Shark Skin Research Articles

Shark skin biomimetic surface modification for plates: influence on free vibration response

Denticle microstructures in shark skin have intrigued researchers since they can be prospective for various marine applications due to their inherent properties which help in reducing drag as well as do not allow barnacles to form on its surface. However, any such alterations to create surface rivulets will invariably lead to changes in mass and stiffness, and thereby natural frequencies. Subsequently, the dynamic response will also be influenced as these hydrodynamic phenomena are strongly dependent on the natural frequencies. Therefore, in this work, an investigation of the effect of shark skin type biomimetic surface modifications on the free vibration response is investigated. The free vibration response of plates with shark skin is computed first using a high-fidelity numerical simulation in a commercial finite element package ABAQUS and then compared against a simplified semi-analytical model proposed herein. Results from both these methods agreed quite well. It is observed that shark skin denticles significantly influence the natural frequency without altering its mode shapes. It appears that adding shark skin like dermal denticles on the surface of a bare plate as an appendage helps in reducing the natural frequencies in each mode. The study also reveals that the reduction in the spacing between denticles reduces natural frequency even further. Therefore, the effect of these wettability altercations on free vibration response should be known apriori for efficient design.

ANALISA USAHA KERUPUK KULIT IKAN HIU DI KABUPATEN LOMBOK TIMUR

The Research purposes: 1). To find out a description of the economic feasibility in terms of product market conditions, technical aspects, management aspects, legal aspects, social, economic, and environmental aspects of the shark skin cracker business. 2). To determine the value of the sensitivity rate of Gross B/C and Payback Periods to the variable of changes in production decline and the variable of changes in the increase in production costs in the shark skin cracker business. The research design used exploratory sequential mixed methods with the Break Even Point method. Data collection techniques were carried out by means of interviews, observation and recording. The research area was chosen, namely East Lombok Regency, sampling was purposive sampling with the consideration that there were 15 units of shark skin cracker business centers located in Keruak District, Sakra District, and Pringgabaya District. By taking the number of respondents as many as 15 respondents. The results showed that: 1). Shark skin cracker business based on the criteria of economic feasibility is feasible. 3). Shark skin cracker business is based on sensitivity criteria, sensitive to changes in production costs and not sensitive to changes in production costs. Keywords: Financial, Economic Feasibility, Shark Skin Crackers, and Sensitivity

Nature‐Inspired Biomimetic Surfaces for Controlling Bacterial Attachment and Biofilm Development

AbstractThe use of antibacterial and antifouling materials is widely being investigated to combat the increasing risk associated with bacterial infections and the evolution of drug‐resistant bacteria. Efficient antibacterial materials can be fabricated by mimicking the topography found on the surface of natural antibacterial materials. Natural materials such as the wings of cicadas and dragonflies have evolved to use the structural features on their surface to attain bactericidal properties. The nanopillars/nanospikes present on these natural materials physically damage the bacterial cells that settle on the nanostructures resulting in cell lysis and death. This article reviews the role of nanostructures found on the surface of some of these natural antibacterial and antifouling materials such as lotus leaf, cicadas and dragonflies wings, shark skin, and rose petals. These natural structures provide guidelines for the design of synthetic bio‐inspired materials. This review article also presents some novel fabrication techniques used to produce biomimetic micro‐ and nano‐structures on synthetic material surfaces. The role of size, shape, aspect ratio, and spacing between the micro/nano‐structures on the bactericidal properties is also discussed. Finally, the review is finished with the author's view on the future of the field.

Shark Skin—An Inspiration for the Development of a Novel and Simple Biomimetic Turbulent Drag Reduction Topology

In this study, a novel but simple biomimetic turbulent drag reduction topology is proposed, inspired by the special structure of shark skin. Two effective, shark skin-inspired, ribletted surfaces were designed, their topologies were optimized, and their excellent drag reduction performances were verified by large eddy simulation. The designed riblets showed higher turbulent drag reduction behavior, e.g., 21.45% at Re = 40,459, compared with other experimental and simulated reports. The effects of the riblets on the behavior of the fluid flow in pipes are discussed, as well as the mechanisms of fluid drag in turbulent flow and riblet drag reduction. Riblets of various dimensions were analyzed and the nature of fluid flow over the effective shark skin surface is illustrated. By setting up the effective ribletted surface on structure’s surface, the shark skin-inspired, biomimetic, ribletted surface effectively reduced friction resistance without external energy support. This method is therefore regarded as the most promising drag reduction technique.

