Surface Chemistry of Biodegradable Polymers for Drug Delivery Systems

Preparation of core-shell CaCO3 capsules via Pickering emulsion templates

Micron size and food grade pristine CaCO(3) particles were used to stabilize an oil in water Pickering emulsion. The particles also acted as nucleation sites for the subsequent crystallization of CaCO(3) with the addition of CaCl(2) and CO(2) gas as precursors. After the controllable crystallization process, a dense CaCO(3) shell with a few microns in thickness was formed. The CaCO(3) shell was proven to be calcite without the presence of crystallization modifiers. The crystallization speed and the shell integrity were controlled by manipulating the addition of CaCl(2) amount during the different crystallization stages; therefore, the homogeneous nucleation in the bulk was almost inhibited, and the heterogeneous nucleation at the oil-water interface on pristine CaCO(3) particles was the main contribution to the growth of the shell. The encapsulated limonene flavor in CaCO(3) capsules showed a prolonged release in neutral water at 85°C, while a burst release at pH 2 water as expected. The method is a simple and scalable process for creating inorganic core-shell capsules and can be used for producing food grade capsules for controlling the flavor release or masking undesirable taste in mouth.

Influence of the pore generator on the evolution of the mechanical properties and the porosity and interconnectivity of a calcium phosphate cement

Porosity and interconnectivity are important properties of calcium phosphate cements (CPCs) and bone-replacement materials. Porosity of CPCs can be achieved by adding polymeric biodegradable pore-generating particles (porogens), which can add porosity to the CPC and can also be used as a drug-delivery system. Porosity affects the mechanical properties of CPCs, and hence is of relevance for clinical application of these cements. The current study focused on the effect of combinations of polymeric mesoporous porogens on the properties of a CPC, such as specific surface area, porosity and interconnectivity and the development of mechanical properties. CPC powder was mixed with different amounts of PLGA porogens of various molecular weights and porogen sizes. The major factors affecting the properties of the CPC were related to the amount of porogen loaded and the porogen size; the molecular weight did not show a significant effect per se. A minimal porogen size of 40 μm in 30 wt.% seems to produce a CPC with mechanical properties, porosity and interconnectivity suitable for clinical applications. The properties studied here, and induced by the porogen and CPC, can be used as a guide to evoke a specific host-response to maintain CPC integrity and to generate an explicit bone ingrowth.

Novel Strategy to Fabricate PLA/Au Nanocomposites as an Efficient Drug Carrier for Human Leukemia Cells in Vitro

Poly (lactic acid) (PLA) polymer has the promising applications in the biomedical field because of its biodegradability and safe elimination. In this study, we have explored the bio-application of new nanocomposites composed with PLA nanofibers and Au nanoparticles as the potential drug carrier for an efficient drug delivery in target cancer cells. The results demonstrated that the anticancer drug daunorubicin could be efficiently self-assembled on the surface of PLA/Au nanocomposites and the synergistic enhancement of PLA/Au nanocomposites conjugated with daunorubicin into drug-sensitive K562 and drug-resistant leukemia K562/AO2 cells could be obviously observed by MTT assay and confocal fluorescence microscopy studies. These observations suggest that the new nanocomposites could readily induce daunorubicin to accumulate and uptake in target leukemia cells and increase the drug's cytotoxicity. Especially, the PLA/Au nanocomposites could significantly facilitate the cellular drug absorbtion of daunorubicin into drug-resistant K562/AO2 cells and efficiently inhibit the cancer cell proliferation. This raised the possibility to utilize the PLA/Au nanocomposites as a new effective additive agent to inhibit the drug resistance and thus as a novel strategy to sensitively track the respective cancer cells.

Synthesis and in vitro characterization of poly(ethylene glycol)-albumin hydrogel microparticles

High water content hydrogel microparticles based on the cross-linking of albumin with activated poly(ethylene glycol) were synthesized. The influence of different synthesis parameters on the physicochemical characteristics of the microparticles, such as the type of oil and of albumin, and the molecular weight of PEG, was evaluated. The water content of the microparticles ranged from 95 to 98%, increasing with an increase of the molecular weight of PEG. At optimal conditions, microparticles with sizes ranging from 3 to 50 μm were prepared. These microparticles showed a negatively charged surface. They were freely dispersed in PBS buffer and they were stable at 4°C for times varying from 0.5 to 10 months. Initial stirring speed and molecular weight of PEG were the 2 main factors that significantly affected microparticle size. High hydrophilicity, good stability and modulable size make this hydrogel an attractive matrix for protein or cell immobilization for biomedical applications.

