Single-digit-micrometer-resolution continuous liquid interface production.

To date, a compromise between resolution and print speed has rendered most high-resolution additive manufacturing technologies unscalable with limited applications. By combining a reduction lens optics system for single-digit-micrometer resolution, an in-line camera system for contrast-based sharpness optimization, and continuous liquid interface production (CLIP) technology for high scalability, we introduce a single-digit-micrometer-resolution CLIP-based 3D printer that can create millimeter-scale 3D prints with single-digit-micrometer-resolution features in just a few minutes. A simulation model is developed in parallel to probe the fundamental governing principles in optics, chemical kinetics, and mass transport in the 3D printing process. A print strategy with tunable parameters informed by the simulation model is adopted to achieve both the optimal resolution and the maximum print speed. Together, the high-resolution 3D CLIP printer has opened the door to various applications including, but not limited to, biomedical, MEMS, and microelectronics.

Injection continuous liquid interface production of 3D objects.

In additive manufacturing, it is imperative to increase print speeds, use higher-viscosity resins, and print with multiple different resins simultaneously. To this end, we introduce a previously unexplored ultraviolet-based photopolymerization three-dimensional printing process. The method exploits a continuous liquid interface-the dead zone-mechanically fed with resin at elevated pressures through microfluidic channels dynamically created and integral to the growing part. Through this mass transport control, injection continuous liquid interface production, or iCLIP, can accelerate printing speeds to 5- to 10-fold over current methods such as CLIP, can use resins an order of magnitude more viscous than CLIP, and can readily pattern a single heterogeneous object with different resins in all Cartesian coordinates. We characterize the process parameters governing iCLIP and demonstrate use cases for rapidly printing carbon nanotube-filled composites, multimaterial features with length scales spanning several orders of magnitude, and lattices with tunable moduli and energy absorption.

A theory for the coexistence of coiled and stretched configurational phases in the extensional flow of entangled polymer melts.

It has recently been demonstrated via nonequilibrium molecular dynamics (NEMD) simulation [M. H. Nafar Sefiddashti, B. J. Edwards, and B. Khomami, J. Chem. Phys. 148, 141103 (2018); Phys. Rev. Lett. 121, 247802 (2018)] that the extensional flow of entangled polymer melts can engender, within a definite strain-rate regime [expressed in terms of the Deborah number (De) based on the Rouse time], the coexistence of separate domains consisting primarily of either coiled or stretched chain-like macromolecules. This flow-induced phase separation results in bimodal configurational distributions, where transitions of individual molecules between the coiled and stretched states occur very slowly by hopping over an apparent energy activation barrier. We demonstrate that the qualitative aspects of this phenomenon can be described via the single-mode Rolie-Poly model including Convective Constraint Release (CCR) and finite extensibility of the chain-like macromolecules. This analysis reveals the physical mechanism for the configurational coexistence, namely, the nonlinear rate of change of the average entropic restoring force of a given entangled chain with extension. Under conditions of significant flow-induced disentanglement, the rate of change of the effective restoring force initially decreases with extension (effective spring softening) and then increases (hardens) as the maximum chain length is approached. When balanced by flow-induced chain stretching, we find that there can be two configuration states within the same De regime, as covered by the NEMD simulations; therefore, a region of conformational coexistence can indeed exist. However, we demonstrate that this coexistence of configurational microstates is only possible when the magnitude of the CCR parameters is consistent with the rate of flow-induced disentanglement, as observed in the NEMD simulations.

Correction: A system for the high-throughput measurement of the shear modulus distribution of human red blood cells.

Correction for 'A system for the high-throughput measurement of the shear modulus distribution of human red blood cells' by Amir Saadat et al., Lab Chip, 2020, 20, 2927-2936, DOI: 10.1039/D0LC00283F.

A system for the high-throughput measurement of the shear modulus distribution of human red blood cells.

