The Structural Dispersity of Oligoethylene Glycol-Containing Polymer Brushes Determines Their Interfacial Properties.

Ought to their bioinert properties and facile synthesis, poly[(oligoethylene glycol)methacrylate]s (POEGMAs) have been raised as attractive alternatives to poly(ethylene glycols) (PEGs) in an array of (bio)material applications, especially when they are applied as polymer brush coatings. However, commercially available OEG-methacrylate (macro)monomers feature a broad distribution of OEG lengths, thus generating structurally polydisperse POEGMAs when polymerized through reversible deactivation radical polymerization. Here, we demonstrate that the interfacial physicochemical properties of POEGMA brushes are significantly affected by their structural dispersity, i.e., the degree of heterogeneity in the length of side OEG segments. POEGMA brushes synthesized from discrete (macro)monomers obtained through chromatographic purification of commercial mixtures show increased hydration and reduced adhesion when compared to their structurally polydisperse analogues. The observed alteration of interfacial properties is directly linked to the presence of monodisperse OEG side chains, which hamper intramolecular and intermolecular hydrophobic interactions while simultaneously promoting the association of water molecules. These phenomena provide structurally homogeneous POEGMA brushes with a more lubricious and protein repellent character with respect to their heterogeneous counterparts. More generally, in contrast to what has been assumed until now, the properties of POEGMA brushes cannot be anticipated while ruling out the effect of dispersity by (macro)monomer feeds. Simultaneously, side chain dispersity of POEGMAs emerges as a critical parameter for determining the interfacial characteristics of brushes.

Polymer Brushes on Nanoparticles for Controlling the Interaction with Protein-Rich Physiological Media.

The interaction of nanoparticles (NPs) with biological environments triggers the formation of a protein corona (PC), which significantly influences their behavior in vivo. This review explores the evolving understanding of PC formation, focusing on the opportunity for decreasing or suppressing protein-NP interactions by macromolecular engineering of NP shells. The functionalization of NPs with a dense, hydrated polymer brush shell is a powerful strategy for imparting stealth properties in order to elude recognition by the immune system. While poly(ethylene glycol) (PEG) has been extensively used for this purpose, concerns regarding its stability and immunogenicity have prompted the exploration of alternative polymers. The stealth properties of brush shells can be enhanced by tailoring functionalities and structural parameters, including the molar mass, grafting density, and polymer topology. Determining correlations between these parameters and biopassivity has enabled us to obtain polymer-grafted NPs with high colloidal stability and prolonged circulation time in biological media.

The Rational Design of Reducing Organophotoredox Catalysts Unlocks Proton-Coupled Electron-Transfer and Atom Transfer Radical Polymerization Mechanisms.

Photocatalysis has become a prominent tool in the arsenal of organic chemists to develop and (re)imagine transformations. However, only a handful of versatile organic photocatalysts (PCs) are available, hampering the discovery of new reactivities. Here, we report the design and complete physicochemical characterization of 9-aryl dihydroacridines (9ADA) and 12-aryl dihydrobenzoacridines (12ADBA) as strong reducing organic PCs. Punctual structural variations modulate their molecular orbital distributions and unlock locally or charge-transfer (CT) excited states. The PCs presenting a locally excited state showed better performances in photoredox defunctionalization processes (yields up to 92%), whereas the PCs featuring a CT excited state produced promising results in atom transfer radical polymerization under visible light (up to 1.21 D, and 98% I*). Unlike all the PC classes reported so far, 9ADA and 12ADBA feature a free NH group that enables a catalytic multisite proton-coupled electron transfer (MS-PCET) mechanism. This manifold allows the reduction of redox-inert substrates including aryl, alkyl halides, azides, phosphate and ammonium salts (Ered up to -2.83 vs SCE) under single-photon excitation. We anticipate that these new PCs will open new mechanistic manifolds in the field of photocatalysis by allowing access to previously inaccessible radical intermediates under one-photon excitation.

Dispersity within Brushes Plays a Major Role in Determining Their Interfacial Properties: The Case of Oligoxazoline-Based Graft Polymers.

