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The goal of bone tissue engineering is to build artificial bone tissue with properties that closely resemble human bone and thereby support the optimal integration of the constructs (biografts) into the body. The development of tissues in 3D scaffolds includes several complex steps that need to be optimized and monitored. In particular, cell–material interaction during seeding, cell proliferation and cell differentiation within the scaffold pores play a key role. In this work, we seeded two types of 3D-printed scaffolds with pre-osteoblastic MC3T3-E1 cells, proliferated and differentiated the cells, before testing and adapting different assays and imaging methods to monitor these processes. Alpha-TCP/HA (α-TCP with low calcium hydroxyapatite) and baghdadite (Ca3ZrSi2O9) scaffolds were used, which had comparable porosity (~50%) and pore sizes (~300–400 µm). Cell adhesion to both scaffolds showed ~95% seeding efficiency. Cell proliferation tests provided characteristic progression curves over time and increased values for α-TCP/HA. Transmitted light imaging displayed a homogeneous population of scaffold pores and allowed us to track their opening state for the supply of the inner scaffold regions by diffusion. Fluorescence labeling enabled us to image the arrangement and morphology of the cells within the pores. During three weeks of osteogenesis, ALP activity increased sharply in both scaffolds, but was again markedly increased in α-TCP/HA scaffolds. Multiphoton SHG and autofluorescence imaging were used to investigate the distribution, morphology, and arrangement of cells; collagen-I fiber networks; and hydroxyapatite crystals. The collagen-I networks became denser and more structured during osteogenic differentiation and appeared comparable in both scaffolds. However, imaging of the HA crystals showed a different morphology between the two scaffolds and appeared to arrange in the α-TCP/HA scaffolds along collagen-I fibers. ALP activity and SHG imaging indicated a pronounced osteo-inductive effect of baghdadite. This study describes a series of methods, in particular multiphoton imaging and complementary biochemical assays, to validly measure and track the development of bone tissue in 3D scaffolds. The results contribute to the understanding of cell colonization, growth, and differentiation, emphasizing the importance of optimal media supply of the inner scaffold regions.

An ongoing challenge in the field of orthopedics is to produce a clinically relevant synthetic ceramic scaffold for the treatment of ‘critical-sized’ bone defects, which cannot heal without intervention. We had developed a bioactive ceramic (baghdadite, Ca₃ZrSi₂O₉) and demonstrated its outstanding bioactivity using traditional manufacturing techniques. Here, we report on the development of a versatile stereolithography printing technology that enabled fabrication of anatomically-shaped and -sized Baghdadite scaffolds. We assessed the in vivo bioactivity of these scaffolds in co-delivering of bone morphogenetic protein-2 (BMP2) and zoledronic acid (ZA) through bioresorbable coatings to induce bone formation and increase retention in a rat model of heterotopic ossification. Micro-computed tomography, histology, mechanical tests pre- and post-implantation, and mechanical modelling were used to assess bone ingrowth and its effects on the mechanics of the scaffolds. Bone ingrowth and the consequent mechanical properties of the scaffolds improved with increasing BMP2 dose. Co-delivery of ZA with BMP2 further improved this outcome. The significant bone formation within the scaffolds functionalized with 10 µg BMP2 and 2 µg ZA made them 2.3 × stiffer and 2.7 × stronger post-implantation and turned these inherently brittle scaffolds into a tough and deformable material. The effects of bone ingrowth on the mechanical properties of scaffolds were captured in a mechanical model that can be used in future clinical studies for non-destructive evaluation of scaffold's stiffness and strength as new bone forms. These results support the practical utilization of our versatile stereolithographic printing methods and BMP2/ZA functionalization to create fit-for-purpose personalized implants for clinical trials. Statement of significance In this study, we addressed a long-standing challenge of developing a ceramic printing technology that enables fabrication of customizable anatomically-shaped and -sized bioceramic scaffolds with precise internal architectures using an inexpensive desktop printer. We also addressed another challenge related to delivery of pharmaceuticals. BMP2, currently available as a bone-inducing bioactive protein, is clinically administered in a collagen scaffold that has limited moldability and poor mechanical properties. The comparably stiffer and stronger 3D printed personalized Baghdadite scaffolds developed here can be readily functionalized with bioresorbable coatings containing BMP2 ± ZA. These innovations considerably improve on the prior art and are scalable for use in human surgery.

