Evaluating Flow-Focused Microfluidic Device Fabrication Techniques for Silk Fibroin Microgel Production

Abstract

Microgels, or micro-scale hydrogels, are versatile emerging biomaterials. They absorb large amounts of water and facilitate molecule transfer. Their tunable size and shape and the capacity to form 3D scaffolds with microscale porosity offer significant advantages over traditional bulk hydrogels. Various strategies exist for fabricating microgels with microfluidic techniques offering the most control of microgel properties. This study compares three microfluidic device fabrication techniques--maskless photolithography, laser engraving, and 3D printing--for making photo-crosslinked silk fibroin microgels. Silk microgels were fabricated via water-in-oil microfluidics and crosslinked through di-tyrosine bonds between native tyrosine residues in silk. Microfluidic devices with target channel depths of 50, 100, or 400 {micro}m were successfully fabricated, with each technique offering unique advantages and limitations. Maskless photolithography provided the highest channel patterning accuracy and smoothest profiles but was costly and required specialized facilities. Laser engraving was affordable but labor-intensive, with lower accuracy in the X-Y plane for deeper channels. 3D printing was user-friendly and affordable but resulted in low accuracy for 50 {micro}m channels and rougher channel profiles, leading to larger microgels and less efficient microgel formation than the other techniques. Spherical silk microgels with diameters between 50 {micro}m and > 400 {micro}m were produced by modulating channel depth and flow rates, while rod-shaped microgels were made by crosslinking in the outlet tubing. Silk molecular weight was adjusted to control surface porosity and compressive modulus, with lower molecular weight silk resulting in higher porosity and softer microgels (modulated between [~] 40 kPa and [~] 590 kPa) while maintaining consistent size. This study demonstrates the versatility and effectiveness of different microfluidic device fabrication techniques for producing photo-crosslinked silk microgels with tunable properties for biomedical applications.