Many synthetic polymers used to form polymer-brush films feature a main backbone with functional, oligomeric side chains. While the structure of such graft polymers mimics biomacromolecules to an extent, it lacks the monodispersity and structural purity present in nature. Here we demonstrate that side-chain heterogeneity within graft polymers significantly influences hydration and the occurrence of hydrophobic interactions in the subsequently formed brushes and consequently impacts fundamental interfacial properties. This is demonstrated for the case of poly(methacrylate)s (PMAs) presenting oligomeric side chains of different length (n) and dispersity. A precise tuning of brush structure was achieved by first synthesizing oligo(2-ethyl-2-oxazoline) methacrylates (OEOXMAs) by cationic ring-opening polymerization (CROP), subsequently purifying them into discrete macromonomers with distinct values of n by column chromatography, and finally obtaining poly[oligo(2-ethyl-2-oxazoline) methacrylate]s (POEOXMAs) by reversible addition-fragmentation chain-transfer (RAFT) polymerization. Assembly of POEOXMA on Au surfaces yielded graft polymer brushes with different side-chain dispersities and lengths, whose properties were thoroughly investigated by a combination of variable angle spectroscopic ellipsometry (VASE), quartz crystal microbalance with dissipation (QCMD), and atomic force microscopy (AFM) methods. Side-chain dispersity, or dispersity within brushes, leads to assemblies that are more hydrated, less adhesive, and more lubricious and biopassive compared to analogous films obtained from graft polymers characterized by a homogeneous structure.

Hydrogels Generated from Cyclic Poly(2-Oxazoline)s Display Unique Swelling and Mechanical Properties.

Cyclic macromolecules do not feature chain ends and are characterized by a higher effective intramolecular repulsion between polymer segments, leading to a higher excluded-volume effect and greater hydration with respect to their linear counterparts. As a result of these unique properties, hydrogels composed of cross-linked cyclic polymers feature enhanced mechanical strength while simultaneously incorporating more solvent with respect to networks formed from their linear analogues with identical molar mass and chemical composition. The translation of topology effects by cyclic polymers into the properties of polymer networks provides hydrogels that ideally do not include defects, such as dangling chain ends, and display unprecedented physicochemical characteristics.

Oxygen Tolerant and Cytocompatible Iron(0)-Mediated ATRP Enables the Controlled Growth of Polymer Brushes from Mammalian Cell Cultures.

The use of zerovalent iron (Fe0)-coated plates, which act both as a source of catalyst and as a reducing agent during surface-initiated atom transfer radical polymerization (SI-ATRP), enables the controlled growth of a wide range of polymer brushes under ambient conditions utilizing either organic or aqueous reaction media. Thanks to its cytocompatibility, Fe0 SI-ATRP can be applied within cell cultures, providing a tool that can broadly and dynamically modify the substrate's affinity toward cells, without influencing their viability. Upon systematically assessing the application of Fe-based catalytic systems in the controlled grafting of polymers, Fe0 SI-ATRP emerges as an extremely versatile technique that could be applied to tune the physicochemical properties of a cell's microenvironments on biomaterials or within tissue engineering constructs.

Brushes, Graft Copolymers, or Bottlebrushes? The Effect of Polymer Architecture on the Nanotribological Properties of Grafted-from Assemblies.

Surface-grafted polyzwitterions (PZW) have gained a foothold in the design of synthetic materials that closely mimic the lubricious properties of articular joints in mammals. Besides their chemical composition, the architecture of PZW brushes strongly determines their morphological, nanomechanical, and nanotribological characteristics. This emerges while comparing the properties of linear poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brushes with those displayed by graft copolymer and bottlebrush brushes, either featuring a low or a high content of PMPC side chains. Surface-initiated atom transfer radical polymerization (SI-ATRP) enabled the synthesis of different branched-brush architectures from multifunctional macroinitiators via multiple grafting steps, and allowed us to modulate their structure by tuning the polymerization conditions. At relatively low grafting densities (σ), long PMPC side segments extend at the interface of bottlebrush and graft copolymer brushes, providing both morphology and lubrication properties comparable to those shown by loosely grafted, linear PMPC brushes. When σ > 0.1 chains nm-2 the effect of the branched-brush architecture on the nanotribological properties of the films became evident. Linear PMPC brushes showed the lowest friction among the studied brush structures, with a coefficient of friction (µ) that reached 1 × 10-4, as measured by atomic force microscopy (AFM). Bottlebrush brushes showed comparatively higher friction, although the high content of hydrophilic PMPC side chains along their backbone substantially improved lubrication compared to that displayed by the more sparsely substituted graft copolymer brushes.

