Genetic network driven control of PHBV copolymer composition

We developed a detailed mathematical model describing the coupling between the molecular weight distribution dynamics of poly(3-hydroxybutyrate-co-3hydroxyvalerate) (PHBV) copolymer chains with those of hydroxybutyrate (HB) and hydroxyvalerate (HV) monomer formation. Sensitivity analysis of the model revealed that both the monomer composition and the molecular weight distribution of the copolymer chains are strongly affected by the ratio between the rates at which the two-monomer units are incorporated into the chains. This ratio depends on the relative HB and HV availability, which in turn is a function of the expression levels of genes encoding enzymes that catalyze monomer formation. Regulation of gene expression was accomplished through the aid of an artificial genetic network, the patterns of expression of which can be controlled by appropriately tuning the concentration of an extracellular inducer. Extensive simulations were used to study the effects of operating conditions and parameter uncertainties on the range of achievable copolymer compositions. Since the predicted conditions fell in the range of feasible bioprocessing manipulations, it is expected that such strategy could be successfully employed. Thus, the presented model constitutes a powerful tool for designing genetic networks that can drive the formation of PHBV copolymer structures with desirable characteristics.     

Physicochemical and biological characterization of polyethylenimine-graft-poly(ethylene glycol) block copolymers as a delivery system for oligonucleotides and ribozymes

Two different series of polyethylenimine (PEI) block copolymers grafted with linear poly(ethylene glycol) (PEG) were investigated as delivery systems for oligodeoxynucleotides (ODN) and ribozymes. The resulting interpolyelectrolyte complexes were characterized with respect to their physicochemical properties, protection efficiency against enzymatic degradation, complement activation, and biological activity under in vitro conditions. The effect of PEG molecular weight and the graft density of PEG blocks on complex characteristics was studied with two different series of block copolymers. The resulting ODN complexes were characterized by photon correlation spectroscopy (PCS) and laser Doppler anemometry (LDA) to determine complex size and zeta potential. Electrophoresis was performed to study the protective effects of the different block copolymers against enzymatic degradation of ODN. Intact ODN was quantified via densitometric analysis. Ribozymes, a particularly unstable type of oligonucleotides, were used to examine the influence of block copolymer structure on biological activity. The stabilization of ribozymes was also characterized in a cell culture model. Within the first series of block copolymers, the grafted PEG chains (5 kDa) had marginal influence on the complex size. Two grafted PEG chains were sufficient to achieve a neutral zeta potential. Within the second series, size and zeta potential increased with an increasing number of PEG chains. A high number of short PEG chains resulted in a decrease in complex size to values comparable to that of the homopolymer PEI 25 kDa and a neutral zeta potential, indicating a complete shielding of the charges. Complement activation decreased with an increasing number of short PEG 550 Da chains. Ribozyme complexes with PEG-PEI block copolymers achieved a 50% down-regulation of the target mRNA. This effect demonstrated an efficient stabilization and biological activity of the ribozyme, which was comparable to that of PEI 25 kDa. PEGylated PEI block copolymers represent a promising new class of drug delivery systems for ODN and ribozymes with increased biocompatibility and physical stability.     

Facile, efficient approach to accomplish tunable chemistries and variable biodistributions for shell cross-linked nanoparticles

The in vivo behavior of shell cross-linked knedel-like (SCK) nanoparticles is shown to be tunable via a straightforward and versatile process that advances SCKs as attractive nanoscale carriers in the field of nanomedicine. Tuning of the pharmacokinetics was accomplished by grafting varied numbers of methoxy-terminated poly(ethylene glycol) (mPEG) chains to the amphiphilic block copolymer precursors, together with chelators for the radioactive tracer and therapeutic agent (64)Cu, followed by self-assembly into block copolymer micelles and chemical cross-linking throughout the shell regions. (64)Cu-radiolabeling was then performed to evaluate the SCKs in vivo by means of biodistribution experiments and positron emission tomography (PET). It was found that the blood retention of PEGylated SCKs could be tuned, depending on the mPEG grafting density and the nanoparticle surface properties. A semiquantitative model of the density of mPEG surface coverage as a function of in vivo behavior was applied to enhance the understanding of this system.     

