The in vitro hydrolysis of poly(ester urethane)s consisting of poly[(R)-3-hydroxybutyrate] and poly(ethylene glycol)

This paper reports the study of the complete degradation process for a series of newly synthesized multi-block poly(ester urethane)s consisting of poly[(R)-3-hydroxybutyrate] (PHB) as hard and hydrophobic block and poly(ethylene glycol) (PEG) as soft and hydrophilic segment. The initial stages of degradation of the poly(PHB/PEG urethane)s were monitored by carrying out the degradation experiments at pH 7.4 and 37 degrees C. The weight loss of the copolymer films was traced, and the degraded copolymer films were characterized by GPC, (1)H NMR, TGA, and SEM. The induction phase of the polymer degradation was characterized by a random chain scission of the ester backbone bonds of the PHB segments and an insignificant decline in the weight of the polymer films. An accelerated degradation process was carried out at pH 11.5 and 37 degrees C to investigate the long-term degradation behaviour. The characterization of the degraded polymer films was similar to that for the experiment at pH 7.4. In addition, the water-soluble degradation products were characterized by GPC, (1)H NMR, and FTIR. The main components of the water-soluble degradation products were found to be PEG blocks (monomeric up to quadmeric), 3-hydroxybutyric acid, and crotonic acid. It was found that the copolymer incorporating the highest amount of PEG degraded at the highest rate of all the copolymers studied. The complete degradation of the poly(PHB/PEG urethane)s was monitored using a combination of the physiological and accelerated hydrolytic degradation.     

Improving hydrophilicity,mechanical properties and biocompatibility of poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate] through blending with poly[(R)-3-hydroxybutyrate]-alt-poly(ethylene oxide)

Natural source poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyvalerate] (PHBV) with a low hydroxyvalerate (HV) content (～8 wt.%) was modified by blending it with poly[(R)-3-hydroxybutyrate]-alt-poly(ethylene oxide) (HE) alternating block copolymer. We hypothesized that the adjoining PHB segments could improve the miscibility of the poly(ethylene oxide) segments of HE with the PHBV matrix and therefore improve the physical properties of the PHBV/HE blends. A differential scanning calorimetry study revealed the improved miscibility of PEO segments of HE characterized by the interference of the crystallization of PHBV. The decrease in water contact angle and the increase in equilibrium water uptake of the PHBV/HE blends indicated that both the surface and bulk hydrophilicity of PHBV could be improved through blending HE. The mechanical properties of the hydrated PHBV/HE blends were assessed by measuring their tensile strength. In contrast to the hydrated natural source PHBV, which failed in a brittle manner, the hydrated PHBV/HE blends were ductile. Their strain at break increased with increasing HE content, reaching a maximum of 394% at an HE content of 15 wt.%. The excellent integrity of the PHBV/HE blends in water is attributed to the strong affinity between the PHB segments of HE and the PHBV matrix. Platelet adhesion on the film surface of the PHBV/HE blends was investigated in vitro to evaluate their blood compatibility. The results demonstrated that the PHBV/HE blends effectively resisted the adhesion of platelets due to the anchored PEO segments from HE on the film surface.     

Biodegradable nanoparticles of amphiphilic triblock copolymers based on poly(3-hydroxybutyrate) and poly(ethylene glycol) as drug carriers

New amorphous amphiphilic triblock copolymers of poly(3-hydroxybutyrate)-poly(ethylene glycol)-poly(3-hydroxybutyrate) (PHB-PEG-PHB) were synthesized using the ring-opening copolymerization of beta-butyrolactone monomer. They were characterized by fluorescence, SEM and (1)H NMR. These triblock copolymers can form biodegradable nanoparticles with core-shell structure in aqueous solution. Comparing to the poly(ethylene oxide)-PHB-poly(ethylene oxide) (PEO-PHB-PEO) copolymers, these nanoparticles exhibited much smaller critical micelle concentrations and better drug loading properties, which indicated that the nanoparticles were very suitable for delivery carriers of hydrophobic drugs. The drug release profile monitored by fluorescence showed that the release of pyrene from the PHB-PEG-PHB nanoparticles exhibited the second-order exponential decay behavior. The initial biodegradation rate of the PHB-PEG-PHB nanoparticles was related to the enzyme amount, the initial concentrations of nanoparticle dispersions and the PHB block length. The biodegraded products detected by (1)H NMR contained 3HB monomer, dimer and minor trimer, which were safe to the body.     

