Microfluidic biochips are being used to perform ever more complex and error-prone bioassays. This results in increasing demand for design automation for such biochips, as these sophisticated designs are beyond the scope of manual design. So far, much research in the field of design automation has been devoted to satisfy this demand from biology, but the gap between design automation and biology is still huge. To narrow this gap, we propose a synthesis method in which sieve valves, which are key components in flow-based microfluidic biochips, are considered for the first time. In addition, we integrate three more constraints into our synthesis that are commonly seen in bioassays but have so far been neglected by design automation: immediate execution, mutual exclusion, and parallel execution. Experiments show that compared with traditional synthesis, this new method shows significant improvements, and the gap between design automation and biology is getting bridged.