Enzymatic degradation of poly(ether urethane) and poly(carbonate urethane) by cholesterol esterase

This study examined the effect of cholesterol esterase (CE) on the degradation of commercial poly(ether urethane) (PEU) and poly(carbonate urethane) (PCU). Unstrained PEU and PCU films were incubated in 400 U/mL CE solution or a buffer control for 36 days. The study used a concentration of cholesterol esterase that was considerably higher than the estimated physiological level in order to accelerate degradation. However, characterization of treated polyurethane films with SEM, attenuated total reflectance Fourier transform infrared (ATR-FTIR) and GPC analysis revealed only a small loss in surface soft segment content. Comparison with implanted PEU and PCU films led to the conclusion that any effect of enzymatic hydrolysis was confined to the immediate surface, and the magnitude of the effect was too small to contribute significantly to in vivo degradation. The study confirmed that oxidation, rather than enzymatic hydrolysis, is the primary mechanism responsible for the observed biodegradation of PEU and PCU. The oxidative H(2)O(2)/CoCl(2) treatment continues to accurately predict the long-term biostability of polyurethanes.     

Poly(carbonate urethane) and poly(ether urethane) biodegradation: in vivo studies

Several strategies have been used to increase the biostability of medical-grade polyurethanes while maintaining biocompatibility and mechanical properties. One approach is to chemically modify or replace the susceptible soft segment. Currently, poly(carbonate urethanes) (PCUs) are being evaluated as a replacement of poly(ether urethanes) (PEUs) in medical devices because of the increased oxidative stability of the polycarbonate soft segment. Preliminary in vivo and in vitro studies have reported improved biostability of PCUs over PEUs. Although several studies have reported evidence of in vitro degradation of these new polyurethanes, there has been no evidence of significant in vivo degradation that validates a degradation mechanism. In this study, the effect of soft segment chemistry on the phase morphology, mechanical properties, and in vivo response of commercial-grade PEU and PCU elastomers was examined. Results from dynamic mechanical testing and infrared spectroscopy suggested that the phase separation was better in PCU as compared with PEU. In addition, the higher modulus and reduced ultimate elongation of PCU was attributed to the reduced flexibility of the polycarbonate soft segment. Following material characterization, the in vivo biostability and biocompatibility of PEU and PCU were studied using a subcutaneous cage implant protocol. The results from the cage implant study and cell culture experiments indicated that monocytes adhere, differentiate, and fuse to form foreign body giant cells on both polyurethanes. It is now generally accepted that the reactive oxygen species released by these adherent macrophages and foreign body giant cells initiate PEU biodegradation. Attenuated total reflectance-Fourier transform infrared analysis of explanted samples provided evidence of chain scission and crosslinking in both polyurethanes. This indicated that the PCU was also susceptible to biodegradation by agents released from adherent cells. These results reinforce the need to evaluate and understand the biodegradation mechanisms of PCUs.     

Oxidative mechanisms of poly(carbonate urethane) and poly(ether urethane) biodegradation: in vivo and in vitro correlations

This study used an in vitro environment that simulated the microenvironment at the adherent cell-material interface to reproduce and accelerate the biodegradation of poly(ether urethane) (PEU) and poly(carbonate urethane) (PCU). Polyurethane films were treated in vitro for 24 days in 20% hydrogen peroxide/0.1 M cobalt chloride solution at 37 degrees C. Characterization with ATR-FTIR and SEM showed soft segment and hard segment degradation consistent with the chemical changes observed after long-term in vivo treatment. Overall, the PCU underwent less degradation and the degraded surface layer was much thinner than PEU. Nevertheless, the results supported a common oxidation mechanism for biodegradation of these polymers. The observed in vitro degradation was inhibited by adding an antioxidant to the polyurethane film. Our findings further support the use of the in vitro H(2)O(2)/CoCl(2) system in evaluating the biostability of polyurethanes under accelerated conditions.     

