The origin of nonlinear stress behaviors in transient networks during oscillatory shear measurements is investigated in this study via two-dimensional rheo-optics observations of a systematically controlled model system comprising tetra-armed polyethylene glycols (Tetra-PEG slime). Transient networks are characterized by their temporary crosslinks. However, the strain scale at which nonlinear viscoelastic responses begin to emerge under strain amplitude sweep in oscillatory shear deformations remains poorly characterized. The uncertainty of nonlinear onset can be attributed, at least in part, to the heterogeneous structures inherent to conventional transient networks, as well as to the limited availability of detailed experimental evaluations. We overcome these limitations by employing Tetra-PEG slime, which possesses a well-defined network structures with uniform strand lengths and functionalities. Rheo-polarization imaging reveals homogeneous deformation in the linear regime and a scaling change of retardation - strain relation around the onset of nonlinearity. Meanwhile, Rheo-SAXS measurements confirmed the absence of nanoscale structural changes. The elastic contribution per network strand at the critical strain (W c,0) approximately collapses onto a single master curve when plotted against the Weissenberg number (Wimax), following the relation W c,0 ∝ Wimax 2. This scaling suggests that the onset of nonlinearity is governed by the balance between molecular relaxation and applied deformation. These findings provide molecular-level insights into the nonlinear elasticity of transient networks and offer a framework to design soft materials with tunable nonlinear responses.

Abstract Percolation in transient polymer networks remains poorly understood because reversible cross-links continuously reorganize the network structure. In this study, we investigated percolation in transient polymer networks by independently controlling network connectivity and polymer concentration in a well-defined Tetra-PEG slime model system. Using diffusing-wave spectroscopy (DWS) microrheology, the Winter–Chambon criterion was identified at high frequencies (>102 rad s–1), despite the system being macroscopically a viscoelastic liquid. In the dilute regime, the critical connectivity required for percolation decreases with decreasing polymer concentration, accompanied by an increase in the fractal dimension of the percolating clusters. While lattice-based percolation models describe the behavior near the overlap concentration, they fail in the dilute regime, for which a reaction-limited particle–cluster aggregation mechanism enabled by reversible cross-links provides a better explanation of network formation. These results reveal that percolation in transient networks follows a mechanism that is fundamentally different from that in permanently cross-linked gels.

Supramolecular polymers (SPs) assembled through non-covalent interactions provide a promising platform for designing soft materials with dynamic and tunable properties. However, the viscoelastic properties of their solutions remain poorly understood, largely due to the tendency of SP chains to bundle into thick supramolecular fibers, often resulting in precipitation or gelation. In this study, we show that bundle formation can be effectively suppressed by appropriate design of the surface of SPs through alteration of their side chains. Structural analyses revealed that the resulting SPs are homogeneously solvated, permitting their rheological characterization. Notably, we found that even subtle modifications, such as positional isomerism of the side chains, can dramatically alter the physical properties of SP solutions. Our findings, therefore, highlight the critical role of side chains in governing hierarchical structures and macroscopic properties of SPs, offering a design strategy for engineering functional supramolecular materials.

2-Pyrone-4,6-dicarboxylic acid (PDC) is a lignin-derived biometabolic intermediate that serves as a versatile monomer for the development of sustainable polymers. In the present study, a bio-based polyesteramide, P-(PDC-BiOx), was successfully synthesized via the atom-economical ring-opening polyaddition of PDC and 2,2'-bis-(2-oxazoline). The thermal, crystalline, and mechanical properties of the resulting polymer were fully characterized, revealing a completely amorphous backbone with a fracture stress of 10.34 MPa. The marine biodegradability behavior was investigated under both surface and deep seawater conditions, demonstrating a clear correlation between material morphology and biodegradation rate. The experimental results demonstrated that, in surface seawater, the electrosprayed microparticles degraded more rapidly than the hot-pressed film, although both eventually reached a comparable extent of degradation. In comparison, in deep seawater, the electrosprayed microparticles achieved approximately 30% biodegradation within 45 days, which was higher than that of the hot-pressed film. Overall, PDC polyesteramides are promising biomass-derived polymers with straightforward accessibility and potential marine biodegradability.

