With ever-increasing demand for more environmentally friendly technologies, companies are searching for safer materials with which to manufacture their products. University researchers have recently provided manufacturers with a potential material from an unlikely source — an oyster.

Jess Cox
Microscopic image of the synthetic nacre University researchers created.
(Courtesy of Nicholas Kotov)
Jess Cox
The process University scientists are using to create artificial nacre.

As more companies demand materials that are biodegradable and use as few toxins as possible in their production, Chemical engineering Prof. Nicholas Kotov and his team have been able to fulfill those objectives by emulating the process by which oysters produce pearls.

Kotov said that nacre (pronounced nac-er), the substance that pearls are made of, has potential uses in fields such as aerospace, biomedical and many others.

Nacre is secreted by oysters when irritants get trapped inside their shell. Those irritants, such as a particles of sand, become coated — layer by layer — with nacre and eventually form a pearl.

Kotov said he and his team are at the forefront of this research effort and have been able to synthesize nacre in their lab at the G.G. Brown Building on North Campus.

The procedure Kotov employs would produce the same amount of nacre in 67 days as an oyster would produce in one year by using a simple technique called layer-by-layer electrostatic assembly.

Layer-by-layer electrostatic assembly involves taking a charged medium, such as glass slide used in microscopes, and immersing it into two chemicals with opposite charges.

The first solution in which the medium is immersed would be positively charged, and the second would be a negatively charged solution. The process continues back and forth, resulting in an ordered structure built by the different layers from the oppositely charged chemicals.

The positively charged solution is PDDA, a synthetic polymer solution that has been shown to be biodegradable. The negatively charged solution is sodium montmorillonite (clay nanoplatelets), which is a form of clay. Both substances are similar to the materials an oyster uses to create nacre, said Rackham chemical engineering student Paul Podsiadlo, who works on the team researching this.

The process of creating the nacre first begins by taking the glass slide and immersing it in a pyranha solution, a corrosive mixture of sulfuric acid and hydrogen peroxide, which will clean the glass, Podsiadlo said.

After drying the glass slide, it is immersed in the PDDA solution for five minutes, followed by a rinse or two minutes. Then, after drying it, the glass slide is immersed into the negatively charged of clay nanoplatelets for 10 minutes, followed by a rinse once again.

After repeating the cycles 50 to 200 times, Podsiadlo said he is able to achieve a thickness of one to four microns. In order to remove the nacre film from the glass slide the researchers immerse the glass slide into hydrofluoric acid, which dissolves the surface of the glass slide beneath the composite, thus releasing the nacre film, Podsiadlo said.

The result of this layer-by-layer deposition process is an ordered “brick-and mortar-arrangement,” due to which the film exhibits high-strength properties. When stress is applied to the artificial nacre, the nacre film with 50 cycles of deposition performs as well as natural nacre.

Possessing high-strength and the potential to be used in different industries this material has a promising future. In biomedical applications, Kotov said, “we could fabricate implantable medical devices coated with nacre, which would be biocompatible.”

By making medical devices more biocompatible with the human body, Kotov said the devices would be safer for use and create less strain for the patient.

Kotov’s team is also working with a few defense contractors with whom they have different levels of collaboration.

The University research team has a close collaboration with Nomadics Inc., a defense contractor based in Stillwater, Okla. Together, they are developing several Homeland Security initiatives, which involve the use of artificially synthesized nacre” Kotov said.

In the future, Kotov said he believes that the artificially synthesized nacre can be strengthened by replacing PDDA with another polymer called Chitosan, which is 70 times stronger than PDDA.

The next step of the team’s research aims to utilize Chitosan. Also, Kotov plans to lower the films’ thermal conductivity, to increase the films ability to disperse heat, which would also increase its potential use. Kotov said that in about two years, we should be able to see objects either made entirely of or coated with artificially synthesized nacre being used in our society.

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