4.2.3. Wall-Coated Reactors

In wall-coated microreactors, the mass transfer resistance was decreased and fluid flow through the channel can occur without difficulties such as pressure drop and channel blocking. Fluid dynamics as well as heat transfer can be controlled more easily than in other microreactors [130]. A disadvantage of wall-coated microreactors is that lower enzyme loadings occur in comparison with other microreactors. In order to increase enzyme loadings, different strategies can be used, including the deposition of nanostructured materials such as gold nanoparticles [107], nanosprings [131], graphene oxide [132] and dopamine [133,134] on the wall as well as using multiple layers immobilised with enzymes attached to the surface of the wall [106,135,136]. For example, Valikhani et al. constructed a wall-coated microreactor with the use of silica nanosprings, comprised of helical silicon dioxide (SiO2) structures grown via a chemical deposition process. The nanosprings were attached on to the channel wall and used to immobilise sucrose phosphorylase. In comparison with the unmodified surface, the loading of enzyme was significantly increased [137]. Bi et al. developed a wall-coated micro reactor in which polyethyleneimine (PEI) and Candida Antarctica lipase B were alternatively absorbed. The loading of the enzyme increased with the increasing number of layers, showing good stability and performance for the synthesis of wax ester [138]. An interesting study was carried out by Britton et al. in which enzymes were attached on the wall in specific separated zones. This type of reactor can be used for multi-step reactions for the synthesis of products such as alpha-d-glucose1-phosphate [139]. For example, Valikhani et al. described the use of sucrose phosphorylase to attach enzymes on the walls of glass microchannels for the synthesis of alpha-d-glucose1-phosphate [140].
