Controlled manipulation of the fibers that are as thin as or even thinner than human hair is a real challenge. Despite the technology development, the precise and reversible change of the microfibers' orientation, like the tweezers, is not easy. The interdisciplinary team of researchers from the Institute of Physical Chemistry, Polish Academy of Sciences, has recently developed a way to control the shape of microfibers with electricity. This brings us closer to a novel technical solution in micromechanics and soft robotics. Their recent work, published in the Nature Communications journal, demonstrates the first proof-of-concept results on the motion of pristine carbon fibers caused by asymmetric electrochemical processes occurring in the material.
Just a few decades ago, developing a technology for producing fibers with a cross-section much smaller than the diameter of a human hair in a controlled manner was a challenge. However, since the development of research techniques that enable the observation of objects at a nanoscale, the progress of miniaturization has accelerated, resulting in the development of many methods for producing microfibers and even nanofibers from many materials. Materials engineering and technological development have delivered many “smart” materials that can change their properties on demand in response to external stimuli. Smart polymers can respond to various stimuli, like electricity, light, heat, pH of the solution, etc., resulting in changes in physicochemical characteristics, such as color, shape, etc., making them very useful in many fields like sensors, textiles, or medicine. These smart materials can be designed to respond only to specific stimuli in a controlled way, returning to the initial state without stimuli. In terms of smart fibers fabrication, many efforts were put to deliver materials with desired shape and size to be used in, e.g., synthetic muscles that respond to electrical signals, drug delivery under a particular pH or temperature, microelectromechanical systems that generate motion under the electrochemical reactions, photoelectrochemical devices under the exposition to electromagnetic waves with defined wavelength, etc. Nevertheless, in many cases, microfibers or nanofibers require specific coatings or the modification of their structure to respond to stimuli in a controlled way, making their fabrication challenging. When it comes to the controlled manipulation of fiber motion, there is still a big gap in the availability of applicable solutions.
Recently, researchers from the Institute of Physical Chemistry, Polish Academy of Sciences (IChF) in Warsaw, led by Dr. Wojciech Nogala, under the international collaboration, demonstrated a breakthrough in the precise control of carbon-based fibers, addressing the challenges of materials manipulation using electricity. In their work, they show that bare carbon fibers can act as miniaturized actuators that change shape on demand by using an electrochemical tool. Why exactly carbon fibers? They are well known for their extraordinary mechanical properties. At the same time, they are not only much stronger but also much lighter than steel or aluminum, being widely used as reinforcement in composites and also for their unique electrical properties.
The key idea demonstrated by the team from IChF is to place a micro-sized diameter single carbon fiber inside an electrochemical setup, specifically a bipolar cell, which has been widely used since ‘70s in the recent century in biosensing, electrochemical reactors, and batteries. The researchers compared two types of carbon fibers: smooth and asymmetrically rough, in which the ions, such as Li + and ClO 4 - , in a supporting electrolyte, containing benzoquinone and hydroquinone as a redox couple, are inserted into the fiber surface under the external voltage application.
Interestingly, in the pristine rough fiber, an asymmetry in the pore distribution was observed, resulting in a different material response compared to the smooth one. As the ion insertion process occurs asymmetrically, the fiber bends in the presence of applied voltage, while its reduction leads to reorientation to the initial position. Such a straightening is an effect of the ions’ expulsion from the fibers’ surface. In other words, ions start moving into and out of the carbon fiber under sufficient applied voltage, inducing the fiber motion in a specific direction. Importantly, this motion is reversible.
Dr. Wojciech Nogala says, “ We successfully used the closed bipolar cell to wirelessly actuate a freestanding carbon fiber electrochemically. An uneven electrical double layer is enabled by the naturally asymmetric groove configuration in the fiber, which is one of the fundamental factors in producing the necessary initial asymmetry. This leads to asymmetric tension and contraction in the fiber. Simultaneous oxidation and reduction reactions in the two compartments of the bipolar cell allow for wireless actuation.”
“ Our findings may open up intriguing possibilities for actuators based on prefabricated asymmetric carbon fibers. ” – remarks Dr. Nogala.
Despite the lack of direct electrical connection to the fibers in the proposed setup, the electrochemical processes, namely oxidation on one end and reduction on the other end of the fiber, can occur. The motion magnitude depends on both the applied voltage and the length of the fiber. Pulses can also be used in cycles, whereby the change in voltage and the duration of each pulse cause the fiber to move up and down repeatedly, working as microscopic tweezers. The proposed system can be used not only for a single fiber but for microactuators that could be used in miniaturized devices covering needs in many fields, from synthetic muscles in microrobotics to controlling the motion of materials at a very small scale.
Their research was funded by the National Science Center (NCN), Poland, through grant 2022/46/E/ST4/00457
Nature Communications