Grabbing and holding objects at the microscale is a complex function, even for microscopic living animals. Inspired by the hominid-type hand, a microscopic equivalent able to catch microelements is engineered. This microhand is light sensitive and can be either remotely controlled by optical illumination or can act autonomously and grab small particles on the basis of their optical properties. Since the energy is delivered optically, without the need for wires or batteries, the artificial hand can be shrunk down to the micrometer scale. Soft material is used, in particular, a custom-made liquid-crystal network that is patterned by a photolithographic technique. The elastic reshaping properties of this material allow finger movement, using environmental light as the only energy source. The hand can be either controlled externally (via the light field), or else the conditions in which it autonomously grabs a particle in its vicinity can be created. This microrobot has the unique feature that it can distinguish between particles of different colors and gray levels. The realization of this autonomous hand constitutes a crucial element in the development of microscopic creatures that can perform tasks without human intervention and self-organized automation at the micrometer scale.

The miniaturization of robots and actuators down to the micrometer length scale constitutes a fascinating technological challenge. Their development faces fabrication issues due to the small dimensions and their design must take into account how physics laws behave on those length scales. Last but not least, a major issue is energy delivery and management. In this scenario, light emerges as a versatile tool for the fabrication and, even more importantly, as an energy source. Optically driven micromachines—in which optical stimuli can be efficiently converted into mechanical work—have been realized in various contexts. This Review collects recent advances in this field, focusing on optical micro robots realized in soft polymers. Starting from an overview of the photoresponsive materials that have been employed, the various designs and realizations of such devices are shown exhibiting tasks and capabilities like swimming, walking, and the manipulation of microscopic objects. In the last part, frontiers studies in the integration of polymeric structures with biological organisms are shown. In many of the reported studies, untethered operation is a key issue, seen as a fundamental requirement toward the development of smart robots that can autonomously perform tasks and respond to their environment.

Light responsive liquid crystalline networks were prepared by photopolymerization of azobenzene-doped mesogen mixtures and applied for production of micro-actuators by a laser writing technique. Adjusting the cross-linker content was found to be an efficient and easy way to control the dynamics of lightinduced deformation from the micro- up to the macro-meter length scales. Starting from a complete characterization of the response of millimeter-sized stripes under irradiation with different sources (LED and laser light), micro-structures based on different monomer mixtures were analyzed for micro-actuator preparation. Double stripes, able to perform a light driven asymmetric movement due to the different mixture properties, were created by a double step process through a laser writing system. These results are a simple demonstration of an optically activated non-reciprocal movement in the microscale by a chemical material manipulation. Moreover, we demonstrate a rapid actuator dynamics that allows a movement in the second time scale for macrostructures and a millisecond actuation in the microscale

light of wavelengths commensurate to their periodicity and, depending on their arrangement and composition, they form photonic structures of different complexity, ranging from 1D gratings (Bragg grating) up to 3D photonic crystals. In all these cases, light-matter interaction leads to characteristic optical features, such as reflection and transmission profiles of diffractive gratings and, even more interestingly, the propagation inhibition of certain frequencies (photonic band gap) in photonic crystals. Surprisingly, nature presents numerous examples of how optical properties can be affected by nanoscale structuration. In fact, many living organisms adopt structural coloration, [1] a nanometric arrangement of dielectric geometric units on their skin or shell, to create vivid and bright structural colors. This way they adapt to the surrounding environment, send warning messages, or mislead their natural enemies through camouflage. [2] Mimic nature, and in particular actual reconfigurable mechanisms, [3] is fundamental also in man-made photonic applications to control the optical response of devices or materials. Current strategies include the use of the electrooptic effect, [4] temperature sensitivity, [5] and carrier injection. [6] Another method relies on the birefringent behavior of liquid crystals (LCs), [7] which can be infiltrated in photonic structures allowing to control the refractive index through temperature, [8] electric, [9] or magnetic [10] fields. Materials with structural color can be effectively-but invasively and slowly-tuned by deforming them, for instance by applying mechanical pressure or stress. [11] This is the only proposed tuning method over the optical response that acts on the unit cell variation instead of relying on refractive index control. Here we wish to explore a different route to achieve tuning and switching of photonic materials, using the light itself as a means to control the mechanical state of a periodic material. By creating a structured material out of photoresponsive polymer, we show that it is possible to induce mechanical deformation-and hence a huge change in the optical response of the material-by simply shining light on it. De facto, this constitutes a nonlinear optical effect, with sub-millisecond time response, using mechanical deformation as an intermediate step. The deformation, and hence the nonlinear response, takes place only in the illuminated region and therefore can be precisely local. The photoresponsive polymers that we use are light sensitive liquid crystal networks (LCN), [12] or elastomers, which have the ability to strongly deform in a reproducible way. [13] It has been demonstrated that LCN can be Materials whose optical response is determined by their structure are of much interest both for their fundamental properties and applications. Examples range from simple gratings to photonic crystals. Obtaining control over the optical properties is of crucial importance in this context, and it is often attempted by electro-optical effect or by using m...