Reversible Switching of Hydrogel-Actuated Nanostructures into Complex Micropatterns
Original entry: Sung Hoon Kang, APPHY 226, Spring 2009
Title: Reversible Switching of Hydrogel-Actuated Nanostructures into Complex Micropatterns
Reference: Alexander Sidorenko,Tom Krupenkin, Ashley Taylor, Peter Fratzl, Joanna Aizenberg, Science 315, 487-490 (2007).
Soft matter keywords
hydrogel, humidity, artificial muscle
Abstract from the original paper
Responsive behavior, which is intrinsic to natural systems, is becoming a key requirement for advanced artificial materials and devices, presenting a substantial scientific and engineering challenge. We designed dynamic actuation systems by integrating high–aspect-ratio silicon nanocolumns, either free-standing or substrate-attached, with a hydrogel layer. The nanocolumns were put in motion by the “muscle” of the hydrogel, which swells or contracts depending on the humidity level. This actuation resulted in a fast reversible reorientation of the nanocolumns from tilted to perpendicular to the surface. By further controlling the stress field in the hydrogel, the formation of a variety of elaborate reversibly actuated micropatterns was demonstrated. The mechanics of the actuation process have been assessed. Dynamic control over the movement and orientation of surface nanofeatures at the micron and submicron scales may have exciting applications in actuators, microfluidics, or responsive materials.
Soft matter example
Adaptive materials and devices that change their properties and functions as a response to external stimuli are actively studied in many fields ranging from medicine and biology to chemistry, physics, materials science, and engineering. A wide range of artificial responsive materials has been reported [1–9]. Among them, hydrogels are prominent examples of such materials.
In this paper, the authors used combination of soft and hard elements for reversible actuation of rigid nano- and microstructures whose motion was controlled by the polymer layer. They combined a hydrogel with an array of isolated high aspect ratio rigid structures (AIRS) into hydrogel-AIRS assemblies (HAIRS) as shown in Fig 1. In this system, the AIRS gives structural rigidity whereas the soft hydrogel gives reponsive behavior to external stimuli.
The details of the fabrication procedure are described in the text and the supplemental information. The geometry of the AIRS used in this paper is consisted of square arrays of nanoposts with diameters d = 100 to 300 nm, heights h = 5 to 8 um, aspect ratios h/d = 15 to 80, and periodicities p = 2 to 4 mm.
Figure 2 shows the microscope images of the HAIRS-1. When the sample was exposed to humidity, the hydrogel swelled and as a result the titled posts moved to upright position. When it was in dry atmosphere, the posts went to back to the tilted state. By adjusting the humidity level, it is possible to control the tilt angle of this nanostructures. The actuation time was ~60 ms.
For the HAIRS-2 design, the hydrogel-embedded nanoposts were attached to the substrate and bent as the polymer film was dried as shown in Fig. 3. The HAIRS-2 was tested using the cycle described for HAIRS-1. Again, exposure to humidity led the hydrogel to swell so the nanoposts stood in the upright position. In this case, the authors visualized this actuation process by placing a drop of water on the surface and taking the image of the drying edge as shown in Fig. 3B. From the image, it was observed that the swelling of the hydrogel gradually reduced across the region which resulted in a continuous increase in the tilt angle of the nanoposts.
Then, the authors discussed a major difference between the HAIRS-1 and HAIRS-2 systemes in the mechanics point, which will not discussed here. Among other data that the authors showed, there was one interesting image which is probably the effect of capillary force. In the Fig. 4, the authors showed that every group of four attached nanocolumns is held together by the hydrogel. Though it was not discussed in the paper, it seems that the capillary force due to the hydrogel between the nanoposts bring them together and the adhesion between the posts make it possible to remain atached.
Finally, the authors expect that these systems can be used for many applications including actuators, controlled reversible-pattern formation, microfluidics, reversible switching of the wetting behavior, tunable photonic structures, artificial muscles, and release systems.
 S. Minko, Responsive Polymer Materials: Design and Applications (Blackwell, Ames, IA, 2006) and references therein.
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