Microwave and millimeter-wave sensing has become one of the most promising non-invasive routes to skin cancer diagnosis. The technique exploits a simple physical fact: malignant lesions such as basal cell carcinoma, squamous cell carcinoma, and melanoma contain more water than healthy skin, and water-rich tissue has a higher dielectric permittivity at microwave frequencies. By placing a transmitter and a receiver antenna in contact with the skin and exciting electromagnetic surface waves that travel along the skin-air interface, the system can detect the extra attenuation and phase shift introduced by a tumor. The approach is non-ionizing, painless, low-cost, and capable of real-time readout, and recent advances in antenna design and deep-learning signal processing have pushed spatial resolution and diagnostic reliability steadily upward.

Yet safety has lagged behind performance. Most previous evaluations of skin-cancer detectors relied on a single-tissue model representing a single anatomical site, typically the forearm or abdomen, and most assessed safety solely through the specific absorption rate, or SAR, the standard metric that quantifies how much electromagnetic energy is dissipated per kilogram of tissue. The McGill team argues that both shortcuts are problematic. Anatomical differences in skin and subcutaneous structure across the body strongly influence the electric-field distribution at microwave frequencies, so a single-site assessment may not reflect the actual risk during clinical deployment. And SAR itself, while codified in regulatory frameworks, does not capture exposure duration, tissue perfusion, or heat diffusion, all of which determine the physiological outcome that actually matters: how much the tissue warms up.

The two antennas under study were both built on 50-micrometer-thick flexible Kapton substrates chosen for biocompatibility and good skin contact. The monopole antenna, the team’s first-generation design, measures 20 by 20 millimeters and operates from 2 to 5 gigahertz, with a standard SMA connector on the reverse side for repeatable measurements. The newer Vivaldi antenna, measuring 26 by 17 millimeters and operating from 3 to 6 gigahertz, was selected for its end-fire radiation pattern and wide bandwidth, which confine energy along the skin surface. Because biological tissue has a high permittivity, both antennas are far smaller than their free-space counterparts, a key advantage for wearable and point-of-care deployment.

To capture anatomical reality, the researchers constructed nine multilayer tissue models in Ansys HFSS, each consisting of skin, fat, muscle, and, where relevant, bone. Layer thicknesses came from published anatomical measurements: skin thicknesses ranged from 0.9 millimeters on the forehead to 1.7 millimeters on the buttock, while fat thickness varied from a mere 0.3 millimeters on the palm to 12.1 millimeters on the buttock. For the forehead, palm, and sole, all four layers were included because soft tissue is thin enough for the waves to reach bone. At the other six sites, where soft tissue exceeds 20 millimeters, the bone layer was omitted, since microwave penetration depth in soft tissue at 2 to 6 gigahertz is less than 20 millimeters and the waves attenuate before reaching bone. Each model shared identical lateral dimensions of 12 by 6 centimeters, large enough to accommodate the compact antennas.

The full-wave simulations revealed a consistent pattern for both antennas: the peak electric field, and therefore the SAR hot spot, always appears in a small area on the skin surface directly beneath the transmitting antenna. But the magnitude of that hot spot varies enormously with anatomy. The forehead model, with only about 5.1 millimeters of soft tissue over bone, produced the strongest fields, because the wave reflects strongly at the bone interface and constructively reinforces the field at the surface. Sites with much thicker soft tissue, roughly 11 to 33 millimeters, showed far weaker surface fields. At 6 gigahertz, the peak SAR in the forehead model approached nearly twice that of the sole or neck models at identical input power, and the monopole results showed the same trend, with all other sites registering 43 to 54 percent lower peak SAR than the forehead.

Frequency matters as well. Across the operating bands, peak SAR rose steadily with frequency in every tissue model. The explanation lies in the dispersive nature of tissue dielectric properties: as frequency increases, conductivity rises while permittivity falls, which strengthens the skin effect and confines the electromagnetic field ever more tightly to the superficial layers. Because SAR is proportional to the square of the local electric field times conductivity, the combination of higher conductivity and a more surface-localized field drives peak absorption upward at higher frequencies.

The temperature analysis, performed by solving Pennes’ bioheat equation with an explicit finite-difference scheme in MATLAB, added a twist that SAR alone could never reveal. Under a 10-dBm Vivaldi scan over 100 seconds, the palm model heated the most, reaching a peak rise of 2.47 degrees Celsius, while the buttock model, despite having the second-highest peak SAR of all nine sites, produced the smallest temperature rise, about 20 percent lower. The ranking of temperature rise simply does not follow the ranking of SAR, because heating depends not only on the absorption hot spot but also on the spatial and temporal distribution of deposited power, the thermal properties of each layer, and heat diffusion. Even in a short 10-second exposure, comparable to a real clinical scan, the differences persisted: the arm model showed the highest rise at 0.516 degrees Celsius, roughly 16 percent above the buttock. The authors note that at most sites the 1-degree-Celsius safety threshold adopted by IEEE Standard C95.1 would be reached within 30 seconds at 10 dBm, meaning that power level is simply too high for practical use.