Structure of poly[(ethylene glycol)-(diethylene glycol) terephthalate] as determined by chemical degradation

The influence of preliminary thermal treatment and diethylene glycol (DEG) content on the rate of degradation, DEG amount in the degradation products, WAXS, relative viscosity, and density of the understructed poly(ethylene terephthalate) (PET) is studied upon aminolysis with hydrazine hydrate at 60 and 85°C. The results indicate that (i) the aminolysis is a two-stage process, corresponding to the degradation of the amorphous and crystalline regions and (ii) the phase boundary is not sharp. A model is proposed, describing the comonomer distribution in the semicrystalline copolyesters. The comonomer units are concentrated mainly in the amorphous regions. The effect of degradation conditions like temperature, concentration, duration, particle size, deposition of low molecular products on the polymer surface, etc. is also discussed.     

Synthesis and water-swelling of thermo-responsive poly(ester urethane)s containing poly(epsilon-caprolactone), poly(ethylene glycol) and poly(propylene glycol)

Thermo-responsive multiblock poly(ester urethane)s comprising poly(epsilon-caprolactone) (PCL), poly(ethylene glycol) (PEG), and poly(propylene glycol) (PPG) segments were synthesized. The copolymers were characterized by GPC, NMR, FTIR, XRD, DSC and TGA. Water-swelling analysis carried out at different temperatures revealed that the bulk hydrophilicity of the copolymers could be controlled either by adjusting the composition of the copolymer or by changing the temperature of the environment. These thermo-responsive copolymer films formed highly swollen hydrogel-like materials when soaked in cold water and shrank when soaked in warm water. The changes are reversible. The mechanical properties of the copolymer films were assessed by tensile strength measurement. These copolymers were ductile when compared to PCL homopolymers. Young's modulus and the stress at break increased with increasing PCL content, whereas the strain at break increased with increasing PEG content. The results of the cytotoxicity tests based on the ISO 10993-5 protocol demonstrated that the copolymers were non-cytotoxic and could be potentially used in biomedical applications.     

Alternative block polyurethanes based on poly(3-hydroxybutyrate-co-4-hydroxybutyrate) and poly(ethylene glycol)

A series of amphiphilic alternative block polyurethane copolymers based on poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P3/4HB) and poly(ethylene glycol) (PEG) were synthesized by a coupling reaction between P3/4HB-diol and PEG-diisocyanate, with different 3HB, 4HB, PEG compositions and segment lengths. Stannous octanoate was used as catalyst. The chemical structure, alternative block arrangement, molecular weight and distribution were systematically characterized by FTIR, ¹H NMR, GPC and composition analysis. The thermal property was studied by DSC and TGA. Platelet adhesion study revealed that the alternative block polyurethanes possess excellent hemocompatibility. CCK-8 assay illuminated that the non-toxic block polyurethanes maintain rat aortic smooth muscle cells (RaSMCs) good viability. The in-vitro degradation of the copolymers in PBS buffer solution and in lipase buffer medium was investigated. Results showed that the copolymer films exhibit different degradation patterns in different media from surface erosion to diffusion bulk collapsing. The synthetic methodology for the alternative block polyurethanes provides a way to control the exact structure of the biomaterials and tailor the properties to subtle requirements.     

pH triggered release of protective poly(ethylene glycol)-b-polycation copolymers from liposomes

