Multilayered Polymersome Formed by Amphiphilic Asymmetric Macromolecular Brushes

Dissipative particle dynamics simulations on the self-assembly of rod-coil asymmetric diblock molecular brushes bearing responsive side chains

The self-assembly behaviors of rod-coil asymmetric diblock molecular brushes (ADMBs) bearing responsive side chains in a selective solvent are investigated via dissipative particle dynamics simulations. By systematically varying the polymerization degree, copolymer concentration, and side chain length, several morphological phase diagrams were constructed. ADMB assemblies exhibited a rich variety of morphologies, including cylindrical micelles, spherical micelles, nanowires, polyhedral micelles, ellipsoid micelles, and large compound micelles. The structures of the representative nanowires were analyzed in detail. A kinetics study revealed that the one-dimensional growth of nanowires follows the step-growth polymerization mechanism. Besides, by calculating the local order parameter of the rigid chains, we found that increasing the lengths of A and C side chains can promote the ordered arrangement of the rigid chains. Moreover, the rod-to-coil conformation transitions were simulated to explore the stimuli-responsive behaviors of ADMBs with responsive rigid side chains. The simulation results indicated that the volume of the assemblies expanded without the support of the rigid chains. The present work not only provides a comprehensive understanding of the self-assembly behaviors of ADMBs but also provides meaningful theoretical support for the development of novel molecular brush materials.

Ultrastable and Redispersible Zwitterionic Bottlebrush Micelles for Drug Delivery

Bottlebrush copolymers are increasingly used for drug delivery and biological imaging applications in part due to the enhanced thermodynamic stability of their self-assemblies. Herein, we discuss the effect of hydrophilic block chemistry on the stability of bottlebrush micelles. Amphiphilic bottlebrushes with zwitterionic poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) and nonionic polyethylene glycol (PEG) hydrophilic blocks were synthesized by "grafting from" polymerization and self-assembled into well-defined spherical micelles. Colloidal stability and stability against disassembly were challenged under high concentrations of NaCl, MgSO4, sodium dodecyl sulfate, fetal bovine serum, and elevated temperature. While both types of micelles appeared to be stable in many of these conditions, those with a PMPC shell consistently surpassed their PEG analogs. Moreover, when repeatedly subjected to lyophilization/resuspension cycles, PMPC micelles redispersed with no apparent variation in size or dispersity even in the absence of a cryoprotectant; PEG micelles readily aggregated. The observed excellent stability of PMPC micelles is attributed to the low critical micelle concentration of the bottlebrushes as well as to the strong hydration shell caused by ionic solvation of the phosphorylcholine moieties. Zwitterionic micelles were loaded with doxorubicin, and higher loading capacity/efficiency, as well as delayed release, was observed with increasing side-chain length. Finally, hemocompatibility studies of PMPC micelles demonstrated no disruption to the red blood cell membranes. The growing concern regarding the immunogenicity of PEG-based systems propels the search for alternative hydrophilic polymers; in this respect and for their outstanding stability, zwitterionic bottlebrush micelles represent excellent candidates for drug delivery and bioimaging applications.

Synthetic Cells from Droplet-Based Microfluidics for Biosensing and Biomedical Applications

Synthetic cells function as biological mimics of natural cells by mimicking salient features of cells such as metabolism, response to stimuli, gene expression, direct metabolism, and high stability. Droplet-based microfluidic technology presents the opportunity for encapsulating biological functional components in uni-lamellar liposome or polymer droplets. Verified by its success in the fabrication of synthetic cells, microfluidic technology is widely replacing conventional labor-intensive, expensive, and sophisticated techniques justified by its ability to miniaturize and perform batch production operations. In this review, an overview of recent research on the preparation of synthetic cells through droplet-based microfluidics is provided. Different synthetic cells including lipid vesicles (liposome), polymer vesicles (polymersome), coacervate microdroplets, and colloidosomes, are systematically discussed. Efforts are then made to discuss the design of a variety of microfluidic chips for synthetic cell preparation since the combination of microfluidics with bottom-up synthetic biology allows for reproductive and tunable construction of batches of synthetic cell models from simple structures to higher hierarchical structures. The recent advances aimed at exploiting them in biosensors and other biomedical applications are then discussed. Finally, some perspectives on the challenges and future developments of synthetic cell research with microfluidics for biomimetic science and biomedical applications are provided.