Reduced deformability of red blood cells (RBCs) can affect the hemodynamics of the microcirculation and reduce oxygen transport efficiency. It is also well known that reduced RBC deformability is a signature of various physical disorders, including sepsis, and that the primary determinant of RBC deformability is the membrane shear modulus. To measure the distribution of an individual's RBC shear modulus with high throughput, we a) developed a high-fidelity computational model of RBCs in confined microchannels to inform design decisions; b) created a novel experimental system combining microfluidic flow, imaging, and image analysis; and c) performed automated comparisons between measured quantities and numerical predictions to extract quantitative measures of the RBC shear modulus for each of thousands of cells. We applied our computational simulation platform to construct the appropriate deformability figure(s) of merit to quantify RBC stiffness based on an experimentally measured, steady-state cell shape in flow through a microchannel. In particular, we determined a shape parameter based on the second moment of the cell shape that is sensitive to the changes in the membrane stiffness and cell size. We then conducted microfluidic experiments and developed custom automated image processing codes to identify and track the position and shape of individual RBCs within micro-constrictions. The fabricated microchannels include a square cross-section imaging region (7 by 7 µm) and we applied order 10 kPa pressure differences to induce order 10 mm s-1 cell velocities. The combination of modeling, microfluidics, and imaging enables, for the first time, quantitative measurement of the shear moduli of thousands of RBCs in human blood samples. We demonstrate the high-throughput features by sensitive quantification of the changes in the distribution of RBC stiffness with aging. This combined measurement and computational platform is ultimately intended to diagnose blood cell disorders in patients.

Three-dimensional simulations of undulatory and amoeboid swimmers in viscoelastic fluids.

Microorganisms often move through viscoelastic environments, as biological fluids frequently have a rich microstructure owing to the presence of large polymeric molecules. Research on the effect of fluid elasticity on the swimming kinematics of these organisms has usually been focused on those that move via cilia or flagellum. Experimentally, Shen (X. N. Shen et al., Phys. Rev. Lett., 2011, 106, 208101) reported that the nematode C. elegans, a model organism used to study undulatory motion, swims more slowly as the Deborah number describing the fluid's elasticity is increased. This phenomenon has not been thoroughly studied via a fully resolved three-dimensional simulation; moreover, the effect of fluid elasticity on the swimming speed of organisms moving via euglenoid movement, such as E. gracilis, is completely unknown. In this study, we discuss the simulation of the arbitrary motion of an undulating or pulsating swimmer that occupies finite volume in three dimensions, with the ability to specify any differential viscoelastic rheological model for the surrounding fluid. To accomplish this task, we use a modified version of the Immersed Finite Element Method presented in a previous paper by Guido and Saadat in 2018 (A. Saadat et al., Phys. Rev. E, 2018, 98, 063316). In particular, this version allows for the simulation of deformable swimmers such that they evolve through an arbitrary set of specified shapes via a conformation-driven force. From our analysis, we observe several key trends not found in previous two-dimensional simulations or theoretical analyses for C. elegans, as well as novel results for the amoeboid motion. In particular, we find that regions of high polymer stress concentrated at the head and tail of the swimming C. elegans are created by strong extensional flow fields and are associated with a decrease in swimming speed for a given swimming stroke. In contrast, in two dimensions these regions of stress are commonly found distributed along the entire body, likely owing to the lack of a third dimension for polymer relaxation. A comparison of swim speeds shows that the calculations in two-dimensional simulations result in an over-prediction of the speed reduction. We believe that our simulation tool accurately captures the swimming motion of the two aforementioned model swimmers and furthermore, allows for the simulation of multiple deformable swimmers, as well as more complex swimming geometries. This methodology opens many new possibilities for future studies of swimmers in viscoelastic fluids.

In Vitro Measurement and Modeling of Platelet Adhesion on VWF-Coated Surfaces in Channel Flow.

