Lipase-catalyzed synthesis of isoamyl acetate in an ionic liquid/n-heptane two-phase system at the microreactor scale

Towards Small Scale: Overview and Applications of Microfluidics in Biotechnology

Thanks to recent and continuing technological innovations, modern microfluidic systems are increasingly offering researchers working across all fields of biotechnology exciting new possibilities (especially with respect to facilitating high throughput analysis, portability, and parallelization). The advantages offered by microfluidic devices-namely, the substantially lowered chemical and sample consumption they require, the increased energy and mass transfer they offer, and their comparatively small size-can potentially be leveraged in every sub-field of biotechnology. However, to date, most of the reported devices have been deployed in furtherance of healthcare, pharmaceutical, and/or industrial applications. In this review, we consider examples of microfluidic and miniaturized systems across biotechnology sub-fields. In this context, we point out the advantages of microfluidics for various applications and highlight the common features of devices and the potential for transferability to other application areas. This will provide incentives for increased collaboration between researchers from different disciplines in the field of biotechnology.

Synthesis of odorants in flow and their applications in perfumery

Continuous flow technology is a key technology for sustainable manufacturing with numerous applications for the synthesis of fine chemicals. In recent years, the preparation of odorants utilizing the advantages of flow reactors received growing attention. In this review, we give an overview of selected methods for the synthesis of odorants in flow, including heterogeneously catalyzed reactions, gas reactions, and photochemical C–H functionalization processes. After a brief introduction on types of odorants, the presented odorant syntheses are ordered according to the main odor families “fruity”, “green”, “marine”, “floral”, “spicy”, “woody”, “ambery”, and “musky” and their use and importance for perfumery is briefly discussed.

Integrating Biocatalysis with Viscous Deep Eutectic Solvents in Lab-On-A-Chip Microreactors

The combination of deep eutectic solvents (DESs, ChCl/glycerol 1 : 2) with buffer (up to 15 % v/v) leads to solvent mixtures that exert viscosities below 25 mPa s^-1 at 45 °C while keeping their non-aqueous nature. This enables the setup of efficient enzymatic esterifications, which can also be applied in different continuous systems. Following those premises, the use of microreactors in biocatalytic reactions was explored using (low-viscous) DES-buffer media, showing that reactions could be performed efficiently. Under non-optimized conditions, the microreactor devices led to specific productivities considerably higher than those observed in the batch reactor (14 vs. 0.24 mg_product  min^-1  mg_biocat ^-1 ) at the same enzyme loadings and conversion of 6 % (to assure a fair comparison). Looking beyond, the combination of several microchannels (e. g., in scale-out fashion) with DES-water media may lead to powerful, sustainable, and efficient tools for industrial synthesis.

Activation and Stabilization of Lipase B from Candida antarctica by Immobilization on Polymer Brushes with Optimized Surface Structure

A reusable support system for the immobilization of lipases is developed using hybrid polymer-inorganic core shell nanoparticles. The biocatalyst core consists of a silica nanoparticle. PMMA is grafted from the nanoparticle as polymer brush via ARGET ATRP (activator regenerated by electron transfer atom transfer radical polymerization), which allows defining the surface properties by chemical synthesis conditions. Lipase B from Candida antarctica is immobilized on the hybrid particles. The activity and stability of the biocatalyst are analyzed by spectroscopic activity analysis. It is shown that the hydrophobic PMMA brushes provide an activating surface for the lipase giving a higher specific activity than the enzyme in solution. Varying the surface structure from disordered to ordered polymer brushes reveals that the reusability of the biocatalyst is more effectively optimized by the surface structure than by the introduction of crosslinking with glutaraldehyde (GDA). The developed immobilization system is highly suitable for biocatalysis in non-native media which is shown by a transesterification assay in isopropyl alcohol and an esterification reaction in n-heptane.

Two-Phase Biocatalysis in Microfluidic Droplets

This Perspective discusses the literature related to two-phase biocatalysis in microfluidic droplets. Enzymes used as catalysts in biocatalysis are generally less stable in organic media than in their native aqueous environments; however, chemical and pharmaceutical compounds are often insoluble in water. The use of aqueous/organic two-phase media provides a solution to this problem and has therefore become standard practice for multiple biotransformations. In batch, two-phase biocatalysis is limited by mass transport, a limitation that can be overcome with the use of microfluidic systems. Although, two-phase biocatalysis in laminar flow systems has been extensively studied, microfluidic droplets have been primarily used for enzyme screening. In this Perspective, we summarize the limited published work on two-phase biocatalysis in microfluidic droplets and discuss the limitations, challenges, and future perspectives of this technology.

