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Thanks to many promising properties, including biocompatibility and the ability to experience large deformations, poly(ethylene glycol) diacrylate (PEGDA) hydrogels are excellent candidate materials for a wide range of applications. Interestingly, the polymerization of PEGDA leads to a network microstructure that is fundamentally different from that of the "classic" polymeric gels. Specifically, PEGDA hydrogels comprise PEG chains that are interconnected by multifunctional densely grafted rod-like polyacrylates (PAs), which serve as cross-linkers. In this work, we derive a microstructurally motivated model that captures the essential features which enable deformation in PEGDA hydrogels: (1) entropic elasticity of PEG chains, (2) deformation of PA rods, and (3) PA-PA interactions. Expressions for the energy-density functions and the stress associated with each of the three contributions are derived. The model demonstrates the microstructural evolution of the network during loading and reveals the role of key microscopic quantities. To validate the model, we fabricate and compress PEGDA hydrogel discs. The model is in excellent agreement with our experimental findings for a broad range of PEGDA compositions. Interestingly, we show that the response of PEGDA hydrogels with short PEG chains and long PA rods is governed by PA-PA interactions, whereas networks with longer PEG chains are dominated by entropy. To enable design, we employ the model to investigate the influence of key microstructural quantities, such as the length of the PEG and the PA chains, on the macroscopic properties and response. The findings from this work pave the way to the efficient design of PEGDA hydrogels with tunable properties and behaviors, which will enable the optimization of their performance in various applications., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Performing conventional mechanical characterization techniques on soft materials can be challenging due to issues such as limited sample volumes and clamping difficulties. Deep indentation and puncture is a promising alternative as it is an information-rich measurement with the potential to be performed in a high-throughput manner. Despite its promise, the method lacks standardized protocols, and open questions remain about its possible limitations. Addressing these shortcomings is vital to ensure consistent methodology, measurements, and interpretation across samples and labs. To fill this gap, we examine the role of finite sample dimensions (and by extension, volume) on measured forces to determine the sample geometry needed to perform and unambiguously interpret puncture tests. Through measurements of puncture on a well-characterized elastomer using systematically varied sample dimensions, we show that the apparent mechanical response of a material is in fact sensitive to near-wall effects, and that additional properties, such as the sliding friction coefficient, can only be extracted in the larger dimension case where such effects are negligible.

We demonstrate that the sessile tunicate Botryllus schlosseri is remarkably resilient to applied loads by attaching the animals to an extensile substrate subjected to quasistatic equiradial loads. Animals can withstand radial extension of the substrate to strain values as high as 20% before they spontaneously detach. In the small to moderate strain regime, we found no relationship between the dynamic size of the external vascular bed and the magnitude of applied stretch, despite known force sensitivities of the vascular tissue at the cellular level. We attribute this resilience to the presence and mechanical properties of the tunic, the cellulose-enriched gel-like substance that encases the animal bodies and surrounding vasculature., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2023. Published by The Company of Biologists Ltd.)

The ability of cells to reorganize in response to external stimuli is important in areas ranging from morphogenesis to tissue engineering. While nematic order is common in biological tissues, it typically only extends to small regions of cells interacting via steric repulsion. On isotropic substrates, elongated cells can co-align due to steric effects, forming ordered but randomly oriented finite-size domains. However, we have discovered that flat substrates with nematic order can induce global nematic alignment of dense, spindle-like cells, thereby influencing cell organization and collective motion and driving alignment on the scale of the entire tissue. Remarkably, single cells are not sensitive to the substrate's anisotropy. Rather, the emergence of global nematic order is a collective phenomenon that requires both steric effects and molecular-scale anisotropy of the substrate. To quantify the rich set of behaviours afforded by this system, we analyse velocity, positional and orientational correlations for several thousand cells over days. The establishment of global order is facilitated by enhanced cell division along the substrate's nematic axis, and associated extensile stresses that restructure the cells' actomyosin networks. Our work provides a new understanding of the dynamics of cellular remodelling and organization among weakly interacting cells.

