Aaron Mena (left) and Jennifer Garcia Rodriguez (right), graduate students in Purdue University’s Department of Chemistry, affix corals using adhesive formulations developed from fully sustainable, bio-based components. Photo: Purdue University photo/Gudrun Schmidt.
Aaron Mena (left) and Jennifer Garcia Rodriguez (right), graduate students in Purdue University’s Department of Chemistry, affix corals using adhesive formulations developed from fully sustainable, bio-based components. Photo: Purdue University photo/Gudrun Schmidt.

Patent-pending adhesive formulations developed at Purdue University from fully sustainable, bio-based components can establish bonds that grow stronger when underwater or exposed to wet conditions.

Researchers led by Gudrun Schmidt, an associate professor of practice in Purdue’s Department of Chemistry, developed the formulations from zein, a protein found in corn, and tannic acid. They report their work in a paper in ACS Applied Materials & Interfaces.

These novel adhesive formulations could be further developed and used in the restoration of coral reefs. They could also have applications in the construction, manufacturing, biomedical, dental, food and cosmetic industries.

Schmidt and her colleagues conducted underwater adhesive experiments on their formulations, using different surfaces and different types of water, including seawater, saline solution, tap water and deionized water.

“Interestingly, the water type does not influence performance a great deal, but the substrate type does,” Schmidt said. “An additional unexpected result was bond strengths increasing over time when exposed to water, contradicting general experiments of working with traditional, petroleum-based glues. Initial adhesion underwater was stronger compared to benchtop adhesion, suggesting that water helps to make the glue stick underwater.”

When placed underwater, a protective skin forms on the surface of the adhesives. This skin immediately keeps water from entering the rest of the material.

“But once the skin was in place, it could be broken to induce faster bond formation,” Schmidt said.

These experiments also revealed that maximum bonding occurred at about 30°C, and that after falling the bonding rate would increase again at higher temperatures.

Schmidt said the process to make the adhesive formulations is a short one. “We can use inexpensive, sustainably sourced, plant-based materials to make gallons of glue within hours,” she explained. “The adhesives are very simple to make in the lab or outdoors, everywhere on the planet.”

According to Schmidt, other researchers are formulating adhesives that mimic the glues used by mussels, barnacles, oysters and sandcastle worms to adhere to the seafloor and other wet surfaces. Those best-performing formulations, however, are fully synthetic.

“Lengthy syntheses with the use of harsh chemicals may hold back their future development,” Schmidt said. “Nontoxicity, sustainably sourced materials and minimal environmental impact are increasingly in demand. Consequently, several groups have turned to developing new and remaking old adhesive systems using bioinspired or bio-based chemistry.”

The increased demand for nontoxic materials has led to the creation of novel adhesives for biomedical applications. The resulting glues have properties similar to soft tissue. But Schmidt said that bio-based adhesives have further applications.

“Once the in vivo and biomedical realm is left behind, there is an entire world of other applications requiring metals, plastics, wood and inorganic substrates that need adhesives to work in the presence of water. Food, oral and cosmetic applications are less restrictive when it comes to purity of starting materials. Food-grade polymers can often be used for making wet adhesives. We also are working on dental applications, trying to make bonds in this wet and challenging environment.”

Another particularly intriguing application for the patent-pending Purdue-developed adhesive formulations is the restoration of coral reefs. “There are several major efforts, worldwide, planting young corals to replace those structures that are already dead,” Schmidt said. “A major hindrance to these efforts is lack of suitable underwater adhesives that work well for this application.”

Schmidt and her research team are working with the Coral Restoration Foundation (CRF), sending various formulations to be tested. “We recently visited the Florida Keys to test a few formulations in bucketsful of ocean water,” Schmidt said. “It is great to see our work outside of the research lab and in the real, wet world.”

According to Phanor Montoya-Maya, coral restoration program manager at the CRF, the foundation constantly searches for the most efficient and effective method of outplanting coral onto reef sites.

“Having different alternatives means different species and habitats can be outplanted with positive results,” Montoya-Maya said. “Science collaborations like this allow us to test and fine-tune methods before mainstream restoration. Field preliminary results are very encouraging, and we'll continue to provide feedback to Purdue researchers to ensure the final product is consistently successful across multiple restoration goals.”

This story is adapted from material from Purdue University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.