(D-144) The Snapping Shrimp: Biomimetic Inspiration for Shockwave Protection

The snapping shrimp (Alpheidae) have developed biological protection against shockwave exposure produced by their own claws, and claws of rival shrimp¹. The shrimp use their claws as a defense by snapping quickly, forming cavitation bubbles that collapse, producing shockwaves². The snapping shrimp are at an increased risk of blast-induced neurotrauma caused by the shockwave exposure from the snaps of rival snapping shrimp or their own.

            The snapping shrimp have evolved a mechanism of self-protection in the form of an orbital hood to limit neurotrauma. The orbital hood is a helmet-like extension of the snapping shrimp’s exoskeleton that works to dampen shockwaves by expelling water out from underneath the orbital hood, redirecting kinetic energy from the shockwave away from the head of the shrimp¹.

            There is little information on the material and microstructural properties of the orbital hoods that allow for the snapping shrimp to be self-protected from shockwaves. To address this problem, the orbital hoods of the snapping shrimp were studied along with a control, the carapace, to compare the data. Mechanical tests, SHG image analyses, and finite element analyses were conducted to study the orbital hoods and carapaces of the snapping shrimp to determine the material properties of the orbital hoods. The long-term goal is to identify the material and structural properties of the orbital hoods and replicate them to improve human helmets to provide more protection against shockwaves and resultant blast-induced neurotrauma.

The orbital hoods and carapaces of snapping shrimp were removed and stored in saltwater before mechanical testing and cryosectioning. An image analysis using ImageJ software was conducted on each sample to obtain the measurements of each sample.

            The Bose® ElectroForce 3230 Test Instrument was used to conduct mechanical testing of the samples. Orbital hood and carapace samples were fixed and subjected to a 0.3 tensile strain over 5 seconds and held in place for 65 seconds. Force-displacement data were acquired using WinTest® Software at a scan rate of 29 scans/second for a total of 70 seconds. The data was converted to stress-strain data and fitted to a Maxwell-Wiechert constitutive equation, containing four-time constants.

The orbital hood and the carapace samples were frozen in a gel compound using the Leica CM1850 Cryostat™ machine. The gel was cryosectioned into slices 25-30 micrometers thick and imaged with second harmonic generation (SHG) using Leica SP8 Multiphoton Confocal™ microscope into stacks of images from each slice. Amira™ Software was used to obtain tortuosity data of the orbital hood and carapace layers for comparative analysis.

LS-DYNA™ software package was used to create a computational model of the shrimp and a blast-induced shockwave for finite element analysis. The software was utilized to mesh and define the material and geometric properties of the models, run a blast-induced shockwave simulation, and extract stress data resulting from the shockwave on a brain model protected by material similar to an orbital hood versus a carapace.

          Orbital hoods and carapaces from the snapping shrimp were tested and analyzed to determine if they exhibited different material properties. The orbital hoods were found to relax approximately twice as much as the carapaces when a strain was applied, signifying the orbital hoods are more viscous than the carapaces. Similarly, the results showed differences in stiffness between the orbital hoods and carapaces. The orbital hoods exhibited infinite, K_∞, and instantaneous, K₀, stiffness values that were approximately half of the values shown by the carapaces. Paired t-tests were run to detect differences in the infinite stiffness, K_∞, instantaneous stiffness, K₀, and ratio of infinite to instantaneous stiffness, g_∞, of the orbital hoods and carapaces. The t-tests displayed p-values of 0.0026, 0.0065, 0.0102 for the K_∞, K_0, and g_∞ values, respectively, indicating there are statistically significant differences between the orbital hoods and carapaces. 

The tortuosity values of the chitin fibers making up the orbital hoods and carapaces extracted from SHG image analysis were greater for the orbital hoods than the carapaces, suggesting the orbital hoods are more viscous than the carapaces. Stiffness values of the orbital hoods and carapaces obtained from mechanical testing were used as material property parameters for finite element analysis. The simulation included a blast originating close to the shrimp model inside a water body model, creating a shockwave that traveled through the water, impacting the shrimp. The stress on the brain from the shockwave simulation was lower when protected by the orbital hood than when protected by the carapace, indicating the orbital hoods dampen shockwaves to protect against neurotrauma.

           Upon conducting mechanical tests and computational analyses, there is data to suggest that the orbital hoods have different material and structural properties than their carapace counterparts, allowing them to better protect the snapping shrimp from shockwave injuries, such as neurotrauma. Such findings imply that biomimetic materials can be used to create shockwave-dampening helmets.

[1] Kingston, A. C., Woodin, S. A., Wethey, D. S., & Speiser, D. I. (2022). Snapping shrimp have helmets that protect their brains by dampening shock waves. Current Biology, 32(16), 3576- 3583.

[2] Lohse, D., Schmitz, B., and Versluis, M. (2001). Snapping shrimp make flashing bubbles. Nature 413, 477–478. https://doi.org/10.1038/ 35097152.

[3] Guimarães, C.F., Gasperini, L., Marques, A.P. et al. The stiffness of living tissues and its implications for tissue engineering. Nat Rev Mater 5, 351–370 (2020). https://doi.org/10.1038/s41578-019-0169-1