Compliant flat interlayers (IL) embedded into a cracked stiff matrix improve damage tolerance in polymeric systems, but typically at the cost of reduced stiffness and lower crack-initiation forces. To mitigate these limitations, a design approach originating from the Earth's lithosphere was adopted. In particular, a symmetric wavy compliant IL was embedded into a stiff matrix, and a numerical model was established in ABAQUS to identify promising geometries via a systematic variation of wave height, wavelength, and the relative position of the wave to the initial crack. Numerical results demonstrate that stiffness, crack-initiation force, and total energy dissipation strongly vary with IL geometry. Compared to the flat IL, wavy ILs continuously enhance stiffness and crack-initiation force, albeit with reduced energy absorption. The crack position relative to the wavy IL shows only minor influence on the global fracture response. Based on the numerical results, seven variants were selected and fabricated using PolyJet technology and experimentally examined. The experimental results confirm the numerical predictions, demonstrating good agreement in the initial force-displacement regime, which validates the numerical model as an efficient tool for preliminary design screening. Beyond crack initiation, linearelastic assumptions lead to deviations, limiting the models applicability to the early fracture stage.

The osteochondral junction is a specialized region ensuring the biomechanical and biological integration of the unmineralized articular cartilage with the subchondral bone through an intermediate layer of mineralized cartilage. This location is of clinical relevance, being a target of osteoarthritis. While aging is considered a risk factor for osteoarthritis, the interplay between microstructural and material changes during aging and predisposing to joint degeneration is not fully clear. This is especially true for mineralized cartilage, which remains understudied despite its critical role in load transfer from unmineralized articular cartilage to bone. We investigate age-related alterations of mineralized cartilage and subchondral bone in rat tibiae of adult and aged animals using a multimodal, high-resolution, correlative analysis. Our approach includes micro-computed tomography to measure microstructural features, second harmonic generation imaging to visualize collagen organization, quantitative backscattered electron imaging to map local mineral content, and nanoindentation to obtain mechanical properties. Mineralized cartilage and subchondral bone exhibited distinct age-related modifications. At the architectural level, the subchondral plate thickened and the trabecular network became coarser, those changes being different from those observed in the metaphysis. At the tissue level, mineralized cartilage was less mineralized than bone but exhibited a greater relative increase in mineral content with age, underlying differences in mineralization. A central observation is that aging led to an abrupt transition in mineral content and mechanical properties across the interface between unmineralized and mineralized cartilage, with a conceivable impact on stress localization. Overall, these changes may alter load transfer and contribute to age-related joint degeneration. STATEMENT OF SIGNIFICANCE: The osteochondral junction is a critical yet poorly understood determinant of joint integrity and a primary site of osteoarthritis. Despite its central biomechanical role, mineralized cartilage remains largely under investigated. Here, we use a high-resolution correlative approach to reveal distinct, age-related changes in microstructure and material properties of mineralized cartilage and subchondral bone. Aging markedly steepens the mineralization gradient at the interface between unmineralized and mineralized cartilage, leading to an abrupt change in mechanical properties that may promote stress concentration and limit energy dissipation. Concurrently, mineralized cartilage becomes more mineralized, stiffer, and more brittle, with some changes more pronounced than in subchondral bone. These findings identify a previously underappreciated mechanism by which aging alters load transfer across the osteochondral interface, with implications for joint degeneration and biomimetic interface design.

