Investigation Of Nmr Properties Of Gel Volume Change With Magnetic Resonance Imaging

Author : Halil M. Oztop
Publisher :
ISBN 13 : 9781267663580
Total Pages : pages
Book Rating : 4.6/5 (635 download)

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Download or read book Characterization of Hydrogels for Controlled Release Applications written by Halil M. Oztop and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Gels prepared from whey proteins can be used for controlled release of nutrients or active ingredients in food systems. These gel systems, exhibit pH-dependent swelling when placed in aqueous solutions. Understanding the physics that govern gel swelling is thus important when designing gel-based delivery platforms. In this study swelling of whey protein gels (WPG) were examined under different conditions. Magnetic resonance imaging (MRI) and NMR relaxometry were used as the main tools of characterizing these gels. For most of the study heat-set WPGs (17% w/w protein) were used. Swelling was monitored in aqueous solutions with pH values of 2.5, 7 and 10. Changes in dimension over time, as characterized by the number of voxels in an MR image, were correlated to gravimetric measurements. Excellent correlations between mass uptake and volume change (R2=0.99) were obtained for the gels in aqueous solutions at pH 7 and 10, but not for gels in the aqueous solution at pH 2.5. To provide insight into the mechanisms for water uptake, NMR relaxation times (T2) were measured in independent experiments. The relaxation spectrum for the spin-spin relaxation time (T2) showed the presence of three proton pools for pH 7 and 10 trials and four proton pools for pH 2.5 trials. Divalent salts are used commonly for gelation of polymer molecules. Calcium, Ca2, is one of the most common divalent ions that is used in WPGs. Manganese, Mn2, also divalent, but paramagnetic, enhancing relaxation decay rates in magnetic resonance imaging (MRI) could be used as a probe to understand the behavior of Ca+2 in these gels. MR Images obtained with gels immersed in MnCl2 solution revealed a relaxation sink region in the gel's surface and the thickness of the region increased with time. These 'no signal' regions in the MR images were attributed to uptake of Mn+2 by the gel. Results obtained with CaCl2 solution indicated that since Ca+2 did not have the paramagnetic effect, the regions where Ca+2 diffused into the gel exhibited a slight decrease in signal intensity. The relaxation spectrums exhibited three populations of protons, for gels immersed in MnCl2 solution, and two populations for gels in CaCl2 solution. No significant change in T2 distributions was observed for the gels immersed in CaCl2 solution. To understand the reverse behavior, -release of the ions-, WPGs loaded with Mn+2 were placed in solutions at pHs 2.40, 6.94 and at pH 10.40 w/wo EDTA. Release of the divalent cation Mn+2 from the gels was enhanced by chelation of free Mn+2 in water containing EDTA (pH>pI) as determined by the relaxation times (T2's) of the WPG. T2 spectrums for Mn+2 loaded gels obtained from multiexponential decays curves differed significantly between two different soaking baths (EDTA (pH>pI vs. pHpI) with respect to the relative concentrations of hydrogen's populating each of the corresponding proton pools and relaxation times. For Ca+2, a strong correlation (R20.99) was found between relative areas of the proton pool's and the amount of calcium released out. In that regard, relaxation spectrum analysis was used to monitor the kinetics of Ca+2 release from the WPG at pH 2.4. At pH 2.40, release of Ca+2 was higher than that of Mn+2. Modeling of mass uptake was also presented in terms of Case I (Fickian diffusion) and Case II (kinetic) models. Due to the extent of swelling, the Fickian diffusion with moving boundaries provided the most realistic reflection of the physics. The average diffusivity was found to be changing between 0.79 x 10−10 m2/s.-1.40 x 10−10 m2/s. The model also yielded instantaneous values of the radius and sample length. The Fickian diffusion with moving boundary model can be extended to evaluate different geometries for controlled release systems. In the final part of the study, release of a model nutrient was examined from whey protein gel particles. Whey proteins in combination with alginate were used to obtain delivery systems. The three whey protein/alginate combination gel solution and beads including riboflavin (0, 50, 100% alginate) were investigated. The three whey protein/alginate combination gel solution and beads (0, 50, 100% alginate) were investigated. Riboflavin release from the beads into a xanthan/sucrose suspending solution was most rapid for the 100% alginate beads, slower for the 100% whey and the 50% Whey-50% Alginate mixture beads. Disintegration of the pure alginate beads was evident in the MRI data at 13 hrs. Multiexponential analysis of proton decay curves yielded two compartments for the beads. The proportion of the second compartment correlated directly with the release rate, the greater the proportion of the 2nd compartment, the rapid the release rate.