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Correction for ‘Liquid marble-based digital microfluidics – fundamentals and applications’ by Chin Hong Ooi et al., Lab Chip, 2021, DOI: 10.1039/d0lc01290d.

Liquid marbles are droplets with volume typically on the order of microliters coated with hydrophobic powder. Their versatility, ease of use and low cost make liquid marbles an attractive platform for digital microfluidics. This paper provides the state of the art of discoveries in the physics of liquid marbles and their practical applications. The paper first discusses the fundamental properties of liquid marbles, followed by the summary of different techniques for the synthesis of liquid marbles. Next, manipulation techniques for handling liquid marbles are discussed. Applications of liquid marbles are categorised according to their use as chemical and biological reactors. The paper concludes with perspectives on the future development of liquid marble-based digital microfluidics.

Single-crystal cubic silicon carbide has attracted great attention for MEMS and electronic devices. However, current leakage at the SiC/Si junction at high temperatures and visible-light absorption of the Si substrate are main obstacles hindering the use of the platform in a broad range of applications. To solve these bottlenecks, we present a new platform of single crystal SiC on an electrically insulating and transparent substrate using an anodic bonding process. The SiC thin film was prepared on a 150 mm Si with a surface roughness of 7 nm using LPCVD. The SiC/Si wafer was bonded to a glass substrate and then the Si layer was completely removed through wafer polishing and wet etching. The bonded SiC/glass samples show a sharp bonding interface of less than 15 nm characterized using deep profile X-ray photoelectron spectroscopy, a strong bonding strength of approximately 20 MPa measured from the pulling test, and relatively high optical transparency in the visible range. The transferred SiC film also exhibited good conductivity and a relatively high temperature coefficient of resistance varying from -12 000 to -20 000 ppm/K, which is desirable for thermal sensors. The biocompatibility of SiC/glass was also confirmed through mouse 3T3 fibroblasts cell-culturing experiments. Taking advantage of the superior electrical properties and biocompatibility of SiC, the developed SiC-on-glass platform offers unprecedented potentials for high-temperature electronics as well as bioapplications.

Improving and optimizing the processes for transfer printing have the potential to further enhance capabilities in heterogeneous integration of various sensing materials on unconventional substrates for implantable and stretchable electronic devices in biosensing, diagnostics, and therapeutic applications. An advanced transfer printing method based on sacrificial layer engineering for silicon carbide materials in stretchable electronic devices is presented here. In contrast to the typical processes where defined anchor structures are required for the transfer step, the use of a sacrificial layer offers enhances versatility in releasing complex microstructures from rigid donor substrates to flexible receiver platforms. The sacrificial layer also minimizes twisting and wrinkling issues that may occur in free‐standing microstructures, thereby facilitating printing onto flat polymer surfaces (e.g., polydimethylsiloxane). The experimental results demonstrate that transferred SiC microstructures exhibit good stretchability, stable electrical properties, excellent biocompatibility, as well as promising sensing‐functions associated with a high level of structural perfection, without any cracks or tears. This transfer printing method can be applied to other classes of wide bandgap semiconductors, particularly group III‐nitrides and diamond films epitaxially grown on Si substrates, thereby serving as the foundation for the development and possible commercialization of implantable and stretchable bioelectronic devices that exploit wide bandgap materials.

Controlled actuation of a floating liquid marble, a liquid droplet coated with hydrophobic particles floating on another liquid surface, is a potential digital microfluidics platform for the transport of aqueous solution with minimal volume loss. This paper reports our recent investigation on the magnetic actuation of floating liquid marbles filled with magnetic particles. The magnetic force and frictional force acting on the floating liquid marble determine the horizontal movement of the marble. We varied the magnetic flux density, flux density gradient, concentration of magnetic particles and speed of the marble to elucidate the relationship between the acting forces. We subsequently determined the suitable operating conditions for the actuation and derived the scaling laws for the actuation parameters.

This work presents the fabrication, mechanical strength characterization, and cell culture demonstration of a high aspect ratio silicon carbide (SiC) membrane. Optimizations in the deposition and fabrication make an ultra-high aspect ratio up to 20,000 SiC membranes with high fracture strength possible. Utilizing the superior properties of SiC material, the ultra-thin SiC membrane is a promising for cell culture/stretching devices, enabling very short optical accesses. The biocompatibility of the SiC membrane was confirmed with the 3T3 fibroblasts cell viability rate of 92.7%, in which the cells flattened and elongated their morphology while maintaining a strong adhesion to the SiC surface.

