Preparation and characterization of multilayer coated microdroplets: droplet deformation simultaneously probed by atomic force spectroscopy and optical detection

Polyelectrolyte Multilayers on Soft Colloidal Nanosurfaces: A New Life for the Layer-By-Layer Method

The Layer-by-Layer (LbL) method is a well-established method for the assembly of nanomaterials with controlled structure and functionality through the alternate deposition onto a template of two mutual interacting molecules, e.g., polyelectrolytes bearing opposite charge. The current development of this methodology has allowed the fabrication of a broad range of systems by assembling different types of molecules onto substrates with different chemical nature, size, or shape, resulting in numerous applications for LbL systems. In particular, the use of soft colloidal nanosurfaces, including nanogels, vesicles, liposomes, micelles, and emulsion droplets as a template for the assembly of LbL materials has undergone a significant growth in recent years due to their potential impact on the design of platforms for the encapsulation and controlled release of active molecules. This review proposes an analysis of some of the current trends on the fabrication of LbL materials using soft colloidal nanosurfaces, including liposomes, emulsion droplets, or even cells, as templates. Furthermore, some fundamental aspects related to deposition methodologies commonly used for fabricating LbL materials on colloidal templates together with the most fundamental physicochemical aspects involved in the assembly of LbL materials will also be discussed.

Changing the Vision in Smart Food Design Utilizing the Next Generation of Nanometric Delivery Systems for Bioactive Compounds

In modern foods, the delivery systems for bioactive compounds play a fundamental role in health promotion, wellbeing, and disease prevention through diet. Nanotechnology has secured a fundamental role in the fabrication of delivery systems with the capability of modulating the in-product and in-body behavior for augmenting bioavailability and activity of bioactive compounds. Structured nanoemulsions and nanoparticles, liposomes, and niosomes can be designed to improve bioactives preservation after ingestion, mucoadhesion, as well as of their release and pathophysiological relevance. In the future, it is expected that the delivery systems will also contribute to augment the efficacy of the bioactive compounds, for example by improving the intestinal absorption and delivery in the bloodstream, as well as promoting the formation of additional bioactive metabolites by regulating the transformations taking place during digestion and the interaction with the intestinal microbiota.

In situ nanomechanical properties of natural oil bodies studied using atomic force microscopy

Natural oil bodies (OBs) from plant organs represent an important category of functional ingredients and materials in a variety of industrial sectors. Their applications are closely related to the membrane mechanical properties on a single droplet level, which remain difficult to determine. In this research, the mechanical properties of the membranes of OBs from soybean, sesame, and peanut were investigated in-situ by atomic force microscopy (AFM). Different regions of the force-deformation curves obtained during compression were analyzed to extract the stiffness Kb or Young's modulus of the OB membranes using Hooke's law, Reissner theory, and the elastic membrane theory. At higher strains (ε = 0.15-0.20), the elastic membrane theory breaks down. We propose an extension of the theory that includes a contribution to the force from interfacial tension based on the Gibbs energy, allowing effective determination of Young's modulus and interfacial tension of the OB membranes in the water environment simultaneously. The mechanical properties of the OBs of different sizes and species, as well as a comparison with other phospholipid membrane materials, are discussed and related to their membrane compositions and structures. It was found that the natural OBs are soft droplets but do not rupture and can fully recover following compressive strains as large as 0.3. The OBs with higher protein/oil ratio, have smaller size and stronger mechanical properties, and thus are more stable. The low interfacial tension due to the existence of phospholipid-protein membrane also contributes to the stability of the OBs. This is the first report measuring the mechanical properties of OB membranes in-situ directly.

Tailoring layer-by-layer capsules for biomedical applications

Polymeric capsules have attracted great interest as versatile carrier systems in the area of medicine and pharmaceutics. These capsules are made by stepwise layer-by-layer adsorption of polymers onto a template core, which can be removed to produce hollow capsules. The cavity of these capsules can host various cargo molecules while the capsules’ wall can be functionalized towards desired properties by embedding specific moieties into the multilayers. Tuning of the capsules’ properties influences their interaction with cells and tissues and paves the way towards the development of stimuli-responsive capsules releasing their payload at a target site. In this review, we describe the generation of tailored layer-by-layer capsules and focus hereby on numerous potential applications of this multifunctional delivery platform in biomedical settings. We review the current status in the field and discuss the opportunities, as well as the hurdles, to be overcome to successfully transfer this technology to therapeutic and diagnostic applications.

Kinetic stability of water-dispersed oil droplets encapsulated in a polyelectrolyte multilayer shell

The original theoretical model of polyelectrolyte adsorption onto water-dispersed colloid particles is extended to the system of polydisperse droplets of sunflower oil. Polycation (poly(allylamine hydrochloride)) and polyanion (poly(sodium 4-styrenesulfonate)) are taken in the theoretically projected concentrations to perform Layer-by-Layer assembly of a multilayer shell on the surface of oil droplets preliminary stabilized with a protein emulsifier (bovine serum albumin). The velocity of gravitational separation in suspension of encapsulated oil droplets is theoretically predicted and experimentally measured depending on the coating shell's thickness, aiming to clarify the mechanism to control over the separation process. Combining the theory and experimental data, the mass density of a polyelectrolyte multilayer shell assembled in a Layer-by-Layer fashion is obtained. Polyelectrolyte multilayer coated oil droplets are characterized by means of ζ-potential, and particle size measurements, and visualized by scanning electron microscopy.

