materials science

Scientists made a stretchable lithium battery you can bend, cut, or stab

aeorogels, batteries, chemistry, electrolytes, materials science, Science / Mike M. / April 15, 2025

The Li-ion batteries that power everything from smartphones to electric cars are usually packed in rigid, sealed enclosures that prevent stresses from damaging their components and keep air from coming into contact with their flammable and toxic electrolytes. It’s hard to use batteries like this in soft robots or wearables, so a team of scientists at the University California, Berkeley built a flexible, non-toxic, jelly-like battery that could survive bending, twisting, and even cutting with a razor.

While flexible batteries using hydrogel electrolytes have been achieved before, they came with significant drawbacks. “All such batteries could [only] operate [for] a short time, sometimes a few hours, sometimes a few days,” says Liwei Lin, a mechanical engineering professor at UC Berkeley and senior author of the study. The battery built by his team endured 500 complete charge cycles—about as many as the batteries in most smartphones are designed for.

Power in water

“Current-day batteries require a rigid package because the electrolyte they use is explosive, and one of the things we wanted to make was a battery that would be safe to operate without this rigid package,” Lin told Ars. Unfortunately, flexible packaging made of polymers or other stretchable materials can be easily penetrated by air or water, which will react with standard electrolytes, generating lots of heat, potentially resulting in fires and explosions. This is why, in 2017, scientists started to experiment with quasi-solid-state hydrogel electrolytes.

These hydrogels were made of a polymer net that gave them their shape, crosslinkers like borax or hydrogen bonds that held this net together, a liquid phase made of water, and salt or other electrolyte additives providing ions that moved through the watery gel as the battery charged or discharged.

But hydrogels like that had their own fair share of issues. The first was a fairly narrow electrochemical stability window—a safe zone of voltage the battery can be exposed to. “This really limits how much voltage your battery can output,” says Peisheng He, a researcher at UC Berkeley Sensor and Actuator Center and lead author of the study. “Nowadays, batteries usually operate at 3.3 volts, so their stability window must be higher than that, probably four volts, something like that.” Water, which was the basis of these hydrogel electrolytes, typically broke down into hydrogen and oxygen when exposed to around 1.2 volts. That problem was solved by using highly concentrated salt water loaded with highly fluorinated lithium salts, which made it less likely to break down. But this led the researchers straight into safety issues, as fluorinated lithium salts are highly toxic to humans.

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Researchers get spiking neural behavior out of a pair of transistors

materials science, neural networks, Science, semiconductor, silicon, spiking neurons / Paul Patrick / March 28, 2025

The team found that, when set up to operate on the verge of punch-through mode, it was possible to use the gate voltage to control the charge build-up in the silicon, either shutting the device down or enabling the spikes of activity that mimic neurons. Adjustments to this voltage could allow different frequencies of spiking. Those adjustments could be made using spikes as well, essentially allowing spiking activity to adjust the weights of different inputs.

With the basic concept working, the team figured out how to operate the hardware in two modes. In one of them, it acts like an artificial synapse, capable of being set into any of six (and potentially more) weights, meaning the potency of the signals it passes on to the artificial neurons in the next layer of a neural network. These weights are a key feature of neural networks like large language models.

But when combined with a second transistor to help modulate its behavior, it was possible to have the transistor act like a neuron, integrating inputs in a way that influenced the frequency of the spikes it sends on to other artificial neurons. The spiking frequency could range in intensity by as much as a factor of 1,000. And the behavior was stable for over 10 million clock cycles.

All of this simply required standard transistors made with CMOS processes, so this is something that could potentially be put into practice fairly quickly.

Pros and cons

So what advantages does this have? It only requires two transistors, meaning it’s possible to put a lot of these devices on a single chip. “From the synaptic perspective,” the researchers argue, “a single device could, in principle, replace static random access memory (a volatile memory cell comprising at least six transistors) in binarized weight neural networks, or embedded Flash in multilevel synaptic arrays, with the immediate advantage of a significant area and cost reduction per bit.”

