New approaches to dissect leaf hydraulics reveal large gradients in living tissues of tomato leaves

The water status of the living tissue in leaves is critical in determining plant function and global exchange of water and CO2. Despite significant advances in the past two decades, persistent questions remain about the tissue-specific origins of leaf hydraulic properties and their dependence on water status. We use a fluorescent nanoparticle reporter that provides water potential in the mesophyll apoplast adjacent to the epidermis of intact leaves to complement existing methods based on the Scholander Pressure Chamber (SPC). Working in tomato leaves, this approach provides access to the hydraulic conductance of the whole leaf, xylem, and outside-xylem tissues. These measurements show that, as stem water potential decreases, the water potential in the mesophyll apoplast can drop below that assessed with the SPC and can fall significantly below the turgor loss point of the leaf. We find that this drop in potential, dominated by the large loss (10-fold) of hydraulic conductance of the outside-xylem tissue, is not however strong enough to significantly limit transpiration. These observations highlight the need to reassess models of water transfer through the outside-xylem tissues, the potential importance of this tissue in regulating transpiration, and the power of new approaches for probing leaf hydraulics.

Localized measurements of water potential reveal large loss of conductance in living tissues of maize leaves

The water status of the living tissue in leaves between the xylem and stomata (outside xylem zone (OXZ) plays a critical role in plant function and global mass and energy balance but has remained largely inaccessible. We resolve the local water relations of OXZ tissue using a nanogel reporter of water potential (ψ), AquaDust, that enables an in situ, nondestructive measurement of both ψ of xylem and highly localized ψ at the terminus of transpiration in the OXZ. Working in maize (Zea mays L.), these localized measurements reveal gradients in the OXZ that are several folds larger than those based on conventional methods and values of ψ in the mesophyll apoplast well below the macroscopic turgor loss potential. We find a strong loss of hydraulic conductance in both the bundle sheath and the mesophyll with decreasing xylem potential but not with evaporative demand. Our measurements suggest the OXZ plays an active role in regulating the transpiration path, and our methods provide the means to study this phenomenon.

Trunk Water Potential Measured with Microtensiometers for Managing Water Stress in "Gala" Apple Trees

The weather variations around the world are already having a profound impact on agricultural production. This impacts apple production and the quality of the product. Through agricultural precision, growers attempt to optimize both yield and fruit size and quality. Two experiments were conducted using field-grown "Gala" apple trees in Geneva, NY, USA, in 2021 and 2022. Mature apple trees (Malus × domestica Borkh. cv. Ultima "Gala") grafted onto G.11 rootstock planted in 2015 were used for the experiment. Our goal was to establish a relationship between stem water potential (Ψ_trunk), which was continuously measured using microtensiometers, and the growth rate of apple fruits, measured continuously using dendrometers throughout the growing season. The second objective was to develop thresholds for Ψ_trunk to determine when to irrigate apple trees. The economic impacts of different irrigation regimes were evaluated. Three different water regimes were compared (full irrigation, rainfed and rain exclusion to induce water stress). Trees subjected the rain-exclusion treatment were not irrigated during the whole season, except in the spring (April and May; 126 mm in 2021 and 100 mm in 2022); that is, these trees did not receive water during June, July, August and half of September. Trees subjected to the rainfed treatment received only rainwater (515 mm in 2021 and 382 mm in 2022). The fully irrigated trees received rain but were also irrigated by drip irrigation (515 mm in 2021 and 565 mm in 2022). Moreover, all trees received the same amount of water out of season in autumn and winter (245 mm in 2021 and 283 mm in 2022). The microtensiometer sensors detected differences in Ψ_trunk among our treatments over the entire growing season. In both years, experimental trees with the same trunk cross-section area (TCSA) were selected (23-25 cm^-2 TCSA), and crop load was adjusted to 7 fruits·cm^-2 TCSA in 2021 and 8.5 fruits·cm^-2 TCSA in 2022. However, the irrigated trees showed the highest fruit growth rates and final fruit weight (157 g and 70 mm), followed by the rainfed only treatment (132 g and 66 mm), while the rain-exclusion treatment had the lowest fruit growth rate and final fruit size (107 g and 61 mm). The hourly fruit shrinking and swelling rate (mm·h^-1) measured with dendrometers and the hourly Ψ_trunk (bar) measured with microtensiometers were correlated. We developed a logistic model to correlate Ψ_trunk and fruit growth rate (g·h^-1), which suggested a critical value of -9.7 bars for Ψ_trunk, above which there were no negative effects on fruit growth rate due to water stress in the relatively humid conditions of New York State. A support vector machine model and a multiple regression model were developed to predict daytime hourly Ψ_trunk with radiation and VPD as input variables. Yield and fruit size were converted to crop value, which showed that managing water stress with irrigation during dry periods improved crop value in the humid climate of New York State.

A signal-like role for floral humidity in a nocturnal pollination system

Previous studies have considered floral humidity to be an inadvertent consequence of nectar evaporation, which could be exploited as a cue by nectar-seeking pollinators. By contrast, our interdisciplinary study of a night-blooming flower, Datura wrightii, and its hawkmoth pollinator, Manduca sexta, reveals that floral relative humidity acts as a mutually beneficial signal in this system. The distinction between cue- and signal-based functions is illustrated by three experimental findings. First, floral humidity gradients in Datura are nearly ten-fold greater than those reported for other species, and result from active (stomatal conductance) rather than passive (nectar evaporation) processes. These humidity gradients are sustained in the face of wind and are reconstituted within seconds of moth visitation, implying substantial physiological costs to these desert plants. Second, the water balance costs in Datura are compensated through increased visitation by Manduca moths, with concomitant increases in pollen export. We show that moths are innately attracted to humid flowers, even when floral humidity and nectar rewards are experimentally decoupled. Moreover, moths can track minute changes in humidity via antennal hygrosensory sensilla but fail to do so when these sensilla are experimentally occluded. Third, their preference for humid flowers benefits hawkmoths by reducing the energetic costs of flower handling during nectar foraging. Taken together, these findings suggest that floral humidity may function as a signal mediating the final stages of floral choice by hawkmoths, complementing the attractive functions of visual and olfactory signals beyond the floral threshold in this nocturnal plant-pollinator system.

Extreme undersaturation in the intercellular airspace of leaves: a failure of Gaastra or Ohm?

BACKGROUND

Recent reports of extreme levels of undersaturation in internal leaf air spaces have called into question one of the foundational assumptions of leaf gas exchange analysis, that leaf air spaces are effectively saturated with water vapour at leaf surface temperature. Historically, inferring the biophysical states controlling assimilation and transpiration from the fluxes directly measured by gas exchange systems has presented a number of challenges, including: (1) a mismatch in scales between the area of flux measurement, the biochemical cellular scale and the meso-scale introduced by the localization of the fluxes to stomatal pores; (2) the inaccessibility of the internal states of CO2 and water vapour required to define conductances; and (3) uncertainties about the pathways these internal fluxes travel. In response, plant physiologists have adopted a set of simplifying assumptions that define phenomenological concepts such as stomatal and mesophyll conductances.

