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Hydrogels generated by the interaction of two different water-soluble polymers offer access to a new group of soft materials. A prototype amino-functionalized polyphosphazene with both tyramine and ferulic acid-based side groups was coupled to aldehyde functionalized-dextrans to form hydrogels crosslinked via Schiff base chemistry. Synthesis of the polyphosphazene was accomplished by macromolecular substitution and protection-deprotection chemistry, with characterization by 1H NMR, 31P NMR, solid state 13C NMR, and DSC techniques. Combination of the aqueous polyphosphazene and aldehyde functionalized-dextran solutions at room temperature caused gelation with different gelation times and crosslink densities dependent on the aldehyde content of the dextran. The hydrogel properties were evaluated using rheology, thermal characterization, and cryo-microscopy. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016

Synthetic bone grafts that promote the natural mineralization process are excellent candidates for the repair and replacement of bone defects. In this study, a series of phosphoester and phosphonic acid containing polyphosphazenes were examined for their ability to mineralize hydroxyapatite (HAp) during exposure to a solution of simulated body fluid (SBF) for a period of four weeks. Although all the polymers showed an initial mineralization response, the amount of deposition and the time scale were dependent upon the side group chemistry of the polymers. After exposure to SBF for one week, all polymers mineralized HAp. After three weeks in SBF, polymers containing phosphoester substituents showed no significant change, with a weight gain of

Phosphazene polymers with N-linked and O-linked amino acid side groups are of biomedical interest, especially for their ability to bioerode under physiological conditions. Polyphosphazenes containing serine and threonine substituents, which contain two different functional sites for attachment to a polyphosphazene backbone (N- and O-terminus), were synthesized using an improved technique, and their hydrolysis behavior was investigated. In aqueous media the solid polymers yield hydrolysis products phosphate, ammonia, the amino acid, and ethanol and have the potential to be used in several different biomedical applications ultimately determined by their hydrolysis behavior. A hydrolysis study in deionized water revealed that all the polymers are hydrolytically sensitive, regardless of the type of linkage to the polyphosphazene backbone, although the hydrolysis rates may be different. Polymers with amino acid ester side groups linked through the N-terminus underwent solid phase hydrolysis between 16 and 60 % within a 6-week period. This is the fastest reported solid state hydrolysis of any amino acid ester substituted polyphosphazene. The mechanism of hydrolysis is by bulk erosion as monitored by environmental scanning electron microscopy. Polymers with the amino acid units linked through the O-terminus are soluble in water; thus their solid state erosion profile in aqueous media could not be determined. However, 31P NMR spectroscopy confirmed their hydrolytic sensitivity in aqueous solution and the formation of phosphorus-containing oligomeric species, the concentrations of which increased during the 6-week hydrolysis period. Complete hydrolysis did not occur within 6 weeks. The O-linked species are possible starting points for bioerodible hydrogel formation.

New polyphosphazenes were designed, synthesized, and characterized to determine their potential as scaffolds for ligament and tendon tissue engineering. The carboxylic acid moiety of the amino acids L-alanine and L-phenylalanine were protected with alkyl esters with increasing chain length from 5 to 8 carbon atoms. This combined the hydrolytic sensitivity of the amino acid ester polyphosphazenes with the elastomeric characteristics induced by the long chain alkoxy polyphosphazenes. Test side group substitution reactions were performed on the cyclic small molecule model, hexachlorocyclotriphosphazene (NPCl2)3, to determine if steric hindrance would inhibit the degree of chlorine replacement by the amino acid ester units. Counterpart polymers were then synthesized by replacement of the chlorine atoms in poly(dichlorophosphazene) (NPCl2)n by the same amino acid esters. The glass transition temperatures of the polymers decreased with increasing alkyl ester chain length, ranging from 11.6 to −24.2 °C. Polymer hydrolysis was studied for solid samples in deionized water at physiological temperature for 12 weeks. The starting pH was 6.3 and the final pH ranged between 5.2 and 6.8. Polymer film mass decreased between ∼8.7 and 26 percent during the 12 week period, while the molecular weights decreased ∼57 to 99 percent.

