Biomedical subjects

Anna C Balazs

Publications and source records attributed to Anna C Balazs.

At least 19 recordsLinked to original sources

PubMed2007

Healing substrates with mobile, particle-filled microcapsules: designing a 'repair and go' system.

We model the rolling motion of a fluid-driven, particle-filled microcapsule along a heterogeneous, adhesive substrate to determine how the release of the encapsulated nanoparticles can be harnessed to repair damage on the underlying surface. We integrate the lattice Boltzmann model for hydrodynamics and the lattice spring model for the micromechanics of elastic solids to capture the interactions between the elastic shell of the microcapsule and the surrounding fluids. A Brownian dynamics model is used to simulate the release of nanoparticles from the capsule and their diffusion into the surrounding solution. We focus on a substrate that contains a damaged region (e.g. a crack or eroded surface coating), which prevents the otherwise mobile capsule from rolling along the surface. We isolate conditions where nanoparticles released from the arrested capsule can repair the damage and thereby enable the capsules to again move along the substrate. Through these studies, we establish guidelines for designing particle-filled microcapsules that perform a 'repair and go' function and thus, can be utilized to repair damage in microchannels and microfluidic devices.

Adhesives↗

PubMed2007

Modeling self-assembly and phase behavior in complex mixtures.

Using a variety of computational techniques, I investigate how the self-assembly of complex mixtures can be guided by surfaces or external stimuli to form spatially regular or temporally periodic patterns. Focusing on mixtures in confined geometries, I examine how thermodynamic and hydrodynamic effects can be exploited to create regular arrays of nanowires or monodisperse, particle-filled droplets. I also show that an applied light source and chemical reaction can be harnessed to create hierarchically ordered patterns in ternary, phase-separating mixtures. Finally, I consider the combined effects of confining walls and a chemical reaction to demonstrate that a swollen polymer gel can be driven to form dynamically periodic structures. In addition to illustrating the effectiveness of external factors in directing the self-organization of multicomponent mixtures, the selected examples illustrate how coarse-grained models can be used to capture both the equilibrium phase behavior and the dynamics of these complex systems.

Journal Article↗

PubMed2006

Modeling the release of nanoparticles from mobile microcapsules.

The authors present a novel computational approach to simulate both the release of nanoparticles from a microcapsule, which is moving through a microchannel, and the adsorption of the released particles onto the channel walls. By integrating the lattice spring model for the micromechanics of elastic solids and the lattice Boltzmann model for fluid dynamics, they simulate the relevant fluid-structure interactions in the system. In particular, they capture the dynamic interactions among the capsule's elastic shell, the encapsulated fluid, and the external, host solution. The nanoparticles are treated as "tracer particles" and their motion is modeled via a Brownian dynamics simulation. An imposed pressure gradient drives the capsule to move along an adhesive substrate and the particles are released from the surface of this mobile capsule. The authors determine how the elasticity of the capsule, the strength of the capsule-surface adhesion and the diffusion coefficient of the nanoparticles affect the relative amount of particles that are adsorbed onto the substrate. In addition to showing that the compliant nature of the capsule can significantly affect the nanoparticle deposition, they isolate a range of parameters for maximizing the adsorbed amount. The findings yield guidelines for optimizing the efficiency of microcapsule carriers in the targeted delivery of nanoparticles.

Journal Article↗

PubMed2006

Nanoparticle polymer composites: where two small worlds meet.

The mixing of polymers and nanoparticles is opening pathways for engineering flexible composites that exhibit advantageous electrical, optical, or mechanical properties. Recent advances reveal routes to exploit both enthalpic and entropic interactions so as to direct the spatial distribution of nanoparticles and thereby control the macroscopic performance of the material. For example, by tailoring the particle coating and size, researchers have created self-healing materials for improved sustainability and self-corralling rods for photovoltaic applications. A challenge for future studies is to create hierarchically structured composites in which each sublayer contributes a distinct function to yield a mechanically integrated, multifunctional material.

Journal Article↗

PubMed2006

Pattern formation and shape changes in self-oscillating polymer gels.

