polymers

hydrogels-can-learn-to-play-pong

Hydrogels can learn to play Pong

active matter, chemistry, hydrogels, materials science, Physics, polymers, Pong, Science, smart materials / /

It’s all about the feedback loops —

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

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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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this-stretchy-electronic-material-hardens-upon-impact-just-like-“oobleck”

This stretchy electronic material hardens upon impact just like “oobleck”

conductive polymers, flexible electronics, kitchen science, materials science, oobleck, Physics, polymers, Science / /

a flexible alternative —

Researchers likened material’s structure to a big bowl of spaghetti and meatballs.

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This flexible and conductive material has “adaptive durability,” meaning it gets stronger when hit.

Enlarge / This flexible and conductive material has “adaptive durability,” meaning it gets stronger when hit.

Yue (Jessica) Wang

Scientists are keen to develop new materials for lightweight, flexible, and affordable wearable electronics so that, one day, dropping our smartphones won’t result in irreparable damage. One team at the University of California, Merced, has made conductive polymer films that actually toughen up in response to impact rather than breaking apart, much like mixing corn starch and water in appropriate amounts produces a slurry that is liquid when stirred slowly but hardens when you punch it (i.e., “oobleck”). They described their work in a talk at this week’s meeting of the American Chemical Society in New Orleans.

“Polymer-based electronics are very promising,” said Di Wu, a postdoc in materials science at UCM. “We want to make the polymer electronics lighter, cheaper, and smarter. [With our] system, [the polymers] can become tougher and stronger when you make a sudden movement, but… flexible when you just do your daily, routine movement. They are not constantly rigid or constantly flexible. They just respond to your body movement.”

As we’ve previously reported, oobleck is simple and easy to make. Mix one part water to two parts corn starch, add a dash of food coloring for fun, and you’ve got oobleck, which behaves as either a liquid or a solid, depending on how much stress is applied. Stir it slowly and steadily and it’s a liquid. Punch it hard and it turns more solid under your fist. It’s a classic example of a non-Newtonian fluid.

In an ideal fluid, the viscosity largely depends on temperature and pressure: Water will continue to flow regardless of other forces acting upon it, such as being stirred or mixed. In a non-Newtonian fluid, the viscosity changes in response to an applied strain or shearing force, thereby straddling the boundary between liquid and solid behavior. Stirring a cup of water produces a shearing force, and the water shears to move out of the way. The viscosity remains unchanged. But for non-Newtonian fluids like oobleck, the viscosity changes when a shearing force is applied.

Ketchup, for instance, is a shear-thickening non-Newtonian fluid, which is one reason smacking the bottom of the bottle doesn’t make the ketchup come out any faster; the application of force increases the viscosity. Yogurt, gravy, mud, and pudding are other examples. And so is oobleck. (The name derives from a 1949 Dr. Seuss children’s book, Bartholomew and the Oobleck.) By contrast, non-drip paint exhibits a “shear-thinning” effect, brushing on easily but becoming more viscous once it’s on the wall. Last year, MIT scientists confirmed that the friction between particles was critical to that liquid-to-solid transition, identifying a tipping point when the friction reached a certain level and the viscosity abruptly increased.

Wu works in the lab of materials scientist Yue (Jessica) Wang, who decided to try to mimic the shear-thickening behavior of oobleck in a polymer material. Flexible polymer electronics are usually made by linking together conjugated conductive polymers, which are long and thin, like spaghetti. But these materials will still break apart in response to particularly large and/or rapid impacts.

So Wu and Wang decided to combine the spaghetti-like polymers with shorter polyaniline molecules and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, or PEDOT:PSS—four different polymers in all. Two of the four have a positive charge, and two have a negative charge. They used that mixture to make stretchy films and then tested the mechanical properties.

Lo and behold, the films behaved very much like oobleck, deforming and stretching in response to impact rather than breaking apart. Wang likened the structure to a big bowl of spaghetti and meatballs since the positively charged molecules don’t like water and therefore cluster into ball-like microstructures. She and Wu suggest that those microstructures absorb impact energy, flattening without breaking apart. And it doesn’t take much PEDOT:PSS to get this effect: just 10 percent was sufficient.

Further experiments identified an even more effective additive: positively charged 1,3-propanediamine nanoparticles. These particles can weaken the “meatball” polymer interactions just enough so that they can deform even more in response to impacts, while strengthening the interactions between the entangled long spaghetti-like polymers.

The next step is to apply their polymer films to wearable electronics like smartwatch bands and sensors, as well as flexible electronics for monitoring health. Wang’s lab has also experimented with a new version of the material that would be compatible with 3D printing, opening up even more opportunities. “There are a number of potential applications, and we’re excited to see where this new, unconventional property will take us,” said Wang.

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