Jonathan Perrin

Summary: Arbuscular mycorrhizal fungi (AMF) contribute to plant nutrient and water uptake via their extraradical hyphal networks. However, in situ methodologies to quantify architectural and morphological traits of these networks in soil are largely lacking, limiting our understanding of AMF‐mediated resource transport. Using synchrotron‐based X‐ray computed microtomography (micro‐CT), we established a workflow to cultivate, noninvasively image, and quantitatively analyze AMF hyphosphere and rhizosphere structures in the interaggregate space across two soil textures and biological contexts. We developed a pipeline for quantitative three‐dimensional (3D) assessment of key architectural and morphological traits including structure counts, hyphal length, branching frequency, volume, and surface area. Our method further permits (1) measurement of AMF–soil and AMF–root interface areas and (2) microscale quantification of pore space occupancy by AMF. Micro‐CT offers a tool for noninvasively visualizing AMF in air‐filled soil pore space. We outline how such quantitative 3D information can be incorporated into image‐based and functional‐structural soil–plant models, thereby supporting a better mechanistic understanding of AMF‐mediated processes in soils and plants.

Understanding and predicting the morphological changes of filler particles within composites under load is critical to design improved materials. Syntactic foams, consisting of microspheres embedded in a solid matrix, have recently emerged as an ultra-lightweight metamaterial with highly tuneable mechanical properties. Glass microspheres are commonly used in syntactic foams, but can fracture, leading to a loss in the bulk mechanical properties. It is thought that hollow thermoplastic microspheres instead buckle, inducing strong, reversible, constitutive nonlinearity in the syntactic foam. Here we demonstrate, in-situ, the buckling of embedded thermoplastic shells under uniaxial compression. The results validate several important predictions from theory; regardless of sphere size, equatorial buckling prevails whenever the shell walls are thin relative to their diameter. Furthermore, the buckling wavelength is shown to be tuneable via different mechanical and geometrical parameters for the microspheres and/or matrix. Now better understood, we anticipate that tuneable, controlled and coordinated microsphere buckling can be exploited in diverse applications including strain sensing.

Le corail rouge de Méditerranée (Corallium rubrum) fait l'objet d'un important commerce depuis des millénaires ; il est traditionnellement utilisé pour la confection de bijoux et d'objets d'art. L'étude de son squelette permet de mieux comprendre les procédés utilisés par la nature pour fabriquer des matériaux résistants avec un coût énergétique minimal (conditions de chimie douce). D'autre part, la caractérisation des taux de croissance vertical et radial est toute aussi importante pour établir des critères à des fins de conservation de l'espèce. Dans ce travail, C. rubrum a été comparé à d'autres espèces de coraux précieux de la même famille (C. elatius et Paracorallium japonicum). Ces espèces présentent deux modes de croissance distincts: (1) un mode 'bloc par bloc' à l'apex de la branche, associé à une croissance axiale rapide (~2 mm/an) et (2) un mode 'couche par couche' se produisant en dessous de l'apex, associé à une croissance radiale lente (~0,2 mm/an). Les structures physiques et chimiques des squelettes des coraux précieux aident à mieux comprendre les mécanismes complexes de la croissance cristalline bio-assistée.

Improving the mechanical performance of flax fibres for textile and biocomposite applications requires reducing kink-band defects, which induce cellulose microfibril misorientation and local pore clusters. While today some studies focus on limiting their formation, no work has quantitatively demonstrated a reduction of existing kink-bands. In this study, tensile testing revealed that a conventional chemical bleaching process based on hydrogen peroxide and sodium hydroxide, widely used in the textile industry, enhances the mechanical properties of elementary fibres. This result is unexpected, as further SEM observations highlight local damage, particularly at kink-band areas. To resolve this contradiction, a multimodal characterization approach was employed. At the cell-wall scale, X-ray microtomography showed that bleaching reduces kink-band porosity by a factor of 8.8, while second harmonic generation microscopy revealed a decrease in microfibrillar misalignment, explaining the improved mechanical properties. At the microfibril scale, nuclear magnetic resonance indicated a reduction in non-cellulosic components, suggesting that a cocrystallization mechanism during bleaching may promote a more stable and oriented cellulose microfibrils arrangement. Overall, these findings represent a significant step toward developing solutions capable of reducing fibre defects.

In this study, we examined how printing temperature affects the microstructure and mechanical properties of polylactic acid (PLA) composite reinforced with iron oxide i.e., magnetite manufactured using a material extrusion technique. The composite was printed at temperatures from 185 °C to 215 °C. Microstructure analysis via synchrotron radiation X-ray microtomography revealed changes in both iron oxide and porosity contents within the printed structures. Mechanical testing results demonstrated a limited effect of the printing temperature on tensile performance. Finite element computation is considered to predict the elasticity behavior of the printed composite by converting 3D images into 3D structural meshes. When implementing a two-phase model, the predictions show a leading role of the iron oxide content, and an overestimation of the stiffness of the composite. A three-phase model demonstrates a better matching of the experimental results suggesting a limited load transfer across the PLA-iron oxide interface with Young’s moduli in the interphase zone as small as 10% of PLA Young’s modulus. Magnetic actuation demonstrates that experiments on PLA-iron oxide plates reveal a pronounced thickness-dependent limitation, with the maximum deflection observed in thin strips of 0.4 mm.

