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Evolution of composition, rheology, and morphology during phase separation in complex fluids is highly coupled to rheological and mass transport processes within the emerging phases, and understanding this coupling is critical for materials design of multiphase complex fluids. Characterizing these dependencies typically requires careful measurement of a large number of equilibrium and transport properties that are difficult to measure

Recent far-reaching advances in synthetic biology have yielded exciting tools for the creation of new materials. Conversely, advances in the fundamental understanding of soft-condensed matter, polymers, and biomaterials offer new avenues to extend the reach of synthetic biology. The broad and exciting range of possible applications have significant implications to address grand challenges in health, biotechnology, and sustainability. Despite the potentially transformative impact that lies at the interface of synthetic biology and biomaterials, the two fields have to date progressed mostly separately. This perspective article provides a review of recent key advances in these two fields, and a roadmap for collaboration at the interface between these two communities. We highlight the near-term applications of this interface to the development of hierarchically structured biomaterials, from bioinspired building blocks to “living” materials that sense and respond based on the reciprocal interactions between materials and embedded cells.

An important but often overlooked feature of Diels-Alder (DA) cycloadditions is the ability for DA adducts to undergo mechanically induced cycloreversion when placed under force. Herein, we demonstrate that the commonly employed DA cycloaddition between furan and maleimide to crosslink hydrogels results in slow gelation kinetics and "mechanolabile" crosslinks that relate to reduced material strength. Through rational computational design, "mechanoresistant" DA adducts were identified by constrained geometries simulate external force models and employed to enhance failure strength of crosslinked hydrogels. Additionally, utilization of a cyclopentadiene derivative, spiro[2.4]hepta-4,6-diene, provided mechanoresistant DA adducts and rapid gelation in minutes at room temperature. This study illustrates that strategic molecular-level design of DA crosslinks can provide biocompatible materials with improved processing, mechanical durability, lifetime, and utility.

Bioinspired iron-catechol cross-links have shown remarkable success in increasing the mechanical properties of polymer networks, in part due to clustering of Fe3+-catechol domains which act as secondary network reinforcing sites. We report a versatile synthetic procedure to prepare modular PEG-acrylate networks with independently tunable covalent bis(acrylate) and supramolecular Fe3+-catechol cross-linking. Initial control of network structure is achieved through radical polymerization and cross-linking, followed by postpolymerization incorporation of catechol units via quantitative active ester chemistry and subsequent complexation with iron salts. By tuning the ratio of each building block, dual cross-linked networks reinforced by clustered iron-catechol domains are prepared and exhibit a wide range of properties (Young's moduli up to ∼245 MPa), well beyond the values achieved through purely covalent cross-linking. This stepwise approach to mixed covalent and metal-ligand cross-linked networks also permits local patterning of PEG-based films through masking techniques forming distinct hard, soft, and gradient regions.

The most widely-used representation of the compressible, isotropic, neo-Hookean hyperelastic model is considered in this paper. The version under investigation is that which is implemented in the commercial finite element software ABAQUS, ANSYS and COMSOL. Transverse stretch solutions are obtained for the following homogeneous deformations: uniaxial loading, equibiaxial loading in plane stress, and uniaxial loading in plane strain. The ground-state Poisson’s ratio is used to parameterize the constitutive model, and stress solutions are computed numerically for the physically permitted range of its values. Despite its broad application to a number of engineering problems, the physical limitations of the model, particularly in the small to moderate stretch regimes, are not explored. In this work, we describe and analyze results and make some critical observations, underlining the model’s advantages and limitations. For example, a snap-back feature of the transverse stretch is identified in uniaxial compression, a physically undesirable behavior unless validated by experimental data. The domain of this non-unique solution is determined in terms of the ground-state Poisson’s ratio and the state of stretch and stress. The analyses we perform are essential to enable the understanding of the characteristics of the standard, compressible, isotropic, neo-Hookean model used in ABAQUS, ANSYS and COMSOL. In addition, our results provide a framework for the parameter-fitting procedure needed to characterize this standard, compressible, isotropic neo-Hookean model in terms of experimental data.

