Cytocompatible, soft and thick brush-modified scaffolds with prolonged antibacterial effect to mitigate wound infections

Biomedical device or implant-associated infections caused by pathogenic bacteria are a major clinical issue, and their prevention and/or treatment remains a challenging task. Infection-resistant antimicrobial coatings with impressive cytocompatibility offer a step towards addressing this problem. Herein, we report a new strategy for constructing highly antibacterial as well as cytocompatible mixed polymer brushes onto the surface of 3D printed scaffold made of biodegradable tartaric acid-based aliphatic polyester blends. The mixed brushes were nothing but a combination of poly(3-dimethyl-(methacryloyloxyethyl) ammonium propane sulfonate) (polyDMAPS) and poly((oligo ethylene glycol) methyl ether methacrylate) (polyPEGMA) with varying chain length (n) of the ethylene glycol unit (n = 1, 6, 11, and 21). Both homo and copolymeric brushes of polyDMAPS with polyPEGMA exhibited antibacterial efficacy against both Gram positive and Gram negative pathogens such as E. coli (Escherichia coli) and S. aureus (Staphylococcus aureus) because of the combined action of bacteriostatic effects originating from strongly hydrated layers present in zwitterionic (polyDMAPS) and hydrophilic (polyPEGMA) copolymer brushes. Interestingly, a mixed polymer brush comprising polyDMAPS and polyPEGMA (ethylene glycol chain unit of 21) at 50/50 ratio provided zero bacterial growth and almost 100% cytocompatibility (tested using L929 mouse fibroblast cells), making the brush-modified biodegradable substrate an excellent choice for an infection-resistant and cytocompatible surface. An attempt was made to understand their extraordinary performance with the help of contact angle, surface charge analysis and nanoindentation study, which revealed the formation of a hydrophilic, almost neutral, very soft surface (99.99% reduction in hardness and modulus) after modification with the mixed brushes. This may completely suppress bacterial adhesion. Animal studies demonstrated that these brush-modified scaffolds are biocompatible and can mitigate wound infections. Overall, this study shows that the fascinating combination of an infection-resistant and cytocompatible surface can be generated on biodegradable polymeric surfaces by modulating the surface hardness, flexibility and hydrophilicity by selecting appropriate functionality of the copolymeric brushes grafted onto them, making them ideal non-leaching, anti-infective, hemocompatible and cytocompatible coatings for biodegradable implants.

d-Amino Acid-Based Antifouling Peptides for the Construction of Electrochemical Biosensors Capable of Assaying Proteins in Serum with Enhanced Stability

The susceptibility of peptides to proteolytic degradation in human serum significantly hindered the potential application of peptide-based antifouling biosensors for long-term assaying of clinical samples. Herein, a robust antifouling biosensor with enhanced stability was constructed based on peptides composed of d-amino acids (d-peptide) with prominent proteolytic resistance. The electrode was electropolymerized with poly(3,4-ehtylenedioxythiophene) and electrodeposited with Au nanoparticles (AuNPs), and the d-peptide was then immobilized onto the AuNPs, and a typical antibody specific for immunoglobulin M (IgM) was immobilized. Because of the effect of d-amino acids, the d-peptide-modified electrode surface showed prominent antifouling capability and high tolerance to enzymatic hydrolysis. Moreover, the d-peptide-modified electrode exhibited much stronger long-term stability, as well as antifouling ability in human serum than the electrode modified with normal peptides. The electrochemical biosensor exhibited a sensitive response to IgM linearly within the range of 100 pg mL^-1 to 1.0 μg mL^-1 and a very low detection limit down to 37 pg mL^-1, and it was able to detect IgM in human serum with good accuracy. This work provided a new strategy to develop robust peptide-based biosensors to resist the proteolytic degradation for practical application in complex clinical samples.

