Can 3D Printing Bring Droplet Microfluidics to Every Lab?-A Systematic Review

A review of the recent achievements and future trends on 3D printed microfluidic devices for bioanalytical applications

3D printing has revolutionized the manufacturing process of microanalytical devices by enabling the automated production of customized objects. This technology promises to become a fundamental tool, accelerating investigations in critical areas of health, food, and environmental sciences. This microfabrication technology can be easily disseminated among users to produce further and provide analytical data to an interconnected network towards the Internet of Things, as 3D printers enable automated, reproducible, low-cost, and easy fabrication of microanalytical devices in a single step. New functional materials are being investigated for one-step fabrication of highly complex 3D printed parts using photocurable resins. However, they are not yet widely used to fabricate microfluidic devices. This is likely the critical step towards easy and automated fabrication of sophisticated, complex, and functional 3D-printed microchips. Accordingly, this review covers recent advances in the development of 3D-printed microfluidic devices for point-of-care (POC) or bioanalytical applications such as nucleic acid amplification assays, immunoassays, cell and biomarker analysis and organs-on-a-chip. Finally, we discuss the future implications of this technology and highlight the challenges in researching and developing appropriate materials and manufacturing techniques to enable the production of 3D-printed microfluidic analytical devices in a single step.

Processes for the 3D Printing of Hydrodynamic Flow-Focusing Devices

Flow focusing is an important hydrodynamic technique for cytometric analysis, enabling the rapid study of cellular samples to identify a variety of biological processes. To date, the majority of flow-focusing devices are fabricated using conventional photolithography or flame processing of glass capillaries. This article presents a suite of low-cost, millifluidic, flow-focusing devices that were fabricated using a desktop sterolithgraphy (SLA) 3D printer. The suite of SLA printing strategies consists of a monolithic SLA method and a hybrid molding process. In the monolithic SLA approach, 1.3 mm square millifluidic channels were printed as a single piece. The printed device does not require any post processing, such as bonding or surface polishing for optical access. The hybrid molding approach consists of printing a mold using the SLA 3D printer. The mold is treated to an extended UV exposure and oven baked before using PDMS as the molding material for the channel. To demonstrate the viability of these channels, we performed a series of experiments using several flow-rate ratios to show the range of focusing widths that can be achieved in these devices. The experiments are validated using a numerical model developed in ANSYS.

A Non-Sacrificial 3D Printing Process for Fabricating Integrated Micro/Mesoscale Molds

Three-dimensional printing technology has been implemented in microfluidic mold fabrication due to its freedom of design, speed, and low-cost fabrication. To facilitate mold fabrication processes and avoid the complexities of the soft lithography technique, we offer a non-sacrificial approach to fabricate microscale features along with mesoscale features using Stereolithography (SLA) printers to assemble a modular microfluidic mold. This helps with addressing an existing limitation with fabricating complex and time-consuming micro/mesoscale devices. The process flow, optimization of print time and feature resolution, alignments of modular devices, and the advantages and limitations with the offered technique are discussed in this paper.

3D printed microfluidics: advances in strategies, integration, and applications

The ability to construct multiplexed micro-systems for fluid regulation could substantially impact multiple fields, including chemistry, biology, biomedicine, tissue engineering, and soft robotics, among others. 3D printing is gaining traction as a compelling approach to fabricating microfluidic devices by providing unique capabilities, such as 1) rapid design iteration and prototyping, 2) the potential for automated manufacturing and alignment, 3) the incorporation of numerous classes of materials within a single platform, and 4) the integration of 3D microstructures with prefabricated devices, sensing arrays, and nonplanar substrates. However, to widely deploy 3D printed microfluidics at research and commercial scales, critical issues related to printing factors, device integration strategies, and incorporation of multiple functionalities require further development and optimization. In this review, we summarize important figures of merit of 3D printed microfluidics and inspect recent progress in the field, including ink properties, structural resolutions, and hierarchical levels of integration with functional platforms. Particularly, we highlight advances in microfluidic devices printed with thermosetting elastomers, printing methodologies with enhanced degrees of automation and resolution, and the direct printing of microfluidics on various 3D surfaces. The substantial progress in the performance and multifunctionality of 3D printed microfluidics suggests a rapidly approaching era in which these versatile devices could be untethered from microfabrication facilities and created on demand by users in arbitrary settings with minimal prior training.

Validation of Easy Fabrication Methods for PDMS-Based Microfluidic (Bio)Reactors

The common method for producing casting molds for the fabrication of polydimethylsiloxane (PDMS) chips is standard photolithography. This technique offers high resolution from hundreds of nanometers to a few micrometers. However, this mold fabrication method is costly, time-consuming, and might require clean room facilities. Additionally, there is a need for non-micromechanics experts, who do not have specialized equipment to easily and quickly prototype chips themselves. Simple, so-called, makerspace technologies are increasingly being explored as alternatives that have potential to enable anyone to fabricate microfluidic structures. We therefore tested simple fabrication methods for a PDMS-based microfluidic device. On the one hand, channels were replicated from capillaries and tape. On the other hand, different mold fabrication methods, namely laser cutting, fused layer 3D printing, stereolithographic 3D printing, and computer numerical control (CNC) milling, were validated in terms of machine accuracy and tightness. Most of these methods are already known, but the incorporation and retention of particles with sizes in the micrometer range have been less investigated. We therefore tested two different types of particles, which are actually common carriers for the immobilization of enzymes, so that the resulting reactor could ultimately be used as a microfluidic bioreactor. Furthermore, CNC milling provide the most reliable casting mold fabrication method. After some optimization steps with regard to manufacturing settings and post-processing polishing, the chips were tested for the retention of two different particle types (spherical and non-spherical particles). In this way, we successfully tested the obtained PDMS-based microfluidic chips for their potential applicability as (bio)reactors with enzyme immobilization carrier beads.

