Acta Biomaterialia

Extracellular matrix (ECM) proteins assemble to form a heterogeneous connective scaffold that supports cells. Physical interactions between cells and the matrix regulate cellular behaviors and influence subsequent tissue construction. However, there is a lack of fundamental understanding regarding the contributions of individual native ECM proteins to the matrix. This gap arises from the need for nanoscopic characterization, which operates on a much smaller length scale than typical assessments in cell and tissue cultures, as well as in tissue reconstruction and clinical implantation. This study aims to systematically investigate how individual ECM proteins affect lipid membranes structurally and mechanically, and how these influences regulate cell migration. Results from Langmuir isotherm analysis, X-ray reflectivity measurements, and cell scratch assays demonstrate that strong collagen adsorption on the membrane surface disrupts lipid packing. However, its rigid network provides a sturdy scaffold for cell adhesion, thereby enhancing cell attachment and promoting cell migration. In contrast, elastin has a minimal structural or mechanical impact on the membrane during both adsorption and compression, but it benefits cells by facilitating migration and reducing the risk of infection. Fibronectin, on the other hand, exhibits complex mechanical responses to compression, characterized by significant structural rearrangements that occur during adsorption. This strong interaction with the membrane can result in excessively high adhesion forces, ultimately limiting cell motility. These findings lay the foundation for the design of artificial scaffolds that can manipulate cellular responses, a critical step toward advancing regenerative medicine and tissue engineering. SignificanceFabricating extracellular matrix (ECM) scaffolds from cells offers advantages over traditional approaches, such as decellularized tissues, which face donor limitations, and artificial scaffolds, which may hinder cellular communication. However, the slow harvesting process of cell-derived ECM has limited its clinical applications. This research is part of a larger mission to engineer ECM prescaffolds on lipid carriers tailored to cell requirements, enhancing ECM production and regulating cell behavior. The first step involves systematically analyzing the structural and mechanical effects of ECM on lipid membranes and how these effects regulate cellular behavior. This work confirms distinct characteristics of ECM proteins, advancing fundamental understanding of cell-matrix interactions and paving the way for scaffold engineering.

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 the 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 exhibits a greater relative increase of 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.

Fibrotic responses at biomaterial-tissue interfaces limit implant integration and regenerative healing, yet how the interaction between biomaterials and the extracellular matrix (ECM) regulates fibroblast activation remains poorly understood. Granular hydrogels including microporous annealed particle scaffolds (MAP) reduce fibrosis, while chemically and mechanically matched hydrogels do not, suggesting a dominant role for scaffold architecture. In this model, MAP scaffolds allow collagen infiltration and form physically continuous composites, whereas hydrogels exclude collagen and generate interfacial slip planes. To isolate how biomaterial architecture influences extracellular matrix (ECM) integration and fibroblast activation, we developed a reductionist in vitro model that integrates collagen type I with either microporous annealed particle (MAP) scaffolds or chemically and mechanically matched bulk hydrogels. This physical integration stabilizes collagen architecture, limits fibroblast-mediated matrix compaction, suppresses contractility, and attenuates myofibroblast transition. Fibroblasts in mechanically integrated environments exhibit reduced expression and nuclear localization of NF-{kappa}B and are enriched for quiescent phenotypes. Together, these findings identify biomaterial-ECM physical continuity as a design principle for limiting fibrotic signaling.

