Gravity-induced Sedimentation Research Articles

Self-Assembly of Atomically Precise Silver Nanoclusters in Crowded Colloids into Ultra-Long Ribbons with Tunable Supramolecular Chirality.

Atomically precise metal nanoclusters (NCs) emerge as fascinating synthons in self-assembled materials. The self-assembly of metal NCs are highly sensitive to the environment because they have an inorganic-organic hybridized structure and a relatively complicated conformation. Here, it is shown that when confined in crowded colloids, a water-soluble Ag9 -cored nanocluster (Ag9 -NC) can self- assemble into ultra-long (up to millimeters) and photoluminescent ribbons with high flexibility. The ribbon contains rectangularly organized columns of Ag9 -NCs and can undergo secondary self-assembly to form bundled and branched structures. Formation of ribbons is observed in all the tested colloids, including lyotropic liquid crystals and disordered, three-dimensional network. The high viscosity/elasticity of the crowded colloids weakens gravity-induced sedimentation of the ribbons, leading to the formation of an interesting class of inorganic-organic composite materials where the hard Ag-containing skeleton strengthens the soft matter. The simultaneously occurring symmetry breaking during the self-assembly of Ag9 -NCs gives uncontrolled supramolecular chirality, which can be tuned through the majority rule and soldier-and-sergeant rule by the introduction of chiral seeds. The regulated chirality and the intrinsic photoluminescence of the Ag9 -NCs ribbons impart the composite material circularly polarized luminescence, opening the door for a variety of potential applications.

Solidification Principle in Large Vertical Steel Casting Under the EMS Effect

The surging demand for large high-quality rotor shafts or similar steel components in heavy industries (energy sector) poses new challenges to steelmakers. Based on the experience of conventional ingot and continuous casting, several new process concepts have been proposed, e.g., vertical continuous casting (VCC), semi-continuous casting (SCC), and segment casting (SC), but none of them are optimally put in operation. The main problems include the control of the as-cast structure and macrosegregation. Electromagnetic stirring (EMS) is necessary to obtain the center equiaxed zone, but EMS-induced multiphase flow can cause severe macrosegregation and uneven distribution of the as-cast structure between equiaxed and columnar. In this study, an advanced mixed columnar-equiaxed solidification model was used to investigate the formation of the as-cast structure and macrosegregation in an example of SCC with a large format (diameter 1 m). The main role of EMS is to create crystal fragments by fragmentation, which is regarded in this work as the only origin of equiaxed grains. The created equiaxed grains are brought by the EMS-induced (primary and secondary) flow and gravity-induced sedimentation to the central/lower part of the casting. The main goal of this study was to understand the solidification principle of SCC. In addition, a numerical parameter study by varying the EMS parameters was also performed to demonstrate the model capability towards the process optimization of SCC.

A reusable single-cell patterning strategy based on an ultrathin metal microstencil.

The ability to arrange distinct cells in specific, predefined patterns at single-cell resolution can have broad applications in cell-based assays and play an important role in facilitating interdisciplinary research for researchers in various fields. However, most existing methods for single-cell patterning are based on the complicated lithography-based microfabrication process, and require professional skills. Thus, exploiting convenient and universal strategies of single-cell preparation while maintaining high-throughput single-cell patterning remains a challenge. Here, we describe a simple approach for rapid and high-efficiency single-cell patterning using an ultrathin metal microstencil (UTmS) and common tools available in any laboratory. In this work, ultrathin steel microstencil plates with only 5 μm thickness could be fabricated with laser drilling and achieve single-cell prototyping on an arbitrary planar substrate under gravity-induced natural sedimentation without requiring additional fixation, reaction pools, and centrifugation procedures. In this method, the UTmS is reusable and single-cell occupancy could easily reach approximately 88% within 30 min on fibronectin-modified substrates under gravity-induced natural sedimentation, and no significant effect on cell viability was observed. To verify this method, the real-time and heterogeneous study of calcium release and apoptosis behaviors of single cells was carried out based on this new strategy. To our knowledge, it is the first time that a UTmS with 5 μm thickness is directly applied to facilitate the micropatterning of high-resolution single cells, which is valuable for researchers in different fields owing to its user-friendly operation.

