Upscaled Production of Satellite-Free Droplets: Step Emulsification with Deterministic Lateral Displacement

Step emulsification is a key technique for achieving scalable production of monodisperse emulsion droplets owing to its resilience to flow fluctuations. However, the persistent issue of satellite droplets, an inherent byproduct of main droplets, poses challenges for achieving truly uniform product sizes. In a previous study, we introduced a module with step-emulsifier nozzles upstream and deterministic lateral displacement (DLD) micropillar arrays downstream to generate satellite-free droplets at a low throughput. In this study, we demonstrate an upscaled parallelized setup with ten modules that were designed to produce satellite-free droplets. Each module integrated 100 step-emulsification nozzles in the upstream region with DLD micropillar arrays downstream. We conducted 3D flow simulations to ensure homogeneous distribution of the input fluids. Uniformly supplying an aqueous polyvinyl alcohol solution and an acrylate monomer as continuous and dispersed phases into the ten modules, the nozzles in each module exhibited a production rate of 539.5 ± 28.6 drop/s (n = 10). We successfully isolated the main droplets with a mean diameter of 66 μm and a coefficient of variation of 3.1% from satellite droplets with a mean diameter of 3 μm. The total throughput was 3.0 mL/h. The high yield and contamination-free features of our approach are promising for diverse industrial applications.

Advanced strategies in high-throughput droplet screening for enzyme engineering

Enzymes, as biocatalysts, play a cumulatively important role in environmental purification and industrial production of chemicals and pharmaceuticals. However, natural enzymes are limited by their physiological properties in practice, which need to be modified driven by requirements. Screening and isolating certain enzyme variants or ideal industrial strains with high yielding of target product enzymes is one of the main directions of enzyme engineering research. Droplet-based high-throughput screening (DHTS) technology employs massive monodisperse emulsion droplets as microreactors to achieve single strain encapsulation, as well as continuous monitoring for the inside mutant library. It can effectively sort out strains or enzymes with desired characteristics, offering a throughput of 108 events per hour. Much of the early literature focused on screening various engineered strains or designing signalling sorting strategies based on DHTS technology. However, the field of enzyme engineering lacks a comprehensive overview of advanced methods for microfluidic droplets and their cutting-edge developments in generation and manipulation. This review emphasizes the advanced strategies and frontiers of microfluidic droplet generation and manipulation facilitating enzyme engineering development. We also introduce design for various screening signals that cooperate with DHTS and devote to enzyme engineering.

Droplet Microfluidics for Current Cancer Research: From Single-Cell Analysis to 3D Cell Culture

Cancer is the second leading cause of death worldwide. Differences in drug resistance and treatment response caused by the heterogeneity of cancer cells are the primary reasons for poor cancer therapy outcomes in patients. In addition, current in vitro anticancer drug-screening methods rely on two-dimensional monolayer-cultured cancer cells, which cannot accurately predict drug behavior in vivo. Therefore, a powerful tool to study the heterogeneity of cancer cells and produce effective in vitro tumor models is warranted to leverage cancer research. Droplet microfluidics has become a powerful platform for the single-cell analysis of cancer cells and three-dimensional cell culture of in vitro tumor spheroids. In this review, we discuss the use of droplet microfluidics in cancer research. Droplet microfluidic technologies, including single- or double-emulsion droplet generation and passive- or active-droplet manipulation, are concisely discussed. Recent advances in droplet microfluidics for single-cell analysis of cancer cells, circulating tumor cells, and scaffold-free/based 3D cell culture of tumor spheroids have been systematically introduced. Finally, the challenges that must be overcome for the further application of droplet microfluidics in cancer research are discussed.

Study of molecular interaction and texture characteristics of hydrocolloid-mixed alginate microspheres: As a shell to encapsulate multiphase oil cores

This work investigates the molecular interaction of hydrocolloids (xanthan gum (XG), hydroxyethyl cellulose (HEC), carbomer (CBM) and hymagic™-4D (HA)) with sodium alginate (SA) in microspheres in detail. The molecular interaction of hydrocolloids with SA are demonstrated by the rheological property analysis of the mixed solutions as well as the morphology structure and texture characteristics studies of the microspheres. It is found that the hydrocolloids (XG, HEC and CBM) with branches or capable to coil are able to form complex networks with SA through molecular interactions which hinders the free diffusion of calcium ions and changes the texture characteristics of microspheres. In addition, the mixed solutions (SA-XG and SA-HEC) with complex networks and do not have a chelating effect on calcium ions are used to form the shell of the microcapsules through droplet microfluidic technology, and stable with soft microcapsules encapsulating multiphase oil cores have been successfully prepared. At the same time, the textural properties of microcapsules are quantized, which are related to human sensory properties. The developed stable and soft microcapsules which have the properties of sensory comfort are expected to be applied in the personal care industry and a variety of fields.

