Literature Review

i) Armored catfish, Pterygoplichthys pardalis

According to Ebenstein et al. (2015), the Pterygoplichthys Pardalis armored catsfish has arrow shape (sandwitch-like structure) dermal plates cover their body to protect them from sharp tooth penetration caused by other predators. He studied the mechanical, chemical and structural properties of the P. pardalis by using tools like differential scanning calorimetry (DSC), scanning electron microscope (SEM)/energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Nanoindentation analysis and statistical analaysis. All these scientific tools are generally explained by the author in the Journal. The interested parts are, unlike most of the common fish have elasmoid fish scales, P. pardalis have a three dimensional “V” shape of dermal plates that 15 mm in length and 1.5 mm in thickness. Below figure shows the Hierarchical structure of the dermal armor of P. pardalis which are provided in the journal. The skin under the exterior protection layer (Dermal Plate) has a smooth texture, meanwhile the dermal plates are covered with the tooth-like elements called tubercles to protect against predators. Song et al. (2010) also reported that the tubercles have both penetration resistance and hydrodynamics ability. 

Figure 1. Hierarchical structure of the dermal armor of P. pardalis.

Dermal armour enables lightweight, flexible and tough ability to the common fish. However, in P. pardalis, the overlapping layers of arrow shape dermal plates containing porous inner matrix which provide toughening mechanism and flexibility that absorbing energy to prevent fracture of the outer lamellar layers. This overlapping structure is also implemented in some military armour due to its lightweight and flexibility. In conclusion, dermal plates provides different protection mechanism such as prevent predator tooth penetration, meanwhile porous inner matrix playing roles that absorbing energy from an attack and increase the fracture toughness (prevent fracture) of the dermal plates (Ebenstein, Calderon, Troncoso, & Torres, 2015). 

ii) Shark’s Skin

By studying the microscopic structure of shark’s skin, it has been found that the surface of the shark skin is formed with many micro-riblets. Upon investigation, it is concluded that the micro-riblets, when aligned in the local flow direction, aid in the drag reduction of the shark hence enabling the shark to be one of the fastest fish in the sea. While aligned in the local flow direction, the micro-riblets acts in a way that it will reduces wall shear stress by altering the distribution of the flow field. Figure below shows the microscopic image of the shark skin with micro-riblets covering the surface.

Figure 2: microscopic image of the shark skin. (Ltd, 2015)

This phenomenon discovered by scientist has since been widely applied at the fuselage of aircrafts and outer surface of ships as it will reduce the drag experienced by the aircrafts and ships while moving through medium such as air or water. By doing so, less power will be needed by both transportation methods and energy could be saved. To put it into perspective, this application will not just save the cost of the transportation by reducing the fuel needed, but also eco-friendly to the environment as less pollutant will be emitted by the engines of both the ships and the aircrafts. Zhao et al. have performed a vacuum casting to replicate the micro-riblets of the shark skin. Through experiment, the prototype shows a promising 9.7% to 18.6% of drag reduction. The same technology was also adopted in the design and fabrication of the Speedo swimming suit. Through the biomimetic of a shark skin like swimsuit, Speedo claims that their Fastskin LZR Racer Elite 2 will have a 6% reduction in drag, which will increase the swimming speed of the swimmer.

Figure 3. Fastskin LZR Racer Elite 2 (Anon., 2015)

iii) Sharklet

In general conception, Sharklet is simply a type of plastic specifically designed to impede bacterial growth. This biomimicry based invention serves great purpose on fields with relatively high potential for bacterial infection such as the hospital. Serving its purpose upon impeding bacterial growth, the Sharklet could be measured as a successful method in purging out the spread of infection.

The inspiration came from observing the Galapagos Shark which does not exhibit signs of being inhibited by barnacle or microscopic algae upon the skin’s surface. Further study shows that the key lies in within the unique skin pattern under microscopic observation which consists of repetitive diamond shaped scales stacked partially amongst each other. Figure 1 portrays visual illustration of the shark’s skin pattern under microscopic pattern. 

