Impact of high-energy electron irradiation on mechanical, structural and chemical properties of agarose hydrogels.

Due to their excellent biocompatibility and biodegradability, natural hydrogels are highly demanded biomaterials for biomedical applications such as wound dressing, tissue engineering, drug delivery or three dimensional cell culture. Highly energetic electron irradiation up to 10 MeV is a powerful and fast tool to sterilize and tailor the material's properties. In this study, electron radiation treatment of agarose hydrogels was investigated to evaluate radiation effects on physical, structural and chemical properties. The viscoelastic behavior, surface hydrophilicity and swelling behavior in a range of typical sterilization doses of 0 kGy to 30 kGy was analyzed. The mechanical properties were determined by rheology measurements and decreased by more than 20% compared to the initial moduli. The number average molecular weight between crosslinks was estimated based on rubber elasticity theory to judge on the radiation degradation. In this dose range, the number average molecular weight between crosslinks increased by more than 6%. Chemical structure was investigated by FTIR spectroscopy to evaluate the radiation resistance of agarose hydrogels. With increasing electron dose, an increasing amount of carbonyl containing species was observed. In addition, irradiation was accompanied by formation of gas cavities in the hydrogels. The gas products were specified for CO2, CO and H2O. Based on the radiolytic products, a radiolysis mechanism was proposed. Electron beam treatment under high pressure conditions was found to reduce gas cavity formation in the hydrogels.

Dependence of light attenuation and backscattering on collagen concentration and chondrocyte density in agarose scaffolds.

Optical coherence tomography (OCT) has been applied for high resolution imaging of articular cartilage. However, the contribution of individual structural elements of cartilage on OCT signal has not been thoroughly studied. We hypothesize that both collagen and chondrocytes, essential structural components of cartilage, act as important light scatterers and that variation in their concentrations can be detected by OCT through changes in backscattering and attenuation. To evaluate this hypothesis, we established a controlled model system using agarose scaffolds embedded with variable collagen concentrations and chondrocyte densities. Using OCT, we measured the backscattering coefficient (µb) and total attenuation coefficient (µt) in these scaffolds. Along our hypothesis, light backscattering and attenuation in agarose were dependent on collagen concentration and chondrocyte density. Significant correlations were found between µt and chondrocyte density (ρ = 0.853, p < 0.001) and between µt and collagen concentration (ρ = 0.694, p < 0.001). µb correlated significantly with chondrocyte density (ρ = 0.504, p < 0.001) but not with collagen concentration (ρ = 0.103, p = 0.422) of the scaffold. Thus, quantitation of light backscattering and, especially, attenuation could be valuable when evaluating the integrity of soft tissues, such as articular cartilage with OCT.

Agarose gel-coated LPG based on two sensing mechanisms for relative humidity measurement.

A relative humidity (RH) sensor based on long-period grating (LPG) with different responses is proposed by utilizing agarose gel as the sensitive cladding film. The spectral characteristic is discussed as the ambient humidity level ranges from 25% to 95% RH. Since increment of RH will result in volume expansion and refractive index increment of the agarose gel, the LPG is sensitive to applied strain and ambient refractive index; both the resonance wavelength and coupling intensity present particular responses to RH within two different RH ranges (25%-65% RH and 65%-96% RH). The coupling intensity decreases within a lower RH range while it increases throughout a higher RH range. The resonance wavelength is sensitive to the higher RH levels, and the highest sensitivity reaches 114.7 pm/% RH, and shares the same RH turning point with coupling intensity response. From a practical perspective, the proposed RH sensor would find its potential applications in high humidity level, temperature-independent RH sensing and multiparameter sensing based on wavelength/power hybrid demodulation and even static RH alarm for automatic monitoring of a particular RH value owing to the nonmonotonic RH dependence of the transmission power within the whole tested RH range.

Tailor-made cell patterning using a near-infrared-responsive composite gel composed of agarose and carbon nanotubes.

Micropatterning is useful for regulating culture environments. We developed a highly efficient near-infrared-(NIR)-responsive gel and established a new technique that enables cell patterning by NIR irradiation. As a new culture substratum, we designed a tissue culture plate that was coated with a composite gel composed of agarose and carbon nanotubes (CNTs). A culture plate coated with agarose only showed no response to NIR irradiation. In contrast, NIR laser irradiation induced heat generation by CNTs; this permitted local solation of the CNT/agarose gel, and consequently, selective cell-adhesive regions were exposed on the tissue culture plate. The solation area was controlled by the NIR intensity, magnification of the object lens and CNT concentration in the gel. Furthermore, we formed circular patterns of HeLa cells and linear patterns of 3T3 cells on the same culture plate through selective and stepwise NIR irradiation of the CNT/agarose gel, and we also demonstrated that individual 3T3 cells migrated along a linear path formed on the CNT/agarose gel by NIR irradiation. These results indicate that our technique is useful for tailor-made cell patterning of stepwise and/or complex cell patterns, which has various biological applications such as stepwise co-culture and the study of cell migration.