Forming ceramics into rationally-designed and complex shapes without compromising their mechanical properties is a major challenge. Here, we demonstrate self-shaping of ceramics through sequential stereolithographic printing of ceramic resins into components with a heterogeneous concentration of ceramic particles, resulting in well-defined anisotropic shrinkage and, consequently, shape changes during sintering. The method is versatile and scalable and results in well-controlled shape changes in ceramics through bending, folding, twisting, and combinations of these mechanisms. The density measurements and mechanical tests show that the stresses resulting from the self-shaping mechanisms do not significantly affect the physical and mechanical properties of the ceramics. Together with the experiments, we developed a material- and scale-independent mechanical model based on linear elasticity that predicted shape changes accurately. The model can serve as a design tool to guide the selection of particle concentrations to realize the desired shapes in a broad range of ceramics.

3D printed bone scaffolds have the potential to replace autografts and allografts because of advantages such as unlimited supply and the ability to tailor the scaffolds' biochemical, biological and biophysical properties. Significant progress has been made over the past decade in additive manufacturing techniques to 3D print bone grafts, but challenges remain in the lack of manufacturing techniques that can recapitulate both mechanical and biological functions of native bones. The purpose of this review is to outline the recent progress and challenges of engineering an ideal synthetic bone scaffold and to provide suggestions for overcoming these challenges through bioinspiration, high-resolution 3D printing, and advanced modeling techniques. The article provides a short overview of the progress in developing the 3D printed scaffolds for the repair and regeneration of critical size bone defects. STATEMENT OF SIGNIFICANCE: Treatment of critical size bone defects is still a tremendous clinical challenge. To address this challenge, diverse sets of advanced manufacturing approaches and materials have been developed for bone tissue scaffolds. 3D printing has sparked much interest because it provides a close control over the scaffold's internal architecture and in turn its mechanical and biological properties. This article provides a critical overview of the relationships between material compositions, printing techniques, and properties of the scaffolds and discusses the current technical challenges facing their successful translation to the clinic. Bioinspiration, high-resolution printing, and advanced modeling techniques are discussed as future directions to address the current challenges.

There is an unmet clinical need for a spinal fusion implant material that recapitulates the biological and mechanical performance of natural bone. We have developed a bioceramic, Sr-HT-Gahnite, which has been identified as a potential fusion device material. This material has the capacity to transform the future of the global interbody devices market, with follow on social, economic, and environmental benefits, rooted in its remarkable combination of mechanical properties and bioactivity. In this study, and in line with FDA requirements, the in vivo preclinical systemic biological safety of a Sr-HT-Gahnite interbody fusion device is assessed over 26 weeks in sheep under good laboratory practice (GLP). Following the in-life phase, animals are assessed for systemic biological effects via blood haematology and clinical biochemistry, strontium dosage analysis in the blood and wool, and histopathology examination of the distant organs including adrenals, brain, heart, kidneys, liver, lungs and bronchi, skeletal muscle, spinal nerves close to the implanted sites, ovaries, and draining lymph nodes. Our results show that no major changes in blood haematology or biochemistry parameters are observed, no systemic distribution of strontium to the blood and wool, and no macroscopic or histopathological abnormalities in the distant organs when Sr-HT-Gahnite was implanted, compared to baseline and control values. Together, these results indicate the systemic safety of the Sr-HT-Gahnite interbody fusion device. The results of this study extend to the systemic safety of other Sr-HT-Gahnite implanted medical devices in contact with bone or tissue, of similar size and manufactured using the described processes. STATEMENT OF SIGNIFICANCE: This paper is considered original and innovative as it is the first that thoroughly reports the systemic biological safety of previously undescribed bioceramic material, Sr-HT-Gahnite. The study has been performed under good laboratory practice, in line with FDA requirements for assessment of a new interbody fusion device, making the results broadly applicable to the translation of sheep models to the human cervical spine; and also the translation of Sr-HT-Gahnite as a biomaterial for use in additional applications. We expect this study to be of broad interest to the readership of Acta Biomaterilia. Its findings are directly applicable to researchers and clinicians working in bone repair and the development of synthetic biomaterials.

