Mechanical Performance of Fish Bones as Inspiration for Lightweight Structures - Research Proposal Example

Contents Contents 2 3 Introduction 3 The osteocytes of fish bones 5 The structure of the fish bone 7 Mechanical properties of fish bones 8 a.Histology and light microscopy 9 b.Distribution of osteocytes and density 10 c.Bone Mineral Density (BMD) 10 d.Young’s modulus 11 e.Water, organic content and ash content 12 Porosity in fish bones 13 Micro-cracking in fish bones 13 Use of fish bones in lightweight plastics 16 Conclusion 19 References 20 Abstract There is a great diversity of the tissue of the skeletons of fishes as compared to other vertebrates but very little research has been conducted in this field hence scanty information is available about the mechanics of the fish bones. According to Bonnuci, since the 1800s it has been widely known among biological scientists that many fishes normally possess acelluar bones within their skeletons but both the mechanical advantages and structural characteristics of these bones have remained unclear for a very long time. Fishes normally comprise about half of all the vertebrates in the world population but the structure and the materials that usually form the fish skeletons are very poorly researched and hence little understanding of the mechanical behavior of this crucial species of vertebrates. It is very interesting that there are many intriguing questions particularly on how the fish bones are able to handle forces and this is opening space for research in this field (Moss, 2005). In this research, focus will be directed towards understanding the structure of the fish bones. We shall conduct a comprehensive analysis on the nature of the fishbone, the mechanical properties, the structural appearance and the use of fish bones in manufacture of lightweight materials. Introduction Bonnuci in his publication, The osteocyte: the underestimated conductor of the bone orchestra, he states that bones from vertebrates can be described as being either cellular or acellular in nature. This difference is usually as a result of whether osteocytes are embedded within the mineralized matrix of the fish bone or not. The existence of osteocytes creates results in the creation of morphologically distinctive materials with a wide ranging and interconnected system of microscopic canals that are associated with the cell lacunae (Bonnuci, 2009). Though the acellular bones lack osteocytes inside the bone matrix, it is still able to recruit periosteal osteocytes to control the remodeling and modeling of the fish bone. A functional important mechanical property of fish bones is stiffness both in the mechanical aspect and the material approach. The mechanical properties of the fish bones are affected by various factors including the porosity of the bones which determines the level of stiffness. According to Curvey, an increase in the bone porosity results into a substantial decrease in the stiffness of the bone. Bones are known to form a tough and protective load that bears the framework of the skeletons of various vertebrates. The magnitude and direction of the loads imparted on the bones usually varies from species to species but, most bones are able to cope with the changing mechanical environment through accumulation of damage and self-repair mechanisms (Draper & Goodship, 2003). These two mechanisms are normally dependent on the ability of the bones to sense local changes that may occur in their mechanical environment. Marek in his publication points out that in the 19th century, scientists observed that majority of the teleost fishes did not contain osteocytes in their bones. This created a great deal of ambiguity in this field as it was already known that the main functions of the bones were mainly mechanically based and absence of osteocytes implied that they were mechanically disabled (Marek, 2003). Several other authors have published reports implying that the acellular bones of neotoleost fishes usually respond to mechanical loads by modeling and remodeling just as any cellular bone would behave. The osteocytic lacuna normally acts as an indicator of stress concentrations on the fish bones and their absence could result into a major decrease in the rate of micro damage accumulated on the bone (Less et al, 2004). Paradigms of the structure of the bones and their associated functions are normally borrowed from the study of the bones of the mammals which have mineralized tissues and consist of extracellular matrices. They are normally reinforced using both intrafibrillar and inter fibrillar hydroxyapatite crystals and are then embedded by cells. Recent studies have established that the fish bones do not contain osteocytes within their structure. The bone tissue is normally acellular and usually lacks the internal cellular components (Ghaly & Ramakrishan, 2005). Bonnuci in his research goes on to state that the teleost species of fish represent between 25-30% of all the vertebrates. The study of the acellular bone tissue in these fishes remains a puzzle in many studies and publications as there are many unanswered questions regarding these animals. The primary purpose for bones in fishes in the bodies of fishes is the mechanical support. Bones arising from a load bearing component of the fish are able to allow the