Bioinspired iron-catechol cross-links have shown remarkable success in increasing the mechanical properties of polymer networks, in part due to clustering of Fe 3+ -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 Fe 3+ -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., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Background: Patient reviews provide an important referral source for physicians and an opportunity to improve practice performance. This study's objective was to characterize the online reviews of hip and knee arthroplasty surgeons published by three of the industry's leading platforms., Methods: A random sample of 1000 hip and knee arthroplasty surgeons across all 50 US states (10 hip and 10 knee surgeons per state) was generated using Google Search. A total of 7842 online reviews posted for those surgeons on Healthgrades, Vitals, and Google were analyzed. A range of surgeons, affiliated hospitals, and reviewer attributes was compared to identify significant predictors of patient satisfaction., Results: The study cohort had 98.1% male surgeons with a mean age of 53.55 ± 8.94 years and mean experience of 26.43 ± 9.21 years. Younger age (p < 0.001), shorter years of experience (p < 0.001), and arthroplasty fellowship training (p < 0.001) were associated with more positive ratings. Reviewer anonymity, observed in 30.93% of all reviews, tended to correlate with more negative ratings (p < 0.001). Overall, 86.93% of patient remarks were positive, and only 74.81% of remarks centered on physician attributes. The five leading components of patient satisfaction were perceptions of physician competence (34.81%, p < 0.001), bedside manner (23.83%, p = 0.002), and communication (16.17%, p = 0.94); interactions with physician extenders (14.75%, p < 0.001); and wait time (2.73%, p < 0.001)., Conclusion: While most ratings of hip and knee arthroplasty surgeons were positive, more than a quarter of reviews were either not directly related to the individual surgeons or were submitted anonymously. Caution is advised regarding overreliance on patient experience surveys as predictors of physician performance., Competing Interests: The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Mohamad J. Halawi reports editorial/governing body memberships with The Journal of Bone and Joint Surgery, The Journal of Arthroplasty, and Arthroplasty Today. He also reports board membership/committee appointments with American Academy of Orthopaedic Surgeons and American Orthopaedic Association.

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.

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., (Copyright © 2022 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Materials that utilize heterogeneous microstructures to control macroscopic mechanical response are ubiquitous in nature. Yet, translating nature's lessons to create synthetic soft solids has remained challenging. This is largely due to the limited synthetic routes available for creating soft composites, particularly with submicron features, as well as uncertainty surrounding the role of such a microstructured secondary phase in determining material behavior. This work leverages recent advances in the development of photocrosslinkable thermogelling nanoemulsions to produce composite hydrogels with a secondary phase assembled at well controlled length scales ranging from tens of nm to tens of μm. Through analysis of the mechanical response of these fluid-filled composite hydrogels, it is found that the size scale of the secondary phase has a profound impact on the strength when at or above the elastofracture length. Moreover, this work shows that mechanical integrity of fluid-filled soft solids can be sensitive to the size scale of the secondary phase.

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.

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 in situ as phase separation proceeds. Here, we propose and demonstrate a high-throughput microscopy platform to achieve simultaneous, in situ mapping of time-evolving morphology and microrheology in phase separating complex fluids over a large compositional space. The method was applied to a canonical example of polyelectrolyte complex coacervation, whereby mixing of oppositely charged species leads to liquid-liquid phase separation into distinct solute-dense and dilute phases. Morphology and rheology were measured simultaneously and kinetically after mixing to track the progression of phase separation. Once equilibrated, the dense phase viscosity was determined to high compositional accuracy using passive probe microrheology, and the results were used to derive empirical relationships between the composition and viscosity. These relationships were inverted to reconstruct the dense phase boundary itself, and further extended to other mixture compositions. The resulting predictions were validated by independent equilibrium compositional measurements. This platform paves the way for rapid screening and formulation of complex fluids and (bio)macromolecular materials, and serves as a critical link between formulation and rheology for multi-phase material discovery.

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 substantial 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, so far, progressed mostly separately. This Perspective provides a review of recent key advances in these two fields, and a roadmap for collaboration at the interface between the 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., (© 2022. Springer Nature Limited.)

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.

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., (© 2021. The Author(s).)

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.

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.

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., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

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.