The process of platelet adhesion is initiated by glycoprotein (GP)Ib and GPIIbIIIa receptors on the platelet surface binding with von Willebrand factor on the vascular walls. This initial adhesion and detachment of a single platelet is a complex process that involves multiple bonds forming and breaking and is strongly influenced by the surrounding blood-flow environment. In addition to bond-level kinetics, external factors such as shear rate, hematocrit, and GPIb and GPIIbIIIa receptor densities have also been identified as influencing the platelet-level rate constants in separate studies, but this still leaves a gap in understanding between these two length scales. In this study, we investigate the fundamental relationship of the dynamics of platelet adhesion, including these interrelating factors, using a coherent strategy. We build a, to our knowledge, novel and computationally efficient multiscale model accounting for multibond kinetics and hydrodynamic effects due to the flow of a cellular suspension. The model predictions of platelet-level kinetics are verified by our microfluidic experiments, which systematically investigate the role of each external factor on platelet adhesion in an in vitro setting. We derive quantitative formulas describing how the rates of platelet adhesion, translocation, and detachment are defined by the molecular-level kinetic constants, the local platelet concentration near the reactive surface determined by red-blood-cell migration, the platelet effective reactive area due to its tumbling motion, and the platelet surface receptor density. Furthermore, if any of these aspects involved have abnormalities, e.g., in a disease condition, our findings also have clinical relevance in predicting the resulting change in the adhesion dynamics, which is essential to hemostasis and thrombosis.

Extravasation of Brownian Spheroidal Nanoparticles through Vascular Pores.

In modern cancer treatment, there is significant interest in studying the use of drug molecules either directly injected into the bloodstream or delivered by nanoparticle (NP) carriers of various shapes and sizes. During treatment, these carriers may extravasate through pores in the tumor vasculature that form during angiogenesis. We provide an analytical, computational, and experimental examination of the extravasation of point particles (e.g., drug molecules) and finite-sized spheroidal particles. We study the advection-diffusion process in a model microvasculature, consisting of a shear flow over and a pressure-driven suction flow into a circular pore in a flat surface. For point particles, we provide an analytical formula [Formula: see text] for the dimensionless Sherwood number S, i.e., the extravasation rate, in terms of the pore entry resistance (Damköhler number κ), the shear rate (Péclet number P), and the suction flow rate (suction strength Q). Brownian dynamics (BD) simulations verify this result, and our simulations are then extended to include finite-sized NPs, in which no analytical solutions are available. BD simulations indicate that particles of different geometries have drastically different extravasation rates in different flow conditions. In general, extreme aspect ratio particles provide a greater flux through the pore because of favorable alignment with streamlines entering the pore and less hindered interaction with the pore. We validate the BD simulations by measuring the in vitro transport of both bacteriophage MS2 (a spherical NP) and free dye (a model drug molecule) across a porous membrane. Despite their vastly different sizes, BD predicts S = 8.53 E-4 and S = 27.6 E-4, and our experiments agree favorably, with Sexp=10.6 E-4± 1.75 E-4 and Sexp=16.3 E-4 ± 3.09 E-4, for MS2 and free dye, respectively, thus demonstrating the practical utility of our simulation framework.

The Effect of Hematocrit on Platelet Adhesion: Experiments and Simulations.

The volume fraction of red blood cells (RBCs) in a capillary affects the degree to which platelets are promoted to marginate to near a vessel wall and form blood clots. In this work we investigate the relationship between RBC hematocrit and platelet adhesion activity. We perform experiments flowing blood samples through a microfluidic channel coated with type 1 collagen and observe the rate at which platelets adhere to the wall. We compare these results with three-dimensional boundary integral simulations of a suspension of RBCs and platelets in a periodic channel where platelets can adhere to the wall. In both cases, we find that the rate of platelet adhesion varies greatly with the RBC hematocrit. We observe that the relative decrease in platelet activity as hematocrit falls shows a similar profile for simulation and experiment.

Experimental observation of the asymmetric instability of intermediate-reduced-volume vesicles in extensional flow.

