NEW MATERIALS FOR ADVANCED BIOMATERIAL APPLICATIONS
Open Access
- Author:
- Weikel, Arlin Lee
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 21, 2010
- Committee Members:
- Professor Harry R Allcock, Dissertation Advisor/Co-Advisor
Harry R Allcock, Committee Chair/Co-Chair
Karl Todd Mueller, Committee Member
Alan James Benesi, Committee Member
James Patrick Runt, Committee Member - Keywords:
- polyphosphazenes
amino acids
polymer blends
block copolymers - Abstract:
- The work described in this thesis focuses on the design, synthesis, and characterization of novel polyphosphazenes for advanced biomedical applications. In addition, the fabrication of polyphosphazene / poly(lactic-co-glycolic acid) (PLGA) blends were examined for their physical properties as hard tissue engineering scaffolds. Chapter 1 outlines the basic fundamentals of polymer chemistry, along with phosphazene chemistry and its application to biomedical materials. Chapter 2 discusses the synthesis of the dipeptides alanyl-glycine ethyl ester, valinyl-glycine ethyl ester, and phenylalanyl-glycine ethyl esters. These dipeptides were synthesized using mixed anhydride solution-phase peptide reactions. The free N-terminus was used as a reactive site for nucleophilic replacement of the chlorine atoms in poly(dichlorophosphazene). The C-terminus was protected with an ethyl ester to prevent side reactions and crosslinking. The alanyl-glycine ethyl ester replaced all the chlorine atoms in poly(dichlorophosphazene). However, replacement of all the chlorine atoms in poly(dichlorophosphazene) by valinyl-glycine ethyl ester or phenylalanyl-glycine ethyl ester polyphosphazenes was prevented by the insolubility of the partially substituted intermediates. To circumvent this problem, co-substitution was carried out using the valinyl- or phenylalanyl esters with glycine ethyl ester or alanine ethyl ester in a 1:1 ratio. Co-substituted polyphosphazenes with alanyl glycine ethyl ester and glycine ethyl ester or alanine ethyl ester were also synthesized with a side group ratio of 1:1. The polymer structures and physical properties were studied using multinuclear NMR, DSC, and GPC techniques. Heterophase hydrolysis experiments in aqueous media at different pH values were carried out to estimate the hydrolytic sensitivity of these polymers. All the polymers were less sensitive to hydrolysis under neutral or basic (pH, 10.0) conditions than at pH 4.0, where rapid hydrolysis occurred. Chapter 3 outlines the preparation of phosphazene tissue engineering scaffolds with bioactive side groups using the biological buffer choline chloride. Mixed-substituent phosphazene cyclic trimers (as model systems) and polymers with choline chloride and glycine ethyl ester, alanine ethyl ester, valine ethyl ester, or phenyl alanine ethyl ester were synthesized. Two different synthetic protocols were examined. A sodium hydride mediated route resulted in polyphosphazenes with a low choline content, while a cesium carbonate mediated process produced polyphosphazenes with higher choline content. The phosphazene structures and physical properties were studied using multinuclear NMR, differential scanning calorimetry (DSC), and GPC techniques. The resultant polymers were then blended with PLGA (50:50) or PLGA (85:15) and characterized by DSC analysis and scanning electron microscopy (SEM). Polymer products obtained via the sodium hydride route produced miscible blends with both ratios of PLGA, while the cesium carbonate route yielded products with reduced blend miscibility. Heterophase hydrolysis experiments in aqueous media revealed that the polymer blends hydrolyzed to near-neutral pH values (~5.8 to 6.8). The effect of different molecular structures on cellular adhesion showed osteoblast proliferation with an elevated osteoblast phenotype expression compared to PLGA over a 21-day culture period. Chapter 4 describes the preparation of phosphazenes that possess reversible cross-linking groups to control mechanical stability and hydrolysis using cysteine and methionine amino acid side groups. Small molecule models and linear polymeric phosphazenes that contain methionine ethyl ester and cysteine ethyl disulfide ethyl ester side groups were synthesized. Protection of the free thiol groups was carried out to circumvent unwanted cross-linking of the phosphazenes through the cysteine ethyl ester N- and