We present an open-volume microfluidic system capable of on-fine modification of a patterned laminar flow by using programmable inlet valves. Each separate solution environment in the flow pattern can be independently exchanged between different preloaded input solutions where each exchange requires 20 s. The number of flow patterns that can be generated by one device is N-n, where N represents the number of valve inlets and n the number of microchannels in the microfluidic system. Furthermore, the system can be operated as a combinatorial mixer, in which mixture of the different input solutions can be obtained independently in each channel. Since the flow patterns are generated in an open volume, they are accessible to many different detection methods and types of probes, e.g., microelectrodes, cells, or cell fragments. This technology offers the possibility to adjust the flow pattern composition in response to an output from a probe. This is the first step toward creating an automated feedback device controlled by, for example, biological cells.