Polymerization within Nanoporous Anodized Alumina Oxide Templates (AAO): A Critical Survey.

In the last few years, the polymerization of monomers within the nanocavities of porous materials has been thoroughly studied and developed, allowing for the synthesis of polymers with tailored morphologies, chemical architectures and functionalities. This is thus a subject of paramount scientific and technological relevance, which, however, has not previously been analyzed from a general perspective. The present overview reports the state of the art on polymerization reactions in spatial confinement within porous materials, focusing on the use of anodized aluminum oxide (AAO) templates. It includes the description of the AAO templates used as nanoreactors. The polymerization reactions are categorized based on the polymerization mechanism. Amongst others, this includes electrochemical polymerization, free radical polymerization, step polymerization and atom transfer radical polymerization (ATRP). For each polymerization mechanism, a further subdivision is made based on the nature of the monomer used. Other aspects of "in situ" polymerization reactions in restricted AAO geometries include: conversion monitoring, kinetic studies, modeling and polymer characterization. In addition to the description of the polymerization process itself, the use of polymer materials derived from polymerization in AAO templates in nanotechnology applications, is also highlighted. Finally, the review is concluded with a general discussion outlining the challenges that remain in the field.

Rheology Applied to Microgels: Brief (Revision of the) State of the Art.

The ability of polymer microgels to rapidly respond to external stimuli is of great interest in sensors, lubricants, and biomedical applications, among others. In most of their uses, microgels are subjected to shear, deformation, and compression forces or a combination of them, leading to variations in their rheological properties. This review article mainly refers to the rheology of microgels, from the hard sphere versus soft particles' model. It clearly describes the scaling theories and fractal structure formation, in particular, the Shih et al. and Wu and Morbidelli models as a tool to determine the interactions among microgel particles and, thus, the viscoelastic properties. Additionally, the most recent advances on the characterization of microgels' single-particle interactions are also described. The review starts with the definition of microgels, and a brief introduction addresses the preparation and applications of microgels and hybrid microgels.

A Brief Overview of Polymers Science and Technology, in Spain.

This Special Issue State-of-the-Art on Polymer Science and Technology in Spain is comprised of a collection of 42 publications/contributions related to very different topics undertaken by the numerous research groups working in Spain in Polymer Science and Technology. This monograph collects the contributions of more than 200 different authors from 24 different national Institutions (>30 different centers/departments) from Universities and CSIC centers distributed throughout the whole of Spain. Two-thirds of the contributions to this Special Issue arise from Institutional collaborations, half of which are international collaborations with European research groups and the other half with other international research groups outside Europe including China, Australia or United States of America among others. This brief overview communication provides a general overview of the research lines in Polymer Science and Technology covered in Spain and show most of the representative polymer groups and their distribution throughout Spain. Most of Spanish polymer groups belong to the Grupo Especializado de Polímeros (GEP) being part of the European Polymer Federation (EPF). It also shows how Spanish science about polymers is positioned at European level.

Polyelectrolyte Multilayer Films Based on Natural Polymers: From Fundamentals to Bio-Applications.

Natural polymers are of great interest in the biomedical field due to their intrinsic properties such as biodegradability, biocompatibility, and non-toxicity. Layer-by-layer (LbL) assembly of natural polymers is a versatile, simple, efficient, reproducible, and flexible bottom-up technique for the development of nanostructured materials in a controlled manner. The multiple morphological and structural advantages of LbL compared to traditional coating methods (i.e., precise control over the thickness and compositions at the nanoscale, simplicity, versatility, suitability, and flexibility to coat surfaces with irregular shapes and sizes), make LbL one of the most useful techniques for building up advanced multilayer polymer structures for application in several fields, e.g., biomedicine, energy, and optics. This review article collects the main advances concerning multilayer assembly of natural polymers employing the most used LbL techniques (i.e., dipping, spray, and spin coating) leading to multilayer polymer structures and the influence of several variables (i.e., pH, molar mass, and method of preparation) in this LbL assembly process. Finally, the employment of these multilayer biopolymer films as platforms for tissue engineering, drug delivery, and thermal therapies will be discussed.

In Situ Synthesis of Poly(butyl methacrylate) in Anodic Aluminum Oxide Nanoreactors by Radical Polymerization: A Comparative Kinetics Analysis by Differential Scanning Calorimetry and 1H-NMR.

