Competition between delamination and tearing in multiple peeling problems

Adhesive attachment systems consisting of multiple tapes or strands are commonly found in nature, for example in spider web anchorages or in mussel byssal threads, and their structure has been found to be ingeniously architected in order to optimize mechanical properties: in particular, to maximize dissipated energy before full detachment. These properties emerge from the complex interplay between mechanical and geometric parameters, including tape stiffness, adhesive energy, attached and detached lengths and peeling angles, which determine the occurrence of three main mechanisms: elastic deformation, interface delamination and tape fracture. In this paper, we introduce a formalism to evaluate the mechanical performance of multiple tape attachments in different parameter ranges, where an optimal (not maximal) adhesion energy emerges. We also introduce a numerical model to simulate the multiple peeling behaviour of complex structures, illustrating its predictions in the case of the staple-pin architecture. Finally, we present a proof-of-principle experiment to illustrate the predicted behaviour. We expect the presented formalism and the numerical model to provide important tools for the design of bioinspired adhesive systems with tuneable or optimized detachment properties.

Surface Phenomena Enhancing the Antibacterial and Osteogenic Ability of Nanocrystalline Hydroxyapatite, Activated by Multiple-Ion Doping

The present work describes a novel nanocrystalline, multidoped hydroxyapatite featuring excellent eukaryotic versus prokaryotic cell selectivity, attested by excellent osteoinductive character and evaluated with human stem cells, and anti-infective ability, tested against different pathogens. Physicochemical analysis and transmission electron microscopy (TEM)/scanning STEM observations highlighted that such enhanced biological features are related to the lower crystallinity level and increased surface charge of hydroxyapatite, both induced by multiple-ion doping. Specifically, the lattice substitution of Ca²⁺ with Zn²⁺ promotes the segregation of Ca²⁺ and doping Mg²⁺ cations to a less-ordered surface layer, thus promoting dynamic ion absorption/release acting as bioactive signals for cells and exerting an antiproliferative effect on all tested pathogens. These findings open the design of new biodevices, combining regenerative ability and effective microbial inhibition without using any antibiotic drugs. This is extremely important to circumvent bacterial resistance to antibiotics, which is today considered as one of the biggest threats to global health.

A CONSTITUTIVE MODEL FOR BOTH LOW AND HIGH STRAIN NONLINEARITIES IN HIGHLY FILLED ELASTOMERS AND IMPLEMENTATION WITH USER-DEFINED MATERIAL SUBROUTINES IN ABAQUS

Strain energy functions (SEFs) are used to model the hyperelastic behavior of rubberlike materials. In tension, the stress–strain response of these materials often exhibits three characteristics: (i) a decreasing modulus at low strains (<20%), (ii) a constant modulus at intermediate strains, and (iii) an increasing modulus at high strains (>200%). Fitting an SEF that works in each regime is challenging when multiple or nonhomogeneous deformation modes are considered. The difficulty increases with highly filled elastomers because the small strain nonlinearity increases and finite-extensibility occurs at lower strains. One can compromise by fitting an SEF to a limited range of strain, but this is not always appropriate. For example, rubber seals in oilfield packers can exhibit low global strains but high localized strains. The Davies–De–Thomas (DDT) SEF is a good candidate for modeling such materials. Additional improvements will be shown by combining concepts from the DDT and Yeoh SEFs to construct a more versatile SEF. The SEF is implemented with user-defined material subroutines in Abaqus/Standard (UHYPER) and Abaqus/Explicit (VUMAT) for a three-dimensional general strain problem, and an approach to overcome a mathematically indeterminate stress condition in the unstrained state is derived. The complete UHYPER and VUMAT subroutines are also presented.