Marine mussels have the remarkable ability to adhere to a variety of natural and artificial surfaces under hostile environmental conditions. Although the molecular composition of mussel adhesives has been well studied, a mechanistic understanding of the physical origins of mussels' impressive adhesive strength remains elusive. Here, we investigated the role of substrate geometry in the adhesive performance of mussels. Experimentally, we created substrates with differing surface properties using 3D printing and laser drilling and introduced these to mussels, which in turn adhered to the engineered surfaces via plaque-thread byssal structures. Tensile testing with in situ imaging was conducted to quantify the adhesion strength of the mussel plaques, and the microstructures of the mechanically deformed plaques were characterized using scanning electron microscopy. Our results reveal that the geometry of the surfaces has no significant impact on the detachment force and the strain, whereas the change in adhesion area leads to a different adhesion stress. Ultrastructural analysis confirms the expected presence of an open-cell foamy network coated with the cuticle. The observed detachment dynamics and failure mechanisms do vary depending on the substrate properties, suggesting the presence of substrate-dependent nonuniform stress distributions at the interface. Together, these results show mussels' remarkable ability to adapt to differing physical conditions and demonstrate the importance of the on-demand and in situ manufacturing of the stiff cuticle and relatively compliant adhesive interlayer. The resultant composite structure avoids the formation of prestress during the formation of the adhesive joint, provides conformability to the surface, and helps compensate for local bending interactions to maintain adhesive strength. Our findings suggest forward design strategies to improve adhesive performance on complex surfaces.

Surface-aligned liquid-crystal networks (LCNs) offer a solution for developing functional materials capable of performing a range of tasks, including actuation, shape memory, and surfaces patterning. Here we show that Diels-Alder cycloaddition can be used to prepare the backbone of planar aligned LCNs under mild ambient conditions without the addition of additives or UV irradiation. The mechanical properties of the networks have robust viscoelastic modulus and stiffness with a reversible local free volume change upon physical aging. This study shows new opportunities to design surface-aligned LCNs based on additive free step-growth Diels-Alder polymerization and enables the potential to incorporate a wider range of photochromic materials into LCNs.

Here we present a new, compact magnetic tweezers design that enables precise application of a wide range of dynamic forces to soft materials without the need to raise or lower the magnet height above the sample. This is achieved through the controlled rotation of the permanent magnet array with respect to the fixed symmetry axis defined by a custom-built iron yoke. These design improvements increase the portability of the device and can be implemented within existing microscope setups without the need for extensive modification of the sample holders or light path. This device is particularly well-suited to active microrheology measurements using either creep analysis, in which a step force is applied to a micron-sized magnetic particle that is embedded in a complex fluid, or oscillatory microrheology, in which the particle is driven with a periodic waveform of controlled amplitude and frequency. In both cases, the motions of the particle are measured and analyzed to determine the local dynamic mechanical properties of the material.

Photosurfactants have shown considerable promise for enabling stimuli-responsive control of the properties and motion of fluid interfaces. Recently, a number of photoswitch chemistries have emerged to tailor the photoresponsive properties of photosurfactants. However, systematic studies investigating how photoresponsive surfactant behavior depends on the photochemical and photophysical properties of the switch remain scarce. In this work, we develop synthetic schemes and surfactant designs to produce a well-controlled library of photosurfactants to comparatively assess the behavior of photoswitch chemistry on interfacial behavior. We employ photoinduced spreading of droplets at fluid interfaces as a model for such studies. We show that although photosurfactant response is largely guided by expected trends with changes in polarity of the photoswitch, interfacial behavior also depends nontrivially and sometimes counter-intuitively on the kinetics and mechanisms of photoswitching, particularly at the interface of two solvents, as well as on complex interactions with other surfactants. Understanding these complexities enables the design of new photosurfactant systems and their optimization toward responsive functions including triggered spreading, dewetting, and destabilization of droplets on solid and fluid surfaces.

Strong and well-engineered interfaces between dissimilar materials are a hallmark of natural systems but have proven difficult to emulate in synthetic materials, where interfaces often act as points of failure. In this work, curing reactions that are triggered by exposure to different wavelengths of visible light are used to produce multimaterial objects with tough, well-defined interfaces between chemically distinct domains. Longer-wavelength (green) light selectively initiates acrylate-based radical polymerization, while shorter-wavelength (blue) light results in the simultaneous formation of epoxy and acrylate networks through orthogonal cationic and radical processes. The improved mechanical strength of these interfaces is hypothesized to arise from a continuous acrylate network that bridges domains. Using printed test structures, interfaces were characterized through spatial resolution of their chemical composition, localized mechanical properties, and bulk fracture strength. This wavelength-selective photocuring of interpenetrating polymer networks is a promising strategy for increasing the mechanical performance of 3D-printed objects and expanding light-based additive manufacturing technologies.

