Supramolecular hydrogels formed by pyrene-terminated poly(ethylene glycol) star polymers through inclusion complexation of pyrene dimers with γ-cyclodextrin.

Novel supramolecular hydrogels were formed between pyrene-terminated poly(ethylene glycol) star polymers and γ-cyclodextrin (γ-CD), through the inclusion complexation of dimers of the pyrene terminals with γ-CD, where γ-CD was directly used as a supramolecular cross-linking reagent without any modification.

Biodegradable thermogelling poly[(R)-3-hydroxybutyrate]-based block copolymers: micellization, gelation, and cytotoxicity and cell culture studies.

A series of biodegradable multiblock poly(ester urethane)s having poly[(R)-3-hydroxybutyrate] (PHB), poly(ethylene glycol) (PEG), and poly(propylene glycol) (PPG) segments was prepared. The critical micellization concentration (CMCs) of these water-soluble poly(ester urethane)s were determined at different temperatures in order to calculate the thermodynamic parameters for the process of micelle formation. The process for micelle formation was found to be entropy-driven. The thermogelling behavior of the aqueous polymer solution was studied by (1)H and (13)C NMR spectroscopy at different temperatures. We obtained valuable molecular level information regarding the state of the copolymer in solution based on the variation of the peak widths. Cytotoxicity studies performed on the extracts of the copolymer gel indicate good cell compatibility. Cells attach on the surface of the gel much better than on the commercially available PEG-PPG-PEG triblock copolymer. These studies indicate a potential for the copolymer gel to be used for tissue engineering applications.

Improving hydrophilicity, mechanical properties and biocompatibility of poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate] through blending with poly[(R)-3-hydroxybutyrate]-alt-poly(ethylene oxide).

Natural source poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate] (PHBV) with a low hydroxyvalerate (HV) content ( approximately 8wt.%) was modified by blending it with poly[(R)-3-hydroxybutyrate]-alt-poly(ethylene oxide) (HE) alternating block copolymer. We hypothesized that the adjoining PHB segments could improve the miscibility of the poly(ethylene oxide) segments of HE with the PHBV matrix and therefore improve the physical properties of the PHBV/HE blends. A differential scanning calorimetry study revealed the improved miscibility of PEO segments of HE characterized by the interference of the crystallization of PHBV. The decrease in water contact angle and the increase in equilibrium water uptake of the PHBV/HE blends indicated that both the surface and bulk hydrophilicity of PHBV could be improved through blending HE. The mechanical properties of the hydrated PHBV/HE blends were assessed by measuring their tensile strength. In contrast to the hydrated natural source PHBV, which failed in a brittle manner, the hydrated PHBV/HE blends were ductile. Their strain at break increased with increasing HE content, reaching a maximum of 394% at an HE content of 15wt.%. The excellent integrity of the PHBV/HE blends in water is attributed to the strong affinity between the PHB segments of HE and the PHBV matrix. Platelet adhesion on the film surface of the PHBV/HE blends was investigated in vitro to evaluate their blood compatibility. The results demonstrated that the PHBV/HE blends effectively resisted the adhesion of platelets due to the anchored PEO segments from HE on the film surface.

Structure-property relationship in polyethylene reinforced by polyethylene-grafted multi-walled carbon nanotubes.

Polyethylene-grafted multiwalled carbon nanotubes (PE-g-MWNT) were used to reinforce polyethylene (PE). The nanocomposites possessed not only improved stiffness and strength, but also increased ductility and toughness. The effects on the structure and morphology of composites due to pristine multiwalled carbon nanotubes (MWNT) and PE-g-MWNT were studied and compared using small angle X-ray scattering (SAXS), wide angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The SAXS long period, crystalline layer thickness and crystallinity of polymer lamellar stacks were found to decrease significantly in MWNT composites, while the decreases were much smaller in PE-g-MWNT composites. PE-g-MWNT allowed a more efficient and unhindered crystallization at a lamellar level, while MWNT disrupted the order of lamellar stacks, probably because of their tendency to aggregate. The SAXS crystallinity and the mechanical properties of the composites showed similar trends as a function of MWNT content. This suggested that the improvement of the interfacial strength between polymer and carbon nanotubes was a result of synergistic effects of better dispersion of the filler, better stress transfer, due to the grafting of polymer and MWNT, and the nucleation of a crystalline phase around MWNT. The latter effect was confirmed by measurements of kinetics of non-isothermal crystallization.