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The seemingly indestructible fists of the mantis shrimp can take a punch

Biomechanics, Mantis shrimp, materials science, Science / Rejus Almole / February 22, 2025

To find out how much force a mantis shrimp’s dactyl clubs can possibly withstand, the researchers tested live shrimp by having them strike a piezoelectric sensor like they would smash a shell. They also fired ultrasonic and hypersonic lasers at pieces of dactyl clubs from their specimens so they could see how the clubs defended against sound waves.

By tracking how sound waves propagated on the surface of the dactyl club, the researchers could determine which regions of the club diffused the most waves. It was the second layer, the impact surface, that handled the highest levels of stress. The periodic surface was almost as effective. Together, they made the dactyl clubs nearly immune to the stresses they generate.

There are few other examples that the protective structures of the mantis shrimp can be compared to. On the prey side, evidence has been found that the scales on some moths’ wings absorb sound waves from predatory bats to keep them from echolocation to find them.

Understanding how mantis shrimp defend themselves from extreme force could inspire new technology. The structures in their dactyl clubs could influence the designs of military and athletic protective gear in the future.

“Shrimp impacts contain frequencies in the ultrasonic range, which has led to shrimp-inspired solutions that point to ultrasonic filtering as a key [protective] mechanism,” the team said in the same study.

Maybe someday, a new bike helmet model might have been inspired by a creature that is no more than seven inches long but literally doesn’t crack under pressure.

Science, 2025. DOI: 10.1126/science.adq7100

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Researchers figure out how to get fresh lithium into batteries

batteries, chemistry, materials science, Science / Mike M. / February 21, 2025

In their testing, they use a couple of unusual electrode materials, such as a chromium oxide (Cr₈O₂₁) and an organic polymer (a sulfurized polyacrylonitrile). Both of these have significant weight advantages over the typical materials used in today’s batteries, although the resulting batteries typically lasted less than 500 cycles before dropping to 80 percent of their original capacity.

But the striking experiment came when they used LiSO₂CF₃ to rejuvenate a battery that had been manufactured as normal but had lost capacity due to heavy use. Treating a lithium-iron phosphate battery that had lost 15 percent of its original capacity restored almost all of what was lost, allowing it to hold over 99 percent of its original charge. They also ran a battery for repeated cycles with rejuvenation every few thousand cycles. At just short of 12,000 cycles, it still could be restored to 96 percent of its original capacity.

Before you get too excited, there are a couple of things worth noting about lithium-iron phosphate cells. The first is that, relative to their charge capacity, they’re a bit heavy, so they tend to be used in large, stationary batteries like the ones in grid-scale storage. They’re also long-lived on their own; with careful management, they can take over 8,000 cycles before they drop to 80 percent of their initial capacity. It’s not clear whether similar rejuvenation is possible in the battery chemistries typically used for the sorts of devices that most of us own.

The final caution is that the battery needs to be modified so that fresh electrolytes can be pumped in and the gases released by the breakdown of the LiSO₂CF₃ removed. It’s safest if this sort of access is built into the battery from the start, rather than provided by modifying it much later, as was done here. And the piping needed would put a small dent in the battery’s capacity per volume if so.

All that said, the treatment demonstrated here would replenish even a well-managed battery closer to its original capacity. And it would largely restore the capacity of something that hadn’t been carefully managed. And that would allow us to get far more out of the initial expense of battery manufacturing. Meaning it might make sense for batteries destined for a large storage facility, where lots of them could potentially be treated at the same time.

Nature, 2025. DOI: 10.1038/s41586-024-08465-y (About DOIs).

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The physics of ugly Christmas sweaters

knitting, materials science, Physics, Science, ugly Christmas sweaters / Paul Patrick / December 27, 2024

In 2018, a team of French physicists developed a rudimentary mathematical model to describe the deformation of a common type of knit. Their work was inspired when co-author Frédéric Lechenault watched his pregnant wife knitting baby booties and blankets, and he noted how the items would return to their original shape even after being stretched. With a few colleagues, he was able to boil the mechanics down to a few simple equations, adaptable to different stitch patterns. It all comes down to three factors: the “bendiness” of the yarn, the length of the yarn, and how many crossing points are in each stitch.

