Reversible Switching of Hydrogel-Actuated Nanostructures into Complex Micropatterns

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By Sung Hoon Kang

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 ==

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 isolatedd high aspect ratio rigid structures (AIRS) into hydrogel-AIRS assemblies (HAIRS) as shown in Fig 1. In this system, the ARIS 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.

Fig. 1 (A) Schematic presentation of the structure and composition of the PAAG film grafted to the Si/SiO2 substrate via the PGMA anchoring layer. (B) Scanning electron micrograph (SEM) of a sample AIRS structure composed of an array of silicon nanocolumns. An inset on the right reveals a banding pattern (scalloping) that is characteristic of the Bosch fabrication process. The band width is ~200 nm. (C) Hydrogel synthesis in the confinement of the AIRS and a secondary substrate (a glass slide or a silicon wafer, either flat or topographically patterned) separated by a spacer of the desired thickness that regulates the size of the polymer film. (D) Hybrid HAIRS-1 design. The nanocolumns, embedded into the hydrogel layer grafted to the PGMA-modified confining surface, break from the original wafer upon the separation of the confining substrates and become fully transferred onto the secondary surface. The surface topography changes from an array of highly tilted to vertically oriented nanocolumns. (E) Hybrid HAIRS-2 design. The hydrogel film is attached directly to the AIRS silicon wafer. Nanocolumns remain attached to the surface and bend upon drying.

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.

Fig. 2 Microscopy study of the HAIRS-1 design. (A) SEM image of a dry sample viewed perpendicular to the surface reveals tilted columns organized in domains with different tilt directions. The top inset shows a zoom-in view perpendicular to the surface. The number of 200-nm bands n on the emerging portion of the nanocolumns and the length of the column projections a were monitored to determine the length l of the exposed nanocolumns (l = 200n, measured in nm) and their tilt angle (sin a = a/l). The SEM of the cross section (bottom inset) clearly shows the tilted nanocolumns partially embedded into the hydrogel layer. Scale bars in insets, 2 mm. (B and C) Optical micrographs, imaging the same region of the HAIRS-1 system in a dry (B) and a wet (C) state, show the reorientation of the nanocolumns from a tilted to a vertical position upon the expansion of the hydrogel.
Fig. 3 Microscopy study of the HAIRS-2 structure. (A) Low-magnification SEM (left) and highmagnification SEM (right) show the hydrogel layer that forms characteristic onionlike or conical features at the bottom of the nanocolumns. (B) Optical micrograph of the drying edge of the HAIRS-2 structure taken perpendicular to the surface. The clarification of the actuation mechanism is shown schematically below the micrograph. A dashed line in the schematic corresponds to the focal plane in the image. The degree of hydration or swelling of the polymer layer decreases gradually across the sample from left to right. Correspondingly, the nanocolumns gradually change their orientation from perpendicular to tilted. The whole range of the orientations is depicted, thus providing a still image of the entire reorientation process.

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.

Fig. 4 An example of a complex HAIRS-2 pattern, showing an array of microtraps, in which every group of four attached nanocolumns is held together by the hydrogel.