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9 Hydrophilic-Hydrophobic Interpenetrating Polymer Networks and Semi-interpenetrating Polymer Networks F. O. Eschbach and S. J. Huang University of Connecticut, Institute of Materials Science, U-136, Storrs, CT 06269-3136
Poly(2-hydroxyethyl methacrylate) (PHEMA) in the water-swollen state is a very soft hydrogel with high water content. Therefore, it is similar to natural tissues and is widely used as a biomaterial. Unfortunately, PHEMA hydrogels have low mechanical strength and tear resistance. The incorporation of hydrophobic polycaprolactone (PCL) improves the mechanical properties of the swollen networks and preserves their biocompatibility. Three ways to combine PHEMA and PCL were investigated. Semi-interpenetrating polymer networks of PHEMA and low molecular weight PCL diol were prepared. Interpenetrating polymer networks of PHEMA and high molecular weight PCL were synthesized using two different methods depending upon the relative composition (PHEMA:PCL). Mechanical, thermal, and swelling properties were investigated.
HYDROGELS ARE POLYMERIC NETWORKS HELD TOGETHER by cross-links or
weaker cohesive forces such as hydrogen or ionic associations. These net works retain a large quantity of water within their structure without dissolv ing. Many natural materials of plant and animal origin are hydrogels. Because of their similarity to natural hydrogels, synthetic hydrogels hold promise for use as biomaterials. The first polymeric synthetic hydrogel intended for use as a biomaterial was poly(2-hydroxyethyl methacrylate) ( P H E M A ) , which was first described and synthesized by L i m and Wichterle in 1960 (1). Currently, P H E M A hydrogels are widely used as biomaterials (2). High water content 0065-2393/94/0239-0205$06.00/0 © 1994 American Chemical Society
In Interpenetrating Polymer Networks; Klempner, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.
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INTERPENETRATING POLYMER NETWORKS
and softness in the water-swollen state impart the quality of natural tissue to these materials (2, 3). Moreover, the high water permeability of these hydrogels allows easy purification before implantation. Likewise, diffusion of biofluids and nutrients through the hydrogel prosthesis after implantation is permitted. Unfortunately, P H E M A hydrogels possess low mechanical strength and tear easily when swollen in water. These adverse properties limit applica tions to low-strength uses such as contact lenses and suture coatings (2). Various modifications of P H E M A hydrogels to improve their mechanical properties in the swollen state have been investigated. Mixed results were obtained from attempts to incorporate particulates such as glass beads (3, 4) and S i 0 (5) as well as to copolymerize P H E M A with small amounts of maleic acid or maleic anhydride (6, 7). These methods resulted in poor water absorption in the modified P H E M A hydrogels. W e successfully incorporated low molecular weight linear polycaprolactone ( P C L ) in the cross-linked P H E M A network. The resulting network showed improved mechanical prop erties in the swollen state (6-8) while high equilibrium water content and high permeability were maintained. Hence, the positive attributes that make P H E M A hydrogels exceedingly biocompatible were maintained. However, the incorporation of biodegradable P C L in the network resulted in a phaseseparated biodegradable network that had mechanical integrity susceptible to change in aqueous media, due to the degradation of the P C L phase. Even though biodegradable hydrogels are of potential interest for some specific applications such as artificial tendons (8), the rate of degradation of the implant must be controlled to closely follow the healing process character ized by the formation of new connective tissue. In this study we improved the mechanical properties of P H E M A hydro gels by independently cross-linking a hydrophobic network within the crosslinked P H E M A network. This mixture of two independently cross-linked polymers that cannot be physically separated is an interpenetrating polymer network (IPN) (9). If only one of the two polymers is cross-linked, the product is called a semi-interpenetrating polymer network (SIPN). The simultaneous interpénétration of two cross-linked polymers imparts the resul tant network with outstanding mechanical properties (toughness, impact strength, tensile modulus). Synergistic mechanical and thermal behaviors also have been observed (10). The influence of polycaprolactone on the proper ties of the resultant modified P H E M A hydrogels SIPNs and IPNs was investigated. The networks were evaluated in terms of structural, mechanical, and thermal properties. 2
Experimental Details Materials. IPNs and SIPNs that consist of a hydrophobic component and a hydrophilic component were investigated. The hydrophilic network was a cross-linked P H E M A network (Structure 1) that was synthesized from the monomer 2-hydroxyethyl methacrylate ( H E M A )
In Interpenetrating Polymer Networks; Klempner, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.
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Downloaded by CHINESE UNIV OF HONG KONG on February 26, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0239.ch009
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