muscles to use skeletal elements as levers for to aid motion (Ghaly & Ramakrishan, 2005). They also give a lightweight and tough layer to the endoskeleton of the fish. The properties of the materials and the architecture of the fish bones are poorly studied and there is need to determine the manner by which fishes respond to mechanical loading (Less et al, 2004). The way in which the fish bones behave mechanically is particularly interesting considering that osteocytes act as sensors of strain thus regulating the process of addition, removal and replacement of bone materials. In this paper, we shall use the scantily available information to review the structure of the fish bones. We shall rely to a large extent on the available reviews that have been carried out in this field and at the same time use our own understanding of mechanical physics to analyze the attributes of fish bones. The osteocytes of fish bones From the study of the mammalian paradigm, it has been proved that the dynamics of the bone are normally dependent on the components that form the cellular part of the fish bones. It is therefore important to understand the consequences that may result from the accelularity of the bones (Draper & Goodship, 2003). Osteocyte bones usually have the ability to monitor the loading on the bodies of the fishes. When loaded repeatedly and in a cyclic manner, the fatigue that accumulates can result into damage of the bone tissues resulting into the formation of micro-cracks (Marek, 2003). The direction and magnitude of loading varies depending on the environment and the behavior of the fish. As the fish grows and increases in size during their lifetime, the process of modeling and remodeling becomes more interwoven and less distinct in their bones (Draper & Goodship, 2003). A deeper study of the fish skeletons has rebutted the previous notion that acellular bones do not undergo remodeling and it has been proven that fish bones are modified by the processes of modeling and remodeling during their lifetime (Marek, 2003). Figure 1: parameter space diagram indicating the material components in fish bones The structure of the fish bone The skeleton of a fish is mostly composed of primary bones that are laid down either as surface layers or as blood vessels. The endoskeleton consists of a mixture of tissues of bones that have been patterned at different angles. The fish bone is therefore primarily made up of type I collagen hydroxyapatite crystals and water molecules. They play a large role in the determination of the material properties of the tissues forming the bone (Marek, 2003). The variations of these materials across the bones have not been determined yet but intensive research is being conducted in this field. The Bone Mineral Density (BMD) of cellular bones has the tendency to appear lower than that of the other vertebrates (Moss, 2005). The nanostructure of the fishes has barely been explored and hence there is need for much more research to be conducted in order to give more insight on the material properties of the bones. Moss studied the difference between accellular and cellular fish bones and he observed that there is a notable disparity in the structural differences in the two types of fish bones though negligible and at a sub-micron scale. The complexity of the structure of the fish bones varies with sex, age, physiological functions, skeletal sites and mechanical loading experience. Figure 2: The dermo bone and cartilage bone in a fish This makes the bone more spongy and compact hence increasing its stiffness (Moss, 2005). The low level of osteocyte combined with the apparent relative density of the blood vessels in the fish bones make them appear more solid and less porous blocks on the bones. Tilapia and carp have low levels of collagen material on the transverse cross section of the opercula and the ribs. Moss described a fish bone as a mixture of both lamellar and woven bone. The dense network of narrow tubules in fishes has remained difficult to interpret for a very long time. It is known that tubules are not usually associated with osteocytes but they closely resemble the Williamson canaliculi which forms a major part of the endoskeleton of the bony fishes (Horton & Summer, 2009). Figure 3: The opercula and rib bones in a fish. Mechanical properties of fish bones Tests for the mechanical properties of fish bones have been carried out in a number of fishes including tilapia, carp, trout and birch. According to Horton and summer, the absence of osteocytic lacunae which is responsible for porosity results into an increase in the stress and a substantial reduction in the stiffness of the cellular bones of the fishes. This results in an increase in the stiffness of the cellular bones (Horton & Summer, 2009). The major function that is distinct in fish bones is its mechanical performance. The young’s modulus of the acellular bones ranges between 3.67 Gpa and 8.40 Gpa. In both tilapia and carp, the mineral density is quite high but both show a stiffness of between 5 Gpa and 8 Gpa. Fish bones normally exhibit adaptive plastic response characteristics to loading in fishes such as sea basses; a lordosis of the spine finally occurs due to loading on the bones (Draper & Goodship, 2003). The changes that occur on the lordotic structure of the fish