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Have you ever wondered how a pine cone decides when to open and release its seeds? Have you ever been amazed by the rapid closure of a Venus flytrap? Nature is full of examples of various systems that automatically change their form and function in response to environmental stimuli or a series of stimuli.
Unlike traditional electromechanical systems that use electronic components and sensors, natural systems embed these functions within the material composition and structural features. For instance, even without a brain or nervous system, a Venus flytrap can make complex decisions: when it senses mechanical stimulation, it can quickly close its leaves to a semi-closed state, then it will determine whether the captured object has sustained mechanical stimulation or is of suitable size, thus “deciding” whether to fully close its leaves to begin digestion or to open its leaves to release an unexpectedly captured inedible object.
A research team from the School of Engineering and Applied Science at the University of Pennsylvania was inspired by such systems in nature, utilizing stimulus-responsive materials and geometric principles to design and create intelligent structures with “embedded logic”.
The research was led by Assistant Professor Jordan Raney and postdoctoral researcher Yijie Jiang from the Department of Mechanical Engineering and Applied Mechanics, with doctoral student Lucia Korpas also contributing. The Penn team’s research Bifurcation-based embodied logic and autonomous actuation was published in Nature Communications on January 10, 2019.
Figure 1 Article homepage
Through multi-material 3D printing, the research team endowed these responsive materials with logical relationships and temporal control, allowing them to respond to corresponding environmental changes with complex mechanical reactions. By using Direct Ink Writing (DIW) 3D printing technology, the team created responsive materials composed of soft materials and anisotropic micro-fiber structures. By combining nonlinear mechanical responses determined by the system’s geometric structure, they achieved precise and orderly stimulus responses. For example, utilizing these principles, they designed a water pollution monitoring device that automatically opens and collects samples when the temperature exceeds a certain threshold and detects oil-based chemical pollution.
Figure 2 Bistable unit automatic activation principle; 3D printing component process and internal fiber arrangement schematic; a simulated Venus flytrap device with preset timing
Raney’s lab is interested in bistable structures (which can maintain two forms without applied external forces) and responsive materials (which can change forms under specific conditions). The two are not inherently related, but “embedded logic” connects them. “The bistable nature depends on the geometric structure, while the response comes from the material’s inherent properties,” Raney said. “Our approach uses multi-material 3D printing to link the two fields, thus leveraging the responsiveness of materials to correctly alter the geometric parameters of the structure.”
In previous work, Raney and his collaborators demonstrated how to 3D print bistable silicone beam lattices. After compression, these beams are locked in a buckled form but can easily stretch back to an uncompressed shape.
“This bistability depends on the angle and aspect ratio of the elastic beams,” Raney said. “When compressing the lattice, elastic energy can be stored in the material. If we can controllably use the environment to change the aspect ratio of the elastic beams, making the structure’s bistability disappear and releasing the stored energy, then we have a drive that does not require electronic components to determine whether or when to trigger.” Shape-changing materials are common, but achieving precise control over their changes is challenging.
Figure 3 Direct Ink Writing (DIW) printing process
“Many materials swell when absorbing water, but the swelling occurs in all directions. This does not meet our needs because the aspect ratio of the beams remains unchanged,” Raney continued. “We need to restrict the swelling to occur only in one direction.” Researchers added glass microfibers or nanocellulose to the 3D printed structures and controlled the arrangement of the fibers along the length of the beams during the printing process. When oil-based liquids or water come into contact with the material, the rigid fibers act like a skeleton, restricting the elastic beams from elongating in the length direction while allowing the material between the fibers to expand laterally, increasing the width of the beams. Through this structural restriction, the researchers employed silicone and hydrogel as base materials, corresponding to the responses of oil-based liquids and water, respectively. Thermal and optical responsive materials can also be designed using the same method for various specific stimulus sources. The paper demonstrates various logic gate instances such as “AND”, “OR”, and “NAND” using different materials’ responses to different stimulus sources.
By changing the aspect ratio of the elastic beams and the content of the internal rigid fibers, the researchers also created driving devices with different response sensitivities. The 3D printing technology used in this research allows for different materials to be used in the same batch of prints, thus enabling different regions to have different morphological responses, even setting the sequence of responses.
Video 1 High-speed camera captures the moment a bistable unit releases energy
“For example,” Dr. Yijie Jiang said, “we designed a box with sequential logic. It automatically opens when it encounters a stimulus and then automatically closes after a fixed time. We also designed a simulated Venus flytrap mechanism that can only close when subjected to mechanical loading within a pre-set time period; a box that requires both oil-based liquid and water to open.”
Video 2 A box with sequential logic that automatically opens upon detecting oil-based liquid and automatically closes after a preset time
This method of embedding logic through material properties and geometric structures is scale-invariant, meaning the same principles can be applied at the microscopic scale. “This could find applications in microfluidics,” Raney explained, “without needing solid-state sensors and microprocessors to constantly read the flow in microfluidic devices; for instance, we could design a gate that automatically closes upon detecting a certain level of pollution.”
Video 3 A “little worm” with silicone components jumps away automatically upon detecting pollutants
Sensors suitable for remote and harsh environments (such as deserts, high mountains, or even other planets) are also a potential application. Since they do not require batteries or computers, these embedded logic sensors can remain dormant for years without human intervention and activate automatically under the right environmental stimuli. In soft robotics, autonomously deploying structures, and medical devices, designs with “embedded logic” also have potential application value.
Video 4 Automatically deployed two-dimensional structure
Click the link in the lower left corner “Read the original text” to view the original paper.
Further Reading
MIT Zhao Xuanhe’s team regenerates folded tissues for human organs
MIT Zhao Xuanhe’s team prints magnetic intelligent soft machines
Zheng Xiaoyu’s team breaks through lattice limitations in 3D printing piezoelectric smart materials
New applications of deionized water in layered semiconductor materials
Chinese scholars discover non-trivial superconducting properties in half-metallic crystals
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