3D printing has established itself as a technology with countless fields of application. Direct laser writing is considered a particularly promising method: A computer-controlled focused laser beam acts as a pen and creates the desired structure in the printer ink, which here is a photoresist. In this way, any three-dimensional form down to a size of a few micrometers can be created. “However, for many applications, particularly in biology and biomedicine, it would be desirable not only to produce rigid structures but also active systems that are still movable after the printing process, e.g., that can change their shapes by an external signal,” emphasizes Professor Martin Bastmeyer from KIT’s Zoological Institute and Institute of Functional Interfaces. In cooperation with Professor Martin Wegener’s group at Institute of Applied Physics and Institute of Nanotechnology at KIT as well as chemists from Karlsruhe and Heidelberg, a printing process for such movable structures has now been developed. Special materials, i.e., stimulus-responsive polymers whose properties can be modified by external signals, are used for the printer ink. The chemical compound poly(N-isopropysycraymide) changes its shape considerably when the temperature is raised only slightly above room temperature. The 3D structures produced in this way are functional in aqueous environments and thus ideal for applications in biology and biomedicine.

“We have developed the method to such an extent that we can also manufacture complex structures in which, as a result of external stimulation, the moving parts do not all react in the same way, but show different but precisely defined reactions,” explains Marc Hippler, first author of the study. This is made possible by grayscale lithography, where the photoresist is not exposed with the same intensity at all points, but is exposed in a graded manner. This allows the desired material properties, and thus the strength of the movement at a certain temperature change, to be set very precisely. With computer simulations, the resulting movements can be precisely predicted and therefore allow a rational design of complex 3D structures.

The working groups around Martin Bastmeyer and Martin Wegener have gone one step further: Instead of temperature, focused light is used as a control signal. For the first time, this allows the targeted control of individual microstructures in a complex, three-dimensional arrangement. This feature could be used, for example, in microfluidic systems. Since the photoresist used can be switched at room temperature, there are additional applications in basic biological research, e.g., the targeted mechanical manipulation of individual cells.

The interdisciplinary study was carried out as part of the “3D Matter Made to Order” excellence cluster, a joint research association of Karlsruhe Institute of Technology and Heidelberg University. PhD students from the Karlsruhe School of Optics & Photonics (KSOP) of KIT were involved also.

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Materials provided by Karlsruher Institut für Technologie (KIT). Note: Content may be edited for style and length.

CAPER-2, short for Cusp Alfvén and Plasma Electrodynamics Rocket-2, is a sounding rocket mission — a type of spacecraft that carries scientific instruments on short, targeted trips to space before falling back to Earth. In addition to their relatively low price tags and quick development time, sounding rockets are ideally suited for launching into transient events — like the sudden formation of the aurora borealis, or northern lights.

For CAPER-2 scientists, flying through an aurora provides a peek into a process as fundamental as it is complex: How do particles get accelerated throughout space? NASA studies this phenomenon in an effort to better understand not only the space environment surrounding Earth — and thus protect our technology in space from radiation — but also to help understand the very nature of stars and atmospheres throughout the solar system and beyond.

“Throughout the universe you have charged particles getting accelerated — in the Sun’s atmosphere, in the solar wind, in the atmospheres of other planets, and in astrophysical objects,” said Jim LaBelle, space physicist at Dartmouth College in Hanover, New Hampshire, and principal investigator for the CAPER-2 mission. “An aurora presents us with a local laboratory where we can observe these acceleration processes close at hand.”

Technically, the CAPER-2 team is interested in what happens just before an aurora starts glowing. Electrons, pouring into our atmosphere from space, collide with atmospheric gases and trigger the aurora’s glow. Somehow, they pick up speed along the way.

“By the time they crash into our atmosphere, these electrons are traveling over 10 times faster than they were before,” said Doug Rowland, space physicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who also studies particle acceleration. “We still don’t understand the fundamental physics of how that happens.”

The CAPER-2 team focused on a special kind of aurora that forms during the day. Unlike the nighttime aurora, the daytime aurora is triggered by electrons that stream in directly from the Sun — and we know far less about them.

“There’s been a huge amount of research done on the regular nighttime aurora, but the daytime aurora is much less studied,” said Craig Kletzing, space physicist at the University of Iowa in Iowa City and coinvestigator for the mission. “There are good indications that there are some similarities and there are also some differences.”