bones are usually an adaptation mechanism due to changes in loading. Studies have shown that loading results into an uneven bone deformation rate on the loaded parts of the bodies of fishes and the less loaded areas exhibit an even bone deformation rate (Horton & Summer, 2009). Due to continuous and cycle loading throughout the life of a fish, the result is an increasing process of failure that occurs as an accumulated matrix micro-cracking. Maximum strains are reached at about 2000 micro strains on the bone (Less et al, 2005). The fundamentals of the mechanicals properties of the fish bones are as outlined below. a. Histology and light microscopy Curvey did a study with the main focus being on the study of the histology and microscopy of the fish bones of different species of can be used to determine the material properties of these structures. Curvey points out that it is important to point out that the mechanical structure of a fish is tightly connected to its structure. In order to determine the material properties, studies have been conducted on the fish opercula which normally appear as plates on the body of the fish and the tubules found on the skeletal components of the fish. b. Distribution of osteocytes and density As indicated earlier, osteocytes usually appear as scattered bits of lacunae within the sections of the operculum and the ribs of the fish. Horton & Summer argue that in many fish species, osteocytes lacunae normally yield a mean density of about 424±33 osteocytes mm-2. The mean density determines the shape of the fish bone but there is very little information that is available in regard to the shape and size of the lacunar for the different species of fish but it is assumed that the shapes are similar to those of mammals such that elliptical with major diameter of 10 micrometers and the minor diameter being 5 micro meters and the area that is taken by the osteocytic lacunae in fish bones is about 1.67 % (Horton & Summer, 2009). c. Bone Mineral Density (BMD) The Bone Mineral Density (BMD) can be determined using the micro-CT. in fish bones, BMD ranges between 735 and 824 mg/cm-3 (Ghaly & Ramakrishan, 2005). It has the tendency to be higher in the opercula bones of the fish as compared to the rib bones. Bone Mineral Density (BMD) in fish bones is higher as compared to all the other vertebrates. The Bone Mineral Density (BMD) of cellular fishes is lower than that of the acellular fishes and it is expected that the Young’s modulus will follow the same pattern as it is normally determined by the content of the mineral density, the architecture and the porosity of the bones (Horton & Summer, 2009). Figure 4: Levels of mineral density in different fishes d. Young’s modulus In order to determine the Young’s modulus of the fish bones, tests have been carried out using beams that are placed in the custom-built micromechanical testing device inside a saline containing test chamber (Ghaly & Ramakrishan, 2005). Bending tests have been done under displacement control at a rate of 500 µmm up to a final displacement of 1500µm; at this displacement the beams were in the plastic post yield region, but had not cracked yet (Marel, 2003). The Young’s modulus can be estimated using the relationship of beam theory as: According to Curvey, the mean value that has been obtained in many studies for the Young’s modulus shows that the values of stiffness in the bones of acellular fishes are almost equal. It ranges between 3.5 and 10.87 Gpa. In carp, the Young’s modulus is 5.7 Gpa suggesting that it is isotropic along the traverse section of the body of the fish. In tilapia, it ranges between 5.3 and10.9 GPa increasing substantially in the dorso ventral direction of the body of the fish (Curvey, 2009). The flexural stiffness of a fish bone is a product of the modulus of elasticity and the moment of inertia. It can be defined as the measure of the ability of an object to resist bending. The flexural stiffness of a fish bone is usually as a result of the geometric arrangement of the molecules rather than its stiffness (Curvey, 2009). However, the stiffness of the bones does not usually explain the adaptive significance of the acellular bones but their acellularity normally has beneficial effects on other properties of the materials. A low value of stiffness is related to a decrease in the level of minerals. A decrease in the content of the minerals increases the compliance of the materials and then limits the propagation of the cracks (Curvey, 2009). More research in the strength, fatigue and the toughness may show the effects of pressure and the prevalence in fish bones. e. Water, organic content and ash content The content of the water, organic matter and ash content in fish bones has been determined previously by subjecting the fish bones to intensive heat to produce ashes (Curvey, 2009). It has been determined that the dry bone ash normally contains about 64.49% of ash in tilapia and 57.58% in carp. In many fishes, the approximate values are 14.2% water content, 49.4% ash content and 3.64 % organic content. Porosity in fish bones Fish bones are known to be tubular in nature forming a compact tissue that has very few vascular spaces. In cellular bones of the fish species, there are conspicuous osteocytes that are filled with tubules from the skeleton of