Large deformation rheology of gelatin gels

Rheological Characterization, Crystallization, and Gelation Behavior of Monoglyceride Gels

Saturated monoglycerides can form firm gels in water. These gels are networks of stiff plate-like beta-crystals of monoglycerides (a "cardhouse"), grown from a space-filling lamellar liquid-crystalline phase. The molecular mechanism of crystallization is discussed in the light of network formation. The concentration dependence of gel development of (shear-cooled) monoglyceride gels has been studied by rheology. A gelation mechanism has been proposed, consisting of two steps: (i) After formation of a nucleus, rapid crystallization in a lateral direction occurs (probably within one bilayer) by which the first space-filling network is formed. (ii) This is followed by reinforcement of the network by which stacks of crystalline bilayers are formed. The plate-like crystals are linked in connective domains or junction zones, probably containing all the material (cosurfactants, diglycerides, etc.) that does not fit in the crystalline array. Small deformation rheology shows that above about 2 wt% monoglyceride a percolating network is formed. The large deformation rheology is typical for a particle gel with a relatively small strain at failure (both in shear deformation and compression). The connective domains or junction zones already fail when relatively small deformations are put on the system.

Biodegradable hydrogels based on stereocomplex formation between lactic acid oligomers grafted to dextran

A novel hydrogel system in which crosslinking is established by stereocomplex formation between lactic acid oligomers of opposite chirality is proposed. To investigate the feasibility of this novel system, we first investigate whether there is an operation window where lactic acid oligomers in either the D- or L-form do not give a crystalline phase, whereas in a blend of the D- and L-form stereocomplex formation occurs. Therefore, D- and L-lactic acid oligomers with different degrees of polymerization (DP) were prepared and analyzed using DSC. It was shown that crystallinity was present in D- or L-oligomers with DP > or = 11. On the other hand, in blends of D- and L-oligomers of lactic acid crystallinity (stereocomplexation) was already observed at a DP > or = 7. In the next step, L- and D-lactic acid oligomers were coupled via their terminal hydroxyl group to dextran, yielding dex-(L)lactate and dex-(D)lactate, respectively. Upon dissolving each product in water separately and mixing the solutions, a hydrogel is formed at room temperature as demonstrated by rheological measurements. The storage modulus of the obtained hydrogel strongly decreased upon heating to 80 degrees C, while it was restored upon cooling to 20 degrees C demonstrating the thermo-reversibility and the physical nature of the cross-links. The storage modulus of the gels depends on the degree of polymerization of the lactate acid grafts and their degree of substitution on dextran. Interestingly, gel formation was favored when one lactic oligomer was coupled via its hydroxyl group whereas the oligomer of opposite chirality was coupled via its carboxylic acid group. This is ascribed to the parallel packing of the oligomers in stereocomplexes.

A modified polymer gel for radiotherapy dosimetry: assessment by MRI and MRS

The characteristics of a new formulation of polymer gel are assessed for MRI-based radiotherapy dosimetry. The gel, based on the first BANG gel formulation, replaces acrylamide with the less toxic monomer sodium methacrylate. The relationship between MR T2 relaxation time and radiation dose for the gel formulation was studied using spin-echo imaging. Proton magnetic resonance spectroscopy was also used to assess the gel composition as a function of dose. The effect of gel pH on the dose-response and baseline R2 was then investigated. A calibration performed on gel without pH modulation (pH = 6.6) revealed a dose-response of 0.14 s(-1) Gy(-1) within the range 0-8 Gy. The baseline R2 increases with pH above neutrality, rising from 1.2 s(-1) at pH = 5.1 to 5.0 s(-1) at pH = 10.1. The dose-response is also pH dependent, having a minimum value of 0.09 s(-1) Gy(-1) at pH = 10.1 and peaking at 0.21 s(-1) Gy(-1) at pH = 7.7. Undertaking proton spectroscopy on the gels enabled resonances associated with the monomer and co-monomer to be studied. By integrating the peaks from the respective monomers and normalizing to the signal at 0 Gy it was shown that only 50% of the methacrylate monomer was used at 10 Gy, whereas 80% of the co-monomer was used at this dose. The data indicate that this gel has a reduced toxicity and a comparable dose response to the previously reported BANG gel. In addition, the performance of the gel can be optimized by controlling the pH. MR spectroscopy revealed that the crosslinking co-monomer is consumed more readily than the monomer, which is in agreement with previous compositional studies.

The reproducibility of polyacrylamide gel dosimetry applied to stereotactic conformal radiotherapy

The reproducibility of polyacrylamide gel (PAG) dosimetry has been evaluated when used to verify two radiotherapy treatment plans of increasing complexity. The plans investigated were a three-field coplanar arrangement, using the linac jaws for field shaping, and a four-field, conformal, non-coplanar plan using precision-cast lead alloy shielding blocks. Each treatment was performed three times using phantoms and calibration gels manufactured in-house. Two phantoms were specially designed for this work to aid accurate positioning of the gels for irradiation and imaging. All gels were imaged post-irradiation using a Siemens Vision 1.5T MR scanner. T2 relaxation images were calibrated to absorbed dose distributions using a number of smaller calibration vessels to produce distribution maps of relative dose. The relative dose distributions were found to be reproducible, with the standard deviation on the mean areas enclosed by the > or = 50% isodose lines measured in three orthogonal planes being 6.4% and 4.1% for the coplanar and non-coplanar plans respectively. The measured distributions were also consistent with those planned, with isodose lines generally agreeing to within a few millimetres. However, the measured absolute doses were on average 23.5% higher than those planned. Although the polyacrylamide gel dosimetry technique has some limitations, particularly when calibrating distributions to absolute dose, the ability to resolve sharp dose gradients in three dimensions with millimetre precision is invaluable when verifying complex conformal treatment plans, where avoidance of proximal, critical structures is a treatment criterion.