Biodegradable microspheres for protein delivery

Role of particle size in phagocytosis of polymeric microspheres

PURPOSE

Polymeric microspheres are extensively researched for applications in drug and vaccine delivery. However, upon administration into the body, microspheres are primarily cleared via phagocytosis by macrophages. Although numerous studies have reported on the biochemical pathways of phagocytosis, relatively little is known about the dependence of phagocytosis on particle size. Here, we investigate the previously unexplained dependence of phagocytosis on particle size.

METHODS

Rat alveolar macrophages and IgG-opsonized and non-opsonized polystyrene microspheres were used as model macrophages and drug delivery particles. Phagocytosis, attachment and internalization were measured by flow cytometry and time-lapse video microscopy.

RESULTS

Particles possessing diameters of 2-3 microm exhibited maximal phagocytosis and attachment. Rate of internalization, however, was not affected significantly by particle size. Maximal attachment of 2-3 microm microspheres is hypothesized to originate from the characteristic features of membrane ruffles in macrophages. Elimination of ruffles via osmotic swelling nearly eliminated the peculiar size-dependence of phagocytosis. A simple mathematical model is presented to describe the dependence of phagocytosis on particle size.

CONCLUSIONS

The dependence of phagocytosis on particle size originated primarily from the attachment step. These results reveal the importance of controlling drug delivery particle size distribution and selecting the size appropriate for avoiding or encouraging phagocytosis.

Effects of process and formulation parameters on characteristics and internal morphology of poly(d,l-lactide-co-glycolide) microspheres formed by the solvent evaporation method

Taking ABT627 as a hydrophobic model drug, poly-(lactic-co-glycolic acid) (PLGA) microspheres were prepared by an emulsion solvent evaporation method. Various process parameters, such as continuous phase/dispersed phase (CP/DP) ratio, polymer concentration, initial drug loading, polyvinyl alcohol concentration and pH, on the characteristics of microspheres and in vitro drug release pattern of ABT627 were investigated. Internal morphology of the microspheres was observed with scanning electron microscopy by stereological method. CP/DP is a critical factor in preparing microspheres and drug loading increased significantly with increasing CP/DP ratios accompanied by a remarkably decreased burst release. At CP/DP ratio 20, microspheres with a core-shell structure were formed and the internal porosity of the microspheres decreased with increasing CP/DP ratio. Increase in PLGA concentration led to increased particle sizes and decreased drug release rates. ABT627 release rate increased considerably with increasing PVA concentrations in the continuous phase from 0.1% to 0.5%. The maximum solubility of ABT627 in PLGA was approximately 30%, under which ABT627 was dispersed in PLGA matrix in a molecular state. Increase in initial drug loading had no significant influence on particle size, drug encapsulation efficiency, burst release and internal morphology. However, drug release rate decreased at higher drug loading. Independent of process parameters, ABT627 was slowly released from the PLGA microspheres over 30 days, by a combination of diffusion and polymer degradation. During the first 13 days, ABT627 was mainly released by the mechanism of diffusion demonstrated by the unchanged internal morphology. In contrast, a core-shell structure of the microspheres was observed after being incubated in the release medium for 17 days, independent of drug loading, implying that the ABT627/PLGA microspheres degraded by autocatalytic effect, starting from inside of the matrix. In conclusion, hydrophobic drug release from the PLGA microspheres is mainly dependent on the internal morphology and drug distribution state in the microspheres.

Synthesis and Characterization of Biodegradable Networks Providing Saturated-Solution Prolonged Delivery

Numerous peptide drugs require continuous and local delivery to obtain optimum therapeutic effect. Herein, we describe the incorporation of a model peptide drug, vitamin B12, as well as goserelin acetate, in biodegradable elastomer cylinders through photo-cross-linking. The elastomer was prepared from acrylated star-poly(epsilon-caprolactone-co-D,L-lactide). Release was manipulated through the incorporation of poly(ethylene glycol) diacrylate (PEGD) into the network at concentrations up to 30% (w/w). The PEGD in the network caused rapid swelling that remained constant throughout the release period. The degree of swelling was low, ranging from 10 to 45% (w/w), and increasing as the PEGD content increased. Release proceeded with a minimal initial burst, and extended periods of nearly constant release, ranging from approximately 5 to 70% mass fraction released, were obtained. The release rate was independent of particle size and increased as the cylinder diameter decreased, as the amount of PEGD increased, as the molecular weight of PEGD increased, and as the agent loading increased. Moreover, goserelin acetate, which has a comparable diffusivity but greater aqueous solubility, was released at a greater rate than vitamin B12. This release behavior is explained as a balance between agent dissolution in the swollen polymer matrix and diffusion through the polymer matrix bulk.

Controlled drug delivery in tissue engineering

The concept of tissue and cell guidance is rapidly evolving as more information regarding the effect of the microenvironment on cellular function and tissue morphogenesis become available. These disclosures have lead to a tremendous advancement in the design of a new generation of multifunctional biomaterials able to mimic the molecular regulatory characteristics and the three-dimensional architecture of the native extracellular matrix. Micro- and nano-structured scaffolds able to sequester and deliver in a highly specific manner biomolecular moieties have already been proved to be effective in bone repairing, in guiding functional angiogenesis and in controlling stem cell differentiation. Although these platforms represent a first attempt to mimic the complex temporal and spatial microenvironment presented in vivo, an increased symbiosis of material engineering, drug delivery technology and cell and molecular biology may ultimately lead to biomaterials that encode the necessary signals to guide and control developmental process in tissue- and organ-specific differentiation and morphogenesis.

Theoretical and experimental investigations on the size of alginate microspheres prepared by dropping and spraying

In order to produce functional microspheres with different ranges of sizes for various applications, the size of alginate droplets prepared by dropping and spraying was studied. It was shown that the mean diameter could be controlled by liquid flow velocity and applied voltage as operating parameters using a conventional dropping and an electrostatic dropping method, separately. The formation mechanism of alginate droplets could be categorized into two different modes: Dripping mode and jetting mode. By employing an effective force analysis, the diameters in each modes showed to be well agreed with the numerical simulation within 7% deviations. It was testified that the initial amount of surface charges had a high impact on droplet diameter and the liquid flow velocity played a more important role on mean diameter of alginate droplets by electrostatic dropping method in dripping mode than in jetting mode. Then, an empirical equation and a semi-empirical model were used to simulate the diameter of droplets obtained by spraying and spraying with electrostatic field (SEF) method, respectively. The decrease in diameter was more sensitive to the increase of gas flow rate than to the decrease of liquid flow rate, and the results of two models fitted well with experimental values. The simulations showed that SEF yielded a 20% lower on droplet diameter than simple spraying method.

