Titanium Phosphate Glass Microspheres as Microcarriers for In Vitro Bone Cell Tissue Engineering - Biointerfaces: Where Material Meets Biology - page 108

in the glass structure without using excessive amounts of P 2 O 5 becomes a
significant technical challenge, and the presence of small amounts of water
can cause crystallisation. 40

d n 3 r 4 n g | 3

TIP: A good way to overcome this challenge is to (1) add some proportion of
phosphorus in the form of a stable oxide that includes one of the modifying
cations, e.g. CaHPO 4 or NaH 2 PO 4 , and then (2) add any remaining minimal
amount as P 2 O 5 .

Water uptake (and also P 2 O 5 loss due to volatilisation) can be minimised by
melting in ampoules or covered crucibles. A useful alternative to the con-
ventional melt-quench process is the sol-gel process, which involves the
mixing of inorganic alkoxide and metal chloride precursors to form a colloid
through hydrolysis and polycondensation reactions and subsequent thermal
treatment to yield a dense, porous, glassy material. This process has a high
level of adaptability, whichmeans that sol-gel glasses can be deposited as thin
films on a substrate or drawn into macro- and micro-sized fibres or obtained
as micrometre- or nanometre-sized particles. The sol-gel process can also be
carried out at temperatures that are up to an order of magnitude lower than in
the melt-quench process, 41,42 which facilitates the introduction of active
biological ingredients such as proteins, antibiotics and chemotherapeutic
molecules into the glass structure. It is worth noting, however, that the
chemistry involved in phosphate sol-gel synthesis presents considerable
technical challenges because alkyl phosphates undergo hydrolysis at con-
siderably slower rates than alkyl silicates, and the synthesis and drying steps
can require several weeks for completion. As a result, sol-gel synthesis of
titaniumphosphate glasses remains a relatively less explored area of research.
The production of microspheres from melt-quenched glasses takes place
using a technique generally known as flame spheroidisation or flame
spheronisation; both names belie the innate simplicity of the underlying
principle, which is essentially the 'moulding' of glass microparticles into
spherical form via surface tension forces whilst in the molten state. 43,44
Various designs for this procedure as well as apparatuses have been
described in the literature, the basic components of which are: (1) a high-
temperature jet, (2) a feed assembly, and (3) collectors, with particles tra-
velling along the flame axis. The high-temperature jet can be a gas/oxygen
flame, with the gas being acetylene or liquid petroleum gas, or it can be a
radio-frequency plasma flame; 45-49 as mentioned in Section 5.2.2, a high-
temperature flame can also be produced by using methylacetylene-
propadiene propane (commercial name: MAPP gas) and can be used for
heating, soldering, brazing and welding due to its high flame temperature of
2925 1C in air. 50 An alternative to the gas flame is a vertical tubular electric
furnace, in which particles dropped at the top end of the furnace are
transformed into microspheres as they fall through the furnace under the
effect of gravity. 48,49,51

.

It is worth noting that the choice of the high-