A New Approach for Creating Polymer Hydrogels with Regions of Distinct Chemical, Mechanical, and Optical Properties

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Abstract

We describe a new approach to creating hybrid polymer hydrogels that comprise two different gel types (gel 1 and gel 2) juxtaposed in predetermined zones or patterns and with the unique properties of each gel being retained. The key to our approach is to ensure that the viscosities of the pregel mixtures are high when they are brought into contact and subsequently polymerized; this limits the diffusion at gel/gel interfaces. The final gel appears as a single, homogeneous, transparent material with smooth, robust interfaces between the dissimilar zones. However, its hybrid nature is revealed by specific tests. In one example, we use the same monomer, N,N-dimethylacrylamide, for gels 1 and 2, but gel 1 is cross-linked by a chemical cross-linker while gel 2 is cross-linked by laponite nanoparticles. In this case, when the hybrid gel is immersed in a mixed solution of cationic and anionic dyes, gel 2 selectively absorbs the cationic dye due to the strong affinity of the nanoparticles in it for cationic species. Additionally, a mechanical test on the above hybrid shows that the gel 2 region is stronger and much more extensible than the gel 1 region. We also make use of the fact that the gel 2 (high-laponite) regions are optically birefringent relative to gel 1 (low or no laponite). This allows us to embed a pattern ("message") of gel 2 within gel 1. While the hybrid gel appears featureless under white light, the hidden "message" becomes visible when the gel is viewed under crossed polarizers. The overall approach described here can be extended in myriad ways for the generation of gels possessing new and unique properties.

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