Reversible Binding Interfaces Made of Microstructured Polymer Brushes.

The polymer brush architecture of the end-tethered polymer molecules is one of the most widely used efficient methods to regulate interfacial interactions in colloidal systems found in live matter and manufactured materials. Emerging applications of polymer brush structures require solutions to new tasks in the control of interfacial interactions. The rapid development of live cell manufacturing relies on scalable and efficient cell harvesting methods. Stimuli-responsive surfaces made of surface-grafted poly(N-isopropylacrylamide) (PNIPAM) can bind and detach the adherent cell upon changes in temperature and have been used for cell growth and harvesting. The applications are limited by the requirement to satisfy a range of PNIPAM coating characteristics that depend on the dimensions of the integrin complex in the cell membrane and the basal surface. The analysis of the microstructured surfaces, when adhesive and disjoining functions of the microdomains are decoupled, shows that many limitations of PNIPAM one-component coatings can be avoided by using a much broader range of structural characteristics of the microstructured interfaces composed of alternating disjoining PNIPAM domains and adhesive polymeric domains with cell-affinity functional groups. Temperature-controlled reversible adhesion to such microstructured interfaces is studied here experimentally with model systems of solid spherical particles and by employing simulations for solid and soft membranes interacting with the microstructured surfaces to mimic interactions with soft and solid disk-like particles.

Tough Bioplastics from Babassu Oil-Based Acrylic Monomer, Hemicellulose Xylan, and Carnauba Wax.

We describe here the fabrication, characterization, and properties of tough bioplastics made of a babassu oil-based acrylic polymer (PBBM), hemicellulose xylan grafted with PBBM chains, and carnauba wax (CW). The plastic was primarily designed to obtain bioderived materials that can replace low-density polyethylene (LDPE) in certain food packaging applications. To obtain plastic, the radical polymerization of an original babassu oil-based acrylic monomer (BBM) in the presence of xylan macromolecules modified with maleic anhydride (X-MA) was conducted. The polymerization resulted in a material (PBBM-X) mostly consisting of highly branched PBBM/X-MA macromolecules. PBBM-X has a glass transition of 42 °C, a storage modulus of 130 MPa (at 25 °C, RT), and a Young's modulus of 30 MPa at RT. To increase the moduli, we blended PBBM-X with carnauba wax, a natural material with a high modulus and a melting temperature of ~80 °C. It was found that PBBM-X is compatible with the wax, as evidenced by the alternation of the material's thermal transitions and the co-crystallization of BBM side alkyl fragments with CW. As a result, the PBBM-X/CW blend containing 40% of the wax had a storage modulus of 475 MPa (RT) and a Young's modulus of 248 MPa (RT), which is close to that of LDPE. As polyethylene, the PBBM-X and PBBM-X/CW bioplastics have the typical stress-strain behavior demonstrated by ductile (tough) plastics. However, the bioplastic's yield strength and elongation-at-yield are considerably lower than those of LDPE. We evaluated the moisture barrier properties of the PBBM-X/(40%)CW material and found that the bioplastic's water vapor permeability (WVP) is quite close to that of LDPE. Our bioderived material demonstrates a WVP that is comparable to polyethylene terephthalate and lower than the WVP of nylon and polystyrene. Taking into account the obtained results, the fabricated materials can be considered as polyethylene alternatives to provide sustainability in plastics production in the packaging areas where LDPE currently dominates.

A magneto-controlled biocatalytic cascade with a fluorescent output.

A biocatalytic cascade based on concerted operation of pyruvate kinase and luciferase with a bioluminescent output was switched reversibly between low and high activity by applying an external magnetic field at different positions or removing it. The enzymes participating in the reaction cascade were bound to magnetic nanoparticles to allow their translocation or aggregation/dispersion to be controlled by the magnetic field. The reaction intensity, measured as the bioluminescent output, was dependent on the effective distances between the enzymes transported on the magnetic nanoparticles controlled by the magnets.

Refining of Particulates at Stimuli-Responsive Interfaces: Label-Free Sorting and Isolation.