Relationship between the structure of amphiphilic copolymers and their ability to disturb lipid bilayers

Nonionic amphiphiles and particularly block copolymers of ethylene oxide and propylene oxide (Pluronics) cause pronounced chemosensitization of tumor cells that exhibit multiple resistance to antineoplastic drugs. This effect is due to inhibition of P-glycoprotein (P-gp) responsible for drug efflux. It was suggested that the inhibition of P-gp might be due to changes in its lipid surrounding. Indeed, high dependence of P-gp activity on the membrane microviscosity was demonstrated [Regev et al. (1999) Eur. J. Biochem. 259, 18-24], suggesting that the ability of Pluronics to affect the P-gp activity is mediated by their effect on the membrane structure. We have found recently that adsorption of Pluronics on lipid bilayers induced considerable disturbance of the lipid packing [Krylova et al. (2003) Chemistry 9, 3930-3936]. In the present paper, we studied 19 amphiphilic copolymers, including newly synthesized hyperbranched polyglycerols, Pluronic and Brij surfactants, for their ability to accelerate flip-flop and permeation of antitumor drug doxorubicin (DOX) in liposomes. It was found that not only bulk hydrophobicity but also the chemical microstructure of the copolymer determines its membrane disturbing ability. Copolymers containing polypropylene oxide caused higher acceleration of flip-flop and DOX permeation than polysurfactants containing aliphatic chains. The effects of copolymers containing hyperbranched polyglycerol "corona" were more pronounced, as compared to the copolymers with linear poly(ethylene oxide) chains, indicating that a bulky hydrophilic block induces additional disturbances in the lipid bilayer. A good correlation between the copolymer flippase activity and a linear combination of copolymer bulk hydrophobicity and the van der Waals volume of its hydrophobic block was found. The relationship between the structure of a copolymer and its ability to disturb lipid membranes presented in this paper may be useful for the design of novel amphiphilic copolymers capable of affecting the activity of membrane transporters in living cells.     

Synthesis and characterization of self-assembling block copolymers containing bioadhesive end groups

3,4-Dihydroxyphenyl-L-alanine (DOPA) is an unusual amino acid found in mussel adhesive proteins (MAPs) that is believed to lend adhesive characteristics to these proteins. In this paper, we describe a route for the conjugation of DOPA moieties to poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) block copolymers. Hydroxyl end groups of PEO-PPO-PEO block copolymers were activated by N,N'-disuccinimidyl carbonate and then reacted with DOPA or its methyl ester with high coupling efficiencies from both aqueous and organic solvents. DOPA-modified PEO-PPO-PEO block copolymers were freely soluble in cold water, and dye partitioning and differential scanning calorimetry analysis of these solutions revealed that the copolymers aggregated into micelles at a characteristic temperature that was dependent on block copolymer composition and concentration in solution. Oscillatory rheometry demonstrated that above a block copolymer concentration of approximately 20 wt %, solutions of DOPA-modified PEO-PPO-PEO block copolymers exhibited sol-gel transitions upon heating. The gelation temperature could be tailored between approximately 23 and 46 degrees C by changing the composition, concentration, and molecular weight of the block copolymer. Rheological measurement of the bioadhesive interaction between DOPA-modified Pluronic and bovine submaxillary mucin indicated that DOPA-modified Pluronic was significantly more bioadhesive than unmodified Pluronic.     

Amphiphilic block copolymer/poly(dimethylsiloxane) (PDMS) blends and nanocomposites for improved fouling-release

Amphiphilic diblock copolymers, Sz6 and Sz12, consisting of a poly(dimethylsiloxane) block (average degree of polymerisation?=?132) and a PEGylated-fluoroalkyl modified polystyrene block (Sz, average degree of polymerisation?=?6, 12) were prepared by atom transfer radical polymerization (ATRP). Coatings were obtained from blends of either block copolymer (1-10 wt%) with a poly(dimethylsiloxane) (PDMS) matrix. The coating surface presented a simultaneous hydrophobic and lipophobic character, owing to the strong surface segregation of the lowest surface energy fluoroalkyl chains of the block copolymer. Surface chemical composition and wettability of the films were affected by exposure to water. Block copolymer Sz6 was also blended with PDMS and a 0.1 wt% amount of multiwall carbon nanotubes (CNT). The excellent fouling-release (FR) properties of these new coatings against the macroalga Ulva linza essentially resulted from the inclusion of the amphiphilic block copolymer, while the addition of CNT did not appear to improve the FR properties.     

