How to design materials with reprogrammable shape and function
Metamaterials — materials whose function is determined by structure, not composition — have been designed to bend light and sound, transform from soft to stiff, and even dampen seismic waves from earthquakes. But each of these functions requires a unique mechanical structure, making these materials great for specific tasks, but difficult to implement broadly.
But what if a material could contain within its structure, multiple functions and easily and autonomously switch between them?
Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute of Biologically Inspired Engineering at Harvard University have developed a general framework to design reconfigurable metamaterials. The design strategy is scale independent, meaning it can be applied to everything from meter-scale architectures to reconfigurable nano-scale systems such as photonic crystals, waveguides and metamaterials to guide heat.
The research is published in Nature.
“In terms of reconfigurable metamaterials, the design space is incredibly large and so the challenge is to come up with smart strategies to explore it,” said Katia Bertoldi, John L. Loeb Associate Professor of the Natural Sciences at SEAS and senior author of the paper. “Through a collaboration with designers and mathematicians, we found a way to generalize these rules and quickly generate a lot of interesting designs.”
Bertoldi and former graduate student Johannes Overvelde, who is the first author of the paper, collaborated with Chuck Hoberman, of the Harvard Graduate School of Design (GSD) and associate faculty at the Wyss and James Weaver, a senior research scientist at the Wyss, to design the metamaterial.
The research began in 2014, when Hoberman showed Bertoldi his original designs for a family of foldable structures, including a prototype of an extruded cube. “We were amazed by how easily it could fold and change shape,” said Bertoldi. “We realized that these simple geometries could be used as building blocks to form a new class of reconfigurable metamaterials but it took us a long time to identify a robust design strategy to achieve this.”
The interdisciplinary team realized that assemblies of polyhedra can be used as a template to design extruded reconfigurable thin-walled structures, dramatically simplifying the design process.
“By combining design and computational modeling, we were able to identify a wide range of different rearrangements and create a blueprint or DNA for building these materials in the future, ” said Overvelde, now scientific group leader of the Soft Robotic Matter group at FOM Institute AMOLF in the Netherlands.
The same computational models can also be used to quantify all the different ways in which the material could bend and how that affected effective material properties like stiffness. This way they could quickly scan close to a million different designs, and select those with the preferred response.
Once a specific design was selected, the team constructed working prototypes of each 3D metamaterial both using laser-cut cardboard and double-sided tape, and multimaterial 3D printing. Like origami, the resulting structure can be folded along their edges to change shape.
“Now that we’ve solved the problem of formalizing the design, we can start to think about new ways to fabricate and reconfigure these metamaterials at smaller scales, for example through the development of 3D-printed self actuating environmentally responsive prototypes,” said Weaver.
This formalized design framework could be useful for structural and aerospace engineers, material scientists, physicists, robotic engineers, biomedical engineers, designers and architects.
“This framework is like a toolkit to build reconfigurable materials,” said Hoberman. “These building blocks and design space are incredibly rich and we’ve only begun to explore all the things you can build with them.”
Learn more: A toolkit for transformable materials
Researchers at Chalmers University of Technology have developed a method that enables them to manipulate light to follow any predetermined path along a surface.
The innovation has now been described as one of the world’s 30 most exciting discoveries within optics and photonics during 2016.
Earlier this year, Chalmers researchers Philippe Tassin and Sophie Viaene published their discovery about how light can be controlled with metamaterials. Amidst strong international competition, their paper has now been chosen as one of the best this year by the influential magazine, Optics & Photonics News. The research is thereby featured in the special edition, Optics in 2016.
The innovation from Chalmers University of Technology makes it possible to manipulate light to follow any predetermined path along a surface. With the help of a mathematical design tool it is possible to create various artificial materials – metamaterials – that guide the light along the path of your choice.
“We don’t have to think about the limitations of natural materials. Instead, we decide what we want to do and then we design a metamaterial waveguide that makes it work. This is beautiful physics, building on Einstein’s general theory of relativity,” says Sophie Viaene, PhD student in the Division of Condensed Matter Physics.
