Molecular Simulations of the Hydration Behavior of a Zwitterion Brush Array and Its Antifouling Property in an Aqueous Environment

Photo-responsive anti-fouling polyzwitterionic brushes: a mesoscopic simulation

The antifouling effects of a toothbrush-shaped photo-responsive polyzwitterionic membrane were studied via dissipative particle dynamics simulations in this work. The results reveal that the membrane modified by spiropyran methacrylate brushes displays photo-switchable and antifouling capability due to the photo-induced ring-opening reaction. Namely, surface morphology and hydrophilicity change in response to visible or UV light irradiation, which can be observed visually by protein adsorption and desorption. Further study indicates that: (1) brush-modification density can influence the structure and properties of the membrane. With low modification density, systems cannot establish an intact selective layer, which hinders the antifouling ability; as the modification density increases, the intact selective layer can be formed, which is conducive to the expression of photo-responsiveness and antifouling capability. (2) Factors of toothbrush-hair length and grafting ratio can influence the establishment of a light-responsive surface: as the grafting ratio and toothbrush-hair length increase, the light-responsive surface is gradually formed, meanwhile, the antifouling ability can be continuously reinforced under UV light irradiation. (3) As the brushes switch into a zwitterionic merocyanine state under UV exposure, the selective layer swelling becomes stronger than that with a hydrophobic spiropyran state under visible exposure. This is owing to the enhanced interaction between zwitterionic brushes and water, which is the root of the antifouling effect. The present work is expected to provide some guidelines for the design and development of novel antifouling membrane surfaces.

Study of Hydration Repulsion of Zwitterionic Polymer Brushes Resistant to Protein Adhesion through Molecular Simulations

The fabrication of antifouling zwitterionic polymer brushes represents a leading approach to mitigate nonspecific adhesion on the surfaces of medical devices. This investigation seeks to elucidate the correlation between the material composition and structural attributes of these polymer brushes in preventing protein adhesion. To achieve this goal, we modeled three different zwitterionic brushes, namely, carboxybetaine methacrylate (CBMA), sulfobetaine methacrylate (SBMA), and (2-(methacryloyloxy)ethyl)-phosphorylcholine (MPC). The simulations revealed that elevating the grafting density enhances the structural stability, hydration strength, and resistance to protein adhesion exhibited by the polymer brushes. PCBMA manifests a more robust hydration layer, while PMPC demonstrates the slightest interaction with proteins. In a comprehensive evaluation, PSBMA polymer brushes emerged as the best choice with superior stability, enhanced protein repulsion, and minimally induced protein deformation, resulting in effective resistance to nonspecific adhesion. The high-density SBMA polymer brushes significantly reduce the level of protein adhesion in AFM testing. In addition, we have pioneered the quantitative characterization of hydration repulsion in polymer brushes by analyzing the hydration repulsion characteristics at different materials and graft densities. In summary, our study provides a nuanced understanding of the material and structural determinants influencing the capacity of zwitterionic polymer brushes to thwart protein adhesion. Additionally, it presents a quantitative elucidation of hydration repulsion, contributing to the advancement and application of antifouling polymer brushes.

High-density zwitterionic polymer brushes exhibit robust lubrication properties and high antithrombotic efficacy in blood-contacting medical devices

High-performance catheters are essential for interventional surgeries, requiring reliable anti-adhesive and lubricated surfaces. This article develops a strategy for constructing high-density sulfobetaine zwitterionic polymer brushes on the surface of catheters, utilizing dopamine and sodium alginate as the primary intermediate layers, where dopamine provides mussel-protein-like adhesion to anchor the polymer brushes to the catheter surface. Hydroxyl-rich sodium alginate increases the number of grafting sites and improves the grafting mass by more than 4 times. The developed high-density zwitterionic polymer brushes achieve long-lasting and effective lubricity (μ<0.0078) and are implanted in rabbits for four hours without bio-adhesion and thrombosis in the absence of anticoagulants such as heparin. Experiments and molecular dynamics simulations demonstrate that graft mass plays a decisive role in the lubricity and anti-adhesion of polymer brushes, and it is proposed to predict the anti-adhesion of polymer brushes by their lubricity to avoid costly and time-consuming bioassays during the development of amphoteric polymer brushes. A quantitative influence of hydration in the anti-adhesion properties of amphiphilic polymer brushes is also revealed. Thus, this study provides a new approach to safe, long-lasting lubrication and anticoagulant surface modification for medical devices in contact with blood. STATEMENT OF SIGNIFICANCE: High friction and bioadhesion on medical device surfaces can pose a significant risk to patients. In response, we have developed a safer, simpler, and more application-specific surface modification strategy that addresses both the lubrication and anti-bioadhesion needs of medical device surfaces. We used dopamine and sodium alginate as intermediate layers to drastically increase the grafting density of the zwitterionic brushes and enabled the modified surfaces to have an extremely low coefficient of friction (μ = 0.0078) and to remain non-bioadhesive for 4 hours in vivo. Furthermore, we used molecular dynamics simulations to gain insight into the mechanisms behind the superior anti-adhesion properties of the high-density polymer brushes. Our work contributes to the development and application of surface-modified coatings.

