Architectural layer-by-layer assembly of drug nanocapsules with PEGylated polyelectrolytes

150-200 nm diameter capsules containing 60-70 wt % of poorly soluble drugs, paclitaxel and camptothecin, were produced by layer-by-layer (LbL) assembly on drug nanocores in a solution containing uncharged stabilizers. Optimization of capsule shell architecture and thickness allowed for concentrated (3-5 mg/mL) colloids that are stable in isotonic salt buffers. Nanoparticle aggregation during the washless LbL-assembly was prevented by using low molecular weight block-copolymers of poly(amino acids) (poly-L-lysine and poly-L-glutamic acid) with polyethylene glycol (PEG) in combination with heparin and bovine serum albumin at every bilayer building step. Minimal amounts of the polyelectrolytes were used to recharge the surface of nanoparticles in this non-washing LbL process. Such PEGylated shells resulted in drug nanocapsules with high colloidal stability in PBS buffer and increased protein adhesion resistance. The washless LbL polyelectrolyte nanocapsule assembly process, colloidal stability and nanoparticle morphology were monitored by dynamic light scattering and electrophoretic mobility measurements, UV-vis spectroscopy, TEM, SEM and laser confocal microscopy imaging.

Introduction to nanocoatings produced by layer-by-layer (LbL) self-assembly

Studies on the adsorption of oppositely charged colloidal particles ultimately resulted in multilayered polyelectrolyte self-assembly. The inception of layer-by-layer constructed particles facilitated the production of multifunctional, stimuli-responsive carrier systems. An array of synthetic and natural polyelectrolytes, metal oxides and clay nanoparticles is available for the construction of multilayered nanocoats on a multitude of substrates or removable cores. Numerous substrates can be encapsulated utilizing this technique including dyes, enzymes, drugs and cells. Furthermore, the outer surface of the particles presents and ideal platform that can be functionalized with targeting molecules or catalysts. Some processing parameters determining the properties of these successive self-assembly constructs are the surface charge density, coating material concentration, rinsing and drying steps, temperature and ionic strength of the medium. Additionally, the simplicity of the layer-by-layer assembly technique and the availability of established characterization methods, render these constructs extremely versatile in applications of sensing, encapsulation and target- and trigger-responsive drug delivery.

Layer-by-layer self-assembled shells for drug delivery

There are several requirements for the safe and effective delivery of therapeutic agents for human use. Direct injection of drugs may cause side effects due to their permeation to other, undiseased regions of the body so that concealment and targeting with appropriate materials is a critical consideration in the design of practical drug delivery systems. In particular, carriers with structures which can be flexibly controlled are more useful since functional structure units can be assembled in component-by-component and/or layer-by-layer fashion. In this review, we focus on preparation of layer-by-layer shells directed at drug delivery applications. After a description of the fundamentals of layer-by-layer (LbL) assembly, recent progress in the field of self-assembled microshells and nanoshells for drug delivery applications are summarized. In addition, concepts developed to solve current difficulties are also described. Encapsulation of insoluble drugs in nanoshells and their delivery can satisfy some of the demands of practical medical use. Thus, aqueous suspensions of insoluble drugs have been subjected to powerful ultrasonic treatment followed by sequential addition of polycations and polyanions to the particle solution leading to assembly of ultra-thin polyelectrolyte shells on the nano-sized drug particles. In another innovative example, stepwise release of drugs from LbL films of mesoporous capsules to the exterior in the absence of external stimuli was demonstrated. It can be regarded as stimuli-free auto-modulated material release.

Converting poorly soluble materials into stable aqueous nanocolloids

Aqueous nanocolloids of poorly soluble materials were produced via sonicated layer-by-layer (LbL) encapsulation with polycation / polyanion shells. Synergy of simultaneous breaking powder particles with ultrasonication and coating them with polycations allowed for the production of 150-200 nm diameter polyelectrolyte coated nanoparticles with sufficient surface electrical potential for colloidal stability. This technique increases water dispersibility of low soluble materials ranging from anticancer drugs to anticorrosion agents, dyes and inorganic salts.

Natural nanocontainer for the controlled delivery of glycerol as a moisturizing agent

Natural halloysite nanotubes with a 15-nm internal lumen and a 50 nm outer diameter were investigated as a nanocontainer for the loading and extended release of glycerol for cosmetic applications. Cytotoxicity testing of the halloysite was conducted on 3T3 and MCF-7 cells, and the tubules showed no toxic effect on the cells for over 48 h. The capability of halloysite for loading glycerol was higher with the USA halloysite than with the New Zealand's, being approximately 20% and 2.3% by weight, respectively. The total elapsed time for releasing glycerol from the nanotubes exceeded 20 h. To further retard the glycerol release rate, the halloysite samples filled with glycerol were coated with several alternate layers of polyethyleneimine and polyacrylic acid. The release rate remained at the same level, however, probably due to the low molecular weight of the polyelectrolytes and the high solubility of glycerol in water.