Zwitterionic Polymer Coatings Enhance Gold Nanoparticle Stability and Uptake in Various Biological Environments

Zwitterionic polymers are a class of materials that have demonstrated utility as non-fouling surfaces for medical devices and drug delivery vehicles. Here, we develop a synthesis protocol to produce zwitterionic polymers as coatings for gold nanoparticles and evaluate nanoparticle stability and biological function after exposure to various biological fluids. Thiol-functionalized polymethacryloyloxyethyl phosphorylcholine polymers (pMPC) were synthesized in nontoxic solvents via photoinitiated free radical polymerization with a radical addition-fragmentation chain transfer (RAFT) agent and coated onto gold nanoparticles. pMPC-coated nanoparticles exhibited reduced particle aggregation, improved suspension stability, and decreased protein adsorption upon exposure to serum and lung lavage fluid (BALF). Cell uptake in A549 cells was greater for pMPC-coated particles than uncoated particles after exposure to serum and BALF, with no observed cell toxicity, but pMPC-coated particles experienced higher levels of cell uptake after serum exposure than BALF exposure, suggesting that differences in the composition of the fluids result in differing impacts on particle fate. These zwitterionic polymers may serve as useful nanoparticle coatings to enhance particle stability and uptake in various biological environments.

Hydrogen-Bonded Structure of Water in the Loop of Anchored Polyrotaxane Chain Controlled by Anchoring Density

Hydrogen-bonded network of water surrounding polymers is expected to be one of the most relevant factors affecting biocompatibility, while the specific hydrogen-bonded structure of water responsible for biocompatibility is still under debate. Here we study the hydrogen-bonded structure of water in a loop-shaped poly(ethylene glycol) chain in a polyrotaxane using synchrotron soft X-ray emission spectroscopy. By changing the density of anchoring molecules, hydrogen-bonded structure of water confined in the poly(ethylene glycol) loop was identified. The XES profile of the confined water indicates the absence of the low energy lone-pair peak, probably because the limited space of the polymer loop entropically inhibits the formation of tetrahedrally coordinated water. The volume of the confined water can be changed by the anchoring density, which implies the ability to control the biocompatibility of loop-shaped polymers.

Cross-linked polymer microparticles with tunable surface properties by the combination of suspension free radical copolymerization and Click chemistry

We propose a general, versatile and broad in scope two-steps approach for the elaboration of cross-linked polymer microparticles (µPs) with tunable functionalities and surface properties. Surface-functionalized cross-linked polymer µPs with diameter in the 80 μm range are prepared by the combination of: 1) suspension free radical copolymerization of styrene, propargyl methacrylate and 1,6-hexanediol dimethacrylate, 2) subsequent covalent tethering of a variety of azide-functionalized moieties (i.e. rhodamine B fluorescent dye or poly(ethylene glycol) (PEG) brush precursor) by copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) and, 3) optional N-alkylation of the 1,2,3-triazole groups followed by anion exchange reaction to afford covalently-tethered 1,2,3-triazolium ionic liquids with iodide or cresol red counter-anions. The resulting µPs are characterized by laser diffraction, differential scanning calorimetry, as well as by optical, confocal fluorescence, scanning electron and atomic force microscopies. Finally, the rheological properties of concentrated suspensions (volume fractions of 0.40 and 0.44) of the different synthesized µPs dispersed in a 1:1 (vol/vol) mixture of polyalkylene glycol and water are studied. The modification of µPs surface properties contributes not only to change the stability of the suspensions against flocculation, but also to significantly modify their rheological behavior at high shear stresses. This represents a clear experimental evidence of the importance of non-hydrodynamic contact forces in the rheology of non-Brownian suspensions (NBSs).

Surface chemical reactions on self-assembled silane based monolayers

In this review, we aim to update our review "Chemical modification of self-assembled silane-based monolayers by surface reactions" which was published in 2010 and has developed into an important guiding tool for researchers working on the modification of solid substrate surface properties by chemical modification of silane-based self-assembled monolayers. Due to the rapid development of this field of research in the last decade, the utilization of chemical functionalities in self-assembled monolayers has been significantly improved and some new processes were introduced in chemical surface reactions for tailoring the properties of solid substrates. Thus, it is time to update the developments in the surface functionalization of silane-based molecules. Hence, after a short introduction on self-assembled monolayers, this review focuses on a series of chemical reactions, i.e., nucleophilic substitution, click chemistry, supramolecular modification, photochemical reaction, and other reactions, which have been applied for the modification of hydroxyl-terminated substrates, like silicon and glass, which have been reported during the last 10 years.

