From personalized jewelry to complex aviation components, 3D printing demonstrates tremendous creativity and application potential. However, the exploration of technology is endless, and a more groundbreaking technology than 3D printing -4D printing, is quietly emerging. It integrates the dimension of time on the basis of 3D printing, endowing objects with the magical ability of "self deformation" and "self-assembly", bringing infinite possibilities for future manufacturing. After 3D printing is completed, the shape of the object is fixed. 4D printing, on the other hand, introduces a temporal dimension, allowing printed objects to change shape, performance, or function over time and in response to environmental changes. Simply put, 3D printing creates static objects, while 4D printing creates dynamic intelligent components resembling "life" that can intelligently respond to the surrounding environment. 4D printed material "code" With 4D printing, scientists can design a flower model made of humidity responsive intelligent hydrogel material. When printing, set the initial shape of the petals and the deformation rules under specific humidity conditions. When this flower model is placed in a damp environment, the petals will gradually unfold according to the preset program, as if a real flower is blooming. The precise control of object deformation process is the core of 4D printing. The implementation of 4D printing relies on intelligent materials and programming design. Smart materials are a type of material that has the ability to perceive and respond to environmental stimuli such as temperature, humidity, magnetic fields, electric fields, etc. In 4D printing, the commonly used smart materials are shape memory alloys, shape memory polymers, magnetic control smart materials, smart hydrogels, etc. Taking shape memory alloys as an example, they can deform at low temperatures and quickly return to their original shape when the temperature rises to a specific value. Shape memory alloy: the "memory magic" of metals. Shape memory alloy is one of the important materials in 4D printing, with nickel titanium alloy being the most representative. It has a unique crystal structure and undergoes phase transitions at different temperatures. In the low-temperature martensitic phase, the alloy is prone to deformation, but when the temperature rises above the austenite phase transition temperature, the alloy quickly returns to the shape of the high-temperature phase. In the 4D printing process, technicians precisely control the distribution and structure of shape memory alloys in different areas according to design requirements, combined with programmed temperature change conditions, to achieve precise control of the object deformation process. For example, in the manufacturing of complex aerospace components, utilizing the characteristic of shape memory alloys to restore preset shapes at high temperatures can achieve self-assembly and adaptive adjustment of components, greatly improving manufacturing accuracy and efficiency, while reducing the number and weight of components and enhancing overall performance. Shape memory polymer: intelligent transformation of plastics. There are various types of shape memory polymers, which have relatively low costs and are easy to process and shape. Its shape memory effect originates from its internal molecular chain structure. When stimulated by external factors, the conformation of the molecular chain changes, resulting in a change in shape. In the biomedical field, shape memory polymers have enormous potential for application. For example, vascular stents made of degradable shape memory polymers can be compressed into small shapes at low temperatures and easily delivered to the site of vascular lesions through catheters. After reaching the designated location, with the action of body temperature, the stent will return to its original shape, stretch the narrow blood vessels, and gradually degrade after completing its mission, avoiding the trouble of secondary surgery for removal. Magnetic Control Intelligent Materials: Future Soft Robots. Magnetron smart materials are an important member of the 4D printing material family, demonstrating unique application value in various fields such as aerospace and biomedicine. This type of material mainly includes magnetic fluids, magnetorheological elastomers, etc., which are composed of a matrix composed of polymers or organic solvents and uniformly dispersed micro/nano magnetic particles. Under the action of a magnetic field, magnetic particles quickly arrange to form specific structures, resulting in significant changes in the mechanical properties of the material, such as stiffness and damping. 4D printing combines these intelligent materials with computer-aided design and manufacturing technology. By accurately designing the distribution and structure of materials in different areas and writing corresponding "programs", it is possible to control the way objects change at different times and in different environments. Soft robots are a new type of soft robot that can adapt to various unstructured environments and interact with humans more safely. Magnetron intelligent materials have the characteristics of fast response speed and high control accuracy. The application of magnetron intelligent materials in soft robots enables them to simulate complex biological movements. For example, soft robots that mimic octopus movements can achieve tentacle bending, stretching, and grasping movements by applying magnetic fields of different strengths and directions to magnetically controlled intelligent materials in different parts. They have broad application prospects in fields such as ocean exploration and underwater rescue. In this regard, the world's first magnetic controlled rheological robot independently developed by the Beijing Jiaotong University team can even mimic the pseudopodic deformation movement of single-cell biological amoebas, and is expected to be applied in the capture and clearance of circulating tumor cells in the future. Intelligent hydrogel: "deformation record" in case of water. Hydrogel is a kind of hydrophilic polymer network material, which can absorb a lot of water and maintain a certain shape. Its application in 4D printing is mainly based on the swelling and shrinking characteristics of this material. Different hydrogels have different responses to different environmental factors (such as pH value, ionic strength, etc.). In the field of drug controlled release, microspheres made of hydrogels can release drugs according to the changes in the body environment. When the microspheres are in the tumor microenvironment (usually with a low pH value), the hydrogel will shrink and slowly release the anti-cancer drugs wrapped in it to achieve precise treatment and reduce damage to normal tissues. 