Sensitive Gels Research Articles

The use of gel dosimetry for verification of electron and photon treatment plans in carcinoma of the scalp

In recent years there has been a large amount of research into the potential use of radiation sensitive gels for three-dimensional verification of clinical radiotherapy doses. In this paper we report the use of a MAGIC gel dosimeter (Fong et al 2001 Phys. Med. Biol. 46 3105) for the verification of a specific patient's radiation therapy dose distribution. A 69-year-old male patient presented with a squamous cell carcinoma extending approximately 180° across the top of the scalp (anterior to posterior) and from just over midline to 90° left of the skull. The patient's treatment was commenced using two electron fields. For gel dosimetry, phantoms were produced in which the outer surface spatially corresponded to the outer contours of the patient's anatomy in the region of irradiation. The phantoms were treated with either electrons or intensity modulated radiation therapy (IMRT) with photons. The results identified a hot spot between the matched electron fields and confirmed the more homogeneous dose distribution produced by the IMRT planning system. The IMRT plan was then clinically implemented. The application of a clinical dose to a phantom shaped to a specific patient as well as the ability to select a slice at will during phantom imaging means that gel dosimetry can no longer be considered to simply have potential alone, but is now in fact a useful dosimetric tool.

Pressure effects on the inhomogeneities of environmentally sensitive polymer gels

AbstractThe effects of pressure at gel preparation on the inhomogeneities were investigated by small‐angle neutron scattering. Poly(N‐isopropylacrylamide) (PNIPA) gels, known as thermosensitive polymer gels undergoing volume transition, were prepared at various pressures ranging from 0.1 to 300 MPa. The scattering intensity increased with increasing Pprep, up to 200 MPa, then decreased by further increase in Pprep. The degree of inhomogeneities evaluated by the ratio of the static and dynamic correlators exhibited a similar behavior to that of the ensemble average light scattered intensity, <I>E. The physical meaning of the gel inhomogeneities is discussed from the viewpoint of swelling thermodynamics. It will be shown that the increase in the swelling degree is due not to an increase in the miscibility of PNIPA with water but to the increase in inhomogeneities.

Optimisation of X-ray micro-tomography for the low-dose analysis of highly-dosed gels

An X-ray micro-tomography system has been designed that is optimised for low dose imaging of radiation sensitive polymer gels. The scanner is based on a third-generation cone-beam X-ray CT scanner. A mini-focus X-ray source (Oxford XTF5011 or an Oxford series 1500) is used in conjunction with a 100mm diameter X-ray image intensifier (Hamamatsu C7336). The sample may be rotated and moved perpendicular to the beam axis.

The use of static light scattering for the structure analysis of radiosensitive polymer gels: a literature survey

Static light scattering (SLS) could be a worthy technique to perform a structure analysis of the polymer structures inside radiation sensitive gels. The information obtained with SLS is a static characterization of the particle structures inside the gel. SLS will be combined with NMR relaxometry and NMR diffusion measurements, which deliver a hydrodynamic characterization of the microstructure of the gels.

Scanning water phantom to optical laser CT scanner conversion kit

Optical computed tomography (CT) scanners have been developed for quantitative reading of dose images written into radiation sensitive gels. Laser equipped, single ray CT scanners, similar to the first generation x-ray CT scanner geometry, provide high quality data because of efficient scatter rejection. These systems are effective for quantitative transmission measurements with both polymerization and radiochromic gels. Widespread research into optical CT gel dosimetry is limited by the lack of readily available commercial instruments. This project demonstrates how scanning water phantoms, which are common equipment in radiotherapy institutions, can easily be modified to perform optical laser CT scanning.

Intelligent polyelectrolytes and gels in oral drug delivery.

The present review concerns smart, or intelligent polymers for oral administration that change conformation in aqueous solutions in response to external stimuli such as pH or temperature. We concentrate on charged polymers and gels with polyelectrolyte properties. Because of the ionization at a certain pH or in response to changes in the ionic composition of the solution, a polyelectrolyte has better chances of displaying smart properties than a neutral polymer. When such smart polyelectrolyte is cross-linked by covalent or hydrogen bonding and/or physical aggregation or is entangled, it forms an environmentally sensitive gel capable of swelling and collapse in an aqueous medium. Varying pH, temperature, and microbial flora are found in the gastrointestinal tract, and thus pH- sensitive polymers and gels that can be degraded by specific enzymes and/or inhibit proteolytic enzymes can be tailored for the efficient site-specific therapy. Smart polymers wield a lot of promise in the targeted, site-specific administration where they can provide advantages in loading of sensitive drugs such as proteins and peptides, while releasing the drug at a specific pH or in response to the presence of certain microbial flora.

