Surface-tension-driven gradient generation in a fluid stripe for bench-top and microwell applications

A simple and inexpensive method is presented employing passive mechanisms to generate centimeters-long gradients of molecules and particles in under a second with only a coated glass slide and a micropipette. A drop of solution is pipetted onto a fluid stripe held in place on a glass slide by a hydrophobic boundary. The resulting difference in curvature pressure drives the flow and creates a concentration gradient by convection. Experiments and theoretical models characterize the flows and gradient profiles and their dependence on the fluid volumes, properties, and stripe geometry. A bench-top rapid prototyping method is outlined to allow the user to design and fabricate the coated slides using only tape and hydrophobic spray. The rapid prototyping method is compatible with microwell arrays, allowing soluble gradients to be applied to cells in shear-protected microwells. The method's simplicity makes it accessible to virtually any researcher or student and its use of passive mechanisms makes it ideal for field use and compatible with point-of-care and global health initiatives.