Recently, carbon dots (CDs) have been widely investigated for biological applications in imaging. One-step hydrothermal synthesis is considered to be one of the most promising methods for the synthesis of CDs, due to its simple and rapid manipulation, flexible selection of ingredients, environmentally friendly conditions, and low-cost. A number of synthetic and post-synthetic parameters, including solvent, heating time, dopant quantity, and particle size distribution, play a crucial role in controlling the size and surface structure of CDs, which ultimately have influence on their photophysical and biological behavior. Despite the crucial role of each of these parameters in defining the yield and nature of synthesized CDs, they have not previously been rigorously optimized, particularly with respect to desired biological applications. Herein, we report our comprehensive optimization of the parameters employed for the hydrothermal synthesis of CDs to gain a better understanding of the effect of these parameters on optical properties, cytotoxicity, and cellular uptake efficiency. Furthermore, this work will open up new pathways toward the design of CDs with physiochemical properties tailored for specific biomedical applications such as bioimaging.

Baghdadite (Ca3ZrSi2O9, BAG), is a Zr-doped calcium silicate that has outstanding bioactivity both in vitro and in vivo. Bioceramic scaffolds should be sufficiently radiopaque to be distinguishable in vivo from surrounding bone structures. To enhance the radiopacity of BAG, this study investigated the effect of incorporating bismuth ions into its crystalline structure (BixCa3-xZrSi2O9, x = 0, 0.1, 0.2, 0.5; BAG, Bi0.1-BAG, Bi0.2-BAG, Bi0.5-BAG, respectively). Monophasic baghdadite was retained after bismuth ion incorporation up to x = 0.2 at calcination temperatures of 1350 °C. When pressed and sintered, energy dispersive x-ray spectroscopy showed that BAG and Bi0.1-BAG retained crystalline homogeneity, but Bi0.2-BAG formed zirconium-rich crystalline regions. BAG, Bi0.1-BAG and Bi0.2-BAG exhibited non-degradation after 56 days of immersion in culture medium. Bi0.1-BAG exhibited the lowest change in culture medium pH (+0.0), compared to BAG (+0.7) and Bi0.2-BAG (+0.2) after 56 days of culture media immersion. Bi0.1-BAG exhibited similar strength and modulus to BAG (σ: 200–290 MPa; E: 4–5 GPa), and significantly higher compressive strength and modulus versus Bi0.2-BAG (σ: 150–200 MPa; E: 3.5–4 GPa) across 56 days of aqueous immersion. In vitro studies using primary human bone derived cells (HOBs) demonstrated a significant increase in HOBs proliferation when cultured on Bi0.1-BAG for seven days compared to BAG and Bi0.2-BAG. Importantly, Bi0.1-BAG showed increased radiopacity by ~33%, when compared to BAG, and by ~115% when compared to biphasic calcium phosphate. The properties of Bi0.1-BAG show promise for its use as a bioactive ceramic with sufficient radiopacity for treatment of bone defects.

Brittleness is a major limitation of polymer-derived ceramics (PDCs). Different concentrations of three nanofillers (carbon nanotubes, Si3N4 and Al2O3 nanoparticles) were evaluated to improve both toughness and modulus of a commercial polysilazane (PSZ) PDC. The PSZs were thermally cross-linked and pyrolyzed under isostatic pressure in nitrogen. A combination of mechanical, chemical, density, and microscopy characterizations was used to determine the effects of these fillers. Si3N4 and Al2O3 nanoparticles (that were found to be active fillers) were more effective than nanotubes and improved the elastic modulus, hardness, and fracture toughness (JIC) of the PDC by ~ 1.5 ×, ~ 3 ×, and ~ 2.5 ×, respectively. Nanotubes were also effective in maintaining the integrity of the samples during pyrolysis. The modulus and hardness of PDCs correlated positively with their apparent density; this can provide a fast way to assess future PDCs. The improvement in fracture toughness was attributed to crack deflection and bridging observed in the micro-indentation cracks in the modified PDCs. The specific toughness of the modified PDCs was 4 × higher than that of high-purity alumina, and its specific modulus reached that of commercially available technical ceramics. These PDCs can also easily take different shapes and therefore are of interest in protective armor, propulsion, thermal protection, device packaging and biomaterial systems.

