Cooperative deformations of periodically patterned hydrogels

Abstract

Nature has shown elegant paradigms of smart deformation, which inspired biomimetic systems with controllable bending, folding, and twisting that are significant for the development of soft electronics and actuators. Complex deformations are usually realized by additively incorporating typical structures in selective domains with little interaction. We demonstrate the cooperative deformations of periodically patterned hydrogel sheets, in which neighboring domains mutually interact and cooperatively deform. Nonswelling disc gels are periodically positioned in a high-swelling gel. During the swelling process, the compartmentalized high-swelling gel alternately bends upward or downward to relieve the in-plane compression, but the overall integrated structure remains flat. The synergy between the elastic mismatch and the geometric periodicity selects the outcome pattern. Both experiment and modeling show that various types of cooperative deformation can be achieved by tuning the pattern geometry and gel properties. Different responsive polymers can also be patterned in one composite gel. Under stimulation, reversible transformations between different cooperative deformations are realized. The principle of cooperative deformation should be applicable to other materials, and the patterns can be miniaturized to the micrometer- or nanometer-scale level, providing the morphing materials with advanced functionalities for applications in various fields.

Fig. 1. Photolithographic patterning of gel and swelling-induced cooperative deformation.

(A) A precursor solution in the reaction cell was exposed to ultraiolet (UV) light irradiation through a mask to produce patterned gels in the light-exposed region. After the residual solution was removed (B), another precursor solution was injected into the interspace between the preformed gels (C). (D) Subsequent photopolymerization without a mask produced an integrated patterned gel. (E) After the periodic patterned gel was swollen in water, it deformed into an alternating concave-convex structure. Blue and red areas correspond to nonswelling and high-swelling gels, respectively. (F) Images of corresponding swollen patterned gel. Scale bars, 1 cm.

Fig. 2. Influence of pattern dimensions on deformations of gels.

(A) Scheme to show the dimensions of the as-prepared patterned gel. The nonswelling disc gels are arranged in a hexagonal lattice. The black dotted line indicates the base unit of deformation (triangular or rhombic). The orange dotted line indicates the shape of a concentric gel disc, which would deform into different configurations (schematic in the right). (B and C) Representative configurations of the swollen composite gels in water: triangular-mode (B) (X = Y = 15 mm, R = 5 mm) and rhombic-mode (C) (X = Y = 15 mm, R = 6 mm) cooperative deformations. Left: Images. Right: Modeled configurations. Scale bars, 1 cm. (D and E) Phase diagram of the deformations of patterned gels with different dimensions: (D) different R (X = Y) and (E) different X and Y (R = 5 mm). Symbols ◊ and Δ indicate the rhombic- and triangle-mode cooperative deformations, respectively. Symbols ○ and × indicate the rolling and random deformation (without cooperativity), respectively. The thickness of the as-prepared gel is 1 mm.

Fig. 3. Switching of cooperative mode by controlled swelling process.

(A) Reversible shape transformation between the triangular- and rhombic-mode cooperative deformation and flat shape with variation in NaCl concentration. Scale bars, 1 cm. (B) Swelling ratio in length λ of PAAm and P(AAm-co-AMPS) gels as a function of saline concentration, C_NaCl. The corresponding modes of cooperative deformation are also shown.

Fig. 4. Control of cooperative deformations by selective preswelling.

A mask with holes was put on top of the patterned gel to selectively swell the regions under the holes, leading to programmed cooperative deformations after the gel was further swollen in water. (A and B) Mask (A) and the corresponding combined triangular- and rhombic-mode cooperative deformations in one composite gel (B). (C and D) Mask (C) and programmed cooperative deformations with desired mode and direction (D). Scale bars, 1 cm.

Fig. 5. Control of cooperative deformations by using different masks.

Periodically patterned gels with square arrangement of square PAAm gels (A), square arrangement of elliptic PAAm gels with identical orientation (B) or orthogonal orientation (C), combined square and rhombic arrangement of disc PAAm gels (D), and kagome arrangement of disc PAAm gels (E). The masks and modeled configurations are above and below the images of deformed patterned gels, respectively. The white and black regions of the mask correspond to nonswelling PAAm gel and high-swelling P(AAm-co-AMPS) gel, respectively. Scale bars, 1 cm.

Fig. 6. Shape transformations of composite gels patterned with multiple responsive polymers.

PAAc and P(AAm-co-VI) gel discs positioned in P(AAm-co-AMPS) gel with square arrangement but different orientation (A) or different periodicity (B) and their distinct cooperative deformations under different pH values. Scale bars, 1 cm.