Contactless acoustic tweezer for droplet manipulation on superhydrophobic surfaces

Droplet manipulation on superhydrophobic surfaces (DMSS) without conventional pipetting is an emerging liquid handling technology, which can be potentially used for diagnostic, analysis, and synthetic processes. Despite notable progress, controlling droplet motion on superhydrophobic surfaces by contactless acoustic waves is rarely reported. Herein, we report a contactless acoustic tweezer (CAT) for DMSS based on establishing ultrasonic standing wave between an ultrasound transducer (UST) and a superhydrophobic substrate to manipulate droplets without physical contact. The CAT utilizes acoustic radiation forces to trap and move droplets on superhydrophobic surfaces, which allows for precise and controllable movement of droplets by controlling the movement of the UST. Small droplets with volume less than 20 μL can be levitated in mid-air for out-plane manipulation, and large droplets with volume up to 500 μL can be trapped for in-plane manipulation. Experimental results demonstrate the versatility of the CAT for manipulating droplets with various compositions and volumes on various superhydrophobic substrates, offering a versatile and cross-contamination-free liquid handling approach for applications, including but not limited to high-throughput surface-enhanced Raman scattering.

Gas-assisted microfluidic step-emulsification for generating micron- and submicron-sized droplets

Micron- and submicron-sized droplets have extensive applications in biomedical diagnosis and drug delivery. Moreover, accurate high-throughput analysis requires a uniform droplet size distribution and high production rates. Although the previously reported microfluidic coflow step-emulsification method can be used to generate highly monodispersed droplets, the droplet diameter (d) is constrained by the microchannel height (b), d ≳ 3 b , while the production rate is limited by the maximum capillary number of the step-emulsification regime, impeding emulsification of highly viscous liquids. In this paper, we report a novel, gas-assisted coflow step-emulsification method, where air serves as the innermost phase of a precursor hollow-core air/oil/water emulsion. Air gradually diffuses out, producing oil droplets. The size of the hollow-core droplets and the ultrathin oil layer thickness both follow the scaling laws of triphasic step-emulsification. The minimal droplet size attains d ≈ 1.7 b , inaccessible in standard all-liquid biphasic step-emulsification. The production rate per single channel is an order-of-magnitude higher than that in the standard all-liquid biphasic step-emulsification and is also superior to alternative emulsification methods. Due to low gas viscosity, the method can also be used to generate micron- and submicron-sized droplets of high-viscosity fluids, while the inert nature of the auxiliary gas offers high versatility.

Scalable Production of Biomedical Microparticles via High-Throughput Microfluidic Step Emulsification

Drug microcarriers are widely used in disease treatment, and microfluidics is well established in the preparation of microcarrier particles. A proper design of the microfluidic platform toward scalable production of drug microcarriers can extend its application values in wound healing, where large numbers of microcarriers are required. Here, a microfluidic step emulsification method for the preparation of monodisperse droplets is presented. The droplet size depends primarily on the microchannel depth rather than flow rate, making the system robust for high-throughput production of droplets and hydrogel microparticles. Based on this platform, basic fibroblast growth factor (bFGF) is uniformly encapsulated in the microparticles, and black phosphorus (BP) is incorporated for controllable release via near-infrared (NIR) stimulation. The microparticles serve as drug carriers to be applied to the wound site, inducing angiogenesis and collagen deposition, thereby accelerating wound repair. These results indicate that the step emulsification technique provides a promising solution to scalable production of drug microcarriers for wound healing as well as tissue regeneration.

