(Nanowerk Spotlight) Artificial muscles hold the promise of revolutionizing fields ranging from robotics and prosthetics to biomedical devices. These lightweight, flexible materials can mimic the behavior of natural muscles, exhibiting high efficiency and rapid responsiveness. However, despite significant advancements in recent years, the widespread adoption of artificial muscles has been hindered by limitations such as low strain, low power density, and high manufacturing costs.
Carbon nanotubes have emerged as a leading material for artificial muscles due to their exceptional performance characteristics, including large strain and high-power density (e.g. this example of a a twisted carbon nanotube yarn. Yet, the fabrication of carbon nanotube-based artificial muscles often relies on the highly oriented growth of carbon nanotube arrays, a process that is both expensive and difficult to scale up for large-scale production. This has led researchers to explore alternative materials that could provide similar performance at a lower cost.
Conductive polymers have long been considered a promising candidate for artificial muscles due to their unique combination of electrical conductivity, mechanical properties, and processability. These materials have found applications in various fields, including flexible electronics, supercapacitors, and optoelectronic devices. However, their inherent low modulus has limited their potential in artificial muscles, resulting in low output stress and work density.
Now, a team of researchers in China has made a significant breakthrough by developing artificial muscles based on pure conductive polymer coiled yarns. This achievement was made possible by the successful fabrication of high-strength conductive polymer microfibers using poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate), or PEDOT:PSS.
As the team reports in Advanced Functional Materials (“Artificial Muscles Based on Coiled Conductive Polymer Yarns”), they systematically investigated the wet-spinning process used to create these microfibers, optimizing parameters such as the coagulation bath and additives in the spinning dope to improve their mechanical properties.
Schematics of the fabrication process of conductive polymer-based artificial muscles (CPAM) and its structure. a) The fabrication of CPAM by twisting multiply PEDOT:PSS microfibers into a coiled structure, inset showing a roll of the PEDOT:PSS microfibers with ≈100 m in length. SEM images of the b) PEDOT:PSS microfibers, d) twisted strand, and f) coiled strand, and c,e,g) their cross-section images. (Reprinted with permssion by Wiley-VCH Verlag)
The resulting PEDOT:PSS microfibers exhibited remarkable mechanical characteristics, including a breaking strength of 250 MPa, a maximum tensile strain of approximately 20%, and a high electrical conductivity of around 2400 S/cm. These properties enabled the researchers to construct artificial muscles by twisting multiple microfibers together to form a distinctive coiled structure.
The researchers also identified a key actuation mechanism underlying the performance of these conductive polymer-based artificial muscles: molecular structural changes that occur during electrochemical processes. These changes endow the microfibers with a significant radial volume expansion, which is then amplified by the coiled yarn structure.
As a result, the artificial muscles achieved a remarkable contractile strain exceeding 11% at a high stress of 5 MPa, equivalent to lifting loads more than 4000 times their own mass, all while operating at a low input voltage of 1 V.
In addition to their electrochemical actuation, the conductive polymer-based artificial muscles also exhibited hydration-induced contraction of up to 33%. This hydro-actuation is attributed to the presence of the highly hygroscopic polyelectrolyte PSS in the PEDOT:PSS microfibers, which enables them to absorb significant amounts of water, leading to volume expansion. The intrinsic high conductivity of the microfibers allows for rapid recovery from the hydrated state through electrical heating, effectively expelling the absorbed water molecules.
The combination of large contraction, high work capacity, and multiple actuation modes makes these conductive polymer-based artificial muscles promising for various smart control systems. The researchers demonstrated their potential by integrating them into an electrochemical actuation unit capable of lifting a load of 1.6 g by 15 mm through a cantilever with a low voltage input. They also constructed a smart window that can automatically close in response to high humidity conditions or sudden rainfall, as well as a smart gripping jaw with a controllable switch that utilizes the contraction of the artificial muscles in water and their elongation when energized.
This groundbreaking work positions high-performance conductive polymer microfibers as a cost-effective alternative to carbon nanotubes in the development of lightweight artificial muscles. By leveraging the unique properties of these materials and the innovative coiled yarn structure, researchers have achieved exceptional performance characteristics that rival those of carbon nanotube-based artificial muscles.
As research in this field continues, further refinements in material composites and engineering approaches hold the potential to unlock even greater possibilities for conductive polymer-based artificial muscles. This breakthrough not only advances the field of artificial muscle technology but also paves the way for more affordable and accessible solutions in robotics, prosthetics, and biomedical devices.
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