By Bill Dockery

Taking some time away from his lab, Mingjun Zhang was watching his young son at play. Nearby he noticed ivy clinging to a wall. It was a robust green, and its vines seemed to defy gravity as they climbed vertically up the smooth surface.

“I wonder how it can cling so tightly?” he mused.

That absentminded question has turned into an important scientific inquiry for Zhang, now a UT associate professor of biomedical engineering; and the answers he is uncovering may have implications for future medical and military technologies.

“In recent years, we’ve seen researchers turn their attention to the climbing ability of the gecko,” he says. “I just wondered what we’d learn if we looked at ivy the same way.”

A New Approach

Zhang was not the first person to pose the question. Charles Darwin, the 19th-century biologist and evolutionary theorist, had raised the issue in an obscure book, Movements and Habits of Climbing Plants, published in 1876. Darwin wrote that the vines exuded a yellowish secretion that allowed the plant to stick to most surfaces, but that was as far as 19th-century scientific technology would let him go.

Mingjun Zhang's curiosity about ivy has led to some interesting discoveries.

To satisfy his curiosity, Zhang was able to turn to 21st-century tools Darwin could not even imagine—equipment like the scanning electron microscope and the atomic force microscope, as well as high-performance liquid chromatography and mass spectrometry.

“The rootlet of a young ivy stem was first inspected using a conventional optical microscope,” Zhang said in an article. “We then let the ivy grow on top of a three-inch silicon wafer and [on] mica for a week. . . .”

When inspected under the optical microscope, a number of disks from the ivy stem showed up sticking to the surface, each with a number of fingerlike growths protruding from the disk.

After the ivy stems were removed, Zhang looked at the residue with an atomic force microscope, an instrument that uses the reactions between a specimen’s surface and an extremely small (nanoscale) probe that produces an image of the surface accurate to a fraction of a nanometer. Those images revealed a collection of nanoparticles on the surface that had been secreted by the disks on the end of the ivy stems.

A Sticky Situation

As Zhang describes the process, when a plant tendril touches a wall or other surface, the mechanical act of touching stimulates the release of nanoparticles across the surface with a polysaccharide cement that creates a film holding the particles together. (Polysaccharides are strings of carbohydrates like starch, glycogen, or cellulose.) His scanning electron microscope shows that the cement forms a sticky layer three millionths of a meter (μm) thick between the surface and the plant cells.

Nanoparticles increase surface contact and adhesive strength.

The nanoparticles are what give the adhesive its particular power. Because they are so small (a nanometer is a billionth of a meter), they fill even the smallest gaps and flaws in the surface they are attaching to, giving the ivy many more points of adhesive contact.

“The adhesion force generated by this mechanism is roughly two million times greater than the weight of the ivy itself,” Zhang says. The adhesive will cling to almost any surface, is strongly water-resistant, and grows stronger after the plant dies.

To understand what the nanoparticles were made of, Zhang turned to high-performance liquid chromatography and mass spectrometry. He and colleagues separated out the particles and analyzed their chemical makeup. So far the group has identified 19 compounds, though they have not yet identified the structures of the molecules.

Zhang has looked at two plants that go by the name of ivy: classic English ivy (from the Araliaceae family native to Western Europe and northwestern Africa) and Boston ivy, which is a deciduous vine related to the grape family. English ivy is regarded as an invasive species in North America and is considered destructive on masonry structures because its adhesive characteristics eventually penetrate and damage mortar.

His adventure with the sticking power of ivy has led to an interest in other notoriously adhesive species like barnacles and marine mussels that affix themselves to ships, docks, and other sea structures.

In future research, Zhang hopes to bring to bear other modern equipment and techniques on the ivy question, including laser-scanning confocal microscopy, which will give him the ability to look at the profiles of his specimens at different depths.

Tiny Technology with Huge Potential

When he thinks about potential applications for his discovery, his imagination turns to military and rescue technologies, anti-damage paint, medical adhesives and drug delivery, and nanoparticle-enhanced imaging.

“Think about soldiers with adhesive gloves that might let them climb a vertical wall. Or paints that would prevent barnacles from fastening onto ship hulls.”

He dreams of nanoparticle materials for medical applications like suturing and drug delivery. He speculates about growing green crops that will produce nanoparticles for harvest, with the chemical content controlled by metals introduced into the soil that grows the plants.

“Biology is nanotechnology by nature,” Zhang says, returning at last to Darwin. “Biological materials achieve their superior properties through billions of years of evolution by adapting to their living environment.”

Zhang’s interests are not limited to bio-nano mechanisms. With colleagues Sharon Bewick and William Hamel, he has explored the parallels between aggressive-passive military defenses against terrorist actions and immune-system responses to viral infections of the liver, using military models to suggest new strategies for fighting Hepatitis B. Another of his research interests is game theory and the biological immune system.