Phosphonate and phosphonic acid containing polymers are of interest for bone tissue engineering because these species have the ability to bind hydroxyapatite [Ca10(PO4)6(OH)2], which comprises 70 wt % of bone. The synthesis of phosphoester [−PO(OEt)2] and phosphonic acid [−PO(OH)2] functionalized polyphosphazenes is described. These polymers could mimic the natural bone healing mechanism, making them excellent candidates for implantable bone grafts. Two synthetic protocols have been developed to obtain the polymers, herein referred to as prior- and post-side-group assembly. Prior assembly required the synthesis of a phosphonate-containing side group before attachment to the polyphosphazene backbone through nucleophilic substitution, whereas post-assembly required the synthesis of a polyphosphazene containing free amino groups to which the phosphonate can be coupled by Michael addition after polymer synthesis. The final step for both routes required the deprotection of the phosphoester to the corresponding...

Synthetic bone grafts that promote the natural mineralization process would be excellent candidates for the repair or replacement of bone defects. In this study, a series of antioxidant-containing polyphosphazenes were evaluated for their ability to mineralize apatite during exposure to a solution of simulated body fluid (SBF). All polymers contained ferulic acid (antioxidant), cosubstituted with different amino acid esters linked to the polyphosphazene backbone. Differences in the side groups determined the hydrophobicity or hydrophilicity of the resulting polymers. All of the polymers mineralized monocalcium phosphate monohydrate, a type of biological apatite. However, the mineralization process (the amount of deposition and length of time) was dependent on the hydrophilicity or hydrophobicity of the polymers. The polymer–apatite composites were examined by electron scanning microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, differential scanning calorimetry, and thermogravametric a...

An account is presented of the development, evaluation, and current status of an unusual series of polymers optimized specifically for biomedical applications. The polymers are based on the polyphosphazene platform with side groups chosen for their ability to sensitize the polymers to hydrolysis to benign small molecules that can be metabolized or excreted from the body. The largest class of these polymers consists of macromolecules with amino acid ester side groups and these are the main focus of the article. However, a variety of polymers with other side groups also show promise as bioerodible species, and these are mentioned later in the article.

The preparation of phosphazene tissue engineering scaffolds with bioactive side groups has been accomplished 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 phenylalanine 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 gel permeation chromatography (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 media (∼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.

Polyphosphazenes were synthesized that contained the antioxidant, ferulic acid, and amino acid esters as co-substituents. The synthesis protocol utilized the replacement of chlorine atoms in poly(dichlorophosphazene) by ferulic acid with either glycine, alanine, valine, or phenylalanine ethyl ester. Ferulic acid protects cells from free radical damage, and its steric characteristics have the potential to generate polymeric materials with high mechanical strength for hard tissue engineering scaffolds. Incorporation of the amino acid esters allows for control over the polymer hydrolysis while releasing the antioxidant at different rates. Macromolecular substitution reactions were carried out utilizing the allyl ester of ferulic acid to prevent side reactions during halogen replacement. The allyl protecting group was then removed using mild conditions. The polymers were characterized by 1H and 31P NMR, GPC, and DSC techniques. Static water contact angles were measured to monitor the change in hydrophobicity/hydrophilicity. Before deprotection, the water contact angles were 82–88° and after deprotection the water contact angles were 56–71°. A pH-dependent hydrolysis study revealed that the polymers are hydrolytically sensitive and decomposed ∼5–25% over 8 weeks yielding a final pH between 6.0–6.8. Polymer hydrolysis resulted in the release of ferulic acid. The polymers were photocrosslinked by a [2 + 2] cycloaddition of the ferulic acid moieties induced by exposure to long-wave UV light. The crosslinking was monitored by UV-spectroscopy via the apparent decrease in the 320 nm absorbance, attributed to the cyclization of the ferulic acid moieties. Increased UV exposure time led to an increase in the level of crosslinking. After 60 s, the polymers were crosslinked ∼41–62%. Hydrolysis experiments of the crosslinked materials showed only a 0–5% weight loss after 8 weeks with a pH between 6.6–7.0.