In this work, we explore the ability to generate well-defined poly(butyl methacrylate) (PBMA) nanostructures by "in situ" polymerization of butyl methacrylate monomer (BMA). PBMA nanostructures of high and low aspect ratios have been successfully obtained through the free radical polymerization (FRP) of a BMA monomer in anodic aluminum oxide (AAO) nanoreactors of suitable size. A polymerization kinetics process has been followed by differential scanning calorimetry (DSC) and proton Nuclear Magnetic Resonance spectroscopy (1H-NMR).The determination of the kinetics of polymerization through DSC is based on a quick and direct analysis of the exothermic polymerization process, whereas the analysis through 1H-NMR also allows the unambiguous chemical analysis of the resulting polymer. When compared to bulk polymerization, both techniques demonstrate confinement effects. Moreover, DSC and 1H-NMR analysis give the same kinetics results and show a gel-effect in all the cases. The number average molecular weight (Mn) of the PBMA obtained in AAO of 60-300 nm are between 30·103-175·103 g/mol. Even if the Mn value is lower with respect to that obtained in bulk polymerization, it is high enough to maintain the polymer properties. As determined by SEM morphological characterization, once extracted from the AAO nanoreactor, the polymer nanostructures show controlled homogeneous aspect/size all throughout the length of nanopillar over a surface area of few cm2. The Young's modulus of low aspect ratio PBMA nanopillars determined by AFM gives a value of 3.1 ± 1.1 MPa. In this work, a 100% of PBMA polymer nanostructures are obtained from a BMA monomer in AAO templates through a quick double process: 30 min of monomer immersion at room temperature and 90 min of polymerization reaction at 60 °C. While the same nanostructures are obtained by polymer infiltration of PBMA at 200 °C in about 6 h, polymerization conditions are much softer than those corresponding to the polymer infiltration process. Furthermore, the 1H-NMR technique has been consolidated as a tool for studying the kinetics of the copolymerization reactions in confinement and the determination of monomer reactivity ratios.

Novel Hydrogels of Chitosan and Poly(vinyl alcohol) Reinforced with Inorganic Particles of Bioactive Glass.

Chitosan (CS) and poly(vinyl alcohol) (PVA) hydrogels, a polymeric system that shows a broad potential in biomedical applications, were developed. Despite the advantages they present, their mechanical properties are insufficient to support the loads that appear on the body. Thus, it was proposed to reinforce these gels with inorganic glass particles (BG) in order to improve mechanical properties and bioactivity and to see how this reinforcement affects levofloxacin drug release kinetics. Scanning electron microscopy (SEM), X-ray diffraction (XRD), swelling tests, rheology and drug release studies characterized the resulting hydrogels. The experimental results verified the bioactivity of these gels, showed an improvement of the mechanical properties and proved that the added bioactive glass does affect the release kinetics.

How Confinement Affects the Nucleation, Crystallization, and Dielectric Relaxation of Poly(butylene succinate) and Poly(butylene adipate) Infiltrated within Nanoporous Alumina Templates.

This work describes the successful melt infiltration of poly(butylene succinate) (PBS) and poly(butylene adipate) (PBA) within 70 nm diameter anodic aluminum oxide (AAO) templates. The infiltrated samples were characterized by SEM, Raman, and FTIR spectroscopy. The crystallization behaviors and crystalline structures of both polymers, bulk and confined, were analyzed by differential scanning calorimetry (DSC) and grazing incidence wide angle X-ray scattering (GIWAXS). DSC revealed that a change in the nucleation process occurred from heterogeneous nucleation for bulk samples to homogeneous nucleation for infiltrated PBA and to surface-induced nucleation for infiltrated PBS. GIWAXS results indicate that PBS nanofibers crystallize in the α-phase, as well as their bulk samples. However, PBA nanofibers crystallize just in the ß-phase, whereas PBA bulk samples crystallize in a mixture of α- and ß-phases. The crystal orientation within the pores was determined, and differences between PBS and PBA were also found. Finally, broadband dielectric spectroscopy was applied to study the segmental dynamics for bulk and infiltrated samples. The glass temperature was found to significantly decrease in the PBS case upon infiltration, while that of PBA remained unchanged. These differences were correlated with the higher affinity of PBS to the AAO walls than PBA, in accordance with their nucleation behavior (surface-induced versus homogeneous nucleation, respectively).

A Way to Predict Gold Nanoparticles/Polymer Hybrid Microgel Agglomeration Based on Rheological Studies.

