4D Printing: promises or operational future?

The concept of 3D printing was patented in 1984 by two patents concerning stereolithography, consisting in the spatially resolved polymerization of a light-sensitive resin. The coordinates of the object to be produced were stored in a computer that controlled galvanometric mirrors to transform a liquid into a solid by photopolymerizing a fluid layer, voxel by voxel, hence the concept of "additive manufacturing". Adding a second layer, then a third, and so on, in principle created the desired prototype part, which then had to be extracted (and washed) from the non-photototransformed fluid. Today, the success of the seven technologies based on the additivity concept is undeniable, with over 150,000 publications, a market worth several tens of billions of euros per year, and a growth rate of 20% per year.

But at the same time, new application niches are emerging, combined with the stabilization and robustness of the 36-year-old technology that serves as their solid base. With the use of more "intelligent" materials and the development of alternative processes, it has been possible to push 3D technologies beyond their fields of excellence to explore new application niches, such as those seeking to modify the form and/or functionality of 3D objects to create new motors, actuators and/or sensors in a single operation, to contribute to the development of flexible robotics, and so on. The biomimetic approach (like the pine cone that opens in the absence of humidity, so that the seeds reach the ground at the "right" moment) serves as a pictorial representation for the presentation and exploitation of the concept.

Some may have thought that we could free ourselves from the determinisms imposed by 3D machines by using chemical modes of material transformation, and play on spontaneous or stimulated self-organization processes (as in the case of bio-printing, a form of 4D printing applied to living organisms). But while proofs of concept may highlight a field of possibilities, in reality and for various reasons recalled in this article (scientific conservatism, satisfying the need, use of commercial 3D machines, etc.), the ball remains in the court of additive manufacturing, even if other difficulties still limit the development of 4D printing. We're still a long way from "Terminator", where matter can be mind-controlled, and it's unlikely that a given shape corresponding to a precise instruction by the designer will be achieved from specific spontaneous, or even controlled, perturbations located in space and time. While it is possible to steer a three-dimensional system at least in part towards a desired shape, by introducing an apparent determinism, sometimes controlled by the engineer, based on certain laws of physics, chemistry and biology, many locks still limit the development...