Tuning the Density of Zwitterionic Polymer Brushes on PET Fabrics by Aminolysis: Effect on Antifouling Performances

Here, we synthesize zwitterionic polymer brushes on polyester fabrics by atom transfer radical polymerization (ATRP) after a prefunctionalization step involving an aminolysis reaction with ethylenediamine. Aminolysis is an easy method to achieve homogeneous distributions of functional groups on polyester fibers (PET) fabrics. Varying the polymerization time and the prefunctionalization conditions of the reaction, it is possible to tune the amount of water retained over the surface and study its effect on protein adhesion. This study revealed that the polymerization time plays a major role in preventing protein adhesion on the PET surface.

Synthetic Semiflexible and Bioactive Brushes

Polymer brushes are extensively used for the preparation of bioactive surfaces. They form a platform to attach functional (bio)molecules and control the physicochemical properties of the surface. These brushes are nearly exclusively prepared from flexible polymers, even though much stiffer brushes from semiflexible polymers are frequently found in nature, which exert bioactive functions that are out of reach for flexible brushes. Synthetic semiflexible polymers, however, are very rare. Here, we use polyisocyanopeptides (PICs) to prepare high-density semiflexible brushes on different substrate geometries. For bioconjugation, we developed routes with two orthogonal click reactions, based on the strain-promoted azide-alkyne cycloaddition reaction and the (photoactivated) tetrazole-ene cycloaddition reaction. We found that for high brush densities, multiple bonds between the polymer and the substrate are necessary, which was achieved in a block copolymer strategy. Whether the desired biomolecules are conjugated to the PIC polymer before or after brush formation depends on the dimensions and required densities of the biomolecules and the curvature of the substrate. In either case, we provide mild, aqueous, and highly modular reaction strategies, which make PICs a versatile addition to the toolbox for generating semiflexible bioactive polymer brush surfaces.

Thermoresponsive Arrays Patterned via Photoclick Chemistry: Smart MALDI Plate for Protein Digest Enrichment, Desalting, and Direct MS Analysis

Sample desalting and concentration are crucial steps before matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) analysis. Current sample pretreatment approaches require tedious fabrication and operation procedures, which are unamenable to high-throughput analysis and also result in sample loss. Here, we report the development of a smart MALDI substrate for on-plate desalting, enrichment, and direct MS analysis of protein digests based on thermoresponsive, hydrophilic/hydrophobic transition of surface-grafted poly(N-isopropylacrylamide) (PNIPAM) microarrays. Superhydrophilic 1-thioglycerol microwells are first constructed on alkyne-silane-functionalized rough indium tin oxide substrates based on two sequential thiol-yne photoclick reactions, whereas the surrounding regions are modified with hydrophobic 1H,1H,2H,2H-perfluorodecanethiol. Surface-initiated atom-transfer radical polymerization is then triggered in microwells to form PNIPAM arrays, which facilitate sample loading and enrichment of protein digests by concentrating large-volume samples into small dots and achieving on-plate desalting through PNIPAM configuration change at elevated temperature. The smart MALDI plate shows high performance for mass spectrometric analysis of cytochrome c and neurotensin in the presence of 1 M urea and 100 mM NaHCO₃, as well as improved detection sensitivity and high sequence coverage for α-casein and cytochrome c digests in femtomole range. The work presents a versatile sample pretreatment platform with great potential for proteomic research.

Antimicrobial Polymers and Surfaces – Natural Mimics or Surpassing Nature?

Fighting pathogenic microbes is one of the great current challenges of mankind. Nature has developed several techniques to counteract microbial attacks. Science has also yielded several technologies, including antimicrobial polymers as biocides and polymers used for microbe killing and repelling surfaces. Recent scientific antimicrobial approaches are mimicking natural concepts. In this chapter, current developments in antimicrobial and antifouling polymers and surfaces are reviewed and discussed regarding the question whether they mimic nature or surpass it.

