NPR's Ari Shapiro talks with Regina Barber and Emily Kwong, hosts of the Short Wave podcast, about the mysteries of multicellular organisms, a house built with diapers, and the physics of gummy candy.

Transcript

ARI SHAPIRO, HOST:

Time now for some science news from our friends at NPR's Short Wave podcast. Emily Kwong and Regina Barber are the hosts, and they are here for the latest science roundup. Hi, you both.

REGINA BARBER, BYLINE: Hey, Ari.

EMILY KWONG, BYLINE: Hey, Ari.

SHAPIRO: What have you got for us this week?

KWONG: So we have three stories for you, and this time, they all have to do with structure - with how things get built and become greater than the sum of their parts.

SHAPIRO: I love a theme.

BARBER: Yeah. I mean, one is about the construction of literal buildings, one is about building the perfect gummy candy, and one is about how single-celled organisms might build themselves into more complex ones.

SHAPIRO: Tantalizing. Regina, kick us off. What have you got first?

BARBER: Yeah, Ari. I'm here to bring you news that will take some guilt away from parents around the world - used disposable diapers can be repurposed to produce concrete and build houses.

SHAPIRO: A diaper house? Why would you build a house out of diapers?

BARBER: Well, 'cause it's cheaper and greener. And aside from building regular houses, researchers think it could be useful in disaster relief when you need to build a new house quickly and at low cost and you need to do it with what's lying around. And at a time when there's actually a shortage of sand used in regular concrete, researchers are interested in finding more sustainable alternatives like dirty diapers.

SHAPIRO: I'm sorry. Do the houses smell like dirty diapers?

BARBER: No, they're sanitized.

SHAPIRO: (Laughter) Tell us about one of these houses.

BARBER: OK, yeah. They're testing this out in Indonesia, a country with a significant housing shortage. And a team of researchers led by Siswanti Zuraida actually built a small prototype house, about 400 square feet, using diaper concrete. They shredded the diapers, added chemicals to sanitize them and mixed them into concrete to replace some of the sand. And they published a paper about it this week in the journal Scientific Reports. And they said you can use about 10% of diapers for external load-bearing walls, but for non-structural walls or floors, it could be up to 40% diapers. And it could also be used for roads.

SHAPIRO: Driving on diapers - so, OK, this sounds like, right now, proof of concept. Could it actually be scaled up and be easy and inexpensive?

BARBER: I mean, that's the hope, right? And these scientists told me that they still need to work with local city governments to work on collecting the diapers like they do for recycling. And any community that wants to take this on will probably need to buy some machinery. And there needs to be some further research to make sure this process can be replicated easily and affordably. But yeah, that's the goal - a process where materials are provided locally, and the benefit is also local - like, everything within the community.

SHAPIRO: OK, so diaper houses is story No. 1 about structure. Emily, I understand story No. 2 is a little bit smaller.

KWONG: Much, much smaller, yes, in scale - Ari, we are going all the way back to biology class to that one chapter about single-celled organisms as you might remember from school.

SHAPIRO: I can picture them under a microscope.

KWONG: Yes. Yes. So at various moments in the history of evolution, single-celled organisms evolved into multicellular organisms, giving rise to complex life forms like my cat, your dogs and, of course, you know, us three. And there's a new paper out in the journal Nature describing how that process may have happened, how single cells started building themselves into a multicellular body capable of moving and metabolizing as one.

SHAPIRO: How did researchers in the present day unlock the secrets of this evolution that happened hundreds of millions of years ago?

KWONG: It's a fascinating story. OK, so it started when this guy Will Ratcliff, an evolutionary biologist at Georgia Tech, was in grad school. He wanted to figure out how to encourage single cells to stick together and set up an experiment with brewer's yeast. And his question was, you know, how do you force multicellular evolution in a lab? Here's Will.

WILL RATCLIFF: We knew that we needed a way to give an advantage to things that form groups of cells 'cause we're starting out with just single cells.

