Hydrogels are squishy, bio-friendly materials that are made mostly of water and a bit of polymer. The Jell-O-like substance is available in the form of medical patches, sprays, and glues, and can be stuck to the skin or implanted in the body to dress wounds, affix implants, and encapsulate and release medicine over time. 

For all their sticky, stretchy, and protective properties, hydrogels lack one key trait: breathability. If worn for too long, a bandage or patch can trap moisture and sweat, which can irritate tissues and reduce the effectiveness of any device that a hydrogel adheres.  

Now MIT engineers have come up with a recipe for a hydrogel that is both hydrated and aerated, or permeable to air. The new material is just as soft, stretchy, and robust as conventional hydrogels, but a network of tiny tunnels running through the gel allows air to pass through. 

The aerated hydrogel can be worn for longer periods of time compared to conventional hydrogels, without causing skin irritation. It can also reduce sweat buildup, even during exercise. In experiments, volunteers wore wireless heart monitors that were attached to their chest with the new breathable hydrogel. After working out regularly for 10 days, the volunteers showed no signs of skin irritation, and the heart monitors maintained clear readings.  

The results, which are reported today in the journal Nature, may enable longer-lasting hydrogel products, such as breathable bandages and dressings, cosmetic face masks, and contact lenses, along with better-performing health monitors and implants. 

“Water and oxygen are both essential for life,” says Xuanhe Zhao, the Uncas (1923) and Helen Whitaker Professor of Mechanical Engineering, and a professor of civil and environmental engineering, and medical engineering and science. “Now that we’ve added air to hydrogels, people can find broad applications.”

Zhao’s MIT co-authors on the study include Xiao-Yun Yan, Shucong Li, Won Jun Song, Runze Li, Bastien Aymon, Jingjing Wu, Gengxi Lu, Jiayi Liu, Shu Wang, Eric Lu, Hyunhee Lee, James Zhang, Casey O’Brien, and Zachary Smith, along with collaborators from multiple other institutions.

Breathing through Jello

Water makes up about 90 percent of a typical hydrogel. The rest of the material consists of polymers. When mixed with water in a chemical process known as “cross-linking,” the polymers settle into a sort of scaffold that holds the water in place, forming a gel that’s both squishy and stretchy. But because hydrogel’s composition is mainly water, it’s inherently challenging for any air to make its way through the material effectively. 

“In general, water is not breathable,” co-lead author Xiao-Yun Yan says. “Hydrogel is 80 to 90 percent water, similar to Jell-O. And you cannot breathe through Jell-O.”

Other groups have tried to design air-permeable hydrogels, mainly taking one of two approaches. The first has been to essentially puncture microscopic holes throughout the gel. Such designs are breathable, but only in air. When they are placed in liquid, the holes quickly clog up. 

Researchers have also tried mixing hydrogel with certain polymers, such as silicone, that naturally allow air through. But this approach requires adding a large amount of polymers to the hydrogel in order to create enough permeable space for air to move through the entire gel. These hydrogels end up having a greater balance of polymer to water, making them less hydrated in general. 

Zhao, who has been a leader in the development and application of hydrogels, looked to make a hydrogel that lets air through without losing its water-heavy makeup. 

“We want to have lots of tiny channels to let air through, while also maintaining lots of water in the gel,” Zhao says. “This was a significant challenge, and something that people thought was impossible to do.”

Highways for air

After several years of investigation, the team hit on an ideal recipe for a breathable hydrogel that minimizes the non-water ingredients needed to let air through. In their new study, they report that the key to the recipe is “phase separation.” A common example of this process is the interaction between oil and water. The difference in the two liquids’ phases cause them to instantly separate. When the two are mixed, oil and water glom to their own kind, while avoiding the other. 

Zhao and his colleagues took advantage of viscoelastic phase separation in concocting a breathable hydrogel. For their new design, they mixed their conventional hydrogel recipe with a very small amount of silica aerogel particles, which are essentially “solid-form” air bubbles. 

“They are like boba beads,” Yan offers. “The particles are made of silica, which is hydrophobic, meaning that water does not want to leak through them, so they are very stable in water.” 

And as it turns out, the particles are similar to oil when mixed with water. The researchers found that when they mixed just a small amount of the particles with a solution of the water-heavy hydrogel, the water molecules glommed together, essentially finding each other faster than the less abundant silica particles. This effect of viscoelastic phase separation created large pockets of water and squeezed the silica particles into skinny, interconnected tunnels. The team observed that after a few hours, this effect formed a network of thin and sturdy, silica-skinned tunnels through which air could flow.

“It’s as if the particles formed a network of connected tunnels, like air-permeable highways within the hydrated hydrogel,” says co-lead author Shucong Li. 

Once they confirmed that the network had formed, the team cross-linked the mixture, essentially freezing the gel, and its breathable network, in place. They then tested the gel’s breathability and mechanical performance over multiple experiments, including one in which they asked several volunteers to wear the gel, attached to a wireless electrocardiogram (ECG) monitor, while exercising for 20 minutes. The volunteers also wore monitors with conventional, commercial hydrogel adhesives.

Throughout the workouts, the researchers observed that the breathable hydrogel maintained a strong ECG signal, in contrast to the conventional gel which exhibited significant signal fluctuations.The researchers observed similar results in an experiment with several volunteers who wore the breathable hydrogel and ECG monitor over 10 days. 

“We reliably saw that after 10 days, the quality of the ECG signal is still pretty good, and after you take off the monitor, there were no noticeable blisters or redness on the skin,” Li says. “This indicates healthy skin conditions.”

The team also exercised the gel itself, putting it through 10,000 cycles of stretching and compression. After these tests, they found the gel still retained the network of air channels, maintaining its breathability. 

“After 10,000 cycles, there was less than a 5 percent drop in oxygen permeability,” Li says. “That matters, because even with your heartbeat, your chest continuously undergoes small strains. So we have to make sure this gel is durable for such daily activity.”

