Sol-Gel Derived Glasses for Medicine - Bio-Glasses: An Introduction - page 39

differently. The surfactant concentration is increased to the point of
saturation, called the critical micelle concentration (CMC). At the CMC
point, the surfactant molecules interact with each other, forming a
sphere. The spheres are all similar size and now do not interact with
the solution. However, the micelles pack themselves automatically (self-
assemble) into hexagonal or cubic arrays and can used as templates, so
the sol flows around them and, after gelation, the micelle spheres are
removed by thermal processing similar to that used in the conventional
sol-gel process.
3.8 SOL-GEL DERIVED BIOACTIVE GLASSES
Chapter 2 describes bioactive glasses. They are important materials
because they bond to bone and dissolve safely in the body and stimulate
bone repair. The original bioactive glass, Bioglass ® , was melt-quenched
and was a four-component system containing silica (SiO 2 ), soda (Na 2 O),
lime (CaO) and phosphate (P 2 O 5 ) [11]. The soda was only in the
composition to reduce the melting point of the glass, but the release
of sodium into the body can be detrimental to cells. Bioactive sol-
gel glasses are usually synthesised via the acid-catalysed route. They
provide the benefit that glasses can be bioactive with much more simple
compositions. Silica, lime, phosphate (58S, 60mol% SiO 2 ,36mol%
CaO, 4mol% P 2 O 5 ) compositions have recently gained approval for
implantation into bone defects in the USA when mixed with NovaBone ® ,
but glasses with 70mol% SiO 2 and 30mol% CaO (70S30C) are also
highly bioactive [12]. The mechanism for bone bonding is similar to that
of the melt-derived glasses (Chapter 2), with cation exchange with the
body fluid (Ca 2 + from the glass and H + from the fluid) on implantation
followed by nucleation of a hydroxycarbonate apatite (HCA) layer,
which forms the bond with bone.
Other active ions, such as silver and strontium, can also be incorpo-
rated into the sol-gel process. Silver has an impact in wound healing
applications, because small amounts of silver are known to kill bacteria
without harming healthy human cells. Silver can be released from the
glass at the rate of degradation of the glass, and therefore sol-gel glasses
can act as controlled delivery devices for silver [13]. Strontium ions have
been shown to be beneficial to patients suffering from osteoporosis, as
the ions inhibit osteoclast activity. Osteoclasts are the cells that remove
bone in the natural remodelling process, but for osteoporosis suffer-
ers the osteoclasts are much more active than the osteoblasts that are