We developed an efficient model for responsive gels that captures large-scale, two-dimensional (2D) deformations and chemical reactions within a swollen polymer network. The 2D calculations allowed us to probe not only volume changes but also changes in sample shape. By focusing on gels undergoing the oscillatory Belousov-Zhabotinsky reaction, we observed traveling waves of local swelling that form a rich variety of dynamic patterns and give rise to distinctive oscillations in the gel's shape. The observed patterns depend critically on the gel's dimensions. The approach provides a useful computational tool for probing the dynamics of chemomechanical processes and uncovering morphological transformations in responsive gels.

Journal Article↗

PubMed2006

Designing a simple ratcheting system to sort microcapsules by mechanical properties.

Using computational modeling, we analyze the fluid-driven motion of compliant particles over a rigid, saw-toothed surface. The particles are modeled as fluid-filled elastic shells and, thus, simulate ex vivo biological cells or polymeric microcapsules. Through the model, we demonstrate how the patterned surface and an oscillatory shear flow can be combined to produce a ratcheting motion, yielding a straightforward method for sorting these capsules by their relative stiffness. Since the approach exploits the capsule's inherent response to the substrate, it does not involve explicit measurement and assessment. Because the process utilizes an oscillatory shear, the sorting can be accomplished over a relatively short portion of the substrate. Due to these factors, this sorting mechanism can prove to be both efficient and relatively low-cost.

Journal Article↗

PubMed2006

Dynamics of ternary mixtures with photosensitive chemical reactions: creating three-dimensionally ordered blends.

Using computer simulations, we establish an approach for creating defect-free, periodically ordered polymeric materials. The system involves ABC ternary mixtures where the A and B components undergo a reversible photochemical reaction. In addition, all three components are mutually immiscible and undergo phase separation. Through the simulations, we model the effects of illuminating a three-dimensional (3D) sample with spatially and temporally dependent light irradiation. Experimentally, this situation can be achieved by utilizing both a uniform background light and a spatially localized, higher intensity light, and then rastering a higher-intensity light over the 3D sample. We first focus on the case where the higher-intensity light is held stationary and focused in a distinct region within the system. The C component is seen to displace the A and B within this region and replicate the pattern formed by the higher-intensity light. In effect, one can write a pattern of C onto the AB binary system by focusing the higher-intensity light in the desired arrangement. We isolate the conditions that are necessary for producing clearly written patterns of C (i.e., for obtaining sharp interfaces between the C and A/B domains). We next consider the effect of rastering a higher-intensity light over this sample and find that this light "combs out" defects in the AB blend as it moves through the system. The resulting material displays a defect-free structure that encompasses both a periodic ordering of the A and B domains and a well-defined motif of C. In this manner, one can create hierarchically patterned materials that exhibit periodicity over two distinct length scales. The approach is fully reversible, noninvasive, and points to a novel means of patterning with homopolymers, which normally do not self-assemble into periodic structures.

Journal Article↗

PubMed2006

Designing compliant substrates to regulate the motion of vesicles.

By integrating mesoscale models for hydrodynamics and micromechanics, we examine fluid-driven motion of vesicles on compliant surfaces. The vesicles, modeled as fluid-filled elastic shells, represent biological cells or polymeric microcapsules. Focusing on nonspecific interactions between these vesicles and synthetic substrates, we isolate mechanically and topographically patterned surfaces that transmit stop and go instructions, causing the vesicles to halt at specific locations, and with an increase in the flow velocity, to resume moving. For surfaces containing arrays of compliant posts, the substrates also affect the vesicles' gait, causing them to "crawl," "walk," or "jump." The latter behavior could promote the intermixing of reactants that are encapsulated within the microcapsules. Such control over vesicle dynamics can facilitate various biological assays and fabrication of arrays of mobile microreactors.

Computer Simulation↗

PubMed2006

Exploiting photoinduced reactions in polymer blends to create hierarchically ordered, defect-free materials.

Computer simulations reveal how photoinduced chemical reactions can be exploited to create long-range order in binary and ternary polymeric materials. The process is initiated by shining a spatially uniform light over a photosensitive AB binary blend, which thereby undergoes both a reversible chemical reaction and phase separation. We then introduce a well-collimated, higher intensity light source. Rastering this secondary light over the sample locally increases the reaction rate and causes formation of defect-free, spatially periodic structures. These binary structures resemble either the lamellar or hexagonal phases of microphase-separated diblock copolymers. We measure the regularity of the ordered structures as a function of the relative reaction rates for different values of the rastering speed and determine the optimal conditions for creating defect-free structures in the binary systems. We then add a nonreactive homopolymer C, which is immiscible with both A and B. We show that this component migrates to regions that are illuminated by the secondary, higher intensity light, allowing us to effectively write a pattern of C onto the AB film. Rastering over the ternary blend with this collimated light now leads to hierarchically ordered patterns of A, B, and C. The findings point to a facile, nonintrusive process for manufacturing high quality polymeric devices in a low-cost, efficient manner.

