From The Swiss Federal Institute of Technology in Zürich [ETH Zürich] [Eidgenössische Technische Hochschule Zürich] (CH): “Stable in all kinds of shapes”

From The Swiss Federal Institute of Technology in Zürich [ETH Zürich] [Eidgenössische Technische Hochschule Zürich] (CH)

Felix Würsten

ETH Zürich researchers have developed a structure that can switch between stable shapes as needed while being remarkably simple to produce. The key lies in a clever combination of base materials

Lightweight, easy to produce, flexibly expandable and reshaped as required – these are the properties of the multi-​stable structure that Giada Risso has developed as part of her doctoral project. (Photograph: G. Risso / ETH Zürich)

For a great many years, researchers have been trying to create structures that can assume different stable shapes as required. The goal of creating these multi-​stable structures, as they are known, is to build three-​dimensional objects that can switch between shapes again and again as needed. This would pave the way for realizing, say, adaptable elements or large objects that can change shape and take less space during transportation.

But the breakthrough has been a long time in coming. This is because previous solutions were either very complex to produce, could be reshaped only once, or required a continuous supply of energy to maintain their new shape.

A remarkably simple solution

Giada Risso, a doctoral student in the Composite Materials and Adaptive Structures Group led by Paolo Ermanni, recently presented a new approach that overcomes these drawbacks in an article in the journal Advanced Science [below]. “One of my main goals was to develop a flat, multi-​stable structure that would be easy to manufacture,” she explains. And the solution is remarkably simple: it involves sticking a flat composite frame onto a pre-​stretched, soft, thermoplastic film of polyurethane. “A flat surface and a clamp to pre-​stretch the film – that’s essentially all that’s needed,” Risso explains.

Holding a structure made this way in your hands, you can bend it from its original flat state into a shape that it will retain without any further assistance. You can then change its shape again and the structure will once again hold this new shape all by itself. Then you can restore the original shape in an equally simple maneuver.

A frame of carbon fibres

But how exactly is it possible to reshape this structure so flexibly into different stable states? Risso discovered that it all hinges on which material you select for the frame: “Our best results have been with a composite material made from carbon fibres. This allows us to produce a structure that can actually take on multiple stable states.” Making a frame out of glass fibres, however, results in far fewer stable shapes. Of all the frame materials tested, steel performed the worst, failing to produce a single other stable state.

The basic element is made by sticking a carbon-​fibre frame onto a pre-​stretched film of polyurethane. Each individual square can take on multiple stable shapes. (Photograph: G. Risso / ETH Zürich)

In her paper, Risso describes the theory behind why the various materials lead to such different results. “Carbon fibres are highly anisotropic, which means they have very different properties along different axes. In other words, they will display differing degrees of rigidity depending on the direction you bend them. It’s this anisotropy that is fundamental in creating a multi-​stable shape.” Unlike carbon fibres, steel is isotropic, which is why it is unsuitable for creating multi-​stable shapes.

Modeling caterpillars

The fundamental component of the new structure is a square element, which can be augmented with other square elements as desired. Since each individual square can take on a variety of stable states, combining them results in a vast number of possible shapes.

The shape of the multi-​stable structure can be changed as needed – and back again. (Photograph: G. Risso / ETH Zürich)

Risso’s next step was to equip a periodic structure comprising 16 squares with what are known as pneumatic actuators. These work in a similar way to a “one-​sided” balloon, expanding only on one side when air is fed in. Forcing air into selected actuators bends the structure to create the desired shape. Through a series of experiments, Risso was able to show that this can recreate the undulating movements of a caterpillar.

Stabel in all kinds of shapes. (Video credit: G. Risso / ETH Zürich)

Risso believes that there are many uses for such structures, such as in the manufacture of reconfigurable building facades, and robots. But she says that the biggest appeal is to the space industry: “This industry is already using lightweight composite materials and relies on having compact materials that are easily adapted.” The new approach could be used to build antennas or solar panels that can be unfolded and configured after arriving in space.

Endless variety

What’s more, the principle can be applied to more than square base elements. In a different paper, Risso proved that it also works with any other polygon. This massively expands the range of potential applications. “Who knows, perhaps we will be using these structures to build cube-​shaped figures that transform into exotic three-​dimensional structures in the blink of an eye,” she says with a smile.

With its myriad of possibilities, there’s no denying that this new concept fires the imagination. “I will not be able to exhaust all the possibilities because I now have to focus on finishing my doctoral studies,” Risso says. She intends to use the remaining time to resolve a couple more research questions, for instance drawing on her background knowledge in applied mathematics to determine how stable a stable state actually is. Another crucial topic she wants to explore in greater detail is the speed at which the structures change shape. “In many applications, it’s important that the shape does not change too abruptly, but rather moves from one state to the next in a controlled way,” she says. “This is why we’re also investigating how to better control the reshaping process.”

And, finally, there is also the question of scale: “We don’t yet know how small we’ll be able to make the individual elements. If we can reduce the size of these elements to within the millimetre range, I could imagine they might be useful for medical applications,” Risso says. “But that kind of thing is still a long way off.”

Science paper:
Advanced Science
See the science paper for detailed material with images.

See the full article here .


