Even without a brain or nervous system, the Venus flytrap (a carnivorous plant) seems to make complex decisions about when to close its mouth to catch potential prey, as well as when to open it when it has caught something it can't eat.
The researchers from the School of Engineering and Applied Science at the University of Pennsylvania have been inspired by these types of plants. Using materials that respond to stimuli, they have designed structures that have “embedded logic”. Through their physical and chemical design alone, they are able to determine which of the multiple possible reactions should occur as a response to their environment.
Despite having no motors, batteries, circuits, or processors of any kind, they can switch between multiple configurations in response to predetermined environmental elements, such as humidity or petroleum-based chemicals.
Using multi-engine 3D printers, researchers can create these active structures, with embedded logic/detection ports and control the schedule of each port, enabling complex mechanical behaviors that respond to simple changes in their environment. For example, by applying these principles, a device for monitoring aquatic pollution could be designed to open and collect a sample only in the presence of an oil-based chemical or when the temperature exceeds a certain threshold.
The researchers published a study that describes their approach in the journal Nature Communications.
The study was conducted by Jordan Raney, a teaching assistant in the Department of Mechanical Engineering and Applied Mechanics at Penn Engineering, and by graduate researcher Yijie Jiang in his lab. Lucia Korpas, an undergraduate student in Raney's lab, also contributed to the study.
The Raney lab is interested in structures that are hesitant, meaning they can hold one of the two compositions indefinitely. It also is interested in «smart» materials that can change their shape under the right conditions.
These abilities are not inherently linked, but the “embedded logic” uses both.
“How flexible a structure is determined by its geometry, while its response comes from the material’s chemical properties”, says Raney. “Our approach uses multi-material 3D printing to bridge these distinct domains, so that we can leverage material response to alter the geometric parameters of our structures in the right ways”.
In previous work, Raney and his colleagues had shown how 3D two-dimensional lattices of silicone beams can be printed at an angle. When pressed together, the beams remain locked in a bracketed configuration, but can easily revert to their previous shape.
“This hesitant behavior depends almost entirely on the angle of the beams and the ratio between their width and length», says Raney.
Materials that change shape are common, but controlling their transformation is more difficult.
«Many materials absorb water and expand, for example, but they expand in all directions. This does not help us, because it means that the ratio between the width and length of the rays remains the same», says Raney. «We needed a way to limit the expansion to a single direction».
The researchers' solution was to inject glass or cellulose fibers into the 3D printed structures, which run parallel to the length of the beams. Like carbon fibers, this non‑elastic skeleton prevents the beams from elongating, but allows the expansion of the space between the fibers, increasing the width of the beams.
With this geometric control, more advanced shape changes can be achieved by altering the material from which the beams are made. The researchers created active structures using silicone, which absorbs oil, and hydrogels that absorb water. Heat- and light-sensitive materials could also be incorporated and designed to respond to even more specific stimuli.
Changing the initial length-to-width ratio of the beams, as well as concentrating the rigid internal fibers, allows researchers to produce actuators with different sensitivity levels. And because the researchers' 3D printing technique allows the use of different materials in the same print, a structure can have multiple shape-change responses in different regions or even be arranged in a sequence.
“For example, Jiang says, we demonstrated sequential logic by designing a box that after exposure to a suitable solvent can automatically open and then close, after a predetermined time interval. We also designed an artificial Venus flytrap that can close only if the mechanical load is applied within a set time limit and a box that opens only in the presence of oil and water».
Both the chemical and geometric elements of this integrated logical approach are scale-independent, meaning that these principles could also be leveraged by structures at microscopic sizes, such as in chips.
Other possible applications could include sensors in remote, harsh environments, such as deserts, mountains, or even other planets. Without the need for batteries or computers, these sensors with embedded logic could remain dormant for years without human interaction and only activate when the appropriate stimuli are present.