The epiphyses of long bones play a critical role in joint integrity by absorbing and redistributing joint forces. While aging induces microstructural and material changes in this anatomical site, their impact on the internal mechanical environment of the epiphysis remains poorly characterized. In this study, we quantified age-related alterations in epiphyseal strain distribution in the knee joints of adult and aged male Wistar rats using in situ mechanical testing combined with micro-computed tomography, digital volume correlation, and finite element analysis. Aging was associated with a pronounced reduction in deformation: mean compressive and tensile principal strains in aged animals decreased by approximately 63% and 67%, respectively, compared with strains in adult animals. In addition to reduced strain magnitude, aged epiphyses exhibited less heterogeneous strain distributions, altered spatial strain patterns, and increased inter-individual variability of the deformation. These mechanical changes are consistent with previously reported age-related thickening of the subchondral bone plate, trabecular coarsening, and increased tissue mineralization. Together, these findings demonstrate that aging leads to substantial stiffening of the proximal tibia and alters its behavior under compression. Such changes may affect the protective mechanical function of the epiphysis and contribute to an unfavorable mechanical environment for articular cartilage, which may predispose to joint degeneration.

At entheses, tendons and bones are bridged by mineralized fibrocartilage, joined to tissues through dedicated interfaces. Tendons and bones are characterized by cells interconnected thanks to their underlying dense networks. Nanotubes connect tenocytes in tendons, allowing cellular crosstalk and providing biomechanical stability. Osteocytes are involved in bone mechanoresponsiveness and mineralization: they are encased into cavities and their cellular processes run through channels, forming the osteocyte lacunocanalicular network. Here, we explore the structural connectivity between fibrocartilage and bone, exploiting rat enthesis as model system and focusing on two specific regions: the Achilles tendon insertion into calcaneus and the periosteal fibrocartilage, facilitating tendon sliding. Those regions are used to characterize the impact of loading environment on tissue connectivity. Central to our approach is rhodamine staining, employed to trace connections between tissues. This information is interpreted using data on tissue microstructure, organization and composition, acquired combining high-resolution imaging methods. At the enthesis, we observe potential connections between trabecular bone marrow and mineralized fibrocartilage through a subchondral channel network perforating the interface. Direct cellular connections between bone and fibrocartilage cells are rare: canaliculi mostly stop or switch direction at the cement line. Yet, we observed a high density of canaliculi around perforating channels, which reach fibrochondrocyte lacunae. Such connections seem practically absent at the periosteal region. Our findings are preliminary but suggest that inter-tissue connectivity is required to support the enthesis load-bearing function. To understand multi-tissue biochemical cellular crosstalk, the physical infrastructure enabling this communication is also a critical feature to investigate.

Developing strategies to control damage and protect vulnerable regions in engineering materials remains a major challenge. Biological materials provide a rich source of inspiration for damage-control based on interfaces, gradients, and hierarchical architectures, principles that have been widely explored also in bioinspired systems. In bone, a lightweight biological material, large canals accommodating delicate blood vessels are found within osteons. These canals are surrounded by a lamellar region and bordered by a thin interface, known to promote crack deflection. Inspired by this construction principle, we investigate whether a circular interlayer can control damage and protect a weak spot (hole) in synthetic systems. Using multimaterial three-dimensional printing, mechanical testing, and numerical simulations, we study how crack propagation is influenced by interlayer properties and crack trajectory. We show that cracks approaching the hole can be redirected either through trapping in a weak interlayer or deflection along a stronger interlayer, with the former being easier to achieve. These results confirm that simple architectural features can program damage pathways. They also provide a controlled model system to examine how interlayer-mediated crack paths may be exploited to protect vulnerable regions, thereby informing the design of damage-tolerant architectured materials. Controlling how material damage develops and propagates is a central challenge for improving the safety and durability of engineering structures. This work shows that damage pathways can be controlled using simple architectural features inspired by the osteons in cortical bone. A single circular interlayer can trap or deflect damage depending on its relative strength compared to the surrounding material, providing design principles for protecting vulnerable regions such as cavities or embedded channels. The results highlight how bioinspired construction principles enable tunable damage, while avoiding overdesigning. These findings are not limited to the model systems studied and prototyped here but are applicable to a broad class of additively manufactured multi-material structures. Local tuning of internal architecture and material contrast between different components shall provide a pathway toward damage-tolerant design, ultimately promoting more resilient and sustainable material systems.