Rainbows in a vial: controlled assembly of 2D colloids in two perpendicular external fields

The ability to control the dynamics of self-assembly of anisotropic nanoparticles over multiple length scales by application of external fields is an important technological and fundamental challenge in colloidal science and soft condensed matter physics. Here we demonstrate the controlled assembly of two-dimensional (2D) colloidal liquid crystals under the influence of gravity and an external magnetic field. Due to the positive anisotropy of diamagnetic susceptibility (▵χ > 0), the colloidal nanoplates of monolayer zirconium phosphate could exhibit intriguing alignment with their normals parallel to the magnetic field, thus transforming the multi-domain nematic phase into a mono-domain superstructure. Interestingly, the quasi-equilibrium states of such 2D soft materials under the prolonged gravity-induced sedimentation were found to generate a rich pattern of interference color bands (referred to as rainbows in a vial) upon under the effect of an aligning magnetic field. The birefringence measurement of the resulting color bands suggested quadratically decreasing volume fraction with sample height. The osmotic pressure (П) as a function of nanoplate volume fraction (ϕ) is found to agree with the theoretical prediction of the equation of state of the nematic phase of hard nanoplates, with a positive deviation at higher volume fractions. To the best of our knowledge, such field-driven dynamic formation of rich interference color bands, imitating almost the full spectrum of Michel-Levy interference bands, which was a pure mathematical representation, is experimentally achieved for the first time in a single sample.

Gold Nanoparticle Labels and Heterogeneous Immunoassays: The Case for the Inverted Substrate.

This paper examines how the difference in the spatial orientation of the capture substrate influences the analytical sensitivity and limits of detection for immunoassays that use gold nanoparticle labels (AuNPs) and rely on diffusion in quiet solution in the antigen capture and labeling steps. Ideally, the accumulation of both reactants should follow a dependence governed by the rate in which diffusion delivers reactants to the capture surface. In other words, the accumulation of reactants should increase with the square root of the incubation time, i.e., t1/2. The work herein shows, however, that this expectation is only obeyed when the capture substrate is oriented to direct the gravity-induced sedimentation of the AuNP labels away from the substrate. Using an assay for human IgG, the results show that circumventing the sedimentation of the gold nanoparticle labels by substrate inversion enables the dependence of the labeling step on diffusion, reduces nonspecific label adsorption, and improves the estimated detection limit by ∼30×. High-density maps of the signal across the two types of substrates also demonstrate that inversion in the labeling step results in a more uniform distribution of AuNP labels across the surface, which translates to a greater measurement reproducibility. These results, which are supported by model simulations via the Mason-Weaver sedimentation-diffusion equation, and their potential implications when using other nanoparticle labels and related materials in diagnostic tests and other applications, are briefly discussed.

Superstructures with diverse morphologies and highly ordered fullerene C60 arrays from 1 : 1 and 2 : 1 adamantane-C60 hybrid molecules.

Superstructures from fullerene C60-containing compounds, especially those tethered to rigid functional groups with defined shapes, remain largely unexplored. Being the smallest diamondoid, adamantane (Ad) can be viewed as a promising building block for the construction of well-defined superstructures. Here, we report the syntheses of 1 : 1 (4a) and 2 : 1 (4b) Ad-C60 hybrid molecules, which were then used to construct superstructures in binary solvent mixtures via a modified liquid/liquid interfacial precipitation (LLIP) method using CHCl3 as a good solvent. Typically in the combination of DMSO/CHCl3 with a final concentration (cf) of 1.0 mmol L-1, 4a successively forms spheres, plates, nanoflowers and plicated particles with increasing content of DMSO while 4b forms cuboid blocks and microparticles with hierarchically organized surfaces. Changing from DMSO to other poor solvents including acetone, MeOH and EtOAc leads to variations of the morphology of the superstructures for both 4a and 4b. At the nanometer length scale, 4a and 4b adopt different organizations within the superstructures. While 4a tends to self-organize into lamellae with highly ordered C60 layers, the hexagonal phase is dominant in the superstructures formed by 4b. Wettability tests indicate that films formed by the superstructures of 4a and 4b show anti-wetting properties. Besides the solvent effect, the morphology of the superstructures can be also tuned by concentration. For example, when cf is lowered to 0.5 mmol L-1, a new form of superstructure, i.e., fibers, was detected for 4a. Our results also indicate that besides the solvent-induced aggregate transition, gravity-induced sedimentation and subsequent structure ripening can have a significant influence on the final morphology of the superstructures and the aggregate transition pathways.

