Porosity-controlled ethylcellulose film coating. II. Spontaneous porous film formation in the spraying process and its solute permeability

New insights on the influence of manufacturing conditions and molecular weight on phase-separated films intended for controlled release

The aim of this work was to investigate how manufacturing conditions influence phase-separated films of ethyl cellulose (EC) and hydroxypropyl cellulose (HPC) with different molecular weights of HPC. Two HPC grades, SSL and M, with weight average molecular weights (M_w) of 30×10³g/mol and 365×10³g/mol, respectively, were combined with EC 10 cps (70:30w/w EC/HPC) and spray-coated from ethanol solutions onto a rotating drum under well-controlled process conditions. Generally, a low spray rate resulted in a more rapid film drying process and, consequently, in smaller HPC-rich domains in the phase-separated film structure. For EC/HPC films with the low M_w HPC (SSL) the most rapid drying process resulted in a shift from a HPC-discontinuous to a partly bicontinuous structure and an increase in the permeability for water. In contrast, films containing the high M_w HPC (M) all showed bicontinuous structures, which resulted in overall higher water permeabilities and polymer release compared to the low M_w films. Interestingly, a maximum in permeability was observed for the high M_w films at intermediate spray rates. Below this spray rate the permeability decreased due to a lower amount of polymer released and at higher spray rates, the permeability decreased due to a loss of pore connectivity (or increased tortuosity). To conclude, this study shows that different M_w systems of EC/HPC can respond differently to variations in manufacturing conditions.

The influence of the molecular weight of the water-soluble polymer on phase-separated films for controlled release

Hydroxypropyl cellulose (HPC) and ethyl cellulose (EC) can be used for extended release coatings, where the water-soluble HPC may act as a pore former. The aim was to investigate the effect of the molecular weight of HPC on the microstructure and mass transport in phase-separated freestanding EC/HPC films with 30% w/w HPC. Four different HPC grades were used, with weight averaged molecular weights (Mw) of 30.0 (SSL), 55.0 (SL), 83.5 (L) and 365 (M) kg/mol. Results showed that the phase-separated structure changed from HPC-discontinuous to bicontinuous with increasing Mw of HPC. The film with the lowest Mw HPC (SSL) had unconnected oval-shaped HPC-rich domains, leaked almost no HPC and had the lowest water permeability. The remaining higher Mw films had connected complex-shaped pores, which resulted in higher permeabilities. The highest Mw film (M) had the smallest pores and very slow HPC leakage, which led to a slow increase in permeability. Films with grade L and SL released most of their HPC, yet the permeability of the L film was three times higher due to greater pore connectivity. It was concluded that the phase-separated microstructure, the level of pore percolation and the leakage rate of HPC will be affected by the choice of HPC Mw grade used in the film and this will in turn have strong impact on the film permeability.

High throughput research and evaporation rate modeling for solvent screening for ethylcellulose barrier membranes in pharmaceutical applications

CONTEXT

Ethylcellulose is commonly dissolved in a solvent or formed into an aqueous dispersion and sprayed onto various dosage forms to form a barrier membrane to provide controlled release in pharmaceutical formulations. Due to the variety of solvents utilized in the pharmaceutical industry and the importance solvent can play on film formation and film strength it is critical to understand how solvent can influence these parameters.

OBJECTIVE

To systematically study a variety of solvent blends and how these solvent blends influence ethylcellulose film formation, physical and mechanical film properties and solution properties such as clarity and viscosity.

MATERIALS AND METHODS

Using high throughput capabilities and evaporation rate modeling, thirty-one different solvent blends composed of ethanol, isopropanol, acetone, methanol, and/or water were formulated, analyzed for viscosity and clarity, and narrowed down to four solvent blends. Brookfield viscosity, film casting, mechanical film testing and water permeation were also completed.

RESULTS AND DISCUSSION

High throughput analysis identified isopropanol/water, ethanol, ethanol/water and methanol/acetone/water as solvent blends with unique clarity and viscosity values. Evaporation rate modeling further rank ordered these candidates from excellent to poor interaction with ethylcellulose. Isopropanol/water was identified as the most suitable solvent blend for ethylcellulose due to azeotrope formation during evaporation, which resulted in a solvent-rich phase allowing the ethylcellulose polymer chains to remain maximally extended during film formation. Consequently, the highest clarity and most ductile films were formed.

CONCLUSION

Employing high throughput capabilities paired with evaporation rate modeling allowed strong predictions between solvent interaction with ethylcellulose and mechanical film properties.

