Controllable growth of hydroxyapatite on electrospun poly(dl-lactide) fibers grafted with chitosan as potential tissue engineering scaffolds

A novel method was exploited to use electrospun poly(dl-lactide) (PDLLA) fibers grafted with chitosan as the induction sites for composite fabrication to suit better the mechanical and biological demands for biomedical applications. The amount of chitosan grafted on the fiber surface could be controlled by the aminolysis time, and the kinetic equations of HA growth were drafted as a function of the incubation time for fibrous scaffolds with different amounts of grafted chitosan. The introduction of amino groups and chitosan on electrospun PDLLA fibers enhanced the cell proliferation due to the improved surface wettability and alleviated dimensional shrinkage. Significantly higher cytoviability and alkaline phosphatase levels were detected on mineralized scaffolds from chitosan grafted fibers than those from aminolyzed fibers, and cells interacted and integrated well with the surrounding fibers. The fibrous nanocomposites should have potential applications as functional coatings on medical devices and as scaffolds for bone tissue engineering.     

Gelation and degradation characteristics of in situ photo-crosslinked poly(l-lactide-co-ethylene oxide-co-fumarate) hydrogels

The objective of this work was to determine the gelation kinetics, extent of swelling, sol fraction, and degradation kinetics of photo-crosslinked poly(l-lactide-co-ethylene oxide-co-fumarate) (PLEOF) hydrogels, with N-vinyl-2-pyrrolidone (NVP) crosslinker, as a function of composition as well as the time and intensity of UV radiation. The gelation process was monitored by in situ rheometry. The crosslinking was shown to be facilitated by increasing NVP concentration up to a certain value above which the hydrogel shear modulus did not increase with additional amount of NVP. Increasing the hydrophobicity of PLEOF macromer resulted in a decrease in the hydrogel swelling ratio and increase in sol fraction which was due to a reduction in the apparent reactivity of the PLEOF fumarate units. The degradation characteristics of PLEOF hydrogels depended on the ratio of PLA to PEG with PLEOF 30/70 (30% lactide) having the highest degradation rate.     

Controlled crystal nucleation in the melt-crystallization of poly(l-lactide) and poly(l-lactide)/poly(d-lactide) stereocomplex

Among the various inorganic nucleators examined, Talc and an aluminum complex of a phosphoric ester combined with hydrotalcite (NA) were found to be effective for the melt-crystallization of poly(l-lactide) (PLLA) and PLLA/poly(d-lactide) (PDLA) stereo mixture, respectively. NA (1.0 phr (per one hundred resin)) can exclusively nucleate the stereocomplex crystals, while Talc cannot suppress the homo crystallization of PLLA and PDLA in the stereo mixture. Double use of Talc and NA (in 1.0 phr each) is highly effective for enhancing the crystallization temperature of the stereo complex without forming the homo crystals. The stereocomplex crystals nucleated by NA show a significantly lower melting temperature (207 °C) than the single crystal of the stereocomplex (230 °C) in spite of recording a large heat of crystallization ΔH_c (54 J/g). Photomicrographic study suggests that the spherulites with a symmetric morphology are formed in the stereo mixture added with NA while the spherulites do not grow in size in the mixture added with Talc. The exclusive growth of the stereocomplex crystals by the melt-crystallization process will open a processing window for the PLLA/PDLA.     

Transcrystalline structures of poly(l-lactide)

Poly(l-lactide) (PLLA) film containing transcrystalline (TC) structures can easily be obtained by placing PLLA films melted between two poly(tetrafluoroethylene) (PTFE) sheets, followed by isothermal crystallization at 122 °C. The fine structures of the PLLA-TC film were studied by various structural techniques such as X-ray diffractometry, optical microscopy and transmission electron microscopy. We also examined the purification effect upon the morphology of PLLA-TC film. The formation of the TC structures revealed that one-dimensional spherulitic growth occurred from the assembling impurities as nucleation agent near the PTFE substrate in the heterogeneous nucleation system. We found that the b-axis of PLLA crystal was parallel to the lamellae growth direction confirmed using X-ray diffraction. The precipitated PLLA film crystallized in a similar process exhibited scanty TC textures, suggesting that the existence of impurity in the PLLA sample was an important factor for the formation of those structures.     

