Just when 3D printing was starting to sound less futuristic, 4D printing has added a new twist. Like 3D printers, the new printing technology turns blueprints into three-dimensional objects by building them up, layer-upon-layer. But unusual materials, designs and external stimuli such as heat, electricity, a magnetic field or water give 4D-printed objects the ability to shape-shift into something new. Practically speaking, this new printing technology could one day lead to airplane wings that morph for take-off and landing, soft robots that can squeeze into and out of tight places, implantable biomedical devices that can maneuver through a blood vessel and even organs for patients in need of a lung, heart or kidney. Although still in the research stage, 4D printing is set to transform the future.
Redefining Ink and Design
When it comes 4D printing, ink and design go hand-in-hand. Unlike 3D printing, which typically relies on liquids that harden into solids, this type of printing uses inks and designs that flex, twist, expand or contract. In some cases, the ink is alive. Bioprinting is one such case. Scientists in this field use custom-built computer-aided-design tools and special printers with microdroplet print heads that can output cells, proteins, DNA, drug particles, growth factors and more. Precisely placed onto a medium in which the living ink can survive, these solutions grow into human skin for burn patients, for instance. One day, they could transform into nerves, blood vessels, cartilage or entire organs, according to News-Medical.
There are plenty of uses for this new printing technology in the industrial world, too. Last year, researchers from the University of Bristol and the University of Bath developed an ink made from cellulose, the main component of plants. They say they were inspired by the design of pine cones, which have scales that close tightly when the weather is cold or wet to protect the seeds inside. When the weather is warm and dry, the scales open to allow the seeds to disperse. The scientists mixed up a paste made of cellulose, plastic and gel, and then used a syringe to squeeze it out into a flat flower. When the paste dried, the petals raised up off the table, and when it was rehydrated, the petals laid down again.
In a separate project from 2018, researchers from the City University of Hong Kong developed ceramic ink from a mixture of polymer and ceramic nanoparticles. The ink is durable and flexible and can be stretched three times beyond its original length. To test how well the ceramic ink worked, the team printed lattice patterns onto flat, pre-stretched bases. The patterns were made up of different shapes with creases for joints. When the stretched bases were allowed to return to their original sizes, the patterns pulled them into forms such as roses, butterflies and even the Sydney Opera House. Heat-treating the finished shape produced a solid ceramic object.
Because ceramics are used widely in industrial applications, including spacecraft, satellites, superconductors and computers, the scientists think that their shape-morphing 4D printing innovation could be used to build components for 5G networks, rockets, electronics and more.
Metal Materials and Liquid-Printed Pneumatics
Ink made from unconventional substances is one thing, but how about printable materials that possess counterintuitive properties not found anywhere in nature? In March 2019, an engineering team from Rutgers University in New Jersey and New Brunswick, U.K. worked together to develop materials that, when heated, morphed between being stiff and soft, changing shape as they did. Picture a cage-like cube with sides made of lattices. Heat triggers the cage to contract upon itself into a tightly twisted shape.
When cooled, the cage expands to its original shape. These so-called metamaterials can be programmed to be stiffer or softer at different temperatures, allow engineers to “program” rigidity into the material, depending on its use. For instance, the material could serve as the framework for a giant space solar panel. At launch time, it could be warmed so that it shrinks down into the size of a tiny box. Deployed into the coldness of space, it would expand to its full size.
One of the most remarkable techniques comes from the scientist that originally coined the term, “4D printing.” Skylar Tibbits, who founded the Self-Assembly Lab at the Massachusetts Institute of Technology, developed a technique called rapid liquid printing, which takes place inside a vat of silicone. There, a needle-thin nozzle squirts a continuous stream of liquid silicone rubber to form shapes that look like small pillows stacked up on top of each other. Once the form is finished, the researchers shine ultraviolet light onto it to cure the liquid into a stretchy material.
Some of the final products are structures with multiple air chambers that expand like bellows when inflated with air. Working in collaboration with BMW, Tibbits and his team developed prototypes that could inspire custom car seats or new ways for building aerodynamic vehicles that morph to reduce drag. In collaboration with Swiss designer Christoph Guberan, Tibbits used the technique to 4D-print a collection of inflatable lamps, vases and vessels, according to Fast Company. Because products made in this manner can be deflated, stored, shipped, and then inflated again, manufacturers would save on shipping costs and space.
When it comes to 4D printing, the applications are numerous, and it’s likely that the most useful products have not yet been imagined. Objects are only limited by the size of the printer and the imagination of the designers and engineers. Once they’re printed, they’ll take on a life of their own.
Northrop Grumman has a long history of research and development resulting in innovation and discovery. We’re always looking for people to join our team and participate in creating the next big thing.