  Magnetic fluid hyperthermia (MFH), the procedure of raising the temperature
of tumor cells using magnetic nanoparticles (MNPs) as heating agents, has
proven successful in treating some types of cancer. However, the low heating
power generated under physiological conditions makes necessary a high local
concentration of MNPs at tumor sites. Here, we report how the in vitro heating
power of magnetically soft MnFe$_2$O$_4$ nanoparticles can be enhanced by
intracellular low-dimensional clusters through a strategy that includes: a) the
design of the MNPs to retain N\'eel magnetic relaxation in high viscosity
media, and b) culturing MNP-loaded cells under magnetic fields to produce
elongated intracellular agglomerates. Our direct in vitro measurements
demonstrated that the specific loss power (SLP) of elongated agglomerates
($SLP=576\pm33$ W/g) induced by culturing BV2 cells in situ under a dc magnetic
field was increased by a factor of 2 compared to the $SLP=305\pm25$ W/g
measured in aggregates freely formed within cells. A numerical mean-field model
that included dipolar interactions quantitatively reproduced the SLPs of these
clusters both in phantoms and in vitro, suggesting that it captures the
relevant mechanisms behind power losses under high-viscosity conditions. These
results indicate that in situ assembling of MNPs into low-dimensional
structures is a sound possible way to improve the heating performance in MFH.
