Monitoring nanoprobe diffusion in osmotically-stressed hydrogels
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1060-LL07-03
Monitoring nanoprobe diffusion in osmotically-stressed hydrogels Hacene Boukari, Candida de Silva, Ralph Nossal, and Ferenc Horkay Lab. of Integrative & Medical Biophysics, National Institutes of Health, Bldg 9/ Rm 1E122, Bethesda, MD, 20892 ABSTRACT We have developed an optical chamber for studying the effect of swelling or shrinking of a gel on the translational diffusion of fluorescent nanoprobes embedded in the gel. On one side of the chamber, the gel is in contact with a hydrating or dehydrating polymeric solution through a porous membrane, allowing control of the rate of hydration or dehydration of the gel. On the other side, a laser beam is focused into the gel to excite the fluorescence of the nanoprobes, which is continuously monitored to reveal possible structural changes of the stressed gel. Using fluorescence correlation spectroscopy we measure correlation functions of the nanoprobes at various times following the contact of the gel with the hydrating or dehydrating solution, and determine changes of both the average concentration and the apparent diffusion time of the nanoprobes as the gel is shrinking or swelling. We have tested the chamber using the fluorophore, TAMRA (MW = 430 Da), embedded in a poly(vinyl-alcohol) gel that is being dehydrated by a solution of poly(vinyl-pyrrolidone) (28% w/w). As expected TAMRA moves slower as the gel shrinks. However, the changes in the diffusion time of TAMRA as a function of the PVA concentration of the shrinking gel appear to be different than those measured on TAMRA diffusing in PVA gels prepared at different PVA concentrations but with the same cross-link density. I. INTRODUCTION Understanding probe diffusion in gels in general and biological gels in particular remains a subject of prime interest to both basic research and applied engineering such as drug delivery and tissue engineering [1-3]. For most applications, the main goal is to understand the relationship between structure and dynamic properties under various conditions (concentration, cross-linking, pH, salt, temperature,..). Recently, we demonstrated that fluorescence correlation spectroscopy (FCS), an optically non-invasive technique, could be successfully used to measure the diffusion of nanoprobes in polymeric solutions and cross-linked hydrogels [4,5]. In particular, we investigated the effects of the concentration and cross-link density in poly(vinyl-alcohol) (PVA) gels on the diffusion of various nanoprobes. In the case of the gels, we found remarkable correlation between the macroscopic elastic modulus and the diffusion of the fluorescent nanoprobe, TAMRA (430 Da) [4]. The mechanisms behind this correlation have yet to be elucidated. In the present study we focus on the effect of swelling and shrinking on the diffusion of the same nanoprobe, TAMRA, in similar PVA gels. Dehydration can cause structural changes, ranging from simple shrinking to cracking. In principle, these changes are likely to affect not only the mechanical properties of the gel (e.g., its elastic modulus) but al
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