Sulfobetaine-Siloxanes: A Class of Self-Destructive Surfactants

The hydrolytic susceptibility of sulfobetaine-siloxane surfactants is investigated by comparison of a homologous series in this subclass of surfactants (R-(CH2)3N+(Me)2(CH2)3SO3-; R = (Me3SiO)3Si-, (Me3SiO)2Si(Me)-, (Me2SiO)3-Si(Me)-) with an analogue series of oxyethylene-siloxane surfactants (R-(CH2)3(OCH2CH2)10.2OH; R = (Me3SiO)3Si-, (Me3SiO)2Si(Me)-, (Me2SiO)3-Si(Me)-). Nuclear magnetic resonance (NMR) monitoring of these surfactants in an aqueous solution shows that the presence of the sulfobetaine head structure greatly enhances the hydrolysis rate of the siloxane tail as compared with oxyethylene-siloxane analogue control experiments. This sulfobetaine effect is confirmed by adding a model compound, (Me)3N+(CH2)3SO3-, to the oxyethylene-siloxane surfactants and observing the large hydrolysis enhancement. Measurements of pH indicate the sulfobetaine presence greatly enhances acidity, but rigorous analysis could discover no source of acid other than the presence of the sulfobetaine structure. Titration measurements confirmed the presence of a tightly bound hydration layer of 4-7 water molecules per sulfobetaine group. It is speculated that the source of acidity may originate from an aqueous exclusion zone nucleated by the hydrated sulfobetaine at the interface of a sulfobetaine-siloxane surfactant bilayer aggregate. Hydrolysis prevention is investigated by addition of a pH 7 phosphate buffer, of an alkyl polyglycoside cosurfactant, and of a combination of both, with a finding of very significant but not complete suppression of the hydrolysis.

Design, preparation, and characterization of lubricating polymer brushes for biomedical applications

The lubrication modification of biomedical devices significantly enhances the functionality of implanted interventional medical devices, thereby providing additional benefits for patients. Polymer brush coating provides a convenient and efficient method for surface modification while ensuring the preservation of the substrate's original properties. The current research has focused on a "trial and error" method to finding polymer brushes with superior lubricity qualities, which is time-consuming and expensive, as obtaining effective and long-lasting lubricity properties for polymer brushes is difficult. This review summarizes recent research advances in the biomedical field in the design, material selection, preparation, and characterization of lubricating and antifouling polymer brushes, which follow the polymer brush development process. This review begins by examining various approaches to polymer brush design, including molecular dynamics simulation and machine learning, from the fundamentals of polymer brush lubrication. Recent advancements in polymer brush design are then synthesized and potential avenues for future research are explored. Emphasis is placed on the burgeoning field of zwitterionic polymer brushes, and highlighting the broad prospects of supramolecular polymer brushes based on host-guest interactions in the field of self-repairing polymer brush applications. The review culminates by providing a summary of methodologies for characterizing the structural and functional attributes of polymer brushes. It is believed that a development approach for polymer brushes based on "design-material selection-preparation-characterization" can be created, easing the challenge of creating polymer brushes with high-performance lubricating qualities and enabling the on-demand creation of coatings. STATEMENT OF SIGNIFICANCE: Biomedical devices have severe lubrication modification needs, and surface lubrication modification by polymer brush coating is currently the most promising means. However, the design and preparation of polymer brushes often involves "iterative testing" to find polymer brushes with excellent lubrication properties, which is both time-consuming and expensive. This review proposes a polymer brush development process based on the "design-material selection-preparation-characterization" strategy and summarizes recent research advances and trends in the design, material selection, preparation, and characterization of polymer brushes. This review will help polymer brush researchers by alleviating the challenges of creating polymer brushes with high-performance lubricity and promises to enable the on-demand construction of polymer brush lubrication coatings.