Thermal degradation of poly(l-lactide): effect of alkali earth metal oxides for selective l,l-lactide formation

To achieve the feed stock recycling of poly(l-lactide) (PLLA) to l,l-lactide, PLLA composites including alkali earth metal oxides, such as calcium oxide (CaO) and magnesium oxide (MgO), were prepared and the effect of such metal oxides on the thermal degradation was investigated from the viewpoint of selective l,l-lactide formation. Metal oxides both lowered the degradation temperature range of PLLA and completely suppressed the production of oligomers other than lactides. CaO markedly lowered the degradation temperature, but caused some racemization of lactide, especially in a temperature range lower than 250 °C. Interestingly, with MgO racemization was avoided even in the lower temperature range. It is considered that the effect of MgO on the racemization is due to the lower basicity of Mg compared to Ca. At temperatures lower than 270 °C, the pyrolysis of PLLA/MgO (5 wt%) composite occurred smoothly causing unzipping depolymerization, resulting in selective l,l-lactide production. A degradation mechanism was discussed based on the results of kinetic analysis. A practical approach for the selective production of l,l-lactide from PLLA is proposed by using the PLLA/MgO composite.     

In situ gelling aqueous solutions of pH- and temperature-sensitive poly(ester amino urethane)s

A series of novel pH- and temperature-sensitive multiblock poly(ester amino urethane)s were synthesized. The copolymers were characterized by ¹H NMR, FT-IR and GPC. In the multiblock copolymers, the tertiary amino groups of the poly(amino urethane) segments act as pH-responsive moieties, while the PCL-PEG-PCL blocks act as biodegradable and temperature-sensitive segments. At a relatively high pH (7.0 or above), the multiblock copolymer aqueous solution showed a sol-to-gel-to-aggregation transition with increasing temperature. In contrast, at a lower pH (below 7.0), the polymer solution always existed as a sol state within the experimental temperature range. The gel window covers the physiological conditions. After subcutaneous injection of the 20 wt% multiblock copolymer solutions into mice, polymeric hydrogels were formed in situ in a short time. The in vitro release of an anticancer drug, paclitaxel, persisted over 1 month under physiological conditions.     

Miscibility and properties of linear poly(l-lactide)/branched poly(l-lactide) copolyester blends

Polymer blends consisting of linear poly(l-lactide) (PLLA) and different proportions of dendritic PLLA-based copolyesters (hb-PLLA) characterized by different degrees of branching (DB) were obtained in melt. The solid-state properties of poly(l-lactide)s and their blends were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), atomic force microscopy (AFM) and stress-strain measurements. DSC and DMA methods proved miscibility of PLLA/hb-PLLA blends for the studied composition range. AFM indicated that no phase separation occurs in PLLA/hb-PLLA blends and that PLLA and hb-PLLA cocrystallize in one single lamellae type. The mechanical characteristics of PLLA/hb-PLLA blends deteriorated with an increase of the DB and with changing blend composition. Susceptibility of the blends to biodegradation was studied by measuring the weight loss in two different biodegradation media. PLLA/hb-PLLA blends showed more pronounced hydrophilic character and higher susceptibility to biodegradation with an increase in the degree of branching.     

Mechanical properties, morphology, and crystallization behavior of blends of poly(l-lactide) with poly(butylene succinate-co-l-lactate) and poly(butylene succinate)

The blends of poly(l-lactide) (PLLA) with poly(butylene succinate) (PBS) and poly(butylene succinate-co-l-lactate) (PBSL) containing the lactate unit of ca. 3 mol% were prepared by melt-mixing and subsequent injection molding, and their mechanical properties, morphology, and crystallization behavior have been compared. Dynamic viscoelasticity and SEM measurements of the blends revealed that the extent of the compatibility of PBSL and PBS with PLLA is almost the same, and that the PBSL and PBS components in the blends with a low content of PBSL or PBS (5-20 wt%) are homogenously dispersed as 0.1−0.4 μm particles. The tensile strength and modulus of the blends approximately followed the rule of mixtures over the whole composition range except that those of PLLA/PBS 99/1 blend were exceptionally higher than those of pure PLLA. All the blends showed considerably higher elongation at break than pure PLLA, PBSL, and PBS. Differential scanning calorimetric analysis of the blends revealed that the isothermal and non-isothermal crystallization of the PLLA component is promoted by the addition of a small amount of PBSL, while the addition of PBS was much less effective.     

