Nanomuscles for prosthetics and robots
As the cucumber grows, its twisted tendrils reach out to the support and wrap around it to lift the plant up. This helps him to get as much sunlight as possible. Researchers from the Massachusetts Institute of Technology have found a way to simulate this twisting and pulling mechanism to produce contracting nanofibers that can be used as artificial muscles in the production of robots, prosthetic limbs and other mechanical and biomedical products.
Many different techniques have been used to create artificial muscles, including hydraulic systems, servo motors, shape memory metals and polymers that respond to stimuli. But they all have drawbacks, including a lot of weight and a long response time to the command. The researchers write that the new fiber-based system is extremely lightweight and can react very quickly.
The new fibers were developed using a thread pulling technique to combine two dissimilar polymers into a strand of fiber. A key role in this process belongs to the connection of two materials that have different coefficients of thermal expansion. In other words, they have different expansion rates when heated. The same principle is applied in thermostats, for example, using a bimetallic strip as a method of measuring temperature. When the strip heats up, the material that should expand faster is held by the second material. As a result, the glued material twists, bending in a direction that expands more slowly.
Source: hereafter – M. Kanik et al.
Using two different polymers (a fairly soft cyclic copolymer-elastomer and a very rigid thermoplastic polyethylene), the authors obtained a fiber that, when stretched several times compared to the original length, naturally turns into a dense spiral, very similar to the tendrils of cucumbers. When one of the researchers took the spiral fiber in his hands, the body heat caused it to curl more tightly. Subsequent tests have shown that even a slight increase in temperature can cause the coil to rise from the surface, creating a surprisingly powerful pulling force. Lowering the temperature returned the fiber to its original length. The group demonstrated that the process of compression and expansion can be repeated 10,000 times without weakening.
It is assumed that one of the reasons for such long-term work is the moderation of exposure, including the low temperature required for fiber activation. An increase of just one degree Celsius may be enough to start a reduction.
Fibers can have a variety of sizes, from a few micrometers (millionths of a meter) to several millimeters (thousandths of a meter) in width. They are easy to produce in batches up to hundreds of meters long. Tests have shown that one fiber is capable of lifting a load 650 times higher than its own. For these experiments, the authors even developed special miniature test installations.
The degree of compression when the fiber is heated can be calculated in advance by determining the degree of initial stretching of the fiber. This allows you to fine-tune the amount of force required and the temperature change required for operation.
The pulling system used in the manufacture allows you to include other components in the fiber itself. First, a large-sized version of the material is created, the so-called preform, which is then heated to a temperature at which the material becomes viscous. Then the workpiece can be pulled out, creating a nanofiber that retains its internal structure, but is a small fraction of the width of the workpiece.
In the tests, the researchers covered the fibers with nanowire grids. These meshes served as sensors of the tension experienced or created by the fiber. In the future, these fibers can also include heating elements that provide heating without an external heat source to activate the reduction of "nanomuscles".
The range of possible applications of fibers is quite wide: actuators in robotic arms, legs or manipulators, as well as in prostheses, where their low weight and short response time will be extremely useful for patients.
Some prostheses today can weigh up to 15 kg, and most of the weight falls on actuators, which are often made pneumatic or hydraulic. Nanofiber-facilitated drives will help simplify the lives of those who need prosthetics. Such fibers can be used in miniature biomedical devices, for example, for a medical robot that is activated when inserted into an artery. The activation time of the fiber is on the order of tens of milliseconds.
To provide the strength to lift heavy loads, fibers can be tied together, just as muscle fibers are tied into muscles. The researchers successfully tested a complex of 100 fibers.
During the manufacturing process, feedback sensors can be integrated into the fibers, which are necessary to ensure the accurate operation of the limb prosthesis.
Article by M. Kanik et al. Strain-programmable fiber-based artificial muscle is published in the journal Science.
Aminat Adzhieva, portal "Eternal Youth" http://vechnayamolodost.ru based on MIT materials: Artificial "muscles" achieve powerful pulling force