**1. Introduction**

Microreactors are usually defined as miniaturized reaction systems fabricated using methods of microtechnology and precision engineering. The term "microreactor" is the proposed name for a wide range of devices, having typically submillimeter channel dimensions which can be further divided into submicron sized components, for example, microparticle and nanoparti‐ cle carriers [1].

Before evolution of microreactor technology, the traditional way to conduct solution phase synthesis and analysis was the batch mode in stationary reactors with stirring or shaking to mix the reactants. Nowadays, microstructured devices offer greatly enhanced performance compared with conventional batch systems due to effects arising from the microscale domain:


**•** Microreactors have a high potential in industry, as developments by microreactors can be faster transferred into production at lower costs than batch processes.

Despite of the rapid development of enzymatic microreactors in the recent decade, impor‐ tant design questions still need to be answered.

Reaction kinetics is a key parameter of device design. Widely used kinetic parameters are deduced from the Michaelis–Menten model, which is valid only in batch reactions. In flow systems, the flow effects should be also considered. Immobilization of the enzymes causes further complication in modeling. Immobilization may affect the intrinsic kinetic parameters and may influence the availability of the enzyme. The kinetic model should also consider that the liquid phase containing the substrate and product is moving compared to the solid phase containing the immobilized enzyme.

When supported catalyst-filled microreactors are used, reproducible filling of the supported catalyst into the reactor space is not always straightforward. Even more challenging is the quantification of the actual load of the carriers.

Long-term stability of the reactor and the reproducibility of the measurements may be affected —among other factors—by the flow rate, the substrate concentration, and the morphology of the immobilized biocatalyst.

This chapter presents results carried out by a microfluidic microreactor system, the so called MagneChip platform including four serial reaction chambers with individually removable permanent magnets. The results were achieved by experiments using phenylalanine ammo‐ nia-lyase (PAL) from *Petroselinum crispum* immobilized on the surface of magnetic nanoparti‐ cles (MNPs) and filled into one or more chamber of the MagneChip.

Biotransformations with PAL under different conditions were performed mostly using the natural substrate L-phenylalanine to study

