Researchers in Germany have used 3D laser lithography to engineer polymer microstructures that mimic the lightweight yet strong properties of cellular materials like wood and bone. The tiny shells and trusses they made have the highest strength-to-weight ratio of any engineered cellular structure yet.
While the technique is currently limited to small samples, the work has answered questions about how control of the dimensions and architecture of key structural elements at the scale of micrometres and nanometres can result in dramatic improvements in material performance.
Jens Bauer and colleagues at the Karlsruhe Institute of Technology (KIT) used a commercial system supplied by Nanoscribe – a spin-out company from KIT – to create a series of complex engineering architectures in a sample of polymer. The technique, 3D direct laser writing, involves focusing a laser onto a sample of photo-curable polymer that is mounted on a computer-controlled stage that can move in three dimensions. The polymer solidifies at the point where the laser hits the sample, so by controlling the movement of the stage complex architectures can be ‘written’ into the polymer.
At the end of the writing process, excess polymer is washed away to reveal the structure. This is then coated with the brittle ceramic alumina by atomic layer deposition. The team produced a series of complex lattice structures in this way, with micrometre-sized trusses and struts, and coatings of alumina from 50 to 200nm thick.
‘We showed that the strength of the alumina layer increases as its thickness decreases,’ says Bauer. This paradoxical phenomenon is known as the mechanical size effect, where some materials become significantly stronger as the governing dimensions in the architecture drop below a certain level.
By designing the architecture of the structures and controlling the thickness of the ceramic layer, the researchers were able to create samples that had a higher strength-to-weight ratio than any other comparable cellular materials, such as metal or ceramic ‘technical foams’. The strength is exceeded only by bulk solid materials.
‘Although size effects have been repeatedly observed in thin films, nano-pillars, nano-fibres and other low-dimensional architectures, exploiting them in a macro-scale material has been so far been extremely difficult,’ says Lorenzo Valdevit, who works on micro-architected materials at the University of California, Irvine, in the US.
He adds that although the manufacturing process is currently too slow to be a commercially viable route to the fabrication of nano-architected macro-scale materials, the work shows that ‘smart control of the topological architecture of a material can dramatically remove its inherent weaknesses – in this case brittleness – and result in the creation of new solid structures with an unprecedented combination of properties’.