Dimitrios G. Papageorgiou

Fabrication of alginate–gelatin crosslinked hydrogel microcapsules and evaluation of the microstructure and physico-chemical properties

Microencapsulation of cells by using biodegradable hydrogels offers numerous attractive features for a variety of biomedical applications including tissue engineering. This study highlights the fabrication of microcapsules from an alginate-gelatin crosslinked hydrogel (ADA-GEL) and presents the evaluation of the physico-chemical properties of the new microcapsules which are relevant for designing suitable microcapsules for tissue engineering. Alginate di-aldehyde (ADA) was synthesized by periodate oxidation of alginate which facilitates crosslinking with gelatin through Schiffs base formation between the free amino groups of gelatin and the available aldehyde groups of ADA. Formation of Schiffs base in ADA-GEL and aldehyde groups in ADA was confirmed by FTIR and NMR spectroscopy, respectively. Thermal degradation behavior of films and microcapsules fabricated from alginate, ADA and ADA-GEL was dependent on the hydrogel composition. The gelation time of ADA-GEL was found to decrease with increasing gelatin content. The swelling ratio of ADA-GEL microcapsules of all compositions was significantly decreased, whereas the degradability was found to increase with the increase of gelatin ratio. The surface morphology of the ADA-GEL microcapsules was totally different from that of alginate and ADA microcapsules, observed by SEM. Two different buffer solutions (with and without calcium salt) have an influence on the stability of microcapsules which had a significant effect on the gelatin release profile of ADA-GEL microcapsules in these two buffer solutions.

β-Nucleated Polypropylene: Processing, Properties and Nanocomposites

Polypropylene (PP) and its allotropic forms is one of the most important polymeric materials due to its wide commercial penetration and academic interest over the last decades. The current review deals with β-polypropylene, one of the most interesting crystalline forms of PP, due to its metastable nature and the various physical properties which it presents. The introduction of β-crystals into the PP structure can be either simple or more complex procedure and several strategies are discussed in the current manuscript. Also, the physical properties of the specific material including structure-property relationships, thermal properties, and supermolecular structure are presented in detail. Additionally, the presence of β-PP in polymeric blends is evaluated and the effect of each component and preparation method on the polymorphic composition is debated. Finally, β-PP has been used as a polymeric matrix in several polymer nanocomposites, therefore the performance, polymorphic composition, and preparation methods have been also discussed in terms of the β-crystalline structure and the ultimate physical properties of the nanocomposites.

Hybrid hydrogels based on keratin and alginate for tissue engineering

Novel hybrid hydrogels based on alginate and keratin were successfully produced for the first time. The self-assembly properties of keratin, and its ability to mimic the extracellular matrix were combined with the excellent chemical and mechanical stability and biocompatibility of alginate to produce 2D and 3D hybrid hydrogels. These hybrid hydrogels were prepared using two different approaches: sonication, to obtain 2D hydrogels, and a pressure-driven extrusion technique to produce 3D hydrogels. All results indicated that the composition of the hydrogels had a significant effect on their physical properties, and that they can easily be tuned to obtain materials suitable for biological applications. The cell-material interaction was assessed through the use of human umbilical vein endothelial cells, and the results demonstrated that the alginate/keratin hybrid biomaterials supported cell attachment, spreading and proliferation. The results proved that such novel hybrid hydrogels might find applications as scaffolds for soft tissue regeneration.

Sustainable, eco-friendly polyesters synthesized from renewable resources: preparation and thermal characteristics of poly(dimethyl-propylene furanoate)

Poly(2,2-dimethyl-1,3-propylene furanoate) (PDMPF), an interesting sustainable biobased polyester based on 2,5-furan dicarboxylic acid (FDCA), was synthesized by applying the two-stage melt polycondensation method. The polyester exhibited a melting temperature of Tm = 198 °C and a glass transition temperature of Tg = 68 °C. Multiple melting was observed for the samples crystallized isothermally at temperatures ranging from 160 to 175 °C. Extensive recrystallization was evidenced by modulated temperature differential scanning calorimetry (MDSC) during heating. The equilibrium melting temperature was found to be and the enthalpy of fusion of the pure crystalline polymer was ΔHf = 133 J g−1. The crystallization rates were analyzed according to the secondary nucleation theory, and a relatively large nucleation constant Kg was obtained, representing the rigidity of the macromolecular chains. Large spherulites were observed during isothermal crystallization tests with the aid of polarized light optical microscopy (PLOM). The polyester showed significant stability during the thermal degradation tests. Finally, the degradation mechanism was investigated by employing a pyrolyzer–gas chromatography–mass spectroscopy (Py-GC-MS) system.