Stable biomimetic superhydrophobic surfaces fabricated by polymer replication method from hierarchically structured surfaces of Al templates

We have developed a simple, efficient, and highly reproducible method to fabricate the large-area biomimetic superhydrophobic polymer surfaces having hierarchical structures of micrometer-sized irregular steps and nanometer-sized fibrils. Commercial Al plates (99.0%) were etched using Beck's dislocation etchant (mixture of HCl and HF) for different time periods in order to alter the structure of the etched Al surfaces from micrometer-sized to highly rough nanometer-sized irregular steps. These hierarchical structures could be easily replicated onto the surface of various thermoplastic polymer plates from the etched Al templates by applying heat and pressure; many polymer replicas without any significant deviations from each other could be duplicated from the same etched Al master templates. All of thermoplastic polymer replicas having hierarchical structures exhibited superhydrophobic properties with water contact angles of larger than 150 degrees. Especially, the surfaces of the high-density polyethylene (HDPE) replicas having nanometer-sized curled strands exhibited superhydrophobicity with a static water contact angle of approximately 160 degrees and a sliding angle of less than 2 degrees. These superhydrophobic HDPE replicas having nanometer-sized curled strands showed excellent stability after being exposed to various organic solvents and aqueous solutions of various pH.

Metal Nanoshell - Capsule for Light-Driven Release of Small Molecule

We report the release of small molecules from the metal nanoshells driven by the laser irradiation. The metal nanoshells were composed of 50 nm silica cores and variable thick silver shells of 10 and 30 nm. The small molecule fluorophores of Rhodamine 123 were physically absorbed in the silica cores of metal nanoshells and released through the metal walls. The release rate was significantly increased with the laser irradiation depending on the metal shell thickness: a thicker metal nanoshell leads to a faster release. The results were interpreted by the photothermal effect of metal nanoshells that could convert the light into the thermal energy via coupling interactions of light with the metal plasmon resonances of shells. The metal nanoshells may be potentially used as the capsules for the controlled release of drugs as other molecules.

A cascade biodegradable polymer based on alternating cyclization and elimination reactions

Polymers that depolymerize by a cascade of intramolecular reactions in response to the removal of a stabilizing end-cap can allow for an unprecedented degree of control over the polymer degradation process. Described here is the development of polymers comprising N,N'-dimethylethylenediamine and 4-hydroxybenzyl alcohol linked by carbamate linkages. The polycarbamate backbone is stable in aqueous solution, but removal of a protective end-cap from the amine terminus allows the diamine to cyclize, forming N,N'-dimethylimidazolidinone and releasing the phenol, which undergoes a 1,6-elimination followed by the release of CO(2) to reveal the next amine to continue the cascade. These polymers therefore degrade by alternating cyclization and elimination reactions. First, a tert-butylcarbamate (Boc) group was introduced as a cleavable end-cap, and the degradation kinetics and mechanism were studied by (1)H nuclear magnetic resonance (NMR) spectroscopy and size exclusion chromatography. Next, to demonstrate the degradability of these polymers under biologically relevant conditions, poly(ethylene oxide) was introduced as an end-cap via an ester linkage, to provide an amphiphilic block copolymer. This copolymer was found to assemble into cascade degradable nanoparticles that were capable of encapsulating and subsequently releasing a fluorescent dye in aqueous solution. This new class of polymers therefore provides highly promising materials that can be used for the development of medical devices, drug delivery vehicles, and tissue engineering scaffolds with unique biodegradation properties.

Use of TOF-SIMS to study adsorption and loading behavior of methylene blue and papain in a nano-porous silicon layer

TOF-SIMS was applied to study the cross-sectional distribution of methylene blue and papain in porous silicon layers. Elemental and molecular information were used to study their distributions in the porous region and the chemistry of their adsorption. Methylene blue (MW = 284 Da) penetrated to the base to the pores. Positive ions (SiCH(3)(+)) suggest methylene blue binds to the substrate via its methyl groups. Negative fragments (SiOSH(3)(-) and SiO(2)SCH(-)) also suggested chemisorption via O bridging of the substrate Si and methylene blue S. The larger Papain molecule (23,406 Da) distributed itself in a similar manner to methylene blue demonstrating larger molecules can be effectively incorporated into such pore structures.