Dermal bone, which forms a variety of skeletal structures and persists in a wide range of extant vertebrates, evolved prior to endochondral bone which forms all mammalian load-bearing bones. Sturgeons are a family of fish which diverged soon after the lobe-finned/ray-finned split. Sturgeon retain a long robust spine at the leading edge of the pectoral fin, called the pectoral fin spine (PFS). Pectoral fin spines are bone elements that are present in many extinct and extant species of non-tetrapod jawed fish. In this study, we characterize the structure (light, polarized, micro-computed tomography and scanning electron microscopy), composition (FTIR, TGA, BMD), and mechanical properties (3-point bending and microindentation) of the pectoral fin spine (PFS) of the Russian sturgeon (Huso gueldenstaedtii). The microstructure of the PFS is highly organized as it is formed by dermal osteonal bone and parallel fibered bone. Its microarchitecture, along with high material toughness, anisotropy, and substantial ash content, enables the PFS to bear loads and function in both locomotion and protection. In addition, we show an interconnected network of neurovascular canals and ornamentations, features also found in pectoral fin spines of other non-tetrapod jawed fish. Collectively, these findings demonstrate that dermal bone can form structurally organized, mechanically competent load-bearing elements and provide new insight into pectoral fin spines in ray-finned fish.

Digital volume correlation (DVC) has become the benchmark experimental technique for full-field strain measurement in bone mechanics. In our previous work we developed a novel data-driven image mechanics (D2IM) approach that learns from DVC data and predicts displacement fields directly from undeformed X-ray computed tomography (XCT) images, deriving strain fields from such predictions. However, strain fields derived through numerical differentiation of displacement fields amplify high-frequency noise, and regularization techniques compromise spatial resolution while incurring substantial computational costs. Here we propose the upgrade D2IM-Strain to predict strain fields directly from XCT images of bone. Two prediction strategies were compared: displacement-derived strain and direct strain prediction. The direct strain prediction model significantly improved accuracy particularly for strain magnitudes below 10000{micro}{varepsilon}, taken as a representative threshold value for bone tissue yielding in compression. In addition, the direct approach reduced false-positive high-strain classifications by 75%. By eliminating numerical differentiation, the approach reduces noise amplification while maintaining computational efficiency. These findings represent a critical step toward developing robust data-driven volume correlation methods for hierarchical materials.

Maintaining a confluent, antithrombotic endothelium on cardiovascular biomaterial surfaces remains a major barrier to long-term hemocompatibility, as endothelial cells (ECs) rapidly denude under supraphysiological shear in prosthetic devices. Here, we hypothesized that mesoscale surface geometry ([~]100-200 {micro}m) could reorganize near-wall hemodynamics, preserving endothelial coverage and function under extreme shear. Engineered microtrenches were introduced onto an implant biomaterial to generate spatially defined shear environments. Under supraphysiological near-wall shear ([~]250 dyn/cm{superscript 2}), microtrenched geometries created attenuated shear and vorticity gradients. Endothelial monolayers were sustained in these flow domains for 120 hours, whereas flat controls rapidly denuded. Endothelial retention in 22.5{degrees} angled trenches increased dramatically, from an EC of 33 to 101 dyn/cm{superscript 2}. 45{degrees} angled trenches further increased endothelial shear resistance to an EC of 207 dyn/cm{superscript 2}. Endothelial monolayers demonstrated collective mechano-adaptation to ultra-high shear through VE-cadherin junction thickening and coordinated cytoskeletal and nuclear alignment. Mechanoadapted monolayers exhibited increased eNOS expression correlated with local shear and elevated nitrite production (45{degrees}: 50.4 {+/-} 6.1 {micro}M; 22.5{degrees}: 35.7 {+/-} 3.3 {micro}M; 0{degrees}: 28.4 {+/-} 6.8 {micro}M). In contrast, interfaces with abrupt shear transitions or elevated rotational flow exhibited reduced coverage, junctional thinning, and re-emergence of VCAM-1 and PAI-1, indicating inflammatory and pro-thrombotic activation. Structural, functional, and inflammatory readouts exhibited peak responses within a shared shear-vorticity regime. Multivariate regression identified shear-vorticity coupling as the dominant predictor of endothelial persistence, with optima clustering within a mechanical range ({approx}0.8-2.9 x 10 dyn{middle dot}cm-{superscript 2}{middle dot}s-{superscript 1}). These findings establish geometry-driven modulation of near-wall flow as a predictive, material-agnostic strategy for endothelialization and vasoprotection of high-shear cardiovascular implants.

