Date of Award

5-2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Program

Biomedical Engineering

Research Advisor

Joo L. Ong, Ph.D.

Committee

Joel Bumgardner, Ph.D.Judith Cole, Ph.D.Warren Haggard, Ph.D.Satoru Nishimoto, Ph.D.Yunzhi Yang, Ph.D.

Keywords

cell signaling, hydroxyapatite, tricalcium phosphate, tissue engineering, osseointegration, ultrasound, perfusion

Abstract

Bone tissue engineering represents a strategy for the repair or regeneration of damaged bone in the body. The science underlying this clinical therapy bridges the traditional fields of cell biology, materials science and mechanical engineering with the aim to identify how cells behave on physiologically relevant materials with natural mechanical stimuli. The objectives of this research were to develop and characterize calcium phosphate ceramic scaffolds matched to the local architecture of natural trabecular bone and to apply tissue engineering strategies for the study of cell behavior in both in vitro and in vivo models.The specific role of environment on cell stress pathways was evaluated on three dimensional (3-D) calcium phosphate scaffolds resembling vertebral trabecular bone. A scaffold foam dipping technique was employed in the fabrication of fully sintered hydroxyapatite and tricalcium phosphate scaffolds. Study of the early cell behavior on two dimension (2-D) controls and scaffolds was performed using human embryonic palatal mesenchyme cells (HEPM), an osteoblast precursor cell line. Cell stress signaling was identified in response to the 3-D architecture using; members of the mitogen activated protein kinase cascade, cell survival signals and adhesion dependant proteins. The application of low intensity pulsed ultrasound (LIPUS) or fluid perfusion further stimulated cell-scaffold hybrids for short and long term in vitro study. Additionally, an animal model was characterized using the scaffolds for the repair of a segmental defect in the canine mandible.Study of the cell stress signaling mechanisms identified high activation of stress pathways on 3-D materials compared to controls with a corresponding increase in anti- apoptosis signaling. Similar trends were found with LIPUS stimulation demonstrating that changes in adhesion proteins during attachment may account for the alteration in stress pathways activated by bone precursors. The absence of cell death and the activation of an anti-apoptosis signal suggest that cells are able to manage these stress levels which may be required for proper function. Supporting this theory, long term in vitro perfusion studies demonstrated that the process of cell transition into a mature bone phenotype was improved with the fluid shear forces of perfusion. Finally, the scaffolds were applied for repair of a segmental defect in the canine mandible and demonstrated extensive bone in-growth and partially-organized, lamellar collagen fiber assembly characteristic of organized bone. The open architecture of the scaffold design also allowed for substantial blood vessel infiltration.This research demonstrated the importance of architecture on bone cell response for in vitro cell study and for clinical application. The scaffold design provides a bridge between laboratory based signaling mechanisms and the development of clinical therapies in regenerative orthopedics.

DOI

10.21007/etd.cghs.2007.0017

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