Strengthening of Dense Bioceramic Samples Using Bioresorbable Polymers – A Statistical Approach

Mechanical properties of bioceramics are poor and need to be improved for biomedical applications. In order to do this, bioceramics may be strengthened by bioresorbable polymers. In this study, the mechanical properties of poly(ε-caprolactone), PCL, coated dense bioceramic pellets made of silica-contained calcium phosphates were studied and analyzed using a statistical experimental design in conjunction with Taguchi methods for optimization. The aim of this experimental work was to maximize the pellet flexural strength and minimize the amount of deposited PCL. The most important factors affecting the strengthening of the ceramic pellets were evaluated. Four independent processing variables (a removal technique of an excess polymer solution, concentration of PCL in the solution, a heat treatment temperature and the number of dipping) with three levels of variability were tested using an L9 (34) orthogonal array. A statistical experimental design using the analysis of means and orthogonal array was applied to optimize the responses of these variables. The optimal conditions for achieving the maximal flexural strength of the coated pellets at the minimal amount of the deposited PCL were determined. A high quality dense bioceramic pellets with ~ 10.5 MPa flexural strength and ~ 80 μm thickness (~ 21 mg weight) of the deposited PCL coating were manufactured as a result.

Alkali etching of a poly(lactide) fiber

Alkali etching of a poly(l-lactic acid) fiber was studied by exposing the fiber surface to sodium hydroxide solutions. The factors examined included the etching time (0-1.5 h), alkali concentration (0.25-3 mol/L), and etching temperature (25-80 degrees C). The extent of etching was determined gravimetrically. Both weight loss and mechanical testing results suggest that alkali etching is strictly a surface hydrolysis reaction, as opposed to a bulk reaction, and thus the weight loss rate decreases with a shrinking fiber radius. A slight increase in the fiber crystallinity observed from thermal analysis was interpreted as a result of surface-limited etching on a sheath-core fiber microstructure. The dependence of the rate on the alkali concentration is nonlinear, suggesting that the fiber weight loss rate is subject to both chemical hydrolysis and transport limitations. The dependence of the rate on the temperature follows the Arrhenius equation. The fiber weight after etching can thus be predicted by an overall expression combining all factors: time, temperature, concentration, and fiber diameter.

Miscibility and hydrolytic behavior of poly(trimethylene carbonate) and poly(L-lactide) and their blends in monolayers at the air/water interface

In this study, two biodegradable polymers, poly(trimethylene carbonate) (PTMC) and poly(L-lactide) (PLLA) along with a series of PTMC/PLLA blends, were used as spreading materials to form LB monolayers at the air/water interface to study hydrolytic reaction kinetics of the monolayers with the Langmuir film balance technique. The pi-A isotherms of each homopolymer and their blends showed that blends of PTMC and PLLA were miscible on the neutral subphase (pH 7.4), whereas there was evidence of phase separation on the basic subphase (pH 10.7). The hydrolysis behavior of each homopolymer was investigated at these two different pH conditions. The PTMC monolayer showed faster hydrolysis on the neutral subphase (pH 7.4) than on the basic subphase (pH 10.7). However, in the case of the PLLA monolayer, the hydrolysis on the basic subphase is faster than that on the neutral subphase. On the basis of this result, hydrolysis mechanisms of PTMC and PLLA, considering a general hydrolysis mechanism and their stereo structures, are proposed. The hydrolysis rates of blends of PTMC and PLLA were much faster than that of each homopolymer on the basic subphase (pH 10.7). This result, which can be explained by a "dilution effect", was supported by the structure based mechanism proposed here.

Toughening of porous bioceramic scaffolds by bioresorbable polymeric coatings

The mechanical properties of poly(ɛ-caprolactone) (PCL)-coated porous bioceramic scaffolds made of calcium phosphates were studied and analysed using a statistical experimental design and Taguchi methods. In this study, both the flexural strength of the coated scaffolds and the amount of deposited PCL were considered as the measured responses. A statistical experimental design using the analysis of means and orthogonal array was applied to optimize these responses. The removal technique of excess polymer solution, the concentration of PCL in the solution, a heat treatment temperature, and the number of times that the scaffolds were dipped in the solution were chosen as the significant processing variables. The removal technique of excess polymer solution and the number of times that the scaffolds were dipped in the solution showed the major effects on the flexural strength, while the technique for removal of excess polymer solution was found to have the major effect on the mass of the deposited PCL. The optimal conditions for achieving the maximal flexural strength of the coated scaffolds at the minimal amount of the deposited PCL were determined and tested. High-quality porous bioresorbable scaffolds with approximately 19 MPa flexural strength and approximately 0.4 g of the total (PCL coating + calcium phosphates) mass with dimensions of 8.5 mm diameter and 13 mm width were manufactured as a result.

