Even good dental fillings go bad. Sometimes fresh cavities are to blame; sometimes it’s the filling itself, worn down by years of fluctuations in acidity and temperature. Now, using star-shaped structures they created, researchers have demonstrated a way to tune thermal expansion that could help fillings, and a host of other items, maintain their shape when they are exposed to heat and cold. The structures exploit a principle known as “negative thermal expansion,” in which a substance—like ice—shrinks instead of expanding when heated. Scientists have been working for years to create materials that have this property. Now, using 3D printing, a group of engineers has created star-shaped structures (pictured) on a millimeter scale that are able to heat more uniformly than other materials have in the past. The interconnecting beams that form the points are made of two materials: a stiff, copper-containing one for the beams that make up the outer frame, and an elastic, polymer-containing one for the inner beams. This configuration enables the structures to shrink inward upon heating. What’s more, this is the first time that scientists have demonstrated negative thermal expansion in all three dimensions, rather than just two, for structures with microscopic architecture, the team reports this month in Physical Review Letters. The researchers even found that by changing the concentration of copper present in the outer beams, they could tune the extent to which these structures shrank. When carefully combined with materials that expand when heated, this shrinking composite material could be used to help products maintain their shape. This could mean longer lasting circuit boards, satellite-mounted cameras, and artificial fillings for your pearly whites.