**7. Conclusions**

Oxidoreductases catalyse redox reactions and have been successfully used to prepare biofuel cells and biosensors. The enzymes undergo electron transfer with the surface of the electrode via mediated or direct electron transfer mechanisms. However, relatively few oxidoreductases can undergo direct electron transfer because their redox active centres are placed deep within their structures. While enzymes have been widely used in batch and flow reactors, the use of oxidoreductases in bioreactors has been mainly confined to cofactor regeneration. A number of strategies that entail the use of high surface area electrodes, enzyme engineering and enzyme cascades have been used to improve the performance of bioelectrocatalysts. As an example of the scope of bioelectrocatalysts, a variety of oxidoreductases has been used in asymmetric synthesis reactions. While the scope of bioelectrocatalysts can be broadened, the successful use of bioelectrocatalysts will lie initially in the development of small-scale reaction systems that focus on high-value fine chemicals. The scale up of the reaction systems to the size required for bulk manufacture will be a significant challenge. Due to this challenge, the development of electrocatalytic bioreactors will likely focus on the preparation of products at a small scale taking advantage of the relatively low-cost systems that can be assembled in a relatively simple and rapid manner. Such systems are adaptable and are particularly suited for systems that may be required in process optimization or in the preparation of products or intermediates on a small (g) scale. Screening the performance of mutant enzymes is also more feasible given the relatively small amounts of enzymes required in such reactors. A particular area of interest lies in the use of multiple enzymes for cascade reactions, where rapid prototyping is particularly useful in screening and selecting the optimal operational conditions.

**Author Contributions:** Writing, S.A.; editing and review, M.N.-A.; writing—review and editing, E.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Science Foundation Ireland (SFI) Research Centre for Pharmaceuticals under Grant Number 12/RC/2275.

**Conflicts of Interest:** The authors declare no conflict of interest.