Background: Total joint arthroplasty is the most common elective orthopedic procedure in the Veterans Affairs hospital system. In 2019, physical medicine and rehabilitation began screening patients before surgery to select candidates for direct transfer to acute rehab after surgery. The primary outcome of this study was to demonstrate that the accelerated program was successful in decreasing inpatient costs and length of stay (LOS). The secondary outcome was to show that there was no increase in complication, reoperation, and readmission rates., Methods: A retrospective review of total joint arthroplasty patients was conducted with three cohorts: 1) control (n = 193), 2) transfer to rehab orders on postop day #1 (n = 178), and 3) direct transfers to rehab (n = 173). To assess for demographic disparities between cohorts, multiple analysis of variance tests followed by a Bonferroni P-value correction were used. Differences between test groups regarding primary outcomes were assessed with analysis of variance tests followed by pairwise t-tests with Bonferroni P-value corrections., Results: There were no significant differences between the cohort demographics or comorbidities. The mean total LOS decreased from 7.0 days in the first cohort, to 6.9 in the second, and 6.0 in the third (P = .00034). The mean decrease in cost per patient was $14,006 between cohorts 1 and 3, equating to over $5.6 million in savings annually. There was no significant change in preintervention and postintervention short-term complications (P = .295)., Conclusions: Significant cost savings and decrease in total LOS was observed. In the current health care climate focused on value-based care, a similar intervention could be applied nationwide to improve Veterans Affair services., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Vascular diseases affect over 1 billion people worldwide and are highly prevalent among the elderly, due to a progressive deterioration of the structure of vascular cells. Most of our understanding of these age-related cellular changes comes from in vitro studies on human cell lines. Further studies of the mechanisms underlying vascular aging in vivo are needed to provide insight into the pathobiology of age-associated vascular diseases, but are difficult to carry out on vertebrate model organisms. We are studying the effects of aging on the vasculature of the invertebrate chordate, Botryllus schlosseri . This extracorporeal vascular network of Botryllus is transparent and particularly amenable to imaging and manipulation. Here we use a combination of transcriptomics, immunostaining and live-imaging, as well as in vivo pharmacological treatments and regeneration assays to show that morphological, transcriptional, and functional age-associated changes within vascular cells are key hallmarks of aging in B. schlosseri , and occur independent of genotype. We show that age-associated changes in the cytoskeleton and the extracellular matrix reshape vascular cells into a flattened and elongated form and there are major changes in the structure of the basement membrane over time. The vessels narrow, reducing blood flow, and become less responsive to stimuli inducing vascular regression. The extracorporeal vasculature is highly regenerative following injury, and while age does not affect the regeneration potential, newly regenerated vascular cells maintain the same aged phenotype, suggesting that aging of the vasculature is a result of heritable epigenetic changes., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Rodriguez, Taketa, Madhu, Kassmer, Loerke, Valentine and Tomaso.)

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.

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.

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.

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.

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.

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.

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'.

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.

Materials often exhibit a trade-off between stiffness and extensibility; for example, strengthening elastomers by increasing their cross-link density leads to embrittlement and decreased toughness. Inspired by cuticles of marine mussel byssi, we circumvent this inherent trade-off by incorporating sacrificial, reversible iron-catechol cross-links into a dry, loosely cross-linked epoxy network. The iron-containing network exhibits two to three orders of magnitude increases in stiffness, tensile strength, and tensile toughness compared to its iron-free precursor while gaining recoverable hysteretic energy dissipation and maintaining its original extensibility. Compared to previous realizations of this chemistry in hydrogels, the dry nature of the network enables larger property enhancement owing to the cooperative effects of both the increased cross-link density given by the reversible iron-catecholate complexes and the chain-restricting ionomeric nanodomains that they form., (Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

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.

Marine mussels use catechol-rich interfacial mussel foot proteins (mfps) as primers that attach to mineral surfaces via hydrogen, metal coordination, electrostatic, ionic, or hydrophobic bonds, creating a secondary surface that promotes bonding to the bulk mfps. Inspired by this biological adhesive primer, it is shown that a ≈1 nm thick catecholic single-molecule priming layer increases the adhesion strength of crosslinked polymethacrylate resin on mineral surfaces by up to an order of magnitude when compared with conventional primers such as noncatecholic silane- and phosphate-based grafts. Molecular dynamics simulations confirm that catechol groups anchor to a variety of mineral surfaces and shed light on the binding mode of each molecule. Here, a ≈50% toughness enhancement is achieved in a stiff load-bearing polymer network, demonstrating the utility of mussel-inspired bonding for processing a wide range of polymeric interfaces, including structural, load-bearing materials., (© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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 cm 2 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., (© 2017 Rodriguez, Braden, et al. This article is distributed by The American Society for Cell Biology under license from the author(s). Two months after publication it is available to the public under an Attribution–Noncommercial–Share Alike 3.0 Unported Creative Commons License (http://creativecommons.org/licenses/by-nc-sa/3.0).)

