Jeremiah Johnson, professor of chemistry at MIT, explained: “The idea is that you could print a material and subsequently, using light, morph the material into something else, or grow the material further.”
Several years ago, prof Johnson and his colleagues set out to create adaptable 3D-printed structures by taking advantage of a technique known as ‘living polymerisation’, which yields materials whose growth can be halted and then restarted later on.
In 2013, the researchers demonstrated that they could use a type of polymerisation stimulated by ultraviolet light to add new features to 3D-printed materials. After printing an object, the researchers used ultraviolet light to break apart the polymers at certain points, creating very reactive molecules called free radicals. These radicals would then bind to new monomers from a solution surrounding the object, incorporating them into the original material.
“The advantage there is you can turn the light on and the chains grow, and you turn the light off and they stop,” prof Johnson continued. “In principle, you can repeat that indefinitely and they can continue growing and growing.”
However, this approach proved to be too damaging to the material and difficult to control, because free radicals are so reactive.
In their next effort, the researchers 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.
“We really have a truly living method where we can take macroscopic materials and grow them in the way we want to,” prof Johnson said.
In their work, the researchers demonstrated that they can incorporate monomers that alter a material’s mechanical properties, such as stiffness, and its chemical properties, including hydrophobicity. 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.
According to the researchers, one limitation of this technique is that the organic catalyst requires an oxygen-free environment. To combat this, they are now testing some other catalysts that can be used in the presence of oxygen.
