Organic Semiconductor Nanotubes Used To Create High-Performance Electrochemical Actuator

University of Houston researchers are reporting a breakthrough in the field of materials science and engineering with the development of an electrochemical actuator that uses specialized organic semiconductor nanotubes (OSNTs).

Currently in the early stages of development, the actuator will become a key part of research contributing to the future of robotic, bioelectronic, and biomedical science.

“Electrochemical devices that transform electrical energy to mechanical energy have potential use in numerous applications, ranging from soft robotics and micropumps to autofocus microlenses and bioelectronics,” said Mohammad Reza Abidian, associate professor of biomedical engineering in the UH Cullen College of Engineering. He’s the corresponding author of the article “Organic Semiconductor Nanotubes for Electrochemical Devices,” published in the journal Advanced Functional Materials, which details the discovery.

Mohammad Reza Abidian
Mohammad Reza Abidian, associate professor of biomedical engineering at the University of Houston Cullen College of Engineering, has announced a breakthrough with the development of an electrochemical actuator. Credit: University of Houston

Significant movement (which scientists define as actuation and measure as deformation strain) and fast response time have been elusive goals, especially for electrochemical actuator devices that operate in liquid. This is because the drag force of a liquid restricts an actuator’s motion and limits the ion transportation and accumulation in electrode materials and structures. In Abidian’s lab, he and his team refined methods of working around those two stumbling blocks.

“Our organic semiconductor nanotube electrochemical device exhibits high actuation performance with fast ion transport and accumulation and tunable dynamics in liquid and gel-polymer electrolytes. This device demonstrates an excellent performance, including low power consumption/strain, a large deformation, fast response and excellent actuation stability,” Abidian said.

This outstanding performance, he explained, stems from the enormous effective surface area of the nanotubular structure. The larger area facilitates the ion transport and accumulation, which results in high electroactivity and durability.

“The low power consumption/strain values for this OSNT actuator, even when it operates in liquid electrolyte, mark a profound improvement over previously reported electrochemical actuators operating in liquid and air,” Abidian said. “We evaluated long-term stability. This organic semiconductor nanotube actuator exhibited superior long-term stability compared with previously reported conjugated polymer-based actuators operating in liquid electrolyte.”

Joining Abidian on the project were Mohammadjavad Eslamian, Fereshtehsadat Mirab, Vijay Krishna Raghunathan and Sheereen Majd, all from the Department of Biomedical Engineering at the UH Cullen College of Engineering.

The organic semiconductors used, called conjugated polymers, were discovered in the 1970s by three scientists – Alan J. Heeger, Alan MacDiarmid and Hideki Shirakawa – who won a Nobel prize in 2000 for the discovery and development of conjugated polymers.

For a new type of actuator to outshine the status quo, the end product must prove not only to be highly effective (in this case, in both liquid and gel polymer electrolyte), but also that it can last.

“To demonstrate potential applications, we designed and developed a micron-scale movable neural probe that is based on OSNT microactuators. This microprobe potentially can be implanted in the brain, where neural signal recordings that are adversely affected, by either damaged tissue or displacement of neurons, may be enhanced by adjusting the position of the movable microcantilevers,” said Abidian.

New Fibers Can Make Breath-Regulating Garments

A new kind of fiber developed by researchers at MIT and in Sweden can be made into clothing that senses how much it is being stretched or compressed, and then provides immediate tactile feedback in the form of pressure, lateral stretch, or vibration. Such fabrics, the team suggests, could be used in garments that help train singers or athletes to better control their breathing, or that help patients recovering from disease or surgery to recover their breathing patterns.

The multilayered fibers contain a fluid channel in the center, which can be activated by a fluidic system. This system controls the fibers’ geometry by pressurizing and releasing a fluid medium, such as compressed air or water, into the channel, allowing the fiber to act as an artificial muscle. The fibers also contain stretchable sensors that can detect and measure the degree of stretching of the fibers. The resulting composite fibers are thin and flexible enough to be sewn, woven, or knitted using standard commercial machines.

The fibers, dubbed OmniFibers, are being presented this week at the Association for Computing Machinery’s User Interface Software and Technology online conference, in a paper by Ozgun Kilic Afsar, a visiting doctoral student and research affiliate at MIT; Hiroshi Ishii, the Jerome B. Wiesner Professor of Media Arts and Sciences; and eight others from the MIT Media Lab, Uppsala University, and KTH Royal Institute of Technology in Sweden.

The new fiber architecture has a number of key features. Its extremely narrow size and use of inexpensive material make it relatively easy to structure the fibers into a variety of fabric forms. It’s also compatible with human skin, since its outer layer is based on a material similar to common polyester. And, its fast response time and the strength and variety of the forces it can impart allow for a rapid feedback system for training or remote communications using haptics (based on the sense of touch).

Afsar says that the shortcomings of most existing artificial muscle fibers are that they are either thermally activated, which can cause overheating when used in contact with human skin, or they have low-power efficiency or arduous training processes. These systems often have slow response and recovery times, limiting their immediate usability in applications that require rapid feedback, she says.

New Fiber Architecture
The key features of the new fiber architecture include its extremely narrow size and use of inexpensive materials, which make it relatively easy to structure the fibers into a variety of fabric forms. Credit: Courtesy of the researchers

As an initial test application of the material, the team made a type of undergarment that singers can wear to monitor and play back the movement of respiratory muscles, to later provide kinesthetic feedback through the same garment to encourage optimal posture and breathing patterns for the desired vocal performance. “Singing is particularly close to home, as my mom is an opera singer. She’s a soprano,” she says. In the design and fabrication process of this garment, Afsar has worked closely with a classically trained opera singer, Kelsey Cotton.

“I really wanted to capture this expertise in a tangible form,” Afsar says. The researchers had the singer perform while wearing the garment made of their robotic fibers, and recorded the movement data from the strain sensors woven into the garment. Then, they translated the sensor data to the corresponding tactile feedback. “We eventually were able to achieve both the sensing and the modes of actuation that we wanted in the textile, to record and replay the complex movements that we could capture from an expert singer’s physiology and transpose it to a nonsinger, a novice learner’s body. So, we are not just capturing this knowledge from an expert, but we are able to haptically transfer that to someone who is just learning,” she says.

Kinesthetic Feedback Garment
As an initial test application of the material, the team made a type of undergarment that singers can wear to monitor and play back the movement of respiratory muscles, to later provide kinesthetic feedback through the same garment to encourage optimal posture and breathing patterns for the desired vocal performance. Credit: Courtesy of the researchers

Though this initial testing is in the context of vocal pedagogy, the same approach could be used to help athletes to learn how best to control their breathing in a given situation, based on monitoring accomplished athletes as they carry out various activities and stimulating the muscle groups that are in action, Afsar says. Eventually, the hope is that such garments could also be used to help patients regain healthy breathing patterns after major surgery or a respiratory disease such as Covid-19, or even as an alternative treatment for sleep apnea (which Afsar suffered from as a child, she says).

The physiology of breathing is actually quite complex, explains Afsar, who is carrying out this work as part of her doctoral thesis at KTH Royal Institute of Technology. “We are not quite aware of which muscles we use and what the physiology of breathing consists of,” she says. So, the garments they designed have separate modules to monitor different muscle groups as the wearer breathes in and out, and can replay the individual motions to stimulate the activation of each muscle group.