Advances in click chemistry for silica-based material construction

Click chemistry is undoubtedly the most powerful 1,3-dipolar cycloaddition reaction in organic synthesis.

Polyethylene Glycol Coatings on Plastic Substrates for Chemically Defined Stem Cell Culture

Human mesenchymal stem cells (hMSCs) are a widely available and clinically relevant cell type with a host of applications in regenerative medicine. Current clinical expansion methods can lead to selective changes in hMSC phenotype potentially resulting from relatively undefined cell culture surfaces. Chemically defined synthetic surfaces can aid in understanding the influence of cell-material interactions on stem cell behavior. Here, a thin copolymer coating for hMSC culture on plastic substrates is developed. The random copolymer is synthesized by living free radical polymerization and characterized in solution before application to the substrate, ensuring a homogeneous coating and limiting the sample-to-sample variations. The ability to coat multiple substrate types and cover large surface areas is reported. Arg-Gly-Asp-containing peptides are incorporated into the coating under aqueous conditions via their lysine or cysteine side chains, resulting in amide and thioester linkages, respectively. Stability studies show amide linkages to be stable and thioester linkages to be labile under standard serum-containing culture conditions. In addition, chemically defined passaging of hMSCs using only ethylenediaminetetraacetic acid on polystyrene dishes is shown. After passage, the hMSCs can be seeded back onto the same plate, indicating potential reusability of the coating.

Ultrathin random copolymer-grafted layers for block copolymer self-assembly

Hydroxyl-terminated P(S-r-MMA) random copolymers (RCPs) with molecular weights (Mn) from 1700 to 69000 and a styrene unit fraction of approximately 61% were grafted onto a silicon oxide surface and subsequently used to study the orientation of nanodomains with respect to the substrate, in cylinder-forming PS-b-PMMA block copolymer (BCP) thin films. When the thickness (H) of the grafted layer is greater than 5-6 nm, a perpendicular orientation is always observed because of the efficient decoupling of the BCP film from the polar SiO2 surface. Conversely, if H is less than 5 nm, the critical thickness of the grafted layer, which allows the neutralization of the substrate and promotion of the perpendicular orientation of the nanodomains in the BCP film, is found to depend on the Mn of the RCP. In particular, when Mn = 1700, a 2.0 nm thick grafted layer is sufficient to promote the perpendicular orientation of the PMMA cylinders in the PS-b-PMMA BCP film. A proximity shielding mechanism of the BCP molecules from the polar substrate surface, driven by chain stretching of the grafted RCP molecules, is proposed.

Controlled decoration of the surface with macromolecules: polymerization on a self-assembled monolayer (SAM)

Polymer functionalized surfaces are important components of various sensors, solar cells and molecular electronic devices. In this context, the use of self-assembled monolayer (SAM) formation and subsequent reactions on the surface have attracted a lot of interest due to its stability, reliability and excellent control over orientation of functional groups. The chemical reactions to be employed on a SAM must ensure an effective functional group conversion while the reaction conditions must be mild enough to retain the structural integrity. This synthetic constraint has no universal solution; specific strategies such as "graft from", "graft to", "graft through" or "direct" immobilization approaches are employed depending on the nature of the substrate, polymer and its area of applications. We have reviewed current developments in the methodology of immobilization of a polymer in the first part of the article. Special emphasis has been given to the merits and demerits of certain methods. Another issue concerns the utility - demonstrated or perceived - of conjugated or non-conjugated macromolecules anchored on a functionally decorated SAM in the areas of material science and biotechnology. In the last part of the review article, we looked at the collective research efforts towards SAM-based polymer devices and identified major pointers of progress (236 references).

Antibiofouling polymer interfaces: poly(ethylene glycol) and other promising candidates

This review highlights antibiofouling polymer interfaces with emphasis on the latest developments using poly(ethylene glycol) and the design new polymeric structures.