Biodegradable thermogelling poly(ester urethane)s consisting of poly(lactic acid)--thermodynamics of micellization and hydrolytic degradation.

Multiblock poly(ether ester urethane)s comprising of poly(lactic acid) (PLA), poly(ethylene glycol) (PEG), and poly(propylene glycol) (PPG) segments were synthesized, and their aqueous solutions exhibited thermogelling behavior at critical gelation concentrations (CGC) ranging from 7 to 9 wt%. The chemical structures and molecular characteristics of the copolymers were studied by GPC, 1H NMR, 13C NMR and FTIR. The thermal stability of the poly(PEG/PPG/PLA urethane)s was studied by thermogravimetry analysis (TGA), and the PLA contents were calculated based on the thermal degradation profile. The results were in good agreement with those obtained from the 1H NMR measurements. The critical micellization concentration (CMC) of these water-soluble poly(ether ester urethane)s was determined at different temperatures using a dye solubilization method. The thermodynamic parameters for micelle formation were calculated, indicating that the process is largely entropy-driven. Interestingly, it appears that there exists a requirement for the system to possess a minimum gain in entropy before the thermogelling effect can be observed. Dilute copolymer solutions showed a lower critical solution temperature (LCST) behavior similar to pNIPAM dissolved in aqueous solutions. The thermogels hydrolytically degraded to polymer fragments corresponding to the constituent segment blocks within 3 months.

Self-organization of double-C60 end-capped poly(ethylene oxide) in chloronaphthalene and benzene solvent mixtures.

Well-defined and narrowly distributed double-C60 end-functionalized poly(ethylene oxide) (C60-PEO-C60) was prepared by reacting azido-terminated PEO with C60. The self-organization behavior of such C60-modified copolymers in different mixtures of chloronaphthalene (CN) and benzene (BN) was studied by a combination of static and dynamic laser light scattering. For C60-PEO-C60 in pure CN, a selective solvent only for C60, self-organization occurred to form a large micelle-like core-shell aggregate, probably with some C60 interlocking with each other with the collapsed PEO chains as the core. The addition of BN, a second selective solvent for core-forming PEO chains, has a significant effect on the structures and compactness of the resultant aggregates because the introduction of BN increases the stretching of the PEO chains inside the core and modifies the interfacial energy of the core-corona interface. On the other hand, for C60-PEO-C60 in pure BN, a non-solvent of C60, several smaller flower-like micelles may self-organize to form a large spherical-like aggregation complex.

Synthesis and characterization of polyrotaxanes consisting of cationic alpha-cyclodextrins threaded on poly[(ethylene oxide)-ran-(propylene oxide)] as gene carriers.

