Figure 3 – Characterization of the eSF microfluidic hydrogel. A) Oscillatory rheological measurements (frequency 1 Hz) for eSF hydrogels at different concentrations (6%, 12%, and 14 %). B) ATR-FTIR spectra for the 3D eSF hydrogel (with 14% eSF) retaining the amorphous protein structure for at least 7 days. C) Tensile tests on 14% eSF microfluidic platform and photograph showcasing the high degree of deformation of the eSF hydrogel.
Next, we evaluate the capability of the eSF chip to recover to other types of mechanical actuation, namely manual tension, torsion, and bending (Figure 4 A). We found that the eSF chip displayed a remarkable flexibility. The chip was stretched, bent, and twisted without breaking or damaging the inner channel for at least 3 days when kept in medium or PBS to avoid dehydration (Figure 4 A) (see supplementary info ). This deformation ability could be optimal in a myriad of tissue models, such as lung, heart and colon, which present dynamic stretching movements (30, 31). Moreover, this opens the way to deepen mechanobiology studies in the field of cancer.
Some concerns were raised due to the hydrophilicity of the material and if it could perfuse water-based solutions efficiently through the channel. Figure 4 B) shows the injection of a blue ink flowing along the whole serpentine channel, demonstrating the ability of the eSF platform to flow fluids. Additionally, the eSF microfluidic chips showed its capacity to generate molecular gradients (see supplementary info ). The slope of such gradients could be accurately tuned by adjusting the concentration of silk: higher concentrations lead to slower gradient formation due to smaller pore size and lower concentration lead to faster gradient formation due to large pore sizes. Indeed, one of the main properties of eSF is the ease of tuning mechanical stiffness (from 1 kPa to 1 MPa) and long-term stability in various environments. Finally, Figure 4 C) shows the flow of micro-sized structures along the curved microchannel, highlighting the laminar flow of the fluid being transported.