(405i) Liquid Crystal Elastomeric Nanowires

The natural world provides many inspiring examples of amazing abilities. For example, the extraordinary ability of geckos to adhere to nearly any surface, regardless of composition, orientation, or geometry, has inspired vast amounts of research in many areas, including adhesion, soft robotics, and surface structures. Geckos are able to quickly climb across many surfaces to escape danger and hunt prey, regardless of the curvature or roughness of the surfaces. Recent research has revealed that this innate ability can be attributed to the hierarchical structure of their foot pads. Specifically, hundreds of thousands of microscopic pillars, called setae (approximately 90 μm long and with an aspect ratio of 9:1:length:diameter), on the gecko’s feet, each ending with hundreds of even smaller spatula shaped structures (approximately 20 μm long with an aspect ratio of 100:1), drastically increase the contact surface area between the surface and the gecko. This large surface area leads to an increase in few other structures use nanoscale materials, like carbon nanotubes, to better mimic the structure of the gecko’s foot. Many of these materials show adhesive characteristics that sometimes surpass those of the gecko. However, nearly all of these materials lack the same curling motion seen with geckos.

To remedy this, many researchers have begun using more responsive materials, such as shape memory polymers, combined with the gecko mimicking adhesive pads. Shape memory polymers are a class of soft polymeric materials that exhibit complex shape transformations in response to an external stimuli from their environment, often a temperature increase, and are commonly used in soft robotics. Liquid crystal elastomers (LCEs) are another kind of shape memory polymers that are able to reversibly switch between their default shape and a deformed shape in response to an external stimulus. This is caused by the interplay between the self-association of liquid crystal (LC) functional groups and the entropic conformations of polymeric backbones and results in phase-dependent shape deformations. The highly controllable nature of the reversible, macroscopic shape deformations has inspired vast amounts of research in the general area of soft robotics. Recently, LCEs have been used in a number of attempts to create materials with a focus on mimicking the unique curling locomotion of a gecko’s foot. In many cases, these materials follow the same process as other shape memory polymers in that they pair the stimuli-responsive deforming characteristics of the materials with the adhesive characteristics of the traditional gecko-inspired tapes. However, to our knowledge, no gecko-inspired materials have been made entirely of LCEs, which increases the complexity and limits the potential applications of these materials. Additionally, most of these materials have mainly focused on creating pillars that are on the milli- or 100’s of micrometers scale and have not attempted to make nanoscale structures.

Here we report a new style of gecko-inspired biomimetic adhesive material consisting of an LCE film with LCE nanowires on one side templated by an anodized aluminum oxide (AAO) mold. The AAO mold creates a dense forest of nanowires, mimicking the surface structure of gecko feet. The nanowires are approximately 10 μm long with an aspect ratio of 25:1, which puts them around the same scale as the nanoscopic spatula on a gecko’s foot. Furthermore, the film exhibits a similar curling motion to what is seen with gecko’s feet and was observed in response to both temperature changes and the presence of specific solvents. The magnitude of the deformations was found to be controlled through the temperature or volume of solvent during the deformation process and through the thickness of the film during the design and creation of the material. Additionally, unique deformations were observed depending on the phase of the solvents, with the film bending towards the nanowire side when exposed to solvent droplets and away from the nanowire side when exposed to solvent vapor. We also provide examples of the gecko-inspired, LCE film adhering to various surfaces and geometries utilizing these unique curling deformations.