Synthesis and characterization of biodegradable polyurethane based on poly(ε-caprolactone) and L-lysine ethyl ester diisocyanate

A biocompatible diisocyanate, lysine ethyl ester diisocyanate, was prepared. Afterwards, biodegradable polyurethane (PU) was synthesized by the stepgrowth polymerization of this diisocyanate with hydroxyl terminated poly(ε-caprolactone) in the presence of 1,4-butanediol as a chain-extender. The resulting PU was characterized by GPC, IR and DSC measurements. Its mechanical strength was found to increase with increasing the hard segment content. The PU microfiber meshes with high porosity were obtained by solution electrospinning technique. Their degradation behavior in the PBS and enzymatic solution was also investigated.     

Synthesis and characterization of biodegradable polyurethane based on poly(?-caprolactone) and L-lysine ethyl ester diisocyanate

A biocompatible diisocyanate, lysine ethyl ester diisocyanate, was prepared. Afterwards, biodegradable polyurethane (PU) was synthesized by the step-growth polymerization of this diisocyanate with hydroxyl terminated poly(?-caprolactone) in the presence of 1,4-butanediol as a chain-extender. The resulting PU was characterized by GPC, IR and DSC measurements. Its mechanical strength was found to increase with increasing the hard segment content. The PU microfiber meshes with high porosity were obtained by solution electrospinning technique. Their degradation behavior in the PBS and enzymatic solution was also investigated.     

pH-stimuli-responsive near-infrared optical imaging nanoprobe based on poly(γ-glutamic acid)/poly(β-amino ester) nanoparticles

pH-stimuli-responsive near-infrared optical imaging nanoprobes are designed and synthesized in this study in a facile one-step synthesis process based on the use of the biocompatible and biodegradable polymer poly(γ-glutamic acid) (γ-PGA)/poly(β-amino ester) (PBAE). PBAE has good transfection efficiency and promotes degradation properties under acidic conditions. This pH-responsive degradability can be used for the effective release of encapsulating materials after cellular uptake. As an optical imaging probe, indocyanine green (ICG) is an FDA-approved near-infrared fluorescent dye with a quenching property at a high concentration. In this regard, we focus here on the rapid degradation of PBAE in an acidic environment, in which the nanoparticles are disassembled. This allows the ICG dyes to show enhanced fluorescence signals after being releasing from the particles. We demonstrated this principle in cellular uptake experiments. We expect that the developed pH-stimuli-responsive smart nanoprobes can be applied in intracellular delivery signaling applications.     

Fabrication and characterization of interconnected porous biodegradable poly(ε-caprolactone) load bearing scaffolds

In this study, poly(ε-caprolactone) (PCL)/poly(ethylene oxide) (PEO) (50:50 wt%) immiscible blend was used as a model system to investigate the feasibility of a novel solventless fabrication approach that combines cryomilling, compression molding and porogen leaching techniques to prepare interconnected porous scaffolds for tissue engineering. PCL was cryomilled with PEO to form blend powders. Compression molding was used to consolidate and anneal the cryomilled powders. Selective dissolution of the PEO with water resulted in interconnected porous scaffolds. Sodium chloride salt (NaCl) was subsequently added to cryomilled powder to increase the porosity of scaffolds. The prepared scaffolds had homogeneous pore structures, a porosity of ~50% which was increased by mixing salt with the blend (~70% for 60% wt% NaCl), and a compressive modulus and strength (ε = 10%) of 60 and 2.8 MPa, respectively. The results of the study confirm that this novel approach offers a viable alternative to fabricate scaffolds.     

Direct imaging of the surfaces of poly(β) hydroxybutyrate and hydroxybutyrate oligomers by atomic force microscopy

Attempts have been made to image the fold surface of a single crystal of polyhydroxybutyrate (PHB) using the relatively new technique of atomic force microscopy (AFM). To overcome the obscuring of the fold surface by loose loops of polymer and chain ends, two different approaches were used. We first studied the single crystals of an oligomer of 32 HB units, which is known to fold once very tightly within a crystal, using AFM. Secondly, studies were made of single crystals of PHB which have been chemically degraded with methylamine to etch away the amorphous layer of loosely folded material, in an attempt to expose the fold surface. The crystals of the 32-mer had a similar morphology to those of the polymer PHB. However, at high magnification, lines of ridges were observed which ran parallel to the crystallographic b axis with a spacing of 0.7 nm, similar to the dimensions of the unit cell (0.58, 1.32, and 0.60 nm). It was not possible to differentiate between chain ends and folds. The partially etched PHB crystals maintained enough integrity to permit imaging by AFM, although surface detail could not be resolved on a molecular scale.     