A simpler stitch

A simplified model of how yarns interact Credit: J. Crassous/University of Rennes

One of the co-authors of that 2018 paper, Samuel Poincloux of Aoyama Gakuin University in Japan, also co-authored this latest study with two other colleagues, Jérôme Crassous (University of Rennes in France) and Audrey Steinberger (University of Lyon). This time around, Poincloux was interested in the knotty problem of predicting the rest shape of a knitted fabric, given the yarn’s length by stitch—an open question dating back at least to a 1959 paper.

It’s the complex geometry of all the friction-producing contact zones between the slender elastic fibers that makes such a system too difficult to model precisely, because the contact zones can rotate or change shape as the fabric moves. Poincloux and his cohorts came up with their own more simplified model.

The team performed experiments with a Jersey stitch knit (aka a stockinette), a widely used and simple knit consisting of a single yarn (in this case, a nylon thread) forming interlocked loops. They also ran numerical simulations modeled on discrete elastic rods coupled with dry contacts with a specific friction coefficient to form meshes.

The results: Even when there were no external stresses applied to the fabric, the friction between the threads served as a stabilizing factor. And there was no single form of equilibrium for a knitted sweater’s resting shape; rather, there were multiple metastable states that were dependent on the fabric’s history—the different ways it had been folded, stretched, or rumpled. In short, “Knitted fabrics do not have a unique shape when no forces are applied, contrary to the relatively common belief in textile literature,” said Crassous.

DOI: Physical Review Letters, 2024. 10.1103/PhysRevLett.133.248201 (About DOIs).

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Generating power with a thin, flexible thermoelectric film

cooling, materials science, power, Science, thermoelectric material / Rejus Almole / December 12, 2024

The No. 1 nuisance with smartphones and smartwatches is that we need to charge them every day. As warm-blooded creatures, however, we generate heat all the time, and that heat can be converted into electricity for some of the electronic gadgetry we carry.

Flexible thermoelectric devices, or F-TEDs, can convert thermal energy into electric power. The problem is that F-TEDs weren’t actually flexible enough to comfortably wear or efficient enough to power even a smartwatch. They were also very expensive to make.

But now, a team of Australian researchers thinks they finally achieved a breakthrough that might take F-TEDs off the ground.

“The power generated by the flexible thermoelectric film we have created would not be enough to charge a smartphone but should be enough to keep a smartwatch going,” said Zhi-Gang Chen, a professor at Queensland University of Technology in Brisbane, Australia. Does that mean we have reached a point where it would be possible to make a thermoelectric Apple Watch band that could keep the watch charged all the time? “It would take some industrial engineering and optimization, but we can definitely achieve a smartwatch band like that,” Chen said.

Manufacturing heaven

Thermoelectric generators producing enough power to run something like an Apple Watch were, so far, made with rigid bulk materials. The obvious problem with them was that nobody would want to wear a metal slab on their wrist or run a power cable from anywhere else to their watch. Flexible thermoelectric devices, on the other hand, were perfectly wearable but offered efficiencies that made them good for low-power health-monitoring electronics rather than more power-hungry hardware like smartwatches.

Back in 2021, generating 35 microwatts per square centimeter in a wristband worn during a typical walk outside was impressive enough to land your research paper in Nature. Today, Chen and his colleagues made a flexible thermoelectric device that performed over 34 times better at room temperature. “To the best of our knowledge, we hold a current record in this field,” Chen says.

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What makes baseball’s “magic mud” so special?

adhesion, baseball, geomaterials, materials science, Non-newtonian fluids, Physics, rheology, Science, soft matter / Tim Belzer / November 7, 2024

“Magic mud” composition and microstructure: (top right) a clean baseball surface; (bottom right) a mudded baseball. Credit: S. Pradeep et al., 2024

Pradeep et al. found that magic mud’s particles are primarily silt and clay, with a bit of sand and organic material. The stickiness comes from the clay, silt, and organic matter, while the sand makes it gritty. So the mud “has the properties of skin cream,” they wrote. “This allows it to be held in the hand like a solid but also spread easily to penetrate pores and make a very thin coating on the baseball.”

