Biomedical Engineering Reference
In-Depth Information
or fibres, drying stresses are small, because the path of evaporation is
short and the stresses can be accommodated by the material. For
monolithic objects greater than about 1 cm in diameter, the path from
the centre of the monolith to the surface is tortuous and the drying
stresses can introduce catastrophic fracture. For gels produced by the
hydrolysis and condensation route, drying has to be carried out with low
heating rates. Increasing pore size and obtaining pores with a narrow
distribution reduces tortuosity.
The difference between the modern development of sol-gel derived
materials, and the classical work of Ebelman is that now drying of the
monolithic silica optics can be achieved in days rather than years.
3.7 MAKING PARTICLES
Particles can be made by crushing and grinding monoliths made by the
acid-catalysed sol-gel process. In fact, when making powders, the drying
process is less critical, because cracks are welcome as they facilitate
grinding. Grinding sol-gel glasses is inherently easier than grinding
melt-derived glasses because the sol-gel glass contains nanoporosity.
A more elegant method for making small particles is what has become
known as the St ober process [8]. The St ober process uses sol-gel pro-
cessing to produce spherical nanoparticles (or a least sub-micrometre
particles). The process is based on colloidal processing. Using ammo-
nium hydroxide as the catalyst rather than using acid tips the pH above
the isoelectric point of soluble silica (silicic acid) [3]. The pH causes
repulsion between newly formed silica particles and causes polyconden-
sation to be terminated. Therefore, after primary particles form due
to hydrolysis, some condensation occurs to form spherical secondary
particles, but bonds do not form between the particles, so the secondary
particles remain as particles (Figure 3.1b). The final size of the spherical
silica powder can be controlled by several process variables: pH, the
types of silicon alkoxide (methyl, ethyl, pentyl, esters, etc.) and alcohol
(methyl, ethyl, butyl and pentyl) mixture used, and reactant tempera-
ture. In biomedical applications, small silica spheres have potential for
cell labelling and drug delivery (see Figure 4 in colour section). This is
because, if they are small enough to enter a cell and do not cause the cell
to change behaviour, they can be used to carry therapeutic agents, for
example small drug molecules.
Of particular benefit are small particles that contain nanopores.
Drugs and growth factors can then be loaded into the particles and
the payload can be delivered into cells by the particles. There is now a
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