The finding could be particularly important to the study and treatment of diseases such as cystic fibrosis, in which the mucus that lines the lungs becomes more concentrated, often causing bacterial infections that take hold in that mucus to become life threatening. This discovery could also have implications in studies of polymer-secreting conglomerations of bacteria known as biofilms — the slippery goo on river rocks, for example — and in industrial applications where they can cause equipment malfunctions and health hazards.

The work is described in a paper published on January 17 in the journal Science Advances.

“We’ve discovered that when many bacteria grow in fluids containing spaghetti-like molecules called polymers, such as mucus in the lungs, they form cable-like structures that intertwine like living gels,” says Sujit Datta, a professor of chemical engineering, bioengineering, and biophysics at Caltech and corresponding author of the new paper. “And, interestingly, there are similarities between the physics of how these structures form and the microscopic physics underlying many nonliving gels, like Purell or Jell-O.”

Datta recently moved to Caltech from Princeton University. One of his graduate students at Princeton, Sebastian Gonzalez La Corte, is lead author of the paper. He and Datta had been interested in how mucus concentration changes in the lungs and guts of cystic fibrosis patients — in whom more polymers than usual are present. Working with mucus samples provided by colleagues at MIT, Gonzalez La Corte grew E. coli bacteria (commonly used in laboratory studies) in regular liquid and in cystic fibrosis-like samples and then observed the specimens under a microscope to watch how the bacterial cells grew in each case.

He focused on cells that had lost the ability to swim, as is the case for many bacteria in nature. Under normal circumstances, when such a cell divides into two, the resulting cells separate and diffuse away from each other. However, Gonzalez La Corte found that in a polymeric solution, the copied cells remained stuck to each other, end to end.

“As cells continue to divide and stick to each other, they start to form these beautiful long structures that we call cables,” Gonzalez La Corte says. “At some point, they actually bend and fold on each other and form an entangled network.”

The team found that the cables continue to elongate and grow as long as the cells have the nutrients they need, eventually creating chains that are thousands of cells long.

Subsequent experiments showed that it does not seem to matter which bacterial species are introduced, nor does the type of organic polymer solution make a difference; once enough polymer surrounds the bacterial cells, the cables grow. The researchers even saw the same result with bacteria in synthetic polymers.

Although the initial motivation for the study was to better understand the growth of infections in patients with cystic fibrosis, the findings are more broadly relevant. Mucus plays an important role in the human body, not only in the lungs but also in the gut and in the cervicovaginal tract. And Datta says the work is also important in the context of biofilms, groupings of bacteria that grow an encapsulating polymer matrix of their own. There are biofilms in the human body, such as dental plaque, but they are also extremely common in soil and in industrial settings, where they can damage equipment and cause health hazards.

“That polymer matrix that they’ve secreted is what makes biofilms so tough to remove from surfaces and treat with antibiotics,” Datta says. “Understanding how cells grow in that matrix could be key to discovering how to better control biofilms.”

Understanding the Physics Behind the Cables

Through carefully designed experiments, the team found that the external pressure exerted by the polymers surrounding the dividing cells is what forces the cells together and holds them in place. In physics, such an attractive force that is under the control of an outside pressure is called a depletion interaction. Gonzalez La Corte used the theory of depletion interaction to create a theoretical model of bacterial cable growth. The model can predict when a cable will survive and grow in a polymeric environment.

“Now we can actually use established theories from polymer physics, which were developed for completely different things, in these biological systems to quantitatively predict when these cables will arise,” Datta says.

Why Do the Bacteria Form These Cables?

“We discovered this interesting, unusual, very unexpected phenomenon,” Datta says. “We can also explain why it happens from a mechanistic, physics perspective. Now the question is: What are the biological implications?”

Interestingly, there are two possibilities: The bacteria could be clumping together to form this network of living gel in an effort to make themselves larger and therefore more difficult for immune cells to engulf and destroy. Alternately, cable formation could actually be harmful to the bacteria. After all, the secretions from the host cause the bacteria to build the cables. “Mucus isn’t static; for example, in the lungs, it’s being constantly swept up by little hairs on the surface of the lungs and propelled upward,” Datta says. “Could it be that when bacteria are all clumped together in these cables, it’s actually easier to get rid of them — to expel them out of the body?”

