A tunable micro filter modulated by pneumatic pressure for cell separation

NanoRidge filters: Fabrication strategies and performance optimization for nano-scale microfluidic particle filtration

Filters with high throughput, minimal dead volume, and greater sensitivity to particle size are needed, which traditional benchtop filtration cannot provide. Leveraging microfabrication techniques developed by the electronics and optics industries, the filters presented here feature a unique serpentine "NanoRidge" structure, offering a continuous filtration gap spanning over three meters on a compact 4 × 14.5 mm2 footprint. This design provides more precise size filtration cut-offs and consistent flow paths compared to traditional membrane filtration systems. Despite challenges associated with glass substrate deformation impacting uniform filter gap sizes, the study provides valuable insights into the development of NanoRidge filters (NRFs) for enhancing filtration efficiency in preparatory techniques and sample analysis. This study describes the fabrication and testing of these new filter types and directly compares the performance to traditional membrane filters using the metrics of particle size cut-off (the smallest difference in particle size which can be filtered vs passed) and particle loss. The NanoRidge filters were characterized using imaging (during fabrication, post-fabrication and use, fluorescent particles captured and small molecule dye), pressure and flow measurements, and a series of particle sizes "filter or pass" studies. Particle capacity (100-250 nm) ranged from 5 × 108 to 7 × 109 in 1 ml samples at a flow rate of 100 μl/min with backpressure in the range of 1-3 Bar. The optimized fabrication procedure for the 150 nm NRF yielded a small particle recovery of 95% while also achieving a large particle filtration of 73%. High filtration efficiency was also proven in the final 60 and 80 nm NRF fabrication procedures at 96% and 91%, respectively.

Tunable microfluidic chip for single-cell deformation study

Microfluidic phenotyping methods have been of vital importance for cellular characterization, especially for evaluating single cells. In order to study the deformability of a single cell, we devised and tested a tunable microfluidic chip-based method. A pneumatic polymer polydimethylsiloxane (PDMS) membrane was designed and fabricated abutting a single-cell trapping structure, so the cell could be squeezed controllably in a lateral direction. Cell contour changes under increasing pressure were recorded, enabling the deformation degree of different types of single cell to be analyzed and compared using computer vision. This provides a new perspective for studying mechanical properties of cells at the single cell level.

Fabrication of a Pneumatic Microparticle Concentrator

We developed a microfluidic platform employing (normally open) pneumatic valves for particle concentration. The device features a three-dimensional network with a curved fluidic channel and three pneumatic valves (a sieve valve (Vs) that concentrates particles and two ON/OFF rubber-seal pneumatic valves that block the working fluid). Double-sided replication employing polydimethylsiloxane (PDMS) was used to fabricate the network, channel, and chamber. Particles were blocked by deformation of the Vs diaphragm, and then accumulated in the curved microfluidic channel. The working fluid was discharged via operation of the two ON/OFF valves. After concentration, particles were released to an outlet port. The Vs pressure required to block solid particles varying in diameter was determined based on the height of the curved microchannel and a finite element method (FEM) simulation of Vs diaphragm displacement. Our method was verified according to the temporal response of the fluid flow rate controlled by the pneumatic valves. Furthermore, all particles with various diameters were successfully blocked, accumulated, and released. The operating pressure, time required for concentration, and concentration ratio were dependent on the particle diameter. The estimated concentration percentage of 24.9 µm diameter polystyrene particles was about 3.82% for 20 min of operation.

One step fabrication of mesh-reinforced hierarchic perforated microporous honeycomb films with tunable filtering property

Highly ordered porous films whose pore size ranges from submicron to micron scale have always been an extensive area of research due to their broad range of application to photonic crystals, cell culturing scaffolds, filtration and separation membranes, just to name a few. However, the fragile nature of such a functional porous film has hindered its implementation to advanced uses. Inspired by the hierarchic structure in nature which offers both robustness and functionality, we created in a single fabrication step a mesh-reinforced hierarchic perforated honeycomb film with highly ordered micron pores using the breath figure method. By using the elastomer 1,2-polybutadiene as the material for the film in the combination of the mesh grid, the pore size of the obtained film can be tuned upon stretching. Tubular structures made from the mesh-reinforced hierarchic perforated porous honeycomb film with tunable pore size have been demonstrated.