SCOPE

Investigators have long been concerned that a failure of basic assumptions could be distorting our understanding of these phenomenological conductances, and the biophysical states inside leaves. Here we review these assumptions and historical efforts to test them. We then explore whether artefacts in analysis arising from the averaging of fluxes over macroscopic leaf areas could provide alternative explanations for some part, if not all, of reported extreme states of undersaturation.

CONCLUSIONS

Spatial heterogeneities can, in some cases, create the appearance of undersaturation in the internal air spaces of leaves. Further refinement of experimental approaches will be required to separate undersaturation from the effects of spatial variations in fluxes or conductances. Novel combinations of current and emerging technologies hold promise for meeting this challenge.

Re-entrant transition as a bridge of broken ergodicity in confined monolayers of hexagonal prisms and cylinders

The entropy-driven monolayer assembly of hexagonal prisms and cylinders was studied under hard slit confinement. At the conditions investigated, the particles have two distinct and dynamically disconnected rotational states: unflipped and flipped, depending on whether their circular/hexagonal face is parallel or perpendicular to the wall plane. Importantly, these two rotational states cast distinct projection areas over the wall plane that favor either hexagonal or tetragonal packing. Monte Carlo simulations revealed a re-entrant melting transition where an intervening disordered Flipped-Unflipped (FUN) phase is sandwiched between a fourfold tetratic phase at high concentrations and a sixfold triangular solid at intermediate concentrations. The FUN phase contains a mixture of flipped and unflipped particles and is translationally and orientationally disordered. Complementary experiments were conducted with photolithographically fabricated cylindrical microparticles confined in a wedge cell. Both simulations and experiments show the formation of phases with comparable fraction of flipped particles and structure, i.e., the FUN phase, triangular solid, and tetratic phase, indicating that both approaches sample analogous basins of particle-orientation phase-space. The phase behavior of hexagonal prisms in a soft-repulsive wall model was also investigated to exemplify how tunable particle-wall interactions can provide an experimentally viable strategy to dynamically bridge the flipped and unflipped states.

Ex Situ and In Situ Measurement of Water Activity with a MEMS Tensiometer

Water acts as the solvent for natural biotic and abiotic processes and in many technological contexts. The availability of water to participate in chemical and physical processes is captured by thermodynamic variables which track the energetic state of water such as water activity and water potential. Our understanding of the energetic state of water in relevant processes is limited by a lack of sensors capable of providing accurate and reliable ex situ and in situ measurements of water activity. To address this technology gap, we present applications of a microtensiometer (μTM): a biomimetic microelectromechanical system (MEMS) sensor capable of measuring water activity in liquid, vapor, and semisolid (e.g., hydrogels, cheese) phases. We developed packaging, measurement systems, and methodology to enable us to make water activity measurements previously inaccessible to tensiometry. We present measurements in two contexts: (1) a small benchtop unit for ex situ measurements and (2) a probe format for in situ measurements. We demonstrate that the μTM can accurately measure water activity in a diversity of complex samples and agrees with chilled mirror hygrometry, an industry standard for water activity measurement.

Adsorption, Desorption, and Crystallization of Aqueous Solutions in Nanopores

Probing nanoconfined solutions in tortuous, mesoporous media is challenging because of pore size, complex pore connectivity, and the coexistence of multiple components and phases. Here, we use optical reflectance to experimentally investigate the wetting and drying of a mesoporous medium with ∼3-nm-diameter pores containing aqueous solutions of sodium chloride and lithium chloride. We show that the vapor activities (i.e., relative humidities) that correspond to optical features in the isotherms for solutions can be used to deduce the thermodynamic state of a nanoscopic solution that undergoes evaporation and crystallization upon drying and condensation and deliquescence when increasing the relative humidity. We emphasize specific equilibrium states of the system: the onset of draining during desorption and the end of filling during adsorption as well as percolation-induced scattering and crystallization. We find that theoretical arguments involving classical thermodynamics (a modified Kelvin-Laplace equation and classical nucleation theory) explain quantitatively the evolution of the optical features and thereby the state of the solution as a function of imposed vapor activity and solute concentration.

How Solutes Modify the Thermodynamics and Dynamics of Filling and Emptying in Extreme Ink-Bottle Pores

We investigate the filling and emptying of extreme ink-bottle porous media-micrometer-scale pores connected by nanometer-scale pores-when changing the pressure of the external vapor, in a case where the pore liquid contains solutes. These phenomena are relevant in diverse contexts, such as the weathering of building materials and artwork, aerosol formation in the atmosphere, and the hydration of soils and plants. Using model systems made of vein-shaped microcavities interconnected by a mesoporous matrix, we show experimentally that the presence of a nonvolatile solute shifts the condensation and evaporation transitions and in a way that is consistent with a modified Kelvin-Laplace equation that takes into account the osmotic pressure of the solution. Emptying occurs far below saturation, when the Kelvin stress, mediated by the large curvature of the liquid-vapor interfaces in the nanopores, is negative enough to induce spontaneous bubble nucleation in the microveins. Filling, on the other hand, occurs close to equilibrium (i.e., at saturation, p_sat for pure water and p_s < p_sat for a solution), driven by the weak capillary pressure of the liquid-vapor interface in the microveins. Interestingly, solutes allow the system to reach situations where the vapor is supersaturated with respect to the solution ( p_s < p < p_sat). We show that in that latter situation, a condensation layer covers the outer surface of the porous system, preventing the generation of Kelvin stresses but inducing osmotic stresses and flows that are vapor pressure-dependent. The timescales and dynamics reflect these different driving forces: emptying proceeds through discrete, stochastic nucleation events with very fast, unsteady bubble growth associated with a poroelastic relaxation process, while filling occurs collectively in all veins of the sample through a slower steady-state process driven by a combination of osmosis and capillarity. The dynamics can however be rendered symmetrical between filling and emptying if bubbles pre-exist during emptying, a case that we explore using cycling of the vapor pressure around equilibrium.

