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 385

P 2 O 5 ). For these glasses, no bonding with bone will occur if the ratio of Ca to P is very low
(Figure 9.4). There is a compositional dependence (in weight percent) of bonding to bone
tissue for the Na 2 O-CaO-P 2 O 5 -SiO 2 glasses (shown in Figure 9.4). All the glasses in Figure
9.4 contain a constant 6 wt.% P 2 O 5 . Region A represents the bioactive bone-bonding bound-
ary as glasses outside this boundary will not form a bond with bone. Region B glasses
include soda-lime-silicate glasses and exhibit first-generation materials responses at their
surface. Glasses within region C are resorbable and hence exhibit second-generation mate-
rials responses. Region D glasses are not technically feasible using traditional melt-derived
glass manufacturing techniques.
Soft tissue bonding occurs within the dashed line of Figure 9.4 via the collagenous con-
stituents strongly adhering to the bioactive silica glasses. For example, in vitro attachment
of collagen to 45S5 surfaces occur after 10 days exposure, closely representing that found
in vivo. In particular, collagen fibers were woven into the surface by growth of the HCA
layer to about 30 to 60 μm of the 100 to 200 μm total interfacial thickness.
Bioactive glasses can be divided into two classes on the basis of their host tissue/mate-
rial surface interface response. Glass compositions that lie within the region bound by the
dashed line in Figure 9.4 have been classified as Class A bioactive glasses. These bioactive
glasses bond to both hard and soft tissues. Class B bioactive glasses lie outside the bound-
aries of the dashed line but within the area of Figure 9.4 described by region A. Class B
bioactive glasses bond to hard tissues only.
The 45S5 bioactive glass composition is a Class A biomaterial. It forms a biological bond
with bone 20 h after implantation. Upon exposure to a physiological solution, biological
proteins and growth factors are absorbed on to the glass surface. The glass structure starts
to degrade within 1 h and hydrated silica has leached into solution. At the surface, silanol
bonds (Si-OH) form and polycondense to form a stable silica-gel surface layer. Between
1 and 2 h, proteins and growth factors are still being absorbed while amorphous calcium,
carbonates, and phosphates migrate from the glass structure and become absorbed into the
silica-gel layer, which results in the crystallization of hydroxyapatite (HA). Between 2 and
10 h, proteins and growth factors are still being absorbed while the HA layer crystallizes

SiO 2

Bioactive
(Second generation)

Inert
(First generation)

B

I B = 8

I B = 10

A-WGC
(high P 2 O 5 ) Resorbable

(Second generation)

C

A

45S5
Bioglass

I B = 2 I B = 5

I B = 0

D

CaO Na 2 O

FIGURE 9.4
Compositional dependence (in wt.%) of tissue response with silica-based bioactive glasses. All compositions
in region A have constant 6 wt.% of P 2 O 5 . AW-GSC (apatite wollastonite glass-ceramic) has a higher P 2 O 5
content (16.3 wt.%). Soft tissue bonding occurs within the area bounded by the dashed line I b gives the index
of bioactivity [8].