Triggered release of adsorbed polymers from liposomes enables protection against immune recognition during circulation and subsequent intracellular delivery of DNA. Polycationic blocks, poly[2-(dimethylamino) ethyl methacrylate] (DMAEMA) (0.8, 3.1, 4.9, or 9.8 kg/mol) or polylysine (K) (3 kg/mol), act as anchors for poly(ethylene glycol) (PEG) (2 or 5 kg/mol) protective blocks. In addition, a copolymer with 15 strictly alternating blocks of PEG (2 kg/mol) and cationic amine sites was evaluated as a protective coating. Incorporation of 1,2-dioleoyl-3-dimethylammonium-propane, a titratable lipid with a pKa of approximately 6.7, allows the liposome's net charge to increase as the pH shifts from 7.4 in the bloodstream to 5.5 in the endosome. The increased net liposome cationicity results in decreased cationic polymer adsorption. The EMPEG113-DMAEMA31 and EMPEG113-DMAEMA62 association constants decrease from 3.1 and 6.2 (mg/m(2))/(mg/ml) at pH 7.4 to 1.7 and 3.2 (mg/m(2))/(mg/ml) at pH 5.5, respectively. However, EMPEG45-DMAEMA5, EMPEG45-DMAEMA20, and EMPEG45-N-DP15 did not show a strong response to changes in pH. Cationic polymer adsorption exceeds calculated values for liposome neutralization, resulting in adsorption profiles in the brush regime.     

Characterization, biodegradability and blood compatibility of poly[(R)-3-hydroxybutyrate] based poly(ester-urethane)s

Poly(ester-urethane)s (PUs) were synthesized using hexamethylene diisocyanate (HDI) or toluene diisocyanate (TDI) to join short chains (M(n) = 2000) of poly(R-3-hydroxybutyrate) (PHB) diols and poly(epsilon-caprolactone) (PCL) diols with different feed ratios under different reaction conditions. The multiblock copolymers were characterized by nuclear magnetic resonance spectrometer (NMR), gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), X-ray diffraction (XRD), and scanning electron microscope (SEM). XRD spectra and second DSC heat thermograms of the multiblock copolymers revealed that the crystallization of both PHB and PCL segments was mutually restricted, and, especially, the PCL segment limited the cold crystallization of the PHB segment. The SEM of platelet adhesion experiments showed that the hemocompatibility was affected to some extent by the chain flexibility of the polymers. Hydrolysis studies demonstrated that the hydrolytic degradation of PUs was generated from the scission of their ester bonds or/and urethane bonds. Simultaneously, the rate of ester bond scission was determined to some extent by the crystallization degree, which was further affected by the configuration of polymer chains. These highly elastic multiblock copolymers combining hemocompatibility and biodegradability may be developed into blood contact implant materials for biomedical applications.     

Synthesis and characterization of a polyrotaxane consisting of β-cyclodextrins and a poly(ethylene glycol)-poly(propylene glycol) triblock copolymer

A polyrotaxane in which many β-cyclodextrins (β-CyDs) are threaded onto a triblock copolymer of poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) capped with fluorescein-4-isothiocyanate (FITC) was synthesized as a model of stimuli-responsive molecular assemblies for nanoscale devices. β-CyDs threaded onto the triblock copolymer enhance the solubility of the polyrotaxane and presumably contribute to the prevention of aggregation between PPG segments. The interaction of β-CyDs with a terminal FITC moiety was observed to be significant at 10°C, however, with increasing temperature, the interaction of β-CyDs with a PPG segment becomes prominent. From these results, it is concluded that the majority of β-CyDs move toward the PPG segment with increasing temperature although some β-CyDs may reside on PEG segments.     