Bioinert and Lubricious Surfaces by Macromolecular Design.

The modification of a variety of biomaterials and medical devices often encompasses the generation of biopassive and lubricious layers on their exposed surfaces. This is valid when the synthetic supports are required to integrate within physiological media without altering their interfacial composition and when the minimization of shear stress prevents or reduces damage to the surrounding environment. In many of these cases, hydrophilic polymer brushes assembled from surface-interacting polymer adsorbates or directly grown by surface-initiated polymerizations (SIP) are chosen. Although growing efforts by polymer chemists have been focusing on varying the composition of polymer brushes in order to attain increasingly bioinert and lubricious surfaces, the precise modulation of polymer architecture has simultaneously enabled us to substantially broaden the tuning potential for the above-mentioned properties. This feature article concentrates on reviewing this latter strategy, comparatively analyzing how polymer brush parameters such as molecular weight and grafting density, the application of block copolymers, the introduction of branching and cross-links, or the variation of polymer topology beyond the simple, linear chains determine highly technologically relevant properties, such as biopassivity and lubrication.

Double-Network Hydrogels Including Enzymatically Crosslinked Poly-(2-alkyl-2-oxazoline)s for 3D Bioprinting of Cartilage-Engineering Constructs.

Double-network (DN) hydrogels are fabricated from poly(2-ethyl-2-oxazoline) (PEOXA)-peptide conjugates, which can be enzymatically crosslinked in the presence of Sortase A (SA), and physical networks of alginate (Alg), yielding matrices with improved mechanical properties with respect to the corresponding PEOXA and Alg single networks and excellent cell viability of encapsulated human auricular chondrocytes (hACs). The addition of a low content of cellulose nanofibrils (CNFs) within DN hydrogel formulations provides the rheological properties needed for extrusion-based three-dimensional (3D) printing, generating constructs with a good shape fidelity. In the presence of hACs, PEOXA-Alg-CNF prehydrogel mixtures can be bioprinted, finally generating 3D-structured DN hydrogel supports showing a cell viability of more than 90%. Expanding the application of poly(2-alkyl-2-oxazoline)-based formulations in the design of tissue-engineering constructs, this study further demonstrates how SA-mediated enzymatic crosslinking represents a suitable and fully orthogonal method to generate biocompatible hydrogels with fast kinetics.

Robust and Biocompatible Functionalization of ZnS Nanoparticles by Catechol-Bearing Poly(2-methyl-2-oxazoline)s.

Zinc sulfide (ZnS) nanoparticles (NPs) are particularly interesting materials for their electronic and luminescent properties. Unfortunately, their robust and stable functionalization and stabilization, especially in aqueous media, has represented a challenging and not yet completely accomplished task. In this work, we report the synthesis of colloidally stable, photoluminescent and biocompatible core-polymer shell ZnS and ZnS:Tb NPs by employing a water-in-oil miniemulsion (ME) process combined with surface functionalization via catechol-bearing poly-2-methyl-2-oxazoline (PMOXA) of various molar masses. The strong binding of catechol anchors to the metal cations of the ZnS surface, coupled with the high stability of PMOXA against chemical degradation, enable the formation of suspensions presenting excellent colloidal stability. This feature, combined with the assessed photoluminescence and biocompatibility, make these hybrid NPs suitable for optical bioimaging.

Poly(2-oxazoline)-Pterostilbene Block Copolymer Nanoparticles for Dual-Anticancer Drug Delivery.

