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Into the Fold

Flat structures pop into 3-D forms, yielding miniature robots and tools

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5:23pm, November 29, 2012
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As a graduate student, roboticist Robert Wood became fascinated with the idea of developing a life-size, flying bee machine. Able to soar above trees or maneuver around obstacles, such a robot, Wood figured, could gather secret information for military missions or monitor hazardous environments without risking human lives.

Later, when he set up his own lab at Harvard University, Wood developed a system to cut materials into well-defined shapes and fold them into insect-sized parts. Creases and pleats gave the structures their three-dimensional forms.

Still, creating a tiny robot proved difficult. Because the structures were so small, the scientists had to use microscopes and tweezers to introduce a ridge or crimp an edge. “Assembly by folding gives you the ability to create all sorts of mechanisms and structures,” Wood says. “But the folding part was tremendously difficult and not very precise.”

One evening while reading to his young son, Wood had a eureka moment: Why not get the bees to fold themselves? Inspired by the intricate engineering found in pop-up books, where structures go from flat to 3-D with the turn of a page, he began investigating ways to get the pieces to swing into place on their own.

Wood’s group has since developed a number of prototypes, combining layers of rigid materials and electronics that, when prompted, self-fold into miniature aerial machines.

His approach, and others like it, may one day lead to all kinds of structures that are hard to create using current manufacturing methods. Applications for the research go far beyond flying machines. Scientists are now seeking ways to use self-folding to add surgical robots to the ends of catheters in order to peer into the darkest recesses of the human body, remove polyps or cauterize tumors. Self-folding also holds promise for delivering drugs and other therapies into the human body, says David Gracias, a chemical and biomolecular engineer at Johns Hopkins University, who outlined the idea in the March Trends in Biotechnology. In addition, antennas and solar cells that unfold automatically may even make their way onto satellites of the future.

Self-folding offers the potential to revolutionize many areas of medicine and electronics, Gracias says. But figuring out how to go about doing it is another thing. Most applications are years away from real-world use, awaiting solutions to several challenging obstacles.

For starters, scientists need to develop materials that can be rolled, folded or molded into various shapes. Another challenge is figuring out where and how to embed the instructions for folding. Ultimately, researchers will have to find ways to guide self-folded devices once they achieve their final form, for example powering the miniature machines so that they can fly on their own. A variety of approaches, many borrowed from the manufacture of printed circuit boards, are at the experimental stage.

Buckle up

A lot of the three-dimensional structures found in nature fold and unfold into well-defined forms. Leaves, for example, unfurl in a predictable pattern from a small, oval bud. Proteins, which start out as formless strings of amino acids, fold and unfold depending on molecular signals provided by their local environment.

Scientists and engineers are working to emulate this type of self-folding to create complex objects too tiny to be assembled by existing machines. The problem, Wood says, is that traditional manufacturing methods fall into two categories. Some methods are designed to assemble large-scale, three-dimensional objects such as cars or refrigerators part by part. Others very precisely etch shrunken components onto two-dimensional surfaces such as computer chips. Neither approach is adequate for assembling the fine-scaled three-dimensional features needed to make a flying robot the size of a bumblebee.

Before developing its “pop-up” technique, Wood’s team had to build each individual robobee manually, adding pieces one at a time. The process was time-consuming, requiring a high level of training and a steady hand. Even then, the results were uneven: Only a small percentage of the bees built in this painstaking fashion worked properly.

Self-folding provided a way to create diminutive structures, on scales of micrometers to centimeters, without having to do a lot of complex assembly, Wood says. By patterning flat structures and then getting them to curl or fold, scientists can produce 3-D forms with the precision of 2-D techniques. Wood’s fully functional robobees, for example, have a wingspan of about 3 centimeters — a little wider than the diameter of a U.S. quarter, and comparable to the wingspan of some bumblebees.

Material matters

While Wood’s robobee assembly relies on a scaffold to trigger the folding of rigid, hinged parts, other approaches demand more bendable materials. So scientists are also in search of substances that can be programmed to achieve a specific shape and then transform into another. Such materials might be used to create the ultimate reconfigurable robot — one that can turn into absolutely anything.

But many of the materials used in manufacturing today don’t easily bend or fold. Some, such as metals and ceramics, are downright brittle and thus prone to cracking. Scientists who want to take full advantage of self-folding’s potential will need to make these materials pliable, or create new ones with built-in flexibility.

That’s what Jennifer Lewis, a materials engineer at the University of Illinois at Urbana-Champaign, is doing. Drawing on a technique used in paper origami, in which artists moisten the paper to make it more malleable, she has found ways to create materials that can be handled and folded without cracking.

Lewis’ fabrication technique depends on direct-write assembly, in which a small nozzle deposits ink containing metal or ceramic particles directly onto a substrate. By mixing solvents with the ink, her team creates sheets that dry only partially. As long as the ink has some solvent, the material remains plasticlike; it can be folded, unfolded and refolded again and again.

Lewis says the approach, outlined in 2010 in Advanced Materials, can be modified to lengthen the material’s window of foldability. Solvents with a high boiling point remain stable at room temperature and barely evaporate at all. Such solvents might allow a material to retain its bendability for months or years.

To date, all of the structures created in Lewis’ lab have been folded by hand. Now the trick is to find ways to make the materials curl up on their own.

One way to go about that would be with “shape-memory” polymers and alloys, materials capable of undergoing phase transformations in response to stimuli such as heat. As the materials transition from one phase to another, they contract, which could drive a folding action.

Such materials were recently used by scientists at North Carolina State University in Raleigh to create self-folding, boxlike structures. A group led by Jan Genzer and Michael Dickey showed how sheets of stretched material called Shrinky Dinks could be programmed to fold into a desired shape with the simple addition of lines printed in black ink. When exposed to a high-heat lamp, the ink absorbed the heat, causing the sheet to shrink down along the lines to its original size. This shrinking created creases that triggered the folding.