In this work, a detailed rheological study of hybrid poly(acrylamide-co-acrylic acid) P(AAm-co-AAc) aqueous microgel dispersions is performed. Our intention is to understand how the presence of gold nanoparticles, AuNP, embedded within the microgel matrix, affects the viscoelastic properties, the colloidal gel structure formation, and the structure recovery after cessation of the deformation of the aqueous microgel dispersions. Frequency sweep experiments confirmed that hybrid microgel dispersions present a gel-like behavior and that the presence of AuNP content within microgel matrix contributes to the elasticity of the microgel dispersions. Strain sweep test confirmed that hybrid microgels aqueous dispersion also form colloidal gel structures that break upon deformation but that can be recovered when the deformation decreases. The fractal analysis performed to hybrid microgels, by applying Shih et al. and Wu and Morbidelli's scaling theories, evidenced that AuNP significantly affects the colloidal gel structure configuration ending up with the formation of agglomerates or microgel clusters with closer structures in comparison to the reference P(AAm-co-AAc) aqueous microgel dispersions.

The application of spatially restricted geometries as a unique route to produce well-defined poly(vinyl pyrrolidones) via free radical polymerisation.

We report, for the first time, the metal-free green synthesis of linear poly(vinyl pyrrolidone) (PVP) homopolymers of molecular weight higher than 100 kg mol-1 and narrow dispersities via thermal and photo-induced free radical polymerisation carried out within alumina nanoporous membranes acting as "nanoreactors".

A Patterned Butyl Methacrylate-co-2-Hydroxyethyl Acrylate Copolymer with Softening Surface and Swelling Capacity.

The tunable swelling and mechanical properties of nanostructures polymers are crucial parameters for the creation of adaptive devices to be used in diverse fields, such as drug delivery, nanomedicine, and tissue engineering. We present the use of anodic aluminum oxide templates as a nanoreactor to copolymerize butyl methacrylate and 2-hydroxyethyl acrylate under radical conditions. The copolymer obtained under confinement showed significant differences with respect to the same copolymer obtained in bulk conditions. Molecular weights, molecular weight dispersities, Young's modulus, and wetting behaviors were significantly modified. The combination of selected monomers allowed us to obtain nanopillar structures with an interesting softening surface and extraordinary swelling capacity that could be of special interest to surface science and specifically, cell culture.

Polysaccharide Coating of Gelatin Gels for Controlled BSA Release.

Self-assembly of natural polymers constitute a powerful route for the development of functional materials. In particular, layer-by-layer (LBL) assembly constitutes a versatile technique for the nanostructuration of biobased polymers into multilayer films. Gelatin has gained much attention for its abundance, biodegradability, and excellent gel-forming properties. However, gelatin gels melt at low temperature, thus limiting its practical application. With respect to the above considerations, here, we explored the potential application of gelatin gels as a matrix for protein delivery at physiological temperature. A model protein, bovine serum albumin (BSA), was encapsulated within gelatin gels and then coated with a different number of bilayers of alginate and chitosan (10, 25, 50) in order to modify the diffusion barrier. The coated gel samples were analyzed by means of Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) and confocal Raman spectroscopy, and it was found that the multilayer coatings onto polymer film were interpenetrated to some extent within the gelatin. The obtained results inferred that the coating of gelatin gels with polysaccharide multilayer film increased the thermal stability of gelatin gels and modulated the BSA release. Finally, the influence of a number of bilayers onto the drug release mechanism was determined. The Ritger-Peppas model was found to be the most accurate to describe the diffusion mechanism.

Local and controlled release of tamoxifen from multi (layer-by-layer) alginate/chitosan complex systems.

Herein, multilayer polysaccharide films were proposed and characterized as biomaterials for the local and controlled release of an antitumoral drug. To that aim, multilayer films of alginate (Alg) and chitosan (Chi) were built up through spray assisted layer-by-layer (LbL) technique employing an automatic equipment. A specific drug against breast cancer, tamoxifen (TMX), was incorporated in different intermediate positions of the multilayer Alg/Chi films. Our findings highlight that Alg/Chi multilayer films can be employed for sustained and local TMX delivery and their therapeutic effect can be modulated and optimized by the number of bilayers deposited over the loaded tamoxifen, the quantity of tamoxifen loaded in several intermediate positions and the total area of the film.

Gecko-like Branched Polymeric Nanostructures from Nanoporous Templates.

Here, we report a simple method to produce hierarchically shaped polymeric one-dimensional nanostructures. More specifically, dual-sized polymer nanowires are fabricated employing multibranched anodic aluminum oxide templates. By fine selection of the anodization conditions, we achieve branched nanopores having a first segment of 400 nm in diameter from which seven further 55 nm in diameter pores arise. Wetting of such nanopores with polymer melts-for example, poly(ε-caprolactone) and polystyrene-allows for the nanomolding of their respective inverse nanostructures, that is, dual-sized multibranched polymer nanowires that, when supported on a flat surface, strongly resemble the spatulae of geckos' toes. The structural features of the dual-sized polymer nanostructures, namely, crystalline phase, crystallinity, texture, and so on, are furthermore characterized and interpreted within the context of polymer phase transitions in confined media. Our work presents a readily applicable approach to produce soft nanomaterials of high morphological complexity, thereby with promising implications in the nanotechnology area, for example, in biomimetic solid adhesion.