Figure 4. Microscopic image of Sharklet pattern

Figure 5. Microscopic image of Sharklet pattern.

Under close observations, it has been discovered that these dermal denticles possess different gradients at a nanoscopic scale, inducing stress gradient upon the lateral surface during initial contact. This simple yet effective mechanism causes the foreign microscopic cells to experience disruption when attempting to settle due to the high requirement of energy needed to be expanded in order to equalize the stresses induced from the fluctuated gradient pattern. So to speak, a large amount of energy is required to inhibit on the surface which was thus deem unfavourable by the settler; from the result of poor thermodynamic efficiency, resulting it to search for an alternate surface with better compatibility to synergise with.

A study done by May et al. shows that the efficiency for these micro-patterned surface demonstrated the ability to reduce bacterial colonization up to 99.9% compared to an un-patterned surface. The aforementioned technology harvested from the holistic approach of biomimicry not only saved precious time and resource for R&D to solve one of the many issues mankind currently faced, but also deemed friendly towards the environment, human and cost efficient as the plastic material currently being widely used would receive minor modifications in terms of surface pattern implementation with varying gradients which is visible under microscopic observation. This breakthrough would be believed as a staggering contribution within the human association in preventing the spread of disease and promote a hygienic society for a better future.

iv) Arapaima Gigas

Figure 6. Arapaima Gigas 

Arapaima Gigas is known as one of the biggest freshwater fishes in the world which can be found in the Amazonian region, reaching a length of 2-2.5m and a mass of over 150kg. Biomimicry of Arapaima’s scales could use to develop new ceramics for armor and panels due to their high toughness and flexibility. The maximum tensile strength and Young’s modulus of the Arapaima’s scales are found to be 53.86Mpa and 1.38Gpa respectively (Torres, Troncoso, Nakamatsu, Grande, & Gómez, 2008. Besides, XRD and FTIR show that the scales are formed by collagen fibres reinforced with a mineral phase of calcium deficient hydroxyapatite. The morphology of the Arapaima Scales show a plywood pattern of collagen layers co-aligned within each individual layer rotating at angles of around 90° between each layer.

Figure 7. Hierarchical structure of the Arapaima gigas scales.

The Arapaima gigas scales have an outer layer that is highly mineralized where the inner layer is a laminate composite of collagen fibers which are formed by fibrils (Lin, Wei, Olevsky, & Meyers, 2011). Ceramic surfaces of constant thickness are strained when forced to follow a curved surface, but the grooves allow the scales to be bent more easily without cracking. The corrugations, the soft but tough internal layer and the hydration of the scales all contribute to their ability to flex while remaining strong. It’s an engineering solution that lets the fish remain mobile while heavily armored, and also allows the scales to bend and deform considerably before breaking.

References:

Ebenstein, D., Calderon, C., Troncoso, O. P., & Torres, F. G. (2015). Characterization of dermal plates from armored catfish Pterygoplichthys pardalis reveals sandwich-like nanocomposite structure. Journal of the Mechanical Behavior of Biomedical Materials, 45, 175–182. http://doi.org/10.1016/j.jmbbm.2015.02.002

Anon., 2015. Speedo. [Online] Available at: www.speedousa.com [Accessed 29 April 2015].

Ltd, M. M., 2015. Insight for the european commercial marine business. [Online]
Available at: http://www.maritimejournal.com/ [Accessed 29 April 2015].

Lin, Y. S., Wei, C. T., Olevsky, E. a., & Meyers, M. a. (2011). Mechanical properties and the laminate structure of Arapaima gigas scales. Journal of the Mechanical Behavior of Biomedical Materials, 4(7), 1145–1156. http://doi.org/10.1016/j.jmbbm.2011.03.024

Torres, F. G., Troncoso, O. P., Nakamatsu, J., Grande, C. J., & Gómez, C. M. (2008). Characterization of the nanocomposite laminate structure occurring in fish scales from Arapaima Gigas. Materials Science and Engineering C, 28(8), 1276–1283. http://doi.org/10.1016/j.msec.2007.12.001