How a mantis shrimp packs a punch

Materials World magazine
,
28 Nov 2018

Robotics could be improved with super strength shrimp design. Ceri Jones reports.

The mantis shrimp has an infamous punch, one so rapid and intense that it produces an underwater shockwave that stuns prey, such as crabs. Several punches renders them immobile and easy to eat.

The biomechanical design of the stomatopod’s dactyl clubs – the fist-like appendages near its jaws and front legs – uses kinematic motion to store and amplify elastic energy, propelling a force of up to 50mph underwater – that’s 1,000 times its own bodyweight – at a speed of up to 23m per second. This has been known for some time, but exactly how this super strength has developed was still a mystery.

Researchers in Singapore used finite element modelling (FEM) and dynamic nano-scale testing to study the shrimp’s dactyl clubs, reporting their findings in the paper Biomechanical design of the mantis shrimp saddle: a biomineralized spring used for rapid raptorial strikes, published in iScience.

The report reveals that the mantis shrimp’s power comes from a small mineralised spring on the back of the head, made of a bioceramic saddle joined to a biopolymer. The team determined to find out how this arrangement could create such a useful tool from what are, essentially, brittle ceramic materials. And further, how to replicate the design, as understanding the mechanics could help improve energy storage capabilities in electronic devices and microrobotics.

The team believes this could increase the use and effectiveness of ceramic materials, with lead author Professor Ali Miserez of Nanyang Technological University, Singapore, saying that ‘More efficient devices could be engineered using stiff and light materials, such as ceramics, since this would enhance the strain energy density that can be stored.

‘In practice, such design is usually avoided due to the intrinsic brittleness of ceramic building blocks. [But] the saddle structure provides a bioinspired design to overcome this limitation.’

Harder, better, faster, stronger

The shrimp’s muscles and connective tissue load energy into the saddle spring in a quasistatic mechanism. Release of this energy provides the speed and power to propel the joints.

To prevent excessive strain and brittle material fracturing, the paper says, stomatopods have evolved their saddle as a ‘bilayer structure at the meso-scale, which is built from a multi-phase biocomposite at the micro-scale with optimised organic/inorganic ratios within each layer’. The outer layer is an amorphous calcium carbonate, with an elastic modulus to manage the compression, and the inner layer contains organic biopolymeric elements to cope with tensile stresses.

Put simply, ceramics can cope with compression and polymers with tension, so the layering intensifies their strengths and reduces weaknesses.

‘Our findings demonstrate that the saddle design [...] indeed leads to a high elastic storage capacity, with minimal viscoelastic dissipation in the highly mineralised layer,’ the paper reads.

‘Furthermore, FEM quantitatively establishes that the saddle geometry and phase distribution confer high safety factors - during saddle loading, maximum stresses within each layer remain in the elastic regime and well below the yield strength of the constitutive materials, thus ensuring that mantis shrimps can safely load their saddle hundreds of times for striking without inducing internal fatigue damage.’

The shape and position of the saddle spring itself is important, as the neutral axis is located near the outer/inner layer interface, to prevent interfacial delamination.

To apply these findings, Miserez says the team is using additive manufacturing to print and test a range of materials in an effort to replicate the bilayer components.


The full paper can be read here: bit.ly/2TlY3ig