Vesicles provide an attractive model system to understand the deformation of living cells in response to mechanical forces. These simple, enclosed lipid bilayer membranes are suitable for complementary theoretical, numerical, and experimental analysis. A recent study [Narsimhan, Spann, Shaqfeh, J. Fluid Mech., 2014, 750, 144] predicted that intermediate-aspect-ratio vesicles extend asymmetrically in extensional flow. Upon infinitesimal perturbation to the vesicle shape, the vesicle stretches into an asymmetric dumbbell with a cylindrical thread separating the two ends. While the symmetric stretching of high-aspect-ratio vesicles in extensional flow has been observed and characterized [Kantsler, Segre, Steinberg, Phys. Rev. Lett., 2008, 101, 048101] as well as recapitulated in numerical simulations by Narsimhan et al., experimental observation of the asymmetric stretching has not been reported. In this work, we present results from microfluidic cross-slot experiments observing this instability, along with careful characterization of the flow field, vesicle shape, and vesicle bending modulus. The onset of this shape transition depends on two non-dimensional parameters: reduced volume (a measure of vesicle asphericity) and capillary number (ratio of viscous to bending forces). We observed that every intermediate-reduced-volume vesicle that extends forms a dumbbell shape that is indeed asymmetric. For the subset of the intermediate-reduced-volume regime we could capture experimentally, we present an experimental phase diagram for asymmetric vesicle stretching that is consistent with the predictions of Narsimhan et al.

In vitro measurement of particle margination in the microchannel flow: effect of varying hematocrit.

It has long been known that platelets undergo margination when flowing in blood vessels, such that there is an excess concentration near the vessel wall. We conduct experiments and three-dimensional boundary integral simulations of platelet-sized spherical particles in a microchannel 30 µm in height to measure the particle-concentration distribution profile and observe its margination at 10%, 20%, and 30% red blood cell hematocrit. The experiments involved adding 2.15-µm-diameter spheres into a solution of red blood cells, plasma, and water and flowing this mixture down a microfluidic channel at a wall shear rate of 1000 s(-1). Fluorescence imaging was used to determine the height and velocity of particles in the channel. Experimental results indicate that margination has largely occurred before particles travel 1 cm downstream and that hematocrit plays a role in the degree of margination. With simulations, we can track the trajectories of the particles with higher resolution. These simulations also confirm that margination from an initially uniform distribution of spheres and red blood cells occurs over the length scale of O(1 cm), with higher hematocrit showing faster margination. The results presented here, from both experiments and 3D simulations, may help explain the relationship between bleeding time in vessel trauma and red blood cell hematocrit as platelets move to a vessel wall.

Examining platelet adhesion via Stokes flow simulations and microfluidic experiments.

While critically important, the platelet function at the high shear rates typical of the microcirculation is relatively poorly understood. Using a large scale Stokes flow simulation, Zhao et al. recently showed that RBC-induced velocity fluctuations cause platelets to marginate into the RBC free near-wall region [Zhao et al., Physics of Fluids, 2012, 24, 011902]. We extend their work by investigating the dynamics of platelets in shear after margination. An overall platelet adhesion model is proposed in terms of a continuous time Markov process and the transition rates are established with numerical simulations involving platelet-wall adhesion. Hydrodynamic drag and Brownian forces are calculated with the boundary element method, while the RBC collisions are incorporated through an autoregressive process. Hookean springs with first order bond kinetics are used to model receptor-ligand bonds formed between the platelet and the wall. The simulations are compared with in vitro microfluidic experiments involving platelet adhesion to Von Willebrand Factor (VWF) coated surfaces.

Singular perturbation theory for predicting extravasation of Brownian particles.

