Microparticles of novel branched copolymers of lactic acid and amino acids: preparation and characterization

Single-Emulsion P(HB-HV) Microsphere Preparation Tuned by Copolymer Molar Mass and Additive Interaction

Herein, we describe the production of poly(hydroxybutyrate-co-hydroxyvalerate) [P(HB-HV)]-based microspheres containing coumarin-6 (C6) or pyrene (Py) fluorophores as additives and models for hydrophobic and hydrophilic drug encapsulation. Their photophysical and morphological properties, as well as encapsulation efficiencies, are studied as this work aims to describe the influence of additive hydrophobicity/hydrophilicity on microparticle formation. These properties were studied by scanning electron microscopy, fluorescence confocal laser scanning microscopy (FCLSM), and steady-state fluorescence spectroscopy. The results show that the surfactant concentration, polymer molar mass, emulsification stirring rate, and the presence of the fluorophore and its nature are determinants of the P(HB-HV) microsphere properties. Also, encapsulation efficiency is shown to be governed by synergic effects of these parameters on the formation of microspheres. Moreover, size distribution is proved to be strongly influenced by the surfactant poly(vinyl alcohol) content. FCLSM showed that the fluorophores were efficiently encapsulated in P(HB-HV) microspheres at distinct distributions within the copolymer matrix. Surprisingly, nanospheres were observed in the microsphere surface, suggesting that microspheres are formed from nanosphere coalescence.

Functionalized poly(lactic-co-glycolic acid) enhances drug delivery and provides chemical moieties for surface engineering while preserving biocompatibility

Poly(lactic-co-glycolic acid) (PLGA) is one of the more widely used polymers for biomedical applications. Nonetheless, PLGA lacks chemical moieties that facilitate cellular interactions and surface chemistries. Furthermore, incorporation of hydrophilic molecules is often problematic. The integration of polymer functionalities would afford the opportunity to alter device characteristics, thereby enabling control over drug interactions, conjugations and cellular phenomena. In an effort to introduce amine functionalities and improve polymer versatility, we synthesized two block copolymers (PLGA-PLL 502H and PLGA-PLL 503H) composed of PLGA and poly(epsilon-carbobenzoxy-l-lysine) utilizing dicyclohexyl carbodiimide coupling. PLGA-PLL microspheres encapsulated approximately sixfold (502H) and threefold (503H) more vascular endothelial growth factor, and 41% (503H) more ciliary neurotrophic factor than their PLGA counterparts. While the amine functionalities were amenable to the delivery of large molecules and surface conjugations, they did not compromise polymer biocompatibility. With the versatile combination of properties, biocompatibility and ease of synthesis, these block copolymers have the potential for diverse utility in the fields of drug delivery and tissue engineering.

Macroporous surface modified microparticles

Novel macroporous and surface functionalized polymeric microparticles were prepared using a modified emulsion technique. The microparticles were able to surface load DNA and have a range of potential applications in drug delivery of biologics and tissue engineering.

Design opportunities for actively targeted nanoparticle vaccines

Vaccines for many infectious diseases are poorly developed or simply unavailable. There are significant technological and practical design issues that contribute to this problem; thus, a solution to the vaccine problem will require a systematic approach to test the multiple variables that are required to address each of the design challenges. Nanoparticle technology is an attractive methodology for optimizing vaccine development because design variables can be tested individually or in combination. The biology of individual components that constitute an effective vaccine is often well understood and may be integrated into particle design, affording optimal immune responses to specific pathogens. Here, we review technological variables and design parameters associated with creating modular nanoparticle vaccine systems that can be used as vectors to protect against disease. Variables, such as the material and size of the core matrix, surface modification for attaching targeting ligands and routes of administration, are discussed. Optimization of these variables is important for the development of nanoparticle-based vaccine systems against infectious diseases and cancer.

Surface property andin vitro biodegradation of microspheres fabricated by poly(ɛ-caprolactone-b-ethylene oxide) diblock copolymers