Tendon or enthesis injuries are a worldwide clinical problem. Along the enthesis, collagen fibrils show a progressive loss of anisotropy and an increase in mineralization reaching the bone. This causes gradients of mechanical properties. The design of scaffolds to regenerate these load-bearing tissues requires validation in vivo in relevant large animal models. The sheep tendon of triceps surae muscle is an optimal animal model for this scope with limited knowledge about its structure and mechanics. We decided to investigate in-depth its structure and full-field mechanics. Collagen fibrils morphology was investigated via scanning electron microscopy revealing a marked change in orientation/dimensions passing from the tendon to the enthesis. Backscatter electron images and nanoindentation at the enthesis/bone marked small gradients of mineralization at the mineralized fibrocartilage reaching 27%wt and indentation modulus around 17-30 GPa. The trabecular bone instead had indentation modulus around 15-22 GPa. Mechanical tensile tests with digital image correlation confirmed the typical non-linear behavior of tendons (failure strain = 8.2 ± 1.0 %; failure force = 1369 ± 187 N) with maximum principal strains reaching mean values of εp1 ∼ 7 %. The typical auxetic behavior of tendon was highlighted by the minimum principal strains (εp2 ∼ 5 %), progressively dampened at the enthesis. Histology revealed that this behavior was caused by a local thickening of the epitenon. Cyclic tests showed a force loss of 21 ± 7 % at the last cycle. These findings will be fundamental for biofabrication and clinicians interested in designing the new generation of scaffolds for enthesis regeneration.

The cement line (CL) is a thin layer, 1-3 μm in width, separating secondary osteons from interstitial bone and other osteons. Despite the possible role for bone quality, the CL is still one of the least understood features of bone. This study aims to investigate how the mineral content of the CL varies not only with osteon age but also with the surrounding environment. Using quantitative backscattered electron imaging to measure the mineral content, we analyzed 35 osteons from femoral bone of 2 male individuals (40 and 81 yr old). We implemented a new approach to investigate the mineral content based on a spatially resolved analysis in layers along the CL and incorporating regions both inside the osteon (formed soon after CL deposition) and outside (already present at the time of CL deposition). We found that the CLs had always higher mineral content than the corresponding osteon (p < .001) and that not only the osteon, but also the CL increases its mineral content with time. Including areas outside the osteon in the analysis improved considerable our understanding of CL mineralization. After a rapid primary phase, where the CL incorporates more mineral than the osteon, secondary mineralization is about 60% lower in the CL than in the osteon. One key finding is that the mineralization of the CL is not universal but depends on the region in which the osteon is formed. This is supported by a strong correlation between the mineral content of the CL and outside the osteon (R = 0.75, p < .001), but not inside. One possible explanation is that mineral released during bone resorption may contribute to the mineralization of the CL, as higher mineral content in resorbed bone was associated with greater mineralization in the CL.

The remarkable mechanical performance of bone arises from its complex hierarchical structure and from the presence of numerous internal interfaces joining the different components. Formed during bone remodeling, the cement line (CL) is a thin interface surrounding osteons. Although often neglected due to its small size, the CL has been suggested to play an important role in bone fracture toughness. However, there is an ongoing debate about its specific structure and mechanical behavior. Here, we investigate the composition-mechanics relationship at the CL and surrounding bone in human osteons using multiple methods in a correlative manner. We found that the CL is stiffer and harder than adjacent osteonal tissue. However, the CL requires more mineral than bone to attain the same stiffness and hardness. Analyzing the nanoscale properties of the mineral, we found thicker but shorter particles at the CL. Using a mechanical model, we interpreted the lower aspect ratio of the mineral particles as a less effective way to reinforce the collagen matrix. Casting our findings into a computational model, we questioned the possible protective role of the CL, which is not a soft interface as traditionally believed.