Multilayer stag beetle elytra perform better under external loading via non-symmetric bending properties

Insect cuticle has drawn a lot of attention from engineers because of its multifunctional role in the life of insects. Some of these cuticles have an optimal combination of lightweight and good mechanical properties, and have inspired the design of composites with novel microstructures. Among these, beetle elytra have been explored extensively for their multilayered structure, multifunctional roles and mechanical properties. In this study, we investigated the bending properties of elytra by simulating their natural loading condition and comparing it with other loading configurations. Further, we examined the properties of their constitutive bulk layers to understand the contribution of each one to the overall mechanical behaviour. Our results showed that elytra are graded, multilayered composite structures that perform better in natural loading direction in terms of both flexural modulus and strength which is likely an adaptation to withstand loads encountered in the habitat. Experiments are supported by analytical calculations and finite element method modelling, which highlighted the additional role of the relatively stiff external exocuticle and of the flexible thin bottom layer in enhancing flexural mechanical properties. Such studies contribute to the knowledge of the mechanical behaviour of this natural composite material and to the development of novel bioinspired multifunctional composites and for optimized armours.

A comparative study of the mechanical properties of a dinosaur and crocodile fossil teeth

Vertebrate teeth are complex structures adapted in terms of shape and structure to serve a variety of functions like biting and grinding. Thus, examining the morphology, composition and mechanical properties of the teeth can aid in providing insights into the feeding behaviour of extinct species. We here provide the first mechanical characterisation of teeth in a spinosaurid dinosaur, Suchomimus tenerensis, and a pholidosaurid crocodylomorph, Sarcosuchus imperator. Our results show that both species have similar macrostructure of enamel, dental and interfacial layers, and similar composition, the main constituent being fluorapatite. Microindentation tests show that Suchomimus teeth have lower elastic modulus and hardness, as compared to Sarchosuchus. On the contrary, Sarcosuchus teeth have lower toughness. Nanoindentation showed the existence of mechanical gradients from dentin to enamel in Suchomimus and, less prominently, in Sarcosuchus. This was also supported by wear tests showing that in Suchomimus the dentin region is more wear-prone than the enamel region. With still scarce information available on the dietary regimes in extinct species, the analysis of micro and nano-mechanical properties of fossils teeth might be a help in targeting specific biological questions. However, much is still unknown concerning the changes underwent by organic material during diagenesis making at present impossible to definitely conclude if the differences in the mechanical properties of Suchomimus and Sarchosuchus here retrieved imply that the two species adopted different strategies when dealing with food processing or are the result of disparate taphonomic histories.

Imaging and mechanical characterization of different junctions in spider orb webs

Spider silk and spider orb webs are among the most studied biological materials and structures owing to their outstanding mechanical properties. A key feature that contributes significantly to the robustness and capability to absorb high kinetic energy of spider webs is the presence of junctions connecting different silk threads. Surprisingly, in spite of their fundamental function, the mechanics of spider web junctions have never been reported. Herein, through mechanical characterization and imaging, we show for the first time that spider orb webs host two different types of junction, produced by different silk glands, which have different morphology, and load bearing capability. These differences can be explained in view of the different roles they play in the web, i.e. allowing for a localized damage control or anchoring the whole structure to the surrounding environment.

Mapping Polymer Molecular Order in the SEM with Secondary Electron Hyperspectral Imaging

Understanding nanoscale molecular order within organic electronic materials is a crucial factor in building better organic electronic devices. At present, techniques capable of imaging molecular order within a polymer are limited in resolution, accuracy, and accessibility. In this work, presented are secondary electron (SE) spectroscopy and secondary electron hyperspectral imaging, which make an exciting alternative approach to probing molecular ordering in poly(3-hexylthiophene) (P3HT) with scanning electron microscope-enabled resolution. It is demonstrated that the crystalline content of a P3HT film is reflected by its SE energy spectrum, both empirically and through correlation with nano-Fourier-transform infrared spectroscopy, an innovative technique for exploring nanoscale chemistry. The origin of SE spectral features is investigated using both experimental and modeling approaches, and it is found that the different electronic properties of amorphous and crystalline P3HT result in SE emission with different energy distributions. This effect is exploited by acquiring hyperspectral SE images of different P3HT films to explore localized molecular orientation. Machine learning techniques are used to accurately identify and map the crystalline content of the film, demonstrating the power of an exciting characterization technique.

