(Nanowerk News) The successful implementation of graphene-based devices invariably requires the precise patterning of graphene sheets at both the micrometer and nanometer scale. As we reported in a previous Nanowerk Spotlight, it appears that graphene-based 3D-printing techniques are an attractive fabrication route towards three-dimensional graphene structures.
3D-printed graphene objects would be highly coveted in certain industries, including batteries, aerospace, separation, heat management, sensors, and catalysis.
Already, researchers have demonstrated 3D-printed graphene objects at a resolution an order of magnitude greater than ever before printed ("Researchers develop novel process to 3D-print graphene"), which unlocks the ability to theoretically create any size or shape of graphene.
In recent work, researchers from Harbin Institute of Technology report the preparation of a moisture sensitive graphene oxide (GO) soft robot (GOSR) by combining direct ink writing (DIW) and constrained drying. To do so, they realized a uniform 3D macrostructure with GO nanosheets highly aligned and compacted inside.
The team points out that all previously reported 3D printing GO structures are highly porous. Indeed, because of the superhydrophilic characteristic of GO, the water content in GO ink is generally higher than 90 wt %, which must be removed following the process of 3D printing.
In order to maintain the structure uniformity, freeze-drying is typically employed. Otherwise, the air drying of the 3D printing GO structure results in a high and nonuniform capillary force, which would easily make the whole structure tortuous. However, the sublimation of ice crystals during the freeze-drying process eventually leaves behind highly porous structures.
In other words, there is a dilemma between macrostructure uniformity and GO compactness.
Because water evaporation can induce the compaction of GO nanosheets, the authors decided to take the advantage of the capillary force instead of avoiding it. In order to maintain the uniformity of the GO structure, they proposed a universal strategy to control the shrinkage process of the 3D printed GO gel with GO highly aligned and densely compacted, by the combination of direct ink writing and constrained drying.
Figure 1. (A) Schematic diagram for the controlled shrinking process for printed GO and GOSR. (B) Shape control process for the printed GO structures. (Reprinted with permission by American Chemical Society)
For the DIW technique, the proper rheological property of the GO ink is necessary to allow both smooth extrusion of ink and shape stability of printed filaments.
To prevent deformation upon drying – resulting from the different spatial water evaporation rates around the printed structure – the researchers employ 3D constraints, which can be arbitrarily designed and easily 3D printed (see Figure 1B).
They also demonstrate the actuation capability of their GOSR by controlling the angle at the corner of the 3D constraints, as well as the local humidity.
By alternatively dropping water droplets on each leg, a 3D-printed graphene-oxide soft robot can move forward or backward without any tethered external power supply.
Concluding their findings, the authors highlight that the reported 3D printing technique combined with the constrained drying process, especially through the delicate design of the angle, location, and orientation of the angle, can provide a versatile platform to develop a graphene-oxide soft robot with complex moisture actuation capability.
By
Michael
Berger
– Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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