Fracture healing is a dynamic biological process involving inflammation, activation of callus-forming skeletal progenitor cells (CaSPCs), and tissue remodeling, in which redox regulation plays a critical role. However, there is no established imaging technique that evaluates CaSPC-related redox activity at fracture sites in vivo, as conventional imaging modalities reflect structural changes rather than cellular activity. In vivo dynamic nuclear polarization magnetic resonance imaging (DNP-MRI) enables visualization of tissue redox status. In this study, we investigated the utility of DNP-MRI to visualize spatiotemporal redox changes at fracture sites using a mouse tibial fracture model. In vivo DNP-MRI with the nitroxyl radical probe carbamoyl-PROXYL (CmP) revealed stage-dependent, accelerated CmP signal decay within the fracture callus during early- and mid-phase healing. Complementary in vitro redox analyses demonstrated that CaSPCs exhibit significantly higher CmP decay rates than intact bone-derived skeletal progenitor cells (IB-SPCs). Ex vivo analyses revealed that CaSPCs exhibited higher mitochondrial ROS production than IB-SPCs. These findings indicate that redox dynamics within the fracture callus are stage-dependent and reflect intrinsic metabolic features associated with CaSPCs. In vivo DNP-MRI represents a functional imaging approach for visualizing redox metabolism during fracture healing, providing metabolic information beyond structural imaging and enabling earlier evaluation of bone regeneration.

Coacervates have emerged as promising systems for achieving dynamic compartmentalization. In particular, synthetic polymeric complex coacervates represent a well-established class. However, they often disassemble at high ionic strength condition due to charge screening effects. In contrast, simple coacervates, which form via phase separation of a single polymer species, serve as complementary systems that can exist under such conditions. Despite this advantage, the design and understanding of synthetic simple coacervates remains limited. Here, we report the thermoresponsive simple coacervation behavior of synthetic copolymers compose of lower critical solution temperature (LCST)-type and hydrophilic monomers. Through systematic investigations, we revealed that a wide variety of combinations of LCST-type monomers and hydrophilic monomers enables thermoresponsive simple coacervation in water in the presence of salts. Our findings provide a guideline for designing thermoresponsive simple coacervate systems based on synthetic LCST-type copolymers. We also highlight the importance of carefully characterizing the microphases that emerge upon phase separation of thermoresponsive copolymers under each specific usage condition.

The elasticity of Tetra-PEG chemical gel deformed by the oscillation of a laser-induced microbubble was obtained through comparison to a bubble dynamics model. An infrared nanosecond laser pulse was focused into the gel to nucleate a spherical microbubble whose subsequent free oscillation (at frequency of order ) was recorded using a high-speed camera. The evolution of the bubble radius is found to be symmetric between the growth and shrinkage phases, suggesting that the gel structure is not damaged by finite-amplitude oscillations of the bubble. The radius evolution in the first collapse phase (from the maximal to the minimal size) was compared with the Rayleigh-Plesset (RP) equation equipped with neo-Hookean and quadratic Kelvin-Voigt (qKV) constitutive models, allowing for the determination of the shear modulus under high-frequency deformation. When the maximum radius of the bubble is sufficiently small, the measured shear modulus agrees with values from previous conventional rheometer studies, indicating frequency-independent behavior of the chemical gel above a certain deformation rate threshold. On the other hand, when the maximum radius exceeds a threshold, the neo-Hookean model fails to capture the elastic response, and comparison with the RP equation with the qKV model, reveals pronounced stiffening under large deformation.

