One of the major hurdles in making artificial muscles for biomechanical devices is to make the muscles bend just like muscle fibers of human limbs and fingers do. However, Massachusetts Institute of Technology (MIT) scientists have come up with a novel approach. They have proposed using a simple and inexpensive solution that involves using nylon fibers to improve workability of artificial muscles.
The designs was created by Seyed M Mirvakili, a doctoral candidate, and Ian W Hunter, a George N Hatsopoulos Professor in Thermodynamics, both from the Department of Mechanical Engineering in MIT.
The new method allows the molding of the nylon fibers by heating and shaping them in certain ways. This method allowed the scientists to bend the synthetic strands in ways just like natural muscle tissues behave.
Previously, scientists had developed ways to make nylon fibers behave in the same way by twisting coils of the filaments to make it behave like linear muscle movements, as in linear actuator devices. The researchers had shown that for a specific weight and size, any movable part of a device can extend forward and backwards by storing and releasing more energy than natural muscle fibers can.
But this method was restricted to just moving parts in a linear motion and not to bending motions, like human limbs do, and this had been challenging for scientists to achieve.
In addition, there have already been methods to make movable parts of machinery bend just like human limbs but they were expensive because carbon nanotubes were used. And although they had high longevity, as they could make about a million linear contraction cycles, they were still too expensive and difficult to make for normal usage.
In contrast, the new method can make use of the same nylon fibers to produce bending motions which can be useful in some biomedical devices, artificial limbs and efficient actuator devices. And in addition, they are inexpensive and easy to manufacture.
The nylon-based method also has high cycling longevity and durability and can be molded in many different shapes and sizes. The reason they can be formed in many ways is that polymer fibers, like nylon, shrink in length but increase in diameter when heated and this property was used for linear motions. In addition, this new system subtracts the need of pulleys, levers or wheels, or any other extra mechanical part, to make it move in a non-linear pattern.
Another limitation of linear actuators was that when moving part was heated to carry out any motion, it had to take some time to cool down which reduces efficiency and hinders productivity. According to Mirvakili, heating one side of the fiber can cause it to contract faster than the heat could reach the other side, therefore causing the bending motion.
Mirvakili says, “The cooling rate can be a limiting factor. But I realized it could be used to an advantage. You need a combination of these properties. High strain and low thermal conductivity.”
But the system is still far from perfect. According to the research team, to make the system work like muscle tissues, the fibers cross-sectional pattern needs to be carefully shaped and aligned. To start with, the team used nylon strings from ordinary fishing lines and compressed them to change their shape from round to rectangular to square. Then heating them from one side caused them to bend while changing the direction of heating produced some complex motions.
The method was detailed in a paper titled “Multidirectional Artificial Muscles from Nylon” in the journal Advanced Materials on 23 November, 2016.
The researchers maintain that the material can bend and retract at least 17 cycles per second. In contrast, when muscle fibers contract, Ca++ concentrations need to be restored in the muscle fibers which happens in 30 milliseconds, which is a mere 3 to 4 times per second.
Hunter applies that this new material could be used to make self-adjusting catheters and other biomedical devices, in addition to its numerous applications in instrumentation.
Beckman Center of the National Academies of Sciences & Engineering in Irvine, California, have stressed the need of artificial systems to support people disabled due to injuries or handicapped with muscular dystrophies and stroke.
Although these devices are nowhere near the ease and comfort of natural limbs and organs, current devices have vastly improved in providing support to physically challenged people. According to them, one major hindrance is the bulkiness and stiffness of the plastics or metals used which interfere with the center of body mass during walking.
Simple supports with one joint are relatively easy to manage but braces spanning from the hip to the foot require much more energy to move.
In addition, it is possible to incorporate bio-electrodes that grow in the tissues with perfect compatibility with the body. Scientists have claimed it is possible to develop a layer of genetically modified neurons on the surface of the cortex of the brain, from which neurons grow into the brain tissue where their activity can be recorded.
The new nylon-based system provides an opportunity to create bio-based hybrid devices that easily interact with the human body and are simple and efficient. Conjugated polymers that are electroactive have been developed and have enormous use in biomedical devices.
Polymers, like nylon, also have numerous uses as electroactive polymers (EAPs), which are polymers that exhibit change to stimulation by an electric field. Electroactive polymer activation systems have vast usages in medical devices. The activation system can be an actuator or a balloon of the device controlling certain parameters, like pressure, fluid control and temperature.
In fact, innovations and advancements in technology have made it possible to create artificial muscular devices that allow people with fecal incontinence to have proper bowel movements. According to suggestions, these muscular devices have been made possible by dielectric electroactive polymers due to their low energy consumption, fast response time, versatility and mobility. In addition, these materials will aid in the fast adaptation when compared to previous materials, and tissue atrophy and erosion can be avoided.