We investigate the effects of transient expression of wild type (WT) and disease-linked mutations of tau (R406W, P301L, ΔN296) on cytoskeletal organization and cargo transport in COS-7 cells, which are natively tau-free. The introduction of tau proteins (either WT or mutant forms) leads to a dramatic restructuring of the microtubule cytoskeleton, as observed using immunofluorescence microscopy. Yet, this microtubule bundling and aggregation has a modest effect on the speed and travel distance of motor-driven cargo transport, as measured by the motions of fluorescently-labeled lysosomes. This suggests that localized transport events are insensitive to the global structure of the microtubule cytoskeleton. Importantly, we also found no evidence that the disease-linked tau mutants were particularly toxic; in fact we found that expression of mutant and WT tau had similar effects on overall microtubule structure and transport phenotypes., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

We report an improved method for calibrating the nonlinear region of a single-beam gradient optical trap. Through analysis of the position fluctuations of a trapped object that is displaced from the trap center by controlled flow we measure the local trap stiffness in both the linear and nonlinear regimes without knowledge of the magnitude of the applied external forces. This approach requires only knowledge of the system temperature, and is especially useful for measurements involving trapped objects of unknown size, or objects in a fluid of unknown viscosity.

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.)

Marine mussels of the genus Mytilus live in the hostile intertidal zone, attached to rocks, bio-fouled surfaces and each other via collagen-rich threads ending in adhesive pads, the plaques. Plaques adhere in salty, alkaline seawater, withstanding waves and tidal currents. Each plaque requires a force of several newtons to detach. Although the molecular composition of the plaques has been well studied, a complete understanding of supra-molecular plaque architecture and its role in maintaining adhesive strength remains elusive. Here, electron microscopy and neutron scattering studies of plaques harvested from Mytilus californianus and Mytilus galloprovincialis reveal a complex network structure reminiscent of structural foams. Two characteristic length scales are observed characterizing a dense meshwork (approx. 100 nm) with large interpenetrating pores (approx. 1 µm). The network withstands chemical denaturation, indicating significant cross-linking. Plaques formed at lower temperatures have finer network struts, from which we hypothesize a kinetically controlled formation mechanism. When mussels are induced to create plaques, the resulting structure lacks a well-defined network architecture, showcasing the importance of processing over self-assembly. Together, these new data provide essential insight into plaque structure and formation and set the foundation to understand the role of plaque structure in stress distribution and toughening in natural and biomimetic materials., (© 2015 The Author(s).)

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., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Mussels are well known for their ability to generate and maintain strong, long-lasting adhesive bonds under hostile conditions. Many prior studies attribute their adhesive strength to the strong chemical interactions between the holdfast and substrate. While chemical interactions are certainly important, adhesive performance is also determined by contact geometry, and understanding the coupling between chemical interactions and the plaque shape and mechanical properties is essential in deploying bioinspired strategies when engineering improved adhesives. To investigate how the shape and mechanical properties of the mussel's plaque contribute to its adhesive performance, we use a custom built load frame capable of fully characterizing the dynamics of the detachment. With this, we can pull on samples along any orientation, while at the same time measuring the resulting force and imaging the bulk deformations of the plaque as well as the holdfast-substrate interface where debonding occurs. We find that the force-induced yielding of the mussel plaque improves the bond strength by two orders of magnitude and that the holdfast shape improves bond strength by an additional order of magnitude as compared to other simple geometries. These results demonstrate that optimizing the contact geometry can play as important a role on adhesive performance as optimizing the chemical interactions as observed in other organisms and model systems.

We examine the bond-breaking dynamics of transiently cross-linked semiflexible networks using a single filament model in which that filament is peeled from an array of cross-linkers. We examine the effect of quenched disorder in the placement of the linkers along the filament and the effect of stochastic bond-breaking (assuming Bell model unbinding kinetics) on the dynamics of filament cross-linker dissociation and the statistics of ripping events. We find that bond forces decay exponentially away from the point of loading and that bond breaking proceeds sequentially down the linker array from the point of loading in a series of stochastic ripping events. We compare these theoretical predictions to the observed trajectories of large beads in a cross-linked microtubule network and identify the observed jumps of the bead with the linker rupture events predicted by the single filament model.