Synthesis of segmented poly(ether urethane)s and poly(ether urethane urea)s incorporating various side-chain or backbone functionalities

A series of linear and branched functionalized high-molecular-weight segmented poly (ether urethane)s and poly(ether urethane urea)s were prepared by chain-extending isocyanate pre-polymers based on poly(tetramethylene oxide) with molecular weight 1000 and 4,4'-diphenylmethane diisocyanate. Different functional groups were incorporated within the polymer backbone or on side-chains by using several chain extenders during the synthesis: glycerol, 2,2-bis (hydroxymethyl) propionic acid, 1H, 1H,2H,3H,3H-perfluoroundecane-1,2-diol, 1H,1H,8H,8H-dodecanefluoro-1,8-octanediol, 1,3-diamino-2-hydroxypropane and 3,5-diaminobenzoic acid, using the method of gradual approach to stoichiometry. In some cases, pendant functional groups were used as reactive sites for the further attachment of side groups. Polymers were characterized using 1H-NMR and FT-IR spectroscopies and GPC in conjunction with chemical structural confirmation by a model compound comparison study of 4,4'-diphenylmethane diisocyanate or trifluoro-p-tolyl isocyanate reacted with 1,3-diamino-2-hydroxypropane and 1,4-butanediol.     

Porogen-induced surface modification of nano-fibrous poly(L-lactic acid) scaffolds for tissue engineering

In addition to suitable pore architecture of a tissue-engineering scaffold, surface modification after scaffolding fabrication is often needed to enhance the interactions between cells and the synthetic material. In this study, a novel one-step process was developed to fabricate surface-modified nano-fibrous poly(L-lactic acid) (NF-PLLA) scaffolds. First, gelatin spheres with smooth surface were prepared by non-surfactant emulsification, solvent extraction, and freeze-drying. A three-dimensional NF-PLLA scaffold was then fabricated by using gelatin spheres as porogen. Gelatin molecules were entrapped onto the scaffold surface during the fabrication process. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy analysis demonstrated the existence of gelatin molecules on the surface of the polymer scaffold. The amount of gelatin on the scaffold surface was controlled by the composition of the solvent mixture of gelatin solution. The compressive modulus of scaffold prepared with gelatin spheres was more than three times higher than that prepared with irregular gelatin particles of the same size range. The surface modification significantly improved initial cell adhesion and proliferation over a 2-week culture. SEM images indicated that cells spread on the gelatin-entrapped scaffolds in contrast to spherical or spindle morphology on the control 1 day after cell seeding. Furthermore, more matrix secretion was observed on the surface-modified scaffolds than on the control after 2 weeks of in vitro cultivation. In conclusion, this approach provides a simple one-step process to fabricate surface-modified collagen-like NF-PLLA scaffolds, which have improved cell adhesion and proliferation.     

Laser surface modification of poly(epsilon-caprolactone) (PCL) membrane for tissue engineering applications

Ultra-thin polycaprolactone (PCL) produced by bi-axial stretching was previously shown to have significant advantage for membrane tissue engineering. However, the permeability of the membrane needs to be enhanced. In this study, ablation experiments using femtosecond laser and excimer laser were carried out to modify the PCL surface. The use of the femtosecond laser produces neat drilled-through holes while the excimer laser is employed to produce blind-holes on the membrane. The modified surface of the membrane was studied and analyzed for different laser parameters (such as pulse energy and pulse repetition rate and characterized using several techniques that include optical microscopy, scanning electron microscopy and water contact angle measurements). Results showed that the morphological surface changes with different laser parameters, and the water contact angle decreases as the surface of the membrane is modified. The decrease in water contact angle suggests that surface of the membrane had become more hydrophilic than the non-laser treated membrane. The present study demonstrated that laser surface modification on the PCL can be achieved with high degree of success and precision. This paved the way for further enhancement in membrane tissue engineering.     