3D printing holds potential as a faster, cheaper alternative compared with traditional photolithography for the fabrication of microfluidic devices by replica molding. However, the influence of printing resolution and quality on device design and performance has yet to receive detailed study. Here, we investigate the use of 3D-printed molds to create staggered herringbone mixers (SHMs) with feature sizes ranging from ∼100 to 500 μm. We provide guidelines for printer calibration to ensure accurate printing at these length scales and quantify the impacts of print variability on SHM performance. We show that SHMs produced by 3D printing generate well-mixed output streams across devices with variable heights and defects, demonstrating that 3D printing is suitable and advantageous for low-cost, high-throughput SHM manufacturing.

We report a visible light-responsive bilayer actuator driven by the photothermal properties of a unique molecular photoswitch: donor-acceptor Stenhouse adduct (DASA). We demonstrate a synthetic platform to chemically conjugate DASA to a load-bearing poly(hexyl methacrylate) (PHMA) matrix via Diels-Alder click chemistry that enables access to stimuli-responsive materials on scale. By taking advantage of the negative photochromism and switching kinetics of DASA, we can tune the thermal expansion and actuation performance of DASA-PHMA under constant light intensity. This extends the capabilities of currently available responsive soft actuators for which mechanical response is determined exclusively by light intensity and enables the use of abundant broadband light sources to trigger tunable responses. We demonstrate actuation performance using a visible light-powered cantilever capable of lifting weight against gravity as well as a simple crawler. These results add a new strategy to the toolbox of tunable photothermal actuation by using the molecular photoswitch DASA.

The electromagnetically actuated MEMS $\mu $ Hammer was used to evaluate the effects of mechanical impact on the membrane permeability, apoptotic induction, and proliferation of human neural progenitor cells. The $\mu $ Hammer enabled application of two strain magnitudes ( $\varepsilon = 42{\%}$ and 69%) for two strain durations ( $10~\mu \text{s}$ and $100~\mu \text{s}$ ) to individual cells at unprecedented high strain rates ( $\dot {\varepsilon }\mathrm { } \boldsymbol {\sim } {200\times }10^{3}\,\,\text{s}^{-1}$ ) and high throughput (up to 36,000 cells/min). This enables large numbers of cells to be analyzed and the effects of strain magnitude and duration to be decoupled. The magnitude of applied strain significantly affected cell membrane permeability shortly after compression, whereas the duration of strain significantly increased early apoptosis in cells 24 hours after compression. Strain magnitude also significantly affected cell quantity over a 70-hour period, despite no significant difference in the cell doubling times. Understanding the relationship between mechanical strain and cellular response will ultimately lead to improved diagnostics and treatment of high strain rate mechanical injury conditions such as Traumatic Brain Injury. [2020-0171]

Epithelial tubules form critical structures in lung, kidney, and vascular tissues. However, the processes that control their morphogenesis and physiological expansion and contraction are not well understood. Here we examine the dynamic remodeling of epithelial tubes in vivo using a novel model system: the extracorporeal vasculature of Botryllus schlosseri, in which the disruption of the basement membrane triggers rapid, massive vascular retraction without loss of barrier function. We developed and implemented 3-D image analysis and virtual reconstruction tools to characterize the cellular morphology of the vascular wall in unmanipulated vessels and during retraction. In both control and regressed conditions, cells within the vascular wall were planar polarized, with an integrin- and curvature-dependent axial elongation of cells and a robust circumferential alignment of actin bundles. Surprisingly, we found no measurable differences in morphology between normal and retracting vessels under extracellular matrix (ECM) disruption. However, inhibition of integrin signaling through focal adhesion kinase inhibition caused disruption of cellular actin organization. Our results demonstrate that epithelial tubes can maintain tissue organization even during extreme remodeling events, but that the robust response to mechanical signals—such as the response to loss of vascular tension after ECM disruption—requires functional force sensing machinery via integrin signaling.

Multimaterial additive manufacturing is an enabling tool for exploring difficult to access structure-property relationships. In this work, a recently developed multimaterial printing approach, solution mask liquid lithography, is used to produce porous polymer-polymer composites inspired by tough, hierarchical structures found in nature. The results demonstrate that varying the size and packing of pores in the core structure leads to significant enhancement in crack deflection. Finite element analysis reveals that this enhancement is linked to geometry-dependent stress distribution.

We investigated the morphology, topology, and mechanical characteristics of a loosely cross-linked epoxy network as a function of the varying content of catechol moieties capable of forming reversi...