Zhao says the new study provides a novel approach for others to fabricate breathable and multifunctional hydrogels, using the concept of visoelastic phase separation as a guide. 

“We’ve discovered that this process can create these air-permeable hydrogels, and we demonstrate one application,” he says. “But we think there can be very broad applications. This is a technology platform.”

This work was carried out in part through the use of MIT.nano’s facilities. This work was supported in part by the MIT Hatsopoulos Faculty Fellowship, the Uncas and Helen Whitaker Professorship, a HEALS seed grant, the National Institutes of Health, the National Science Foundation, and the Department of Defense Congressionally Directed Medical Research Programs.

In 2024, a U.S. government official warned that Russia could be developing a new satellite designed to carry nuclear weapons into space. The statement followed the launch of a suspicious Russian satellite into low-Earth orbit in 2022, just a few weeks before the country’s full-scale invasion of Ukraine.

A nuclear detonation in low-Earth orbit — the region about 100 miles to 1,200 miles above Earth’s surface — would release trillions of highly energetic electrons that would destroy many of the satellites in space, disrupting telecommunications networks, GPS, space-based internet, and more.

The 1967 Outer Space Treaty bans the placement of nuclear weapons in space, but there’s currently no way to verify satellites don’t contain nuclear weapons. In fact, no verification methods have even been proposed in unclassified, peer-reviewed literature.

Now, MIT Professor Areg Danagoulian is proposing a way to determine if a satellite orbiting Earth contains a nuclear weapon. In a new paper published in Nature, Danagoulian describes his idea for a satellite-based sensor system that could orbit close by a suspect satellite and detect neutrons generated by high-energy protons colliding with radioactive material.

In the paper, Danagoulian calculates that a sensor system the size of a large encyclopedia could detect a nuclear weapon with 99 percent accuracy if it orbited within 4,000 meters of the suspect satellite for about a week. He also estimates that the detection time could be cut to a matter of hours if multiple satellite sensors were used or the sensor satellite was able to get within 1,000 meters of the suspect satellite.

“If we eventually have some verification mechanisms for the Outer Space Treaty, that will put pressure on countries to respect the treaty or disclose what they are doing, because they know if they try to violate it, we will find out,” Danagoulian says. “I very much hope this will turn into a real system, or proof-of-concept system, but the goal right now is to get national labs to use this work for their own research, and to get policymakers to seriously consider this technology as a potential part of national technical means.”

Protecting space

In 1962, the U.S. detonated a 1.4-megaton thermonuclear warhead in space, which unintentionally destroyed many of the early satellites of the era. The blast released enormous volumes of highly energized electrons, and many became trapped in Earth’s magnetic field, where they damage any electronics in their path.

“When you have a nuclear detonation in outer space, basically the whole body of the bomb becomes ionized, and nearly every single electron in the weapon’s mass becomes free,” Danagoulian explains. “It gets injected into what’s called the inner Van Allen radiation belt. Once there, the electrons start hitting everything flying through those belts, causing ionization, radiation damage, and more. As you go further out into space, you create these thick belts around Earth populated by highly energetic protons and electrons.”

The 1967 Outer Space Treaty declared space the “province of all mankind” and banned nuclear weapons in space, among other safeguards. It has since been signed by 118 countries including the U.S., China, and Russia.

Monitoring compliance with the treaty has taken on increased urgency since Russia’s 2022 launch of a suspicious satellite, Cosmos2553, which Russia claims is used for surveillance and sensing. However, U.S. authorities believe it may carry components of a nuclear device undergoing testing, with the possible future goal of fielding an actual nuclear anti-satellite weapon. The detonation of a nuclear weapon at that orbit could destroy many of the U.S. reconnaissance satellites, international communication satellite platforms, as well as the Starlink satellites.

“The Russians launched this satellite in a very strange and unusual orbit because it goes through the most hostile environment possible around the planet,” Danagoulian explains. “No one puts satellites there because it’s highly radioactive. Why would you put a satellite in that orbit? Well, that location is likely the best point for trapping electrons if you were to detonate a thermonuclear weapon.”

Danagoulian notes most research on nuclear detection is highly classified, making it hard to know how much progress has been made in national labs. But he wanted to show that scientifically proving the presence of a nuclear weapon in space is possible.

Particle bombardment

The approach Danagoulian developed centers on a reaction known as spallation, caused by highly energetic protons in radioactive environments.

“When an energetic proton slams into elements with a high atomic number, like uranium and plutonium, each proton may knock out something like 40 neutrons,” he explains. “That’s a ridiculously large number. We’re talking about millions of protons per second per square centimeter, with many of them generating 40 neutrons. The question is can you detect some of those neutrons?”

Normal satellites wouldn’t emit nearly as many neutrons, but there are still naturally occurring protons, neutrons, and electrons in the atmosphere, especially in low-Earth orbit. Danagoulian’s concept uses two panels made up of pixels of neutron sensors known as scintillators that interact with radiation and emit light. The panels are sandwiched between synthetic crystal diamond detectors that allow the system to distinguish between neutrons coming from radioactive materials and natural protons and electrons. The two-panel construction then can be used to estimate the direction of the neutron, allowing it to differentiate between natural atmospheric neutrons and those coming from a suspected satellite. 

“Most neutron detectors are very sensitive to protons, so you have to come up with some smart ways to reject protons but keep neutrons,” Danagoulian says. “You also have to tell the difference between naturally occurring neutrons and neutron spallation from the satellite.”

He believes the system, placed inside of an inspector satellite, would be strong enough to survive the harsh environment of low-Earth orbit while also being fast enough to process the protons, electrons, and neutrons that bombard it.