Cytocompatibility and uptake of halloysite clay nanotubes

Halloysite is aluminosilicate clay with hollow tubular structure of 50 nm external diameter and 15 nm diameter lumen. Halloysite biocompatibility study is important for its potential applications in polymer composites, bone implants, controlled drug delivery, and for protective coating (e.g., anticorrosion or antimolding). Halloysite nanotubes were added to different cell cultures for toxicity tests. Its fluorescence functionalization by aminopropyltriethosilane (APTES) and with fluorescently labeled polyelectrolyte layers allowed following halloysite uptake by the cells with confocal laser scanning microscopy (CLSM). Quantitative Trypan blue and MTT measurements performed with two neoplastic cell lines model systems as a function of the nanotubes concentration and incubation time indicate that halloysite exhibits a high level of biocompatibility and very low cytotoxicity, rendering it a good candidate for household materials and medicine. A combination of transmission electron microscopy (TEM), scanning electron microscopy (SEM), and scanning force microscopy (SFM) imaging techniques have been employed to elucidate the structure of halloysite nanotubes.

Layer-by-layer nano-encapsulation of microbes: controlled cell surface modification and investigation of substrate uptake in bacteria

LbL nano self-assembly coating of A. vinosum with different polyelectrolyte combinations is presented as an example to investigate substrate uptake in bacteria. The effects of surface charge and the formation of a physical barrier provides new insights in the contact mechanisms between the cell surface and insoluble elemental sulfur. Furthermore, uptake of sulfide by encapsulated cells was investigated. Growth experiments of coated cells showed that surface charge did neither affect sulfide uptake nor the contact formation between the cells and solid sulfur. However, increasing layers slowed or inhibited the uptake of sulfide and elemental sulfur. This work demonstrates how defining surface properties of bacteria has potential for microbiological and biotechnological applications.

Porous biocompatible three-dimensional scaffolds of cellulose microfiber/gelatin composites for cell culture

Physiological tissues, including brain and other organs, have three-dimensional (3-D) aspects that need to be supported to model them in vitro. Here we report the use of cellulose microfibers combined with cross-linked gelatin to make biocompatible porous microscaffolds for the sustained growth of brain cell and human mesenchymal stem cells (hMSCs) in 3-D structure. Live imaging using confocal microscopy indicated that 3-D microscaffolds composed of gelatin or cellulose fiber/gelatin both supported brain cell adhesion and growth for 16days in vitro. Cellulose microfiber/gelatin composites containing up to 75% cellulose fibers can withstand a higher mechanical load than gelatin alone, and composites also provided linear pathways along which brain cells could grow compared to more clumped cell growth in gelatin alone. Therefore, the bulk cellulose microfiber provides a novel skeleton in this new scaffold material. Cellulose fiber/gelatin scaffold supported hMSCs growth and extracellular matrix formation. hMSCs osteogenic and adipogenic assays indicated that hMSCs cultured in cellulose fiber/gelatin composite preserved the multilineage differentiation potential. As natural, biocompatible components, the combination of gelatin and cellulose microfibers, fabricated into 3-D matrices, may therefore provide optimal porosity and tensile strength for long-term maintenance and observation of cells.

Encapsulation of bacterial spores in nanoorganized polyelectrolyte shells

Layer-by-layer assembly uses alternating charged layers of polyionic polymers to coat materials sequentially in a sheath of functionalized nanofilms. Bacterial spores were encapsulated in organized ultrathin shells using layer-by-layer assembly in order to assess the biomaterial as a suitable core and determine the physiological effects of the coating. The shells were constructed on Bacillus subtilis spores using biocompatible polymers polyglutamic acid, polylysine, albumin, lysozyme, gelatin A, protamine sulfate, and chondroitin sulfate. The assembly process was monitored by measuring the electrical surface potential (zeta-potential) of the particles at each stage of assembly. Fluorescent laser confocal microscopy and scanning electron microscopy confirmed the formation of uniform coatings on the spores. The coating surface charge and thickness (20-100 nm) could be selectively tuned by using appropriate polymers and the number of bilayers assembled. The effect of each coating type on germination was assessed and compared to native spores. The coated spores were viable, but the kinetics and extent of germination were changed from control spores in all instances. The results and insight gained from the experiments may be used to design various bioinspired systems. The spores can be made dormant for a desired amount of time using the LbL encapsulation technique and can be made active when appropriate.