Ion diffusion modelling of Fricke-agarose dosemeter gels.

In Fricke-agarose gels, an accurate determination of the spatial dose distribution is hindered by the diffusion of ferric ions. In this work, a model was developed to describe the diffusion process within gel samples of finite length and, thus, permit the reconstruction of the initial spatial distribution of the ferric ions. The temporal evolution of the ion concentration as a function of the initial concentration is derived by solving Fick's second law of diffusion in two dimensions with boundary reflections. The model was applied to magnetic resonance imaging data acquired at high spatial resolution (0.3 mm) and was found to describe accurately the observed diffusion effects.

Spatial resolution of magnetic resonance imaging Fricke-gel dosimetry is improved with a honeycomb phantom.

The spatial accuracy of magnetic resonance imaging (MRI) Fricke-gel dosimetry is limited by diffusion of ferric ions. This paper describes a honeycomb structure to limit diffusion of Fe3+ ions in a three-dimensional phantom. Such a phantom containing the dosimeter gel was irradiated to a known dose distribution. Maps of dose distributions were produced from the MR images acquired at 2 and 24 hours after the dose was given. The dose distribution maps verified that the honeycomb structure precludes ion diffusion from one honeycomb cell to another, thus improving the usefulness of MRI Fricke-gel dosimetry.

The role of dose distribution gradient in the observed ferric ion diffusion time scale in MRI-Fricke-infused gel dosimetry.

Ferric ion diffusion is a detrimental factor in MRI-Fricke-infused gel dosimetry. In this study, a novel approach involving MR image subtraction and a fast image-based dosimetry technique to study ferric ion diffusion effects is presented. The fast image-based approach allows studying dose profile degradation within minutes post-irradiation. The relationship between the rate of dose profile deterioration and dose distribution gradients can be elucidated with the improved imaging temporal resolution also. Our results showed that for a dose distribution with gradient 4 Gy/mm or higher, ferric ion diffusion causes apparent dose profile degradation in 0.5-1 h post-irradiation. For a gradual dose gradient change of 2.1 Gy/mm or smaller, dose profile degradation appears insignificant for a two-hour elapsed diffusion time. These observations agree well with the theoretical analysis of a square dependence between dose profile degradation and dose distribution gradient. Because all stereotactic radiosurgery procedures produce steep dose distributions and because the ideal "snapshot" of MR scanning cannot be achieved, knowledge of the ferric ion diffusion time scale is important in experimental designs in order to avoid potential measurement errors in MRI-Fricke-agarose gel dosimetry.

Dose reconstruction in irradiated Fricke-agarose gels by means of MRI and optical techniques: 2D modelling of diffusion of ferric ions.

Fricke-agarose gels have elicited much interest in the field of radiation dosimetry, as tissue-equivalent dosemeters. magnetic resonance (MR) relaxation rates are measured for dose reconstruction. A major problem of Fricke-agarose gels is the diffusion of the ferric ions formed after irradiation. Knowledge of the diffusion coefficient of ferric ions may be necessary. Xylene orange, a dye that specifically chelates ferric ions, was added to the Fricke gel system to reduce ion diffusion and, as the binding gives a coloured complex, to allow optical detection of ferric ions. Diffusion of ferric ions in two dimensions and time evolution of ion concentration were evaluated. MR images were obtained at different times after irradiation. Ferric ion distribution, the corresponding images and the doses at different times after irradiation were reconstructed taking into account the calculated diffusion coefficients. Diffusion was then estimated by means of two different optical methods. The agreement obtained supports the reliability of the MRI method and of the model.

Radiation dosimetry using Fricke-infused gels and magnetic resonance imaging.

We have witnessed the advancements of MRI-Fricke-infused gel dosimetry since its commencing in 1984. Over the years, many efforts have been spent to improve the method's efficacy, i.e., to improve its dose-response sensitivity, reproducibility and measurement accuracy. In this article, we give a review of the development of this relatively new dosimetric method. An example of applying this method to gamma knife stereotactic radiosurgery dose distribution mapping is also given.

Dose distribution measurements in superficial x-ray beams using NMR dosimetry.

