Switch and stick: A new reversible adhesive that allows its adhesive effect to be switched on and off with ease

Glued to the spot: Max Planck scientists from Stuttgart have used gallium as an adhesive to grip a glass sphere with a movable punch model. The metal is located at the bottom of the punch model. Once it touches the glass sphere, the researchers heat the gallium and then cool it off again, so that it connects to the glass. This way, they can retract the sphere from the surface. © MPI for Intelligent Systems

Glued to the spot: Max Planck scientists from Stuttgart have used gallium as an adhesive to grip a glass sphere with a movable punch model. The metal is located at the bottom of the punch model. Once it touches the glass sphere, the researchers heat the gallium and then cool it off again, so that it connects to the glass. This way, they can retract the sphere from the surface.
© MPI for Intelligent Systems

The chemical element gallium could be used as a new reversible adhesive that allows its adhesive effect to be switched on and off with ease

Some adhesives may soon have a metallic sheen and be particularly easy to unstick. Researchers at the Max Planck Institute for Intelligent Systems in Stuttgart are suggesting gallium as just such a reversible adhesive. By inducing slight changes in temperature, they can control whether a layer of gallium sticks or not. This is based on the fact that gallium transitions from a solid state to a liquid state at around 30 degrees Celsius. A reversible adhesive of this kind could have applications everywhere that temporary adhesion is required, such as industrial pick-and-place processes, transfer printing, temporary wafer bonding, or for moving sensitive biological samples such as tissues and organs. Switchable adhesion could also be suitable for use on the feet of climbing robots.

The principle is actually quite simple: Above 30 degrees Celsius, gallium metal is liquid, and below 30 degrees it is solid. So if a drop of liquid gallium is introduced between two objects and then cooled to less than 30 degrees, the gallium layer solidifies and sticks the two objects together. When it is time to separate the objects, the temperature is raised to transition the gallium layer to its liquid state and they can be pulled apart with a small amount of unloading force. As an adhesive, gallium works in a similar fashion to hot glue, widely used in DIY applications. The difference is that far less heating and cooling are sufficient in the case of gallium, it lifts much more easily and cleanly from the surface, it is highly repeatable, and it is electrically conductive.

For their experiments, scientists working with Metin Sitti, Director at the Max Planck Institute for Intelligent Systems, wet the tip of a cylindrical elastomer rod with liquid gallium. They then brought the gallium droplet into contact with different materials such as glass, plastic and gold. After cooling the tip to 23 degrees, they found that the solidified gallium formed a strong bond between the elastomer and each of the materials.

Tests on particularly rough or damp surfaces

The researchers also took direct measurements of the effective binding power of gallium in both its liquid and solid phases. “The behaviour of these two values tells us something about the true reversibility and switchability of the adhesion process,” explains Metin Sitti. The greater the difference of the binding power between the liquid and solid state, the easier it is to reverse and switch the adhesive effect.

The team deliberately tested gallium on particularly rough and damp surfaces as well. “These are surface conditions that showed up as major weaknesses of reversible micro/nanostructured adhesives proposed recently,” says Sitti. How so? Adhesives that have yielded strong binding values on rough or wet surfaces to date have always had poor reversibility. Not so with the new gallium approach. The Stuttgart-based team have become convinced of its effectiveness in damp conditions, even testing it under water. Its binding power and reversibility when wet were reduced compared to dry conditions, but they still remained relatively strong for a wide range of applications.

Application wherever careful and reversible adhesion is required

Metin Sitti emphasises that gallium’s performance in damp conditions makes it ideal for biological applications. The scientist and engineer foresees a time when gallium may be used to move individual cells, tissue samples or even organs, for example in laboratory or hospital settings.

Another possible field of application is industrial manufacturing, especially where fragile components such as ultra-thin graphene membranes or tiny electronic chips are involved. These components could be picked up by gallium-coated grippers and then set down at the precise location where they are required, e.g. a circuit board. In technical jargon, this kind of assembly technology is called “pick and place”. It already exists today, but is generally based on the use of vacuum suction.

Metin Sitti believes the temperature-controlled gallium adhesive has two advantages. “Wetting an object with a metallic liquid such as gallium that forms a bond when cooled slightly is a far more gentle process for fragile materials than sucking them up using a vacuum,” he expounds, adding that the new methods are also more energy-efficient. Once an object adheres to the gallium layer, no more energy is required to sustain the adhesive bond. Only when it is time to reverse the adhesion must the metal be quickly heated to 30 degrees. The vacuum technique, however, requires the constant use of suction in order to maintain the adhesive effect.

