Author: Gina Policastro

Publisher:

Published: 2016

Total Pages: 272

ISBN-13:

Both naturally occurring and synthetic polymeric materials are currently under investigation for use as tissue engineering constructs for bone defect repair. While, naturally occurring proteins and polysaccharides provide the inherent biochemical signaling necessary for proper bone regeneration, these constructs often lack mechanical and chemical properties suitable for this application. On the contrary, synthetic polymers, such as poly-¿-caprolactone (PCL) and polypropylene fumarate (PPF), are advantageous because of their tunable mechanical, degradation and chemical properties. However, these polymers tend to degrade into acidic byproducts that cause chronic inflammation in vivo and overall failure of the implant. Independently, both naturally occurring and synthetic materials fail to meet all of the necessary requirements for osteogenic tissue engineering. More recently, research has focused its attention on the synthesis of polymeric materials functionalized with bioactive peptides to overcome the previously mentioned limitations. Poly(ester urea)s are a high modulus, non-toxic, amino acid-based class of polymers that has been thoroughly investigated by the Becker group. Both its tunable properties and ease of functionality, makes this polymer ideal for bone tissue engineering applications. Phenylalanine-based poly(ester urea)s (poly(PHE)) have been reported to have tensile moduli in a range comparable to healthy bone tissue (~7 GPa), as well as non-acidic degradation byproducts. Poly(PHE)s can be easily functionalized with bioactive peptides, such as osteogenic growth peptide (OGP) for enhancement of its osteoinductive potential. OGP[10-14] is the active subunit of the naturally occurring tetradecapeptide that is known to upregulate proliferation, differentiation and matrix mineralization of osteoblast cell lines. In this dissertation, both OGP-tethered and OGP-crosslinked poly(PHE) materials were synthesized using efficient `click' chemistry techniques. These materials were tested in vitro and in vivo to reveal the enhanced osteoinductive ability of poly(PHE) materials for tissue engineering applications. With just 1% tethered OGP incorporation into the poly(PHE) network, osteogenic lineage commitment of human mesenchymal stem cells (hMSCs) was enhanced at both 2 and 4 weeks. In vivo results confirmed the osteoinductive ability of OGP[10-14]-tethered poly(PHE). Crosslinking OGP[10-14] into the poly(PHE) network was intended to enhance mechanical and osteoinductive properties of the polymer construct, simultaneously. Results show that toughness of the polymeric constructs increased with OGP-crosslinking, however a decrease in tensile modulus was observed, possibly due to low crosslinking density. Early osteogenic differentiation of mouse preosteoblast cells (MC3T3-E1) was also enhanced on these constructs. Osseointegration of metallic implants is also a major cause for concern in the medical world. Titanium is one of the most widely used implants for bone repair, however, it is not uncommon for these implants to fail due to a lack of tissue-metal integration in vivo. Here, the surface functionalization of titanium oxide (TiO2) substrates with catechol-bearing dendritic OGP[10-14] modular peptides is reported. Functionalized OGP[10-14] modular peptides were synthesized and non-covalently bound to the surface of TiO2. Osteogenic differentiation of MC3T3-E1 cells was tested to result in enhanced expression of osteogenic genes compared to cells cultured without OGP-functionalization. Finally, to keep with the theme of enhancing osteogenesis for bone tissue engineering applications, several other well-known bioactive peptides were studied in this dissertation. Bone morphogenetic protein 2 (BMP-2) is a peptide fragment responsible for upregulation of osteogenic differentiation of multipotent cell lines via enhanced expression of bone specific genes. The Arg-Gly-Asp (RGD) amino acid sequence is the well-known adhesion peptide found in extracellular matrix proteins such as vinculin and fibronectin. RGD has been shown in many studies, to influence cell lineage commitment through the promotion of focal adhesion and cell spreading on a surface. Here, the concentration-dependent synergistic effect of BMP-2 and RGD was studied on 2D gradient substrates. In nature, peptides do not exist and act as a single entity, therefore this study reveals a glimpse at what the future holds for studying concentration effects of multiple bioactive factors simultaneously for enhanced bone tissue regeneration. Each of these studies reveals the importance of blending synthetic materials with naturally occurring peptides for optimal tissue engineering constructs. There is a clear need for polymeric constructs with both the mechanical and biological properties necessary for bone tissue engineering, and this dissertation provides a solid foundation for the future of bone regeneration biomaterials in the Becker group.