A new wrinkle

Whether working with rigid or bendable materials, getting the final product right means making sure the angles of the folds are correct. That requires precision in positioning hinges or creases, Gracias says.

Gracias and his group are strategically placing tiny hinges on polymer films and other materials so that they fold into capsules, similar to the way viruses construct their shells. The goal is to make dust-sized devices, such as drug-carrying containers that can circulate through the bloodstream to deliver medicine only where needed. The researchers envision a hollow shell — a simplified geodesic dome of sorts — with a precisely patterned surface that is not only porous, but also capable of flexing to control the release of medication over long periods of time.

Last year, the group got a good start, creating a self-assembling 12-sided polyhedron with faces angled at 116.6 degrees.

To figure out what folding pattern would turn a flat sheet into a desired shape, Gracias joined efforts with mathematician Govind Menon of Brown University. Using a computer program, the scientists first “cut” a virtual version of the desired shape apart along its faces and flattened it to find all the possible 2-D arrangements, called “nets,” that might work. A single 3-D shape may have thousands of different nets that could re-create it when folded. After sifting through the possibilities, the scientists selected a handful to try, fabricating those versions in the lab. Through trial and error, the researchers found that the more compact nets — those with faces that linked up in more places — were better at self-folding. The compact nature forced a net to fold in a particular way, decreasing the chance of misfolding, Menon says.

To get the hinges to lift, the scientists deposited solder along the edges of each piece. When heated, the solder melted and balled up, causing the faces to fold upward. The edges pulled together to fuse the structure shut.

“That these structures fold is not all that interesting, but that they fold only into a specific shape with such precision is fascinating, because that is the meaning of self-assembly,” Gracias says. Details of the study were reported last year in the Proceedings of the National Academy of Sciences.

Gracias and Menon are now studying ways to generate more complicated structures. One effort aims to create self-assembling buckyball-type structures that contain 50 to 60 faces each, as opposed to 12.

“We’re beginning to uncover the rules of folding,” Gracias says.

Gracias’ group is also designing hinged structures that open and close, acting as microsurgery tools that, once swallowed or injected, could cut away damaged tissue or excise a tumor. In lab experiments, the researchers have used a microgripper guided by a magnet to retrieve animal cells placed in a glass tube. Recently, the team successfully performed a biopsy-like procedure with the microtool, gathering cells from the bile duct of a live pig. Results were reported online October 9 in Advanced Materials.

But hurdles, such as finding ways to include biocompatible materials in the self-folding structures, stand in the way of such devices becoming commonplace. Many of the manufacturing methods designed to create small structures were developed for the microelectronics industry, Gracias says, and simply aren’t suitable for working with biological materials.

Layers of complexity

While some groups figure out the best routes for folding by studying a single flattened structure, this approach doesn’t always deliver on a desired function.

“Having multiple layers gives us a huge range of design options,” Wood says. Wood’s robotic bee, for example, starts as a stack of 18 sheets of strong but lightweight material sandwiched together. This structure allows researchers to add or etch electrical components onto the layers while the device is flat, using the same techniques chip companies rely on to make circuit boards.

To date, most of the bee’s individual electronic components — such as vision and horizon-detection sensors that help the bees maintain stability while in flight — have been added manually. But the researchers are working on ways to integrate the components into the 2-D form by including “circuit layers.”

Such additions make placing the folds a more challenging question. Each of the 18 sheets — carbon fiber for the body, titanium and polymer films for wings, and plastic for the joints — is cut to a certain pattern by a laser and aligned appropriately in the stacked sandwich. As the structure pops into shape, flexor hinges in the design push out, allowing the bee’s wings and other tiny parts to join into a body. Later, the hinges allow the parts to bend and flex.

Figuring out where and how to place the more than 100 hinges that will create the pop-up bee continues to be grueling. Currently done by “paper and pen,” the process requires months of work. That’s because the hinges must be laid out through multiple layers of thickness. An incorrect arrangement might result in a configuration that cannot self-assemble or that causes the robot to freeze in action.

To tackle this challenge, Wood and his group have turned to Erik Demaine of the Massachusetts Institute of Technology. Demaine has long been fascinated by the mathematical problems that develop naturally in origami, and is an expert in a branch of science called origami mathematics. His studies include efforts to discern how the thickness of material — whether paper, polymer or metal — affects the ability to create different shapes by folding.

Wood is drawing upon such insights to develop a computer-based program that can help cut through the folding process. The new software will automate many aspects of the layout, showing where and how key mechanisms must be placed.

After developing shortcuts for making the intricate structures and electronics for their bee-sized robots, the scientists hope to tackle other challenges associated with the flying machines.

Currently, the prototype bees run on electricity transmitted through thin wiring from high-voltage amplifiers. Wood aims to add an onboard power source, such as a built-in battery. A thin battery could be included as an additional layer, or a micro–fuel cell could be glued to one of the existing layers during manufacturing.

Eventually, Wood says, he hopes to design a crew of fully functional robobees that act as a group, communicating and working together to explore places that are dangerous or hard to reach.

But it may be a decade or so before swarms of the bees are flying about. Before that time, other self-folding robots could find real-world applications.

Wood’s team is experimenting with a 20-legged centipede-inspired robot to perform search-and-rescue operations. And Gracias’ group is designing a wide array of self-folding devices — developing wireless, reconfigurable surgical tools that respond to light, pH, enzymes and temperature. Such abilities may allow the devices to reconfigure themselves in response to specific disease markers or conditions in the body.

One thing is certain: It will be fun to watch the developments unfold.

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