Modeling the hyperelasticity of magnetic field sensitive gels

The continuum theory, previously developed to quantitatively account for the large deformations observed in gels endowed with electric properties, is extended to magnetic field sensitive gels (ferromagnetic or diamagnetic in origin). The derived analytical formula for the dependence of the gel displacement on the magnetic field can be applied either to control recently developed biomimetic valves and possible artificial muscles constructions or to interpret similar phenomena in biophysics.

Novel pH, ion sensitive polyampholyte gels based on carboxymethyl chitosan and gelatin

AbstractNatural pH and ion sensitive polyampholyte gels were successfully prepared by blending carboxymethyl chitosan (CM‐chitosan) and gelatin using glutaraldehyde as crosslinking agent. Their swelling behaviour under the influence of pH and ionic strength of the solution has been studied, and molecular interaction and, morphology of the gels were investigated by infrared spectroscopy and scanning electron microscopy. At the isoelectric point (IEP) the blend gels shrunk most, and when pH deviated from the IEP they behaved as polycations or polyanions. A decrease in swelling degree (Ds) with rising NaCl concentration up to 0.15 M was observed for all gels, while addition of Ca2+ made them shrink due to formation of Ca2+ crosslinked bridges. The blended gels showed increasing sensitivity to pH and ionic strength as the weight fraction of CM‐chitosan increased. Two essential components seem to regulate the swelling behaviour of blended gel: the first is related to osmotic pressure difference caused by the redistribution of mobile ions; the second is related to the possible formation of interactions corresponding to hydrogen bonding and hydrophobic interactions.© 2003 Society of Chemical Industry

Polymer Gels that Memorize Structures of Mesoscopically Sized Templates. Dynamic and Optical Nature of Periodic Ordered Mesoporous Chemical Gels

A periodically ordered interconnecting porous structure could be imprinted in thermosensitive hydrogels by using silica colloidal crystals as mesoscopically sized templates. The interconnecting porous structure provides a fast response to the change in temperature for reversible swelling and shrinking of the hydrogels, while the periodically ordered mesoscopical structure endows a structural color to the gel. The structural color imprinted in the gel is quickly synchronized with the change in the volume of the gels. The newly invented mesoporous hydrogels possess optical properties as temperature sensitive smart gels.

MUSCULAR CONTRACTION MIMICED BY MAGNETIC GELS

The ability of magnetic-field-sensitive gels to undergo a quick controllable change of shape can be used to create an artificially designed system possessing sensor- and actuator functions internally in the gel itself. The peculiar magneto-elastic properties may be used to create a wide range of motion and to control the shape change and movement, that are smooth and gentle similar to that observed in muscle. Magnetic field sensitive gels provide attractive means of actuation as artificial muscle for biomechanics and biomimetic applications.

Reversible adsorption of calcium ions by imprinted temperature sensitive gels

With the aim of developing polymeric gels sensitive to external stimuli and able to reversibly adsorb and release divalent ions, copolymer gels of N-isopropylacrylamide (NIPA) and methacrylic (MAA) monomers were prepared. We chose calcium as a target divalent ion. Two MAAs form a complex with a calcium ion, and the NIPA component allows the polymers to swell and shrink reversibly in response to temperature. The adsorbing site develops an affinity to target ions when the adsorbing molecules come into proximity, but when they are separated, the affinity diminishes. To enhance the affinity to calcium, an imprinting technique was applied using Ca2+ and Pb2+ ions as templates in methylsulfoxide and dioxane media, respectively. The adsorption capacity of the imprinted gels was compared with that of the nonimprinted gels, and the effects of the templates, the solvents, and the amount of methacrylic monomers used in the synthesis and the medium temperature over the Ca2+ adsorption capacity of the gels from aqueous solutions were evaluated. The analysis of the adsorption revealed that (a) the adsorption can be described by the Langmuir isotherms; (b) there is an approximately linear relationship between saturation and methacrylic monomer concentration; (c) the affinity depends on the degree of gel swelling or shrinkage that can be switched on and off by temperature; (d) in the shrunken state, the affinity depends approximately linearly on the MAA concentration in the imprinted gels, whereas in the nonimprinted gels it is proportional to the square of MAA concentration; (e) the imprinted gels adsorb more than the nonimprinted gels when MAA concentration is less than that of permanent cross linkers. The success of imprinting of CaMAA2 and PbMAA2 complex is evidence for memory of such complex onto the weakly cross-linked gel.

