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When a complex item is created using 3-D printing, the item is printed in parts that must be assembled. The purpose of 4-D printing is to reduce the total time needed to create a finished product by printing with materials that are capable of changing form or self-assembling with minimal human interaction. The "D" in 4-D printing stands for time -- more specifically, time saved.
The materials in a 4-D printed item are chosen to respond to a certain stimulus such as the transfer of kinetic energy from one medium to another. In such an example, the particles in printed material would start to bond together and change form when heat is introduced. Another approach to 4-D printing involves programming physical and biological materials to change shape and change properties. 4-D printing is closely associated with nanotechnology, a branch of engineering that is also called molecular manufacturing.
While 4-D printing is still very much in the experimental phase, it has the potential to eventually save a lot more than just time by opening the door for new kinds of assemble-at-home products. Because unassembled items created with 4-D printing would be flatter and easier to ship in large quantities, they would also save on transportation costs. The recipient would simply introduce the needed stimulus and assemble the end product without requiring directions.
This video from FW: Thinking explains more about the possibilities of 4-D printing.
See also: Amazon Prime air drones