Browsing by Subject "Hydrogels"

Now showing 1 - 3 of 3
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    Effect of chemical crosslinking on the swelling and shrinking properties of thermal and pH-responsive Chitosan hydrogels
    (Wiley, 2003-10-07) Heras, Ángeles; Aranaz, Inmaculada; Galed, Gemma; Fernández Valle, María E.; Argüelles Monal, Waldo; Goycoolea Valencia, Francisco Martín; Biología Celular e Histología
    The ability to form a gel through the physical or chemical crosslinking of chitosan has been well documented. In an attempt to mimic biological systems, thermal and pH-sensitive chitosan cylindrical hydrogels were produced by a combination of physical and chemical crosslinking processes. To this end, chitosan hydrogels prepared from alkali chitin were molded in cylinders and, once washed, were further crosslinked with glutaraldehyde at stoichiometric ratios, R (-[–CH=O]/[–NH2]), of 1.61 and 3.22 x10^-2. Variation in swelling as a result of stepwise changes in temperature between 40 and 2 °C at pH values of 7.0, 7.6, and 8.0 revealed that the system responds in markedly different manners dependent upon the pH. At pH 7.0, cooling from 40 to 2 °C results in contraction of the gel network structure. While raising the temperature from 2 to 40 °C leads to a rapid swelling response (i.e., ca. a twofold increase in the amount of solvent uptake). Subsequent cooling to 2 °C is accompanied by a new contraction cycle. At pH 7.6 the temperature dependence of the swelling–contraction behavior is exactly the opposite of that observed at pH 7.0.Very similar trends were observed for the gels at both degrees of crosslinking. The swelling–shrinking behavior observed in gels of pH 7.6, is similar in kind to that of uncrosslinked gels and is interpreted in terms of a lower critical solution temperature (LCST) volume phase transition, driven by hydrophobic association, presumably involving residual acetyl groups in the chitin. The results at pH 7.0 suggest that the slight ionization of the –NH3+ groups leads to the destruction of the hydrophobic hydration thus effectively reversing the negative thermal shrinking.
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    Kinetics of gelation and thermal sensitivity of N-Isobutyryl Chitosan hydrogels
    (American Chemical Society, 2005-06-25) Félix, Leticia; Hernández, Javier; Argüelles Monal, Waldo M.; Goycoolea Valencia, Francisco Martín; Biología Celular e Histología
    N-Acylation of chitosan with carboxylic anhydrides in dilute acetic acid/methanol has been a well documented strategy to selectively modify chitosan. Although this reaction is known to lead to irreversible gel formation, the kinetics and mechanism of this process have not so far been addressed. To this purpose, gel formation during the N-isobutyrylation of chitosan was investigated as a function of the reaction stoichiometry (R), chitosan concentration, and temperature by small deformation oscillatory rheology. Gel formation follows closely the chemical reaction and it proceeds predominantly under second-order kinetics as established from the dependence of critical gel time, tgel, on R and concentration. The activation energy value derived from tgel vs 1/T data (Ea ) 68.29 ( 1.80 kJ/mol) was almost identical to values reported for the chitosan N-acetylation reaction in previous studies. An excess isobutyric anhydride is suggested to be necessary for nucleation and hydrophobic association. The potential application of N-isobutyrylchitosan (NIBC) hydrogels in the design of thermally sensitive materials is also demonstrated.
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    pH and temperature – sensitive chitosan hydrogels: swelling and magnetic resonance imaging (MRI) studies
    (Wiley, 2011-02-25) Goycoolea Valencia, Francisco Martín; Fernández Valle, María E.; Aranaz, Inmaculada; Heras, Ángeles; Biología Celular e Histología; Facultades de la UMU::Facultad de Biología
    MRI and swelling experiments are used to probe the state of water and infer the microstructure of chitosan hydrogels. SEM reveals a porous open scaffold-type structure for hydrogels that were equilibrated at 2 °C before freezing as compared to those equilibrated at 37 °C. ADC MRI measurements reveal an anisotropy in the microstructure of these gels. T1 relaxation MRI values were larger as the pH increased from 7.6 to 12.0, the result of a lower rate of exchange between protons of the hydration sphere of the polymer and bulk water. The thermosensitive and pH-sensitive properties of these hydrogels can be utilized in the development of innovative materials for biotechnological and biomedical applications, including criobiocatalysis and bioremediation as well as in programmed drug delivery.