ReviewDesign and applications of interpenetrating polymer network hydrogels. A review
Introduction
Hydrogels are three-dimensional, hydrophilic, polymeric networks capable to retain large amounts of water, or biological fluids, characterized by a soft and rubbery consistence, being thus similar with living tissues [1], [2]. Hydrogels may be chemically stable or “reversible” (physical gels) stabilized by molecular entanglements, and/or secondary forces including ionic, H-bonding or hydrophobic interactions, these hydrogels being nonhomogeneous [1], [2]. Examples of reversible hydrogels are “ionotropic” hydrogels formed by the interaction between a polyelectrolyte and an oppositely charged multivalent ion, and the polyelectrolyte complexes (complex coacervates) formed by the interaction between two oppositely charged polyelectrolytes. Physical gels can be disintegrated by changes in the environment conditions such as ionic strength, pH, and temperature. Physical hydrogels have numerous biomedical applications in drug delivery, wound dressing, tissue engineering and so on. Covalently cross-linked networks form permanent or chemical gels [1]. “Smart” hydrogels are able to significantly change their volume/shape in response to small alterations of certain parameters of the environment. Responsive hydrogels have numerous applications, the most of them being focused on biological and therapeutic demands [3], [4], [5], and sensing applications [6]. However, single-network hydrogels have weak mechanical properties and slow response at swelling. To enhance the mechanical strength and swelling/deswelling response, multicomponent networks as interpenetrating polymer networks (IPNs) have been designed.
IPNs are “alloys” of cross-linked polymers, at least one of them being synthesized and/or cross-linked within the immediate presence of the other, without any covalent bonds between them, which cannot be separated unless chemical bonds are broken [7], [8], [9]. The combination of the polymers must effectively produce an advanced multicomponent polymeric system, with a new profile [10]. According to the chemistry of preparation, IPN hydrogels can be classified in: (i) simultaneous IPN, when the precursors of both networks are mixed and the two networks are synthesized at the same time by independent, noninterfering routs such as chain and stepwise polymerization [7], [9], [11] (Fig. 1a), and (ii) sequential IPN, typically performed by swelling of a single-network hydrogel into a solution containing the mixture of monomer, initiator and activator, with or without a cross-linker (Fig. 1b). If a cross-linker is present, fully-IPN result, while in the absence of a cross-linker, a network having linear polymers embedded within the first network is formed (semi-IPN) [7], [8], [12], [13].
When a linear polymer, either synthetic or biopolymer, is entrapped in a matrix, forming thus a semi-IPN hydrogel, fully-IPN can be prepared after that by a selective cross-linking of the linear polymer chains [14], [15], [16] (Fig. 1c).
By their structure, IPN hydrogels can be classified in: (i) IPNs, formed by two networks ideally juxtaposed, with many entanglements and physical interactions between them; (ii) homo-IPNs, which are a special case of IPN, where the two polymers which form the independent networks have the same structure; (iii) semi- or pseudo-IPNs, in which one component has a linear instead of a network structure. Mechanically enhanced IPN hydrogels as “double networks”, promoted by Gong et al., have attracted attention by their potential for biomaterials, mainly as a replacement of natural cartilage [17], [18], [19]. The particular feature of this new type of IPN hydrogels, characterized by high resistance to wear and high fracture strength, consists of the preparation first of a densely cross-linked ionic hydrogel, the second network being a neutral loosely cross-linked network [17], [18].
This review aims to give an overview on the preparation and applications of semi- and fully-IPN hydrogels based on the most recent publications in the field. In the first part, the main synthesis strategies of IPN hydrogels, their relevant properties and biomedical applications will be presented. In the second part, an overview on the most specific applications of the IPN hydrogels in separation processes will be given.
Section snippets
Design, characterization, and biomedical applications of IPN hydrogels
A wide variety of hydrophilic polymers or their precursors have been used to synthesize hydrogels, the main classes consisting of natural polymers and their derivatives (polysaccharides and proteins), and synthetic polymers containing hydrophilic functional groups such as –COOH, –OH, –CONH2, SO3H, amines and R4N+, and ether [1]. By the combination of polymers coming from these two classes, IPN composite hydrogels can be prepared by the three routes presented in Fig. 1. The most often
Characterization of sorption properties
To evaluate the sorption capacity for ionic species, the behavior of various IPN hydrogels in the presence of ionic dyes or heavy metal ions has been evaluated, usually in batch mode. From the determination of the residual concentration of the dye or metal ion as a function of various parameters, the adsorption or binding capacity, q, expressed by Eq. (1), and the removal efficiency, R, %, expressed by Eq. (2) can be calculated.where Co and Ce are the initial
Summary of the benefits of semi-IPN compared to single-network hydrogels, and of the influence of the second network on the properties of IPN hydrogels
IPNs, as a particular class of polymer blends, have been developed with the aim to improve at least one property of the constituent networks. The main advantages of IPNs are that relatively dense hydrogel matrices can be produced, which feature stiffer and tougher mechanical properties, more widely controllable physical properties, and (frequently) more efficient drug loading compared to single-network hydrogels [4], [7], [13]. The loading capacity of the CS/PNIPAAm IPN with DS significantly
Conclusions and perspectives
Interpenetrating polymer network hydrogels, as a particular category of composite materials, received a great attention last decade owing to their improved responsiveness and mechanical properties, which differentiate them on the single network hydrogels. Even if the fabrication of IPN hydrogels by the simultaneous strategy has the advantage to be time- and cost-saving, it was found that the sequential technique allows a better control of the properties of the gels compared to the simultaneous
Acknowledgement
This work was supported by CNCSIS-UEFISCSU by the project PN-II-ID-PCE-2011-3-0300.
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