Amphiphilic block copolymer/poly(dimethylsiloxane) (PDMS) blends and nanocomposites for improved fouling-release

Amphiphilic diblock copolymers, Sz6 and Sz12, consisting of a poly(dimethylsiloxane) block (average degree of polymerisation = 132) and a PEGylated-fluoroalkyl modified polystyrene block (Sz, average degree of polymerisation = 6, 12) were prepared by atom transfer radical polymerization (ATRP). Coatings were obtained from blends of either block copolymer (1–10 wt%) with a poly(dimethylsiloxane) (PDMS) matrix. The coating surface presented a simultaneous hydrophobic and lipophobic character, owing to the strong surface segregation of the lowest surface energy fluoroalkyl chains of the block copolymer. Surface chemical composition and wettability of the films were affected by exposure to water. Block copolymer Sz6 was also blended with PDMS and a 0.1 wt% amount of multiwall carbon nanotubes (CNT). The excellent fouling-release (FR) properties of these new coatings against the macroalga Ulva linza essentially resulted from the inclusion of the amphiphilic block copolymer, while the addition of CNT did not appear to improve the FR properties.     

Bacterial synthesis of PHA block copolymers

Polyhydroxyalkanoates (PHA) containing block copolymers were synthesized in Cupriavidus necator using periodic substrate addition. Poly(3-hydroxybutyrate) (PHB) segments were formed during fructose utilization. Pulse feeds of pentanoic acid resulted in the synthesis of 3-hydroxyvalerate monomers, forming poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) random copolymer. PHA synthesis was controlled using analysis of oxygen uptake and carbon evolution rates from the bioreactor off-gas. A combination of characterization techniques applied to the polymer batches strongly suggests the presence of block copolymers: (i) Thermodynamically stable polymer samples obtained by fractionation and analyzed by differential scanning calorimetry (DSC) and nuclear magnetic resonance spectroscopy (NMR) indicate that some fractions, representing approximately 30% of the total polymer sample, exhibit melting characteristics and nearest-neighbor statistics indicative of block copolymers, (ii) preliminary rheology experiments indicate additional mesophase transitions only found in block copolymer materials, (iii) dynamic mechanical analysis shows extension of the rubbery plateaus in block copolymer samples, and (iv) uniaxial extension tests result in differences in mechanical properties (modulus and elongation at failure) expected of similarly prepared block copolymer and single polymer type materials.     

Monomolecular assembly of siRNA and poly(ethylene glycol)-peptide copolymers

In this work, we design and investigate the complex formation of highly uniform monomolecular siRNA complexes utilizing block copolymers consisting of a cationic peptide moiety covalently bound to a poly(ethylene glycol) (PEG) moiety. The aim of the study was to design a shielded siRNA construct containing a single siRNA molecule to achieve a sterically stabilized complex with enhanced diffusive properties in macromolecular networks. Using a 14 lysine-PEG (K14-PEG) linear diblock copolymer, formation of monomolecular siRNA complexes with a stoichiometric 1:3 grafting density of siRNA to PEG is realized. Alternatively, similar PEGylated monomolecular siRNA particles are achieved through complexation with a graft copolymer consisting of six cationic peptide side chains bound to a PEG backbone. The hydrodynamic radii of the resulting complexes as measured by fluorescence correlation spectroscopy (FCS) were found to be in good agreement with theoretical predictions using polymer brush scaling theory of a PEG decorated rodlike molecule. It is furthermore demonstrated that the PEG coating of the siRNA-PEG complexes can be rendered biodegradable through the use of a pH-sensitive hydrazone or a reducible disulfide bond linker between the K14 and the PEG blocks. To model transport under in vivo conditions, diffusion of these PEGylated siRNA complexes is studied in various charged and uncharged matrix materials. In PEG solutions, the diffusion coefficient of the siRNA complex is observed to decrease with increasing polymer concentration, in agreement with theory of probe diffusion in semidilute solutions. In charged networks, the behavior is considerably more complex. FCS measurements in fibrin gels indicate complete dissociation of the diblock copolymer from the complex, while transport in collagen solutions results in particle aggregation.     

Temperature sensitization of liposomes by use of thermosensitive block copolymers synthesized by living cationic polymerization: effect of copolymer chain length