Multi-scale computational study of the Warburg effect, reverse Warburg effect and glutamine addiction in solid tumors

Cancer metabolism has received renewed interest as a potential target for cancer therapy. In this study, we use a multi-scale modeling approach to interrogate the implications of three metabolic scenarios of potential clinical relevance: the Warburg effect, the reverse Warburg effect and glutamine addiction. At the intracellular level, we construct a network of central metabolism and perform flux balance analysis (FBA) to estimate metabolic fluxes; at the cellular level, we exploit this metabolic network to calculate parameters for a coarse-grained description of cellular growth kinetics; and at the multicellular level, we incorporate these kinetic schemes into the cellular automata of an agent-based model (ABM), iDynoMiCS. This ABM evaluates the reaction-diffusion of the metabolites, cellular division and motion over a simulation domain. Our multi-scale simulations suggest that the Warburg effect provides a growth advantage to the tumor cells under resource limitation. However, we identify a non-monotonic dependence of growth rate on the strength of glycolytic pathway. On the other hand, the reverse Warburg scenario provides an initial growth advantage in tumors that originate deeper in the tissue. The metabolic profile of stromal cells considered in this scenario allows more oxygen to reach the tumor cells in the deeper tissue and thus promotes tumor growth at earlier stages. Lastly, we suggest that glutamine addiction does not confer a selective advantage to tumor growth with glutamine acting as a carbon source in the tricarboxylic acid (TCA) cycle, any advantage of glutamine uptake must come through other pathways not included in our model (e.g., as a nitrogen donor). Our analysis illustrates the importance of accounting explicitly for spatial and temporal evolution of tumor microenvironment in the interpretation of metabolic scenarios and hence provides a basis for further studies, including evaluation of specific therapeutic strategies that target metabolism.

Cancer metabolism is an emerging hallmark of cancer. In the past decade, a renewed focus on cancer metabolism has led to several distinct hypotheses describing the role of metabolism in cancer. To complement experimental efforts in this field, a scale-bridging computational framework is needed to allow rapid evaluation of emerging hypotheses in cancer metabolism. In this study, we present a multi-scale modeling platform and demonstrate the distinct outcomes in population-scale growth dynamics under different metabolic scenarios: the Warburg effect, the reverse Warburg effect and glutamine addiction. Within this modeling framework, we confirmed population-scale growth advantage enabled by the Warburg effect, provided insights into the symbiosis between stromal cells and tumor cells in the reverse Warburg effect and argued that the anaplerotic role of glutamine is not exploited by tumor cells to gain growth advantage under resource limitations. We point to the opportunity for this framework to help understand tissue-scale response to therapeutic strategies that target cancer metabolism while accounting for the tumor complexity at multiple scales.

Enhanced Oxygen Solubility in Metastable Water under Tension

Despite its relevance in numerous natural and industrial processes, the solubility of molecular oxygen has never been directly measured in capillary-condensed liquid water. In this article, we measure oxygen solubility in liquid water trapped within nanoporous samples, in metastable equilibrium with a subsaturated vapor. We show that solubility increases two fold at moderate subsaturations (relative humidity ∼0.55). This evolution with relative humidity is in good agreement with a simple thermodynamic prediction using properties of bulk water, previously verified experimentally at positive pressure. Our measurement thus verifies the validity of this macroscopic thermodynamic theory to strong confinement and large negative pressures, where significant nonidealities are expected. This effect has strong implications for important oxygen-dependent chemistries in natural and technological contexts.

Phloem Loading through Plasmodesmata: A Biophysical Analysis

In many species, Suc en route out of the leaf migrates from photosynthetically active mesophyll cells into the phloem down its concentration gradient via plasmodesmata, i.e. symplastically. In some of these plants, the process is entirely passive, but in others phloem Suc is actively converted into larger sugars, raffinose and stachyose, and segregated (trapped), thus raising total phloem sugar concentration to a level higher than in the mesophyll. Questions remain regarding the mechanisms and selective advantages conferred by both of these symplastic-loading processes. Here, we present an integrated model-including local and global transport and kinetics of polymerization-for passive and active symplastic loading. We also propose a physical model of transport through the plasmodesmata. With these models, we predict that (1) relative to passive loading, polymerization of Suc in the phloem, even in the absence of segregation, lowers the sugar content in the leaf required to achieve a given export rate and accelerates export for a given concentration of Suc in the mesophyll and (2) segregation of oligomers and the inverted gradient of total sugar content can be achieved for physiologically reasonable parameter values, but even higher export rates can be accessed in scenarios in which polymers are allowed to diffuse back into the mesophyll. We discuss these predictions in relation to further studies aimed at the clarification of loading mechanisms, fitness of active and passive symplastic loading, and potential targets for engineering improved rates of export.

Passive phloem loading and long-distance transport in a synthetic tree-on-a-chip

Vascular plants rely on differences in osmotic pressure to export sugars from regions of synthesis (mature leaves) to sugar sinks (roots, fruits). In this process, known as Münch pressure flow, the loading of sugars from photosynthetic cells to the export conduit (the phloem) is crucial, as it sets the pressure head necessary to power long-distance transport. Whereas most herbaceous plants use active mechanisms to increase phloem sugar concentration above that of the photosynthetic cells, in most tree species, for which transport distances are largest, loading seems, counterintuitively, to occur by means of passive symplastic diffusion from the mesophyll to the phloem. Here, we use a synthetic microfluidic model of a passive loader to explore the non-linear dynamics that arise during export and determine the ability of passive loading to drive long-distance transport. We first demonstrate that in our device, the phloem concentration is set by the balance between the resistances to diffusive loading from the source and convective export through the phloem. Convection-limited export corresponds to classical models of Münch transport, where the phloem concentration is close to that of the source; in contrast, diffusion-limited export leads to small phloem concentrations and weak scaling of flow rates with hydraulic resistance. We then show that the effective regime of convection-limited export is predominant in plants with large transport resistances and low xylem pressures. Moreover, hydrostatic pressures developed in our synthetic passive loader can reach botanically relevant values as high as 10 bars. We conclude that passive loading is sufficient to drive long-distance transport in large plants, and that trees are well suited to take full advantage of passive phloem loading strategies.

Imbibition Triggered by Capillary Condensation in Nanopores

We study the spatiotemporal dynamics of water uptake by capillary condensation from unsaturated vapor in mesoporous silicon layers (pore radius r_p ≃ 2 nm), taking advantage of the local changes in optical reflectance as a function of water saturation. Our experiments elucidate two qualitatively different regimes as a function of the imposed external vapor pressure: at low vapor pressures, equilibration occurs via a diffusion-like process; at high vapor pressures, an imbibition-like wetting front results in fast equilibration toward a fully saturated sample. We show that the imbibition dynamics can be described by a modified Lucas-Washburn equation that takes into account the liquid stresses implied by Kelvin equation.

Capillarity-driven flows at the continuum limit

We experimentally investigate the dynamics of capillary-driven flows at the nanoscale, using an original platform that combines nanoscale pores (⋍3 nm in diameter) and microfluidic features. In particular, we show that drying involves a fine coupling between thermodynamics and fluid mechanics that can be used to generate precisely controlled nanoflows driven by extreme stresses - up to 100 MPa of tension. We exploit these tunable flows to provide quantitative tests of continuum theories (e.g. Kelvin-Laplace equation and Poiseuille flow) across an unprecedented range and we isolate the breakdown of continuum as a negative slip length of molecular dimension. Our results show a coherent picture across multiple experiments including drying-induced permeation flows, imbibition and poroelastic transients.