4D printing is bringing revolutionary changes to fields such as aerospace, healthcare, and smart wearables. Manufacturing traditional aviation components requires a large number of processing steps and high costs, and in space, complex environmental changes place extremely high demands on the performance of the components. 4D printing technology can manufacture aviation components with adaptive capabilities. For example, the skin of aircraft wings can be manufactured using 4D printing, using shape memory alloys or smart composite materials. When the aircraft flies at different altitudes and speeds, the wing skin can automatically adjust its shape according to aerodynamic changes, optimize the lift and drag of the wing, thereby reducing fuel consumption and improving flight efficiency. In the field of space, 4D printed foldable structures have significant advantages. During the launch phase, the components of the spacecraft can be folded into a compact shape, saving space and reducing launch costs. After entering space, these components can automatically unfold and assemble into the desired shape under specific environmental stimuli, such as solar panels, antennas, etc. This not only reduces the difficulty and risk of space assembly, but also improves the reliability and maintainability of spacecraft. In 2021, China's Tianwen-1 probe successfully completed the controllable dynamic unfolding of the Chinese national flag using a shape memory polymer structure. This deployment mechanism was independently designed and developed by a team from Harbin Institute of Technology, making China the world's first country to apply shape memory polymer intelligent structures to deep space exploration engineering. In the biomedical field, the application of 4D printing brings new hope for the development of medical technology. In terms of tissue engineering, scientists use 4D printing to manufacture scaffolds with biological activity. These scaffolds not only have a three-dimensional porous structure that facilitates cell adhesion, growth, and proliferation, but also undergo changes in shape and mechanical properties as cells grow and tissues repair, providing an ideal microenvironment for tissue regeneration. For example, printed bone tissue scaffolds can gradually adjust their mechanical properties according to the growth of bones, promoting the formation of new bone. In the field of personalized medicine, 4D printing has demonstrated its unique advantages. Doctors can customize 4D printed medical devices based on the specific condition and physical characteristics of patients. For example, printing personalized heart valves for children with congenital heart disease, the valves can adaptively adjust their shape and function as the heart beats and grows, improving treatment effectiveness. In addition, the 4D printed drug delivery system can also achieve precise drug release, accurately controlling the amount of drug release according to the needs and time nodes of different parts of the body, improving the efficacy of drugs, and reducing side effects. In the field of flexible clamping, 4D printing has also demonstrated unique application value. In industrial production, traditional rigid clamping tools are prone to damage when facing irregularly shaped, soft or fragile items. The flexible gripper made of intelligent materials in 4D printing can effectively solve this problem. For example, a flexible gripper made of shape memory polymer has a certain degree of flexibility at low temperatures and can easily wrap around the clamped object. When the temperature rises, the gripper will tighten according to the preset program, firmly grasp the object, and can adaptively adjust the gripping force according to the shape of the object to avoid damage to the object. In the process of electronic chip manufacturing, high-precision and gentle clamping operations are required for the handling and assembly of small chips. The flexible gripper using magnetic control intelligent material 4D printing can accurately complete tasks, improve production efficiency and product quality. With the increasing demand for smart wearable devices, 4D printing technology has injected new vitality into this field. Traditional smart wearable devices often have shortcomings in comfort and personalization, while 4D printing technology can customize smart wearable products with unique functions based on each person's physical characteristics and usage needs. For example, 4D printed smart clothes can be made of materials such as shape memory polymers or hydrogels. When the human body sweats or the body temperature rises, the clothes can automatically adjust the permeability and humidity to maintain the comfort of wearing. At the same time, this smart clothing can also integrate various sensors and electronic components to achieve real-time interaction with the human body, such as monitoring physiological indicators such as heart rate and blood pressure, and transmitting data to devices such as mobile phones. In addition, 4D printed wearable devices also have self-healing functions, which can automatically repair under certain conditions when the device is slightly damaged, extending its service life. The development prospects are very broad. Although 4D printing has made significant progress, it still faces some technical challenges at present. Firstly, the performance of smart materials still needs to be further improved. For example, the response speed and memory accuracy of shape memory alloys cannot fully meet the needs of some high-end applications; The mechanical properties of hydrogel are relatively weak, which limits its application in some fields with high load bearing requirements. Secondly, the technology and equipment for 4D printing are not yet mature enough, and further optimization is needed in terms of printing accuracy, speed, and material compatibility to achieve more complex and efficient printing processes. Again, precise control and programming of the deformation process of objects also require more advanced algorithms and software support. Meanwhile, due to the high cost of smart materials and the complexity of printing equipment and processes, the cost of 4D printing technology is relatively high. The high cost limits its application in large-scale production, and currently it mainly focuses on high-end fields and customized products. To achieve widespread application, it is necessary to reduce costs, improve production efficiency, and achieve