3D printing has given designers a lot of freedom to create objects by their own design that suit their purpose exactly. However, once an object is printed, it keeps the same shape until it is (hopefully) recycled. MIT chemists have now developed a 3D printing technique that allows them to print objects and then add new polymers that alter the materials’ chemical composition and mechanical properties. These new adaptable polymers can be reactivated by light. It is also possible to fuse two or more printed objects together to form more complex structures.

The idea is to print a material and subsequently morph that material into something else, or grow the material further using light.

One of the most common techniques used for 3D printing is stereolithography. By shining light onto a liquid solution of monomers, the building blocks of plastic and other materials, stereolithography devices can form layers of solid polymers until the final shape is completed.

In an earlier study, the researchers found that they could let a 3D printed material grow when it was shone upon by ultra violet light. This light breaks apart the polymers at certain points, creating reactive molecules called free radicals. These radicals would bind to new monomers from a solution surrounding the object, incorporating the original material. Once the light turned off, the growing process stops. However, this method proved to be damaging for the material and difficult to control, because free radicals are so reactive.

In their new research, the team designed new polymers that are also reactivated by light, but in a slightly different way. Each of the polymers contains chemical groups that act like a folded up accordion. These chemical groups, known as TTCs, can be activated by organic catalysts that are turned on by light. When blue light from an LED shines on the catalyst, it attaches new monomers to the TTCs, making them stretch out. As these monomers are incorporated uniformly throughout the structure, they give the material new properties.

The researchers demonstrated that they can incorporate monomers that alter a material’s mechanical properties, such as stiffness, and its chemical properties, including hydrophobicity (affinity for water). They also showed that they could make materials swell and contract in response to temperature by adding a certain type of monomer.

One limitation of this technique is that the organic catalyst requires an oxygen-free environment. The researchers are now testing some other catalysts that have been reported to catalyse similar polymerisations, but can be used in the presence of oxygen.

Image: Demin Liu and Jeremiah Johnson