m a te ri a l wo rl d
A novel response materializes A hybrid material made of silicon and hydrogel allows reversible actuation and complex nanopatterning.
P
olymer networks such as hydrogels have been a favorite of researchers seeking adaptive materials capable of changing their properties and functions in response to external stimuli. But it can be difficult to control actuation in these materials, and they lack the structural rigidity required for many applications. Joanna Aizenberg and colleagues at Bell Laboratories/Alcatel-Lucent and the Max Planck Institute of Colloids and Interfaces (Germany) have now engineered a unique combination of rigid silicon nanocolumns embedded in a hydrogel layer (Science 2007, 315, 487– 490). The result, says Aizenberg, is “reversibly actuated surface nanostructures that dynamically change their orientation in response to external stimuli and form highly controlled, complex microstructures.” “This research opens a new direction in the responsive-materials field,” says Sergiy Minko of Clarkson University. “The hard, inorganic skeleton brings order, shape, and the physical properties of solids. The soft, responsive gel brings the capability to switch between different arrangements of the skeleton and thus to switch between different properties of the device.” The materials started out as an array of isolated, high-aspect-ratio rigid structures (AIRS)—in this case, square arrays of vertically oriented, uniform nanocolumns—etched from a silicon wafer substrate. The silicon nanocolumns had diameters of 100–300 nm and heights of 5–8 µm. A polyacrylamide gel (PAAG) layer was then formed in the space between the AIRS and a secondary substrate to make a hydrogel–AIRS (HAIRS) assembly. The researchers produced two different configurations. In the first, the PAAG layer was bonded to the original silicon substrate, so the nanocolumns were embedded in the hydrogel and attached to the substrate (HAIRS-2). In © 2007 AMERICAN CHEMICAL SOCIETY
the second, the PAAG layer was bonded to the secondary substrate, and shear stress was applied to break the nanocolumns at their bases (HAIRS-1); the result was a hydrogel with an arrangement of freestanding nanocolumns. In both instances, changes in humidity caused
Hydrogel
An optical micrograph (top) and schematic (bottom) of the HAIRS-2 structure show changes in the orientation of the nanocolumns as the hydrogel dries. The left side of the structure is wet, and the right side is dry. (Adapted with permission. Copyright 2007 American Association for the Advancement of Science.)
the hydrogel to swell and contract, resulting in the coordinated, reversible movement of the nanocolumn arrays into a variety of detailed, reproducible surface patterns. “We believe that the ability to form reversibly actuated micropatterns with complex geometries is a major strength” of HAIRS devices, Aizenberg says. Given the mechanical properties of the hybrid material and the wide variety of external stimuli (including pH, light, and temperature) to which they respond, HAIRS devices can be designed to provide tunable, directed actuation. Patterning templates onto the primary or secondary substrates can add a further dimension of control over the final
geometry of the HAIRS arrays. Highly detailed nano- and micropatterning have recently been shown to provide important adhesive, self-cleaning, water-repelling, and photonic properties to living organisms. “The design was inspired by biological structures such as carnivorous plants,” which use reversible movement of rigid hairs to capture insects, says Aizenberg. “But we took it from the macroscale to a nanometer scale.” And like biological systems, says George Whitesides of Harvard University, the HAIRS materials are hierarchical in nature, converting nanoscale structure into mesoscale function. “We’ve got all kinds of ways to make nanoscale structures now,” Whitesides says, “but this is a big step towards new and useful functions.” Responsive materials are being investigated for applications that include tissue engineering, artificial muscles, valves, actuators, and shape-memory materials. Aizenberg says the HAIRS systems seem particularly well suited to applications such as actuators, reversible switches, tunable photonics, and microfluidics. “The approach could be used to fabricate smart and responsive microfluidic devices where stirring, separation, and mixing can be either self-regulated or regulated and controlled by external signals,” notes Minko. Whitesides emphasizes their potential use as reporters in analytical microfluidic applications. “Response is the basis of most of analytical chemistry, after all,” he says. “And the cooperative response of the nanostructures allows you to visualize changes in the gel” as it responds to external stimuli. No matter what the final applications, says Whitesides, “this is great discovery research that gives us a new kind of system to work with. The fact that it works so well is going to have a lot of people thinking about it.” a —Thomas Hayden
A P R I L 1 , 2 0 0 7 / A N A LY T I C A L C H E M I S T R Y
2613