the fish (Moss, 2005). Large spaces are normally developed around the osteocytes and appear to be in the lacunae of the bone. The tubules are not lenticular in the transverse section but circular surrounding many of the osteocytes there are lightly stained areas of the bone matrix and clear areas of various sizes are evident (Bonnuci, 2009). Coalescing of the clear areas around the osteocytes usually produce small spaces which finally develop into larger spaces. The bars from matrices between the smaller spaces usually lose detail and become lighter and then the centers of the bars disappear and the ends of the protrude into the larger spaces (Marel, 2003). This causes the osteocytes to relocate from the matrix to the margins and centers of the developing area. Micro-cracking in fish bones Micro-cracks usually result from fracturing of the fish bones. They normally form at the interfaces of the bones that appear weak and are usually very important for the mechanisms that are used in toughening of the fish bones including both crack bridging and crack deflection which are common methods of monitoring cracking (Less et al, 2004). Micro-cracking usually plays a major role in the processes of modeling and remodeling of the bone. The toughness of a bone is usually a function of the path followed by a crack on the bone tissue. The main structural feature that controls the process of micro-cracking is the osteon of the fish bone. Just like the phenomenon in the biological occurrences, structural features usually control the growth of cracks rather than the initiation of the process of cracking (Moss, 2005). A fracture can be described as a competition between the mechanism of intrinsic damage and the extrinsic crack shielding mechanisms that normally act behind the tip of the bone. In order to inhibit the process of cracking, toughening mechanisms normally come in to limit the damage of the microstructures of the bones. For instance the plasticity ahead of the crack normally dissipates energy and reduces stresses by blunting the tip of the crack (Curvey, 2009). All the other mechanisms do not usually increase the inherent resistance of the bone but rater shield the tip of the crack from the force responsible for inducing cracking. According to Marel, extrinsic mechanisms of micro-cracking include crack bridging. In this mechanism, intact fibers present in composite materials, collagen fibers present in bones and interlocking grains present in ceramics can be used to bridge a developing crack and carry the load that would have otherwise been used to drive the crack deeper into the material. The deflection is then imparted onto the cracks by osteon. The ultimate result is the slowing down of the progress of the crack (Marel, 2003). Micro-cracking occurs in several ways including the following: a. Crack deflection and twist: As a crack emerges from a notch in the bone of the fish, it normally encounters micro-cracks at the weak boundaries of the structures of the osteons. Micro-cracks consume energy causing the growing crack to grossly deviate from the direction of the maximum tensile stress increasing the toughness of the bones. Figure 4: Crack deflection and twist b. Constrained micro-cracking: The natural tendency of fish bones to develop and then repair micro-cracks also increases in toughness of the bone by a much smaller amount of strain. Figure 5: Constrained micro-cracking c. Uncracked ligament bridging: An unbroken region formed between a primary growing crack and another initiated ahead of it. It is capable of carrying significant amounts of loads that would otherwise be used to propagate the crack. The effect of uncracked ligament bridging adds 1-2 Mpa to the toughness of the fracture. Figure 6: Uncracked ligament bridging d. Collagen-fibril bridging: These are unbroken collagen fibrils that usually bridge the gap that is formed in a crack and as a result its propagates the crack by about 0.1 mpa. Figure 7: Collagen fibril bridging Use of fish bones in lightweight plastics Draper and Goodship have conducted a wide study of the uses of the fish bones. From their publication, A novel technique for four-point bending of small bone samples with semi-automatic analysis, fish bones are normally used to create lightweight structures for use in different fields of the manufacturing industry. The presence of collagen in the structure of the fish bones gives it make it appropriate for use in manufacture of lightweight plastics. There are a number of factors that make fish bones more appropriate for use as lightweight materials as compared to the other materials. The body of a fish bone is primarily build up by lightweight frameworks that are composed of collagen molecules, crystals of minerals and ions dissolved in water. The elastic modulus of the stiffness of fish bones during the process of elastic deformation ranges between a span of 15 Gpa and 25 Gpa (Michael et al, 2013). The applied stress that is experienced on the bones is found to be less by approximately 100 mega Pascal of the strain (Moss, 2003). In comparison to the ceramics that are created from aluminium which normally undergo fracturing at about a range of 3mpa/m and10mpa/m. A major advantage of using fish bones in lightweight structures is that they have the capability for self-repair making