Perspectives on Theoretical Models and Molecular Simulations of Polymer Brushes

Polymer brushes have witnessed extensive utilization and progress, driven by their distinct attributes in surface modification, tethered group functionality, and tailored interactions at the nanoscale, enabling them for various scientific and industrial applications of coatings, sensors, switchable/responsive materials, nanolithography, and lab-on-a-chips. Despite the wealth of experimental investigations into polymer brushes, this review primarily focuses on computational studies of antifouling polymer brushes with a strong emphasis on achieving a molecular-level understanding and structurally designing antifouling polymer brushes. Computational exploration covers three realms of thermotical models, molecular simulations, and machine-learning approaches to elucidate the intricate relationship between composition, structure, and properties concerning polymer brushes in the context of nanotribology, surface hydration, and packing conformation. Upon acknowledging the challenges currently faced, we extend our perspectives toward future research directions by delineating potential avenues and unexplored territories. Our overarching objective is to advance our foundational comprehension and practical utilization of polymer brushes for antifouling applications, leveraging the synergy between computational methods and materials design to drive innovation in this crucial field.

Relationships between Water's Structure and Solute Affinity at Polypeptoid Brush Surfaces

Excellent antifouling surfaces are generally thought to create a tightly bound layer of water that resists solute adsorption, and highly hydrophilic surfaces such as those with zwitterionic functionalities are of significant current interest as antifoulant strategies. However, despite significant proofs-of-concept, we still lack a fundamental understanding of how the nanoscopic structure of this hydration layer translates to reduced fouling, how surface chemistry can be tuned to achieve antifouling through hydration water, and why, in particular, zwitterionic surfaces seem so promising. Here, we use molecular dynamics simulations and free energy calculations to investigate the molecular relationships among surface chemistry, hydration water structure, and surface-solute affinity across a variety of surface-decorated chemistries. Specifically, we consider polypeptoid-decorated surfaces that display well-known experimental antifouling capabilities and that can be synthesized sequence specifically, with precise backbone positioning of, e.g., charged groups. Through simulations, we calculate the affinities of a range of small solutes to polypeptoid brush surfaces of varied side-chain chemistries. We then demonstrate that measures of the structure of surface hydration water in response to a particular surface chemistry signal solute-surface affinity; specifically, we find that zwitterionic chemistries produce solute-surface repulsion through highly coordinated hydration water while suppressing tetrahedral structuring around the solute, in contrast to uncharged surfaces that show solute-surface affinity. Based on the relationship of this structural perturbation to the affinity of small-molecule solutes, we propose a molecular mechanism by which zwitterionic surface chemistries enhance solute repulsion, with broader implications for the design of antifouling surfaces.

Molecular Dynamics Study of the Antifouling Mechanism of Hydrophilic Polymer Brushes

We perform all-atom molecular dynamics simulations of the adsorption of amino acid side-chain analogues on polymer brushes. The analogues examined are nonpolar isobutane, polar propionamide, negatively charged propionate ion, and positively charged butylammonium ion. The polymer brushes consist of a sheet of graphene and strongly hydrophilic poly(carboxybetaine methacrylate) (PCBMA) or weakly hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA). The effective interactions between isobutane and polymer chains are repulsive for PCBMA and attractive for PHEMA. Gibbs energy decomposition analysis shows that this is due to the abundance of water in the PCBMA brush, which increases the steric repulsion and decreases the Lennard-Jones attraction. The affinity of the hydrophilic analogues is low for both PCBMA and PHEMA chains, but the balance between the components of the Gibbs energy is different for the two polymers. The simulations are performed at several θ, where θ is the degree of overlap of polymer chains. The antifouling performance against the neutral analogues is better for PCBMA than for PHEMA in the low and high θ regimes. However, in the middle θ regime, the antifouling performance of PHEMA is close to or better than that of PCBMA. This is attributed to the formation of a dense layer of PHEMA on the graphene surface that inhibits direct adsorption of analogue molecules on graphene. The charged analogues do not bind to either the PHEMA or PCBMA brush irrespective of θ.

Implantable biomedical materials for treatment of bone infection

The treatment of bone infections has always been difficult. The emergence of drug-resistant bacteria has led to a steady decline in the effectiveness of antibiotics. It is also especially important to fight bacterial infections while repairing bone defects and cleaning up dead bacteria to prevent biofilm formation. The development of biomedical materials has provided us with a research direction to address this issue. We aimed to review the current literature, and have summarized multifunctional antimicrobial materials that have long-lasting antimicrobial capabilities that promote angiogenesis, bone production, or "killing and releasing." This review provides a comprehensive summary of the use of biomedical materials in the treatment of bone infections and a reference thereof, as well as encouragement to perform further research in this field.