We designed and fabricated, using low-cost 3D printing technologies, a device that enables direct control of cell density in epithelial monolayers. The device operates by varying the tension of a silicone substrate upon which the cells are adhered. Multiple devices can be manufactured easily and placed in any standard incubator. This allows long-term culturing of cells on pretensioned substrates until the user decreases the tension, thereby inducing compressive forces in plane and subsequent instantaneous cell crowding. Moreover, the low-profile device is completely portable and can be mounted directly onto an inverted optical microscope. This enables visualization of the morphology and dynamics of living cells in stretched or compressed conditions using a wide range of high-resolution microscopy techniques.

Although high strain and strain-rate impacts to the human body have been the subject of substantial research at both the systemic and tissue levels, little is known about the cell-level ramifications of such assaults. This is largely due to the lack of high throughput, dynamic compression devices capable of simulating such traumatic loading conditions on individual cells. To fill this gap, we developed and characterized a high speed, high actuation force, magnetically driven MEMS chip to apply stress to biological cells at unprecedented strain (10% to 90%), strain rate (30,000 to 200,000 s−1), and throughput (12,000 cells/min). To demonstrate the capabilities of the $\mu $ Hammer, we applied biologically relevant strains and strain rates to human leukemic K562 cells and then monitored their viability for up to 8 days. We observed significantly repressed proliferation of the hit cells compared to both unperturbed and sham-hit control cells, accompanied by minimal cell death. This indicates success in applying cellular damage without compromising the overall viability of the population, allowing us to conclude that this device is well suited to study the subtle effects of impact on large populations of inherently heterogeneous cells. [2019-0132]

We present a new approach to measuring cell-generated forces from the deformations of elastic microspheres embedded within multicellular aggregates. By directly fitting the measured sensor deformation to an analytical model based on experimental observations and invoking linear elasticity, we dramatically reduce the computational complexity of the problem, and directly obtain the full 3D mapping of surface stresses. Our approach imparts extraordinary computational efficiency, allowing tractions to be estimated within minutes and enabling rapid analysis of microsphere-based traction force microscopy data.

Background Quantification of individual species in microbial co-cultures and consortia is critical to understanding and designing communities with prescribed functions. However, it is difficult to physically separate species or measure species-specific attributes in most multi-species systems. Anaerobic gut fungi (AGF) (Neocallimastigomycetes) are native to the rumen of large herbivores, where they exist as minority members among a wealth of prokaryotes. AGF have significant biotechnological potential owing to their diverse repertoire of potent lignocellulose-degrading carbohydrate-active enzymes (CAZymes), which indirectly bolsters activity of other rumen microbes through metabolic exchange. While decades of literature suggest that polysaccharide degradation and AGF growth are accelerated in co-culture with prokaryotes, particularly methanogens, methods have not been available to measure concentrations of individual species in co-culture. New methods to disentangle the contributions of AGF and rumen prokaryotes are sorely needed to calculate AGF growth rates and metabolic fluxes to prove this hypothesis and understand its causality for predictable co-culture design. Results We present a simple, microplate-based method to measure AGF and methanogen concentrations in co-culture based on fluorescence and absorbance spectroscopies. Using samples of < 2% of the co-culture volume, we demonstrate significant increases in AGF growth rate and xylan and glucose degradation rates in co-culture with methanogens relative to mono-culture. Further, we calculate significant differences in AGF metabolic fluxes in co-culture relative to mono-culture, namely increased flux through the energy-generating hydrogenosome organelle. While calculated fluxes highlight uncertainties in AGF primary metabolism that preclude definitive explanations for this shift, our method will enable steady-state fluxomic experiments to probe AGF metabolism in greater detail. Conclusions The method we present to measure AGF and methanogen concentrations enables direct growth measurements and calculation of metabolic fluxes in co-culture. These metrics are critical to develop a quantitative understanding of interwoven rumen metabolism, as well as the impact of co-culture on polysaccharide degradation and metabolite production. The framework presented here can inspire new methods to probe systems beyond AGF and methanogens. Simple modifications to the method will likely extend its utility to co-cultures with more than two organisms or those grown on solid substrates to facilitate the design and deployment of microbial communities for bioproduction and beyond.