Synthesis and characterization of shape-memory poly carbonate urethane microspheres for future vascular embolization

Two types of shape memory poly carbonate urethanes (PCUs) microspheres were synthesized by pre-polymerization and suspension polymerization, based on Polycarbonate diol (PCDL) as the soft segment, Isophorone diisocyanate (IPDI) and 1,6-hexamethylene diisocyanate (HDI) as the hard segments and 1,4-butanediol (BDO) as the chain expanding agent. The structure, crystallinity, and thermal property of the two synthesized PCUs were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Differential scanning calorimetery (DSC), respectively. The results showed that the two types of PCUs exhibited high thermal stability with phase separation and semi-crystallinity. Also, the results of the compression test displayed that the shape fixity and the shape recovery of two PCUs were more than 90% compared to the originals, indicating their similar bio-applicability and shape-memory properties. The tensile strength, elongation at break was enhanced by introducing and increasing content of HDI. The water contact angles of PCUs decreased and their surface tension increased by surface modified with Bovine serum albumin (BSA). Furthermore, the biological study results of two types of PCUs from the platelet adhesion test and the cell proliferation inhibition test indicated they had some biocompatibilites. Hence, the PCU microspheres might represent a smart and shape-memory embolic agent for vascular embolization.     

Nanohydroxyapatite/poly(ester urethane) scaffold for bone tissue engineering

Biodegradable viscoelastic poly(ester urethane)-based scaffolds show great promise for tissue engineering. In this study, the preparation of hydroxyapatite nanoparticles (nHA)/poly(ester urethane) composite scaffolds using a salt-leaching-phase inverse process is reported. The dispersion of nHA microaggregates in the polymer matrix were imaged by microcomputed X-ray tomography, allowing a study of the effect of the nHA mass fraction and process parameters on the inorganic phase dispersion, and ultimately the optimization of the preparation method. How the composite scaffold’s geometry and mechanical properties change with the nHA mass fraction and the process parameters were assessed. Increasing the amount of nHA particles in the composite scaffold decreased the porosity, increased the wall thickness and consequently decreased the pore size. The Young’s modulus of the poly(ester urethane) scaffold was improved by 50% by addition of 10 wt.% nHA (from 0.95 ± 0.5 to 1.26 ± 0.4 MPa), while conserving poly(ester urethane) viscoelastic properties and without significant changes in the scaffold macrostructure. Moreover, the process permitted the inclusion of nHA particles not only in the poly(ester urethane) matrix, but also at the surface of the scaffold pores, as shown by scanning electron microscopy. nHA/poly(ester urethane) composite scaffolds have great potential as osteoconductive constructs for bone tissue engineering.     

Synthesis and characterization of fluorocarbon chain end-capped poly(carbonate urethane)s as biomaterials: a novel bilayered surface structure

Poly(carbonate urethane)s (PCUs) are usually considered as biostable elastomers for long-term implantation. However, their hydrolytic stability is still questionable. The biodegradation appears to be initiated by oxidative and hydrolytic substances released by inflammatory cells. Therefore, the biostability of polyurethane might be improved with control of surface structure to reduce inflammatory response. A new type of PCUs end-capped with perfluoro chains was synthesized to explore a new avenue. A fluorinated alcohol, 2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluoro-1-octanol (PDFOL), was end-capped to the backbones of PCUs by reaction of the --OH in PDFOL with the --NCO end groups in PCU backbones. Contact angle measurement, X-ray photoelectron spectroscopy, atomic force microscopy, and attenuated total reflectance-Fourier transform infrared spectroscopy were used to examine their surface structure and properties. Elemental analysis, gel permeation chromatography, differential scanning calorimetry, and tensile testing were used to assess bulk chemistry and properties. The fluorocarbon end-capped poly (carbonate urethane)s (FPCUs) maintained the high mechanical properties (about 40 MPa tensile strength) and typical microphase separation structure of polyurethane elastomers. Results from surface analyses revealed the presence of a double-layered structure at the surfaces of the FPCUs. The first one was composed of fluorocarbon tails rising up on the uppermost layer and the second one made up of hard-segments. This novel bilayered surface structure could protect the weak carbonate linkages in soft segments, and consequently, may potentially increase the biostability of this kind of polyurethanes.     