Instability of membranes containing ionizable cationic lipids: Effects of the repulsive range of headgroups and tail structures

The stability of membranes formed by ionizable cationic lipids, which constitute the primary components in lipid nanoparticles capable of endosomal escape, is explored using coarse-grained dissipative particle dynamics. Three types of ionizable model lipids with different tail structures are considered. Endosome acidification causes the ionization of lipids, leading to an increased repulsive range between their headgroups. When electrostatic repulsion is modeled as a conservative force with a long-range cutoff distance (rc,HH), the membrane and vesicle experience a loss of structural integrity and develop holes as rc,HH is beyond a critical value, which varies with the tail structure. When Coulombic repulsion is explicitly incorporated and intensified, a fully ionized lipid membrane undergoes a loss of structural integrity, displaying a qualitative similarity to the effect observed with the increase in rc,HH on the membrane stability. Qualitatively similar results are obtained for partially ionized membranes as the fraction of charged lipids increases. The stability of a mixed lipid membrane containing both ionizable and conventional lipids is also investigated. The disruption of the bilayer structure occurs for a sufficiently high charged fraction. The membrane instability can be attributed to the decrease in the packing parameter, which significantly deviates from unity as the interaction range increases.

Self-assembly of amphiphilic asymmetric comb-like copolymers with responsive rigid side chains

Amphiphilic asymmetric comb-like copolymers (AACCs) exhibit distinct self-assembly behaviours due to their unique architecture. However, the synthetic difficulties of well-defined AACCs have prohibited a systematic understanding of the architecture-morphology relationship. In this work, we conducted dissipative particle dynamics simulations to investigate the self-assembly behaviours of AACCs with responsive rigid side chains in selective solvents. The effects of side chain length, number of branches, and spacers on the morphology of aggregates were investigated by mapping out morphology diagrams. Besides, the numbers and surface areas of aggregates clearly depicted the morphological transitions during the self-assembly process. Moreover, the rod-to-coil conformation transitions were simulated to explore the stimuli-responsive behaviour of the AACCs with responsive rigid side chains by adjusting the bond angle parameter of the rigid chains. The results indicated that without the support of the rigid chains, the assembly structure collapsed, leading to the tube-to-channelized micelles and one-compartment-to-multicompartment vesicle morphology transformations. The simulation results are consistent with earlier experimental results, which can provide theoretical guidance for assembly toward desired nanostructures.

Amphiphilic Heterograft Copolymers Bearing Biocompatible/Biodegradable Grafts

The amphiphilic heterograft copolymers bearing biocompatible/biodegradable grafts [poly(2-methyl-2-oxazoline-co-2-pentyl-2-oxazoline)-g-poly(d-l-lactic acid)/poly(2-ethyl-2-oxazoline)] were synthesized successfully by the combination of cationic ring-opening polymerization and click chemistry via the ⟨"grafting to"⟩ approach. The challenge of this synthesis was to graft together hydrophobic and hydrophilic chains on a hydrophilic platform based on PMeOx. The efficiency of grafting depends on the chemical nature of the grafts and of the length of the macromolecular chains. The self-assembly of these polymers in aqueous media was investigated by DLS, cryo-TEM, and SANS. The results demonstrated that different morphologies were obtained from nanospheres and vesicles to filaments depending on the hydrophilic weight ratio in the heterograft copolymer varying from 0.38 until 0.84. As poly(2-ethyl-2-oxazoline) is known to be thermoresponsive, the influence of temperature rise on the nanoassembly stability was studied in water and in a physiological medium. SANS and DLS measurements during a temperature ramp allowed to show that nanoassemblies start to self-assemble in "raspberry like" primary structures at 50 °C, and these structures grow and get denser as the temperature is increased further. These amphiphilic heterograft copolymers may include hydrophobic drugs and should find important applications for biomedical applications which require stealth properties.

