Temperature dosimetry using MR relaxation characteristics of poly(vinyl alcohol) cryogel (PVA-C)

Magnetic cryogels as a shape-selective and customizable platform for hyperthermia-mediated drug delivery

Cryogels consisting of polyvinyl alcohol and iron (II, III) oxide magnetic nanoparticles coated with a model drug—acetaminophen, were developed as a tunable platform for thermally triggered drug release, based on shape-selective heat transfer. Two different shapes of cryogels; discs and spherical caps, were formed via adding polymer-nanoparticle-drug mixtures into 3D printed molds, followed by freeze-thawing five times. No additional chemical crosslinking agents were used for gel formation and the iron oxide nanoparticles were coated with acetaminophen using only citric acid as a hydrogen-bonding linker. The two gel shapes displayed varying levels of acetaminophen release within 42–50 °C, which are ideal temperatures for hyperthermia induced drug delivery. The amount and time of drug-release were shown to be tunable by changing the temperature of the medium and the shape of the gels, while keeping all other factors (ex. gel volume, surface area, polymer/nanoparticle concentrations and drug-loading) constant. The discs displayed higher drug release at all temperatures while being particularly effective at lower temperatures (42–46 °C), in contrast to the spherical caps, which were more effective at higher temperatures (48–50 °C). Magnetic hyperthermia-mediated thermal imaging and temperature profiling studies revealed starkly different heat transfer behavior from the two shapes of gels. The disc gels retained their structural integrity up to 51 °C, while the spherical caps were stable up to 59 °C, demonstrating shape-dependent robustness. The highly customizable physicochemical features, facile synthesis, biocompatibility and tunable drug release ability of these cryogels offer potential for their application as a low cost, safe and effective platform for hyperthermia-mediated drug delivery, for external applications such as wound care/muscle repair or internal applications such as melanoma treatment.

Effect of Cryogenic Treatment on the Rheological Properties of Gelatin Hydrogels

Gelatin has the ability to form a gel when cooled below the sol—gel temperature. In this study, the effects of various cryo-parameters, including the number of freeze—thaw cycles, cooling rate, thawing rate, and gelatin concentration, on the material properties were examined. The rheological properties of the cryogels improved with increasing cryo-cycles and decreasing cooling and thawing rates as well as were superior to those of the corresponding hydrogels formed at room temperature. In addition, the critical gelation concentration decreased after repeated cryo-treatments. Methacrylamide-modified gelatin was also treated cryogenically, followed by in situ UV irradiation to enable radical cross-linking. The cross-linking efficiency of specific gelatin concentrations improved with freeze—thawing. Cryogelation can be used to fine-tune the mechanical properties of hydrogels. This is of relevance to tissue engineering where porous gelatin hydrogels are used as biomaterials.

Review of tissue simulating phantoms for optical spectroscopy, imaging and dosimetry

Optical spectroscopy, imaging, and therapy tissue phantoms must have the scattering and absorption properties that are characteristic of human tissues, and over the past few decades, many useful models have been created. In this work, an overview of their composition and properties is outlined, by separating matrix, scattering, and absorbing materials, and discussing the benefits and weaknesses in each category. Matrix materials typically are water, gelatin, agar, polyester or epoxy and polyurethane resin, room-temperature vulcanizing (RTV) silicone, or polyvinyl alcohol gels. The water and hydrogel materials provide a soft medium that is biologically and biochemically compatible with addition of organic molecules, and are optimal for scientific laboratory studies. Polyester, polyurethane, and silicone phantoms are essentially permanent matrix compositions that are suitable for routine calibration and testing of established systems. The most common three choices for scatters have been: (1.) lipid based emulsions, (2.) titanium or aluminum oxide powders, and (3.) polymer microspheres. The choice of absorbers varies widely from hemoglobin and cells for biological simulation, to molecular dyes and ink as less biological but more stable absorbers. This review is an attempt to indicate which sets of phantoms are optimal for specific applications, and provide links to studies that characterize main phantom material properties and recipes.

Multi-modality tissue-mimicking phantom for thermal therapy

A tissue-mimicking phantom material has been developed for use with thermal therapy devices and techniques. This material has magnetic resonance properties (primarily T2) which change drastically upon thermal coagulation, enabling its use for device characterization and treatment verification using simple T2-weighted imaging techniques. The coagulation temperature of the phantom can be changed from 50-60 degrees C by adjusting the pH from 4.3 to 4.7. The energy absorption properties can be adjusted to match the acoustical and optical properties of tissues. T2 relaxation measurements are provided as a function of temperature, along with T2-weighted MR images to illustrate the visualization of heating patterns. A complete recipe for fabricating phantoms is provided.

Investigation and analysis of ferrous sulfate polyvinyl alcohol (PVA) gel dosimeter

Ferrous sulfate (Fe(SO4)2) PVA gels were investigated for a range of absorbed doses up to 20 Gy using both magnetic resonance imaging (MRI) and spectrophotometry to determine R1 and optical density (OD) dose responses and G values. It was found that R1- and OD-dose sensitivities increased with O2 saturation or by the introduction of a freeze-thaw cycle during preparation of the PVA gel. The storage temperature of the Fe(SO4)2 PVA gel at -18 degrees C increased R1-dose sensitivity above that of gels stored at 5 degrees C. The addition of sucrose to the formulation was found to result in the largest increase in both R1- and OD-dose sensitivities. Fe(SO4)2 PVA gel with and without the addition of xylenol orange was demonstrated to have a G value of approximately 20 ions/100 eV and with sucrose approximately 24 ions/100 eV.