Cationic polymers have been receiving growing attention as gene delivery carriers. Herein, a series of novel cationic supramolecular polyrotaxanes with multiple cationic alpha-cyclodextrin (alpha-CD) rings threaded and blocked on a poly[(ethylene oxide)-ran-(propylene oxide)] (P(EO-r-PO)) random copolymer chain were synthesized and investigated for gene delivery. In the cationic polyrotaxanes, approximately 12 cationic alpha-CD rings were threaded on the P(EO-r-PO) copolymer with a molecular weight of 2370 Da and an EO/PO molar ratio of 4:1, while the cationic alpha-CD rings were grafted with linear or branched oligoethylenimine (OEI) of various chain lengths and molecular weights up to 600 Da. The OEI-grafted alpha-CD rings were only located selectively on EO segments of the P(EO-r-PO) chain, while PO segments were free of complexation. This increased the mobility of the cationic alpha-CD rings and the flexibility of the polyrotaxanes, which enhanced the interaction of the cationic alpha-CD rings with DNA and/or the cellular membrane. All cationic polyrotaxanes synthesized in this work could efficiently condense plasmid DNA to form nanoparticles that were suitable for delivery of the gene. Cytotoxicity studies showed that the cationic polyrotaxanes with all linear OEI chains of molecular weights up to 423 Da exhibited much less cytotoxicity than high-molecular-weight branched polyethylenimine (PEI) (25 kDa) in both HEK293 and COS7 cell lines. The cationic polyrotaxanes displayed high gene transfection efficiencies in a variety of cell lines including HEK293, COS7, BHK-21, SKOV-3, and MES-SA. Particularly, the gene delivery capability of the cationic polyrotaxanes in HEK293 cells was much higher than that of high-molecular-weight branched PEI (25 k).

The self-assembly of biodegradable cationic polymer micelles as vectors for gene transfection.

Cationic micelles self-assembled from a biodegradable amphiphilic copolymer, poly{(N-methyldietheneamine sebacate)-co-[(cholesteryl oxocarbonylamido ethyl) methyl bis(ethylene) ammonium bromide] sebacate} (P(MDS-co-CES)) have recently been reported for efficient gene delivery and co-delivery of drug and nucleic acid. In this study, poly(ethylene glycol) (PEG) of various molecular weights (Mn=550, 1100 and 2000) was conjugated to P(MDS-co-CES) having different cholesterol grafting degrees to improve the stability of micelle/DNA complexes in the blood for systemic in vivo gene delivery. DNA binding ability, gene transfection efficiency and cytotoxicity of P(MDS-co-CES), PMDS, PEGylated PMDS and PEGylated P(MDS-co-CES) micelles were studied and compared. As with P(MDS-co-CES), PEG-P(MDS-co-CES) polymers could also self-assemble into stable micelles of small size. However, PMDS and PEG-PMDS without cholesterol could not form stable micelles but formed large particles. PEGylation of polymers significantly decreased their gene transfection efficiency in HEK293, HepG2, HeLa, MDA-MB-231 and 4T1 cells. However, increasing N/P ratio promoted gene transfection. An increased cholesterol grafting degree led to greater gene expression level possibly because of the more stable core-shell structure of the micelles. PEG550-P(MDS-co-CES) micelles induced high gene transfection level, comparable to that provided by P(MDS-co-CES) micelles. PEGylated polymers were much less cytotoxic than P(MDS-co-CES). PEGylated P(MDS-co-CES) micelles may provide a promising non-viral vector for systemic in vivo gene delivery.

Hydrolytic degradation and protein release studies of thermogelling polyurethane copolymers consisting of poly[(R)-3-hydroxybutyrate], poly(ethylene glycol), and poly(propylene glycol).

This paper reports the hydrolytic degradation and protein release studies for a series of newly synthesized thermogelling tri-component multi-block poly(ether ester urethane)s consisting of poly[(R)-3-hydroxybutyrate] (PHB), poly(propylene glycol) (PPG), and poly(ethylene glycol) (PEG). The poly(PEG/PPG/PHB urethane) copolymer hydrogels were hydrolytically degraded in phosphate buffer at pH 7.4 and 37 degrees C for a period of up to 6 months. The mass loss profiles of the copolymer hydrogels were obtained. The hydrogel residues at different time periods of hydrolysis were visualized by scanning electron microscopy, which exhibited increasing porosity with time of hydrolysis. The degradation products in the buffer were characterized by GPC, (1)H NMR, MALDI-TOF, and TGA. The results showed that the ester backbone bonds of the PHB segments were broken by random chain scission, resulting in a decrease in the molecular weight. In addition, the constituents of degradation products were found to be 3-hydroxybutyric acid monomer and oligomers of various lengths (n=1-5). The protein release profiles of the copolymer hydrogels were obtained using BSA as model protein. The results showed that the release rate was controllable by varying the composition of the poly(ether ester urethane)s or by adjusting the concentration of the copolymer in the hydrogels. Finally, we studied the correlation between the protein release characteristics of the hydrogels and their hydrolytic degradation. This is the first example that such a correlation has been attempted for a biodegradable thermogelling copolymer system.