Motivated by recent studies on tumor treatments using the drug delivery of nanoparticles, we provide a singular perturbation theory and perform Brownian dynamics simulations to quantify the extravasation rate of Brownian particles in a shear flow over a circular pore with a lumped mass transfer resistance. The analytic theory we present is an expansion in the limit of a vanishing Péclet number (P), which is the ratio of convective fluxes to diffusive fluxes on the length scale of the pore. We state the concentration of particles near the pore and the extravasation rate (Sherwood number) to O(P1/2). This model improves upon previous studies because the results are valid for all values of the particle mass transfer coefficient across the pore, as modeled by the Damköhler number (κ), which is the ratio of the reaction rate to the diffusive mass transfer rate at the boundary. Previous studies focused on the adsorption-dominated regime (i.e., κ → ∞). Specifically, our work provides a theoretical basis and an interpolation-based approximate method for calculating the Sherwood number (a measure of the extravasation rate) for the case of finite resistance [κ ~ O(1)] at small Péclet numbers, which are physiologically important in the extravasation of nanoparticles. We compare the predictions of our theory and an approximate method to Brownian dynamics simulations with reflection-reaction boundary conditions as modeled by κ. They are found to agree well at small P and for the κ ≪ 1 and κ ≫ 1 asymptotic limits representing the diffusion-dominated and adsorption-dominated regimes, respectively. Although this model neglects the finite size effects of the particles, it provides an important first step toward understanding the physics of extravasation in the tumor vasculature.

Shear-induced platelet margination in a microchannel.

The lateral migration of platelets in a microchannel is studied numerically where the hydrodynamic interactions between red cells, platelets, and vessel walls are resolved by the Stokes flow boundary integral equations. The simulations provide a clear physical description of the expulsion of the platelets by the velocity fluctuations in the core cellular flow toward the cell-depleted layer near the wall. The lateral migration is shown to be diffusional, and we further demonstrate, in agreement with previous experiments, that the diffusivity scales sublinearly with the shear rate if the relevant capillary number, Ca<1, as a result of its intrinsic dependence on the deformation of red cells.

The shear flow processing of controlled DNA tethering and stretching for organic molecular electronics.

DNA has been recently explored as a powerful tool for developing molecular scaffolds for making reproducible and reliable metal contacts to single organic semiconducting molecules. A critical step in the process of exploiting DNA-organic molecule-DNA (DOD) array structures is the controlled tethering and stretching of DNA molecules. Here we report the development of reproducible surface chemistry for tethering DNA molecules at tunable density and demonstrate shear flow processing as a rationally controlled approach for stretching/aligning DNA molecules of various lengths. Through enzymatic cleavage of λ-phage DNA to yield a series of DNA chains of various lengths from 17.3 µm down to 4.2 µm, we have investigated the flow/extension behavior of these tethered DNA molecules under different flow strengths in the flow-gradient plane. We compared Brownian dynamic simulations for the flow dynamics of tethered λ-DNA in shear, and found our flow-gradient plane experimental results matched well with our bead-spring simulations. The shear flow processing demonstrated in our studies represents a controllable approach for tethering and stretching DNA molecules of various lengths. Together with further metallization of DNA chains within DOD structures, this bottom-up approach can potentially enable efficient and reliable fabrication of large-scale nanoelectronic devices based on single organic molecules, therefore opening opportunities in both fundamental understanding of charge transport at the single molecular level and many exciting applications for ever-shrinking molecular circuits.

The conformational dynamics of lambda-DNA in the anti-Brownian electrokinetic trap: Brownian dynamics and Monte Carlo simulation.