Synthesis and Aggregation Behavior of Hexameric Quaternary Ammonium Salt Surfactant Tz-6C12QC

A hexameric quaternary ammonium salt surfactant Tz-6C12QC featuring a rigid triazine spacer and six ammonium groups was synthesized. The molecular structure and aggregation behavior of Tz-6C12QC were characterized by nuclear magnetic resonance spectroscopy, surface tension, conductivity, dynamic light scattering, and transmission electron microscopy, etc. Dissipative particle dynamics (DPD) simulation was employed to investigate the self-assembly behavior of Tz-6C12QC at different concentrations. The rheological behavior of the polyacrylamide/Tz-6C12QC system was characterized by shear rheology. The results indicated that Tz-6C12QC exhibited superior surface activity and lower surface tension compared to conventional surfactants. Rheology analysis revealed that Tz-6C12QC had a significant viscosity reduction effect on polyacrylamide. DLS and TEM indicated that, as the concentration of Tz-6C12QC increased, monomer associations, spherical aggregations, vesicles, tubular micelles, and bilayer vesicles were sequentially formed in the solution. This study presents a synthetic approach for polysurfactants with a rigid spacer and sheds light on the self-assembly process of micelles.

Morphological Transitions in Micelles of Amphiphilic Bottlebrushes upon the Adsorption and Compression at the Liquid Interface

Densely grafted comb-like macromolecules (bottlebrushes) with alternating solvophobic and solvophilic side chains were studied in a selective solvent and at the liquid interface using mesoscopic computer simulations. The effects of backbone length and copolymer composition were considered. While self-assembly in solution revealed only spherical aggregates for all ar-chitectures studied, adsorption onto the liquid interface in particular cases resulted in morpho-logical changes, with worm-like aggregates or a continuous monolayer observed. In turn, the compression of macromolecules at the interface also leads to morphological transitions, includ-ing the formation of a mesh-like percolated structure. The obtained results may be useful for the preparation of solid nanoparticles of anisotropic shape or nanostructured ultra-thin copolymer films.

Atypical vesicles and membranes with monolayer and multilayer structures formed by graft copolymers with diblock side-chains: nonlamellar structures and curvature-enhanced permeability

Graft copolymers with diblock side-chains A_m(-graft-B₃A_y)_n in a selective solvent have been reported to self-assemble into vesicles, but the structure is expected to differ distinctly from those of lipid bilayers. Surprisingly, the number of alternating hydrophobic A-block and hydrophilic B-block layers in the vesicle can vary from a monolayer to multilayers such as the hepta-layer, subject to the same copolymer concentration. The area density of the copolymer layer is not uniform across the membrane. This structural difference among different layers is attributed to the neighboring environment and the curvature of the layer. Because of the unusual polymer conformations, nonlamellar structures of polymersomes are formed, and they are much more intricate than those of liposomes. In fact, a copolymer can contribute to a single or two hydrophilic layers, and it can provide up to three hydrophobic layers. The influence of the backbone length (m) and side-chain length (y) and the permeation dynamics are also studied. The thickness of hydrophobic layers is found to increase with increasing side-chain length but is not sensitive to the backbone length. Although the permeation time increases with the layer number for planar membranes, the opposite behavior is observed for spherical vesicles owing to the curvature-enhanced permeability associated with Laplace pressure.