Design and Preparation of Carbon Nitride-Based Amphiphilic Janus N-Doped Carbon/MoS2 Nanosheets for Interfacial Enzyme Nanoreactor

Janus amphiphilic particles have gained much attention for their important application value in areas as diverse as interfacial modification, sensors, drug delivery, optics, and actuators. In this work, we prepared Janus amphiphilic nanosheets composed of nitrogen-doped stratiform meso-macroporous carbons (NMC) and molybdenum sulfide (MoS₂) for hydrophilic and hydrophobic sides, respectively. The dicyandiamide and glucose were used as precursors for synthesizing two-dimensional nitrogen-doped meso-macroporous carbons, and the molybdate could be anchored by the functional groups on the surface of carbon layers and then transform into uniformly MoS₂ to form the Janus amphiphilic layer by layer NMC/MoS₂ support. Transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy are used to demonstrate the successful preparation of Janus materials. As the typical interfacial enzyme, Candida rugosa lipase (CRL) immobilized on the Janus amphiphilic NMC/MoS₂ support brought forth to improvement of its performance because the Janus nanosheets can be easily attached on the oil-aqueous interface for better catalytic activity (interfacial activation of lipases). The obtained immobilized lipase (NMC/MoS₂@CRL) exhibited satisfactory lipase loading (193.1 mg protein per g), specific hydrolytic activity (95.76 U g^-1), thermostability (at 55 °C, 84% of the initial activity remained after 210 min), pH flexibility, and recyclability (60% of the initial activity remained after nine runs). In terms of its application, the esterification rate of using NMC/MoS₂@CRL (75%) is higher than those of NMC@CRL (20%) and MoS₂@CRL (11.8%) in the "oil-water" biphase and CRL as well as NMC/MoS₂@CRL in the one-phase. Comparing with the free CRL, NMC@CRL, and MoS₂@CRL, the Janus amphiphilic NMC/MoS₂ served as a carrier that exhibited more optimal performance and practicability.

Synergy of Microtechnology and Biotechnology: Microreactors as an Effective Tool for Biotransformation Processes§: §The paper was presented at European Biotechnology Congress, 25-27 May 2017, Dubrovnik, Croatia

Despite the fact that microreactors have been present for more than 40 years now and that their potential has been extensively exploited in chemical synthesis, analytics and screening, to date very few biocatalytic processes have been explored in microreactors. It is claimed that enzymatic microreactor technology is exactly in the same place where chemical microreactors were 15 years ago. However, general opinion is that the efforts devoted to the research of micro-enzymatic reactors will inaugurate a new breakthrough in bio-based processing. The aim of this review is to explore the synergy between microtechnology, mainly microreactors, and biotechnology, and to assess its potential, opportunities, challenges and future application in biotechnology.

Moving and unsinkable graphene sheets immobilized enzyme for microfluidic biocatalysis

Enzymatic catalysis in microreactors has attracted growing scientific interest because of high specific surface enabling heat and mass transfer and easier control of reaction parameters in microreactors. However, two major challenges that limit their application are fast inactivation and the inability to the biocatalysts in microchannel reactors. A fluid and unsinkable immobilized enzyme were firstly applied in a microchannel reactor for biocatalysis in this study. Functionalized forms of graphene-immobilized naringinase flowing in microchannels have yielded excellent results for isoquercitrin production. A maximum yield of 92.24 ± 3.26% was obtained after 20 min in a microchannel reactor. Ten cycles of enzymatic hydrolysis reaction were successively completed and an enzyme activity above 85.51 ± 2.76% was maintained. The kinetic parameter V _m/K _m increased to 1.9-fold and reaction time was decreased to 1/3 compared with that in a batch reactor. These results indicated that the moving and unsinkable graphene sheets immobilized enzyme with a high persistent specificity and a mild catalytic characteristic enabled the repetitive use of enzyme and significant cost saving for the application of enzyme catalysis. Thus, the developed method has provided an efficient and simple approach for the productive and repeatable microfluidic biocatalysis.

Biosynthesis of Flavour-Active Esters via Lipase-Mediated Reactions and Mechanisms

Flavour active esters belong to one group of fine aroma chemicals that impart desirable fruity flavour notes and are widely applied in the flavour and fragrance industry. Due to the increasing consumer concern about health, natural products are attracting more attention than chemically synthesized substances. The biosynthesis of flavour-active esters via lipase-catalyzed reactions is one of the most important biotechnological methods for natural flavour generation. To proceed with the industrial production of esters on a large scale, it is critical to understand the enzyme properties and behaviours under different reaction conditions. In this short review, the lipase-catalyzed reactions in various systems and their mechanisms for synthesis of the esters are summarized and discussed.

Rapid synthesis of propyl caffeate in ionic liquid using a packed bed enzyme microreactor under continuous-flow conditions

Propyl caffeate has the highest antioxidant activity among caffeic acid alkyl esters, but its industrial production via enzymatic transesterification in batch reactors is hindered by a long reaction time (24h). To develop a rapid process for the production of propyl caffeate in high yield, a continuous-flow microreactor composed of a two-piece PDMS in a sandwich-like microchannel structure was designed for the transesterification of methyl caffeate and 1-propanol catalyzed by Novozym 435 in [B mim][CF3SO3]. The maximum yield (99.5%) in the microreactor was achieved in a short period of time (2.5h) with a flow rate of 2 μL/min, which kinetic constant Km was 16 times lower than that of a batch reactor. The results indicated that the use of a continuous-flow packed bed enzyme microreactor is an efficient method of producing propyl caffeate with an overall yield of 84.0%.