Acute synovitis and continuous mechanical stress are critical factors of Post-traumatic Osteoarthritis (PTOA); however, the effect on cartilage degeneration and its underlying mechanisms remains unclear. Here, we established a novel biomaterial-based method for synovial fluid analysis in mice to uncover the mechanisms underlying the onset of PTOA, focusing on synovitis and mechanical stress. Twelve-week-old C57BL/6J males were divided into the ACL-Transection (ACL-T), ACL-rupture (ACL-R), and Intact groups. We performed a joint instability test and histological analysis for cartilage degeneration and synovitis at 2, 6, and 10 weeks. Real-time PCR was performed on articular cartilage at 2 weeks and synovium at 2, 6, and 10 weeks. Tetra-slime was injected into the knee joint, and solidified slime containing synovial fluid molecules was analyzed in digital PCR at 2 weeks. Acute synovitis and cartilage degeneration were induced in the ACL-T group at 2 weeks, although no difference was observed in joint instability between the two ACL injury models. Real-time PCR showed a significant increase in Mmp-3, Tnf-α, Ifn-γ, and inos in the synovium of the ACL-T group. During this tissue interaction, although there were no significant differences, miR145-5p and miR149-5p in synovial fluid were also upregulated in the ACL-T group compared to the ACL-R group. Unlike the early stage, no histological or biological differences were observed between groups at 6 and 10 weeks. In conclusion, acute synovitis caused secondary cartilage degeneration via MMP-3 and TNF-α in the synovium during the early stage and may involve M1 macrophage activation. Whereas it was suggested that synovial fluid miRNA might be produced to suppress the secondary cartilage degeneration. Furthermore, mechanical stress was the dominant factor in the late stage of OA, regardless of the initial synovitis condition.

Practical applications of viral- and cell-based therapeutics require precise targeted delivery to minimize off-target effects. Conventional hydrogel-based drug delivery carriers may undergo a sol-gel phase transition upon in vivo degradation, leading to a burst release of encapsulated substances. Transient-network materials have been proposed to overcome this challenge. However, the relationship between the network structure and release mechanisms remains unclear, mainly due to the lack of control over structural heterogeneity in typical transient networks. This study aimed to elucidate the mechanism underlying the release of micrometer-scale particles from transient networks using a systematically controlled model system composed of tetra-armed polyethylene glycol (Tetra-PEG slime). The system features a well-defined structure with uniform strand lengths and consistent functionalities. Our results demonstrate that particle release is driven by the dissolution of the matrix and that the release barrier depends on the surrounding network topology. This release behavior is primarily determined by network connectivity and is independent of the polymer concentration and strand length. These insights advance our understanding of the sustainable release of microparticles from transient networks and provide broadly applicable guidelines for the development of effective drug delivery systems.

Understanding the interplay between polymer adsorption and colloidal interactions is essential for designing advanced materials with tailored properties. This study investigates the adsorption-driven aggregation and rheological transitions in semidilute mixtures of silica nanoparticles and high-molecular-weight poly(ethylene oxide) (PEO) in the protein limit, where the polymer's size exceeds that of the particles. By systematically varying the ratio of the particle hydrodynamic size to the polymer's hydrodynamic screening length (Rh,silica/ξh,PEO), distinct regimes of adsorption suppression, aggregation onset, and saturation were identified. Below Rh,silica/ξh,PEO = 1, adsorption was suppressed due to the entropic penalty of polymer distortion, resulting in negligible viscosity changes and stable particle dispersions. Near Rh,silica/ξh,PEO = 1, the adsorption energy overcame the entropy loss, triggering rapid aggregation and a sharp increase in viscosity, accompanied by the emergence of a slow relaxation mode in dynamic light scattering. At higher ratios (Rh,silica/ξh,PEO > 2), adsorption saturated, forming dense PEO-silica aggregates, as confirmed by small-angle neutron scattering. These findings challenge conventional theories of polymer adsorption and emphasize the critical role of polymer conformational entropy and adsorption energy balance. This study provides a framework for understanding polymer-mediated colloidal interactions in semidilute regimes, with implications for the rational design of polymer-colloid composites in materials science, biophysics, and industrial formulations.