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The Swiss Federal Institute of Technology in Zürich [ETH Zürich] [Eidgenössische Technische Hochschule Zürich] (CH) is a public research university in the city of Zürich, Switzerland. Founded by the Swiss Federal Government in 1854 with the stated mission to educate engineers and scientists, the school focuses exclusively on science, technology, engineering and mathematics. Like its sister institution The Swiss Federal Institute of Technology in Lausanne [EPFL-École Polytechnique Fédérale de Lausanne](CH) , it is part of The Swiss Federal Institutes of Technology Domain (ETH Domain)) , part of the The Swiss Federal Department of Economic Affairs, Education and Research [EAER][Eidgenössisches Departement für Wirtschaft, Bildung und Forschung] [Département fédéral de l’économie, de la formation et de la recherche] (CH).

The university is an attractive destination for international students thanks to low tuition fees of 809 CHF per semester, PhD and graduate salaries that are amongst the world’s highest, and a world-class reputation in academia and industry. There are currently 22,200 students from over 120 countries, of which 4,180 are pursuing doctoral degrees. In the 2021 edition of the QS World University Rankings ETH Zürich is ranked 6th in the world and 8th by the Times Higher Education World Rankings 2020. In the 2020 QS World University Rankings by subject it is ranked 4th in the world for engineering and technology (2nd in Europe) and 1st for earth & marine science.

As of November 2019, 21 Nobel laureates, 2 Fields Medalists, 2 Pritzker Prize winners, and 1 Turing Award winner have been affiliated with the Institute, including Albert Einstein. Other notable alumni include John von Neumann and Santiago Calatrava. It is a founding member of the IDEA League and the International Alliance of Research Universities (IARU) and a member of the CESAER network.

ETH Zürich was founded on 7 February 1854 by the Swiss Confederation and began giving its first lectures on 16 October 1855 as a polytechnic institute (eidgenössische polytechnische schule) at various sites throughout the city of Zurich. It was initially composed of six faculties: architecture, civil engineering, mechanical engineering, chemistry, forestry, and an integrated department for the fields of mathematics, natural sciences, literature, and social and political sciences.

It is locally still known as Polytechnikum, or simply as Poly, derived from the original name eidgenössische polytechnische schule, which translates to “federal polytechnic school”.

ETH Zürich is a federal institute (i.e., under direct administration by the Swiss government), whereas The University of Zürich [Universität Zürich ] (CH) is a cantonal institution. The decision for a new federal university was heavily disputed at the time; the liberals pressed for a “federal university”, while the conservative forces wanted all universities to remain under cantonal control, worried that the liberals would gain more political power than they already had. In the beginning, both universities were co-located in the buildings of the University of Zürich.

From 1905 to 1908, under the presidency of Jérôme Franel, the course program of ETH Zürich was restructured to that of a real university and ETH Zürich was granted the right to award doctorates. In 1909 the first doctorates were awarded. In 1911, it was given its current name, Eidgenössische Technische Hochschule. In 1924, another reorganization structured the university in 12 departments. However, it now has 16 departments.

ETH Zürich, EPFL (Swiss Federal Institute of Technology in Lausanne) [École polytechnique fédérale de Lausanne](CH), and four associated research institutes form The Domain of the Swiss Federal Institutes of Technology (ETH Domain) [ETH-Bereich; Domaine des Écoles polytechniques fédérales] (CH) with the aim of collaborating on scientific projects.

Reputation and ranking

ETH Zürich is ranked among the top universities in the world. Typically, popular rankings place the institution as the best university in continental Europe and ETH Zürich is consistently ranked among the top 1-5 universities in Europe, and among the top 3-10 best universities of the world.

Historically, ETH Zürich has achieved its reputation particularly in the fields of chemistry, mathematics and physics. There are 32 Nobel laureates who are associated with ETH Zürich, the most recent of whom is Richard F. Heck, awarded the Nobel Prize in chemistry in 2010. Albert Einstein is perhaps its most famous alumnus.

In 2018, the QS World University Rankings placed ETH Zürich at 7th overall in the world. In 2015, ETH Zürich was ranked 5th in the world in Engineering, Science and Technology, just behind the Massachusetts Institute of Technology, Stanford University and University of Cambridge (UK). In 2015, ETH Zürich also ranked 6th in the world in Natural Sciences, and in 2016 ranked 1st in the world for Earth & Marine Sciences for the second consecutive year.

In 2016, Times Higher Education World University Rankings ranked ETH Zürich 9th overall in the world and 8th in the world in the field of Engineering & Technology, just behind the Massachusetts Institute of Technology, Stanford University, California Institute of Technology, Princeton University, University of Cambridge(UK), Imperial College London(UK) and University of Oxford(UK) .

In a comparison of Swiss universities by swissUP Ranking and in rankings published by CHE comparing the universities of German-speaking countries, ETH Zürich traditionally is ranked first in natural sciences, computer science and engineering sciences.

In the survey CHE Excellence Ranking on the quality of Western European graduate school programs in the fields of biology, chemistry, physics and mathematics, ETH Zürich was assessed as one of the three institutions to have excellent programs in all the considered fields, the other two being Imperial College London (UK) and the University of Cambridge (UK), respectively.