S-termini. Cyclic trimeric cysteine ethyl disulfide ethyl ester model compounds were deprotected by S-S bond cleavage using β-mercaptoethanol, dithiothreitol (DTT), and zinc in aqueous hydrochloric acid. For the high polymeric derivatives, the extent of S-S bond cleavage varied depending on the deprotection method used. With the exception of the Zn / HCl method, the resultant deprotected polymers were soluble in common organic solvents and underwent minimal chain cleavage during the reaction sequence. The protected or deprotected high polymers are candidates for reversible cross-linking in drug delivery systems and for cross-link stabilization of tissue engineering scaffolds. Chapter 5 evaluates the first reported synthesis of a completely hydrolysable polyphosphazene-containing block co-polymer. The synthesis of poly(lactic acid)-co-poly[(bis-alanine ethyl ester phosphazene)], poly(lactic acid)-co-poly[(bis-valine ethyl ester phosphazene)], and poly(lactic acid)-co-poly[(bis-phenylalanine ethyl ester phosphazene)] has been accomplished. These block co-polymers were used as blend compatibalizers to form composites of PLAGA (50:50) or PLAGA (85:15) with poly[(bis-alanine ethyl ester phosphazene)], poly[(bis-valine ethyl ester phosphazene)], or poly[(bis-phenylalanine ethyl ester phosphazene)]. The effect of the block copolymers on the composites formed from previously immiscible blends was studied. The resultant composites were characterized using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) to investigate systems miscibility. The rates of hydrolysis and pH of the hydrolysis media were also investigated. Chapter 6 discusses a unique polymer erosion process for biodegradable biomaterials through which the polymer changes from a solid coherent film to an assemblage of microspheres with interconnected porous structures. The polymer system was developed on the highly versatile platform of self-neutralizing polyphosphazene-polyester blends. Co-substituting a polyphosphazene backbone with both glycylglycine dipeptide and with side groups that can retard the polymer degradation, such as hydrophobic 4-phenylphenoxy, generated a unique polymer with strong hydrogen bonding ability and a slow degradation rate. During the blend hydrolysis in aqueous media, the relatively fast degradation rate of the polyester favored 3D void space formation characterized by macropores (10-100 µm) between polyphosphazene spheres as well as micro and nanopores on the sphere surface. The blend degradation was further investigated in vivo using a rat subcutaneous implantation model. A 12-week degradation resulted in a 3D porous structure with 82-87% porosity and with 100% interconnectivity. This in situ-formed 3D interconnected porous structure enabled cell infiltration and collagen tissue in-growth. Thus, the dynamic pore formation process accompanying the matrix erosion provides a new strategy in regenerative medicine for developing solid matrices that allows tissue integration as the matrix degrades. Chapter 7 describes a series of closely related polyphosphazenes with propoxy, pentoxy, hexoxy, octoxy, isostearyloxy, and 2-(2-methoxyethoxy)ethoxy (MEE) side groups, together with co-substituent species with both the alkoxy and MEE side chains. These were studied for their morphology and miscibility with oligoisobutylene (OIB). All the pure polymers except one had a single glass transition temperature. The exception was the species with both isostearyloxy and MEE side groups, which underwent two low temperature second order transitions, even though 31P NMR spectra indicated the absence of a block-type structure. For the single-substituent macromolecules, the solubility at 80 ˚C in OIB increased as the length of the unbranched alkoxy side groups rose from propoxy to octoxy (from 1 to 11 wt/wt%). However, the polymer with two isostearyloxy side chains per repeat unit had a low solubility in OIB (3 wt/wt%) and the species with the two MEE side groups on every repeat unit was totally insoluble. When both alkoxy and MEE side groups were present, the solubility in OIB was also low (0-3%), except for the species with both isostearyloxy and MEE side groups, which was soluble in OIB at a level of 21 wt/wt% at 80 ˚C, and showed Tg evidence of polymer/oligomer miscibility even at -80 ˚C. Explanations are suggested for the unusual behavior of this polymer.