Macromolecules ARTICLE Figure 5. (a) Scheme for photopatterning elevated features by decreasing the cross-linking density in exposed regions, which subsequently swell in aqueous media. (b) Photomask used to pattern the hydrogel. (c) Hydrogel exhibiting elevated features from photopatterning. Scale bar = 1 mm. (d) Proﬁlometry indicating height and breadth of raised features (approximately 100 and 1200 μm, respectively) on both the photomask side (solid line) and the reverse side (dotted line). Figure 6. (a) Schematic for the photoimpression patterning of photoadaptable PEG-disulﬁde hydrogels wherein a LAP-infused hydrogel is pressed against a topographically textured surface and exposed to light. (b) The surface against which the gel was pressed for this demonstration. Scale bar = 1.5 mm. Height of features is ∼750 μm. (c) The resulting pattern transferred to a photoadaptable PEG-disulﬁde hydrogel. Scale bar = 1.5 mm. network relaxation as well.23 Figure 7a depicts how this transfer was accomplished. A disulﬁde cross-linked hydrogel was placed on a ﬂat surface. On top of the hydrogel was placed a photomask (Figure 7b) of transparent squares, 500 μm per side separated by 400 μm. A normal force of 100 g/cm2, resulting in a deformation of 50% (i.e., the gel was compressed to half its original height), was applied to the hydrogel, whereupon the gel was exposed, through the mask, to 10 mW/cm2 365 nm light. Where exposed, the gel adapts to the applied strain by breaking and re-forming cross-links in such a way as to minimize the stress. Upon removal of the force that was causing the elastic strain, the unexposed regions recovered their original dimensions while exposed regions remained in their deformed, compressed state as seen in Figure 7. Unlike the features obtained by unstrained photo- patterning (Figure 5), the features obtained here are recessed. The depth of the recessed features, determined by proﬁlometry and shown in Figure 7d, was, on average, 90 μm, less than that the 250 μm which would be expected with complete relaxation of exposed regions and 100% recovery of unexposed regions. One likely cause for this discrepancy is the counteraction of increased swelling of exposed regions due to lower cross-linking density. The photoinduced disulﬁde rearrangement also enables the healing and welding of hydrogel materials. Two gels that were swollen in 0.1% LAP were pressed together under a transparent plate with a force of 50 g/cm2 and exposed to light. Upon relief of the pressure the gels were annealed (Figure 8). Similar experi- ments in the absence of light or radical initiators did not result in any appreciable welding of materials. Mechanical properties of gels prior to and following photohealing were measured by parallel plate rheometry. The elastic modulus of two unhealed gels did not measurably register on the instrument due to slippage between the two gels. Following photohealing, an elastic modulus of 2.2 kPa was measured. This number is consistent with the photorheometry degradation experiments as reported in Figure 2, where full exposure of a gel with 0.1 wt % LAP (or 3.4 mM) results in a 65% decrease in modulus. This shear elastic modulus is an average mechanical property measurement, and while it conﬁrms the photohealing of the hydrogels, it should not be extrapolated as a measurement of the strength of the welded interface. While the radical cleavage of disulﬁdes by radicals is well established, this research demonstrates that carbon- and phos- phorus-centered radical products of photocleavage can eﬀect disulﬁde lysis for photdegradation and disulﬁde rearrangement. This outcome is particularly relevant as degradation, shape, and property control are all desirable in a wide range of hydrogel 2448 dx.doi.org/10.1021/ma200202w |Macromolecules 2011, 44, 2444–2450