In an effort to reduce the amount of harmful plastic waste entering oceans, scientists have created biodegradable flip-flops made from algae.
Scientists at the University of California San Diego (UCSD) created polyurethane foams, made from algae oil, to meet commercial specifications for flip-flops and other footwear usually made from flexible plastic.
This commercial-grade flip-flops can degrade in compost and soil after 16 weeks, implying that even if they entered the oceans, they would break down. The foam meets the commercial requirements for the foot-bed of flip-flops and the cushioning mid-sole section of shoes.
One of the most popular and affordable footwear, flip-flops account for a large percentage of plastic waste that ends up in landfills, on seashores and in oceans. Since these are commonly used for a short period, they are discarded after a few uses, and later end up in oceans. These take hundreds of years to decompose, killing marine life and contaminating water supplies.
The findings of the study, which was a collaboration between UC San Diego and startup company Algenesis Materials - a materials science and technology company - are published in Bioresource Technology Reports.
The research was co-led by graduate student Natasha Gunawan from the labs of professors Michael Burkart, Division of Physical Sciences, and Stephen Mayfield, Division of Biological Sciences, and by Marissa Tessman from Algenesis.
"The paper shows that we have commercial-quality foams that biodegrade in the natural environment. After hundreds of formulations, we finally achieved one that met commercial specifications," Mayfield said.
The researchers worked with Algenesis to not only make the shoes but to degrade them as well. Mayfield said that while scientists have shown that commercial products like polyesters, bioplastics (PLA) and fossil-fuel plastics (PET) can biodegrade, it is only possible in the lab tests or industrial composting.
"We redeveloped polyurethanes with bio-based monomers from scratch to meet the high material specifications for shoes, while keeping the chemistry suitable, in theory, so the shoes would be able to biodegrade," Mayfield said.
The scientists extracted oils from algae, which has fast growth rates under photosynthetic conditions and 'possess unique metabolic pathways' for producing hydrocarbons that can be converted into plastic-making chemicals, reported MailOnline.
Bio content, either living or dead organisms, constitute over 50 per cent of the foam.
"These foams are 52 per cent biocontent – eventually we'll get to 100 per cent. We use algae oil to make what is called the polyols, which is about half of polyurethane – the other half of a polyurethane comes from what is called an isocyanate, and we still get that from petroleum," Mayfield told the media.
"Even though isocyanate comes from petroleum it is still biodegradable and eaten by microorganisms once degraded into molecules," he added.
Using the oil, the researchers made the foot-bed of the sandal, the soft part under the foot, and the out-sole. While the strap is also made from biodegradable material, the researchers have not yet tried to make it from their own polyurethane foam.
In the last 60 years, human beings have generated over 6 billion metric tonnes of plastic waste. Of this, while only nine per cent was recycled, 12 per cent was incinerated and a whopping 79 per cent was left to accumulate in landfills or the natural environment.
According to Mayfield, on some islands in the Indian ocean, an estimated 25 per cent of the ocean plastic trash is made up of flip flops and other simple shoes. If the plastic pollution is not addressed, it will lead to 13 billion metric tons of plastic in landfills or the natural environment by 2050.
Therefore, UCSD is collaborating with Algenesis Materials to develop biodegradable polyurethane products.
"The life of material should be proportional to the life of the product. We don't need material that sits around for 500 years on a product that you will only use for a year or two," Mayfield said.
The foam is also harmless to marine life which often ends up swallowing plastic debris that enters the ocean, mistaking it for prey.
"Some organisms can live on just our foam and some salts, so our foam is actually food for microorganisms," said Mayfield.
In order to test the foams, the researchers immersed them in traditional compost and soil. Cubes of them foam were incubated in two separate environment - one in soil, and the other in compost - to expose them to microorganisms.
Before incubating the cubes, they were weighed and compression-tested in containers filled with either compost or soil material, and placed at 30 degree Celcius under continuous high humidity.
Another set of control cubes were also tested under the same conditions to confirm that the foam degraded as a result of exposure to compost or soil and not just due to high humidity.
Following the test, the researchers discovered that the materials degraded after just 12 weeks, before decomposing in 16 weeks.
Further, to account for any toxicity during the decomposition period, a team of scientists led by UC San Diego's Skip Pomeroy measured every molecule shed from the biodegradable materials as well as identified the organisms that degraded the foams.
The researchers then took the enzymes from these organisms and showed that they could "use them to depolymerize these polyurethane products, and then identified the intermediate steps that take place in the process." This meant that that the bacteria and fungi broke down the polymeric bonds that give the foam its solid structure.
"We then showed that we could isolate the depolymerized products and use those to synthesize new polyurethane monomers, completing a 'bioloop', Mayfield said.
This full recyclability of commercial products is the next step in the scientists' continued mission to fight the present plastic production and waste management.
"If you could turn back the clock and re-envision how you could make the petroleum polymer industry, would you do it the same today that we did it years ago? There's a bunch of plastic floating in every ocean on this planet that suggests we shouldn't have done it that way," said Pomeroy.
The scientists are currently working out ways to commercially produce the foam in an economical way.Also Read: How Masks Made From Banana-Tree Species Could Help Cut COVID-19 Plastic Waste