Stability Limit of Water by Metastable Vapor-Liquid Equilibrium with Nanoporous Silicon Membranes

Liquid can sustain mechanical tension as its pressure drops below the vapor-liquid coexistence line and becomes less than zero, until it reaches the stability limit-the pressure at which cavitation inevitably occurs. For liquid water, its stability limit is still a subject of debate: the results obtained by researchers using a variety of techniques show discrepancies between the values of the stability limit and its temperature dependence as temperature approaches 0 °C. In this work, we present a study of the stability limit of water by the metastable vapor-liquid equilibrium (MVLE) method with nanoporous silicon membranes. We also report on an experimental system which enables tests of the temperature dependence of the stability limit with MVLE. The stability limit we found increases monotonically (larger tension) as temperature approaches 0 °C; this trend contradicts the centrifugal result of Briggs but agrees with the experiments by acoustic cavitation. This result confirms that a quasi-static method can reach stability values similar to that from the dynamic stretching technique, even close to 0 °C. Nevertheless, our results fall in the range of ∼ -20 to -30 MPa, a range that is consistent with the majority of experiments but is far less negative than the limit obtained in experiments involving quartz inclusions and that predicted for homogeneous nucleation.

Endothelial cell dynamics during anastomosis in vitro

Vascular anastomosis - the fusion of vessels from two distinct branches of the vascular system - represents a critical step in vascular growth under both healthy and pathological conditions, in vivo, and presents an important target for engineering of vascularized tissues, in vitro. Recent works in animal models have advanced our understanding of the molecular and cellular players in vascular anastomosis, but questions remain related to cellular dynamics and control of this process, in vitro. In this study, we exploited a three-dimensional (3-D) culture platform to examine the dynamics of endothelial cell (EC) during and after vascular anastomosis by allowing angiogenesis and vasculogenesis to proceed in parallel. We show that anastomosis occurs between sprouts formed by angiogenesis from an endothelium and tubes formed by vasculogenesis in the bulk of a 3-D matrix. This fusion leads to highly connected vessels that span from the surface of the matrix into the bulk in a manner that depends on cell density and identity. Further, we observe and analyze intermixing of endothelial cells of distinct origin (surface versus bulk) within the vessels structures that are formed; we provide evidence that the cells migrate along pre-existing vessels segments as part of this intermixing process. We conclude that anastomosis can occur between vessels emerging by angiogenesis and vasculogenesis and that this process may play an important role in contexts such as wound healing.

Drying by cavitation and poroelastic relaxations in porous media with macroscopic pores connected by nanoscale throats

We investigate the drying dynamics of porous media with two pore diameters separated by several orders of magnitude. Nanometer-sized pores at the edge of our samples prevent air entry, while drying proceeds by heterogeneous nucleation of vapor bubbles--cavitation--in the liquid in micrometer-sized voids within the sample. We show that the dynamics of cavitation and drying are set by the interplay of the deterministic poroelastic mass transport in the porous medium and the stochastic nucleation process. Spatiotemporal patterns emerge in this unusual reaction-diffusion system, with temporal oscillations in the drying rate and variable roughness of the drying front.

A microtensiometer capable of measuring water potentials below -10 MPa

Tensiometers sense the chemical potential of water (or water potential, Ψw) in an external phase of interest by measuring the pressure in an internal volume of liquid water in equilibrium with that phase. For sub-saturated phases, the internal pressure is below atmospheric and frequently negative; the liquid is under tension. Here, we present the initial characterization of a new tensiometer based on a microelectromechanical pressure sensor and a nanoporous membrane. We explain the mechanism of operation, fabrication, and calibration of this device. We show that these microtensiometers operate stably out to water potentials below -10 MPa, a tenfold extension of the range of current tensiometers. Finally, we present use of the device to perform an accurate measurement of the equation of state of liquid water at pressures down to -14 MPa. We conclude with a discussion of outstanding design considerations, and of the opportunities opened by the extended range of stability and the small form factor in sensing applications, and in fundamental studies of the thermodynamic properties of water.

How a "pinch of salt" can tune chaotic mixing of colloidal suspensions

Efficient mixing of colloids, particles or molecules is a central issue in many processes. It results from the complex interplay between flow deformations and molecular diffusion, which is generally assumed to control the homogenization processes. In this work we demonstrate on the contrary that despite fixed flow and self-diffusion conditions, the chaotic mixing of colloidal suspensions can be either boosted or inhibited by the sole addition of a trace amount of salt as a co-mixing species. Indeed, this shows that local saline gradients can trigger a chemically driven transport phenomenon, diffusiophoresis, which controls the rate and direction of molecular transport far more efficiently than the usual Brownian diffusion. A simple model combining the elementary ingredients of chaotic mixing with diffusiophoretic transport of the colloids allows rationalization of our observations and highlights how small-scale out-of-equilibrium transport bridges to mixing at much larger scales in a very effective way. Considering chaotic mixing as a prototypal building block for turbulent mixing suggests that these phenomena, occurring whenever the chemical environment is inhomogeneous, might bring interesting perspectives from micro-systems to large-scale situations, with examples ranging from ecosystems to industrial contexts.

Leaf hydraulics II: vascularized tissues

Current models of leaf hydration employ an Ohm's law analogy of the leaf as an ideal capacitor, neglecting the resistance to flow between cells, or treat the leaf as a plane sheet with a source of water at fixed potential filling the mid-plane, neglecting the discrete placement of veins as well as their resistance. We develop a model of leaf hydration that considers the average conductance of the vascular network to a representative areole (region bounded by the vascular network), and represent the volume of tissue within the areole as a poroelastic composite of cells and air spaces. Solutions to the 3D flow problem are found by numerical simulation, and these results are then compared to 1D models with exact solutions for a range of leaf geometries, based on a survey of temperate woody plants. We then show that the hydration times given by these solutions are well approximated by a sum of the ideal capacitor and plane sheet times, representing the time for transport through the vasculature and tissue respectively. We then develop scaling factors relating this approximate solution to the 3D model, and examine the dependence of these scaling factors on leaf geometry. Finally, we apply a similar strategy to reduce the dimensions of the steady state problem, in the context of peristomatal transpiration, and consider the relation of transpirational gradients to equilibrium leaf water potential measurements.

Leaf hydraulics I: scaling transport properties from single cells to tissues

In leaf tissues, water may move through the symplast or apoplast as a liquid, or through the airspace as vapor, but the dominant path remains in dispute. This is due, in part, to a lack of models that describe these three pathways in terms of experimental variables. We show that, in plant water relations theory, the use of a hydraulic capacity in a manner analogous to a thermal capacity, though it ignores mechanical interactions between cells, is consistent with a special case of the more general continuum mechanical theory of linear poroelasticity. The resulting heat equation form affords a great deal of analytical simplicity at a minimal cost: we estimate an expected error of less than 12%, compared to the full set of equations governing linear poroelastic behavior. We next consider the case for local equilibrium between protoplasts, their cell walls, and adjacent air spaces during isothermal hydration transients to determine how accurately simple volume averaging of material properties (a 'composite' model) describes the hydraulic properties of leaf tissue. Based on typical hydraulic parameters for individual cells, we find that a composite description for tissues composed of thin walled cells with air spaces of similar size to the cells, as in photosynthetic tissues, is a reasonable preliminary assumption. We also expect isothermal transport in such cells to be dominated by the aquaporin-mediated cell-to-cell path. In the non-isothermal case, information on the magnitude of the thermal gradients is required to assess the dominant phase of water transport, liquid or vapor.