Crystallization, structure and properties of plasticized poly(l-lactide)

Poly(l-lactide) (PLA) was plasticized with poly(ethylene glycol)s having M_w of 400 and 600 g/mol. In addition to poly(ethyne glycol)s with hydroxyl end groups, monomethyl ethers of poly(ethylene glycol) having M_w of 550 and 750 g/mol, with chains terminated with hydroxyl groups and methyl groups, were used. The effect of different end groups on the plasticization of both amorphous and semicrystalline PLA was studied. The crystallization, structure, thermal and tensile properties of PLA and PLA with 5 and 10 wt% of plasticizers were explored. No marked effect induced by different end groups of plasticizers was found. All the plasticizers used decreased T_g and increased the ability of PLA to cold crystallization. While an amorphous plasticized PLA could be deformed to about 550%, a semicrystalline PLA with the same total plasticizer content exhibited nonuniform plasticization of the amorphous phase and less ability to the plastic deformation. Nevertheless, a 20% elongation at break was achieved for a semicrystalline PLA with 10 wt% of the plasticizer. The plastic deformation of both neat and plasticized PLA was associated with crazing.     

Mechanical properties, morphologies, and crystallization behavior of plasticized poly(l-lactide)/poly(butylene succinate-co-l-lactate) blends

The blends of poly(l-lactide) (PLLA) with poly(butylene succinate-co-l-lactate) (PBSL) containing the lactate unit of ca. 3 mol% and Rikemal PL710 (RKM) which is a plasticizer mainly composed of diglycerine tetraacetate were prepared by melt-mixing and subsequent injection molding. The studied RKM content of the PLLA/PBSL/RKM blends was 0-20 wt%, and the PLLA/PBSL weight ratio was 100/0 to 80/20. Although elongation at break in the tensile test did not increase by the addition of 10 wt% RKM to PLLA, the addition of a small amount of PBSL to the PLLA/RKM blend caused a considerable increase of the elongation. The SEM and DSC analyses revealed that all the PLLA/PBSL/RKM blends are immiscible blends where the PBSL particles are finely dispersed, and that there is some compatibility between PLLA-rich phase and PBSL-rich phase in the amorphous state when the RKM content is 20 wt%. As a result of investigation of the crystallization behavior by DSC and polarized optical microscopic measurements, it was revealed that the addition of RKM causes the acceleration of crystalline growth rate at a lower annealing temperature, and the addition of PBSL mainly enhances the formation of PLLA crystal nucleus.     

Melt intercalation of poly(l-lactide) chains into clay galleries

The melt intercalation of poly(l-lactide) (PLLA) chains into silicate galleries has been investigated via a melting process without any shearing force at elevated temperature. Under the melting process, the incorporation of various types of organo-modified montmorillonites into PLLA matrix lead to the increase in the basal spacing of clay particles in different manner without delamination into individual layers. The changes in layer-stacked structures of the clay particles in the PLLA matrix were examined by use of wide-angle X-ray diffraction and transmission electron microscopy. The effects of clay content in PLLA matrix and clay surfactants on the melt intercalation of PLLA were discussed in terms of chain mobility.     

Unique crystallization behavior of poly(l-lactide)/poly(d-lactide) stereocomplex depending on initial melt states

A unique crystallization behavior of poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) stereocomplex was observed when a PLLA/PDLA blend (50/50) was subjected to specific melting conditions. PLLA and PDLA were synthesized by ring opening polymerization of l- or d-lactide using zinc lactate as catalyst. PLLA/PDLA blend was prepared through solution mixing followed by vacuum drying. The blend was melted under various melting conditions and subsequent crystallization behaviors were analyzed by using DSC, XRD, NMR and ESEM. Stereocomplex was exclusively formed from the 50/50 blend of PLLA and PDLA with relatively low molecular weights. Surprisingly, stereocomplex crystallization was distinctly depressed when higher melting temperature and longer melting period were applied, in contrast to homopolymer crystallization. Considering predominant interactions between PLLA and PDLA chains, a novel model of melting process is proposed to illustrate this behavior. It is assumed that PLLA and PDLA chain couples would preserve their interactions (melt memory) when the stereocomplex crystal melts smoothly, thus resulting in a heterogeneous melt which can easily crystallize. The melt could gradually become homogeneous at higher temperature or longer melting time. The strong interactions between PLLA and PDLA chain segments are randomly distributed in a homogeneous melt, thus preventing subsequent stereocomplex crystallization. However, the homogeneous melt can recover its ability to crystallize via dissolution in a solvent.     

