Could This King-Size Citrus Fruit Inspire a Better Mobile Phone Case?
Boston University engineers are studying the pomelo’s remarkable shock absorbency—including by throwing it off a campus roof
It’s the world’s biggest citrus fruit, native to Southeast Asia. The grapefruit-like pomelo, officially Citrus maxima, is a popular festival food with a mild, slightly sweet taste. It also has a hefty peel that protects the delicate flesh inside—and that could help inspire more shock absorbent cell phone cases, packaging, and even naval ships.
At Boston University College of Engineering, researchers are studying the pomelo’s remarkable ability to fall from great heights—pomelo trees can grow up to 50 feet—without smashing into pieces. With a US Navy grant, they’re combining biology, materials science, and computational mechanics to engineer novel, lightweight materials that replicate this unique fruit’s energy absorption mechanisms, on a larger scale.
“I’ve always been intrigued by bioinspired engineering,” says project lead J. Gregory McDaniel, an ENG associate professor of mechanical engineering and of materials science and engineering. “Nature keeps building things and testing them all the time, right in front of our eyes.”
The Pomelo’s “Magical Impact Resistance”
The team’s Naval Engineering Education Consortium grant is aimed at developing materials that mitigate not only impact, but shock and blast as well. “Those are three different things, but they all happen on a very short timescale of high stress or force on something that might break,” says McDaniel.
Searching for solutions in the biology literature, he found a host of papers praising the pomelo. “It’s regarded as having this kind of magical impact resistance,” says McDaniel. “There seem to be a lot of design features in the pomelo fruit that are there for a reason, and are not random.”
Every season, ripe pomelos plummet to the forest floor, traveling at 30 miles per hour. But instead of bursting upon impact and then laying around rotting, the fruit survives the fall and keeps its enticing appearance so that wandering animals will gobble it up and leave its seeds far away.
The key lies in the peel’s three-layered construction: inner and outer membranes encase a spongy, foamlike layer, itself consisting of up to 18 mini layers and pockmarked throughout with pores, or voids.
“It’s what’s called viscoelastic—kind of a rubbery elastic,” says McDaniel. “If you squish it, it deforms itself to absorb the shock, then slowly rebounds,” not unlike a memory foam mattress.
From Peels to Protective Coverings
Working with BU PhD student Aidan Jimenez (ENG’30), McDaniel’s task is to figure out precisely what the pomelo peel is doing right, and translate that knowledge into building a protective covering for a ship’s hull. Any advances could later have broad civilian uses too.
“There are interesting questions we need to answer,” in designing the covering, says McDaniel. “What should be the size of the voids in the foam in the inner layer? What should be the thicknesses and stiffnesses of the membranes?”
The team will ultimately fabricate and test a first-of-its-kind “peel,” made of engineered rubbers and polymers, that the Navy can use to protect ships.
“We’re absolutely not the pioneers in identifying the qualities of the pomelo fruit,” says McDaniel, “but what we are trying to do is go a little further in finding the optimal parameters that will allow us to steal [these qualities], and take this all the way to [creating] a coating or layer that actually goes on a structure and works to absorb impact.”
In the video above, watch as BU researchers throw a pomelo off a high campus roof, then examine the peel’s different layers in the lab.
A version of this article originally appeared on the BU College of Engineering’s news page.
