(335q) Magnetic Sorting of Unlabeled Red Blood Cells (RBC)

A novel high-gradient magnetic sorter (HGMS) has been used to separate up to 90% of unlabeled RBC from a mixed cell population. Gradient-centrifugation is the current method of choice, and scalability makes centrifugation the most convenient technique for the large volumes handled by blood banks. Magnetic techniques are potentially less stressful to the cells and mechanically simpler; until now only very small volumes and complex magnetic system have been able to achieve unlabeled, magnetic RBC separation.

Emerging cell sorting applications are poised to expand the role of magnetic separations in RBC sorting. Powerful yet sensitive separation technologies will be required to capture extremely rare tumor cells (1:1E9) and progenitor cells that circulate in peripheral blood. Miniaturized permanent magnets may also be utilized in the next generation of microfluidic devices via deposition or lithography.

RBC are uniquely suited to magnetic separation due to their inherently magnetic ferro-heme proteins. In the presence of oxygen, the electron spin of the oxygen-heme complex and adjacent globin bonds has a negligible paramagnetic response to an applied field. The oxygenated RBC is thus diamagnetic like most other cells, having a small SI magnetic susceptibility of -9.22E-6. Under anoxic conditions the net electron spin of the four ferro-heme groups is significantly positive to produce a paramagnetic response in the RBC that is greater that water (-5.70E-6 vs. -9.05E-6, respectively). This means that an oxygenated RBC is repelled by the magnet while a deoxygenated RBC is attracted to the magnet.

However, even the deoxygenated RBC is only slightly magnetic; most magnetic separations are performed on cells that have been magnetically labeled to 10 or 100 times this level of susceptibility. In order to take advantage of the ferro-heme group to dispense with additional costly labeling, a very powerful HGMS is required. A field gradient can always be increased by decreasing the gap between magnetic poles but this has the drawback of reducing the area which the magnetic field covers and thus the volume of cells that may be processed. The challenge for magnetic RBC separation is to design an HGMS that is sufficiently powerful but amenable to scale-up by optimization and parallelization. At the same time, the HGMS has to quickly deoxygenate the cell suspension just prior to separation and then to immediately reoxygenate the sample following separation.

Our new HGMS incorporates a special beveled quadrupole arrangement of off-the-shelf Neodymium Iron Boride (NeFeB) magnets. Over a 2 mm gap it generates a uniform magnetic gradient of 1,600 T/m and a maximum field induction of about 1 T. By using small, modular NeFeB briquettes, devices based on this design can quickly be assembled at very low cost. A sample reservoir with N2 headspace and hollow fiber membrane modules have been incorporated into the HGMS sample to deoxygenate and reoxygenate the sample. The HGMS is compatible with thin-walled tubes, annuli, as well as packed tubes.

Ongoing research aims to finalize an HGMS system capable of reducing the RBC concentration of a whole blood sample to 0.01% of its initial value in fifteen minutes and of processing 1.8E6 RBC per second.