Impact of electroviscosity on the hydraulic conductance of the bordered pit membrane: a theoretical investigation

In perfusion experiments, the hydraulic conductance of stem segments ( ) responds to changes in the properties of the perfusate, such as the ionic strength ( ), pH, and cationic identity. We review the experimental and theoretical work on this phenomenon. We then proceed to explore the hypothesis that electrokinetic effects in the bordered pit membrane (BPM) contribute to this response. In particular, we develop a model based on electroviscosity in which hydraulic conductance of an electrically charged porous membrane varies with the properties of the electrolyte. We use standard electrokinetic theory, coupled with measurements of electrokinetic properties of plant materials from the literature, to determine how the conductance of BPMs, and therefore , may change due to electroviscosity. We predict a nonmonotonic variation of with with a maximum reduction of 18%. We explore how this reduction depends on the characteristics of the sap and features of the BPM, such as pore size, density of chargeable sites, and their dissociation constant. Our predictions are consistent with changes in observed for physiological values of sap and pH. We conclude that electroviscosity is likely responsible, at least partially, for the electrolyte dependence of conductance through pits and that electroviscosity may be strong enough to play an important role in other transport processes in xylem. We conclude by proposing experiments to differentiate the impact of electroviscosity on from that of other proposed mechanisms.

Formation of microvascular networks in vitro

This protocol describes how to form a 3D cell culture with explicit, endothelialized microvessels. The approach leads to fully enclosed, perfusable vessels in a bioremodelable hydrogel (type I collagen). The protocol uses microfabrication to enable user-defined geometries of the vascular network and microfluidic perfusion to control mass transfer and hemodynamic forces. These microvascular networks (μVNs) allow for multiweek cultures of endothelial cells or cocultures with parenchymal or tissue cells in the extra-lumen space. The platform enables real-time fluorescence imaging of living engineered tissues, in situ confocal fluorescence of fixed cultures and transmission electron microscopy (TEM) imaging of histological sections. This protocol enables studies of basic vascular and blood biology, provides a model for diseases such as tumor angiogenesis or thrombosis and serves as a starting point for constructing prevascularized tissues for regenerative medicine. After one-time microfabrication steps, the system can be assembled in less than 1 d and experiments can run for weeks.

Physicochemical regulation of endothelial sprouting in a 3D microfluidic angiogenesis model

Both physiological and pathological tissue remodeling (e.g., during wound healing and cancer, respectively) require new blood vessel formation via angiogenesis, but the underlying microenvironmental mechanisms remain poorly defined due in part to the lack of biologically relevant in vitro models. Here, we present a biomaterials-based microfluidic 3D platform for analysis of endothelial sprouting in response to morphogen gradients. This system consists of three lithographically defined channels embedded in type I collagen hydrogels. A central channel is coated with endothelial cells, and two parallel side channels serve as a source and a sink for the steady-state generation of biochemical gradients. Gradients of vascular endothelial growth factor (VEGF) promoted sprouting, whereby endothelial cell responsiveness was markedly dependent on cell density and vessel geometry regardless of treatment conditions. These results point toward mechanical and/or autocrine mechanisms that may overwhelm pro-angiogenic paracrine signaling under certain conditions. To date, neither geometrical effects nor cell density have been considered critical determinants of angiogenesis in health and disease. This biomimetic vessel platform demonstrated utility for delineating hitherto underappreciated contributors of angiogenesis, and future studies may enable important new mechanistic insights that will inform anti-angiogenic cancer therapy.

Impact of chaos and Brownian diffusion on irreversibility in Stokes flows

We study a reversal process in Stokes flows in the presence of weak diffusion in order to clarify the distinct effects that chaotic flows have on the loss of reversibility relative to nonchaotic flows. In all linear flows, including a representation of the baker's map, we show that the decay of reversibility presents universal properties. In nonlinear chaotic and nonchaotic flows, we show that this universality breaks down due to the distribution of strain rates. In the limit of infinitesimal diffusivity, we predict qualitatively distinct behavior in the chaotic case.

Multiscale models of breast cancer progression

Breast cancer initiation, invasion and metastasis span multiple length and time scales. Molecular events at short length scales lead to an initial tumorigenic population, which left unchecked by immune action, acts at increasingly longer length scales until eventually the cancer cells escape from the primary tumor site. This series of events is highly complex, involving multiple cell types interacting with (and shaping) the microenvironment. Multiscale mathematical models have emerged as a powerful tool to quantitatively integrate the convective-diffusion-reaction processes occurring on the systemic scale, with the molecular signaling processes occurring on the cellular and subcellular scales. In this study, we reviewed the current state of the art in cancer modeling across multiple length scales, with an emphasis on the integration of intracellular signal transduction models with pro-tumorigenic chemical and mechanical microenvironmental cues. First, we reviewed the underlying biomolecular origin of breast cancer, with a special emphasis on angiogenesis. Then, we summarized the development of tissue engineering platforms which could provide high-fidelity ex vivo experimental models to identify and validate multiscale simulations. Lastly, we reviewed top-down and bottom-up multiscale strategies that integrate subcellular networks with the microenvironment. We present models of a variety of cancers, in addition to breast cancer specific models. Taken together, we expect as the sophistication of the simulations increase, that multiscale modeling and bottom-up agent-based models in particular will become an increasingly important platform technology for basic scientific discovery, as well as the identification and validation of potentially novel therapeutic targets.

Exploring water and other liquids at negative pressure

Water is famous for its anomalies, most of which become dramatic in the supercooled region, where the liquid is metastable with respect to the solid. Another metastable region has been hitherto less studied: the region where the pressure is negative. Here we review the work on the liquid in the stretched state. Characterization of the properties of the metastable liquid before it breaks by nucleation of a vapour bubble (cavitation) is a challenging task. The recent measurement of the equation of state of the liquid at room temperature down to  - 26 MPa opens the way to more detailed information on water at low density. The threshold for cavitation in stretched water has also been studied by several methods. A puzzling discrepancy between experiments and theory remains unexplained. To evaluate how specific this behaviour is to water, we discuss the cavitation data on other liquids. We conclude with a description of the ongoing work in our groups.

