This research examined the effect of particle size, microcracking and grain-boundary grooving in hydroxyapatite (HA) ceramics on osteoblast (OB) attachment, with the overall goal of understanding the role of physical characteristics in optimized scaffolds for bone tissue engineering.

Bimodally porous HA scaffolds were fabricated by foaming and sintering either micron-scale or nano-scale HA powder, yielding two sets with average grain diameters of 8.6 ± 1.9 pm and 588 ± 55 nm, respectively. OBs were seeded onto these scaffolds and counted at 0.5, 1, 2 and 4 hours for attachment and 1, 3 and 5 days for proliferation using a hemacytometer. Results showed that OB attachment and proliferation was not significantly affected by the change in grain size and may depend more on the bimodal porosity of the implant. However, as our attempt to reduce the error in the hemacytometer counts was not fully successful, a more accurate method of counting the OBs, such as a quantifiable dye, must be used to verify this trend.

While microcracks occur as a result of thermal processing of HA, these TEA-induced cracks are not easily controlled. For our studies we used Vickersinduced microcracks to quantify the effect of microcracking on OB attachment in HA. OB attachment was not significantly affected at one hour, but increased at four hours to 61% higher than on non-microcracked control specimens. This increase indicates that microcracking does have an effect on OB attachment and should be studied further, to assess its effect on OB proliferation and differentiation. It is not surprising that microcracks have a positive effect on OB attachment, as this mimics the natural process of bone remodeling. However, they are not likely to occur in nano-grained HA as a result of processing, as its small grain size falls below the known values of critical grain size for microcracking (G_CR) in HA.

grain boundary grooving in dense HA is also investigated in this dissertation. OBs were seeded onto grain boundary grooved and control dense HA discs and counted at one and four hours. At both times, the presence of grain-boundary grooves did not have a significant effect on OB attachment.

These findings eliminate microcracking and grain boundary grooving as contributing factors in increased OB attachment on HA substrates with nanoscale grains. Having ruled out these two phenomena, the observed increase in OB attachment on nano-grained HA is likely due to either the associated increase in surface roughness or the presence of nano-scale charge variations at grain boundaries. The latter is the subject of a current study by our group.