Synthesis and characterization of segmented poly(esterurethane urea) elastomers for bone tissue engineering

Segmented polyurethanes have been used extensively in implantable medical devices, but their tunable mechanical properties make them attractive for examining the effect of biomaterial modulus on engineered musculoskeletal tissue development. In this study, a family of segmented degradable poly(esterurethane urea)s (PEUURs) were synthesized from 1,4-diisocyanatobutane, a poly(epsilon-caprolactone) (PCL) macrodiol soft segment and a tyramine-1,4-diisocyanatobutane-tyramine chain extender. By systematically increasing the PCL macrodiol molecular weight from 1100 to 2700Da, the storage modulus, crystallinity and melting point of the PCL segment were systematically varied. In particular, the melting temperature, T(m), increased from 21 to 61 degrees C and the storage modulus at 37 degrees C increased from 52 to 278MPa with increasing PCL macrodiol molecular weight, suggesting that the crystallinity of the PCL macrodiol contributed significantly to the mechanical properties of the polymers. Bone marrow stromal cells were cultured on rigid polymer films under osteogenic conditions for up to 21 days. Cell density, alkaline phosphatase activity, and osteopontin and osteocalcin expression were similar among PEUURs and comparable to poly(d,l-lactic-coglycolic acid). This study demonstrates the suitability of this family of PEUURs for tissue engineering applications, and establishes a foundation for determining the effect of biomaterial modulus on bone tissue development.     

Biodegradable poly(ether ester urethane)urea elastomers based on poly(ether ester) triblock copolymers and putrescine: synthesis, characterization and cytocompatibility

Polymers with elastomeric mechanical properties, tunable biodegradation properties and cytocompatibility would be desirable for numerous biomedical applications. Toward this end a series of biodegradable poly(ether ester urethane)urea elastomers (PEEUUs) based on poly(ether ester) triblock copolymers were synthesized and characterized. Poly(ether ester) triblock copolymers were synthesized by ring-opening polymerization of epsilon-caprolactone with polyethylene glycol (PEG). PEEUUs were synthesized from these triblock copolymers and butyl diisocyanate, with putrescine as a chain extender. PEEUUs exhibited low glass transition temperatures and possessed tensile strengths ranging from 8 to 20MPa and breaking strains from 325% to 560%. Increasing PEG length or decreasing poly(caprolactone) length in the triblock segment increased PEEUU water absorption and biodegradation rate. Human umbilical vein endothelial cells cultured in a medium supplemented with PEEUU biodegradation solution suggested a lack of degradation product cytotoxicity. Endothelial cell adhesion to PEEUUs was less than 60% of tissue culture polystyrene and was inversely related to PEEUU hydrophilicity. Surface modification of PEEUUs with ammonia gas radio-frequency glow discharge and subsequent immobilization of the cell adhesion peptide Arg-Gly-Asp-Ser increased endothelial adhesion to a level equivalent to tissue culture polystyrene. These biodegradable PEEUUs thus possessed properties that would be amenable to applications where high strength and flexibility would be desirable and exhibited the potential for tuning with appropriate triblock segment selection and surface modification.     

Surface modification of poly(ether urethane urea) with modified dehydroepiandrosterone for improved in vivo biostability

In this study, a fatty acid urethane derivative of dehydroepiandrosterone (DHEA) was synthesized and evaluated as a polyurethane additive to increase long-term biostability. The modification was hypothesized to reduce the water solubility of the DHEA and physically anchor the additive in the polyurethane during implantation. Polyurethane film weight loss in water as a function of time was studied to determine the polymer retention of the modified DHEA. The polyurethane film with unmodified DHEA had significant weight loss in the first day (10%) that was previously correlated to rapid leaching of the additive. The polyurethane film with modified DHEA had significantly less weight loss at all time points indicating improved polymer retention. The effect of the modified DHEA additive on the biostability of a poly(ether urethane urea) was examined after 5 weeks of subcutaneous implantation in Sprague-Dawley rats. Optical micrographs and infrared analysis of the specimens indicated that the modified DHEA bloomed to the surface of the film forming a crystalline surface layer approximately 10-15 microns thick. After explantation, this surface layer was intact without measurable differences in surface chemistry as monitored by attenuated total reflectance-Fourier transform infrared spectroscopy. There was no evidence of degradation of the polyurethane underneath the modified DHEA surface layer as compared with the polyurethane control. We have concluded that the modified DHEA self-assembled into a protective surface coating that inhibited degradation of the polyurethane. The roughness of the modified DHEA surface layer prevented adherent cell analysis to determine if the additive retained the ability to down-regulate macrophage activity. Subsequent studies will investigate the ability of surface-modifying additives to modulate cellular respiratory bursts in addition to the formation of an impermeable barrier. This bimodal approach to improving biostability holds great promise in the field of polyurethane biomaterials.     

