Organocatalytic Polymers from Affordable and Readily Available Building Blocks for the Cycloaddition of CO2 to Epoxides.

The catalytic cycloaddition of CO2 to epoxides to afford cyclic carbonates as useful monomers, intermediates, solvents, and additives is a continuously growing field of investigation as a way to carry out the atom-economic conversion of CO2 to value-added products. Metal-free organocatalytic compounds are attractive systems among various catalysts for such transformations because they are inexpensive, nontoxic, and readily available. Herein, we highlight and discuss key advances in the development of polymer-based organocatalytic materials that match these requirements of affordability and availability by considering their synthetic routes, the monomers, and the supports employed. The discussion is organized according to the number (monofunctional versus bifunctional materials) and type of catalytically active moieties, including both halide-based and halide-free systems. Two general synthetic approaches are identified based on the postsynthetic functionalization of polymeric supports or the copolymerization of monomers bearing catalytically active moieties. After a review of the material syntheses and catalytic activities, the chemical and structural features affecting catalytic performance are discussed. Based on such analysis, some strategies for the future design of affordable and readily available polymer-based organocatalysts with enhanced catalytic activity under mild conditions are considered.

Osteoinduction of stem cells by collagen peptide-immobilized hydrolyzed poly(butylene succinate)/ß-tricalcium phosphate scaffold for bone tissue engineering.

Bone substitute is a therapeutic approach to treat bone abnormalities. A scaffold serves mainly as osteoconductive elements. To facilitate a better biological performance, short collagen peptide was immobilized onto hydrolyzed poly(butylene succinate)/ß-tricalcium phosphate (HPBSu/TCP) scaffolds. PBSu/TCP (80:20) scaffolds were fabricated by a supercritical CO2 technique, hydrolyzed with 0.6 M NaOH and conjugated with short collagen peptide tagged with or without red fluorescence. The surface morphology and porous structure of scaffolds were characterized by scanning electron microscopy and micro-computed tomography. Human mesenchymal stem cells were cultured onto the scaffolds and examined for osteogenic differentiation and biomineralization in vitro by means of alkaline phosphatase activity, alizarin red staining, and reverse transcription-polymerase chain reaction. The PBSu/TCP and HPBSu/TCP scaffolds were successfully prepared. Scanning electron microscopy and micro-computed tomography results showed that the porosity was distributed throughout the scaffolds with the pore sizes in the range of 250-900 µm. Fluorescence microscopy demonstrated retention of tagged short collagen peptide on the scaffold. Mesenchymal stem cells adhered and grew well on the material. Under osteogenic induction, cells cultured on the short collagen peptide -immobilized scaffold significantly produced a greater amount of alkaline phosphatase activity and positive mineralization than those cultured on the control scaffold. The present results have shown that the short collagen peptide-immobilized HPBSu/TCP scaffold enhanced osteoinduction and biomineralization of stem cell-derived osteoblasts, possibly via stimulation of alkaline phosphatase activity. This suggests the potential use of osteogenic peptide-immobilized material in bone tissue engineering for correcting bone defects.

In vitro human chondrocyte culture on plasma-treated poly(glycerol sebacate) scaffolds.

Porous poly(glycerol sebacate) (PGS) scaffolds were prepared using a salt leaching technique and subsequently surface modified by a low oxygen plasma treatment prior to the use in the in vitro culture of human chondrocytes. Condensation polymerization of glycerol and sebacic acid used at various mole ratios, i.e. 1:1, 1:1.25, and 1:1.5, was initially conducted to prepare PGS prepolymers. Porous elastomeric PGS scaffolds were directly fabricated from the mixtures of each prepolymer and 90% (w/w) NaCl particles and then subjected to the plasma treatment to enhance the surface hydrophilicity of the materials. The properties of both untreated and plasma-treated PGS scaffolds were comparatively evaluated, in terms of surface morphology, surface chemical composition, porosity, and storage modulus using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy, micro-computed tomography, and dynamic mechanical analysis, respectively. The responses of chondrocytes cultured on individual PGS scaffolds were assessed, in terms of cell proliferation and ECM production. The results revealed that average pore sizes and porosity of the scaffolds were increased with an increasing sebacic acid concentration used. The storage moduli of the scaffolds were raised after the plasma treatment, possibly due to the further crosslinking of PGS upon treatment. Moreover, the scaffold prepared with a higher sebacic acid content demonstrated a greater capability of promoting cell infiltration, proliferation, and ECM production, especially when it was plasma-treated; the greatest HA, sGAG, uronic acid, and collagen contents were detected in matrix of this scaffold. The H & E and safranin O staining results also strongly supported this finding. The storage modulus of the scaffold was intensified after incubation with the chondrocytes for 21 days, indicating the accretion and retention of matrix ECM on the cell-cultured scaffold.

Stem cell adhesion and proliferation on hydrolyzed poly(butylene succinate)/ß-tricalcium phosphate composites.

Although poly(butylene succinate)/ß-tricalcium phosphate (PBSu/TCP) composites are biocompatible and allow the growth and osteogenic differentiation of stem cells, cell attachment and adhesion to the PBSu-based substrates is often limited. To enhance cell adhesion and proliferation, we used a sodium hydroxide (NaOH) hydrolysis technique to generate a different degree of roughness on PBSu/TCP substrates with different PBSu:TCP ratios. The results showed that NaOH hydrolysis increased surface roughness of PBSu/TCP substrates in a concentration-dependent manner. Substrates with higher ratios of TCP:PBSu provided more porous topography after NaOH hydrolysis, with a substrate containing 40 wt % TCP (PBSu/TCP-6040) hydrolyzed with 1.5M NaOH (HPBSu/TCP-6040-1.5) showing the highest degree of roughness. As with the roughness, PBSu/TCP surface hydrophilicity was positively affected by the increasing NaOH concentration and TCP incorporation. Stem cells adhered best on HPBSu/TCP-6040-1.5 with three-dimensionally elongated cell extensions. Moreover, the HPBSu/TCP-6040-1.5 substrate most significantly facilitated stem cell actin cytoskeleton reorganization and vinculin-positive focal adhesion formation when compared with the other substrates tested. HPBSu/TCP-6040-1.5 also demonstrated the greatest increase in cell proliferation when compared with the other substrates studied. In conclusion, the results have shown that among various substrates tested, HPBSu/TCP-6040-1.5 provided the best support for stem cell adhesion and proliferation, suggesting its potential use in bone engineering.