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).