Relevance of the poly(ethylene glycol) linkers in peptide surfaces for proteases assays

Poly(ethylene glycol)s (PEGs) with different lengths were used as linkers during the preparation of peptide surfaces for protease detection. In the first approach, the PEG monolayers were prepared using a "grafting to" method on 3-aminopropyltrietoxysilane (APTES)-modified silicon wafers. Protected peptides with a fluorescent marker were synthesized by Fmoc solid phase synthesis. The protected peptide structures enabled their site-specific immobilization onto the PEG surfaces. Alternatively, the PEG-peptide surface was obtained by immobilizing a PEG-peptide conjugate directly onto the modified silicon wafer. The surfaces (composition, grafting density, hydrophilicity, and roughness) were characterized by time-of-flight-secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS), contact angle (CA), and atomic force microscopy (AFM). Introducing the PEG linker between the peptide and surface increased their resistance toward nonspecific protein adsorption. The peptide surfaces were examined as analytical platforms to study the action of trypsin as a representative protease. The products of the enzymatic hydrolysis were analyzed by fluorescence spectroscopy, electrospray ionization-mass spectrometry (ESI-MS), and ToF-SIMS. Conclusions about the optimal length of the PEG linker for the analytical application of PEG-peptide surfaces were drawn. This work demonstrates an effective synthetic procedure to obtain PEG-peptide surfaces as attractive platforms for the development of peptide microarrays.

Nanofiber Polyplex Formation Based on the Morphology Elongation by the Intrapolyplex PEG Crowding Effect

We prepared a multiarm poly(ethylene glycol)-poly(l-lysine) block copolymer (maPEG-PLL) with a size-controllable maPEG head and a cationic PLL tail for the evaluation of the effect of maPEG crowding to the polyplex formation with plasmid DNA. maPEG-PLLs of various compositions were synthesized and the formation of a polyplex was confirmed by gel retardation assay. The maPEG-PLL exhibited noncooperative polyplex formation behavior, suggesting the effective hydration of the polyplex. Also, an increase in the size of the maPEG head induces the elongation of polyplex morphology from spherical aggregates to nanorods and nanofibers because of the intrapolyplex PEG crowding effect. Furthermore, an increase in the size of the maPEG head also improves the effective inhibition of the decrease in cell-free gene expression, indicating the importance of the control of pDNA packaging in the polyplex.

Grafting of poly(ethylene glycol) on click chemistry modified Si(100) surfaces

Poly(ethylene glycol) (PEG) is one of the most extensively studied antifouling coatings due to its ability to reduce protein adsorption and improve biocompatibility. Although the use of PEG for antifouling coatings is well established, the stability and density of PEG layers are often inadequate to provide optimum antifouling properties. To improve on these shortcomings, we employed the stepwise construction of PEG layers onto a silicon surface. Acetylene-terminated alkyl monolayers were attached to nonoxidized crystalline silicon surfaces via a one-step hydrosilylation procedure with 1,8-nonadiyne. The acetylene-terminated surfaces were functionalized via a copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction of the surface-bound alkynes with an azide to produce an amine terminated layer. The amine terminated layer was then further conjugated with PEG to produce an antifouling surface. The antifouling surface properties were investigated by testing adsorption of human serum albumin (HSA) and lysozyme (Lys) onto PEG layers from phosphate buffer solutions. Detailed characterization of protein fouling was carried out with X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) combined with principal component analysis (PCA). The results revealed no fouling of albumin onto PEG coatings whereas the smaller protein lysozyme adsorbed to a very low extent.

Molecularly functionalized silicon substrates for orientation control of the microphase separation of PS-b-PMMA and PS-b-PDMS block copolymer systems

The use of block copolymer (BCP) thin films to generate nanostructured surfaces for device and other applications requires precise control of interfacial energies to achieve the desired domain orientation. Usually, the surface chemistry is engineered through the use of homo- or random copolymer brushes grown or attached to the surface. Herein, we demonstrate a facile, rapid, and tunable approach to surface functionalization using a molecular approach based on ethylene glycol attachment to the surface. The effectiveness of the molecular approach is demonstrated for the microphase separation of PS-b-PMMA and PS-b-PDMS BCPs in thin films and the development of nanoscale features at the substrate.