TLDR: 4D printing is a type of 3D printing that creates objects that can change shape over time in response to environmental conditions. It uses special materials and techniques to achieve this shape-morphing effect.

4D printing, also known as 4D bioprinting, active origami, or shape-morphing systems, is an advanced form of 3D printing. Like traditional 3D printing, it involves depositing material layer by layer to create a three-dimensional object. However, in 4D printing, the resulting object has the ability to change its shape over time in response to external stimuli, such as temperature, humidity, or voltage. This fourth dimension of time allows the printed object to morph into different forms after the printing process is complete.

The printing techniques used in 4D printing are often based on stereolithography, which uses photopolymerization to bind layers of material together. The materials used in 4D printing are typically designed to have specific properties, such as anisotropy, which allows for controlled transformations under certain conditions. For example, hydro-reactive polymers or hydrogels can swell or contract in response to changes in humidity or temperature, while thermo-reactive polymers can change shape when exposed to heat.

One of the key applications of 4D printing is in the field of biomedical engineering. Researchers have developed 4D-printable resins that are biocompatible and can be used to create scaffolds for tissue engineering. These scaffolds can change shape to better mimic natural tissues and promote cell growth. Other potential applications of 4D printing include responsive architecture, where buildings can adapt to changing environmental conditions, and smart materials for commerce and transportation, such as packaging that can self-assemble or self-repair.

Overall, 4D printing is an exciting and innovative technology that has the potential to revolutionize manufacturing and design. By adding the element of time to 3D printing, it opens up new possibilities for creating objects that can adapt and respond to their surroundings.