Stimuli responsive gels based on interpenetrating network of chitosan and poly(vinylpyrrolidone)
Introduction
Hydrogels (networks of hydrophilic polymers) have the capability of absorbing large amounts of water, without losing their three dimensional structure. When discontinuous volume variations upon changes of environmental parameters, like solvent composition, temperature, pH, etc., occur, hydrogels are named ‘stimuli responsive gels’ or ‘intelligent materials’ [1]. Their main applications concern the medical industry (contact lenses, artificial corneas, dressing and coating for sutures, electrode sensors) and systems for drug delivery. Most parts of these uses require biocompatible and biodegradable polymers.
Chitosan, [poly-β(1-4)-2-amino-2-deoxy-d-glucose], a polymer which derives from chitin deacetylation, has both these properties. Moreover, the presence of –NH2 groups, whose amount is related to the chitin deacetylation degree, makes the polymer soluble in acid solutions. The solubilisation pH lowers as the degree of acetylation increases: as an example, chitosan, having acetylation degree about 15% (i.e. the original chitin without 85% of its acetyl groups) dissolves at pH 6,2 [2]. When the pH exceeds that value, the polymer forms a hydrated gel-like precipitate. Therefore, the lack of solubility at higher pH, requires how to improve the hydrophilicity of the system.
A first method consists in modifying the chitosan chain, for example by carboxymethylation before the crosslinking (these products captured noticeable interest due to their antibacterial activity [3], [4]). A second way is to add a suitable amount of a hydrosoluble polymer. After crosslinking of chitosan, a semi-interpenetrate network (semi-IPN) can be obtained [3], [4], [5], [6] with improved swelling behaviour in the complete pH range. Moreover, even the free polymer may be crosslinked giving a network interpenetrate with that of chitosan. Some examples are available in the literature [5], [6], [7], [8]. Risbud et al [6] studied a semi-interpenetrate network chitosan–polyvinylpyrrolidone. Chitosan and polyvinylpyrrolidone dissolved in acetic acid 0.1 M and pure water, respectively, were mixed in suitable ratio (w/w about 1). Addition of gluteraldehyde causes the crosslinking of chitosan giving a semi-interpenetrate network, which may be used to prepare membranes having swelling properties in a wide pH range.
Thermal and IR analysis performed on PVP–chitosan blends have recently demonstrated that the two polymers are miscible at the solid state [9], probably because hydrogen-bond between the biological polymer and the synthetic one easily occurs. However, when a solvent such as water is added, it becomes competitive with respect to the hydrogen bond formation causing immiscibility in aqueous solution.
This conclusion makes questionable the structural reproducibility of any process based on the transition solution–solid state of the polymer mixture.
Moreover, the uncrosslinked polymer (PVP) of the semi-interpenetrated gel is always subject to diffuse in the solvent, especially in swelled condition.
We believe that an interpenetrate gel may overcome the problem: so, we prepared sequential interpenetrate networks of varying chitosan/polyvinylpyrrolidone ratios and tested their behaviour.
Section snippets
Materials
Commercial chitosan (CHT) Antartich Krill Euphasia Superba was produced by Riber Fisheries Central Board, Szcecin, Poland It was refined twice by dissolution in dilute acetic acid, filtration and precipitation with 70 vol % ethanol and 30 vol % ammonia aqueous solution (35% w/w) and finally freeze dried. CHT sample, used in this work, was characterized [10], [11] using different techniques. Values indicating the degree of acetylation were perfectly coincident (42.4 from NMR [10] and 42 from IR
Results and discussion
Before discussing the chitosan–PVP IPN behaviour we need to know the properties of the networks of single polymers. Concerning chitosan we deal with a sample having a relatively high molecular weight and less than an half of acetylated amino groups: as a consequence, polyelectrolyte properties are expected and hence a strict control of pH is necessary when swelling measurements are performed.
Conclusions
Sequential IPNs synthesized in this work show some peculiar properties: the first one is the capability of reversible swelling in water at room temperature at pH 7, unlike neat CHT gels which swell only in acidic environments. Moreover, IPN's swell quickly even starting from the dry state and this could be connected to the thermodynamics interaction between the two polymers. In fact, previous studies [9] demonstrated that CHT and PVP are miscible at the solid state and not in aqueous solutions.
Acknowledgements
Financial support from NATO GRANT LST. CLG: 978834 is gratefully acknowledged.
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