A solid-state NMR method to determine domain sizes in multi-component polymer formulations

Polymer domain sizes are related to many of the physical properties of polymers. Here we present a solid-state NMR experiment that is capable of measuring domain sizes in multi-component mixtures. The method combines selective excitation of carbon magnetization to isolate a specific component with proton spin diffusion to report on domain size. We demonstrate the method in the context of controlled release formulations, which represents one of today's challenges in pharmaceutical science. We show that we can measure domain sizes of interest in the different components of industrial pharmaceutical formulations at natural isotopic abundance containing various (modified) cellulose derivatives, such as microcrystalline cellulose matrixes that are film-coated with a mixture of ethyl cellulose (EC) and hydroxypropyl cellulose (HPC).

Nanostructure of Materials Determined by Relayed Paramagnetic Relaxation Enhancement

Particle and domain sizes strongly influence the properties of materials. Here we present an NMR approach based on paramagnetic relaxation enhancement (PRE) relayed by spin diffusion (SD), which allows us to determine lengths in the nm-μm range. We demonstrate the method on multicomponent organic polymer mixtures by selectively doping one component with a paramagnetic center in order to measure the domain size in a second component. Using this approach we determine domain sizes in ethyl cellulose/hydroxypropyl cellulose film coatings in pharmaceutical controlled release formulations. Here we measure particle sizes ranging from around 50 to 200 nm.

Novel Film Modifiers to Alter the Physical Properties of Composite Ethylcellulose Films

PURPOSE

Polyvinylpyrrolidone (PVP), molecular-composite PVP, and Plasdone S-630 copolyvidonum are potential polymeric film modifiers for achieving improved drug release. The aim of this study was to investigate how these polymeric additives would affect the physicomechanical properties of composite ethylcellulose films.

METHODS

The miscibility of these polymeric additives with ethylcellulose was determined from the differential scanning calorimetry (DSC) thermograms of various polymer blends formed from organic solvents. It was found that ethylcellulose (EC) was miscible with the polymeric additives up to a concentration of 50%. Ten percent to 30% w/w polymeric additives were then added to aqueous ethylcellulose dispersion to form composite films. The morphology, film transparency, dynamic mechanical analysis (DMA) thermograms, and mechanical properties of the composite ethylcellulose films were studied. In addition, puncture strength and % elongation of the dry and wet films were also compared from indentation test.

RESULTS

Significant reduction and change in film transparency and morphology was obtained for EC films blended with PVP of higher molecular weight (MW). The composite EC films also showed higher Tg, greater elastic modulus, tensile and puncture strength depending on the concentration and type of additives present.

CONCLUSIONS

The interaction between ethylcellulose and the polymeric additives is dependent on the MW and concentration of additives. The composite films offer new opportunities for the use of ethyl-cellulose as modified release coatings for dosage forms.

Drying of poloxamer hydrogel films

The drying of hydrogel films formed by Poloxamer 407 poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) amphiphilic block copolymer was investigated at various air relative humidity (RH) conditions ranging from 11 to 97%. Initially, the amount of water lost increased linearly with the drying time. After this linear region (stage I), a nonlinear behavior was observed (stage II). The drying rate increased with decreasing RH, thus greatly shortening the drying time. A decrease of the film thickness also shortened the drying time; however, the drying mechanism did not change. Three models for one-dimensional water diffusion were used to fit the experimental results at different RH conditions and film thicknesses. Model 1 assumes semi-infinite medium and constant diffusion coefficient, and fits very well the data in stage I of the drying process. The fitted water diffusion coefficient (D) is 5 x 10(-10) m(2)/s, whereas the effects of the RH are captured by a proportionality constant (alpha) that appears in the boundary condition. Model 2 considers a finite (constant) film thickness and captures the experimental observations over the whole drying period for the same D and alpha as in Model 1. The analytical solutions available for Models 1 and 2, used together with the experimentally derived model parameters D and alpha, allow for easy estimation of drying time and water loss from Poloxamer hydrogel films of various compositions and thicknesses and at different relative humidities. Numerical solutions for water diffusion under conditions of decreasing film thickness and diffusion coefficient being a function of concentration are also presented (Model 3). It becomes apparent from the fit of the data to the different models that the drying rate is more sensitive to the boundary condition at the film-air interface (represented by alpha) than to the diffusion in the film. It is notable that the alpha values obtained from the fits of the Poloxamer hydrogel drying rate are comparable to those obtained from drying of water films under the same experimental conditions.

Aqueous starch acetate dispersion as a novel coating material for controlled release products

The aim of this study was to evaluate film-formation properties of a novel, organic solvent-free aqueous dispersion of potato starch acetate (SA; degree of substitution 2.8) and its ability to control drug release from a coated tablet. Initially, film-formation mechanisms and drug permeabilities of both organic solvent and dispersion-based SA free films (prepared by cast or spraying techniques) were investigated. The SA dispersion was suitable for the fluid-bed coating process, forming strong films with complete coalescent polymeric spheres. The model compounds predominantly permeated via the micro-pores of SA free films, which resulted from the leaching of water-soluble excipients from the dispersion. Thus, the permeation rate depended on the film structure rather than the physico-chemical properties of the penetrant. In the case of SA-coated tablet, drug release was sustained when the coating level was increased (from 12% to 20%, stated as a weight gain), and also as lipophilicity of the drug increased. When compared to the reference polymer dispersion (Surelease), SA coatings showed better mechanical properties against the osmotic pressure caused by a hydrophilic core tablet. These results clearly demonstrate that SA dispersion has high utility as a novel aqueous coating material for controlled release products.