Bone fractures and critical-sized bone defects present significant health threats for the elderly who have limited capacity for regeneration due to the presence of functionally compromised senescent cells. A wide range of synthetic materials has been developed to promote the regeneration of critical-sized bone defects, but it is largely unknown if a synthetic biomaterial (scaffold) can modulate cellular senescence and improve bone regeneration in aged scenarios. The current study investigates the interaction of Baghdadite (Ca3ZrSi2O9) ceramic scaffolds with senescent human primary osteoblast-like cells (HOBs) and its bone regeneration capacity in aged rats. A senescent HOB model was established by repeatedly passaging HOBs till passage 7 (P7). Compared to the clinically used hydroxyapatite/tricalcium phosphate (HA/TCP), Baghdadite prevented senescence induction in P7 HOBs and markedly negated the paracrine effect of P7 HOB secretomes that mediated the up-regulations of cellular senescence-associated gene expression levels in P2 HOBs. We further demonstrated that conditioned media extracted from Baghdadite corrected the dysfunctional mitochondria in P7 HOBs. In vivo, the bone regeneration capacity was enhanced when 3D printed Baghdadite scaffolds were implanted in a calvaria critical-sized bone defect model in both young and aged rats compared to HA/TCP scaffolds, but a better effect was observed in aged rats than in young rats. This study suggests that Baghdadite ceramic represents a novel and promising biomaterial approach to promote bone regeneration capacity in the elderly by providing an anti-senescent microenvironment.

Precise material architectures and interfaces can generate unusual and attractive combinations of mechanisms and properties. For example, the segmentation into blocks of finite size and well-defined geometries can turn brittle ceramics into tough, deformable and impact resistant material systems. This strategy, while scarcely used in engineering, has been successfully used for millions of years in biological materials such as bone, nacre or tooth enamel. In this work, the precise relationships between architecture, mechanics, and properties in architectured ceramic panels are explored using a combination of mechanical testing with stereo-imaging, 3D reconstruction, and finite-element/analytical modeling. In particular, this work shows that a fine balance of interlocking and block size generates controlled frictional sliding and rotation of blocks, minimizes damage to individual blocks and optimizes performance. These ceramic architectured panels have 1/4 to 1/2 of the strength of monolithic ceramic panels, but they can absorb 5 to 20 times more mechanical energy, making them very attractive for applications where high surface hardness or high resistance to temperature must be combined with resistance to impact and toughness.

Designing a two-photon ratiometric pH-sensitive nanoprobe for real time monitoring of intracellular pH in biological environment is of great importance for better understanding the pathogenesis of diseases and the design of intracellular drug delivery-based system. However, the development of such probe remains a challenge. Here, we report for the first time the design and development of the two-photon ratiometric carbon dot-based nanoprobe and demonstrate its capability to monitor and quantify the intracellular cytoplasmic pH value, in real time. The nanoprobe comprised of a fluorescent carbon dot functionalized with a pH-responsive fluorescein dye (FACD). With increasing pH, FACD exhibits a clear ratiometric change in the emission intensity ratio, with sensitivity across a wide pH range in both extracellular and intracellular compartments. FACD is non-toxic to adipose stem cells in cell imaging experimental conditions and exhibits remarkable thermal stability, photostability, and two-photon near-infrared excitation capability. Using real-time dual-channel two-photon confocal microscopy we demonstrate the great potential of FACD as an efficient nanoprobe with high accuracy for the intracellular sensing of pH in living adipose stem cells seeded on either cell-culture dishes or on a 3D printed bioactive scaffold.

Topologically interlocked materials (TIMs) are an emerging class of architectured materials based on stiff building blocks of well-controlled geometries which can slide, rotate, or interlock collectively providing a wealth of tunable mechanisms, precise structural properties, and functionalities. TIMs are typically 10 times more impact resistant than their monolithic form, but this improvement usually comes at the expense of strength. Here we used 3D printing and replica casting to explore 15 designs of architectured ceramic panels based on platonic shapes and their truncated versions. We tested the panels in quasi-static and impact conditions with stereoimaging, image correlation, and 3D reconstruction to monitor the displacements and rotations of individual blocks. We report a design based on octahedral blocks which is not only tougher (50×) but also stronger (1.2×) than monolithic plates of the same material. This result suggests that there is no upper bound for strength and toughness in TIMs, unveiling their tremendous potential as structural and multifunctional materials. Based on our experiments, we propose a nondimensional "interlocking parameter" which could guide the exploration of future architectured systems.

