Microcellular materials, forms

Microcellular materials exist in many forms. Methods for production of these materials are as varied as potential applications. This chapter reviews the technology of one class of microcellular materials, microcellular foams, which are sought for biomedical applications. Included is a description of several methods of foam production, foam morphologies, and present uses for microcellular foam materials. New methods of microcellular foam production and potential uses for the resultant foam materials are important to those interested in biomaterials and contemporary biomedical applications. It is for this reason that advances in microcellular foam formation are emphasized in the final section of this chapter. Increasingly, it is becoming evident that microcellular foams can be used effectively in many medical applications, particularly polymeric foam materials which are being investigated in this laboratory. For this reason, the focus of this chapter pertains to possible biomedical applications of polymeric microcellular foams. 

Open-pore microcellular aluminium foams can be produced by a process known as replication . This consists in infiltration of NaCl powder preforms by a melt, which is then solidified to form a composite. The NaCl is subsequently leached out with water, to leave a network of open pores, of volume fraction roughly varying between 65 and 90% [15], The foams can be produced to feature good microstructural homogeneity over a comparatively wide range of metal alloy compositions, pore size and component shape. They furthermore serve as attractive model materials for the investigation of microstructure/property relations in metal foams because of their macroscopically uniform and fine-scale microstructure, and because the metal making the foam can be varied with relatively wide latitude and produced free of internal defects. 

Porous materials can also be coated with zeolite films by direct synthesis. For example, microcellular SiOC ceramic foams in the form of monoliths were coated on their cell walls with thin films of silicalite-1 and ZSM-5 using a concentrated precursor solution for in situ hydrothermal growth (Fig. 9).[62] The zeolite-coated monoliths show a bimodal pore system and are thermally stable to at least 600 °C. A related strategy is based on the conversion of macroporous Vycor borosilicate glass beads, having pores of about 100 nm, to MFI-type zeolite-containing beads retaining the same macroscopic shape.[63] This conversion was achieved by hydrothermal treatment with an aluminium source and a template such as TPABr. 

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