(Nanowerk Spotlight) The next generation of robots may not look anything like the rigid, mechanical machines we’re used to. Instead, imagine a robot that moves more like an octopus or a human hand, with parts that can change from soft to rigid at will. Researchers have been working toward this vision, but creating robots with such versatility has been a massive challenge. Traditional methods for building soft robots, those inspired by the flexibility of living creatures, have often fallen short when it comes to performing complex, delicate tasks.
However, traditional manufacturing techniques have proven limited, requiring complex, multi-step processes that offer little flexibility in materials. As a result, fabricating hybrid robots with both soft and rigid elements has remained a formidable task.
Enter the next generation of soft robotics research. This new frontier is made possible through the convergence of advanced materials and additive manufacturing techniques, allowing for the creation of hybrid robots with both soft and rigid parts. These developments are not only overcoming the limitations of existing technology but also opening doors to new possibilities in robotics. At the center of this breakthrough is the use of shape-transformable liquid metal nanoparticles (LMNPs) integrated into polymers, enabling the direct 3D printing of complex, multi-functional robots.
Recent advances in liquid metal-based technologies have been instrumental in this shift. Gallium-based liquid metals, such as eutectic gallium-indium (EGaIn), have emerged as versatile materials with unique properties such as softness, high electrical conductivity, and excellent responsiveness to external stimuli like heat or light. When integrated into polymers for 3D printing, these materials facilitate the creation of hybrid robots with components that can switch between soft and rigid states depending on the task. Such materials can be programmed to remember and return to their original shapes when exposed to certain stimuli, such as near-infrared (NIR) light, giving rise to what is termed 4D printing.
The latest research, published in Advanced Materials (“4D Printing Hybrid Soft Robots Enabled by Shape-Transformable Liquid Metal Nanoparticles”), pushes the boundaries of 4D printing through the development of a hybrid material toolkit composed of shape-transformable liquid metal nanoparticles and gallium-based nanorods. This approach allows the creation of robots that can perform tasks impossible for purely soft or rigid robots alone. By integrating different shapes and compositions of nanoparticles into polymers, researchers are now able to finely tune the mechanical properties of the robot components. These innovations hold great promise for fields such as medical rehabilitation, precision engineering, and autonomous systems.
Schematic illustration of the preparation of spherical liquid metal nanoparticles and rod-like gallium-based nanorods. (Image: Reproduced from DOI:10.1002/adma.202409789, CC BY)
The core of this research lies in its novel 3D printing technique, which allows for the creation of hybrid robots in a single step. By combining the flexibility of liquid metals and the rigidity of nanorods, the research team developed materials that can be programmed to transform their shapes when subjected to external stimuli. This is achieved through the use of spherical liquid metal nanoparticles (SLMNPs), which possess soft, deformable properties, and rod-like gallium-based nanorods (RGNDs), which are crystalline and rigid. Together, these components create a hybrid robot structure that can adapt to various tasks with unparalleled precision.
To fabricate their devices, the team employed a type of 3D printing known as stereolithography, which uses a photoinitiated polymerization process. This method involves the use of a liquid resin containing the nanoparticles, which solidifies when exposed to a specific wavelength of light. The direct printing of hybrid robots eliminates the need for complex multi-step processes, reducing production time and increasing the efficiency of material use. This approach also allows for greater control over the shape memory and mechanical properties of the robot, as the type and amount of nanoparticles can be easily adjusted to create different functionalities within the same structure.
One of the key innovations in this research is the precise control over the shape transformation of the LMNPs. When subjected to a hydrothermal treatment, the spherical nanoparticles can be transformed into rod-like shapes, significantly altering their mechanical properties. This bidirectional adjustment allows the robot to be both stiff and flexible, depending on the task. For example, in one demonstration, the team created a gripper with high-precision capabilities, able to delicately handle objects and then return to its original shape once the task was completed.
The versatility of this material toolkit extends beyond just mechanical properties. The LMNPs exhibit excellent photothermal effects, meaning they can convert light into heat. This capability is harnessed in 4D printing, where the robot’s components can return to their programmed shapes when exposed to NIR light. In practical applications, this means that a robot could be programmed to change its shape to perform a task, and then revert to its original form once the task is complete, without any manual intervention. For instance, the team developed a bioinspired motor that mimics natural muscle movements, offering new possibilities for creating robots that can move and function autonomously in real-world environments.
The significance of this research lies in its potential applications. In the field of medical rehabilitation, for example, the ability to create hybrid robots that can be programmed to assist with physical therapy could revolutionize the way we approach recovery from injury. Robots designed with this technology could be used to create wearable devices that help patients regain movement by mimicking natural limb motions. The researchers demonstrated this with a hand rehabilitation device that could assist patients in regaining dexterity by precisely controlling the shape and movement of the robot’s components.
In industrial settings, these hybrid robots could be used for tasks that require both soft and rigid functionalities, such as assembling delicate electronic components or handling fragile materials in manufacturing processes. The ability to switch between different states of flexibility also opens up new possibilities for autonomous robots that can adapt to their environments, performing a wide range of tasks without needing to be reprogrammed or retooled.
While this research represents a significant step forward, there is still work to be done. The team notes that further advancements in 3D printing technologies and material design strategies will be necessary to fully unlock the potential of hybrid soft robots. In particular, increasing the concentration of metal-based nanoparticles within the composite materials could lead to even greater responsiveness and improved mechanical properties. Moreover, refining the process of integrating these nanoparticles into 3D printing resins will be crucial for scaling up the production of hybrid robots for commercial applications.
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