Pharmaceutical particle engineering via spray drying

This review covers recent developments in the area of particle engineering via spray drying. The last decade has seen a shift from empirical formulation efforts to an engineering approach based on a better understanding of particle formation in the spray drying process. Microparticles with nanoscale substructures can now be designed and their functionality has contributed significantly to stability and efficacy of the particulate dosage form. The review provides concepts and a theoretical framework for particle design calculations. It reviews experimental research into parameters that influence particle formation. A classification based on dimensionless numbers is presented that can be used to estimate how excipient properties in combination with process parameters influence the morphology of the engineered particles. A wide range of pharmaceutical application examples—low density particles, composite particles, microencapsulation, and glass stabilization—is discussed, with specific emphasis on the underlying particle formation mechanisms and design concepts.

Controlled delivery of a hydrophilic drug from a biodegradable microsphere system by supercritical anti-solvent precipitation technique

The purpose of this study was to prepare microspheres loaded with hydrophilic drug, bupivacaine HCl using poly(D,L-lactic-co-glycolic acid) (PLGA) and poly(L-lactic acid) (PLLA). Microspheres were prepared with varying the PLGA/PLLA ratio with two different levels of bupivacaine HCl (5 and 10%) using a supercritical anti-solvent (SAS) technique. Microspheres ranging from 4-10 microm in geometric mean diameter could be prepared, with high loading efficiency. Powder X-ray diffraction (PXRD) revealed that bupivacaine HCl retained its crystalline state within the polymer and was present as a dispersion within the polymer phase after SAS processing. The release of bupivacaine HCl from biodegradable polymer microspheres was rapid up to 4 h, thereafter bupivacaine HCl was continuously and slowly released for at least 7 days according to the PLGA/PLLA ratio and the molecular weight of PLLA.

Effect of excipients on the encapsulation efficiency and release of human growth hormone from dextran microspheres

The possibility was investigated to modulate the encapsulation efficiency and release of human growth hormone (hGH) from hydroxyl ethyl methacrylated dextran (dex-HEMA) hydrogel microspheres by using excipients. Microspheres were prepared by polymerization of dex-HEMA in an aqueous two-phase system of this polymer and PEG with or without excipients (Tween 80, pluronic F68, sucrose, NaCl, urea or methionine). High hGH encapsulation efficiencies (50-70%) were obtained for microspheres prepared without excipients and with Tween 80, NaCl or methionine. Substantially lower encapsulation efficiencies (27% and 19%, respectively) were obtained for microspheres prepared in the presence of sucrose and urea, which was attributed to the more favoured partitioning of hGH over the PEG-phase due to higher hydrophobicity of the (partly) denatured hGH. Likely, differences in precipitate size of the encapsulated hGH resulted in different release profiles between microspheres prepared without excipients (biphasic release: 2 days delay time followed by 6 days release) and the release profile for microspheres prepared with Tween 80, pluronic F68, sucrose, NaCl and urea (release over a period of 6-8 days (without a delay time)). Microspheres prepared with methionine showed a concentration-dependent delay time varying from 0 to 2 days followed by almost zero-order release over 6 days, attributed to the effect of methionine on the polymerization of dex-HEMA. Especially, Tween 80 and methionine are attractive excipients since hGH was encapsulated in high yield (60-70%) and the protein was released from the microspheres mainly in its monomeric form without a delay time and with an almost zero-order release over 6-8 days.

Controlled release of insulin from PLGA nanoparticles embedded within PVA hydrogels

A simple and versatile delivery platform for peptide and protein based on physically cross-linked poly (vinyl alcohol) (PVA) hydrogels containing insulin-loaded poly (lactic-co-glycolic acid) (PLGA) nanoparticles was successfully fabricated. The particle morphology and size were characterized by SEM and laser light scattering method, respectively. Results showed that these particles had a mean diameter of 615 nm with a narrow size distribution and homogeneous particle production. The protein encapsulation efficiency was 72.6%. When insulin-loaded PLGA nanoparticles were administered intraperitoneally as a single dose (20 U/kg) to streptozotocin-induced diabetic mouse, blood glucose levels of these mice decreased and it could be sustained at such levels over 24 h. In vitro release further indicated that entrapment of the nanoparticles into the PVA hydrogels causes a reduction in both the release rate and the total amount of insulin released, which suggesting that PLGA nanoparticles entrapped into the PVA hydrogels showed more suitable controlled release kinetics for protein delivery.

Characterization of nanomaterial dispersion in solution prior to in vitro exposure using dynamic light scattering technique

The need to characterize nanoparticles in solution before assessing the in vitro toxicity is a high priority. Particle size, size distribution, particle morphology, particle composition, surface area, surface chemistry, and particle reactivity in solution are important factors which need to be defined to accurately assess nanoparticle toxicity. Currently, there are no well-defined techniques for characterization of wet nanomaterials in aqueous or biological solutions. Previously reported nanoparticle characterization techniques in aqueous or biological solutions have consisted of the use of ultra-high illumination light microscopy and disc centrifuge sedimentation; however, these techniques are limited by the measurement size range. The current study focuses on characterizing a wide range of nanomaterials using dynamic light scattering (DLS) and transmission electron microscopy, including metals, metal oxides, and carbon-based materials, in water and cell culture media, with and without serum. Cell viability and cell morphology studies were conducted in conjunction with DLS experiments to evaluate toxicological effects from observed agglomeration changes in the presence or absence of serum in cell culture media. Observations of material-specific surface properties were also recorded. It was also necessary to characterize the impact of sonication, which is implemented to aid in particle dispersion and solution mixture. Additionally, a stock solution of nanomaterials used for toxicology studies was analyzed for changes in agglomeration and zeta potential of the material over time. In summary, our results demonstrate that many metal and metal oxide nanomaterials agglomerate in solution and that depending upon the solution particle agglomeration is either agitated or mitigated. Corresponding toxicity data revealed that the addition of serum to cell culture media can, in some cases, have a significant effect on particle toxicity possibly due to changes in agglomeration or surface chemistry. It was also observed that sonication slightly reduces agglomeration and has minimal effect on particle surface charge. Finally, the stock solution experienced significant changes in particle agglomeration and surface charge over time.

