Microstructured poly(2-hydroxyethyl methacrylate)/poly(glycerol monomethacrylate) interpenetrating network hydrogels: UV-scattering induced accelerated formation and tensile behavior

Highlights

• Microstructured PHEMA-PGMA interpenetrating networks (MIPNs) were synthesized.

• Scattering of irradiation by microstructure of networks enhanced polymerization rate.

• The PHEMA network exhibited high swelling when incorporated into the MIPN with PGMA.

• The MIPNs revealed superior mechanical properties.

Abstract

Methacrylate hydrogels are unique synthetic materials known for their capability to serve as multifunctional eye-implants, practically without duration and compatibility limits. We introduce a novel strategy consisting in toughening of a macroporous microstructure using the interpenetrating network concept, which improves the commonly preferred preparation way of hydrogel based on photopolymerization. The method proceeds at ambient temperature and can be used in situ. Scattering of irradiation generated by the microstructure considerably enhances the polymerization rate. This acceleration effect was quantified by careful optical analysis and is important for in situ applications. Crosslinked IPN hydrogels of 2-hydroxyethyl methacrylate (HEMA) as the first network and glycerol methacrylate (GMA) as the second network based on this new design were studied and compared with IPNs prepared from non-porous PHEMA gels. Surprisingly, a relatively high swelling capacity was achieved with this new design and the Young’s modulus increased from 4 kPa for parent PHEMA network to 380 kPa for the PHEMA–PGMA IPNs and to 980 kPa for the PHEMA–PHEMA IPNs. The IPN hydrogels were strong and resisted mechanical load. The reinforcement of the mechanically poor macroporous network by swelling in another hydrophilic monomer and subsequent polymerization presents a new concept of preparation of strong microstructured IPNs (MIPNs).

Introduction

Interpenetrating networks (IPN), including double networks (DN) are based on permanent “blending” of crosslinked polymer structures in which the network chains are of different or the same chemical nature and different or the same state of coiling. The chains of either network can be molecularly dispersed, or phase-separated macroscopically or on nanoscale by spinodal decomposition. The chains of either network are not covalently bonded to each other (IUPAC definition of IPN), or the chains of different nature and/or state of coiling are chemically bonded one to another by grafting. The IPN principle can change the properties appreciably. There is a vast literature on IPNs (cf., refs. [1], [2]) but the complexity of structural features and interactions and their effect on physical properties are still not well understood. Permanent pre-stretching of chains of the first network caused by its swelling in second monomer in sequential IPNs is another special feature which adds to rigidity and toughness of these materials.

In the hydrogel applications, the double network concept was used especially in ophthalmology where the interpenetration served as a joint between tissue and implant (cf., e.g., refs. [3], [4], [5], [6]). In the last decade, a special group of IPN (DN) hydrogels was developed by Gong and collaborators [7] in which the sacrificing (rupture) of the first stiff network could increase the gel toughness enormously. Since this pioneering work, many types of IPN hydrogels employing polymers providing special features such as pH-responsibility were explored; cf., recent work of Kitiri et al. [8] on DN with the homogeneous first network structure.

In this article, we are dealing with application of the IPN concept to hydrophilic methacrylate hydrogels. We focus on phenomena observed during IPN hydrogel formation as well as on the mechanical properties of sequential IPNs prepared from 2-hydroxyethyl methacrylate (HEMA) and glycerol methacrylate (GMA). Both polymers prepared by crosslinking polymerization of these monomers have been widely used in ophthalmologic applications such as soft contact lenses [9] and intraocular lenses [10]. These polymers swell in aqueous media, are biocompatible, non-irritating and non-toxic. Also, poly-HEMA (PHEMA) and poly-GMA (PGMA) were used in development of corneal implants [11] and vitreous substitutes [12]. Although certain applications of gels in ophthalmology demand well-defined and optically clear materials (cf., e.g., [13]), even non-transparent, translucent, or opaque materials have a high potential for applications in eye implants and tissue engineering [14]. However, for ophthalmologic applications, the water content, refractive index, and mechanical properties of the gels should vary within certain ranges in order to serve for long time in the intraocular environment.

The concept of double networks prepared sequentially in PHEMA-containing systems has been already used, mainly by Chirila and co-workers. For instance, a new type of artificial cornea consisting of a transparent homogeneous optical core and a porous skirt was designed; the transparent cornea core was bonded to the porous skirt by monomer penetration and subsequent polymerization [15]. The core remained optically clear presumably due to selective sorption of the monomer from the monomer-porogen mixture. Improvement of mechanical properties of PHEMA macroporous hydrogel “sponges” is another example of application of the double network principle. PHEMA gels prepared in the presence of 80 wt.% of water are macroporous and are characterized by fused-sphere morphology [15], [16], [17]. The mechanical properties of such gels are rather poor and were improved only moderately by the “squeeze-soak-squeeze” procedure [18]. Water was first squeezed out of the sponge, the sponge was then soaked with the monomers and interstitial monomers were removed again by squeezing; then, the remaining monomer was thermally polymerized. Driven by capillary forces, the monomer filled narrow interstices between spheres and did not practically change the chain conformations of the first polymer.

No systematic study of the factors affecting properties of interpenetrating network hydrogels, especially chain blending and pre-stretching, is available at present. Due to the dominant position of PHEMA and PGMA in ophthalmologic applications and the potential of the IPN concept to improve the performance of hydrogel materials, we focused in this study on preparation of PHEMA–PHEMA and PHEMA–PGMA sequential interpenetrating networks and on characterization of their swelling and mechanical properties. We investigated whether the crosslinking polymerization of the second monomer in the first network was complete as it is necessary condition for successful application of the final IPN hydrogel. Photopolymerization is the preferred way of hydrogel preparation because it proceeds at ambient temperature and can be used in situ. Scattering of irradiation before and during photopolymerization is characteristic for some IPN systems. By careful optical analysis we have explained why this seeming obstacle can turn into an advantage and accelerate the IPN hydrogel preparation. To enhance the IPN hydrogel properties, we exploited both principles of the IPN concept: (1) an improvement of water absorption by incorporating the more hydrophilic covalently lightly crosslinked PGMA chains into the PHEMA network and (2) an increase of hydrogel toughness by pre-stretching the PHEMA first network chains by swelling in second network monomers. Both homogeneous and macroporous PHEMA were employed as the first networks.

In the following text, the preparation of the second network is described in detail: the respective monomer reaction mixture including crosslinker and UV-initiator was swollen into the first network gel and polymerized using UV radiation. During the second network formation, the conversion of double bonds was followed by FTIR spectroscopy and the change of UV light absorption and scattering was monitored using UV-VIS spectroscopy. Then, the prepared IPNs were swollen in water to their final swelling degree and the mechanical properties of IPN hydrogels were studied using tensile tests.