From the SAR results, the team derived site-specific power limits that keep the peak spatial-average SAR below the 1.6 watts-per-kilogram threshold mandated by both the U.S. FCC guidelines and Health Canada’s Safety Code 6, averaged over 1 gram of tissue. For the Vivaldi antenna, allowable power ranged from as low as 2.2 dBm at the forehead up to 4.9 dBm at the sole, so the researchers adopted the strictest value, 2.2 dBm, as the global limit for full-body use. Thermal verification confirmed the conservativeness of that choice: after a 10-second exposure at 2.2 dBm, the highest temperature rise anywhere was only about 0.068 degrees Celsius, negligible by any clinical standard. For the monopole antenna, the corresponding global limit was 2.9 dBm, again set by the forehead, and the worst-case 10-second temperature rise at that power stayed below 0.09 degrees Celsius, occurring at the palm.

The authors are careful to note the limitations of their thermal model, which omits blood perfusion, metabolic heat, and convective cooling at the skin-air interface, making the temperature predictions deliberately conservative upper bounds. Future work will incorporate perfusion and heterogeneous boundary effects. Even so, the study delivers something the field has lacked: a standardized workflow, from electric-field mapping and SAR calculation to temperature-rise evaluation and power-limit derivation, that bridges electromagnetic design and biomedical safety. For a technology racing toward clinical integration, the message is clear. A safety assessment based on a single body site is not a safety assessment at all, and the device that scans your forehead may need very different power settings from the one that scans your sole.

Subject of Research: Radiation safety analysis of microwave surface-wave antennas for skin cancer detection across nine anatomical sites

Article Title: Site-dependent numerical safety analysis in microwave skin spectroscopy: comparison of monopole and vivaldi antennas

Article References: Shang, S., & Popović, M. (2025). Site-dependent numerical safety analysis in microwave skin spectroscopy: comparison of monopole and vivaldi antennas. Frontiers in Antennas and Propagation, 3, Article 1714578. https://doi.org/10.3389/fanpr.2025.1714578

Image Credits: AI Generated

DOI: 10.3389/fanpr.2025.1714578

Keywords: microwave sensing, skin cancer detection, specific absorption rate, antenna safety, temperature rise, bioheat equation, Vivaldi antenna, monopole antenna, wearable devices, electromagnetic simulation, radiation exposure, tissue modeling

Magnetic hyperthermia works on a deceptively simple principle. When magnetic nanoparticles are exposed to an alternating magnetic field, or AMF, they absorb energy from the field and dissipate it as heat. Because tumor cells are generally more vulnerable to elevated temperatures than healthy tissue, maintaining temperatures in the range of approximately 42 to 46 degrees Celsius can trigger apoptosis, the programmed death of cancer cells, while largely sparing surrounding tissue. The technique has been investigated as a minimally invasive local treatment and as a complementary approach to conventional cancer therapies, and its application has been explored across a wide range of tumor models and treatment settings, from breast cancer scaffolds to glioblastoma and prostate cancer models.

The heating capability of a nanoparticle dispersion is typically described by the specific absorption rate, or SAR, which quantifies the power absorbed per unit mass of magnetic material. Crucially, SAR is not a fixed property of a material. It depends on external factors such as the amplitude and frequency of the applied field and the viscosity of the medium, as well as intrinsic particle characteristics including particle size, magnetic anisotropy, and concentration. This complexity is why materials scientists obsess over structural details that might seem trivial, such as the thickness of an insulating shell wrapped around a magnetic core.

Iron oxide nanoparticles remain the favorite candidates for clinical magnetic hyperthermia because they combine favorable heating capability with a relatively acceptable safety profile. Alternative materials such as cobalt and manganese nanoparticles offer stronger magnetic performance but are limited by poor chemical stability and potential toxicity. The catch with iron oxide is colloidal stability. In physiological media, unmodified iron oxide nanoparticles tend to aggregate, particularly smaller particles with large surface areas that clump together to reduce surface energy. Proteins also readily adsorb onto their surfaces, forming a protein corona that can induce toxicity. Surface modification is therefore essential, and silica has long been favored for this role owing to its low toxicity, chemical robustness, and ease of processing.

In the new study, the team synthesized magnetic iron oxide nanocrystals approximately eleven nanometers in diameter using chemical co-precipitation, then grew silica layers onto them through a sol-gel method based on the hydrolysis and condensation of tetraethyl orthosilicate, or TEOS. By adjusting the amount of TEOS precursor added, they produced three samples with silica layers of approximately two, four, and six nanometers, designated MIONs/SiO₂-1, MIONs/SiO₂-2, and MIONs/SiO₂-3 respectively. Transmission electron microscopy revealed that the products were not isolated single-core core-shell particles but rather clusters in which multiple iron oxide cores were embedded within continuous, amorphous silica-rich regions, an architectural detail that shaped the entire interpretation of the study.