Danagoulian’s calculations on how long the detector satellite would have to be near the suspect satellite give him confidence in the feasibility of the system. If a detector satellite were able to get within 1,000 meters of the suspect satellite, it could accurately detect nuclear weapons in about one hour. That would amount to a single flyby.

Danagoulian calls the paper a feasibility study of the concept.

“I say in the paper this isn’t a completely proven system,” he says. “The purpose of the paper is to show the scientific community that it’s scientifically possible to do this. But there are many more practical considerations to be made to actually build these detectors.”

Danagoulian hopes the study will stimulate further research and development. He is also working with researchers in MIT’s Center for Nuclear Security and Policy (CNSP) to understand the policy landscape around this issue.

If a version of his system is eventually developed, Danagoulian believes it could encourage the nonproliferation that has helped preserve satellites so far. He notes that while adversarial countries are naturally suspicious of each other’s claims, scientific evidence would strengthen trust.

“You can fake intelligence,” he says, “but you can’t fake physics.”

The work was supported, in part, by the National Nuclear Security Administration, the Carnegie Foundation, and Longview Philanthropy.

If you are crossing an unfamiliar room in the dark, you may grope around a bit to get a sense of your space.

But for many animals, feeling out a space comes more naturally. A mouse, for instance, can efficiently navigate in the dark just by grazing its whiskers against walls and other obstacles.

Fan Wang, a professor of brain and cognitive sciences and an investigator at the McGovern Institute for Brain Research at MIT, has discovered how neurons in a mouse’s brainstem use signals from the animal’s touch-sensitive whiskers to estimate an object’s distance from the face.

Her team’s findings, published June 25 in the journal Neuron, unlock key circuitry the brain uses to represent the space immediately surrounding the body.

Mapping space

The circuit the team discovered is part of the brain’s system for creating an egocentric map of space — that is, understanding where things are relative to one’s own body. Neuroscientists know that the brain calls on specialized circuits to understand space in this way, which are different from its system for mapping space using external landmarks.

In their study, Wang and her team explored how the brain maps the space closest to the body, known as the peripersonal space. This is the space in which we move, and it is vital that we understand where things are in relationship to our bodies so we can reach, step, avoid hazards, and otherwise interact effectively with our environment.

Wang says mice were an appealing model for investigating how the brain understands objects’ distance within the peripersonal space, because a rodent’s whiskers seem so much like a built-in set of rulers. These whiskers, which vary in length, are swept back and forth as the animals explore their environment. As whiskers bend and vibrate, the mechanical sensations are relayed to the brain by sensory neurons at their base. Those neurons fire more when a whisker bends close to the face than they do in response to contact near the whisker’s tip, communicating information about the proximity of the touch.

Wang’s team wanted to know if the brain uses these signals to build an internal ruler-like representation of distance more precise than “near” or “far.” To find out, graduate student Wenxi Xiao and Research Scientist Kyle Severson monitored neural activity in a small sensory-processing region in the brainstem where tactile signals from the whiskers first arrive in the brain. They studied what happened there as mice walked on a treadmill while brushing their whiskers against a wall that passed by at different distances.

Many neurons in the region were sensitive to the whisker bending triggered by the wall. Some behaved similarly to the sensory neurons they were getting their information from, firing more when the wall was closer to the face and thus serving as a proximity-based distance code. But other cells were tuned in to discrete distances, firing only when the distance of the wall the whiskers had touched was within a specific range.

The whiskers rule

For some neurons, activity peaked when the wall was 23 millimeters away from the face, near the tips of the longest whiskers. Others responded most when the wall was at intermediate distances. “Each of these neurons represents a specific distance, and together they span the full range reached by the longest whisker, like tick marks on the ruler,” Wang explains. “We call that the map code.”

The team wanted to know how the brain converts proximity signals from different whiskers into accurate map code of object’s distances from the head. “You cannot just listen to individual whisker neurons, because a contact at the tip of a short whisker would be in the middle of a long whisker. You need a brain circuit to build a unified distance map,” Wang says.

Through computational modeling and by exploring what happened when they manipulated neural signaling in specific ways, Wang’s team showed how distances can be calculated by comparing inputs from different sensory neurons. Their findings suggest that each brainstem neuron that makes up the map code receives both direct excitatory inputs from proximity-sensitive whisker neurons and inhibitory inputs from neurons driven by proximity-dependent whisker touch signals.

“Essentially, the inhibitory pathway allows the brainstem to compare two inputs by subtraction,” Wang explains. “If one input signals ‘this is how far it is’ and the other signals ‘this is how far I estimate it to be,’ subtracting one from the other yields an intermediate value. We think it’s a simple and elegant way to transform tactile input into a representation of discrete distance.”

Wang notes that despite their importance, the brain’s body-centered representations of space have so far received little attention from neuroscientists, who know much more about how we understand locations in space relative to landmarks (an allocentric map). She is eager to investigate how the egocentric map code her team discovered is integrated with other brain systems to guide movement, social interactions, and other behavior, and hopes the findings will further exploration from other groups.

The study was funded by grants from the National Institutes of Health.

In today's world, artificial intelligence chatbots such as ChatGPT and Claude can perform many functions, such as composing work emails and planning travel itineraries. These chatbots are systems built around large vision-language models (VLMs): AI trained on a massive dataset that includes books, websites, code, and images. 

The AI algorithms are then refined on massive amounts of human-generated feedback to follow instructions and avoid harmful or unwanted output, and use that "knowledge" to produce text or images based on input from a user. Although chatbots have clear limitations, they can be very helpful for a wide range of tasks, including in some areas that traditionally require specialized skills, like computer programming.

As part of a project for the U.S. Department of the Air Force–MIT AI Accelerator's Phantom Program, U.S. Air Force cadet Joshua Lynch — with the help of his mentor, Laura Niss, a technical staff member in the Embedded and AI Systems Group at MIT Lincoln Laboratory — wanted to determine if, as a complete novice to coding, he could develop a fully functional program. He used a process called "vibe-coding," in which a user relies entirely on prompts to guide a generative AI chatbot to write and refine code. 