Transport phenomena in chaotic laminar flows

In many important chemical processes, the laminar flow regime is inescapable and defines the performance of reactors, separators, and analytical instruments. In the emerging field of microchemical process or lab-on-a-chip, this constraint is particularly rigid. Here, we review developments in the use of chaotic laminar flows to improve common transport processes in this regime. We focus on four: mixing, interfacial transfer, axial dispersion, and spatial sampling. Our coverage demonstrates the potential for chaos to improve these processes if implemented appropriately. Throughout, we emphasize the usefulness of familiar theoretical models of transport for processes occurring in chaotic flows. Finally, we point out open challenges and opportunities in the field.

Membraneless, room-temperature, direct borohydride/cerium fuel cell with power density of over 0.25 W/cm2

The widespread adoption and deployment of fuel cells as an alternative energy technology have been hampered by a number of formidable technical challenges, including the cost and long-term stability of electrocatalyst and membrane materials. We present a microfluidic fuel cell that overcomes many of these obstacles while achieving power densities in excess of 250 mW/cm(2). The poisoning and sluggish reaction rate associated with CO-contaminated H(2) and methanol, respectively, are averted by employing the promising, high-energy density fuel borohydride. The high-overpotential reaction of oxygen gas at the cathode is supplanted by the high-voltage reduction of cerium ammonium nitrate. Expensive, ineffective membrane materials are replaced with laminar flow and a nonselective, porous convection barrier to separate the fuel and oxidant streams. The result is a Nafion-free, room-temperature fuel cell that has the highest power density per unit mass of Pt catalyst employed for a non-H(2) fuel cell, and exceeds the power density of a typical H(2) fuel cell by 50%.

Phosphorescent nanoparticles for quantitative measurements of oxygen profiles in vitro and in vivo

We present the development and characterization of nanoparticles loaded with a custom phosphor; we exploit these nanoparticles to perform quantitative measurements of the concentration of oxygen within three-dimensional (3-D) tissue cultures in vitro and blood vessels in vivo. We synthesized a customized ruthenium (Ru)-phosphor and incorporated it into polymeric nanoparticles via self-assembly. We demonstrate that the encapsulated phosphor is non-toxic with and without illumination. We evaluated two distinct modes of employing the phosphorescent nanoparticles for the measurement of concentrations of oxygen: 1) in vitro, in a 3-D microfluidic tumor model via ratiometric measurements of intensity with an oxygen-insensitive fluorophore as a reference, and 2) in vivo, in mouse vasculature using measurements of phosphorescence lifetime. With both methods, we demonstrated micrometer-scale resolution and absolute calibration to the dissolved oxygen concentration. Based on the ease and customizability of the synthesis of the nanoparticles and the flexibility of their application, these oxygen-sensing polymeric nanoparticles will find a natural home in a range of biological applications, benefiting studies of physiological as well as pathological processes in which oxygen availability and concentration play a critical role.

Ideal rate of collision of cylinders in simple shear flow

The collision of particles influences the behavior of suspensions through the formation of aggregates for adhesive particles or through the contributions of solid-body contacts to the stress for nonadhesive particles. The simplest estimate of the collision rate, termed the ideal collision rate, is obtained when particles translate and rotate with the flow but have no hydrodynamic or colloidal interactions. Smoluchowski calculated the ideal collision frequency of spherical particles in 1917. So far, little work has been done to understand rate of collision for nonspherical particles. In this work, we calculate the ideal collision rate for cylindrical particles over a broad range of particle aspect ratios r defined as the ratio of length to diameter. Monte Carlo simulations are performed with initial relative positions and orientations that model the rate of approach of noninteracting particles following Jeffery orbits with several choices of the orbit distribution. The role of rotational motion of particles on collision frequency is elucidated by comparing the ideal collision rate calculations with similar calculations for nonrotating particles. It is shown that the ratio of the collision rate of cylinders to that of spheres that circumscribe the cylinders is proportional to 1/rr(e) for r ≫ 1 and r(e) for r ≪ 1. Here, r(e) is the effective aspect ratio defined as the aspect ratio of a spheroid having the same period of rotation as the cylinder. The effective aspect ratio of the cylindrical particles was determined using finite element calculations of the torque on nonrotating cylinders with their axes parallel to the velocity and velocity gradient directions. In addition to deriving the total collision rate, we categorize collisions as side-side, edge-side, and face-edge based on the initial point of contact. Most collisions are found to be side-edge for r ≫ 1 and face-edge for r ≪ 1, suggesting that nonlinear aggregates will develop if particles stick at the point of first contact.

Oxygen-controlled three-dimensional cultures to analyze tumor angiogenesis

Tumor angiogenesis is controlled by the integrated action of physicochemical and biological cues; however, the individual contributions of these cues are not well understood. We have designed alginate-based microscale tumor models to define the distinct importance of oxygen concentration, culture dimensionality, and cell-extracellular matrix interactions on the angiogenic capability of oral squamous cell carcinoma, and have verified the relevance of our findings with U87 glioblastoma cells. Our results revealed qualitative differences in the microenvironmental regulation of vascular endothelial growth factor (VEGF) and interleukin-8 (IL-8) secretion in three-dimensional (3D) culture. Specifically, IL-8 secretion was highest under ambient conditions, whereas VEGF secretion was highest in hypoxic cultures. Additionally, 3D integrin engagement by RGD-modified alginate matrices increased IL-8 secretion independently of oxygen, whereas VEGF secretion was only moderately affected by cell-extracellular matrix interactions. Using two-dimensional migration assays and a new 3D tumor angiogenesis model, we demonstrated that the resulting angiogenic signaling promotes tumor angiogenesis by increasing endothelial cell migration and invasion. Collectively, tissue-engineered tumor models improve our understanding of tumor angiogenesis, which may ultimately advance anticancer therapies.

Microfluidic culture models of tumor angiogenesis

Blood vessels control all stages of tumor development and therapy by defining the physicochemical and cellular state of the tumor microenvironment. However, no pathologically relevant culture systems currently exist that recapitulate the associated cellular and convective mass transfer processes that are implicated in tumor angiogenesis. By integrating tissue engineering and microfluidic technologies, it will be possible to develop tumor-mimetic culture environments with embedded microvascular structures. Utilization of these microfluidic tumor models will help reveal the importance of the transport of chemical and cellular factors in tumor angiogenesis, and provide a test bed that may ultimately improve current strategies to antiangiogenic therapy.