Cationic star polymers consisting of alpha-cyclodextrin core and oligoethylenimine arms as nonviral gene delivery vectors.

A series of novel cationic star polymers were synthesized by conjugating multiple oligoethylenimine (OEI) arms onto an alpha-cyclodextrin (alpha-CD) core as nonviral gene delivery vectors. The molecular structures of the alpha-CD-OEI star polymers, which contained linear or branched OEI arms with different chain lengths ranging from 1 to 14 ethylenimine units, were characterized by using size exclusion chromatography, 13C and 1H NMR, and elemental analysis. The alpha-CD-OEI star polymers were studied in terms of their DNA binding capability, formation of nanoparticles with plasmid DNA (pDNA), cytotoxicity, and gene transfection in cultured cells. All the alpha-CD-OEI star polymers could inhibit the migration of pDNA on agarose gel through formation of complexes with pDNA, and the complexes formed nanoparticles with sizes ranging from 100 to 200 nm at N/P ratios of 8 or higher. The star polymers displayed much lower in vitro cytotoxicity than that of branched polyethylenimine (PEI) of molecular weight 25K. The alpha-CD-OEI star polymers showed excellent gene transfection efficiency in HEK293 and Cos7 cells. Generally, the transfection efficiency increased with an increase in the OEI arm length. The star polymers with longer and branched OEI arms showed higher transfection efficiency. The best one of the star polymers for gene delivery showed excellent in vitro transfection efficiency that was comparable to or even higher than that of branched PEI (25K). The novel alpha-CD-OEI star polymers with OEI arms of different chain lengths and chain architectures can be promising new nonviral gene delivery vectors with low cytotoxicity and high gene transfection efficiency for future gene therapy applications.

Synthesis and characterization of cationic micelles self-assembled from a biodegradable copolymer for gene delivery.

We have recently reported biodegradable cationic micelles self-assembled from an amphiphilic copolymer, poly{(N-methyldietheneamine sebacate)-co-[(cholesteryl oxocarbonylamido ethyl)methyl bis(ethylene)ammonium bromide]sebacate} (P(MDS-co-CES)), which were utilized to deliver a drug and nucleic acid simultaneously, and a synergistic effect was achieved. In this paper, synthesis and characterization of the polymer is presented in details, focusing on micelle formation and DNA binding under various conditions, cytotoxicity, in-vitro degradation, and gene transfection in various cell lines. The polymer was degradable and formed micelles at very low concentrations even in an environment with high salt concentration. These micelles fabricated at pH 4.6 had an average size of less than 82 nm and zeta potential of up to 84 +/- 5 mV, displaying strong DNA binding ability. They induced high gene expression efficiency in various cell lines, which was significantly greater than poly(ethylenimine) (PEI) especially in 4T1 mouse and MDA-MB-231 human breast cancer cell lines, but they were less cytotoxic. These cationic micelles may provide a promising nonviral vector for gene delivery.

New biodegradable thermogelling copolymers having very low gelation concentrations.