Flax fiber reinforcements weaken with aging and microstructural changes, limiting their applications. Here, we examine the effects of microstructure and aging on flax fiber elements’ performance by using 4000-year-old and modern Egyptian flax as references through multi-scale numerical modeling. This study introduces a novel investigation into the tensile stress distribution behavior of archaeological and modern flax yarns. The finite element (FE) model is derived from 3D volumes obtained via X-ray microtomography and tensile testing in the elastic domain. At the microscale, fibers exhibit higher axial stress concentrations around surface defects and pores, particularly in regions with kink bands and lumens. At the mesoscale, fiber bundles show increased stress concentrations at inter-fiber voids and lumen, with larger bundles exhibiting greater stress heterogeneity, especially around pores and surface roughness. At the macroscale, yarns display significant stress heterogeneity, especially around microstructural defects like pores and fiber–fiber cohesion points. Aged fibers from ancient Egyptian cultural heritage in particular demonstrate large fiber discontinuities due to long-term degradation or aging. These numerical observations highlight how porosity, surface imperfections, and structural degradation increase stress concentration, leading to fiber rupture and mechanical failure. This insight reveals how aging and defects impact flax fiber performance and durability.

Efficient design strategies found in nature have led to innovative materials and technologies. One promising blueprint for lightweight applications is the hymenium of Fomes fomentarius, a cellular, hierarchically structured, light, strong, and failure‐resistant biological tissue. Phase‐contrast enhanced micro‐computed tomography is used to investigate deformation and failure during in situ compression testing of the tubular honeycomb structure. Specimens are loaded parallel or transverse to the long axis of the tubes , in wet or dry condition. Displacements are evaluated between defined loading steps with three‐dimensional optical flow and morphological image analysis is used to correlate the micro‐ and mesostructure with local strains and structural damage. Key findings include that displacements are about five times larger in transverse than parallel loading, with greater displacements in regions of lower density. Higher hyphae density is found in struts compared to vertices, which leads to uniform displacements without significant local strain concentration. While cracks are observed in both struts and vertices, surprisingly, fracture is more catastrophic in the struts. Parallel loading results in plastic buckling and delamination in dry samples, while their wet counterparts show telescopic shortening from the first loading step and fewer fatal cracks, probably due to greater hyphae elasticity.

Harlequin frogs, genus Atelopus, are a species-rich group of bufonid anurans from the Neotropics with more than 100 species. For nearly four decades now, this group has suffered from massive population declines. Almost all species are threatened with extinction, and many populations and several species are considered extinct or possibly extinct. This results in a limited sampling available for studies on harlequin frog systematics, especially in terms of molecular genetic information. However, efficient conservation of harlequin frogs requires an improved taxonomy. This is further complicated through the circumstance that many Atelopus species are relatively poor in external morphological characters combined with a high level of intra-specific character variation (e.g. coloration and body size). At the same time, cryptic diversity exists with well differentiated species (supported by osteology and molecular genetics) almost indistinguishable by external morphology. We compiled the largest dataset to date for mitochondrial (12S, 16S, cyt b) and nuclear (POMC, RAG1) markers and present a phylogeny (likelihood and Bayesian inference methods) including 152 samples from 104 populations scattered over the entire geographic range of the genus. Four allo- or parapatric main clades are distinguished: I. Sierra Nevada; II. Venezuelan-Andean; III. Andean-Chocó-Central American (with the ignescens and the varius-longirostris clades); and IV Amazonian (containing the tricolor and the flavescens-spumarius clades). The phylogenetic relationships within these clades remain to be resolved. Taxonomic implications included both splitting and lumping, but taxonomic action is here only taken for populations related to A. spumarius from western Amazonia. Besides redescriptions of A. spumarius sensu stricto and A. colomai, we describe two new species based on morphology, skull osteology and bioacoustics. Additional yet understudied populations from Amazonia may be allocated to these species or may represent additional undescribed taxa.

Flax fibers, while offering numerous benefits, are susceptible to mechanical weakening due to the presence of kink-bands within their structure. The novelty of this study lies in linking mechanical behavior to fiber morphology and defects at multiple scales by utilizing X-ray microtomography to generate detailed 3D images of elementary flax fibers, enabling the creation of accurate finite element (FE) models for analysis. Aging reduces flax fibers’ strength, so both modern and ancient fibers were analyzed to understand their structural evolution over time. Static X-ray microtomography images were converted into 3D FE models for tensile simulations, and tensile tests provided essential properties for numerical modeling. Morphological analysis for both fiber types revealed that kink-bands contain multiple pores oriented ~45° to the fiber/lumen axis, with ancient fibers showing higher porosity (5.6%) and kink-band density (20.8 mm⁻¹) than modern fibers (3.3% and 16.6 mm⁻¹). SEM images confirmed that the intricate lumen and kink-bands lead to fiber failure under tensile loading. Numerical analysis highlighted higher stress concentrations at the kink-band region, particularly at pores in the kink-band region, which can initiate cracks and lead to rupture.