Functional block copolymers based on poly(2-oxazoline)s are versatile building blocks for the fabrication of dual-drug delivery nanoparticles (NPs) for anticancer chemotherapy. Core-shell NPs are fabricated from diblock copolymers featuring a long and hydrophilic poly(2-methyl-2-oxazoline) (PMOXA) block coupled to a relatively short and functionalizable poly(2-methylsuccinate-2-oxazoline) (PMestOXA) segment. The PMOXA block stabilizes the NP dispersions, whereas the PMestOXA segment is used to conjugate pterostilbene, a natural bioactive phenolic compound that is used as lipophilic model-drug and constitutes the hydrophobic core of the designed NPs. Subsequent loading of the NPs with clofazimine (CFZ), an inhibitor of the multidrug resistance pumps typically expressed in a large variety of cancer cells, provides an additional function to their formulation. Optimization of the copolymer composition allows the design of polymer scaffolds showing low toxicity and capable of assembling into highly stable NPs dispersions at physiologically relevant pH. In addition, the incorporation of CFZ increases the stability of the NPs and stimulates their internalization by RAW 264.7 cells.

Hairy and Slippery Polyoxazoline-Based Copolymers on Model and Cartilage Surfaces.

Comb-like polymers presenting a hydroxybenzaldehyde (HBA)-functionalized poly(glutamic acid) (PGA) backbone and poly(2-methyl-2-oxazoline) (PMOXA) side chains chemisorb on aminolized substrates, including cartilage surfaces, forming layers that reduce protein contamination and provide lubrication. The structure, physicochemical, biopassive, and tribological properties of PGA-PMOXA-HBA films are finely determined by the copolymer architecture, its reactivity toward the surface, i.e. PMOXA side-chain crowding and HBA density, and by the copolymer solution concentration during assembly. Highly reactive species with low PMOXA content form inhomogeneous layers due to the limited possibility of surface rearrangements by strongly anchored copolymers, just partially protecting the functionalized surface from protein contamination and providing a relatively weak lubrication on cartilage. Biopassivity and lubrication can be improved by increasing copolymer concentration during assembly, leading to a progressive saturation of surface defects across the films. In a different way, less reactive copolymers presenting high PMOXA side-chain densities form uniform, biopassive, and lubricious films, both on model aminolized silicon oxide surfaces, as well as on cartilage substrates. When assembled at low concentrations these copolymers adopt a "lying down" conformation, i.e. adhering via their backbones onto the substrates, while at high concentrations they undergo a conformational transition, assuming a more densely packed, "standing up" structure, where they stretch perpendicularly from the substrate. This specific arrangement reduces protein contamination and improves lubrication both on model as well as on cartilage surfaces.

Quasi-3D-Structured Interfaces by Polymer Brushes.

The fabrication of polymer brushes via surface-initiated controlled radical polymerizations has progressively developed beyond a simple surface functionalization technique, enabling the design of complex polymer interfaces with a quasi-3D molecular organization. The modulation of polymer brush structure has led to an extremely broad tuning potential for technologically relevant interfacial, physicochemical properties, allowing one to precisely tune swelling, nanomechanical, and nanotribological characteristics of polymer films. In addition, the synthesis of multilayer brush interfaces with hierarchical architecture has been exploited to control biological phenomena on modified platforms, such as cell adhesion and settlement, or to fully prevent biological contamination from bacteria. In this feature article, the most recent developments in the synthesis and application of quasi-3D structured polymer brushes are summarized, placing particular attention on how the tuning of grafted-polymer architecture could translate into a variation of interfacial characteristics.

Cyclic Polymer Grafts That Lubricate and Protect Damaged Cartilage.

Tissue-reactive graft copolymers were designed to protect the cartilage against enzymatic degradation and restore its lubrication properties during the early stages of osteoarthritis (OA). The copolymers feature a poly(glutamic acid) (PGA) backbone bearing hydroxybenzaldehyde (HBA) functions and cyclic poly(2-methyl-2-oxazoline) (PMOXA) side chains. PGA-PMOXA-HBA species chemisorb on the degraded tissue via Schiff bases and expose the biopassive and lubricious PMOXA cyclic grafts at the interface. The smaller hydrodynamic radius by cyclic PMOXA side chains coupled to the intrinsic absence of chain ends generate denser and more lubricious films on cartilage when compared to those produced by copolymers bearing linear PMOXA. Topology effects demonstrate how the introduction of cyclic polymers within tissue-reactive copolymers substantially improve their tribological and biopassive properties, suggesting a plethora of possible applications for cyclic macromolecules in biomaterials formulations.

