The development of bioabsorbable hydrogels on the basis of polyester grafted poly(vinyl alcohol) (2006)

The present work describes the synthesis and characterization of amorphous and covalently crosslinked polymer systems based on poly(vinyl alcohol) and polyesters. The hydrogels prepared are biocompatible and hydrolytically degradable. The macroscopic properties, primarily the degradation rate, mass loss, water uptake, mechanical properties of the hydrogels can be tailored by variation of the polyester composition and the network structure. Covalently crossliked polymer networks were synthesized via a three step reaction. Short polyester chains were initially prepared by ring opening polymerization of lactide and glycolide. Hydroxyethyl methacrylate was used as an initiator which enables the simultaneous introduction of double bonds into the system. In the second step the hydroxy end groups of the polyesters were transferred into carboxylic groups by reaction with succinic anhydride. The third step was the grafting of the polyester chains onto the poly(vinyl alcohol) chain. Finally, crosslinking was accomplished through reaction of the double bonds using a free radical initiator. The chemical composition of the networks was investigated by means of IR and NMR spectroscopy, whereas NMR was used to pursue each step of synthesis and indicated its good control. The IR spectroscopy gave insight into the composition of the networks by means of the characteristic bands at 3300 cm-1 (OH), 1750 cm-1 (C=O), as well as in the fingerprint region. DSC measurements showed only one characteristic transition temperature in networks, the glass transition temperature Tg, in the range between 51 and 71°C .The surface properties of hydrogel films were investigated using the captive-bubble method. Statistic contact angles were found to be between 28° and 45°. Mechanical testing showed Young’s modulus E to have values between 0.01 and 103 MPa. Biocompatibility was tested on hydrogel type P with the assistance of primary human dermal fibroblasts (hF) cells, which after four days of incubation displayed good adhesion and viability, confirming the good biocompatibility of the material. Hydrolytical degradatio n experiments were carried out in an aqueous phosphate buffer solution at pH 7.4 and room temperature. The mass loss that accompanies the degradation of hydrogels was determined gravimetrically. More hydrophilic hydrogels, as a result of shorter polyester grafts or a fewer number of grafts, show a faster mass loss. Glycolide in the polyester chains additionally contributes to a faster mass loss due to its more hydrophilic nature. All hydrogels exhibit an increase in swelling in the course of hydrolytical degradation although at different rate. Within the first eight weeks of degradation, the hydrogels display a weight related degree of swelling, S, in the range from less than 2% up to 30%. The morphology change of hydrogels during hydrolytical degradation was examined by means of scanning electron microscopy. This method enables the surface and the cross section of the sample as a consequence of different mechanisms of degradation to be followed. The decrease of the E modulus as a result of hydrolytical degradation is immanent to all hydrogels. It is evident that hydrogels with the shortest polyester grafts show the lowest E modulus decrease during degradation, apart from having the lowest modulus initially. The contact angles of degraded hydrogels were measured. After eight weeks of degradation all hydrogels showed about the same value of 20°. A difference was seen only when the time is considered within this value is reached. The investigation of the degradation process by means of IR spectroscopy was possible through the observation of characteristic IR bands. The area ratio of the bands OH/C-H and OH/C=O which increases indicates the decrease of the polyester content. The existence and relative intensity of characteristic bands in the fingerprint region give evidence for different compositions of the networks as well as for the change that occurs in networks during the degradation. All networks show an increase of the glass transition temperature with degradation time. All the analyses and tests performed confirm the possibility of tailoring the properties, predisposition and tendency of the materials to hydrolyze depending on their composition and structure.