Natural materials such as nacre, collagen, and spider silk are composed of staggered stiff and strong inclusions in a softer matrix. This type of hybrid microstructure results in remarkable combinations of stiffness, strength, and toughness and it now inspires novel classes of high-performance composites. However, the analytical and numerical approaches used to predict and optimize the mechanics of staggered composites often neglect statistical variations and inhomogeneities, which may have significant impacts on modulus, strength, and toughness. Here we present an analysis of localization using small representative volume elements (RVEs) and large scale statistical volume elements (SVEs) based on the discrete element method (DEM). DEM is an efficient numerical method which enabled the evaluation of more than 10,000 microstructures in this study, each including about 5,000 inclusions. The models explore the combined effects of statistics, inclusion arrangement, and interface properties. We find that statistical variations have a negative effect on all properties, in particular on the ductility and energy absorption because randomness precipitates the localization of deformations. However, the results also show that the negative effects of random microstructures can be offset by interfaces with large strain at failure accompanied by strain hardening. More specifically, this quantitative study reveals an optimal range of interface properties where the interfaces are the most effective at delaying localization. These findings show how carefully designed interfaces in bioinspired staggered composites can offset the negative effects of microstructural randomness, which is inherent to most current fabrication methods.

Internal architecture of tissue scaffolds plays a significant role in their ability to heal critical-size bone defects. Many studies have investigated these effects but lack isolating architectural features in 3D space, hindering optimization of pore shape to improve bone ingrowth and consequently clinical outcome. To address this challenge, we developed a systematic design strategy and a high-fidelity and -precision ceramic printing technique using a stereolithography desktop printer. We used these techniques to print 5 scaffold architectures with different surface convexities/concavities, pore interconnectivities, and permeabilities, while maintaining the same porosity, average pore size, and surface area. We determined the mechanical effects of the architecture using mechanical tests with in-situ imaging, finite element, and computational fluid dynamic simulations. The effects of architecture on bioactivity and bone ingrowth were determined in a rabbit calvarial critical-size defect model at 12-week implantation, using µ-computed tomography, and histology. The results showed that bone ingrowth is significantly affected by pore interconnectivity in 3D space and maximum fluid permeability in 3D regardless of flow direction, but not permeability in one or few directions. Surface convexity/concavity did not affect bone formation in our 3D scaffolds. Bone ingrowth in scaffolds with highly interconnected pores resulted in a significantly tougher and stronger bioceramic/bone composites, compared to the inherently brittle scaffolds pre-implantation. Our findings provide a rational design of 3D scaffolds architectures for effective translation to the clinic and could be used to predict the tissue regeneration capacity of scaffolds with other architectures or made of other materials.

Bone's outstanding biomechanical performance is derived from cooperative interactions between its composition and microarchitecture. Towards developing bioceramic scaffolds with similar biomechanical performance for repairing large bone defects under load, we have developed 13 new bioceramic compositions by doping various concentrations of iron and magnesium into Baghdadite (a Zr-Ca-Silicate: Ca3ZrSi2O9). The resulting bioceramics were printed into scaffolds with precisely controlled internal and external shapes using a versatile photopolymerization-based stereolithography technique. The biomechanical performance of new compositions and scaffolds were determined using mechanical tests with in situ imaging, in vitro cell study, an in vivo animal study, histological analysis, and microcomputed tomography. Mg-doped Baghdadite with composition Ca3Mg0.1Zr0.9Si2O8.9 demonstrated superior bioactivity and mechanical properties, compared to Baghdadite. 3D printed Mg-doped Baghdadite scaffolds with 35% porosity and designed architecture matched the stiffness and strength of cortical bone. These scaffolds were 2–5 times stronger than other bioceramic and bioglass scaffolds with the same porosity made with photopolymerization techniques. In vivo bone ingrowth was 2.2 times higher in Mg-doped Baghdadite than Baghdadite, effectively transforming these mechanically brittle scaffolds into deformable and tough ceramic-bone composites. Mg-doped Baghdadite scaffolds demonstrate a combination of favorable mechanical properties and bone regeneration capacity that show their potential for clinical success.