Microspheres fabricated by biodegradable polymers with tunable surface properties show great potentials as microcarriers in in vitro cell cultivation and tissue engineering. Herein we reported a new method to regulate the surface property and morphology of microspheres via the synthesis of biodegradable amphiphilic block copolymers with adjustable compositions. The poly(epsilon-caprolactone-b-ethylene oxide) diblock copolymers with functional amino end groups bonding to the PEO block (PCL-b-PEO-NH(2)) were synthesized by sequential ring-opening polymerization with potassium bis(trimethylsilyl) amide as initiator. The copolymers were characterized by gel permeation chromatography (GPC) and (1)H NMR, and then used to fabricate microspheres by w/o/w double emulsion solvent evaporation technique. The surface properties of microspheres were studied by means of scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). The results indicated that both the fabrication conditions and copolymer composition have great influences on the surface morphology and property of microspheres. The reactive amino functional groups are dominantly located on the surface of microspheres. The in vitro degradation of microspheres was studied by following the morphological changes of microspheres. The influences of hydrophilic PEO out-layers on the enzymatic degradation of microspheres were discussed. These microspheres with controllable surface morphology and amino functional groups are expected to be promising alternatives for the further biomimetic modification to promote cell growth on materials.

One-step surface modification of poly(lactide-co-glycolide) microparticles with heparin

PURPOSE

The aim of this study was to modify the surface of poly(lactide-co-glycolide) (PLGA) microparticles with heparin. The heparin-coated PLGA may enhance blood and tissue compatibility of PLGA devices and provide a novel approach to deliver growth factors.

MATERIALS AND METHODS

A one-step method using heparin to replace traditional emulsifiers (e.g., PVA) during emulsion-solvent evaporation process was employed to surface-entrap heparin in PLGA microspheres. The emulsifying activity of heparin was modified via varying counter ion form, including univalent (Na(+), K(+), Li(+), and [Formula: see text]) and divalent (Ca(2+), Mg(2+), Ba(2+), and Zn(2+)) cations, and complexation with amino acids (Arg, Lys, Leu, Val, Gly and Glu). Surface accessible and total heparin loading were determined by a modified toluidine blue assay and elemental analysis, respectively.

RESULTS

Heparin bound with univalent counter ions and amino acids exhibited emulsifying activity to varying degrees, whereas divalent heparin salts tended to cause complete aggregation of the PLGA o/w emulsion. Increasing pH (>or=7.4) of hardening medium enhanced heparin adsorption and significantly stabilized the PLGA o/w emulsion. The initial surface density of heparin on the PLGA microspheres prepared using univalent heparin salts was around 8-33 mg/m(2). Surface associated heparin desorbed quickly; potassium heparin showed the best retention, with approximately 0.2 and 0.1 mg/m(2) detected on PLGA microsphere surface following 1- and 14-day incubation in PBST at 37 degrees C, respectively.

CONCLUSIONS

PLGA microparticles were successfully surface-modified with heparin. Univalent salts and amino acid complexes of heparin, as effective emulsifiers, can become surface-immobilized in PLGA microspheres.

Novel Amphiphilic Poly(ε-caprolactone)-g-poly(l-lysine) Degradable Copolymers

As part of the search of novel degradable polymers, amphiphilic and cationic poly(epsilon-caprolactone)-g-poly(l-lysine) (PCL-g-PlL) copolymers have been synthesized following a grafting "onto" or a grafting "from" method both applied to a macropolycarbanionic PCL derivative. The first approach led to PCL-g-PZlL containing 36% of epsilon-caprolactone and 64% of N-epsilon-Z-l-lysine units, by reaction of activated poly(N-epsilon-Z-l-lysine) on the macropolycarbanion derived from PCL. The second route was based on the anionic ring opening polymerization of N-carboxyanhydride of N-epsilon-benzyloxycarbonyl-l-lysine initiated by the macropolycarbanion derived from PCL and led to a similar copolymer containing 45% of of epsilon-caprolactone and 55% of N-epsilon-Z-l-lysine units. After deprotection of the lysine units, PCL-g-PlL copolymers were obtained. These copolymers are water-soluble and form nanometric micelle-like objects with mean diameters between 60 and 500 nm in distilled water depending on the synthesis route.

Stable cationic microparticles for enhanced model antigen delivery to dendritic cells

The objectives of this work were (i) to prepare physically stable cationic microparticles and (ii) to study the impact of the surface properties on microparticle phagocytosis and the phenotype of dendritic cells (DC). Protein loaded biodegradable microparticles from poly(lactic-co-glycolic acid) [PLGA] were produced in a micromixer-based w/o/w solvent evaporation procedure. Anionic particles were obtained by using polyvinyl alcohol (PVA) as stabilizing agent; for cationic surfaces cetyltrimethylammonium bromide (CTAB) and chitosan/PVA or DEAE-dextran/PVA blends were evaluated. In phagocytosis studies human monocytes and monocyte-derived DC were incubated with microparticles and analysed by flow cytometry. While CTAB modified microparticles lost their positive charge and aggregated due to CTAB desorption from the particle surface, the modification with chitosan and DEAE-dextran resulted in stable microparticles without cell toxicity. Due to a very low endotoxin content, phagocytosis of anionic and cationic microparticles did not induce an upregulation of maturation-associated surface markers on DC. DEAE-dextran modified microparticles showed an enhanced model protein delivery into phagocytic cells. Overall, PLGA microparticles are suitable vehicles for protein delivery to DC, which might be used for DC-based cell therapies.