Bionicomposites

Incorporating nanomaterials in living systems could force the latter to produce "bionicomposites". We report a review of the first attempts with such bionicomposites, e.g. showing how the control of the eating and dormant states of microorganisms can provide nano-architectures with novel mechanical and functional properties, and how introducing nanomaterials in the diets of animals producing silks (spiders or silkworms) leads to intrinsically reinforced fibers with strengths higher than those of their natural counterparts, as well as those of synthetic polymer fibers or carbon fiber-reinforced polymeric composites.

Secondary electron emission and yield spectra of metals from Monte Carlo simulations and experiments

In this work, we present a computational method, based on the Monte Carlo statistical approach, for calculating electron energy emission and yield spectra of metals, such as copper, silver and gold. The calculation of these observables proceeds via the Mott theory with a Dirac-Hartree-Fock spherical potential to deal with the elastic scattering processes, and by using the Ritchie dielectric approach to model the electron inelastic scattering events. In the latter case, the dielectric function, which represents the starting point for the evaluation of the energy loss, is obtained from experimental reflection electron energy loss spectra. The generation of secondary electrons upon ionization of the samples is also implemented in the calculation. A remarkable agreement is obtained between both theoretical and experimental electron emission spectra and yield curves.

Cleaning interfaces in layered materials heterostructures

Heterostructures formed by stacking layered materials require atomically clean interfaces. However, contaminants are usually trapped between the layers, aggregating into randomly located blisters, incompatible with scalable fabrication processes. Here we report a process to remove blisters from fully formed heterostructures. Our method is over an order of magnitude faster than those previously reported and allows multiple interfaces to be cleaned simultaneously. We fabricate blister-free regions of graphene encapsulated in hexagonal boron nitride with an area ~ 5000 μm², achieving mobilities up to 180,000 cm² V^-1 s^-1 at room temperature, and 1.8 × 10⁶ cm² V^-1 s^-1 at 9 K. We also assemble heterostructures using graphene intentionally exposed to polymers and solvents. After cleaning, these samples reach similar mobilities. This demonstrates that exposure of graphene to process-related contaminants is compatible with the realization of high mobility samples, paving the way to the development of wafer-scale processes for the integration of layered materials in (opto)electronic devices.

Modeling and simulation of the impact behavior of soft polymeric-foam-based back protectors for winter sports

OBJECTIVES

Winter sports are high-energy outdoor activities involving high velocities and acrobatic maneuvers, thus raising safety concerns. Specific studies on the impact mechanics of back protectors are very limited. In this study analytical and numerical models are developed to rationalize results of impact experiments and propose new design procedures for this kind of equipment.

DESIGN

Different soft-shell solutions currently available on the market are compared. In particular, the role of dynamic material constitutive properties and of environmental temperature (which affects mainly material stiffness) on energy absorption capability are evaluated.

METHODS

Starting from dynamic mechanical-thermal characterization of the closed-cell polymeric foams constituting the protectors, we exploited analytical modeling and Finite Element Method simulations to interpret experimental data from drop weight impact test and to characterize protectors at different temperatures and after multiple impacts.

RESULTS

The temperature and frequency dependent properties of these materials characterize their impact behavior. Modeling results are in good agreement with impact tests. Results demonstrate how ergonomic soft-shell solution provides an advantage with respect to traditional hard-shell in terms of impact protection. Moreover, it can maintain nearly unaltered its protective properties after multiple impacts on the same point.

CONCLUSIONS

The coupled analytical-simulation approach here presented could be extensively used to predict the impact behavior of such equipment, starting from material characterization, allowing to save costs and time for physical prototyping and tests for design and optimization.

A Graded Multifunctional Hybrid Scaffold with Superparamagnetic Ability for Periodontal Regeneration

The regeneration of dental tissues is a still an unmet clinical need; in fact, no therapies have been completely successful in regenerating dental tissue complexes such as periodontium, which is also due to the lack of scaffolds that are able to guide and direct cell fate towards the reconstruction of different mineralized and non-mineralized dental tissues. In this respect, the present work develops a novel multifunctional hybrid scaffold recapitulating the different features of alveolar bone, periodontal ligament, and cementum by integrating the biomineralization process, and tape casting and electrospinning techniques. The scaffold is endowed with a superparamagnetic ability, thanks to the use of a biocompatible, bioactive superparamagnetic apatite phase, as a mineral component that is able to promote osteogenesis and to be activated by remote magnetic signals. The periodontal scaffold was obtained by engineering three different layers, recapitulating the relevant compositional and microstructural features of the target tissues, into a monolithic multifunctional graded device. Physico-chemical, morphological, and ultrastructural analyses, in association with preliminary in vitro investigations carried out with mesenchymal stem cells, confirm that the final scaffold exhibits a good mimicry of the periodontal tissue complex, with excellent cytocompatibility and cell viability, making it very promising for regenerative applications in dentistry.