Reactive nitrogen and phosphorus removal from aquaculture wastewater effluents using polymer hydrogels

We have developed poly(allyl amine hydrochloride) (PAA · HCl) polymer hydrogels, that efficiently remove nitrate (NO 3 −), nitrite (NO 2 −), and orthophosphate (PO 4 3−) nutrient anions from the aquaculture wastewater. The hydrogels were prepared by chemically crosslinking linear PAA · HCl chains with epichlorohydrin (EPI). The anion binding capacity of the pH sensitive polymer gels was measured in standard solutions and studied as a function of gel synthesis parameters. Equilibrium NO 3–N, NO 2–N, and PO 4–P loading of 15, 1.6, and 17 mg/g of dry gel, respectively, were calculated from the measurement of decrease in anion concentration in aqueous solutions using UV–vis spectrophotometry. Batch experiments showed that nutrient concentrations in aquaculture wastewater effluents decreased with regard to PO 4–P by 98+%, NO 3–N by 50+% and NO 2–N by 85+% within 3 h of reaction. The regeneration of the hydrogels was demonstrated by the release of bound nutrient anions upon washing the gels with a 1 N NaOH solution. These results have demonstrated that the hydrogels are appropriate materials for treating aquaculture wastewater effluents, and reducing the nutrient anion concentrations to levels, less than 10 mg/l NO 3–N, 0.08 mg/l NO 2–N, and 0.3 mg/l PO 4–P, suitable for discharge into natural surface waters.

Tissue-equivalent gel for non-invasive spatial radiation dose measurements

In almost every single application of ionising radiation, the accurate determination of absorbed radiation energy is of great importance. Radiation sensitive gels were first proposed for non-destructive and non-invasive measurements of the spatial distribution of radiation absorbed dose in 1984 [1]. Two different systems were developed but neither of these systems is widely used in practice due to some technical drawbacks. To overcome the problems associated with previous systems a gelatin gel of different composition has been devised and characterised for radiation dosimetry. This system is based on the changes in the optical properties of a transparent gel medium when it is exposed to ionising radiation. The detection material was carefully investigated and the role of each constituent of this system has been studied in order to determine the optimum composition. Gel with this formula has a linear dose response in the range 0.1–30 Gy, this range can be altered to suit other applications by changing the concentration of the active chemicals. The gel sensitivity, defined as the change in optical density per unit radiation dose, was found to be 70×10−3 cm−1 Gy−1 and the system stability was studied for both pre- and post-irradiation effects. This new formulation allows a convenient, rapid and economic readout method using an optical tomography system operating at visible wavelengths.

Optimized MR imaging for polyacrylamide gel dosimetry

Polyacrylamide gels are a powerful tool to measure radiation dose by quantifying the NMR T2 relaxation times of the irradiated gel. The exploitation of these radiation sensitive gels in clinical radiotherapy requires accurate mapping of T2 values. This paper describes the optimization strategy used to identify accurate and practical methods of measuring the range of T2 values typical of gel dosimeters (140-700 ms). The MR imaging techniques used to measure T2 values and the choice of image acquisition parameters are described. Four sequences are compared and the results are analysed in terms of accuracy, signal-to-noise ratio and acquisition time. A multiple spin echo sequence was found to yield the most accurate results (98.9%). Single spin echo sequences, such as Hahn spin echo and EPI spin echo, were found to measure gel T2 values with an accuracy of 90.1%. This paper reports the importance of careful selection and optimization of the MR imaging sequences for accurate and reliable polyacrylamide gel dosimetry.

Adhesion of Triblock Copolymer-Based Thermoreversible Gels and Pressure Sensitive Adhesives

ABSTRACTTriblock copolymers with poly (methyl methacrylate) (PMMA) end blocks and a poly (n-butyl acrylate) (PnBA) midblock have been synthesized as model pressure sensitive adhesives and thermoreversible gels. These materials dissolve in a variety of alcohols at temperatures above 60 °C to form freely flowing liquids. At lower temperatures the PMMA end-blocks associate so that the solutions form ideally elastic solids. In our case the solvent is 2-ethylhexanol, polymer volume fractions vary from 0.05 to 0.3, and the elastic moduli are close to 10,000 Pa. We have conducted three types of experiments to elucidate the origins of adhesion and bulk mechanical properties of these materials: 1) Weakly adhering gels: The adhesive properties of the gels are dominated by the solvent. Very little adhesion hysteresis is observed in this case, although we do observe hysteresis associated with the frictional response of the layers. 2) Strongly adhering gels. By heating the gels in contact with a PMMA surface, it is possible to bond the gels to the surface. Development of adhesion as the PMMA blocks penetrate into the PMMA substrate can be probed in this case. The cohesive strengths of the gels are found to be substantially greater than their elastic moduli, so that these materials can be reversibly extended to very high strains. These properties have enabled us to probe the origins of elastic shape instabilities that play a very important role in the behavior of thin adhesive layers. 3) Dried gels – model pressure sensitive adhesives. By removing the solvent at low temperatures, the underlying structure of the gel is preserved, giving a thin elastic layer with excellent performance as a pressure sensitive adhesive. Resistance to adhesive failure, expressed as a velocity-dependent fracture energy, greatly exceeds the thermodynamic work of adhesion. This energy is further magnified by ‘bulk’ energy dissipation when the stress applied to the adhesive layer exceeds its yield stress.

