Fig. 1: Microfluidic oxygen gradient with spatial hydrogel sensor.

a The microfluidic device contains multiple layers. The gas layer balances oxygen diffusion (red) with nitrogen convection (blue) to create a 0–20% gradient as measured with a fiber optic oxygen probe. Note that the gradient is symmetrical, so either half can be used. The 200 µM polydimethylsiloxane (PDMS) membrane that defines the gas channels is highly permeable to oxygen and diffuses the gradient to the hydrogel and cell culture seeded atop. On top of the oxygen gradient, a porous hydrogel sensor was bonded to the microchannels. This hydrogel encapsulates anti-insulin capture antibodies, 20k polyethylene glycol (PEG) porogen, and arginylglycylaspartic acid (RGD) peptides to provide cell attachment and insulin detection. The open top aqueous reservoir allows normal cell culture processes and fluorescence microscopy. b The fabrication process of the multilayered microfluidics begins with normal photolithography and PDMS molding. During the chemical surface modifications, a nitrogen chamber is used to prevent oxygen inhibition of hydrogel cross-linking at the PDMS–gel interface. The resultant sandwich is washed with buffer to remove uncross-linked reagents and porogens to complete the porosity-enhanced hydrogel sensor. c Moreover, attachment of the functionalized hydrogel is achieved through both topological and chemical surface modifications. Microtextures are patterned on the cell facing the side of the PDMS membrane to increase the surface area in contact. Then, the sandwich is incubated with acrylated silane to create Si–O-bonds to tether the hydrogel backbones