Almost all solid materials, be it steel, wood, or others, expand when heated. This can cause damages such as cracks, especially if they are combined with other materials that expand at a different rate. A team of researchers in the US has developed a method to make materials shrink with the use of a 3D printer. Because the structure determines the qualities of the material rather than the material itself, the shrinking material falls in the category of metamaterials.

The team has designed tiny, star-shaped structures that consist of interconnected beams (trusses). These structures are made from materials that expand when heated. In certain architectures, such as the star-shapes, the trusses can pull the structure inward, causing it to shrink. The stars are bout the size of a sugar cube.

The researchers used a 3D printing technique called microstereolithography. This method uses light from a projector to print very small structures in liquid resin, layer by layer.

The outer beams of the star are made from a stiff, slow-to-expand copper-containing material. This inner trusses are made from a more elastic, fast-expanding polymer substance. When the stars are heated to a temperature of about 282 degrees Celsius (540 degrees Fahrenheit), the beams gradually bend inward. This causes the construction to shrink with 0.6 per cent. This may seem little, but the team stresses that it is more important that the material does not expand rather than that it shrinks.

It would be possible to control the amount of shrinkage by either changing the dimensions of the trusses or the relative stiffness of the materials. The technique could even produce metamaterials that do not change size at all when heated.

The team sees a bright and divers future for the printing technique: from dental fillings to the filling of gaps in buildings and bridges that are normally left open to account for thermal expansion.

The study involved researchers from Lawrence Livermore National Laboratory, Massachusetts Institute of Technology, University of Southern California and University of California and was led by Nicholas Fang, associate professor of mechanical engineering at MIT.