New biodegradable multiblock amphiphilic and thermosensitive poly(ether ester urethane)s consisting of poly[(R)-3-hydroxybutyrate] (PHB), poly(ethylene glycol) (PEG), and poly(propylene glycol) (PPG) blocks were synthesized, and their aqueous solutions were found to undergo a reversible sol-gel transition upon temperature change at very low copolymer concentrations. The multiblock poly(ether ester urethane)s were synthesized from diols of PHB, PEG, and PPG using 1,6-hexamethylene diisocyanate as a coupling reagent. The chemical structures and molecular characteristics of the copolymers were studied by GPC, 1H NMR, 13C NMR, and FTIR. The thermal stability of the poly(PEG/PPG/PHB urethane)s was studied by thermogravimetry analysis (TGA), and the PHB contents were calculated based on the thermal degradation profile. The results were in good agreement with those obtained from the 1H NMR measurements. The poly(PEG/PPG/ PHB urethane)s presented better thermal stability than the PHB precursors. The water soluble poly(ether ester urethane)s had very low critical micellization concentration (CMC). Aqueous solutions of the new poly(ether ester urethane)s underwent a sol-gel-sol transition as the temperature increased from 4 to 80 degrees C, and showed a very low critical gelation concentration (CGC) ranging from 2 to 5 wt %. As a result of its multiblock architecture, a novel associated micelle packing model can be proposed for the sol-gel transition for the copolymer gels of this system. The new material is thought to be a promising candidate for injectable drug systems that can be formulated at low temperatures and forms a gel depot in situ upon subcutaneous injection.

Synthesis, characterization, and morphology studies of biodegradable amphiphilic poly[(R)-3-hydroxybutyrate]-alt-poly(ethylene glycol) multiblock copolymers.

Novel biodegradable amphiphilic alternating block copolymers based on poly[(R)-3-hydroxybutyrate] (PHB) as biodegradable and hydrophobic block and poly(ethylene glycol) (PEG) as hydrophilic block (PHB-alt-PEG) were successfully synthesized through coupling reaction. Their chemical structures have been characterized by using gel permeation chromatography, (1)H nuclear magnetic resonance, and Fourier transform infrared spectroscopy. Differential scanning calorimetry (DSC) analysis revealed that both PHB and PEG blocks in PHB-alt-PEG block copolymers can crystallize to form separate crystalline phase except in those with a short PEG block (M(n) 600) only PHB crystalline phase has been observed. However, due to the mutual interference from each other, the melting transition of both PHB and PEG crystalline phases shifted to lower temperature with lower crystallinity in comparison with corresponding pure PHB and PEG. The crystallization behavior of PHB block and PEG block has also been studied by X-ray diffraction, and the results were in good agreement with those deduced from DSC study. The surface morphologies of PHB-alt-PEG block copolymer thin films spin-coated on mica have been visualized by atomic force microscopy with tapping mode, indicating formation of laterally regular lamellar surface patterns. Static water contact angle measurement revealed that surface hydrophilicity of these spin-coated thin films increases with increasing PEG block content.

Hyperbranched poly(amino ester)s with different terminal amine groups for DNA delivery.

Hyperbranched poly(amino ester)s containing tertiary amines in the core and primary, secondary, and tertiary amines in the periphery, respectively, were evaluated for DNA delivery in vitro. The same core structure facilitated the investigation on the effects of the terminal amine type on the properties of hyperbranched poly(amino ester)s for DNA delivery. The hydrolysis of the poly(amino ester)s was monitored using (1)H NMR. The results reflected that the terminal amine type had negligible effects on the hydrolysis rate but was much slower than that of linear poly(amino ester)s, probably due to the compact hyperbranched spatial structure preventing the accessibility of water. In comparison with PEI 25 K, the hyperbranched poly(amino ester)s showed much lower cytotoxicity in Cos7, HEK293, and HepG2 cells. Gel electrophoresis indicated that poly(amino ester)s could condense DNA efficiently, and the zeta potentials and sizes of the complexes formed with different weight ratios of hyperbranched poly(amino ester)s and DNA were measured. Remarkably, all the hyperbranched poly(amino ester)s showed DNA transfection efficiency comparable to PEI 25 K in Cos7, HEK293, and HepG2 cells regardless of the terminal amine type. Therefore, the terminal amine type had insignificant effects on the hydrolysis rate, cytotoxicity, DNA condensation capability, and in vitro DNA transfection efficiency of the hyperbranched poly(amino ester)s.