Precision-cut lung slices (PCLS) retain the native cells and extracellular matrix that contribute to the structural and functional integrity of lung tissue. This technique enables the study of cell-matrix interactions and is particularly useful for pre-clinical pharmacological studies. More specifically, PCLS are widely used to model the complex pathophysiology of pulmonary fibrosis, an uncurable and progressive interstitial lung disease. Current ex vivo pulmonary fibrosis models expose PCLS to pro-fibrotic biochemical cues over a short timeframe (hours to days) and quickly collect samples for analysis due to viability concerns. This condensed timeline is a limitation to understanding chronic disease mechanisms. To extend the utility of ex vivo pulmonary fibrosis models, PCLS were embedded in engineered hydrogels and exposed to pro-fibrotic biochemical and biophysical cues. Hydrogel-embedded PCLS maintained greater than 80% total cell viability over 3 weeks in culture. Gene expression patterns in samples exposed to pro-fibrotic cues matched trends measured in human fibrotic lung tissue. Finally, treatment with Nintedanib, a Food and Drug Administration approved pulmonary fibrosis drug, moderately reduced fibroblast activation and influenced epithelial cell differentiation. Collectively, these results show that hydrogel-embedded PCLS models of pulmonary fibrosis extend our ability to study fibrotic processes ex vivo and, when applied to human tissues, present a new approach methodology for studying lung disease and treatment.

The thymus plays a pivotal role in the generation of immunocompetent T cells. Although its function is dependent on its complex extracellular matrix, its 3D architecture and mechanical properties remain poorly characterised This knowledge gap limits efforts to model and engineer the organ, which is a critical step towards the development of strategies for the treatment of many haematological and autoimmune diseases. Here, we provide the first comprehensive multiscale dataset of bovine thymic extracellular matrix architecture and viscoelastic behaviour, including quantitiative descriptors such as relaxation times, instantaneous and equilibrium elastic moduli, storage and loss moduli, and spatial mechanical heterogeneity. Taken together, our data define the thymus as a compliant, highly dissipative viscoelastic organ with a fibrillar architecture. They also represent a unique database, which, for the first time, paves the way for quantitative thymus tissue engineering.

Biomimetic tubular scaffolds hold great promise for tackling unmet clinical needs thanks to their biocompatibility and recapitulation of cellular microenvironments, conferring the ability to promote regeneration. Potential applications include small-diameter vascular implants and grafts for airway repair, for which no viable off-the-shelf solutions currently exist. The tubular materials (4 and 8 mm internal and external diameters) presented here consist purely of type I collagen, contain no chemical crosslinkers, and reproduce the multi-scale architecture of the native tissue including the presence of collagen fibrils. A novel two-step protocol provides materials with distinct concentric layers. A porous external structure, obtained by means of ice templating combined with collagen topotactic fibrillogenesis, favours oriented cell colonization. A smooth and much less porous internal layer provides mechanical and water-tightness properties relevant for in vivo implantation and promotes the formation of an endothelial monolayer under both static and flow conditions. The compliance of the double-layered materials under physiological pressure is close to that of piglet carotid arteries. The materials are also determined to be sufficiently flexible to provide the ability to perform ex vivo anastomosis with bronchi, although the relatively low value of suture retention strength remains a limitation for in vivo suturing.

