As the world gets smaller and smaller, microrobots and nanomachines become more important for fabrication, assembly and other tasks. Controlling such tiny machines can be problematic – cable-based controls don’t work at such small scales or even, sometimes, in liquids. Piezoelectric crystals have been shown to move when exposed to electricity, but electricity also relies on cables and can cause problems with liquids. The solution? A new cable-free microrobotic arm that can be controlled with light. New microrobotic arms made of crystals show reversible curling to a hairpin shape upon irradiation with ultraviolet light. The crystals return to the original straight shape upon irradiation with visible light. Courtesy of Masahiro Irie, Rikkyo University. The robotic arms from Masahiro Irie and colleagues at Rikkyo University are made of crystals shaped like micron- or millimeter-size flat rods. When exposed to ultraviolet light, the rods bend toward the light source; when irradiated with visible light, they stretch back to their original shape. The molecules in the crystals are an organic ring system containing five rings. The central structural unit is a diarylethene group. UV light induces rearrangement of the chemical bonds and causes a ring closure within the molecule, resulting in a shape change of each molecule. This leads to a geometry change of the crystal, which then contracts, but only where it is exposed to the UV light – that is, on the outer layer of the irradiated side of the rod. This causes bending similar to that of a bimetallic strip. Visible light causes the reaction to reverse. The newly formed sixth ring opens, the original crystal structure is restored, and the crystal straightens out. Reversible bending of a rodlike crystal in water upon irradiation with UV and visible light. The crystal contains two slightly different diarylethene derivatives that must be present in just the right ratio. In this type of mixed crystal, the interactions between the individual molecules are weaker than those in a homogeneous crystal. They can withstand more than 1000 bending cycles without any evidence of fatigue. Depending on the irradiation, it could be possible to induce extreme bending, to the point of a hairpin shape. This research was published online in Angewandte Chemie (doi: 10.1002/ange.201107570). This new approach – a contrast to previous concepts for “molecular muscles” – offers the possibility of translating the motion of individual molecules to the macroscopic level. In addition, unlike synthetic micromuscles based on polymers, the new microrobotic arm is wireless and responds quickly, even in water and at low temperatures. A gearwheel rotation operated by a light-driven molecular crystal actuator. The 3.2-mm-diameter gear was rotated by the crystal, which showed reversible bending upon alternate irradiation with UV and visible light. “The light-driven actuators can be remote-controlled even in water, and the size can be reduced to micro- or nanometers,” Irie said. “The robust actuators can find versatile applications in biological and medical fields as light-driven mechanical tools.” The scientists also observed that if one end of the crystal rod is anchored, alternating irradiation with UV and visible light could be used to induce the loose end to cause a small gear to turn. It could also be used to work as a freight elevator: If attached to a ledge, the rod can lift a weight more than 900 times as heavy as the crystal itself. This makes it equivalent to piezoelectric crystals and stronger than polymer muscles. Next up, Irie said his team will work toward how to efficiently prepare the rodlike crystals with controlled size.