Double oil-in-oil-in-oil emulsions stabilised solely by particles

The stability of vegetable oil-in-100cS silicone oil-in-vegetable oil V/S/V emulsions to sedimentation and coalescence has been studied. The emulsions contained two types of silica particle of different surface silanol content, 70% and 20% SiOH, which prefer to stabilise 100cS silicone oil-in-vegetable oil S/V and vegetable oil-in-100cS silicone oil V/S emulsions respectively, in systems containing equal proportion of the oils. The emulsions were prepared in a two-step process and were characterised using the drop test and optical microscopy. The emulsions were completely stable to coalescence for over a month. However, they underwent sedimentation which decreases as the concentration of 70% SiOH silica particles stabilising the silicone oil globules increases. The gravity-induced sedimentation halted at a relatively high particle concentration (2wt.%) and negligible amount of the vegetable oils was released even after a month. Their average droplet diameter also decreases as the concentration of 70% SiOH silica particles increases. Values of oil-oil-solid particle contact angles θoo, measured on microscope glass slides composed of the particles, are in good agreement with the type of simple emulsions and hence double emulsions formed by the particles.

Critical Point in Space: A Quest for Universality

The behavior of matter near a 2nd order phase transition is expected to obey universal features. In particular, fluids, liquid mixtures, polymers, which belong to the same class of universality (the class of fluids) should exhibit the same universal scaling laws for many thermodynamics and kinetic parameters. Critical point slowing down is the most notorious. Such divergence or convergence makes the class of fluids extremely sensitive to even minute external disturbances and especially gravity: on earth the fluid becomes compressed under its own weight. Compensating for these effects by space experiments and/or magnetic forces or isotopic density matching has led to enlarge our vision of universality for phase transition. New phenomena have been discovered by suppressing gravity effects, as the thermal “Piston Effect”, which leads to a paradoxical critical point speeding up and the apparent violation of the 2nd thermodynamic law. Another finding is concerned with the use of critical slowing down and weightlessness to investigate the dynamics of phase separation with no gravity-induced sedimentation. The key role of the coalescence of domains makes valid only two simple growth laws. The latter can be successfully applied to a quite different situation, the evolution laws in the well-known biological problem of sorting of embryonic cells. Due to the extreme sensitivity of fluids near their critical point, the effect of vibration can be investigated in much detail, using only one fluid, which represents the whole class of fluids. The investigation of the above thermal and phase transition problems under vibrations indeed suggests that a periodic excitation can act as a kind of artificial gravity, which induces thermal convection, speeds up phase transition and localizes the liquid and vapor phases perpendicular to it.

Emulsions Stabilized by Microfibrillated Cellulose: The Effect of Hydrophobization, Concentration and O/W Ratio

The properties of emulsions stabilized by nanosized cellulose particles (denominated also microfibrillated cellulose; MFC) were investigated. The wettability (hydrophobicity) of the MFC was changed through surface modification with chlorodimethyl isopropylsilane. The modified MFC was characterized with Fourier transform infrared spectroscopy (FTIR), and x-ray photoelectron spectroscopy (XPS). Using MFC of intermediate hydrophobicity resulted in water-in-oil emulsions within a range of oil/water ratios. The emulsifying capacity of the MFC was found to be proportional to both the relative amount of MFC used and its wettability (degree of surface substitution; DSS). The emulsions separated rapidly from the excess oil phase; the separated emulsion layers were highly stable towards gravity-induced sedimentation and, once a stable MFC network was formed, were also stable towards coalescence. The influence of the oil/water ratio, the concentration and DSS of the MFC on the stability of the emulsions toward coalescence, and the inversion to oil-in-water emulsions was also investigated. The results verify that MFC can be efficiently used as a stabilizer of oil/water emulsions within a range of oil/water ratios and particle concentrations.