Differential dynamic microscopy (DDM) is a form of video image analysis that combines the sensitivity of scattering and the direct visualization benefits of microscopy. DDM is broadly useful in determining dynamical properties including the intermediate scattering function for many spatiotemporally correlated systems. Despite its straightforward analysis, DDM has not been fully adopted as a routine characterization tool, largely due to computational cost and lack of algorithmic robustness. We present statistical analysis that quantifies the noise, reduces the computational order and enhances the robustness of DDM analysis. We propagate the image noise through the Fourier analysis, which allows us to comprehensively study the bias in different estimators of model parameters, and we derive a different way to detect whether the bias is negligible. Furthermore, through use of Gaussian process regression (GPR), we find that predictive samples of the image structure function require only around 0.5%-5% of the Fourier transforms of the observed quantities. This vastly reduces computational cost, while preserving information of the quantities of interest, such as quantiles of the image scattering function, for subsequent analysis. The approach, which we call DDM with uncertainty quantification (DDM-UQ), is validated using both simulations and experiments with respect to accuracy and computational efficiency, as compared with conventional DDM and multiple particle tracking. Overall, we propose that DDM-UQ lays the foundation for important new applications of DDM, as well as to high-throughput characterization. We implement the fast computation tool in a new, publicly available MATLAB software package., Published in Physical Review E. 24 pages, 12 figures. Typos in Section 2B are corrected

Anaerobic gut fungi (AGF) are lignocellulose degraders that naturally form biofilms in the rumen of large herbivores and in standard culture techniques. While biofilm formation enhances biomass degradation and carbohydrate-active enzyme (CAZyme) production in some bacteria and aerobic fungi, gene expression and metabolism in AGF biofilms have not been compared to non-biofilm cultures. Here, using the tunable morphology of the non-rhizoidal AGF, Caecomyces churrovis, the impacts of biofilm formation on AGF gene expression, metabolic flux, growth rate, and xylan degradation rate are quantified to inform future industrial scale-up efforts. Contrary to previous findings, C. churrovis upregulated catabolic CAZymes in stirred culture relative to biofilm culture. Using a de novo transcriptome, 197 new transcripts with predicted CAZyme function were identified. Stirred cultures grew and degraded xylan significantly faster than biofilm-forming cultures with negligible differences in primary metabolic flux, offering a way to accelerate AGF biomass valorization without altering the fermentation product profile. The rhizoidal AGF, Neocallimastix lanati, also grew faster with stirring on a solid plant substrate, suggesting that the advantages of stirred C. churrovis cultures may apply broadly to other AGF.

Experimental evidence suggests that suction may play a role in the attachment strength of mushroom-tipped adhesive structures, but the system parameters which control this effect are not well established. A fracture mechanics-based model is introduced to determine the critical stress for defect propagation at the interface in the presence of trapped air. These results are compared with an experimental investigation of millimeter-scale elastomeric structures. These structures are found to exhibit a greater increase in strength due to suction than is typical in the literature, as they have a large tip diameter relative to the stalk. The model additionally provides insight into differences in expected behavior across the design space of mushroom-shaped structures. For example, the model reveals that the suction contribution is length-scale dependent. It is enhanced for larger structures due to increased volume change, and thus the attainment of lower pressures, inside of the defect. This scaling effect is shown to be less pronounced if the tip is made wider relative to the stalk. An asymptotic result is also provided in the limit that the defect is far outside of the stalk, showing that the critical stress is lower by a factor of 1/2 than the result often used in the literature to estimate the effect of suction. This discrepancy arises as the latter considers only the balance of remote stress and pressure inside the defect and neglects the influence of compressive tractions outside of the defect.

Marine mussel plaques are an exceptional model for wet adhesives. Despite advances in understanding their protein composition and strategies for molecular bonding, the process by which these soluble proteins are rapidly processed into load-bearing structures remains poorly understood. Here, we examine the effects of seawater pH on the time evolution of the internal microstructures in plaques harvested from Mytilus californianus. Experimentally, plaques deposited by mussels on glass and acrylic surfaces were collected immediately after foot retraction without plaque separation from the surface, placed into pH-adjusted artificial seawater for varying times, and characterized using scanning electron microscopy and tensile testing. We found a pH dependent transition from a liquid-like state to a porous solid within 30 min for pH ≥ 6.7; these plaques are load-bearing. By contrast, samples maintained at pH 3.0 showed no porosity and no measurable strength. Interestingly, we found cuticle development within 15 min regardless of pH, suggesting that cuticle formation occurs prior to pore assembly. Our results suggest that sea water infusion after deposition by and disengagement of the foot is critical to the rapid formation of internal structures, which in turn plays an important role in the plaques' mechanical performance.