One-step preparation of titanium sharkskin bionic antibacterial surface

In recently, titanium and its alloys have been used in a large number of medical implants due to their excellent biological qualities. However, infections brought on by their biological inertness have emerged as a challenging issue for clinicians across a range of specialties. On the other hand, improper use of antibiotics has increased bacterial resistance, which has made the issue of infections brought on by titanium implants worse. The preparation of implants with antimicrobial capabilities has emerged as a key research area in the effort to address this issue at its source. However, present techniques for preparing antimicrobial surfaces are pricy, ineffective, and harmful to the environment, thus cannot be used extensively in industrial manufacturing. In this study, the bionic shark skin's antibacterial rhombic surface was prepared in a single step using the WEDM processing. The samples were then described, and the outcomes were examined using XRD, FE-SEM, and EDS. By using the plate coating method and SBF immersion assays, the samples' bioactivity and antibacterial capacity were assessed. Due to the high temperature produced during the WEDM process, which enables the tool electrode to melt and metallurgically link with the titanium alloy substrate surface, the primary crystalline phase of the antibacterial surface is CuTi2. After 1 h of incubation on the sample surface, the samples' antibacterial rate was 93%. The antibacterial property of the sample also attained an ideal 98.4% when acid etching was carried out using a 20%wt HNO3 solution. The Ti on the sample surface had no discernible impact on the effectiveness of the antibacterial. Additionally, the simulated liquid immersion findings demonstrated that the sample pieces made using this approach exhibited good bioactivity.

A biomimetic structured bio-based flame retardant coating on flexible polyurethane foam with low smoke release and antibacterial ability

Flexible polyurethane foam (FPUF) is widely used in our life, but it is inherent flammable. The demand for environmental-friendly multi-functional FPUF has been increasing rapidly in the last decade. In this work, a novel bio-based flame retardant coating was constructed by chemically reacting sodium alginate (OSA) and polydopamine (PDA) on the FPUF, followed by depositing nanorod-like β-FeOOH molecules through complexation reaction to form a biomimetic structure. The limiting oxygen index of the coated FPUF samples reached 25.5%. The peak heat release rate was reduced by 45.0%, and the smoke density of the coated sample was decreased by 69.1% compared to that of the control FPUF sample. It was proposed that the OSA-PDA-β-FeOOH decomposed during combustion to promote the formation of compact crosslinked char and released inert gases to dilute the combustible gases, and the β-FeOOH transferred to Fe2O3 to settled the smoke particles reducing the smoke release. Furthermore, the coating with shark skin like structure endowed FPUF antibacterial ability because of its good superoleophobicity underwater. This work provided a novel strategy to construct a biomimetic multifunctional coating on the FPUF.

Ultrastable Super-Hydrophobic Surface with an Ordered Scaly Structure for Decompression and Guiding Liquid Manipulation.

Directional droplet manipulation is very crucial in microfluidics, intelligent liquid management, etc. However, excessive liquid pressure tends to destroy the solid-gas-liquid (SAL) composite interface, creating a highly adhesive surface, which is not conducive to liquid transport. Herein, we propose a strategy to enhance the surface durability, in which the surface cannot withstand liquid pressure only by "blocking" but must instead guide liquid transport for "decompression". Learning from the water resistance of water strider legs and the drag reduction of shark skin, we present a continuous integrated system to obtain an ultrastable super-hydrophobic surface with a highly ordered scaly structure via a liquid flow-induced alignment method for lossless unidirectional liquid transport. The nonwetting scaly structure can both buffer liquid pressure and drive droplet motion to further reduce the vertical pressure of the liquid. Moreover, droplets can be manipulated unidirectionally using a voice. This work could aid in manufacturing scalable anisotropic micro-nanostructure surfaces, which inspires efforts in realizing lossless continuous liquid control on demand and related microfluidic applications.

