Macromolecules ARTICLE Figure 7. (a) Scheme for creating topographical patterns via strained exposure. A photomask is placed on the gel. The gel is compressed by application of normal stress and exposed to masked UV irradiation. Upon relief of force, the gel exhibits topographical features on both sides due to relaxation in UV- exposed regions. (b) Photomask for patterning hydrogel by method described. White squares are 500 μm separated by 400 μm. (c) Resulting recessed patterns on surface of hydrogel. (d) Proﬁlometry of the surface of the patterned gel. The mask-side proﬁle is the solid line while the reverse side is represented by the dotted line. Figure 8. (a) Scheme for photohealing of disulﬁde hydrogel. Gels are pressed together to ensure adequate contact. Light exposure and accompanying disulﬁde rearrangement result in the annealing of the two hydrogels. (b) Two hydrogels that have been covalently annealed by photoinitiated disulﬁde rearrangement. applications. Disulﬁde degradation could have substantial poten- tial in the development of stimuli-responsive materials. Many researchers have used disulﬁdes to stabilize self-assembling micro- and nanostructures.31,32 Among the most widely imple- mented and cited of these materials are those developed by Stupp and co-workers wherein amphiphilic, cysteine-bearing peptides self-assemble into nanoﬁbers33 and macroscopic materials such as hydrogels and membranes. The inclusion of radical generators could add to the capabilities of these disulﬁde-stabilized materi- als, yielding another dimension of stimulus-responsive character. Disulﬁdes have also been incorporated into synthetic polymers for drug delivery.8,34,35 Photoinitiated radical degradation and disulﬁde rearrangement provide a route to exogenous control over the release of therapeutic agents. Also worth noting is that photoinitiations in the presence of proteins,36 many of which are held in their ternary conformation by disulﬁdes,37 are becoming more and more common as researchers explore covalently cross-linked hydrogels for the delivery of therapeutically active enzymes and growth factors. Additionally, disulﬁde formation is frequently used to create cyclic peptides29,38 which may be incorporated in tissue engi- neering constructs for the direction of cellular behavior. These results should not suggest that photoinitiation cannot be used in the presence of peptides and proteins containing disulﬁde bonds. Polymerization is most often a chain process with a single initiating radical resulting in the formation of many covalent bonds, whereas it is demonstrated herein that the scission of a single disulﬁde bond and subsequent termination of thiyl radicals 2449 dx.doi.org/10.1021/ma200202w |Macromolecules 2011, 44, 2444–2450