From the capacitors in our electronics to avoiding a fogged-up airplane windshield, complex ceramic materials are essential to the modern world. There’s only been so many ways to manufacture these products, however, which is what makes a recent project out of the City University of Hong Kong so exciting. They call it 4D printing.
Lead by mechanical engineering professor Jian Lu, a team at CityU has developed a 3D-printing technique employing a soft, and slightly elastic, ceramic precursor material known as the “ink.” That 3D-printed object can then harden into a variety of intricate ceramic shapes after being heated, stretched, magnetically stimulated, folded, or otherwise altered via the passage of time. It’s the fourth dimension in what’s described by researchers as 4D-printing.
For their ink, the group started out with a polydimethylsiloxane (PDMS) polymer: a putty-like form of silicone sometimes used in a biomedical microelectronics manufacturing process called soft-lithography. The PDMS precursor has major advantages over starting immediately with ceramic materials, which tend to be difficult to work into complex shapes or deposit via conventional laser printing, because they get brittle quickly.
“The whole process sounds simple, but it’s not,” as Lu noted in a statement from CityU. “From making the ink to developing the printing system, we tried many times and different methods. Like squeezing icing on a cake, there are a lot of factors that can affect the outcome, ranging from the type of cream and the size of the nozzle, to the speed and force of squeezing, and the temperature.”
Details of the CityU team’s novel “ceramic ink” were published last week in the journal Science Advances.
After posing, shaping, and heat-setting, the final ceramic structures proved to be remarkably strong: a tiny, light-weight structure, 1.6 grams-per-cubic-centimeter in density, could withstand attempts to squish or compress it up 547 megapascals of force, making it almost as compression resistant as metals and metal alloys, stronger than porous ceramic materials, but not as strong as traditional ceramics.
“Since ceramic is a mechanically robust material that can tolerate high temperatures,” Lu said, “the 4D-printed ceramic has high potential to be used as a propulsion component in the aerospace field.” (Currently, for example, ceramics play a key role in ultra-heat resistant components of rocket exhaust systems, protective re-entry panels, and the like.)
The elastic properties of the PDMS ceramic precursor has interesting potential applications as well; since 3D-printed structures with this material can be stretched up to three times their initial length, the opportunity exists for folded and tightened structures that are origami-like in their complexity.
The group made an interesting proof-of-concept of this, designed to resemble the Sydney Opera House — promising not only a revolution in Australian tourism tchotchkes, but also (one can only assume) novelty mugs for storing your pens at the office.
Four-dimensional (4D) printing involves conventional 3D printing followed by a shape-morphing step. It enables more complex shapes to be created than is possible with conventional 3D printing. However, 3D-printed ceramic precursors are usually difficult to be deformed, hindering the development of 4D printing for ceramics. To overcome this limitation, we developed elastomeric poly(dimethylsiloxane) matrix nanocomposites (NCs) that can be printed, deformed, and then transformed into silicon oxycarbide matrix NCs, making the growth of complex ceramic origami and 4D-printed ceramic structures possible. In addition, the printed ceramic precursors are soft and can be stretched beyond three times their initial length. Hierarchical elastomer-derived ceramics (EDCs) could be achieved with programmable architectures spanning three orders of magnitude, from 200 μm to 10 cm. A compressive strength of 547 MPa is achieved on the microlattice at 1.6 g cm−3. This work starts a new chapter of printing high-resolution complex and mechanically robust ceramics, and this origami and 4D printing of ceramics is cost-efficient in terms of time due to geometrical flexibility of precursors. With the versatile shape-morphing capability of elastomers, this work on origami and 4D printing of EDCs could lead to structural applications of autonomous morphing structures, aerospace propulsion components, space exploration, electronic devices, and high-temperature microelectromechanical systems.