We present the design methodology for arrays of neodymium iron boron (NdFeB)-based magnets for use in magnetic tweezers devices. Using finite element analysis (FEA), we optimized the geometry of the NdFeB magnet as well as the geometry of iron yokes designed to focus the magnetic fields toward the sample plane. Together, the magnets and yokes form a magnetic array which is the basis of the magnetic tweezers device. By systematically varying 15 distinct shape parameters, we determined those features that maximize the magnitude of the magnetic field gradient as well as the length scale over which the magnetic force operates. Additionally, we demonstrated that magnetic saturation of the yoke material leads to intrinsic limitations in any geometric design. Using this approach, we generated a compact and light-weight magnetic tweezers device that produces a high field gradient at the image plane in order to apply large forces to magnetic beads. We then fabricated the optimized yoke and validated the FEA by experimentally mapping the magnetic field of the device. The optimization data and iterative FEA approach outlined here will enable the streamlined design and construction of specialized instrumentation for force-sensitive microscopy.

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.

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., (© 2013 The Authors Journal of Microscopy © 2013 Royal Microscopical Society.)

Using a suite of biophysical tools, we assess the mechanical, structural, and functional properties of microtubules (MTs) stabilized by the chemotherapeutic compounds epothilone-A, epothilone-B, and taxol in vitro. We demonstrate that MTs stabilized by epothilone-A or epothilone-B are competent to bind tau proteins and support kinesin translocation. Kinesin speed is sensitive not only to the type of small molecule stabilizer used but also to the presence of the essential MT-associated protein tau. Epothilone-stabilized MTs are substantially less stiff than taxol-stabilized MTs. The addition of tau proteins to MTs stabilized by either epothilone compound or taxol further reduces stiffness. Taken together, these results suggest that small molecule stabilizers do not simply stabilize a "native" MT structure, but rather they modulate the structure, function, and mechanics of the MTs they bind. This may have important consequences to the therapeutic use of these agents in cancer chemotherapies., (Copyright © 2012 Wiley Periodicals, Inc.)

Cellular biopolymers can exhibit significant compositional heterogeneities as a result of the non-uniform binding of associated proteins, the formation of microstructural defects during filament assembly, or the imperfect bundling of filaments into composite structures of variable diameter. These can lead to significant variations in the local mechanical properties of biopolymers along their length. Existing spectral analysis methods assume filament homogeneity and therefore report only a single average stiffness for the entire filament. However, understanding how local effects modulate biopolymer mechanics in a spatially resolved manner is essential to understanding how binding and bundling proteins regulate biopolymer stiffness and function in cellular contexts. Here, we present a new method to determine the spatially varying material properties of individual complex biopolymers from the observation of passive thermal fluctuations of the filament conformation. We develop new statistical mechanics-based approaches for heterogeneous filaments that estimate local bending elasticities as a function of the filament arc-length. We validate this methodology using simulated polymers with known stiffness distributions, and find excellent agreement between derived and expected values. We then determine the bending elasticity of microtubule filaments of variable composition generated by repeated rounds of tubulin polymerization using either GTP or GMPCPP, a nonhydrolyzable GTP analog. Again, we find excellent agreement between mechanical and compositional heterogeneities.

We determine the time- and force-dependent viscoelastic responses of reconstituted networks of microtubules that have been strongly crosslinked by biotin-streptavidin bonds. To measure the microscale viscoelasticity of such networks, we use a magnetic tweezers device to apply localized forces. At short time scales, the networks respond nonlinearly to applied force, with stiffening at small forces, followed by a reduction in the stiffening response at high forces, which we attribute to the force-induced unbinding of crosslinks. At long time scales, force-induced bond unbinding leads to local network rearrangement and significant bead creep. Interestingly, the network retains its elastic modulus even under conditions of significant plastic flow, suggesting that crosslinker breakage is balanced by the formation of new bonds. To better understand this effect, we developed a finite element model of such a stiff filament network with labile crosslinkers obeying force-dependent Bell model unbinding dynamics. The coexistence of dissipation, due to bond breakage, and the elastic recovery of the network is possible because each filament has many crosslinkers. Recovery can occur as long as a sufficient number of the original crosslinkers are preserved under the loading period. When these remaining original crosslinkers are broken, plastic flow results.