For now, no one knows which possibility is correct, and Datta says that is what makes this project remain interesting. “Now that we have found this phenomenon, we can frame these new questions and design further experiments to test our suspicions,” he says.

“Most people don’t realize how often they rely on touch instead of vision — typing, walking, picking up a flimsy cup of water,” said Charles Greenspon, PhD, a neuroscientist at the University of Chicago. “If you can’t feel, you have to constantly watch your hand while doing anything, and you still risk spilling, crushing or dropping objects.”

Greenspon and his research collaborators recently published papers in Nature Biomedical Engineering and Science documenting major progress on a technology designed to address precisely this problem: direct, carefully timed electrical stimulation of the brain that can recreate tactile feedback to give nuanced “feeling” to prosthetic hands.

The science of restoring sensation

These new studies build on years of collaboration among scientists and engineers at UChicago, the University of Pittsburgh, Northwestern University, Case Western Reserve University and Blackrock Neurotech. Together they are designing, building, implementing and refining brain-computer interfaces (BCIs) and robotic prosthetic arms aimed at restoring both motor control and sensation in people who have lost significant limb function.

On the UChicago side, the research was led by neuroscientist Sliman Bensmaia, PhD, until his unexpected passing in 2023.

The researchers’ approach to prosthetic sensation involves placing tiny electrode arrays in the parts of the brain responsible for moving and feeling the hand. On one side, a participant can move a robotic arm by simply thinking about movement, and on the other side, sensors on that robotic limb can trigger pulses of electrical activity called intracortical microstimulation (ICMS) in the part of the brain dedicated to touch.

For about a decade, Greenspon explained, this stimulation of the touch center could only provide a simple sense of contact in different places on the hand.

“We could evoke the feeling that you were touching something, but it was mostly just an on/off signal, and often it was pretty weak and difficult to tell where on the hand contact occurred,” he said.

The newly published results mark important milestones in moving past these limitations.

Advancing understanding of artificial touch

In the first study, published in Nature Biomedical Engineering, Greenspon and his colleagues focused on ensuring that electrically evoked touch sensations are stable, accurately localized and strong enough to be useful for everyday tasks.

By delivering short pulses to individual electrodes in participants’ touch centers and having them report where and how strongly they felt each sensation, the researchers created detailed “maps” of brain areas that corresponded to specific parts of the hand. The testing revealed that when two closely spaced electrodes are stimulated together, participants feel a stronger, clearer touch, which can improve their ability to locate and gauge pressure on the correct part of the hand.

The researchers also conducted exhaustive tests to confirm that the same electrode consistently creates a sensation corresponding to a specific location.

“If I stimulate an electrode on day one and a participant feels it on their thumb, we can test that same electrode on day 100, day 1,000, even many years later, and they still feel it in roughly the same spot,” said Greenspon, who was the lead author on this paper.

From a practical standpoint, any clinical device would need to be stable enough for a patient to rely on it in everyday life. An electrode that continually shifts its “touch location” or produces inconsistent sensations would be frustrating and require frequent recalibration. By contrast, the long-term consistency this study revealed could allow prosthetic users to develop confidence in their motor control and sense of touch, much as they would in their natural limbs.

Adding feelings of movement and shapes

The complementary Science paper went a step further to make artificial touch even more immersive and intuitive. The project was led by first author Giacomo Valle, PhD, a former postdoctoral fellow at UChicago who is now continuing his bionics research at Chalmers University of Technology in Sweden.

“Two electrodes next to each other in the brain don’t create sensations that ’tile’ the hand in neat little patches with one-to-one correspondence; instead, the sensory locations overlap,” explained Greenspon, who shared senior authorship of this paper with Bensmaia.