Layer-by-Layer-Coated Gelatin Nanoparticles as a Vehicle for Delivery of Natural Polyphenols

Natural polyphenols with previously demonstrated anticancer potential, epigallocatechin gallate (EGCG), tannic acid, curcumin, and theaflavin, were encased into gelatin-based 200 nm nanoparticles consisting of a soft gel-like interior with or without a surrounding LbL shell of polyelectrolytes (polystyrene sulfonate/polyallylamine hydrochloride, polyglutamic acid/poly-l-lysine, dextran sulfate/protamine sulfate, carboxymethyl cellulose/gelatin, type A) assembled using the layer-by-layer technique. The characteristics of polyphenol loading and factors affecting their release from the nanocapsules were investigated. Nanoparticle-encapsulated EGCG retained its biological activity and blocked hepatocyte growth factor (HGF)-induced intracellular signaling in the breast cancer cell line MBA-MD-231 as potently as free EGCG.

(-)-Epigallocatechin gallate/gelatin layer-by-layer assembled films and microcapsules

A new type of protein/polyphenol microcapsules on the basis of naturally occurring polyphenol (-)-epigallocatechin gallate (EGCG) and gelatin, type A, was obtained using the layer-by-layer (LbL) assembly method. The microcapsules show a more pronounced dependence of permeability on molecular weight of permeating substances than commonly used polyallylamine/polystyrene sulfonate capsules. The regularities of EGCG adsorption in alternation with type A and B gelatins have been investigated using quartz crystal microbalance and electrophoretic mobility measurements on microparticles and found to be dependent on gelatin properties. EGCG in the LbL assemblies retains its antioxidant activity. The kinetics of the reaction of 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) cation-radicals with films consisting of 1-10 gelatin/EGCG bilayers is affected by film structure. The EGCG content in the protein/polyphenol film material is as high as 30% w/w. Encapsulation of EGCG via its alternated adsorption with gelatins can be a perspective way to new formulations containing the polyphenol for drug delivery applications.

Organized shells on clay nanotubes for controlled release of macromolecules

The use of tubular halloysite clay as a nanotemplate for layer-by-layer (LbL) shell assembly and its utilization for controlled release of drug macromolecules are studied. The LbL nanoshell allowed additional control for the sustained release of drug loaded halloysite tubes. The number of polymeric layers in the shell and molecular weight of the assembled polymers influences the drug release rate. Three bilayer shells of chitosan and gelatin of 15 nm thicknesses gave the best encapsulation and retardation in the release rate of dexamethasone. An encapsulation of the macromolecules inside the lumen of the biocompatible clay nanotubes coupled with the polyelectrolyte shell formation provides a novel formulation for the controlled release of bioactive agents.

Halloysite clay nanotubes for controlled release of protective agents

Halloysite aluminosilicate nanotubes with a 15 nm lumen, 50 nm external diameter, and length of 800 +/- 300 nm have been developed as an entrapment system for loading, storage, and controlled release of anticorrosion agents and biocides. Fundamental research to enable the control of release rates from hours to months is being undertaken. By variation of internal fluidic properties, the formation of nanoshells over the nanotubes and by creation of smart caps at the tube ends it is possible to develop further means of controlling the rate of release. Anticorrosive halloysite coatings are in development and a self-healing approach has been developed for repair mechanisms through response activation to external impacts. In this Perspective, applications of halloysite as nanometer-scale containers are discussed, including the use of halloysite tubes as drug releasing agents, as biomimetic reaction vessels, and as additives in biocide and protective coatings. Halloysite nanotubes are available in thousands of tons, and remain sophisticated and novel natural nanomaterials which can be used for the loading of agents for metal and plastic anticorrosion and biocide protection.

Nanoencapsulation of Stem Cells within Polyelectrolyte Multilayer Shells

Mouse mesenchymal stem cells have been individually encased by polyelectrolyte layers of poly (L-lysine) and hyaluronic acid using the electrostatic layer-by-layer assembly technique, resulting in a shell consisting of nanolayers of thickness around 6-9 nm. Maintenance of cell morphology and viability were demonstrated for up to one week. Further adjustments to shell permeability and flexibility will facilitate the use of these encapsulated cells in tissue engineering and targeted-delivery applications.

Nanoencapsulation of stem cells within polyelectrolyte multilayer shells

Mouse mesenchymal stem cells have been individually encased by polyelectrolyte layers of poly (L-lysine) and hyaluronic acid using the electrostatic layer-by-layer assembly technique, resulting in a shell consisting of nanolayers of thickness around 6-9 nm. Maintenance of cell morphology and viability were demonstrated for up to one week. Further adjustments to shell permeability and flexibility will facilitate the use of these encapsulated cells in tissue engineering and targeted-delivery applications.

