3D and 4D lithography of untethered microrobots

In the last decades, additive manufacturing (AM), also called three-dimensional (3D) printing, has advanced micro/nano-fabrication technologies, especially in applications like lightweight engineering, optics, energy, and biomedicine. Among these 3D printing two-photon polymerization (TPP) offers highest resolution (even at nanometric scale), reproducibility possibility to create monolithically complex structures with a variety of materials (e.g. organic inorganic, passive active). Such active change their shape upon an applied stimulus or degrade over time certain conditions making them dynamic reconfigurable (also 4D printing). This is particularly interesting field medical microrobotics as functions such gentle interactions biological samples, adaptability when moving small capillaries, controlled cargo-release profiles, protection encapsulated cargoes, are required. Here we review physics, chemistry engineering principles TPP, some innovations that include use micromolding microfluidics, explain how this fabrication schemes provide microrobots additional features application opportunities. The using smart materials, nano- biomaterials, for situ chemical reactions, biofunctionalization, imaging put into perspective. We categorize based on motility mechanisms, function, architecture, finally discuss future directions research.