Chemical Design of Non-Ionic Polymer Brushes as Biointerfaces: Poly(2-oxazine)s Outperform Both Poly(2-oxazoline)s and PEG.

The era of poly(ethylene glycol) (PEG) brushes as a universal panacea for preventing non-specific protein adsorption and providing lubrication to surfaces is coming to an end. In the functionalization of medical devices and implants, in addition to preventing non-specific protein adsorption and cell adhesion, polymer-brush formulations are often required to generate highly lubricious films. Poly(2-alkyl-2-oxazoline) (PAOXA) brushes meet these requirements, and depending on their side-group composition, they can form films that match, and in some cases surpass, the bioinert and lubricious properties of PEG analogues. Poly(2-methyl-2-oxazine) (PMOZI) provides an additional enhancement of brush hydration and main-chain flexibility, leading to complete bioinertness and a further reduction in friction. These data redefine the combination of structural parameters necessary to design polymer-brush-based biointerfaces, identifying a novel, superior polymer formulation.

Loops and Cycles at Surfaces: The Unique Properties of Topological Polymer Brushes.

Grafting synthetic polymers to inorganic and organic surfaces to yield polymer "brushes" has represented a revolution in many fields of materials science. Polymer brushes provide colloidal stabilization to nanoparticles (NPs), prevent and/or regulate the adsorption of proteins on biomaterials, and significantly reduce friction when applied to two surfaces sheared against each other. Can the performance of polymer brushes as steric stabilizers and boundary lubricants be improved? The answer to this question encompasses the application of polymer grafts presenting different chain topologies, beyond linearity. In particular, grafted polymers forming loops and cycles at the surface have been recently demonstrated to enable the modulation of interfacial physicochemical properties, including nanomechanical and nanotribological, to an extent that is difficultly addressed by using their linear counterparts. Loop and cyclic polymer brushes provide enhanced steric stabilization to surfaces, increase their biopassivity and show superlubricious behavior. Their distinctive structure, the methods applied to fabricate them and their application in several technologically relevant fields of materials science are reviewed in this contribution.

Effects of Lateral Deformation by Thermoresponsive Polymer Brushes on the Measured Friction Forces.

The nanotribological properties of hydrophilic polymer brushes are conveniently analyzed by lateral force microscopy (LFM). However, the measurement of friction for highly swollen and relatively thick polymer brushes can be strongly affected by the tendency of the compliant brush to be laterally deformed by the shearing probe. This phenomenon induces a "tilting" in the recorded friction loops, which is generated by the lateral bending and stretching of the grafts. In this study we highlight how the brush lateral deformation mainly affects the friction measurements of swollen PNIPAM brushes (below LCST) when relatively short scanning distances are applied. Under these conditions, the energy dissipation recorded by LFM is almost uniquely determined by stretching and bending of the compliant brush back and forth along the scanning direction, and it is not correlated to dynamic friction between two sliding surfaces. In contrast, when the scanning distance applied during LFM is relevantly longer than the brush lateral deformation, sliding of the probe on the brush interface becomes dominant, and a correct measurement of dynamic friction can be accomplished. By increasing the temperature above the LCST, the PNIPAM brushes undergo dehydration and assume a collapsed morphology, thereby hindering their lateral deformation by scanning probe. Hence, at 40 °C in water the recorded friction loops do not show any tilting and LFM accurately describes the dynamic friction between the probe and the polymer surface.

Easy to Apply Polyoxazoline-Based Coating for Precise and Long-Term Control of Neural Patterns.

