Fikret Aydin

The design of shape-tunable hairy vesicles.

Via the use of a mesoscopic simulation technique called dissipative particle dynamics, we design sterically stable biocompatible vehicles through the self-assembly of a binary mixture composed of amphiphilic molecular species, such as PEGylated lipids, and phospholipids. We examine the factors controlling the shape of the hairy vesicle, and report the shape to change with molecular stiffness, and dissimilarity in the hydrocarbon tail groups, along with the relative concentration of the species, and the functional group length. We also draw correspondence with experimental studies on the shape transformations of the hairy vesicles through phase diagrams of the reduced volume, the ratio of the minimum and maximum radii, and the interfacial line tension, as a function of the concentration of the hairy lipids and the hydrocarbon tail molecular chain stiffness. Results from our investigations can be used for the design and prediction of novel hybrid soft materials for applications in the encapsulation and delivery of therapeutic agents.

Bioinspired Vesicles Encompassing Two-Tail Phospholipids: Self-Assembly and Phase Segregation via Implicit Solvent Coarse-Grained Molecular Dynamics

Via implicit solvent molecular dynamics simulations, we demonstrate the self-assembly of stable single and binary vesicles composed of two-tail phospholipid molecules. The amphiphilic lipid molecules are composed of a hydrophilic headgroup and two hydrophobic tails and are represented by a reduced coarse-grained model which effectively captures the key chemical and geometric attributes of phospholipid molecules. We report our measurements of the bilayer thickness to be consistent with experimental values reported in the literature. We have probed the role of temperature on the physical properties of single component lipid vesicles and found our results to concur with experimental results. Our investigations on the phase segregation in binary vesicles demonstrate that the degree of distinction between the tail groups of the lipid species can be used to tune their phase segregation in the vesicle bilayer. Finally, our measurements of the scaling exponents for macroscopically phase-segregated systems have been found to be in good agreement with theoretical and simulation studies. Our results can be used for the design of responsive biomaterials for applications in drug delivery, sensing, and imaging.

Self-Assembly and Critical Aggregation Concentration Measurements of ABA Triblock Copolymers with Varying B Block Types: Model Development, Prediction, and Validation

The dissipative particle dynamics (DPD) simulation technique is a coarse-grained (CG) molecular dynamics-based approach that can effectively capture the hydrodynamics of complex systems while retaining essential information about the structural properties of the molecular species. An advantageous feature of DPD is that it utilizes soft repulsive interactions between the beads, which are CG representation of groups of atoms or molecules. In this study, we used the DPD simulation technique to study the aggregation characteristics of ABA triblock copolymers in aqueous medium. Pluronic polymers (PEG-PPO-PEG) were modeled as two segments of hydrophilic beads and one segment of hydrophobic beads. Tyrosine-derived PEG5K-b-oligo(desaminotyrosyl tyrosine octyl ester-suberate)-b-PEG5K (PEG5K-oligo(DTO-SA)-PEG5K) block copolymers possess alternate rigid and flexible components along the hydrophobic oligo(DTO-SA) chain, and were modeled as two segments of hydrophilic beads and one segment of hydrophobic, alternate soft and hard beads. The formation, structure, and morphology of the initial aggregation of the polymer molecules in aqueous medium were investigated by following the aggregation dynamics. The dimensions of the aggregates predicted by the computational approach were in good agreement with corresponding results from experiments, for the Pluronic and PEG5K-oligo(DTO-SA)-PEG5K block copolymers. In addition, DPD simulations were utilized to determine the critical aggregation concentration (CAC), which was compared with corresponding results from an experimental approach. For Pluronic polymers F68, F88, F108, and F127, the computational results agreed well with experimental measurements of the CAC measurements. For PEG5K-b-oligo(DTO-SA)-b-PEG5K block polymers, the complexity in polymer structure made it difficult to directly determine their CAC values via the CG scheme. Therefore, we determined CAC values of a series of triblock copolymers with 3-8 DTO-SA units using DPD simulations, and used these results to predict the CAC values of triblock copolymers with higher molecular weights by extrapolation. In parallel, a PEG5K-b-oligo(DTO-SA)-b-PEG5K block copolymer was synthesized, and the CAC value was determined experimentally using the pyrene method. The experimental CAC value agreed well with the CAC value predicted by simulation. These results validate our CG models, and demonstrate an avenue to simulate and predict aggregation characteristics of ABA amphiphilic triblock copolymers with complex structures.

Harnessing Nanoscale Confinement to Design Sterically Stable Vesicles of Specific Shapes via Self-Assembly.

We design sterically stable biocompatible vehicles with tunable shapes through the self-assembly of a binary mixture composed of amphiphilic molecular species, such as PEGylated lipids, and phospholipids under volumetric confinement. We use a molecular dynamics-based mesoscopic simulation technique called dissipative particle dynamics to resolve the aggregation dynamics, structure, and morphology of the hybrid aggregate. We examine the effect of confinement on the growth dynamics and shape of the hybrid aggregate, and demonstrate the formation of different morphologies, such as oblate and prolate shaped vesicles and bicelles. We perform these investigations by varying the degree of nanoscale confinement, for different relative concentrations of the species and the length of the functional groups. Results from our investigations can be used for the design and prediction of novel hybrid soft materials for applications requiring the encapsulation of therapeutic agents in micro- or nanofluidic channels.