KWONG: So every day, Will would swirl the yeast cells in their test tube and extract the ones that sank to the bottom the quickest. He then used that population to grow the next day's population of yeast and repeat it and repeat it and throw out all the other cells.

SHAPIRO: What was so special about the yeast cells that sank to the bottom?

KWONG: Well, it's because they stayed together. Basically, he's hacking biology, creating a selective pressure where yeasts that stick together survive. And within two months, the yeast cells created this branching structure of dozens of cells that looked like a snowflake. Notably, Will had this breakthrough while snow was falling down from the sky.

RATCLIFF: This was sort of an homage to the fact that this started in Minnesota in the middle of winter. Big snowflakes were falling down.

KWONG: And he continued this work with yeast snowflakes, as he calls them, for years. A colleague at Georgia Tech, Ozan Bozdag, determined that if you deprive generations of yeast oxygen, they grow even bigger and stronger, each cell becoming more entangled, the bonds as tough as wood. And that is the kind of development that gives rise to true multicellularity.

SHAPIRO: So what does this tell us about how single-celled organisms became your cat or my dogs?

KWONG: (Laughter) It's a very good question. So that kind of evolution happened dozens of times. Our ancestors are different than yeast, but what these experiments do show is that multicellularity is possible not just because cells stick together. It's because the bonds between them are strong and lasting.

SHAPIRO: There's a metaphor there about strength as a collective. OK, we've saved dessert for last. You've got research about gummy candy. What's that?

BARBER: Yeah, I mean, we saved the most compelling structure for last.

KWONG: Yeah. Researchers at Ozyegin University and Middle East Technical University in Turkey basically wanted to know, how do you keep gummy candies optimally gummy?

SHAPIRO: And that's why we call this show ALL THINGS CONSIDERED.

(LAUGHTER)

SHAPIRO: How do you define optimally gummy?

KWONG: OK, optimally gummy - what do we mean by that? Yeah, you know, we mean shelf-stable and chewy. No one likes stiff gummies, right? So...

SHAPIRO: Right.

KWONG: ...These Turkish researchers - they published their paper in the journal Physics of Fluids this week detailing a bunch of gummy candy experiments. They wanted to know how changing up, say, the glucose-syrup-to-sucrose ratio or storage or temperature conditions would change the end result. And this matters for candy quality. You want to get the best product possible.

BARBER: Yeah. I mean, Ari, as a physicist and as a candy lover, I love this research. They had so many combinations of gummy creation that they had to use statistical modeling to describe it all. They even measured the average length of the bonds between molecules in the candy to make a judgment call about which candy-making method produced the best structure. This is material science at its finest.

SHAPIRO: All right. You're burying the lead. What's the conclusion here? What did they learn?

KWONG: So the best gummy combination, according to this research, for a stable candy with a long shelf life involves reducing the cornstarch and increasing the gelatin in the mix, and to keep them soft, storing them at, like, a warm room temperature 'cause if it gets too cold or too hot, they get stiff.

SHAPIRO: I'm going to take my gummy bears out of the refrigerator as soon as I get home.

BARBER: There is a fun fact, though, Ari. From a material science perspective, this actually totally makes sense because gummy candies are long chains of molecules. And they undergo something called the glass transition meaning that when they get cold, they get harder and more brittle, like glass, and they start to lose some of that flexibility and chewiness that we love in our candy.

SHAPIRO: I think we all get a little less flexible in the cold, wouldn't you say? Emily, Regina, thank you so much for bringing us this cutting-edge research.

KWONG: This was really fun. Thanks for having us.

BARBER: Thank you.

SHAPIRO: Emily Kwong and Regina Barber host NPR's science podcast, Short Wave, where you can learn about new discoveries, everyday mysteries and the science behind the headlines.

(SOUNDBITE OF BABY BASH SONG, "SUGA SUGA") Transcript provided by NPR, Copyright NPR.