Aerosols porous particles

Ben-Jebria A, Eskew ML, Edwards DA (2000) Inhalation systems for pulmonary aerosol delivery in rodents using large porous particles. Aerosol Sci Technol 32 421-433. 

In an attempt to increase the amount of particles retained in the lungs, large porous particles with low density (p < 0.1 g/cm2) have been designed (Edwards et al. 1997). The particles were composed of 50% lactide and 50% glycolide. Porous and nonporous particles loaded with testosterone were aerosolized into a cascade impactor system from a dry powder inhaler (DPI) and the respirable fraction was measured. Nonporous particles (d = 3.5 pm, p = 0.8 g/cm3) exhibited a respirable fraction of 20.5 3.5%, whereas 50 + 10% of porous particles (d = 8.5 pm, p = 0.1 g/cm3) were respirable, even though the aerodynamic diameter of the two particle types were nearly identical. Porous particles as a consequence of their large size and low mass density can... 

Lastly, particle engineering as a method to improve suspension stability may be an alternative. Weers et al. and Dellamary et al. describe the use of hollow porous particles to decrease the attractive forces between particles in suspension (43,51). The similarities between the particles and the dispersing medium (the propellant system enters and fills the porous particles) reduces the effective Hamaker constant that corresponds to forces of attraction, and also makes the density difference between the propellant and the particles smaller. The FPF of these aerosols was reported to be around 70%. 

As explained in Sect. 38.2, to produce particles using a conventional spray pyrolysis (CSP) process, the precursor is first atomized into a reactor where the aerosol droplets undergo evaporation and solute condensation drying and thermolysis of the precipitate particles at higher temperature forms micro- or meso-porous particles, and, finally, sintering of these porous particles forms dense particles. However, sub-micrometer to micrometer-sized particles traditionally are formed using the CSP process based on the one-droplet-to-one-particle (ODOP) principle due to the difficulty of generating very fine droplets (below 1 pm) [1-3]. 

Mosleh M. Preparation of micro porous ceramic membranes by flame generated aerosol nano-particles [dissertation]. Lyngby Technical University of Denmark, 2004. 

An interesting example of a large specific surface which is wholly external in nature is provided by a dispersed aerosol composed of fine particles free of cracks and fissures. As soon as the aerosol settles out, of course, its particles come into contact with one another and form aggregates but if the particles are spherical, more particularly if the material is hard, the particle-to-particle contacts will be very small in area the interparticulate junctions will then be so weak that many of them will become broken apart during mechanical handling, or be prized open by the film of adsorbate during an adsorption experiment. In favourable cases the flocculated specimen may have so open a structure that it behaves, as far as its adsorptive properties are concerned, as a completely non-porous material. Solids of this kind are of importance because of their relevance to standard adsorption isotherms (cf. Section 2.12) which play a fundamental role in procedures for the evaluation of specific surface area and pore size distribution by adsorption methods. 

Gebhart, J. and J. Heyder, Removal of Aerosol Particles from Stationary Air within Porous Media, J. Aerosol Sci. 16 175-187 (1985). 

Maa et al. [3.84] used spray drying and spray freeze-drying (see Chapter 5, [5.13, 5.14]) to produce protein powders for inhalation from deoxyribonuclease (rhDNase) and anti-IgE monoclonal antibody (anti-IgE Mab) with lactose as carrier. Spray freezedrying produced light and porous protein particles with superior aerosol performance. 

Collection on porous filter media is perhaps the most efficient means of particle removal. Aerosol filtration is an effective means of air purification, while at the same time it has been widely used for sampling airborne material for mass and chemical composition determination. A wide variety of filter media is available, ranging from fibrous mats of relatively inert material to porous membranes. Fibrous mats and model filter arrays appear microscopically as stacks of overlaid cylinders, where the cylinders may be smooth or rough. In contrast, the membrane media are plastic films with microscopic holes of nearly uniform size nuclepore filters, for example, are produced of sheets of polyester, and the holes are introduced by neutron bombardment. 

A dry packed column with porous material was used for the characterization according to size of the PVAc latex samples. The packing employed was CPG (Controlled Pore Glass), 2000 A, 200-400 mesh size. Deionized water with 0.8 gr/lit Aerosol O.T. (dioctyl sodium sulphosuccinate), 0.8 gr/lit sodium nitrate and 0.4 gr/lit sodium azide served as the carrier fluid under a constant flowrate. The sample loop volume was 10 pC A Beckman UV detector operating at 254 nm was connected at the column outlet to monitor particle size. A particle size-mean retention volume calibration curve was constructed from commercially available polystyrene standards. For reasons of comparison, the samples previously characterized by turbidity spectra were also characterized by SEC. A number of injections were repeated to check for the reproducibility of the method. 

The collection of the pyrolysis oils is difficult due to their tendency to form aerosols and also due to the volatile nature of many of the oil constituents. As the aerosols agglomerate into larger droplets, they can be removed by cyclonic separators. However, the submicron aerosols cannot be efficiently collected by cyclonic or inertial techniques, and collection by impact of the aerosols due to their Brownian or random motion must be utilized. A coalescing filter is relatively porous, but it contains a large surface area for the aerosol particles to impact by Brownian motion as they are swept through by the pyrolysis gases. Once the aerosol droplets impact the filter fibers, they are captured and coalesce into large drops that can flow down the fibers and be collected. 

Figure 2 illustrates the kinds of particles made in this study. Here, polystyrene nanoparticles (170-nm diameter) were prepared in an ethanol cosolvent and spray-dried to produce large thin-walled particles with a wall thickness of approximately 400 nm, or 3 layers of nanoparticles. The study showed that such particles aerosolize effectively from a small inhaler and redisperse into nanoparticles once in solution. Nanoparticle aggregates were made with a variety of different materials and through many different spraydrying conditions, suggesting that these large porous nanoparticle systems are robust and functional as aerosols. 

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