In Situ-Forming Oleogel Implant for Rivastigmine Delivery

PURPOSE

To provide a simplified dosing schedule and potentially reduce side effects associated to peak plasma concentrations, an in situ-forming oleogel implant was studied for the sustained-release of rivastigmine.

MATERIALS AND METHODS

The gel was prepared by dissolving 5-10% (w/w) N-stearoyl L: -alanine methyl ester (SAM) organogelator in safflower oil containing either dissolved rivastigmine or its dispersed hydrogen tartrate salt. Rheological analysis, differential scanning calorimetry, and infrared spectroscopy were carried out to assess the impact of drug incorporation on the oleogel; this was followed by in vitro and in vivo release studies.

RESULTS

A weakening of intermolecular interactions was suggested by gel-sol transition temperature drops of 10-15 degrees C upon incorporation of dissolved drug. Meanwhile, the dispersed drug salt induced minimal or no changes in transition temperature. Gels containing dispersed rivastigmine had the lowest burst in vitro (<15% in 24 h). In vivo, the 10% SAM formulation containing dispersed rivastigmine provided prolonged drug release within the therapeutic range for 11 days, with peak plasma levels well below the toxic threshold and up to five times lower than for the control formulation.

CONCLUSIONS

This study established SAM gels to be a promising option for sustained-release formulations in the treatment of Alzheimer's Disease.

Immobilization and bioactivity of glucose oxidase in hydrogel microspheres formulated by an emulsification–internal gelation–adsorption–polyelectrolyte coating method

The purpose of this study was to develop a novel microsphere formulation of glucose oxidase (GOX) with high drug loading, encapsulation efficiency and bioactivity. GOX was encapsulated in alginate/chitosan microspheres (ACMS) using an emulsification-internal gelation, followed by GOX adsorption and polyelectrolyte coating method. The factors influencing GOX loading, encapsulation efficiency and activity of the loaded GOX were investigated. The resultant ACMS in wet state were spherical with a mean diameter of about 138 microm. GOX loading was found to be pH dependent. High GOX loading and encapsulation efficiency were achieved when the pH of the adsorption medium was lower than the isoelectric point (pI) of GOX. GOX loading and encapsulation efficiency increased with increasing GOX concentration in the loading solution, but decreased with increasing chitosan concentration in the coating solution. The activity of loaded GOX increased and then decreased with increasing chitosan concentration. The activity of GOX in ACMS was maintained and showed sustained production of H(2)O(2) as compared to free GOX. Around 90% of the original activity of immobilized GOX remained after lyophilization and storage at -20 degrees C for a month. These results suggest that the ACMS and the fabrication method are suitable for microencapsulation of proteins like GOX.

Improvement of a bovine serum albumin microencapsulation process by screening design

The first objective of this study was to prepare microspheres containing a model protein by double emulsion-solvent evaporation/extraction method. This method was modified to consider the fragile nature of proteins. These modifications related to the reduction of polymer loss, of agitation duration and of contact time between protein and solvent. The polymer used was poly(epsilon-caprolactone) and the model protein was bovine serum albumin. The control of the microsphere properties constituted a second objective of this project. A screening design methodology was used to evaluate the effects of the process and formulation variables on microsphere properties. Twelve operating factors were retained, and the particle properties considered were the mean size, the encapsulation efficiency, and the surface state. The statistical analysis of the results allowed determining the most influent factors. Considering the whole results, it appeared that the polymer concentration, the osmotic pressure equilibrium and the volume of the inner, outer and organic phases were the most important parameters. Following this screening study, it was possible to produce particles of small size with high entrapment efficiency (near to 80%) and smooth surface. A good batch to batch reproductibility was obtained.

PLA Nano- and Microparticles for Drug Delivery: An Overview of the Methods of Preparation

The controlled release of medicaments remains the most convenient way of drug delivery. Therefore, a wide variety of reports can be found in the open literature dealing with drug delivery systems. In particular, the use of nano- and microparticles devices has received special attention during the past two decades. PLA and its copolymers with GA and/or PEG appear as the preferred substrates to fabricate these devices. The methods of fabrication of these particles will be reviewed in this article, describing in detail the experimental variables associated with each one with regard to the influence of them on the performance of the particles as drug carriers. An analysis of the relationship between the method of preparation and the kind of drug to encapsulate is also included. Furthermore, certain issues involved in the addition of other monomeric substrates than lactic acid to the particles formulation as well as novel devices, other than nano- and microparticles, will be discussed in the present work considering the published literature available.

Glycol chitosan as a stabilizer for protein encapsulated into poly(lactide-co-glycolide) microparticle

Glycol chitosan (GC), a chitosan derivative conjugated with ethylene glycol, is soluble in water at a neutral/acidic pH and is viscous. This GC was incorporated into poly(lactide-co-glycolide) (PLGA) microparticles (prepared by the multi-emulsion W(1)/O/W(2) (water-in-oil-in-water) method) to stabilize lysozyme (Lys) used as a model protein. Herein, GC's viscous property helped to improve Lys encapsulation efficacy and reduce Lys denaturaton at the water/organic solvent interface. When the GC concentration in the W(1) phase increased, the formation of non-covalent Lys aggregates decreased. This may be because the aqueous microdroplets surrounded by the firm viscous interface protect Lys from the degrading environment formed by the water/organic solvent interface. In an in vitro Lys release test, 40mg incorporation of GC led to continuous Lys release of up to 78wt.% for 1 month and presented bioactivity of more than 95% for Lys released from microparticles. In addition, there was negligible immune response in the tissue treated with the GC-incorporated PLGA microparticles, whereas there was a moderate foreign body reaction in the muscle layer and many configurations of neutrophils in the tissue treated with the PLGA microparticles without GC. It is expected that GC facilitates a decrease in immune responses exacerbated as a consequence of PLGA degradation.