Synthesis and characterization of photocrosslinkable,degradable poly(vinyl alcohol)-based tissue engineering scaffolds

Hydrogels have many advantages that make them prime candidates for tissue engineering applications: high water content, tissue-like elasticity, and relative biocompatibility. We aim to tissue engineer heart valves using a hydrogel scaffold based on poly(vinyl alcohol) (PVA), and the design parameters for a suitable tissue engineering scaffold are quite stringent. In this research, we develop degradable and photocrosslinkable poly(lactic acid)-g-PVA multifunctional macromers that can be reacted in solution to form degradable networks. The mass loss profiles and bulk properties of the resulting scaffolds are easily tailored by modifying the structure of the starting macromers. Specifically, altering the number of lactide repeat units per crosslinking side chain, percent substitution, molecular weight of PVA backbone, and macromer solution concentration, the rate of mass loss from these degradable networks is controlled. In addition, by increasing the network's hydrophobicity, valve interstitial cell adhesion is improved.     

Synthesis and characterization of macroporous poly(ethylene glycol)-based hydrogels for tissue engineering application

Peptide activated poly(ethylene glycol) (PEG)-based hydrogels have received wide attention as material for tissue engineering application. However, the close structure of these materials may pose severe barriers to tissue invasion and nutrient transport. The aim of this work was to synthesize highly interconnected macroporous PEG hydrogels, suitable for use as tissue engineering scaffolds, by combining the photocrosslinking reaction with a foaming process. In particular, various porous samples, differing for both the polymer molecular weight and concentration in the starting precursor solution, have been prepared and characterized by means of scanning electron microscopy and mercury porosimetry. Moreover, water swelling properties have been evaluated and compared with those of the conventional nonporous ones, by performing both equilibrium and kinetic swelling measurements in distilled water. Results indicated that foamed hydrogels display a well-interconnected porous network, suitable for tissue invasion and free molecular trafficking within them. Pores dimension as well as swelling rate can be modulated by polymer concentrations and bubbling agent composition in the precursor solution.     

Synthesis,characterization, and cytocompatibility of elastomeric,biodegradable poly(ester-urethane)ureas based on poly(caprolactone) and putrescine

The engineering of tissue for mechanically demanding applications in the cardiovascular system is likely to require mechanical conditioning of cell-scaffold constructs prior to their implantation. Scaffold properties amenable to such an application include high elasticity and strength coupled with controllable biodegradative and cell-adhesive properties. To fulfill such design criteria, we have synthesized a family of poly(ester-urethane)ureas (PEUUs) from polycaprolactone and 1,4-diisocyanatobutane. Lysine ethyl ester (Lys) or putrescine was used as chain extenders. To encourage cell adhesion, PEUUs were surface modified with radio-frequency glow discharge followed by coupling of Arg-Gly-Asp-Ser (RGDS). The synthesized PEUUs were highly flexible, with breaking strains of 660-895% and tensile strengths from 9.2-29 MPa. Incubation in aqueous buffer for 8 weeks resulted in mass loss, from >50% (Lys chain extender) to 10% (putrescine chain extender). Human endothelial cells cultured for 4 days with medium containing the degradation products from PEUUs with either the Lys or putrescine chain extender showed no toxic effects. Cell adhesion was 85% of that measured on tissue-culture polystyrene for unmodified PEUU surfaces (p < 0.01) and >160% (p < 0.001) of polystyrene on RGDS-modified PEUUs. These biodegradable PEUUs demonstrate potential for future application as cell scaffolds in cardiovascular tissue-engineering or other soft-tissue applications.     

Synthesis and characterization of aliphatic-aromatic poly(ether amide)s

Four aromatic diamines containing aliphatic spacers and Meta and para oriented oxyphenylene rings, and their corresponding hydrochlorides, were combined with isophthaloyl chloride (IPC) and terephthaloyl chloride (TPC) to give high molecular weight polyamides by interfacial and low-temperature solution methods. The synthesis and characterization of monomers and polymers are reported, and the differences observed in polycondensation yields, molecular weights and molecular weight distributions, as a function of the method of synthesis, are discussed. Values of number-average molecular weight (M?_n) up to 8 × 10⁴ g/mol and weight-average molecular weight (M?_w) up to 1 × 10⁵ g/mol could be measured by gel permeation chromatography using aromatic polyamide standards, and values of M?_n up to 2 × 10⁵ g/mol and M?_w up to 3.6 × 10⁵ g/mol by using polystyrene standards.     