Ultrasmall Cu3(PO4)2 Nanoparticles Reinforced Hydrogel Membrane for Super-antifouling Oil/Water Emulsion Separation

Membrane fouling is a persistent and crippling challenge for oily wastewater treatment due to the high susceptibility of membranes to contamination. A feasible strategy is to design a robust and stable hydration layer on the membrane surface to prevent contaminates. A hydrogel illustrates a distinct category of materials with outstanding antifouling performance but is limited by its weak mechanical property. In this research, we report a reinforced hydrogel on a membrane by in situ growing ultrasmall hydrophilic Cu₃(PO₄)₂ nanoparticles in a copper alginate (CuAlg) layer via metal-ion-coordination-mediated mineralization. The embeddedness of hydrophilic Cu₃(PO₄)₂ nanoparticle with a size of 3-5 nm endows the CuAlg/Cu₃(PO₄)₂ composite hydrogel with enhanced mechanical property as well as reinforced hydrate ability. The as-prepared CuAlg/Cu₃(PO₄)₂ modified membrane exhibits a superior oil-repulsive property and achieves a nearly zero flux decline for separating surfactant stabilized oil-in-water emulsions with a high permeate flux up to ∼1330 L m^-2 h^-1 bar^-1. Notably, it is capable of keeping similar permeate flux for both pure water and oil-in-water emulsions during filtration, which is superior to the currently reported membranes, indicating its super-antifouling properties.

Zwitterionic Biomaterials

The term "zwitterionic polymers" refers to polymers that bear a pair of oppositely charged groups in their repeating units. When these oppositely charged groups are equally distributed at the molecular level, the molecules exhibit an overall neutral charge with a strong hydration effect via ionic solvation. The strong hydration effect constitutes the foundation of a series of exceptional properties of zwitterionic materials, including resistance to protein adsorption, lubrication at interfaces, promotion of protein stabilities, antifreezing in solutions, etc. As a result, zwitterionic materials have drawn great attention in biomedical and engineering applications in recent years. In this review, we give a comprehensive and panoramic overview of zwitterionic materials, covering the fundamentals of hydration and nonfouling behaviors, different types of zwitterionic surfaces and polymers, and their biomedical applications.

Molecular dynamics study of the interactions between a hydrophilic polymer brush on graphene and amino acid side chain analogues in water

We perform all-atom molecular dynamics simulations of poly(2-hydroxyethyl methacrylate) (PHEMA) brushes in aqueous solutions of isobutane, propionamide, and sodium propionate. These solutes are side chain analogues to leucine, glutamine, and glutamic acid, respectively. We compute the Gibbs energy profile of the solute's adsorption to the polymer brush and decompose it into the contributions from the steric repulsion, van der Waals interaction, and Coulomb interaction to reveal the energetic origin of repulsion or attraction of the solute by the polymer brush. The Henry adsorption constant is the amount of adsorption normalized by the concentration in aqueous solution. We examine the dependence of this quantity on the grafting density and chain length. Our results suggest that the concurrent primary and ternary adsorption mechanism may be more important than previously expected when the solute is hydrophobic.

Interactions on Proteins Arising from the Self-Assembly of a Polyelectrolyte Brush

Surfaces grafted with polyelectrolyte chains for excellent performance in protein antifouling are highly desired in many applications, such as biomedical implants and devices. In general, the adsorbing/resisting behaviors of proteins can be mainly attributed to the electrostatic interactions that are associated with the charge properties of proteins and polyelectrolytes. By coarse-grained molecular dynamics simulations, we examined the self-assembled structures of polyanion and polyzwitterion brushes as well as the interactions on negatively and positively charged proteins. We found that in addition to charges, the structural polarization induced by self-assembly with a certain charge distribution shows significant influences on protein behavior. The large-scale dipole-dipole interactions between brushes and proteins can dominate the behavior of proteins on the brushes under certain circumstances. To ensure simulation accuracy, we compared two models and found a polar Martini model that explicitly treats electrostatic interactions as long-ranged ones, giving a more reasonable structural description compared with the normal Martini model that truncates electrostatic interactions.

Inducing pH control over the critical micelle concentration of zwitterionic surfactants via polyacids adsorption: Effect of chain length and structure

HYPOTHESIS

The critical concentration above which micelles form from zwitterionic surfactant solutions and their thermodynamic stability is affected by the interaction with weak Brønsted polyacid chains (A_n) via the formation of charged hydrogen bonds between the latter and anionic moieties.

EXPERIMENTS

The interaction between zwitterionic micelles and polyacids capable of forming hydrogen bonds, and its dependence on the environmental pH and polymer structure, has been studied with constant-pH simulations and a restricted primitive model for all electrolytes.

FINDINGS

At low pH, the formation of polyacid/micelle complexes is witnessed independently of the polymer size or structure, so that the concentration above which micelles form is substantially decreased compared to polyacid-free cases. Upon rising pH, polymer desorption takes place within a narrow range of pH values, its location markedly depending on the size and structure of polyacids, and on the relative disposition between headgroup charged moieties. Thus, the desorption onset for long linear polyacids (A₆₀) interacting with sulphobetainic headgroups is roughly two pH units higher than for six decameric chains (6A₁₀) adsorbed onto micelles bearing phosphorylcholinic headgroups. This effect, together with the preferential desorption of chain ends at intermediate pH, may be exploited for drug delivery purposes or building advanced metamaterials.

