Dense Gas Processing Research Articles

Loading of 5-fluorouracil onto Halloysite nanotubes for targeted drug delivery using a subcritical gas antisolvent process (GAS)

Halloysite clay nanotubes are efficient biocompatible nanocarriers for pharmaceutical products. A gas antisolvent (GAS) process employing subcritical carbon dioxide delivers a product with high loading (43 %) and free of residual solvent. An anticancer agent, 5-fluorouracil, was loaded on nanoclay particles. A quantitative distinction is made between tightly-bound (adsorbed) and loosely-bound (co-precipitated) drug. The physical stages of the dense gas process are clearly outlined. The strongly attached drug is released completely at pH = 7.4 but only up to 30 % in pH = 1.2. The release follows first-order kinetics with a characteristic time of 2 h. The mass ratio of clay nanotubes to 5-fluorouracil affects the loading with adsorbed drug as well as the ratio of loose to tightly attached drug molecules. The system Halloysite-5-fluorouracil can be implemented in drug delivery systems to achieve a desired release profile. The GAS process is a one-pot green process with a low solvent consumption and high-purity product.

Development of a novel continuous dense gas process for the production of residual solvent-free self-assembled nano-carriers

A continuous dense gas process has been developed based on the principles of process intensification. The continuous process is designed for the production of self-assembled nano-carriers for pharmaceutical applications such as liposomes, polymersomes, and micelles. Carbon dioxide (CO2) is being used during the dense gas process. The dense gas CO2 process is regarded as a green technology since the process does not generate CO2 as a product, but uses the existing CO2 from the environment for the process. Removal of residual solvent is a key aspect of pharmaceutical formulation and an effective single-step process to achieve that endpoint has been demonstrated. Ethanol and dichloromethane were investigated as residual organic solvents to be removed by CO2. Dichloromethane was eliminated to a level undetectable by gas chromatography, while ethanol content was reduced down to 1.70 wt % with an extraction efficiency of 95.7%. The average hydrodynamic diameter of polymersomes produced by the continuous process was 300 ± 10 nm. The present study provides a basic platform for up-scaling the production of self-assembled nano-carriers.

Process intensification: Nano-carrier formation by a continuous dense gas process

Formation of nano-carriers such as vesicles and micelles using dense gas processing has been under extensive research for decades. Several dense gas processes have been developed to produce nano-carriers, most of them being batch processes. In the present study, a novel continuous dense gas, known as nano-carrier by a continuous dense gas (NADEG) process was developed as an evolution of a dense gas batch process known as the Depressurization of an Expanded Solution into Aqueous Media (DESAM) process. Transforming a batch process into a continuous process is a main aspect of process intensification. The NADEG process developed in this work enhances the production output of the batch process while producing nano-carriers free of harmful residual organic solvent. The NADEG process is a one-step process for the production of nano-carriers with lower size and higher encapsulation efficiency than the nano-carriers produced by other batch processes. Encapsulation efficiencies as high as 15% were achieved using liposomes to encapsulate a model hydrophilic compound (isoniazid) while encapsulation efficiencies of 10% were achieved in polymersomes for the same model compound.

Formation of nanocarrier systems by dense gas processing.

Nanocarrier systems, such as liposomes, polymersomes, and micelles, find applications in the delivery of a wide range of compounds, including targeted delivery of pharmaceuticals. Nanocarrier systems have the ability to increase the bioavailability, reduce toxicity, and avoid undesirable interactions of active pharmaceutical ingredients. In this work, a novel dense gas technique known as depressurization of an expanded solution into aqueous media (DESAM) was used to produce different types of nanocarrier systems. The effects of using different types of dense gases and different operating temperatures were investigated. Encapsulation of hydrophilic compounds in the vesicles (liposomes and polymersomes) was also studied. The highest encapsulation efficiencies in liposomes and polymersomes achieved were 10.2 and 9.7%, respectively. The DESAM process was also able to reduce the residual solvent content in the product to 2.2% (v/v), which is significantly lower than the solvent residual levels reported for conventional processing.

