Hedgehog Particles Could Mean Safer Water-Repellant Paints, Better Drug Delivery

An unexpected find in a University of Michigan chemical engineering lab could lead to safer, cheaper water-repellant paints and coatings that don’t use hazardous volatile solvents.

Typical coatings and paints that are formulated to shed water use oily pigments that are hydrophobic, meaning they don’t readily mix with safe solvents composed mainly of water. To help these color particles stay suspended and distributed so they can be applied in even coats on surfaces, they are often dissolved in chemicals like hexane and toluene, hydrocarbons refined as part of gasoline production. Such compounds impact the respiratory and central nervous systems of humans and animals. They are also air and water pollutants.

A team led by biomedical and chemical engineering professor Nicholas Kotov says they have discovered a new process to make the surface-coating particles that could cut out the dangerous solvents. They have been able to modify oily, hydrophobic microspheres of polystyrene by growing microscopic spikes on their surfaces. The spikes, which can be grown on many types of material, let the so-called “hedgehog” particles stably disperse in water-based solutions. When applied to fabrics and paper in a water-based solution, they created superhydrophobic coatings.

“We thought we’d made a mistake,” Kotov said in a statement. “We saw these particles that are supposed to hate water dispersing in it and we thought maybe the particles weren’t hydrophobic, or maybe there was a chemical layer that was enabling them to disperse. But we double-checked everything and found that, in fact, these particles defy the conventional chemical wisdom that we all learned in high school."

In lab experiments, the spikes proved to decrease the contact points and attractive forces between individual hedgehog particles, preventing them from clumping together and separating out of solution. The team revealed their work in yesterday’s issue of the journal Nature.

(Hedgehog particle coating on fabric demonstrating superhydrophobic properties. Courtesy Joseph Xu, Michigan Engineering.)

If it proves to be safe and scalable to industrial production, the modified particles could let paint manufacturers add fewer nasty surfactants and volatile organic solvents to their products while achieving the same water-shedding properties. Cutting out these chemicals would be an achievement for public health and the environment. Other paint formulations using no or low amounts of volatiles are on the market, Kotov says, but these are more complicated and expensive to produce.

He says the same technique could potentially make a better drug delivery vehicle for pharmaceuticals that aren’t soluble in water.

"Anytime you need to dissolve an oily particle in water, there’s a potential application for hedgehog particles,” he said. “We’re only just beginning to explore the uses for these particles, and I think we’re going to see a lot of applications in the future.”

All images and gifs created from materials provided with Nature paper, courtesy Bahng et al./Nature.