Effect of repulsive interactions on the rate of doublet formation of colloidal nanoparticles in the presence of convective transport

In this work, we have performed a systematic investigation of the effect of electrostatic repulsive interactions on the aggregation rate of colloidal nanoparticles to from doublets in the presence of a convective transport mechanism. The aggregation rate has been computed by solving numerically the Fuchs–Smoluchowski diffusion–convection equation. Two convective transport mechanisms have been considered: extensional flow field and gravity-induced relative sedimentation. A broad range of conditions commonly encountered in the applications of colloidal dispersions has been analyzed. The relative importance of convective to diffusive contributions has been quantified by using the Peclet number Pe. The simulation results indicate that, in the presence of repulsive interactions, the evolution of the aggregation rate as a function of Pe can always be divided into three distinct regimes, no matter which convective mechanism is considered. At low Pe values the rate of aggregation is independent of convection and is dominated by repulsive interactions. At high Pe values, the rate of aggregation is dominated by convection, and independent of repulsive interactions. At intermediate Pe values, a sharp transition between these two regimes occurs. During this transition, which occurs usually over a 10–100-fold increase in Pe values, the aggregation rate can change by several orders of magnitude. The interval of Pe values where this transition occurs depends upon the nature of the convective transport mechanism, as well as on the height and characteristic lengthscale of the repulsive barrier. A simplified model has been proposed that is capable of quantitatively accounting for the simulations results. The obtained results reveal unexpected features of the effect of ionic strength and particle size on the stability of colloidal suspensions under shear or sedimentation, which have relevant consequences in industrial applications.

Low-Shear Modelled Microgravity Environment Maintains Morphology and Differentiated Functionality of Primary Porcine Hepatocyte Cultures

Hepatocytes cultured in conventional static culture rapidly lose polarity and differentiated function. This could be explained by gravity-induced sedimentation, which prevents formation of complete three-dimensional (3D) cell-cell/cell-matrix interactions and disrupts integrin-mediated signals (including the most abundant hepatic integrin α₅β₁), important for cellular polarity and differentiation. Cell culture in a low fluid shear modelled microgravity (about 10^–2g) environment promotes spatial colocation/self-aggregation of dissociated cells and induction of 3D differentiated liver morphology. Previously, we demonstrated the utility of a NASA rotary bioreactor in maintaining key metabolic functions and 3D aggregate formation of high-density primary porcine hepatocyte cultures over 21 days. Using serum-free chemically defined medium, without confounding interactions of exogenous bioscaffolding or bioenhancing surface materials, we investigated features of hepatic cellular polarity and differentiated functionality, including expression of hepatic integrin α₅, as markers of functional morphology. We report here that in the absence of exogenous biomatrix scaffolding, hepatocytes cultured in serum-free chemically defined medium in a microgravity environment rapidly (<24 h) form macroscopic (2–5 mm), compacted 3D hepatospheroid structures consisting of a shell of glycogen-positive viable cells circumscribing a core of eosinophilic cells. The spheroid shell layers exhibited ultrastructural, morphological and functional features of differentiated, polarized hepatic tissue including strong expression of the integrin α₅ subunit, functional bile canaliculi, albumin synthesis, and fine ultrastructure reminiscent of in vivo hepatic tissue. The low fluid shear microgravity environment may promote tissue-like self-organization of dissociated cells, and offer advantages over spheroids cultured in conventional formats to delineate optimal conditions for enhanced directed tissue self-assembly.

Adhesion Receptors Mediate Efficient Non-viral Gene Delivery

For a variety of reasons, including production limitations, potential unanticipated side effects, and an immunological response upon repeated systemic administration, virus-based vectors are as yet not ideal gene delivery vehicles, justifying further research into alternatives. Unlike viral vectors, non-viral vectors pose minimal health risks, but to meet therapeutic requirements their efficacy needs major improvement. This goal may be accomplished by better defining the mechanism of non-viral gene delivery and exploiting specific cellular properties. Here we demonstrate that transfection of epithelial cells with lipoplexes is almost exclusively mediated by the beta1 integrin cell surface receptor. More important, we show that in general, adhesion receptors can be exploited by lipoplexes to gain access to cells, including difficult-to-transfect primary neural stem cells and suspension cells, thereby leading to productive transfection. We propose that adhesion receptors serve as "natural" receptors for lipoplexes. As no natural cellular receptors for lipoplexes have previously been identified, our results are an important step forward in understanding the mechanisms of non-viral gene delivery. Moreover, the finding that adhesion receptors mediate efficient non-viral gene delivery paves the way for the optimization of (standard) transfection procedures as well as ex vivo gene therapy protocols using non-viral vectors.

