3. Results and discussion
PDMS has long been utilized in microfluidic assays due to its high transparency (allowing the monitoring of cells by optical microscopy, elasticity (making valve integration through channel deformation possible), cost (less expensive than other materials), and gas permeability (enabling cell culture). Nevertheless, it possesses some drawbacks such as minimal mechanical and biochemical properties that limits the study of certain biological events (23). To address this, hydrogels have been proposed as more relevant materials with the advantage of using the whole three-dimensional hydrogel as a working/testing platform to evaluate this. The material must fulfil several conditions: i) handle the fabrication methods to make the hydrogel microfluidic device without degenerating; ii) promote cellular adhesion and homeostasis; iii) contain chemical groups for hypothetical chemical modification; iv) exhibit slow or tunable degradation rates for in vitro orin vivo applications; v) flexible, but with robust mechanical properties; and vi) transparent to allow microscope observations.
In this study, we took advantage of our experience with the use of silk fibroin in tissue engineering, microfluidics modelling, and colorectal cancer (14, 24) to design a unique human colorectal tumor model built on soft microfluidics platform. The model was designed to comprise a traditional serpentine channel for HCoMECS seeding, media perfusion, and for the injection of anti-cancer drugs while colorectal cancer cells were embedded in the hydrogel (Figure 2 A). The choice of design was made to mimic simply the native tumor environment, which is characterized by displaying a tortuous vasculature. Figures 2 B-C show the procedure to fabricate the eSF microfluidic chip using a combination of microfabrication techniques: photolithography and soft lithography (double replica molding), already patented by the group. SEM images in Figure 2D demonstrate the eSF hydrogel maintained the microfluidic features with excellent fidelity after the entire fabrication process and crosslinking. Since it is not possible to observe wet samples in SEM, the eSF hydrogel microfluidic platform was subjected to critical point drying using CO2. Indeed, this result is of significant since hydrogels are mostly made of water and thus may not retain the nano-scaled features required by microfluidics (13). Many hydrogels have been reported in microfluidics, such as collagen (25), gelatin (26) or agarose (27). However, hydrogels as a base for microfluidics can only work if, after the processing/crosslinking, the microfluidics chambers and channels remain unchanged, with extreme fidelity regarding the mold. This will depend on the inherent features of this type of material, e.g. change in size and distortion owed to swelling or drying, that decreases/increases their mechanical properties. Moreover, in some cases, such as Matrigel®, a limited range of stiffness can be achieved that is simply not enough to withstand microfabrication (28).