His motivation was to empower anyone familiar with the military problem space, regardless of their technical background, to advance their ideas for useful software applications, essentially bypassing the time and cost constraints of the traditional military software development pipeline. Lynch aimed to build his own application while Niss monitored his experience with the technology.

"The Phantom student wanted to see if he could create a useful application through self-identified vibe-coding, without any previous experience," Niss says. "Within this project, I wanted to understand how his perception of AI changed over time with use. We both wanted to understand better where and how AI could be used by nontechnical users in the military."

Lynch set out to see if, starting with no coding skills and using chatbots, he could create an application specific to his type of tactical team to help reduce collateral damage while enhancing survivability in the broader mission. This application would offer capabilities including AI-assisted target recognition; modular intelligence, surveillance, and reconnaissance; autonomous striking; and communication management on the battlefield. 

During the project, Lynch completed several professional development courses in AI and familiarized himself with both military and nonmilitary uses of the technology. For the basis for his code generation, he used the paid models of three AI chatbots: Anthropic's Claude, OpenAI's ChatGPT, and Google's Gemini. Most of this work was done only through the chatbots' main chat function on a web browser, not as an integrated system within a development environment, as is standard now. The final application was produced using Google AI Studio App, which can create applications that interface with the Gemini application programming interface and has AI integrated in the development environment. 

Over three months, Lynch worked with these models to build his application, called the Remote Operating Modular Augmentation Device (ROMAD-AI). During this time, he learned several methods to improve the code output. For example, he often encountered difficulties with the AI chatbots lacking hierarchical focus and modifying unrelated code sections. He discovered it was important to break problems into small parts, frame questions clearly, and steer conversations back on topic when they stray too far from the objective. 

Learning to recognize the chatbots' limitations and effectively work around them took up most of the project timeline. As Lynch gained more experience with the chatbots, limitations in the AI capabilities and time for development caused him to re-scope the project, moving it from an application that could assist on the battlefield to one that could perform basic document processing, such as analyzing tactical maps of battlefields and generating mission-planning documents through an interface with a VLM-powered chatbot. While the resulting prototype did not perform all capabilities Lynch originally set out to include (and in its current iteration was not secure for the desired use case), it proved the capability and usefulness of such an application for service members.

"I was quite impressed with this final product, and it showed me how powerful these systems can be at prototyping designs from nonexperts," Niss says. "I'm now of the opinion that these can be powerful tools for nontechnical experts to convey problems and possible solutions to technical experts, and aid in communicating desired outcomes."

Niss observed the change in Lynch's perspective of AI language models during his experience. After starting with an impressive goal, Lynch gained understanding of the capabilities of current technology and significantly scoped down his expectations by the end of the project period. Measures of his perceptions of the different AI systems over time and across system updates were particularly interesting to Lynch and Niss, with Claude showing more stability than ChatGPT across traits such as likeability, anthropomorphism, and perceived intelligence. Lynch found AI to be a helpful tutor, but noted its inaccuracies on topics he knew well.

The project showed that AI chatbots can empower nontechnical service members to produce viable software applications for their unique problems, although it works better as a prototyping assistant than as a full production tool when handling sensitive information and for critical applications. Improper vetting of code may lead to security risks, as demonstrated by an instance where Lynch didn't realize that the final application was sending the input documents to a Gemini AI model to analyze, rather than parsing the documents locally on his computer. Although AI can generate significant amounts of functional code, code review remains a bottleneck in this space.

"For me, this project reinforced the expanse between experts in different fields," Niss says. "No matter how good AI gets, I think we'll always need to collaborate to get to the best solutions for the most important problems."

Research was sponsored by the Department of the Air Force Artificial Intelligence Accelerator and was accomplished under Cooperative Agreement Number FA8750-19-2-1000.

When a star dies, a black hole is born. This has been the textbook origin story for most black holes. At the end of a massive star’s life, its outer layers blast away in a brilliant supernova, and its core collapses into a gravitationally tight and dense region, forming a black hole.

Recent discoveries from gravitational-wave detectors have revealed hundreds of merging black holes across the universe. Many of them have been thought to come directly from exploding stars. But black holes can also come from other, smaller black holes. The products of previous black hole mergers can, in principle, merge again, creating a more massive black hole. This alternative, black-holes-birthing-black-holes pathway is known as “hierarchical merging.”

Now MIT scientists are finding that a good number of merging black holes may have indeed merged before. They carried out a new analysis of recent data from the LIGO, Virgo, and KAGRA observatories, containing 155 pairs of binary black holes, and found about 14 percent of merging black holes in the universe may in fact be second-generation black holes that formed from the previous merging of two smaller black holes. 

The results, which the team reports this week in Physical Review Letters, suggest that repeated hierarchical merging is a significant pathway by which black holes form. 

“We’re finding that, for some of these merging black holes, it’s not their first rodeo,” says the study’s first author, Cailin Plunkett, a graduate student in MIT’s Department of Physics. “Overall in the universe, black holes are merging all the time. The question of how often are they repeatedly merging was pretty uncertain. Now we’re seeing a relatively consistent picture where there’s a decent percentage of black holes that are coming from this repeated pathway.”

The study’s co-authors are Salvatore Vitale, associate professor of physics at MIT; Thomas Callister of Williams College; and Michael Zevin of Adler Planetarium and Northwestern University.

Lopsided pairs

When a massive star collapses and dies, the resulting black hole should have very little spin. In addition to losing a huge amount of mass when it explodes, the star should also lose much of its inherent spin, or angular momentum. The black hole left over should then have little to no spin. 

In contrast, when two black holes merge, the collision should create a new, wildly spinning second-generation black hole. 