Central nervous system (CNS) disease or injury might be treated by implanted devices, tissue regenerative scaffolds, or drug delivery platforms. However, inflammatory CNS responses limit these interventions and may worsen outcomes following damage to the CNS. Via the foreign body reaction (FBR), macrophages and glial cells trigger a "glial scar" around implants, reducing device performance, scaffold regenerative ability, or drug delivery potential. Previous studies have shown that stiffness of CNS implants significantly affects glial encapsulation, but few studies have investigated materials that truly match brain tissue stiffness. Porous precision-templated scaffolds (PTS) with uniform, interconnected, 40 {micro}m pores have shown favorable healing outcomes and a reduced FBR in numerous soft and hard tissue applications. To quantify the effects of both hydrogel compliance (stiffness) and pore size on glial encapsulation, we implanted poly(2-hydroxyethyl methacrylate-co-glycerol methacrylate) (pHEMA/GMA) PTS of varying stiffness and pore size for 4 weeks in rat brain. We observed reduced astrocyte encapsulation around PTS compared to solid hydrogel rods, reduced pro-inflammatory macrophage polarization for softer hydrogels versus stiffer hydrogels, and the presence of neuronal markers and neurogenesis within the pores. Utilizing soft, precision-porous hydrogels could provide a strategy for mitigating glial scarring and improving implant-based CNS treatments.

The bone matrix is precisely maintained and optimized to resist fractures. However, aging and disease deteriorate the bone matrix and increase fragility. Individuals with type 2 diabetes (T2D) have an elevated risk of bone fracture despite apparently normal bone mass. The chronic hyperglycemia in T2D promotes the formation of advanced glycation end-products (AGEs) in the bone tissue and modify the matrix mechanics. AGEs also bind to its receptor, RAGE, to activate inflammation and alter homeostasis. Using a leptin-receptor deficient mouse model of diabetes, we used a combination of high-resolution methods across multiple scales to evaluate the microarchitectural-, material- and cellular- level changes affected by the modulation of RAGE. To demonstrate the relevance of RAGE, we genetically ablated RAGE (RAGE-null) before the onset of diabetes; and to demonstrate the potency of RAGE as a disease modifying therapy, a RAGE antagonist (FPS-ZM1) was administered after prolonged diabetes. Diabetes impaired bone microstructure, the homeostatic actions of bone cells, the bone matrix nanomechanics, and whole- bone strength. The constitutive ablation of RAGE in diabetic animals prevented AGEs accumulation and the decline of trabecular connectivity; protected against the loss of osteocyte lacunae density and morphology; and maintained the matrix nanomechanics and bone strength. The inhibition of RAGE after the onset of diabetes reversed AGE accumulation and loss of bone volume; rescued osteocyte lacunae density and osteoclast activity; and restored matrix nanomechanics and bone strength. These results suggest that RAGE is a viable therapeutic target for diabetes-mediated impairments of bone quality. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=129 SRC="FIGDIR/small/716153v1_ufig1.gif" ALT="Figure 1000"> View larger version (26K): org.highwire.dtl.DTLVardef@6d8a5borg.highwire.dtl.DTLVardef@1967b5borg.highwire.dtl.DTLVardef@7cb1edorg.highwire.dtl.DTLVardef@85491d_HPS_FORMAT_FIGEXP M_FIG C_FIG

BACKGROUND: Cranioplasty is an essential procedure to restore cranial integrity, protect neural structures, and improve cosmetic outcomes. However, commercially available implants are often costly, limiting their accessibility in public healthcare systems. Three dimensional (3D) printing offers a low cost alternative for producing patient-specific solutions. METHODS: A retrospective case series of eight patients undergoing cranioplasty using customized polymethylmethacrylate (PMMA) implants fabricated with 3D printed molds was conducted. Computed tomography (CT) scans were used for segmentation and digital modeling. Patient specific molds were designed and printed preoperatively. Variables analyzed included design time, printing time, intraoperative workflow, and clinical outcomes. RESULTS: Design time ranged from approximately 1 hour for small defects to 3 hours for larger defects. Printing time ranged from 2 3 hours for smaller defects and up to 8 10 hours for larger reconstructions. Satisfactory aesthetic outcomes were achieved in 7 of 8 patients (87.5%). No major implant related complications were observed. CONCLUSION: Low cost 3D printing for PMMA cranioplasty is a feasible, accessible, and effective technique for cranial reconstruction, particularly in resource limited settings. Keywords: Cranioplasty; 3D printing; Cranial defect reconstruction; Low cost surgery; Patient specific implants; Polymethylmethacrylate; Skull reconstruction