Flexible surfaces prepared through direct ink writing with drag reduction and antifouling

The riblet structure on the surface of the shark skin provides significant drag reduction and fouling protection. Inspired by the structure of the shark skin, we present a direct ink writing protocol by modifying the rheological properties of polydimethylsiloxane to make it printable, and a series of flexible surfaces have been created with drag reduction and antifouling. The shear stress transport k–omega model was used as a suitable turbulence model for the numerical study to compare with results of drag reduction tests. Simulations and tests proved that printed non-smooth surfaces effectively reduced water drag, where the maximum drag reduction rate reached 18.16 % at 0.06 m/s. Through the analysis of the characteristics of the flow field near the wall, the drag reduction mechanism of the riblet surface was examined. Moreover, the antifouling tests revealed that the printed surface exhibited excellent fouling resistance and inhibited the spread of dirt on the surface. Our findings may provide a new surface skinning strategy for ships, underwater vehicles and wearable devices to reduce drag and contaminant adhesion and improve navigation efficiency and longevity.

Bionic non-smooth epoxy resin coating with corrosion inhibitor for drag-reduction and durability

Ship hulls and subsea equipment are plagued by current resistance, seawater particles impact, corrosion and other challenges in the ocean. Consequently, developing sustainable nontoxic antifouling coatings is highly urgent. Discoveries from nature have inspired us to design high-efficiency, environmentally friendly, and cost-effective antifouling coatings. In the study, a strategy of combining microstructures and Benzotriazole (BTA) was proposed through constructing shark skin shield scale structure, which improved drag reduction and increased abrasion resistance as well. On the synergistic effect of BTA as corrosion inhibitor, the friction coefficient of the coating was significantly reduced and the anti-corrosion performance was greatly improved. Compared to the smooth pure epoxy coating, the anti-corrosion performance and the drag reduction rate of Texture-BTA-3 % coating was improved by 30 % and about 10 %, respectively. Furthermore, the effect of coating structure on corrosion resistance was revealed. This investigation will broaden the applications in marine navigation with the favorable performance.

Ridges and riblets: Shark skin surfaces versus biomimetic models

Shark skin has been an inspiration for biomimetic materials and structures due to its role in reducing drag and enhancing thrust, properties believed to be due to the textured surface composed of ridges on the surface of individual tooth-like scales (denticles). Attempts to replicate the hydrodynamic performance of shark skin have involved manufacturing both engineered riblets and fabrics with textured surfaces. However, there are no studies that compare the surface ornamentation of shark denticles to bioinspired materials. Using three-dimensional surface profilometry we analyzed the cross-sectional profile of the surface of shark denticles at two locations on 17 species and compared these data to values obtained from engineered structures (e.g., riblets) and competition swimsuits that are often proposed as having a comparable surface texture to shark skin. Of the variables measured, crown aspect ratio (p = 0.007), ridge height, ridge spacing, ridge aspect ratio, and ridge bumpiness (all p < 0.001) differed among the three materials. Overall, engineered riblet surfaces were very different than biological shark skin. Some of the competition swimsuit materials were more shark-like, with the fabric texture having similar height variation, but with irregular ridge spacing. Cross-sectional profile, which includes pathlength and aspect ratio in addition to ridge spacing and height, is an important feature of the skin’s surface, affecting water flow over the individual denticles, and future research will address these parameters. Quantitative 3D analysis of the surface of real shark denticle ridges enables the design of more biomimetic engineered shark skin surfaces.

Cellulose-Based Cryogel Microspheres with Nanoporous and Controllable Wrinkled Morphologies for Rapid Hemostasis.