Computational design of Janus polymersomes with controllable fission from double emulsions

Janus polymer vesicles (polymersomes) with biphasic membranes have special properties and potential applications in many fields. The big barrier for the preparation of Janus polymersomes lies in the difficulty of complete lateral microphase separation of polymers along the vesicle membrane due to the limited mobility. Herein, we present a systematic simulation study to provide a new strategy for the fabrication of Janus polymersomes based on water-in-oil-in-water double emulsions. Two incompatible block copolymers of AB and AC completely separate into two hemispheres of the polymersome driven by the dewetting of double emulsions, followed by the stabilization of the Janus structure with the block copolymers BC at the interface between AB and AC hemispheres. The simulation results demonstrate the formation of Janus polymersomes in a wide range of the incompatibility between blocks B and C. In addition, the morphologies of the Janus polymersomes can be readily regulated by changing the number of copolymers BC, the ratio of AB to AC, and the dewetting rate of organic solvents. Both the Janus and patchy polymersomes can be obtained through the adjustment of the dewetting rate. Besides, by introducing stimulus-cleavable copolymers of BC, the Janus polymersomes can perform controllable fission. Further comparison with similar experiments has also demonstrated the feasibility of our strategy. We believe the present work will be useful for the fabrication of polymersomes with controlled patches in a large quantity, and the stimulus-responsive fission process will also make the polymersomes promising in some applications like controlled drug delivery and cytomimetic membrane communication.

Effect of side-chain length on solute encapsulation by amphiphilic heterografted brush copolymers

Anisotropic nanomaterials are non-spherical structures that possess unique shape-dependent physicochemical properties and functionalities. Inspired by the abundance of filamentous entities in nature, cylindrical nanostructures have gained significant attention due to their unique performance. Herein, we discuss the effect of side-chain length on the encapsulation properties of amphiphilic heterografted bottlebrushes. We observed that by grafting a long hydrophilic block to the double-brush, we were able to restrict solute-induced conformational changes, thus producing drug-loaded anisotropic carriers. Unimolecular encapsulation in brushes was solute-dependent as shown here for probucol and rose bengal lactone. Stabilization with an amphiphilic diblock copolymer-consisting of the same type of blocks as those comprising the heterografted brush-served to explain the solute-dependent behavior observed for brushes, suggesting that solutes with a higher propensity to nucleation could be more effectively stabilized by the anisotropic carrier in a unimolecular worm-like construct.

Design, Synthesis, and Self-Assembly of Janus Bottlebrush Polymers

Janus bottlebrush polymers are a class of special molecular brushes, which have two immiscible side chains on the repeating unit of the backbone. The characteristic architectures of Janus bottlebrush polymers enable unique self-assembly properties and broad applications. Recently, remarkable advances of Janus bottlebrush polymers have been achieved for polymer chemistry and material science. This review summarizes the synthetic strategies of Janus bottlebrush polymers, and highlights the self-assembly applications. Finally, the challenges and opportunities are proposed for the further development.

Self-assembly of amphiphilic polymers of varying architectures near attractive surfaces

We use coarse-grained molecular dynamics simulations to investigate the assembly of A-B amphiphilic polymers near/on surfaces as a function of polymer architecture and surface attraction to the solvophobic B-block in the polymer. We study four polymer architectures: linear, bottlebrush with a backbone that is longer than each of the side chains, bottlebrush where the solvophobic backbone is similar in length to each of the side chains, and 'star-like' architectures where the backbone is significantly shorter than the side chain lengths. For each architecture and surface-B attraction, we quantify the assembled aggregate structure (i.e., aggregation number, domain shapes and sizes) and the chain conformations (i.e., components of the chain radius of gyration) on and away from the surface. For all the architectures and surface-B attraction strengths, the assembled structure away from the surface is similar to the assembly observed in bulk systems without surfaces. Near/on the surface, the assembled B-blocks form domains whose shapes and sizes are dependent on the surface-B attraction strength and the ability of the B-block in the polymer architecture to change conformations and pack on the surface. Domain sizes formed from linear and 'star-like' polymer architectures show the highest sensitivity to surface-B-block attraction strength, transitioning from hemispherical to disordered domains with increasing attraction strength. In contrast, bottlebrushes with long backbones and short side chains transition from hemispherical to striped to continuous domains with increasing surface-B attraction strength. Bottlebrushes with similar solvophobic backbone and side chain lengths form hemispherical domains that do not change significantly with the surface-B-block attraction strength. These computational results can guide experimentalists in their choices of surface chemistry and polymer architecture to achieve desired assembled domain shapes and sizes on the surface.