Protein complexed with chondroitin sulfate in poly(lactide-co-glycolide) microspheres

Chondroitin sulfate (CsA) is an acidic mucopolysaccharide, which is able to form ionic complexes with positively charged proteins. In this study, a protein-CsA complex was constructed to nano-sized particles. Zeta potential measurements revealed that a CsA-to-protein fraction of greater than 0.1 results in a neutralization of the positive charge on lysozyme (Lys). Based on this preliminary study, we have prepared poly(lactide-co-glycolide) (PLGA) microspheres harboring Lys/CsA complexes via the multi-emulsion method. Protein stability in the PLGA microspheres was preserved during both microsphere preparation and protein release. The profiles of Lys release from the PLGA microspheres evidenced nearly zero-order kinetics, depending on the quantity of CsA. An in vivo fluorescent image of experimental mouse tissue showed that the PLGA microspheres with the Lys/CsA complex had released the entirety of their Lys without no residual amount after 23 days, but microspheres without the complex harbored a great deal of residual Lys, which is attributable to its degradation by acidic PLGA degradates. The tissue reaction evidenced by the PLGA microspheres stabilized with CsA showed minimal foreign body reaction and little configuration of immune cells including neutrophils and macrophages, but the reactions of the PLGA microspheres without CsA were characterized by a relatively elevated inflammation. These results show that CsA is a viable candidate for long-acting micro-particular protein delivery.

Mathematical modeling of guided neurite extension in an engineered conduit with multiple concentration gradients of nerve growth factor (NGF)

Neurotrophic factors such as nerve growth factor (NGF) provide essential cues to navigate growing axon toward their targets. Concentration and concentration gradient of NGF are key parameters affecting the growth rate and direction of neurites and axons. However, the maximum distance for guided nerve growth under stimulation of a single concentration gradient is limited and is thus unfavorable in nerve regeneration. Since the sensitivity of PC12 cells to NGF signals is restorable even after brief removal of the factors, exposure to multiple concentration gradients of the factor can achieve longer distances and greater rates of guided growth. In this study, a mathematical model simulating nerve growth in a virtually constructed nerve conduit incorporating multiple NGF concentration gradients is established. Using a genetic algorithm, optimized initial profiles of NGF able to achieve 4.5 cm of guided growth with a significantly improved growth rate has been obtained. The model also predicts an inverse relationship between the diffusion coefficient of the factor and the neurite growth rate. This model provides a useful tool for evaluating various conduit designs before fabrication and evaluation.

HRP-Loaded Bioresorbable Microspheres: Effect of Copolymer Composition and Molecular Weight on Microstructure and Release Profile

Poly(DL-lactic-co-glycolic acid) microspheres are prepared using a double-emulsion technique and are loaded with the model enzyme horseradish peroxidase (HRP). These microspheres can be used alone or as coatings for bioresorbable fibers that may be used as scaffolds for tissue regeneration applications. The present study focuses on the effect of the copolymer's composition and initial molecular weight on the microsphere structure, encapsulation efficiency, and cumulative protein release for 12 weeks. The release profiles generally exhibits an initial burst effect accompanied by slow release over an extended period of time, during which diffusion rather than degradation controlled HRP release from these structures. An increase in the initial molecular weight or in the copolymer's lactic acid content results in larger microspheres with smoother surfaces, and a decrease in the burst release and in the total HRP release. Molecular weight is found to have a stronger effect than copolymer composition. We demonstrate that it is possible to obtain versatile release profiles, which can be tailored for specific applications by choosing the right initial molecular weight and copolymer composition.

Microspheres of Collagen-Apatite Nanocomposites with Osteogenic Potential for Tissue Engineering

Microparticulate systems have attracted a great deal of attention over the past few years as a carrier for the delivery of cells and proteins in the treatment of defective tissues. The composition of microparticulates is regarded as being of utmost importance for the successful recruitment of the cells involved in the tissue regeneration process. Collagen-apatite nanocomposites mimicking the extracellular bone matrix are thus considered to be a potential vector for bone regeneration, either directly or through the delivery of osteogenic cells. In this study, we developed microspheres constituted of collagen and apatite for the treatment of skeletal defects. The apatite-precipitated collagen solution (30% apatite) was formed into microspheres under a water-in-oil emulsion condition. Spherical particles with diameters of tens to hundreds of micrometers (average of approximately 166 microm) were successfully produced. The internal structure of the microspheres featured a typical nanocomposite wherein apatite nanocrystalline precipitates were organized evenly within the reconstituted collagen matrix. The nanocomposite microspheres were observed to recruit favorable adhesion and growth of rat bone marrow derived stem cells. The cells supported on the nanocomposite microspheres stimulated the expression of a series of bone-associated genes. The osteogenic marker, alkaline phosphatase, was secreted to a significantly higher level on the nanocomposite microspheres than on the pure collagen counterpart. The present finding suggests that the collagen-apatite nanocomposite microspheres have high osteogenic potential and are useful for tissue-engineering applications.

Confocal Microscopy for the Elucidation of Mass Transport Mechanisms Involved in Protein Release from Lipid-based Matrices

PURPOSE

It was the aim of this study to identify the governing mechanisms during protein release from cylindrical lipid matrices by visualizing mass transport and correlating the data with in vitro dissolution testing.

MATERIALS AND METHODS

Glyceryl trimyristate cylinders of 2 mm diameter, 2.2 mm height and 7 mg weight were manufactured by compression of a protein-lipid powder mixture prepared by a polyethylene glycol (PEG) co-lyophilization technique. BSA was fluorescence-labeled and the distribution visualized and quantified at different stages of the release process by confocal microscopy in parallel to the quantification in the release buffer. The impact of matrix loading and protein molecular weight was assessed with the model proteins lysozyme, BSA, alcohol dehydrogenase and thyroglobulin.

RESULTS

Buffer penetration and protein release occurred simultaneously from the outer regions of the cylinder progressing towards the center. Release from the top and bottom of the matrix was not negligible but much slower than penetration from the side, probably due to an oriented arrangement of lipid flakes during compression. The different quantification strategies were found to yield identical results. At 6% protein loading, buffer penetration was complete after 4 days, while only 60% of the protein was liberated in that time and release continued up to day 63. Protein release kinetics could be described using the power law equation M ( t ) /M ( infinity ) = kt ( n ) with an average time exponent n of 0.45 (+/-0.04) for loadings varying between 1 and 8%. A percolation threshold at 5% pure protein loading and 3-4% mixed loading (PEG and protein at a 1:1 mass ratio) could be identified. Release rate was found to decrease with increasing molecular weight.

CONCLUSIONS

Protein release from lipid-based matrices is a purely diffusion controlled mechanism. Potential protein stabilization approaches should address the time span between complete buffer penetration of the matrix and 100% release of the remaining loading, which would be exposed to an aqueous environment before leaving the matrix.

