Microvalves and pumps provide precise fluid handling in microfluidic devices for biomedical applications, yet achieving flow stability and controlled storage and release of fluid remains a significant challenge due to the pulsatile nature of the actuation. We present the development and characterization of a programmable microfluidic capacitor (PMC) designed for controlled storage and release of fluid in microfluidic systems. The PMC was designed with a resistor–capacitor system consisting of a cell culture chamber that accumulates fluid in response to the pressure-driven actuation of lifting-gate valves, which stabilizes fluid flow. The system's fluidic capacitance (Cf) was determined to be 4.537 nl/kPa, demonstrating a linear volume–pressure relationship. The charging and discharging dynamics were characterized and modeled, yielding a charging actuation time constant of 327.5 ms and a discharging actuation time of 763.94 ms. Additionally, we observed a significant reduction in cell detachment, with only 3% of cells removed during actuation. To validate the PMC's functionality, we performed immunocytochemical staining for microglia and astrocytes under inflammatory conditions. Astrocytes tested complement-C3 positive, while microglia were identified by transmembrane protein 119 (TMEM119). The PMC optimized reagent delivery and stabilized flow around cells, making it a reliable system for automated cell culture and organ-on-a-chip assays.
Metastatic breast cancer has been shown to be aggressive and drug resistant, yet it is unclear what factors drive this altered phenotype at the secondary site. During metastatic spread, cancer cells are exposed to fluid shear stress (FSS), the force acting on the cell parallel to the endothelium. Previous studies have shown that exposure to FSS induces proliferation, so it was hypothesized that exposure to FSS can induce additional altered phenotypes such as enhanced migratory capabilities. This study developed a modular microfluidic approach to expose triple-negative breast cancer (TNBC) cells to FSS and study one-dimensional (1D) migration. 1D confined migration was selected as studies have shown that the underlying mechanisms are more similar between 1D and 3D migration when compared to 2D. TNBC cells were first sheared in a 1 m long serpentine channel that was paired with a migratory microfluidic device containing 7 μm channels to allow for 1D migration. Single-cell trajectories were tracked to compare speed, distance, and displacement between sheared and non-sheared cohorts. Initially, population-averaged analysis revealed no statistically significant difference between the cohorts; however, stratifying the cells into subpopulations based on displacement revealed distinct migratory behaviors. Sheared cells in the high displacement subpopulation exhibited a greater average distance and directional persistence compared to the non-sheared cohort, while sheared cells in the intermediate displacement subpopulation exhibited reduced migration distance. These findings suggest that FSS selectively enhances the migratory behavior of select subpopulations, providing a new insight into how biomechanical cues like FSS alter 1D migration.
Due to its non-contact, label-free, penetrable nature and favorable biocompatibility, acoustic manipulation has become an essential technique for handling microparticles, cells, droplets, soft materials, and microtissues. Existing approaches largely rely on acoustic radiation-force models for Rayleigh-scale particles. When object dimensions approach or exceed the acoustic wavelength, higher-order scattering, many-body coupling and boundary effects markedly reshape the local field. This leads to deviations between the prescribed acoustic environment and the actual force landscape. Focusing on the resulting bottleneck in complex biological systems, this work systematically examines the distinct acoustic responses across Rayleigh, Mie, and geometrical regimes and outlines a design strategy for a cross-scale acoustic manipulation platform. This provides a reference for a unified Rayleigh–Mie-geometrical co-manipulation model spanning continuous length scales and holds promise for delivering new engineering modalities for organoid microenvironment construction.
Rapid microscopic examination of suspended microorganisms, cells, and micrometer-scale particles is often limited by the time required for samples to reach a stable and observable distribution. Conventional counting chambers rely mainly on passive sedimentation, which can require tens of minutes before reliable imaging and counting, whereas automated imaging or flow-based systems improve throughput at the cost of increased equipment complexity. Here, we introduce acoustic planar concentration (APC), a compact pulsed bulk-acoustic strategy for improving the single-plane observability of suspended micro-objects. APC is implemented in a conventional platelet-counting chamber with a 100 μm-high observation cavity, without redesigning the chamber into a microfluidic resonator. An approximately 4 MHz pulsed acoustic field is coupled into the standard slide-based chamber, where acoustically induced particle redistribution improves single-plane observation. Pulsed excitation was used to avoid prolonged uninterrupted acoustic driving, while an external acoustic waveguide defines the actuation region. Using 5 and 2 μm polystyrene fluorescent particles as model objects, we demonstrate improved single-plane observability within seconds under the tested conditions. For 2 μm particles, the fraction of detected particle images classified as in focus increased from 76.1 ± 6.5% to 90.6 ± 2.5% within approximately 5 s. APC was further demonstrated using Chlorella sorokiniana cells of comparable size, for which the detected cell count within the selected observation plane increased by approximately 2.6-fold after actuation. This approach provides a compact, contact-free route compatible with conventional counting chambers and an inverted fluorescence microscopy setup.
Microfluidic devices containing artificial cellular membranes have been developed for biology, biosensors, and bioengineering. Although forming a lipid bilayer between two microchannels has advantages for integrating cellular membrane functions into continuous chemical processing and evaluating cellular membrane responses under fluid-flow-induced shear stress, it typically suffers from complex operation of external pumps and contamination of the lipid bilayer by residual organic solvent. This study presents a simplified formation process of an ultrasmall lipid bilayer between parallel micrometer channels leveraging dominant surface tension effects. Utilizing 4 μm channels with partial hydrophobic modification, the simple operation of a parallel two-phase flow involving an aqueous phase and a lipid-containing organic phase (chloroform) was achieved under a controlled 101 kPa Laplace pressure. After parallel two-phase flow was formed to create a lipid monolayer at the aqueous/organic interface, the organic phase was replaced with the aqueous phase, maintaining the interface position via Laplace pressure. A 2.2 × 3.7 μm2 lipid bilayer with a thickness of 4.2–6.4 nm and reduced contamination was successfully formed, as verified by fluorescence and capacitance measurements. To demonstrate the functionality of this parallel-channel platform, the stability of the lipid bilayer was evaluated under shear stress induced by pressure-driven flows. The results indicated that the bilayer partially ruptured at a shear rate higher than 0.27 μs−1, suggesting a critical shear rate that induces membrane instability. This work demonstrates the feasibility of the developed in vitro platform for studying cellular membranes at the subcellular level and conducting fL-chemical operations using cellular membrane functions.