Microfluidic Methods for Generation of Submicron Droplets: A Review

Submicron droplets are ubiquitous in nature and widely applied in fields such as biomedical diagnosis and therapy, oil recovery and energy conversion, among others. The submicron droplets are kinetically stable, their submicron size endows them with good mobility in highly constricted pathways, and the high surface-to-volume ratio allows effective loading of chemical components at the interface and good heat transfer performance. Conventional generation technology of submicron droplets in bulk involves high energy input, or relies on chemical energy released from the system. Microfluidic methods are widely used to generate highly monodispersed micron-sized or bigger droplets, while downsizing to the order of 100 nm was thought to be challenging because of sophisticated nanofabrication. In this review, we summarize the microfluidic methods that are promising for the generation of submicron droplets, with an emphasize on the device fabrication, operational condition, and resultant droplet size. Microfluidics offer a relatively energy-efficient and versatile tool for the generation of highly monodisperse submicron droplets.

Microfluidic Coupling of Step Emulsification and Deterministic Lateral Displacement for Producing Satellite-Free Droplets and Particles

Step emulsification, which uses a geometry-dependent mechanism for generating uniformly sized droplets, has recently gained considerable attention because of its robustness against flow fluctuations. However, like shear-based droplet generation, step emulsification is susceptible to impurities caused by satellite droplets. Herein, we demonstrate the integration of deterministic lateral displacement (DLD) to separate the main and satellite droplets produced during step emulsification. Step-emulsification nozzles (16 μm deep) in the upstream region of the proposed device were arrayed on the sidewalls of the main channel (91 μm deep). In the downstream region, the DLD micropillars were arrayed periodically with a critical diameter (cut-off value for size-based separation) of 37 μm. When an acrylate monomer and aqueous polyvinyl alcohol solution were infused as the dispersed and continuous phases, respectively, the nozzles produced monodisperse main droplets in the dripping regime, with an average diameter of ~60 μm, coefficient of variation (CV) value below 3%, and satellite droplets of ~3 μm. Upon entering the DLD region near the sidewall, these main and satellite droplets were gradually separated through the pillars based on their sizes. Finally, off-chip photopolymerization yielded monodisperse polymeric microspheres with an average diameter of 55 μm and a CV value of 2.9% (n = 202).

Advances in Microscale Droplet Generation and Manipulation

Microscale droplet generation and manipulation have widespread applications in numerous fields, from biochemical assays to printing and additive manufacturing. There are several techniques for droplet handling. Most techniques, however, can generate and work with only a limited range of droplet sizes. Furthermore, there are constraints regarding the workable variety of fluid properties (e.g., viscosity, surface tension, mass loading, etc.). Recent works have focused on developing techniques to overcome these limitations. This feature article discusses advances in this area that cover a wide range of droplet sizes from subpicoliter to microliter.

Bent-Capillary-Centrifugal-Driven Monodisperse Droplet Generator with Its Application for Digital LAMP Assay

We developed a bent-capillary-centrifugal-driven (BCCD) monodisperse droplet generator, which could achieve a perfect combination of driving and segmentation for the dispersed phase only using a rotating bent capillary immersed in the continuous phase (mineral oil). The sample could flow continuously to the bent-capillary outlet to form the droplet precursors, which were segmented into homogeneous droplets in the continuous phase. Through the investigation of influence factors on droplet size and stability, we found that the droplet size could be conveniently controlled by the rotational speed of the bent capillary. The droplet volumes could be adjusted with the range from 34 pL to 1 μL, and the coefficient variations (CVs) were less than 3%. Meanwhile, the BCCD droplet generator could realize the controllable droplet output with a high-efficiency sample utilization of 99.75 ± 1.15%, which offered a significant advantage in reducing the waste of precious samples in the droplet generation process. We validated this system with a digital loop-mediated isothermal amplification (dLAMP) assay for the absolute quantification of Mycobacterium tuberculosis complex nucleic acids. The results demonstrated that the BCCD droplet generator was easy to build, was of low cost, and was convenient to operate, as well as avoided sample loss and cross-contamination by coupling with a 96-well plate. Overall, the present platform, as a simple chip-free droplet generator, will provide an especially valuable droplet generation solution for biochemical applications based on droplets.