Uniform Biodegradable Hydrogel Microspheres Fabricated by a Surfactant-Free Electric-Field-Assisted Method

Uniform biodegradable hydrogel microspheres (HMS) with precisely controlled size have been fabricated using an electric-field-assisted precision particle fabrication technique. Particle agglomeration was prevented by charging the hydrogel drops and allowing Coulomb repulsion to separate them. As a result, surfactant-free and non-toxic particle fabrication was possible and the resulting microspheres were most suitable for biomedical and food-related applications. Due to the size uniformity, the present HMS may serve as a convenient yet most accurate vehicle for controlled delivery of therapeutic agents and other active ingredients.

Nanohybrids for controlled antibiotic release in topical applications

New polymeric composite materials containing a nanohybrid to be used for the controlled release of an antibiotic molecule, chloramphenicol succinate, have been formulated, prepared and characterised. The nanohybrid consists of a layered double hydroxide of Mg-Al hydrotalcite-type, in which the nitrate anions present in the host galleries were replaced with chloramphenicol succinate anions (CFS(-)) by a simple ion-exchange reaction. Different amounts of the hybrid material were incorporated in polycaprolactone and processed as films of 0.15mm thickness. The composite materials were analysed by X-ray diffractometry and thermogravimetry and their mechanical properties were determined. They showed properties even better than those of the pristine polymer. The release process of the antibiotic molecules was found to be very interesting and promising for tuneable drug delivery. It consists of two stages: an initial stage of a very rapid burst, in which a small fraction of drug is released; and a second stage that is much slower, extending for a longer and longer time. This behaviour is profoundly different and much slower than that of a sample in which the antibiotic molecule is directly incorporated into the polymeric matrix. The parameters influencing drug release have been individuated and discussed.

Mechanisms controlling protein release from lipidic implants: Effects of PEG addition

Different types of tristearin-based implants for controlled rh-interferon alpha-2a (IFN-alpha) release were prepared by compression and thoroughly characterised in vitro. Hydroxypropyl-beta-cyclodextrin (HP-beta-CD) was added as a co-lyophilisation agent for protein stabilisation and different amounts of polyethylene glycol (PEG) as efficient protein release modifier. To get deeper insight into the underlying mass transport mechanisms, the release of IFN-alpha, HP-beta-CD and PEG into phosphate buffer pH 7.4 was monitored simultaneously and appropriate analytical solutions of Fick's second law of diffusion were fitted to the experimental results. Importantly, the addition of only 5-20% PEG to the lipidic implants significantly altered the resulting protein release rates and the relative importance of the underlying mass transport mechanisms. The release of IFN-alpha from PEG-free implants was purely diffusion controlled. In contrast, in PEG-containing devices other phenomena were also involved in the control of protein release: the IFN-alpha release rate remained about constant over prolonged periods of time and the total amounts of mobile IFN-alpha increased. Interestingly, the release of PEG itself as well as of HP-beta-CD from the implants remained purely diffusion controlled, irrespective of the amount of added PEG. Thus, different mass transport mechanisms govern the release of the drug, co-lyophilisation agent and release modifier out of the lipidic implants.

New Insight into the Role of Polyethylene Glycol Acting as Protein Release Modifier in Lipidic Implants

PURPOSE

It has recently been shown that the addition of polyethylene glycol 6000 (PEG) to lipidic implants fundamentally affects the resulting protein release kinetics and moreover, the underlying mass transport mechanisms (Herrmann, Winter, Mohl, F. Siepmann, & J. Siepmann, J. Control. Release, 2007). However, it is yet unclear in which way PEG acts. It was the aim of this study to elucidate the effect of PEG in a mechanistic manner.

MATERIALS AND METHODS

rh-interferon alpha-2a (IFN-alpha)-loaded, tristearin-based implants containing various amounts of PEG were prepared by compression. Protein and PEG release was monitored in phosphate buffer pH 4.0 and pH 7.4. IFN-alpha solubility and stability were assessed by reverse phase and size exclusion HPLC, SDS PAGE, fluorescence and FTIR.

RESULTS

Importantly, in presence of PEG IFN-alpha was drastically precipitated at pH 7.4. In contrast, at pH 4.0 up to a PEG concentration of 20% no precipitation occurred. These fundamental effects of PEG on protein solubility were reflected in the release kinetics of IFN-alpha from the tristearin implants: At pH 7.4 the protein release rates remained nearly constant over prolonged periods of time, whereas at pH 4.0 high initial bursts and continuously decreasing release rates were observed. Interestingly, it could be shown that IFN-alpha release was governed by pure diffusion at pH 4.0, irrespective of the PEG content of the matrices. In contrast, at pH 7.4 both--the limited solubility of the protein as well as diffusion through tortuous liquid-filled pores--are dominating.

CONCLUSIONS

For the first time it is shown that the release of pharmaceutical proteins can be controlled by an in-situ precipitation within inert matrices.

Mesoporous silicon: a platform for the delivery of therapeutics

Nanostructuring materials can radically change their properties. Two interesting examples highlighted here are nanoscale porosity inducing biodegradability, and nanoscale confinement affecting the physical form of an entrapped drug. Mesoporous silicon is under increasing study for drug-delivery applications, and is the topic of this review. The authors focus on those properties of most relevance to this application, as well as those recent studies published on small molecule and peptide/protein delivery.

Extended release of high pI proteins from alginate microspheres via a novel encapsulation technique

Alginate has potential as a matrix for controlled delivery of protein-based drugs that require site-specific long-term delivery. In the current work albumin, lysozyme and chymotrypsin were encapsulated into alginate microspheres using a novel method that involved soaking the microspheres in a protein-containing NaCl solution. This was followed by recrosslinking with calcium chloride. High pI proteins also appeared to physically crosslink the sodium alginate which resulted in more sustained release. Release was affected by the nature of the releasate solution. In TRIS buffered saline, the high pI proteins chymotrypsin and lysozyme showed sustained release lasting over 150 h. Release into 0.15% NaCl led to relatively constant release of lysozyme and chymotrypsin over more than 2000 h; reduction of the releasate volume lengthened the lysozyme release to greater than 8 months. Released lysozyme was shown to remain active for at least 16 days, in some cases with activity greater than 100% of the active control. This encapsulation technique can therefore be used to rapidly load alginate microspheres with proteins, with high isoelectric point proteins showing particular promise. Furthermore, the interactions between the high pI proteins and the alginate gel could potentially be exploited to generate new protein delivery systems.

