Novel microwell with a roof capable of buoyant spheroid culture

Controlling bead and cell mobility in a recirculating hanging-drop network

Integrating flowing cells, such as immune cells or circulating tumour cells, within a microphysiological system is crucial for body-on-a-chip applications. However, ensuring unimpeded recirculation of cells is a significant challenge. Closed microfluidic devices have a no-slip boundary condition along channel walls and a defined chip geometry (laminar flow) that hinders the ability to freely control cell flow. Open microfluidic devices, where the bottom device boundary is an air-liquid interface (ALI), e.g., hanging drop networks (HDNs), offer the advantage of an easily-actuatable fluid-phase geometry, where cells can either flow or stagnate. In this paper, we optimized a hanging-drop-integrated pneumatic-pump system for closed-loop recirculation of particles (i.e., beads or cells). Experiments with both beads and cells in cell culture medium initially resulted in particle stagnation, which was suggestive of a pseudo-no-slip boundary condition at the ALI. Transmission electron microscopy and dynamic light scattering measurements of the ALI suggested that aggregation of submicron-scale cell-culture-medium components is the cause of the pseudo-no-slip boundary condition. We used the finite element method to study the forces on particles at the ALI and to optimize HDN design (drop aperture) and operation (drop height) parameters. Based on this analysis, we report a phase diagram delineating the conditions for free flow or stagnation of particles at the ALI of hanging drops. Using our experimental setup with 3.5 mm drop apertures, we conducted particle flow experiments while actuating drop heights. We confirmed the ability to control the flow or stagnation of particles by actuating the height of hanging drops: a drop height over 300 μm led to particle stagnation and a drop height under 300 μm allowed for particle flow. This particle-flow control, combined with the ease of integrating scaffold-free organ models (microtissues or organoids) in HDNs, constitutes the basis for an experimental setup enabling the control of the residence time of single cells around 3D organ models.

Spheroid Engineering in Microfluidic Devices

Two-dimensional (2D) cell culture techniques are commonly employed to investigate biophysical and biochemical cellular responses. However, these culture methods, having monolayer cells, lack cell-cell and cell-extracellular matrix interactions, mimicking the cell microenvironment and multicellular organization. Three-dimensional (3D) cell culture methods enable equal transportation of nutrients, gas, and growth factors among cells and their microenvironment. Therefore, 3D cultures show similar cell proliferation, apoptosis, and differentiation properties to in vivo. A spheroid is defined as self-assembled 3D cell aggregates, and it closely mimics a cell microenvironment in vitro thanks to cell-cell/matrix interactions, which enables its use in several important applications in medical and clinical research. To fabricate a spheroid, conventional methods such as liquid overlay, hanging drop, and so forth are available. However, these labor-intensive methods result in low-throughput fabrication and uncontrollable spheroid sizes. On the other hand, microfluidic methods enable inexpensive and rapid fabrication of spheroids with high precision. Furthermore, fabricated spheroids can also be cultured in microfluidic devices for controllable cell perfusion, simulation of fluid shear effects, and mimicking of the microenvironment-like in vivo conditions. This review focuses on recent microfluidic spheroid fabrication techniques and also organ-on-a-chip applications of spheroids, which are used in different disease modeling and drug development studies.

Ceiling culture chip reveals dynamic lipid droplet transport during adipocyte dedifferentiation via actin remodeling

Adipocyte dedifferentiation has recently gained attention as a process underpinning adipocyte plasticity; however, a lack of suitable experimental platforms has hampered studies into the underlying mechanisms. Here, we developed a microscope-mountable ceiling culture chip that provides a stable yet tunable culture environment for long-term live-imaging of dedifferentiating adipocytes. A detailed spatiotemporal analysis of mature adipocyte dedifferentiation utilizing the culture platform and Cre-recombinase tracers revealed the involvement of dynamic actin remodeling for lipid droplet (LD) secretion during adipocyte dedifferentiation. Additionally, Hippo, Hedgehog, and PPARγ signaling pathways were identified as potent regulators of adipocyte dedifferentiation. Contrary to the belief that adult adipocytes are relatively static, we show that adipocytes are very dynamic, relying on actin-driven mechanical forces to execute LD extrusion and intercellular LD transfer processes.

Fabrication of Concave Microwells and Their Applications in Micro-Tissue Engineering: A Review

At present, there is an increasing need to mimic the in vivo micro-environment in the culture of cells and tissues in micro-tissue engineering. Concave microwells are becoming increasingly popular since they can provide a micro-environment that is closer to the in vivo environment compared to traditional microwells, which can facilitate the culture of cells and tissues. Here, we will summarize the fabrication methods of concave microwells, as well as their applications in micro-tissue engineering. The fabrication methods of concave microwells include traditional methods, such as lithography and etching, thermal reflow of photoresist, laser ablation, precision-computerized numerical control (CNC) milling, and emerging technologies, such as surface tension methods, the deformation of soft membranes, 3D printing, the molding of microbeads, air bubbles, and frozen droplets. The fabrication of concave microwells is transferring from professional microfabrication labs to common biochemical labs to facilitate their applications and provide convenience for users. Concave microwells have mostly been used in organ-on-a-chip models, including the formation and culture of 3D cell aggregates (spheroids, organoids, and embryoids). Researchers have also used microwells to study the influence of substrate topology on cellular behaviors. We will briefly review their applications in different aspects of micro-tissue engineering and discuss the further applications of concave microwells. We believe that building multiorgan-on-a-chip by 3D cell aggregates of different cell lines will be a popular application of concave microwells, while integrating physiologically relevant molecular analyses with the 3D culture platform will be another popular application in the near future. Furthermore, 3D cell aggregates from these biosystems will find more applications in drug screening and xenogeneic implantation.

Flipped Well-Plate Hanging-Drop Technique for Growing Three-Dimensional Tumors

Three-dimensional (3D) tumor culture techniques are gaining popularity as in vitro models of tumoral tissue analogues. Despite the widespread interest, need, and present-day effort, most of the 3D tumor culturing methodologies have not gone beyond the inventors’ laboratories. This, in turn, limits their applicability and standardization. In this study, we introduce a straightforward and user-friendly approach based on standard 96-well plates with basic amenities for growing 3D tumors in a scaffold-free/scaffold-based format. Hanging drop preparation can be easily employed by flipping a universal 96-well plate. The droplets of the medium generated by the well-plate flip (WPF) method can be easily modified to address various mechanisms and processes in cell biology, including cancer. To demonstrate the applicability and practicality of the conceived approach, we utilized human colorectal carcinoma cells (HCT116) to first show the generation of large scaffold-free 3D tumor spheroids over 1.5 mm in diameter in single-well plates. As a proof-of-concept, we also demonstrate matrix-assisted tumor culture techniques in advancing the broader use of 3D culture systems. The conceptualized WPF approach can be adapted for a range of applications in both basic and applied biological/engineering research.