“They would be spinning very fast, at about 70 percent their maximum possible spin,” Vitale says. 

Scientists suspect that hierarchical mergers occur in dense stellar environments, where stars are so tightly packed together that multiple neighboring stars could die and collapse to form black holes that are then close enough to merge with each other to form second-generation black holes. 

“You might have a ton of stars whizzing around each other, and if some are massive and explode, they become black holes. The black holes continue to whizz around, and can capture each other and merge,” Plunkett says. “This process can repeat potentially ad infinitum, by virtue of the fact that you have a ton of stars and black holes in this really dense environment.”

One sign of a hierarchical merger is that one black hole in a pair of merging black holes has a much higher spin, and higher mass, than the other. Such a lopsided duo would signal that at least one of the black holes came from the collision of two previous black holes. 

In 2024, scientists detected two such lopsided mergers in signals recorded by the LIGO, Virgo, and KAGRA observatories. The observatories detect incoming gravitational waves — incredibly small wobbles in the fabric of space and time — that are the reverberations from distant cosmic phenomena, such as colliding black holes. 

The observatories detected two gravitational-wave signals, labeled GW241011 and GW241110, each of which likely contain a black hole spinning much faster than its partner. The hierarchical mergers were discovered by analyzing each signal in detail to tease out the specific masses and spins of the black holes involved in each merger.

That work inspired Plunkett and Vitale to do a search of similar hierarchical mergers using all the gravitational-wave signals that the observatories have captured to date. 

A pattern of wobbles

For their new study, the team analyzed the LIGO-Virgo-KAGRA Gravitational Wave Transient Catalog 4.0 (GWTC-4.0), which comprises gravitational-wave detections from the observatories’ fourth observing run. Rather than analyze each gravitational-wave signal one by one, which is what scientists did for GW241011 and GW241110, Plunkett and Vitale searched for a characteristic pattern of hierarchical mergers across the data overall, to see if any matching signals popped out.

The pattern they searched for represents a range of orbital “wobbles.” Just before they merge, two black holes spiral toward each other in a disk-like, orbital plane. When the spins of the pair are perpendicular to the plane, this remains relatively steady. But when one or both spins are not perpendicular to the plane, the disk will wobble. The degree to which the whole plane wobbles, or “precesses,” can tell scientists about the balance of masses and spins between the two spiraling black holes. 

Plunkett and Vitale developed a model for the range of wobbling that should be a sign of a hierarchical merger, specifically between a first-generation and a second-generation black hole. 

The team applied the model to the entire GWTC-4.0 catalog, which comprises gravitational-wave signals from 153 black hole mergers, in addition to the signals from GW241011 and GW241110. Their analysis revealed that a number of mergers fit the pattern for orbital wobbling that was likely caused by the colliding of first- and second-generation black holes. 

Specifically, they found that roughly 14 percent of merging black holes in the universe may have merged before, and that these second-generation black holes had very particular masses: Black holes of around 10 solar masses (10 times the mass of the sun) and 30 solar masses were run-of-the-mill star-born black holes, while second-generation black holes had masses of around 20 solar masses or 40 solar masses and above. 

“One of the reasons why the 40-and-above regime is interesting is, stellar evolution theory predicts you shouldn’t be able to form black holes in that mass range at all from just a supernova,” Plunkett says. “We think supernovae from really massive stars end up being so violent that they leave no black holes at all above roughly 45 solar masses. Yet we have seen black holes that are that massive. And the question is: Where did they come from?”

The team’s new analysis provides support for the idea that black holes can form from the repeated merging of other black holes, and that this alternate origin story could explain some of the curious black holes that we can detect today. 

This work was supported, in part, by the National Science Foundation, and the Brinson Foundation.

Many experts refer to hydrogen as “the fuel of the future.” It is expected to help decarbonize the global economy in two main ways: burning it or feeding it into a fuel cell produces storable energy with no carbon emissions, just water. And it can be used in place of fossil fuels or as a chemical feedstock in hard-to-decarbonize industrial processes such as steel and cement production.

But for hydrogen to realize its potential, two challenges must be overcome. Researchers worldwide are now working to address the first: finding a method of producing pure hydrogen that’s both cheap and low in carbon emissions.

Just as critical is finding a good means of transporting and storing hydrogen. A team led by researchers at the MIT Energy Initiative (MITEI) has been tackling that less-discussed but important challenge. The location where the pure hydrogen is produced is likely to be far away from where it will be used, so moving it will be critical — and difficult.

The problem stems from two characteristics of hydrogen: It’s the lightest gas there is, and it has low energy density per volume. Therefore, delivering a given amount of energy requires a large volume of hydrogen and a container that’s sealed so tightly that the hydrogen molecules can’t escape. Suffice it to say, moving a liquid fuel such as gasoline is easier. And without a good means of storing and transporting hydrogen, it can’t fulfill its promise as the world’s clean fuel of the future.

In 2024, with funding provided by ExxonMobil Technology and Engineering Co. through MITEI, a team of MITEI researchers and their Exxon colleagues began examining various approaches to transporting hydrogen. The researchers have now concluded that there’s no single answer; the cost and carbon emissions from a given transportation method will vary from one location to another. Therefore, instead of presenting a table showing the “best” outcome, the team created a tool that enables users to understand the various options and choose the best option for their particular use case. 

The researchers present their study and the tool they developed in a new paper published in the journal Fuel.    

The study was led by former MITEI postdocs Gasim Ibrahim, now an R&D engineer/scientist at Honeywell, and Guiyan Zang, former MITEI group lead who is now an associate professor at Washington State University. Additional MIT co-authors include former postdocs Bosong Lin, Jacqueline Garrido, Woojae Shin, and Haoxiang Lai.

The hydrogen challenge and hydrogen “carriers” that can help

The team’s starting assumption was that for hydrogen to become a viable fuel for the world, it would need to be transported over long distances — specifically, overseas, across continents, or across large water bodies. Given the properties of hydrogen gas, it would be best to convert it to some liquid form before shipping.

