginate viscosity of 0.2 Pa.s or less, the AirJet and JetCutter technology were less sensitive to higher viscosities. Optimum parameters for production of beads with specific characteristics were defined for the three methods. In conclusion, this study describes optimized methods for alginate microencapsulation of genetically modified mammalian cells, which may be used for treatment of human diseases in vivo. High frequency Scanning Acoustic Microscopy is used for mechanical characterization of the microspheres as well as for investigation of surface properties. The mechano-elastical properties are measured in terms of acoustic impedance. Additionally, 2- and 3D images show the surface of the microspheres with a spatial resolution of 1.5 mum. Mechanical stiffness is obtained from bulk measurements of acoustic velocity and mass density.