Layer-by-layer nanoassembly of polyelectrolytes using formamide as the working medium

Formamide, in its pure state, has been used as a working solvent for layer-by-layer (LbL) polyelectrolyte self-assembly. Polystyrene sulfonate (PSS) and polyallylamine hydrochloride (PAH) polyelectrolyte films were deposited onto planar substrates and colloidal particles. Film deposition was confirmed using quartz crystal microbalance and zeta potential measurements. Formamide was used as an alternative to the water-based working solvents commonly used for LbL self-assembly. Few LbL self-assembly studies using nonaqueous solvents have been reported. Most studies performed with nonaqueous solvents have required the addition of small volumes of water to dissolve the polyelectrolytes. Conversely, the high dielectric constant of pure formamide led to the dissolution and transport of PSS and PAH. Using formamide, it is possible to deposit nanometer thick polyelectrolyte films onto water-sensitive surfaces. Formamide can be thus be used for encapsulating water sensitive hydrogen storage materials within polyelectrolyte films.

Cellulose Fiber−Enzyme Composites Fabricated through Layer-by-Layer Nanoassembly

Cellulose microfibers were coated with enzymes, laccase and urease, through layer-by-layer assembly by alternate adsorption with oppositely charged polycations. The formation of organized polyelectrolyte and enzyme multilayer films of 15-20 nm thickness was demonstrated by quartz crystal microbalance, zeta-potential analysis, and confocal laser scanning microscopy. These biocomposites retained enzymatic catalytic activity, which was proportional to the number of coated enzyme layers. For laccase-fiber composites, around 50% of its initial activity was retained after 2 weeks of storage at 4 degrees C. The synthesis of calcium carbonate microparticles on urease-fiber composites confirmed urease functionality and demonstrated its possible applications. This strategy could be employed to fabricate fiber-based composites with novel biological functions.

Stem cell attachment to layer-by-layer assembled TiO2 nanoparticle thin films

Surface topography is one of the most important factors influencing the attachment and spreading of cells. In the present study, layer-by-layer assembled titanium dioxide (TiO2) nanoparticle thin films were chosen for attachment, proliferation and spreading studies on mouse mesenchymal stem cells (MSC). Increasing surface roughness was observed with increasing number of layer-by-layer assembled TiO2 thin films. Four layer TiO2 thin film showed higher number of attached cells than a one layer thin film and control surfaces. MSCs experienced no cytotoxic effects after culture on the TiO2 coated substrates as observed from the cytotoxicity tests. Cell spreading, visualized with scanning electron microscopy, showed a faster rate of spreading on a rougher surface. Cells on a four-layer substrate, at 12 h showed complete spreading, where as most of the cells on a control surface and a one-layer surface, at 24 h, retained a rounded morphology. In conclusion, TiO2 nanoparticle thin films were successfully assembled in alternation with polyelectrolytes and in-vitro studies with MSC showed an increase in the attachment and faster spreading of cells on rougher surfaces.

Layer-by-Layer Enzyme/Polyelectrolyte Films as a Functional Protective Barrier in Oxidizing Media

The influence of a catalase (Cat) layer located at different depths in the layer-by-layer hemoglobin/polystyrene sulfonate films with an (Hb/PSS)(20)(-)(x)/(Cat/PSS)/(Hb/PSS)(x) (x = 0-20) architecture on kinetics of hemoglobin degradation under treatment with hydrogen peroxide solutions of different concentrations and features of H(2)O(2) decay in surrounding solutions has been studied. While assembled on the top of the multilayers, the catalase layer shows the highest activity in hydrogen peroxide decomposition. Hemoglobin in such films retains its nativity for a longer period of time. The effect of catalase layers is compared with that of protamine, horseradish peroxidase, and inactivated catalase. Positioning an active layer with catalytic properties as an outer layer is the best protection strategy for layer-by-layer assembled films in aggressive media.

Nano-engineered microcapsules of tannic acid and chitosan for protein encapsulation

Tannic acid (TA), a high molecular weight polyphenol of natural origin, was assembled in alternation with chitosan (CH) using a layer-by-layer technique. The deposition of tannic acid and chitosan layers on flat supports was monitored by quartz crystal microbalance, UV-vis spectroscopy, and electrophoretic mobility measurements on microparticles. Hollow (TA/CH)4 capsules were built and their permeability as a function of pH and molecular weight of a penetrating compound was investigated. The pH-permeability threshold for TA/CH capsules is shifted to lower pH for 2 pH units, as compared with commonly used polyallylamine/polystyrene sulfonate capsules. A more pronounced dependence of the TA/CH capsules' permeability on molecular weight of encapsulated substances allows better control over their release properties. Bovine serum albumin was loaded into (TA/CH)4 capsules using a pH-driven method and released by decreasing pH. Biocompatible tannic acid/chitosan films and capsules have advantages toward capsules made of synthetic polyelectrolytes for drug encapsulation and as delivery and depot systems. Incorporating a layer of tannic acid with proved antioxidant and antimicrobial properties into capsule walls, provides defense for encapsulated materials.