The mechanism of separation methods, for example, liquid chromatography, is realized through rapid multiple adsorption-desorption steps leading to the dynamic equilibrium state in a mixture of molecules with different partition coefficients. Sorting of colloidal particles, including protein complexes, cells, and viruses, is limited due to a high energy barrier, up to millions kT, required to detach particles from the interface, which is in dramatic contrast to a few kT for small molecules. Such a strong interaction renders particle adsorption quasi-irreversible. The dynamic adsorption-desorption equilibrium is approached very slowly, if ever attainable. This limitation is alleviated with a local oscillating repulsive mechanical force generated at the microstructured stimuli-responsive polymer interface to switch between adsorption and mechanical-force-facilitated desorption of the particles. Such a dynamic regime enables the separation of colloidal mixtures based on the particle-polymer interface affinity, and it could find use in research, diagnostics, and industrial-scale label-free sorting of highly asymmetric mixtures of colloids and cells.

Polyethylene Glycol Crowder's Effect on Enzyme Aggregation, Thermal Stability, and Residual Catalytic Activity.

Protein stability and performance in various natural and artificial systems incorporating many other macromolecules for therapeutic, diagnostic, sensor, and biotechnological applications attract increasing interest with the expansion of these technologies. Here we address the catalytic activity of lysozyme protein (LYZ) in the presence of a polyethylene glycol (PEG) crowder in a broad range of concentrations and temperatures in aqueous solutions of two different molecular mass PEG samples (Mw = 3350 and 10000 g/mol). The phase behavior of PEG-protein solutions is examined by using dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS), while the enzyme denaturing is monitored by using an activity assay (AS) and circular dichroism (CD) spectroscopy. Molecular dynamic (MD) simulations are used to illustrate the effect of PEG concentration on protein stability at high temperatures. The results demonstrate that LYZ residual activity after 1 h incubation at 80 °C is improved from 15% up to 55% with the addition of PEG. The improvement is attributed to two underlying mechanisms. (i) Primarily, the stabilizing effect is due to the suppression of the enzyme aggregation because of the stronger PEG-protein interactions caused by the increased hydrophobicity of PEG and lysozyme at elevated temperatures. (ii) The MD simulations showed that the addition of PEG to some degree stabilizes the secondary structures of the enzyme by delaying unfolding at elevated temperatures. The more pronounced effect is observed with an increase in PEG concentration. This trend is consistent with CD and AS experimental results, where the thermal stability is strengthened with increasing of PEG concentration and molecular mass. The results show that the highest stabilizing effect is approached at the critical overlap concentration of PEG.

Gravity Drawing of Micro- and Nanofibers for Additive Manufacturing of Well-Organized 3D-Nanostructured Scaffolds.

This work introduces a gravity fiber drawing (GFD) method of making single filament nanofibers from polymer solutions and precise alignment of the fibers in 3D scaffolds. This method is advantageous for nanofiber 3D alignment in contrast to other known methods. GFD provides a technology for the fabrication of freestanding filament nanofibers of well-controlled diameter, draw ratio, and 3D organization with controllable spacing and angular orientation between nanofibers. The GFD method is capable of fabricating complex 3D scaffolds combining fibers with different diameters, chemical compositions, mechanical properties, angular orientations, and multilayer structures in the same construct. The scaffold porosity can be as high as 99% to secure transport of nutrients and space for cell infiltration and differentiation in tissue engineering and 3D cell culture applications.

Enhanced neuronal differentiation of neural stem cells with mechanically enhanced touch-spun nanofibrous scaffolds.

We studied NE-4C neural cells differentiation on 2D polycaprolactone (PCL) nanofibrous scaffolds with systematically varied mechanical characteristics of nanofibers while retaining an unchanged fiber alignment, diameter, and chemical composition. Our experiments demonstrated that the nanofibers with enhanced mechanical properties are beneficial for the preferential development of neuronal cells vs. glial cells. Electrospun (ES) and touch-spun (TS) nanofibers were fabricated with Young's modulus in the range of 10 MPa to 230 MPa and a fraction of crystallinity from 30% to 80%. The TS fibers undergo a greater drawing ratio and thus approach a greater polymer chain stretching and alignment that resulted in an increased crystallinity. The TS scaffolds demonstrated improved stability in the aqueous cell culture environment, resisting misalignment and entanglement after a period of 2 weeks of swelling followed by 14 days of neural differentiation. The results confirmed that the neurites on the TS fibers have a preferred orientation even after swelling.