Heparin Immobilized Porous PLGA Microspheres for Angiogenic Growth Factor Delivery

PURPOSE

Heparin immobilized porous poly(D,L-lactic-co-glycolic acid) (PLGA) microspheres were prepared for sustained release of basic fibroblast growth factor (bFGF) to induce angiogenesis.

MATERIALS AND METHODS

Porous PLGA microspheres having primary amine groups on the surface were prepared using an oil-in-water (O/W) single emulsion method using Pluronic F-127 as an extractable porogen. Heparin was surface immobilized via covalent conjugation. bFGF was loaded into the heparin functionalized (PLGA-heparin) microspheres by a simple dipping method. The bFGF loaded PLGA-heparin microspheres were tested for in vitro release and in vivo angiogenic activity.

RESULTS

PLGA microspheres with an open-porous structure were formed. The amount of conjugated amine group onto the microspheres was 1.93+/-0.01 nmol/mg-microspheres, while the amount of heparin was 95.8 pmol/mg-microspheres. PLGA-heparin microspheres released out bFGF in a more sustained manner with a smaller extent of initial burst than PLGA microspheres, indicating that surface immobilized heparin controlled the release rate of bFGF. Subcutaneous implantation of bFGF loaded PLGA-heparin microspheres in mice significantly induced the formation of new vascular microvessels.

CONCLUSIONS

PLGA microspheres with an open porous structure allowed significant amount of heparin immobilization and bFGF loading. bFGF loaded PLGA-HP microspheres showed sustained release profiles of bFGF in vitro, demonstrating reversible and specific binding of bFGF to immobilized heparin. They also induced local angiogenesis in vivo in an animal model.

Vesicle-to-micelle transition in aqueous solutions of amphiphilic calixarene derivatives

Structure and conformation of spontaneous self-assembled calix[8]arenes derivatives are studied by means of static and dynamic light scattering and electrophoretic mobility. These amphiphilic molecules are in the aggregated form in aqueous solution, in a wide range of pH; they take a vesicle structure in neutral and basic pH environment, but, in relatively strong acidic conditions (below pH=4.5), a transition from vesicle to micelle occurs. The structural change is driven by the surface charge density. At neutral pH calix[8]arenes take a negative surface charge, which prevents coagulation and ensures stability; at acidic pH the surface charge tend to become positive because of the protonation of the hydrophilic head. These pH-responsive aggregates, able to release an encapsulated hydrophilic guest, are promising systems for application as nanocarriers.

Surface modification of biodegradable polyesters with fatty acid conjugates for improved drug targeting

We describe a general method for incorporating target ligands into the surface of biocompatible polyester poly(lactic-co-glycolic acid) (PLGA) 50/50 materials using fatty acids. Avidin-fatty acid conjugates were prepared and efficiently incorporated into PLGA. Avidin was chosen as an adaptor protein to facilitate the attachment of a variety of biotinylated ligands. We show that fatty acid preferentially associates with the hydrophobic PLGA matrix, rather than the external aqueous environment, facilitating a prolonged presentation of avidin over several weeks. We successfully applied this approach in both microspheres encapsulating a model protein, bovine serum albumin, and PLGA scaffolds fabricated by a salt-leaching method. Because of its ease, generality and flexibility, this strategy promises widespread utility in modifying the surface of PLGA-based materials for applications in drug delivery and tissue engineering.

