Nano and Micro Technologies
Kate McGown (she/her/hers)
Student
University of Kentucky
Lexington, Kentucky, United States
Sheng Tong
Associate Professor
University of Kentucky, United States
Sub-track: Micro/Nano Technologies In Molecular and Cellular Bioengineering, Medicine and Biology
Magnetic fluid hyperthermia (MFH) uses the magnetic properties of iron oxide nanoparticles to generate heat under an alternating magnetic field (AMF), as shown in Figure 1. The mechanism of heat generation is based on Brownian relaxation—rotation of the particle with a fixed magnetic moment, and Néel relaxation—rotation of the magnetic moment inside the particle. MFH has applications in destroying tumor cells and rewarming preserved organs; both are processes that require rapid heating of a precise area. This study aims to characterize the heating properties of magnetite (Fe3O4) iron oxide nanoparticles, both experimentally and mathematically. We show the relationship between particle size, frequency (f), and field strength (H) with the specific absorbance rate (SAR), with respect to the accepted clinical tolerance of f × H < 5 × 109 Am-1s-1.
Synthesized 15 nm and 20 nm magnetite nanoparticles were coated with polyethylene glycol (PEG) and dispersed in DI water to a concentration of 1 mg/mL. The heating efficiency of magnetite particles was measured using the nanoTherics magneTherm machine. 1 mL samples (n = 3) at room temperature (22℃) were tested using a 25 mm high field strength coil under five different frequencies, ranging from 169 to 983 kHz, and field strengths ranging from 8 to 42 kA/m. The heat generated by the particles under the AMF is analyzed as SAR according to:
SAR = (1/mFe)Csolmsol(dT/dt)
Where mFe is the mass of Fe, Csol is the specific heat of the solvent, msol is the mass of the solvent, and dT/dt is the change in solution temperature over time.
Experimental results were compared to a mathematical Monte Carlo MATLAB simulation with the same parameters. In the simulation, the SAR is calculated as the product of the area of the hysteresis loops, A, with the frequency, f:
SAR = A × f.
The experimental results show linear correlation (R2 > 0.97) between field strength and SAR (Figures 2 & 3), and a quadratic relationship (R2 > 0.998) between frequency and SAR (Figures 4 & 5). The highest SAR values achieved were 206 W/gFe (15 nm) and 517 W/gFe (20 nm), using a frequency of 983 kHz and field strength of 16 kA/m (f × H = 1.57 × 109 Am-1s-1). Based on the accepted clinical tolerance, f × H < 5 × 109 Am-1s-1, we selected a frequency of 605 kHz and field strength of 8 kA/m (f × H = 4.84 × 109 Am-1s-1) to simulate conditions acceptable in human studies. Under this condition, 15 and 20 nm particles generated SAR values of 30.2 W/gFe and 110 W/gFe.
The mathematical results show similar trends (Figures 6-9). The theoretical 15 and 20 nm particles reach a maximum SAR of 436 W/gFe and 886 W/gFe, respectively. This is a relative error of 53% and 42%, respectively, from the corresponding experimental result. The clinically applicable condition generated SAR values of 44 W/gFe (15 nm, 32% RE) and 141 W/gFe (20 nm, 22% RE). We suggest that the large relative error is due to running the program through only 100 cycles to generate preliminary results and that using a larger cycle number (n > 1000) will significantly decrease the relative error.
This study compares the heating ability of 15 and 20 nm magnetite particles. Both experimental and mathematical results suggest a linear increase in SAR with increasing field strength and a quadratic increase in SAR with increasing frequency. We also note an increase in SAR with increasing particle size, which shows promise for large temperature increases even at lower frequencies. Also of note is the change in the hysteresis loops between the two particle sizes (Figure 10). The thin, S-shaped curves produced by the 15 nm simulation suggest superparamagnetic character, whereas the wider curves of the 20 nm simulation suggest more ferromagnetic character. These results show promise for future applications in both cancer treatment and organ preservation, providing a fast, efficient alternative to conventional methods.