The team is focusing on how the electrons that create daytime auroras are jostled around by waves, in ways that may or may not differ from nighttime auroras. Two kinds of waves are of special interest, and have opposite effects. Alfvén waves, named after Swedish Nobel laureate Hannes Alfvén who first predicted their existence in 1942, are thought to accelerate the electrons. These huge waves — measuring tens to hundreds of miles long from peak to peak — propagate along Earth’s magnetic field lines, whipping electrons to and fro.

On the other side are Langmuir waves, which are generated by the electrons themselves — a process that steals some of the electrons’ energy and slows them down. CAPER-2 will carry a high-resolution wave-particle correlator to measure them, the first sounding rocket mission to do so for the daytime aurora.

“This is very data-intensive,” said LaBelle. “It’s unique to sounding rockets to be able to look at this mechanism in this level of detail.”

For the launch, the CAPER-2 team traveled to northern Norway, one of the few places that can put a rocket within range of the daytime aurora. Every day, northern Norway rotates under an opening in Earth’s magnetic field known as the northern polar cusp, where particles from the Sun can funnel into our upper atmosphere.

Meeting the aurora right where they form is the best way to understand physical processes that are far too large to replicate in a lab.

“It’s a kind of natural laboratory,” LaBelle added. “We take our experiment to two different environments, where the variables are different, and then test the theory and answer the questions.”

The buckyball, formally known as buckminsterfullerene, is extremely complex. Due to its enormous 60-atom size, the overall molecule has a staggeringly high number of ways to vibrate — at least 100,000,000,000,000,000,000,000,000 vibrational quantum states when the molecule is warm. That’s in addition to the many different energy states for the buckyball’s rotation and other properties.

As described in the January 4 issue of Science, the JILA team used an updated version of their frequency comb spectroscopy and cryogenic buffer gas cooling system to observe isolated, individual energy transitions among rotational and vibrational states in cold, gaseous buckyballs. This is the first time anyone has been able to prepare buckyballs in this form to analyze its rotations and vibrations at the quantum level.

JILA is jointly operated by the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder.

Buckyballs, first discovered in 1984, have created great scientific excitement. But high-resolution spectroscopy, which can reveal the details of the molecule’s rotational and vibrational properties, didn’t work at ordinary room temperatures because the signals were too congested, NIST/JILA Fellow Jun Ye said. Low temperatures (about -138 degrees Celsius, which is -216 degrees Fahrenheit) enabled researchers to concentrate the molecules into a single rotational-vibrational quantum state at the lowest energy level and probe them with high-resolution spectroscopy.

The buckyball is the most symmetric molecule known, with a soccer-ball-like shape known as a modified icosahedron. It is small enough to be fully understood with basic quantum mechanics principles. Yet it is large enough to reveal insights into the extreme quantum complexity that emerges in huge systems.

As an example of practical applications, buckyballs could act as a pristine network of 60 atoms. The core of each atom possesses an identical property known as “nuclear spin,” which enables it to interact magnetically with its environment. Therefore, each spin could act as a magnetically controlled quantum bit or “qubit” in a quantum computer.

“If we had a buckyball made of pure isotopic carbon-13, each atom would have a nuclear spin of 1/2, and each buckyball could serve as a 60-qubit quantum computer,” Ye said. “Of course, we don’t have such capabilities yet; we would need to first capture these buckyballs in traps.”

A key part of the new quantum revolution, a quantum computer using qubits made of atoms or other materials could potentially solve important problems that are intractable using today’s machines. NIST has a major stake in quantum science.

“There are also a lot of astrophysics connections,” Ye continued. “There are abundant buckyball signals coming from remote carbon stars,” so the new data will enable scientists to better understand the universe.

After they measured the quantum energy levels, the JILA researchers collected statistics on buckyballs’ nuclear spin values. They confirmed that all 60 atoms were indistinguishable, or virtually identical. Precise measurements of the buckyball’s transition energies between individual quantum states revealed its atoms interacted strongly with one another, providing insights into the complexities of its molecular structure and the forces between atoms.

For the experiments, an oven converted a solid sample of material into gaseous buckyballs. These hot molecules flowed into a cell (container) anchored to a cryogenic cold apparatus, such that the molecules were cooled by collisions with cold argon gas atoms. Then laser light at precise frequencies was aimed at the cold gas molecules, and researchers measured how much light was absorbed. The observed structure in the infrared spectrum encoded details of the quantum-mechanical energy-level structure.

The laser light was produced by an optical frequency comb, or “ruler of light,” and aimed into an optical cavity surrounding the cold cell to enhance the absorption signals. The comb contained about 1000 “teeth” at optical frequencies spanning the full band of buckyball vibrations. The comb light was generated from a single fiber laser.