The conversion of Fe2+ to Fe3+ in a Fricke solution due to ionizing radiation can be detected using nuclear magnetic resonance (NMR) imaging. The aim of the present study was to develop a suitable system for the study of dose distributions in superficial radiation beams making use of the water equivalence of the dosimetric gel system. Agarose gels (1.5%) doped with 0.5 mM ammonium ferrous sulphate and 125 mM sulphuric acid were exposed to x-rays from a superficial radiotherapy treatment unit (HVT 1.4, 2.4 and 7.25 mm A1). In the experiments doses between 10 Gy and 20 Gy were given, and the gel surface was in direct contact with the lead glass cone applicators (diameters 1 cm and 2.5 cm) at an FSD of 10 cm. Images were obtained within 4 h after irradiation in the head coil of a 1.5 T clinical MR scanner. Using spin-echo sequences with seven different repetition times between 120 ms and 4 s, the spin-lattice relaxation time (T1) was calculated for specified regions of interest with about 1 mm spacing. The inverse of T1 was shown to be proportional to the given dose and 1/T1 maps were obtained for all three superficial radiation qualities. The depth-dose curves determined with NMR dosimetry compare well with those obtained with a thin-window parallel plate ion chamber and thermoluminescence dosimetry in the same radiation beams.

NMR relaxation enhancement in gels polymerized and cross-linked by ionizing radiation: a new approach to 3D dosimetry by MRI.

A new type of tissue-equivalent medium for magnetic resonance imaging of the dose distributions produced by ionizing radiation has been developed. Agarose gel is infused with acrylamide and N,N'-methylene-bis-acrylamide (Bis) comonomers, which are readily polymerized by free radical initiators in de-aerated aqueous solutions. Polymerization and cross-linking induced locally by free radical products of water radiolysis increase the rate of water proton spin relaxation gradually up to doses of about 15 Gy. The slopes of the dose-response curves at 64 MHz are 0.015 and 0.28 s-1 Gy-1 for R1 and R2, respectively. The agarose matrix as well as the high (50% by weight) relative concentration of the cross-linker (Bis) per total comonomer limit the spread of polymerization so that the spatial distribution of the radiation dose is faithfully represented in the resultant spatial distribution of relaxation rates. The gel can be imaged with conventional magnetic resonance imaging devices with high spatial resolution and accuracy. In addition, due to the well established effect of the precipitation of insoluble agglomerates of highly cross-linked acrylamide, the optical turbidity of the gel increases gradually with the absorbed dose. This may provide an additional means of visualizing the dose distribution in three dimensions. The major advantage of the acrylamide-Bis-agarose gels over those that depend on ionic chemical dosimeters, for example, Fricke-infused gels, lies in the lack of diffusion of radiation-induced chemical changes subsequent to or concurrent with irradiation.

Dose-response curves for Fricke-infused agarose gels as obtained by nuclear magnetic resonance.

The radiation-response characteristics of agarose gels prepared with Fricke dosemeter solution have been studied. The response mechanism is an increase in the NMR longitudinal relaxation rate of protons caused by ferric ions. It has been observed that: (i) oxygen saturation assures consistent and maximum sensitivity; (ii) agarose concentrations in the range 1.0-2.0% have no effect upon sensitivity; (iii) the initial G value is 150 Fe3+/100 eV for gels containing 0.5 mM Fe2+ ions; (iv) increasing NMR frequencies only causes a moderate increase in sensitivity; (v) the gel dosemeters are dose rate independent in the range 4.7-24.2 Gy min-1; (vi) sensitivity is pH dependent, being zero at pH 7; (vii) freshly prepared gels are slightly more sensitive than those more than 24 h old; and (ix) the diffusion coefficient for ferric ions in a 1.0% agarose gel containing 0.0125 M H2SO4 is 1.83 x 10(-2) cm2 h-1, and this will require consideration for the NMR imaging of dose distributions.

MR imaging of absorbed dose distributions for radiotherapy using ferrous sulphate gels.

The measurement of absorbed dose distributions using dosemeter gel and magnetic resonance imaging (MRI) in a standard geometry has been investigated. Absorbed depth-dose curves and profiles measured with this new technique show good agreement with corresponding measurements using diodes. This was proven in a 60Co beam as well as an electron beam. The dosemeter gel is made of agarose and ferrous sulphate solution. The dose response is linear (r = 0.9996) in the investigated dose interval, 0-40 Gy. The sensitivity is a factor of about six higher compared to ordinary ferrous sulphate solution, known as 'Fricke'. This is a true 3D dose measurement technique which will have a number of applications in radiation therapy, since it is possible to mould the gel to arbitrary geometries, mix different radiation qualities and integrate the absorbed dose from different kinds of fields.