We prepared block copolymers of (2-ethoxy)ethoxyethyl vinyl ether (EOEOVE) and octadecyl vinyl ether (ODVE) with the number average molecular weights of 6900, 9300, and 16 700 by living cationic polymerization. The poly(EOEOVE) block acts as a temperature-sensitive moiety, and the poly(ODVE) block acts as an anchor moiety. We also investigated the effect of chain length of the copolymer poly(EOEOVE) block on the ability to sensitize liposomes. The copolymers underwent a coil-globule transition at approximately 36 degrees C in the presence of a membrane of egg yolk phosphatidylcholine (EYPC), detected using differential scanning calorimetry (DSC). Liposomes encapsulating calcein, a water-soluble fluorescent dye, were prepared from mixtures of dioleoylphosphatidylethanolamine, EYPC, and the copolymers. While the copolymer-modified liposomes released little calcein below 30 degrees C, release was enhanced above 35 degrees C, indicating that dehydrated copolymer chains destabilized the liposome membrane. In addition, copolymers with a longer poly(EOEOVE) block induced a more drastic enhancement of contents release in a narrow temperature region near the transition temperature of the poly(EOEOVE) block. As a result, the copolymer with an average molecular weight of 16 700 generated highly sensitive liposomes that produced rapid and dramatic release of the contents in response to temperature.     

Drug release from and hydrolytic degradation of a poly(ethylene glycol) grafted poly(3-hydroxyoctanoate)

Monoacrylate-poly(ethylene glycol)-grafted poly(3-hydroxyoctanoate) (PEGMA-g-PHO) copolymers were synthesized to develop a swelling-controlled release delivery system for ibuprofen as a model drug. The in vitro hydrolytic degradation of and the drug release from a film made of the PEGMA-g-PHO copolymer were carried out in a phosphate buffer saline (pH 7.4) medium. The hydrolytic degradation of the copolymer was strongly dependent on the degree of grafting (DG) of the PEGMA group. The degradation rate of the copolymer films in vitro increased with increasing DG of the PEGMA group on the PHO chain. The copolymer films showed a controlled delivery of ibuprofen to the medium in periods of time that depend on the composition, hydrophilic/hydrophobic characteristics, initial drug loading amount and film thickness of the graft copolymer support. The drug release rate from the grafted copolymer films was faster than the rate of weight loss of the films themselves. In particular, a combination of the low DG of the PEGMA group in the PHO chains with the low ibuprofen solubility in water led to long-term constant release from these matrices in vitro.     

Preparation of magnetic microspheres from water-in-oil emulsion stabilized by block copolymer dispersant

A new method for the preparation of magnetic microspheres is reported. The preparation involved first the dispersion of an aqueous phase, containing magnetite nanoparticles and a water-soluble homopolymer, into droplets in an organic medium using an amphiphilic block copolymer as the dispersant. This was followed by water distillation at a raised temperature from the aqueous droplets to yield polymer/magnetite particles. The structure of the particles was then locked in by a reagent being added to cross-link the water-soluble copolymer block and homopolymer. Since the hydrophobic block of the copolymer consisted of a protected polyester, the removal of the protective moieties from the coronal chains yielded poly(acrylic acid) or other functional polymers to render water dispersibility to the spheres and to enable biomolecule immobilization.     

Graph theoretical approach to the mechanical strength of polymers

In this research work, a graph theoretical approach has been introduced to find a mathematical model for the change of hardness of weakly cross-linked polymer network systems with the change of configurations. The polymers studied are (i) poly(methyl methacrylate), (ii) styrene–acrylonitrile copolymer and (iii) polystyrene. In very weakly cross-linked polymer network systems, both the chain entanglement and the network rigidity competitively contribute to the hardness of polymer network. The average length (i.e. molecular weight), and length distribution of chains between the cross-linking sites both play important roles in network rigidity. The former can be changed by the mole fraction of the cross-linker, and the latter by the molecular weight of the prepolymer formed, which can be controlled by the late addition of the cross-linker. Graph theoretical approach introduces simplifications to the dynamics of polymer chains and polymer network, and thus can explain the change of hardness with the change of chain statistics. It was shown that there was a very good agreement between the theoretical equations and the experimental hardness values of polymer network systems studied. It was also found that there were scaling relations between the parameters used in the theoretical equation.     

Size control of styrene oxide-ethylene oxide diblock copolymer aggregates with classical surfactants: DLS, TEM, and ITC study

