Cleaving to Connect: Non-Free Radical Photopolymerization for Orthogonal Multi Material Volumetric Bioprinting of Hydrogels

Abstract

The Biofabrication field seeks ways of producing organs by Bioprinting which is the 3D printing of cells and other tissue components. The materials used for this 3D printing are network forming molecules that can trap water, forming hydrogels. Examples of these are gelatin, hyaluronic acid, or the synthetic polymer polyethylene glycol (PEG). Networks are formed by bonding interactions named cross-links of chemicals groups having binding interactions. Blends of these hydrogels with cells and other tissue molecules are called bio-inks. Hydrogels are important in Bioprinting as they offer structural integrity and provide cells with aqueous environments. Traditionally, the more aqueous, the easier cellular interactions and tissue culture medium uptake, which are fundamental for tissue growth.

Older Bioprinting techniques involve contact Bioprinting, meaning that bio-inks are in physical contact with the printers. This is not beneficial for cells, especially in techniques where bioinks pass through a nozzle where they are compressed and stressed. Contact Bioprinting is done by printing multiple layers on top of one another. Limitations are needs for supports in case of overhang structures and slow printing speeds. The latter is suboptimal for cells since they are separated from nutrient containing culture medium during printing.

Contactless Bioprinting generally is better for cells, as it does not stress cells physically and usually much faster. Volumetric Bioprinting (VBP) uses stimuli responsive bio-inks, commonly gelatin based, that respond to light. These bio-inks contain an additional photo-initiator to start the cross-linking of the hydrogel. These photo-initiators are cleaved by light irradiation to produce free radicals. The hydrogels have reactive groups that cross-link because of these free radicals, forming solid constructs. VBP works by light irradiation through bio-inks in 2D fashion, using tomographic projections which can be compared to a CT scan. Rotating the platform results in 360-degree irradiation and printing of 3D constructs. Though showing promise, free radicals are uncontrollable and perform side reactions with adverse effects, such as cell damage. In case of printing with different hydrogels, selective cross-linking is not possible because they react with multiple hydrogels simultaneously. As native tissues comprise different materials, multi material printing methods are essential.

This thesis demonstrates cross-linking without free radicals by use of maleimides and thiols, that cross-link spontaneously. Two hydrogels with respectively either thiols or maleimides were used. For controlled cross-linking, a green light absorbing photocleavable protecting group (PPG) was placed onto the thiols of a PEG hydrogel. A PPG is a molecule that is attached to a reactive group, making it unavailable to fulfill its original function. Depending on their molecular structure, PPGs can absorb specific light wavelengths. Only when they are irradiated with wavelengths that they absorb, they are released, thus making the reactive groups available. This hydrogel was incubated with a maleimide containing hyaluronic acid hydrogel and due to the PPG, selective green light-controlled cross-linking was achieved using VBP.

Future VBP developments lie in using multiple different PPGs combinations with different hydrogels that use different spontaneous cross-linking mechanisms. A requirement is that the PPGs do not have overlapping absorbance and that the cross-linking mechanisms are selective.