EnGRAM: Engineering Open-Space Microfluidic Gradient Modulators

We report an approach for dynamically generating and reconfiguring microscale gradients to directly operate on surfaces of traditional labware, along with a toolbox of such devices, named EnGRAM. We merge principles of fluid mixing in closed microchannels with microfluidic scanning probe technology to transform the generated gradient profile out of plane, in which the gradient is confined in a single stream on top of a liquid-immersed open surface. This offers temporal, spatial, and spatiotemporal control over gradient formation, in real-time and guided by the user. Using an accessible 3D printing-based rapid prototyping workflow, we demonstrate a range of device designs that generate diverse concentration profiles, including linear, exponential, Gaussian, and stepwise, with feature dimensions as small as 75 μm, representing a 2-fold improvement compared to other reports. With the provided analytical model for predicting the concentration profile and the device design guidelines, functional prototypes can be fabricated within approximately 20 minutes. We demonstrate device operation in surface patterning and floating modes, opening new avenues for user-directed, real-time modulation of biomimetic gradients in studies that adapt to the evolving chemical and biological systems.