Multiscale simulations of polyzwitterions in aqueous bulk solutions and brush array configurations

Zwitterionic polymers are very promising candidates for antifouling materials that exhibit high chemical stability as compared to polyethylene glycol-based systems. A number of simulation and experimental studies have emerged over recent years for the investigation of sulfobetaine-based zwitterionic polymers. Investigating the structural and thermodynamic properties of such polymers requires access to broad time and length regimes, thus necessitating the development of multiscale simulation strategies. The present article advocates a mesoscopic dissipative particle dynamics (DPD) model capable of addressing a wide range of time and length scales. The mesoscopic force field was developed hand-in-hand with atomistic simulations based on the OPLS force field through a bottom-up parameterization procedure that matches the atomistically calculated strand-length, strand-angle and pair distribution functions. The DPD model is validated against atomistic simulations conducted in this work, and against relevant atomistic simulation studies, theoretical predictions and experimental correlations from the literature. Properties examined include the conformations of SPE polymers in dilute bulk aqueous solution, the density profile and thickness of brush arrays as functions of the grafting density and chain length. In addition, we compute the potential of mean force of an approaching hydrophilic or hydrophobic foulant via umbrella sampling as a function of its position relative to the poly-zwitterion-covered surface. The aforementioned observables lead to important insights regarding the conformational tendencies of grafted polyzwitterions and their antifouling properties.

Molecular simulations and understanding of antifouling zwitterionic polymer brushes

Zwitterionic materials are an important class of antifouling biomaterials for various applications. Despite such desirable antifouling properties, molecular-level understanding of the structure-property relationship associated with surface chemistry/topology/hydration and antifouling performance still remains to be elucidated. In this work, we computationally studied the packing structure, surface hydration, and antifouling property of three zwitterionic polymer brushes of poly(carboxybetaine methacrylate) (pCBMA), poly(sulfobetaine methacrylate) (pSBMA), and poly((2-(methacryloyloxy)ethyl)phosporylcoline) (pMPC) brushes and a hydrophilic PEG brush using a combination of molecular mechanics (MM), Monte Carlo (MC), molecular dynamics (MD), and steered MD (SMD) simulations. We for the first time determined the optimal packing structures of all polymer brushes from a wide variety of unit cells and chain orientations in a complex energy landscape. Under the optimal packing structures, MD simulations were further conducted to study the structure, dynamics, and orientation of water molecules and protein adsorption on the four polymer brushes, while SMD simulations to study the surface resistance of the polymer brushes to a protein. The collective results consistently revealed that the three zwitterionic brushes exhibited stronger interactions with water molecules and higher surface resistance to a protein than the PEG brush. It was concluded that both the carbon space length between zwitterionic groups and the nature of the anionic groups have a distinct effect on the antifouling performance, leading to the following antifouling ranking of pCBMA > pMPC > pSBMA. This work hopefully provides some structural insights into the design of new antifouling materials beyond traditional PEG-based antifouling materials.

Surface modification of polyamide reverse osmosis membranes with small-molecule zwitterions for enhanced fouling resistance: a molecular simulation study

Surface modification with small-molecule zwitterions is experimentally proved to be an effective solution to improve the antifouling performance of polyamide membranes. However, there is no comprehensive understanding of their microscopic mechanism. In order to address this issue, in this work we constructed two atomistic models, PA (a pure polyamide membrane) and QDAP-PA (a polyamide membrane surface-modified with QDAP), where QDAP was a zwitterion that was prepared by 2,6-daaminopyridine quaternized with 3-bromopropionic acid experimentally. Density functional theory was adopted to elucidate the variations in the electrostatic potential before and after modification. Then, equilibrium molecular dynamics (EMD) simulations were conducted to investigate the structure and hydrophobic/hydrophilic nature of the membrane surface in the two models. Finally, we introduced two typical organic foulants, sodium dodecyl sulfonate (SDS) and dodecyl trimethyl ammonium chloride (DTAC), to evaluate the antifouling performance of the membranes with the umbrella sampling method. The analyses of the membrane structure and properties show that surface modification with small-molecule zwitterions can densify the membrane surface as well as enlarge the distribution of electrostatic potential on the membrane surface. Water molecules tend to have more interactions with the membrane and more hydrogen bonds near the membrane surface are observed in QDAP-PA. The antifouling test supports that QDAP-PA shows a better antifouling performance, as the surface-modified membrane exhibits a stronger resistance to SDS and DTAC. Even if the foulant is adsorbed to the membrane surface, the denser interface region can prevent a further pollution of the foulant. Also, the free energy needed during the process for QDAP-PA to desorb a foulant is relatively small, indicating that this kind of membrane is easy to clean. The current work might provide a comprehensive understanding of the enhanced fouling resistance of polyamide membranes after surface modification with small-molecule zwitterions.

