Search

Fish scales can be considered as a laminated composite based on collagen fibrils arranged in a cross-plywood structure. This collagen-based composite is often partially mineralized (primarily hydroxyapatite) in the scale exterior in order to resist penetration and hence to enhance protection. Together with the overlapping assembly, the fish scales offer an excellent model system for developing fiber composite materials and flexible armor systems. The primary objective of this thesis is to characterize the distribution of the mineral phase within individual scale and to investigate the corresponding mechanical consequences of the scale as a whole and its different fields through experimental and computational approaches. In this thesis, we chose the scales from the black drum (Pogonias cromis) fish as a model system. First of all, the exterior surface morphology of individual scales was systematically studied, from which several distinct structural regions are identified, including focus field (central), lateral field (dorsal and ventral), rostral field (anterior), and caudal field (posterior). In the focus field, the classic two-layer design, i.e., mineralized exterior layer and collagen-based interior layer, was observed, and nanoindentation results revealed that the high mineral exterior layer results in a much higher hardness (800 vs 450 MPa). Moreover, macroscopic tensile tests indicate that the mechanical removal of mineralized layer did not lead to reduction in strength values, whereas acid-treated demineralized scales showed reduced mechanical properties. Finally, we identified a previously unreported mineral distribution pattern in the rostral field, in which the mineral phase is segregated into long strips along the anterior-posterior direction (width, ~300 μm). In addition, towards the interior of the scale, it appears that the mineral deposition is highly correlated with the collagen orientation, resulting a unique mineralized-unmineralized collagen-based composite structure. We built finite element models to compare this unique structure to two other mineral phases in different fields at the individual scale. This unique structure demonstrates a larger deformation displacement when load was applied, indicating that it provides further flexibility in anterior end of an individual scale. The mineralized phases and structures of various fields within a single scale provide different mechanical characteristics and properties. The structural and mechanical analysis of the various regions of the fish scale can further investigate the flexibility and protective capacity of the individual scale.

Mother nature is an ingenious master on developing efficient structural and functional materials, which manifest various fascinating properties that are superior to synthetic materials. Here, we systematically investigate two topics: the materials design for piscine defense and hydro-actuated reversible deformation of plant organs. In each topic, we selected several types of biological materials to study and unravel the connection between their intelligent hierarchical structure and superb mechanical properties. We selected three fish and fully investigated the structure and mechanical properties of their scales. The first is the legendary lobe-finned fish coelacanth, which is extant for 400 million years. This defense is provided by primitive elasmoid scales having a double-twisted Bouligand structure of collagenous lamellae. The collagen fibrils in coelacanth scale form bundles which are embedded in a matrix composed of fibers arranged perpendicular to the layered structure, providing added rigidity and resistance to deformation. The second one we studied is a more evolved scale from common carp. Carp has typical modern elasmoid fish scales which are commonly found on most current teleosts. Like coelacanth scale, the outer surface of carp scale is composed of mineralized layers and the inner core is comprised of collagenous lamellae. The lamella orientations in carp scale follows a single twisted Bouligand pattern, with a rotation angle of 36°. Moreover, perpendicular to the lamellae, thinner collagen fibrils, which are called threading fibrils here, form a “sheet-like” structure oriented from the basal part to the external layer. Using in situ synchrotron small-angle x-ray scattering during uniaxial tensile testing, the deformation mechanisms of the collagen in these two scales are identified in terms of fibril stretching, reorientation, sliding, bending and delamination. The third scale we studied is from one of the largest freshwater fish, Arapaima, which has successfully survived in piranha-infested seasonal lakes of the Amazon. To quantitatively investigate the scale’s fracture toughness, we developed a new fixture and measured the J-integral based fracture toughness of the scale and find that the crack-growth toughness as high as ~200 kJm-2, which is one of the toughest flexible biological materials. This toughness is primarily the result of multiple mechanisms which are identified by in situ SEM observation. Our results may bring some critical thinking for developing novel armor materials. Instead of evolving body armor to protect themselves passively, some fish develop powerful weapons to defend actively. We fully investigated the thorny catfish (Doradidae; order Siluriforme), which has barbed pectoral fin spines and mid-lateral scutes that work in concert to provide an active mechanical defense capability. The structural design of these two weapons is very impressive, including a hollow structure, porous components, and gradient transitions, leading to an outstanding performance by maintaining strength, toughness and light weight synergistically. These designs can provide inspiration for developing new structural materials.The second topic we studied is the hydro-actuated reversible deformation in plant organs. Plants have developed many intelligent strategies to respond to external stimuli, significantly benefiting their survival. Here we studied a classic: pine cone, which opens to release the pods upon drying and can reclose upon rehydrating. We unraveled a novel mechanism of reversible deformation actuated by hydration. Our findings provide an interdisciplinary perspective combining materials science, structural engineering and biology, as well as offering some design models for development of novel smart responsive materials which have improved mechanical properties and biocompatibility yet simpler strategy.

Australian underutilised fish species, such as silver warehou (Seriollela punctata) and eastern school whiting (Sillago flindersi) as well as fish by-products may be potent sources of bioactive peptides. These species and/or their by-products are often wasted due to their poor technological and textural properties, and not acceptable for consumption by the Australian consumers. Interestingly, these fish wastes possess important nutritional value and physiological benefits, owing to their high protein content. Technological processing and the presence of endogenous enzymes in the fish muscle have the potential to release the so-called bioactive peptides during storage or digestion in the gastrointestinal tract. In the new era of the field of food science and technology, the production of bioactive peptides released from fish wastes is becoming increasingly important to preserve the marine sustainable environment and develop essential functional food to maintain human health as well as preventing the risks of developing certain types of metabolic diseases, like obesity and/or metabolic syndrome. As a result, the main focus of this project was to investigate the controlled hydrolysis of fish muscle proteins from by-catch fish species as a means of producing bioactive peptides with beneficial physiological properties. Important in-vitro bioactivities investigated could be related to the prevention of obesity and associated health complications. They include i) angiotensin-converting enzyme (ACE – hypertension) inhibition, ii) trypsin inhibition (satiety-induced) and iii) anti-oxidative (oxidative stress and inflammation-related conditions) activity.