Efficient Terpene Synthase Catalysis by Extraction in Flow

Flowing enzymes: Continuous extraction of products enhances the enzymatic productivity of sesquiterpenes. Even unnatural substrates are tolerated leading to valuable unnatural target molecules in superior yields compared with batch protocols.

Continuous steroid biotransformations in microchannel reactors

The use of microchannel reactor based technologies within the scope of bioprocesses as process intensification and production platforms is gaining momentum. Such trend can be ascribed a particular set of characteristics of microchannel reactors, namely the enhanced mass and heat transfer, combined with easier handling and smaller volumes required, as compared to traditional reactors. In the present work, a continuous production process of 4-cholesten-3-one by the enzymatic oxidation of cholesterol without the formation of any by-product was assessed. The production was carried out within Y-shaped microchannel reactors in an aqueous-organic two-phase system. Substrate was delivered from the organic phase to aqueous phase containing cholesterol oxidase and the product formed partitions back to the organic phase. The aqueous phase was then forced through a plug-flow reactor, containing immobilized catalase. This step aimed at the reduction of hydrogen peroxide formed as a by-product during cholesterol oxidation, to avoid cholesterol oxidase deactivation due to said by-product. This setup was compared with traditional reactors and modes of operation. The results showed that microchannel reactor geometry outperformed traditional stirred tank and plug-flow reactors reaching similar conversion yields at reduced residence time. Coupling the plug-flow reactor containing catalase enabled aqueous phase reuse with maintenance of 30% catalytic activity of cholesterol oxidase while eliminating hydrogen peroxide. A final production of 36 m of cholestenone was reached after 300 hours of operation.

Microfluidic devices: useful tools for bioprocess intensification

The dawn of the new millennium saw a trend towards the dedicated use of microfluidic devices for process intensification in biotechnology. As the last decade went by, it became evident that this pattern was not a short-lived fad, since the deliverables related to this field of research have been consistently piling-up. The application of process intensification in biotechnology is therefore seemingly catching up with the trend already observed in the chemical engineering area, where the use of microfluidic devices has already been upgraded to production scale. The goal of the present work is therefore to provide an updated overview of the developments centered on the use of microfluidic devices for process intensification in biotechnology. Within such scope, particular focus will be given to different designs, configurations and modes of operation of microreactors, but reference to similar features regarding microfluidic devices in downstream processing will not be overlooked. Engineering considerations and fluid dynamics issues, namely related to the characterization of flow in microchannels, promotion of micromixing and predictive tools, will also be addressed, as well as reflection on the analytics required to take full advantage of the possibilities provided by microfluidic devices in process intensification. Strategies developed to ease the implementation of experimental set-ups anchored in the use of microfluidic devices will be briefly tackled. Finally, realistic considerations on the current advantages and limitation on the use of microfluidic devices for process intensification, as well as prospective near future developments in the field, will be presented.

Micromixing within microfluidic devices

Micromixing is a crucial process within microfluidic systems such as micro total analysis systems (μTAS). A state-of-art review on microstructured mixing devices and their mixing phenomena is given. The review first presents an overview of the characteristics of fluidic behavior at the microscale and their implications in microfluidic mixing processes. According to the two basic principles exploited to induce mixing at the microscale, micromixers are generally classified as being passive or active. Passive mixers solely rely on pumping energy, whereas active mixers rely on an external energy source to achieve mixing. Typical types of passive micromixers are discussed, including T- or Y-shaped, parallel lamination, sequential, focusing enhanced mixers, and droplet micromixers. Examples of active mixers using external forces such as pressure field, electrokinetic, dielectrophoretic, electrowetting, magneto-hydrodynamic, and ultrasound to assist mixing are presented. Finally, the advantages and disadvantages of mixing in a microfluidic environment are discussed.

Enzyme-functionalized polymer brush films on the inner wall of silicon-glass microreactors with tunable biocatalytic activity

The lipase from Candida Rugosa was immobilized to a poly(methacrylic acid) polymer brush layer, grown on the inner wall of silicon-glass microreactors. The hydrolysis of 4-nitrophenyl acetate was used as a model reaction to study the activity of this biocatalytic system. The amount of bound lipase could be tuned by changing the polymerization time of the brush formation. The Michaelis-Menten constants and V(max) values, determined for immobilized and free lipase, are similar, demonstrating that the lipase's substrate affinity and its activity remain unchanged upon immobilization to the microchannel wall.

Miniaturization in biocatalysis

The use of biocatalysts for the production of both consumer goods and building blocks for chemical synthesis is consistently gaining relevance. A significant contribution for recent advances towards further implementation of enzymes and whole cells is related to the developments in miniature reactor technology and insights into flow behavior. Due to the high level of parallelization and reduced requirements of chemicals, intensive screening of biocatalysts and process variables has become more feasible and reproducibility of the bioconversion processes has been substantially improved. The present work aims to provide an overview of the applications of miniaturized reactors in bioconversion processes, considering multi-well plates and microfluidic devices, update information on the engineering characterization of the hardware used, and present perspective developments in this area of research.