them superior over other materials. Changes that are related to aging are tied to the structure of the muscuskeleton of the fish and give them susceptibility to fracture. The physical concept of fracturing is due to dissipation of plastic energy that is acquired from the applied load on the fish bone (Ghaly & Ramakrishan, 2005). Fish bones are basically made up of proteins which contain molecules of tropocollogens built from three polypeptides arranged in triple layers of helical geometry which are stabilized by building blocks of amino acids and hydrogen (Curvey, 2009). Due to these components, the fish bones are typically elastic and able to dissipate energy high loading capacity experienced by fish under water. Some of the major strengths of fish bones over other materials are as outlined below. a. Strength and deformation The mechanical properties of the bone are largely controlled by strength and plasticity found in the formation of the bones. Various microscopic and stereoscopic methods can be used to determine the sale and the mechanical properties of the fish bones including Raman stereoscopy, x-ray microscopy and spectra-spectroscopy (Moss, 2005). These are used to analyze the properties of the bones. Bones usually have different structural levels of deformation. Molecules of collagen normally undergo deformation by stretching and unwinding due to entropic and energetic mechanisms which involve breaking of hydrogen bonds (Marel, 2003). This results into sliding of atoms which help fish bones to overcome large elastic strains without undergoing failure. Hydroxyapatite is very important to the stiffness of a fish bone. Minerals such as collagen and tropocollagen bonded by hydroxyapatite help to increase the young’s modulus of the bone which assists in increasing the stiffness (Draper & Goodship, 2003). The toughness of the fish bones can also be attributed to the presence of sacrificial bonds within the bone structure. b. Fracture and toughness Toughness develops in fish bones from different scales of length. The deformation of mineralized collagen results in toughening of the tissue of the fish bone and forms plastic zones around areas that seem to have cracks and other types of defects. These zones usually protect the structural integrity of the fish bones by allowing energy to flow into the localized area of the bone (Draper & Goodship, 2003). Collagen fibrils that have been mineralized are able to tolerate micro-cracking. Deterioration of the quality fish bones with increase in age The exposure and risk of a fracture on a fish bone normally increases with age due to the loss of bone mass and degradation in the quality of the bone. The toughness of the fracture measurements on the fish bones that are 3-9 years old show a decrease of about 40% in the toughness of the bone (Draper & Goodship, 2003). An analysis of the changes at micro-scale shows that the properties of the fibrils and collagen normally deteriorate as aging progresses. Another process that results from aging is the initiation of modeling and remodeling of the fish bone. Draper and Goodship continue to argue that accumulation of cement normally results into the death of the remaining cells in the fish bones. This causes the density of the osteons to increase causing the rate of micro-cracking to increase. Cracking that is induced by age can result into reduction in the resistance of the fish bone to fracturing. Conclusion From the research and reviews carried out above, it is clear that the field of the mechanics of fish bones remains unexplored and much information is lacking in this field. There is need for researchers and biologists to put more emphasis in this field so that they untapped potential available in fish bones can be utilized for sustainable economic development. Fish bones have shown to possess a wide range of mechanical advantages over the bones from vertebrates. The stiffness of the fish bones increase with age and hence become stiffer with time. This is a major advantage as compared to other types of materials used in engineering. The fact that fish bones have a self-repairing mechanism and are able to bridge the micro-cracks that appear on the weaker sections of the bones give make them mechanically superior over all the other materials. The advances that have been achieved in technologies such as atomic microscopy will help in much deeper understanding of the structural appearance of the fish bones. However, a multidisciplinary approach in research will give a more comprehensive analysis in this field. References Bonucci, E.,The osteocyte: the underestimated conductor of the bone orchestra. Rend. Lincei Sci. Fis, (2009). Currey, J. D, The design of mineralized hard tissues for their mechanical functions. J. Exp. Biol. 202, 3285-3294 (2009). Draper, E. R. C. and Goodship, A. E. , A novel technique for four-point bending of small bone samples with semi-automatic analysis. J. Biomech, (2003). Ghaly AE, Ramakrishnan VV, Brooks MS, Budge SM and Dave D, Fish Processing Wastes as a Potential Source of Proteins, Amino Acids and Oils: A Critical Review, Retrieved from: http://omicsonline.org/fish-processing-wastes-as-a-potential-source-of-proteins-amino-acids-and-oils-a-critical-review-1948-5948.1000110.pdf. 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