MIT chemists have developed a technique allowing them to add new polymers to already-printed objects that alter the materials’ chemical composition and mechanical properties. The researchers can also fuse two or more printed objects together to form more complex structures.

This technique, the researchers say, could greatly expand the complexity of objects that can be created with 3-D printing.

3D-printing with multiple polymers Credit: Demin Liu and Jeremiah Johnson/MITThe technique, called “living polymerization,” yields materials whose growth can be halted and then restarted. Initial research attempts to restart polymerization proved to be too damaging to the material and difficult to control.

The researchers next designed new polymers that are also reactivated by light. Each of the polymers contains chemical groups—called TTCs—that can be activated by light-reactive organic catalyst. The TTCs fold and stretch like an accordion.

Blue light from an LED activates the catalyst, then attaches new monomers to the TTCs, making them stretch out. As these monomers are incorporated 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.

The researchers also used this approach to fuse two structures together, by shining light on the regions where they come in contact with each other.

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 catalyze similar polymerizations but can be used in the presence of oxygen.