2. Systems in which an organic reverse micellar solution is in equilibrium with
a conjugated aqueous phase.
Both the systems are based on the differential partitioning of biomolecules between the immiscible phases. However, the principles of solubilization differ
greatly. These two developing techniques will be discussed briefly here.
3.1
Existing Methods – Brief Analysis
Even though aqueous two-phase extraction (ATPE) has been known for quite
some time, it has gained importance for industrial exploitation only recently.
Unlike conventional liquid-liquid extraction involving organic and aqueous
phases or pairs of organic phases, ATPE employs two aqueous phases. ATPE has
been successful to a large extent in overcoming the limitations of conventional
extraction such as poor solubility of proteins in organic solvents and the
tendency of organic solvents to denature proteins/enzymes. ATPE has been
recognized as a versatile technique for downstream processing of biomolecules
such as proteins, enzymes, viruses, cells, cell organelles, and other biological materials [70, 80–82]. Its applications for cells and particles was discussed in the
earlier sections. ATPE offers many advantages such as biocompatible environment, low interfacial tension, low energy, easy scale-up, and continuous operation. More importantly, partitioning depends on differences in surface
properties and does not depend on size, shape, or density of the separand except
under specific conditions [78, 83]. Further, the equipment and methods of conventional organic aqueous phase extraction used in the chemical industry can be
easily adapted to ATPE. However, ATPE is not selective enough to provide the extreme purity usually desired. The main reasons for ATPE not reaching industry
are perhaps the high cost of the phase forming polymers and slow demixing rate
of the phases. Thus, ATPE has been recognized as a potential primary purification step in the overall protein/enzyme recovery train [84, 85] in which the final
purification is achieved by methods such as chromatography or crystallization.
ATPE is also effective and efficient for the removal of contaminating materials
and undesirable byproducts such as nucleic acids and polysaccharides [83].
Temperature induced phase separation, proposed by Galaev and Mattiasson
[86] and studied extensively by Tjerneld and coworkers [87–90] enables the
recovery and recycling of the polymers so the economics have improved to a
large extent. It also improves the yield, degree of selectivity, etc. Methods for
faster demixing of the phases were developed by employing electric fields [91,
92] and by acoustic fields [93]. In the near future ATPE is expected to enjoy
further commercial adaptation.
Some recent developments in this area, which resulted in either increase in
selectivity or improvement in yield, are briefly discussed here. Affinity partitioning (AP) is based on the preferential/biospecific interaction between the
molecule and affinity polymer derivative which results in a biomoleculepolymer derivative complex which selectively partitions to one of the phases
leaving the contaminating substances or proteins in the other phase. Most of the
reported investigations regarding affinity partitioning pertain to polymer/
Multistage Magnetic and Electrophoretic Extraction of Cells, Particles and Macromolecules
171
a conjugated aqueous phase.
Both the systems are based on the differential partitioning of biomolecules between the immiscible phases. However, the principles of solubilization differ
greatly. These two developing techniques will be discussed briefly here.
3.1
Existing Methods – Brief Analysis
Even though aqueous two-phase extraction (ATPE) has been known for quite
some time, it has gained importance for industrial exploitation only recently.
Unlike conventional liquid-liquid extraction involving organic and aqueous
phases or pairs of organic phases, ATPE employs two aqueous phases. ATPE has
been successful to a large extent in overcoming the limitations of conventional
extraction such as poor solubility of proteins in organic solvents and the
tendency of organic solvents to denature proteins/enzymes. ATPE has been
recognized as a versatile technique for downstream processing of biomolecules
such as proteins, enzymes, viruses, cells, cell organelles, and other biological materials [70, 80–82]. Its applications for cells and particles was discussed in the
earlier sections. ATPE offers many advantages such as biocompatible environment, low interfacial tension, low energy, easy scale-up, and continuous operation. More importantly, partitioning depends on differences in surface
properties and does not depend on size, shape, or density of the separand except
under specific conditions [78, 83]. Further, the equipment and methods of conventional organic aqueous phase extraction used in the chemical industry can be
easily adapted to ATPE. However, ATPE is not selective enough to provide the extreme purity usually desired. The main reasons for ATPE not reaching industry
are perhaps the high cost of the phase forming polymers and slow demixing rate
of the phases. Thus, ATPE has been recognized as a potential primary purification step in the overall protein/enzyme recovery train [84, 85] in which the final
purification is achieved by methods such as chromatography or crystallization.
ATPE is also effective and efficient for the removal of contaminating materials
and undesirable byproducts such as nucleic acids and polysaccharides [83].
Temperature induced phase separation, proposed by Galaev and Mattiasson
[86] and studied extensively by Tjerneld and coworkers [87–90] enables the
recovery and recycling of the polymers so the economics have improved to a
large extent. It also improves the yield, degree of selectivity, etc. Methods for
faster demixing of the phases were developed by employing electric fields [91,
92] and by acoustic fields [93]. In the near future ATPE is expected to enjoy
further commercial adaptation.
Some recent developments in this area, which resulted in either increase in
selectivity or improvement in yield, are briefly discussed here. Affinity partitioning (AP) is based on the preferential/biospecific interaction between the
molecule and affinity polymer derivative which results in a biomoleculepolymer derivative complex which selectively partitions to one of the phases
leaving the contaminating substances or proteins in the other phase. Most of the
reported investigations regarding affinity partitioning pertain to polymer/
Multistage Magnetic and Electrophoretic Extraction of Cells, Particles and Macromolecules
171