Modeling Biofilm Formation on Dynamically Reconfigurable Composite Surfaces

We augment the dissipative particle dynamics (DPD) simulation method to model the salient features of biofilm formation. We simulate a cell as a particle containing hundreds of DPD beads and specify <i>p</i>, the probability of breaking the bond between the particle and surface or between the particles. At the early stages of film growth, we set <i>p</i> = 1, allowing all bonding interactions to be reversible. Once the bound clusters reach a critical size, we investigate scenarios where <i>p</i> = 0, so that incoming species form irreversible bonds, as well as cases where <i>p</i> lies in the range of 0.1–0.5. Using this approach, we examine the nascent biofilm development on a coating composed of a thermoresponsive gel and the embedded rigid posts. We impose a shear flow and characterize the growth rate and the morphology of the clusters on the surface at temperatures above and below <i>T</i>_c, the volume phase transition temperature of a gel that displays lower critical solubility temperature (LCST). At temperatures above <i>T</i>_c, the posts effectively inhibit the development of the nascent biofilm. For temperatures below <i>T</i>_c, the swelling of the gel plays the dominant role and prevents the formation of large clusters of cells. Both these antifouling mechanisms rely on physical phenomena and, hence, are advantageous over chemical methods, which can lead to unwanted, deleterious effects on the environment

Flow-Driven Assembly of Microcapsules into Three-Dimensional Towers

By harnessing biochemical signaling and chemotaxis, unicellular slime molds can aggregate on a surface to form a long, vertical stalk. Few synthetic systems can self-organize into analogous structures that emerge out of the plane. Through computational modeling, we devise a mechanism for assembling tower-like structures using microcapsules in solution as building blocks. In the simulations, chemicals diffusing from a central patch on a surface produce a concentration gradient, which generates a radially directed diffusioosmotic flow along the surface toward the center. This toroidal roll of a fluid pulls the microcapsules along the surface and lifts them above the patch. As more capsules are drawn toward the patch, some units are pushed off the surface but remain attached to the central microcapsule cluster. The upward-directed flow then draws out the cluster into a tower-like shape. The final three-dimensional (3D) structure depends on the flow field, the attractive capsule–capsule and capsule–surface interaction strengths, and the sedimentation force on the capsules. By tuning these factors, we can change the height of the structures that are produced. Moreover, by patterning the areas of the wall that are attractive to the capsules, we can form multiple vertical strands instead of a single tower. Our approach for flow-directed assembly can permit the growth of reconfigurable, 3D structures from simple subunits

Coassembly of Nanorods and Photosensitive Binary Blends: “Combing” with Light To Create Periodically Ordered Nanocomposites

Using computational modeling, we establish a means of controlling structure formation in nanocomposites that encompass nanorods and a photosensitive binary blend. The complex cooperative interactions in the system include a preferential wetting interaction between the rods and one of the phases in the blend, steric repulsion between the coated rods, and the response of the binary blend to light. Under uniform illumination, the binary mixture undergoes both phase separation and a reversible chemical reaction, leading to a morphology resembling that of a microphase-separated diblock copolymer. When a second, higher intensity light source is rastered over the sample, the binary blend and the nanorods coassemble into regular, periodically ordered structures. In particular, the system displays an essentially defect-free lamellar morphology, with the nanorods localized in the energetically favorable domains. By varying the speed at which the secondary light is rastered over the sample, we can control the directional alignment of the rods within the blend. Our approach yields an effective route for achieving morphological control of both the polymeric components and nanoparticles, providing a means of tailoring the properties and ultimate performance of the composites

Coassembly of Nanorods and Photosensitive Binary Blends: “Combing” with Light To Create Periodically Ordered Nanocomposites

Using computational modeling, we establish a means of controlling structure formation in nanocomposites that encompass nanorods and a photosensitive binary blend. The complex cooperative interactions in the system include a preferential wetting interaction between the rods and one of the phases in the blend, steric repulsion between the coated rods, and the response of the binary blend to light. Under uniform illumination, the binary mixture undergoes both phase separation and a reversible chemical reaction, leading to a morphology resembling that of a microphase-separated diblock copolymer. When a second, higher intensity light source is rastered over the sample, the binary blend and the nanorods coassemble into regular, periodically ordered structures. In particular, the system displays an essentially defect-free lamellar morphology, with the nanorods localized in the energetically favorable domains. By varying the speed at which the secondary light is rastered over the sample, we can control the directional alignment of the rods within the blend. Our approach yields an effective route for achieving morphological control of both the polymeric components and nanoparticles, providing a means of tailoring the properties and ultimate performance of the composites