Arranging cultured cells in patterns via surface modification is a tool used by biologists to answer questions in a specific and controlled manner. In the past decade, bottom-up neuroscience emerged as a new application, which aims to get a better understanding of the brain via reverse engineering and analyzing elementary circuitry in vitro. Building well-defined neural networks is the ultimate goal. Antifouling coatings are often used to control neurite outgrowth. Because erroneous connectivity alters the entire topology and functionality of minicircuits, the requirements are demanding. Current state-of-the-art coating solutions such as widely used poly(l-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) fail to prevent primary neurons from making undesired connections in long-term cultures. In this study, a new copolymer with greatly enhanced antifouling properties is developed, characterized, and evaluated for its reliability, stability, and versatility. To this end, the following components are grafted to a poly(acrylamide) (PAcrAm) backbone: hexaneamine, to support spontaneous electrostatic adsorption in buffered aqueous solutions, and propyldimethylethoxysilane, to increase the durability via covalent bonding to hydroxylated culture surfaces and antifouling polymer poly(2-methyl-2-oxazoline) (PMOXA). In an assay for neural connectivity control, the new copolymer's ability to effectively prevent unwanted neurite outgrowth is compared to the gold standard, PLL-g-PEG. Additionally, its versatility is evaluated on polystyrene, glass, and poly(dimethylsiloxane) using primary hippocampal and cortical rat neurons as well as C2C12 myoblasts, and human fibroblasts. PAcrAm-g-(PMOXA, NH2, Si) consistently outperforms PLL-g-PEG with all tested culture surfaces and cell types, and it is the first surface coating which reliably prevents arranged nodes of primary neurons from forming undesired connections over the long term. Whereas the presented work focuses on the proof of concept for the new antifouling coating to successfully and sustainably prevent unwanted connectivity, it is an important milestone for in vitro neuroscience, enabling follow-up studies to engineer neurologically relevant networks. Furthermore, because PAcrAm-g-(PMOXA, NH2, Si) can be quickly applied and used with various surfaces and cell types, it is an attractive extension to the toolbox for in vitro biology and biomedical engineering.

Controlling Enzymatic Polymerization from Surfaces with Switchable Bioaffinity.

The affinity of surfaces toward proteins is found to be a key parameter to govern the synthesis of polymer brushes by surface-initiated biocatalytic atom transfer radical polymerization (SI-bioATRP). While the "ATRPase" hemoglobin (Hb) stimulates only a relatively slow growth of protein repellent brushes, the synthesis of thermoresponsive grafts can be regulated by switching the polymer's attraction toward proteins across its lower critical solution temperature (LCST). Poly(N-isopropylacrylamide) (PNIPAM) brushes are synthesized in discrete steps of thickness at temperatures above LCST, while the biocatalyst layer is refreshed at T < LCST. Multistep surface-initiated biocatalytic ATRP demonstrates a high degree of control, results in high chain end group fidelity and enables the synthesis of multiblock copolymer brushes under fully aqueous conditions. The activity of Hb can be further modulated by tuning the accessibility of the heme pocket within the protein. Hence, the multistep polymerization is accelerated at acid pH, where the enzyme undergoes a transition from its native to a molten globule conformation. The controlled synthesis of polymer brushes by multistep SI-bioATRP highlights how a biocatalytic synthesis of grafted polymer films can be precisely controlled through the modulation of the polymer's interfacial physicochemical properties, in particular of the affinity of the surface toward proteins. This is not only of importance to gain a predictive understanding of surface-confined enzymatic polymerizations, but also represents a new way to translate bioadhesion into a controlled functionalization of materials.

Next-Generation Polymer Shells for Inorganic Nanoparticles are Highly Compact, Ultra-Dense, and Long-Lasting Cyclic Brushes.

Cyclic poly-2-ethyl-2-oxazoline (PEOXA) ligands for superparamagnetic Fe3 O4 nanoparticles (NPs) generate ultra-dense and highly compact shells, providing enhanced colloidal stability and bio-inertness in physiological media. When linear brush shells fail in providing colloidal stabilization to NPs, the cyclic ones assure long lasting dispersions. While the thermally induced dehydration of linear PEOXA shells cause irreversible aggregation of the NPs, the collapse and subsequent rehydration of similarly grafted cyclic brushes allow the full recovery of individually dispersed NPs. Although linear ligands are densely grafted onto Fe3 O4 cores, a small plasma protein such as bovine serum albumin (BSA) still physisorbs within their shells. In contrast, the impenetrable entropic shield provided by cyclic brushes efficiently prevents nonspecific interaction with proteins.