Micromachined membranes are promising platforms for cell culture thanks to their miniaturization and integration capabilities. Possessing chemical inertness, biocompatibility, and integration, silicon carbide (SiC) membranes have attracted great interest toward biological applications. In this paper, we present the batch fabrication, mechanical characterizations, and cell culture demonstration of robust ultrathin epitaxial deposited SiC membranes. The as-fabricated ultrathin SiC membranes, with an ultrahigh aspect ratio (length/thickness) of up to 20 000, possess high a fracture strength up to 2.95 GPa and deformation up to 50 μm. A high optical transmittance of above 80% at visible wavelengths was obtained for 50 nm membranes. The as-fabricated membranes were experimentally demonstrated as an excellent substrate platform for bio-MEMS/NEMS cell culture with the cell viability rate of more than 92% after 72 h. The ultrathin SiC membrane is promising for in vitro observations/imaging of bio-objects with an extremely short optical access.

Liquid marble as a bioreactor platform for cell-based studies has received significant attention, especially for developing 3D cell-based assays. This platform is particularly suitable for 3D in-vitro modeling of cell-cell interactions. For the first time, we demonstrated the interaction of olfactory ensheathing cells (OECs) with nerve debris and meningeal fibroblast using liquid marbles. As the transplantation of OECs can be used for repairing nerve injury, degenerating cell debris within the transplantation site can adversely affect the survival of transplanted OECs. In this paper, we used liquid marbles to mimic the hostile 3D environment to analyze the functional behavior of the cells and to form the basis for cell-based therapy. We show that OECs interact with debris and enhanced cellular aggregation to form a larger 3D spheroidal tissue. However, these spheroids indicated limitation in biological functions such as the inability of cells within the spheroids to migrate out and adherence to neighboring tissue by fusion. The coalescence of two liquid marbles allows for analyzing the interaction between two distinct cell types and their respective environment. We created a microenvironment consisting of 3D fibroblast spheroids and nerve debris and let it interact with OECs. We found that OECs initiate adherence with nerve debris in this 3D environment. The results suggest that liquid marbles are ideal for developing bioassays that could substantially contribute to therapeutic applications. Especially, insights for improving the survival and adherence of transplanted cells.

A rigid spherical particle floating on a liquid is a known problem with well-defined solutions. Under the combined effect of gravity and surface tension, the rigid particle deforms the liquid surface. However, in the case of a floating soft particle such as a liquid marble, not only the liquid surface but also the particle itself deforms. In this paper, we investigate the deformation of a floating liquid marble and characterise its height as well as aspect ratio. The experimental results show that theoretical models for a rigid spherical particle suit well for small liquid marbles. Larger marbles require an oblate liquid spheroid model. We will discuss the limitations of the two models and characterise the deformation of these marbles.

A three-dimensional (3D) tissue model has significant advantages over the conventional two-dimensional (2D) model. A 3D model mimics the relevant in-vivo physiological conditions, allowing a cell culture to serve as an effective tool for drug discovery, tissue engineering, and the investigation of disease pathology. The present reviews highlight the recent advances and the development of microfluidics based methods for the generation of cell spheroids. The paper emphasizes on the application of microfluidic technology for tissue engineering including the formation of multicellular spheroids (MCS). Further, the paper discusses the recent technical advances in the integration of microfluidic devices for MCS-based high-throughput drug screening. The review compares the various microfluidic techniques and finally provides a perspective for the future opportunities in this research area.

Three-dimensional (3D) printing of human tissues and organ has been an exciting research topic in the past three decades. However, existing technological and biological challenges still require a significant amount of research. The present review highlights these challenges and discusses their potential solutions such as mapping and converting a human organ onto a 3D virtual design, synchronizing the virtual design with the printing hardware. Moreover, the paper discusses in details recent advances in formulating bio-inks and challenges in tissue construction with or without scaffold. Next, the paper reviews fusion processes effecting vascular cells and tissues. Finally, the paper deliberates the feasibility of organ printing with state-of-the-art technologies.