The morphology and phase mixing studies on poly(ester–urethane) during shape memory cycle

Three series of shape memory poly(ester–urethane) with varying hard-segment contents were synthesized. The materials were designed to display a three-phase structure consisting of a disperse phase formed by crystallites and hard domains embedded in an amorphous matrix. The initial undeformed morphology was investigated using techniques such as modulated differential scanning calorimetry, Fourier transform infrared spectroscopy, and wide angle X-ray scattering. These techniques were used to determine the phase separation, hydrogen-bonding structure, and crystalline fraction of the specimens prior to thermo-mechanical treatments. The obtained information was correlated with small angle X-ray scattering investigations of morphological changes that occurred during shape memory cycling. The deformation cycle led to the formation of an oriented nanostructure derived from chain alignment. The nanostructure recovered was observed to be triggered by the melting of the crystallites and bulk incompatibility. A relationship between the ability of the studied poly(ester–urethane) specimens to recover their original shape and their original nanostructure was determined.     

Preparation and mechanical property of poly(ε-caprolactone)–matrix composites containing nano-apatite fillers modified by silane coupling agents

This study aims to improve the tensile strength and elastic modulus of nano-apatite/poly(ε-caprolactone) composites by silane-modification of the nano-apatite fillers. Three silane coupling agents were used to modify the surfaces of nano-apatite particles and composites of silanized apatite and PCL were prepared by a technique incorporating solvent dispersion, melting-blend and hot-pressing. The results showed that the silane coupling agents successfully modified the surfaces of nano-apatite fillers, and the crystallization temperatures of the silanized apatite/PCL composites were the higher than that of the non-silanized control material, although the melting temperature of the composites remained almost unaffected by silanization. The ultimate tensile strength and elastic modulus of the silanized composites reached 22.60 MPa and 1.76 GPa, as a result of the improved interfacial bonding and uniform dispersion of nano-apatite fillers. This study shows that the processing technique and silanization of nano-apatite particles can effectively improve the tensile strength and elastic modulus of nano-apatite/PCL composites.     

Morphology of solution-grown poly(β-propiolactone) single crystals

Single crystals of poly(-propiolactone) (PPL) with different molecular weights (M _w = 70000 and M _w = 2300) were grown from four kinds of solvents under isothermal crystallization condition. The morphologies and crystal structures of PPL single crystals were investigated by means of transmission electron microscopy and atomic force microscopy. The single crystals of high-molecular-weight PPL (HMW-PPL) grown from cyclohexanone appeared elongated with dimensions of around 0.2–0.8 m along the short and 5–10 m along the long axes. Single crystals of low-molecular-weight PPL grown from cyclohexanone showed three to five elliptical-shaped lamellae, from central nucleus like petals. The long axes of both single crystals corresponded to the crystallographic b axis. The reciprocal lattice parameters: a* = 2.045 nm^–1, b* = 1.420 nm^–1 and * = 90° could be determined from electron diffractograms. Decoration of the crystals with polyethylene suqqested that the single crystals of HMW-PPL have regular chain-folding along their long axis in the [010] direction with consecutive folds in the [110] and [1 0] directions. Accordingly, it is deduced that HMW-PPL has the anti-parallel chain-packing structure.     

Synthesis and properties of poly(ethylene glycol) macromer/β-chitosan hydrogels

Semi-interpenetrating polymer network (semi-IPN) hydrogels composed of -chitosan and poly(ethylene glycol) diacrylate macromer (PEGM) were synthesized and characterized for the application as potential biomedical materials. The mixture of PEGM and -chitosan, dissolved in water including a small amount of acetic acid, was cast to prepare hydrogel films, followed by a subsequent crosslinking with 2,2-dimethoxy-2-phenylacetophenone as a non-toxic photoinitiator by ultraviolet irradiation. Photocrosslinked hydrogels exhibited relatively high equilibrium water content in the range 77–83% which is mainly attributed to the free water content rather than to the bound water, hydrogen bonded with components in semi-IPN hydrogels. The crystallinity, thermal properties and mechanical properties of semi-IPN hydrogels were studied. All the photocrosslinked hydrogels revealed a remarkable decrease in crystallinity. The glass transition temperatures, Tg, of crosslinked PEGM segment in semi-IPNs increased compared with poly(ethylene glycol) itself. However, with increasing -chitosan content their Tg decreased owing to the higher degree of crosslinking. The tensile strengths of semi-IPNs in dry state were rather high, but those of hydrogels in wet state decreased drastically.     

