Using hydrogels to help corneal cells survive

With limited traditional treatments available for corneal diseases, alternative approaches must be explored, such as enhanced biomaterial options, which can replicate the microenvironment of the stroma – the largest layer of tissue in the cornea. This is exactly what Annika Vrehen explored for her PhD research.

Hydrogels

Along with her colleagues, Vrehen’s goal was to design hydrogels with the ability to mimic the microenvironment of the stroma, given that the water content of hydrogels allow them to closely mimic the natural environment of the stroma

Thanks to this approach, the supramolecular hydrogels, bioactivity, and mechanical properties can be varied to create hydrogels that can support stroma cells – referred to as keratocytes.

Synthetic biomaterials based on supramolecular chemistry were designed such that broken connections can be reconnected (self-healing), thus resulting in dynamic materials. Within her work, Vrehen used polymers based on ureido-pyrimidinone (UPy) motifs as building blocks. These motifs are able to form hydrogen bonds that induce self-assembly in the supramolecular polymers, and ultimately allowing to generate supramolecular hydrogels.

Comparison

A design based on these UPy motifs combined with a bioactive additive resulted in the creation of a fully synthetic and injectable supramolecular hydrogel. The biological and mechanical properties of this synthetic hydrogel were compared with a hybrid hydrogel consisting of the same synthetic base mixed with natural collagen (instead of using the bioactive additive).

Both hydrogels demonstrated the ability to deliver the necessary biochemical signals for cells to successfully move through the gel (migrate), multiply (proliferate), and develop (differentiate) within the hydrogels.

New biomaterials

The application of a supramolecular approach allowed for the creation of a large numbers of different hydrogels. Vrehen and her collaborators synthesized various bioactive additives, ranging from collagen binding peptides to collagen mimicking peptides, and included them in the hydrogels.

Besides these additives, different versions of the building blocks were also used to formulate a range of hydrogel variations. The introduction of a supramolecular polyaminoacid library, consisting of polymers with the same UPy building block but with different amino acid monomers as end groups, resulted in the creation of multiple hydrogel variations which allowed for 2D and 3D cell culture.

In addition, Vrehen successfully combined multiple polyaminoacids in one hydrogel to mimic the natural protein presentation in a minimalistic manner. The introduction of this supramolecular polyaminoacid library contributes to the design of a large number of new functional biomaterials.

Insights

By combining supramolecular hydrogels with microfluidics, Vrehen succeeded in making very small supramolecular gels, known as microgels. During culture, a mix of microgels, consisting of microgels with and without an encapsulated single cell, started to assemble and resulted in the formation of interesting porous microstructures into which cells could easily migrate and infiltrate.

Due to the small size of the microgels and the reversible bonds between microgels and the cells, these microgels have the potential to contribute to the development of minimally invasive therapies.

Vrehen’s results provide important insights on the different designs of supramolecular hydrogels capable of mimicking microenvironments of the stroma.

In the future, these hydrogels systems have the potential to contribute to minimally invasive treatment methods for patients suffering from corneal injury.

However, the knowledge gathered in Vrehen’s research extends beyond application in the corneal field. It may also be applicable to other biomedical applications involving interactions between cells and materials.

Title of PhD thesis: Designing corneal stromal microenvironments based on supramolecular hydrogels. Supervisors: Patricia Dankers and Carlijn Bouten.