Adhesion and Stability of Nanocellulose Coatings on Flat Polymer Films and Textiles.

Renewable nanocellulose materials received increased attention owing to their small dimensions, high specific surface area, high mechanical characteristics, biocompatibility, and compostability. Nanocellulose coatings are among many interesting applications of these materials to functionalize different by composition and structure surfaces, including plastics, polymer coatings, and textiles with broader applications from food packaging to smart textiles. Variations in porosity and thickness of nanocellulose coatings are used to adjust a load of functional molecules and particles into the coatings, their permeability, and filtration properties. Mechanical stability of nanocellulose coatings in a wet and dry state are critical characteristics for many applications. In this work, nanofibrillated and nanocrystalline cellulose coatings deposited on the surface of polymer films and textiles made of cellulose, polyester, and nylon are studied using atomic force microscopy, ellipsometry, and T-peel adhesion tests. Methods to improve coatings' adhesion and stability using physical and chemical cross-linking with added polymers and polycarboxylic acids are analyzed in this study. The paper reports on the effect of the substrate structure and ability of nanocellulose particles to intercalate into the substrate on the coating adhesion.

Magneto-Controlled Biocatalytic Cascades with Logically Processed Input Signals - Substrate Channeling versus Free Diffusion.

Magnetic nanoparticles (MNPs) functionalized with various enzymes (amyloglucosidase, glucose oxidase and horseradish peroxidase) were used to perform biocatalytic cascades in two different states, solute suspension or aggregated, produced in the absence or presence of an external magnetic field. The biocatalytic reactions proceeded through bulk solution diffusion of intermediate substrates or substrate channeling, when the systems were dispersed or aggregated, respectively. The both pathways have shown very similar kinetics, unless the intermediate substrate was consumed by an additional biocatalytic process called "filter" for brevity. In the presence of the "filter" process, the diffusional process in the bulk solution was significantly inhibited, while the process based on the substrate channeling was still active. The systems were switched reversibly between the inhibited dispersed state and the active aggregated state by removing and applying the external magnetic field, respectively. The signal-controlled biocatalytic cascades were considered as Boolean logic circuits with the inputs consisting of biomolecules and the magnetic field on-off.

Designing Highly Thermostable Lysozyme-Copolymer Conjugates: Focus on Effect of Polymer Concentration.

Designing biomaterials capable of functioning in harsh environments is vital for a range of applications. Using molecular dynamics simulations, we show that conjugating lysozymes with a copolymer [poly(GMA- stat-OEGMA)] comprising glycidyl methacrylate (GMA) and oligo(ethylene glycol) methyl ether methacrylate (OEGMA) results in a dramatic increase of stability of these enzymes at high temperatures provided that the concentration of the copolymer in the close vicinity of the enzyme exceeds a critical value. In our simulations, we use triads containing the same ratio of GMA to OEGMA units as in our recent experiments (N. S. Yadavalli et al., ACS Catalysis, 2017, 7, 8675). We focus on the dynamics of the conjugate at high temperatures and on its structural stability as a function of the copolymer/water content in the vicinity of the enzyme. We show that the dynamics of phase separation in the water-copolymer mixture surrounding the enzyme is critical for the structural stability of the enzyme. Specifically, restricting water access promotes the structural stability of the lysozyme at high temperatures. We identified critical water concentration below which we observe a robust stabilization; the phase separation is no longer observed at this low fraction of water so that the water domains promoting unfolding are no longer formed in the vicinity of the enzyme. This understanding provides a basis for future studies on designing a range of enzyme-copolymer conjugates with improved stability.

Magnetic Field-Activated Sensing of mRNA in Living Cells.

