At the mention of nylon, you may think of shirts, stockings, fishing line, and sutures. A recent discovery, first reported in February 2014 in Science, is that the strong, tough, and inexpensive threads that make up fabrics and filaments can actively contract and expand—with much greater force than muscle . This newly discovered musclelike action creates opportunities in medical devices—among other areas—that are just beginning to be explored.
Our own skeletal muscle, like that of other mammals, contracts by 20% in length, and each square centimeter can just about lift a gallon of milk (3.78 L). Amazingly, nylon can also contract by a similar fraction, and it can lift a hundred times more, operating at stresses in the tens of megapascals. This means that nylon thread may provide an alternative to conventional motors, creating the possibility of more natural, simpler, inexpensive, and compact devices.
Nylon threads, when heated, contract by almost 2% in length. At the same time, they expand in diameter by 5%. Remarkably, the two effects can be combined to create a 20% or more change in length by creating a coil, the helical structure shown above. This discovery was made by Carter Haines, a 23-year-old Ph.D. student at the University of Texas at Dallas, working with artificial muscle pioneer Ray Baughman, Welch Professor of chemistry and director of the Alan G. MacDiarmid NanoTech Institute. This team, which also included researchers from the University of Wollongong, Australia; Hanyang University, South Korea; Namık Kemal University, Turkey; and our group at the University of British Columbia in Canada made rapid progress in characterizing and understanding how the anisotropy in thermal expansion combined with the helical structure leads to the remarkable contractile properties in this ubiquitous material. Other plastics, including polyethylene, commonly found in plastic bags and containers, also show this remarkable response. How does this work, and what can we do with these fibers?
Driving an active cycle is a matter of heating and then cooling. To transform a sewing thread or fishing line into artificial muscle, the filament is twisted until it forms a coil. This is achieved by hanging a weight from the line and spinning it. Eventually, the twist induces coiling. Activation is achieved through heating by any means—hot air, hot water, or electric current, for example. A hair dryer or heat gun blowing hot air is sufficient to create significant contraction. Placing the coiled fibers within tubing allows hot and cold water to be alternately pumped through, producing an activation cycle. Nylon with a thin metal coating can be used, through which a current is applied to generate heat, as electrical heating allows good control. Several volts per centimeter of length are required to achieve the required heating. Maximum displacement occurs when heating to just below the melting point, so full displacement requires reaching temperatures of about 180 °C in nylon, for example. A change of several percent in length can be achieved over a more comfortable range of room temperature to 50 °C. This can be improved by increasing the coil diameter, with changes in length of 35% observable for this relatively small temperature range, albeit at reduced load.
How fast can these materials actuate? Up to about five times a second so far. Fast current pulses can heat thin multistranded filaments in milliseconds and produce a power output of 5,000 W/kg—higher than the internal combustion engine! It is the cooling time that limits average rate and power. Water can be used to carry away the heat. A faster response should be possible using thinner filaments.
Applications are just starting to be explored, and medical applications are among the first to be considered. Tools for minimally invasive surgery are one example. Long, thin tools such as the forceps shown in the figure above generate high forces, and the jaws move through a large angle. Nylon actuators can readily generate the displacements needed to move the jaws, and they can do it from within the housing of the tool, where a connecting rod would normally sit. Based on the inner diameter and the high operating stresses of the actuator, there should be enough space to generate the needed forces, though this has yet to be demonstrated. But what is the best way to get heat in and out? Electrical heating combined with compressed air or water for cooling are natural choices in the operating room.
With an aging population, it is natural to envision the use of artificial muscle to assist the infirm with basic motions such as getting out of bed or moving from sitting to standing. Márcio D. Lima at the University of Texas at Dallas has demonstrated that polyethylene-based actuators in particular can lift large weights, as seen above, including Márcio himself, paving the way for human assist applications.