Breaking the Nanoparticle Loading-Dispersion Dichotomy in Polymer Nanocomposites with the Art of Croissant-Making

The intrinsic properties of nanomaterials offer promise for technological revolutions in many fields, including transportation, soft robotics, and energy. Unfortunately, the exploitation of such properties in polymer nanocomposites is extremely challenging due to the lack of viable dispersion routes when the filler content is high. We usually face a dichotomy between the degree of nanofiller loading and the degree of dispersion (and, thus, performance) because dispersion quality decreases with loading. Here, we demonstrate a potentially scalable pressing-and-folding method (P & F), inspired by the art of croissant-making, to efficiently disperse ultrahigh loadings of nanofillers in polymer matrices. A desired nanofiller dispersion can be achieved simply by selecting a sufficient number of P & F cycles. Because of the fine microstructural control enabled by P & F, mechanical reinforcements close to the theoretical maximum and independent of nanofiller loading (up to 74 vol %) were obtained. We propose a universal model for the P & F dispersion process that is parametrized on an experimentally quantifiable " D factor". The model represents a general guideline for the optimization of nanocomposites with enhanced functionalities including sensing, heat management, and energy storage.

Bone matrix development in steroid-induced osteoporosis is associated with a consistently reduced fibrillar stiffness linked to altered bone mineral quality

Glucocorticoid-induced osteoporosis (GIOP) is a major secondary form of osteoporosis, with the fracture risk significantly elevated - at similar levels of bone mineral density - in patients taking glucocorticoids compared with non-users. The adverse bone structural changes at multiple hierarchical levels in GIOP, and their mechanistic consequences leading to reduced load-bearing capacity, are not clearly understood. Here we combine experimental X-ray nanoscale mechanical imaging with analytical modelling of the bone matrix mechanics to determine mechanisms causing bone material quality deterioration during development of GIOP. In situ synchrotron small-angle X-ray diffraction combined with tensile testing was used to measure nanoscale deformation mechanisms in a murine model of GIOP, due to a corticotrophin-releasing hormone promoter mutation, at multiple ages (8-, 12-, 24- and 36 weeks), complemented by quantitative micro-computed tomography and backscattered electron imaging to determine mineral concentrations. We develop a two-level hierarchical model of the bone matrix (mineralized fibril and lamella) to predict fibrillar mechanical response as a function of architectural parameters of the mineralized matrix. The fibrillar elastic modulus of GIOP-bone is lower than healthy bone throughout development, and nearly constant in time, in contrast to the progressively increasing stiffness in healthy bone. The lower mineral platelet aspect ratio value for GIOP compared to healthy bone in the multiscale model can explain the fibrillar deformation. Consistent with this result, independent measurement of mineral platelet lengths from wide-angle X-ray diffraction finds a shorter mineral platelet length in GIOP. Our results show how lowered mineralization combined with altered mineral nanostructure in GIOP leads to lowered mechanical competence.

SIGNIFICANCE STATEMENT

Increased fragility in musculoskeletal disorders like osteoporosis are believed to arise due to alterations in bone structure at multiple length-scales from the organ down to the supramolecular-level, where collagen molecules and elongated mineral nanoparticles form stiff fibrils. However, the nature of these molecular-level alterations are not known. Here we used X-ray scattering to determine both how bone fibrils deform in secondary osteoporosis, as well as how the fibril orientation and mineral nanoparticle structure changes. We found that osteoporotic fibrils become less stiff both because the mineral nanoparticles became shorter and less efficient at transferring load from collagen, and because the fibrils are more randomly oriented. These results will help in the design of new composite musculoskeletal implants for bone repair.

