Temperature and Impedance Variations During Tumor Treating Fields (TTFields) Treatment.

Tumor Treating Fields (TTFields) is an FDA-approved cancer treatment technique used for glioblastoma multiforme (GBM). It consists in the application of alternating (100-500 kHz) and low-intensity (1-3 V/cm) electric fields (EFs) to interfere with the mitotic process of tumoral cells. In patients, these fields are applied via transducer arrays strategically positioned on the scalp using the NovoTAL™ system. It is recommended that the patient stays under the application of these fields for as long as possible. Inevitably, the temperature of the scalp increases because of the Joule effect, and it will remain above basal values for most part of the day. Furthermore, it is also known that the impedance of the head changes throughout treatment and that it might also play a role in the temperature variations. The goals of this work were to investigate how to realistically account for these increases and to quantify their impact in the choice of optimal arrays positions using a realistic head model with arrays positions obtained through NovoTAL™. We also studied the impedance variations based on the log files of patients who participated in the EF-14 clinical trial. Our computational results indicated that the layouts in which the arrays were very close to each other led to the appearance of a temperature hotspot that limited how much current could be injected which could consequently reduce treatment efficacy. Based on these data, we suggest that the arrays should be placed at least 1 cm apart from each other. The analysis of the impedance showed that the variations seen during treatment could be explained by three main factors: slow and long-term variations, array placement, and circadian rhythm. Our work indicates that both the temperature and impedance variations should be accounted for to improve the accuracy of computational results when investigating TTFields.

Femtosecond Laser-Induced Vanadium Oxide Metamaterial Nanostructures and the Study of Optical Response by Experiments and Numerical Simulations.

Surface patterning is a popular approach to produce photonic metasurfaces that are tunable when electro-optic, thermo-optic, or magneto-optic materials are used. Vanadium oxides (VyOx) are well-known phase change materials with many applications, especially when used as tunable metamaterial photonic structures. Particularly, VO2 is a well-known thermochromic material for its near-room-temperature phase transition from the insulating to the metallic state. One-dimensional (1D) VO2 nanograting structures are studied by numerical simulation, and the simulation results reveal that the VO2 nanograting structures could enhance the luminous transmittance (Tlum) compared with a pristine flat VO2 surface. It is worth mentioning that Tlum is also polarization-dependent, and both larger grating height and smaller grating periodicity give enhanced Tlum, particularly at TE polarization in both insulating (20 °C) and metallic (90 °C) states of VO2. Femtosecond laser-patterned VO2 films exhibiting nanograting structures with an average periodicity of ≈500-700 nm have been fabricated for the first time to enhance thermochromic properties. Using X-ray photoelectron spectroscopy, it is shown that at the optimum laser processing conditions, VO2 dominates the film composition, while under extra processing, the existence of other vanadium oxide phases such as V2O3 and V2O5 increases. Such structures show enhanced transmittance in the near-infrared (NIR) region, with an improvement in NIR and solar modulation abilities (ΔTNIR = 10.8%, ΔTsol = 10.9%) compared with a flat VO2 thin film (ΔTNIR = 8%, ΔTsol = 10.2%). The slight reduction in transmittance in the visible region is potentially due to the scattering caused by the imperfect nanograting structures. This new patterning approach helps understand the polarization-dependent optical response of VO2 thin films and opens a new gateway for smart devices.

Training artificial neural network for optimization of nanostructured VO2-based smart window performance.

In this work, we apply for the first time a machine learning approach to design and optimize VO2 based nanostructured smart window performance. An artificial neural network was trained to find the relationship between VO2 smart window structural parameters and performance metrics-luminous transmittance (Tlum) and solar modulation (ΔTsol), calculated by first-principle electromagnetic simulations (FDTD method). Once training was accomplished, the combination of optimal Tlum and ΔTsol was found by applying classical trust region algorithm on the trained network. The proposed method allows flexibility in definition of the optimization problem and provides clear uncertainty limits for future experimental realizations.

Two-Dimensional SiO2/VO2 Photonic Crystals with Statically Visible and Dynamically Infrared Modulated for Smart Window Deployment.

Two-dimensional (2D) photonic structures, widely used for generating photonic band gaps (PBG) in a variety of materials, are for the first time integrated with the temperature-dependent phase change of vanadium dioxide (VO2). VO2 possesses thermochromic properties, whose potential remains unrealized due to an undesirable yellow-brown color. Here, a SiO2/VO2 core/shell 2D photonic crystal is demonstrated to exhibit static visible light tunability and dynamic near-infrared (NIR) modulation. Three-dimensional (3D) finite difference time domain (FDTD) simulations predict that the transmittance can be tuned across the visible spectrum, while maintaining good solar regulation efficiency (ΔTsol = 11.0%) and high solar transmittance (Tlum = 49.6%). Experiments show that the color changes of VO2 films are accompanied by NIR modulation. This work presents a novel way to manipulate VO2 photonic structures to modulate light transmission as a function of wavelength at different temperatures.

Vanadium dioxide nanogrid films for high transparency smart architectural window applications.

This study presents a novel approach towards achieving high luminous transmittance (T(lum)) for vanadium dioxide (VO(2)) thermochromic nanogrid films whilst maintaining the solar modulation ability (ΔT(sol)). The perforated VO(2)-based films employ orderly-patterned nano-holes, which are able to favorably transmit visible light dramatically but retain large near-infrared modulation, thereby enhancing ΔT(sol). Numerical optimizations using parameter search algorithms have implemented through a series of Finite Difference Time Domain (FDTD) simulations by varying film thickness, cell periodicity, grid dimensions and variations of grid arrangement. The best performing results of T(lum) (76.5%) and ΔT(sol) (14.0%) are comparable, if not superior, to the results calculated from nanothermochromism, nanoporosity and biomimic nanostructuring. It opens up a new approach for thermochromic smart window applications.

Relaxation dynamics in glycerol-water mixtures: I. Glycerol-rich mixtures.

We discuss the relaxation dynamics of glycerol-water mixtures, as studied by dielectric spectroscopy in the frequency range from 1 Hz to 250 MHz and at temperatures between 173 and 323 K. The experimental results obtained for the glycerol-rich mixtures suggest that the main dielectric relaxation process, as well as the so-called high-frequency "excess wing" (EW) and dc conductivity, follow the same temperature dependence. This result indicates that all of these processes are induced by the same molecular origin. A new phenomenological function is proposed to describe the whole dielectric spectrum in the covered frequency range, and some possible mechanisms of dielectric behaviors through the dc conductivity, the main relaxation process, and the EW are discussed.

Relaxation dynamics in glycerol-water mixtures. 2. Mesoscopic feature in water rich mixtures.

The relaxation dynamics of water-rich glycerol-water mixtures is studied by broadband dielectric spectroscopy (BDS) at 173-323 K and differential scanning calorimetry (DSC) at 138-313 K. These data indicate the existence of the critical concentration of 40 mol % glycerol. In the studied temperature range for water-rich glycerol mixtures, the two states of water (ice and interfacial water) are observed in addition to water in the mesoscopic 40 mol % glycerol-water domains. The possible kinetics of water exchange between different water states is discussed in order to explain the mechanism of the broad melting behavior observed by DSC.