Abstract by Zishuo Yuan

Drug development is a slow and costly business with often low success rates in clinical trials. One important reason that causes this are ambiguous or even false predictions provided by preclinical studies. Although animal models and 2D monolayer in vitro cell models have been playing a critical role in drug development and provide a wealth of valuable data, they are either not able to accurately predict drug toxicity and efficacy in humans in preclinical studies (because their genetic makeups are very different from human), or provide no predictive or even misleading information regarding in vivo response because they fail to accurately mimic and reconstruct the complex microenvironment that cells encounter in vivo. On the other hand, 3D cell culture systems using human cells and more powerful formats based on microfluidic technology, so called organ-on-a-chip (OOC), have been well accepted as promising tools that are able to “mimic humans” more accurately and provide more predictive information for clinical trials in drug development research.

While poly(dimethylsiloxane) (PDMS) still is the most frequently used polymer material for the fabrication of bio-microdevices, thiol-ene (TE) polymers are emerging as a new promising biomaterial, not least in order to bridge the gap between laboratory concepts and commercial production of microfluidic devices. In this thesis, I evaluated the potential of TE polymers regarding three aspects related to 3D cell cultures and organ- or tissue-on-a-chip applications, namely a) their potential use as 3D cell culture scaffold material; b) the application of oxygen gradients generated in TE microchannel for pharmaceutical studies and OOCs systems; and c) their application as substrate material for blood brain barrier-on-a-chip (BBB-on-a-chip).

Firstly, the biocompatibility of different fully polymerized TE material surfaces (50% thiol excess, 50% allyl excess, and a stoichiometric mixture of the monomers that make up thiol-enes), as well as the potential use of TE monoliths made by a single emulsion method as 3D scaffolds was tested by culturing astrocytes and endothelial cells (ECs) on these materials and morphologies. The results showed that, except for TE surfaces with a 50% thiol excess, fully polymerized TE polymers provide good biocompatibility. However, TE monoliths may be not suitable as a polymeric scaffold for 3D cell culture. The main reason could be that the monolith lacks a highly porous and fully interconnected structure, which is an essential and basic characteristic that scaffolds for 3D cell culture should have.

Secondly, chips designed for the generation of controlled oxygen gradients (made from TE polymers that are not fully polymerized and thus able to scavenge dissolved oxygen from solutions they are in contact with) were tested, and kinetics studies of TE-induced oxygen depletion over time were performed. Dissolved oxygen concentrations inside microchannels were measured and imaged with a sensing system relying on an oxygen-sensitive dye immobilized in a foil that constituted the bottom of the microchannels. The results indicated that at least two processes are involved in the oxygen scavenging process, and water physisorbed by TE polymers plays an important role. This scavenging ability of TE polymers can be used to generate oxygen gradients in microchannels, and the results showed that a higher flow rate (2.0 µL/min) generates a shallower gradient while a lower flow rate (0.7 µL/min) generates a steeper gradient. These obtained results provide us further insight into the mechanisms behind the oxygen scavenging process, but, so far, the mechanism is still poorly understood and more studies and data is needed. Additionally, cell culture experiments were performed on the TE chips with the oxygen scavenging ability as well. However, the results showed that cells were not able to survive in the chip, which is likely mainly due to the poor biocompatibility of non-fully polymerized TE polymers (a pre-requisite for oxygen scavenging, however).

Finally, a BBB-on-a-chip was designed and fabricated by using fully polymerized stoichiometric TE polymers, and astrocytes in hydrogel (3D culture environment) and hCMEC/D3 cells were co-cultured in the chip. The results showed that both astrocytes and hCMEC/D3 are able to grow in the chip based on TE polymer, which indicates the large potential of TE polymers as microfluidic device substrate for organ-on-a-chip applications.

In summary, the results shown in this thesis demonstrate that TE polymers have huge potential to be used in 3D cultures and OOCs systems.