Synthesis, properties and thermal behavior of poly(decylene-2,5-furanoate): a biobased polyester from 2,5-furan dicarboxylic acid

In the present study, an interesting, eco-friendly polyester, poly(decylene-2,5-furanoate) (PDeF) was synthesized from 2,5-furan dicarboxylic acid with a variation of the well-known two-step melt polycondensation method. The crystallization and melting behavior of PDeF, was evaluated with different calorimetric methods; conventional, fast and temperature modulated scanning calorimetry. The results showed that PDeF is a fast crystallizing polyester, with a glass transition close to 1 °C and an equilibrium melting temperature equal to 129.8 °C. Various crystallization temperatures and rates were employed in order to evaluate in detail the thermal characteristics of PDeF. Isothermal and non-isothermal crystallization kinetics were also investigated by means of Avrami, Lauritzen–Hoffman theories and model-free kinetics. The structural features of PDeF were also studied by X-ray diffraction (XRD) and nuclear magnetic resonance (1H-NMR) while the size and density of spherulites was observed by polarized optical microscopy (POM) after crystallization in a wide range of temperatures. Finally, from tensile testing it was realized that PDeF has similar mechanical properties like tensile strength and Youngs modulus to that of low density polyethylene (LDPE).

Competitive crystallization of a propylene/ethylene random copolymer filled with a β-nucleating agent and multi-walled carbon nanotubes. Conventional and ultrafast DSC study.

A propylene/ethylene polymeric matrix was reinforced by the simultaneous addition of a β-nucleating agent (calcium pimelate) and multi-walled carbon nanotubes (MWCNTs) in various concentrations. The present manuscript explores the competitive crystallization tendency that is caused by the presence of the two fillers. On the one hand, calcium pimelate forces the material to crystallize predominantly in the β-crystalline form, while, on the other, the strong α-nucleating ability of MWCNTs compels the material to develop higher α-crystalline content. An in-depth study has been performed on the nanocomposite samples by means of conventional, temperature-modulated, and differential fast scanning calorimetry (DFSC) under various dynamic and isothermal conditions. The results showed that β-crystals are predominant at low MWCNT content (<2.5 wt %), while, at high MWCNT content, the material crystallizes mainly in the α-form. The recrystallization phenomenon during melting was confirmed with step-scan DSC, and the use of very high cooling rates by UFDSC made it possible to achieve and study the nucleation of the samples. The presence of MWCNTs enabled the nanocomposites to crystallize faster under both isothermal and dynamic conditions. The activation energy of the samples was also calculated according to Friedmans theory.

Biobased poly(ethylene furanoate-co-ethylene succinate) copolyesters: solid state structure, melting point depression and biodegradability

Poly(ethylene furanoate) (PEF) is a fully bio-based polyester with unique gas barrier properties, considered an alternative to poly(ethylene terephthalate) (PET) in food packaging applications. However, it is not biodegradable. For this reason, copolymerization with an aliphatic succinic acid monomer was investigated. The respective poly(ethylene furanoate-co-ethylene succinate) (PEFSu) copolymers were prepared via melt polycondensation from 2,5-dimethylfuran-dicarboxylate, succinic acid and ethylene glycol at different ratios. 1HNMR spectroscopy showed the copolymers are random. The crystallization and melting of the copolymers were thoroughly evaluated. Isodimorphic cocrystallization was concluded from both the WAXD patterns and the minimum in the plots of melting temperature versus composition. The pseudo-eutectic melting point corresponded to an ethylene succinate content of about 30 mol%. The enzymatic hydrolysis tests using Rhizopus delemar and Pseudomonas cepacia lipase revealed that the copolymers with up to 50 mol% ES units show measurable weight loss rates. For higher ES content, the copolymers showed fast hydrolysis.