When the mud dries on the baseball, however, the residue left behind is not like skin cream. That’s due to the angular sand particles bonded to the baseball by the clay, which can increase surface friction by as much as a factor of two. Meanwhile, the finer particles double the adhesion. “The relative proportions of cohesive particulates, frictional sand, and water conspire to make a material that flows like skin cream but grips like sandpaper,” they wrote.

Despite its relatively mundane components, the magic mud nonetheless shows remarkable mechanical behaviors that the authors think would make it useful in other practical applications. For instance, it might replace synthetic materials as an effective lubricant, provided the gritty sand particles are removed. Or it could be used as a friction agent to improve traction on slippery surfaces, provided one could define the optimal fraction of sand content that wouldn’t diminish its spreadability. Or it might be used as a binding agent in locally sourced geomaterials for construction.

“As for the future of Rubbing Mud in Major League Baseball, unraveling the mystery of its behavior does not and should not necessarily lead to a synthetic replacement,” the authors concluded. “We rather believe the opposite; Rubbing Mud is a nature-based material that is replenished by the tides, and only small quantities are needed for great effect. In a world that is turning toward green solutions, this seemingly antiquated baseball tradition provides a glimpse of a future of Earth-inspired materials science.”

DOI: PNAS, 2024. 10.1073/pnas.241351412 (About DOIs).

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“Impact printing” is a cement-free alternative to 3D-printed structures

3D printing, construction, materials science, Science / Mike M. / October 29, 2024

Recently, construction company ICON announced that it is close to completing the world’s largest 3D-printed neighborhood in Georgetown, Texas. This isn’t the only 3D-printed housing project. Hundreds of 3D-printed homes are under construction in the US and Europe, and more such housing projects are in the pipeline.

There are many factors fueling the growth of 3D printing in the construction industry. It reduces the construction time; a home that could take months to build can be constructed within days or weeks with a 3D printer. Compared to traditional methods, 3D printing also reduces the amount of material that ends up as waste during construction. These advantages lead to reduced labor and material costs, making 3D printing an attractive choice for construction companies.

A team of researchers from the Swiss Federal Institute of Technology (ETH) Zurich, however, claims to have developed a robotic construction method that is even better than 3D printing. They call it impact printing, and instead of typical construction materials, it uses Earth-based materials such as sand, silt, clay, and gravel to make homes. According to the researchers, impact printing is less carbon-intensive and much more sustainable and affordable than 3D printing.

This is because Earth-based materials are abundant, recyclable, available at low costs, and can even be excavated at the construction site. “We developed a robotic tool and a method that could take common material, which is the excavated material on construction sites, and turn it back into usable building products, at low cost and efficiently, with significantly less CO₂ than existing industrialized building methods, including 3D printing,” said Lauren Vasey, one of the researchers and an SNSF Bridge Fellow at ETH Zurich.

How does impact printing work?

Excavated materials can’t be used directly for construction. So before beginning the impact printing process, researchers prepare a mix of Earth-based materials that has a balance of fine and coarse particles, ensuring both ease of use and structural strength. Fine materials like clay act as a binder, helping the particles stick together, while coarser materials like sand or gravel make the mix more stable and strong. This optimized mix is designed such that it can move easily through the robotic system without getting stuck or causing blockages.

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Graphene-enhanced ceramic tiles make striking art

chemistry, graphene, materials science, Science, science and art / Paul Patrick / October 28, 2024

In recent years, materials scientists experimenting with ceramics have started adding an oxidized form of graphene to the mix to produce ceramics that are tougher, more durable, and more resistant to fracture, among other desirable properties. Researchers at the National University of Singapore (NUS) have developed a new method that uses ultrasound to more evenly distribute graphene oxide (GO) in ceramics, according to a new paper published in the journal ACS Omega. And as a bonus, they collaborated with an artist who used the resulting ceramic tiles to create a unique art exhibit at the NUS Museum—a striking merger of science and art.

As reported previously, graphene is the thinnest material yet known, composed of a single layer of carbon atoms arranged in a hexagonal lattice. That structure gives it many unusual properties that hold great promise for real-world applications: batteries, super capacitors, antennas, water filters, transistors, solar cells, and touchscreens, just to name a few.