Investigation of the stability of polyelectrolyte multilayer coatings in open-tubular capillary electrochromatography using laser scanning confocal microscopy

A simple polyelectrolyte multilayer (PEM) coating procedure was used for the development of stable modified capillaries. PEM coatings were constructed in fused-silica capillaries using alternating rinses of cationic and anionic polyelectrolytes. The multilayer coatings investigated in this study consisted of two and twenty layer pairs, or bilayers. A bilayer is one layer of a cationic polymer and one layer of an anionic polymer. Poly(diallyldimethylammonium chloride) was used as the cationic polymer, and the polymeric surfactant poly(sodium N-undecanoyl-L-leucylvalinate) was used as the anionic polymer. Previous studies for both chiral and achiral separations have shown that PEM-coated capillaries have excellent reproducibilities, remarkable endurance, and strong stabilities against extreme pH values when used in open-tubular capillary electrochromatography (OT-CEC). In this study, the stability of the coatings was further investigated after exposure to 0.1 M and 1.0 M NaOH. Structural changes of these coatings were monitored using laser scanning confocal microscopy (LSCM) after flushing the capillaries with NaOH. This technique allowed observation of the degradation of the coatings. Observations are discussed in terms of separations using OT-CEC. Electropherograms obtained from the chiral separation of 1,1'-binaphthyl-2,2'-dihydrogenphosphate in OT-CEC showed a decrease in selectivity and an increase in electroosmotic mobility after long exposure to NaOH. The ability to recover the capillaries by exposure to NaOH was also demonstrated. Measurements of electroosmotic mobility and selectivity showed that 2-bilayer and 20-bilayer PEM coatings could be completely removed from the capillary surface after approximately 3.5 and 9.5 h, respectively, of continuous exposure to 1 M NaOH.

A simple synthetic route to the formation of a block copolymer of poly(lactic-co-glycolic acid) and polylysine for the fabrication of functionalized, degradable structures for biomedical applications

This article documents the formation of a block copolymer of poly(lactic-co-glycolic acid) and polylysine via a simple coupling technique using dicyclohexyl carbodiimide (DCC). The resulting polymer has been characterized via UV-Vis spectroscopy, GPC, (1)H NMR, and elemental analysis, is soluble in a wide variety of solvents, and is easily processable, making the technique a simple and practical one for the formation of functionalized, degradable block copolymers for the fabrication of functionalized structures for biomedical applications.

Development of highly fluorescent detection reagents for the construction of ultrasensitive immunoassays

We developed two kinds of highly fluorescent streptavidin-based conjugates for use as universal detection reagents in ultrasensitive immunoassays. The direct conjugate was produced by covalently linking streptavidin to poly(Glu: Lys) which was labeled heavily with Eu chelates; the indirect conjugate was made by first conjugating bovine serum albumin (BSA) to poly(Glu:Lys) labeled heavily with Eu chelates and then further linking streptavidin to the conjugate of BSA-poly(Glu:Lys)-Eu chelate. Both direct and indirect conjugates were used to construct a highly sensitive time-resolved fluorometric assay for prostate-specific antigen (PSA). Of two monoclonal antibodies used in the assay, one was coated on the well surface of the microtitration strips, and the other was biotinylated. When 10 microL of sample volume was used, we found that the assay using the indirect conjugate had a detection limit of 0.006 microg/L, which was approximately 5.6-fold more sensitive than the one using Eu chelate directly labeled detection antibody and 6.8-fold more sensitive than the one using Eu chelate-labeled streptavidin. However, the assay that used the direct conjugate was 1.5-fold more sensitive than the one that utilized the indirect conjugate. When 45 microL of sample volume was used, a detection limit of 0.001 microg/L was achieved by using the direct conjugate. This improvement in sensitivity should be equally obtainable for the analytes other than PSA. We further demonstrated that the final immunoassay performance was affected not only by the quality of the streptavidin-based conjugate used but also by the quality of the biotinylated antibody reagent. The universal detection reagents described here are believed to be particularly useful for the construction of ultrasensitive time-resolved fluorometric immunoassays and are potentially applicable in other fields such as immunohistochemistry and nucleic acid detection.

Structural analysis of microparticles by confocal laser scanning microscopy

This study demonstrates the potential of confocal laser scanning microscopy (CLSM) as a characterization tool for different types of microparticles. Microparticles were prepared by various methods including complex coacervation, spray drying, double emulsion solvent evaporation technique, and ionotropic gelation. Protein drugs and particle wall polymers were covalently labeled with a fluorescent marker prior to particle preparation, while low molecular weight drugs were labeled by mixing with a fluorescent marker of similar solubility properties. As was demonstrated in several examples, CLSM allowed visualization of the polymeric particle wall composition and detection of heterogeneous polymer distribution or changes in polymer matrix composition under the influence of the drug. Furthermore, CLSM provides a method for three-dimensional reconstruction and image analysis of the microparticles by imaging several coplanar sections throughout the object. In conclusion, CLSM allows the inspection of internal particle structures without prior sample destruction. It can be used to localize the encapsulated compounds and to detect special structural details of the particle wall composition.