Detection of specific mRNA in living cells has attracted significant attention in the past decade. Probes that can be easily delivered into cells and activated at the desired time can contribute to understanding translation, trafficking and degradation of mRNA. Here we report a new strategy termed magnetic field-activated binary deoxyribozyme (MaBiDZ) sensor that enables both efficient delivery and temporal control of mRNA sensing by magnetic field. MaBiDZ uses two species of magnetic beads conjugated with different components of a multicomponent deoxyribozyme (DZ) sensor. The DZ sensor is activated only in the presence of a specific target mRNA and when a magnetic field is applied. Here we demonstrate that MaBiDZ sensor can be internalized in live MCF-7 breast cancer cells and activated by a magnetic field to fluorescently report the presence of specific mRNA, which are cancer biomarkers.

Reactive Magnetospinning of Nano- and Microfibers.

Reactive spinning of nano- and microfibers that involves very fast chemical reactions and ion exchange is a challenge for the common methods for nanofiber formation. Herein, we introduce the reactive magnetospinning method. This procedure is based on the magnetic-field-directed collision of ferrofluid droplets with liquid droplets that contain complementary reactants. The collision, start of the chemical reaction, and the fiber drawing are self-synchronized. The method is used to synthesize, cross-link, and chemically modify fiber-forming polymers in the stage of fiber formation. The method provides new opportunities for the fabrication of nanofibers for biomedical applications.

Plasmonic nanobiosensor with chain reaction amplification mechanism.

Herein we demonstrate a plasmonic nanobiosensor that explores chain reaction amplification mechanisms to transduce chemical signals released in biocatalytic reactions, turning optical signals into a visual spectral range. The sensor has a very simple design: gold nanoparticles resting in the surface of a grafted P2VP film. Changes in the gold nanoparticles' position causes changes in the plasmon coupling mode. This is detected by means of a maximum absorbance shift.

Highly efficient phase boundary biocatalysis with enzymogel nanoparticles.

The enzymogel nanoparticle made of a magnetic core and polymer brush shell demonstrates a novel type of remote controlled phase-boundary biocatalysis that involves remotely directed binding to and engulfing insoluble substrates, high mobility, and stability of the catalytic centers. The mobile enzymes reside in the polymer brush scaffold and shuttle between the enzymogel interior and surface of the engulfed substrate in the bioconversion process. Biocatalytic activity of the mobile enzymes is preserved in the enzymogel while the brush-like architecture favors the efficient interfacial interaction when the enzymogel spreads over the substrate and extends substantially the reaction area as compared with rigid particles.

Field-directed self-assembly with locking nanoparticles.

A reversible locking mechanism is established for the generation of anisotropic nanostructures by a magnetic field pulse in liquid matrices by balancing the thermal energy, short-range attractive and long-range repulsive forces, and dipole-dipole interactions using a specially tailored polymer shell of nanoparticles. The locking mechanism is used to precisely regulate the dimensions of self-assembled magnetic nanoparticle chains and to generate and disintegrate three-dimensional (3D) nanostructured materials in solvents and polymers.

Strategy for Nonenzymatic Harvesting of Cells via Decoupling of Adhesive and Disjoining Domains of Nanostructured Stimulus-Responsive Polymer Films.

The nanostructured polymer film introduces a novel mechanism of nonenzymatic cell harvesting by decoupling solid cell-adhesive and soft stimulus-responsive cell-disjoining areas on the surface. The key characteristics of this architecture are the decoupling of adhesion from detachment and the impermeability to the integrin protein complex of the adhesive domains. This surface design eliminates inherent limitations of thermoresponsive coatings, namely, the necessity for the precise thickness of the coating, grafting or cross-linking density, and material of the basal substrate. The concept is demonstrated with nanostructured thermoresponsive films made of cell-adhesive epoxy photoresist domains and cell-disjoining poly(N-isopropylacrylamide) brush domains.

Superomniphobic magnetic microtextures with remote wetting control.

Universal remote control of wetting behavior enabling the transition from a superomniphobic to an omniphilic wetting state in an external magnetic field via the alternation of reentrant curvature of a microstructured surface is demonstrated. This reconfigurable microtexture made of Ni micronails repels water, water-surfactant solutions, and practically all organic liquids, whereas it gets wetted by all of these liquids after a magnetic field pulse is applied.