The functional integration of soft tissues into bone requires complex adaptation strategies [1]. Tendon and bone are both characterized by cells (tenocytes and osteocytes) that communicate extensively thanks to their dense networks: nanotubes in tendons and the osteocyte lacunocanalicular network (LCN) in bone. These networks serve multiple functions: nanotubes in tendon are believed to provide biomechanical stability [2] while the LCN is involved in bone mechanoresponsiveness [3] and mineralization [4]. At the interface between theses tissues lies the enthesis, a specialized insertion site often featuring fibrocartilage (FC), a fibrous form of cartilage reinforced with minerals before anchoring to bone. Our research focuses specifically on the Achilles tendon insertion, comparing enthesis with periosteal FC, two contiguous tissues sustaining different loading conditions: tension at enthesis and compression/shear at periosteal FC. We have previously revealed microstructural [5] and material heterogeneity [6] of mineralized FC. The aim of this study is to investigate the behavior of fibrochondrocytes (FC cells) and their physical interaction pathways with bone functional porosity. Rat samples were stained with rhodamine and then micro-computed tomography, quantitative backscattered electron imaging, second harmonic generation imaging and confocal laser scanning microscopy (CLSM) were combined on the same locations to highlight functional porosity at multiple length scales, as well as its link with mineral content and matrix organization. At the enthesis, we identified a connection path between trabecular bone marrow space and FC through the anisotropic subchondral channel network, perforating the bone-FC interface at several locations. Communication between bone and FC cells seems to be primarily indirect: osteocytes are highly connected to perforating channels through many canaliculi, these channels crossing the bone-FC interface and reaching the fibrochondrocyte rows. The striking absence of such connections at the adjacent periosteal region suggests that communication between bone and fibrocartilage may be needed for a proper enthesis biomechanical function.

Cement lines (CLs) are thin interphases separating osteons from the surrounding bone. They are deposited during bone remodeling, preceding the formation of the osteocyte lacunocanalicular network (LCN) in secondary osteons. Of central interest for bone mechanobiology is the possible interaction between the osteocytes inside an osteon and the cells of the surrounding environment. This interaction depends on the canaliculi crossing the CL and connecting to the osteocytes outside the osteon. However, the extent to which osteocytes can communicate across CLs remains debated: some studies reported a complete network disruption at CLs [1], while others found intact inter-osteon canalicular connections [2]. This work focuses on the characteristics of the LCN along the CLs. Human femoral cortical bone samples from two male individuals (40 and 81 y.o.) were obtained and rhodamine stained. The LCN was visualized using confocal laser scanning microscopy and analyzed using a dedicated software (TINA) [3]. Initial data revealed reduced canalicular density near the osteon boundary (Fig. 1A). The orientation of the canaliculi shifted from predominantly radial within the osteon to predominantly lateral at the outer periphery of the osteons (Fig. 1B). Visual examination of this region showed that most radially oriented canaliculi are interrupted at the CL. Yet, a few canaliculi could cross the CL. This is usually observed between adjacent osteons rather than between osteons and interstitial bone (Fig. 2A-D). While crossings exist, the limited occurrence suggests minimal inter-osteon communication, raising questions about the functional significance. Future analysis will explore how osteon mineral content (a surrogate of tissue age) may affect inter-osteon connectivity. For any figures or tables, please contact the authors directly.

The cement line (CL) is a thin layer separating secondary osteons from interstitial bone and other osteons. It is assumed to play a significant role in bone fracture resistance, owing to its ability to deflect or arrest microcracks. Despite the possible role for bone quality, the CL is still one of the least understood microstructural features of bones, with unknowns on CL composition, mineralization, and mechanical properties. This study, focusing on CL mineralization, aims to elucidate the interplay between the mineral content of the CL and of adjacent bone tissue. Using quantitative backscattered electron imaging, osteons with different degrees of mineralization coming from human femoral samples were analyzed. We implemented a spatially resolved analysis of the mineral content in layers along the CL, considering both regions inside the osteon (i.e., formed soon after CL deposition) and outside (i.e., already present at the time of CL deposition). We found that the mineral content of the CL correlates strongly with the mineral content outside of the osteon, but not inside. Assuming the mineral content of the osteon as a proxy of its age, we demonstrate that not only the osteon, but also the CL increases its mineral content with time. However, the rate of increase is lower in the CL. Importantly, the specific value of the high initial mineral content of the CL depends on the mineral content of the local surrounding, in which the osteon was formed. Our findings highlight the central role of the local degree of mineralization of the bone around the osteon for building the CL. ### Competing Interest Statement The authors have declared no competing interest.

