Hair is a product of evolution that can
only be found in mammals. Hairs can have various functions to mammals, which
include signaling, sensing the environment, camouflage, to retain body warmth
as wells as a signal to ward of predators, or to find a mate. Quills and spines
are a form of modified hair which are common in four groups of mammals. These
groups are:
1. Hedgehogs
(Erinaceomorpha: Erinaceidae, Erinaceinae)
2. Tenrecs
(Afrosoricida: Tenrecidae, Tenrecinae)
3. Echidnas
(Monotremata: Tachyglossidae)
4. Rodents
(Rodentia)
Quills and spines both serve different
purposes although they have a hard and thick outer layer of keratin. Quills act
as a defensive armor for porcupines, as they are long, highly irregular in
length and they break and separate from the host’s body quite easily. Spines on
the other hand serve as a shock absorber for hedgehogs as they are all of the
same length all over hedgehog, embedded well in the skin, curved slightly and
firm.
The spines of the hedgehog consists of many contained air pockets that are separated by a regular spacing of septum coupled with longitudinal stringers. The cross section of the structure of hedgehog spines can be viewed from Figure 1. The existence of the network of air pockets in the spines give them a high strength to weight ratio. Besides that, the air pockets helps prevent breakage of spines as well as buckling.
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| Figure 1: Cross section of the hedgehog spine. |
The mechanical properties of spines across different species may vary depending on the microstructure of the spine. Figure 2 shows the microstructure of the Erinaceus europaeus spine, whereas figure 3 shows the microstructure of the Hemiechinus sp spine. The septa of the former is much more widely spaced, and has longitudinal stringers that are much deeper as compared to the latter. This piece knowledge is crucial, as the internal structure of the hedgehog is essential in order to successfully design knee-pads which can absorb any excess shock. By varying the depth of the longitudinal spine, as well as the distribution and thickness of the septum, the amount of shock that can be absorbed by the aforementioned knee-pads can change tremendously.
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| Figure 3: Transverse (a) and longitudinal (b) section of Hemiechinus sp spine. Scale is 100 µm |
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| Figure 2: Transverse (a) and longitudinal (b) section of Erinaceus europaeus spine. Scale is 100 µm |
By using the concept of bio-mimicry, the hedgehog’s spines can be model as a potential for impact resistant materials and structures. There were many experimental studies that were conducted in the past on the mechanical properties for the spines of the hedgehog. From those studies, the critical Euler bulking force for a single hedgehog spine was identified to be about 6N and this was found by conducting bending and compression test on the spines of the hedgehog. Moreover, tests for the consistency of the durability performance of the hedgehog’s spine were also studied upon as it is important for various applications such as concussion mitigation for the football helmets [4]. Among the tests that were conducted was the dynamic impact test by using a dual-arm crash pendulum. Figure 4 shows the schematic of the pendulum apparatus during the trial of collision on the hedgehog’s spine that has been retrieved from a naturally deceased hedgehog.
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| Figure 4: Impact test on hedgehog spine using pendulum |
After the test was conducted, it was found that the spines in the humid sample has minor damage and remains intact after a consecutive of 5 collisions of each being 8 Joules. However, the spine which was of the dry sample shows a significant amount of breakage after just 2 collisions of each being 8 Joules.
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| Figure 5 shows the results of the hedgehog spine after impact testing [5]. |
Hence, from this literature study, this concept of using the hedgehog’s spines or quills that acts as the animal’s safety mechanism can provide some promising applications in future materials. In this study, the design hedgehog’s spines are used to redesign elbow pads and knee pads for motorcyclist as improve the mechanical properties and reduce impact of motorcyclist when acted upon.
References
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