Researchers from Case Western Reserve University, Dayton Air Force Research Laboratory and China have developed a new dry adhesive that bonds in extreme temperatures—a quality that could make the product ideal for space exploration and beyond.
The gecko-inspired adhesive loses no traction in temperatures as cold as liquid nitrogen or as hot as molten silver, and actually gets stickier as heat increases, the researchers report.
The research, which builds on earlier development of a single-sided dry adhesive tape based on vertically aligned carbon nanotubes, is published in the journal Nature Communications. As far as the researchers know, no other dry adhesive is capable of working at such temperature extremes.
Liming Dai, professor of macromolecular science and engineering at Case Western Reserve and an author of the study teamed with Ming Xu, a senior research associate at Case School of Engineering and visiting scholar from Huazhong University of Science and Technology; Feng Du, senior research associate in Case Western Reserve’s Department of Macromolecular Science and Engineering; and Sabyasachi Ganguli and Ajit Roy, of the Materials and Manufacturing Directorate, Air Force Research Laboratory.
Vertically aligned carbon nanotubes with tops bundled into nodes replicate the microscopic hairs on the foot of the wall-walking reptile and remain stable from -320 degrees Fahrenheit to 1,832 degrees, the scientists say.
“When you have aligned nanotubes with bundled tops penetrating into the cavities of the surface, you generate sufficient van der Waal’s forces to hold,” Xu said. “The dry adhesive doesn’t lose adhesion as it cools because the surface doesn’t change. But when you heat the surface, the surface becomes rougher, physically locking the nanotubes in place, leading to stronger adhesion as temperatures increase.”
Because the adhesive remains useful over such a wide range of temperatures, the inventors say it is ideally suited for use in space, where the shade can be frigid and exposure to the sun blazing hot.
In addition to range, the bonding agent offers properties that could add to its utility. The adhesive conducts heat and electricity, and these properties also increase with temperature. “When applied as a double-sided sticky tape, the adhesive can be used to link electrical components together and also for electrical and thermal management,” Roy said.
“This adhesive can thus be used as connecting materials to enhance the performance of electronics at high temperatures,” Dai said. “At room temperature, the double-sided carbon nanotube tape held as strongly as commercial tape on various rough surfaces, including paper, wood, plastic films and painted walls, showing potential use as conducting adhesives in home appliances and wall-climbing robots.”
In testing, a double-sided tape made with the carbon nanotubes (CNTs) applied between two layers of copper foil had an adhesive strength of about 37 newtons per cm-2 at room temperature, about the same as a commercial double-sided sticky tape.
Unlike the commercial tape, which loses adhesion as it freezes or is heated, the CNT adhesive maintained its strength down to -320 degrees Fahrenheit. The adhesive strength more than doubled at 785 degrees Fahrenheit and was about six times as strong at 1891 degrees.
Surprised by the increasing adhesive strength, the researchers used a scanning electron microscope to search for the cause. They found that, as the bundled nodes penetrate the surface cavities, the flexible nanotubes no longer remain vertically aligned but collapse into web-like structures. The action appears to enhance the van der Waal’s forces due to an increased contact surface area with the collapsed nanotubes.
Looking further, the researchers found that as the temperature increased above 392 degrees Fahrenheit, the surface of the copper foil became increasingly rough. The bundled ends and collapsed nanotubes appear to penetrate deeper into the heat-induced irregularities in the surface, increasing adhesion. The researchers dub this adhesion mechanism “nano-interlocking.”
The adhesive held strong during hundreds of temperature transition cycles between ambient temperature and -320 degrees then up to 1891 degrees and between the cold extreme and ambient temperature.
Copper foil, which was used for many of the tests to demonstrate the potential for thermal management, is not unique. The surface of many other materials, including polymer films and other metal foils, roughen when heat is applied, making them good targets for this kind of adhesive, the team suggests.
Wearable integrated thermocells based on gel electrolytes use body heat
Electronics integrated into textiles are gaining in popularity: Systems like smartphone displays in a sleeve or sensors to detect physical performance in athletic wear have already been produced. The main problem with these systems tends to be the lack of a comfortable, equally wearable source of power. Chinese scientists are now aiming to obtain the necessary energy from body heat. In the journal Angewandte Chemie, they have introduced a flexible, wearable thermocell based on two different gel electrolytes.
Our muscle activity and metabolism cause our bodies to produce constant heat, some of which is released through the skin into the environment. Because of the relatively small temperature difference between skin (approximately 32°C) and the temperature of our surroundings, it is not so easy to make use of body heat. Previous thermoelectric generators, such as those based on semiconductors, produce too little energy, are costly, or are too brittle for use in wearable systems. Thermocells with electrolyte solutions are difficult to integrate into extensive wearable systems. A team led by Jun Zhou at Huazhong University of Science and Technology (Wuhan, China) has now found a solution to this problem: thermocells with gel-based electrolytes.
The researchers are making use of the thermogalvanic effect: if two electrodes in contact with an electrolyte solution—or an electrolyte gel—are kept at different temperatures, a potential difference is generated. The ions of a redox pair in the electrolyte can rapidly switch between two different charge states, accepting or releasing electrons at electrodes with different temperature. In order to use this to produce a current, the scientists combined two types of cells containing two different redox pairs. Each cell consists of two tiny metal plates that act as electrodes, with an electrolyte gel in between. The first cell type contains the Fe2+/Fe3+ redox pair. The second type of cell contains the complex ions [Fe(CN)6]3?/[Fe(CN)6]4?. Because of the choice of these redox pairs, in cell type 1, the cold end gives a negative potential, while in type 2, the cold end gives a positive potential.