Polysaccharides are valuable building blocks for constructing functional hydrogel materials for diverse applications. Although many polysaccharides with high water solubility require cross-linking to be formulated into hydrogels, physically cross-linking ionic polysaccharides with biocompatible substances under mild aqueous conditions remains a significant challenge. Herein, we report the physical cross-linking of hyaluronic acid with crystalline cello-oligosaccharides via bottom-up coassembly at the molecular level. Neutralization of alkaline mixtures of cello-oligosaccharides and hyaluronate resulted in the facile preparation of translucent hydrogels with equilibrium moduli of 1-10 Pa. Structural analyses suggested that hyaluronate formed the backbone of the gel networks, while crystalline cello-oligosaccharides acted as physical cross-linkers. In vitro cytotoxicity assays demonstrated the excellent cytocompatibility of the coassembled hydrogels. Furthermore, the cationic antibiotic polymyxin B was successfully incorporated into the hydrogels, yielding antibacterial composite hydrogels. This study provides a promising strategy for the development of advanced polysaccharide-based biomaterials through physical cross-linking with crystalline oligosaccharides.

Transient polymer networks, formed by polymer chains linked through reversible bonds, exhibit time-dependent viscoelasticity, that bridges solid-like elasticity and liquid-like flow. These materials have attracted increasing attention due to their potential for self-healing, toughness, and recyclability. Classical models, such as the Green–Tobolsky and Tanaka–Edwards theories, primarily describe stress relaxation by bond dissociation kinetics, assuming homogeneous equilibrium structures and small deformations. However, real-world applications often involve large, dynamic strains, where nonlinear viscoelasticity, finite extensibility, and spatial flow heterogeneity dominate. Conventional systems frequently suffer from structural and dynamic heterogeneities, complicating efforts to connect molecular dynamics with bulk mechanical behavior. To address these challenges, recent studies have developed a model transient network—Tetra-PEG slime—constructed from tetrafunctional polyethylene glycol (PEG) precursors connected via dynamic covalent bonds between phenylboronic acid and diol groups. This system allows for precise control over network connectivity, bond lifetime, and strand architecture, while minimizing unwanted heterogeneities. Leveraging this model, researchers have applied a multimodal approach, combining surface plasmon resonance (SPR), macroscopic rheology, two-dimensional rheo-optics, and particle-tracking microrheology to investigate relaxation behavior across linear and nonlinear regimes. These efforts have uncovered clear correlations between molecular kinetics and viscoelastic relaxation, time–strain separability under large deformations, damping mechanisms tied to strand pullout, and emergent spatial heterogeneity near the percolation threshold. This review summarizes these findings and explores their implications for the rational design of transient networks with programmable mechanical properties, while offering perspectives on future integrations with theory and simulation.

In this study, a model transient network system, Tetra-PEG slime, was employed to elucidate the influence of heterogeneous strand length distribution on nonlinear stress relaxation behavior. Transient networks, characterized by their temporary crosslinks, are notable examples of viscoelastic liquids. Despite their widespread applications, the molecular mechanisms governing their viscoelastic responses under significant deformations remain unclear. The heterogeneous distribution of network strand lengths is particularly significant; however, controlling this parameter poses a considerable challenge owing to the limited availability of suitably designed experimental materials. Therefore, this study investigated the relationship between nonlinear stress relaxation and network strand distribution using model transient networks with controlled heterogeneous strand length distributions consisting of two size-mismatched tetra-armed precursors (bimodal Tetra-PEG slime).The strain at damping significantly decreased with the increasing degree of heterogeneity, which indicated that the strain energy per network strand decreased upon the introduction of heterogeneous strand length distributions. This behavior was attributed to stress concentration, which induced heterogeneous flow at an earlier stage and thereby triggered damping more readily. These insights provide a strong framework for understanding the complex rheological properties of transient networks.

