The synthesis of micro or nano-patterns has become of great use in material science as well as technology. Nano or micro patterns have huge applications in flexible electronics, micro imprinting, mechanical property measurements, cell culture bio interfaces, and surface-enhanced Raman scattering. As a highly efficient and cost effective method, mechanical-driven buckling patterns such as creases, folds of thin films and wrinkles on compliant substrates have been researched widely. However, there are still some challenges despite the bold step that have been made in theoretical and experimental analyses of the buckling patterns.
Most previous research works have focused on the mechanical self-assembly on planar substrates while the synthesis of the 3-D micro/nano-patterns on curved surfaces was scarcely researched. Even is if researched to a limited extent, the low mismatched strain between the film and the substrates has limited the variation of the patterns. The synthesis of multiscale topographies remain complicated because the sequential mechanical deformation or sequential transfer of the films are necessary in the synthesis process. This in turn makes it difficult to tune the deformation or maintain the patterns after multistep manipulation. It is therefore important to come up with a facile one-step as well as transfer free method to develop highly buckling patterns with hierarchical microstructures.
Researchers led by Professor Zengyong Chu from the National University of Defense Technology in China demonstrated the actuation attribute of the highly folded graphene oxide/latex bilayer composite. The researchers introduced a 3-D shrinking approach to generate the cortex-like patterns implementing 2-D graphene oxide as the building block. Their research work is published in peer-reviewed journal, ACS Nano.
The researchers prepared the graphene oxide suspensions consistent with the Hummers approach. The graphene oxide suspension was ration-dip-coated and air dried on the surface of air charged latex balloon, resulting in a uniform graphene oxide coating. Highly folded graphene oxide surfaces could be initiated by discharging the air slowly. The authors could control the thickness of the graphene oxide film by changing the concentration of the graphene oxide suspension from 2.0 to 7.0 mg/mL. They controlled the tensile prestrains of the latex substrate by varying the air-charged balloon diameters from 5cm to approximately 24cm.
Gyrification in the human brain is driven by the compressive stress owing to the tangential expansion of the cortical layer. Here, it is initiated by the compressive stress owing to tangential shrinkage of the soft substrate. The graphene oxide topography is identical to the cerebral cortex at the microscale level. Wrinkling-to-folding transition was noted and the folding could easily be regulated by varying the prestrain of the substrate as well as the thickness of the graphene oxide film.
Initiated by the residue stresses in the system, sheet-to-tube actuating occurred once the bilayer system was cut into slices. The square bilayer actuator indicated superior reversible, large deformational, and bidirectional curling features on wetting and drying. A record-large curvature of 2.75mm-1 was observed in 18 seconds from the initial negative bending to the final positive bending reference to the tetrahydrofuran. Base on this marvelous performance, the authors have demonstrated versatile actuating applications using the bilayer system, such as a swimming worm, smart package, mechanical hand, bionic mimosa, in addition to an interesting dynamic oil collector.
Quote from the author (Zengyong Chu, National University of Defense Technology , Changsha, China)
“The 3-D shrinking strategy proposed in this work is very easy and universal to developing highly buckling patterns. The record-large curvature observed in our experiments were, on the one side, based on the pre-curved structure of the graphene oxide-latex bilayer system, and on the other side, derived from the asymmetric bilayer system in which graphene oxide acts as a molecular barrier on wetting. The rich functional groups of the graphene oxide enable its versatile surface modifications for multifunctional applications, such as dynamic oil spill collectors.
In addition, graphene oxide can also be easily reduced to graphene, which further widens its applications. For example, in a very recent further-going report (DOI: 10.1021/acsnano.7b05961), Po-Yen Chen et al revisited the fabrication process and reduced the graphene oxide to graphene. This enhances the molecular barrier performance in addition to a stretching-sensitive electrical conductivity. So the highly-folded graphene oxide (or graphene)-polymer bilayer structures can be used as ultra deforming, ultra stretchable sensors and actuators, which may find many new applications in the fields of chemical collecting, chemical protection, stretchable electronics, or soft robotics. ”
Yinlong Tan, Zengyong Chu, Zhenhua Jiang, Tianjiao Hu, Gongyi Li, and Jia Song. Gyrification-Inspired Highly Convoluted Graphene Oxide Patterns for Ultra large Deforming Actuators. ACS Nano 2017, 11, 6843−6852.
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