Evaluation of soft tissue response to a poly(urethane urea).

Variations in the performance of vascular prostheses constructed of polyurethanes, and some evidence which suggested that these variations could be due not to the properties of the polymer itself, but to differences in the cellular response to the various microstructures of porous polyurethanes require investigation. Experiments were performed to evaluate quantitatively the extent of the cell behaviour adjacent to a series of polyurethane samples. It was shown that, with Biomer, a polyurethane urea, the profile of cell behaviour as a function of distance from the implant surface and of time following implantation, the response of cells in general and macrophages in particular, varied considerably with different internal microstructure. This supports the suggestion that the cellular response to different structures and susceptibility to degradation are related.     

Studies of poly(ether)urethane pacemaker lead insulation oxidation

Published reports suggest that silver ions may catalyze the oxidation of poly(ether)urethane soft-segments resulting in the failure of urethane insulations of specific models of pacemaker leads. Attempted oxidation of soft-segment models, poly(tetra-methylene ether)glycols, by silver nitrate has shown that metal-ion catalyzed oxidative-reduction (MICOR) does not adequately explain observed failures unless antioxidants are removed in process. Such cracking can, however, be explained in terms of a metal ion enhanced environmental stress cracking.     

Synthesis and characterization of poly(arylene ether sulfone amide)s

Methyl-substituted or unsubstituted aromatic dicarboxylic acids and diamines containing ether and sulfone linkages between phenylene units were used alone or in combination with p-phenylenediamine, m-phenylenediamine, terephthalic acid, and isophthalic acid to prepare flexible or semirigid aromatic polyamides by direct polycondensation activated by triphenyl phosphite and pyridine. The inherent viscosities of the obtained polymers ranged from 0.40 to 1.01 dL/g. The wholly flexible polyamides are amorphous, are readily soluble in N,N-dimethylacetamide, dimethyl sulfoxide, and m-cresol, and can afford transparent, flexible, and tough films by solution-casting. The polyamides prepared from p-phenylenediamine or terephthalic acid are partially crystalline and melt around 410°C. Differential scanning calorimetry shows glass transition temperature in the 202–255°C range for the wholly flexible polyamides. All the polyamides are thermally stable in excess of 400°C. The methyl-substituted polyamides had higher glass transition temperatures, but lower initial decomposition temperatures, than the corresponding unsubstituted polyamides.     

Antioxidant inhibition of poly(carbonate urethane) in vivo biodegradation

This study compared the effect of an antioxidant on the in vivo biodegradation of a poly(carbonate urethane) (PCU) and a poly(ether urethane) (PEU). Unstrained PEU and PCU films with and without Santowhite were implanted subcutaneously into 3-month-old Sprague-Dawley rats for 3, 6, and 12 months. Characterization of unstabilized PEU and PCU with ATR-FTIR and SEM showed soft-segment and hard-segment degradation consistent with previous studies. In particular, evidence of chain scission and crosslinking of the surface was present in the ATR-FTIR spectra of explanted specimens. Addition of 2.2 wt % antioxidant inhibited the in vivo degradation of both PCU and PEU. Although the antioxidant probably improved polyurethane biostability by decreasing the susceptibility of the polymer to degradation, modulation of the cellular response to prevent the release of degradative agents was also possible. To differentiate the effects, the foreign-body response was investigated with the use of a standard cage implant protocol. Polyurethane films were implanted in wire mesh cages subcutaneously in rats for 4, 7, and 21 days. There were no statistical differences among materials in the inflammatory exudate cell counts, adherent cell densities, or percent fusion of macrophages into foreign-body giant cells (FBGCs). Therefore, it was concluded that the antioxidant inhibited degradation by capturing oxygen radicals that would otherwise cause polyurethane chain scission and crosslinking.