Ceramic matrix composites with gradient concentration of metal particles

The design of ceramic–metal composites with a gradient concentration of the metal particles fabricated by the slip casting method is presented. The gradient concentration of the metal particles was achieved through their sedimentation under the action of the gravitational force and a magnetic field. The paper describes the method of calculating the distance between the ceramic particles and indicates the correlation between this distance and the size of the metal particles. Preliminary experimental results of fabricated Al 2O 3–Fe composites confirmed the calculations. The metal particles, whose density is higher than that of ceramic particles, can undergo gravity-induced sedimentation only when the distance between the ceramic particles is greater than the diameter of these metal particles. With a slip casting containing 50 vol.% of solid phase, no concentration gradient of metal particles occurred. The Fe gradient concentration was achieved when the sedimentation was aided by a magnetic field.

Aerosol aspiration into a cylindrical sampler from a low-velocity downward flow and from calm air

The problem of aerosol aspiration into a two-dimensional cylindrical sampler from a low-velocity downward flow and from calm air is solved. A simple analytical model for the velocity field of the carrier medium in the vicinity of the sampler with allowance for the finite size of the input orifice is proposed. Parametric studies of the aspiration factor as a function of the Stokes number for different ratios of the free-stream and aspiration velocities and different gravity-induced sedimentation velocities for two positions of the sampler are performed. Sedimentation of particles on the lower side of the cylinder for the sampler with a downward-oriented orifice is discussed.

Kinetics of gravity-induced amyloplast sedimentation in statocytes of cress roots grown under fast clino-rotation, 1 g and after 180° inversion

In order to detail the relation between the initial positioning of statoliths and root graviresponsiveness, the movement of amyloplasts within root statocytes of cress ( Lepidium sativum L.) seedlings grown under fast clino-rotation (50 rpm) and vertically at 1 g or 180° inverted after growth at 1 g has been analyzed during a subsequent 6-min period of lateral (reoriented 90°) gravistimulation. The main parameters of statolith motion kinetics such as relative positions and motion velocities in transverse and longitudinal directions have been evaluated taking linear measurements of organelle position with respect to the lower longitudinal and distal wall in the cells of the 2nd–5th columella storeys. A significant discrepancy in the direction and velocity of statolith movement has been determined within the first and second minutes of root horizontal placement. Centrally or proximally located statoliths in clino-rotated or inverted root statocytes moved transversely to the lower longitudinal cell wall more quickly as compared with distally located ones in 1- g root cells. At the same time, the longitudinal displacement of proximally located statoliths towards the cell center has been determined, while this displacement of distally located plastids was significantly more pronounced. The longitudinal motion of amyloplasts grouped in the central cell part was negligible within this 2-min period. After the 6th minute of stimulation, the total downward shift of the mean relative position of statoliths amounted to 16.3% from the total cell width in clino-rotated root statocytes and 17.1% in the cells of inverted roots, while it equalled 13.3% in 1- g root statocytes. The total change of the relative longitudinal position of amyloplasts with respect to the distal cell wall remained almost unchanged in roots grown on the clinostat. In inverted roots, the initial longitudinal position of 56.2% from the total cell length has been shifted significantly to 47.8% and from 27.7% to 38.8% in 1- g roots. The data imply that a more pronounced sedimentation of centrally or proximally located amyloplasts during the first minutes of root stimulation could be responsible for the earlier generation and transmission of a directional signal which triggers the subsequent events of gravitropic reaction in clino-rotated or inverted roots as compared to 1- g roots.

Tip-localized actin polymerization and remodeling, reflected by the localization of ADF, profilin and villin, are fundamental for gravity-sensing and polar growth in characean rhizoids.

Polar organization and gravity-oriented, polarized growth of characean rhizoids are dependent on the actin cytoskeleton. In this report, we demonstrate that the prominent center of the Spitzenkörper serves as the apical actin polymerization site in the extending tip. After cytochalasin D-induced disruption of the actin cytoskeleton, the regeneration of actin microfilaments (MFs) starts with the reappearance of a flat, brightly fluorescing actin array in the outermost tip. The actin array rounds up, produces actin MFs that radiate in all directions and is then relocated into its original central position in the center of the Spitzenkörper. The emerging actin MFs rearrange and cross-link to form the delicate, subapical meshwork, which then controls the statolith positioning, re-establishes the tip-high calcium gradient and mediates the reorganization of the Spitzenkörper with its central ER aggregate and the accumulation of secretory vesicles. Tip growth and gravitropic sensing, which includes control of statolith positioning and gravity-induced sedimentation, are not resumed until the original polar actin organization is completely restored. Immunolocalization of the actin-binding proteins, actin-depolymerizing factor (ADF) and profilin, which both accumulate in the center of the Spitzenkörper, indicates high actin turnover and gives additional support for the actin-polymerizing function of this central, apical area. Association of villin immunofluorescence with two populations of thick undulating actin cables with uniform polarity underlying rotational cytoplasmic streaming in the basal region suggests that villin is the major actin-bundling protein in rhizoids. Our results provide evidence that the precise coordination of apical actin polymerization and dynamic remodeling of actin MFs by actin-binding proteins play a fundamental role in cell polarization, gravity sensing and gravity-oriented polarized growth of characean rhizoids.

