Zoe Boekelheide and Cindi Dennis
Magnetic nanoparticles in colloidal liquid form are used for damping in vehicle suspensions, MRI contrast agents, heat transfer materials, and even in art installations. One particular application we are interested in is magnetic nanoparticle hyperthermia (MNH) cancer treatment. MNH is a promising method of cancer therapy in which magnetic nanoparticles are targeted to a tumor, an alternating magnetic field is applied, and the magnetic moments of the nanoparticles switch directions, releasing heat and killing tumor cells. This treatment has proven effective in many mouse models and some small clinical trials on humans, and measurement science can help guide commercialization. The evaluation of nanoparticles for use in MNH therapy includes characterization of basic magnetic properties as well as direct measurements of the heat released under application of an alternating magnetic field.
Techniques for measuring basic magnetic properties, such as magnetization (M), are well-developed for small solid samples such as bulk crystals and thin films, but special issues arise in measurement of fluid samples. First, the effects of the sample vessel must be taken into account. Often, the vessel must be vacuum-tight; care must be taken that the sealing process does not physically change the properties of the fluid. Then, the portion of the signal due to the sample vessel should be subtracted from the total, not a trivial subtraction as the sample vessel has a different geometry from the sample (in contrast to, e.g., a thin film sample and substrate). In addition, the sample must be centered, adding an additional degree of difficulty when the material is fluid and the center position may be a dynamic property. Our results show that incorrect centering can lead to not only incorrect values of M, but to a change in the shape of M(H).
There are many issues complicating the accurate measurement of heat output of magnetic nanoparticles under an alternating magnetic field. One issue is temperature measurement, as bulk metallic temperature sensors experience eddy current heating, causing errors, and remote measurement by infrared detects only the surface temperature. The best method currently available is measurement by fiber optic sensor. Additionally, departures from adiabicity must be quantified and accounted for and the results scaled by the demagnetization factor of the magnetic sample. We are building a calorimeter to accurately measure the heat output by nanoparticle samples designed for MNH. Once these measurement and data analysis issues are overcome and standard measurement and reporting techniques are adopted by the scientific community, measurements of nanoparticles by different research groups can be compared directly, leading to improved understanding of the mechanisms responsible for producing heat and optimization of nanoparticles for MNH treatment.