Clogging-free microfluidics for continuous size-based separation of microparticles

In microfluidic filtration systems, one of the leading obstacles to efficient, continuous operation is clogging of the filters. Here, we introduce a lateral flow microfluidic sieving (μ-sieving) technique to overcome clogging and to allow continuous operation of filter based microfluidic separation. A low frequency mechanical oscillation was added to the fluid flow, which made possible the release of aggregated unwanted polystyrene (PS) particles trapped between the larger target PS particles in the filter demonstrating continuous μ-sieving operation. We achieved collection of the target PS particles with 100% separation efficiency. Also, on average, more than 98% of the filtered target particles were retrieved after the filtration showing high retrieval rates. Since the oscillation was applied to the fluid but not to the microfluidic filter system, mechanical stresses to the system was minimized and no additional fabrication procedures were necessary. We also applied the μ-sieving technique to the separation of cancer cells (MDA-MB-231) from whole blood and showed that the fluidic oscillations prevented the filters from being blocked by the filtered cancer cells allowing continuous microfluidic separation with high efficiency.

Tunable Microfluidic Devices for Hydrodynamic Fractionation of Cells and Beads: A Review

The adjustable microfluidic devices that have been developed for hydrodynamic-based fractionation of beads and cells are important for fast performance tunability through interaction of mechanical properties of particles in fluid flow and mechanically flexible microstructures. In this review, the research works reported on fabrication and testing of the tunable elastomeric microfluidic devices for applications such as separation, filtration, isolation, and trapping of single or bulk of microbeads or cells are discussed. Such microfluidic systems for rapid performance alteration are classified in two groups of bulk deformation of microdevices using external mechanical forces, and local deformation of microstructures using flexible membrane by pneumatic pressure. The main advantage of membrane-based tunable systems has been addressed to be the high capability of integration with other microdevice components. The stretchable devices based on bulk deformation of microstructures have in common advantage of simplicity in design and fabrication process.

Size and deformability based separation of circulating tumor cells from castrate resistant prostate cancer patients using resettable cell traps

The enumeration and capture of circulating tumor cells (CTCs) are potentially of great clinical value as they offer a non-invasive means to access tumor materials to diagnose disease and monitor treatment efficacy. Conventional immunoenrichment of CTCs may fail to capture cells with low surface antigen expression. Micropore filtration presents a compelling label-free alternative that enriches CTCs using their biophysical rather than biochemical characteristics. However, this strategy is prone to clogging of the filter microstructure, which dramatically reduces the selectivity after processing large numbers of cells. Here, we use the resettable cell trap (RCT) mechanism to separate cells based on their size and deformability using an adjustable aperture that can be periodically cleared to prevent clogging. After separation, the output sample is stained and analyzed using multi-spectral analysis, which provides a more sensitive and unambiguous method to identify CTC biomarkers than traditional immunofluorescence. We tested the RCT device using blood samples obtained from 22 patients with metastatic castrate-resistant prostate cancer while comparing the results with the established CellSearch® system. The RCT mechanism was able to capture ≥5 CTCs in 18/22 (82%) patients with a mean count of 257 in 7.5 ml of whole blood, while the CellSearch system found ≥5 CTCs in 9/22 (41%) patients with a mean count of 25. The ~10× improvement in the CTC capture rate provides significantly more materials for subsequent analysis of these cells such as immunofluorescence, propagation by tissue culture, and genetic profiling.

On-chip sample preparation for complete blood count from raw blood

This paper describes a monolithic microfluidic device capable of on-chip sample preparation for both RBC and WBC measurements from whole blood. For the first time, on-chip sample processing (e.g. dilution, lysis, and filtration) and downstream single cell measurement were fully integrated to enable sample preparation and single cell analysis from whole blood on a single device. The device consists of two parallel sub-systems that perform sample processing and electrical measurements for measuring RBC and WBC parameters. The system provides a modular environment capable of handling solutions of various viscosities by adjusting the length of channels and precisely controlling mixing ratios, and features a new 'offset' filter configuration for increased duration of device operation. RBC concentration, mean corpuscular volume (MCV), cell distribution width, WBC concentration and differential are determined by electrical impedance measurement. Experimental characterization of over 100,000 cells from 10 patient blood samples validated the system's capability for performing on-chip raw blood processing and measurement.

