Here in the Pacific Northwest we are heading into a possibly historic heatwave. Water — the importance of hydration — is on everyone’s mind. Nature beautifully anticipated our needs.

In The Wonder of Water, Michael Denton showed how the characteristics of Earth’s crust and atmosphere, and the properties of the water molecule itself, make our planet profoundly well suited for life. Once an organism has water available, though, it needs to handle it properly. Cells are equipped to take in the individual molecules of H₂O with exquisite perfection. Larger organisms also are endowed with techniques to gather droplets of water even in dry climates. Scientists are learning how to imitate their expertise.

Cactus Cooler

Many parts of the world have a water crisis. Despite living on a water planet, where three-quarters of the earth’s surface is ocean, only 0.5 percent of that water is fresh water. An article from Caltech reminds one of the old “Acres of Diamonds” motivational speech in which Russell Conwell related the story of a man searching the world to buy diamonds, going broke in the process. The buyer of his land discovered that the property was full of diamonds. He was surrounded by his dream but couldn’t see it. Similarly, humans are surrounded by fresh water that is right in the air they breathe! Even the driest deserts like Atacama in Chile contain water brought in by the atmosphere. The problem is how to gather it into drinkable form. Cacti do it. Can humans?

Cacti have spines shaped for hoarding water droplets and directing them where needed. Inspired by that success, the Caltech team manufactured microscopic spines made of hydrogel. Cactus spines work because “the hierarchically assembled conical structures of Cactus spine are able to continuously harvest fog by driving directional movement of droplets.” In tests, Caltech’s fractal spines, shaped like miniature trees, succeeded in capturing nighttime water from condensation. Fog condensation has been attempted before, but this system offers another feature: daytime water collection from steam. It can work 24×7. As Caltech explains,

The spines are built out of a hydrogel; that is, a network of hydrophilic (water-loving) polymers that naturally attract water. Due to their tiny size, they can be printed onto a wafer-thin membrane. During the day, the hydrogel membrane absorbs sunlight to heat up water trapped beneath it, which becomes steam. The steam then recondenses onto a transparent cover, where it can be collected. During the night, the transparent cover folds up and the hydrogel membrane is exposed to humid air to capture fog. As such, the material can harvest water from both steam and fog. [Emphasis added.]

Engineers Ye Shi, Julia Greer and two colleagues wrote about this in Nature Communications. They had no need of evolution in their work but mentioned design 14 times in the paper. In these two sentences, the word “design” appears four times:

We designed and demonstrated an all-day fresh water harvesting device that combines fog collection and interfacial solar steam generation enabled by 3D micro-topologies engineered on the surface of PVA/PPy hydrogel membrane. We utilized bio-inspired design principles and other design considerations for efficient fog capture, directional droplet transport, and quick drainage to guide our design and fabrication of 3D micro-tree arrays comprised of self-similar branched micro-cones that mimic the structure of spines on Cactus stem.

Nano-Directed Water

Another master water handler in every living cell is the aquaporin channel. Aquaporins are a family of membrane channels that direct individual water molecules in or out of the membrane as needed, one molecule at a time. Staff at the Cockrell School of Engineering, part of the University of Texas at Austin, studied aquaporins to imitate their supreme performance. For applications like desalination, the team wanted to improve water filtration and reduce energy cost. UTA says,

This is the first instance of an artificial nanometer-sized channel that can truly emulate the key water transport features of these biological water channels. And it could improve the ability of membranes to efficiently filter out unwanted molecules and elements, while speeding up water transport, making it cheaper to create a clean supply.

They constructed channels 3 nanometers in width that “can pass roughly 80 kilograms of water per second per square meter of membrane” — better than any commercial desalination membrane. An analogy relates how the new design imitates aquaporins:

Aquaporin-based channels are so small that they only allow a single molecule of water through at a time, like a single-lane road. A unique structural feature in these new channels is a series of folds in the channels that create additional “lanes,” so to speak, allowing water molecules to be transported faster.

“You’re going from a country road to a highway in terms of water transport speed, while still keeping out other things by putting little bumps in the road,” said Aleksei Aksimentiev, a professor of biological physics at the University of Illinois at Urbana-Champaign who collaborated on the research.

Their paper was published Nature Nanotechnology, “Foldamer-based ultrapermeable and highly selective artificial water channels that exclude protons.”

The paper mentions “outstanding capacity of aquaporins (AQPs) for mediating highly selective superfast water transport.” If their bio-inspired project comes to market successfully, some of that vast ocean water could enter drinking cups of the poor in Third World countries. Thank you, aquaporins! 

Quick Takes

The Eastern states recently experienced the deafening of sounds of cicadas after Brood X awakened from its 17-year slumber. While the noise and locust-like plague may be off-putting to many residents, engineers at the University of Illinois at Urbana-Champaign had more sanguine feelings about the insects. Last October they were hard at work imitating the cicadas’ remarkable ability to shed water off their wings.

“We chose to work with wings of this species of cicada because our past work demonstrates how the complex nanostructures on their wings provide an outstanding ability to repel water. That is a highly desirable property that will be useful in many materials engineering applications, from aircraft wings to medical equipment,” [said one of the engineers].

For ships and other craft that must plow through water, other creatures provide inspiration. An article from the American Institute of Physics summarized a paper from their journal Physics of Fluids. Which creatures get the honors for this ability?

Fish and seaweed secrete a layer of mucus to create a slippery surface, reducing their friction as they travel through water. A potential way to mimic this is by creating lubricant-infused surfaces covered with cavities. As the cavities are continuously filled with the lubricant, a layer is formed over the surface.

The paper tells how engineers studied the mucus layer under the skin of these creatures (one an alga and one a vertebrate) and how the organisms secrete it onto their exteriors. With the measurements in hand, they can help the shipping industry substantially, because “Long-distance cargo ships lose a significant amount of energy due to fluid friction.” Fish glide through the water, and algae wave gracefully. Any questions? “Looking to the drag reduction mechanisms employed by aquatic life can provide inspiration on how to improve efficiency.”

Real-World Problems

We see once again that living things know how to solve real-world problems that humans want to solve. We’ve seen seaweed, fish, cicadas, cacti and human cells teach scientists their tricks. Water is an abundant resource that is only as useful when it is properly handled. Because of exceptional designs in life, we can look forward to more efficient ways to capture, filter, attract, repel and glide through the wonder of water.