Modular Microfluidics: Current Status and Future Prospects

This review mainly studies the development status, limitations, and future directions of modular microfluidic systems. Microfluidic technology is an important tool platform for scientific research and plays an important role in various fields. With the continuous development of microfluidic applications, conventional monolithic microfluidic chips show more and more limitations. A modular microfluidic system is a system composed of interconnected, independent modular microfluidic chips, which are easy to use, highly customizable, and on-site deployable. In this paper, the current forms of modular microfluidic systems are classified and studied. The popular fabrication techniques for modular blocks, the major application scenarios of modular microfluidics, and the limitations of modular techniques are also discussed. Lastly, this review provides prospects for the future direction of modular microfluidic technologies.

Contribution of omics to biopreservation: Toward food microbiome engineering

Biopreservation is a sustainable approach to improve food safety and maintain or extend food shelf life by using beneficial microorganisms or their metabolites. Over the past 20 years, omics techniques have revolutionised food microbiology including biopreservation. A range of methods including genomics, transcriptomics, proteomics, metabolomics and meta-omics derivatives have highlighted the potential of biopreservation to improve the microbial safety of various foods. This review shows how these approaches have contributed to the selection of biopreservation agents, to a better understanding of the mechanisms of action and of their efficiency and impact within the food ecosystem. It also presents the potential of combining omics with complementary approaches to take into account better the complexity of food microbiomes at multiple scales, from the cell to the community levels, and their spatial, physicochemical and microbiological heterogeneity. The latest advances in biopreservation through omics have emphasised the importance of considering food as a complex and dynamic microbiome that requires integrated engineering strategies to increase the rate of innovation production in order to meet the safety, environmental and economic challenges of the agri-food sector.

Stereolithography 3D Printer for Micromodel Fabrications with Comprehensive Accuracy Evaluation by Using Microtomography

Micromodels are important for studying various pore-scale phenomena in hydrogeology. However, the fabrication of a custom micromodel involves complicated steps with cost-prohibitive equipment. The direct fabrication of micromodels with a 3D printer can accelerate the fabrication steps and reduce the cost. A stereolithography (SLA) 3D printer is one of the best options because it has sufficient printing performance for micromodel fabrication and is relatively inexpensive. However, it is not without drawbacks. In this report, we explored the capability of an SLA 3D printer for micromodel fabrication. Various parameters affecting the printing results, such as the effects of geometries, dimensions, printing axis configurations, printing thickness resolutions, and pattern thicknesses were investigated using microtomography for the first time. Eventually, the most optimal printing configuration was then also discussed. In the end, a complete micromodel was printed, assembled, and used for fluid displacement experiments. As a demonstration, viscous and capillary fingerings were successfully performed using this micromodel design.

A Monolithic 3D Printed Axisymmetric Co-Flow Single and Compound Emulsion Generator

We report a microfluidic droplet generator which can produce single and compound droplets using a 3D axisymmetric co-flow structure. The design considered for the fabrication of the device integrated a user-friendly and cost-effective 3D printing process. To verify the performance of the device, single and compound emulsions of deionized water and mineral oil were generated and their features such as size, generation frequency, and emulsion structures were successfully characterized. In addition, the generation of bio emulsions such as alginate and collagen aqueous droplets in mineral oil was demonstrated in this study. Overall, the monolithic 3D printed axisymmetric droplet generator could offer any user an accessible and easy-to-utilize device for the generation of single and compound emulsions.

Time-efficient fabrication method for 3D-printed microfluidic devices

Recent developments in 3D-printing technology have provided a time-efficient and inexpensive alternative to the fabrication of microfluidic devices. At present, 3D-printed microfluidic systems face the challenges of post-processing, non-transparency, and being time consuming, limiting their practical application. In this study, a time-efficient and inexpensive fabrication method was developed for 3D-printed microfluidic devices. The material for 3D-printed microfluidic chips is Dowsil 732, which is used as a sealant or encapsulant in various industries. The curing time and surface hydrophobicity of the materials were evaluated. The results indicated that the surface of Dowsil 732 is hydrophilic. An optimization model of the direct ink writing method is proposed to establish a time-efficient and accurate fabrication method for microfluidic devices. The results indicate that the optimization model can effectively describe the change trend between printing speed, printing pressure, and channel wall accuracy, and the model accuracy rate exceeds 95%. Three examples-a micromixer, concentration gradient generator, and droplet generator-were printed to demonstrate the functionality and feasibility of the fabrication method.