What makes a good spring

My research is driven by a simple question: which aspects of a spring govern its mechanical behavior? Solid mechanics, a major branch of engineering, shows that the mechanical behavior of a structure is dependent on both its structure (the shape it takes) and its material (what its made of). Although these design principles are successfully used to create man-made springs, there are lessons that we can learn from biological springs about efficiency and multifunctionality. Spring systems are everywhere in nature from the muscle-tendon system in kangaroos to the exoskeletal legs of fleas. Which strategies has nature employed over the evolutionary history of life to make these multifunctional springs more efficient?

Why study mantis shrimp

Mantis shrimp contain exoskeletal springs in their appendages that power the fastest strike in the animal kingdom. Some species of mantis shrimp have the ability to move their arms with the acceleration of a bullet, reaching top speeds of 55 mph in less than 0.0002 seconds under water. The end product is a punch that emits heat, light, and can generate over 1000 Newtons of force! This powerful spring is also small and lightweight. Unraveling the secrets behind this spring could teach us how to better design strong, efficient springs. The video below gives more information about mantis shrimp, as well as some footage of the strike in action.

Comparative biomechanics

Mantis shrimp appendages come in many different shapes and sizes as seen below. Additionally, the degree to which different species of mantis shrimp rely on a spring-loaded system also varies.

My research involves understanding how underlying morphological and material variation translates into differences in the mantis shrimp strike. By taking advantage of the evolutionary history of mantis shrimp, my dissertation will take steps towards understanding the evolution of the spring systems that have made elastically-loaded animal movements so spectacularly powerful.

Measuring mechanical response

To characterize the spring paremeters between species, I performed mechanical tests on the mantis shrimp appendages using an Instron materials testing machine. I was interested in measuring key aspects of elastic behavior such as spring stiffness, spring energy, and resilience of mantis shrimp appendages when the spring was compressed. To do so, I embedded the springs in epoxy, and collected force and displacement data while loading the appendages in compression. I found that although mantis shrimp are able to store more energy as they become larger, the stiffness of their springs doesn't increase. These results were published in the Journal of Experimental Biology. Below is a video from a single trial showing spring compression using the materials testing machine.

Measuring effect of shape

I use a combination of 2-dimensional and 3-dimensional computational models in order to understand the effect of shape on the spring. The strength of using these computational models is that I can remove the effect of differences in materials, and focus solely on any functional differences that are results of shape alone. All of my computational models incorporate shape data from micro-CT scans. For example, the finite element model seen below involves converting a micro-CT scan into a volume model that is comprised of thousands of connected tetrahedral elements. The model is then given information about compression distance, and the displacement, strain, and stress are mathematically calculated for all tetrahedrals. By removing the effect of materials, we can see that the model behaves similarly to the mechanical tests. Red colored regions in the model indicate regions of high elastic energy storage.