In microgravity, the coalescence of dispersed phase droplets results in a
decrease in dispersed phase volume. Since the continuous phase is incompressible, the decrease in droplet volume induces a flow of the continuous phase
toward the droplet. In the absence of buoyancy forces, assuming the dispersed
phase droplets are uniformly distributed, there would be an increase in the collision efficiency of droplets due to the effect of local flow. Thus in microgravity,
as in unit gravity, coalescence and phase separation is spontaneous and enhanced by the local flow induced by droplet coalescence. This would help to
explain why phase separation was possible for some of the systems in STS-26
using the PPE module but not for others and why low viscosity and surface
tension had such a large positive effect on demixing rates [107]. Due to the
combined effects of Brownian motion and the local flow induced by droplet
coalescence there is a greater probability that PEG-rich phase droplets
(typically the dispersed phase in PEG/DX systems) would impact the container
walls and remain there. If the continuous phase were DX it would prevent the
dispersed phase from contacting the container walls. The sole effect of surface
wetting forces in microgravity would seem to be giving the phase separation
directionality.
Reverse micellar extraction (RME) has been gaining popularity as an attractive liquid-liquid extraction process [108–111]. This is mainly due to the
fact that enzymes can be solubilized in organic solvents with the aid of reverse
micellar aggregates [112, 113]. Their inner core contains an aqueous microphase, which is able to solubilize polar substances, e.g., hydrophilic enzymes
[114]. In many cases not only the enzymes retained their activity in organic
environment; in some cases they seem to perform even better if they are
entrapped into reverse micellar aggregates [111]. One of the remarkable
findings that gave this field a major boost is that the solubilization of different
proteins into micellar solutions is a selective process [112].
3.2
Magnetic and Electro-Extraction Methods
3.2.1
Magnetic Extraction
Affinity techniques appear currently to be among the most powerful tools available for downstream processing both in terms of their selectivity and recovery.
Conventional porous affinity supports are mostly applicable for work in
clarified solutions and not suitable for work in early stages where suspended
solids and other fouling compounds are present in the system. Non-porous support particles, which are easier to clean and less prone to fouling, are more
useful for the purification from feed streams [29, 115]. However, in order to
obtain similar surface area of typical microporous particles of 100 mm, the size
of non-porous support particles have to be in the range of 0.1–1 mm [116]. It
appears that the only feasible method for the recovery of such small particles in
the presence of biological debris of similar size is magnetic separation, which
eliminates many time-consuming steps and is easy to carry out. The direct and
Multistage Magnetic and Electrophoretic Extraction of Cells, Particles and Macromolecules
173
decrease in dispersed phase volume. Since the continuous phase is incompressible, the decrease in droplet volume induces a flow of the continuous phase
toward the droplet. In the absence of buoyancy forces, assuming the dispersed
phase droplets are uniformly distributed, there would be an increase in the collision efficiency of droplets due to the effect of local flow. Thus in microgravity,
as in unit gravity, coalescence and phase separation is spontaneous and enhanced by the local flow induced by droplet coalescence. This would help to
explain why phase separation was possible for some of the systems in STS-26
using the PPE module but not for others and why low viscosity and surface
tension had such a large positive effect on demixing rates [107]. Due to the
combined effects of Brownian motion and the local flow induced by droplet
coalescence there is a greater probability that PEG-rich phase droplets
(typically the dispersed phase in PEG/DX systems) would impact the container
walls and remain there. If the continuous phase were DX it would prevent the
dispersed phase from contacting the container walls. The sole effect of surface
wetting forces in microgravity would seem to be giving the phase separation
directionality.
Reverse micellar extraction (RME) has been gaining popularity as an attractive liquid-liquid extraction process [108–111]. This is mainly due to the
fact that enzymes can be solubilized in organic solvents with the aid of reverse
micellar aggregates [112, 113]. Their inner core contains an aqueous microphase, which is able to solubilize polar substances, e.g., hydrophilic enzymes
[114]. In many cases not only the enzymes retained their activity in organic
environment; in some cases they seem to perform even better if they are
entrapped into reverse micellar aggregates [111]. One of the remarkable
findings that gave this field a major boost is that the solubilization of different
proteins into micellar solutions is a selective process [112].
3.2
Magnetic and Electro-Extraction Methods
3.2.1
Magnetic Extraction
Affinity techniques appear currently to be among the most powerful tools available for downstream processing both in terms of their selectivity and recovery.
Conventional porous affinity supports are mostly applicable for work in
clarified solutions and not suitable for work in early stages where suspended
solids and other fouling compounds are present in the system. Non-porous support particles, which are easier to clean and less prone to fouling, are more
useful for the purification from feed streams [29, 115]. However, in order to
obtain similar surface area of typical microporous particles of 100 mm, the size
of non-porous support particles have to be in the range of 0.1–1 mm [116]. It
appears that the only feasible method for the recovery of such small particles in
the presence of biological debris of similar size is magnetic separation, which
eliminates many time-consuming steps and is easy to carry out. The direct and
Multistage Magnetic and Electrophoretic Extraction of Cells, Particles and Macromolecules
173