After discovering that a natural product has a particular, useful biological activity, it is desirable to prepare analogs of the natural product. However, natural products and their analogs are typically complex molecules requiring multi-step syntheses that are usually laborious, costly, and time consuming. This invention is directed to a microfluidics device that can be used to prepare natural products and their analogs. The device comprises the enzymes of a biosynthetic pathway immobilized thereon and a means for sequentially directing a starting material and each ensuing reaction product to the enzymes of the biosynthetic pathway in the order corresponding to the steps of the biosynthetic pathway. The device can thus be used to prepare the natural product using the natural starting material of the biosynthetic pathway or analogs of the natural product using an unnatural starting material. Alternatively, artificial pathways can be created by immobilizing an appropriate selection of enzymes on the device in an order whereby each subsequent enzyme can catalyze a reaction with the product of the prior enzyme. Novel chemical entities can be prepared from these artificial pathways.