Self-diffusion of poly (ethylene oxide) – modified paclitaxel in dilute aqueous solutions

In order to make the hydrophobic anti-cancer drug Paclitaxel watersoluble, it was coupled to highly uniform poly (ethylene oxide) (PEO) with the molar mass M_w = 5,000 g/mol with a self-immolating succinate linker. The concentration and temperature dependence of the unrestricted molecular mobility of the molecules (long-time self-diffusion) in homogeneous aqueous (D₂O) solution was studied by gradient field NMR around body temperature in the highly dilute region. The concentration dependence of the friction coefficient, and the self-diffusion coefficient is unexpectedly strong and probably caused by peculiarities of the shape and/or the flexibility of the molecules rather than their size. Dedicated to Prof. Dr. W. S. Veeman, Gerhard-Mercator-University, Duisburg, on the occasion of his 60th birthday     

Preparation and properties of polyrotaxane from α-cyclodextrin and poly(ethylene glycol) with poly(vinyl alcohol)

α–Cyclodextrin (α-CD) was found to form inclusion complexes with poly(ethylene glycol) (PEG) having a crystalline state in high yields, which have been investigated extensively in the past. Formation of an inclusion complex depends strongly on structure, molecular weight and geometry of the polymer. Development of a dicomponent inclusion complex (DIC) of PEG and α-CD in the presence of poly(vinyl alcohol) (PVA) and initiation of hexagonal crystals upon sonication have exhibited various microstructures. Formation of the new inclusion complex in PVA heavily depends on the concentration of PVA, temperature and sonication time. The complexes produced are characterized by FTIR, HNMR spectra and powder X-ray. ¹HNMR of the complexes demonstrate that their stoichiometric ratio is 2:1 (two ethylene glycol units and one α-CD). X-ray patterns of PEG–α-CD complex indicate that the α-CD forms channels whereas PEG/α-CD/PVA creates cage-type structures.     

Enzymatic degradation of the hydrogels based on synthetic poly(α-amino acid)s

Biodegradable hydrogels are studied as potential scaffolds for soft tissue regeneration. In this work biodegradable hydrogels were prepared from synthetic poly(α-amino acid)s, poly(AA)s. The covalently crosslinked gels were formed by radical copolymerization of methacryloylated poly(AA)s, e.g. poly[N ⁵-(2-hydroxy-ethyl)-l-glutamine-ran-l-alanine-ran-N ⁶-methacryloyl-l-lysine], as a multifunctional macro-monomer with a low-molecular-weight methacrylic monofunctional monomer, e.g. 2-hydroxyethyl methacrylate (HEMA). Methacryloylated copolypeptides were synthesized by polymerization of N-carboxyanhydrides of respective amino acids and subsequent side-chain modification. Due to their polypeptide backbone, synthetic poly(AA)s are cleavable in biological environment by enzyme-catalyzed hydrolysis. The feasibility of enzymatic degradation of poly(AA)s alone and the hydrogels made from them was studied using elastase, a matrix proteinase involved in tissue healing processes, as a model enzyme. Specificity of elastase for cleavage of polypeptide chains behind the l-alanine residues was reflected in faster degradation of l-alanine-containing copolymers as well as of hydrogels composed of them.     

Biodegradation trend of poly(ε-caprolactone) and nanocomposites

PCL nanocomposites based on two organically modified montmorillonites at 5% clay loading were biodegraded in a mature compost. All samples showed an effective degradation in compost but nanoclays were found to partially delay the process. Biodegradation carried out by microorganisms isolated from the compost showed that the bacterium Bacillus licheniformis was able to degrade the studied systems without considerable differences in the polymer degradation trend due to the presence of nanoclays.     

Non-conventional injection molding of poly(lactide) and poly(ε-caprolactone) intended for orthopedic applications

Biodegradable polymers such as poly(lactide) (PLA) and poly(epsilon-caprolactone) (PCL) are increasingly used in biomedical applications as temporary implants. However, melt processing of these materials in particular of PLA is difficult due to the temperature sensitivity. Within this study, PLA and PCL were injection molded conventionally and by using the process shear controled orientation in injection molding (SCORIM) in order to investigate the effect of processing parameters on the physical properties of the moldings. Therefore, flexural testing, differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), molecular weight (MW) and orientation measurements were performed. PLA showed high sensitivity to melt temperature. In the case of amorphous poly(DL-lactide), the molecular weight and subsequently the ductility is substantially reduced by processing at higher melt temperatures. In the case of crystallizable poly(L-lactide), higher melt temperatures and shear induced by the SCORIM process resulted in enhanced crystallinity, which compromised the mechanical properties. Generally, SCORIM processing improved the mechanical properties, in particular the ductility, by orientating the molecular structure. PCL was shown to be less sensitive to shear and temperature than PLA. Stress at yield and stiffness are more improved by SCORIM processing. However, the processing temperature in combination with the grade used proved to be influential for the mechanical properties of resulting moldings.     