First-aid hemostatic agents for acute bleeding can save lives in emergency situations. However, rapid hemostasis remains challenging when uncontrolled hemorrhage occurs on lethal noncompressible and irregular wounds. Herein, cellulose-based cryogel microspheres with deliberately customized micromorphologies for ultrafast water transportation and diffusion, including the shark skin riblet-inspired wrinkled surface with low fluid drag and the hydrophilic nanoporous 3D networks, are developed to deal with the acute noncompressible bleeding within seconds. These cryogel microspheres can rapidly absorb a large amount of blood over 6 times their own weight in 10 s and form a robust barrier to seal a bleeding wound without applying pressure. Remarkably, massive bleeding from a cardiac penetrating hole is effectively stopped using the microspheres within 20 s and no blood leakage is observed after 30 min. Additionally, these microspheres could be readily removed without rebleeding and capillary thrombus, which is highly favorable to rapid hemostasis in emergency rescue.

Preparation of Composite Multi-Iron Material 0.2NiFe2O4/0.8BaTiO3 and Its Application in Sports Practice

Competitive sports has always been the front line of high-tech applications, from electronic timers to "shark skin" swimsuits, from "bird's nest" and "water cube" to blood lactic acid detectors; from electronic science to material science, from architecture to biology chemistry. To find more suitable sports equipment materials, we prepared the composite multiferroic material 0.2NiFe2O4/0.8BaTiO3 by solution casting, spin coating, and hot pressing, and used various instruments to understand its various properties. The study found that the two exothermic peaks of the material in the range of 110–220 °C are attributed to the large amount of adsorbed water and the degradation of hydroxyl compounds. The 20% weight loss associated with the strong exothermic peak at 300 °C is due to the degradation of inorganic salts and citric acid. What is more worth mentioning is that there is almost no weight loss when the temperature exceeds 340 °C, which proves that 80% of the total mass of organic species (such as: citric acid), inorganic nitrates, and other adsorbents (such as: water, ethanol, etc.) have been completely removed. In addition, the heat treatment process can change the crystallinity and physical and chemical properties of the material. In competition sports, 0.2NiFe2O4/0.8BaTiO3 material has the characteristics of good bounce and excellent balance. It can be applied to ski poles. Its good ductility and light weight make it very useful in bow, arrow and hull design space. In folk sports, 0.2NiFe2O4/0.8BaTiO3 composite sports equipment can be widely used in various sports and various types of sports equipment.

Morphological pattern of the dermal denticles of the Southern sawtail catshark Galeus mincaronei Soto, 2001.

Dermal denticles cover the skin of sharks for protection and may have additional functions, depending on the environment. The aim of the present study was to analyze the morphological pattern of dermal denticles across the body of the poorly known deep-water catshark Galeus mincaronei, which is endemic to Southern Brazil. Additionally, we aimed to identify differences between sexes and stages of sexual maturity. For this purpose, we analyzed patches of eight skin samples from body regions, that is, the head, body, and tail. Dermal denticles were imaged using scanning electron microscopy. Measurements of crown morphological variations (length and width) and density of dermal denticles were performed. Nine characters of dermal denticle morphology were selected to describe a morphological pattern using principal component analysis (PCA). The first PCA axis explained 45.2% of the variation, the second explained 33%, and the third explained 14.3%, together explaining 92.5% of the total variation in denticle morphology. We identified three groups of dermal denticle crown morphologies and their possible functions. The first was composed of nostril and snout denticles, which might suggest abrasion strength functions; the second was composed of most body denticles, such as those in the cranium, on the left side of the body, and on the pectoral, dorsal, and caudal fins, which might be related to generalized functions; and the third was composed of only caudal crest denticles, which might indicate defense functions with hydrodynamic properties in G. mincaronei. Likewise, our results reveal variations in dermal denticle morphology and arrangement along the three maturity stages considered in both sexes. Similarly, sexual dimorphism in the crown width and morphology of dermal denticles was observed in developing and mature individuals. Our results are the first to reveal the morphological variation of dermal denticles in relation to function, sex, and ontogeny of individual G. mincaronei.