Modelling of drug release from coated granular pellets

A mathematical model of drug release from coated pellets with a granular core has been developed. The model includes a dynamic description of all three main processes contributing to drug release from such a system, i.e. liquid inflow, drug dissolution, and liquid efflux caused by diffusion across the coating. The cumulative fraction of released drug has been shown to be determined by three rate constants, one for each process mentioned above, together with two dimensionless parameters. These parameters are related to the porosity of the pellet core and the solubility of the drug in the dissolution medium. The model has been validated by comparison with experimentally determined release profiles for pellets consisting of a granular core of microcrystalline cellulose containing dispersed salicylic acid, coated by a thin layer of ethyl cellulose.

Influence of film additives on stabilizing drug release rates from pellets coated with acrylic polymers

The objective of this study was to investigate the influence of talc and triethyl citrate (TEC) on stabilizing the drug release rates following curing and storage at elevated temperature of pellets coated with an aqueous acrylic polymeric dispersion. Core pellets containing anhydrous theophylline (20%), microcrystalline cellulose, and polyvinylpyrrolidone were prepared by extrusion-spheronization. The aqueous dispersions were prepared by adding up to 30% TEC as a plasticizer and talc up to 200% as an antiadherent to a mixture of Eudragit RS 30D/RL 30D (95:5). The theophylline pellets were coated in a fluidized-bed coating unit and then cured at elevated temperatures. Theophylline pellets were successfully coated with the Eudragit dispersions that contained up to 200% talc, based on the dry polymer weight, and the coating efficiency was greater than 93%. Our results demonstrated that the polymer, which was plasticized by TEC, was able to function as a film-forming agent for dispersions containing high levels of talc. No sticking of the coated pellets was observed during the coating process or during the curing or equilibrating phase, even with high levels of TEC in the film. The dissolution rate of theophylline from the coated pellets was delayed when the film coating dispersion contained high levels of talc. Additionally, the stability of the drug release profiles from the coated pellets after storage was significantly improved. Furthermore, a modified dissolution testing used to simulate mechanical stresses that may be encountered in vivo showed the film coated pellets would have sufficient strength. The results of this study demonstrated that high levels of film additives in the acrylic dispersion contributed to the stabilization of the drug release rates as well as the reproducibility of the coating process.

Suppression of agglomeration in fluidized bed coating. IV. Effects of sodium citrate concentration on the suppression of particle agglomeration and the physical properties of HPMC film

We previously reported that sodium citrate (Na citrate), which is a high order salt in the Hofmeister's series, greatly suppressed particle agglomeration in fluidized bed coating (Pharm. Res., 16 (1999), 1616-1620). In this paper, we studied the effects of Na citrate concentration on the particle agglomeration in fluidized bed coating and on the structure of coated film on the particles. Spherical granules made of crystalline cellulose (Celphere) containing phenacetin were coated in a fluidized bed with the aqueous coating solution of hydroxypropylmethyl cellulose (HPMC) containing Na citrate at various concentrations. The particle diameter and drug release profile of coated particles, and the physical properties, i.e. tensile strength, elongation percentage at break, porosity and pore size distribution, of the HPMC cast film were investigated. The particle agglomeration was suppressed with the increasing Na citrate concentration. It is considered that the increase in the suppression effect was caused by the salting-out effect of the increased Na citrate. In the HPMC cast film system, the tensile strength and elongation percentage decreased and the porosity and cumulative pore volume increased with an increase in Na citrate concentration. It is considered that the increase in the porosity by adding Na citrate resulted from a phase separation due to the salting-out during the film forming process. The drug release rate from coated particles also increased with the increasing Na citrate concentration. It can be concluded that the increase in the release rate was due to the increase in porosity of the HPMC coated film caused by the increased Na citrate concentration.

Factors affecting membrane-controlled drug release for an osmotic pump tablet (OPT) utilizing (SBE)(7m)-beta-CD as both a solubilizer and osmotic agent

PURPOSE

The purpose of this study was to define membrane controlling factors responsible for drug release from a controlled-porosity osmotic pump tablet (OPT) that utilizes a sulfobutyl ether-beta-cyclodextrin, (SBE)(7m)-beta-CD, as both a solubilizing and osmotic agent.