Tendon/enthesis injuries are a worldwide clinical problem. Along the enthesis, collagen fibrils show a progressive loss of anisotropy and an increase in mineralization reaching the bone. This causes gradients of mechanical properties. The design of scaffolds to regenerate these load-bearing tissues requires of being validated in vivo in relevant large animal models. The sheep tendon of triceps surae muscle is an optimal animal model for this scope with limited knowledge about its structure and mechanics. We decided to understand in-depth its structure and full-field mechanics. Collagen fibrils morphology was investigated via scanning electron microscopy revealing a marked change in orientation/dimensions passing from tendon to enthesis. Backscatter electron images and nanoindentation at the enthesis/bone marked small gradients of mineralization at the mineralized fibrocartilage reaching 27%wt and indentation modulus around 17-30 GPa. The trabecular bone instead had indentation modulus around 15-22 GPa. Mechanical tensile tests with digital image correlation confirmed the typical non-linear behavior of tendons (failure strain = 8.2 ± 1.0%; failure force = 1369 ± 187 N) with maximum principal strains reaching mean values of ep1~7%. The typical auxetic behavior of tendon was highlighted by the minimum principal strains (ep2~5%), progressively dampened at the enthesis. Histology revealed that this behavior was caused by a local thickening of the epitenon. Cyclic tests showed a force loss of 21 ± 7 % at the last cycle. These findings will be fundamental for biofabrication and clinicians interested in designing the new generation of scaffolds for enthesis regeneration. ### Competing Interest Statement The authors have declared no competing interest.

Characterizing the mechanics of periodic scaffolds used for bone tissue repair, while maintaining their structural integrity, is a challenging problem. By leveraging concepts arising from the bulk phononic crystal community, here we investigate the reflection of elastic waves propagating through a water-immersed biphasic architectured medium in the ultrasonic regime. Towards this goal, Bloch–Floquet analysis is applied on a 2D unit cell made of a soft inclusion embedded in a hard matrix, to recover its corresponding phononic band structure. Exploring the modal conversion at the boundary between the homogeneous incident medium and the architectured one allows identifying a bandgap within the considered frequency range, which exhibits a significant sensitivity to varying volume fraction of the soft phase. Conducting further numerical analyzes, which account for the viscoelasticity of the two constituent phases, along with the finite-size and bounded nature of the architectured medium, shows that a sudden amplitude rise of the reflection coefficient takes place at a frequency that is closely related to the upper limit of this bandgap. This hypothesis is experimentally verified on 3D-printed bio-mimicking samples, which exhibit an in-plane periodicity at a length scale of a few hundred micrometers. Altogether, the reported results suggest that tracking prohibited frequency bands via the measurement of the reflection coefficient allows for the monitoring of micro-architectured media like scaffolds.

Assessing the mechanical characteristics of periodic architectured scaffolds employed in bone tissue repair, while preserving their structural integrity, poses a significant challenge from both theoretical and experimental viewpoints. By leveraging concepts arising from the phononic crystals community, here we investigate the reflection and transmission of elastic waves propagating through water-immersed biphasic architectured samples in the MHz regime, which exhibit a periodic organization at a length scale of a few hundred micrometers. Experimental outcomes are systematically compared with modeled predictions, spanning a range of scaffold-like samples engineered to mimic the expected variations that take place in a biological environment. A particular attention is given to critical modeling considerations such as the behavior in reflection, the interaction of Bloch waves with the viscoelastic properties of the constituent phases, as well as the role of modal conversion at the interface between the homogeneous incident medium and the architectured one. Altogether, the reported results suggest that specific ultrasonic signatures associated with elastic wave propagation in periodic media, encompassing phenomena such as bandgaps and dispersion, offer valuable insights towards the nondestructive monitoring of micro-architectured media like scaffolds.