The ability to form fabrics from these coiled threads, as in the figure below, invites one to imagine wearable artificial muscle. In fact, the term artificial muscle conjures up more dramatic applications such as powered prosthetic limbs and exoskeletons providing superhuman strength. Such fantastic devices face some practical hurdles, however. There has been little exploration of basic properties such as the number of cycles that can be achieved without failure, for example. So far, 1 million cycles have been demonstrated without failure. The efficiency of converting heat to work is not high, so sustained high power in an autonomous device will require a lot of stored energy. Because of this, the fiber and fabric actuators will not replace the motors in our cars—but they may take advantage of the waste heat from them. More immediately, promising applications include those where occasional high force and large displacements are needed, such as helping someone out of the bath.
Adding twist to nylon or polyethylene creates both rotational and torsional actuation. Rotation can be strong, with the work done in torsion reaching 2 kJ/kg, a value similar to that seen in tensile actuation and about 100 times larger than can be extracted from the same mass of muscle. How does the twisting and coiling help amplify the thermal expansion in the threads? In many materials, the expansion with temperature is isotropic. If you heat a rod or a helical spring, both cast from the same metal, the expansion with temperature will typically be the same in all directions and the same for both structures. There is no amplification of the thermal expansion, whereas in the nylon, the coiling induces a 17-fold increase in linear contraction. The key is the combination of anisotropy of expansion with coiling.
To create artificial muscle, take nylon—which contracts along its length and expands in diameter when heated—and start by twisting it. With a weight hanging from a length of fishing line or sewing thread, spin the weight. Use a permanent marker to draw a line along one side to see the extent of twist. During twisting, the line that was originally straight now forms a helix, as seen in (a) below. This helix is in the direction that contracts when heated. Keep twisting until that line forms an angle of almost 45° with the fiber axis. Heat the twisted fiber with a hair dryer and it will untwist, while during cooling it will twist back again. This untwist with heating is expected because of shortening in the helical direction as the temperature rises. Meanwhile, the radius has a tendency to expand as the temperature is increased. Both these processes lead to a magnification of untwist. Think of a string helically wound onto a spool, and then imagine that the string is trying to shorten while the spool diameter is also expanding. The torque on the spool from both of these processes tends to make it untwist—and so it is with the anisotropic thermal expansion of the polymer line. Untwist resulting from heating is reversed upon cooling.
The active twist is converted to linear motion by coiling the twisted fiber. Coiling is induced by continuing to twist. The helical coiling is similar to that produced in the highly twisted rubber band that drives a balsa-wood airplane. This coiling converts the torsional displacement to linear displacement, such that when the filament untwists, the coil also shortens, as shown in (b) (representing the cold state) and (c) (representing the hot, contracted state). As a stretched coil is heated, the filament untwists to compress the coils and produce linear actuation.
The thermally activated artificial muscle based on anisotropic thermal expansion exploited through twist and coiling— more simply known as Baughman muscle—is one of a number of artificial muscle technologies . Others include highly stretchable materials to which very strong electric fields are applied, which in turn enables very large deformations (can be greater than 100%). Polymers that swell and contract as ions are inserted or removed are so-called ionic artificial muscle. Shape-memory alloys are metals that can undergo a large (up to about 10%) change in length as they are heated and cooled. What makes the Baughman muscle special is its large strain and its work density that is only exceeded by shape-memory alloys. This performance, combined with low-cost, easy controllability (especially compared to shape-memory alloys), and wide availability, make these artificial muscles promising for application in medical devices.
- C. S. Haines, M. D. Lima, N. Li, G. M. Spinks, J. Foroughi, J. D. W. Madden, and R. H. Baughman, “Artificial muscles from fishing line and sewing thread,” Science, vol. 343, no. 6173, pp. 868–872, 2014.’
- J. D. Madden, N. A. Vandesteeg, P. A. Anquetil, P. G. Madden, A. Takshi, R. Z. Pytel, and I. W. Hunter, “Artificial muscle technology: Physical principles and naval prospects,” IEEE J. Ocean. Eng., vol. 29, no. 3, pp. 706–728, 2004.