Antioxidant coating of micronsize droplets for prevention of lipid peroxidation in oil-in-water emulsion

Fast lipid peroxidation in emulsified oils results in carcinogens formation and product rancidity. Prevention of oxidative degradation in oil-in-water emulsion has been achieved by encapsulating of each droplet of dispersed phase in antioxidant multilayer coating shell. The fabrication comprised placing a surface-active ionic emulsifier at the oil/water interface followed by stepwise alternate adsorption a biocompatible polyelectrolyte and antioxidant layers. Uncoupled polyelectrolyte macromolecules and antioxidant were thoroughly removed from formulation, thus the protection was entirely attributed to the droplets' shell. The experiments were performed using linseed oil, the richest source of highly unstable omega-3 alpha linolenic essential fatty acid. Bovine serum albumin (BSA) was exploited as an anionic emulsifier. The biodegradable coating shell was formed of poly-l-arginine (PARG) and dextran sulfate (DS) applied as a polycation and a polyanion respectively. Tannic acid (TA) known as a natural antioxidant and possessing antimicrobial properties was used as a protective remedy. Oil microdroplets coated with TA-containing shell displayed physical-chemical and mechanical stability in aqueous phase and over freeze-drying process as determined by ζ-potential measurements, dynamic light scattering (DLS), and confocal laser scanning microscopy (CLSM). Oxidation of emulsified oil was monitored by formation of malondialdehyde (MDA) in the samples quantified by Thiobarbituric Acid Reactive Substances (TBARS) assay. Coating shell with an incorporated layer of TA effectively suppressed oxidation in water-dispersed oil droplets and affected iron-catalyzed oxidation over 15 days of incubation at 37 °C in 0.3 mM FeBr2 solution. Antioxidant activity of TA-containing shell assembled around each oil droplet was found to be higher than that of mixed tocopherols (MT) added to linseed oil in concentration of 10000 ppm.

Multi-layer films of block copolymer micelles adsorbed to colloidal templates

Alternating layers of cationic and anionic block copolymer micelles have been deposited onto colloidal silica particles using a layer-by-layer approach. The resulting films have been investigated using a number of characterization techniques including zeta potential measurements, dynamic light scattering, thermo-gravimetric analysis and microscopy. The micelles used here demonstrate pH-responsive behaviour both in solution and when adsorbed at interfaces. It has been shown that block copolymer micelles can selectively encapsulate and release hydrophobic materials; therefore, the incorporation of such responsive species within films has the potential to offer increased functionality. The formation of films onto colloidal particles is of great interest as it can provide pathways to direct encapsulation of materials along with surface modification. This study aims to provide new insights into the nature and properties of responsive films. Such studies will allow for the future development of novel delivery systems that have potential application within a number of industrial sectors including personal care products, pharmaceuticals and agro-chemicals.

Polymeric microcapsules assembled from a cationic/zwitterionic pair of responsive block copolymer micelles

Using a layer-by-layer (LbL) approach, this work presents the preparation of hollow microcapsules with a membrane constructed entirely from a cationic/zwitterionic pair of pH-responsive block copolymer micelles. Our previous work with such systems highlighted that, in order to retain the responsive nature of the individual micelles contained within the multilayer membranes, it is important to optimize the conditions required for the selective dissolution of the sacrificial particulate templates. Consequently, here, calcium carbonate particles have been employed as colloidal templates as they can be easily dissolved in aqueous environments with the addition of chelating agents such as ethylenediaminetetraacetic acid (EDTA). Furthermore, the dissolution can be carried out in solutions buffered to a desirable pH so not to adversely affect the pH sensitive micelles forming the capsule membranes. First, we have deposited alternating layers of anionic poly[2-(dimethylamino)ethyl methacrylate-block-poly(2-(diethylamino)ethyl methacrylate)] (PDMA-PDEA) and cationic poly(2-(diethylamino)ethyl)methacrylate-block-poly(methacrylic acid) (PDEA-PMAA) copolymer micelles onto calcium carbonate colloidal templates. After deposition of five micelle bilayers, addition of dilute EDTA solution resulted in dissolution of the calcium carbonate and formation of hollow polymer capsules. The capsules were imaged using atomic force microscopy (AFM) and scanning electron microscopy (SEM), which shows that the micelle/micelle membrane is sufficiently robust to withstand dissolution of the supporting template. Quartz crystal microbalance studies were conducted and provide good evidence that the micelle multilayer structure is retained after EDTA treatment. In addition, a hydrophobic dye was incorporated into the micelle cores prior to adsorption. After dissolution of the particle template, the resulting hollow capsules retained a high concentration of dye, suggesting that the core/shell structure of the micelles remains intact. Finally, thermogravimetric analysis (TGA) of dried capsules confirmed complete removal of the sacrificial inorganic template. As far as we are aware, this is the first demonstration of LbL assembled capsules composed entirely from responsive block copolymer micelles. The results presented here when combined with our previous findings demonstrate that such systems have potential application in the encapsulation and triggered release of actives.