3. 4D printing state of art
4D printing, at the time it was firstly brought to life by Skylar Tibbits, was defined as 3D printing with the addition of time, meaning featuring transformation by time in the object manufactured.

3.1 Stimulus-responsive materials SSM

Often defined as smart materials, according to Axel Ritter SSM can be classified on the basis of the effect produced by the stimulus

3.2 Shape shifting and principal investigations

4. Main Challenges