Colloidal characteristics and formulation of pure protein particulate vaccines

We recently reported that dense gas processing of the protein ovalbumin (OVA) resulted in the formation of particles that were insoluble in water and which retained their immunogenicity in vivo. In the present study, the colloidal properties of these pure protein particles were investigated to in part inform rational formulation approaches. The colloidal properties of the particles, in terms of size, zeta potential and pH-dependent surface and solution properties, were examined. In phosphate-buffered saline (pH7.4), flocculation of the particles was observed, which was prevented when particles were suspended in acetate buffer at pH lower than 4. The resulting particle size was 300nm with low polydispersity and zeta potential of 22.9±3.1mV (mean±SEM, n=3) at pH3. Dense gas OVA particles were also prevented from flocculation using steric stabilisation with Pluronic F127. In this form the particles were stable in Krebs-Henseleit solution for 48h at room temperature. These findings indicate that insoluble pure protein particles produced by dense gas processing have desirable characteristics as particulate vaccines, including consistency of particle size under controlled conditions and high colloid stability.

Development of a continuous dense gas process for the production of liposomes

A new process called Continuous Anti-Solvent (CAS) process was developed for the production of liposomes using supercritical CO 2. Unlike the current dense gas technologies, CAS method is a single step and continuous process. Preliminary experiments were conducted in semi-batch mode to determine the most suitable operating conditions (stirring speed = 225 rpm; water/lecithin mass ratio = 21) to ensure an efficient phase mixing in the autoclave. Then, two procedures were developed for the CAS process in the continuous mode. According to the results, the single exit procedure enhances the phase mixing in the autoclave with the formation of a CO 2-in-water emulsion which is a good precursor to liposome formation. Liposomes prepared with the CAS method ( P = 9 MPa; T = 308 K; CO 2 flow rate = 0.3 kg h −1; organic solution flow rate = 240 mL h −1; water flow rate = 180 mL h −1) are spherical and multilamellar with a medium diameter included between 10 and 100 μm.

Dense Gas Processing of Polymers

There is a growing global awareness about environmental pollution, and many sanctions and sustainable practices have been implemented. In particular, the use of volatile organic compounds (VOCs) is a practice that is being limited and minimized world-wide. These VOCs are not only damaging to the environment, but are also an occupational hazard. The polymer processing industry is known to use VOCs extensively for polymerization, fractionation, plasticization, degradation, extraction and purification. More environmentally-friendly methods to circumvent the use of these toxic and hazardous compounds are being explored. The use of dense gases in polymer processing can respond to the need for more environmentally-friendly industrial processes. Products with high-purity, sterility, and porosity can be achieved using dense gas technology (DGT). Currently, DGT has been used for different aspects of polymer processing including polymerization, micronization, and impregnation. Due to its high solubility in polymers and diffusivity, dense CO2 can penetrate and plasticize polymers, whilst impregnating them with low-molecular weight CO2-soluble compounds. The dense CO2 properties of inertness, non-toxicity, and affinity for various therapeutic compounds are specifically advantageous to the medical and biomedical industries. Biodegradable polymers and other medical-grade polymers have benefited from the application of DGT. The aim of this review was to show the versatility of dense CO2 for polymer processing applications, specifically polymerization, polymer blend preparation, drug loading and sterilization.

The Depressurization of an Expanded Solution into Aqueous Media for the Bulk Production of Liposomes

A new dense gas process for the formation of liposomes has been developed: depressurization of an expanded solution into aqueous media (DESAM). The technique provides a fast and simple process for bulk liposome formation. As an alternative to current dense gas technologies, the DESAM process reduces the pressure requirements for liposome formation. Liposomes with diameters between 50 and 200 nm were formed. For all samples produced using ethanol as the solvent, the average effective diameter ranged from 119 to 207 nm. When chloroform was used as the solvent, the average effective diameter increased to 387 nm. The residual solvent volume fraction in the liposomal product was less than 4% v/v, which is approximately one-quarter of the value reported for some other dense gas liposome formation methods. The liposomal samples were stored after formation at 5 degrees C for up to 8 months, with the average effective diameter and polydispersity increasing by only 13% and 7%, respectively, indicating high stability of the formulations.