Machine Learning-Enabled Design and Prediction of Protein Resistance on Self-Assembled Monolayers and Beyond

The rational design of highly antifouling materials is crucial for a wide range of fundamental research and practical applications. The immense variety and complexity of the intrinsic physicochemical properties of materials (i.e., chemical structure, hydrophobicity, charge distribution, and molecular weight) and their surface coating properties (i.e., packing density, film thickness and roughness, and chain conformation) make it challenging to rationally design antifouling materials and reveal their fundamental structure-property relationships. In this work, we developed a data-driven machine learning model, a combination of factor analysis of functional group (FAFG), Pearson analysis, random forest (RF) and artificial neural network (ANN) algorithms, and Bayesian statistics, to computationally extract structure/chemical/surface features in correlation with the antifouling activity of self-assembled monolayers (SAMs) from a self-construction data set. The resultant model demonstrates the robustness of Q_CV² = 0.90 and RMSE_CV = 0.21 and the predictive ability of Q_ext² = 0.84 and RMSE_ext = 0.28, determines key descriptors and functional groups important for the antifouling activity, and enables to design original antifouling SAMs using the predicted antifouling functional groups. Three computationally designed molecules were further coated onto the surfaces in different forms of SAMs and polymer brushes. The resultant coatings with negative fouling indexes exhibited strong surface resistance to protein adsorption from undiluted blood serum and plasma, validating the model predictions. The data-driven machine learning model demonstrates their design and predictive capacity for next-generation antifouling materials and surfaces, which hopefully help to accelerate the discovery and understanding of functional materials.

On the distribution of hydrophilic polyelectrolytes and their counterions around zwitterionic micelles: the possible impact on the charge density in solution

Despite their charge neutrality, micelles composed of surfactants with zwitterionic headgroups selectively accumulate anions at their hydrophobic core/solution interphase due to electrostatic interactions if headgroup positive moieties are the innermost. This tendency may be markedly enhanced if polyions substitute simple ions. To investigate this possibility, solutions composed of zwitterionic micelles and hydrophilic polyanions have been investigated with Monte Carlo simulations representing the studied systems via primitive electrolyte models. Structural and energetic properties are obtained to highlight the impact of connecting simple ions into polyions on the interactions between electrolytes and micelles. Despite the latter, polyanions conserve their conformational properties. A marked increase in the concentration of charged species inside the micellar corona is, instead, found when polyions are present independently of their charge sign or the headgroup structure. Thus, polyelectrolytes act as "shuttle" for all charged species, with the potential of increasing reactions rates involving the latter due to mass effects. Besides, results for the polyions/micelles mixing free energy and Helmholtz energy profiles indicate that the critical micelle concentration is impacted minimally by hydrophilic polyelectrolytes, an outcome agreeing with experiments. This finding is entirely due to weak enthalpic effects while mixing hydrophilic polyions and micelles. A strong reduction in the screening of the micelle negative charge, acquired following the adsorption of anions in the corona and due to counterions layering just outside it (the so called "chameleon effect"), is forecasted when polyanions substitute monovalent anions.

Hydrophilic polymer-based anti-biofouling coatings: Preparation, mechanism, and durability

Anti-biofouling materials that combat microorganism attachment have been intensively studied due to the ever-growing demand on smart and durable coatings. Although various hydrophilic polymer surfaces demonstrated superior anti-biofouling properties, their practical application was hampered by the undesired mechanical vulnerability and complicated fabrication process. In this review, we summarized the mechanically and chemically robust anti-biofouling coatings into six strategies namely (i) 3D-grafted coatings, (ii) hierarchical spheres-based coatings, (iii) inorganic nanomaterials-reinforced coatings, (iv) hydrolysis-based coating, (v) semi-interpenetrating structure-based coatings, and (vi) layer-by-layer (LbL) assembled coatings. The anti-biofouling efficacy and durability of these coatings over a series of challenges were also comprehensively presented. The purpose of this review is to inspire researchers to develop novel anti-biofouling coatings for future practical applications.

