Titanium Phosphate Glass Microspheres as Microcarriers for In Vitro Bone Cell Tissue Engineering - Biointerfaces: Where Material Meets Biology - page 122

osteogenic markers, to increase osteoblast proliferation and/or to induce
early osteoblasts towards an osteogenic differentiation pathway. 2,14,67 On the
other hand, enhanced endothelial cell proliferation, the formation of cell
tubules networks or the stimulated secretion of angiogenic growth factors
(i.e., vascular endothelial growth factor, VEGF) are some of the angiogenic
responses that have been observed on cells exposed to trace elements such
as Cu and Co. 68 Some metal ions were also found to present antimicrobial
activity by altering the culture pH or by compromising cell integ-
rity. 12,27-29,31,34-36 More generally, most metal ions play a key role in meta-
bolic pathways, by interacting with proteins and in particular with enzymes.
When investigating the biocompatibility of phosphate glasses containing
TiO 2 , researchers have tended to use one or more of the following three
approaches. The first and most common involves immersing glass speci-
mens in artificially prepared fluids such as citric acid, Tris buffer, Tris-HCl,
or simulated body fluid (SBF) for example, and then using techniques such
as scanning electron microscopy (SEM) or X-ray diffraction (XRD) spec-
troscopy to detect the formation of apatites on the glass surface. 67,69 This
approach is quite popular among biomaterials researchers but doubts exist
regarding the ability of the approach to confirm whether a material that
forms hydroxyapatite on its surface will elicit a favourable response from
cells and tissues cultured on the material surface. Some materials are cap-
able of directly bonding with bone tissue without apatite formation; 70,71
several titanium glass compositions have shown favourable results in cell
culture studies with upregulation of a number of bone-related genes, and yet
they have been found to show no evidence of apatite formation. 14,72,73
Consequently, in vitro cell culture studies are a better means to investigate
material biocompatibility since they yield qualitative and quantitative data
on cell viability, adhesion, proliferation, differentiation and toxicity as well
as gene expression, thereby providing us with an improved understanding of
the ability of cells to adhere and proliferate on the material surface. In the
case of titanium phosphate glasses, increasing the titanium oxide content
has been found to improve cell viability, attachment, and proliferation, as
confirmed by gene expression studies, which reveal significant upregulation
of important bone cell markers such as COLIA1, ALP, osteonectin and
Cbfa-1. 14-16,59,74,75 A threshold concentration of TiO 2 , beyond which there is
no further improvement in glass biocompatibility, has also been observed. 15
The morphology of the titanium glass surface may exert significant influence
on cell proliferation, with rough and highly porous samples inhibiting cell
proliferation over relatively short time-scales. 59
There is a relative lack of in vivo implantation studies concerning titanium
phosphate glasses. The few studies where in vivo implantation has been
performed have used either murine or lapine models with the defect site
being filled with glass granules of predefined size over periods of 5-12
weeks. Titanium glass solubility has been found to exert a significant effect
on in vivo glass behaviour with glasses that are less soluble undergoing
degradation to a much lesser extent in physiological environments.
A combination of solution-mediated and cell-mediated processes is

d n 3 r 4 n g | 3

.