In 2021, scientists found that this wonder material might also provide a solution to the fading of colors of many artistic masterpieces. For instance, several of Georgia O’Keeffe’s oil paintings housed in the Georgia O’Keeffe Museum in Santa Fe, New Mexico, have developed tiny pin-sized blisters, almost like acne, for decades. Conservators have found similar deterioration in oil-based masterpieces across all time periods, including works by Rembrandt.

Van Gogh’s Sunflower series has been fading over the last century due to constant exposure to light. A 2011 study found that chromium in the chrome yellow Van Gogh favored reacted strongly with other compounds like barium and sulfur when exposed to sunlight. A 2016 study pointed the finger at the sulfates, which absorb in the UV spectrum, leading to degradation.

Even contemporary art materials are prone to irreversible color changes from exposure to light and oxidizing agents, among other hazards. That’s why there has been recent work on the use of nanomaterials for conservation of artworks. Graphene has a number of properties that make it attractive for art-conservation purposes. The one-atom-thick material is transparent, adheres easily to various substrates, and serves as an excellent barrier against oxygen, gases (corrosive or otherwise), and moisture. It’s also hydrophobic and is an excellent absorber of UV light.

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Octopus suckers inspire new tech for gripping objects underwater

adhesives, Biology, biomimicry, cephalopods, materials science, octopuses, Science / Rejus Almole / October 9, 2024

Over the last few years, Virginia Tech scientists have been looking to the octopus for inspiration to design technologies that can better grip a wide variety of objects in underwater environments. Their latest breakthrough is a special switchable adhesive modeled after the shape of the animal’s suckers, according to a new paper published in the journal Advanced Science.

“I am fascinated with how an octopus in one moment can hold something strongly, then release it instantly. It does this underwater, on objects that are rough, curved, and irregular—that is quite a feat,” said co-author and research group leader Michael Bartlett. “We’re now closer than ever to replicating the incredible ability of an octopus to grip and manipulate objects with precision, opening up new possibilities for exploration and manipulation of wet or underwater environments.”

As previously reported, there are several examples in nature of efficient ways to latch onto objects in underwater environments, per the authors. Mussels, for instance, secrete adhesive proteins to attach themselves to wet surfaces, while frogs have uniquely structured toe pads that create capillary and hydrodynamic forces for adhesion. But cephalopods like the octopus have an added advantage: The adhesion supplied by their grippers can be quickly and easily reversed, so the creatures can adapt to changing conditions, attaching to wet and dry surfaces.

From a mechanical engineering standpoint, the octopus has an active, pressure-driven system for adhesion. The sucker’s wide outer rim creates a seal with the object via a pressure differential between the chamber and the surrounding medium. Then muscles (serving as actuators) contract and relax the cupped area behind the rim to add or release pressure as needed.

There have been several attempts to mimic cephalopods when designing soft robotic grippers, for example. Back in 2022, Bartlett and his colleagues wanted to go one step further and recreate not just the switchable adhesion but also the integrated sensing and control. The result was Octa-Glove, a wearable system for gripping underwater objects that mimicked the arm of an octopus.

Improving the Octa-Glove

Grabbing and releasing underwater objects of different sizes and shapes with an octopus-inspired adhesive. Credit: Chanhong Lee and Michael Bartlett

For the adhesion, they designed silicone stalks capped with a pneumatically controlled membrane, mimicking the structure of octopus suckers. These adhesive elements were then integrated with an array of LIDAR optical proximity sensors and a micro-control for the real-time detection of objects. When the sensors detect an object, the adhesion turns on, mimicking the octopus’s nervous and muscular systems. The team used a neoprene wetsuit glove as a base for the wearable glove, incorporating the adhesive elements and sensors in each finger, with flexible pneumatic tubes inserted at the base of the adhesive elements.

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Natural piezoelectric effect may build gold deposits

chemistry, geology, gold, materials science, Quartz, Science / Rejus Almole / September 3, 2024

Building the bling —

How does an unreactive, barely soluble metal end up forming giant chunks?

