Cell-free biology using remote-controlled digital microfluidics for individual droplet control

Digital microfluidics for biological analysis and applications

Digital microfluidics (DMF) is an emerging liquid-handling technology based on arrays of microelectrodes for the precise manipulation of discrete droplets. DMF offers the benefits of automation, addressability, integration and dynamic configuration ability, and provides enclosed picoliter-to-microliter reaction space, making it suitable for lab-on-a-chip biological analysis and applications that require high integration and intricate processes. A review of DMF bioassays with a special emphasis on those actuated by electrowetting on dielectric (EWOD) force is presented here. Firstly, a brief introduction is presented on both the theory of EWOD actuation and the types of droplet motion. Subsequently, a comprehensive overview of DMF-based biological analysis and applications, including nucleic acid, protein, immunoreaction and cell assays, is provided. Finally, a discussion on the strengths, challenges, and potential applications and perspectives in this field is presented.

Digital microfluidics as an emerging tool for bacterial protocols

Bacteria are widely studied in various research areas, including synthetic biology, sequencing and diagnostic testing. Protocols involving bacteria are often multistep, cumbersome and require access to a long list of instruments to perform experiments. In order to streamline these processes, the fluid handling technique digital microfluidics (DMF) has provided a miniaturized platform to perform various steps of bacterial protocols from sample preparation to analysis. DMF devices can be paired/interfaced with instrumentation such as microscopes, plate readers, and incubators, demonstrating their versatility with existing research tools. Alternatively, DMF instruments can be integrated into all-in-one packages with on-chip magnetic separation for sample preparation, heating/cooling modules to perform assay steps and cameras for absorbance and/or fluorescence measurements. This perspective outlines the beneficial features DMF offers to bacterial protocols, highlights limitations of current work and proposes future directions for this tool's expansion in the field.

Surface-Enhanced Raman Spectroscopic Probing in Digital Microfluidics through a Microspray Hole

We report a novel approach for surface-enhanced Raman spectroscopy (SERS) detection in digital microfluidics (DMF). This is made possible by a microspray hole (μSH) that uses an electrostatic spray (ESTAS) for sample transfer from inside the chip to an external SERS substrate. To realize this, a new ESTAS-compatible stationary SERS substrate was developed and characterized for sensitive and reproducible SERS measurements. In a proof-of-concept study, we successfully applied the approach to detect various analyte molecules using the DMF chip and achieved micro-molar detection limits. Moreover, this technique was exemplarily employed to study an organic reaction occurring in the DMF device, providing vibrational spectroscopic data.

Rolling microswarms along acoustic virtual walls

Rolling is a ubiquitous transport mode utilized by living organisms and engineered systems. However, rolling at the microscale has been constrained by the requirement of a physical boundary to break the spatial homogeneity of surrounding mediums, which limits its prospects for navigation to locations with no boundaries. Here, in the absence of real boundaries, we show that microswarms can execute rolling along virtual walls in liquids, impelled by a combination of magnetic and acoustic fields. A rotational magnetic field causes individual particles to self-assemble and rotate, while the pressure nodes of an acoustic standing wave field serve as virtual walls. The acoustic radiation force pushes the microswarms towards a virtual wall and provides the reaction force needed to break their fore-aft motion symmetry and induce rolling along arbitrary trajectories. The concept of reconfigurable virtual walls overcomes the fundamental limitation of a physical boundary being required for universal rolling movements.

Electrically-driven handling of gametes and embryos: taking a step towards the future of ARTs

Electrical stimulation of gametes and embryos and on-chip manipulation of microdroplets of culture medium serve as promising tools for assisted reproductive technologies (ARTs). Thus far, dielectrophoresis (DEP), electrorotation (ER) and electrowetting on dielectric (EWOD) proved compatible with most laboratory procedures offered by ARTs. Positioning, entrapment and selection of reproductive cells can be achieved with DEP and ER, while EWOD provides the dynamic microenvironment of a developing embryo to better mimic the functions of the oviduct. Furthermore, these techniques are applicable for the assessment of the developmental competence of a mammalian embryo in vitro. Such research paves the way towards the amelioration and full automation of the assisted reproduction methods. This article aims to provide a summary on the recent developments regarding electrically stimulated lab-on-chip devices and their application for the manipulation of gametes and embryos in vitro.

