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A Small, Sharp Eye
Hans Zappe is developing an automatic, autonomous microcamera with chemical “muscles” which could be used in endoscopes and other applications
Trained muscle molecules – In the excellence cluster livMatS - Living, Adaptive and Energy-autonomous Materials Systems, Professor Hans Zappe is teaching particular chemical compounds to extend and contract in a certain direction. These liquid crystalline elastomers are called LCEs for short. They are shape-changing polymer networks – also called micromuscles. Zappe plans to use them to build autonomous, fully automatic microcameras, for example for use in endoscopes. The material will harvest energy and at the same time take over all the tasks that light sensors, control units and motors in conventional cameras have to perform.
Hans Zappe is head of the Gisela-und-Erwin-Sick Professorship for Microoptics at the Institute for Microsystems Technology (IMTEK) at the University of Freiburg. Artificial eyes the size of a matchhead is what he has in mind. “The system should be able to adapt and focus automatically to changing light conditions,” he explains. The aperture and focal length have to adjust themselves, just as they do in modern cameras. This requires light sensors, control units, motors and energy. “We integrate all these capabilities into the material,” says Zappe. For the livMatS project, his team is using LCEs which can contract like tiny muscles.
No opposition necessary
In the human eye, muscles control how much light the iris admits. “It opens or closes depending on the brightness,” says Zappe. As we know from high school biology, if conditions are bright, a muscle constricts the ring-shaped iris and its central opening. When it gets darker, the opposing, fan-shaped muscle, pulls the iris back. The mechanical aperture diaphragms of conventional cameras also function on that principle. But for Zappe’s purposes, they are far too large. “We're planning a greatly miniaturized system,” he says. His LCE iris is intended to become a part of tiny, autonomous camera systems, to be used wherever space is at a premium - in robots, satellites and particularly in endoscopes. These devices allow doctors to look inside very small spaces in the body. Such medical “eyes” need to be tiny.
Zappe's team uses LCE molecules to reconstruct the iris in miniature. This can be done with a chemical micro-muscular system alone. LCE do not need opposing “muscles” - unlike real muscles, they can actively contract and stretch themselves. This happens when LCEs change their state. In one case, the liquid crystal molecule chains appear to be arrayed chaotically in all directions; in the other, they all point one way in an orderly fashion. LCEs shorten when their state changes from chaotic to well-ordered. Conversely, from ordered to chaotic, they stretch again to their full extent. Zappe explains that “their length changes by up to 50 percent when their state changes.”
Shrinking in the right direction
The transition is caused by energy, for example in the form of heat. It can also be used to direct how the LCEs are aligned. “This allows us to determine the direction of their shrinkage and stretching,” explains Zappe. His doctoral student, Yannick Folwill, demonstrates this on glass slides. Triangular and star-shaped fields can be seen on them through a polarizing filter - uniform structures due to liquid crystals in their aligned form. Folwill has coated one side of the slides with a thin layer of LCEs and has given them different heat treatments. He can adjust the direction of the liquid crystals to an accuracy of almost a hundredth of a millimeter.
In the iris-style aperture Zappe is planning, the LCEs will be arranged radially out from a central point. Heating or cooling will then narrow or widen the opening at its center. “This gives us a variable hole based on the iris principle,” the microsystems engineer says. Thermally, this works wonderfully. But for many applications, other energy sources such as light would be better suited. In the simplest case, according to Zappe, the liquid crystals of the LCEs would only need to absorb light and generate heat from it. “That would be exactly the same as thermal activation.”
But often the devil in in the details. Another Zappe doctoral student, Jasleen Kaur Lall, knows that all too well. She coats both sides of small pieces of brown plastic foil with light-sensitive LCEs. The intention is that, when exposed to light, the liquid crystals on one side should contract while the LCEs on the other side should stretch. Their medium, i.e. the brown pieces of film, would then bend in the desired direction. Kaur Lall has a small bowl containing bent plastic pieces. They are curved: U-shaped, S-shaped, slightly twisted. “Unfortunately the LCEs move unusually here,” Zappe says, “we are not yet able to control their direction perfectly.” But Kaur Lall is working on getting the liquid crystals to bend into the right shapes.
Prototype in two years
“We’re pioneers,” Zappe enthuses. “Our experiments on putting energy other than heat into LCEs place us at the cutting edge of research.” The manufacturing technology, the physics and chemistry of liquid crystals are difficult, says Zappe. Professor Jürgen Rühe's research group at the IMTEK is responsible for the latter. “He is in charge of the entire chemical side of things,” says Zappe, “so we can already adjust various properties of the systems.” Zappe's research group focuses on LCE structures and components. “New challenges are constantly emerging in the micro field,” he says. Yet he hopes to be able to bring out a prototype in two years at the latest.
Zappe's second cooperative project in the excellence cluster is developing a self-aligning solar cell. “It always turns to where the light is coming from. In other projects, his team is researching other adaptive optical systems, for example for liquids. Zappe refers to the microcamera with the LCE eye as a “secondary activity.” But he combines two wishes - "We want to be able to demonstrate for the first time an energy-autonomous, adaptive, artificial micromuscular system - without wires, controllers and control elements. Under constantly changing light conditions, the microcamera should deliver constantly sharp images. Wish number two is a concrete application. “We have many projects with the Freiburg Medical Center. I would be very interested in building a completely autonomous microcamera for endoscopes which constantly adapts to the changing conditions in the body and focuses automatically.”
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