Conformational transitions of flexible hydrophobic polyelectrolytes in solutions of monovalent and multivalent salts and their mixtures.

Conformations of cationic polyelectrolytes (PEs), a weak poly(2-vinylpyridine) (P2VP) and a strong poly(N-methyl-2-vinylpyridinium iodide) (qP2VP), adsorbed on mica from saline solutions in the presence of counterions of different valences are studied using in situ atomic force microscopy (AFM). Quantitative characteristics of chain conformations are analyzed using AFM images of the adsorbed molecules. The results of the statistical analysis of the chain contour reveal collapse of the PE coils when ionic strength is in a range from tens to hundreds of millimoles per kilogram and re-expansion of the coils with a further increase of ionic strength up to a region of the saturated saline solutions. The competition between monovalent and multivalent counterions simultaneously present in solutions strongly affects conformations of PE chains even at a very small fraction of multivalent counterions. Shrinkage of PE coils is steeper for multivalent counterions than for monovalent counterions. However, the re-expansion is only incremental in the presence of multivalent counterions. Extended adsorbed coils at low salt concentrations and at very high concentrations of monovalent salt exhibit conformation corresponding to a 2D coil with 0.95 fraction of bound segments (segments in "trains") in the regime of diluted surface concentration of the PE. Shrunken coils in the intermediate range of ionic strength resemble 3D-globules with 0.8 fraction of trains. The incrementally re-expanded PE coils at a high ionic strength remain unchanged at higher multivalent salt concentrations up to the solubility limit of the salt. The formation of a strong PE complex with multivalent counterions at high ionic strength is not well understood yet. A speculative explanation of the observed experimental result is based on possible stabilization of the complex due to hydrophobic interactions of the backbone.

Emerging applications of stimuli-responsive polymer materials.

Responsive polymer materials can adapt to surrounding environments, regulate transport of ions and molecules, change wettability and adhesion of different species on external stimuli, or convert chemical and biochemical signals into optical, electrical, thermal and mechanical signals, and vice versa. These materials are playing an increasingly important part in a diverse range of applications, such as drug delivery, diagnostics, tissue engineering and 'smart' optical systems, as well as biosensors, microelectromechanical systems, coatings and textiles. We review recent advances and challenges in the developments towards applications of stimuli-responsive polymeric materials that are self-assembled from nanostructured building blocks. We also provide a critical outline of emerging developments.

AFM imaging of adsorbed Nafion polymer on mica and graphite at molecular level.

Perfluorosulfonic acid ionomer (PFSA, specifically Nafion at EW = 975 g/mol) was visualized at the single molecule level using atomic force microscopy (AFM) in liquid. The diluted commercial Nafion dispersion shows an apparent M(w) = 1430 kg/mol and M(w)/M(n) = 3.81, which is assigned to chain aggregation. PFSA aggregates, imaged on mica and HOPG during adsorption from EtOH-H(2)O solvent at pH(e) 3.0 (below isoelectric point), showed a stable, segmented rod-like conformation. This structure is consistent with earlier NMR, SAXS/SANS, and TEM results that support a stiff helical Nafion conformation with long persistence length, a sharp solvent-polymer interface, and an extension of the sulfonated side chain into solution. Adsorption of Nafion structures on HOPG was observed at even higher pH(e) from EtOH due to screening of the repulsive electrostatic interaction in lower dielectric constant solvent, while the chain adopted an expanded coil conformation. These measurements provided direct evidence of the chain aggregation in EtOH-H(2)O solution and revealed their equilibrium conformations for adsorption on two model surfaces, highly ordered pyrolitic graphite (HOPG) and mica. The commercial Nafion dispersion was autoclaved at 0.10% w/w in nPrOH/H(2)O = 4:1 v/v solvent at 230 °C for 6 h to give a single-chain dispersion with M(w) = 310 kg/mol and M(w)/M(n) = 1.60. The autoclaved chains adopt an electrostatically stabilized compact globule conformation as observed by AFM imaging of the single PFSA molecules after rapid deposition on mica and HOPG at a low surface coverage.