Surface-grafting of phosphates onto a polymer for potential biomimetic functionalization of biomaterials

In the human body, phosphate groups play important roles in signaling and the biological functions of proteins and peptides. Despite the importance of phosphate groups, polymer surfaces have not been directly grafted with phosphate groups by chemical reactions because the usual organic solvents used to graft phosphate groups can dissolve or swell polymers. We focused this study on grafting phosphate groups onto a poly(ethylene-co-acrylic acid) (PEAA) surface in an aqueous solution. O-phospho L-serine and O-phosphoethanolamine were grafted on PEAA surfaces to introduce phosphate groups by activating carboxylic acid groups of PEAA using N-hydroxysuccinimide (NHS) and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) in an aqueous environment. X-ray photoelectron spectroscopy (XPS) was used to elucidate the process by which surface grafting occurs and the process that the phosphate group is cleaved into a phosphate ion and a hydrolyzed molecule at high pH. It was found that under appropriate reaction conditions the phosphate groups could be successfully grafted on the polymer surfaces. The phosphate-grafted polymer surfaces showed lower water contact angles than the initial polymer surfaces likely due to their highly mobile and hydrophilic phosphate side groups. This work demonstrates a technique to successfully graft phosphate groups onto organic polymer surfaces in a biocompatible aqueous environment, which may open new avenues to functionalizing synthetic polymeric and natural macromolecule derived biomaterials.

PEGylation as a tool for the biomedical engineering of surface modified microparticles

Microparticles are of considerable interest for drug delivery, vaccination and diagnostic imaging. In order to obtain microparticles with long circulation times, or to provide the prerequisite for tissue specific targeting through decoration with suitable ligands, their surfaces need to be modified such that they become repellent to the adsorption of opsonic proteins and resistant to unspecific phagocytosis. The currently most considered strategy relies on the immobilisation of a poly(ethylene glycol) (PEG) corona onto the microparticles' surface. In the first chapter of this review, we discuss the unique physicochemical properties of PEG, which make it the polymer of choice to render the surfaces of microparticles repellent to the adsorption of proteins and resistant to cellular recognition. Furthermore, we present various technologies for the preparation of microparticles with PEGylated surfaces. Another aspect is the decoration of the PEGylated surfaces with suitable ligands for cell specific recognition and targeting. Finally, we review miscellaneous applications of PEGylated microparticles, mainly focusing on the fields of drug delivery, targeting and vaccination. Although still in its infancy, the PEGylation of microparticles holds promise towards future biomedical applications.

Bioeliminable nanohydrogels for drug delivery

One of the most significant obstacles for systematic delivery of nanopayloads is the foreign particle clearance by the mononuclear phagocyte system (MPS). The majority of biocompatible nanopayloads with charged groups on their surface cannot fully evade the clearance by MPS during systemic circulation. For safe and effective targeted nanodrug delivery in vivo, we describe a novel approach for evading the macrophage clearance. We demonstrate that neutral and hydrophilic materials can effectively evade the macrophage uptake and also quickly degrade into bioeliminable fragments. We show that there is no opsonization effect and no toxic effect on living cells. In addition, the payloads are stable in an aqueous environment, and they can release drugs in a cellular environment. These results suggest that the unique properties of this kind of payloads may make them useful for designing new drug delivery systems.