Chitosan-g-PEG/DNA complexes deliver gene to the rat liver via intrabiliary and intraportal infusions.

BACKGROUND: Chitosan has been shown to be a non-toxic and efficient vector for in vitro gene transfection and in vivo gene delivery through pulmonary and oral administrations. Recently, we have shown that chitosan/DNA nanoparticles could mediate high levels of gene expression following intrabiliary infusion 1. In this study, we have examined the possibility of using polyethylene glycol (PEG)-grafted chitosan/DNA complexes to deliver genes to the liver through bile duct and portal vein infusions. METHODS: PEG (Mw: 5 kDa) was grafted onto chitosan (Mw: 47 kDa, deacetylation degree: 94%) with grafting degrees of 3.6% and 9.6% (molar percentage of chitosan monosaccharide units grafted with PEG). The stability of chitosan-g-PEG/DNA complexes was studied by measuring the change in particle size and by agarose gel electrophoresis against bile or serum challenge. The influence of PEG grafting on gene transfection efficiency was evaluated in HepG2 cells using luciferase reporter gene. Chitosan and chitosan-g-PEG/DNA complexes were delivered to the liver through bile duct and portal vein infusions with a syringe pump. Gene expression in the liver and the distribution of gene expression in other organs were evaluated. The acute liver toxicity of chitosan and chitosan-g-PEG/DNA complexes was examined by measuring serum alanine aminotranferase (ALT) and aspartate aminotransferase (AST) activities as a function of time. RESULTS: Both chitosan and chitosan-g-PEG displayed comparable gene transfection efficiency in HepG2 cells. After challenge with serum and bile, chitosan-g-PEG/DNA complexes, especially those prepared with chitosan-g-PEG (GD = 9.6%), did not form large aggregates like chitosan/DNA complexes but remained stable for up to 30 min. In addition, chitosan-g-PEG prevented the degradation of DNA in the presence of serum and bile. On day 3 after bile duct infusion, chitosan-g-PEG (GD = 9.6%)/DNA complexes mediated three times higher gene expression in the liver than chitosan/DNA complexes and yielded background levels of gene expression in other organs. On day 1 following portal vein infusion, gene expression level induced by chitosan/DNA complexes was hardly detectable but chitosan-g-PEG (GD = 9.6%) mediated significant transgene expression. Interestingly, transgene expression by chitosan-g-PEG/DNA complexes in other organs after portal vein infusion increased with increasing grafting degree of PEG. The ALT and AST assays indicated that grafting of PEG to chitosan reduced the acute liver toxicity towards the complexes. CONCLUSION: This study demonstrated the potential of chitosan-g-PEG as a safe and more stable gene carrier to the liver.

Evaluation of hyperbranched poly(amino ester)s of amine constitutions similar to polyethylenimine for DNA delivery.