Despite being porous, natural materials such as bone, nacre and tooth enamel possess interesting combinations of stiffness, strength and toughness, an outstanding performance which is hard to achieve in engineering materials. Recent studies show that this performance is rooted in their architectures in which stiff and strong inclusions are arranged in a staggered structure and placed within a softer matrix. These studies however focus on void-free staggered composites. Here, we use finite element analysis and exhaustive exploration of the design space to show that the specific modulus and yield strength of staggered composites can be improved by removing the non-load bearing regions of the matrix material and to show the morphologies emerging from this process. This improvement increases with aspect ratio of the inclusions and with mismatch of properties between the phases and decreases with inclusion content. Particularly, 20–30% increase in the specific modulus and 10–20% increase in specific yield strength can be achieved considering the actual properties of carbon nanotube (or ceramic/mineral platelet) reinforced polymers. The morphologies emerging from our exploration can be used to develop lightweight composites of interest in aerospace, defence, and packaging industries.

Structural biological materials such as bone, teeth or mollusk shells draw their remarkable performance from a sophisticated interplay of architectures and weak interfaces. Pushed to the extreme, this concept leads to sutured materials, which contain thin lines with complex geometries. Sutured materials are prominent in nature, and have recently served as bioinspiration for toughened ceramics and glasses. Sutures can generate large deformations, toughness and damping in otherwise all brittle systems and materials. In this study we examine the design and optimization of sutures with a jigsaw puzzle-like geometry, focusing on the non-linear traction behavior generated by the frictional pullout of the jigsaw tabs. We present analytical models which accurately predict the entire pullout response. Pullout strength and energy absorption increase with higher interlocking angles and for higher coefficients of friction, but the associated high stresses in the solid may fracture the tabs. Systematic optimization reveals a counter-intuitive result: the best pullout performance is achieved with interfaces with low coefficient of friction and high interlocking angle. We finally use 3D printing and mechanical testing to verify the accuracy of the models and of the optimization. The models and guidelines we present here can be extended to other types of geometries and sutured materials subjected to other loading/boundary conditions. The nonlinear responses of sutures are particularly attractive to augment the properties and functionalities of inherently brittle materials such as ceramics and glasses.

Mechanisms associated with geometric interlocking are critical in engineering materials such as adhesives, metals or composites, and also in natural materials such as diatoms or mollusk shells. In this work we developed a new type of sutured material based on jigsaw-like morphologies which can be programmed to geometrically lock into two distinct stable positions. Using 3D printing and design exploration, we show that the mechanical response of these bistable materials can be tailored by fine tuning of the architecture of the tabs. The system can be cycled between the two equilibrium positions without apparent damage accumulation. We also show that the second equilibrium position can be made more stable than the first, effectively providing a geometric hardening mechanism to delay localization and spread nonlinear deformations over large volumes of material. The resulting materials are up to 10 times tougher than the polymer they are made of and they offer attractive properties such as large and reversible deformations, damage tolerance and re-manufacturability, offering promising perspectives in the development of new architectured materials.

Combining high strength, hardness and high toughness remains a tremendous challenge in materials engineering. Interestingly nature overcomes this limitation, with materials such as bone which display unusual combinations of these properties in spite of their weak constituents. In these materials, highly mineralized “building-blocks” provide stiffness and strength, while weak interfaces between the blocks channel non-linear deformation and trigger powerful toughening mechanisms. This strategy is also exploited in multilayered ceramics, fiber-reinforced composites, and more recently in topologically-interlocked materials. In this work we apply these concepts to the toughening of glass panels by incorporating internal architectures carved within the material using three-dimensional laser engraving. Glass is relatively stiff and hard but it has no microstructure, no inelastic deformation mechanism, low toughness and poor resistance to impacts. We demonstrate how introducing controlled architectures in glass completely changes how this material deforms and fails. In particular, our new architectured glass panels can resist about two to four times more impact energy than plain glass. Our architectured glass also displays non-linear deformation, progressive damage and failure contained within a few building blocks. This work demonstrates how micro-architecture, bio-inspiration and top-down fabrication strategies provide new pathways to transform the mechanics and performance of materials and structures.

