Buckling, symmetry breaking, and cavitation in periodically micro-structured hydrogel membranes

Abstract

We investigated swelling induced instabilities in porous membranes with a square array of micron-sized circular holes prepared from a pH and temperature dual-responsive hydrogel, poly(2-hydroxyethyl methacrylate-co-N-isopropylacrylamide-co-acrylic acid) (PHEMA-co-PNIPAAm-co-PAA). At room temperature (25 °C), the hydrogel swelled to ∼1.5 to 8 times of its dried volume when pH was increased from 2 to 7. Within this regime, we observed four distinctive morphologies of the hydrogel membrane, including a "breathing" mode of the membrane having circular pore arrays, a buckled pore array of alternating mutually orthogonal ellipses, twisted snap-shut pores forming "S" shaped slits, and cusps formed in local regions that perturbed the 2D periodicity of the hydrogel membrane. Using a 3D confocal imaging technique, we followed the post-buckling behaviors of the porous membranes and investigated the pattern evolution process as a function of pH. Amplification of buckling and symmetry breaking were observed when we increased the pH of the buffer solutions from pH 4.0 to 5.0, leading to the transition from an achiral buckled state (pH 4.0) to a chiral twisted state (pH 5.0) driven by the compaction of the hydrogel domains within the space to completely close the pores. When the pH of the aqueous environment was further increased to 7, star-shaped patterns appeared randomly in the film, where the hydrogel domains were compressed by the adjacent neighbors, thus resulting in out-of-plane deformation. Finally, we demonstrated the temperature-dependent reversible switching of the hydrogel membrane among the chiral twisted state, buckled state, and circular state via changing the temperature between 20 °C and 45 °C.

Exploiting Microstructural Instabilities in Solids and Structures: From Metamaterials to Structural Transitions

“…Beyond the instability threshold, rapid and dramatic changes of the structural geometry occur, and a careful design of the initial architecture may lead to the formation of new and homogeneous periodic patterns [159][160][161][162]. Interestingly, it has been recently shown that such dramatic geometric rearrangements induced by instabilities can be exploited to rapidly tune the macroscopic response and functionalities of the structures [22,34,[163][164][165][166][167][168][169][170][171][172][173][174].…”

“…11). Moreover, while in most of the cases mechanical loading is used to deform the structures and trigger the instabilities, it has also been shown that other stimuli including swelling [160,[165][166][167][168]183] (see Fig. 12(a)), capillary pressure [172] (see Fig.…”

“…12(a)), reversible encapsulation systems [22] (see Fig. 11(d)), structures capable of switching between achiral and chiral configurations [166][167][168] (see Fig. 13(a)), soft actuators [169] (see Fig.…”

Chiral structures from achiral liquid crystals in cylindrical capillaries

“…Recently and in a different vein, identification and elucidation of pathways by which achiral building blocks spontaneously organize to create chiral structures have become an area of active study. Examples of these pathways include packing with multiple competing length scales (8-10, 23, 24), reconfiguration through mechanical instabilities of periodic structures (20,25,26), and helix formation of flexible cylinders through inter-and intracylinder interactions (27,28). In addition, the system of a broken chiral symmetry often consists of domains of opposite handedness with defects separating the domains.…”

Optomechanics of Soft Materials

“…Thus, the deformation of a dielectric elastomer links a voltage to a force [26], the deformation of a hydrogel links a change in acidity in a solution to a change in the focal length of a lens [27], the swelling of an elastomer seals an oil well for hydraulic fracture [28,29], and the deformation of a gel and an elastomer enables ionic music and ionic skin [30,31]. Soft materials can be fabricated with feature sizes as small as several micrometers [32][33][34][35] and moduli as low as ~00Pa 1 [35][36][37]. It is conceivable that optical forces will enable devices of soft materials, complex shapes, and small features.…”