Phosphate binding polymeric hydrogels for aquaculture wastewater remediation

Methods that will remove conventional nutrient pollutants such as reactive phosphorus, even at extremely low concentrations, from wastewater effluents are a major need in the aquaculture industry. In this study, novel phosphate binding crosslinked poly(allylamine), PAA · HCl, polymeric hydrogel materials were developed, which efficiently bind phosphate anions in aquaculture wastewater effluents. The polymeric hydrogels were synthesized by chemically crosslinking linear PAA · HCl chains with epichlorohydrin. The phosphate binding capacity of the synthesized pH sensitive polymer gels was studied as a function of various gel processing parameters. Equilibrium orthophosphate (PO 4 3-) loadings of 47 mg g −1 polymer, were calculated from direct measurement of the decrease in phosphate concentration in aqueous solutions using UV spectroscopy. Experiments showed that pollutant concentrations in aquaculture wastewater effluents decreased with regard to PO 4 3- by more than 99%. The ability of the gels to bind phosphates was not fouled by particulate or dissolved complex organics and inorganics, or counterions which are present in aquaculture wastewater effluents. The hydrogels can be regenerated by release of the bound phosphates upon washing with a 1 N NaOH solution. Results demonstrated that the novel crosslinked polymeric hydrogels are appropriate materials for treating aquaculture wastewater effluents, and reducing the phosphorus concentrations to levels, less than 0.01 ppm, suitable for discharge to natural surface waters.

Kinetics of the shape change of magnetic field sensitive polymer gels

Abstract This study focuses on the dynamical behaviour of magnetic-field-sensitive polymer gels. These gels have been prepared by introducing nanosized magnetic particles into polymer gels homogeneously. We have studied the kinetics of the motion of magnetic gels. The highest working frequency of magnetic gels has been established, and a simple, completed Kelvin model was suggested to describe the dynamical behaviour.

Shape Transition of Magnetic Field Sensitive Polymer Gels

Magnetic field sensitive gels, called ferrogels, are chemically cross-linked polymer networks swollen by a ferrofluid. The monodomain magnetic particles, with a typical size of about 10 nm, couple the shape of the polymer gel to the nonuniform external magnetic field. Shape distortion occurs instantaneously and disappears abruptly when an external magnetic field is applied or removed, respectively. This work provides a thermodynamic analysis of shape transition based on the free energy of the swollen network that includes the elasticity of network chains as well as magnetic interactions of finely dispersed solid particles with the external field. It is shown that noncontinuous shape transition is due to a shift of equilibrium state from one local minimum to another one, similar to a first-order phase transition. The discussions presented here may be useful for the design of magnetically active soft polymeric actuators.

Small-Angle Neutron Scattering Study on Charged Gels in Deformed State

The structure of weakly charged temperature sensitive gels has been investigated in deformed state by means of small-angle neutron scattering (SANS). At low temperatures where the gel is in a good ...

Comparison of the Experimental and Theoretical Structure Factors of Temperature Sensitive Polymer Gels

The comparison of the experimental and theoretical structure factors, S(q), of weakly charged polymer gels has been carried out as a function of the gel parameters, i.e., temperatures at preparation and at observation, polymer concentration, and degrees of cross-linking and of polymerization, where q is the scattering vector. Experimental structure factors at two different q regimes, i.e., small-angle neutron scattering (SANS, 0.005 ≤ q ≤ 0.1 A-1) and light scattering (LS, q ≈ 0.001 A-1) regimes, were employed for the comparison with the Rabin−Panyukov (RP) theory for weakly charged gels. In the case of SANS, a quantitative agreement was obtained for S(q) in the poor solvent regime. For LS, S(q) was decomposed to two contributions from dynamic thermal fluctuations and static frozen inhomogeneities. The experimental results on their parameter dependence of S(q) were successfully reproduced by the RP theory.