Comparison of Physicochemical and Structural Properties of Acid-Soluble and Pepsin-Soluble Collagens from Blacktip Reef Shark Skin.

Fish collagen has been widely used in tissue engineering (TE) applications as an implant, which is generally transplanted into target tissue with stem cells for better regeneration ability. In this case, the success rate of this research depends on the fundamental components of fish collagen such as amino acid composition, structural and rheological properties. Therefore, researchers have been trying to find an innovative raw material from marine origins for tissue engineering applications. Based on this concept, collagens such as acid-soluble (ASC) and pepsin-soluble (PSC) were extracted from a new type of cartilaginous fish, the blacktip reef shark, for the first time, and were further investigated for physicochemical, protein pattern, microstructural and peptide mapping. The study results confirmed that the extracted collagens resemble the protein pattern of type-I collagen comprising the α1, α2, β and γ chains. The hydrophobic amino acids were dominant in both collagens with glycine and hydroxyproline as major amino acids. From the FTIR spectra, α helix (27.72 and 26.32%), β-sheet (22.24 and 23.35%), β-turn (21.34 and 22.08%), triple helix (14.11 and 14.13%) and random coil (14.59 and 14.12%) structures of ASC and PSC were confirmed, respectively. Collagens retained their triple helical and secondary structure well. Both collagens had maximum solubility at 3% NaCl and pH 4, and had absorbance maxima at 234 nm, respectively. The peptide mapping was almost similar for ASC and PSC at pH 2, generating peptides ranging from 15 to 200 kDa, with 23 kDa as a major peptide fragment. The microstructural analysis confirmed the homogenous fibrillar nature of collagens with more interconnected networks. Overall, the preset study concluded that collagen can be extracted more efficiently without disturbing the secondary structure by pepsin treatment. Therefore, the blacktip reef shark skin could serve as a potential source for collagen extraction for the pharmaceutical and biomedical applications.

Marine origin biomaterials using a compressive and absorption methodology as cell-laden hydrogel envisaging cartilage tissue engineering.

In the recent decade, marine origin products have been growingly studied as building blocks complying with the constant demand of the biomedical sector regarding the development of new devices for Tissue Engineering and Regenerative Medicine (TERM). In this work, several combinations of marine collagen-chitosan-fucoidan hydrogel were formed using a newly developed eco-friendly compressive and absorption methodology to produce hydrogels (CAMPH), which consists of compacting the biopolymers solution while removing the excess of water. The hydrogel formulations were prepared by blending solutions of 5% collagen from jellyfish and/or 3% collagen from blue shark skin, with solutions of 3% chitosan from squid pens and solutions of 10% fucoidan from brown algae, at different ratios. The biopolymer physico-chemical characterization comprised Amino Acid analysis, ATR-FTIR, CD, SDS-PAGE, ICP, XRD, and the results suggested the shark/jellyfish collagen(s) conserved the triple helical structure and had similarities with type I and type II collagen, respectively. The studied collagens also contain a denaturation temperature of around 30-32°C and a molecular weight between 120 and 125kDa. Additionally, the hydrogel properties were determined by rheology, water uptake ability, degradation rate, and SEM, and the results showed that all formulations had interesting mechanical (strong viscoelastic character) and structural stability properties, with a significant positive highlight in the formulation of H3 (blending all biopolymers, i.e., 5% collagen from jellyfish, 3% collagen from skin shark, 3% chitosan and 10% of fucoidan) in the degradation test, that shows a mass loss around 18% over the 30days, while the H1 and H2, present a mass loss of around 35% and 44%, respectively. Additionally, the in vitro cellular assessments using chondrocyte cells (ATDC5) in encapsulated state revealed, for all hydrogel formulations, a non-cytotoxic behavior. Furthermore, Live/Dead assay and Phalloidin/DAPI staining, to assess the cytoskeletal organization, proved that the hydrogels can provide a suitable microenvironment for cell adhesion, viability, and proliferation, after being encapsulated. Overall, the results show that all marine collagen (jellyfish/shark)-chitosan-fucoidan hydrogel formulations provide a good structural architecture and microenvironment, highlighting the H3 biomaterial due to containing more polymers in their composition, making it suitable for biomedical articular cartilage therapies.

