A review on 4D printing of polymers and polymer composites

With the development and expansion of 3D printing and upgrading to 4D printing, it became possible for static 3D printed structures to turn them into a dynamic structure over time by applying one or more stimuli simultaneously. 4D printing was able to transform a simple geometry into a complex geometry without the need for printing the entire geometry from the beginning. Also, 4D printing reduced production costs, wasted fewer materials, increased production speed, and added features such as self-assembly, self-compatibility, and self-healing to the features related to 3D printed structures. In this study, first, different types of smart materials (such as shape memory polymers, shape memory alloys, hydrogels, etc.) and activation mechanisms (such as water, heat, pH, etc.) used in 4D printing were evaluated. Then, focusing on polymers and polymer composites, the properties and characteristics of 4D printing were described. In the following, the types of 3D printing methods utilized in the 4D printing of polymers were explained, and in a separate section, the effect of the printing process parameters using an FDM printer on the response of the materials used to the applied stimulus is discussed. Furthermore, the expansion of 4D printing in various industries such as medicine, aerospace, sensors, robotics, and other fields of application in which 4D printing entered was discussed and at the end of this study, the existing challenges and future opportunities were profoundly discussed.