Implicit solvent coarse‐grained model of polyamidoamine dendrimers: Role of generation and pH

Highly branched polymers such as polyamidoamine (PAMAM) dendrimers are promising macromolecules in the realm of nanobiotechnology due to their high surface coverage of tunable functional groups. Modeling efforts of PAMAM can provide structural and morphological properties, but the inclusion of solvents and the exponential growth of atoms with generations make atomistic simulations computationally expensive. We apply an implicit solvent coarse‐grained model, called the Dry Martini force field, to PAMAM dendrimers. The reduced number of particles and the absence of a solvent allow the capture of longer spatiotemporal scales. This study characterizes PAMAM dendrimers of generations one through seven in acidic, neutral, and basic pH environments. Comparison with existing literature, both experimental and theoretical, is done using measurements of the radius of gyration, moment of inertia, radial distributions, and scaling exponents. Additionally, ion coordination distributions are studied to provide insight into the effects of interior and exterior protonation on counter ions. This model serves as a starting point for future designs of larger functionalized dendrimers.

Flow-Induced Shape Reconfiguration, Phase Separation, and Rupture of Bio-Inspired Vesicles

The structural integrity of red blood cells and drug delivery carriers through blood vessels is dependent upon their ability to adapt their shape during their transportation. Our goal is to examine the role of the composition of bio-inspired multicomponent and hairy vesicles on their shape during their transport through in a channel. Through the dissipative particle dynamics simulation technique, we apply Poiseuille flow in a cylindrical channel. We investigate the effect of flow conditions and concentration of key molecular components on the shape, phase separation, and structural integrity of the bio-inspired multicomponent and hairy vesicles. Our results show the Reynolds number and molecular composition of the vesicles impact their flow-induced deformation, phase separation on the outer monolayer due to the Marangoni effect, and rupture. The findings from this study could be used to enhance the design of drug delivery and tissue engineering systems.

Surface Reconfiguration of Binary Lipid Vesicles via Electrostatically Induced Nanoparticle Adsorption

We demonstrate the adsorption of nanoparticles (NPs) with charged patches onto a binary vesicle encompassing polar neutral and polar zwitterionic lipids via an implicit solvent coarse-grained model and molecular dynamics simulations. Our investigations on the interactions between NPs and a binary vesicle demonstrate that the adsorption of charged NPs onto a binary vesicle surface can induce structural reorganization of the lipid bilayer. The approach of the NP to the vesicle surface is accompanied by spatial reorganization of the zwitterionic lipids, and the degree of reorganization is found to depend on the NP patch size. Interfacial adsorption of the NP is observed to promote a group of zwitterionic lipids to cluster at the adsorption site. Spatial reorganization of the zwitterionic lipids is activated by favorable electrostatic interactions with the NP and not between the lipids. The favorable electrostatic interaction between oppositely charged lipid headgroup moieties increases and assists the clustering process as the NP approaches the vesicle surface. In addition, the availability of zwitterionic lipids in the vesicle affects the adsorption dynamics of multiple NPs. Our results can be used for the design of reconfigurable biomaterials for applications in drug delivery, sensing, and imaging.

Modeling Interactions between Multicomponent Vesicles and Antimicrobial Peptide-Inspired Nanoparticles

We examine the interaction between peptide-inspired nanoparticles, or nanopins, and multicomponent vesicles using the dissipative particle dynamics simulation technique. We study the role of nanopin architecture and cholesterol concentration on the binding of the nanopins to the lipid bilayer, their insertion, and postembedding self-organization. We find the insertion to be triggered by enthalpically unfavorable interactions between the hydrophilic solvent and the lipophilic components of the nanopins. The nanopins are observed to form aggregates in solution, insert into the bilayer, and disassemble into the individual nanopins following the insertion process. We examine factors that influence the orientation of the nanopins in the host vesicle. We report the length of the hydrophilic segment of the nanopins to regulate their orientation within the clusters before the embedding process and in the bilayer, after the postinsertion disassembly of the aggregates. The orientation angle distribution for a given nanopin architecture is found to be driven by energy minimization. In addition, higher concentration of cholesterol is observed to constrain the orientation of the nanopins. We also report thermal fluctuations to induce transverse diffusion of nanopins with specific architectures. The incidence of transverse diffusion is observed to decrease with the concentration of cholesterol. Our results can provide guidelines for designing peptide-inspired nanoparticles or macromolecules that can interface with living cells to serve as sensors for applications in medicine, sustainability, and energy.

Harnessing steric hindrance to control interfacial adsorption of patchy nanoparticles onto hairy vesicles.

Via the Dissipative Particle Dynamics simulation technique we investigate the interfacial adsorption of nanoparticles with a binding site onto a hairy vesicle encompassing phospholipids and lipids functionalized with oligo ethylene glycol (OEG) chain. The functionalized nanoparticles are modeled as patchy spherical particles. We examine the relation between the relative concentration and size of the OEG chains, the adsorption kinetics, life-time and post-adsorption dynamics of the nanoparticles. We also draw correspondence with experimental studies on the adsorption of proteins onto the surface of colloidal particles. Results from our investigations can elucidate the fundamental factors and mechanisms controlling the adsorption of functionalized nanoparticles onto colloidal particles.