Stabilization of recombinant human growth hormone against emulsification-induced aggregation by Pluronic surfactants during microencapsulation

Protein aggregation upon exposing to the water/organic solvent interface is one of the most significant obstacles in developing poly(lactic-co-glycolic acid) (PLGA) microspheres with double emulsion process. The aim of present study is to devise a formulation strategy to prevent recombinant human growth hormone (rhGH) from aggregation during microencapsulation. The excipients used for stabilizing rhGH were selected from sugars, nonionic surfactants, polyol, and protein. Among the candidates, surfactants exhibited potentialities in protecting rhGH against emulsification-induced aggregation. It was also found that Pluronic F127 showed an outstanding as well as concentration-dependent stabilizing effect on rhGH, which was different to Pluronic F68 and Tween 20. After the rhGH solution comprising F127 and sucrose was emulsified with methylene chloride, the recovery of monomeric protein achieved 99.0%, principally attributed to the presence of F127. This solution was subsequently encapsulated as inner aqueous phase in the PLGA microspheres by a conventional double emulsion process, with the encapsulation efficiency higher than 98%. Improvement in the release of rhGH was observed for the microspheres co-encapsulating Pluronic F127 regardless in the presence or absence of sucrose, compared to the microspheres containing rhGH alone. The result further implied that co-encapsulation of Pluronic F127 in the microspheres played an important role in the stabilization of rhGH.

Heparin conjugated polymeric micelle for long-term delivery of basic fibroblast growth factor

Heparin conjugated amphiphilic block copolymer, Tetronic-PCL-heparin (TCH), was developed and its polymeric micelles (PMs) were prepared as an injectable vehicle for long-term delivery of bFGF, which is one of the heparin-binding growth factors (HBGF). TCH PMs were fabricated by a single emulsion and solvent evaporation method. The structural properties of TCH were confirmed by (1)H NMR, FT-IR and GPC. The contents of bound heparin were 0.44 micro g/micro g and the heparin activity by APTT assay was 43.6% when compared to free heparin. The critical micelle concentration (CMC) of TCH PMs was approximately 0.11 g/l. The diameter of TC micelle was approximately 25 nm and its size after conjugation of heparin was increased to 114 nm due to the heparin molecules on the shell of the micelle. The bFGF loading amount of TCH PMs was considerably higher than that of TC, caused by specific interactions between heparin and bFGF. In vitro study, bFGF was released from TCH PMs in a controlled manner over 2 months. The results demonstrated that TCH PMs become a novel candidate for the long-term delivery of various growth factors with heparin-binding domain in tissue engineering.

Intradermal immunization with ovalbumin-loaded poly-?-caprolactone microparticles conferred protection in ovalbumin-sensitized allergic mice

BACKGROUND

Although immunotherapy has been reported as the only treatment able to revert the T-helper type 2 (Th2) response, its administration has some disadvantages such as the requirement of multiple doses, possible side-effects provoked by conventional adjuvants and the risk of suffering an anaphylactic shock. For these reasons, drug-delivery systems appear to be a promising strategy due to its ability to (i) transport the allergens, (ii) protect them from degradation, (iii) decrease the number of administrations and (iv) act as immuno-adjuvants.

OBJECTIVE

The aim of this work was to evaluate the properties of poly-epsilon-caprolactone (PCL) microparticles as adjuvants in immunotherapy using ovalbumin (OVA) as an allergen model. For this purpose, the protection capacity of these microparticles (OVA PCL) against OVA allergy was studied in a murine model.

METHODS

The humoral and cellular-induced immune response generated by OVA encapsulated into PCL microparticles was studied by immunizing BALB/c mice intradermically. Also, OVA-sensitized mice were treated with OVA PCL and OVA adsorbed to aluminium hydroxide (OVA-Alum). Fifteen days after therapy, animals were challenged with OVA and different signs of anaphylactic shock were evaluated.

RESULTS

One single shot by an intradermal route with OVA PCL resulted in a Th2-type immune response. In OVA-sensitized mice, treatment with OVA PCL elicited high OVA-specific IgG but low levels of IgE. Furthermore, OVA PCL mice group displayed lower levels of serum histamine and higher survival rate in comparison with the positive control group.

CONCLUSION

The anaphylactic shock suffered by OVA PCL-treated mice was weaker than the one induced in the OVA-Alum group. Hence, the intradermal immunization with OVA PCL microparticles induced hyposensitization in OVA-allergic mice.

Controlled Release Biopolymers for Enhancing the Immune Response

Controlled release of biologically active compounds in the context of drug and vaccine delivery is an important area of research with broad implications in many areas of medicine. In particular, the challenges of oral delivery are of specific interest to reduce the cost and potential health risks related to parenteral administration of pharmaceuticals and vaccine formulations. We discuss the biological activities of two biopolymers, beta-glucans and emulsans, both of which offer significant potential for individual formulations related to drug impact, while in combination offer synergistic opportunities in terms of formulation and delivery. beta-Glucans have been established as potent immunomodulatory and biologically active compounds with application in a wide range of disease systems. The emulsan family of biopolymers also has significant potential in vaccine and drug delivery based on recent studies. Each of these biopolymers offers exciting opportunities to modulate biological responses via control of chemistry and physical properties achieved during biosynthesis or postsynthesis modifications. When combined into a delivery system for controlled release, synergistic outcomes may be achieved that offer new and exciting opportunities as described in the present paper. These outcomes represent the combined improvements of solubility in physiological environments and immunomodulation due to the specific chemistry and structures involved. Overall, this approach provides a new direction in controlled release wherein the biomaterial carrier, in this case emulsan, and the drug, in this case beta-glucan, play an active role both in biological activation as well as in delivery profiles.

Encapsulation of proteins in biodegradable polymeric microparticles using electrospray in the Taylor cone-jet mode

Solvent extraction (or evaporation from a W(1)/O/W(2)-dispersion), coacervation, and spray drying methods are commonly employed to encapsulate protein drugs in polymeric microparticles for sustained delivery applications. To overcome the limitations of these methods, a novel electrospray method was developed to encapsulate a model protein drug-bovine serum albumin (BSA) in biodegradable polymeric microparticles and examine the feasibility of the process in not denaturing the protein. Microparticles of approximately 20 microm diameter with corrugated surfaces and smooth surfaces were observed by scanning electron microscope. Confocal laser scanning microscope images showed that BSA was distributed evenly in microparticles. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was employed to investigate the protein integrity of BSA released from the polymer matrix after 38 days. No protein degradation was observed during the 38 days release. The secondary structure of released BSA was characterized by Fourier transform infrared (FTIR) and circular dichroism (CD), which suggested that the released BSA was almost identical to native BSA. The encapsulation efficiency could reach 76% by adjusting the amount of the additive Pluronic F127 and processing parameters. The release profile could be tailored by the fabrication process and the sustained release of BSA could endure for more than 1 month. More than 80% of the bioactivity of BSA (evaluated by BSA ELISA kit) could be maintained after releasing from polymer matrix. Findings of the present study demonstrate that this novel electrospray method is a promising approach to encapsulate bioactive materials such as proteins, enzymes, antibiotics, and DNA fragments in biodegradable polymeric particles.