The researchers decided to test whether they could use this overlapping nature to create sensations that could let users feel the boundaries of an object or the motion of something sliding along their skin. After identifying pairs or clusters of electrodes whose “touch zones” overlapped, the scientists activated them in carefully orchestrated patterns to generate sensations that progressed across the sensory map.

Participants described feeling a gentle gliding touch passing smoothly over their fingers, despite the stimulus being delivered in small, discrete steps. The scientists attribute this result to the brain’s remarkable ability to stitch together sensory inputs and interpret them as coherent, moving experiences by “filling in” gaps in perception.

The approach of sequentially activating electrodes also significantly improved participants’ ability to distinguish complex tactile shapes and respond to changes in the objects they touched. They could sometimes identify letters of the alphabet electrically “traced” on their fingertips, and they could use a bionic arm to steady a steering wheel when it began to slip through the hand.

These advancements help move bionic feedback closer to the precise, complex, adaptive abilities of natural touch, paving the way for prosthetics that enable confident handling of everyday objects and responses to shifting stimuli.

The future of neuroprosthetics

The researchers hope that as electrode designs and surgical methods continue to improve, the coverage across the hand will become even finer, enabling more lifelike feedback.

“We hope to integrate the results of these two studies into our robotics systems, where we have already shown that even simple stimulation strategies can improve people’s abilities to control robotic arms with their brains,” said co-author Robert Gaunt, PhD, associate professor of physical medicine and rehabilitation and lead of the stimulation work at the University of Pittsburgh.

Greenspon emphasized that the motivation behind this work is to enhance independence and quality of life for people living with limb loss or paralysis.

“We all care about the people in our lives who get injured and lose the use of a limb — this research is for them,” he said. “This is how we restore touch to people. It’s the forefront of restorative neurotechnology, and we’re working to expand the approach to other regions of the brain.”

The approach also holds promise for people with other types of sensory loss. In fact, the group has also collaborated with surgeons and obstetricians at UChicago on the Bionic Breast Project, which aims to produce an implantable device that can restore the sense of touch after mastectomy.

Although many challenges remain, these latest studies offer evidence that the path to restoring touch is becoming clearer. With each new set of findings, researchers come closer to a future in which a prosthetic body part is not just a functional tool, but a way to experience the world.

Resembling the interlocking links in chainmail, the nanoscale material exhibits exceptional flexibility and strength. With further work, it holds promise for use in high-performance, light-weight body armor and other uses that demand lightweight, flexible and tough materials.

Publishing on Friday (Jan. 17) in the journal Science, the study marks several firsts for the field. Not only is it the first 2D mechanically interlocked polymer, but the novel material also contains 100 trillion mechanical bonds per 1 square centimeter — the highest density of mechanical bonds ever achieved. The researchers produced this material using a new, highly efficient and scalable polymerization process.

“We made a completely new polymer structure,” said Northwestern’s William Dichtel, the study’s corresponding author. “It’s similar to chainmail in that it cannot easily rip because each of the mechanical bonds has a bit of freedom to slide around. If you pull it, it can dissipate the applied force in multiple directions. And if you want to rip it apart, you would have to break it in many, many different places. We are continuing to explore its properties and will probably be studying it for years.”

Dichtel is the Robert L. Letsinger Professor of Chemistry at the Weinberg College of Arts and Sciences and a member of the International Institute of Nanotechnology (IIN) and the Paula M. Trienens Institute for Sustainability and Energy. Madison Bardot, a Ph.D. candidate in Dichtel’s laboratory and IIN Ryan Fellow, is the study’s first author.

Inventing a new process

For years, researchers have attempted to develop mechanically interlocked molecules with polymers but found it near impossible to coax polymers to form mechanical bonds.

To overcome this challenge, Dichtel’s team took a whole new approach. They started with X-shaped monomers — which are the building blocks of polymers — and arranged them into a specific, highly ordered crystalline structure. Then, they reacted these crystals with another molecule to create bonds between the molecules within the crystal.