The characterization work confirmed the coating chemistry in multiple independent ways. X-ray diffraction showed that silica encapsulation preserved the original spinel iron oxide crystal structure, while indicating a partially oxidized phase containing both magnetite-like and maghemite-like components. Fourier-transform infrared spectroscopy revealed the characteristic silicon-oxygen vibrations of the silica framework, and X-ray photoelectron spectroscopy showed a growing silicon signal and attenuating iron signal with increasing silica content, alongside largely unchanged iron oxidation states. Zeta potential measurements became substantially more negative after coating, reflecting the negatively charged hydroxyl groups generated on the silica surface, which provided strong electrostatic stabilization in water and improved resistance to rapid sedimentation compared with uncoated particles.

Colloidal behavior, however, proved highly medium-dependent. Over seven days, the coated clusters remained relatively stable in deionized water and in cell culture medium supplemented with fetal bovine serum, where serum proteins apparently formed a stabilizing corona. In phosphate-buffered saline, by contrast, all three coated samples aggregated severely, with hydrodynamic diameters climbing beyond 1.5 micrometers, because the high ionic strength screened the surface charges that held the hydrated assemblies apart. The results suggest that while silica coating markedly improves dispersion compared with bare iron oxide, salt-induced aggregation remains a genuine challenge for these cluster-type materials in physiological ionic environments.

When the samples were exposed to an alternating magnetic field of 100 kilohertz and 140 oersted, the ultrathin two-nanometer coating emerged as the clear winner. At an iron concentration of 6 milligrams per milliliter in water, MIONs/SiO₂-1 achieved a specific absorption rate of 80.11 watts per gram and an intrinsic loss power of 6.454 nanohenry square meters per kilogram, values that were substantially higher than those of the uncoated nanoparticles, which reached only 13.55 watts per gram, and of the four- and six-nanometer coated samples, which recorded 31.44 and 42.50 watts per gram respectively. The temperature rise of the two-nanometer coated sample nearly doubled that of the bare particles under identical conditions.

Mechanistically, the outcome makes physical sense. Increasing the silica thickness inevitably dilutes the magnetic material, and indeed saturation magnetization fell from 74.6 emu per gram for bare nanoparticles to 40.8, 32.7, and 25.3 emu per gram for the two-, four-, and six-nanometer coated samples. But silica does more than dilute. It modifies surface charge, aggregation state, hydrodynamic size, and the spacing between magnetic cores, all of which influence dipolar interactions and collective magnetic relaxation. Thicker shells also enlarge the hydrodynamic diameter, restricting Brownian rotation, one of the two main relaxation channels by which superparamagnetic particles dissipate field energy, the other being Néel relaxation, the thermally assisted reorientation of the magnetic moment within the crystal lattice.

To probe which relaxation pathways were actually at work, the researchers performed viscosity-dependent heating experiments using glycerol-water mixtures and immobilized their best sample in agarose gels. Heating in viscous glycerol solutions and gelled samples fell below the value in pure water, consistent with a partial contribution from Brownian-type rotation of the whole assembly, yet substantial heat was still generated in the gels, indicating that internal magnetic relaxation remained the dominant mechanism. The authors are appropriately cautious, noting that differences in heating cannot be attributed exclusively to silica thickness, that the applied field conditions exceeded the classical Brezovich safety criterion and should be regarded as a material-screening condition rather than a clinically optimized one, and that cytotoxicity, hemocompatibility, and cell-based hyperthermia studies remain necessary before any clinical translation. Still, the message is clear: when it comes to coating magnetic nanoparticles for heat therapy, thinner silica may be the sweet spot.

Subject of Research: Effect of ultrathin silica coating thickness on the magnetothermal heating performance of silica-coated magnetic iron oxide nanoparticle clusters for magnetic hyperthermia

Article Title: Ultrathin SiO₂-coated magnetic iron oxide nanoparticle clusters for magnetic hyperthermia: effect of silica layer thickness on magnetothermal performance

Article References: Chen, T., Im, P. W., Kim, H., Hong, H., Zhang, W., Quan, B., Paek, S. H., & Piao, Y. (2026). Ultrathin SiO₂-coated magnetic iron oxide nanoparticle clusters for magnetic hyperthermia: effect of silica layer thickness on magnetothermal performance. Journal of Nanoparticle Research, 28(9), Article 240. https://doi.org/10.1007/s11051-026-06767-y

Image Credits: AI Generated

DOI: 10.1007/s11051-026-06767-y

Keywords: magnetic hyperthermia, iron oxide nanoparticles, silica coating, nanoparticle clusters, specific absorption rate, intrinsic loss power, alternating magnetic field, colloidal stability, sol-gel synthesis, Néel relaxation, Brownian relaxation, cancer therapy