Procedural integration already exists in nature: "The human bone re-accommodates its formal organization (“geometry”), through material distribution (material) according to the magnitude and direction of loads applied to it at any given moment (structure)." \cite{Oxman2010}

2. 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 \cite{hirsch2014} . The last researches on the field have emphasized the importance of materials in the process, hence a more accurate formulation could be: "additive manufacturing of stimulus-responsive materials".