Microfluidic patterning of protein gradients on biomimetic hydrogel substrates

This protocol describes a versatile microfluidic method to generate tethered protein gradients of virtually any user-defined shape on biomimetic hydrogel substrates. It can be applied to test, in a microenvironment of physiologically relevant stiffness, how cells respond to graded biomolecular signals, for example to elucidate how morphogen proteins affect stem cell fate. The method is based on the use of microfluidic flow focusing to rapidly capture in a step-wise manner tagged biomolecules via affinity binding on the gel surface. The entire patterning process can be performed in <1 h. We illustrate one application of this method, namely, the spatial control of mouse embryonic stem cell self-renewal in response to gradients of the self-renewal-promoting signal leukemia inhibitory factor.