Living organisms use functional gradients (FGs) to interface hard and soft materials (e.g., bone and tendon), a strategy with engineering potential. Past attempts involving hard (or soft) phase ratio variation have led to mechanical property inaccuracies because of microscale-material macroscale-property nonlinearity. This study examines 3D-printed voxels from either hard or soft phase to decode this relationship. Combining micro/macroscale experiments and finite element simulations, a power law model emerges, linking voxel arrangement to composite properties. This model guides the creation of voxel-level FG structures, resulting in two biomimetic constructs mimicking specific bone-soft tissue interfaces with superior mechanical properties. Additionally, the model studies the FG influence on murine preosteoblast and human bone marrow-derived mesenchymal stromal cell (hBMSC) morphology and protein expression, driving rational design of soft-hard interfaces in biomedical applications.

AbstractLiving organisms have developed design principles, such as functional gradients (FGs), to interface hard materials with soft ones (e.g., bone and tendon). Mimicking such design principles can address the challenges faced when developing engineered constructs with soft-hard interfaces. To date, implementing these FG design principles has been primarily performed by varying the ratio of the hard phase to that of the soft phase. Such design approaches, however, lead to inaccurate mechanical properties within the transition zone. That is due to the highly nonlinear relationship between the material distribution at the microscale and the macroscale mechanical properties. Here, we 3D print micro-bricks from either a soft or a hard phase and study the nonlinear relationship between their arrangements within the transition zone and the resulting macroscale properties. We carry out experiments at the micro- and macroscales as well as finite element simulations at both scales. Based on the obtained results, we develop a co-continuous power-law model relating the arrangement of the micro-bricks to the local mechanical properties of the micro-brick composites. We then use this model to rationally design FGs at the individual micro-brick level and create two types of biomimetic soft-hard constructs, including a specimen modeling bone-ligament junctions in the knee and another modeling the nucleus pulposus-annulus fibrosus interface in intervertebral discs. We show that the implemented FGs drastically enhance the stiffness, strength, and toughness of both types of specimens as compared to non-graded designs. Furthermore, we hypothesize that our soft-hard FGs regulate the behavior of murine preosteoblasts and primary human bone marrow-derived mesenchymal stromal cells (hBMSCc). We culture those cells to confirm the effects of soft-hard interfaces on cell morphology as well as on regulating the expression of focal adhesion kinase, subcellular localization, and YAP nuclear translocation of hBMSCs. Taken together, our results pave the way for the rational design of soft-hard interfaces at the micro-brick level and (biomedical) applications of such designs.

Biominerals are organic–mineral composites formed by living organisms. They are the hardest and toughest tissues in those organisms, are often polycrystalline, and their mesostructure (which includes nano‐ and microscale crystallite size, shape, arrangement, and orientation) can vary dramatically. Marine biominerals may be aragonite, vaterite, or calcite, all calcium carbonate (CaCO 3 ) polymorphs, differing in crystal structure. Unexpectedly, diverse CaCO 3 biominerals such as coral skeletons and nacre share a similar characteristic: Adjacent crystals are slightly misoriented. This observation is documented quantitatively at the micro‐ and nanoscales, using polarization‐dependent imaging contrast mapping (PIC mapping), and the slight misorientations are consistently between 1° and 40°. Nanoindentation shows that both polycrystalline biominerals and abiotic synthetic spherulites are tougher than single‐crystalline geologic aragonite. Molecular dynamics (MD) simulations of bicrystals at the molecular scale reveal that aragonite, vaterite, and calcite exhibit toughness maxima when the bicrystals are misoriented by 10°, 20°, and 30°, respectively, demonstrating that slight misorientation alone can increase fracture toughness. Slight‐misorientation‐toughening can be harnessed for synthesis of bioinspired materials that only require one material, are not limited to specific top‐down architecture, and are easily achieved by self‐assembly of organic molecules (e.g., aspirin, chocolate), polymers, metals, and ceramics well beyond biominerals.