Clog-free cell filtration using resettable cell traps

The separation of cells by filtration through microstructured constrictions is limited by clogging and adsorption, which reduce selectivity and prevent the extraction of separated cells. To address this key challenge, we developed a mechanism for simply and reliably adjusting the cross-section of a microfluidic channel to selectively capture cells based on a combination of size and deformability. After a brief holding period, trapped cells can then be released back into flow, and if necessary, extracted for subsequent analysis. Periodically clearing filter constrictions of separated cells greatly improves selectivity and throughput, and minimizes adsorption of cells to the filter microstructure. This mechanism is capable of discriminating cell-sized polystyrene microspheres with <1 μm resolution. Rare cancer cells doped into leukocytes can be enriched ~1800× with ~90% yield despite a significant overlap in size between these cell types. An important characteristic of this process is that contaminant leukocytes are captured by non-specific adsorption and not mechanical constraint, enabling repeated filtration to improve performance. The throughput of this mechanism is 900,000 cells per hour for 32 multiplexed microchannels, or ~1,200,000 cells cm⁻² h⁻¹ on a per area basis, which exceeds existing micropore filtration mechanisms by a factor of 20.

High-purity and label-free isolation of circulating tumor cells (CTCs) in a microfluidic platform by using optically-induced-dielectrophoretic (ODEP) force

Negative selection-based circulating tumor cell (CTC) isolation is believed valuable to harvest more native, and in particular all possible CTCs without biases relevant to the properties of surface antigens on the CTCs. Under such a cell isolation strategy, however, the CTC purity is normally compromised. To address this issue, this study reports the integration of optically-induced-dielectrophoretic (ODEP) force-based cell manipulation, and a laminar flow regime in a microfluidic platform for the isolation of untreated, and highly pure CTCs after conventional negative selection-based CTC isolation. In the design, six sections of moving light-bar screens were continuously and simultaneously exerted in two parallel laminar flows to concurrently separate the cancer cells from the leukocytes based on their size difference and electric properties. The separated cell populations were further partitioned, delivered, and collected through the two flows. With this approach, the cancer cells can be isolated in a continuous, effective, and efficient manner. In this study, the operating conditions of ODEP for the manipulation of prostate cancer (PC-3) and human oral cancer (OEC-M1) cells, and leukocytes with minor cell aggregation phenomenon were first characterized. Moreover, performances of the proposed method for the isolation of cancer cells were experimentally investigated. The results showed that the presented CTC isolation scheme was able to isolate PC-3 cells or OEC-M1 cells from a leukocyte background with high recovery rate (PC-3 cells: 76-83%, OEC-M1 cells: 61-68%), and high purity (PC-3 cells: 74-82%, OEC-M1 cells: 64-66%) (set flow rate: 0.1 μl min(-1) and sample volume: 1 μl). The latter is beyond what is currently possible in the conventional CTC isolations. Moreover, the viability of isolated cancer cells was evaluated to be as high as 94 ± 2%, and 95 ± 3% for the PC-3, and OEC-M1 cells, respectively. Furthermore, the isolated cancer cells were also shown to preserve their proliferative capability. As a whole, this study has presented an ODEP-based microfluidic platform that is capable of isolating CTCs in a continuous, label-free, cell-friendly, and particularly highly pure manner. All these traits are found particularly meaningful for exploiting the harvested CTCs for the subsequent cell-based, or biochemical assays.

Cell separation based on size and deformability using microfluidic funnel ratchets

The separation of biological cells by filtration through microstructured constrictions is limited by unpredictable variations of the filter hydrodynamic resistance as cells accumulate in the microstructure. Applying a reverse flow to unclog the filter will undo the separation and reduce filter selectivity because of the reversibility of low-Reynolds number flow. We introduce a microfluidic structural ratchet mechanism to separate cells using oscillatory flow. Using model cells and microparticles, we confirmed the ability of this mechanism to sort and separate cells and particles based on size and deformability. We further demonstrate that the spatial distribution of cells after sorting is repeatable and that the separation process is irreversible. This mechanism can be applied generally to separate cells that differ based on size and deformability.

