Glass is unyielding and brittle. Ceramics are smooth and hold up well to corrosion. Synthetics can be as flexible as the sole of a sneaker or as rigid as a CD. With so many materials to choose from, one can almost always be found to suit any product's intended purpose. But each has its limitations. Countless materials that perform well under normal circumstances fail to meet the requirements of more demanding operating environments. Many synthetic plastics cannot withstand high temperatures and not all steels are suitable for use in aggressive environments. Of course, there are ways to enhance a material’s properties and fine-tune its performance or composition to fit specifications – you could come up with a smarter design, modify the production process or even swap out the atoms in alloys. But this only works to a limited extent, because every material possesses inherent properties that restrict its use in certain applications.
Scientists at the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg aim to push these limits a little further than nature wants them go by lending old materials new capabilities - not by altering the material itself but by modifying its structure. "We do use legacy materials, but we fabricate them in a different way. Plastics, for example, are usually cast in one piece, but we take a different approach in which we start by designing precisely ordered lattice structures that prompt the material to behave in different ways," says Christoph Eberl, micromechanics expert and deputy head of IWM. He and his colleagues Matthew Berwind, Hamideh Jafarpoorchekab and Felix Schiebel used this method to create microscale polymer lattice structures. One such polymer is designed to enable the thin walls of its lattice structure to give under pressure, stretching in a certain direction when a certain amount of force is applied. This action carries over from one cell in the lattice to the next, enabling the polymer’s response to pressure to be defined precisely.
A synthetic material that acts like a system
Elastic polymers have been on the market for decades, but Eberl is taking this to the next level. "With our process, we can structure and modify a small area and specific points of a component made from synthetic material.” One of the applications he has in mind is a new type of vehicle dashboard that is mostly rigid but contains areas that have been micro-structured to be flexible. These areas could serve as pushbuttons. Piezoelectric ceramic switches and other controls that generate electrical pulses require an entire switching system consisting of a receiver, conductor and actuator to trigger an action. Eberl says, “But we want the material itself to be the switch." In that way, the material itself becomes part of the operating system.
Another application that researchers are currently looking into is a lining for the inner surface of prosthetic limbs. For this purpose, the team is designing a material that remains pliable under normal conditions but stiffens when subjected to a higher load. “This is another promising application of microstructured polymers,” says Eberl. “Prostheses need to be padded with a soft material to prevent pain at the point of contact between the prosthesis and the stump of the amputated limb. On the other hand, if the wearer lifts a heavy load, it’s better if the material is sufficiently rigid to resist the pressure of being pressed against the soft tissue of the stump. This kind of material could also be used as a shock buffer in exoskeletons. These robotic suits help people with mobility issues to walk or could be worn by workers to help them lift heavy objects.
Fabricating materials with a 3D nanoprinter
Christoph Eberl uses the term "programmable materials” a lot when talking about how he intends to integrate new functions in legacy materials. His tool of choice for this kind of project is a 3D nanoprinter, with which is is possible to design and program tools to an accuracy of a few hundred nanometers. The Karlsruhe Institute of Technology developed this printer. "This device is the perfect complement to our micro-structuring skills. We can use it to grow 3D structures or burn structures into polymer layers," says Eberl.
These researchers work with jaw-dropping precision. They recently developed a 40-micrometer wide polymer damper that expands under pressure, stretching to different degrees in different directions. When the pressure decreases, the elastic component snaps back into its original shape. With the benefit of this property, it could relay pressure-related information in any given direction. “I can imagine many potential applications for this technology," says Christoph Eberl. One of these is micro-joints.
An erector kit chock-full of structures
Right now, he is less concerned about where exactly this or that component will end up being used. As Eberl explains it, "We're upending the development process with our method. Usually, you define a goal and then consider what materials you'll need to achieve it. Instead, in the years ahead, we will be offering a lot of programmable materials and developing a whole erector kit of structures that take over system functions and let you come up with entirely new ideas for products" – like the dashboard with the integrated button.
A vast playground
The Freiburg team is not just thinking of micro-structured components. Smart materials can also be structured on a larger scale. “In many applications, it is sufficient if the cells of a lattice structure measure one or more centimeters in diameter. We want to see to how far we can upscale structures; that is, we want to learn at what dimensions certain effects occur and use these to integrate functions," says Eberl. At present, the IWM experts are working mainly with polymers, but intend to move on to ceramics and metals later down the line. "We have several variables that we can tweak: the material, the structure and the scale," notes Eberl, adding that this is a vast playground for creating new functions with legacy materials. "I can't wait to see what promising applications the future holds in store."