Temperature control for phase change of gallium

To achieve rapid heating and cooling as required in their tests, the team in Stuttgart connected a Peltier element to their experiment set-up. This element can either release or absorb heat when an electric current is applied. However, for practical applications in the future, the scientists anticipate that the adhesive bond could also be reversed using infrared radiation remotely or using electrical Joule heating by integrating conductive wiring to the surface.

Metin Sitti sees robotics as another possible application for this adhesive. For example, climbing robots such as those that may one day ascend wind turbines for maintenance purposes could benefit from reversible adhesives. By activating the adhesive, the robot foot would be fixed to the wall of the turbine, and for the next step, the adhesive layer between the foot and the wall would be briefly heated by means of an integrated heating element.

An adhesive that doesn’t run out

A consideration of major importance for practical applications is that the material should be able to be used for as many cycles as possible without the need to replace it. Gallium conforms to this requirement, because the liquid metal lifts completely from the substrate with proper loading and unloading conditions. No residues are left on the surface, and the adhesive loses none of its own substance. This is by no means something to be taken for granted. “Good adhesives are generally hard to separate from the substrate,” states Sitti, explaining that in gallium’s case, the material forms a fine oxide layer in air. This shell of gallium oxide retains the gallium and ensures that no residues are left behind when the adhesion is reversed.

And that’s not all. Gallium has even more to offer: “We can use it at different scales, from the nanometre range to microelectronics, and right up to larger applications,” says Sitti with a smile. In theory, it could even be used to lift a fully-grown person as long as the contact surface was sufficiently large. However, it would be most cost-effective, energy efficient, and practical with smaller objects.

Metin Sitti believes that this method could be used in practical applications in the near future. And his team has started exploring some of these potential applications already. At the same time, they are working to optimize the technique. Until now, for example, the gallium was applied to an elastomer rod around two millimetres in diameter for all tests. “We want to test other elastomer geometries and designs with different length scales and see if we can enhance the binding strength as we do so,” says Sitti. The scientists also plan to study alloys of gallium with other metals such as indium, but they will be watching closely to ensure that the melting point is close to normal ambient temperature.

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No humans would be harmed in the operation of these robots

Elastic machines: Membranes surrounding sealed, air-filled chambers can be used as actuators, facilitating risk-free contact between humans and robots. Compliant electrodes are attached to each side of the membrane and cause it to stretch when voltage is applied. The membranes are bistable, meaning that they can enclose two different volumes at the same air pressure. A membrane switches from its more compact state to its stretched state when voltage is applied to its electrodes. Even in the case of three or more linked, bubble-shaped chambers, one can be controlled in this way so that it inflates to a larger volume, thereby exerting force. [less] © Alejandro Posada

Elastic machines: Membranes surrounding sealed, air-filled chambers can be used as actuators, facilitating risk-free contact between humans and robots. Compliant electrodes are attached to each side of the membrane and cause it to stretch when voltage is applied. The membranes are bistable, meaning that they can enclose two different volumes at the same air pressure. A membrane switches from its more compact state to its stretched state when voltage is applied to its electrodes. Even in the case of three or more linked, bubble-shaped chambers, one can be controlled in this way so that it inflates to a larger volume, thereby exerting force. 
Photo: Alejandro Posada

A soft actuator using electrically controllable membranes could pave the way for machines that are no danger to humans

In interacting with humans, robots must first and foremost be safe. If a household robot, for example, encounters a human, it should not continue its movements regardless, but rather give way in case of doubt. Researchers at the Max Planck Institute for Intelligent Systems in Stuttgart are now presenting a motion system – a so-called elastic actuator – that is compliant and can be integrated in robots thanks to its space-saving design.

The actuator works with hyperelastic membranes that surround air-filled chambers. The volume of the chambers can be controlled by means of an electric field at the membrane. To date, elastic actuators that exert a force by stretching air-filled chambers have always required connection to pumps and compressors to work. A soft actuator such as the one developed by the Stuttgart-based team means that such bulky payloads or tethers may now be superfluous.

Many robots have become indispensable, and it is accepted that they may be dangerous to humans in their workspace. In the automotive industry, for example, they assemble cars with speed and reliability, but are well shielded from direct contact with humans. These robots go through their motions precisely and relentlessly, and anyone who gets in the way could be seriously injured.

Robots with soft actuators that cannot harm humans, on the other hand, are tethered by pneumatic hoses and so their radius of motion is restricted. This may be about to change. “We have developed an actuator that makes large changes in form possible without an external supply of compressed air”, says Metin Sitti, Director at the Max Planck Institute for Intelligent Systems.

The new device consists of a dielectric elastomer actuator (DEA): a membrane made of hyperelastic material like a latex balloon, with flexible (or ‘compliant’) electrodes attached to each side. The stretching of the membrane is regulated by means of an electric field between the electrodes, as the electrodes attract each other and squeeze the membrane when voltage is applied. By attaching multiple such membranes, the place of deformation can be shifted controllably in the system.