Although microfluidic micro-electromechanical systems (MEMS) are well suited to investigate the effects of mechanical force on large populations of cells, their high-throughput capabilities cannot be fully leveraged without optimizing the experimental conditions of the fluid and particles flowing through them. Parameters such as flow velocity and particle size are known to affect the trajectories of particles in microfluidic systems and have been studied extensively, but the effects of temperature and buffer viscosity are not as well understood. In this paper, we explored the effects of these parameters on the timing of our own cell-impact device, the μHammer, by first tracking the velocity of polystyrene beads through the device and then visualizing the impact of these beads. Through these assays, we find that the timing of our device is sensitive to changes in the ratio of inertial forces to viscous forces that particles experience while traveling through the device. This sensitivity provides a set of parameters that can serve as a robust framework for optimizing device performance under various experimental conditions, without requiring extensive geometric redesigns. Using these tools, we were able to achieve an effective throughput over 360 beads/s with our device, demonstrating the potential of this framework to improve the consistency of microfluidic systems that rely on precise particle trajectories and timing.

We identify unique features of a highly-absorbing negatively photochromic molecular switch, donor acceptor Stenhouse adduct (DASA), that enable its use for self-regulating light-activated control of fluid flow. Leveraging features of DASA’s chemical properties and solvent-dependent reaction kinetics, we demonstrate its use for photo-controlled Rayleigh-Bénard convection to generate dynamic, self-regulating flows with unparalleled fluid velocities (~mm s−1) simply by illuminating the fluid with visible light. The exceptional absorbance of DASAs in solution, uniquely controllable reaction kinetics and resulting spatially-confined photothermal flows demonstrate the ways in which photoswitches present exciting opportunities for their use in optofluidics applications requiring tunable flow behavior., Autonomous control of liquid motion is vital to the development of new actuators and pumps in fluid systems but autonomous control of fluid motion is inaccessible in current systems. Here, the authors identify unique features of a photochromic molecular switch that enables its use for self-regulating light activated control of fluid flow.

Hydrogels are hydrophilic polymer networks that swell upon submersion in water. Thanks to their bio-compatibility, compliance, and ability to undergo large deformations, hydrogels can be used in a wide variety of applications such as in situ sensors for measuring cell-generated forces and drug delivery vehicles. In this work we investigate the equilibrium mechanical responses that can be achieved with hydrogel-based shells filled with a liquid core. Two types of gel shell geometries are considered - a cylinder and a spherical shell. Each shell is filled with either water or oil and subjected to compressive loading. We illustrate the influence of the shell geometry and the core composition on the mechanical response of the structure. We find that all core–shell structures stiffen under increasing compressive loading due to the load-induced expulsion of water molecules from the hydrogel shell. Furthermore, we show that cylindrical core–shell configurations are stiffer then their spherical equivalents. Interestingly, we demonstrate that the compression of a core–shell structure with an aqueous core leads to the transportation of water molecules from the core into the hydrogel. These results will guide the design of novel core–shell structures with tunable properties and mechanical responses.

Combining stiffness and stretchiness There is usually a trade-off between making a material stretchy, so that it can absorb energy on deformation, and making a material stiff, so that it does not extend very much when stretched. Mussels have long been an inspiration for developing adhesives that work when wet. Filippidi et al. produced an extensible polymeric material containing catechol groups whose mechanical properties were augmented when dry through the addition of iron ions (see the Perspective by Winey). The iron ions lead to sacrificial metal coordination bonds, creating a reversible load-bearing network that does not trade extensibility for stiffness. Science , this issue p. 502 ; see also p. 449

We report here that a dense liquid formed by spontaneous condensation, also known as simple coacervation, of a single mussel foot protein-3S-mimicking peptide exhibits properties critical for underwater adhesion. A structurally homogeneous coacervate is deposited on underwater surfaces as micrometer-thick layers, and, after compression, displays orders of magnitude higher underwater adhesion at 2 N m-1 than that reported from thin films of the most adhesive mussel-foot-derived peptides or their synthetic mimics. The increase in adhesion efficiency does not require nor rely on post-deposition curing or chemical processing, but rather represents an intrinsic physical property of the single-component coacervate. Its wet adhesive and rheological properties correlate with significant dehydration, tight peptide packing and restriction in peptide mobility. We suggest that such dense coacervate liquids represent an essential adaptation for the initial priming stages of mussel adhesive deposition, and provide a hitherto untapped design principle for synthetic underwater adhesives.

