materials science

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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building-robots-for-“zero-mass”-space-exploration

Building robots for “Zero Mass” space exploration

materials science, robotics, Science, Space, Space exploration / /
A robot performing construction on the surface of the moon against the black backdrop of space.

Sending 1 kilogram to Mars will set you back roughly $2.4 million, judging by the cost of the Perseverance mission. If you want to pack up supplies and gear for every conceivable contingency, you’re going to need a lot of those kilograms.

But what if you skipped almost all that weight and only took a do-it-all Swiss Army knife instead? That’s exactly what scientists at NASA Ames Research Center and Stanford University are testing with robots, algorithms, and highly advanced building materials.

Zero mass exploration

“The concept of zero mass exploration is rooted in self-replicating machines, an engineering concept John von Neumann conceived in the 1940s”, says Kenneth C. Cheung, a NASA Ames researcher. He was involved in the new study published recently in Science Robotics covering self-reprogrammable metamaterials—materials that do not exist in nature and have the ability to change their configuration on their own. “It’s the idea that an engineering system can not only replicate, but sustain itself in the environment,” he adds.

Based on this concept, Robert A. Freitas Jr. in the 1980s proposed a self-replicating interstellar spacecraft called the Von Neumann probe that would visit a nearby star system, find resources to build a copy of itself, and send this copy to another star system. Rinse and repeat.

“The technology of reprogrammable metamaterials [has] advanced to the point where we can start thinking about things like that. It can’t make everything we need yet, but it can make a really big chunk of what we need,” says Christine E. Gregg, a NASA Ames researcher and the lead author of the study.

Building blocks for space

One of the key problems with Von Neumann probes was that taking elements found in the soil on alien worlds and processing them into actual engineering components was resource-intensive and required huge amounts of energy. The NASA Ames team solved that with using prefabricated “voxels”—standardized reconfigurable building blocks.

The system derives its operating principles from the way nature works on a very fundamental level. “Think how biology, one of the most scalable systems we have ever seen, builds stuff,” says Gregg. “It does that with building blocks. There are on the order of 20 amino acids which your body uses to make proteins to make 200 different types of cells and then combines trillions of those cells to make organs as complex as my hair and my eyes. We are using the same strategy,” she adds.

To demo this technology, they built a set of 256 of those blocks—extremely strong 3D structures made with a carbon-fiber-reinforced polymer called StattechNN-40CF. Each block had fastening interfaces on every side that could be used to reversibly attach them to other blocks and form a strong truss structure.

A 3×3 truss structure made with these voxels had an average failure load of 900 Newtons, which means it could hold over 90 kilograms despite being incredibly light itself (its density is just 0.0103 grams per cubic centimeter). “We took these voxels out in backpacks and built a boat, a shelter, a bridge you could walk on. The backpacks weighed around 18 kilograms. Without technology like that, you wouldn’t even think about fitting a boat and a bridge in a backpack,” says Cheung. “But the big thing about this study is that we implemented this reconfigurable system autonomously with robots,” he adds.

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a-locally-grown-solution-for-period-poverty

A locally grown solution for period poverty

agave, agriculture, materials science, Science, Sustainability / /

Absorbant agave —

A Kenyan tinkerer and Stanford engineer team up to make maxi pads from agave fibers.

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Image of rows of succulents with long spiky leaves and large flower stalks.

Enlarge / Sisal is an invasive species that is also grown agriculturally.

Women and girls across much of the developing world lack access to menstrual products. This means that for at least a week or so every month, many girls don’t go to school, so they fall behind educationally and often never catch up economically. 

Many conventional menstrual products have traditionally been made of hydrogels made from toxic petrochemicals, so there has been a push to make them out of biomaterials. But this usually means cellulose from wood, which is in high demand for other purposes and isn’t readily available in many parts of the globe. So Alex Odundo found a way to solve both of these problems: making maxi pads out of sisal, a drought-tolerant agave plant that grows readily in semi-arid climates like his native Kenya.

Putting an invasive species to work

Sisal is an invasive plant in rural Kenya, where it is often planted as livestock fencing and feedstock. It doesn’t require fertilizer, and its leaves can be harvested all year long over a five- to seven-year span. Odundo and his partners in Manu Prakash’s lab at Stanford University developed a process to generate soft, absorbent material from the sisal leaves. It relies on treatment with dilute peroxyformic acid (1 percent) to increase its porosity, followed by washing in sodium hydroxide (4 percent) and then spinning in a tabletop blender to enhance porosity and make it softer. 

They tested their fibers with a mixture of water mixed with glycerol—to make it thicker, like blood—and found that it is as absorbent as the cotton used in commercially available maxi pads. It was also as absorbent as wood pulp and more absorbent than fibers prepared from other biomaterials, including hemp and flax. Moreover, their process is less energy-intensive than conventional processing procedures, which are typically performed at higher temperatures and pressures. 

In a cradle-to-gate carbon footprint life cycle analysis, including sisal cultivation, harvesting, manufacturing, and transportation, sisal cellulose microfiber production fared roughly the same as production of cellulose microfiber from wood and much better than that from cotton in terms of both carbon footprint and water consumption, possibly because cotton requires so much upstream fertilizer. Much of the footprint comes from transportation, highlighting how useful it can be to make products like this in the same communities that need them.

Science for the greater good

This is not Odundo’s first foray into utilizing sisal; at Olex Techno Enterprises in Kisumu, Kenya, he has been making machines to turn sisal leaves into rope for over 10 years. This benefits local farmers since sisal rope and even sisal fibers sell for ten times as much as sisal leaves. In addition to making maxi pads, Odundo also built a stove that burns sawdust, rice husks, and other biodegradable waste products. 

By reducing wood stoves, he is reducing deforestation and improving the health of the women who breathe in the smoke of the cookfires. Adoption of such stoves have been a goal of environmentalists for years, and although a number of prototypes have been developed by mostly male engineers in developed countries, they have not been widely used because they are not that practical or appealing to the mostly female cooks in developing countries—the people who actually need to cook with them, yet were not consulted in their design.

Manu Prakash’s lab’s website proclaims that “we are dedicated toward inventing and distributing ‘frugal science’ tools to democratize access to science.” Partnering with Alex Odundo to manufacture menstrual products in the low-income rural communities that most need them seems like the apotheosis of that goal.

Communications Engineering, 2023. DOI:  10.1038/s44172-023-00130-y

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