Designed by a group at the University of Pennsylvania's School of Engineering and Applied Science, the structures - despite having no motors, batteries, circuits or processors - are able to respond in a variety of ways to a number of pre-determined environmental cues.
This artificial Venus flytrap only closes when a weight is inside and the actuator is exposed to a solvent. Structures with "embodied logic" can have even more complicated behaviors, all without motors or computers. CREDIT: University of Pennsylvania
Produced using multi-material 3D printers they mimic the behaviour of the Venus flytrap, which despite having no brain or nervous systems, appears to be able to make decisions about when to snap shut on potential prey, as well as to open when it has accidentally caught something it can't eat.
The group claims that the principles demonstrated by its work could be used to develop sensors for use in remote, harsh environments such as deserts or even other planets. Without a need for batteries or computers, such sensors could remain dormant for years without human interaction, only springing into action when presented with the right environmental cue.
The study was led by Professor Jordan Raney, whose group has a particular interest in bistable structures (meaning they can hold one of two configurations indefinitely) as well as responsive materials, which can change their shape under the correct circumstances.
Raney explained that "embodied logic" draws on both of these properties. "Bistability is determined by geometry, whereas responsiveness comes out of the material's chemical properties," he said. "Our approach uses multi-material 3D printing to bridge across these separate fields so that we can harness material responsiveness to change our structures' geometric parameters in just the right ways."
In previous work, Raney and colleagues had demonstrated how to 3D print bistable lattices of angled silicone beams. When pressed together, the beams stay locked in a buckled configuration, but can be easily pulled back into their expanded form.
This behaviour depends almost entirely on the angle of the beams and the ratio between their width and length," Raney said. "Compressing the lattice stores elastic energy in the material. If we could controllably use the environment to alter the geometry of the beams, the structure would stop being bistable and would necessarily release its stored strain energy. You'd have an actuator that doesn't need electronics to determine if and when actuation should occur."
Shape-changing materials are common, but fine-grained control over their transformation is harder to achieve.
The researchers' solution was to infuse their 3D-printed structures with glass or cellulose fibres running in parallel to the length of the beams. This prevents the beams from elongating, but allows the space between the fibres to expand, increasing the beams' width.
With this geometric control in place, more sophisticated shape-changing responses can be achieved by altering the material the beams are made of.
The researchers made active structures using silicone, which absorbs oil, and hydrogels, which absorb water. The group claims that heat- and light-sensitive materials could also be incorporated, and materials responsive to even more specific stimuli could be designed.
What’s more, changing the beams' starting length/width ratio, as well as the concentration of the stiff internal fibres, allows the researchers to produce actuators with different levels of sensitivity. And because the 3D-printing technique allows for the use of different materials in the same print, a structure can also have multiple shape-changing responses in different areas, or even arranged in a sequence.
The research – which is described in the journal journal Nature Communications was supported by the US Army Research Office.
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