Remodeling of the extracellular matrix plays an important role in vascular homeostasis in the basal chordate Botryllus schlosseri, whose large, transparent, extracorporeal vascular network makes it an ideal system for studies of vascular mechanotransduction., We investigated the physical role of the extracellular matrix (ECM) in vascular homeostasis in the basal chordate Botryllus schlosseri, which has a large, transparent, extracorporeal vascular network encompassing an area >100 cm2. We found that the collagen cross-linking enzyme lysyl oxidase is expressed in all vascular cells and that in vivo inhibition using β-aminopropionitrile (BAPN) caused a rapid, global regression of the entire network, with some vessels regressing >10 mm within 16 h. BAPN treatment changed the ultrastructure of collagen fibers in the vessel basement membrane, and the kinetics of regression were dose dependent. Pharmacological inhibition of both focal adhesion kinase (FAK) and Raf also induced regression, and levels of phosphorylated FAK in vascular cells decreased during BAPN treatment and FAK inhibition but not Raf inhibition, suggesting that physical changes in the vessel ECM are detected via canonical integrin signaling pathways. Regression is driven by apoptosis and extrusion of cells through the basal lamina, which are then engulfed by blood-borne phagocytes. Extrusion and regression occurred in a coordinated manner that maintained vessel integrity, with no loss of barrier function. This suggests the presence of regulatory mechanisms linking physical changes to a homeostatic, tissue-level response.

Using single-molecule fluorescence microscopy, we investigated the dynamics of dye-labeled EB1, a +TIP microtubule binding protein. To promote EB1 binding along the entire microtubule length, we formed microtubules using the nonhydrolyzable GTP analogs GMPCPP and GTPγS. Through precise tracking of the motions of individual dye-labeled proteins, we found EB1 to be highly dynamic and continuously diffusive while bound to a microtubule, with a diffusion coefficient and characteristic binding lifetime that were sensitive to both the choice of GTP analog and the buffer ionic strength. Using fluorescence-based equilibrium binding measurements, we found EB1 binding to be cooperative and also sensitive to GTP analog and ionic strength. By tracking the motion of a small number of individually-labeled EB1 proteins within a bath of unlabeled EB1 proteins, we determined the effects of increasing the total EB1 concentration on binding and dynamics. We found that the diffusion coefficient decreased with increasing EB1 concentration, which may be due at least in part, to the cooperativity of EB1 binding. Our results may have important consequences for the assembly and organization of the growing microtubule plus-end. © 2015 Wiley Periodicals, Inc.

In this article, we will summarize recent progress in understanding the mechanical origins of rigidity, strength, resiliency and stress transmission in the MT cytoskeleton using reconstituted networks formed from purified components. We focus on the role of network architecture, crosslinker compliance and dynamics, and molecular determinants of single filament elasticity, while highlighting open questions and future directions for this work.

The proteinaceous byssal plaque-thread structures created by marine mussels exhibit extraordinary load-bearing capability. Although the nanoscopic protein interactions that support interfacial adhesion are increasingly understood, major mechanistic questions about how mussel plaques maintain toughness on supramolecular scales remain unanswered. This study explores the mechanical properties of whole mussel plaques subjected to repetitive loading cycles, with varied recovery times. Mechanical measurements were complemented with scanning electron microscopy to investigate strain-induced structural changes after yield. Multicyclic loading of plaques decreases their low-strain stiffness and introduces irreversible, strain-dependent plastic damage within the plaque microstructure. However, strain history does not compromise critical strength or maximum extension compared with plaques monotonically loaded to failure. These results suggest that a multiplicity of force transfer mechanisms between the thread and plaque-substrate interface allow the plaque-thread structure to accommodate a wide range of extensions as it continues to bear load. This improved understanding of the mussel system at micron-to-millimeter lengthscales offers strategies for including similar fail-safe mechanisms in the design of soft, tough and resilient synthetic structures.

Using the nonhydrolyzable GTP analog GMPCPP and the slowly hydrolyzable GTPγS, we polymerize microtubules that recapitulate the end binding behavior of the plus end interacting protein (+TIP) EB1 along their entire length, and use these to investigate the impact of EB1 binding on microtubule mechanics. To measure the stiffness of single filaments, we use a spectral analysis method to determine the ensemble of shapes adopted by a freely diffusing, fluorescently labeled microtubule. We find that the presence of EB1 can stiffen microtubules in a manner that depends on the hydrolysis state of the tubulin-bound nucleotide, as well as the presence of the small-molecule stabilizer paclitaxel. We find that the magnitude of the EB1-induced stiffening is not proportional to the EB1-microtubule binding affinity, suggesting that the stiffening effect does not arise purely from an increase in the total amount of bound EB1. Additionally, we find that EB1 binds cooperatively to microtubules in manner that depends on tubulin-bound nucleotide state. © 2014 Wiley Periodicals, Inc.