Harnessing Interfacially-Active Nanorods to Regenerate Severed Polymer Gels

With newly developed computational approaches, we design a nanocomposite that enables self-regeneration of the gel matrix when a significant portion of the material is severed. The cut instigates the dynamic cascade of cooperative events leading to the regrowth. Specifically, functionalized nanorods localize at the new interface and initiate atom transfer radical polymerization with monomers and cross-linkers in the outer solution. The reaction propagates to form a new cross-linked gel, which can be tuned to resemble the uncut material

Harnessing Interfacially-Active Nanorods to Regenerate Severed Polymer Gels

With newly developed computational approaches, we design a nanocomposite that enables self-regeneration of the gel matrix when a significant portion of the material is severed. The cut instigates the dynamic cascade of cooperative events leading to the regrowth. Specifically, functionalized nanorods localize at the new interface and initiate atom transfer radical polymerization with monomers and cross-linkers in the outer solution. The reaction propagates to form a new cross-linked gel, which can be tuned to resemble the uncut material

Harnessing Interfacially-Active Nanorods to Regenerate Severed Polymer Gels

With newly developed computational approaches, we design a nanocomposite that enables self-regeneration of the gel matrix when a significant portion of the material is severed. The cut instigates the dynamic cascade of cooperative events leading to the regrowth. Specifically, functionalized nanorods localize at the new interface and initiate atom transfer radical polymerization with monomers and cross-linkers in the outer solution. The reaction propagates to form a new cross-linked gel, which can be tuned to resemble the uncut material

Optimizing Micromixer Surfaces To Deter Biofouling

Using computational modeling, we show that the dynamic interplay between a flowing fluid and the appropriately designed surface relief pattern can inhibit the fouling of the substrate. We specifically focus on surfaces that are decorated with three-dimensional (3D) chevron or sawtooth “micromixer” patterns and model the fouling agents (e.g., cells) as spherical microcapsules. The interaction between the imposed shear flow and the chevrons on the surface generates 3D vortices in the system. We pinpoint a range of shear rates where the forces from these vortices can rupture the bonds between the two mobile microcapsules near the surface. Notably, the patterned surface offers fewer points of attachment than a flat substrate, and the shear flows readily transport the separated capsules away from the layer. We contrast the performance of surfaces that encompass rectangular posts, chevrons, and asymmetric sawtooth patterns and thereby identify the geometric factors that cause the sawtooth structure to be most effective at disrupting the bonding between the capsules. By breaking up nascent clusters of contaminant cells, these 3D relief patterns can play a vital role in disrupting the biofouling of surfaces immersed in flowing fluids

Convective Self-Sustained Motion in Mixtures of Chemically Active and Passive Particles

We develop a model to describe the behavior of a system of active and passive particles in solution that can undergo spontaneous self-organization and self-sustained motion. The active particles are uniformly coated with a catalyst that decomposes the reagent in the surrounding fluid. The resulting variations in the fluid density give rise to a convective flow around the active particles. The generated fluid flow, in turn, drives the self-organization of both the active and passive particles into clusters that undergo self-sustained propulsion along the bottom wall of a microchamber. This propulsion continues until the reagents in the solution are consumed. Depending on the number of active and passive particles and the structure of the self-organized cluster, these assemblies can translate, spin, or remain stationary. We also illustrate a scenario in which the geometry of the container is harnessed to direct the motion of a self-organized, self-propelled cluster. The findings provide guidelines for creating autonomously moving active particles, or chemical “motors” that can transport passive cargo in microfluidic devices

Self-Healing Vesicles Deposit Lipid-Coated Janus Particles into Nanoscopic Trenches

Using dissipative particle dynamics (DPD) simulations, we model the interaction between nanoscopic lipid vesicles and Janus nanoparticles localized on an adhesive substrate in the presence of an imposed flow. The system is immersed in a hydrophilic solution, and the hydrophilic substrate contains nanoscopic trenches, which are either step- or wedge-shaped. The fluid-driven vesicle successfully picks up Janus particles on the substrate, transports these particles as cargo along the surface, and then drops off the particles into the trenches. For Janus particles with a relatively large hydrophobic region, lipids from the bilayer membrane become detached from the vesicle and bound to the hydrophobic domain of the deposited particle. While the detachment of these lipids rips the vesicle, it provides a coating that effectively shields the hydrophobic portion of the nanoparticle from the outer solution. After the particle has been dropped off, the torn vesicle undergoes structural rearrangement, reforming into a closed structure that resembles its original shape. In effect, the vesicle displays pronounced adaptive behavior, shedding lipids to form a protective coating around the particle and undergoing a self-healing process after the particle has been deposited. This responsive, adaptive behavior is observed in cases involving both the step- and wedge-shaped trenches, but the step trench is more effective at inducing particle drop off. The results reveal that the introduction of grooves or trenches into a hydrophilic surface can facilitate the targeted delivery of amphiphilic particles by self-healing vesicles, which could be used for successive delivery events