Materials with dense architectures are composed of stiff and strong building blocks that are arranged to interact through energy-dissipative interfaces. Examples of these materials include engineered constructions such as the Abeille vault and highly mineralized natural materials such as tooth enamel. Compared with synthetic materials, natural materials with dense architectures exhibit outstanding mechanical performance, serving as a continued source of inspiration and study. This review details the status of advances in the fabrication and mechanics of bioinspired materials with dense architectures. The fabrication methods include freeze-casting, mineralization, 3D printing, coating–assembling, and laser engraving. Micromechanics of the resulting materials are discussed in tension, flexion, fracture, puncture, and impact. The discussion shows that strength of these materials can be improved by decreasing the size of their building blocks to the nm–µm range. However, interface hardening mechanisms that are crucial to the spread of deformation and toughness have not yet been implemented at nm–μm scales although they have been successfully realized for materials with larger building blocks. Future directions to address this and other unmet challenges are discussed throughout the text.

Strong and tough ceramics using architecture and topological interlocking

The remarkable mechanical performance of biological materials such as bone, nacre, and spider silk stems from their staggered microstructure in which stiff and strong reinforcements are elongated in the direction of loading, separated by softer interfaces, and shifted relative to each other. This structure results in useful combinations of modulus, strength and toughness and therefore is increasingly mimicked in bio-inspired engineering composites. Here, we report the use of a simple and versatile technique based on doctor-blading to fabricate staggered composites of microscopic alumina tablets with high alignment in a chitosan matrix. Tensile tests on these nacre-like materials show that the modulus and strength of the composite films are enhanced by the incorporation of ceramic tablets, but only up to 15vol% after which all properties degrade. This phenomenon, also reported in the past for most of nacre-like materials, composed of micro/nano tablets, obtained from different techniques, has been limiting our ability to produce large volumes of high-performance nacre-like materials. Examination of the structure of the films revealed that at lower tablet concentrations the tablets are well-aligned and well dispersed thorough the volume of the film. At 15vol% and beyond, we observed tablet misalignment and clustering. In order to investigate the impact of these imperfections on material performance we developed large scale finite element models representative of the structure of the composite films. These models show that the mechanical performance significantly degrades with tablet misalignment, and especially at high tablet concentrations. The simulations along with the SEM images therefore quantitatively explain the experimental trends, e.g. the degradation of mechanical properties at high tablet contents.

Self-assembly produces materials with highly organized microstructures and attractive properties for a variety of applications. Self-assembly is a process which typically involves molecules or nano-scale objects, with only a few reports of successful self-assembly of objects with larger dimensions (greater than 1 μm). Self-assembly at this length scale is however important, and may find different technological applications because of the possibility to incorporate different functionalities to the building blocks by for example lithographic and microfabrication techniques. Meso-scale self-assembly is also particularly promising to duplicate the structure of natural materials such as nacre (mother of pearl). Here, we fabricated 10 μm sized hexagonal tablets of silicon which self-assembled into a well-packed periodically arranged structure at a water–air interface. The microstructure was secured in a PDMS thin film, which made it stable and more organized compared to the similar large scale assemblies reported in the past. The self-assembled films can serve as building blocks for biomimetic materials, protective coatings, flexible electronics, or tunable optical devices.

Highly mineralized natural materials such as teeth or mollusk shells boast unusual combinations of stiffness, strength and toughness currently unmatched by engineering materials. While high mineral contents provide stiffness and hardness, these materials also contain weaker interfaces with intricate architectures, which can channel propagating cracks into toughening configurations. Here we report the implementation of these features into glass, using a laser engraving technique. Three-dimensional arrays of laser-generated microcracks can deflect and guide larger incoming cracks, following the concept of 'stamp holes'. Jigsaw-like interfaces, infiltrated with polyurethane, furthermore channel cracks into interlocking configurations and pullout mechanisms, significantly enhancing energy dissipation and toughness. Compared with standard glass, which has no microstructure and is brittle, our bio-inspired glass displays built-in mechanisms that make it more deformable and 200 times tougher. This bio-inspired approach, based on carefully architectured interfaces, provides a new pathway to toughening glasses, ceramics or other hard and brittle materials.