The new technique has a wide field of application. For example, it can be used in optical chips to achieve reliable data delivery on the internet, or to speed up routers.
“Our method opens up the toolbox of transformation optics to a plethora of waveguide-based devices,” says Philippe Tassin, Professor in the Division of Condensed Matter Physics.
In the future, the researchers hope that they can even improve how light is produced.
“For example, an LED lamp is far from perfect when it comes to energy efficiency. Metamaterials could extract light in a more efficient way – and maybe we can also manipulate the colour of light,” says Tassin.
Counterintuitive “metamaterial” may enable heat-resistant circuit boards.
Almost all solid materials, from rubber and glass to granite and steel, inevitably expand when heated. Only in very rare instances do certain materials buck this thermodynamic trend and shrink with heat. For instance, cold water will contract when heated between 0 and 4 degrees Celsius, before expanding.
Engineers from MIT, the University of Southern California, and elsewhere are now adding to this curious class of heat-shrinking materials. The team, led by Nicholas X. Fang, an associate professor of mechanical engineering at MIT, has manufactured tiny, star-shaped structures out of interconnected beams, or trusses. The structures, each about the size of a sugar cube, quickly shrink when heated to about 540 degrees Fahrenheit (282 C).
Each structure’s trusses are made from typical materials that expand with heat. Fang and his colleagues realized that these trusses, when arranged in certain architectures, can pull the structure inward, causing it to shrink like a Hoberman sphere — a collapsible toy ball made from interconnecting lattices and joints.
The researchers consider the structures to be “metamaterials” — composite materials whose configurations exhibit strange, often counterintuitive properties that are not normally found in nature.
In some cases, these structures’ resistance to expanding when heated — rather than their shrinking response per se — may be especially useful. Such materials could find applications in computer chips, for example, which can warp and deform when heated for long periods of time.
“Printed circuit boards can heat up when there’s a CPU running, and this sudden heating could affect their performance,” Fang says. “So you really have to take great care in accounting for this thermal stress or shock.”
The researchers have published their results in the journal Physical Review Letters. Fang’s co-authors include former MIT postdoc Qi Ge, along with lead author Qiming Wang of the University of Southern California, Jonathan Hopkins of the University of California at Los Angeles, and Julie Jackson and Christopher Spadaccini of Lawrence Livermore National Laboratory (LLNL).
In the mid-1990s, scientists proposed theoretical structures whose arrangement should exhibit a property called “negative thermal expansion,” or NTE. The key to the arrangement was to build three-dimensional, lattice-like structures from two types of materials, each with a different NTE coefficient, or rate of expansion upon heating. When the whole structure is heated, one material should expand faster and pull the other material inward, shrinking the entire structure as a result.
“These theoretical papers were talking about how these types of structures could really break the conventional limit of thermal expansion,” Fang says. “But at the time, they were limited by how things were made. That’s where we saw this as a very good opportunity for microfabrication to demonstrate this concept.”
Fang’s lab has pioneered a 3-D printing technique called microstereolithography, in which the researchers use light from a projector to print very small structures in liquid resin, layer by layer.
“We can now use the microstereolithography system to create a thermomechanical metamaterial that may enable applications not possible before,” said Spadaccini, who is the director of LLNL’s Center for Engineered Materials and Manufacturing. “It has thermomechanical properties not achievable in conventional bulk materials.”
“We can take the same idea as an inkjet printer, and print and solidify different ingredients, all on the same template,” Fang says.
Taking inspiration from the general framework proposed previously by theorists, Fang and his colleagues printed small, three-dimensional, star-shaped structures made from interconnecting beams. They fabricated each beam from one of two ingredients: a stiff, slow-to-expand copper-containing material, and a more elastic, fast-expanding polymer substance. The internal beams were made from the elastic material, while the outer trusses were composed of stiff copper.
“If we have proper placement of these beams and lattices, then even if every individual component expands, because of the way they pull each other, the overall lattice could actually shrink,” Fang says.
