The butterfly’s first flight inspired new ways to generate force and

Butterfly wings are made of chitin, an organic polymer that is a major component of the shells of arthropods such as crustaceans and other insects. As a butterfly emerges from its cocoon in the final stages of metamorphosis, it will slowly spread its wings to full splendor. During opening, the chitin material becomes dehydrated as blood is pumped through the butterfly’s blood vessels, generating forces that rearrange the material’s molecules to give it the unique strength and rigidity needed for flight. This natural combination of force, water movement and molecular organization is the inspiration behind Associate Professor Javier G. Fernandez’s research.

Together with research colleagues from the Singapore University of Technology and Design (SUTD), Assoc Prof Fernandez have explored the use of chitinous polymers as sustainable materials for engineering applications. In their recent study, ‘Reorientation of secondary and hygroscopic forces in chitinous biopolymers and their use in passive actuation and biochemistry’ was published in Advanced Material Technologythe research team explained the adaptability and changes of the molecular material of chitin in response to changes in the environment.

“We have shown that even after being extracted from natural sources, chitinous polymers retain their natural ability to link different forces, molecular organization, and water content to produce mechanical motion and generate electricity without the need for an external power source or control system,” says Assoc Prof Fernandez, highlighting the unique features that make chitinous polymers energy-efficient and biocompatible smart materials.

Chitin is the second most abundant organic polymer in nature after cellulose and is a part of every ecosystem. It can be easily and sustainably sourced from a variety of organisms, and the same SUTD research team has shown that it can be sourced even from urban waste.

In the current study, the researchers extracted chitinous polymer from discarded shrimp shells to create films 130.5 micrometers thick. They investigated the effects of external forces on these chitin films, focusing on changes in molecular organization, moisture content and mechanical properties. The researchers observed that similar to the wings of a butterfly opening, the stretching of the chitin film rearranged the crystal structure — the molecules became denser and the water content decreased.

Initially with characteristics similar to commodity plastics, the chitinous film is converted into a plastic-like material for special and sophisticated engineering purposes. In contrast to the inert nature of synthetic polymers, the rearranged chitinous film can independently relax and contract in response to changes in the environment, similar to how some insects adapt their shells to different situations. This ability allows the chitin film to lift objects weighing more than 4.5 kilograms vertically.

To demonstrate the applicability of the biocompatible film technique, the research team assembled it in mechanical hands. By controlling the film’s intermolecular water through environmental changes and biochemical processes, the team created enough force for the hand to perform a gripping motion. Impressively, its grip strength is equivalent to 18 kilograms—more than half the grip strength of the average adult. The ability to generate such forces through biochemical means also demonstrates the potential for seamless integration of chitinous films into biological systems and their suitability for biomedical applications, such as artificial muscles and medical implants.

In a different demonstration, the team showed that a material’s response to changes in humidity could be used to harvest energy from changes in the environment and convert it into electricity. By attaching a film to a piezoelectric material, the mechanical motion of the film in response to changes in humidity is converted into an electrical current suitable for powering small electronics.

Assoc Prof Fernandez’s proof-of-concept study illustrates how the native mechanical characteristics and embedded functionality of chitin can be reproduced, emphasizing the potential use of chitin in engineering and biomedical applications. He argues that materials like chitin play an important role in the transition to a more sustainable paradigm, which he calls the biomaterial age.

“Chitin is used for many complex functions in nature, from making insect wings to forming the hard protective shells of mollusks, and has direct engineering applications. Our ability to understand and use chitin in its native form is critical to enabling the application of new techniques and developing them within the paradigm of ecological and low-energy integration,” concluded Assoc Prof Fernandez.