A new and simple approach for surface wrinkling inspired by polymer assemblies in wood fibers is introduced. A hard skin is synthesized on a linear polysaccharide support that resembles the structural units of the cell wall. This skin, a wood mimetic layer, is produced through immersion in a solution containing phenolic precursor and subsequent surface reaction by horseradish peroxidase. A patterned surface with micron-scale wrinkles is formed upon drying and as a result of inhomogeneous shrinkage. We demonstrate that the design of the wrinkled surfaces can be controlled by the molecular structure of the phenolic precursor, temperature, and drying stress. It is noteworthy that this is a totally bio-based system involving green materials and processes.

We previously developed biobased wrinkled surfaces based on wood mimetic skins in which microscopic wrinkles were fabricated on a chitosan film by immersion in a phenolic acid solution, horseradish peroxidase-catalyzed surface reaction, and drying. Here, we prepared a diverse range of wrinkled films by immersion treatment at 30, 40, 50, and 60 °C in p-coumaric acid and then investigated the correlation between wrinkle morphology and mechanical properties. Wrinkle wavelengths gradually decreased as the immersion temperature increased as well as the previous report. In order to clarify the mechanisms responsible for the different wrinkle morphologies, the films were subjected to elastic moduli measurement and GPC analysis after immersion treatment. These experiments provided evidence that the chitosan around the film surface decomposed along with the immersion process. The decomposition was accelerated by higher immersion temperature, suggesting that higher temperatures led to the formation of softer skins, inducing smaller wrinkles. In fact, wrinkle morphologies with this system were predominately determined by the hardness of the wood mimetic skins. This phenomenon is consistent with the fundamentals of surface wrinkling in nature. This study is the first to demonstrate that artificial wrinkling triggered by water evaporation can be controlled by precise control of the surface hardness of soft material.

Microscopic surface architectures that can be easily manufactured have been in demand as mechano-structural cues for tissue engineering. Microscopic surface reliefs synthesized by wrinkling were expected as cell culture scaffolds for cell proliferation, control of cellular alignment and differentiation, and spheroid generation. We previously developed bio-based wrinkled films prepared via lignification-mimetic reactions and drying. Although these films are expected as a candidate for cell culture scaffolds, stability and morphology of the wrinkled surfaces in aqueous buffer solutions were not explored. Here, we investigate the surface morphologies of the wrinkled films in phosphate-buffered saline, and their application to 3T3 cell culture. The wrinkled film prepared with the immersion treatment at 40 • C maintained its wrinkled structure in phosphate-buffered saline even after five days, although the wrinkles were broadened by hydration of the skin layer. Interestingly, higher cell numbers were observed in the 3T3 cell culture using the wrinkled film than using flat film with the same surface composition. In addition, the high biocompatibility of the wrinkled film was confirmed by in vivo experiments. These results strongly encourage application of the wrinkled film as a mechano-structural cue. Studies of the advanced applications for the wrinkled films are now in progress.

We previously developed bio-based wrinkled surfaces induced by wood-mimetic skins upon drying in which microscopic wrinkles were fabricated on a chitosan (CS) film by immersing it in a phenolic acid solution, followed by horseradish peroxidase (HRP)-catalyzed surface reaction and drying. However, the detailed structure of the resulting wood-mimetic skins, including crosslinking mode and thickness, has not been clarified due to the difficulty of the analysis. Here, we prepare wrinkled films using ferulic acid (FE), vanillic acid (VA), and homovanillic acid (HO) and characterize their structures to clarify the unknown characteristics of wood-mimetic skin. Chemical and structural analyses of wood-mimetic skins prepared using VA and HO indicate that the crosslinking structure in the skin is composed of ionic bonds between CS and an oligophenolic residue generated by the HRP-catalyzed reaction on the CS surface. Moreover, the quantity of these ionic bonds is related to the skin hardness and wrinkle size. Finally, SEM and TOF-SIMS analyses indicate that the skin thickness is on the submicron order (<200nm).