Rheological properties and mechanical stability of new gel-entrapment systems applied in bioreactors

The mechanical stability of gels applied for entrapment and retention of bi ocatalysts in bioreactors is of crucial importance for successful scale-up applications. Gel abrasion in agitated reactors will depend on liquid shear , bubble shear, and wall shear, as well as collisions between the gel parti cles. As a simplified standardized model system, abrasion of gel beads was studied in 1-m-high bubble columns with controlled aeration, and quantified by measuring the loss of gel material into solution. Gel beads were also t aken out to measure stress-strain response during controlled compression. M ore general rheological properties of different gels were studied by applyi ng a variety of regimes of controlled compression of standardized gel cylin ders: Gel strength was measured by recording the fracture properties and th e Young's modulus. Viscoelastic properties were revealed by recording creep during compression as well as recovery after compression. Oscillation test s up to 1000 cyclic compressions were applied to compare the fatigue of dif ferent gels. Results obtained for Ca-alginate gels, gels of chemically modi fied polyvinyl alcohol with stilbazolium groups (PVA-SbQ) as well as mixed gels of Ca-alginate and PVA-SbQ are compared with previously published data for K-carrageenan, agar, and polyethylene glycol (PEG) gels. It is conclud ed that material fatigue rather than mechanical properties such as stiffnes s or fracture stress should be considered when selecting a suitable gel mat erial on the basis of abrasion resistance. The very soft and superelastic P VA-SbQ gel showed no significant fatigue in mechanical tests and no abrasio n was detected in the standardized model system used. Ca-alginate gels, how ever, showed severe:irreversible changes due to fatigue at oscillating load s and creep at constant load. Due to their similarities with K-carrageenan gels in mechanical tests, it is likely that Ca-alginate would also be sensi tive to abrasion. Mixed gels of Ca-alginate and PVA-SbO represent a complex system with intermediate properties, showing significant fatigue and creep , but elastic properties from the PVA-SbQ gel make it less sensitive than t he pure Ca-alginate gel. (C) 2000 John Wiley & Sons.