Journal Article↗

PubMed2005

Modeling the flow of fluid/particle mixtures in microchannels: encapsulating nanoparticles within monodisperse droplets.

We develop a hybrid computational approach for simulating mixtures of binary fluids and mobile, submicron particles. The model couples a lattice Boltzmann method for the binary fluid with a Brownian dynamics model for the particles. The particles can exhibit preferential wetting interactions with the different components of the fluid. As an illustration of the method, we carry out simulations in two dimensions to compare the spinodal decomposition of a pure binary fluid with the phase separation of binary blends that contain either fixed or mobile particles. We then isolate conditions where the flow of a binary fluid/particle mixture past surfaces with well-defined asperities leads to the formation of monodisperse droplets, which encapsulate the nanoparticles. The findings provide guidelines for creating multiphase emulsions with well-controlled morphologies.

Journal Article↗

PubMed2005

Local control of periodic pattern formation in binary fluids within microchannels.

Simulations show that a local perturbation (a "hot spot") at the inlet of a microchannel can be a source of traveling concentration waves in a binary fluid and can be exploited to create periodic, oscillatory patterns within the channel. We isolate two distinctly different types of traveling periodic patterns and estimate the wavelength of the structures. Our findings provide the first example of how a local perturbation can be used to create well-defined periodic patterns in two-phase flow within a confined geometry and can lead to new routes for creating gradient materials.

Journal Article↗

PubMed2005

Harnessing light to create defect-free, hierarchically structured polymeric materials.

Computer simulations reveal how photoinduced chemical reactions in polymeric mixtures can be exploited to create long-range order in materials with features that range from the submicron to the nanoscale. The process is initiated by shining a spatially uniform light on a photosensitive AB binary blend, which thereby undergoes both a reversible chemical reaction and a phase separation. When a well-collimated, higher intensity light is rastered over the sample, the system forms defect-free, spatially periodic structures. If a nonreactive homopolymer C is added to the system, this component localizes in regions that are irradiated with a higher intensity light, and one can effectively "write" a pattern of C onto the AB film. Rastering over the ternary blend with the collimated light now leads to hierarchically ordered patterns of A, B, and C. Because our approach involves homopolymers, it significantly expands the range of materials that can be fashioned into a periodic pattern. The findings point to a facile process for manufacturing high-quality polymeric components in an efficient manner.

Letter↗

PubMed2005

Pattern formation arising from condensation of a homogeneous gas into a binary, phase-separating liquid.

We examine the nucleated growth of a binary, immiscible liquid drop within a homogeneous gas. The system couples the growth of the liquid drop with the phase separation of the immiscible components and, thus, can potentially reveal novel pattern formation. To carry out this study, we first characterize the thermodynamic properties of the system in terms of an appropriate Ginzburg-Landau free energy density. By minimizing this free energy, we construct the equilibrium phase diagram for the system. We then use a lattice Boltzmann algorithm to solve the hydrodynamic equations describing the dynamical evolution of the fluid. We observe intriguing tentaclelike structures within the nucleation and growth regime and explore how the formation of these structures depends on the thermodynamic and transport properties of the system. We give scaling laws describing domain growth in both the diffusion- and flow-limited regimes. The results highlight the novel physics that can emerge when there is interplay between the ordering of a density and a concentration field.

Journal Article↗

PubMed2005

Newtonian fluid meets an elastic solid: coupling lattice Boltzmann and lattice-spring models.