Amphiphilic multicomponent molecular brushes

Multicomponent molecular brushes containing amphiphilic polymer moieties are promising objects of research of macromolecular chemistry. The development of stimulus-responsive systems sensitive to changes in environmental parameters, based on the molecular brushes, opens up new possibilities for their applications in medicine, biochemistry and microelectronics. The review presents the current understanding of the structures of main types of amphiphilic multicomponent brushes, depending on the chemical nature and type of coupling of the backbone and side chains. The approaches to the controlled synthesis of multicomponent molecular brushes of different architecture are analyzed. Self-assembly processes of multicomponent molecular brushes in selective solvents are considered.

The bibliography includes 259 references.

Synthesis and characterization of amphiphilic branched silica derivatives associated with oligomeric medium

Amphiphilic branched silica derivatives associated with oligomeric medium (ASiP) were obtained using tetraethoxysilane, polyoxyethylene glycol and low molecular weight polydimethylsiloxane. The creation of a silica core was based on tetraethoxysilane hydrolysis and condensation reactions by using water and a potassium diethylene glycolate as the catalyst. These reactions proceeded with the sequential participation of polyoxyethylene glycol and polydimethylsiloxane in parallel transetherification reactions. Microporous polymer film based on 2,4-toluene diisocyanate and block copolymers of propylene and ethylene oxides with terminal potassium-alcoholate groups were modified by ASiP. It has been shown that ASiP at the phase interface between thermodynamically incompatible macrochains performs the function of a link. It leads to a significant increase of intermolecular interactions and the supramolecular organization of the modified microporous polymers.

The scheme of ASiP formation.

Molecular dynamics simulation study of linear, bottlebrush, and star-like amphiphilic block polymer assembly in solution

In this study we investigate the effect of varying branched polymer architectures on the assembly of amphiphilic block polymers in solution using coarse-grained molecular dynamics simulations. We quantify assembly structure (e.g., aggregation number, assembly morphology, and micelle core size) and thermodynamics (e.g., unimer to micelle transition conditions) as a function of increasing solvophobicity of the solvophobic block in the copolymer for three broad categories of polymer architectures: linear, 'bottlebrush' (with many short side chains on a long backbone), and 'star-like' (with few long side chains on a short backbone). Keeping the total number of coarse-grained beads in each polymer (or polymer molecular weight) constant, as we go from either linear or 'star-like' to 'bottlebrush' polymer architectures, the micelle aggregation number and micelle core size decrease, and the solvophobicity required for assembly (i.e., transition solvophobicity) increases. This trend is linked to the topological/steric hinderance for making solvophobic bead contacts between neighboring polymers for the 'bottlebrush' polymer architecture compared to the linear or 'star-like' architectures. We are able to identify some universal trends in assembly by plotting the assembly structure and thermodynamics data as a function of branching parameter defined as the ratio of the branched chain to the linear chain radius of gyration in the unimer state, and the relative lengths of the backbone versus side chain. The results in this paper guide how one could manipulate the amphiphilic block polymer assembly structure and thermodynamics by choosing appropriate polymer architecture, block sequence, and composition.

Self-assembly and stimuli-responsive behaviours of side-chain liquid crystalline copolymers: a dissipative particle dynamics simulation approach

Side-chain liquid crystalline copolymers are able to self-assemble into various aggregates in selective solvents, in particular, deformed structures.