Relationship Between Skin Scales and the Main Flow Field Around the Shortfin Mako Shark Isurus oxyrinchus.

The aim of this study was to reveal potential relationship between the main flow field around a shortfin mako shark and the surface morphology of shark skin. Firstly, a numerical simulation using the large eddy simulation (LES) method was conducted to obtain the main flow field around a smooth shark model. Then, the surface morphology characteristics of a shark (Isurus oxyrinchus) at different positions were characterized by scanning electron microscope (SEM), which showed that the morphology, riblet size, and density of scales at different positions on the shark were significantly different. At positions where the surfaces face into the water flow direction (i.e., nose and leading edge of fins), the scales were flat and round, with a lower density, and the pressure or wall shear stress (WSS) was greater. Scales with three longitudinal riblets ending in three tips were found on the middle and trailing edges of the first dorsal fin and caudal fin, where water flow states progress from transitional to turbulent. The ranges of the ratio of riblet depth to spacing (RD/RS) in the anterior zone, middle zone and posterior zone of the shark were 0.05–0.17, 0.08–0.23, and 0.32–0.33, respectively. The riblet angle generally followed the flow direction, but it varied across different areas of the body. The turbulence intensity increased gradually across the first dorsal fin, pectoral fin, caudal fin, and the shark body overall. In summary, it was found that the microstructure riblets on the shark skin surface, generally thought to be drag reduction structures, were only located in transitional and turbulent regions at the middle and trailing edge of the shark body and fin surfaces, and there were almost no microstructural grooves in the laminar flow regions along the leading edge. These findings can provide design guidance for engineering applications of bionic riblet surfaces. Riblets placed in transitional and fully turbulent regions can be used to effectively reduce drag. The riblet direction should be consistent with the direction of flow.

Superhydrophobic SLA 3D printed materials modified with nanoparticles biomimicking the hierarchical structure of a rice leaf

ABSTRACT The rice leaf, combining the surface properties of lotus leaves and shark skin, presents outstanding superhydrophobic properties motivating its biomimesis. We created a novel biomimetic rice-leaf superhydrophobic surface by a three-level hierarchical structure, using for a first time stereolithographic (SLA) 3D printed channels (100µm width) with an intrinsic roughness from the printing filaments (10µm), and coated with TiO2 nanoparticles (22 and 100nm). This structure presents a maximum advancing contact angle of 165° characterized by lower both anisotropy and hysteresis contact angles than other 3D printed surfaces, due to the presence of air pockets at the surface/water interface (Cassie-Baxter state). Dynamic water-drop tests show that the biomimetic surface presents self-cleaning, which is reduced under UV-A irradiation. The biomimetic surface further renders an increased floatability to 3D printed objects meaning a drag-reduction due to reduced water/solid contact area. Numerical simulations of a channel with a biomimetic wall confirm that the presence of air is essential to understand our results since it increases the average velocity and decreases the friction factor due to the presence of a wall-slip velocity. Our findings show that SLA 3D printing is an appropriate approach to develop biomimetic superhydrophobic surfaces for future applications in anti-fouling and drag-reduction devices.