Soft-matrices based on silk fibroin and alginate for tissue engineering

Soft tissue regeneration requires the use of matrices that exhibit adequate mechanical properties as well as the ability to supply nutrients and oxygen, and to remove metabolic bio-products. In this work, we describe the development of hydrogels based on the blend between alginate (Alg) and silk fibroin (SF). Herein, we report two main strategies to combine cells with biomaterials: cells are either seeded onto prefabricated hydrogels films (2D), or encapsulated during hydrogel microcapsules formation (3D). Both geometries were successfully produced and characterized. FTIR results indicated a change of conformation of SF from random coil to β-sheet after hydrogel formation. The thermal degradation behavior of films and microcapsules fabricated from Alg, and Alg/SF was dependent on the hydrogel composition and on the geometry of the samples. The presence of SF caused decreased water uptake ability and affected the degradation behavior. Mechanical tests showed that addition of SF promotes an increase in storage modulus, leading to a stiffer material as compared with pure Alg (6 times higher stiffness). Moreover, the in vitro cell-material interaction on Alg/SF hydrogels of different geometries was investigated using human umbilical vein endothelial cells (HUVECs). Viability, attachment, spreading and proliferation of HUVECs were significantly increased on Alg/SF hydrogels compared to neat Alg. These findings indicate that Alg/SF hydrogel is a promising material for the biomedical applications in tissue-engineering and regeneration.

Amino-Functionalized Multiwalled Carbon Nanotubes Lead to Successful Ring-Opening Polymerization of Poly(ε-caprolactone): Enhanced Interfacial Bonding and Optimized Mechanical Properties.

In this work, the synthesis, structural characteristics, interfacial bonding, and mechanical properties of poly(ε-caprolactone) (PCL) nanocomposites with small amounts (0.5, 1.0, and 2.5 wt %) of amino-functionalized multiwalled carbon nanotubes (f-MWCNTs) prepared by ring-opening polymerization (ROP) are reported. This method allows the creation of a covalent-bonding zone on the surface of nanotubes, which leads to efficient debundling and therefore satisfactory dispersion and effective load transfer in the nanocomposites. The high covalent grafting extent combined with the higher crystallinity provide the basis for a significant enhancement of the mechanical properties, which was detected in the composites with up to 1 wt % f-MWCNTs. Increasing filler concentration encourages intrinsic aggregation forces, which allow only minor grafting efficiency and poorer dispersion and hence inferior mechanical performance. f-MWCNTs also cause a significant improvement on the polymerization reaction of PCL. Indeed, the in situ polymerization kinetics studies reveal a significant decrease in the reaction temperature, by a factor of 30-40 °C, combined with accelerated the reaction kinetics during initiation and propagation and a drastically reduced effective activation energy.

Thermal degradation kinetics and decomposition mechanism of PBSu nanocomposites with silica-nanotubes and strontium hydroxyapatite nanorods

Novel poly(butylene succinate) (PBSu) nanocomposites containing 5 and 20 wt% mesoporous strontium hydroxyapatite nanorods (SrHNRs) and silica nanotubes (SiNTs) were prepared by melt-mixing. A systematic investigation of the thermal stability and decomposition kinetics of PBSu was performed using pyrolysis-gas chromatography-mass spectroscopy (Py-GC-MS) and thermogravimetry (TG). Thorough studies of evolving decomposition compounds along with the isoconversional and model-fitting analysis of mass loss data led to the proposal of a decomposition mechanism for PBSu. Moreover, the effects of SrHNRs and SiNTs on the thermal stability and decomposition kinetics of PBSu were also examined in detail. The complementary use of these techniques revealed that the incorporation of SiNTs in PBSu does not induce significant effects neither on its thermal stability nor on its decomposition mechanism. In contrast, the addition of SrHNRs resulted in the catalysis of the initial decomposition steps of PBSu and also in modified decomposition mechanisms and activation energies. The evolving gaseous products of PBSu and their evolution pattern in the SiNT nanocomposites were the same as in neat PBSu, while they were slightly modified for the SrHNR nanocomposites, confirming the findings from thermogravimetric analysis.