Low-field magnetic separation of monodisperse Fe3O4 nanocrystals

Nov 20 2006

Whether the goal is to separate different types of cells or molecules, methods that rely on the age-old principle of magnetism are a staple among researchers.

Now, two reports show that the use of magnetic nanoparticles in bioseparations could have a significant impact on both clinical oncology and basic cancer research.

Reporting its work in the journal Biotechnology and Bioengineering, a research team headed by Maciej Zborowski, Ph.D., demonstrated that magnetic nanoparticles, combined with antibodies, successfully enriches peripheral blood progenitor cells (PBPCs) in samples of whole blood. Clinical trials have shown that PBPCs are more effective than bone marrow transplantation at restoring an individual's blood cells population following high-dose chemotherapy or radiation therapy.

Current magnetic separation methods, while effective, operate in batch mode rather than flow mode, making them too slow and inefficient for optimal use in a high-throughput clinical setting. In previous work, Zborowski and his colleagues had developed a quadruple magnetic flow sorter (QMS) similar to a fluorescence activated cell sorter (FACS) but with up to a 1,000-fold faster throughput than this widely used device, on the order of 10 million cells per second.

PBPCs are known as CD34+ cells because they express a protein, known as CD34, on their cell surfaces. CD34 is a well-studied molecule and antibodies that bind to this marker are commercially available. In a long series of experiments, the investigators tested a wide variety of methods to develop a procedure for labeling an anti-CD34 antibody with magnetic nanoparticles and using that labeled antibody to bind to CD34+ cells.

Next, the investigators conducted a second set of experiments aimed at optimizing flow conditions and other instrument parameters to maximize the ability of their QMS instrument to separate CD34+ cells from other blood cells. The researchers noted that theoretical modeling of cell flow in magnetic fields was critical to the success of this phase of their study.

Finally, the investigators tested their optimized protocol on human blood samples. These experiments demonstrated that this technique was able to recover between 18 percent and 60 percent of the PBPCs in human blood samples, while the purity of the CD34+ cells ranged from 60 percent to 90 percent. Both parameters fall well within the clinical useful range. The researchers note that they will now develop a sterile protocol as the next step toward clinical trials.

Meanwhile, investigators at Rice University have discovered that magnetic nanoparticles behave much differently than expected in weak magnetic fields, and that they can exploit this unusual behavior to separate mixtures of molecules quickly and inexpensively. The research team, led by Vicki Colvin, Ph.D., at Rice University, published its results in the journal Science.

Given that the magnetic force that operates on bulk-scale magnetic particles decreases dramatically as the particle gets smaller, it appears counterintuitive that a weak magnetic field would have much effect at all on magnetic nanoparticles. But theoretical and experimental work by Colvin and her colleagues suggested that magnetic particles smaller than 20 nanometers in diameter might interact with weak magnetic fields in a way that essentially amplifies the magnetic force operating on these particles.

In fact, that's exactly what the researchers observed when they held a small hand-held magnet to the side of a glass vessel containing a solution of iron oxide nanoparticles. Within minutes, the formerly rust-colored solution became clear as the nanoparticles migrated to the magnet. But the investigators found, too, that the velocity with which the nanoparticles moved toward the magnet depended on the exact size of the nanoparticles.

The investigators note that this latter finding suggests that it should be possible to use iron oxide nanoparticles to separate mixtures of biological molecules by labeling capture reagents, such as antibodies or aptamers, for specific biomolecules with iron oxide nanoparticles of a specific size. When combined with microfluidic technology, low-field magnetic separations could enable faster and less expensive processing of tissue samples for biomarker detection, among other applications.

The work on blood cell separation, which was funded by the National Cancer Institute, is detailed in a paper titled, "Blood progenitor cell separation from clinical leukapheresis product by magnetic nanoparticle binding and magnetophoresis." This paper was published online in advance of print publication. An abstract is available through PubMed. View abstract.

The work on multiplexed separations is detailed in a paper titled, "Low-field magnetic separation of monodisperse Fe3O4 nanocrystals." An abstract of this paper is available at the journal's website. View abstract.