Folding Large Graphene-on-Polymer Films Yields Laminated Composites with Enhanced Mechanical Performance

A folding technique is reported to incorporate large-area monolayer graphene films in polymer composites for mechanical reinforcement. Compared with the classic stacking method, the folding strategy results in further stiffening, strengthening, and toughening of the composite. By using a water-air-interface-facilitated procedure, an A5-size 400 nm thin polycarbonate (PC) film is folded in half 10 times to a ≈0.4 mm thick material (1024 layers). A large PC/graphene film is also folded by the same process, resulting in a composite with graphene distributed uniformly. A three-point bending test is performed to study the mechanical performance of the composites. With a low volume fraction of graphene (0.085%), the Young's modulus, strength, and toughness modulus are enhanced in the folded composite by an average of 73.5%, 73.2%, and 59.1%, respectively, versus the pristine stacked polymer films, or 40.2%, 38.5%, and 37.3% versus the folded polymer film, proving a remarkable mechanical reinforcement from the combined folding and reinforcement of graphene. These results are rationalized with combined theoretical and computational analyses, which also allow the synergistic behavior between the reinforcement and folding to be quantified. The folding approach could be extended/applied to other 2D nanomaterials to design and make macroscale laminated composites with enhanced mechanical properties.

Combining Living Microorganisms with Regenerated Silk Provides Nanofibril-Based Thin Films with Heat-Responsive Wrinkled States for Smart Food Packaging

Regenerated silk (RS) is a protein-based “biopolymer” that enables the design of new materials; here, we called “bionic” the process of regenerated silk production by a fermentation-assisted method. Based on yeast’s fermentation, here we produced a living hybrid composite made of regenerated silk nanofibrils and a single-cell fungi, the Saccharomyces cerevisiae yeast extract, by fermentation of such microorganisms at room temperature in a dissolution bath of silkworm silk fibers. The fermentation-based processing enhances the beta-sheet content of the RS, corresponding to a reduction in water permeability and CO₂ diffusion through RS/yeast thin films enabling the fabrication of a mechanically robust film that enhances food storage durability. Finally, a transfer print method, which consists of transferring RS and RS/yeast film layers onto a self-adherent paraffin substrate, was used for the realization of heat-responsive wrinkles by exploiting the high thermal expansion of the paraffin substrate that regulates the applied strain, resulting in a switchable coating morphology from the wrinkle-free state to a wrinkled state if the food temperature overcomes a designed threshold. We envision that such efficient and smart coatings can be applied for the realization of smart packaging that, through such a temperature-sensing mechanism, can be used to control food storage conditions.

Emergence of the interplay between hierarchy and contact splitting in biological adhesion highlighted through a hierarchical shear lag model

Contact unit size reduction is a widely studied mechanism as a means to improve adhesion in natural fibrillar systems, such as those observed in beetles or geckos. However, these animals also display complex structural features in the way the contact is subdivided in a hierarchical manner. Here, we study the influence of hierarchical fibrillar architectures on the load distribution over the contact elements of the adhesive system, and the corresponding delamination behaviour. We present an analytical model to derive the load distribution in a fibrillar system loaded in shear, including hierarchical splitting of contacts, i.e. a "hierarchical shear-lag" model that generalizes the well-known shear-lag model used in mechanics. The influence on the detachment process is investigated introducing a numerical procedure that allows the derivation of the maximum delamination force as a function of the considered geometry, including statistical variability of local adhesive energy. Our study suggests that contact splitting generates improved adhesion only in the ideal case of extremely compliant contacts. In real cases, to produce efficient adhesive performance, contact splitting needs to be coupled with hierarchical architectures to counterbalance high load concentrations resulting from contact unit size reduction, generating multiple delamination fronts and helping to avoid detrimental non-uniform load distributions. We show that these results can be summarized in a generalized adhesion scaling scheme for hierarchical structures, proving the beneficial effect of multiple hierarchical levels. The model can thus be used to predict the adhesive performance of hierarchical adhesive structures, as well as the mechanical behaviour of composite materials with hierarchical reinforcements.