New hyperbranched poly(amino ester)s were synthesized via A3 + 2BB'B' ' approach, represented by the Michael addition polymerization of trimethylol-propane triacrylate (TMPTA) (A3-type monomers) with a double molar 1-(2-aminoethyl)piperazine (AEPZ) (BB'B''-type monomer) performed in chloroform at ambient temperature. The results obtained by in situ monitoring the polymerization using NMR and MS indicated that hyperbranched poly(TMPTA1-AEPZ2) was formed via a A(B'B'')2 intermediate, and the B' ' (the formed 2 degrees amine) was kept intact in the reaction. Therefore, poly(TMPTA1-AEPZ2) contained secondary and tertiary amines in the core and primary amines in the periphery similar to polyethylenimine (PEI). The chemistry of protonated poly(TMPTA1-AEPZ2) was further confirmed by 13C NMR, and the molecular weight, the radius of gyration (Rg), and the hydrodynamic radius (Rh) were determined using GPC, small-angle X-ray scattering (SAXS), and laser dynamic light scattering (LDLS), respectively. The ratio of Rg/Rh of ca. 1.1 verified the hyperbranched structure. Protonated hyperbranched poly(TMPTA1-AEPZ2) is degradable and less cytotoxic as compared with PEI (25 K). Gel electrophoresis reflected that stable complexes could be formed from protonated hyperbranched poly(TMPTA1-AEPZ2) and DNA, and the size and xi-potential of the complexes were characterized. Remarkably, protonated hyperbranched poly(TMPTA1-AEPZ2) showed transfection efficiency comparable to PEI (25 k) for in vitro DNA delivery.

Double walled POE/PLGA microspheres: encapsulation of water-soluble and water-insoluble proteins and their release properties.

The poly(orthoester) (POE)-poly(D,L-lactide-co-glycolide) (50:50) (PLGA) double-walled microspheres with 50% POE in weight were loaded with hydrophilic bovine serum albumin (BSA) and hydrophobic cyclosporin A (CyA). Most of the BSA and CyA was entrapped within the shell and core, respectively, because of the difference in their hydrophilicity. The morphologies and release mechanisms of proteins-loaded double-walled POE/PLGA microspheres were investigated. Scanning electron microscope studies revealed that the CyA-BSA-loaded double-walled POE/PLGA microspheres yielded a more porous surface and PLGA shell than those without BSA. The neat POE and PLGA yielded slow and incomplete CyA and BSA release. In contrast, nearly complete BSA and more than 95% CyA were released in a sustained manner from the double-walled POE/PLGA microspheres. Both the BSA- and CyA-BSA-loaded POE/PLGA microspheres yielded a sustained BSA release over 5 days. The CyA release pattern of the CyA-loaded double-walled POE/PLGA microspheres was biphasic, characterized by a slow release over 15 days followed by a sustained release over 27 days. However, the CyA-BSA-loaded double-walled POE/PLGA microspheres provided a more constant and faster CyA release due to their more porous shell. In the CyA-BSA-loaded double-walled POE/PLGA microspheres system, the PLGA layer acted as a carrier for BSA and mild reservoir for CyA. During the first 5 days, most BSA was released from the shell but only 14% CyA was left from the microspheres. Subsequently, more than 80% CyA were released in the next 25 days. The distinct structure of double-walled POE/PLGA microspheres would make an interesting device for controlled delivery of therapeutic agents.

POE/PLGA composite microspheres: formation and in vitro behavior of double walled microspheres.

The poly(ortho ester) (POE) and poly(D,L-lactide-co-glycolide) 50:50 (PLGA) composite microspheres were fabricated by a water-in-oil-in-water (w/o/w) double emulsion process. The morphology of the composite microspheres varied depending on POE content. When the POE content was 50, 60 or 70% in weight, the double walled microspheres with a dense core of POE and a porous shell of PLGA were formed. The formation of the double walled POE/PLGA microspheres was analysed. Their in vitro degradation behavior was characterized by scanning electron microscopy, gel permeation chromatography, Fourier-transform infrared microscopy and nuclear magnetic resonance spectroscopy (NMR). It was found that compared to the neat POE or PLGA microspheres, distinct degradation mechanism was achieved in the double walled POE/PLGA microspheres system. The degradation of the POE core was accelerated due to the acidic microenvironment produced by the hydrolysis of the outer PLGA layer. The formation of hollow microspheres became pronounced after the first week in vitro. 1H NMR spectra showed that the POE core was completely degraded after 4 weeks. On the other hand, the outer PLGA layer experienced slightly retarded degradation after the POE core disappeared. PLGA in the double walled microspheres kept more than 32% of its initial molecular weight over a period of 7 weeks.