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.

Introducing Hyaluronic Acid into Supramolecular Polymers and Hydrogels.

The use of supramolecular polymers to construct functional biomaterials is gaining more attention due to the tunable dynamic behavior and fibrous structures of supramolecular polymers, which resemble those found in natural systems, such as the extracellular matrix. Nevertheless, to obtain a biomaterial capable of mimicking native systems, complex biomolecules should be incorporated, as they allow one to achieve essential biological processes. In this study, supramolecular polymers based on water-soluble benzene-1,3,5-tricarboxamides (BTAs) were assembled in the presence of hyaluronic acid (HA) both in solution and hydrogel states. The coassembly of BTAs bearing tetra(ethylene glycol) at the periphery (BTA-OEG4) and HA at different ratios showed strong interactions between the two components that led to the formation of short fibers and heterogeneous hydrogels. BTAs were further covalently linked to HA (HA-BTA), resulting in a polymer that was unable to assemble into fibers or form hydrogels due to the high hydrophilicity of HA. However, coassembly of HA-BTA with BTA-OEG4 resulted in the formation of long fibers, similar to those formed by BTA-OEG4 alone, and hydrogels were produced with tunable stiffness ranging from 250 to 700 Pa, which is 10-fold higher than that of hydrogels assembled with only BTA-OEG4. Further coassembly of BTA-OEG4 fibers with other polysaccharides showed that except for dextran, all polysaccharides studied interacted with BTA-OEG4 fibers. The possibility of incorporating polysaccharides into BTA-based materials paves the way for the creation of dynamic complex biomaterials.

Exploring the Potential of Benzene-1,3,5-tricarboxamide Supramolecular Polymers as Biomaterials.

The fast dynamics occurring in natural processes increases the difficulty of creating biomaterials capable of mimicking Nature. Within synthetic biomaterials, water-soluble supramolecular polymers show great potential in mimicking the dynamic behavior of these natural processes. In particular, benzene-1,3,5-tricaboxamide (BTA)-based supramolecular polymers have shown to be highly dynamic through the exchange of monomers within and between fibers, but their suitability as biomaterials has not been yet explored. Herein we systematically study the interactions of BTA supramolecular polymers bearing either tetraethylene glycol or mannose units at the periphery with different biological entities. When BTA fibers were incubated with bovine serum albumin (BSA), the protein conformation was only affected by the fibers containing tetraethylene glycol at the periphery (BTA-OEG4). Coarse-grained molecular simulations showed that BSA interacted with BTA-OEG4 fibers rather than with BTA-OEG4 monomers that are present in solution or that may exchange out of the fibers. Microscopy studies revealed that, in the presence of BSA, BTA-OEG4 retained their fiber conformation although their length was slightly shortened. When further incubated with fetal bovine serum (FBS), both long and short fibers were visualized in solution. Nevertheless, in the hydrogel state, the rheological properties were remarkably preserved. Further studies on the cellular compatibility of all the BTA assemblies and mixtures thereof were performed in four different cell lines. A low cytotoxic effect at most concentrations was observed, confirming the suitability of utilizing functional BTA supramolecular polymers as dynamic biomaterials.

Anchoring Supramolecular Polymers to Human Red Blood Cells by Combining Dynamic Covalent and Non-Covalent Chemistries.

Understanding cell/material interactions is essential to design functional cell-responsive materials. While the scientific literature abounds with formulations of biomimetic materials, only a fraction of them focused on mechanisms of the molecular interactions between cells and material. To provide new knowledge on the strategies for materials/cell recognition and binding, supramolecular benzene-1,3,5-tricarboxamide copolymers bearing benzoxaborole moieties are anchored on the surface of human erythrocytes via benzoxaborole/sialic-acid binding. This interaction based on both dynamic covalent and non-covalent chemistries is visualized in real time by means of total internal reflection fluorescence microscopy. Exploiting this imaging method, we observe that the functional copolymers specifically interact with the cell surface. An optimal fiber affinity towards the cells as a function of benzoxaborole concentration demonstrates the crucial role of multivalency in these cell/material interactions.

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.

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.

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.

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.

Topological Polymer Chemistry Enters Surface Science: Linear versus Cyclic Polymer Brushes.

The cyclic polymer topology strongly alters the interfacial, physico-chemical properties of polymer brushes, when compared to the linear counterparts. In this study, we especially concentrated on poly-2-ethyl-2-oxazoline (PEOXA) cyclic and linear grafts assembled on titanium oxide surfaces by the "grafting-to" technique. The smaller hydrodynamic radius of ring PEOXAs favors the formation of denser brushes with respect to linear analogs. Denser and more compact cyclic brushes generate a steric barrier that surpasses the typical entropic shield by a linear brush. This phenomenon, translates into an improved resistance towards biological contamination from different protein mixtures. Moreover, the enhancement of steric stabilization coupled to the intrinsic absence of chain ends by cyclic brushes, produce surfaces displaying a super-lubricating character when they are sheared against each other. All these topological effects pave the way for the application of cyclic brushes for surface functionalization, enabling the modulation of physico-chemical properties that could be just marginally tuned by applying linear grafts.

Introducing carbohydrate patterning in mannose-decorated supramolecular assemblies and hydrogels.