Tissue engineering scaffolds of mesoporous magnesium silicate and poly(ε-caprolactone)–poly(ethylene glycol)–poly(ε-caprolactone) composite

Mesoporous magnesium silicate (m-MS) and poly(ε-caprolactone)–poly(ethylene glycol)–poly(ε-caprolactone) (PCL–PEG–PCL) composite scaffolds were fabricated by solvent-casting and particulate leaching method. The results suggested that the incorporation of m-MS into PCL–PEG–PCL could significantly improve the water adsorption of the m-MS/PCL–PEG–PCL composite (m-MPC) scaffolds. The in vitro degradation behavior of m-MPC scaffolds were determined by testing weight loss of the scaffolds after soaking into phosphate buffered saline (PBS), and the result showed that the degradation of m-MPC scaffolds was obviously enhanced by addition of m-MS into PCL–PEG–PCL after soaking for 10 weeks. Proliferation of MG63 cells on m-MPC was significantly higher than MPC scaffolds at 4 and 7 days. ALP activity on the m-MPC was obviously higher than MPC scaffolds at 7 days, revealing that m-MPC could promote cell differentiation. Histological evaluation showed that the introduction of m-MS into PCL–PEG–PCL enhanced the efficiency of new bone formation when the m-MPC scaffolds implanted into bone defect of rabbits. The results suggested that the inorganic/organic composite of m-MS and PCL–PEG–PCL scaffolds exhibited good biocompatibility, degradability and osteogenesis.     

Tailoring impact toughness of poly(L-lactide)/poly(ε-caprolactone) (PLLA/PCL) blends by controlling crystallization of PLLA matrix

Melt blending poly(L-lactide) (PLLA) with various biodegradable polymers has been thought to be the most economic and effective route to toughen PLLA without compromising its biodegradability. Unfortunately, only very limited improvement in notched impact toughness can be achieved, although most of these blends show significant enhancement in tensile toughness. In this work, biodegradable poly(ε-caprolactone) (PCL) was used as an impact modifier to toughen PLLA and a nucleating agent was utilized to tailor the crystallization of PLLA matrix. Depending on the nucleating agent concentrations in the matrix and mold temperatures in injection molding, PLLA/PCL blends with a wide range of matrix crystallinity (10-50%) were prepared by practical injection molding. The results show that there is a linear relationship between PLLA matrix crystallinity and impact toughness. With the increase in PLLA crystalline content, toughening becomes much easier to achieve. PLLA crystals are believed to provide a path for the propagation of shear yielding needed for effective impact energy absorption, and then, excellent toughening effect can be obtained when these crystals percolate through the whole matrix. This investigation provides not only a new route to prepare sustainable PLLA products with good impact toughness but also a fresh insight into the importance of matrix crystallization in the toughening of semicrystalline polymers with a flexible polymer.     

Poly(acrylonitrile-co-itaconic acid)–poly(3,4-ethylenedioxythiophene) and poly(3-methoxythiophene) nanoparticles and nanofibres

This work aimed to produce poly(acrylonitrile-co-itaconic acid) (P(AN-co-IA)) nanocomposites with poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3-methoxythiophene) (PMOT). An anionic surfactant sodium dodecyl benzene sulphonate was used in emulsion polymerization for nanocomposite production. Incorporations of PEDOT and PMOT on the nanoparticles were characterized by scanning electron microscopy (SEM), atomic force microscopy, Fourier transform infrared-attenuated total reflectance spectroscopy and ultra-violet spectroscopy. These nanoparticles were blended with PAN and the blends were electrospun to produce P(AN-co-IA)–polythiophene-derivative-based nanofibres, and the obtained nanofibres were characterized by SEM and energy dispersive spectroscopy. In addition, electrochemical impedance studies conducted on nanofibres showed that PEDOT and PMOT in matrix polymer P(AN-co-IA) exhibited capacitive behaviour comparable to that of ITO–PET. Their capacitive behaviour changed with the amount of electroactive polymer.