We integrate the lattice Boltzmann model (LBM) and lattice spring model (LSM) to capture the coupling between a compliant bounding surface and the hydrodynamic response of an enclosed fluid. We focus on an elastic, spherical shell filled with a Newtonian fluid where no-slip boundary conditions induce the interaction. We calculate the "breathing mode" oscillations for this system and find good agreement with analytical solutions. Furthermore, we simulate the impact of the fluid-filled, elastic shell on a hard wall and on an adhesive surface. Understanding the dynamics of fluid-filled shells, especially near adhesive surfaces, can be particularly important in the design of microcapsules for pharmaceutical and other technological applications. Our studies reveal that the binding of these capsules to specific surfaces can be sensitive to the physical properties of both the outer shell and the enclosed fluid. The integrated LBM-LSM methodology opens up the possibility of accurately and efficiently capturing the dynamic coupling between fluid flow and a compliant bounding surface in a broad variety of systems.

Journal Article↗

PubMed2005

Modeling the morphology and mechanical properties of sheared ternary mixtures.

Through a combination of simulation techniques, we determine both the structural evolution and mechanical properties of blends formed from immiscible ternary mixtures. In this approach, we first use the lattice Boltzmann method to simulate the phase separation dynamics of A/B/C fluid mixtures for varying compositions within the spinodal region. We also investigate the effect of an imposed shear on the phase ordering of the mixture. We assume that the fluid is quenched sufficiently rapidly that the phase-separated structure is preserved in the resultant solid. Then, the output from our morphological studies serves as the input to the lattice spring model, which is used to simulate the elastic response of solids to an applied deformation. These simulations reveal how the local stress and strain fields and the global Young's modulus depend on the composition of the blend and the stiffness of the components. By comparing the results for the sheared and unsheared cases, we can isolate optimal processing conditions for enhancing the mechanical performance of the blends. Overall, the findings provide fundamental insight into the relationship between structure, processing, and properties for heterogeneous materials and can yield guidelines for formulating blends with the desired macroscopic mechanical behavior.

Journal Article↗

PubMed2005

Convection-driven pattern formation in phase-separating binary fluids.

Using a thermal-lattice Boltzmann model, we examine the rich phase behavior that develops when partially miscible fluids evolve in the presence of a vertical temperature gradient, which encompasses the critical temperature T(c) of the mixture. In particular, a binary AB fluid is confined between two plates in a gravitational field. The upper plate is fixed below T(c) and hence, the nearby fluid phase separates into A-rich and B-rich domains. The lower plate is fixed above the temperature T(c), and the surrounding fluid is in the homogeneous phase. A coupling between convection (driven by the temperature gradient) and phase separation gives rise to unique pattern formation. A number of regimes are identified: regularly spaced stripes, convective steady-state columns, the periodic disturbance of these columns, and finally, chaotic dripping from the upper surface. These results highlight dynamical behavior in partially miscible mixtures.

Journal Article↗

PubMed2004

Theoretical model of interfacial polymerization.

We present a theoretical description for the creation of a thin polymeric layer through the interfacial polymerization of two immiscible, low molecular weight liquids. The theory specifically takes into account the effects of polydispersity on the formation of the polymer film at the liquid-liquid interface. Consequently, we can describe the structure of the growing film and the molecular weight distribution of the resultant polymer chains. We focus on a model system where alternating AB copolymers are formed at the interface between phase-separated, low molecular weight species A and B. It is assumed that any A(B) unit can reversibly attach to any available B(A) unit or B(A)-ended chain. The formation of the copolymer layer is described by a system of reaction-diffusion equations, which detail the chemical evolution and diffusive dynamics of the polydisperse mixture of AB copolymers around the interface, and the evolution of the interface itself. Using this model, we determine the effects of the chemical reaction rates and the initial conditions on the kinetics of forming the AB copolymer layer and the structure of this film.

Journal Article↗

PubMed2004

Pattern formation in binary fluids confined between rough, chemically heterogeneous surfaces.

Using a mesoscale model for hydrodynamics, we simulate driven flow of AB binary fluids past surfaces that contain well-defined roughness or asperities. The geometry and wetting properties of the asperities are found to have a dramatic effect on the flow patterns. We isolate conditions where the A fluid forms vertical bands that bridge the asperities and an imposed shear (or pressure gradient) drives the system to form monodisperse droplets of A within the B fluid. The size of the droplets can be tailored by varying the morphology of the asperities. The surfaces needed to create this rich dynamical behavior are used as the stamps in microcontact printing; thus, the parameter space can readily be accessed experimentally, and the predictions suggest an efficient method for forming emulsions with well-controlled morphologies.

Journal Article↗