Tailoring Pseudo-Zwitterionic Bifunctionalized Silica Nanoparticles: From Colloidal Stability to Biological Interactions

Zwitterionic molecules are known to resist nonspecific protein adsorption and have been proposed as an alternative to the widely used polyethylene glycol. Recently, zwitterionic-like nanoparticles were created from the coimmobilization of positive and negative ligands, resulting in surfaces that also prevent protein corona formation while keeping available sites for bioconjugation. However, it is unclear if they are able to keep their original properties when immersed in biological environments while retaining a toxicity-free profile, indispensable features before considering these structures for clinics. Herein, we obtained optimized zwitterionic-like silica nanoparticles from the functionalization with varying ratios of THPMP and DETAPTMS organosilanes and investigated their behavior in realistic biological milieu. The generated zwitterionic-like particle was able to resist single-protein adsorption, while the interaction with a myriad of serum proteins led to significant loss of colloidal stability. Moreover, the zwitterionic particles presented poor hemocompatibility, causing considerable disruption of red blood cells. Our findings suggest that the exposure of ionic groups allows these structures to directly engage with the environment and that electrostatic neutrality is not enough to grant low-fouling and stealth properties.

Molecular Understanding of Ion Effect on Polyzwitterion Conformation in an Aqueous Environment

Polyzwitterions (PZs) are promising materials for the antifouling in reverse osmosis and nanofiltration membrane technology for water treatment. Fundamental understanding of the structure and molecular interactions involving zwitterions is crucial to the optimal design of antifouling in membrane separation. Here we employ the umbrella sampling and molecular dynamics simulations to investigate molecular interactions between sulfobetaine/carboxybetaine zwitterions and different metal ions (Na⁺, K⁺, and Ca²⁺) in an aqueous solution. The simulation results show that these ions can form stable or metastable contact ionic/solvent-shared-ionic pairs with zwitterions. Simulations at different grafting densities of PZ brush arrays reveal complex competitive association mechanisms, which are attributed to nonbonded electrostatic and van der Waals interactions among zwitterions, water molecules, and different metal ions in an aqueous environment. While the high-grafting density of the PZ brush array leads to a strong branch association between different zwitterions in water, this association is decreased at intermediate- and low-grafting densities due to strong zwitterion-water interactions. More importantly, adding ions into water at intermediate- and low-grafting densities further breaks down the zwitterion branch association, resulting in a randomly oriented and dispersed branch configuration with significant swelling of the polymers. The degree of swelling depends on the type of ions, which further changes the surface electrostatic potential of PZ coatings.

Computational Investigation of Antifouling Property of Polyacrylamide Brushes

Antifouling materials and coatings have broad fundamental and practical applications. Strong hydration at polymer surfaces has been proven to be responsible for their antifouling property, but molecular details of interfacial water behaviors and their functional roles in protein resistance remain elusive. Here, we computationally studied the packing structure, surface hydration, and protein resistance of four poly(N-hydroxyalkyl acrylamide) (PAMs) brushes with different carbon spacer lengths (CSLs) using a combination of molecular mechanics (MM), Monte Carlo (MC), and molecular dynamics (MD) simulations. The packing structure of different PAM brushes were first determined and served as a structural basis for further exploring the CSL-dependent dynamics and structure of water molecules on PAM brushes and their surface resistance ability to lysozyme protein. Upon determining an optimal packing structure of PAMs by MM and optimal protein orientation on PAMs by MC, MD simulations further revealed that poly(N-hydroxymethyl acrylamide) (pHMAA), poly(N-(2-hydroxyethyl)acrylamide) (pHEAA), and poly(N-(3-hydroxypropyl)acrylamide) (pHPAA) brushes with shorter CSLs = 1-3 possessed a much stronger binding ability to more water molecules than a poly(N-(5-hydroxypentyl)acrylamide) (pHPenAA) brush with CSL = 5. Consequently, CSL-induced strong surface hydration on pHMAA, pHEAA, and pHPAA brushes led to high surface resistance to lysozyme adsorption, in sharp contrast to lysozyme adsorption on the pHPenAA brush. Computational studies confirmed the experimental results of surface wettability and protein adsorption from surface plasmon resonance, contact angle, and sum frequency generation vibrational spectroscopy, highlighting that small structural variation of CSLs can greatly impact surface hydration and antifouling characteristics of antifouling surfaces, which may provide structural-based design guidelines for new and effective antifouling materials and surfaces.

Effect of the Self-Assembled Structures of Hydrated Polyzwitterionic and Polyanionic Brushes on Their Self-Cleaning Capabilities

The capability of polyelectrolyte brushes to spontaneously clean oil fouling via water is determined by factors including water wettability and the self-assembled structures of hydrated polyelectrolytes. Although the charged groups of polyelectrolytes provide the original source of water wettability, the self-assembled structures play a significant role in the self-cleaning performances. Here, we employ coarse-grained molecular dynamics simulations to study the general self-cleaning characteristics of two types of surface-grafted polyelectrolyte brushes (i.e., zwitterionic and anionic polyelectrolytes). It has been found that the high grafting density is favorable to fouling reduction for both polyzwitterions and polyanions. To be specific, the hydrated polyzwitterions form an intermolecular cross-linked network via zwitterionic complexes, resulting in better self-cleaning capabilities than the polyanions at lower grafting densities. However, polyanions form bundles with each consisting of several chains via hydrophobic interactions and electrostatic repulsions presenting better self-cleaning performances than the polyzwitterions at higher grafting densities.