Coherently aligned nanoparticles within a biogenic single crystal: A biological prestressing strategy

In contrast to synthetic materials, materials produced by organisms are formed in ambient conditions and with a limited selection of elements. Nevertheless, living organisms reveal elegant strategies for achieving specific functions, ranging from skeletal support to mastication, from sensors and defensive tools to optical function. Using state-of-the-art characterization techniques, we present a biostrategy for strengthening and toughening the otherwise brittle calcite optical lenses found in the brittlestar Ophiocoma wendtii This intriguing process uses coherent nanoprecipitates to induce compressive stresses on the host matrix, functionally resembling the Guinier-Preston zones known in classical metallurgy. We believe that these calcitic nanoparticles, being rich in magnesium, segregate during or just after transformation from amorphous to crystalline phase, similarly to segregation behavior from a supersaturated quenched alloy.

Serpentine locomotion through elastic energy release

A model for serpentine locomotion is derived from a novel perspective based on concepts from configurational mechanics. The motion is realized through the release of the elastic energy of a deformable rod, sliding inside a frictionless channel, which represents a snake moving against lateral restraints. A new formulation is presented, correcting previous results and including situations never analysed so far, as in the cases when the serpent's body lies only partially inside the restraining channel or when the body has a muscle relaxation localized in a small zone. Micromechanical considerations show that propulsion is the result of reactions tangential to the frictionless constraint and acting on the snake's body, a counter-intuitive feature in mechanics. It is also experimentally demonstrated that the propulsive force driving serpentine motion can be directly measured on a designed apparatus in which flexible bars sweep a frictionless channel. Experiments fully confirm the theoretical modelling, so that the presented results open the way to exploration of effects, such as variability in the bending stiffness or channel geometry or friction, on the propulsive force of snake models made up of elastic rods.

Tuning the strain-induced resonance shift in silicon racetrack resonators by their orientation

In this work, we analyze the role of strain on a set of silicon racetrack resonators presenting different orientations with respect to the applied strain. The strain induces a variation of the resonance wavelength, caused by the photoelastic variation of the material refractive index as well as by the mechanical deformation of the device. In particular, the mechanical deformation alters both the resonator perimeter and the waveguide cross-section. Finite element simulations taking into account all these effects are presented, providing good agreement with experimental results. By studying the role of the resonator orientation we identify interesting features, such as the tuning of the resonance shift from negative to positive values and the possibility of realizing strain insensitive devices.

Micromechanical model for protein materials: From macromolecules to macroscopic fibers

We propose a model for the mechanical behavior of protein materials. Based on a limited number of experimental macromolecular parameters (persistence and contour length) we obtain the macroscopic behavior of keratin fibers (human, cow, and rabbit hair), taking into account the damage and residual stretches effects that are fundamental in many functions of life. We also show the capability of our approach to describe the main dissipation and permanent strain effects observed in the more complex spider silk fibers. The comparison between our results and the data obtained experimentally from cyclic tests demonstrates that our model is robust and is able to reproduce with a remarkable accuracy the experimental behavior of all protein materials we tested.

The influence of substrate roughness, patterning, curvature, and compliance in peeling problems

Biological adhesion, in particular the mechanisms by which animals and plants 'stick' to surfaces, has been widely studied in recent years, and some of the structural principles have been successfully applied to bioinspired adhesives. However, modelling of adhesion, such as in single or multiple peeling theories, has in most cases been limited to ideal cases, and due consideration of the role of substrate geometry and mechanical properties has been limited. In this paper, we propose a numerical model to evaluate these effects, including substrate roughness, patterning, curvature, and deformability. The approach is validated by comparing its predictions with classical thin film peeling theoretical results, and is then used to predict the effects of substrate properties. These results can provide deeper insight into experiments, and the developed model can be a useful tool to design and optimize artificial adhesives with tailor-made characteristics.

Ice-regenerated flame retardant and robust film of Bombyx mori silk fibroin and POSS nano-cages

In this study, we present a simple method to prepare and control the structure of regenerated hybrid silkworm silk films through icing.