sP (EO-stat-PO) - glycosaminoglycans (GAGs) hybrid-hydrogels for medical applications

The primary focus of this work is to develop a highly stable hydrogel system as a depot for drug delivery and as material for tissue engineering scaffolds. Two separate cross-linkers namely six arm star shaped (EO-stat-PO) prepolymers with isocyanate endgroups [NCO-sP(EO-stat-PO)] and six arm star shaped (EO-stat-PO) prepolymers with acrylate endgroups [Acr-sP(EO-stat-PO)] were prepared to gel unmodified and thiol-modified bio-polysaccharides respectively. In particular we discuss three types of hydrogel systems for the above mentioned bio-applications namely, hydrogels formed by gelation of NCO-sP(EO-stat-PO) only, i.e., the hydrogel system-1, hydrogel formed by addition of hyaluronic acid to NCO-sP(EO-stat-PO), i.e. the hydrogel system-2 and hydrogel formed by addition of thiol-modified hyaluronic acid to Acr-sP(EO-stat-PO), i.e. the hydrogel system-3. Different than hydrophobic particles or layers, hydrogels are open to water. Water molecules can diffuse in and out and thus extract a drug, that has been loaded to the hydrogel rather unrestricted. In systemic application, the organism does not easily recognize hydrogel particles as foreign and circulation times are rather long. Water content and the size of the pores increases with a decrease in cross-linking density and it is the other way around with the increase in the cross-linking density. As the network is formed from the hexafunctional sP(EO-stat-PO) as a cross-linker and hyaluronic acid as a linear polysaccharide, the degree of cross-linking depends on the relative concentration of the two components. We also studied the effect of different molecular weight of the unmodified hyaluronic acid. Higher molecular weight (longer chains) HA yielded gels with a higher water content, bigger pore size and lower mechanical strength than the lower molecular weight (shorter chains). Within the time of the experiment, i.e. up to several weeks, the hydrogel system-1 showed no degradation. The equilibrium degree of swelling and internal micro-porous structures are inversely proportional to the concentration of NCO-sP(EO-stat-PO) of the gelling solution. The hydrogel system-2 showed slow degradation and the equilibrium degree of swelling and internal micro-porous structures could be varied by the relative concentrations of the two components. The hydrogel system-3, which is also semi-synthetic, showed complete degradation. Here cross-links are formed by Michael-type addition of thiols to acrylates, but degradation occurs via hydrolysis of the acrylate ester groups. Testing these hydrogels as drug carrying and delivering vehicles showed for a hydrophilic drug, i.e. Dexamethasone 21 phosphate disodium salt, a quick and complete release by a diffusion controlled mechanism. Actually 90% of the drug is released within 8 hours (swelling controlled burst release) followed by a slow release of the rest of the drug by a diffusion controlled mechanism. It was shown that the release can be retarded by encapsulating the hydrogel by a silicone tube with one end closed and the other end open for the controlled release of the drug. In order to use these hydrogels as cell scaffolds, the primary requirement apart from high water content and high porous structures is the incorporation of cell attachment sites. This can be achieved by linking RGD peptides to our hydrogel system. It is a particular advantage of using NCO-sP(EO-stat-PO) as cross-linker system that the isocyanates can react not only with the hydroxyl groups of the polysaccharides but also with the amine functional groups of the peptide sequence. Thus co-addition of the peptides enables biofunctionalisation in the cross-linking step. Cyto-compatibility tests showed that the bio-functionalised and non-bio-functionalised cross-linked hydrogels were non-toxic provided complete conversion of the NCO-groups. The cell experiments using mouse fibroblast L929 showed clearly the cells adhering on to the RGD decorated sites of the hydrogel coated surfaces. When we added the cells to the hydrogel precursor prior to complete conversion the NCO-sP(EO-stat-PO) based hydrogel systems were toxic to cells. In contrast, the Acrylate sP(EO-stat-PO) based hydrogel system were non-toxic to cells irrespective of the full conversion of the acrylate groups. Core shell hydrogel particles and multi-layer hydrogel coatings have been prepared in an attempt to enable sequential drug release. It has been possible to combine hydrogels of different composition, cross-linking and with different drug loadings. In the case of acrylate-thiol gels, different drugs can be incorporated in order to prepare scaffolds with multiple types of seed cells even in a locally controlled manner. In tissue engineering, this is particular important for the structuring of the regenerated tissue