Pine cone inspires new development in shape-shifting materials
By Ben Coxworth
April 17, 2013
While there are already memory materials that are able to change to a given shape when exposed to certain stimuli, researchers from ETH Zurich have created something a little different. Taking inspiration from the humble pine cone, they’ve developed a process that allows a wider variety of materials to be used, that can in turn attain a wider variety of shapes.
Conducted by a group led by Prof. André Studart, the project began with a review of the manner in which pine cones are able to close their scales when wet, then open them when dry. It turns out that each scale consists of two layers made of the same material. Rigid fibers within those layers are aligned in one direction in one layer, and in another direction in the other.
When exposed to moisture, both of the layers swell, although the direction of alignment of the fibers causes them to swell in different directions. The upshot is that one side of the scale extends longitudinally while the other side bends, allowing the scales to close in on one another when wet.
Applying that knowledge to man-made materials, the scientists took a swell-able base material – gelatin – added ultrafine aluminum oxide platelets to take the place of the fibers, then poured the mixture into square molds. Because the platelets were pre-coated with iron oxide nanoparticles, it was possible to then get them all aligned in one direction using a weak rotating magnetic field.
Once that first batch of the gelatin mixture had set, they poured another layer over top of it. That layer was identical to the first, except for the fact that the researchers aligned its platelets in a different direction.
After that second layer had also set, they cut the resulting two-layered squares of material into strips. Depending on the direction of the cut as compared to the alignment of the platelets, those strips behaved differently from one another when exposed to moisture – according to ETH, “some coiled lengthwise like a pig’s tail, others turned loosely or very tightly on their own axis to form a helix reminiscent of spiral pastries.”
Some longer strips were also created that behaved differently in different sections, while others (made from another polymer) were able to respond to changes in both moisture and temperature, by twisting in different directions.
Studart claims that any material that responds to external stimuli could be used for the base, depending on the desired application – the platelets could also be made from just about anything, since they’re coated with iron oxide anyway. As far as what the applications could be, he envisions things like ceramic parts that pull themselves into shape, or precisely-fitting biodegradable implants that only become active once they’re in the correct part of the body.
Source: ETH Zurich