Insect cuticle is one of the most common and versatile materials on the planet. Comprising limbs, joints, wings and even transparent eye sections, cuticle adapts to take on almost any role that insects require. But little was known about the mechanical properties of this remarkable material. ‘There have been a few studies to understand its stiffness and strength but none have ever studied the fracture mechanics’, explains Jan-Henning Dirks from Trinity College Dublin, Ireland. Intrigued by the material’s extraordinary versatility, Dirks and his colleague, bone fracture mechanics expert David Taylor, decided to find out just how strong and stiff locust legs are (p. 1502).
However, instead of just investigating the mechanical properties of isolated sections of leg cuticle, Dirks and Taylor decided to analyse the strength and stiffness of intact leg segments. ‘If you want to understand the biomechanical implications of a structure, you don’t only want to know the material properties but how is it implemented in the structure’, says Dirks.
Quickly removing a locust tibia and inserting it into a tensile testing machine, Dirks filmed the leg while compressing it along its length until it buckled and snapped. But when he calculated the leg’s stiffness – the resistance to bending – based on the force at which the leg failed, Dirks was surprised. The stiffness was only 3.05 GPa, a fraction of the 9.0 GPa measured by Martin Jensen and Torkel Weis-Fogh 50 years earlier. Perplexed, Dirks checked the equipment and samples and repeated the stiffness measurements using a different method, but the stiffness was always 3.05 GPa. Then he realised that it had taken Jensen and Weiss-Fogh an hour to make their stiffness measurements. Could the leg have dried out and stiffened in the additional time?
Repeating the stiffness measurements anything from 15 to 180 min after removal of the leg, Dirks noticed that the leg dried quickly and as it dried it stiffened, reaching a maximum stiffness of 8.94 GPa 2 h after removal. So the stiffness of active kicking and jumping locusts’ legs is far less than had been believed. And when the duo measured the hydrated leg’s strength, it was also significantly lower (72.05 MPa) than that of the desiccated legs (217.41 MPa). Essentially, the stiffness and strength of insect cuticle can be tuned, depending on the material’s moisture content.
Next, Dirks and Taylor decided to test the limb’s toughness – its resistance to fracture. ‘Locusts have to withstand leg defects so that they can still jump even if they have a small notch from fighting’, explains Dirks.
Making a small incision in the limb and bending it until the leg fractured, Dirks measured the force at which the limb failed and calculated the toughness. It was an incredible 4.12 MPa m1/2. And when he calculated the amount of energy required to tear insect cuticle apart, it was an impressive 5.56 kJ m–2. ‘That is as high as antler and higher than bone, and cuticle achieves this without using a mineral phase. It is basically just chitin and protein’, says Dirks.
Admitting that he is amazed that insect cuticle is so tough, Dirks says, ‘Usually if you want a high fracture toughness you have a high stiffness’. However, he suspects that there is a trade off between the various mechanical properties, allowing the leg to be strong enough to weather the extreme forces of take-offs and kicking while resisting fractures. ‘You could make the leg much stiffer but it would probably become less strong’, he says. And, having discovered that humidity is a factor that contributes to the remarkable versatility of the material, Dirks is now keen to understand how altering the moisture content of cuticle fine tunes its astonishing mechanical properties.