Liquid crystals: Printed actuators in a flap.

news & views possibilities for such devices in general. In particular, omnidirectional retroreflectors are likely to find use wherever back-reflecting mirrors are required whose alignment is hard to fix. Although perhaps less striking in its physical effect than a perfect lens or an invisibility cloak, the work of Ma et al. demonstrates that other devices hitherto confined to theory can be practically constructed. Of course, the field of metamaterials is so vibrant partly because old theoretical suggestions, such as that of Eaton here, or Veselago for negative refractive index 11 , are now being physically implemented by a synthesis of new theory and experiment. Many other possibilities for metamaterials have been suggested. Now that even singularities in refractive index are no longer an obstacle towards their implementation, only time will tell as to which of these will be engineered into optical devices. I n spite of impressive demonstrations of the potential of soft polymeric materials for actuator applications 1 , key obstacles to the construction of practical all-polymer devices remain. The continuing need for large shape changes and complex motion, which in turn requires precise control of sample shape, material structure and composition, as well as effective addressability and efficient energy transfer, all impede further development. However, innovations that make fresh use of existing technologies can be remarkably effective in advancing a field, as shown by Galileo's use of optical lenses to make a telescope. On page 677 of this issue 2 , the team of van Oosten, Bastiaansen and Broer at Eindhoven University of Technology propose an elegant scheme using self-organizing liquid-crystal inks and inkjet printers together with photoaddressing to produce miniature actuators. These 'artificial cilia' can be made to flap by irradiating them with light to drive flow in microfluidic devices. Liquid crystals are characterized by the long-range orientational order of their constituents; typically, rod-like molecules align parallel to one another in a liquid phase. As temperature is increased, thermotropic liquid crystals undergo a phase transition to an isotropic fluid phase as orientational order is lost. In 1975, de Gennes suggested that combining liquid crystallinity with polymer networks 3 would result in a coupling between orientational order and mechanical strain. In 1981, Finkelmann produced the first liquid-crystal elastomer 4 , an elastic solid, consisting of a weakly crosslinked polymer network with mesogenic side chains having nematic order. As predicted by de Gennes, increasing the temperature decreases the degree …