Po-Yu Chen

Shark dermal denticles have been extensively investigated with respect to their morphology, elemental composition, and mechanical properties. Among these features, the denticle crown is recognized as the principal structure driving functional adaptations, such as drag reduction, anti-fouling, bioluminescence, and protection. However, research on the denticle base has generally been overlooked its morphology and the way it embeds within the skin remain insufficiently explored. In this study, we extracted the base of the longfin catshark (Apristurus herklotsi) from a 3D-reconstructed model to investigate the mechanical behavior of its root, which is characterized by asymmetrical branching. Symmetrical bases were also generated by assembling components from the original model to provide a comparative framework, thereby enabling verification of whether the natural asymmetry represents an optimized structural trait. For further investigation, denticle arrays based on the longfin catshark model were constructed with varying penetration levels, enabling observation of distinct mechanical behavior. Our samples were fabricated by a multi-material 3D printer, followed by pull-out tests on single denticle bases and compression tests on denticle arrays. This study deepens our understanding of the underexplored morphology–function relationships in shark denticle roots, offering insights that could exploit new micro-scale anchoring systems for biological tissues or underwater gripping devices.

Bone strength depends on factors such as mineral quantity and quality; together, these characteristics determine the mineralised tissue properties. How this quality is organised across the tissue and recovered during regeneration is still not well understood. Using the zebrafish (Danio rerio) caudal fin as a regenerative model, we applied Raman spectroscopy, field-emission electron probe microanalysis, atomic force microscopy, nanoindentation and scanning electron microscopy to characterise mineral quality in native and regenerated hemi-ray tissue, sometimes using a split-amputation design to pair native and regenerated tissue within the same individual. Bone composition, its ultrastructure and its mechanical properties converged consistently across techniques: mineral-to-matrix ratio, elemental Ca and P content, and nanoindentation reduced elastic modulus (Er) and hardness (H) all decreased from proximal to distal regions and from the centre to the edge of each hemi-ray. At 14 dpa, regenerated tissue showed a significant reduction in Er relative to native tissue (p = 0.0286), while hardness did not differ significantly (p = 0.343), and regenerated distal Ca/P ratio remained far below native proximal values (p = 0.0065), indicating partial but incomplete recovery of mineral quality at this regenerative stage. This agreement across techniques points to location, both along the fin and across the hemi-ray, as an important factor in bone mineralisation during regeneration. These findings contribute to understanding the relationship between mineral content and mechanical properties during de novo bone mineralisation and demonstrate a multi-technique approach to assessing bone quality recovery that may extend to other models of bone healing and repair.

Hierarchical hard–soft interfaces offer a route to fracture-resistant polymer composites without relying solely on intrinsically tough constituents. Here, Koch-curve-derived interfacial networks with fractal orders n = 0–3 were embedded as a compliant photopolymer phase within a rigid matrix and fabricated by multimaterial PolyJet printing. Pre-notched tensile tests, an eccentrically loaded single-edge-notch tension framework, digital image correlation, laser confocal microscopy, scanning electron microscopy, and crack-path box-counting analysis were used to determine how fractal order and pattern orientation govern fracture. At 0°, the n = 2 design increased peak nominal stress, nominal strain at fracture, tensile toughness, and crack-path length by approximately 2.2-, 1.7-, 3.6-, and 2.1-fold, respectively, relative to n = 0. Hierarchy also raised the conditional stress-intensity factor K_C from 9.78 to 13.3–13.6 MPa·m^1/2 for n = 2–3. The representative n = 2 specimen exhibited a broader strain-redistribution zone than the representative n = 0 and n = 3 specimens, while the n = 2 architecture showed the most tortuous fracture morphology, whereas added hierarchy at n = 3 yielded diminishing benefits and coincided with reduced printing fidelity. Pattern orientation altered the balance between load transfer, deformation, and crack guiding. Toughening arose primarily from interfacial crack deflection and branching, assisted by crack-tip blunting, strain delocalization, and uncracked-ligament bridging. Overall, the second-order fractal architecture printed at 0° provides the best balance of load-bearing capacity, deformability, and fracture resistance, establishing fractal order and pattern orientation as practical design variables for tough multimaterial printed composites.

TiZrNbTaMo and six TiZrNbTaMoNx high entropy alloy (HEA) nitride coatings with controlled nitrogen contents were synthesized by high power impulse magnetron sputtering to elucidate the effects of nitrogen incorporation on phase evolution, microstructure, and functional performance. Increasing nitrogen content induced a phase transformation from a body-centered cubic (BCC) solid-solution structure to a NaCl-type face-centered cubic (FCC) nitride structure, with the critical transition occurring between 17.4 and 24.9 at. % N. The mechanical properties showed a non-monotonic dependence on nitrogen content, with the highest hardness of 29.1 GPa achieved at 33.4 at. % N. Tribological and electrochemical results further revealed that the HEA nitride coating containing 36.8 at. % N exhibited the best overall wear and corrosion resistance. X-ray photoelectron spectroscopy confirmed sequential nitridation of constituent elements and stabilization of metal nitrogen bonding at higher nitrogen levels. This study establishes a nitrogen-controlled BCC to FCC phase transformation and identifies an optimal nitridation regime that provides the best balance of hardness, wear resistance, and anticorrosion performance in TiZrNbTaMoNₓ HEA coatings.

Sharks have roamed the oceans since the Devonian, surviving multiple mass extinctions. Today, sharks are agile, streamlined swimmers, ambush predators hiding beneath the sand, and giants cruising in slow motion. Impressively, they are all covered in an armour of tiny skin teeth (dermal denticles). We quantify dermal denticle disparity using deep learning on scanning electron microscopy images, discerning ten morphogroups across modern sharks (Selachii). Seven of the ten morphogroups display ridged denticles. Through nanoindentation, electron probe micro-analysis, and computational fluid dynamics, we establish mechanical properties, elemental composition, and swimming performance, and find that denticle ridges are highly variable and present a spectrum of functional properties beyond drag reduction. Further, we identify a potential correlation between a radiation event within the ground sharks Carcharhiniformes around 170 MYA, resulting in the enormous diversity of the clade, and ridged dermal denticles.

Stalked barnacles represent an evolutionarily earlier lineage than acorn barnacles. They not only rely on rigid shell plates but also a unique flexible yet protective stalk to withstand powerful and repeated wave-induced mechanical forces in intertidal environments. However, their mechanical properties remain poorly understood. In this study, the gooseneck barnacle (Capitulum mitella) was investigated by the Materials Science approaches. Its hierarchical structure and elemental composition were characterized using PXM, SEM, EPMA, XRD, and FTIR. Tensile testing and nanoindentation were further conducted to evaluate its mechanical performance. The results revealed pronounced tensile anisotropy in the stalk, with transverse tensile specimens exhibiting approximately 1.9 times higher elastic modulus and ultimate tensile stress than longitudinal specimens, accompanied by distinct crack propagation and fracture behaviors. Notably, this study provided the first systematic evidence of soft-hard material integration within the scales, which were embedded on the stalk. Nanoindentation results indicated that the mineralized core exhibits a hardness more than an order of magnitude higher than that of the surrounding non-mineralized region, despite the absence of a distinct graded transition layer. This sharp mechanical contrast suggested a direct soft–hard integration strategy, in which the compliant organic outer layer contributes to energy dissipation and mitigates stress concentration in the stiff mineralized core. This work elucidated the hierarchical structure and mechanical adaptation mechanisms of C. mitella, which offered new insights for bio-inspired protective armor design.