Bioactive Glass-Based Coatings and Modified Surfaces: Strategies for the Manufacture, Testing, and Clinical Applications for Regenerative Medicine - Biological and Biomedical Coatings: Processing and Characterization - page 388

The ideal bioactive glass will degrade at a controlled rate over time and be replaced
with natural, healthy host tissue. However this is no easy task because the rate of tissue
ingrowth varies with tissue type, tissue age, location, and patient [8].
Bioactive glasses have been developed and applied as coatings, foams, powders, and
combined with polymeric binders to form novel composite materials.
Figure 9.4 shows that tissue response is directly related to materials compositions for
four-component silicate glasses. For melt-derived glasses, increasing silica content results
in decreasing dissolution rates. As silica content reaches 60%, bioactivity is eliminated.
It is now accepted that silica and not Na 2 O is the active component of bioactive glasses.
Na 2 O is a network disrupter. Melt-derived glasses are not inherently porous. Introduction
of pores increases surface area, potential bioactivity, and surfaces for tissue ingrowth. Pores
have been introduced into melt-derived glasses but with limited success. Monoliths with
pore diameters 200 to 300 μm with a total porosity of 21% have been produced; however,
the pores were not interconnected. These materials did not mimic trabecular bone [8].
It is possible to extend the compositional range of bioactive glasses beyond 60 mol%
silica content via manufacturing the glasses via the sol-gel method.
Sol-gel bioactive glasses (Figure 9.5) have many advantages over melt-derived glass.
These are:

• Lower manufacturing temperatures (600-700°C)
• Higher silica and low alkali content (up to 90 mol% silica) compositions that are
bioactive
• Better control of bioactivity via changes in composition and or processing
temperature
• Inherently porous with porosity content changed via processing temperatures
• Can be foamed resulting in significantly higher surface areas (ranging from 150 to
600 m 2 g 1) ratios, including pore interconnectivity and enhanced bioactivity
• Fewer components required to make bioactive glasses (e.g., SiO 2 -CaO-P 2 O 5 ; SiO 2 -
CaO; pure silica compositions) [8]

Bioactivity is higher because the increased surface area results in more silanol groups on
the surface which in turn increases the number of nucleation sites for HCA layer forma-
tion. Resorbtion is enhanced and can be controlled via changing porosity.
Sol-gel bioactive glasses follow the same mechanisms of surface dissolution as melt-
derived glasses (Table 9.3). The degree of bioactivity and thickness of the tissue boundary
layer varies with chemical composition and textural characteristics. For example, for 58S
composition, sol-gel glass has significantly faster dissolution rates and faster HA surface
layer formation after 6 h of immersion in simulated body fluid (SBF) in vitro. After this
time, Si dissolution from the glass network continues and stabilizes after 4 days.
Dissolution rate of Si from glass network exhibits a time-dependent concentration pro-
file response. Dissolution is rapid at short times (e.g., up to 1 h of immersion) and slow after
a time t * (e.g., greater than 6 h of immersion). This can be described by the Douglas and
El-Shamy equation:

Q = kty (9.1)

where Q is the concentration of silicon ions in solution, k is the rate constant, t is time, and
y is ½ for first stage (Table 9.3) of silicon dissolution.