Figure 1: Photolithographic patterned hydrogels with controllable swelling mismatch and resulting internal-stress-directed orientation of rigid macromolecules.

(a) Experimental set-up for synthesis of programmed superstructure in the hydrogels using photolithographic method. As an example, two identical photo masks of stripe pattern are placed in phase on two surfaces of the reaction cell containing precursor solution of gel (i). The width of masked regions (black) and unmasked regions (white) is 2 and 0.6 mm, respectively. UV was irradiated from both sides of the cell to initiate polymerization. After gelation, the as-prepared gel shows morphology characteristic to the mask pattern (ii). (b) Schematic to show the built-up of internal stress because of the swelling mismatch, and the resultant superstructure of rigid molecules in the photolithographic patterned gels. In the gel, composition gradient between the masked and unmasked regions leads to swelling mismatch. For stripe-patterned hydrogel, as shown in a, the large swelling region experiences a compression from the less swelling region. Rigid molecules embedded in the gel intend to orient along the tensile direction while they orient perpendicularly to the compressive direction. Thus, the patterned swelling mismatch induces programmed internal stress, which directs the orientation of the rigid macromolecules. Using oppositely charged rigid molecules and polymer network, the structure is memorized by polyion complex formation after dialysis of their counterions.