Spectral similarity is often judged with a single metric such as RMSE, yet this can be misleading: physically different errors can produce similar scores. This is a critical limitation for computational biomechanics, where spectral agreement underpins both model validation and machine-learning loss design. Here, we develop a multi-metric framework for objective spectral biofidelity and test whether it better captures meaningful disagreement across complex frequency-domain responses. We evaluated 12 complementary similarity metrics, including CORA and ISO/TS 18571, using controlled spectral perturbations that mimic common real-world deviations such as resonance shifts, localized spikes, and broadband tilts. We then applied the framework to an SBI-tuned finite-element middle-ear model to assess convergence with training dataset size and robustness to measurement noise across repeated stochastic runs. No single metric performed reliably across all distortion types. Shape-based metrics tracked resonance morphology but could miss vertical scaling, whereas MaxError remained important for narrowband anomalies that smoother metrics underweighted. CORA and ISO 18571 did not consistently outperform simpler metrics. Rank aggregation using Borda count provided a robust consensus across metrics, enabling objective identification of training-data saturation and noise thresholds beyond which similarity rankings became unstable. These results show that spectral biofidelity cannot be reduced to a single norm. A multi-metric consensus provides a clearer and more physically meaningful basis for comparing experimental and simulated spectra, and offers a more defensible foundation for data-fidelity terms in physics-informed and simulation-based machine learning.

Direct ink writing is compatible with an expansive materials palette. While enabling diverse applications, this materials versatility brings significant bottlenecks in ink formulation, often requiring the mixing, printing, and testing of dozens to hundreds of ink compositions over the course of a project. To accelerate ink-space exploration, we introduce gradient embedded multinozzle (GEM) printheads that combine the high-throughput parallelized printing of multinozzles with combinatorial ink mixing. These printheads allow simultaneous mixing of two-, three-, and four-input inks which are distributed to printer nozzles to create complex 3D structures with graded compositions of inks. Using a two-way GEM printhead, we vali-date cell compatibility by printing scaffolds containing various concentrations of fibroblasts and observing non-linear compaction behaviours. We next test a three-way GEM multinozzle to print ten compositions of di- and multi-functionalized poly(ethylene-glycol) diacrylate hydrogel tri-leaflet valves, optimizing for stiffness, swelling ratio, and toughness. Our GEM multinozzles are compatible with open-source printers and either pressure- or volume-driven extrusion systems and promise to accelerate iterative ink design and testing.

Extracellular matrix (ECM) mechanical properties regulate tissue homeostasis and disease progression, with persistent ECM stiffening serving as a hallmark of fibrosis; yet, the early transition from healthy to diseased tissue remains poorly understood. Dynamic three-dimensional (3D) tissue models that capture early-stage stiffening are needed to investigate cellular responses during disease initiation. This work presents an innovative platform for studying cell responses in 3D environments undergoing active matrix stiffening. A bioinspired synthetic ECM incorporates collagen-mimetic peptides and employs sequential, non-terminal strain-promoted azide-alkyne cycloaddition (SPAAC) reactions to enable controlled increases in matrix stiffness over physiologically relevant timescales. Alternating polymer incubations produce a 2.5-fold increase in storage modulus over 72 hours, modeling the mechanical transition from healthy to early-stage fibrotic lung tissue. Live-cell reporter fibroblasts enable real-time monitoring of alpha-smooth muscle actin (SMA) expression, revealing significant upregulation during matrix stiffening that remains transient and difficult to detect via traditional endpoint assays. Active stiffening also modulates fibroblast motility, transiently increasing migration speed while persistently enhancing directional persistence. Complementary computational reaction-diffusion modeling provides mechanistic insight into modulus gradient formation and reaction kinetics. This versatile toolbox enables investigation of early mechanobiological responses to matrix stiffening and may aid identification of markers of fibrotic disease onset.