Droplet microfluidics for CTC-based liquid biopsy: a review

Circulating tumor cells (CTCs) are important biomarkers of liquid biopsy. The number and heterogeneity of CTCs play an important role in cancer diagnosis and personalized medicine. However, owing to the low-abundance biomarkers of CTCs, conventional assays are only able to detect CTCs at the population level. Therefore, there is a pressing need for a highly sensitive method to analyze CTCs at the single-cell level. As an important branch of microfluidics, droplet microfluidics is a high-throughput and sensitive single-cell analysis platform for the quantitative detection and heterogeneity analysis of CTCs. In this review, we focus on the quantitative detection and heterogeneity analysis of CTCs using droplet microfluidics. Technologies that enable droplet microfluidics, particularly high-throughput droplet generation and high-efficiency droplet manipulation, are first discussed. Then, recent advances in detecting and analyzing CTCs using droplet microfluidics from the different aspects of nucleic acids, proteins, and metabolites are introduced. The purpose of this review is to provide guidance for the continued study of droplet microfluidics for CTC-based liquid biopsy.

Monodisperse Micro-Droplet Generation in Microfluidic Channel with Asymmetric Cross-Sectional Shape

Micro-droplets are widely used in the fields of chemical and biological research, such as drug delivery, material synthesis, point-of-care diagnostics, and digital PCR. Droplet-based microfluidics has many advantages, such as small reagent consumption, fast reaction time, and independent control of each droplet. Therefore, various micro-droplet generation methods have been proposed, including T-junction breakup, capillary flow-focusing, planar flow-focusing, step emulsification, and high aspect (height-to-width) ratio confinement. In this study, we propose a microfluidic device for generating monodisperse micro-droplets, the microfluidic channel of which has an asymmetric cross-sectional shape and high hypotenuse-to-width ratio (HTWR). It was fabricated using basic MEMS processes, such as photolithography, anisotropic wet etching of Si, and polydimethylsiloxane (PDMS) molding. Due to the geometric similarity of a Si channel and a PDMS mold, both of which were created through the anisotropic etching process of a single crystal Si, the microfluidic channel with the asymmetric cross-sectional shape and high HTWR was easily realized. The effects of HTWR of channels on the size and uniformity of generated micro-droplets were investigated. The monodisperse micro-droplets were generated as the HTWR of the asymmetric channel was over 3.5. In addition, it was found that the flow direction of the oil solution (continuous phase) affected the size of micro-droplets due to the asymmetric channel structures. Two kinds of monodisperse droplets with different sizes were successfully generated for a wider range of flow rates using the asymmetric channel structure in the developed microfluidic device.

Parallelization of Microfluidic Droplet Junctions for Ultraviscous Fluids

The parallelization of multiple microfluidic droplet junctions has been successfully achieved so that the production throughput of the uniform microemulsions/particles has witnessed considerable progress. However, these advancements have been observed only in the case of a low viscous fluid (viscosity of 10^-2 -10^-3 Pa s). This study designs and fabricates a microfluidic device, enabling a uniform micro-emulsification of an ultraviscous fluid (viscosity of 3.5 Pa s) with a throughput of ≈330 000 droplets per hour. Multiple T-junctions of a dispersed oil phase, split from a single inlet, are connected into the single post-crossflow channel of a continuous water phase. In the proposed device, the continuous water phase undergoes a series circuit, wherein the resistances are continuously accumulated. The independent corrugations of the dispersed oil phase channel, under the theoretical guidance, compromise such increased resistances; the ratio of water to oil flow rates at each junction becomes consistent across T-junctions. Owing to the design being based on a fully 2D interconnection, single-step soft lithography is sufficient for developing the full device. This easy-to-craft architecture contrasts with the previous approach, wherein complicated 3D interconnections of the multiple junctions are involved, thereby facilitating the rapid uptake of high throughput droplet microfluidics for experts and newcomers alike.

Simplifying the complex: accessible microfluidic solutions for contemporary processes within in vitro diagnostics

In vitro diagnostics (IVDs) form the cornerstone of modern medicine. They are routinely employed throughout the entire treatment pathway, from initial diagnosis through to prognosis, treatment planning, and post-treatment surveillance. Given the proven links between high quality diagnostic testing and overall health, ensuring broad access to IVDs has long been a focus of both researchers and medical professionals. Unfortunately, the current diagnostic paradigm relies heavily on centralized laboratories, complex and expensive equipment, and highly trained personnel. It is commonly assumed that this level of complexity is required to achieve the performance necessary for sensitive and specific disease diagnosis, and that making something affordable and accessible entails significant compromises in test performance. However, recent work in the field of microfluidics is challenging this notion. By exploiting the unique features of microfluidic systems, researchers have been able to create progressively simple devices that can perform increasingly complex diagnostic assays. This review details how microfluidic technologies are disrupting the status quo, and facilitating the development of simple, affordable, and accessible integrated IVDs. Importantly, we discuss the advantages and limitations of various approaches, and highlight the remaining challenges within the field.