The interactions between the diblock copolymer S(15)E(63) and the surfactants sodium dodecyl sulfate (SDS), sodium decyl sulfate (SDeS), and sodium octyl sulfate (SOS) have been investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM), and isothermal titration calorimetry (ITC). The surfactants with the same headgroup differentiate in their chain length. At 20 degrees C, the block copolymer is associated into micelles with a hydrodynamic radius of 11.6 nm, which is composed of a hydrophobic styrene oxide (S) core and a water-swollen oxypolyethylene (PEO or E) corona. The different copolymer/surfactant systems have been studied at a constant copolymer concentration of 2.5 g dm(-3) and in a vast range of surfactant concentrations, from 7.5 x 10(-6) up to 0.75 M. When SDS and SDeS are added to the block copolymer solution, different regions are observed in the DLS data: at low surfactant concentrations (c < 1.0 x 10(-4) M), single surfactant molecules associate with the copolymer micelle, probably the former being solubilized in the micelle core, leading to a certain disruption of the mixed micelle due to repulsive electrostatic interactions between surfactant headgroups followed by a stabilization of the mixed micelle. At higher concentrations (1.0 x 10(-4) < c < 0.1 M), two types of copolymer-surfactant complexes coexist: one large copolymer-rich/surfactant complex and one small complex consisting of one or a few copolymer chains and rich in surfactants. At higher SDS and SDeS concentrations, complete disintegration of mixed micelles takes place. In contrast, SOS-S(15)E(63) interactions are less important up to surfactant concentrations of 0.05 M due to its higher hydrophilicity, reducing the hydrophobic interactions between surfactant alkyl chains and copolymer micelles. At concentration larger than the critical aggregation concentration (cac) of the system, 0.05 M, disruption of copolymer micelles occurs. These regions have been confirmed by transmission electron microscopy. On the other hand, the titration calorimetric data for SDS and SDeS present an endothermic increase indicating the formation of mixed copolymer-rich-surfactant micelles. From that point, important differences in the ITC plot for both surfactants are present. However, the ITC curve obtained after titration of a SOS solution in the copolymer solution is quite similar to that of its titration in water.     

Biosynthesis and characterization of polyhydroxyalkanoate block copolymer P3HB-b-P4HB

Polyhydroxyalkanoates (PHA) synthesis genes phbC and orfZ cloned from Ralstonia eutropha H16 were transformed into beta-oxidation weakened Pseudomonas putida KTOY08ΔGC, a mutant of P. putida KT2442. The recombinant P. putida strain termed KTHH06 was able to produce a short-chain-length PHA block copolymer consisting of poly(3-hydroxybutyrate) (P3HB) as one block and poly(4-hydroxybutyrate) (P4HB) as another block. One-dimensional and two-dimensional nuclear magnetic resonance (NMR) clearly indicated the polymer was a diblock copolymer consisting of 20 mol % P3HB as one block and 80 mol % P4HB as another one. Differential scanning calorimetric (DSC) showed that P3HB block melting temperatures (T(m)) in the block copolymer P3HB-b-P4HB was shift to low temperature compared with homopolymer P3HB and a blend of P3HB and P4HB. The block copolymer with a number average molecular weight of 50000 Da and a polydispersity of 3.1 demonstrated a better yield and tensile strength compared with that of its related random copolymer and blend of homopolymers of P3HB and P4HB.     

Synthesis and characterization of a head-tail type polycation block copolymer as a nonviral gene vector

A head-tail type polycation block copolymer, which is composed of the polyamidoamine (PAMAM) dendron and poly(L-lysine) (PLL) blocks, was newly designed as a nonviral gene vector in this study. This block copolymer (PAMAM dendron-PLL) was successfully synthesized in two steps: the synthesis of the PAMAM dendron block and the polymerization of the PLL block from the PAMAM dendron block. PAMAM dendron and PLL blocks in block copolymer showed independent deprotonation behavior, and their pK(a) were determined to be 6.8 and 9.0, respectively. The complexation with pDNA was evaluated by gel retardation assay and dye exclusion assay, and both assays indicated that pDNA was selectively complexed with PLL block of block copolymer. Also, the PAMAM dendron-PLL poplyplexes showed 10(2) fold higher transfection efficiency to HeLa cells as that for PLL polyplexes. This might be due to the buffering effect of the PAMAM dendron block. This block copolymer could produce a function share in each block, i.e., tail block complexed with pDNA and head block showed a buffering effect. This molecular design of the head-tail type block copolymer might provide a new approach for realizing in vivo gene therapy.     

Expression of a cascading genetic network within liposomes

Liposomes have long been used as possible compartments for artificial cells, and it has been shown that liposomes can sustain various types of biochemical reactions. To elevate the degree of molecular complexity of the system in liposomes, we have constructed a two-stage genetic network encapsulated in liposomes. This two-stage genetic network was constructed with the plasmid pTH, in which the protein product of the first stage (T7 RNA polymerase) is required to drive the protein synthesis of the second stage (GFP). We show that the two-stage genetic network constructed in a cell-free expression system is functional within liposomes.     