“I give a lot of credit to Madison because she came up with this concept for forming the mechanically interlocked polymer,” Dichtel said. “It was a high-risk, high-reward idea where we had to question our assumptions about what types of reactions are possible in molecular crystals.”

The resulting crystals comprise layers and layers of 2D interlocked polymer sheets. Within the polymer sheets, the ends of the X-shaped monomers are bonded to the ends of other X-shaped monomers. Then, more monomers are threaded through the gaps in between. Despite its rigid structure, the polymer is surprisingly flexible. Dichtel’s team also found that dissolving the polymer in solution caused the layers of interlocked monomers to peel off each other.

“After the polymer is formed, there’s not a whole lot holding the structure together,” Dichtel said. “So, when we put it in solvent, the crystal dissolves, but each 2D layer holds together. We can manipulate those individual sheets.”

To examine the structure at the nanoscale, collaborators at Cornell University, led by Professor David Muller, used cutting-edge electron microscopy techniques. The images revealed the polymer’s high degree of crystallinity, confirmed its interlocked structure and indicated its high flexibility.

Dichtel’s team also found the new material can be produced in large quantities. Previous polymers containing mechanical bonds typically have been prepared in very small quantities using methods that are unlikely to be scalable. Dichtel’s team, on the other hand, made half a kilogram of their new material and assume even larger amounts are possible as their most promising applications emerge.

Adding strength to tough polymers

Inspired by the material’s inherent strength, Dichtel’s collaborators at Duke University, led by Professor Matthew Becker, added it to Ultem. In the same family as Kevlar, Ultem is an incredibly strong material that can withstand extreme temperatures as well as acidic and caustic chemicals. The researchers developed a composite material of 97.5% Ultem fiber and just 2.5% of the 2D polymer. That small percentage dramatically increased Ultem’s overall strength and toughness.

Dichtel envisions his group’s new polymer might have a future as a specialty material for light-weight body armor and ballistic fabrics.

“We have a lot more analysis to do, but we can tell that it improves the strength of these composite materials,” Dichtel said. “Almost every property we have measured has been exceptional in some way.”

Steeped in Northwestern history

The authors dedicated the paper to the memory of former Northwestern chemist Sir Fraser Stoddart, who introduced the concept of mechanical bonds in the 1980s. Ultimately, he elaborated these bonds into molecular machines that switch, rotate, contract and expand in controllable ways. Stoddart, who passed away last month, received the 2016 Nobel Prize in Chemistry for this work.

“Molecules don’t just thread themselves through each other on their own, so Fraser developed ingenious ways to template interlocked structures,” said Dichtel, who was a postdoctoral researcher in Stoddart’s lab at UCLA. “But even these methods have stopped short of being practical enough to use in big molecules like polymers. In our present work, the molecules are held firmly in place in a crystal, which templates the formation of a mechanical bond around each one.

“So, these mechanical bonds have deep tradition at Northwestern, and we are excited to explore their possibilities in ways that have not yet been possible.”

The study, “Mechanically interlocked two-dimensional polymers,” was primarily supported by the Defense Advanced Research Projects Agency (contract number HR00112320041) and Northwestern’s IIN (Ryan Fellows Program).

Fifteen years of a persistent, devastating megadrought — the longest lasting in a thousand years — have nearly dried out Chile’s water reserves, even affecting the country’s vital mining output. This is but one blatant example of how the warming climate is causing multi-year droughts and acute water crises in vulnerable regions around the globe. However, droughts tend only to be noticed when they damage agriculture or visibly affect forests. Thus, some pressing questions arise: Can we consistently identify extreme multi-year droughts and examine their impacts on ecosystems? And what can we learn from the drought patterns of the past forty years?

To answer these questions, researchers from the Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL) and the Institute of Science and Technology Austria (ISTA) have analyzed global meteorological data and modeled droughts between 1980 and 2018. They demonstrated a worrying increase in multi-year droughts that became longer, more frequent, and more extreme, covering more land. “Each year since 1980, drought-stricken areas have spread by an additional fifty thousand square kilometers on average — that’s roughly the area of Slovakia, or the US states of Vermont and New Hampshire put together — , causing enormous damage to ecosystems, agriculture, and energy production,” says ISTA Professor Francesca Pellicciotti, the Principal Investigator of the WSL-funded EMERGE Project, under which the present study was conducted. The team aims to unveil the possible long-lasting effects of persistent droughts around the globe and help inform policy preparing for more frequent and severe future megadroughts.

