Controllable Hierarchical Surface Patterns of Supramolecular Hydrogels: Harnessing Buckling Instability by Confinement

Abstract

Patterned surfaces of responsive polymers find applications in diverse fields. However, it is still a great challenge to fabricate hierarchical patterns with long-range orders. Herein controllable hierarchical surface patterns that can be fabricated by combining nanoembossing techniques with the surface instability of supramolecular hydrogels are presented. Nanoembossed nanostripe arrays of polyethylene glycol (PEG)-based polyurethane-urea supramolecular hydrogels are fabricated and exposed to water, whereby the lateral expansion of nanostripes is confined and leads to the formation of folded in-plane or out-of-plane patterns depending on the aspect ratios. The direction of folds is perpendicular to the nanostripes. Both the amplitude and the wavelength of out-of-plane folds are proportional to the thickness of nanostripes. Therefore, hierarchical structures, in which one periodicity is defined by the nanoembossing processes and the other is determined by surface buckling, can be quickly fabricated in supramolecular hydrogel thin films.

Smart Polymer Surfaces with Complex Wrinkled Patterns: Reversible, Non-Planar, Gradient, and Hierarchical Structures

“…On the other hand, Hu et al [ 138 ] demonstrate that controllable hierarchical surface patterns can be fabricated by combining nano embossing techniques and surface instability of supramolecular hydrogel. Their article used polyethylene glycol (PEG) based hydrogels to fabricate nanostripe arrays upon water exposure.…”

Multi-scale structuring of cell-instructive cellulose nanocrystal composite hydrogel sheets via sequential electrospinning and thermal wrinkling

“…However, all these methods have limitations in terms of their capacity to create structures with adequate resolution and/or hierarchical/multi-scale structures commensurate in size with the features of the native ECM. More precise patterns can be achieved using microcontact printing, [13][14][15][16] soft embossing/contact lithography, [17][18][19][20] and photolithography; [21][22][23][24] however, these 2D techniques cannot fully reproduce the 3D cell environments of native ECM. As such, new strategies are required to facilitate controllable multi-scale structuring of hydrogels with the length scale and dimensionality required for precise ECM mimicry.…”

2.5D Hierarchical Structuring of Nanocomposite Hydrogel Films Containing Cellulose Nanocrystals

“…There are currently only a few published methods for preparing structured thin-film hydrogels, almost all of which employ UV-mediated gelation and small molecule cross-linkers for film curing . Physical templating methods, such as microcontact printing − and soft embossing/contact lithography, − have demonstrated some success; however, these techniques require additional fabrication steps to prepare molds and typically yield hydrogels with limited feature size resolution and weak mechanical properties. Photolithography has also been used, although feature size resolution is limited by the photomask construction. − It should be noted that high-energy irradiation techniques, such as electron beam patterning, have been demonstrated to improve feature size resolution below the tens of microns length scale; however, the achieved improvements in feature size resolution are offset by the lengthier and more complicated preparation process as well as limitations to the thickness of the film etched (typically to a maximum of hundreds of nanometers). − Finally, hydrogels with swelling-induced wrinkling patterns have been prepared by photopolymerizing a hydrogel film affixed on top of a rigid substrate; − as the surface-attached hydrogel layer swells, wrinkles on the order of hundreds of microns are formed as a mechanism of stress dissipation. − However, due to the necessity for photopolymerization and the inherently limited mechanical properties of most hydrogels, the currently reported structured thin-film hydrogel preparation methods are highly limited in terms of both feature size resolution and shape fidelity.…”