Understanding the coupling between the dynamics of microstructural deformation and the bulk flow behavior of colloidal suspensions is crucial for both fundamental studies and practical applications of this important class of soft matter systems. In this study, we investigated the flow behavior of cellulose nanofibril (CNF) suspensions─renewable, sustainable materials with low environmental impact─using the "Rheo-Iris," a rheo-polarized imaging system we developed to visualize two-dimensional microstructural changes in fluid under applied stress. Creep tests under constant shear stress revealed an initial elastic material response, a yield transition, followed by viscoplastic flow at long times. Simultaneous polarized imaging identified three distinct retardation patterns, depending on the applied shear stress. At low stresses (≤10 Pa), or small strain immediately after stress application, the phase retardation remained uniformly low, and the orientation axis of the microstructure was randomly distributed, indicating that the homogeneously dispersed CNFs form an isotropic, entangled network structure. At a stress near the yield point (40 Pa), a spiral-shaped region of high retardation appeared, and the orientation axis shifted to 60-85° away from the flow direction. This corresponds to the formation of rosary-like structures aligned in the vorticity direction, exhibiting spatially nonuniform birefringence and an oriented microstructure. At higher stresses far above the yield point (200 Pa), this "log-rolling" mesostructure collapsed, and smaller CNF aggregates became aligned in the flow direction, leading to spatially uniform oriented birefringence across the entire field. Both cases represent distinct fiber orientation phenomena, and our noninvasive rheo-polarization method clearly distinguishes how the spatial orientation distribution in the field changes with applied stress. The Rheo-Iris system enables real-time, quantitative analysis of internal microstructural evolution under imposed shear strain or stress and offers a powerful tool for exploring the orientation dynamics in soft matter systems, opening a new eye on complex fluid rheology in colloidal dispersions.

Objective Synovial fluid micro-RNAs have been investigated to clarify the Osteoarthritis (OA) development, however, analyzing them in the murine models had not been established. The purpose of this study was to develop a novel biomaterial-based method to collect murine synovial fluid and elucidate onset mechanism of OA with acute synovitis and chronic mechanical stress. Method Twelve-week-old C57BL/6J males (n = 72) were divided to the ACL-Transection (ACL-T), ACL-rupture (ACL-R), and Intact groups. We performed joint instability test and histological analysis for cartilage degeneration and synovitis at 2, 6, and 10 weeks. Real time PCR was conducted for articular cartilage at 2 weeks and synovium at 2, 6, and 10 weeks. Tetra-slime was injected into the knee joint and solidified slime including synovial fluid was analyzed in digital PCR at 2 weeks. Results No difference was observed in joint instability between both ACL injury models. Unlike the Control and ACL-R groups, acute synovitis and secondary cartilage degeneration were induced in the ACL-T group at 2 weeks. The ACL-T group also exhibited an increase of Mmp-3, Tnf-α, Ifn-γ, and inos in synovium, and miR145-5p and miR149-5p in synovial fluid compared to the ACL-R group. However, no histological and biological differences were observed at 6 and 10 weeks. Conclusion We successfully analyzing synovial fluid miRNAs in mice model using Tetra-Slime. Acute synovitis caused secondary cartilage degeneration through MMP-3, TNF-α, and M1 macrophage activation in synovium during the early stage, whereas miR145-5p and miR149-5p in synovial fluid were upregulated to prevent the progression. However, mechanical stress had a much greater impact on cartilage degeneration in the long run rather than synovitis. ### Competing Interest Statement The authors have declared no competing interest. Grant-in-Aid for JSPS Research Fellows, 22J23384 Grant-in-Aid for JSPS Challenging Research (Exploratory), 21K19724