Thermo-responsive PNIPAm nanopillars displaying amplified responsiveness through the incorporation of nanoparticles.

The possibility of combining more than one stimulus-responsive property into a single material holds interesting potential for the creation of adaptive devices to be used in diverse fields such as drug delivery, nanomedicine and tissue engineering. This paper describes a novel material based on thermo-responsive PNIPAm nanopillars with amplified surface properties through the incorporation of Fe3O4 nanoparticles. The incorporation of magnetic nanoparticles into the nanopillars, prepared via surface-initiated atom-transfer radical polymerization in anodized aluminum oxide templates, sharply increased their stiffness and hydrophobicity when increasing the temperature above the volume phase transition temperature. Furthermore, their magnetic response turned out to be proportional to the amount of the incorporated nanoparticles. The possibility of sharply increasing the stiffness with a temperature variation close to the human body temperature paves the way to the application of these substrates as "smart" scaffolds for cell culture. Additionally, the presence of superparamagnetic nanoparticles in the nanopillars offers the possibility of using these nanostructured systems for magnetic hyperthermia.

Nanostructured fumarate copolymer-chitosan crosslinked scaffold: An in vitro osteochondrogenesis regeneration study.

In the tissue engineering field, the design of the scaffold inspired on the natural occurring tissue is of vital importance. Ideally, the scaffold surface must promote cell growth and differentiation, while promote angiogenesis in the in vivo implant of the scaffold. On the other hand, the material selection must be biocompatible and the degradation times should meet tissue reparation times. In the present work, we developed a nanofibrous scaffold based on chitosan crosslinked with diisopropylfumarate-vinyl acetate copolymer using anodized aluminum oxide (AAO) templates. We have previously demonstrated its biocompatibility properties with low cytotoxicity and proper degradation times. Now, we extended our studies to demonstrate that it can be successfully nanostructured using the AAO templates methodology, obtaining a nanorod-like scaffold with a diameter comparable to those of collagen fibers of the bone matrix (170 and 300 nm). The nanorods obtained presented a very homogeneous pattern in diameter and length, and supports cell attachment and growth. We also found that both osteoblastic and chondroblastic matrix production were promoted on bone marrow progenitor cells and primary condrocytes growing on the scaffolds, respectively. In addition, the nanostructured scaffold presented no cytotoxicity as it was evaluated using a model of macrophages on culture. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 570-579, 2018.

Hybrid Surface Patterns Mimicking the Design of the Adhesive Toe Pad of Tree Frog.

Biological materials achieve directional reinforcement with oriented assemblies of anisotropic building blocks. One such example is the nanocomposite structure of keratinized epithelium on the toe pad of tree frogs, in which hexagonal arrays of (soft) epithelial cells are crossed by densely packed and oriented (hard) keratin nanofibrils. Here, a method is established to fabricate arrays of tree-frog-inspired composite micropatterns composed of polydimethylsiloxane (PDMS) micropillars embedded with polystyrene (PS) nanopillars. Adhesive and frictional studies of these synthetic materials reveal a benefit of the hierarchical and anisotropic design for both adhesion and friction, in particular, at high matrix-fiber interfacial strengths. The presence of PS nanopillars alters the stress distribution at the contact interface of micropillars and therefore enhances the adhesion and friction of the composite micropattern. The results suggest a design principle for bioinspired structural adhesives, especially for wet environments.

Manipulating Semicrystalline Polymers in Confinement.

Because final properties of nanoscale polymeric structures are largely determined by the solid-state microstructure of the confined polymer, it is imperative not only to understand how the microstructure of polymers develops under nanoscale confinement but also to establish means to manipulate it. Here we present a series of processing strategies, adapted from methods used in bulk polymer processing, that allow us to control the solidification of polymer nanostructures. First, we show that supramolecular nucleating agents can be readily used to modify the crystallization kinetics of confined poly(vinylidene fluoride) (PVDF). In addition, we demonstrate that microstructural features that are not traditionally affected by nucleating agents, such as the orientation of crystals, can be tuned with the crystallization temperature applied. Interestingly, we also show that high crystallization temperatures and long annealing periods induce the formation of the γ modification of PVDF, hence enabling the simple production of ferro/piezoelectric nanostructures. We anticipate that the approaches presented here can open up a plethora of new possibilities for the processing of polymer-based nanostructures with tailored properties and functionalities.