Monodispersed Sirolimus-Loaded PLGA Microspheres with a Controlled Degree of Drug–Polymer Phase Separation for Drug-Coated Implantable Medical Devices and Subcutaneous Injection

Monodispersed sirolimus (SRL)-loaded poly(lactic-co-glycolic acid) microspheres with a diameter of 1.8, 3.8, and 8.5 μm were produced by high-throughput microfluidic step emulsification—solvent evaporation using single crystal silicon chips consisted of 540–1710 terraced microchannels with a depth of 2, 4, or 5 μm arranged in 10 parallel arrays. Uniform sized droplets were generated over 25 h across all channels. Nearly 15% of the total drug was released by the initial burst release during an accelerated drug release testing performed at 37 °C using a hydrotropic solution containing 5.8 M N,N-diethylnicotinamide. After 24 h, 71% of the drug was still entrapped in the particles. The internal morphology of microspheres was investigated by fluorescence microscopy using Nile red as a selective fluorescent stain with higher binding affinity toward SRL. By increasing the drug loading from 33 to 50 wt %, the particle morphology evolved from homogeneous microspheres, in which the drug and polymer were perfectly mixed, to patchy particles, with amorphous drug patches embedded within a polymer matrix to anisotropic patchy Janus particles. Janus particles with fully segregated drug and polymer regions were achieved by pre-saturating the aqueous phase with the organic solvent, which decreased the rate of solvent evaporation and allowed enough time for complete phase separation. This approach to manufacturing drug-loaded monodisperse microparticles can enable the development of more effective implantable drug-delivery devices and improved methods for subcutaneous drug administration, which can lead to better therapeutic treatments.

Materials and methods for droplet microfluidic device fabrication

Since the first reports two decades ago, droplet-based systems have emerged as a compelling tool for microbiological and (bio)chemical science, with droplet flow providing multiple advantages over standard single-phase microfluidics such as removal of Taylor dispersion, enhanced mixing, isolation of droplet contents from surfaces, and the ability to contain and address individual cells or biomolecules. Typically, a droplet microfluidic device is designed to produce droplets with well-defined sizes and compositions that flow through the device without interacting with channel walls. Successful droplet flow is fundamentally dependent on the microfluidic device - not only its geometry but moreover how the channel surfaces interact with the fluids. Here we summarise the materials and fabrication techniques required to make microfluidic devices that deliver controlled uniform droplet flow, looking not just at physical fabrication methods, but moreover how to select and modify surfaces to yield the required surface/fluid interactions. We describe the various materials, surface modification techniques, and channel geometry approaches that can be used, and give examples of the decision process when determining which material or method to use by describing the design process for five different devices with applications ranging from field-deployable chemical analysers to water-in-water droplet creation. Finally we consider how droplet microfluidic device fabrication is changing and will change in the future, and what challenges remain to be addressed in the field.

High-Aspect-Ratio Microfluidic Channel with Parallelogram Cross-Section for Monodisperse Droplet Generation

Droplet-based microfluidics has been widely used as a potent high-throughput platform due to various advantages, such as a small volume of reagent consumption, massive production of droplets, fast reaction time, and independent control of each droplet. Therefore, droplet microfluidic systems demand the reliable generation of droplets with precise and effective control over their size and distribution, which is critically important for various applications in the fields of chemical analysis, material synthesis, lab-on-a-chip, cell research, diagnostic test, and so on. In this study, we propose a microfluidic device with a high-aspect-ratio (HAR) channel, which has a parallelogram cross-section, for generating monodisperse droplets. The HAR channel was fabricated using simple and cheap MEMS processes, such as photolithography, anisotropic wet etching, and PDMS molding, without expensive equipment. In addition, the parallelogram cross-section channel structure, regarded as a difficult shape to implement in previous fabrication methods, was easily formed by the self-alignment between the silicon channel and the PDMS mold, both of which were created from a single crystal silicon through an anisotropic etching process. We investigated the effects of the cross-sectional shape (parallelogram vs. rectangle) and height-to-width ratio of microfluidic channels on the size and uniformity of generated droplets. Using the developed HAR channel with the parallelogram cross-section, we successfully obtained smaller monodisperse droplets for a wider range of flow rates, compared with a previously reported HAR channel with a rectangular cross-section.