In vitro study of lysozyme in poly(lactide-co-glycolide) microspheres with sucrose acetate isobutyrate

This study investigated the suitability of microsphere formulations for extended protein delivery and complete protein release. These microspheres were prepared by a multi-emulsion method and prepared using a mixture of poly(lactide-co-glycolide) (PLGA), RG 502H (lactide:glycolide=50:50, M(W) 9300) and sucrose acetate isobutyrate (SAIB). SAIB embedded into the microspheres and mixed with PLGA, improved the efficiency of enzyme encapsulation. The in vitro release rate of lysozyme (Lys) from the microspheres was reduced due to the high viscosity of the added SAIB and less degradation of PLGA by SAIB. These properties enabled prolonged release of Lys for up to 2 months, characterized by a minimal initial burst of Lys and nearly zero-order protein release kinetics result from co-administration of sorbitan monooleate 80. When it is considered that degradation products of SAIB are inactive for labile proteins, SAIB may be regarded as a promising candidate for long-acting protein delivery.

Hydrogels in controlled release formulations: network design and mathematical modeling

Over the past few decades, advances in hydrogel technologies have spurred development in many biomedical applications including controlled drug delivery. Many novel hydrogel-based delivery matrices have been designed and fabricated to fulfill the ever-increasing needs of the pharmaceutical and medical fields. Mathematical modeling plays an important role in facilitating hydrogel network design by identifying key parameters and molecule release mechanisms. The objective of this article is to review the fundamentals and recent advances in hydrogel network design as well as mathematical modeling approaches related to controlled molecule release from hydrogels. In the first section, the niche roles of hydrogels in controlled release, molecule release mechanisms, and hydrogel design criteria for controlled release applications are discussed. Novel hydrogel systems for drug delivery including biodegradable, smart, and biomimetic hydrogels are reviewed in the second section. Several mechanisms have been elucidated to describe molecule release from polymer hydrogel systems including diffusion, swelling, and chemically-controlled release. The focus of the final part of this article is discussion of emerging hydrogel delivery systems and challenges associated with modeling the performance of these devices.

Core/Shell nanoparticles with lecithin lipid cores for protein delivery

Core/shell nanoparticles with lipid core, were prepared and characterized as a sustained delivery system for protein. The lipid core is composed of protein-loaded lecithin and the polymeric shell is composed of Pluronics (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer, F-127). Based on the preparation method in the previous report by us, the freeze-drying of protein-loaded lecithin was performed in the F-127 aqueous solution containing trehalose used as a cryoprotectant to form stabilized core/shell nanoparticles. Cryo-TEM (transmittance electron microscopy) and a particle size analyzer were used to observe the formation of stabilized core/shell nanoparticles. For the application of core/shell nanoparticles as a protein drug carrier, lysozyme and vascular endothelial growth factor (VEGF) were loaded into the core/shell nanoparticles by electrostatic interaction, and the drug release pattern was observed by manipulating the polymeric shell.

In Vitro Degradation Behavior of Microspheres Based on Cross-Linked Dextran

The aim of this study was to investigate the in vitro degradation of hydroxyl ethyl methacrylated dextran (dex-HEMA) microspheres. Dextran microspheres were incubated in phosphate buffer pH 7.4 at 37 degrees C, and the dry mass, mechanical strength, and chemical composition of the microspheres were monitored in time. The amount and nature of the formed degradation products were established for microspheres with different cross-link densities by FT-IR (Fourier transformed infrared spectroscopy), NMR, mass spectrometry, SEC analysis, and XPS (X-ray photoelectron microscopy). The dex-HEMA microspheres DS 12 (degree of HEMA substitution; the number of HEMA groups per 100 glucose units) incubated at pH 7.4 and 37 degrees C showed a continuous mass loss, leaving after 6 months a residue of about 10% (w/w) of water-insoluble products. NMR, mass spectrometry, and SEC showed that the water-soluble degradation products consisted of dextran, low molecular weight pHEMA (M(n) approximately 15 kg/mol), and small amounts of unreacted HEMA and HEMA-DMAP (intermediate reaction product of the Baylis-Hillman reaction of HEMA with DMAP (4-dimethyl aminopyridine)). Microscopy revealed that the water-insoluble residue consisted of particles with shape and size similar to that of nondegraded microspheres. However, these particles had lost their mechanical strength as evidenced from micromanipulation experiments. FT-IR and XPS (X-ray photoelectron microscopy) revealed that these particles consisted of pHEMA, of which a small fraction was soluble in methanol (M(n) ranging between 27 and 82 kg/mol). The insoluble material likely consisted of lightly cross-linked pHEMA. In conclusion, in vitro degradation of dex-HEMA microspheres results in the formation of water-soluble degradation products (mainly dextran), leaving a small water-insoluble residue mainly consisting of pHEMA.

Sustained release of bioactive therapeutic proteins from a biodegradable elastomeric device

Effective localized delivery of a therapeutic protein requires a biodegradable device capable of delivering active protein at a sustained rate, and at a concentration within its therapeutic window. The objective of this study was to demonstrate that a biodegradable elastomeric device can be made in a cylindrical geometry, and still retain the ability to release a variety of therapeutic proteins at a nearly constant rate in nanomolar concentration with high bioactivity. The elastomers were prepared with cylindrical geometry by photo-cross-linking an acrylated star-poly(epsilon-caprolactone-co-d,l-lactide) macromer. Vascular endothelial growth factor (VEGF), interferon-gamma (IFN-gamma), and interleukin-2 (IL-2) were co-lyophilized with excipients, then entrapped within the elastomer matrix by photo-polymerization. Under identical formulation conditions, these proteins were released at the same, nearly constant rate for a significant part of the release profile (until 70%-80% release depending on formulation characteristics). Decreasing the molecular weight of the acrylated macromer increased the rate of protein release, but did not alter the zero order nature of the release kinetics. Cell based bioactivity assays showed only that 57% of the VEGF released was bioactive. By contrast, both IL-2 and IFN-gamma showed relatively high bioactivity and over 80% of the released proteins were bioactive. The elastomer formulation has potential as a regio-specific protein delivery device.