We tested the hypothesis that mutant tau proteins that cause neurodegeneration and dementia differentially alter kinesin translocation along microtubules (MTs) relative to normal tau in vitro. We employed complementary in vitro motility assays using purified recombinant kinesin, purified recombinant tau, and purified bovine brain α:β tubulin to isolate interactions among these components without any contribution by cellular regulatory mechanisms. We found that kinesin translocates slower along MTs assembled by any of three independent tau mutants (4-repeat P301L tau, 4-repeat ΔN296 tau, and 4-repeat R406W tau) relative to its translocation rate along MTs assembled by normal, 4-repeat wild type (WT) tau. Moreover, the R406W mutation exhibited isoform specific effects; while kinesin translocation along 4-repeat R406W tau assembled MTs is slower than along MTs assembled by 4-repeat WT tau, the R406W mutation had no effect in the 3-repeat tau context. These data provide strong support for the notion that aberrant modulation of kinesin translocation is a component of tau-mediated neuronal cell death and dementia. Finally, we showed that assembling MTs with taxol before coating them with mutant tau obscured effects of the mutant tau that were readily apparent using more physiologically relevant MTs assembled with tau alone, raising important issues regarding the use of taxol as an experimental reagent and novel insights into therapeutic mechanisms of taxol action.

The byssi of sessile mussels have the extraordinary ability to adhere to various surfaces and withstand static and dynamic loadings arising from hostile environmental conditions. Many investigations aimed at understanding the unique properties of byssal thread–plaque structures have been conducted and have inspired the enhancement of fibre coatings and adhesives. However, a systems-level analysis of the mechanical performance of the composite materials is lacking. In this work, we discuss the anatomy of the byssus and the function of each of the three components (the proximal thread portion, the distal thread portion and the adhesive plaque) of its structures. We introduce a basic nonlinear system of springs that describes the contribution of each component to the overall mechanical response and use this model to approximate the elastic modulus of the distal thread portion as well as the plaque, the response of which cannot be isolated through experiment alone. We conclude with a discussion of unresolved questions, highlighting areas of opportunity where additional experimental and theoretical work is needed. This article is part of the theme issue ‘Transdisciplinary approaches to the study of adhesion and adhesives in biological systems’.

Summary We propose a novel and algorithmically simple Hough transform method that exploits the geometric properties of ellipses to enable the robust determination of the ellipse position and properties. We make use of the unique features of the evolute created by Hough voting along the gradient vectors of a two-dimensional image to determine the ellipse centre, orientation and aspect ratio. A second one-dimensional voting is performed on the minor axis to uniquely determine the ellipse size. This reduction of search space substantially simplifies the algorithmic complexity. To demonstrate the accuracy of our method, we present analysis of single and multiple ellipsoidal particles, including polydisperse and imperfect ellipsoids, in both simulated images and electron micrographs. Given its mathematical simplicity, ease of implementation and reasonable algorithmic completion time, we anticipate that the proposed method will be broadly useful for image processing of ellipsoidal particles, including their detection and tracking for studies of colloidal suspensions, and for applications to drug delivery and microrheology.

The attachment of marine mussels to each other, to rocks, as well as to a variety of pristine and bio-fouled surfaces (glass, metals or plastics) while under seawater is impressive. The attachment is accomplished via a wide adhesive plaque interpenetrated by the collagen-rich fibers of a thin long thread that connects the plaque at the distal end to the mussel at the proximal end. While the robust and versatile adhesion of the plaque has been attributed to the molecular adhesive properties, we propose that microscopic geometry, interior structure and the penetration (or lack thereof) of the thread are critical in load distribution and enhancement of adhesive performance.We present novel results on the structure of plaques from a variety of genera (Modiolus, Mytilus and Septifer) and species (M. californianus, M. galloprovincialis) studied via electron microscopy and neutron scattering. Despite living in the same coastal area, these species live in micro-environments subjected to different wave motion, allowing us to investigate how ecology might influence plaque structure and load-bearing capacity. Their architecture is reminiscent of various structural foams, ranging from closed-cell foams with one relevant length scale, to open-cell foams characterized by two lengthscales. Similar foam architectures have been found in other marine secretions, such as the sandcastle worm's tube cement. For the mussel plaques, the most complex structure we observe exhibits a collection of pores having an inner network, further connected with an outer mesh network. We reveal a robust structure, largely unaffected by guanidine hydrochloride treatment, and study the effect of temperature on the plaque's mechanical properties. Finally, we develop models to demonstrate the load distribution in the plaque and explore the potential benefits of such structures in wet environments.