John Timmer – Sep 3, 2024 6: 37 pm UTC

Image of a white rock with gold and black deposits speckled throughout it. — Enlarge / A lot of gold deposits are found embedded in quartz crystals.

One of the reasons gold is so valuable is because it is highly unreactive—if you make something out of gold, it keeps its lustrous radiance. Even when you can react it with another material, it’s also barely soluble, a combination that makes it difficult to purify away from other materials. Which is part of why a large majority of the gold we’ve obtained comes from deposits where it is present in large chunks, some of them reaching hundreds of kilograms.

Those of you paying careful attention to the previous paragraph may have noticed a problem here: If gold is so difficult to get into its pure form, how do natural processes create enormous chunks of it? On Monday, a group of Australian researchers published a hypothesis, and a bit of evidence supporting it. They propose that an earthquake-triggered piezoelectric effect essentially electroplates gold onto quartz crystals.

The hypothesis

Approximately 75 percent of the gold humanity has obtained has come from what are called orogenic gold deposits. Orogeny is a term for the tectonic processes that build mountains, and orogenic gold deposits form in the seams where two bodies of rock are moving past each other. These areas are often filled with hot hydrothermal fluids, and the heat can increase the solubility of gold from “barely there” to “extremely low,” meaning generally less than a single milligram in a liter of water.

The other striking thing about these deposits is that they’re generally associated with the mineral quartz, a crystalline form of silicon dioxide. And that fact formed the foundation for the new hypothesis, which brings together a number of topics that are generally considered largely unrelated.

It turns out that quartz is the only abundant mineral that’s piezoelectric, meaning that it generates a charge when it’s placed under strain. While you don’t need to understand why that’s the case to follow this hypothesis, the researchers’ explanation of the piezoelectric effect is remarkably cogent and clear, so I’ll just quote it here for people who want to come away from this having learned something: “Quartz is the only common mineral that forms crystals lacking a center of symmetry (non-centrosymmetric). Non-centrosymmetric crystals distorted under stress have an imbalance in their internal electric configuration, which produces an electrical potential—or voltage—across the crystal that is directly proportional to the applied mechanical force.”

Quartz happens to be an insulator, so this electric potential doesn’t easily dissipate on its own. It can, however, be eliminated through the transfer of electrons to or from any materials that touch the quartz crystals, including fluids. In practice, that means the charge can drive redox (reduction/oxidation) reactions in any nearby fluids, potentially neutralizing any dissolved ions and causing them to come out of solution.

This has the potential to be self-reinforcing. Once a small metal deposit forms on the surface of quartz, it will ease the exchange of electrons with the fluid in its immediate vicinity, meaning more metal will be deposited in the same location. This will also lower the concentration of the metal in the nearby solution, which will favor the diffusion of additional metal ions into the location, meaning that the fluid itself doesn’t need to keep circulating past the same spot.

Finally, the concept also needs a source of strain to generate the piezoelectric effect in the first place. But remember that this is all happening in an active fault zone, so strain is not in short supply.

And the evidence

Figuring out whether this happens in active fault zones would be extremely challenging for all sorts of reasons. But it’s relatively easy to dunk some quartz crystals in a solution containing gold and see what happens. So the latter is the route the Australians took.

The gold came in the form of either a solution of gold chloride ions or a suspension of gold nanoparticles. Quartz crystals were either pure quartz or obtained from a gold-rich area and already contained some small gold deposits. The crystals themselves were subject to strain at a frequency similar to that produced by small earthquakes, and the experiment was left to run for an hour.

An hour was enough to get small gold deposits to form on the pure quartz crystals, regardless of whether it was from dissolved gold or suspended gold nanoparticles. In the case of the naturally formed quartz, the gold ended up being deposited on the existing sites where gold metal is present, rather than forming additional deposits.

The researchers note that a lot of the quartz in deposits is disordered rather than in the form of single crystals. In disordered material, there are lots of small crystals oriented randomly, meaning the piezoelectric effect of any one of these crystals is typically canceled out by its neighbors. So, gold will preferentially form on single crystals, which also helps explain why it’s found in large lumps in these deposits.