Unveiling extreme droughts that flew under the radar

The international team used the CHELSA climate data prepared by WSL Senior Researcher and study author Dirk Karger, which goes back to 1979. They calculated anomalies in rainfall and evapotranspiration — water evaporation from soil and plants — and their impact on natural ecosystems worldwide. This allowed them to determine the occurrence of multi-year droughts both in well-studied and less accessible regions of the planet, especially in areas like tropical forests and the Andes, where little observational data is available. “Our method not only mapped well-documented droughts but also shed light on extreme droughts that flew under the radar, such as the one that affected the Congo rainforest from 2010 to 2018,” says Karger. This discrepancy is likely due to how forests in various climate regions respond to drought episodes. “While temperate grasslands have been most affected in the past forty years, boreal and tropical forests appeared to withstand drought more effectively and even displayed paradoxical effects during the onset of drought.” But how long can these forests resist the harsh blow of climate change?

Contrasting impacts on ecosystems

The persistently rising temperatures, extended droughts, and higher evapotranspiration ultimately lead to dryer and browner ecosystems, despite also causing heavier precipitation episodes. Thus, scientists can use satellite images to monitor the effect of drought by tracking changes in vegetation greenness over time. While this analysis works well for temperate grasslands, the changes in greenness cannot be tracked as easily over dense tropical forest canopies, leading to underestimated effects of drought in such areas. Thus, to ensure consistent results worldwide, the team developed a multistep analysis that better resolves the changes in high-leaf regions and ranked the droughts by their severity since 1980. Unsurprisingly, they showed that megadroughts had the highest immediate impact on temperate grasslands. ‘Hotspot’ regions included the western USA, central and eastern Mongolia, and particularly southeastern Australia, where the data overlapped with two well-documented multi-year ecological droughts. On the other hand, the team shed additional light on the paradoxical effects observed in the tropical and boreal forests. While tropical forests can offset the expected effects of drought as long as they have enough water reserves to buffer the decrease in rainfall, boreal forests and tundra react in their distinct way. It turns out that the warming climate extends the boreal growth season since vegetation growth in these regions is limited by lower temperatures rather than water availability.

Droughts evolve in time and space

The results show that the trend of intensifying megadroughts is clear: The team generated the first global — and globally consistent — picture of megadroughts and their impact on vegetation at high resolution. However, the long-term effects on the planet and its ecosystems remain largely unknown. Meanwhile, the data already agrees with the observed widely greening pan-Arctic. “But in the event of long-term extreme water shortages, trees in tropical and boreal regions can die, leading to long-lasting damage to these ecosystems. Especially, the boreal vegetation will likely take the longest to recover from such a climate disaster,” says Karger. Pellicciotti hopes the team’s result will help change our perception of droughts and how to prepare for them: “Currently, mitigation strategies largely consider droughts as yearly or seasonal events, which stands in stark contrast to the longer and more severe megadroughts we will face in the future,” she says. “We hope that the publicly available inventory of droughts we are putting out will help orient policymakers toward more realistic preparation and prevention measures.” As a glaciologist, Pellicciotti also seeks to examine the effects of megadroughts in the mountains and how glaciers can buffer them. She leads a collaborative project titled “MegaWat — Megadroughts in the Water Towers of Europe — From Process Understanding to Strategies for Management and Adaptation.”

Project and funding information The present study was conducted within the scope of the EMERGE Project of the Swiss Federal Institute for Forest, Snow, and Landscape Research (WSL) with Professor Francesca Pellicciotti from the Institute of Science and Technology Austria (ISTA) serving as its Principal Investigator. The research was supported by funding from the Extreme Program of the WSL for the EMERGE project.