New hydrogels based on the interpenetration of physical gels of agarose and chemical gels of polyacrylamide

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Abstract

In this paper we report a novel method for preparing interpenetrating polymer hydrogels of agarose and polyacrylamide (PAAm) in three steps. The procedure consists in (i) formation of physical hydrogels of agarose, (ii) diffusion of acrylamide, N,N′-methylene-bis-acrylamide and potassium persulfate (the initiator) from aqueous solutions inside the gel of agarose, and (iii) cross-linking copolymerization reaction of the aforementioned reactants to produce PAAm chemical gels interpenetrated with the agarose physical gels. Viscoelasticity measurements and thermal analysis have been performed in order to follow the kinetics of copolymerization. The viscoelastic, swelling and thermal properties of the resulting hydrogels confirm the formation of an interpenetrated system. Further evidence of interpenetration is obtained from inspection with atomic force microscopy. The improvement of the agarose and PAAm gel properties in the resulting interpenetrated hydrogel is analyzed in view of the results.

Introduction

Hydrogels are hydrophilic polymer networks that have a large water absorbing capacity and that are characterized by the presence of cross-links, entanglements, coexistence of crystalline and amorphous regions, and rearrangements of hydrophobic and hydrophilic domains [1], [2], [3].

The ability of polymer gels to undergo substantial swelling and collapsing as a function of their environment, up to 1000 times in volume [4], is one of the most notable properties of these materials [5]. Gel volume transitions can be induced by temperature, pH, or ionic strength, among other stimuli. The phenomenon has prompted researchers to investigate gels as potential actuators, artificial muscles, sensors, controllable separation membranes, and vehicles for drug delivery [6], [7], [8], [9], [10], [11]. Gels are particularly appropriate for biomedical applications because of their ability to simulate biological tissues [12].

Agarose is one of the two major components of the polygalactoside agar, together with agaropectine. Agarose is the gelling fraction of agar. It consists of repeating units of alternating β-d-galactopyranosil and 3,6-anhydro-α-l-galactopyranosil groups (an individual unit is named agarobiose) [13]. Agarose forms thermoreversible gels when dissolved in water, and is normally insoluble in organic solvents, save a few exceptions such as DMF and DMSO. In these latter solvents, it cannot form gels unless a certain amount of water is added. Apparently, the structure of water plays a decisive role to induce gelation [14], [15].

A characteristic feature of agarose is that the gels show a large thermal hysteresis, attributed to the formation of large aggregates that remain stable at temperatures much higher than those at which the individual helices reform on cooling [16]. Gain of understanding in the gelation mechanism of agarose has been subject of many scientific publications, justified by its importance as typical model for gelling materials, as texture modifier in the food industry, or as bacterial medium in the biomedical field [17], [18].

In recent years, polyacrylamide-based hydrogels have received considerable attention. These gels are also used in many applications: as specific sorbents, support carriers in biomedical engineering, aggregating agents, soil improvement agents, polymer processing or improving textiles, paper strengthening agents, adhesives, paints, oil salvaging agents, etc. [19], [20], [21], [22]. The kinetics of network formation in free-radical copolymerization of acrylamide and N,N′-methylene-bis-acrylamide in aqueous solution has been extensively studied. It has been reported that the polyacrylamide network exhibits an inhomogeneous cross-link distribution [23]. Partially hydrolysed polyacrylamide gels experience volumetric phase transitions when changing the temperature, pH, and solvent composition [24]. Polyacrylamide gels are extensively used as matrix for electrophoresis, and to encapsulate drugs, enzymes and proteins for application in drug delivery systems and biosensors [25], [26].

Combinations of agarose and PAAm polymers can be prepared in the form of blends, copolymers, and interpenetrating polymer networks or gels. Interpenetrating polymer hydrogels (IPHs) are combinations of two or more polymer hydrogels synthesized in juxtaposition [27]. They can also be described as polymer hydrogels held together by topological bonds due to permanent entanglements, essentially without covalent bonds between polymeric chains of different type. By definition, an IPH structure is obtained when at least one polymer gel is synthesized independently in the immediate presence of another. IPHs constitute an important class of materials. They are attracting broad interest both from fundamental and applications viewpoints [28], [29], [30], [31], [32].

The present study aims at the preparation of agarose-PAAm interpenetrating hydrogels with improved properties, in-between those of the agarose and PAAm systems. The swelling and viscoelastic properties have been investigated to corroborate the formation of interpenetrating hydrogels. Further evidence of the formation of interpenetrating systems has been obtained by atomic force microscopy (AFM).

Section snippets

Materials

The agarose (D-1 LE) used in this study was kindly supplied by Hispanagar, Spain, and was stored under reduced pressure at room temperature prior to use. The molecular weight of an agarose sample was 102,000 g/mol as determined by viscometry measurements. Sulphate percentage was 0.081%. Solutions were prepared using deionised water (milli-Q grade).

The acrylamide (AAm) monomer solution, the initiator potassium persulfate (K2S2O8) and the crosslinker N,N′ methylene bisacrylamide (BMAAm), were

Results and discussion

Interpenetrating hydrogels based on a physical gel of agarose and a chemical gel of PAAm were prepared by the sequential procedure describe in the experimental section. First, agarose hydrogels were prepared by quenching agarose aqueous solutions obtained at high temperature. Secondly, pieces of these gels were immersed in aqueous solutions of AAm, BMAAm and initiator, and the diffusion of reactants within the agarose hydrogels was allowed to proceed to obtain pre-IPHs. Finally, the

Conclusions

A sequential method has been developed to obtain new hydrogels based on the interpenetration of a physical gel of agarose and a chemical gel of polyacrylamide.

The method allows us to prepare controlled and interpenetrating hydrogels of agarose and PAAm with defined concentrations and cross-linking degrees.

IPHs of agarose and PAAm provide materials with intermediate properties between those of the separate systems. In this way, it is possible to obtain a material with the good elastic properties

Acknowledgments

Financial support from CICYT (MAT 2005-1179) is gratefully acknowledged. This work has been performed in the frame of NoE Nanofun-Poly EU contract No. NMP3-CT-2004-500361. E. Fernández is grateful to the Spanish Ministry of Science and Innovation for a grant in aid.

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