Acid-labile core cross-linked micelles for pH-triggered release of antitumor drugs

Micelles of a model amphiphilic block copolymer, poly(hydroxyethyl acrylate)-block-poly(n-butyl acrylate) (PHEA-b-PBA), synthesized via the RAFT polymerization were cross-linked by copolymerization of a degradable cross-linker from the living RAFT-end groups of PBA chains, yielding a cross-linked core without affecting significantly the original micelle size. The cross-linker incorporation into the micelles was evidenced via physicochemical analysis of the copolymer unimers formed upon acidic cleavage of the cross-linked micelles. High doxorubicin loading capacities (60 wt %) were obtained. Hydrolysis of less than half of the cross-links in the core was found to be sufficient to release doxorubicin faster at acidic pH compared to neutral pH. The system represents the first example of core-cross-linked micelles that can be destabilized (potentially both above and below CMC) by the pH-dependent cleavage of the cross-links and the subsequent polarity change in the core to enable the release of hydrophobic drugs entrapped inside the micelle.     

The use of Hydrophobic Dye Molecules in Monitoring the Liposome Bilayer Microenvironment and Locating Block Copolymers Added to Enhance Liposome Steric Stability

Abstract

The dispersion of soybean lecithin in water leads to the formation of multilamellar vesicles (MLVs), which on sonication (4hrs approx.) break down to small unilamellar vesicles of ～ 50nm diameter. The addition of polymeric molecules in the liposomal system provides increased steric stabilization. The molecules used were (tri-)block copolymers (Synperonics) containing a central hydrophobic part (polypropylene oxide-PPO) and two hydrophillic chains (polyethylene oxide-PEO) extending from either side. The interaction of these molecules with the vesicle bilayer is thought to be of upmost importance to the mechanical stress, thermodynamic restrictions and steric stability that may be induced. The exact localisation of the copolymer molecules was attempted using a multiprobe technique. The full spectrum of two hydrophobic dyes, namely Nile red (NIL) and Pinacyanol chloride (PCYN), were compared while solubilized inside the liposome bilayer. The sensitivity of their spectral characteristics to polarity and molecular mobility produced a monitor of the bilayer micropolarity and fluidity. The relatively high hydrophobicity of Nile red (NIL) provides an accurate polarity sensor of the bilayer microenvironment. The formation of Pinacyanol chloride (PCYN) dimers (and their respective peak) was directly related to the distance between the dye molecules. Shifts of the maximum absorbance (X_max) for both dyes showed that the bilayer environment was becoming more apolar with increasing copolymer concentration. The absorbance intensity decreased with increasing copolymer concentration, denoting a reduction in the solubilization of both dyes and therefore of the bilayer population. The absorbance peak of Pinacyanol chloride (PCYN) due to dimer formation increased at moderate copolymer concentrations, showing signs of possible incorporation inside the bilayer. These experiments provided information about the bilayer structure. Adding block copolymers at an optimum concentration may increase the stability of the liposome by incorporation, following various models proposed. However, at high content of copolymer some bilayer solubilization and mixed micelle formation may occur.     

Addition of Block Copolymers to Liposomes Prepared Using Soybean Lecithin. Effects on Formation,Stability and the Specific Localization of the Incorporated Surfactants Investigated

Abstract

Soybean lecithin disperses into water forming multilamellar liposomes, which on sonication produce vesicles of the order of 40–50nm (diameter), as determined by Photon Correlation Spectroscopy (PCS). The effect of concentration of lecithin and sonication time was systematically investigated. Vesicles were then prepared by incorporation of A – B – A block copolymers of polyethylene oxide (PEO) and polypropylene oxide(PPO), i.e.(PEO-PPO-PEO), in order to construct systems of increased steric stability. The effect of the molecular weight of the PEO and PPO chains on the vesicle size was systematically studied by using various molecules to prepare the vesicles. Initial addition of these (tri-)block copolymers causes an increase in the size of the vesicles. This increase continues until a certain concentration of block copolymer is reached, after which a decrease in size is observed. The initial increase was thought to be due to the incorporation of the block copolymer onto the vesicle bilayer. The reduction at high surfactant concentration is thought to be due to solubilization of the bilayer and the ultimate breakdown of the vesicles. Electrophoresis experiments showed a reduction in the ξ-potential of the vesicles on incorporation of the block copolymer which can be attributed to the shift of the shear plane. Various models are presented to describe this incorporation. The vesicles prepared using the block copolymers are believed to enhance the steric effects and so lead to more stable and pharmaceutically optimum systems.