Effect of the Hydrodynamic Interaction of a Large Number of Particles on Their Sedimentation Rate in a Viscous Fluid

The gravity-induced sedimentation of a large number of identical spherical particles in a viscous fluid is considered. The hydrodynamic interaction of all the particles is taken into account. The problem is modeled numerically for random locations of 1–100 particles forming different configurations. A dependence of the mean sedimentation rate on the particle number and concentration in the cloud is obtained and compared with previous results.

Gravity-induced sedimentation during melting and liquid phase sintering of Pb–Sn alloys

Gravity-induced phase separation during melting of Pb–Sn alloys results in a macroscopic concentration gradient that must be eliminated before phase equilibrium is established. When Pb–Sn alloys are heat treated in solid-plus-liquid regions (akin to liquid phase sintering), recession of the mushy (solid-plus-liquid) region accompanies elimination of the concentration gradient. Sedimentation apparently ceases as phase equilibrium is approached. Some solid-plus-liquid alloys do not sediment. These have a solid volume fraction sufficient to sterically hinder the gravity-induced phase separation. This critical volume fraction is about 0.50 for hypoeutectic (Pb-rich) alloys and about 0.75 for hypereutectic (Sn-rich) alloys.

Gravity perception requires statoliths settled on specific plasma membrane areas in characean rhizoids and protonemata.

The noninvasive infrared laser micromanipulation technique (optical tweezers, optical trapping) and centrifugation were used to study susception and perception, the early events in the gravitropic pathway of tip-growing characean rhizoids and protonemata. Reorientation of the growth direction in both cell types was only initiated when at least 2-3 statoliths settled on specific areas of the plasma membrane. This statolith-sensitive plasma membrane area is confined to the statolith region (10-35 microns behind the tip) in positively gravitropic rhizoids, whereas in negatively gravitropic protonemata, this area is limited to the apical plasma membrane (0-10 microns). Statolith sedimentation towards the sensitive plasma membrane areas is mediated by the concerted action of actin and gravity. The process of sedimentation, the pure physical movement, of statoliths is not sufficient to initiate graviresponses in both cell types. It is concluded that specific statolith-sensitive plasma membrane areas play a crucial role in the signal transduction pathway of gravitropism. These areas may represent the primary sites for gravity perception and may transform the information derived from the gravity-induced statolith sedimentation into physiological signals which trigger the molecular mechanisms of the opposite graviresponses in characean rhizoids and protonemata.

Prospects for use of microgravity‐based bioreactors to study three‐dimensional host—tumor interactions in human neoplasia

Microgravity offers unique advantages for the cultivation of mammalian tissues because the lack of gravity-induced sedimentation supports three-dimensional growth in batch culture in aqueous medium. Bioreactors that simulate microgravity but operate in unit gravity provide conditions that permit human epithelial cells to grow to densities approaching 10(7) cells/ml on microcarriers in suspension, in masses up to 1 cm in diameter, and under conditions of low shear stress. While useful for many different applications in tissue culture, this culture system is especially useful for the analysis of the microenvironment in which host matrix and cells interact with infiltrating tumor cells. Growth in the microgravity-based bioreactor has supported morphological differentiation of human colon carcinoma cells when cultured with normal human stromal cells. Furthermore, these co-cultures produced factors that stimulated goblet cell production in normal colon cells in an in vivo bioassay. Early experiments also suggest that the microgravity environment will not alter the ability of epithelial cells to recognize and associate with each other and with constituents of basement membrane and extracellular matrix. These findings suggest that cells grown in bioreactors that simulate aspects of microgravity or under actual microgravity conditions will produce tissues and substances in sufficient quantity and at high enough concentration to promote characterization of molecules that control differentiation and neoplastic transformation.