A biological material that grows stronger when wet could replace single-use plastics

Plastics became ubiquitous because of specific properties: durability, low cost, and resistance to water. Those same properties make them environmental liabilities. Plastics that escape waste streams accumulate in soils, oceans, and food chains, persisting for centuries. The search for biological alternatives has proceeded for decades, but nearly every candidate material has stumbled on a fundamental constraint: biological polymers weaken when they get wet.

Cellulose, chitosan, proteins - the most abundant biological building blocks tend to absorb water, and as they do, the bonds holding their structures together relax and the material softens. This has forced engineers who want to use biomaterials in real-world wet environments to rely on chemical modifications or protective coatings that undermine the sustainability rationale for choosing biological materials in the first place.

A study published in Nature Communications from the Institute for Bioengineering of Catalonia (IBEC), in collaboration with the Singapore University of Technology and Design, reports a material that inverts this principle. Rather than weakening when wet, it gets stronger - up to 50 percent stronger after immersion in water. The key is a trace element: nickel.

A serendipitous observation about worm fangs

The research was inspired by an observation about the sandworm Nereis virens. This marine worm possesses extraordinarily hard fangs made from a protein-based material reinforced with zinc ions. When zinc is removed chemically, the fangs soften and become susceptible to hydration. This suggested that metal ions play a key structural role - not just strengthening the material, but controlling how it responds to water.

The IBEC team focused on chitosan, derived from chitin - the second most abundant organic molecule on Earth after cellulose, found in crustacean shells, insect exoskeletons, and fungal cell walls. Chitosan is biocompatible, biodegradable, and industrially available as a byproduct of shrimp processing. To test whether metal ions could control its hydration response, the researchers incorporated nickel - a naturally occurring trace element that interacts with chitin and dissolves readily in water.

The result was unexpected. When the chitosan-nickel films were immersed in water, rather than softening, they became stiffer and stronger. Strength increased by up to 50 percent after immersion.

Water as an active structural component

The mechanism differs from the passive structural role that metal crosslinks play in most engineered materials. In the new material, water becomes an active participant in the structure rather than a threat to it. Nickel ions and surrounding water molecules form a dynamic network of weak, reversible bonds that continuously break and reform. This constant microscopic reconfiguration enables the material to absorb and redistribute mechanical stress rather than concentrating it at fracture points.

"A material where being soft at the molecular scale actually makes it stronger," is how principal investigator Javier G. Fernandez, ICREA Research Professor at IBEC, summarized it. The approach directly contradicts over a century of materials engineering philosophy, which has sought to make structural materials resistant to environmental conditions. This material interacts with its environment and draws strength from that interaction.

Zero-waste production and scalability

The manufacturing process addresses a common barrier to adopting novel biomaterials: waste. When the chitosan-nickel films are first immersed in water, the majority of nickel ions that are not contributing to structural bonds leach out. Rather than discarding this nickel-rich solution, the team designed a closed loop in which it becomes the input for producing the next batch of material. The result is 100 percent efficiency in the use of nickel.

Chitosan itself can be produced locally rather than depending on a single global supply chain. While shrimp shells are the main industrial source, chitosan can also be obtained through bioconversion of organic waste including urban food residues and fungal byproducts. Each year, the world produces an estimated 100 billion tonnes of chitin in nature - equivalent to roughly three centuries of global plastic production.

Applications and limitations

The researchers identified early applications in agriculture, fishing gear, and packaging - sectors where biodegradable, water-resistant materials are urgently needed. The material has been demonstrated forming watertight containers, including cups and large sheets. Both nickel and chitosan are individually approved by the FDA for certain medical uses, opening a potential path toward medical applications such as waterproof coatings for biomaterials.

The study is a first demonstration of the principle. Nickel, while naturally occurring, is an allergen for some people and raises concerns in food-contact applications that will require regulatory evaluation. The long-term biodegradation behavior of the nickel-chitosan composite in different environments has not been fully characterized. The researchers acknowledge that nickel is probably not the only metal capable of producing this water-strengthening effect - finding other combinations is an identified next step.

"This is the first study. Now that we know this effect exists, we and others can search for new materials and new ways to achieve it," said Fernandez.