Dense type I collagen matrices that support cellular remodeling and microfabrication for studies of tumor angiogenesis and vasculogenesis in vitro

Type I collagen is a favorable substrate for cell adhesion and growth and is remodelable by many tissue cells; these characteristics make it an attractive material for the study of dynamic cellular processes. Low mass fraction (1.0-3.0 mg/ml), hydrated collagen matrices used for three-dimensional cell culture permit cellular movement and remodeling, but their microstructure and mechanics fail to mimic characteristics of many extracellular matrices in vivo and limit the definition of fine-scale geometrical features (<1 mm) within scaffolds. In this study, we worked with hydrated type I collagen at mass fractions between 3.0 and 20 mg/ml to define the range of densities over which the matrices support both microfabrication and cellular remodeling. We present pore and fiber dimensions based on confocal microscopy and longitudinal modulus and hydraulic permeability based on confined compression. We demonstrate faithful reproduction of simple pores of 50 μm-diameter over the entire range and formation of functional microfluidic networks for mass fractions of at least 10.0 mg/ml. We present quantitative characterization of the rate and extent of cellular remodelability using human umbilical vein endothelial cells. Finally, we present a co-culture with tumor cells and discuss the implications of integrating microfluidic control within scaffolds as a tool to study spatial and temporal signaling during tumor angiogenesis and vascularization of tissue engineered constructs.

Stability limit of liquid water in metastable equilibrium with subsaturated vapors

A pure liquid can reach metastable equilibrium with its subsaturated vapor across an appropriate membrane. This situation is analogous to osmotic equilibrium: the reduced chemical potential of the dilute phase (the subsaturated vapor) is compensated by a difference in pressure between the phases. To equilibrate with subsaturated vapor, the liquid phase assumes a pressure that is lower than its standard vapor pressure, such that the liquid phase is metastable with respect to the vapor phase. For sufficiently subsaturated vapors, the liquid phase can even assume negative pressures. The appropriate membrane for this metastable equilibrium must provide the necessary mechanical support to sustain the difference in pressure between the two phases, limit nonhomogeneous mechanisms of cavitation, and resist the entry of the dilutant (gases) into the pure phase (liquid). In this article, we present a study of the limit of stability of liquid water--the degree of subsaturation at which the liquid cavitates--in this metastable state within microscale voids embedded in hydrogel membranes. We refer to these structures as vapor-coupled voids (VCVs). In these VCVs, we observed that liquid water cavitated when placed in equilibrium with vapors of activity aw,vapair<or=0.85 (relative humidity<or=85%). When expressed as a pressure in the liquid based on thermodynamic calculations, the liquid cavitated at pressures P<or=-22 MPa (-220 bar). This limiting pressure is smaller in magnitude than the limit predicted by homogeneous nucleation theory or molecular simulations (Pcav=-140 to -180 MPa). To determine the cause of the disparity between the observed and predicted stability limit, we examine experimentally the likelihood of several nonhomogeneous mechanisms of nucleation: (i) heterogeneous nucleation caused by hydrophobic patches on void walls, (ii) nucleation caused by the presence of dissolved solute, (iii) nucleation caused by the presence of pre-existing vapor nuclei, and (iv) invasion of air through the hydrogel membrane into the voids. We conclude that, of these possibilities, (i) and (ii) cannot be discounted, whereas (iii) and (iv) are unlikely to play a role in determining the stability limit.

Experimental investigation of selective colloidal interactions controlled by shape, surface roughness, and steric layers

Photolithography can be used to form monodisperse colloids of well-defined, nonspherical shape in a negative photoresist, SU-8. In aqueous suspension, in the presence of dextran as a depletant, we showed previously that the aggregation of these particles was highly selective for the end-to-end configuration: cylinders assembled into linear aggregates that could extend to lengths of tens of units without significant lateral aggregation. This article presents an in-depth study of the mechanisms by which these particles aggregate. In particular, we focus on the roles of global shape, roughness, and adsorbed layers of surfactants in mediating depletion, van der Waals (vdW), and electrostatic interactions between these particles. We describe in detail the fabrication and characterization of the particles. To allow for the interpretations of the experiments, we present predictions for the interactions between mathematically ideal cylinders with smooth surfaces, and a statistical thermodynamic model for the linear assemblies. We present experimental observations of the state of aggregation as a function of concentration of dextran and ionic strength for typical particles that present roughness of larger amplitude on their rounded side walls than on their flat ends. We compare this behavior to that of particles that lack this contrast in roughness; this comparison indicates that roughness can serve to attenuate strongly the attractive depletion interactions. To achieve a more quantitative measure of this effect, we analyze size distributions of linear aggregates to calculate the energies of the end-to-end "bonds" on the basis of our statistical model. We find that both the depletion and vdW interactions are attenuated approximately 20 fold relative to predictions for smooth surfaces. We conclude with an assessment of outstanding questions and opportunities to exploit shape and roughness to direct the self-assembly of colloids.

The transpiration of water at negative pressures in a synthetic tree

Plant scientists believe that transpiration-the motion of water from the soil, through a vascular plant, and into the air-occurs by a passive, wicking mechanism. This mechanism is described by the cohesion-tension theory: loss of water by evaporation reduces the pressure of the liquid water within the leaf relative to atmospheric pressure; this reduced pressure pulls liquid water out of the soil and up the xylem to maintain hydration. Strikingly, the absolute pressure of the water within the xylem is often negative, such that the liquid is under tension and is thermodynamically metastable with respect to the vapour phase. Qualitatively, this mechanism is the same as that which drives fluid through the synthetic wicks that are key elements in technologies for heat transfer, fuel cells and portable chemical systems. Quantitatively, the differences in pressure generated in plants to drive flow can be more than a hundredfold larger than those generated in synthetic wicks. Here we present the design and operation of a microfluidic system formed in a synthetic hydrogel. This synthetic 'tree' captures the main attributes of transpiration in plants: transduction of subsaturation in the vapour phase of water into negative pressures in the liquid phase, stabilization and flow of liquid water at large negative pressures (-1.0 MPa or lower), continuous heat transfer with the evaporation of liquid water at negative pressure, and continuous extraction of liquid water from subsaturated sources. This development opens the opportunity for technological uses of water under tension and for new experimental studies of the liquid state of water.

Bridging kinematics and concentration content in a chaotic micromixer

We analyze the mixing properties of the microfluidic herringbone configuration introduced to mix scalar substances in a narrow channel at low Reynolds but large Péclet numbers. Because of the grooves sculpted on the channel floor, substantial transverse motions are superimposed onto the usual longitudinal Poiseuille dispersion along the channel, whose impact on both the mixing rate and mixture content is quantified. We demonstrate the direct link between the flow kinematics and the deformation rate of the mixture's concentration distribution, whose overall shape is also determined.

Adhesive properties of laminated alginate gels for tissue engineering of layered structures

A significant challenge in tissue engineering is the creation of tissues with stratified morphology or embedded microstructures. This study investigated methods to fabricate composite gels from separately deposited alginate layers and examined the effects of processing methods on the mechanics of adhesion. Laminated alginate gels were created through a three step process which included: treatment of the interfaces with citrate; annealing of the gels to allow for molecular rearrangement of the alginate chains; and exposure to a CaCl(2) to crosslink the alginate sheets. Process variables included volume and concentration of applied citrate, annealing time, incubation time in CaCl(2), and CaCl(2) concentration. Laminated sheets were tested in lap-shear geometry to characterize failure phenomena and mechanical properties. The site of failure within the gel depended on the integrity of the interface, with weaker gels delaminating and gels with mechanical properties similar to that of bulk gels failing randomly throughout the thickness. Citrate volume, citrate concentration, CaCl(2) incubation time, and CaCl(2) concentration altered the mechanical properties of the laminated alginate sheets, while annealing time had little effect on all measured parameters. This study demonstrates the integration of separately fabricated alginate layers to create mechanically or chemically anisotropic or heterogeneous structures.