An EPR, ENDOR and EIE study of gamma-irradiated poly (lactide-co-glycolide) polymers

Gamma radiation of poly (lactide-co-glycolide) raw polymers and processed microspheres under vacuum and at 77 K results in the formation of a series of free radicals. The resulting powder electron paramagnetic resonance (EPR) spectrum contains a distribution of several different radicals, depending on the annealing temperature, and is therefore difficult to interpret. By utilising the selectivity of the electron nuclear DOuble resonance (ENDOR) and associated ENDOR induced EPR (EIE) techniques, a more direct approach for the deconvolution of the EPR spectrum can be achieved. Using this approach, the radiolytically induced CH3 *CHC(O)R- chain scission radical was identified at 120 K by simulation of the EIE spectrum. At elevated temperatures (250 K), this radical decays considerably and the more stable radicals -O*CHC(O)-, CH3 *C(OR)C(O)- and CH3 *C(OH)C(O)- predominate. This work demonstrates the utility of the EIE approach to supplement and aid the interpretation of powder EPR spectra of radicals in a polymer matrix.

NIR spectroscopy—a non-destructive analytical tool for protein quantification within lipid implants

Lipid implants have been proposed as promising sustained release devices for the parenteral application of pharmaceutical proteins. Near infrared spectroscopy (NIRS) has been reported in the literature to be a non-destructive tool for drug quantification within controlled release matrix systems based on poly-(lactic-co-glycolic) acid (PLGA). The objective of this study was to evaluate the potential application of NIRS for protein content determination within lipid matrices containing stabilizing and release modifying additives. Bovine serum albumin (BSA) and rh-interferon alpha-2a (IFN alpha-2a) were initially lyophilized with trehalose and then blended with tristearin (matrix material) and optionally with polyethylene glygol 6000 (PEG, release modifier). Implants were prepared by compression. NIR transmittance spectra were measured on a NIRTab spectrometer. Partial least squares regression (PLSR) calibration models were developed to predict protein content in implants from the NIRS results. Additional samples were measured after performing release studies. It could be shown that NIRS allowed protein quantification in complex matrix systems with good accuracy after implant manufacture and during release studies [e.g., standard error of prediction (SEP) between 57 microg-176 microg]. In addition, small protein amounts down to 70 microg of incorporated protein per implant could be determined, thus demonstrating the low detection limit of NIRS.

Synthetic Biodegradable Ionomers that Engulf, Store, and Deliver Intact Proteins

Telechelic anionic and cationic biodegradable ionomers capable of loading, storing, and releasing proteins are presented. Two different ionomers have been synthesized with either anionic or cationic end groups. The reaction was done quantitatively as shown by (1)H NMR. The swelling properties of the hydrophobic poly(trimethylene carbonate) polymer are contributed to the ionic end groups that display hydrophilic properties. Depending on the molecular weight of the ionomer, and also on the ionic charge, the materials swell differently in water, from approximately 50% for M(w) = 12 000 g/mol to approximately 500% when dealing with 2000 g/mol. The high swelling led us to believe that it would be possible to load and release proteins preferably in a still active form. As models, two different proteins were chosen: hemoglobin and cytochrome c. The swelling and release study shows that both ionomers possess the capability to adsorb and later release the proteins with retained structure. Release measurements from both the swollen and dried states have been evaluated with similar results, showing that the dried state seems to release a little bit less than the swollen one. These kinds of materials should be interesting for a wide variety of applications where drug and protein release is wanted, as well as in applications such as protein separation media.

Synthesis and characterization of biocompatible poly(organophosphazenes) aiming for local delivery of protein drugs

Biocompatible and thermosensitive poly(organophosphazenes) with a lower critical solution temperature (LCST) below body temperature have been designed with the aim for the local delivery of peptide and protein drugs. These polymers could be synthesized by introducing short chain tri- or tetraethylene glycol as a hydrophilic group and a dipeptide, GlyGluEt2 as a hydrophobic group into the polyphosphazene backbone. The local tolerance tests using rabbits have shown that our polymers are biocompatible. Using the amphiphilic properties of these polymers, in vitro studies were performed for loading and releasing of a human growth hormone (hGH) as a model drug. The entrapment efficiency (%) of hGH by the polymer decreased as its polymer concentration increased, but exhibited high efficiency of more than 95% even at 20% hGH concentration in the polymer. The entrapped hGH has shown to be controlled releasing for 3-4 days.

Microdroplet dissolution into a second-phase solvent using a micropipet technique: test of the Epstein-Plesset model for an aniline-water system

The Epstein-Plesset model was originally derived for the dissolution of a single gas bubble in an infinite aqueous solution (Epstein, P. S.; Plesset, M. S. J. Chem. Phys. 1950, 18, 1505-1509). The micropipet manipulation technique was previously shown to test this theory on air microbubbles and air-filled lipid-coated microparticles accurately and appropriately (Duncan, P. B.; Needham, D. Langmuir 2004, 20, 2567-2578). This same theory is now tested to model liquid microdroplet dissolution in a well-defined solution environment. As presented previously for the gas-bubble system, holding a single microparticle at the end of a micropipet was not shown to affect the dissolution profile and allowed isotropic diffusion significantly, a necessary condition for the validation of the theory. Here, an aniline-water system with an initial droplet diameter of 50 microm was used as a model liquid-liquid system. A microdroplet of aniline in an aqueous solution presatureated with aniline at distinct levels was tested, as was the reverse system of a water droplet in an aniline solution. The dissolution lifetime was shown to increase with increasing medium saturation fraction according to the Epstein-Plesset time-dependent theory (including the time required to establish the stationary layer) neglecting interfacial tension. The droplet lifetime can be increased by an order of magnitude (from about 10 to 100 s) by increasing the saturation fraction from 0 to 0.9 and by another order of magnitude by increasing from 0.9 to 0.99. The technique proved to be an accurate and appropriate method to test the dissolution of single liquid microdroplets in a second liquid solution and establishes a systematic experimental and theoretical approach to the investigation of the formation of polymer and other microparticles.