New plastic-like materials that dissolve in the sea developed

Artistic rendering of the new plastic. Cross linked salt bridges visible in the plastic outside the seawater give it its structure and strength.

Microplastics—tiny plastic particles less than 5mm in size—have now spread across the entire planet, from the depths of the ocean and the Arctic to the air we inhale.

These microscopic pollutants are increasingly being detected within human bodies, including in our bloodstream and brain tissue. While the full extent of their impact on both the environment and human health remains uncertain, it is well-documented that they disrupt marine and land-based ecosystems. Among their harmful effects, microplastics have been linked to slower animal growth, reduced fertility, and organ dysfunction.

A Potential Solution from Seawater

Scientists at RIKEN are working to address microplastic pollution in oceans through the development of a new material that naturally biodegrades in saltwater.

Possessing similar strength and weight characteristics to conventional plastics, this innovative material could help curb plastic pollution and reduce greenhouse gas emissions generated from plastic incineration, says Takuzo Aida, a materials scientist leading the Emergent Soft Matter Function Research Group at the RIKEN Center for Emergent Matter Science in Wako, Japan.

This breakthrough is the result of Aida’s decades-long research into supramolecular polymers—materials that differ from traditional plastics in how their molecular structures are held together. Unlike conventional polymers, which are made up of tightly bound molecules through energy-intensive covalent bonds, supramolecular polymers rely on weaker, reversible bonds, comparable to the adhesion of sticky notes that can be attached and removed.

This unique bonding structure enables supramolecular polymers to self-repair when pressed back together after breaking. Additionally, they can be easily recycled, as specialized solvents can dismantle their molecular bonds, making reuse and repurposing highly feasible.

Overcoming Plastic’s Challenges

Aida acknowledges the widespread use of plastics due to their unmatched versatility. For example, polyethylene terephthalate (PET), commonly used in bottles, is both strong and flexible, durable, and recyclable. These qualities make it difficult to replace.

Although biodegradable plastics have been proposed as an alternative, they degrade too slowly under natural conditions, posing a significant challenge. For instance, polylactic acid (PLA), a plastic designed to break down in soil, often remains intact in marine environments due to slow decomposition. Over time, such materials fragment into microplastics, which persist in nature as they cannot be broken down by microbes or enzymes.

Determined to find a more effective solution, Aida and his team explored ways to enhance supramolecular materials for better environmental compatibility. However, the inherent reversibility of supramolecular polymer bonds posed a challenge, as it made the materials too fragile for practical applications.

To address this, the team sought a combination of compounds that would enhance the material’s mechanical strength while ensuring rapid breakdown into harmless substances under the right conditions. Aida envisioned a mechanism where the molecular bonds would remain stable but could be selectively unlocked by a specific trigger—salt.

After testing various compounds, they discovered that a mixture of sodium hexametaphosphate (a common food additive) and guanidinium ion-based monomers (used in fertilizers and soil conditioners) could form ‘salt bridges.’ These bridges create strong cross-linked bonds, acting as molecular locks that provide both flexibility and durability.

A Breakthrough in Plastic Innovation

“Finding the right molecular combination can be as difficult as searching for a needle in a haystack,” Aida remarks. However, his team’s early success suggested that their approach was viable.

The researchers synthesized a sheet of the supramolecular material by dissolving the components in water. Unexpectedly, the solution separated into two layers—a viscous lower layer and a watery upper layer. The bottom layer, which contained the salt-bridged compounds, was extracted and dried to form a plastic-like sheet.

The resulting material was not only as strong as traditional plastics but also transparent, colorless, and non-flammable, offering great versatility. Crucially, the material decomposed into its raw components when submerged in salt water. The salt ions acted as a key to unlock the molecular bonds, allowing the material to dissolve completely within approximately 8.5 hours.

Additionally, the material could be made waterproof with a hydrophobic coating. Even with this coating, scratches on the surface allowed salt water to penetrate, triggering the degradation process at the same rapid rate.

Moving Towards Sustainable Plastic Use

Beyond being biodegradable, Aida envisions that the remnants of the material could serve a useful purpose. Upon breaking down, the material releases nitrogen and phosphorus—nutrients that microbes can metabolize and plants can absorb.

However, he cautions that careful management is necessary. While these nutrients can enhance soil fertility, excessive amounts in coastal ecosystems could trigger algal blooms, which may disrupt aquatic life. Aida suggests that controlled recycling in seawater treatment facilities could provide an optimal solution, allowing raw materials to be recovered and reused for producing new supramolecular plastics.

While alternatives to petroleum-based plastics are essential, Aida emphasizes that government policies, industries, and research efforts must work together to drive significant change. Without decisive action, global plastic production and its associated carbon emissions could more than double by 2050.

“The plastics industry is deeply entrenched, with well-established infrastructure and production lines, making change incredibly difficult,” Aida notes. “However, a tipping point will eventually come, and when it does, we will need innovative technologies like this to drive the transition.”