emerging fabrication and synthesis techniques have recently enabled their extension to micro and nanoscale objects. Miniaturization of shape-changing structures allows for their integration into new application areas. As soft actuators are being developed at the micrometer scale, their applications are being realized in biomedical science such as in minimally invasive biopsies, [10,11] targeted drug and cell delivery, [12][13][14] and responsive microfluidics. [15] Since the size of shape-changing structures are approaching the wavelength of visible light, novel approaches to modulate light-matter interactions are being explored with applications in tunable photonics. [16][17][18] Small-scale shapechanging architectures are now being considered as smart building blocks of materials that can be assembled and reconfigured on demand. [19,20] In this review, we discuss mechanisms that facilitate shape changes in micro-and nanostructures as well as their usefulness in several emerging applications. We refer to these structures as "shape-changing particles." We examine a variety of shape-transformations, except the simple case of isotropic volume-change. In Section 2, we review numerous mechanisms that have been recently exploited to change the shape of particles in response to stimuli such as temperature, pH, light, and magnetic fields. We discuss the properties of commonly used materials, including liquid crystal elastomers, [21] responsive hydrogels, [22] and shape-memory polymers [23] as well as techniques for fabricating particles from these materials. We highlight various types of shape transformations such as changing the aspect ratio of microand nanoscale anisotropic particles, [24] bending of microcylinders and particle chains, [25,26] twisting of microribbons, [27] and folding of microscale sheets. [28] Finally, we discuss the challenges and opportunities for programmable shape-morphing and the kinetics of shape change that are controlled by particle size and the mode of stimulation. In Section 3, we review recent applications of shape-changing particles, including photonics, microfluidics, microrobotics, drug delivery, and surgery. In each of these categories, we discuss seminal advances that have been enabled by shape-changing particles, and we discuss their benefits compared to conventional approaches. In Section 4, we identify key unresolved challenges in current applications and areas of research that hold promise for advancing the current state of knowledge of dynamic materials design. Demands for next-generation soft and responsive materials have sparked recent interest in the development of shape-changing particles and particle assemblies. Over the last two decades, a variety of mechanisms that drive shape change have been explored and integrated into particulate systems. Through a combination of top-down fabrication and bottom-up synthesis techniques, shape-morphing capabilities extend from the microscale to the nanoscale. Consequently, shape-morphing particles are rapidly emerging in a variety of co...
