Literature Review

Hedgehog spines are naturally impact resistant. If you ever notice, hedgehogs tend to fall out of trees from heights exceeding ten meters, either by accident or in order to avoid being captured by a predator. The hedgehog will roll into a ball, and surround itself with “spines” that will absorb the impact. The animal survives unscathed despite the velocity at impact, due to the shock-absorbing capabilities of its spines.  

Based on the research, hedgehog spines possess a unique internal morphology. When force is applied axially to an individual spine, it begins to “bow” laterally until the critical buckling load is achieved. However, the testing for longitudinal strength of hedgehog spines discovered that, it failed under far less axial load than with septa present. It is believed that circumferential septa resist tensile load and reinforce the spine’s cylindrical shape, hence, the endurance of longitudinal strength is way less than axial strength in hedgehog spines.

Figure 1: (a) Photography of hedgehog spine, showing the bulbed end on the left, which attaches to the animal; (b) SEM of a spine’s lateral cross-section; (c) CT scan of a spine’s longitudinal cross-section

 A few experiments were conducted to study about the mechanical properties of hedgehog spines. Results showed that the critical Euler buckling force for a single spine is roughly 6 N. Next, it was noticed that humidity softens the spines, although it became more durable but less energy absorbent. In terms of spines arrangement and orientation study, the study found that in certain conditions, hedgehog spines can absorb as much, or more than industry standard impact absorption foam. However, there is a definite balance between impact absorption and multi-hit durability, as samples that absorb greater amount of impact energy remain intact for fewer hits due to greater damage, while other samples that absorb less impact energy have greater resilience and can endure significantly more collisions. Though, these results only confirm the capabilities of the spine system as a whole. Further studies are still needed to understand the specific roles under different conditions.

References

Swift, N.B. et al., 2016. Dynamic impact testing of hedgehog spines using a dual-arm crash pendulum. Journal of the Mechanical Behavior of Biomedical Materials, 61, pp.271–282. Available at: http://dx.doi.org/10.1016/j.jmbbm.2016.03.019.

(Swift et al. 2016)