Hybrid 3D structure of poly(d,l-lactic acid) loaded with chitosan/chondroitin sulfate nanoparticles to be used as carriers for biomacromolecules in tissue engineering

In the tissue engineering (TE) field, the concept of producing multifunctional scaffolds, capable not only of acting as templates for cell transplantation but also of delivering bioactive agents in a controlled manner, is an emerging strategy aimed to enhance tissue regeneration. In this work, a complex hybrid release system consisting in a three-dimensional (3D) structure based on poly(d,l-lactic acid) (PDLLA) impregnated with chitosan/chondroitin sulfate nanoparticles (NPs) was developed. The scaffolds were prepared by supercritical fluid foaming at 200 bar and 35 °C, and were then characterized by scanning electron microscopy (SEM) and micro-CT. SEM also allowed to assess the distribution of the NPs within the structure, showing that the particles could be found in different areas of the scaffold, indicating a homogeneous distribution within the 3D structure. Water uptake and weight loss measurements were also carried out and the results obtained demonstrated that weight loss was not significantly enhanced although the entrapment of the NPs in the 3D structure clearly enhances the swelling of the structure. Moreover, the hybrid porous biomaterial displayed adequate mechanical properties for cell adhesion and support. The possibility of using this scaffold as a multifunctional material was further evaluated by the incorporation of a model protein, bovine serum albumin (BSA), either directly into the PDLLA foam or in the NPs that were eventually included in the scaffold. The obtained results show that it is possible to achieve different release kinetics, suggesting that this system is a promising candidate for dual protein delivery system for TE applications.     

Stereocomplex crystallization and spherulite growth behavior of poly(l-lactide)-b-poly(d-lactide) stereodiblock copolymers

The non-isothermally and isothermally crystallized stereodiblock copolymers of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) with equimolar l-lactyl and d-lactyl units and different number-average molecular weights (M_n) of 3.9 × 10³, 9.3 × 10³, and 1.1 × 10⁴ g mol⁻¹, which are abbreviated as PLLA-b-PDLA copolymers, contained only stereocomplex crystallites as crystalline species, causing higher melting temperatures of the PLLA-b-PDLA copolymers compared to those of PLLA homopolymers. In the case of non-isothermal crystallization, the cold crystallization temperatures of the PLLA-b-PDLA copolymers during heating and cooling were respectively lower and higher than those of PLLA homopolymers, indicating accelerated crystallization of PLLA-b-PDLA copolymers. In the case of isothermal crystallization, in the crystallizable temperature range, the crystallinity (X_c) values of the PLLA-b-PDLA copolymers were lower than those of the PLLA homopolymers, and were susceptible to the effect of crystallization temperature in contrast to that of homopolymers. The radial growth rate of the spherulites (G) of the PLLA-b-PDLA copolymers was the highest at the middle M_n of 9.3 × 10³ g mol⁻¹. This trend is different from that of the PLLA homopolymers where the G values increased monotonically with a decrease in M_n, but seems to be caused by the upper critical M_n values of PLLA and PDLA chains as in the case of PLLA/PDLA blends (in other papers), above which homo-crystallites are formed in addition to stereocomplex crystallites. The disturbed crystallization of PLLA-b-PDLA copolymers compared to that of the PLLA/PDLA blend is attributable to the segmental connection between the PLLA and PDLA chains, which interrupted the free movement of those chains of the PLLA-b-PDLA copolymers during crystallization. The crystallite growth mechanism of the PLLA-b-PDLA copolymers was different from that of the PLLA/PDLA blend.     

Broadband dielectric spectroscopic characterization of the hydrolytic degradation of carboxylic acid-terminated poly(d,l-lactide) materials

Broadband dielectric spectroscopy was used to examine carboxylic acid-terminated poly(d,l-lactide) samples that were hydrolytically degraded in 7.4 pH phosphate buffer solutions at 37 °C. The dielectric spectral signatures of degraded samples were considerably more distinct than those of undegraded samples and a T_g-related relaxation associated with long range chain segmental mobility was seen. For both degraded and undegraded samples, a relaxation peak just beneath a DSC-based T_g was observed, which shifts to higher frequency with increasing temperature. Thus, this feature is assigned as the glass transition as viewed from the dielectric relaxation perspective. Linear segments on log-log plots of loss permittivity vs. frequency, in the low frequency regime, are attributed to d.c. conductivity. An upward shift in relaxation peak maximum, f_max, observed especially after 145 d of immersion in buffer, implies a decrease in the time scale of long range segmental motions with increased degradation time.Permittivity data for degraded and undegraded materials were fitted to the Havriliak-Negami equation with subtraction of the d.c. conductivity contribution to uncover pure relaxation peaks. Parameters extracted from these fits were used to construct Vogel-Fulcher-Tammann-Hesse (VFTH) curves and distribution of relaxation time, G(τ), curves for all samples. It was seen that the relaxation times for the α-transition in both degraded and undegraded samples showed VFTH temperature behavior. G(τ) curves showed a general broadening and shift to lower τ with degradation, which can be explained in terms of a broadening of molecular weight within degraded samples and faster chain motions.     

