In 1959, Feynman gave a talk titled "There’s Plenty of Room at the Bottom" , in which he addressed the issues of controlling and guiding things in the micro/nano scale, as well as the huge potential of this field. There has been an explosion of continuing and in-depth research on materials, manufacturing, manipulation, and characterization in the micro/nano scale since then. Using the micro/nano technologies, physicists and chemists can view the consequences and even the process of reactions in the microscale, biologists can handle a single cell, and engineers can construct integrated circuits with a resolution of several nanometers. However, static architecture is becoming more and more difficult to meet the future demands of complex environment adaptation and multi-functional integration in the micro/nano domain.

4D printing was first proposed by Tibbits at a TED (technology, entertainment, and design) talk in 2013. Although there is no precise definition, 4D printing is often interpreted as "3D printing+ time, " which means that the qualities of a static object (shape, property, etc.) change in response to a specific external stimulus. From the macro-field to the micro/nano field, the reaction time of the micro/nano devices is substantially shortened, the response sensitivity is much enhanced, and the demand for actuation energy is much lowered. In view of the potential of the micro/nano 4D printing technologies as mentioned above, it is necessary to make a review of the recent research progress on the micro/nano 4D printing techniques.

First of all, we summarize the commonly used micro/nanofabrication technologies, including direct ink writing, digital light processing, projection micro-stereolithography, and two-photon polymerization (Table 1). Among these various techniques, two-photon polymerization has become the most popular technology in the field of micro/nano 4D printing because of its excellent processing precision of tens of nanometers and true 3D processing ability. Second, we introduce the common material systems in the micro/nano 4D printing field, including intelligent hydrogel, liquid crystal elastomer, shape memory polymer, and biological-based materials (Fig. 2). The mechanisms of the stimuli-responsiveness of four kinds of intelligent materials are introduced. Third, we summarize the recent research in the micro/nano 4D printing field from the perspective of stimulus response. The first is the magnetic response and we summarize the recent works on how magnetic field changes the shape of one body. For example, the magnetic field drives the cilia to swing, which makes the "Paramecium" move. The varying magnetic field drives the complex deformed "bird" (Fig. 3). The solvent response is a direct mode of actuation. The opening and closing of stomata and the opening and closing of flowers are caused by the adsorption/desorption of water in the environment by the polymer (Fig. 4). The pH response is a widely used actuation method. At present, the pH response has been used to achieve certain complex deformation, such as the panda expression change (Fig. 5). The temperature response is also a widely used actuation mode. The temperature response can induce three different mechanisms, including hydrogel phase transition, liquid crystal phase transition, and material thermal expansion deformation. Figure 7 shows the work of the deformation caused by different principles. The light response is a more efficient mode of actuation, which can be precisely controlled by adjusting the power, position and duration time of a laser beam. Pure photothermal response is easiest to achieve (Fig. 8). More complicated motion control can be realized by the photothermal effect combined with the photochemical effect, Marangoni effect and others. Research based on the photoelectric effect has also been reported recently, by which the surface electrochemical effect is excited (Fig. 9). At the end of this section, we summarize the characteristics of the above-mentioned actuation methods (Table 2). Finally, we show some typical applications of 4D printing. In the field of biomedicine, the micro-helix can transport cells in a magnetic field, and the micro-drug-loaded fish can release drugs in a targeted way. In the field of micro-mechanics, the synthetic micro-walker can dynamically respond to the walking behavior of micro-devices, and the micro-gripper can accurately grasp and transport tiny particles. The micro/nano 4D printing technology can also be used in the micro-optical field to achieve focal length tunable diffraction gratings and artificial compound eyes.

Micro/nano 4D printing is the most advanced manufacturing technology for dynamic response of micro/nano devices, and it is expected to have great applications in many frontier fields such as biomedicine, microelectromechanical systems, flexible electronics, reconfigurable surfaces, and metamaterials. We believe that the future research can make breakthroughs in the following areas: 1) developing more accurate and efficient micro/nano processing methods; 2) developing high performance responsive materials with better response sensitivity, better actuation, and better biocompatibility; 3) developing and optimizing the control methods to achieve more efficient, more accurate individual control, and more intelligent cluster control; 4) enhancing the integration of scientific research with the application scenario. In the future, we believe that the micro/nano 4D printing technology should and must play a greater role in human cognitive exploration of the micro-world and make a greater contribution to improve human life.