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 p, the probability of breaking the bond between the particle and surface or between the particles. At the early stages of film growth, we set p = 1, allowing all bonding interactions to be reversible. Once the bound clusters reach a critical size, we investigate scenarios where p = 0, so that incoming species form irreversible bonds, as well as cases where p 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 T_c, the volume phase transition temperature of a gel that displays lower critical solubility temperature (LCST). At temperatures above T_c, the posts effectively inhibit the development of the nascent biofilm. For temperatures below T_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.

Systematic design and application of unimolecular star-like block copolymer micelles: a coarse-grained simulation study

Unimolecular polymeric micelles have several features, such as thermodynamic stability, small particle size, biocompatibility, and the ability to internalize hydrophobic molecules. These micelles have recently attracted significant attention in various applications, such as nano-reactors, catalysis, and drug delivery. However, few attempts have explored the formation mechanisms and conditions of unimolecular micelles due to limited experimental techniques. In this study, a unimolecular micelle system formed from β-cyclodextrin-graft-{poly(lactide)-block-poly(2-(dimethylamino) ethyl multimethacrylate)-block-poly[oligo (2-ethyl-2-oxazoline) methacrylate]} β-CD-g-(PLA-b-PDMAEMA-b-PEtOxMA) star-like block copolymers in aqueous media was investigated by dissipative particle dynamics (DPD) to explore the formation process of unimolecular micelles. The simulation results showed that using longer hydrophobic or pH-sensitive chains, shorter hydrophilic backbones, smaller hydrophilic side chain grafting density, and fewer polymer arms resulted in micellar aggregation. Furthermore, this unimolecular polymeric micelle could be used for encapsulating gold nanoparticles, whose mesoscopic structure was also explored. The gold nanoparticles tended to distribute in the middle layer formed by PDMAEMA, and the unimolecular micelles were capable of impeding gold nanoparticle aggregation. This study could help understand the formation mechanism of unimolecular micelles formed from star-like block copolymers in dilute solutions and offer a theoretical guide to the design and preparation of promising unimolecular polymeric micelles with targeting properties.

Nanoparticle encapsulation in vesicles formed by amphiphilic diblock copolymers

We investigated the co-assembly of nanoparticles P and amphiphilic diblock copolymers AB in selective solvents using a dissipative particle dynamics (DPD) method. By controlling the nanoparticle concentration and the interaction parameter between the hydrophobic blocks and the solvents, we found that the aggregation morphology can be changed from rod-like micelles to disk-like micelles and further to vesicles. The ratio of the hydrophobic/hydrophilic block and the nanoparticle concentration largely affects the structural characteristics of vesicles and the dispersion of nanoparticles. Copolymers with a longer hydrophobic block length are more likely to form vesicles with a smaller aqueous cavity size and vesicle size as well as a thicker wall. At the same time, the nanoparticles in the hydrophobic membrane tend to locate closer to the center of the vesicle and they become more compactly organized. A significant discovery has found that the larger the nanoparticle concentration, the smaller the aqueous cavity and the larger the vesicle size. We can also locate the nanoparticles at the center of spherical micelles or the hydrophobic membranes of vesicles by varying the nanoparticle concentration. This provides an effective and simple method to prepare size-controlled vesicles containing nanoparticles, project the localization of nanoparticles within the vesicles, and even tune the distance between the nanoparticles.