This chapter investigates the concept of hierarchy widely found in biological materials. First, natural hierarchical materials are explored in terms of their high order structures formed from universal building blocks. Hierarchical arrangement is claimed to give rise to remarkable mechanical properties of biological structures. Therefore at the next step, the significance of hierarchical structuring on mechanical properties is investigated through available analytical models. Finally, fabrication methods which could potentially lead to artificial hierarchical structures are briefly reviewed in the domain of biomimetics.

Materials such as bone, teeth, and seashells possess remarkable combinations of properties despite the poor structural quality of their ingredients (brittle minerals and soft proteins). Nacre from mollusk shells is 3,000 times tougher than the brittle mineral it is made of, a level of toughness amplification currently unmatched by any engineering material. For this reason, nacre has become the model for bio-inspiration for novel structural materials. The structure of nacre is organized over several length scales, but the microscopic brick-and-mortar arrangement of the mineral tablets is prominent. This staggered structure provides a universal approach to arranging hard building blocks in nature and is also found in bone and teeth. Recent models have demonstrated how an attractive combination of stiffness, strength, and toughness can be achieved through the staggered structure. The fabrication of engineering materials that duplicate the structure, mechanics, and properties of natural nacre still present formidable challenges to this day.

Highly mineralized biological materials such as nacre (mother of pearl), tooth enamel or conch shell boast unique and attractive combinations of stiffness, strength and toughness. The structures of these biological materials and their associated mechanisms are now inspiring new types of advanced structural materials. However, despite significant efforts, no bottom up fabrication method could so far match biological materials in terms of microstructural organization and mechanical performance. Here we present a new 'top down' strategy to tackling this fabrication problem, which consists in carving weak interfaces within a brittle material using a laser engraving technique. We demonstrate the method by fabricating and testing borosilicate glasses containing nacre-like microstructures infiltrated with polyurethane. When deformed, these materials properly duplicate the mechanisms of natural nacre: combination of controlled sliding of the tablets, accompanied with geometric hardening, strain hardening and strain rate hardening. The nacre-like glass is composed of 93 volume % (vol%) glass, yet 700 times tougher and breaks at strains as high as 20%.

Crocodiles, armadillo, turtles, fish and many other animal species have evolved flexible armored skins in the form of hard scales or osteoderms, which can be described as hard plates of finite size embedded in softer tissues. The individual hard segments provide protection from predators, while the relative motion of these segments provides the flexibility required for efficient locomotion. In this work, we duplicated these broad concepts in a bio-inspired segmented armor. Hexagonal segments of well-defined size and shape were carved within a thin glass plate using laser engraving. The engraved plate was then placed on a soft substrate which simulated soft tissues, and then punctured with a sharp needle mounted on a miniature loading stage. The resistance of our segmented armor was significantly higher when smaller hexagons were used, and our bio-inspired segmented glass displayed an increase in puncture resistance of up to 70% compared to a continuous plate of glass of the same thickness. Detailed structural analyses aided by finite elements revealed that this extraordinary improvement is due to the reduced span of individual segments, which decreases flexural stresses and delays fracture. This effect can however only be achieved if the plates are at least 1000 stiffer than the underlying substrate, which is the case for natural armor systems. Our bio-inspired system also displayed many of the attributes of natural armors: flexible, robust with 'multi-hit' capabilities. This new segmented glass therefore suggests interesting bio-inspired strategies and mechanisms which could be systematically exploited in high-performance flexible armors. This study also provides new insights and a better understanding of the mechanics of natural armors such as scales and osteoderms.

How to arrange soft materials with strong but brittle reinforcements to achieve attractive combinations of stiffness, strength and toughness is an ongoing and fascinating question in engineering and biological materials science. Recent advances in topology optimization and bioinspiration have brought interesting answers to this question, but they provide only small windows into the vast design space associated with this problem. Here, we take a more global approach in which we assess the mechanical performance of thousands of possible microstructures. This exhaustive exploration gives a global picture of structure–property relationships and guarantees that global optima can be found. Landscapes of optimum solutions for different combinations of desired properties can also be created, revealing the robustness of each of the solutions. Interestingly, while some of the major hybrid designs used in engineering are absent from the set of solutions, the microstructures emerging from this process are reminiscent of materials, such as bone, nacre or spider silk.