New healable, recyclable polymer enables highly adaptable soft robots
(Nanowerk Spotlight) Soft robotics is a rapidly growing field that aims to create machines that mimic the flexibility, adaptability, and complex movements found in living organisms. Soft robots, made of flexible, stretchable materials, can perform tasks that are challenging for traditional rigid robots. They can squeeze through tight spaces, adapt to complex environments, and safely interact with humans and delicate objects. This makes them ideal for a wide range of applications, from search and rescue missions in disaster-stricken areas to delicate surgical procedures and targeted drug delivery within the human body.
However, the development of soft robotics has been constrained by a major challenge: finding a polymer that’s soft enough for rapid deformation yet also recyclable. The ideal softness requires a polymer matrix with a Young’s modulus of 0.1-10 MPa and over 200% stretchability. This enables smooth motions like bending and twisting, similar to the movements of natural muscles. It also allows repeated deformation without permanent damage, which is crucial for the longevity and functionality of the robots. However, most soft polymers are either not stretchable enough or not recyclable, limiting the sustainability and adaptability of soft robotics.
The ideal softness requires a polymer matrix with a Young’s modulus of 0.1-10 MPa and over 200% stretchability. This enables smooth motions like bending and twisting. It also allows repeated deformation without permanent damage. However, most soft polymers are either not stretchable enough or not recyclable.
For instance, silicone rubbers like polydimethylsiloxane (PDMS) have suitable softness but cannot be recycled. Hydrogel polymers are recyclable but often not stretchy enough. This mismatch between softness and recyclability has hampered sustainable soft robotics development. It also limits how easily robots can be reshaped for different tasks, as most today have a fixed shape and function.
Schematic illustration of fully recyclable and healable magnetic soft robots. a) Mechanical properties of diverse materials and of recyclable or healable materials before and after recycling or healing. b) Polymerization of monomers to prepare dynamic covalently crosslinked polyimine (PI). c) Schematic illustration of dynamic covalent PI network based on reversible imine bonds containing magnetic NdFeB microparticles. d–h) The recycling process of magnetic soft robots: d) original flower-shaped magnetic soft robot, e) destroyed robot, f) depolymerized solution, g) recycled butterfly-shaped magnetic soft robot and h) recycled gripper-shaped robot. Photos of i) shape deformation and j) locomotion of the recycled gripper-shaped magnetic soft robot under magnetic stimulation. k,l) Healing process of magnetic soft robot: (k) damaged robot and (l) healed robot. (Reprinted with permission by Wiley-VCH Verlag)
The key innovation is a polymer matrix of dynamic polyimine (PI). Polymers are large molecules made up of smaller, repeating units called monomers. In this case, the researchers used a monomer called methylated diamine to create the PI. This PI material can be laser cut into soft robots of varying geometries. When embedded with magnetic particles, the robots can wirelessly perform complex moves like bending, twisting, rolling, and folding within seconds under magnetic control.
The researchers specially designed the dynamic PI matrix using methylated diamine as the monomer, which decreased the crosslinking density. Crosslinking refers to the process of chemically joining the polymer chains together, and the density of these crosslinks can affect the material's properties. In this case, reducing the crosslinking density made the material softer and more stretchable. The researchers also reduced the intermolecular hydrogen bonding, which further enhanced the material's flexibility.
Yet unlike other soft polymers, the PI can be fully broken down into monomers at room temperature. This enables the robots to be completely recycled and reprocessed into new designs after completing a task. The recycling process is demonstrated by transforming a flower robot into a butterfly robot and then into a gripper robot. Despite recycling, the gripper could still quickly roll and deform under magnetic stimuli, thanks to the PI’s ideal softness being completely restored.
The researchers also demonstrated the material's self-healing capabilities. The dynamic bonds in the PI network allow the material to self-repair rips and tears within minutes at room temperature. This healing process is realized by adding a small amount of a mixed solution consisting of bis(3-aminopropyl) methylamine, tris(2-aminoethyl)amine, and ethanol to the cut, followed by physical adhesion. This healing process recovers over 90% of the mechanical properties of the material, effectively extending the lifespan of the soft robots.
“Our recyclable polyimines are soft and stretchable enough to produce reprogrammable soft robots beyond the fixed geometries of traditional soft robots,” said Dr. Guangda Zhu, leader of the study.
According to the researchers, this pioneering development of fully recyclable, self-healing, highly maneuverable soft robots opens up new possibilities for sustainable and adaptable soft robotics. They believe their dynamic PI innovation could inspire the creation of a new generation of intelligent soft robots. These robots, with their recyclability and self-healing capabilities, represent a significant step towards reducing the environmental impact of robotics, contributing to a more sustainable future in robotics technology.