Air is displaced between two chambers

The researchers are helped in this by the fact that their membrane material knows two stable states. In other words, it can have two different volume configurations at a given pressure without the need to minimize the larger volume. This is a little like letting the air out of an inflated balloon; it does not shrink back to its original size, but remains significantly larger. Thanks to this bi-stable state, the researchers are able to move air between a more highly inflated chamber and a less inflated one. They do this by applying an electric current to the membrane of the smaller chamber which responds by stretching and sucking air out of the other bubble. When the power supply is switched off the membrane contracts, but not to its original volume; it remains larger, corresponding to its stretched state.

“It is important to find suitable hyperelastic polymers that will enable strong and fast deformation and be durable,” points out Metin Sitti. With this in mind, the team has tested different membrane materials and also used models to systematically record the behaviour of the elastomer in the actuator.

Thus far, the elastomers tested by Sitti’s team have each had a mix of advantages and disadvantages. Some show strong deformation, but at a slow rate. Others work fast, but their deformation is more limited. “We will combine different materials with a view to combining different properties in a single membrane,” says Sitti. This is, however, just one of the next steps he and his team have in mind. They also plan to integrate their actuator in a robot so that it can, for instance, move its legs but still give way if it happens to come across a human. Only then can machine-human interactions be risk-free.

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Gecko-inspired Adhesion: Self-cleaning and Reliable

Microhairs similar to the gecko’s setae before and after cleaning through lateral friction contact with a smooth surface. (Photograph with scanning electron microscope: Michael Röhrig, KIT)

Researchers of KIT and the Carnegie Mellon University Developed a Reusable Adhesive Tape Modeled on Nature

Geckos outclass adhesive tapes in one respect: Even after repeated contact with dirt and dust do their feet perfectly adhere to smooth surfaces. Researchers of the KIT and the Carnegie Mellon University, Pittsburgh, have now developed the first adhesive tape that does not only adhere to a surface as reliably as the toes of a gecko, but also possesses similar self-cleaning properties. Using such a tape, food packagings or bandages might be opened and closed several times.

When moving forwards, the gecko‘s toes drag across a part of the surface. As a result of this lateral friction contact, larger dirt particles are removed. Smaller particles deposit among the setae on the sole and in the skinfolds below. In an experiment, the researchers have proved that both mechanisms provide for 95% of the self-cleaning effect. “This effect is determined by the ratio between particle size and setae diameter“, Dr. Hendrik Hölscher of KIT’s Institute of Microstructure Technology (IMT) says.

For their experiments, the scientists used elastic microhairs of variable size. Instead of dirt particles, they employed glass spheres of micrometer size (10-6 meters) and distributed them on a smooth plate. To simulate the steps made by a gecko, they pressed an artificial adhesive tape covered by microhairs onto the plate, shifted it laterally, and lifted the tape off again. This “load-drag-unload“ cycle was repeated several times. In parallel, adhesive force was measured.

When the diameter of the spheres exceeded that of the microhairs, the adhesive force disappeared after the first contact (”load“) – as in case of an ordinary adhesive tape. After eight to ten test cycles, however, the gecko-inspired adhesive tape reached 80 to 100 percent of its original power again. “In the long term, this effect might be used to develop a low-cost alternative to hook and loop fasteners,“ Hölscher says. “Such a tape might be applied in the sports sector, in medicine, automotive industry or aerospace technology,“ Metin Sitti, Professor of the Carnegie Mellon University, adds.

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Major Advance in tissue engineering and 3D printing

English: Principle of tissue engineering (Photo credit: Wikipedia)

Researchers at Brigham and Women’s Hospital (BWH) and Carnegie Mellon University have introduced a unique micro-robotic technique to assemble the components of complex materials, the foundation of tissue engineering and 3D printing.

Described in the Jan. 28, 2014, issue of Nature Communicationsthe research was conducted by Savas Tasoglu, PhD, MS, research fellow in the BWH Division of Renal Medicine, and Utkan Demirci, PhD, MS, associate professor of Medicine in the Division of Biomedical Engineering, part of the BWH Department of Medicine, in collaboration with Eric Diller, PhD, MS, and Metin Sitti, PhD, MS, professor in the Department of Mechanical Engineering, Carnegie Mellon University.

Tissue engineering and 3D printing have become vitally important to the future of medicine for many reasons. The shortage of available organs for transplantation, for example, leaves many patients on lengthy waiting lists for life-saving treatment. Being able to engineer organs using a patient’s own cells can not only alleviate this shortage, but also address issues related to rejection of donated organs. Developing therapies and testing drugs using current preclinical models have limitations in reliability and predictability. Tissue engineering provides a more practical means for researchers to study cell behavior, such as cancer cell resistance to therapy, and test new drugs or combinations of drugs to treat many diseases.