METHOD

The OPT was spray coated with cellulose acetate solutions varying the amount and size of micronized lactose, the amount of triethyl citrate (TEC) and the composition ratio of dichlormethane to ethanol. Chlorpromazine (CLP) was used as a model drug. The release of CLP from the OPTs was studied using the Japanese Pharmacopoeia dissolution method. The membrane surface area of the OPTs were measured with multi-point analysis by the gas absorption method.

RESULTS

The release rate of CLP from OPTs containing (SBE)(7m)-beta-CD increased with increasing amounts of micronized lactose and decreasing amounts of TEC and lactose particle size in the membrane. Also, the CLP release rates from the spray-coated OPTs using mixtures of varying ratios of dichlormethane to ethanol were almost identical. The membrane surface area of the OPTs following release of membrane components had a linear relationship to CLP release rates from the OPTs.

CONCLUSION

The present results confirmed that the membrane controlling factors responsible for the drug release were the amount and size of micronized lactose and the amount of TEC in the membrane.

Effect of polysorbates on atenolol release from film-coated tablets

The effects of different concentrations of various polysorbates on the release rate of atenolol from film-coated tablets were evaluated. The release profile of atenolol showed that increasing the concentration of polysorbate resulted in an increase in the release rate of atenolol. The type of polysorbate had less effect on the release rate of atenolol. This study revealed that the release kinetic of atenolol from these film-coated tablets was a function of polysorbate concentration. Correlation coefficients of kinetic models could not solely determine the suitability of the models; the sum of the least square of differences also should be calculated when different kinetic models have similar correlation coefficients.

Controlled-release behavior of diphenhydramine hydrochloride loaded neutral microgranules and coated using ethylcellulose water dispersion

The development of a loading method of a water-soluble drug using aqueous binding solution to produce microgranules that were then coated with an aqueous ethylcellulose dispersion to sustain drug release is described. The results, in terms of drug used, showed that besides the fluidized bed parameters, the amount of drug dissolved in the binder solution plays an important role in obtaining a satisfying result during the spraying process. Thus, it seems necessary to determine the critical concentration above which the material started to adhere to the interior of the fluidization column, and the possibility of drug layering onto carrier material is aggravated. ANOVA of the time parameter for release of 63.2% of total drug (td) value showed significant influence of ethylcellulose (Aquacoat ECD-30) and dibutyl sebacate concentration on diphenhydramine hydrochloride (DPH) release. The dissolution rate decreased with an increase in polymer concentration. The diffusional exponent n of the Peppas equation indicated that the DPH release kinetic was non-Fickian but approached Fickian diffusion, particularly at higher coating levels.

Effect of fractal dimension on drug permeation through porous ethylcellulose films

Fractal geometry was applied to quantify the complexity of an internal structure of a porous film prepared with ethylcellulose (EC) and diethylphthalate (DEP) as a plasticizer. EC was dissolved together with DEP in a water-ethanol mixture solution, and then evaporated on Teflon petri dishes in order to make porous EC films. Boundary lines of the porous structures in the EC film cross section were taken by a confocal laser microscope as image data, and these images were fed into a computer to estimate the fractal dimension. The porous structure in EC film was observed to be a typical fractal and its complexity was quantified as a non-integral fractal dimension. No clear correlation was observed between the fractal dimension and the porosity of EC films, suggesting that they were mutually independent parameters representing the porous structure in the EC films. The permeation of theophylline through the EC films was determined by using two-chamber diffusion cells. A fairly good relationship between the permeability coefficient of theophylline and the fractal dimensions was observed, suggesting the usefulness of the fractal dimension as a novel parameter for evaluating drug permeation through porous films.

Microencapsulation of drugs by the coacervation technique using ethylcellulose and acrylate-methacrylate copolymer as wall materials

In this study, ethylcellulose and acrylate-methacrylate copolymer (Eudragit RL 100, Eudragit RS 100) membranes were prepared by using appropriate types and amounts of plasticizers. Thirty percent and 20% of triacetin based on polymer weight were found to be appropriate plasticizers for ethylcellulose membranes and acrylate-methacrylate copolymer membranes, respectively. The ratios of 3:2 and 2:3 Eudragit RL 100: Eudragit RS 100 also gave transparent and flexible membranes. Cephalexin was chosen as a model drug. The coacervation technique was investigated for the preparation of cephalexin microcapsules. Ethylcellulose and acrylate-methacrylate copolymer corresponding to the above ratios were selected as wall materials of the microcapsules. The effects of core-to-wall ratios on the surface characteristics and dissolution of the microcapsules were also studied. The coacervation technique with ethylcellulose as wall material gave the higher yield (90%) of microcapsules. The release of cephalexin from ethylcellulose walled microcapsules was slow whilst the release from those of acrylate-methacrylate copolymer was faster. The increase of deposition of wall materials due to the decrease of the core-to-wall ratio resulted in a decrease of dissolution rate.