Biofouling and Mitigation Methods: A Review

Biofouling accumulates living organisms on surfaces in contact with the water and causes significant economic, structural, and microbial problems on ship hulls, piers, oil rigs, power plants, pipework, water treatment facilities as well as medical devices. In order to mitigate problems associated with biofouling, many toxic and non-toxic antifouling methods have been developed. Unfortunately, most of the methods used to control biofouling are either harmful to the environment or, in some cases, considered effective. Thus, antifouling research's main objective is to develop green, sustainable, viable, widely applicable, and environmentally friendly antifouling technology. In this review, chemical, physical, and biological mitigation methods to prevent biofilm formation employed in the past and present have been discussed along with the current literature. Chemical antifouling methods generally contain antifouling (AF) paints with biocides including copper, silver, thiocyanate, Copper powder, Irgarol 1051, Zinc pyrithione, and Tributyltin (TBT). The physical antifouling control methods employ physical force or surface modifications such as low drag, low adhesion, wettability (super hydrophobicity or super hydrophilicity), as well as microtextured structures that minimize microorganism adhesion and/or accumulation on contact surfaces, hindering the formation of biofouling. The use of nature-inspired antibiological and biomimetic surfaces like shark skin, whale skin, dolphin skin, and lotus leaves are promising for the effective control of biofouling and present opportunities for developing non-polluting technologies.

Laminar Flow Control using an Optimised Forward-Facing Step - An Experimental Approach

Abstract With the rapid increase of global aviation emissions, biomimetics has proven to be a promising avenue to achieve greener aviation. This paper aims to explore the influence of the shark skin denticle inspired two-dimensional Forward-Facing Steps (FFS) on boundary-layer transition delay and drag reduction. Previous computation simulations indicate that the presence of FFS inside the laminar boundary layer can damp disturbances and lead to transition delay and drag reduction. The purpose of this paper is to find the optimal configuration of the FFS through experimental tests and correlate results to previously concluded simulations. Conditional results indicate that the presence of the FFS within the laminar boundary layer is effective in transition delay and drag reduction.

Sensational shark skin and dazzling dermal denticles

Shark skin has a unique structure which has served as a source of inspiration for engineers interested in reducing drag in water or preventing parasites from becoming attached. The skin is covered in tooth-like structures called dermal denticles, made of an enamel-type tissue and dentine on the outermost portion and containing a central cavity filled with pulp. Their general shape is similar to a pointy canine tooth where the base is wider than the edge and is thought to reduce the amount of drag pulling on the animal when moving through water. Interestingly, previous work suggests a wide range of denticle shapes on the skin around the gills of dogfish, where the denticles closer to the head are rounded and smooth compared with the ridged and sharp denticles toward the tail. But does this diversity in denticle shape near the gills also occur in other shark species? And what purpose might these different shapes serve?To study these questions, Molly Gabler-Smith and George Lauder from Harvard University, USA, Dylan Wainwright from Yale University, USA, and colleagues from Harvard University compared the shape and size of the denticles around the gills of 13 shark species with various lifestyles. They aimed to better understand whether the shape of these denticles differs between species and whether these differences might help water to flow over the gills.The researchers collected the sharks from fishing surveys and museums, ranging from the thresher shark, known for its massive tail, enabling it to jump out of water like a dolphin, to the spiny dogfish, which are smaller and known to live in deep sea beds. They then took 3D measurements of denticle length and width and used electron and light microscopy to look at differences in denticle shape across the skin around the gills of individual sharks and between different species.Comparing the structures, it turned out that the denticle shape varied across different areas of the skin surrounding the gills in all of the species, with the short broad denticles arranged toward the front of the shark and longer, pointier denticles occurring toward to the tail of the shark. In fact, the denticles found closer to the tail had pronounced ridges on their surfaces and three spiky spines, in contrast to the smoother almond-shaped denticles found towards the head. Some species, such as the chain catshark, had extreme changes in denticle shape over a distance of a few millimeters. They also found that the shark species that continuously swim to breathe had larger denticles toward the head compared with shark species that breathe by opening and closing their mouths to pump water over the gills.The extreme differences in shape between the denticles found towards the head versus those toward the tail may serve to reduce friction and prevent damage to the skin when the gills flap against each other while breathing. The dramatic shape difference may also reduce drag from water moving through the gills, which could prevent water from flowing in the opposite direction, stopping the shark from getting enough oxygen from the water. This study gives us a foundation for future research on how denticle shape can help sharks to breathe and if there are differences in how water moves over the denticles surrounding the gills of active or passively breathing species. Understanding the great diversity and function of denticles in shark skin can help us to learn how they evolved and even allow us to better imitate their amazing ability to reduce drag in water.