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Materials Science Advance Brings A Rose Petal’s Natural Texture To Metallic Structures

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Nature and it’s mechanisms have been a source of inspiration for scientific innovation throughout history. Take, for example, hook and loop fastenings, often blanketly referred to as “Velcro” after the private company and brand which first commercialized the product. Velcro is ubiquitous — it’s found on everything from shoes to clothing to bags. But first, it was inspired by the burdock seeds which stuck to the fur of Velcro inventor George des Mestral’s dog on a hike through the woods in 1941. 

“Nature is the ultimate engineer,” says Martin Thuo, an assistant professor of Materials Science and Engineering at Iowa State University. “She has had the luxury of time and unlimited trials and as such has made amazing structures. I am therefore always interested in making structures that nature has perfected.”

Thuo leads a team of researchers at Iowa State University who are studying environmentally friendly methods of creating materials which are biomimetic, or, in other words, mimic nature. Their latest paper, published in Angewandte Chemie, details how they used a heat free technology to recreate the texture of a rose petal in metal. 

Rose petals have long captured the attention of scientists because they exhibit a property called superhydrophobicity. When a drop of water falls onto a rose petal, it doesn’t spread out across the surface. Instead, the droplet remains in the shape of a tiny ball, which clings to the petal. Superhydrophobicity is sometimes called “the lotus effect” because the lotus leaf shows similar behavior. However, where the two plants differ is that if you tip a lotus leaf, the droplet of water will easily slide off. On a rose petal, you could turn it over and the droplet will still stick.

Material scientists are interested in mimicking the rose petal’s texture, and therefore it’s superhydrophobicity, for a variety of applications. Self-cleaning coatings, non-icing surfaces, and even air conditioning surfaces are among the potential uses which Thuo names. However, recreating the texture of delicate natural surfaces has previously been limited to replica molding with stretchy, soft materials. Replica molding with a hard material like metal is more difficult.

“To shape metals we either use mechanical force or melting,” Thuo explains. “These processes are not compatible with delicate surfaces like a rose petal, a soft tissue or living organism.”

Previously, Thuo headed a team of researchers in developing a method of heat free soldering for microelectronics, in an effort to prevent high temperatures from damaging the sensitive devices. His team encapsulated particles of molten metal into a balloon-like oxide shell which keeps the liquid metal from solidifying.

The researchers realized the undercooled liquid metal particles would be perfect for soft-lithography, except for with metal instead an elastomeric material as the mold. The particles have the ability to conform and tightly pack, which makes them good for shaping to complex surfaces like that of a rose petal.

They call their replication process biomimetic metal patterning, or BIOMAP for short. The BIOMAP process is just three steps: Depositing the liquid metal particles onto the surface of the substrate and tightly packing them, allowing the metal to solidify, and then lifting off the final, replicated structure.

“The BIOMAP process is simple, it utilizes a process called self-filtration, where smaller particles would essentially fill gaps created by larger particles until it is jammed, locking it in place,” explains co-author Andrew Martin, a doctoral student at the university. “After it is jammed, a chemical sintering process is deployed to induce solidification on the metal, further maintaining and stabilizing its form factor.”

Julia Chang, another co-author and doctoral student, further describes the BIOMAP process. “We deposit the liquid metal particles on templates either by hand-painting, spin-coating or spraying. Then we drop some acidic solution on the particles and wait until dried.” 

Once dried, the researchers can easily lift the metal structure off the rose petal using tape.

Not only did the BIOMAP process not damage the rose petal substrate, it also proved to be so precise the researchers could identify which metallic structure was from a red rose versus a pink rose based on the sub-micrometer petal patterns.

According to Thuo, recreating a rose petal is just the beginning of what he says is a long-term effort to create structures with metal, including medical devices. 

“We are preparing a second body of work based on picking up 3D fingerprints or impressions, beetles and eventually the skin texture of some reptiles,” says Thuo. 

While this research marks a major advance in soft-lithography, BIOMAP has uses beyond replicating natural structures. It can also be used to tightly pack metal onto the surface of a structured system for use as an electrode.

Their second paper on the topic will be published in the near future in the journal Advanced Materials Interfaces.

“We are using this method to create conformal electrical contacts in biological tissues like the brain — across both smooth and rough regions — and the heart.”

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