28 June 2021

[Polymers] – Sustainable polymer nanocoatings : An innovative concept

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MD simulations of polymer chains - Codex International

The modification of surface properties, such as metals, medical devices, and glass mirrors, represents a vital opportunity to inhibit the buildup of proteins or contaminants (i.e., fouling) for biomedical materials/devices, food packaging, many membrane filtration applications. Surface coatings with sustainable anti-fouling, bactericidal or the killing of bacterial membrane cells, and self-cleaning surface properties are particularly desirable and are receiving great attentions.
Clinical results have depicted the development of catheter-related infections, such as thrombosis, caused by numerous types of bacteria which adhere onto a fibrin and a vascular catheter, form colonies, and produce coagulating enzymes, leading to mural thrombosis1.
Additionally, the adsorption of blood proteins onto biomedical devices (such as sensors or implants) may reduce effectiveness or even prompt unwanted biological responses2.
Although an anticoagulant drug treatment offers an opportunity to utilize these internal devices, the drug therapy often consists of undesirable side effects including hemorrhage, neutropenia, and thrombocytopenia3.
Polymer-based coatings involving fabrications of nanostructured surfaces over conventional chemical-based antimicrobial agents have become major research efforts designed to greatly prevent protein/bacteria adsorption as well as to promote self-cleaning surface abilities for numerous industry applications.
A better understanding of the molecular-level mechanism by which polymer coatings lead to anti-fouling surface properties is vital. It has been believed that “interfacial water molecules”, which strongly interact with a hydrophilic polymer surface, act as a “barrier” against protein adsorption. Hence, as Berg4 and Vogler et al.5 proposed, a water contact angle can be used as a gauge to connect the process of protein adsorption.
Generally, hydrophilic surfaces are those in which the water contact angle θ<90°, while hydrophobic surfaces are characterized by water contact angles of θ>90° 6.
Quantitatively, Berg’s limit defines a critical contact angle of θ≅65° in which hydrophilic surfaces with any angle θ less than the critical value are able to resist protein absorption as the protein cannot displace the water from the surface3,4.
In essence, the hydration forces found in the water layer bound to a surface repels protein adhesion5, and if the hydration forces are enhanced through greater attraction from a modified surface such as involving carefully structured polymers, the repulsion force can also be increased.
Understanding this relationship prompts the development of sustainable surface modifications especially involving optimally designed polymers which must resist protein absorption both chemically and mechanically and represents a sustainable option over a long duration of time.
The design of protein-resistant self-assembled monolayers (SAMs) follows a set of rules termed the Whitesides rules, which describe optimal functional groups aimed towards discouraging protein adsorption7,8.
As simplified by Haag et al.3, the Whitesides rules proposed that 1) the presence of polar functional groups or hydrophilicity, 2) the presence of hydrogen bond acceptor groups, 3) the absence of hydrogen bond donor groups, and 4) the absence of net charge within the molecular structure of polymer prevents protein adsorption7,8.
Thereafter, various optimally designed polymers, such as polyglycerol (PG)10 and zwitterionic polymers9, exhibited excellent resistance to protein adsorption. Importantly, although the molecular structure of PG contains many hydroxyl groups acting as hydrogen bond donors seemingly violating the third Whitesides rule, the remarkable hydrophilicity of a surface coated with PG makes up for an exception3.
Particularly, zwitterionic molecules are chemically composed of an equal number of charged functional groups, which promotes ion-ion or ion-dipole bonding with water rather than simply weaker hydrogen bonding found in other polymers like PG3. As aforementioned, this greater attraction between the polymers atop the modified surface and water results in greater repulsion forces.
Recently, Endoh and Koga11 reported that the nanometer-scale structural barrier associated with polymer conformations governs protein resistance of polymer surfaces instead of interfacial chemical interactions.
The anti-fouling polymer coating they designed is composed of non-charged, hydrophilic or hydrophobic homopolymer chains physically adsorbed onto a solid, resulting in a few nanometer-thick polymer layer (“polymer nanolayer”).
To understand the mechanism behind the undiscovered protein resistant property of the adsorbed polymer chains, they investigated the chain conformations of this polymer nanolayer in water using sum frequency generation spectroscopy (SFG) and explicit solvent coarse-grained molecular dynamics (MD) simulations.The SFG results clarified the non-significant role of the interfacial water in the emergence of an anti-fouling property and the two-dimensional chain architecture commonly shared among the polymers in water. The MD results further allowed us to establish the generality of the self-organized polymer architecture in water: outer high-density “loops” (sequences of free segments connecting successive trains) and “tails” (non-adsorbed chain ends) and inner densely packed “trains” (adsorbed segments) across homopolymer systems with different interactions among a polymer, substrate, and water.
Hence, it is hypothesized that the loops/tails act as high-density polydispersed brushes and the trains behave as a molecular level “rigid-wall”, such that most of the proteins are not allowed to penetrate into the nanolayer.
Thus, their findings not only provide a molecular level understanding of the mechanism behind protein adsorption but also facilitates a universal structure-based design of anti-fouling surfaces, which is currently lacking, using common synthetic polymers. In addition, it is expected that the initial adhesion of bacteria, whose mechanism is generally protein-mediated, can be prevented by these antifouling materials.
Endoh and Koga17 further extended their studies on ultrathin homopolymer films (up to 60 nm thick) composed of polystyrene (PS), poly(2-vinyl pyridine) (P2VP), poly(methyl methacrylate) (PMMA), and polybutadiene (PB). Protein adsorption test against two model plasma proteins were subject: bovine serum albumin (BSA) and fibrinogen. The two proteins have significant differences in their size, shape, and internal stability.
Their experimental results reveal that protein adsorption within all the ultrathin films is almost prohibited when the film thickness (h) is less than a critical thickness hc≅ 20 nm), while at h>hc, the amount of protein adsorption exhibits very strong thickness dependence (αh2).
Molecular dynamics simulations identified a correlation between protein adsorption and highly packed conformations of polymer chains, which either adsorb on the substrate or do not adsorb but are in contact with the adsorbed polymer chains, resulting in a protein repellent “dense layer” at h

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