Words by Lluís Martinez, founder and CSO of SEPMAG’s BLOG
A magnetic nanoparticle is typically less than 1 micrometer in size, which puts it into the nanometer realm. To put that into perspective, one magnetic nanoparticle is less than 100th of the size of a human hair. The cores of the particles are spheres most commonly made of iron. The particles are so small that though they are made of a ferromagnetic material they are paramagnetic, which means that they are not magnetic unless an external magnetic field gradient is applied. Applying an external magnetic field gradient affects the behavior of magnetic nanoparticles. In the magnetic field gradient the particles tend to aggregate and overcome drag force to move in the direction of the gradient.This property provides a way to control their movement. This behavior combined with their tiny size makes them an important tool for biotechnology and biomedicine.
These tiny particles have been demonstrating their worth over the past decade in the fields of biotechnology and biomedicine. Some recent applications are magnetic activated cell sorting, magnetic DNA purification, directing tissue growth and graft localization, bacteria and virus detection, drug delivery systems, cancer treatment, and enhanced magnetic resonance imaging (MRI). Additionally, new and creative applications for magnetic nanoparticles are constantly being published in scientific journals.
Magnetic activated cell sorting is a tool commonly used in research to isolate and study a desired cell population. As explained in the recent article “Beyond the compass: using magnets in biological research,” the particles are coated with materials that allow them to specifically bind to cells of interest. The application of an external magnetic field gradient via a permanent magnet causes the magnetic nanoparticles to aggregate together, move in the direction of the gradient, and be isolated from the solution. Magnetic DNA purification is based on a similar concept. Here, the magnetic particles are coated with carboxylic acid and the amount of salt in the solution allows for reversible binding of the magnetic particles to the DNA.
Another exciting application for magnetic nanoparticles is in the realm of tissue engineering. There is a lot of talk in the biomedical field around directing stem cell growth to a site of injury. This is beneficial because the healing would be initiated by a patient’s own cells and the possibility of graft rejection low. The cells can be enticed to locate to a particular location by chemical signals coming from nearby cells, but another method offering more external control would be to incorporate magnetic nanoparticles into the cells and direct them to the desired site by applying a magnetic field gradient. This idea has been shown to work outside of the body in cell cultures. The shape of the tissue that stem cells form when they multiply in a culture has also been controlled by an external magnetic field gradient.
These particles have also been used to detect bacteria and viruses. The systems combine magnetic activated cell sorting with flow cytometry or fluorescence activated cell sorting. The important concept here is that the magnetic beads are surface functionalized. The term surface functionalized means that the beads are coated with materials that allows them to bind to the bacteria or virus and only to that bacteria or virus. This functionalization can be accomplished by using polymer coatings, functional organic chemistry groups, and/or peptides. Once the particles bind with the bacteria or virus there is another mechanism that allows the researcher to identify the particles and sometimes even quantify how much of them are bound. This provides a way to identify the presence of the microbe and to determine how much of it there is. This use of magnetic particles is desirable because other methods of bacteria or virus detection take a long time and need to be performed in a laboratory. These new techniques using magnetic nanoparticles could possibly be scaled down to a portable size and used in locations where laboratory access is limited.
Development of drug delivery systems is another area of intense research. There are diverse ideas for how to do this and all of them differ based on the type of drug, the location it needs to be delivered to, and the timescale of delivery. Magnetic nanoparticles have been used in some of these applications.
Another interesting use for magnetic nanoparticles is for cancer therapy. It is called magnetic hyperthermia. The basic idea is that magnetic nanoparticles are localized to the site of a tumor and an external magnetic field is applied. This magnetic field causes the magnetic particles to “vibrate,” which creates heat. This excess heat (hyperthermia) is damaging to the tumor cells.
Finally, another application for magnetic nanoparticles is enhanced magnetic resonance imaging (MRI). The presence of magnetic nanoparticles causes local magnetic fields that influence nearby water molecules in the body. The MRI machine detects the excitation and relaxation of those water molecules and creates the image. In certain circumstances the magnetic nanoparticles could be more useful than traditional contrast dyes.
Many of the uses of magnetic nanoparticles mentioned here are explained in more detail on the Sepmag blog. This list of applications is by no means complete. The potential of these tiny particles is only limited by the imaginations of the engineers and scientists who decide to use them. There is always more to understand and more to discover about these exciting particles that have already become an indispensable tool for modern research and medicine.