Microfluidic scaffolds for tissue engineering

Most methods to culture cells in three dimensions depend on a cell-seedable biomaterial to define the global structure of the culture and the microenvironment of the cells. Efforts to tailor these scaffolds have focused on the chemical and mechanical properties of the biomaterial itself. Here, we present a strategy to control the distributions of soluble chemicals within the scaffold with convective mass transfer via microfluidic networks embedded directly within the cell-seeded biomaterial. Our presentation of this strategy includes: a lithographic technique to build functional microfluidic structures within a calcium alginate hydrogel seeded with cells; characterization of this process with respect to microstructural fidelity and cell viability; characterization of convective and diffusive mass transfer of small and large solutes within this microfluidic scaffold; and demonstration of temporal and spatial control of the distribution of non-reactive solutes and reactive solutes (that is, metabolites) within the bulk of the scaffold. This approach to control the chemical environment on a micrometre scale within a macroscopic scaffold could aid in engineering complex tissues.

An active wound dressing for controlled convective mass transfer with the wound bed

Conventional wound dressings-gauze, plastic films, foams, and gels-do not allow for spatial and temporal control of the soluble chemistry within the wound bed, and are thus limited to a passive role in wound healing. Here, we present an active wound dressing (AWD) designed to control convective mass transfer with the wound bed; this mass transfer provides a means to tailor and monitor the chemical state of a wound and, potentially, to aid the healing process. We form this AWD as a bilayer of porous poly(hydroxyethyl methacrylate) (pHEMA) and silicone; the pHEMA acts as the interface with the wound bed, and a layer of silicone provides a vapor barrier and a support for connecting to external reservoirs and pumps. We measure the convective permeability of the pHEMA sponge, and use this value to design a device with a spatially uniform flow profile. We quantify the global coefficient of mass transfer of the AWD on a dissolvable synthetic surface, and compare it to existing theories of mass transfer in porous media. We also operate the AWD on model wound beds made of calcium alginate gel to demonstrate extraction and delivery of low molecular weight solutes and a model protein. Using this system, we demonstrate both uniform mass transfer over the entire wound bed and patterned mass transfer in three spatially distinct regions. Finally, we discuss opportunities and challenges for the clinical application of this design of an AWD.

Integration of layered chondrocyte-seeded alginate hydrogel scaffolds

Motivated by the necessity to engineer appropriately stratified cartilage, the shear mechanics of layered, bovine chondrocyte-seeded 20mg/mL alginate scaffolds were investigated and related to the structure and biochemical composition. Chondrocyte-seeded alginate scaffolds were exposed to a calcium-chelating solution, layered, crosslinked in CaCl(2), and cultured for 10 weeks. The shear mechanical properties of the layered gels were statistically similar to those of the non-layered controls. Shear modulus of layered gels increased by approximately six-fold while toughness and shear strength increased by more than two-fold during the culture period. Hydroxyproline content in both layered gels and controls had statistically significant increases after 6 weeks. Glycosaminoglycan (GAG) content of controls increased throughout culture while GAG content in layered gels leveled off after 4 weeks. Hematoxylin and eosin histological staining showed tissue growth at the interface over the first 4 weeks. Shear mechanical properties in the engineered tissues showed significant correlations to hydroxyproline content. Dependence of interfacial mechanical properties on hydroxyproline content was most evident for layered gels when compared to controls, especially for toughness and shear strength. Additionally, interfacial properties showed almost no dependence on GAG content. These findings demonstrate the feasibility of creating stratified engineered tissues through layering and that collagen deposition is necessary for interfacial integrity.

A general method for patterning gradients of biomolecules on surfaces using microfluidic networks

This report outlines a general method for the fabrication of immobilized gradients of biomolecules on surfaces. This method utilizes a microfluidic network that generates a gradient of avidin in solution and immobilizes this protein on the surface of glass or poly(dimethylsiloxane) by physical adsorption. The immobilized gradient of avidin is then translated into gradients of biotinylated ligands (e.g., small molecules, oligomers of DNA, polysaccharides) using the specific interaction between biotin and avidin. This method can also generate immobilized gradients of certain proteins and artificial polymers by a direct transfer of gradients from solution onto the surface. The major advantage of this method is that almost any type of molecule can, in principle, be immobilized in a well-defined surface gradient of arbitrary shape with dimensions of a few micrometers to a few centimeters. It is possible to tailor the precise shapes of gradients on surfaces from gradients in solution, either kinetically or competitively. Kinetic methods rely on controlling the time that the surface is exposed to the gradient in solution: when a single protein adsorbs from solution, the amount that adsorbs depends both on its concentration in solution and on the time allowed for adsorption. Competitive methods rely on exposure of the surface to a complementary gradient of two proteins in solution (In these experiments, the sum of the concentrations of the proteins in solution is independent of positions although the concentration of each, individually, depends on the position. In this procedure, the relative amount of each protein, at saturation on the surface, depends only on its concentration.).

Nanobiotechnology: protein-nanomaterial interactions

We review recent research that involves the interaction of nanomaterials such as nanoparticles, nanowires, and carbon nanotubes with proteins. We begin with a focus on the fundamentals of the structure and function of proteins on nanomaterials. We then review work in three areas that exploit these interactions: (1) sensing, (2) assembly of nanomaterials by proteins and proteins by nanomaterials, and (3) interactions with cells. We conclude with the identification of challenges and opportunities for the future.

Protein translocation through a tunnel induces changes in folding kinetics: a lattice model study

Compaction of a nascent polypeptide chain inside the ribosomal exit tunnel, before it leaves the ribosome, has been proposed to accelerate the folding of newly synthesized proteins following their release from the ribosome. Thus, we used Kinetic Monte Carlo simulations of a minimalist on-lattice model to explore the effect that polypeptide translocation through a variety of channels has on protein folding kinetics. Our results demonstrate that tunnel confinement promotes faster folding of a well-designed protein relative to its folding in free space by displacing the unfolded state towards more compact structures that are closer to the transition state. Since the tunnel only forbids rarely visited, extended configurations, it has little effect on a "poorly designed" protein whose unfolded state is largely composed of low-energy, compact, misfolded configurations. The beneficial effect of the tunnel depends on its width; for example, a too-narrow tunnel enforces unfolded states with limited or no access to the transition state, while a too-wide tunnel has no effect on the unfolded state entropy. We speculate that such effects are likely to play an important role in the folding of some proteins or protein domains in the cellular environment and might dictate whether a protein folds co-translationally or post-translationally.