In this work, we examine the conformational dynamics of long polymer molecules under confinement, as in the recently developed anti-Brownian electrokinetic (ABEL) trap [A. Cohen and W. Moerner, Proc. Natl. Acad. Sci. USA. 103, 4362 (2006)]. We analyze polymer motion using Brownian dynamics simulations (bead-spring and bead-rod models) and via Monte Carlo methods. We first verify Cohen and Moerner's (2007) single molecule observations regarding the existence of short time correlations [Phys. Rev. Lett. 98, 116001 (2007)] in the motion of a polymer's center of mass, which arise due to fluctuating hydrodynamic interactions. Thereafter, following Cohen and Moerner, we use principal component analysis to extract the principal modes governing polymer conformation and find that confinement and backbone bending only affect small polymers and should not play a significant role in the dynamics of long polymers such as lambda-DNA. We find excellent agreement between our principal component analysis modes and those measured by Cohen and Moerner [Proc. Natl. Acad. Sci. U.S.A. 104, 12622 (2007)]. Finally, to explore the effect of excluded volume, in particular, the effect of the excluded volume parameter (z), we use image-image correlations to examine its relation to polymer dynamics. Image-image correlation measurements performed on lambda-DNA in the ABEL trap did not display a simple exponential-type behavior and motivated the use of stretched exponential functions to determine the characteristic timescale (tau) governing conformational dynamics. We show that tau scales with polymer length as N(2) and decreases with increasing z. Furthermore, we can collapse a variety of data when tauN(-2) is plotted with respect to N/z(m) (m=0.14 for freespace and 0.366 for walls).

Hydrodynamic interactions in metal rodlike-particle suspensions due to induced charge electroosmosis.

We present a theoretical and experimental study of the role of hydrodynamic interactions on the motion and dispersion of metal rodlike particles in the presence of an externally applied electric field. In these systems, the electric field polarizes the particles and induces an electroosmotic flow relative to the surface of each particle. The simulations include the effect of the gravitational body force, buoyancy, far-field hydrodynamic interactions, and near-field lubrication forces. The particles in the simulations and experiments were observed to experience repeated pairing interactions in which they come together axially with their ends approaching each other, slide past one another until their centers approach, and then push apart. These interactions were confirmed in measurements of particle orientations and velocities, pair distribution functions, and net dispersion of the suspension. For large electric fields, the pair distribution functions show accumulation and depletion regions consistent with many pairing events. For particle concentrations of 10;{8}particles/mL and higher, dispersion within the suspension dramatically increases with increased field strength.

Brownian demixing and wall effects in sedimenting suspensions of orientable particles.

We describe the Brownian demixing of sedimenting suspensions, a recently discovered phenomenon in which increases in the thermal energy can destabilize a system of orientable particles subjected to a torque to fluctuations in concentration. Through use of Brownian dynamics simulation and a mean-field analysis, we demonstrate that demixing occurs in a model system composed of slender rigid rods sedimenting between no-slip walls. Additionally, we describe the effects of wall separation distance on suspension stability.

Dynamics of DNA tumbling in shear to rotational mixed flows: pathways and periods.

The tumbling dynamics of DNA have been examined via experiments and Brownian dynamics (BD) simulations in mixed flows that vary from pure shear to pure rotation. In shear, tumbling pathways and periods agree well with earlier studies; in rotation-dominated flows, a new tumbling pathway is identified and experimentally observed. Based on these results, we have developed robust scaling laws for DNA tumbling in both shear and rotational flows and have found a critical flow-type parameter for transition from the shearlike flow regime to the rotation-dominated one.

Ergodicity breaking and conformational hysteresis in the dynamics of a polymer tethered at a surface stagnation point.

We study the dynamics of long chain polymer molecules tethered to a plane wall and subjected to a stagnation point flow. Using a combination of theory and numerical techniques, including Brownian dynamics (BD), we demonstrate that a chain conformation hysteresis exists even for freely draining (FD) chains. Hydrodynamic interactions (HI) between the polymer and the wall are included in the BD simulations. We find qualitative agreement between the FD and HI simulations, with both exhibiting simultaneous coiled and stretched states for a wide range of fixed flow strengths. The range of state coexistence is understood by considering an equivalent projected equilibrium problem of a two state reaction. Using this formalism, we construct Kramers rate theory (from the inverse mean first passage time for a Markov process) for the hopping transition from coil to stretch and stretch to coil. The activation energy for this rate is found to scale proportionally to chain length or Kuhn step number. Thus, in the limit of infinite chain size the hopping rates at a fixed value of the suitably defined Deborah number approach zero and the states are "frozen." We present the results that demonstrate this "ergodicity breaking."