A demanding task of the musculoskeletal system is the attachment of tendon to bone at entheses. This region often presents a thin layer of fibrocartilage (FC), mineralized close to the bone and unmineralized close to the tendon. Mineralized FC deserves increased attention, owing to its crucial anchoring task and involvement in enthesis pathologies. Here, we analyzed mineralized FC and subchondral bone at the Achilles tendon-bone insertion of rats. This location features enthesis FC anchoring tendon to bone and sustaining tensile loads, and periosteal FC facilitating bone-tendon sliding with accompanying compressive and shear forces. Using a correlative multimodal investigation, we evaluated potential specificities in mineral content, fiber organization and mechanical properties of enthesis and periosteal FC. Both tissues had a lower degree of mineralization than subchondral bone, yet used the available mineral very efficiently: for the same local mineral content, they had higher stiffness and hardness than bone. We found that enthesis FC was characterized by highly aligned mineralized collagen fibers even far away from the attachment region, whereas periosteal FC had a rich variety of fiber arrangements. Except for an initial steep spatial gradient between unmineralized and mineralized FC, local mechanical properties were surprisingly uniform inside enthesis FC while a modulation in stiffness, independent from mineral content, was observed in periosteal FC. We interpreted these different structure-property relationships as a demonstration of the high versatility of FC, providing high strength at the insertion (to resist tensile loading) and a gradual compliance at the periosteal surface (to resist contact stresses). STATEMENT OF SIGNIFICANCE: Mineralized fibrocartilage (FC) at entheses facilitates the integration of tendon in bone, two strongly dissimilar tissues. We focus on the structure-function relationships of two types of mineralized FC, enthesis and periosteal, which have clearly distinct mechanical demands. By investigating them with multiple high-resolution methods in a correlative manner, we demonstrate differences in fiber architecture and mechanical properties between the two tissues, indicative of their mechanical roles. Our results are relevant both from a medical viewpoint, targeting a clinically relevant location, as well as from a material science perspective, identifying FC as high-performance versatile composite.

Spearing mantis shrimps are aggressive crustaceans using specialized appendages with sharp spikes to capture fishes with a fast movement. Each spike is a biological tool that have to combine high toughness, as required by the initial impact with the victim, with high stiffness and strength, to ensure sufficient penetration while avoid breaking. We performed a multimodal analysis to uncover the design strategies of this harpoon based on chitin. We found that the spike is a slightly hooked hollow beam with the outer surface decorated by serrations and grooves to enhance cutting and interlocking. The cuticle of the spike resembles a multilayer composite: an outer heavily mineralized, stiff and hard region (with average indentation modulus and hardness of 68 and 3 GPa), providing high resistance to contact stresses, is combined with a less mineralized region, which occupies a large fraction of the cuticle (up to 50%) and features parallel fibers oriented longitudinally, enhancing stiffness and strength. A central finding of our work is the presence of a tiny interphase (less than 10 μm in width) based on helical fibers and showing a spatial modulation in mechanical properties, which has the critical task to integrate the stiff but brittle outer layer with the more compliant highly anisotropic parallel fiber region. We highlighted the remarkable ability of this helicoidal region to stop nanoindentation-induced cracks. Using three-dimensional multimaterial printing to prototype spike-inspired composites, we showed how the observed construction principles can not only hamper damage propagation between highly dissimilar layers (resulting in composites with the helical interphase absorbing 50% more energy than without it) but can also enhance resistance to puncture (25% increase in the force required to penetrate the composites with a blunt tool). Such findings may provide guidelines to design lightweight harpoons relying on environmentally friendly and recyclable building blocks.