Soft Magnetic Microrobot Doped with Porous Silica for Stability-Enhanced Multimodal Locomotion in a Nonideal Environment

As an emerging field of robotics, magnetic-field-controlled soft microrobot has broad application prospects for its flexibility, locomotion diversity, and remote controllability. Magnetic soft microrobots can perform multimodal locomotion under the control of a magnetic field, which may have potential applications in precision medicine. However, previous research studies mainly focus on new locomotion in a relatively ideal environment, lacking exploration on the ability of magnetic microrobot locomotion to resist external disturbances and proceed in a nonideal environment. Here, a porous silica-doped soft magnetic microrobot is constructed for enhanced stability of multimodal locomotion in the nonideal biological environment. Porous silica spheres are doped into a NdFeB-silicone elastomer base, improving adhesion properties and refining the comprehensive mechanical properties of the microrobot. Multimodal locomotions are achieved, and the influence of porous silica doping on the stability of each locomotion in a nonideal environment is explored in depth. Motions in nonideal circumstances such as climbing, loading, current rushing, wind blowing, and obstacle hindering are conducted successfully with porous silica doping. Such a stability-enhanced multimodal locomotion system can be used in biocatalysis and thrombus removal, and its prospect for precision medicine is highlighted by in vivo demonstration of multimodal locomotion with nonideal disturbance.

3D Magnetic Field-Controlled Synthesis, Collective Motion, and Bioreaction Enhancement of Multifunctional Peasecod-like Nanochains

Magnetic field-induced synthesis and biocatalysis of magnetic materials have inspired great interest due to the flexible controllability of morphologies and unique magnetoelectrical properties. However, the interaction of the magnetic field and the reaction kinetics during the synthesis of magnetic nanochains has not been revealed. The collective motions in fluids and the multifunctional enhancements for bioreaction of 3D magnetic-controlled nanochains have not been systematically researched. Here, an integrated 3D magnetic control method was reported for the synthesis, collective motion, and multifunctional bioreaction enhancement of peasecod-like nanochains. The interactions of magnetic field and reaction kinetics were rationally controlled to synthesize magnetic nanochains of different morphologies. Collective motions of nanochains under alternating magnetic fields were studied to provide insights into the disturbance on confined fluids. Three mechanisms of reaction enhancement of nanostir, magnetic agent, and nanocatalyst were achieved simultaneously via 3D magnetic-controlled nanochains using a glucose oxidase-horseradish peroxidase multi-enzyme system. The peasecod-like nanochain also exhibited excellent reaction enhancement in cell-free protein synthesis reaction, which is desired for effective high-throughput screening. The integrated 3D magnetic control method through the whole process from fabrication to applications of magnetic nanomaterials could be extended to multifunctional biocatalysis and multi-task biomedicine.

Physical stimuli-responsive cell-free protein synthesis

Cell-free protein synthesis has been developed as a critical platform in synthetic biology. Unlike the cell-based synthesis system, cell-free system activates transcriptional and translational mechanisms in vitro, and can control protein synthesis by artificially adding components or chemicals. However, the control method puts forward higher requirements in terms of accurate and non-toxic control, which cannot be achieved by chemical substances. For cell-free system, physical signal is a kind of ideal spatiotemporal control approach to replace chemical substances, realizing high accuracy with little side effect. Here we review the methods of using physical signals to control gene expression in cell-free systems, including studies based on light, temperature, electric field, and magnetic force. The transfer of these switches into cell-free system further expands the flexibility and controllability of the system, thus further expanding the application capability of cell-free systems. Finally, existing problems such as signal source and signal transmission are discussed, and future applications in pharmaceutical production, delivery and industrial production are further looked into.