“The problem we’re treating is a thermal mismatch problem,” Wang says. “These materials have different thermal expansion coefficients, so once we increase the temperature, they interact with each other and pull inward, so the overall structure’s volume decreases.”
“Room to experiment”
The researchers put their composite structures to the test by placing them within a small glass chamber and slowly increasing the chamber’s temperature, from room temperature to about 540 degrees Fahrenheit. They observed that as the structure was heated, it first maintained its initial shape, then gradually bent inward, shrinking in size.
“It shrinks by about one part in a thousand, or about 0.6 percent,” Fang says. While that may not seem significant, Fang adds that “the very fact that it shrinks is impressive.” For most applications, Fang says designers may simply prefer structures that do not expand when heated.
“Normal materials have positive thermal expansion, and this leads to challenging problems in engineering when a device needs to maintain its shape and work in a wide range of temperature,” says Xiaoming Mao, an assistant professor of physics at the University of Michigan, who was not involved in the study. “NTE materials are a great solution because they can be combined with normal materials and cancel the stress coming from thermal expansion. This leads to wide applications in space technology, bridges, piping systems, et cetera.”
In addition to their experiments, the researchers developed a computational model to characterize the relationships between the interconnecting beams, the spaces between the beams, and the direction and degree to which they expand with heat. The researchers can control how much a structure will shrink by tuning two main “knobs” in the model: the dimensions of the individual beams, and their relative stiffness, which is directly related to a material’s rate of heat expansion.
“We now have a tuning method for digitally placing individual components of different stiffness and thermal expansion within a structure, and we can force a particular beam or section to deflect or extend in a desired fashion,” Fang says. “There is room to experiment with other materials, such as carbon nanotubes, which are stronger and lighter. Now we can have more fun in the lab exploring these different structures.”
Invisibility cloaks have less to do with magic than with metamaterials. These human-engineered materials have properties that don’t occur in nature, allowing them to bend and manipulate light in weird ways. For example, some of these materials can channel light around an object so that it appears invisible at a certain wavelength. These materials are also useful in applications such as smaller, faster, and more energy efficient optics, sensors, light sources, light detectors and telecommunications devices.
Now researchers have designed a new kind of metamaterial whose properties can be changed with a flick of a switch. In their proof-of-principle experiment, the researchers used germanium antimony telluride (GST) — the kind of phase-change material found in CDs and DVDs — to make an improved switchable metasurface that can block or transmit particular wavelengths of light at the command of light pulses. The researchers describe the metamaterial this week in Applied Physics Letters, from AIP Publishing, and how its ability to switch properties can be used in a range of sophisticated optical devices.
“Technologies based upon the control and manipulation of light are all around us and of fundamental importance to modern society,” said Kevin MacDonald, a researcher at the University of Southampton in the U.K. “Metamaterials are part of the process of finding new ways to use light and do new things with it — they are an enabling technology platform for 21st century optics.”
By dynamically controlling the optical properties of materials, you can modulate, select, or switch characteristics of light beams, such as intensity, phase, color and direction — an ability that’s essential to many existing and potential devices, he said.
Switchable metamaterials in general aren’t new. MacDonald and many others have made such materials before by combining metallic metamaterials with so-called active media such as GST, which can respond to external stimuli like heat, light or an electric field. In these hybrid materials, the metal component is structurally engineered at the nanometer scale to provide the desired optical properties. Incorporating the active medium provides a way to tune or switch those properties.
The problem is that metals tend to absorb light at visible and infrared wavelengths, making them unsuitable for many optical device applications. Melting points are also suppressed in nanostructured metals, making the metamaterials susceptible to damage from laser beams. In addition, a typical metal is gold, which isn’t compatible with the CMOS technology that’s ubiquitous in making today’s integrated devices.
In the new work, MacDonald and his colleagues at Southampton’s Optoelectronics Research Centre & Centre for Photonic Metamaterials have made a switchable metamaterial that doesn’t use metal at all. “What we’ve done now is structure the phase-change material itself,” MacDonald said. “We have created what is known as an all-dielectric metamaterial, with the added benefit of GST’s nonvolatile phase-switching behavior.”