The researchers arranged many of these two types of cells into a checkerboard pattern. The cells were connected to each other by metal plates alternating above and below, to link them into a series. They then integrated this “checkerboard” into a glove. When the glove is worn, the desired temperature difference results between the upper and lower plates. This produces a voltage between neighboring cells, and the voltage adds up. This makes it possible to generate current to power a device or charge a battery.
In an environment at 5 °C, it was possible to produce 0.7 volts and about 0.3 µW. By optimizing this system, it should be possible to improve the power, even with smaller temperature gradients.
Learn more: Body Heat as a Power Source
Researchers in China report that air plasma can be used to kill biofilms found on the surfaces of perishable fruits and foods — significantly extending their shelf life, and reducing the world’s ‘food waste’ problem as well
Seeing fruit “turn bad and going to waste” inspired a team of researchers in China to explore using atmospheric pressure nonequilibrium plasma — already widely used for medical purposes — as a novel solution to extend the shelf life of fruit and other perishable foods.
When bacteria attaches to food surfaces, it can extract nutrients and continue to proliferate in the form of “biofilms.” Bacterial biofilms on food and food-processing surfaces diminish the food’s quality and safety.
But plasma sources are capable of killing bacteria such as Salmonella and E.coli on apples, as well as other types of spoilage microorganisms on mangos and melons, and Listeria on meat.
Now, researchers from China’s Shanghai Jiao Tong University and Huazhong University of Science and Technology report this week in the journal Physics of Plasmas, from AIP Publishing, about their computational study of how air plasma interacts with bacterial biofilms on an apple’s surface suggests that plasma technology could be used to decontaminate food in the future.
The fundamental concept behind the team’s work is to harness the reactive species generated by plasma to kill bacterial biofilms, which are notoriously difficult to wipe out.
“A biofilm consists of groups of microorganisms in which cells stick to each other, and these cells often adhere to a surface,” explained Xinpei Lu, a professor in the College of Electrical and Electronic Engineering at Huazhong University of Science and Technology. “These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance, which forms in different shapes and acts to protect the bacteria.”
For this work, the team simulated how the structure of the biofilm affects the discharge dynamics and then zeroed in on how the reactive species generated by the plasma are distributed on the biofilm’s surface — because it can later kill the bacteria within the biofilm.
“Plasma is formed when enough energy is added to a gas to ‘free up’ electrons from a significant number of atoms or molecules,” Lu said. “This process, known as ‘ionization,’ creates a mixture of positively charged particles, negatively charged particles, and various uncharged particles.”
High concentrations of so-called free radicals — very chemically reactive atomic or molecular fragments — often exist among these particles.
“These free radicals can quickly overwhelm the natural defenses of living organisms, which leads to their destruction,” he added.
Because plasma can easily produce more than a trillion free radicals per cubic centimeter of volume, it can serve as an efficient decontamination agent.
“Free radicals are one type of germ-killing agent generated via plasmas,” Lu pointed out. “Plasmas also produce other agents such as ultraviolet light, which sterilizes by causing DNA damage.”
Scientists previously observed that bacterial cell membranes sometimes rupture when exposed to plasma. This may be caused by charged particles attaching to the outer surface of the cell — inducing an electrostatic force that can overcome the tensile strength of the cell’s membrane by rupturing it.
So the team decided to explore how plasma interacts with biofilms and how the reactive species generated by the plasma are able to penetrate the cavity of the biofilm.
“Technically, we wanted to simulate the discharge (in millimeter gap distance) while capturing the effect of the biofilm’s mushroom shape (within a micrometer range) — an extremely challenging task,” said Lu.
What did they find?
“We discovered that the structure of the biofilm results in non-uniform distribution of reactive species during the plasma-on period,” he explained. “The mean free path of charged species at micron-scale permitted the plasma to penetrate into the cavity of the biofilm. This means that although the density of reactive species decreased by 6 to 7 orders of magnitude, the diffusion caused a uniform distribution of reactive species inside the cavity during its pulse-off period.”
In terms of applications, the team’s work indicates that air plasma can be used to kill bacteria within biofilms, which could “significantly prolong the amount of time fruit remains edible,” said Lu. Such a technique could be on the market within a few years, “once a low-cost plasma source is developed.”
The next step toward using low-temperature plasma technology for the decontamination of fruit is “to generate a uniform plasma over the irregular surface of the fruit, or to use a plasma jet to scan the surface of the fruit,” Lu noted. “We’re currently working on the latter method to achieve this goal.”
The Huazhong University of Science and Technology (HUST; simplified Chinese: 华中科技大学; traditional Chinese: 華中科技大學; pinyin: Huázhōng Kējì Dàxué; literally: “Central China University of Science and Technology”) is a public, coeducational research university located in Wuhan, Hubei province, China.
As a national key university, HUST is directly affiliated to the Ministry of Education of China. HUST has been referred as the flagship of China’s higher education system after Chinese Civil War. HUST manages Wuhan National Laboratories for Opto-electronics (WNLO) at Wuchang, which is one of the five national laboratories in China.