Dec 5 2018
The pads in the astonishingly sticky feet of geckos are covered with setae—microscopic, hairlike structures with special physical and chemical composition and high flexibility that enable the lizard to grip ceilings and walls with ease.
Liquid crystal elastomers deform in response to heat, and the shape they take depends on the alignment of their internal crystalline elements, which can be determined by exposing them to different magnetic fields during formation. (Image credit: Wyss Institute at Harvard University)
Researchers have made efforts to reproduce such dynamic microstructures in the laboratory with different materials, such as liquid crystal elastomers (LCEs)—rubbery polymers that consist of liquid crystalline compounds that govern the directions in which the LCEs can stretch and move. To date, synthetic LCEs have largely been able to deform in just one or two dimensions, restricting the ability of the structures to move throughout space and take on a range of shapes.
Currently, a team of researchers from Harvard’s Wyss Institute for Biologically Inspired Engineering and John A. Paulson School of Engineering and Applied Sciences (SEAS) has taken advantage of magnetic fields to manipulate the molecular structure of LCEs and develop microscopic three-dimensional polymer shapes that can be programmed to move in any direction in response to multiple types of stimuli. The study, published in PNAS, could open the path to the development of various useful devices, such as solar panels that turn to follow the sun for optimized energy capture.
What’s critical about this project is that we are able to control the molecular structure by aligning liquid crystals in an arbitrary direction in 3D space, allowing us to program nearly any shape into the geometry of the material itself.
Yuxing Yao, Study First Author, Wyss Institute at Harvard University.
Yao is a graduate student in the lab of Wyss Founding Core Faculty Member Joanna Aizenberg, PhD.
The microstructures developed by Yao and Aizenberg’s group are formed by casting LCEs into arbitrary shapes that can deform in response to light, heat, and humidity, where the specific reconfiguration of the LCEs can be controlled by their own material and chemical properties. The researchers exposed the LCE precursors to a magnetic field during the synthesis and discovered that all the liquid crystalline elements within the LCEs aligned along the magnetic field and preserved this molecular alignment after solidification of the polymer. The researchers were able to manipulate the deformation of the ensuing LCE shapes, when heated to a temperature that upset the orientation of their liquid crystalline structures, by altering the direction of the magnetic field during this process. Upon returning to room temperature, the deformed structures regained their initial, internally oriented shape.
Programmed shape changes such as these can be applied to create encrypted messages that are revealed only when heated to a particular temperature, adhesive materials the stickiness of which can be switched on and off, or actuators for tiny soft robots. The system could also make shapes to autonomously bend in directions that would normally require the input of some energy to accomplish.
For instance, it was demonstrated that an LCE plate undergoes not just “traditional” out-of-plane bending but also in-plane bending or twisting, contraction, and elongation. Moreover, distinctive motions could be realized through the exposure of different regions of an LCE structure to multiple magnetic fields at the time of polymerization, which subsequently deformed in various directions upon being heated.
The researchers were also able to program the LCE shapes to reconfigure on their own in response to light by integrating light-sensitive cross-linking molecules into the structure at the time of polymerization. Then, illumination of the structure from a specific direction led to the contraction of the side facing the light, resulting in the entire shape being bent toward the light. This kind of self-regulated motion enables LCEs to deform in response to their environment and constantly reorient on their own to autonomously follow the light.
In addition, it is possible to create LCEs with both heat- and light-responsive properties, such that a single-material structure now has the ability to carry out multiple forms of movement and response mechanisms.
An interesting application of these multiresponsive LCEs is the development of solar panels covered with microstructures that turn to follow the sun when it moves across the sky, similar to a sunflower, thereby leading to highly efficient light capture. The technology could even turn out to be the foundation for autonomous source-following radios, smart buildings, sensors, and multilevel encryption.
Our lab currently has several ongoing projects in which we’re working on controlling the chemistry of these LCEs to enable unique, previously unseen deformation behaviors, as we believe these dynamic bioinspired structures have the potential to find use in a number of fields.
Joanna Aizenberg, Founding Core Faculty Member, Wyss Institute at Harvard University.
Aizenberg is also the Amy Smith Berylson Professor of Material Science at SEAS.
Asking fundamental questions about how Nature works and whether it is possible to replicate biological structures and processes in the lab is at the core of the Wyss Institute’s values, and can often lead to innovations that not only match Nature’s abilities, but improve on them to create new materials and devices that would not exist otherwise.
Donald Ingber, Founding Director, Wyss Institute at Harvard University.
Ingber, MD, PhD, is also the Judah Folkman Professor of Vascular Biology at Harvard Medical School and the Vascular Biology Program at Boston Children’s Hospital, as well as Professor of Bioengineering at SEAS.
James Waters, PhD and Anna Balazs, PhD from the University of Pittsburgh; Anna Schneidman, PhD, Jiaxi Cui, PhD, Xioguang Wang, PhD, and Nikolaj Mandzberg from Harvard SEAS; and Shucong Li from Harvard’s chemistry department are the additional authors of the paper.
The Department of Energy and DARPA supported this study.