Generic construction of single component particles that elicit humoural and cellular immune responses without the need for adjuvants

Insoluble, pure protein particles could be advantageous as single-entity vaccines or as carriers for small peptide epitopes. Dense gas anti-solvent precipitation was employed to produce pure protein particles which were found to be insoluble in water. As particulate and multimerized antigens are more immunogenic and hence more advantageous for vaccination, particles were produced via this method using ovalbumin as a model antigen. The particles produced had a mean diameter of ∼300 nm, and remained as discrete particles at low pH. At neutral pH or in the presence of electrolyte, the particles exhibited predictable flocculation behaviour to produce aggregates 1–5 μm in diameter. Immunisation of mice with these flocculates elicited specific ovalbumin antibody production, T-cell proliferation and a cytotoxic T-cell response, all in the absence of adjuvant. Thus, dense gas processing could be used as a generic method to produce pure protein particulate vaccines.

The fabrication of elastin-based hydrogels using high pressure CO 2

The aim of this study was to investigate the effect of high pressure CO 2 on the crosslinking of elastin-based polymers and the characteristics of the fabricated hydrogels. A hydrogel was fabricated by chemically crosslinking α-elastin with glutaraldehyde at high pressure CO 2. The effects of pressure, reaction time, and crosslinker concentration on the characteristics of the fabricated hydrogels were determined. The reaction time had negligible effect on either the swelling ratio or the pore size of the fabricated hydrogels. Increasing the processing pressure from 30 bar to 150 bar resulted in a 60% increase in the hydrogel swelling ratio. The crosslinked hydrogels displayed stimuli-responsive characteristics towards temperature and salt concentration. The dense gas process facilitated coacervation, expedited the crosslinking reaction, and dramatically affected the micro- and macrostructures of pores within the sample. The results of micro-CT scan and SEM images demonstrated that pore interconnectivity was substantially enhanced for α-elastin hydrogels fabricated using high pressure CO 2. Dense gas CO 2 reduced the wall thickness and size of the pores and importantly induced channels within the structure of the α-elastin hydrogels. In vitro cell culture studies demonstrated that the channels facilitated fibroblast penetration and proliferation within α-elastin structures.

Dense Gas Processing of Micron-Sized Drug Formulations Incorporating Hydroxypropylated and Methylated Beta-Cyclodextrin

Because of their importance in pharmaceutical applications, hydroxypropyl-beta-cyclodextrin and methyl-beta-cyclodextrin have been selected to study the formation of micronized complexes incorporating active pharmaceutical ingredients (APIs) and cyclodextrins (CDs) by dense gas (DG) processing. A single-step DG technique was used as an alternative to conventional methods for the manufacturing of API/CD complexes. The DG technology is highly attractive in the pharmaceutical industry because of its potential to generate micronized particles with controlled particle size distributions at moderate operating conditions. The effect of the aerosol solvent extraction system (ASES) processing on the dissolution performance of naproxen (NPX) was examined. The CDs were produced as microspheres smaller than 3 microm. The coprecipitation of each CD with NPX resulted in the production of microparticles with enhanced dissolution rates. The ASES was operated under mild conditions and generated micron-sized spherical particles that could be of particular interest in formulations for pulmonary delivery. Particular advantages of the technique are that (1) nontoxic solvents are used, and (2) it is suitable for the processing of thermally labile compounds. The proposed process can create opportunities to improve current administration routes and exploit novel delivery systems for drug formulations incorporating CDs.