Reverse thermo-responsive poly(ethylene oxide) and poly(propylene oxide) multiblock copolymers

Aiming at developing new reverse thermo-responsive polymers, poly(ethylene oxide)-poly(propylene oxide) multiblock copolymers were synthesized by covalently binding the two components using carbonyl chloride and diacyl chlorides as the coupling molecules. The appropriate selection of the various components allowed the generation of systems displaying much enhanced rheological properties. For example, 15 wt% aqueous solutions of an alternating poly(ether-carbonate) comprising PEO6000 and PPO3000 segments, achieved a viscosity of 140,000 Pas, while the commercially available Pluronic F127 displayed 5,000 Pas only. Furthermore, the structure of the chain extender played a key role in determining the sol-gel transition. While poly(ether-ester)s containing therephtaloyl (para) and isophtaloyl (metha) coupling units failed to gel at any concentration, a 15 wt% aqueous solution of the polymer chain-extended with phtaloyl chloride (ortho) gelled at 43 degrees C. The water solutions were also studied by dynamic light scattering and a clear influence of the PEO/PPO ratio on the aggregate size was observed. By incorporating short aliphatic oligoesters into the backbone, prior to the chain extension stage, reverse thermal gelation-displaying biodegradable poly(ether-ester-carbonate)s, were generated.     

Biodegradable thermogelling poly(ester urethane)s consisting of poly(lactic acid)--thermodynamics of micellization and hydrolytic degradation

Multiblock poly(ether ester urethane)s comprising of poly(lactic acid) (PLA), poly(ethylene glycol) (PEG), and poly(propylene glycol) (PPG) segments were synthesized, and their aqueous solutions exhibited thermogelling behavior at critical gelation concentrations (CGC) ranging from 7 to 9 wt%. The chemical structures and molecular characteristics of the copolymers were studied by GPC, 1H NMR, 13C NMR and FTIR. The thermal stability of the poly(PEG/PPG/PLA urethane)s was studied by thermogravimetry analysis (TGA), and the PLA contents were calculated based on the thermal degradation profile. The results were in good agreement with those obtained from the 1H NMR measurements. The critical micellization concentration (CMC) of these water-soluble poly(ether ester urethane)s was determined at different temperatures using a dye solubilization method. The thermodynamic parameters for micelle formation were calculated, indicating that the process is largely entropy-driven. Interestingly, it appears that there exists a requirement for the system to possess a minimum gain in entropy before the thermogelling effect can be observed. Dilute copolymer solutions showed a lower critical solution temperature (LCST) behavior similar to pNIPAM dissolved in aqueous solutions. The thermogels hydrolytically degraded to polymer fragments corresponding to the constituent segment blocks within 3 months.     

Control of vitamin B12 release from poly(ethylene glycol)/poly(butylene terephthalate) multiblock copolymers

The release of vitamin B12 (1355 Da) from matrices based on multiblock copolymers was studied. The copolymers were composed of hydrophilic poly(ethylene glycol)-terephthalate (PEGT) blocks and hydrophobic poly(butylene terephthalate) (PBT) blocks. Vitamin B12 loaded films were prepared by using a water-in-oil emulsion method. The copolymer properties, like permeability, could be varied by increasing the PEG-segment length from 300 up to 4,000 g/mol and by changing the wt% of PEGT. From permeation and release experiments. the diffusion coefficient of vitamin B12 through PEGT/PBT films of different compositions was determined. The diffusion coefficient of Vitamin B12 was strongly dependent on the composition of the copolymers. Although an increased wt% of PEGT (at a constant PEG-segment length) resulted in a higher diffusion coefficient, a major effect was observed at increasing PEG-segment length. By varying the copolymer composition, a complete release of vitamin B12 in 1 day up to a constant release for over 12 weeks was obtained. The release rate could be effectively tailored by blending copolymers with different PEG-segment lengths. The swelling and the crystallinity of the matrix could explain the effect of the matrix composition on the release behavior.     