Understanding the Antifouling Mechanism of Zwitterionic Monomer-Grafted Polyvinylidene Difluoride Membranes: A Comparative Experimental and Molecular Dynamics Simulation Study

The antifouling process of the membrane is very vital for the highly efficient treatment of industrial wastewater, especially high salinity wastewater containing oil and other pollutants. In the present work, the dynamical antifouling mechanism is explored via molecular dynamics simulations, while the corresponding experiments about surface properties of the zwitterionic monomer-grafted polyvinylidene difluoride membrane are designed to verify the simulated mechanism. Water can form a stable hydration layer at the grafted membrane surface, where all the simulated radial distribution function of water/membrane, hydrogen bond number, water diffusivity, and experimental oil contact angles are stable. However, the water flux across the membrane will increase first and then decrease as the grafting ratio increases, which not only depends on the reduced pore size of the zwitterionic monomer-grafted membrane but also results from water diffusion. Furthermore, the dynamical fouling processes of pollutants (taking sodium alginate as an example) on the grafted membrane in water and brine solution are investigated, where both the high grafting ratio and electrolyte CaCl₂ can enhance the fouling energy barrier of the pollutant. The results show that both the enhanced hydrophilic property and the electrostatic repulsion can affect the antifouling capability of the grafted membrane. Finally, the ternary synergistic antifouling mechanisms among the zwitterionic membrane, electrolyte, and pollutant sodium alginates are discussed, which could be helpful for the rational design and preparation of new and highly efficient zwitterionic antifouling membranes.

Blood-Compatible Surfaces with Phosphorylcholine-Based Polymers for Cardiovascular Medical Devices

For the acquisition of blood-compatible materials, various hydrophilic polymers for surface modification have been examined. Among them, polymers with a representative phospholipid polar group, the phosphorylcholine (PC) group, are a successful example. These polymers were designed from inspiration of the cell membrane surface and provide protein adsorption resistance even following contact with plasma. This important property is based on the unique hydration state of water molecules surrounding hydrated polymer; in other words, water molecules weakly interact with the polymers and maintain their favorable cluster structure through hydrogen bonding. These polymers are not only hydrophilic, but also electrically neutral, important characteristics which make hydrogen bonding with water molecules less likely to occur and avoid hydrophobic interactions. Phosphorylcholine groups and other zwitterionic structures are significant as hydrophilic functional groups meeting these important requirements. In this review, blood compatibility of a polymer having a PC group is introduced in relation to its hydration structure, followed by a description of the applications of this polymer to cardiovascular medical devices.

Measuring the Interactions between Protein-Coated Microspheres and Polymer Brushes in Aqueous Solutions

Hydrophilic or zwitterionic polymer-functionalized surfaces have become attractive biomaterials in bioscience and technology due to their excellent protein-resistant ability. Understanding the fundamental interactions between proteins and polymers plays an essential role in the surface design of biomaterials. In this work, we studied the interactions between bovine serum albumin (BSA) and two sorts of polymer brushes including zwitterionic poly(carboxybetaine methacrylate) (PCBMA) and hydrophilic poly[oligo(ethylene glycol) methyl ether methacrylate] (POEGMA) in NaCl aqueous solutions directly with a self-established total internal reflection microscope (TIRM) to provide a better understanding of the underlying nonfouling mechanism of polymers. Our results indicate that both the surface charge and brushes length can affect protein adsorption through electrostatic and steric repulsions, respectively. Both PCBMA- and POEGMA-coated surfaces display negative charge properties due to incomplete coverage and ionic adsorption. As a result, strong electrostatic repulsions between proteins and negatively charged polymer-coated surfaces could contribute to the resistance of protein-coated particles in solutions with low ionic strength (0.1, 0.5, and 1 mM) and disappear in solutions with high ionic strength (10 mM). The measured interaction profiles demonstrate that PCBMA brushes could provide apparent steric forces only at high ionic strength (10 mM), where zwitterionic brushes exhibit a relatively extended conformation with a lack of electrostatic forces between intra- and interpolymers. In contrast, the steric repulsion between proteins and POEGMA brushes appears when particles diffuse at low positions in all salt concentrations (0.1-10 mM) with similar steric decay lengths, which results from the unresponsiveness of POEGMA brushes to the salt stimulus.