Single Particle Layer Research Articles

Selective through-drying in fixed beds

In a laboratory apparatus, drying experiments were carried out with fixed beds consisting of porous aluminum silicate particles wetted with the binary mixture 2-propanol-water. The air was either dry or preloaded with one of the components of the mixture. The drying rate and the composition curves were determined by analyzing the exhaust air by means of two infrared gas analyzers. The local distribution of the moisture and its composition within the fixed bed were measured by sheering the bed into single particle layers and determining the moisture content and the composition of the moisture of each particle layer with a combined vaporization and extraction method. Experimental results show that the selectivity of the drying process can be effected in any direction by choosing appropriate drying conditions. The axial moisture profile, which moves through the fixed bed with a constant pattern, as is known from the drying of a fixed bed wetted with a pure substance, only exists if the drying process is nonselective. In the case of selective drying, the moisture profile moves through the bed with a variable pattern due to considerable changes in the composition of the particle moisture over the whole fixed bed. While evaporation of both components takes place at the drying front, condensation of that component which is enriched in the fixed bed and evaporation of the other component occur some particle layers downstream. By this mechanism, it may even be possible that one of the components is totally displaced by the other. Therefore, in the case of selective drying, the components of the moisture are nonuniformly distributed in the bed.

Release of drugs from fatty suppository bases II. A rate-limiting interfacial process

In this report the effect of particle size on the release rate of sodium salicylate (a highly water-soluble drug) from fatty suppository bases is studied. In vitro it could be shown by comparing sedimentation measurements and release measurements that a rate-limiting process which is operating on the lipid side of the interface is causing the particle size effect. An equation is proposed describing the release flow as a function of particle size. It is shown that the time needed for the drainage of liquified lipid between a particle and the interface, a process which is essential for the drug to arrive at its dissolution site, is of the same order of magnitude as the time required for a wetted particle of a highly water-soluble drug to dissolve in an aqueous phase. This may imply that, in addition to the dissolution process, the drainage effect under certain conditions could become rate-limiting in the release of drugs from suppositories and thus for rectal absorption rate. Although the drainage time for a single particle layer appeared to be rather independent of particle size, reduction of particle size leads to an increase in the number of layers and consequently to a summation of drainage times for the same weight of drug. In vivo it is shown that probably an increase in ‘concentration viscosity’ (increasing the concentration of particles in the base) does not have a negative effect on absorption rate from the rectum, whereas an increase in the ‘fluid viscosity’, does have a negative effect on absorption rate for suppository bases and liquid-paraffin enemas. Since the drainage time is not influenced by the concentration of particles in the base but only by the viscosity of the lipid phase while the bulk transport of particles through the base is influenced by both effects, it is concluded that in line with in vitro data the drainage time of particles determines the release flow of highly water-soluble drugs also in vivo.

The Effect of Porous Surface Configuration on the Tensile Strength of Fixation of Implants by Bone Ingrowth

In an attempt to gain information that could be directly applied to the design of clinical porous-surfaced prostheses intended for biological attachment by bone ingrowth, the tensile strength of the bone-implant interface was expressed as a function of 2 fundamentally different porous-surface configurations. Using powder metallurgy techniques, standard 3-hole fracture fixation plates were prepared with both a single and a multiple layer of spherically shaped metal powder particles on the bone-contacting surface to produce implants with different porous surfaces. These plates were implanted onto the lateral aspect of canine femurs for periods of 4, 6, 8, 12, 18, and 24 weeks. Mechanical tests were performed to measure the tensile strength of fixation of the implants by the ingrowth of bone. The results of the mechanical tests indicated that implants with the multiple particle layer surfce configuration develop a greater tensile strength of fixation than do implants with the single particle layer surface configuration. In addition, this fixation strength develops more quickly if the cortical bone is petaled prior to implantation. These findings should be considered when designing porous-surfaced implants intended for fixation by bone ingrowth.