There are known ways to do that, but what would be best for shipping? How much would various methods cost, and how much would they add to the carbon intensity of the delivered hydrogen?

“There hasn’t been a lot of attention paid to addressing those questions,” Ibrahim says. While some studies have been done, their conclusions are inconsistent and many uncertainties remain, both because the cost and carbon emissions will differ from place to place and because there’s not a lot of data to inform how the large-scale transportation of hydrogen will work.

“So we decided the best thing to do was to develop an adaptive tool that would enable users to perform their own assessments — a tool that could be updated very easily,” Ibrahim explains. “And we would make it open source, so anyone can see and update the numbers that we used in formulating and testing it. As the industry develops, and as scale becomes more a factor, the assumptions made in [our initial] assessments of the economics and the carbon intensity [of different shipping methods] will need to be updated.”

To focus on the transportation and storage issues, their model — called the Hydrogen Carrier Analysis Tool, or HyCAT — doesn’t consider how the starting hydrogen is produced, or how the hydrogen is used after it’s delivered. HyCAT focuses on determining the costs and carbon emissions incurred as the hydrogen is transported and delivered. In addition, while a full life-cycle assessment would include all environmental impacts, HyCAT focuses on emissions of greenhouse gases (GHGs).

The tool is easy to use, says Ibrahim. Built into it is a user interface with drop-down menus for inputting assumptions, and results from an analysis are presented in simple bar charts that include links to tables presenting the details.

Ibrahim clarifies that, while HyCAT has a well-defined boundary — “incoming hydrogen to outgoing hydrogen” — in an analysis of a specific situation, the user will input various factors about the local situation, including the carbon intensity and cost associated with production of the incoming hydrogen. “So that will inform the final values that come out of a HyCAT analysis,” says Ibrahim, and in part explains why the results vary from place to place.

Based on the user’s assumptions, HyCAT calculates the cost and GHG emissions at five steps in the “supply chain”:

• converting the hydrogen into liquid form at the “export” terminal;
• storing the hydrogen-rich liquid;
• shipping it when an empty tanker becomes available;
• storing it at the “import” terminal; and
• releasing the hydrogen as a gas suitable for burning or being fed into a pipeline for distribution.  

Options for liquifying hydrogen gas

The main decision in analyzing the cost and emissions of a proposed hydrogen transport plan is how to convert the gaseous hydrogen to a liquid, and then how to recover the hydrogen gas at the end.

One approach is to simply change the gaseous hydrogen into an easily transportable liquid. But turning hydrogen gas into a liquid requires making it very, very cold. Indeed, notes Ibrahim, “you would need to consume about a third of the energy content of the hydrogen to make the gaseous hydrogen cold enough to liquify.” A further problem arises as the liquified hydrogen is being stored and moved. Unless the vessel containing the liquid hydrogen is properly insulated, the liquid hydrogen can re-gasify and escape. The upside of hydrogen liquefaction is that no chemical reactions are required.

Other options involve using a hydrogen “carrier.” Some liquid chemical compounds will absorb hydrogen atoms under certain conditions, and under other conditions will release them. Therefore, one approach to solving the hydrogen transportation problem is to make a carrier compound absorb the hydrogen where it’s made and then release it when it reaches its destination. This approach therefore involves two chemical reactions — one to bind the hydrogen to the carrier and the other to release it.  

In their demonstration runs, the researchers looked at the hydrogen carriers involving three potential compounds, each of which has known advantages and disadvantages.

One of those carriers is produced by adding hydrogen to toluene. That chemical reaction hasn’t been studied a lot, but there’s one known drawback: the source of toluene is typically the oil and gas industry, so the toluene itself has a relatively high carbon intensity when it picks up the hydrogen. Moreover, over time some of the toluene is lost, so more toluene must be added.
    
The researchers also looked at “synthetic methane,” which is made by reacting hydrogen with carbon dioxide. That reaction has been known for some time. Ibrahim notes that making synthetic methane actually consumes carbon dioxide, often captured from the atmosphere. On the negative side, however, one of the products of the reaction is water, so some of the hydrogen is lost each time the reaction occurs.

The final option they analyzed is ammonia, which forms when hydrogen reacts with nitrogen from the air. That reaction is very well-studied and is used commercially. “We’ve been producing ammonia for a long time,” says Ibrahim. And the infrastructure for transporting and storing it is well established. While Ibrahim refers to ammonia as the “most promising option,” the reaction needed to release the hydrogen has not received much attention.

Varying conclusions and future plans

Based on their sample runs, the researchers observed that the best path to follow will vary from place to place and from situation to situation. “As we developed the tool, we saw that the ‘best’ carrier was very specific to the supply chain at hand,” says Ibrahim. “It’s a function of how far you’re trying to ship your hydrogen, energy and shipping costs at your exporting and importing countries, the capital cost of building the needed facilities at both ends, and more.”

Ibrahim and his team are now planning a follow-up study in which they use HyCAT to analyze specific supply chains under certain conditions. They’ll then select assumptions that are highly uncertain and look at the range of possible values for those assumptions. “Then we’ll be able to say, ‘under these conditions, this carrier is better than that one,’ or ‘this carrier is better at cost, but worse at carbon intensity,’” says Ibrahim.

For now, the main conclusion of the study, says Ibrahim, is that “there’s no conclusion.” He warns decision-makers not to assume that anything they see in the literature can easily be generalized or extrapolated to their specific conditions. Instead, decision-makers should use HyCAT to explore the options available to them. Guided by their results and the objectives and values of their company, they will be able to optimize their supply chains and make clean-burning hydrogen a reality.