Controlled delivery of growth factors and viable cells remains a significant challenge in bone tissue engineering. In this study, a 3D-printed hydrogel scaffold system was developed for the co-delivery of bone morphogenetic protein-9 (BMP-9) and preosteoblasts to enhance bone regeneration. The system consisted of a 3D-printed base scaffold containing BMP-9-coated calcium sulfate (CaS) microparticles and a photocurable hydrogel coating layer encapsulating viable cells. The scaffold design exploited electrostatic interactions between BMP-9 and gelatin matrices by incorporating gelatin type B in the base scaffold and gelatin type A in the coating layer. Differences in the isoelectric points of these gelatin types were utilized to regulate protein binding and release. Release studies demonstrated that CaS microparticles alone exhibited rapid burst release, with nearly 80% of BMP-9 released within 24 h. Encapsulation of BMP-9 coated CaS particles in the 3D-printed scaffolds reduced the release rate, while the addition of the coating layer significantly improved sustained release, limiting BMP-9 release to approximately 50-60% by day 5. Bioactivity studies showed enhanced cell attachment in BMP-9 containing scaffolds compared with controls. Live/Dead cytotoxicity assays demonstrated high cell viability (>80%) within the coating layer over the culture period, confirming that the encapsulation and photocuring processes did not adversely affect cell survival. Cell proliferation and differentiation were further evaluated using WST-1 and alkaline phosphatase assays. The results demonstrate that electrostatic interactions governed by gelatin type selection can regulate BMP-9 release while maintaining high cell viability, providing a promising platform for growth factors and cell delivery in bone tissue engineering.

Humans are tight-skinned mammals who typically fail to regenerate large full-thickness skin wounds, instead healing with substantial scarring and concomitant loss of function. Mechanical context is a major determinant of this outcome: elevated tissue tension or stiffness promotes fibrotic repair associated with hypertrophic or keloid scarring. Accordingly, regenerative medicine research has relied on diverse animal models to understand scar development and skin regeneration. Loose-skinned mammals exhibit greater regeneration ability. Furthermore, spiny mouse skin is significantly less stiff and associated with enhanced regenerative ability. Interestingly, this skin wound stiffness can be modulated to shift healing toward more regenerative or more fibrotic trajectories. Despite of this progress, the restoration of normal skin architecture after large-full thickness injury has not been elucidated in tight-skinned mammals. Can large full-thickness wounds regenerate with minimal scarring in tight-skinned mammals? Here we show the tight-skinned mammal Frasers Dolphin regenerates de novo a complex rete ridge architecture with associated vasculature and minimal scar following large full-thickness wound healing. Counterintuitively, this skin regeneration occurs in an aqueous, high-shear stress and high-tension environment. Complete rete ridge regeneration in tight-skinned mammals has not been documented and not observed in humans except in utero. This unique ability to rebuild elaborate rete ridges under tension is an opportunity to uncover molecular, cellular, and tissue-level mechanisms that enable regenerative wound healing in a mechanical regime typically associated with fibrosis.

The spatial arrangement of cells is fundamental to the mechanical and functional integrity of three-dimensional (3D) tissues, yet engineering spatially well-controlled tissue architectures remains challenging. Here, we computationally investigated how layered tissue architectures can be designed by modulating cell-cell interfacial tension. We performed simulations using a 3D vertex model and systematically varied interfacial tension magnitudes. The simulations generated a range of layered tissue architectures, including planar monolayers, bilayers, and structurally stratified states. In homogeneous cell populations, increasing interfacial tension drove transitions from monolayer to structurally stratified configurations. In heterogeneous populations, differential interfacial tensions induced out-of-plane cell sorting and the formation of compositionally sorted multilayers. Moreover, a recursive tension design strategy enabled hierarchical organization of multiple cell types into separate layers. Notably, this recursive scheme uses only two tension levels (high vs. low) assigned across interfaces and can, in principle, be extended to specify layered architectures with an arbitrary number of layers. Together, these results identify cell-cell interfacial tension as a tunable mechanical parameter for regulating layered tissue architecture and provide design principles for layered tissue engineering and regenerative medicine.