A challenge in gel science is the construction of hydrogels with high stiffness, particularly at ultralow solid contents. In this study, we are inspired by the biosynthesis of plant cell walls. In plants, cellulose chains synthesized by enzymes crystallize in situ into nanofibers, which coassemble with cellulose-binding polysaccharides to form cell walls with remarkable mechanical properties. Here, we enzymatically synthesize low-molecular-weight (LMW) cellulose in vitro in the presence of cellulose-binding polysaccharides. Various cellulose-binding polysaccharides facilitate the formation of hydrogels, with carboxymethyl cellulose (CMC) yielding the stiffest hydrogels. Remarkably, the hydrogels formed with 0.5 It is difficult to construct hydrogels with high stiffness at ultralow solid content. Here, low-molecular-weight cellulose was synthesized in the presence of cellulose-binding polysaccharides to construct stiff hydrogels that exhibit Young’s modulus of 386 kPa at a low solid content of 1.34

We investigated the rheological behavior of mixtures of microgel particle (Hereinafter referred to as MGP) dispersions and aqueous solutions of hydrophobically modified ethoxylated urethane (Hereinafter referred to as HEUR), which are high-performance polymeric thickeners. Despite their intrinsic immiscibility and phase separation, the combination of MGP and HEUR produced a synergistic increase in the complex modulus and notable changes in relaxation behavior. Dynamic viscoelastic and rheo-optical (dynamic birefringence) measurements revealed that the addition of a small amount of HEUR to MGP dispersions significantly increased the modulus values and shifted the maximum relaxation frequency to lower values. The application of the modified stress optical rule (MSOR) enabled separation of the elastic moduli of the MGP and HEUR components, confirming that HEUR preferentially localizes between MGPs, counterbalances MGP-specific concentration fluctuations and induces cooperative relaxation in the MGP phase. Micromechanical modeling, combined with a parallel model incorporating connected HEUR domains, reproduced the slow HEUR relaxation and MGP deformation behavior, highlighting stress coupling between the two phases as the origin of the retarded relaxation—without requiring specific interactions such as chain entanglement. This study reports that combined rheological and rheo-optical analysis, supported by micromechanical modeling, provides insight into the viscoelastic behavior and structural organization of immiscible thickener mixtures, with implications for designing high-performance formulations such as cosmetics. The rheological behavior of immiscible mixtures of microgel particle (MGP) dispersions and hydrophobically modified ethoxylated urethanes (HEUR) were investigated. Despite phase separation, mixing MGP and HEUR synergistically enhanced the modulus and altered relaxation behavior. Rheo-optical analysis revealed that HEUR localizes between MGPs, counterbalancing concentration fluctuations and inducing cooperative relaxation. Micromechanical modeling reproduced the observed behavior, highlighting stress coupling between phases as the key mechanism. This study provides insights into the viscoelastic properties of thickener mixtures, offering guidance for designing advanced formulations like cosmetics.

The stress relaxation test is an effective and facile method for clarifying the nonlinear rheological behavior of soft materials. A thorough analysis of the stress relaxation behavior offers valuable insights into the molecular dynamics. However, the stress relaxation behavior and underlying molecular dynamics of polymer/particle mixtures remain poorly understood, despite their widespread industrial application. In this study, we systematically investigated the nonlinear stress relaxation behavior of a simple-structured poly(ethylene oxide) (PEO)/silica nanoparticle aqueous mixture. Time‒strain separability was observed at high polymer concentrations, with the stress relaxation attributable to the relaxation of the polymer matrix. At lower polymer concentrations, the time‒strain separability was no longer valid, and changes in absorbance over time suggested that stress relaxation originated from the relaxation of the aggregated structures. A transition from time‒strain separability to inseparability was observed when the estimated number of PEO molecules forming interparticle bridges was less than 1; this suggests that structural changes during shear loading occur only when new interparticle polymer bridges are formed, leading to the development of clustered structures. These results provide a basic understanding of the relationship between deformation and relaxation, which is crucial for systematically understanding the nonlinear rheology of polymeric materials. Normalized relaxation time as a function of the estimated number of interparticle polymer bridges (nbridge). Images show the molecular dynamics pertaining to nbridge < 1 and 1 < nbridge.