Convenient synthesis and properties of polypropyleneimine dendrimer-functionalized polymer nanoparticles

A simple synthesis of polymer core-dendrimer shell nanoparticles (NPs) in the 15-20-nm-diameter range is presented. Amine-terminated polypropyleneimine (PPI) dendrimers DAB-dendri-(NH(2))(4) and DAB-dendri-(NH(2))(16) (DAB4 and DAB16) are covalently attached to the surface of primary polystyrene-based NPs bearing reactive chlorobenzyl groups produced by microemulsion polymerization in the presence of a cationic surfactant. The grafting readily proceeds under mild conditions and leads to translucent aqueous suspensions of core-shell-type NPs with a high density of peripheral amine groups that have been characterized relative to their size and chemical composition. The dendritic shell acts as a protective ionizable outer layer and provides an improvement of the colloidal stability in neutral and acidic media. The metal-binding capacity of the PPI dendrimers is retained, and spectrophotometric titrations show that the dendrimer-grafted NPs can trap a large number of Cu(2+) ions (more than 900 Cu per NP-DAB16). These properties make them potentially valuable templates for the elaboration of hybrid nanomaterials. The reactivity of the external amine groups is used to link covalently azobenzene chromophores (disperse Red 1 residues) through aza-Michael addition in aqueous suspension. This simple method gives access to colored NPs with high dye contents in the outer layer (up to 1000-1500 dye molecules per NP), which indicates that dendrimer-functionalized NPs are valuable building blocks for the construction of multifunctional nanomaterials.

Biological selection of peptides for poly(l-lactide) substrates

Short peptides that recognize the alpha form of poly( l-lactide) (PLLA) crystalline films were identified from a phage-displayed peptide library. An enzyme-linked immunosorbent assay (ELISA) revealed that the apparent binding constants of the phage clones for the alpha form of PLLA were greater than those of the unselected phage library. The specificity index for the alpha form of PLLA referred to a structurally similar atactic poly(methyl methacrylate) (at-PMMA), supporting the alpha form of PLLA specific binding of the selected phage. Amino acid residues with proton-donor lateral groups and hydrophobic alkyl groups were relatively enriched in a sequence of heptapeptides on the specific phage clones, thereby suggesting the presence of hydrogen bonding as well as hydrophobic interactions between the alpha form of PLLA and the peptides. Surface plasmon resonance (SPR) analysis revealed that the binding constant of the freed c22 heptapeptide (Gln-Leu-Met-His-Asp-Tyr-Arg) for the alpha form of PLLA was greater than those for reference at-PMMA, amorphous PLLA, and the beta form of PLLA. It was found that c22 peptide can recognize slight differences in PLLA polymorphs such as a crystalline state and an arrangement of PLLA functional groups.

Ionic polymeric amphiphiles with cholesterol mesogen: adsorption and organization characteristics at the air/water interface from Langmuir film balance studies

Ionic polymeric amphiphiles consisting of cholesterol mesogen were investigated for the interfacial adsorption characteristics at the air/water interface using a Langmuir film balance with an aim to understand the influence of ionic segment from 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) on the packing behavior of cholesterol at the interface. From surface pressure (pi)-area (A) isotherm characteristics, it is demonstrated that the homopolymer and the copolymer C consisting of 0.15 mol fraction CAB segments exhibit the most expanded structures contributing to surface area of about 84 A(2)/molecule. It is shown that the copolymer B with 0.1 mol fraction CAB provides optimum hydrophilic liphophilic balance to form the most compact structures contributing to a surface area of 35.75 A(2)/molecule. The high surface pressure, >40 mN/m, in contrast to that of PAMPS demonstrates significant adsorption of the copolymers at the interface. An interesting correlation among interfacial packing characteristics, thermal behavior, and solution structures is demonstrated. From molecular models developed for CAB, it is shown that the horizontal orientation of the linker group with respect to cholesterol chain in CAB underlies the expanded structures observed in PCAB and copolymer C.

Poly(lactic acid) (PLA) based nanocomposites--a novel way of drug-releasing

In this communication, poly(lactic acid) nanofibers have been fabricated by electrospinning and then poly(lactic acid) (PLA) based nanocomposites have been prepared by accumulating anticancer drug daunorubicin on PLA nanofibers combined with TiO2 nanoparticles. Our atomic force microscopy (AFM) and laser-scanning confocal microscope (LSCM) studies demonstrate that the respective drug molecules could be readily self-assembled on the surface of the blends of nano-TiO2 with PLA polymer nanocomposites, which could further efficiently facilitate the drug permeation and accumulation on the target leukemia K562 cells. Besides, the respective new nanocomposites have good biocompatibility, ease of surface chemistry modification and very high surface area, which may afford the possibility for their promising application in pharmacology and biomedical engineering areas.