Pulsatile peptide release from multi-layered hydrogel formulations consisting of poly(ethylene glycol)-grafted and ungrafted dextrans

Multi-layered hydrogel formulations consisting of poly(ethylene glycol)-grafted dextran (PEG-g-Dex) and ungrafted Dex were investigated as a model of Pulsatile drug release. In these formulations, it is considered that the grafted PEG domains act as a drug reservoir dispersed in the Dex matrix based on aqueous polymer two-phase systems. The formulations exhibited surface-controlled degradation by dextranase, and insulin release was observed in a pulsatile manner because of the multi-layered structure, PEG-g-Dex hydrogel layers containing insulin and insulin-free Dex hydrogel layers. Thus, it is suggested that the multi-layered hydrogel formulations using PEG-g-Dex and Dex are feasible for chronopharmacological drug delivery systems.     

Pulsatile peptide release from multi-layered hydrogel formulations consisting of poly(ethylene glycol)-grafted and ungrafted dextrans

Multi-layered hydrogel formulations consisting of poly(ethylene glycol)-grafted dextran (PEG-g-Dex) and ungrafted Dex were investigated as a model of pulsatile drug release. In these formulations, it is considered that the grafted PEG domains act as a drug reservoir dispersed in the Dex matrix based on aqueous polymer two-phase systems. The formulations exhibited surface-controlled degradation by dextranase, and insulin release was observed in a pulsatile manner because of the multi-layered structure: PEG-g-Dex hydrogel layers containing insulin and insulin-free Dex hydrogel layers. Thus, it is suggested that the multi-layered hydrogel formulations using PEG-g-Dex and Dex are feasible for chronopharmacological drug delivery systems.     

Double-stimuli-responsive degradation of hydrogels consisting of oligopeptide-terminated poly(ethylene glycol) and dextran with an interpenetrating polymer network

Biodegradable hydrogels consisting of oligopeptide-terminated poly(ethylene glycol) (PEG) and dextran (Dex) with an interpenetrating polymer network (IPN) structure were prepared as models of novel biomaterials exhibiting a double-stimuli-response function. The IPN-structured hydrogels were synthesized by sequential cross-linking reaction of N-methacryloyl-glycylglycylglycyl-terminated PEG and Dex. In vitro degradation of the IPN-structured hydrogels was examined using papain and dextranase as model enzymes of hydrolyzing oligopeptide and Dex, respectively. Specific degradation in the presence of papain and dextranase was observed in the IPN-structured hydrogel with a particular composition of oligopeptide-PEG and Dex. This same hydrogel was not degraded by one of the two enzymes. The IPN-structured hydrogels were characterized by water content, thermal mechanical analysis, and wide-angle X-ray diffraction, and the results were compared with those of co-cross-linked hydrogels consisting of N-methacryloyl-glycylglycylglycyl-terminated PEG and methacryloyl Dex. The results suggest that the IPN-structured hydrogels contain physical chain entanglements between networks as well as chemical cross-linked networks. It is concluded that the double-stimuli-responsive degradation observed in the IPN-structured hydrogel is achieved by controlling the chain entanglements between the two biodegradable polymers. Such degradation property of the IPN-structured hydrogel can be useful as a fail-safe system for guaranteed drug delivery and/or medical micromachines.     

In vivo and in vitro degradation of poly(ether ester) block copolymers based on poly(ethylene glycol) and poly(butylene terephthalate)

Two in vivo degradation studies were performed on segmented poly(ether ester)s based on polyethylene glycol (PEG) and poly(butylene terephthalate) (PBT) (PEOT/PBT). In a first series of experiments, the in vivo degradation of melt-pressed discs of different copolymer compositions were followed up for 24 weeks after subcutaneous implantation in rats. The second series of experiments aimed to simulate long-term in vivo degradation. For this, PEOT/PBT samples were pre-degraded in phosphate buffer saline (PBS) at 100 degrees C and subsequently implanted. In both series, explanted materials were characterized by intrinsic viscosity measurements, mass loss, proton nuclear magnetic resonance spectroscopy (1H-NMR) and differential scanning calorimetry (DSC). In both studies the copolymer with the higher PEO content degraded the fastest, although all materials degraded relatively slowly. To determine the nature of the degradation products formed during hydrolysis of the copolymers, 1000 PEOT71PBT29 (a copolymer based on PEG with a molecular weight of 1000 g/mol and 71 wt% of PEO-containing soft segments) was degraded in vitro at 100 degrees C in phosphate buffer saline (PBS) during 14 days. The degradation products present in PBS were analyzed by 1H-NMR and high performance liquid chromatography/mass spectroscopy (HPLC/MS). These degradation products consisted of a fraction with high contents of PEO that was soluble in PBS and a PEOT/PBT fraction that was insoluble at room temperature. From the different in vitro and in vivo degradation experiments performed, it can be concluded that PEOT/PBT degradation is a slow process and generates insoluble polymeric residues with high PBT contents.     