Asymmetric Polymersomes from an Oil-in-Oil Emulsion: A Computer Simulation Study

Asymmetric vesicles are valuable for understanding and mimicking cell and practical biomedicine applications. Recently, a very straightforward methodology for fabricating asymmetric polymersome was developed by Lodge's group through the coassembly of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) and polybutadiene-b-poly(ethylene oxide) (PB-b-PEO) block copolymers at the interface of a polystyrene/polybutadiene/chloroform (PS/PB/CHCl₃) emulsion. However, the in-depth microscopic mechanism for the formation of asymmetric polymersomes remains unclear. To address this issue, in this article, the coassembly process for the formation of the asymmetric polymersomes in Asano's experimental system was systematically investigated by employing a dissipative particle dynamics (DPD) simulation. Our results definitely demonstrate the formation of the asymmetric polymersomes such as that in the experiments and that the bilayer formed through the folding and crossing of the PEO blocks. Besides, from the microscopic view, three stages can be discerned in the formation process: (1) the formation of micelles, (2) the micelle diffusion to the interface, and (3) the micelle rearrangement at the interface to form an asymmetric polymersome. Meanwhile, the incompatibility among PS, PB, and PEO is proven to be the main driving force for asymmetric polymersome formation. Moreover, the effects of the order of addition of copolymers and the volume fraction of PEO blocks on the structure of the asymmetric polymersomes are also investigated. We find that the formation process is affected severely by the order of addition, and adding PS-b-PEO first can make the asymmetric bilayer more perfect. Not only that, but perfect asymmetric polymersomes can be formed only when the volume fraction of PEO (f_PEO) is greater than 0.55. We believe the present work can extend the knowledge of the self-assembly of asymmetric polymersomes, especially with respect to the self-assembly mechanism.

Self-Assembly of Human Serum Albumin: A Simplex Phenomenon

Spontaneous self-assemblies of biomolecules can generate geometrical patterns. Our findings provide an insight into the mechanism of self-assembled ring pattern generation by human serum albumin (HSA). The self-assembly is a process guided by kinetic and thermodynamic parameters. The generated protein ring patterns display a behavior which is geometrically related to a n-simplex model and is explained through thermodynamics and chemical kinetics.

Harnessing Cooperative Interactions between Thermoresponsive Aptamers and Gels To Trap and Release Nanoparticles

We use computational modeling to design a device that can controllably trap and release particles in solution in response to variations in temperature. The system exploits the thermoresponsive properties of end-grafted fibers and the underlying gel substrate. The fibers mimic the temperature-dependent behavior of biological aptamers, which form a hairpin structure at low temperatures (T) and unfold at higher T, consequently losing their binding affinity. The gel substrate exhibits a lower critical solution temperature and thus, expands at low temperatures and contracts at higher T. By developing a new dissipative particle dynamics simulation, we examine the behavior of this hybrid system in a flowing fluid that contains buoyant nanoparticles. At low T, the expansion of the gel causes the hairpin-shaped fibers to extend into the path of the fluid-driven particle. Exhibiting a high binding affinity for these particles at low temperature, the fibers effectively trap and extract the particles from the surrounding solution. When the temperature is increased, the unfolding of the fiber and collapse of the supporting gel layer cause the particles to be released and transported away from the layer by the applied shear flow. Since the temperature-induced conformational changes of the fiber and polymer gel are reversible, the system can be used repeatedly to "catch and release" particles in solution. Our findings provide guidelines for creating fluidic devices that are effective at purifying contaminated solutions or trapping cells for biological assays.

Fluorescent amphiphilic heterografted comb polymers comprising biocompatible PLA and PEtOx side chains

The comb polymers are synthesized in three independent steps by ROP, CROP, and RAFT.

Assembly of Lock-and-Key Colloids Mediated by Polymeric Depletant

Polymer-mediated lock-and-key assembly via depletion attraction is purely a shape-recognition process without any molecular bonding. Since the depletion attraction relates to osmotic pressure and excluded volume, the binding tendency in a dispersion of lock-and-key colloids can be controlled by adjusting the characteristics of polymeric depletants. In this work, dissipative particle dynamics accounting for explicit solvents, polymers, and multiple lock-key pairs are performed to investigate the influences of the polymer concentration, chain length, solvent quality, and chain stiffness. As the polymer concentration associated with osmotic pressure is increased, the binding free energy (E(b)) between a lock-key pair rises linearly and the binding fraction (θ(LK)) in the dispersion grows sigmoidally. Moreover, the increases in the chain length, solvent quality, and chain stiffness lead to the expansion of the polymer size associated with excluded volume and thus both E(b) and θ(LK) rise accordingly. However, E(b) and θ(LK) grow to be insensitive to the chain length for long enough polymer coils but still can be enhanced if the polymer becomes rod-like. As the solvent quality is varied, θ(LK) can be dramatically altered, although the radius of gyration of polymers is slightly changed.