The presented approach uses untethered magnetic micro-robotic coding for precise construction of individual cell-encapsulating hydrogels (such as cell blocks). The micro-robot, which is remotely controlled by magnetic fields, can move one hydrogel at a time to build structures. This is critical in tissue engineering, as human tissue architecture is complex, with different types of cells at various levels and locations. When building these structures, the location of the cells is significant in that it will impact how the structure will ultimately function. “Compared with earlier techniques, this technology enables true control over bottom-up tissue engineering,” explains Tasoglu.

Tasoglu and Demirci also demonstrated that micro-robotic construction of cell-encapsulating hydrogels can be performed without affecting cell vitality and proliferation. Further benefits may be realized by using numerous micro-robots together in bioprinting, the creation of a design that can be utilized by a bioprinter to generate tissue and other complex materials in the laboratory environment.

“Our work will revolutionize three-dimensional precise assembly of complex and heterogeneous tissue engineering building blocks and serve to improve complexity and understanding of tissue engineering systems,” said Metin Sitti, professor of Mechanical Engineering and the Robotics Institute and head of CMU’s NanoRobotics Lab.

“We are really just beginning to explore the many possibilities in using this micro-robotic technique to manipulate individual cells or cell-encapsulating building blocks.” says Demirci. “This is a very exciting and rapidly evolving field that holds a lot of promise in medicine.”

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Medical Capsule Robots take pictures and much more

Sitti and Yim wanted to engineer an active robotic capsule that could be controlled while inside the gastrointestinal tract.

When someone mentions robots, popular representations such as R2-D2, Wall-E, or the Terminator may come to mind. But what if scientists could make robots so tiny they could navigate inside the human body? Metin Sitti, Carnegie Mellon professor in mechanical engineering and head of the Nanorobotics Lab, and Sehyuk Yim, a Ph.D. student in mechanical engineering, recently received funding for their research on medical capsule robots.

Pill cameras that take pictures of the gastrointestinal tract after being swallowed have already been around for 11 years. These miniature cameras are FDA-approved and widely used in hospitals — but they do not have any functions beyond imaging.

“Imaging is good,” Sitti said, “but it’s a passive tablet you swallow that moves through your body naturally. If you miss something, you cannot go back. If you want to stay at one place longer, you can’t. If you want to do drug delivery or a biopsy, there is no way to do that because the capsule is not controlled.”

Sitti and Yim wanted to engineer an active robotic capsule that could be controlled while inside the gastrointestinal tract. Some scientists utilize wired endoscopes, which are tubes with an attached light source, to illuminate an organ. While endoscopes can perform camera imaging, inject drugs, take tissue samples, and have heat probes, they are also highly invasive.

“We want to combine the advantages of wired endoscopes with all its functions and try to put that into the pill camera so we can have an active pill that can do all of these functions in a minimally invasive manner,” Sitti said.

A popular idea in medical robots has been tiny rigid robots that can move inside the body using legs. To overcome the safety hazard of rigid robots, Yim decided to explore the idea of soft capsule robots made of a squishy material called elastomer. In addition to being safe inside the body, the shape of a soft robot can be easily changed. “Combining soft robotics with nanorobotics is a novelty,” Sitti said.

Sitti and Yim designed a robot with two heads that has an internal magnet at each end. Using an external magnet at a certain distance away from the robot, the capsule can be remotely controlled. By using magnetic fields to contract and expand the soft capsule after it has been swallowed, the tiny robot can roll and twist inside a patient’s stomach.

The researchers hope to improve this concept by increasing the friction on the capsule’s surface — perhaps with adhesive fibers inspired by the fibers on geckos’ feet — so that it can move more easily on the mucus-covered stomach lining.

“You can put a drug chamber in the middle, and by changing the formation of the capsule, you can inject a drug,” Sitti said. Another function of the capsule is to inject biopsy microgrippers that come out and grab tissue by folding and contracting when exposed to body temperature. After the capsule leaves the body naturally, this tissue sample can be taken out and examined.

The building process for these tiny capsule robots takes a high level of precision. “I use a molding process,” Yim said. “These capsule robots are made of polymer material, so we have to use a 3-D printer, which is a rapid prototyping machine that works by layering of material. Using this, we make a mold that I can then pour a polymer into. After seven or eight hours, the elastomer is cured and I detach it from the mold and assemble the parts.”

By using this molding process, Yim is able to make many copies of the capsule robot in an inexpensive manner.

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