When a mantis shrimp uses shock waves to strike and kill its prey, how does it prevent those shock waves from damaging its own tissues? Researchers at Northwestern University in the US have answered this question by identifying a structure within the shrimp that filters out harmful frequencies. Their findings, which they obtained by using ultrasonic techniques to investigate surface and bulk wave propagation in the shrimp’s dactyl club, could lead to novel advanced protective materials for military and civilian applications.
Dactyl clubs are hammer-like structures located on each side of a mantis shrimp’s body. They store energy in elastic structures similar to springs that are latched in place by tendons. When the shrimp contracts its muscles, the latch releases, releasing the stored energy and propelling the club forward with a peak force of up to 1500 N.
This huge force (relative to the animal’s size) creates stress waves in both the shrimp’s target – typically a hard-shelled animal such as a crab or mollusc – and the dactyl club itself, explains biomechanical engineer Horacio Dante Espinosa, who led the Northwestern research effort. The club’s punch also creates bubbles that rapidly collapse to produce shockwaves in the megahertz range. “The collapse of these bubbles (a process known as cavitation collapse), which takes place in just nanoseconds, releases intense bursts of energy that travel through the target and shrimp’s club,” he explains. “This secondary shockwave effect makes the shrimp’s strike even more devastating.”
Protective phononic armour
So how do the shrimp’s own soft tissues escape damage? To answer this question, Espinosa and colleagues studied the animal’s armour using transient grating spectroscopy (TGS) and asynchronous optical sampling (ASOPS). These ultrasonic techniques respectively analyse how stress waves propagate through a material and characterize the material’s microstructure. In this work, Espinosa and colleagues used them to provide high-resolution, frequency-dependent wave propagation characteristics that previous studies had not investigated experimentally.
The team identified three distinct regions in the shrimp’s dactyl club. The outermost layer consists of a hard hydroxyapatite coating approximately 70 μm thick, which is durable and resists damage. Beneath this, an approximately 500 μm-thick layer of mineralized chitin fibres arranged in a herringbone pattern enhances the club’s fracture resistance. Deeper still, Espinosa explains, is a region that features twisted fibre bundles organized in a corkscrew-like arrangement known as a Bouligand structure. Within this structure, each successive layer is rotated relative to its neighbours, giving it a unique and crucial role in controlling how stress waves propagate through the shrimp.
“Our key finding was the existence of phononic bandgaps (through which waves within a specific frequency range cannot travel) in the Bouligand structure,” Espinosa explains. “These bandgaps filter out harmful stress waves so that they do not propagate back into the shrimp’s club and body. They thus preserve the club’s integrity and protect soft tissue in the animal’s appendage.”
The team also employed finite element simulations incorporating so-called Bloch-Floquet analyses and graded mechanical properties to understand the phonon bandgap effects. The most surprising result, Espinosa tells Physics World, was the formation of a flat branch around the 450 to 480 MHz range, which correlates to frequencies arising from bubble collapse originating during club impact.
Evolution and its applications
For Espinosa and his colleagues, a key goal of their research is to understand how evolution leads to natural composite materials with unique photonic, mechanical and thermal properties. In particular, they seek to uncover how hierarchical structures in natural materials and the chemistry of their constituents produce emergent mechanical properties. “The mantis shrimp’s dactyl club is an example of how evolution leads to materials capable of resisting extreme conditions,” Espinosa says. “In this case, it is the violent impacts the animal uses for predation or protection.”
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The properties of the natural “phononic shield” unearthed in this work might inspire advanced protective materials for both military and civilian applications, he says. Examples could include the design of helmets, personnel armour, and packaging for electronics and other sensitive devices.
In this study, which is described in Science, the researchers analysed two-dimensional simulations of wave behaviour. Future research, they say, should focus on more complex three-dimensional simulations to fully capture how the club’s structure interacts with shock waves. “Designing aquatic experiments with state-of-the-art instrumentation would also allow us to investigate how phononic properties function in submerged underwater conditions,” says Espinosa.
The team would also like to use biomimetics to make synthetic metamaterials based on the insights gleaned from this work.