Designing a gel-fiber composite to extract nanoparticles from solution

The extraction of nanoscopic particulates from flowing fluids is a vital step in filtration processes, as well as the fabrication of nanocomposites. Inspired by the ability of carnivorous plants to use hair-like filaments to entrap species, we use computational modeling to design a multi-component system that integrates compliant fibers and thermo-responsive gels to extract particles from the surrounding solution. In particular, hydrophobic fibers are embedded in a gel that exhibits a lower critical solution temperature (LCST). With an increase in temperature, the gel collapses to expose fibers that self-assemble into bundles, which act as nanoscale "grippers" that bind the particles and draw them into the underlying gel. By varying the relative stiffness of the fibers, the fiber-particle interaction strength and the shear rate in the solution, we identify optimal parameters where the particles are effectively drawn from the solution and remain firmly bound within the gel layer. Hence, the system can be harnessed in purifying fluids and creating novel hybrid materials that integrate nanoparticles with polymer gels.

Dissipative particle dynamics simulation study on self-assembly of amphiphilic hyperbranched multiarm copolymers with different degrees of branching

Hyperbranched multiarm copolymers (HMCs) have shown great potential to be excellent precursors in self-assembly to form various supramolecular structures in all scales and dimensions in solution. However, theoretical studies on the self-assembly of HMCs, especially the self-assembly dynamics and mechanisms, have been greatly lagging behind the experimental progress. Herein, we investigate the effect of degree of branching (DB) on the self-assembly structures of HMCs by dissipative particle dynamics (DPD) simulation. Our simulation results demonstrate that the self-assembly morphologies of HMCs can be changed from spherical micelles, wormlike micelles, to vesicles with the increase of DBs, which are qualitatively consistent with the experimental observations. In addition, both the self-assembly mechanisms and the dynamic processes for the formation of these three aggregates have been systematically disclosed through the simulations. These self-assembly details are difficult to be shown by experiments and are very useful to fully understand the self-assembly behaviors of HMCs.

Designing Composite Coatings That Provide a Dual Defense against Fouling

Inspired by marine organisms that utilize spines and shape changes to prevent the biofouling of their surfaces, we use computational modeling to design a gel-based composite coating that provides a two-pronged defense mechanism against the fouling of the underlying substrate. Using dissipative particle dynamics (DPD) simulations, we construct a coating that encompasses rigid posts embedded in a thermoresponsive gel, which exhibits a lower critical solution temperature (LCST). When the gel is heated above its LCST, it collapses to expose the buried posts, which act as spines or spikes that prevent a solid particle from penetrating the layer. Moreover, we show that an imposed shear flow readily dislodges these particles and washes them away from the coated substrate. As the system dissipates heat and cools, the LCST gel expands, and this dynamic morphological change can also be harnessed to dislodge the adsorbed particles. Thus, both the exposed posts and the swelling gels can provide barriers to the penetration of particulates through the coating. In this manner, the coating provides a dual mechanism against the fouling of the substrate. This physical approach can be particularly beneficial because it does not require the release of any chemical substances that could have detrimental consequences to the environment.

Brush macromolecules with thermo-sensitive coil backbones and pendant polypeptide side chains: synthesis, self-assembly and functionalization

Macromolecular brushes with thermo-sensitive coil backbones and pendant poly(γ-benzyl-l-glutamate) side chains were synthesized by reversible addition–fragmentation chain transfer and ring-opening polymerization. Functionalization and self-assembly of the macromolecules were investigated.