Pulses of laser light can change the structure of GST between a random, amorphous one and a crystalline one. For GST, this behavior is nonvolatile, which means it will stay in a particular state until another pulse switches it back. In rewritable CDs and DVDs, this binary laser-driven switching is the basis for data storage.
The researchers created metamaterial grating patterns in an amorphous GST film only 300 nm thick, with lines 750 to 950 nanometers apart. This line spacing allows the surfaces to selectively block the transmission of light at near-infrared wavelengths (between 1300 and 1600 nm). But when a green laser converts the surfaces into a crystalline state, they become transparent at these wavelengths.
The research team is now working to make metamaterials that can switch back and forth over many cycles. They’re also planning increasingly complex structures to deliver more sophisticated optical functions. For example, this approach could be used to make switchable ultra-thin metasurface lenses and other flat, optical components.
Distance Wireless Charging Without Being Close to the Charging Base Enhanced by Magnetic Metamaterials
Universitat Autònoma de Barcelona researchers have developed a system which efficiently transfers electrical energy between two separate circuits. The system, made with a shell of metamaterials which concentrates the magnetic field, could transmit energy efficiently enough to charge mobile devices without having to place them close to the charging base.
The research was published in the journal Advanced Materials.
Wireless charging of mobile devices is possibly one of the most desired technological milestones. Some devices can already be charged wirelessly by placing the mobile device on top of a charging base. The next step, charging devices without the need of taking them out of one’s pocket, might be just around the corner.
A group of researchers from the Department of Physics of Universitat Autònoma de Barcelona has developed a system which can efficiently transfer electrical energy between two separated circuits thanks to the use of metamaterials. This system is still in the experimental stage, but once it has been perfected and can be applied to mobile devices, it will be able to charge them wirelessly and at a longer distance than currently possible.
Today’s wireless devices make use of induction to charge through a special case adapted to the device and a charging base connected to an electrical socket. When the device is placed on top of the base, this generates a magnetic field which induces an electric current inside the case and, without the need of using any cables, the device is charged. If the device is separated from the base, the energy is not transferred efficiently enough and the battery cannot be charged.
The system created by UAB researchers overcomes these limitations. It is made up of metamaterials which combine layers of ferromagnetic materials, such as iron compounds, and conductor materials such as copper. The metamaterials envelop the emitter and receiver and enable transferring energy between the two, at a distance and with unprecedented efficiency.
With the use of metamaterial crowns researchers were able in the lab to increase the transmission efficiency 35-fold, “and there is much more room for improvement, since theoretically the efficiency can be increased even more if conditions and the design of the experiment are perfected” explains Àlvar Sánchez, director of the research.
“Enveloping the two circuits with metamaterial shells has the same effect as bringing them close together; it’s as if the space between them literally disappears”, states Jordi Prat, lead author of the paper.
Moreover, the materials needed to construct these crowns such as copper and ferrite are easily available. The first experiments conducted with the aim of concentrating static magnetic fields required the use of superconductor metamaterials, unfeasible for everyday uses with mobile devices. “In contrast, low frequency electromagnetic waves – the ones used to transfer energy from one circuit to the other – only need conventional conductors and ferromagnets”, Carles Navau explains.
A group of UK researchers has discovered a new type of optical activity by breaking the symmetry of metamaterials with reflected light
Optical activity–rotation of the polarization of light–is well known to occur within materials that differ from their mirror image. But what happens if this symmetry is broken by the direction of illumination rather than the material itself?
Curiosity about this question has led to the discovery of a new type of optical activity. As a group of University of Southampton researchers report in Applied Physics Letters, from AIP Publishing, breaking the symmetry of metamaterials with reflected light will enable novel applications because it causes optical activity of unprecedented magnitude–far exceeding previously known specular or “mirror-like” optical activity.
At the heart of the group’s work are metamaterials–materials constructed with unique shapes and symmetries that generate properties which don’t occur in their natural counterparts.