Many will agree that muscles are essential in our lives. However, have you ever thought about the structure and mechanism behind these incredible tissues that allow us to perform heavy yet delicate tasks? The skeletal muscle is a complex tissue of aligned fibers and proteins orchestrated by electrical signals from our brains. The signals promote a cascade of biochemical processes that make the muscle contract and expand, following the direction of the fibers.
Given their crucial role, it severely impacts our quality of life if our muscles are compromised due to injury or genetic disorders. That is why there is a growing interest in developing prosthetic devices with artificial muscles that mimic natural muscle tissues' functions.
Unlike natural muscles, artificial muscles can be engineered to expand and contract upon a change in environmental conditions or stimuli. Relying on this ability to respond with a controlled motion upon external stimulation, artificial muscles hold immense promise for various applications, including devices such as prosthetics that can replace lost limbs and exoskeletons that can enhance the body’s capability after loss of mobility or paralysis. Another area is soft robotics, where they can improve current robots by making their movements look more lifelike or by even allowing them to perform delicate surgical tasks. In the future, it might even be possible to use these materials as implants to replace damaged muscle tissue.
Our EU-funded Pathfinder project INTEGRATE aims to establish new artificial muscle designs. The disruptive aspect of our artificial muscles is that we want them to be powered by the energy produced by our bodies, exactly like in the case of natural muscles. The experimental work on this project is mainly conducted by two teams at the Adolphe Merkle Institute, University of Fribourg in Switzerland, where one team focuses on the design and development of artificial muscles, while the other is working on building a device that produces energy leveraging biochemical processes occurring in our bodies. The design of our artificial muscle architectures is guided by theoretical studies conducted by partners in the INTEGRATE consortium.
To make a material responsive to external stimuli, we use stimuli-responsive polymers. These polymers can alter their ability to interact with the surrounding liquid upon application of a stimulus, for example, a shift in temperature or exposure to light of a specific color, and thereby undergo a change from being fully stretched to completely coiled. To mimic the directionality of skeletal muscles, the design also includes rigid nanoparticles that, when aligned, promote the material to contract in one direction. The nanoparticles' rigidity will also help reinforce the material, just like the fibrous structure in natural muscles does.
Further, we have developed chemical processes that allow us to (1) have the polymers attached to the nanoparticles, and (2) connect these polymers together by exposing the materials to UV-light. This last step results in the formation of a network. If the network is obtained in the presence of a liquid, a gel is formed that, due to the responsive nature of the polymer and alignment of the nanoparticles, should shrink in one direction.
One exciting aspect of our project involves the 3D printing of our artificial muscle material. This is performed by our partner at the University of Rome, Tor Vergata, who, through extrusion 3D printing, aims to align and bond our polymer-nanoparticle units into complex muscle architecture. Another equally exciting aspect is the collaboration with our partners at the University Paris-Saclay and TU Eindhoven, whose theoretical studies and computational simulations play a crucial role in guiding our experimental work and helping us optimize the design and functionality of our artificial muscles.