Professor Jesse Thaler has been named director of the MIT Laboratory for Nuclear Science (LNS), effective Aug. 1. He succeeds Professor Bolek Wyslouch, who directed LNS for the past decade. Thaler is a theoretical particle physicist who combines techniques from quantum field theory and machine learning to address outstanding questions in fundamental physics. 

“In his research, Jesse has done pioneering work on particle jets at the Large Hadron Collider and is a leader in combining AI and machine learning with fundamental particle physics,” says Nergis Mavalvala, dean of the MIT School of Science and the Curtis and Kathleen Marble Professor of Astrophysics. “The collaborative nature of his research programs will serve the Laboratory for Nuclear Science as science enters a new era of AI-driven discovery.”

Thaler is the William and Emma Rogers Professor of Physics in the MIT Center for Theoretical Physics — a Leinweber Institute (CTP-LI). Since 2020, he has served as inaugural director of the National Science Foundation (NSF) AI Institute for Artificial Intelligence and Fundamental Interactions, or IAIFI, which was recently renewed for another five years. Mike Williams, professor of physics, will succeed Thaler as IAIFI director. LNS is also poised to pursue new research projects through the Department of Energy’s Genesis Mission, which has a focus on AI-enabled scientific discovery.

“In my own field of particle physics, researchers are developing cutting-edge AI algorithms to handle the data deluge from collider experiments and to perform heroic theoretical calculations. This work has direct implications for discovering new physics, but the algorithms themselves turn out to be valuable well beyond our field,” says Thaler. “I’m excited to bring LNS into the next wave of discoveries supported by AI-driven capabilities.”

At IAIFI, Thaler has championed education and research activities at the intersection of physics and AI. With the MIT Institute for Data, Systems, and Society, IAIFI leadership created a doctoral program in physics, statistics, and data science. IAIFI also created dedicated postdoctoral fellowships to give early-career researchers the freedom to pursue interdisciplinary work. 

“Giving young scientists space to build connections across domains, universities, and career stages has been transformative within IAIFI,” says Thaler, who hopes to bring this type of framework to LNS. Established in 1946 to support nuclear and particle physics, LNS now encompasses research spanning cosmology, gravity, field theory, and quantum information science.

As head of LNS, Thaler will also oversee his home center of CTP-LI, which last year received a donation from the Leinweber Foundation to establish a network of theoretical physics research institutes. According to the Science Philanthropy Alliance, a nonprofit organization that promotes philanthropy for science, this constitutes the largest philanthropic commitment ever for this field.

Thaler received his PhD in physics from Harvard University in 2006, and his BS in math/physics from Brown University in 2002. From 2006 to 2009, he was a fellow at the Miller Institute for Basic Research in Science at the University of California at Berkeley. He joined the MIT faculty in 2010.

As advanced medical technology gets closer to hitting consumer markets, the need for guardrails on protected usage should increase. What might begin as a neural implant to aid in communication could become a device used to police one’s innermost thoughts.

Intrigued by the far-reaching benefits and risks of neural implants, Rachel Sava, a PhD candidate in the Harvard-MIT Program in Health Sciences and Technology, explores how a life-changing medical device can become a tool for surveillance by corporations and government entities in her winning submission, “Superintelligence, Superintimate,” for the fourth annual Envisioning the Future of Computing Prize. 

Sava’s concept was inspired by an internship at IBM, where she worked on a project with the PACE Center in London. “A mentor on the project was Kevin Brown, who had himself designed one of the earliest brain decoders — an EEG-based system he built for a colleague who had suffered a stroke that left him with locked-in syndrome,” she says. “It was this patient population for whom the body has become an unreliable vehicle for the mind that motivated my writing about neuroprostheses some six years later.”

Sava explains that research and applications right now are at a “watershed moment in neurotechnology.” Using examples like companies taking advantage of neural implants to monitor mental productivity, or authorities policing a population for “thought crimes,” Sava said that as this tech hits consumer markets, there is a genuine fear that what starts as a revolutionary medical device could transition into more dystopian usages.

Presented by the Social and Ethical Responsibilities of Computing (SERC), a cross-campus initiative of the MIT Schwarzman College of Computing, in collaboration with the School of Humanities, Arts, and Social Sciences and with support from MAC3 Philanthropies, the competition invited MIT students to identify, in 3,000 words or fewer, which sector stands to gain the highest net positive impact from artificial intelligence. Students were encouraged to explore realistic technological deployments while considering potential risks and ethical concerns. All submissions were eligible for cash awards with the grand prize set at $10,000.

During a live awards ceremony hosted by Caspar Hare, former associate dean of SERC and professor of philosophy, who founded the prize in 2023, three finalists each gave a 20-minute presentation on their concepts and took questions from a panel of judges and audience members.

“SERC and the donors who make this prize possible year after year are asking us, the next generation of scientists: ‘what world do you want to see?’ I think it’s worth taking the time to ask yourself the same,” Sava said. “And if, as it did for me, the sentiment grows bright enough to motivate further action — then it’s worth giving yourself permission to explore it as deeply as you do your other academic work.”

Each year, the Envisioning the Future of Computing Prize asks students to look beyond technological advancement and consider the societal benefits and costs of their work from the outset. From its inception, the competition has consistently attracted undergraduate and graduate students from across a wide range of disciplines.

“This year’s submissions were amazing and included essays on brain-computer interfaces, AI and religion, AI for scientific discovery, finding efficiencies in the power grid, and many more,” says Brian Hedden, co-associate dean of SERC and a professor of philosophy, who holds an MIT Schwarzman College of Computing shared position with the Department of Electrical Engineering and Computer Science. “They showed the breadth and depth of thinking going on at MIT on the social and ethics impacts of technologies.”

Nikos Trichakis, co-associate dean of SERC and the J.C. Penney Professor of Management, adds “what is most striking about these essays is the breadth of imagination they display: the students move fluidly across medicine, neurotechnology, law, ethics, and public institutions, while keeping human agency at the center. Their work is creative, rigorous, and deeply thoughtful, showing a remarkable ability to envision not only what AI can do, but what it should do.”