Functionalization of poly(ethylene glycol) and monomethoxy-poly(ethylene glycol)

Simple methods are described for the substitution of poly(ethylene glycol) and monomethoxy-poly(ethylene glycol) substitution. Affinity ligands, coenzymes, or enzymes can be covalently attached to the substitution product or they can be used as liquid ionexchangers.     

Thermodynamics of depolymerization of biotechnological poly[(R)-3-hydroxybutyrate] with formation of (R,R,R)-4,8,12-trimethyl-1,5,9-trioxadodeca-2,6,10-trione and of its polymerization with ring-opening to highly isotactic poly[(R)-3-hydroxybutyrate] from 0 to 500 K at standard pressure

In adiabatic vacuum and dynamic calorimeters the temperature dependence of the heat capacity C of (R,R,R-4,8,12-trimethyl-1,5,9-trioxadodeca-2,6,10-trione a twelve-membered cyclic trilactone), biotechnological poly[(R)-3-hydroxybutyrate] and highly isotactic poly[(R)-3-hydroxybutyrate] was studied between 5 K and 500 K; temperatures and enthalpies of melting of the above mentioned substances were measured. In a calorimeter with a static bomb and an isothermal shield the energy of combustion of the same substances was measured. From the results the thermodynamic functions C (T), H⁰(T) ? H⁰ (0), S⁰ (T), G⁰ (T) ? H⁰ (0) were calculated in the range of 0 K to 500 K and thermochemical parameters ΔH, ΔH, ΔS, ΔG were estimated at T = 298,15 K and standard pressure. The thermodynamic parameters of depolymerization of the biotechnological polymer to the 12-membered trilactone ΔH, ΔS ΔG and of the polymerization of the monomer formed in the highly isotactic poly[(R)-3-hydroxybutyrate] ΔH, ΔS, ΔG were calculated for 0 K to 500 K.     

Enzymatic degradation of microbial poly(3-hydroxybutyrate) films

The effects of crystallinity and spherulite size on the enzymatic degradation of microbial poly(3-hydroxybutyrate) (PHB) films have been studied at 37°C and pH 7,4 in aqueous solutions of an extracellular PHB depolymerase from Alcaligenes faecalis T1. The rate of enzymatic degradation of PHB films decreases with an increase in crystallinity, but it is little influenced by the size of PHB spherulites. It was suggested that the PHB depolymerase firstly hydrolyzes the PHB chains in the amorphous state on the surface of the films and subsequently erodes the PHB chains in the crystalline state.     

Adaptable poly(ethylene glycol) microspheres capable of mixed-mode degradation

A simple, degradable poly(ethylene glycol) (PEG) microsphere system formed from a water-in-water emulsion process is presented. Microsphere network degradation and erosion were controlled by adjusting the number of hydrolytically labile sites, by varying the PEG molecular weight, and by adjusting the emulsion conditions. Microsphere size was also controllable by adjusting the polymer formulation. Furthermore, it is demonstrated that alternative degradation and erosion mechanisms, such as proteolytic degradation, can be incorporated into PEG microspheres, resulting in mixed-mode degradation. Owing to the adaptability of this approach, it may serve as an attractive option for emerging tissue engineering, drug delivery and gene delivery applications.