So, this is a pretty compelling hypothesis—it explains something puzzling, relies on well-established processes, and has a bit of experimental support. Given that activity in active faults is likely to remain both slow and inaccessible, the next steps are probably going to involve getting longer-term information on the rate of deposition through this process and a physical comparison of these deposits with those found in natural settings.

Nature Geoscience, 2024. DOI: 10.1038/s41561-024-01514-1 (About DOIs).

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Hydrogels can learn to play Pong

active matter, chemistry, hydrogels, materials science, Physics, polymers, Pong, Science, smart materials / Paul Patrick / August 22, 2024

It’s all about the feedback loops —

Work could lead to new “smart” materials that can learn and adapt to their environment.

Jennifer Ouellette – Aug 22, 2024 6: 32 pm UTC

This electroactive polymer hydrogel “learned” to play Pong. Credit: Cell Reports Physical Science/Strong et al.

Pong will always hold a special place in the history of gaming as one of the earliest arcade video games. Introduced in 1972, it was a table tennis game featuring very simple graphics and gameplay. In fact, it’s simple enough that even non-living materials known as hydrogels can “learn” to play the game by “remembering” previous patterns of electrical stimulation, according to a new paper published in the journal Cell Reports Physical Science.

“Our research shows that even very simple materials can exhibit complex, adaptive behaviors typically associated with living systems or sophisticated AI,” said co-author Yoshikatsu Hayashi, a biomedical engineer at the University of Reading in the UK. “This opens up exciting possibilities for developing new types of ‘smart’ materials that can learn and adapt to their environment.”

Hydrogels are soft, flexible biphasic materials that swell but do not dissolve in water. So a hydrogel may contain a large amount of water but still maintain its shape, making it useful for a wide range of applications. Perhaps the best-known use is soft contact lenses, but various kinds of hydrogels are also used in breast implants, disposable diapers, EEG and ECG medical electrodes, glucose biosensors, encapsulating quantum dots, solar-powered water purification, cell cultures, tissue engineering scaffolds, water gel explosives, actuators for soft robotics, supersonic shock-absorbing materials, and sustained-release drug delivery systems, among other uses.

In April, Hayashi co-authored a paper showing that hydrogels can “learn” to beat in rhythm with an external pacemaker, something previously only achieved with living cells. They exploited the intrinsic ability of the hydrogels to convert chemical energy into mechanical oscillations, using the pacemaker to apply cyclic compressions. They found that when the oscillation of a gel sample matched the harmonic resonance of the pacemaker’s beat, the system kept a “memory” of that resonant oscillation period and could retain that memory even when the pacemaker was turned off. Such hydrogels might one day be a useful substitute for heart research using animals, providing new ways to research conditions like cardiac arrhythmia.

For this latest work, Hayashi and co-authors were partly inspired by a 2022 study in which brain cells in a dish—dubbed DishBrain—were electrically stimulated in such a way as to create useful feedback loops, enabling them to “learn” to play Pong (albeit badly). As Ars Science Editor John Timmer reported at the time:

Pong proved to be an excellent choice for the experiments. The environment only involves a couple of variables: the location of the paddle and the location of the ball. The paddle can only move along a single line, so the motor portion of things only needs two inputs: move up or move down. And there’s a clear reward for doing things well: you avoid an end state where the ball goes past the paddles and the game stops. It is a great setup for testing a simple neural network.

Put in Pong terms, the sensory portion of the network will take the positional inputs, determine an action (move the paddle up or down), and then generate an expectation for what the next state will be. If it’s interpreting the world correctly, that state will be similar to its prediction, and thus the sensory input will be its own reward. If it gets things wrong, then there will be a large mismatch, and the network will revise its connections and try again.

There were a few caveats—even the best systems didn’t play Pong all that well—but the approach mostly worked. Those systems comprising either mouse or human neurons saw the average length of Pong rallies increase over time, indicating they might be learning the game’s rules. Systems based on non-neural cells, or those lacking a reward system, didn’t see this sort of improvement. The findings provided some evidence that neural networks formed from actual neurons spontaneously develop the ability to learn. And that could explain some of the learning capabilities of actual brains, where smaller groups of neurons are organized into functional units.

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