Surface Instability of Bilayer Hydrogel Subjected to Both Compression and Solvent Absorption

Ultrafast and facile construction of programmable, multidimensional wrinkled-patterned polyacrylamide/sodium alginate hydrogels for human skin-like tactile perception

Customizable structures and patterns are becoming powerful tools for biomimetic design and application of soft materials. The construction of long-range ordered self-wrinkled structures on multi-dimensional and complex-shaped surfaces with facile, fast and efficient strategies still faces serious challenges. During the stretch-recovery process, the carboxyl groups in the polyacrylamide/sodium alginate dual network gel form robust coordination with Fe3+ to achieve a hard shell layer, resulting in a modulus mismatch between the inner soft layer and the outer hard layer, thereby forming a wrinkled surface. This flexible strategy allows simultaneous construction of complex topologies from 1D to 3D wits well-organized microstructure and controllable dimensions. The mechanism of the influence of ion treating time and pre-stretching ratio on wrinkle wavelength was explored in detail. The finite element simulations matched well with the experimental results. Due to the unique surface and dual crosslinking network, the self-wrinkled hydrogel maintains a high sensitivity of up to 67.47 kPa-1 in 1000 compression cycles. As a high-sensitivity pressure sensor integrated into the detection system, it can be efficiently applied to the contact dynamic tactile perception and monitoring of various movement behaviors of the human body.

Mechanical Adaptability of Patterns in Constrained Hydrogel Membranes

Pattern formation and dynamic restructuring play a vital role in a plethora of natural processes. Understanding and controlling pattern formation in soft synthetic materials is important for imparting a range of biomimetic functionalities. Using a three-dimensional gel Lattice spring model, we focus on the dynamics of pattern formation and restructuring in thin thermoresponsive poly(N-isopropylacrylamide) membranes under mechanical forcing via stretching and compression. A mechanical instability due to the constrained swelling of a polymer network in response to the temperature quench results in out-of-plane buckling of these membranes. The depth of the temperature quench and applied mechanical forcing affect the onset of buckling and postbuckling dynamics. We characterize formation and restructuring of buckling patterns under the stretching and compression by calculating the wavelength and the amplitude of these patterns. We demonstrate dynamic restructuring of the patterns under mechanical forcing and characterize the hysteresis behavior. Our findings show that in the range of the strain rates probed, the wavelength prescribed during the compression remains constant and independent of the sample widths, while the amplitude is regulated dynamically. We demonstrate that significantly smaller wavelengths can be prescribed and sustained dynamically than those achieved in equilibrium in the same systems. We show that an effective membrane thickness may decrease upon compression due to the out-of-plane deformations and pattern restructuring. Our findings point out that mechanical forcing can be harnessed to control the onset of buckling, postbuckling dynamics, and hysteresis phenomena in gel-based systems, introducing novel means of tailoring the functionality of soft structured surfaces and interfaces.