Monday, February 15, 2010

Adhesion based Immobilization Techniques

Each immobilization method has specific properties and advantages. Therefore, the selection of a cell delivery technique depends heavily on the intended application.

Adhesion: Adhesion to a three-dimensional structure is used to immobilize cells for culture or analytical procedures as well as to provide a structural template directing cell growth and differentiation. Adhesion alone does not offer immunoisolation. For in vivo investigations, adhesion-based immobilization must be used in conjunction with either a polymeric membrane or matrix entrapment methods. This method is effective for surface binding, either on top of gel films or within hydrogel foams. Several hydrogels can be engineered with bioadhesive properties by methods which include interfacial polymerization, phase separation, interfacial precipitation and polyelectrolyte complexation. Factors affecting cell affinity and behavior on hydrogels include the general chemistry of the monomers and the crosslinks, hydrophilic and hydrophobic properties, and the surface charge and functionality. One method to enhance cell adhesion is by adding immobilized cell-adhesive proteins or oligopeptides, such as the arginine-glycine-aspartic acid sequence, in the hydrogel. The physical characteristics of the hydrogel also govern the adhesion affinity. Therefore, altering the pore size and network structure can modify cell adhesion as well as morphology and function. For some adhesion applications the mechanical strength is also important with a lower fractional porositygenerally creating stronger networks. Furthermore, closed pore systems make stronger hydrogels than open pore ones. With the adhesion approach, cells are generally plated onto the hydrogel and allowed to attach and migrate. Supplemented culture media provide the cells with essential nutrients for growth and development as well as a means of oxygen and metaboli product transport while in vitro.

Macroporous hydrogel membranes are manufactured by several techniques. One method of constructing pores large enough for cell growth is by phase separation in the polymer and solvent mixture. The “freeze thaw” and the porosigen techniques are two other approaches. The hydrogel is polymerized around a crystalline matrix made from freezing the aqueous solvent (freeze-thaw technique) or around a porosigen of desired size (porosigen technique). With the “freeze-thaw” method, the ice-based crystalline matrix is then thawed after UV polymerization, leaving a macroporous foam. The porosigen technique also requires removal of the crystalline porosigens, in this case usually by leaching or dispersion after polymerizing of the hydrogel with free-radical initiators has taken place. Another method for constructing hydrogel foams uses gas bubbles from sodium bicarbonate to create the macroporous network. Bubbles are trapped during the gelation stage. Thus, the foam morphology is dependent on the polymerization kineics and varies for different hydrogel compositions.

Tags: Bio Technology, Bio Genetics, Cell Encapsulation

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