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Environmentally sensitive hyaluronan hydrogel adjustable by physical and chemical cross-links for biomedical applications

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Erikci, S. (2020). Environmentally sensitive hyaluronan hydrogel adjustable by physical and chemical cross-links for biomedical applications. PhD Thesis.

Cite as: http://hdl.handle.net/21.11116/0000-0007-4C93-E
In recent years, hydrogels developed to promising tools for biomedical and industrial applications. For biomedical approaches hydrogels, possess the capacity to immobilize and release cells, they offer the desired 3D environment to induce cell specific behaviour or serves as a drug delivery system. Moreover, they can be used for tissue engineering approaches by mimicking the ECM. In this thesis, a novel hybrid double cross-linked hydrogel is presented and designed based on the bottom-up approach of synthetic biology. It consists of simultaneously formed chemical and physical cross-links and made out of two components: (1) thiol functionalized HA (74 kDa) (HA-DTPH) and (2) ionic crosslinker (Cl+). HA-DTPH provides the chemical cross-link by forming disulphide bonds and the ionic cross-linker forms physical cross-links, such as hydrogen bonds and salt bridges. Three different ionic cross-linker were used: (1) deacetylated disaccharide unit of HA (dHA+) (2) charged glucosamine (GluA+) and (3) ammonium chloride (NH4+). These ionic cross-linker were chosen due to their biocompatibility and ability to form physical cross-links, such as hydrogen bonds and salt bridges. The increasing capacity to form hydrogen bonds from NH4+ to dHA+ enabled us to study the influence of the physical cross-link on the hydrogel properties. I could show that the disulphide bond formation was enhanced, by adding an ionic cross-linker and led to the formation of stable hydrogels. Under the same reaction conditions, HA-DTPH without an ionic cross-linker, needed further oxidation with hydrogen peroxide to result in a stable hydrogel (HA-DTPH-Ox.). By varying the degree of thiolation on HA and additionally by varying the type and concentrations of the used ionic cross-linker, the mechanical stiffness, swelling properties and response to external stimuli were tuneable. Varying the degree of modification and used ionic cross-linker enables a specific adjustment of the hydrogels specifically the hydrogel suitable for cell studies with mechanical range of 0.1 Pa to 8 kPa. Furthermore, swelling ratios of HA-DTPH-Cl+ hydrogels are highly influenced by the ionic strength and pH. Remarkably HA- DTPH- dHA+ hydrogels upon incubation in a solution of pH 7 showed a feedback loop swelling behaviour. At the swollen state of the hydrogel, the ionic cross-linker dHA+, leaked out of the hydrogel network, acidified the solution, which resulted in shrinking of the hydrogel. Biological properties like enzymatic degradability showed that the half-live of HA-DTPH-Cl+ hydrogels are increasing with increasing capacity of the ionic cross-linker to form hydrogen bonds. Moreover, due to the absence of any toxic agent during the hydrogel formation the hydrogel system was used for live cell applications such as cell encapsulation or cell adhesion studies. To conclude, a hybrid double cross-linked hydrogel system could be presented, mimicking the ECM, in a minimal model and a critical influence of physical cross-links is observed from results obtained by characterizing the physical and biochemical properties by investigating the gels’ swelling capability, response to environmental changes and sensitivity to hyaluronidases. Depending on the desired biomedical application, these hydrogel systems can be tuned in regards to their stiffness, swelling behavior and degradability enabling applications in 3D tissue engineering, drug delivery and regenerative medicine.