The bone marrow haematopoietic niche is composed of a diverse array of cell types and extracellular matrix components that together support healthy haematopoiesis. However, live imaging of the bone marrow microenvironment is hampered by tissue accessibility limitations. Using intravital imaging through a titanium imaging window, we investigated the dynamics of human haematopoietic cells and mesenchymal stromal cells within an ectopically implanted humanised scaffold in an immunodeficient murine host. These cell populations expand and differentiate over time, accompanied by progressive remodelling of the scaffold. We observe migration of murine endothelial cells into the scaffold, leading to the formation of a vascular network during the initial development of the humanised niche. Subsequently, the dense collagen matrix that makes up the implanted niche is altered and larger gaps form in regions populated by mesenchymal stroma cells. Collectively, our findings demonstrate dynamic remodelling of the extracellular milieu that supports haematopoietic cell development and establish a platform for longitudinal, in vivo investigation of these processes. Altogether, we describe a novel model that aligns with the 3R guiding principles and enables real-time assessment of bone marrow cell dynamics in vivo. Summary statementRatcliffe and Mian et al. image in vivo dynamics of a bone marrow haematopoietic niche model.

IntroductionTrabecular bone exhibits brittle behaviour governed by microscale deformation and damage processes, yet quantitative characterisation of crack progression remains challenging because classical fracture mechanics approaches do not apply to architecturally discontinuous porous tissues. This study evaluates whether synchrotron X-ray computed tomography (XCT) combined with digital volume correlation (DVC) can provide a practical experimental approach for quantifying crack opening behaviour in human trabecular bone. MethodSemicylindrical specimens harvested from femoral heads of hip-fracture donors (n = 5) and non-fracture controls (n = 5) underwent stepwise three-point-bending during XCT imaging. Full-field displacement maps enabled direct measurement of crack mouth opening displacement (CMOD), crack length (a), and their ratio, CMOD/a, used here as a geometry-normalised comparative descriptor of brittle response. Automated crack segmentation using phase-congruency crack detection (PCCD) was compared against manual measurements. ResultsXCT-DVC successfully resolved three-dimensional displacement discontinuities during crack initiation and propagation in all specimens. Hip-fracture donors exhibited significantly lower critical crack-opening ratios (CMOD/a)* than Controls (0.31 vs 0.47; p = 0.008) and reached mechanical instability at lower applied loads, consistent with a more brittle structural response under this test configuration. Despite these differences, total crack extension ({Delta}a*) was similar between groups. Automated crack tracking using phase-congruency-based segmentation showed excellent agreement with manual measurements (r{superscript 2} = 0.98), confirming reliable extraction of crack geometry from DVC displacement fields. DiscussionThese results indicate that XCT-DVC can provide a practical approach for quantifying crack-opening behaviour in trabecular bone when classical fracture-mechanics parameters are not applicable in anatomically constrained specimens. The reduced critical crack-opening ratios and earlier instability observed in Hip-fracture donors are consistent with a more brittle comparative mechanical response that is not captured by crack extension alone. The strong agreement between automated and manual crack measurements further supports displacement-based descriptors as reliable comparative indicators of brittle behaviour in porous, architecturally discontinuous tissues. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=76 SRC="FIGDIR/small/714043v1_ufig1.gif" ALT="Figure 1"> View larger version (28K): org.highwire.dtl.DTLVardef@31c5d7org.highwire.dtl.DTLVardef@1b3d9a4org.highwire.dtl.DTLVardef@95df7borg.highwire.dtl.DTLVardef@1834216_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOGraphical abstractC_FLOATNO C_FIG