Integrated three-dimensional filter separates nanoscale from microscale elements in a microfluidic chip

We report on the integration of a size-based three-dimensional filter, with micrometre-sized pores, in a commercial microfluidic chip. The filter is fabricated inside an already sealed microfluidic channel using the unique capabilities of two-photon polymerization. This direct-write technique enables integration of the filter by post-processing in a chip that has been fabricated by standard technologies. The filter is located at the intersection of two channels in order to control the amount of flow passing through the filter. Tests with a suspension of 3 μm polystyrene spheres in a Rhodamine 6G solution show that 100% of the spheres are stopped, while the fluorescent molecules are transmitted through the filter. We demonstrate operation up to a period of 25 minutes without any evidence of clogging. Preliminary validation of the device for plasma separation from whole blood is shown. Moreover, the filter can be cleaned and reused by reversing the flow.

The application of an optically switched dielectrophoretic (ODEP) force for the manipulation and assembly of cell-encapsulating alginate microbeads in a microfluidic perfusion cell culture system for bottom-up tissue engineering

This study reports the utilisation of an optically switched dielectrophoretic (ODEP) force for the manipulation and assembly of cell-encapsulating alginate microbeads in a microfluidic perfusion cell culture system for bottom-up tissue engineering. One of the key features of this system is the ODEP force-based mechanism, which allows a commercial projector to be coupled with a computer to manipulate and assemble cell-encapsulating microbeads in an efficient, manageable, and user-friendly manner. Another distinctive feature is the design of the microfluidic cell culture chip, which allows the patterned cell-encapsulating microbeads to be cultivated on site under culture medium perfusion conditions. For demonstrating its application in bottom-up cartilage tissue engineering, chondrocyte-encapsulating alginate microbeads varying in encapsulated cell densities were generated. The manipulation forces associated with operating the alginate microbeads were experimentally evaluated. The results revealed that the measured manipulation forces increased with increases in both the applied electric voltage and the number of cells in the alginate microbeads. Nevertheless, the observed manipulation force was found to be independent of the size of the cell-free alginate microbeads. It can be speculated that the friction force may influence the estimation of the ODEP force within the experimental conditions investigated. In this study, chondrocyte-encapsulating alginate microbeads with three different cell densities were manipulated and assembled in the proposed microfluidic system to form a compact sheet-like cell culture construct that imitates the cell distribution in the cross-section of native articular cartilage. Moreover, the demonstration case also showed that the cell viability of the cultured cells in the microfluidic system remained as high as 96 ± 2%. In this study, four sheet-like cell culture constructs were stacked to create a larger assembled cell culture construct. The cell distribution inside the cell culture construct was further confirmed by a confocal microscopy observation, which showed that the distribution was similar to that in native articular cartilage. As a whole, the proposed system holds great promise as a platform for engineering tissue constructs with easily tunable inner cell distributions.

System Integration - A Major Step toward Lab on a Chip

Microfluidics holds great promise to revolutionize various areas of biological engineering, such as single cell analysis, environmental monitoring, regenerative medicine, and point-of-care diagnostics. Despite the fact that intensive efforts have been devoted into the field in the past decades, microfluidics has not yet been adopted widely. It is increasingly realized that an effective system integration strategy that is low cost and broadly applicable to various biological engineering situations is required to fully realize the potential of microfluidics. In this article, we review several promising system integration approaches for microfluidics and discuss their advantages, limitations, and applications. Future advancements of these microfluidic strategies will lead toward translational lab-on-a-chip systems for a wide spectrum of biological engineering applications.

Stem cells in microfluidics

Microfluidic techniques have been recently developed for cell-based assays. In microfluidic systems, the objective is for these microenvironments to mimic in vivo surroundings. With advantageous characteristics such as optical transparency and the capability for automating protocols, different types of cells can be cultured, screened, and monitored in real time to systematically investigate their morphology and functions under well-controlled microenvironments in response to various stimuli. Recently, the study of stem cells using microfluidic platforms has attracted considerable interest. Even though stem cells have been studied extensively using bench-top systems, an understanding of their behavior in in vivo-like microenvironments which stimulate cell proliferation and differentiation is still lacking. In this paper, recent cell studies using microfluidic systems are first introduced. The various miniature systems for cell culture, sorting and isolation, and stimulation are then systematically reviewed. The main focus of this review is on papers published in recent years studying stem cells by using microfluidic technology. This review aims to provide experts in microfluidics an overview of various microfluidic systems for stem cell research.