A new study led by the Institute for Bioengineering of Catalonia (IBEC), located at the University of Barcelona Science Park (PCB-UB), shows that a single biomaterial can be locally “programmed” to be rigid in some areas and flexible in others, mimicking a strategy long used by insects and other arthropods. As a proof of concept, described in the Journal of Materials Chemistry A, the researchers built articulated structures combining rigid segments and flexible joints to enable controlled movement within a single continuous sheet.

Researchers from the Biointegrated Materials and Engineering group at the IBEC have shown that adding trace amounts of different metal ions to chitosan can locally tune how stiff, strong or flexible the material becomes, without changing its underlying chemistry. Chitosan is a biopolymer derived from chitin, commonly sourced from crustacean shells but also obtainable from fungi, insects and other renewable waste. The approach, published recently in the Journal of Materials Chemistry A, borrows a strategy long used by nature — such as the flexible joints and rigid plates that make up a single insect exoskeleton — to explore how a single sustainable material could do the mechanical work of several.

Programming one material, several ways

Modern products are usually built by combining several different materials, each chosen for a specific job — think of a bottle and its cap, or a rigid casing around a flexible seal. That strategy meets short-term performance needs, but it comes at a cost at the end of a product’s life.

“Today, the main bottleneck in recycling isn’t the material itself — it’s recovering and sorting the different materials that make up a single object,” says Javier G. Fernández, ICREA Research Professor at IBEC and leader of the study. “If a single material can be tuned to behave very differently from one region to another, it can still be recycled or composted as one material, with no sorting needed first.” Plastics alone come in hundreds of different types, each requiring its own recycling stream.

In contrast with this multimaterial approach, living organisms achieve different mechanical functions within a single structure by locally tuning a limited set of building blocks. Insects, for example, build their exoskeletons around chitin — the natural polymer from which chitosan is derived — adapting different regions for support, movement, or protection.

his economy of materials reflects an evolutionary strategy known as the “survival of the cheapest”: achieving broad functional diversity without multiplying the materials needed. “Nature routinely changes the local properties of a single material to gain efficiency,” Fernández explains. “An insect builds its whole exoskeleton as one continuous piece, yet it includes parts that are very flexible, like the joints in its legs, and parts that are very rigid, like the thorax or the wings. That’s efficient to produce, because the animal only has to fine-tune a single process, and it’s mechanically efficient too, because a seamless structure is far stronger than one built by joining separate pieces.”

To put this principle into practice, the IBEC team used chitosan, a derivative of chitin, to capture metal ions within the structure of thin films. They introduced copper, zinc, or nickel ions while the polymer was dissolved, trapping them within the material as it dried. An alkaline treatment then made chitosan’s natural binding sites more available to engage with the embedded ions. These interactions reshape how the polymer chains associate, giving researchers a way to tune the material’s mechanical behavior by choosing the metal while preserving its underlying chemical backbone.

The result is the same base polymer with markedly different mechanical personalities depending on which metal was added. Under dry conditions, zinc-doped chitosan became more than 50% stronger and stiffer than plain chitosan, though less stretchy; copper-doped chitosan became more compliant, stretching much further before breaking, at the cost of strength; nickel-doped chitosan fell in between. The most distinctive result appeared once the films were wetted: while plain chitosan and the copper- and zinc-doped versions weakened in water, as most biological and biology-inspired materials do, the nickel-doped version became stronger and stiffer instead — a reversal the team had already observed, and studied in more mechanistic depth, in a related chitosan-nickel material described in an earlier IBEC study published in Nature Communications.

To demonstrate how useful this local “programming” can be, the team built a fan-shaped, origami-inspired structure that combines a stiff, nickel-doped chitosan with a flexible, plain chitosan in a single continuous sheet. “We wanted to show, in a single object, that you can fold and unfold it in a controlled way — like an insect wing — without any glue, seams or separate parts,” says Akshayakumar Kompa, a postdoctoral researcher in Fernández’s group at IBEC and the study’s first author.

The team also tested how the different versions of the material broke down in soil. Over three weeks, the zinc-doped films degraded completely, the copper-doped films lost about half their mass, and the nickel-doped films — the toughest of the three when wet — degraded the slowest, losing about a quarter of their mass over the same period.

» Article of reference: Akshayakumar Kompa and Javier G. Fernández. Artificial reproduction of chitinous mechanical versatility through metal doping and its use in resource-efficient product design. Journal of Materials Chemistry A (2026). DOI: 10.1039/D6TA04260K

» Link to the news: IBEC website [+]