Benzene-1,3,5-tricarboxamide (BTA) glyco-monomers containing one, two or three mannose units are synthesized and formulated into differently patterned supramolecular glycopolymers through homo-assembly or co-assembly with non-functionalized BTAs. Unfortunately, no cellular activity could be detected. Excitingly, these glyco-BTA monomers could be formulated into hydrogels, paving the way for (immune) cell culture.

Polymer Topology Determines the Formation of Protein Corona on Core-Shell Nanoparticles.

Linear and cyclic poly(2-ethyl-2-oxazoline) (PEOXA) adsorbates provide excellent colloidal stability to superparamagnetic iron oxide nanoparticles (FexOy NPs) within protein-rich media. However, dense shells of linear PEOXA brushes cannot prevent weak but significant attractive interactions with human serum albumin. In contrast, their cyclic PEOXA counterparts quantitatively hinder protein adsorption, as demonstrated by a combination of dynamic light scattering and isothermal titration calorimetry. The cyclic PEOXA brushes generate NP shells that are denser and more compact than their linear counterparts, entirely preventing the formation of a protein corona as well as aggregation, even when the lower critical solution temperature of PEOXA in a physiological buffer is reached.

Topological Polymer Chemistry Enters Materials Science: Expanding the Applicability of Cyclic Polymers.

Polymer-topology effects can alter technologically relevant properties when cyclic macromolecules are applied within diverse materials formulations. These include coatings, polymer networks, or nanostructures for delivering therapeutics. While substituting linear building blocks with cyclic analogues in commonly studied materials is itself of fundamental interest, an even more fascinating observation has been that the introduction of physical or chemical boundaries (e.g., a grafting surface or cross-links) can amplify the topology-related effects observed when employing cyclic polymer-based precursors for assembling multidimensional objects. Hence, the application of cyclic polymers has enabled the fabrication of coatings with enhanced biorepellency and superior lubricity, broadened the tuning potential for mechanical properties of polymer networks, increased the thermodynamic stability, and altered the capability of loading and releasing drugs within polymeric micelles.

Molecularly Engineered Biolubricants for Articular Cartilage.

Lubrication within articular joints plays a crucial role in daily life, providing an extremely low coefficient of friction and preventing wear at the surface of the articular cartilage. Natural biomacromolecules responsible for lubrication are part of the synovial fluid and their degradation is associated with the onset of degenerative diseases, such as osteoarthritis (OA). The current absence of effective treatments for OA has captured the attention of chemists and material scientists over the last two decades, triggering the development of partially or fully synthetic biolubricants aimed to reduce friction within the joints and restore cartilage functions. Although there is still a long way to go before synthetic replacements of natural biolubricants can be applied clinically, this review highlights those formulations that meet the fundamental requirements for being efficient lubricants for articular cartilage.

Mixing Poly(ethylene glycol) and Poly(2-alkyl-2-oxazoline)s Enhances Hydration and Viscoelasticity of Polymer Brushes and Determines Their Nanotribological and Antifouling Properties.

Poly(2-alkyl-2-oxazoline)s (PAOXAs) have progressively emerged as suitable alternatives for replacing poly(ethylene glycol) (PEG) in a variety of biomaterial-related applications, especially in the designing of polymer brush-based biointerfaces because of their stealth properties and chemical robustness. When equimolar mixtures of PEG and PAOXAs are assembled on surfaces to yield mixed polymer brushes, the interfacial physicochemical properties of the obtained films are significantly altered, in some cases, surpassing the biopassive and lubricious characteristics displayed by single-component PAOXA and PEG counterparts. With a combination of variable angle spectroscopic ellipsometry, quartz crystal microbalance with dissipation, and atomic force microscopy-based methods, we demonstrate that mixing of PEG brushes with equimolar amounts of PAOXA grafts determines an increment in film's hydration and viscoelasticity. In the case of mixtures of PEG and poly(2-methyl-2-oxazoline) or poly(2-ethyl-2-oxazoline), brushes displaying full inertness toward serum proteins and improved lubricity with respect to the corresponding single-component layers can be generated, while providing a multifunctional surface that substantially enlarges the applicability of the designed coatings.

Surface Density Variation within Cyclic Polymer Brushes Reveals Topology Effects on Their Nanotribological and Biopassive Properties.

While topology effects by cyclic polymers in solution and melts are well-known, their translation into the interfacial properties of polymer "brushes" provides new opportunities to impart enhanced surface lubricity and biopassivity to inorganic surfaces, above and beyond that expected for linear analogues of identical composition. The impact of polymer topology on the nanotribological and protein-resistance properties of polymer brushes is revealed by studying linear and cyclic poly(2-ethyl-2-oxazoline) (PEOXA) grafts presenting a broad range of surface densities and while shearing them alternatively against an identical brush or a bare inorganic surface. The intramolecular constraints introduced by the cyclization provide a valuable increment in both steric stabilization and load-bearing capacity for cyclic brushes. Moreover, the intrinsic absence of chain ends within cyclic adsorbates hinders interpenetration between opposing brushes, as they are slid over each other, leading to a reduction in the friction coefficient (µ) at higher pressures, a phenomenon not observed for linear grafts. The application of cyclic polymers for the modification of inorganic surfaces generates films that outperform both the nanotribological and biopassive properties of linear brushes, significantly expanding the design possibilities for synthetic biointerfaces.