Hydrolytic degradation of poly(d,l-lactide) as a function of end group: Carboxylic acid vs. hydroxyl

d,l-Lactide was initiated with 1,4-butanediol in the presence of stannous octoate catalyst to provide hydroxyl-terminated poly(d,l-lactide) at 5000 and 20,000 g/mol. Portions of these materials were reacted with succinic anhydride in the presence of 1-methylimidazole to convert the hydroxyl functionality to succinic acid-terminated polymers in relatively high yield. The four materials were placed in a 7.4 pH buffered saline solution at 37 °C and monitored up to 180 days for their relative moisture uptake and weight loss behaviors. Carboxylic acid functionality displayed a dramatic effect on the moisture uptake behaviors for the 5000 and 20,000 g/mol polymers when compared to their respective hydroxyl functional materials. Carboxylic acid functionality significantly increased the hydrolytic degradation rate and mass loss behavior for the 5000 g/mol material, but did not affect the hydrolytic degradation rate for the higher molecular weight sample. These results suggest that moisture uptake is not the rate limiting step for the hydrolytic degradation high molecular weight poly(d,l-lactide).     

Intercalated structure of polypropylene/in situ polymerization-modified talc composites via melt compounding

Talc was modified with methyl methacrylate (MMA) or butyl acrylate (BA) via in situ polymerization. The talc/isotactic polypropylene (PP) composites with nano-sized intercalated structure were formed by melt compounding of PP with the modified talc. The results showed that the talc layers were partially delaminated, aligned along the flow direction, and uniformly dispersed in the PP matrix. The thickness of the PMMA-modified talc layers in the PP matrix was in the range 80-240 nm, while the PBA-modified talc was even thinner. PMMA or PBA macromolecules attached on the surface of talc layers hindered the crystallization of the PP component. Moreover, the aligned pristine talc layers promoted the orientation of the PP crystals. However, the extent of PP crystal orientation decreased in the presence of PMMA or PBA-modified talc.     

Temperature dependent variations in the lamellar structure of poly(l-lactide)

Time- and temperature-dependent SAXS and WAXS experiments on poly(l-lactide) were used (i) to establish the relationships between the crystallization temperature, the crystal thickness and the melting point, (ii) to follow recrystallization processes during heating, and (iii) to detect perturbations of the crystalline order. The studies showed several peculiarities: (i) although no solid state thickening occurs during a crystallization, crystal thicknesses are with values between 11 and 20 nm very large (ii) crystal thicknesses and long spacings have a minimum at 120 °C and increase for both higher and lower crystallization temperatures. The anomalous behavior at low crystallization temperatures is to be related with a disordering of the crystal lattice (iii) there exists an extended temperature range where crystal thicknesses change in controlled manner by recrystallization processes (iv) as it appears, a triple point where the fluid, the crystalline and a mesomorphic phase coexist is located near to normal pressure and a temperature of 190 °C.     

Preparation in supercritical CO2 of porous poly(methyl methacrylate)-poly(l-lactic acid) (PMMA-PLA) scaffolds incorporating ibuprofen

Partially biodegradable porous scaffolds incorporating bioactive molecules prepared by clean techniques posses an enormous interest in tissue engineering applications. Poly(methyl methacrylate)-poly(l-lactic acid) (PMMA-PLA) blends were submitted to CO₂ supercritical conditions (P = 160-260 bar, T = 60 °C) after certain time and then rapidly depressurized to obtain porous structures that have been related with the supercritical parameters and to the polymer blend composition. In some cases ibuprofen was also incorporated to the formulations previously to the CO₂ treatment and studied the appropriate conditions for avoiding its extraction in SCCO₂. Scaffolds purity, thermal transitions, swelling and degradation behaviour, and the ibuprofen release were also studied to determine the appropriate scaffolds with a desired porosity for cell seeding. Cell culture was performed on the selected porous scaffolds using human fibroblast examined by scanning electron microscopy (SEM).