Manufacture and characterization of chitosan/PLGA nanoparticles nanocomposite buccal films.

Oral bioavailability of C-glycosyl flavonoid enriched fraction of Cecropia glaziovii (EFF-Cg) is limited due to its chemical complexity. The purpose of this study is the prospective evaluation of chitosan buccal films impregnated with EFF-Cg-loaded nanospheres as a drug delivery system for labial herpes treatment or for buccal administration. EFF-Cg-loaded PLGA nanospheres were prepared by double emulsion solvent evaporation technique. Nanoparticles were embedded into buccoadhesive chitosan films in different concentrations in order to obtain nanocomposite films. Films were characterized in term of morphology, mechanical properties and water absorption test. Furthermore a cytotoxicity assay was analyzed to evaluate the biocompatibility of systems. The results obtained from these analyses revealed that nanocomposite films present transparent appearance in all composition and Scanning Electron Microscopy (SEM) images show a continuous and compact section structure. Compared to the control film, mechanical responses of nanocomposites presented lower tensile strength values and no significant effect on the elongation at break. Dynamic Mechanical Analysis (DMA) tests indicated that increasing of NP concentration caused decreased stiffness and an increased of glass transition temperature values. Direct cytotoxicity test shows that nanoparticles and chitosan films not induce cytotoxic effect. Given the promising results, the study concludes that the developed buccal film impregnated with EFF-Cg-loaded nanospheres could be a promising approach for effective delivery of EFF-Cg.

In vitro antiherpes effect of C-glycosyl flavonoid enriched fraction of Cecropia glaziovii encapsulated in PLGA nanoparticles.

In this work is reported a novel and promising approach for the preparation of C-glycosylflavonoid enriched fraction of Cecropia glaziovii (EFF-Cg) loaded PLGA nanoparticles (NP) with antiherpes properties. The purpose of this study was to evaluate and to compare the effect of two nonionic surfactants (poloxamer 188 (PLU) and polyvinyl alcohol (PVA)), and also an emulsion stabilized by solid particles of cellulose nanocrystal (CNC) in place of surfactants. The characterization of these nanoparticles was in terms of size, polydispersity index, zeta potential, morphology, thermogravimetric analysis (TGA), loading capacity and percent yield. Since TGA analysis revealed thermo stability especially for NP-PLU, this formulation was selected for the evaluation of drug release profile, cytotoxicity and antiherpes activity. The drug delivery profile demonstrated a sustained release through the polymer structure and a significant reduction of the polymer molecular weight at 21-day period. The cytotoxicity of these nanoparticles was determined on Vero cells, and the selected formulation did not exhibit cytotoxicity even at the highest tested concentration. The results demonstrated a potential antiherpetic effect of the EFF-Cg loaded NP at 48h of testing. In summary, EFF-Cg loaded NP exhibited a promising system for the effective drug delivery in the treatment of herpes infections.

Thermally-induced softening of PNIPAm-based nanopillar arrays.

The surface properties of soft nanostructured hydrogels are crucial in the design of responsive materials that can be used as platforms to create adaptive devices. The lower critical solution temperature (LCST) of thermo-responsive hydrogels such as poly(N-isopropylacrylamide) (PNIPAm) can be modified by introducing a hydrophilic monomer to create a wide range of thermo-responsive micro-/nano-structures in a large temperature range. Using surface initiation atom-transfer radical polymerization in synthesized anodized aluminum oxide templates, we designed, fabricated, and characterized thermo-responsive nanopillars based on PNIPAm hydrogels with tunable mechanical properties by incorporating acrylamide monomers (AAm). In addition to their LCST, the incorporation of a hydrophilic entity in the nanopillars based on PNIPAm has abruptly changed the topological and mechanical properties of our system. To gain an insight into the mechanical properties of the nanostructure, its hydrophilic/hydrophobic behavior and topological characteristics, atomic force microscopy, molecular dynamics simulations and water contact angle studies were combined. When changing the nanopillar composition, a significant and opposite variation was observed in their mechanical properties. As temperature increased above the LCST, the stiffness of PNIPAm nanopillars, as expected, did so too, in contrast to the stiffness of PNIPAm-AAm nanopillars that decreased significantly. The molecular dynamics simulations proposed a local molecular rearrangement in our nanosystems at the LCST. The local aggregation of NIPAm segments near the center of the nanopillars displaced the hydrophilic AAm units towards the surface of the structure leading to contact with the aqueous environment. This behavior was confirmed via contact angle measurements below and above the LCST.