In addition to awarding Sava the $10,000 grand prize, the judges recognized two runners-up with $5,000 each: Cordiana Cozier, a PhD candidate in the Department of Chemistry, for her paper on the use of AI as a cognitive buffer for public defenders; and Strahinja Janjusevic, a graduate student in the Technology and Policy Program in the Institute for Data, Systems, and Society, for his submission on agency and ownership in the field of neural-controlled prosthetics. The judges also named four honorable mentions, each of whom received a $500 cash prize.

Next generation batteries that use new electrolyte materials could achieve far higher energy density than today’s lithium-ion batteries, without many of the safety concerns. But advanced batteries, such as those that use solid or almost-solid electrolytes, have been plagued by the formation of tiny spikes of lithium metal called dendrites that cause the batteries to lose efficiency and fail.

Exactly how those dendrites form is still up for debate. While the interface between the battery’s electrolyte and electrodes has been the focus of most research, another culprit is the boundary where two grains of electrolyte in a solid material meet. Researchers know these boundaries can seed dendrites within electrolytes, although the effects have been difficult to study.

Now researchers at MIT and the Technical University of Munich have uncovered why such boundaries can lead to dendrites: Hidden electrical imbalances across the boundaries affect how the electrolyte conducts electrical charges, which influences how the ions and electrons move through the material during battery operation. In a paper published today in Nature Nanotechnology, the researchers characterized the electrical and chemical behavior of the boundaries and showed that adjusting how the electrolyte is processed enhances the movement of ions while reducing electron leakage. This adjustment can increase critical current density by more than 300 percent, which could enable solid-state batteries that charge faster and last longer.

“Grain boundaries are like the weather: Everyone talks about it, but nobody does anything about it,” says senior author Harry Tuller, a professor in MIT’s Department of Materials Science and Engineering. “In this paper, we’ve decided to do something about grain boundaries, and by doing something we’ve shown improved performance and demonstrated the importance of grain boundaries more broadly.”

Joining Tuller on the paper are first author Hyunwon Chu PhD ’25; former MIT professor Jennifer Rupp, the Electrochemical Material Professor at the Technical University of Munich (TUM), who led the study; TUM researchers Waldemar Kaiser, Lukas Wolz, Fran Kurnia, Kun Joong Kim, David Egger, and Johanna Eichhorn; Thomas Defferriere PhD ’22; Willis O’Leary PhD ’24; and University of Antwerp researchers Proloy Nandi, Johan Verbeeck, Sara Bals, and Thomas Altantzis.

Investigating grain boundaries

Rupp’s research group, which moved from MIT to TUM during this research, has spent years studying the behavior of next-generation electrolyte materials. Electrolytes in solid-state batteries are made of many tiny crystals of material packed together.

“What we call a grain, like a grain of salt, is actually a single crystal, but it might only be on the order of 1 micron in size,” explains Tuller. “Under high temperature processes, the best materials essentially consolidate to be void or pore-free and can be nearly 100 percent dense, but each of those crystallites is separated from its neighbor by a grain boundary.”

Solid-state battery researchers have increasingly focused on grain boundaries as the source of the lithium metal dendrites that cause them to short circuit. It’s been suspected that grain boundaries have different chemical and electrical properties from the grains, which interact with the ions and electrons shuttling between electrodes during battery charging and discharging. However, the exact mechanisms by which the boundaries slowed the ions down, leaked electrons, and led to dendrites was unknown.

“Grain boundaries are like defects,” Tuller says. “The boundaries have a higher level of defects than in the grains themselves, and generally that means as carriers of charge approach the boundary, whether electrons or ions, there’s some kind of blockage to overcome.”

To better understand that interference, the researchers developed a model to explain how local electrical imbalances at grain boundaries change the movement of lithium ions and electronic charge carriers. They tested the model in a common solid electrolyte material called lithium lanthanum zirconate, or LLZO, using techniques including electron microscopy, machine learning modeling, and electrochemical impedance spectroscopy, which measures how easily a charge moves through a material.

They found the cores of the boundaries carry a local electrical charge, building up local electric fields that lead to enhanced ionic resistance while causing a build-up of electrons in the boundary region, where they can reduce lithium ions, leading to lithium metal dendrite formation.

“For the last 30 years, the world has been dominated by lithium-ion batteries, but there is a growing recognition that other battery types are needed for batteries used in a variety of uses,” Rupp explains. “This work gives us the fundamental understanding of the space charge interface at the grain boundary. If understood properly, we can come up with engineering concepts to increase cycle life, transference of ions over electrons at these interfaces, and ultimately a better battery.”

Better battery materials

The researchers used their observations to adjust the material processing conditions of the LLZO electrolyte material and minimize the negative charges at the boundaries, finding they could ease the movement of lithium ions and reduce the leakage of electrons.

The modifications allowed them to create an electrolyte that had a critical current density more than 300 percent higher than a baseline sample. Higher current density allows for faster charging and discharging. It should also delay short circuiting to extend the life of batteries.

“Fires are currently a huge issue in the battery industry,” Rupp says. “By showing how to engineer these space charges in a controlled way, which is new in the field, we can have a strong impact on safety. It’s a new way to turn up the notch and get these batteries to charge faster and last longer before they break.”

The findings, along with the researchers’ engineering work, present a roadmap for battery researchers to accelerate the development of high-performance, longer lasting solid-state batteries.

“We showed we can control the initiation of these dendrites to maximize solid state batteries’ high performance,” Chu says. “In this paper, we started with a theory for how these dendrites form, then we did the material characterization to support that theory, then we did the engineering to apply the findings and actually improve battery performance.”

The work was supported, in part, by the National Science Foundation and the U.S. Department of Homeland Security.