Scaling up the throughput of microfluidic droplet-based materials synthesis: A review of recent progress and outlook

The last two decades have witnessed tremendous progress in the development of microfluidic chips that generate micrometer- and nanometer-scale materials. These chips allow precise control over composition, structure, and particle uniformity not achievable using conventional methods. These microfluidic-generated materials have demonstrated enormous potential for applications in medicine, agriculture, food processing, acoustic, and optical meta-materials, and more. However, because the basis of these chips' performance is their precise control of fluid flows at the micrometer scale, their operation is limited to the inherently low throughputs dictated by the physics of multiphasic flows in micro-channels. This limitation on throughput results in material production rates that are too low for most practical applications. In recent years, however, significant progress has been made to tackle this challenge by designing microchip architectures that incorporate multiple microfluidic devices onto single chips. These devices can be operated in parallel to increase throughput while retaining the benefits of microfluidic particle generation. In this review, we will highlight recent work in this area and share our perspective on the key unsolved challenges and opportunities in this field.

(R)evolution-on-a-chip

Billions of years of Darwinian evolution has led to the emergence of highly sophisticated and diverse life forms on Earth. Inspired by natural evolution, similar principles have been adopted in laboratory evolution for the fast optimization of genes and proteins for specific applications. In this review, we highlight state-of-the-art laboratory evolution strategies for protein engineering, with a special emphasis on in vitro strategies. We further describe how recent progress in microfluidic technology has allowed the generation and manipulation of artificial compartments for high-throughput laboratory evolution experiments. Expectations for the future are high: we foresee a revolution on-a-chip.

Droplet breakup mechanisms in premix membrane emulsification and related microfluidic channels

Premix membrane emulsification (PME) is a pressure driven process of droplet breakup, caused by their motion through membrane pores. The process is widely used for high-throughput production of sized-controlled emulsion droplets and microparticles using low energy inputs. The resultant droplet size depends on numerous process, membrane, and formulation factors such as flow velocity in pores, number of extrusions, initial droplet size, internal membrane geometry, wettability of pore walls, and physical properties of emulsion. This paper provides a comprehensive review of different mechanisms of droplet deformation and breakup in membranes with versatile pore morphologies including sintered glass and ceramic filters, SPG and polymeric membranes with sponge-like structures, micro-engineered metallic membranes with ordered straight-through pore arrays, and dynamic membranes composed of unconsolidated particles. Fundamental aspects of droplet motion and breakup in idealized pore networks have also been covered including droplet disruption in T-junctions, channel constrictions, and obstructed channels. The breakup mechanisms due to shear interactions with pore walls and localized shear (direct breaking) or due to interfacial tension effects and Rayleigh-Plateau instability (indirect breaking) are systematically discussed based on recent experimental and numerical studies. Non-dimensional droplet size correlations based on capillary, Weber, and Ohnesorge numbers are also presented.

A Double-Deck Self-Digitization Microfluidic Chip for Digital PCR

In this work, a double-deck microfluidic chip was presented for digital PCR application. This chip consists of two reverse-placed micro-patterned polydimethylsiloxane (PDMS) layers between the top and bottom glass substrates. Each micropatterned PDMS layer contains more than 20,000 cylindrical micro-chambers to hold the partitioned droplets. The double-deck designs can double the number of chambers and reagent capacity without changing the planar area of the chip. In addition, carbon black was mixed into the pure PDMS gel to obstruct the passage of fluorescence from the positive chambers between the two layers of the chip. Thus, the fluorescence signal of micro-chambers in different layers of the chip after PCR can be collected without mutual interference. The quantitative capability of the proposed chip was evaluated by measuring a 10-fold serial dilution of the DNA template. A high accuracy of the absolute quantification for nucleic acid with a dynamic range of 10⁵ was demonstrated by this chip in this work. Owing to its characteristics of small planar area, large capacity, and sensitivity, the double-deck microfluidic chip is expected to further promote the extensive applications of digital PCR.