Skeletal stem cell isolation: A review on the state-of-the-art microfluidic label-free sorting techniques

Sorting Technology for Mesenchymal Stem Cells from a Single Tissue Source

Mesenchymal stem cells (MSCs) are adult stem cells that can be obtained, enriched and proliferated in vitro. They owned enormous potential in fields like regenerative medicine, tissue engineering and immunomodulation. However, though isolated from the same origin, MSCs are still essentially heterogeneous cell populations with different phenotypes and functions. This heterogeneity of MSCs significantly affects their therapeutic efficacy and brings obstacles to scientific research. Thus, reliable sorting technology which can isolate or purify MSC subpopulations with various potential and differentiation pathways is urgently needed. This review summarized principles, application status and clinical implications for these sorting methods, aiming at improving the understanding of MSC heterogeneity as well as providing fresh perspectives for subsequent clinical applications.

Scaling up stem cell production: harnessing the potential of microfluidic devices

Stem cells are specialised cells characterised by their unique ability to both self-renew and transform into a wide array of specialised cell types. The widespread interest in stem cells for regenerative medicine and cultivated meat has led to a significant demand for these cells in both research and practical applications. Despite the growing need for stem cell manufacturing, the industry faces significant obstacles, including high costs for equipment and maintenance, complicated operation, and low product quality and yield. Microfluidic technology presents a promising solution to the abovementioned challenges. As an innovative approach for manipulating liquids and cells within microchannels, microfluidics offers a plethora of advantages at an industrial scale. These benefits encompass low setup costs, ease of operation and multiplexing, minimal energy consumption, and the added advantage of being labour-free. This review presents a thorough examination of the prominent microfluidic technologies employed in stem cell research and explores their promising applications in the burgeoning stem cell industry. It thoroughly examines how microfluidics can enhance cell harvesting from tissue samples, facilitate mixing and cryopreservation, streamline microcarrier production, and efficiently conduct cell separation, purification, washing, and final cell formulation post-culture.

The Yin and Yang of exosome isolation methods: conventional practice, microfluidics, and commercial kits

Exosomes are a subset of extracellular vesicles released from various cells, and they can be found in different bodily fluids. Exosomes are used as biomarkers to diagnose many diseases and to monitor therapy efficiency as they represent the status and origin of the cell, which they are released from. Considering that they co-exist in bodily fluids with other types of particles, their isolation still remains challenging since conventional separation methods are time-consuming, user-dependent, and result in low isolation yield. This review summarizes the conventional strategies and microfluidic-based methods for exosome isolation along with their strengths and limitations. Microfluidic devices emerge as a promising approach to overcome the limitations of the conventional methods due to their inherent characteristics, such as the need for minute sample volume and rapid operation, in order to isolate exosomes with a high yield and a high purity in a short amount of time, which make them unprecedented tools for molecular biology and clinical applications. This review elaborates on the existing microfluidic-based exosome isolation methods and denotes their benefits and drawbacks. Herein, we also introduce various commercially available platforms and kits for exosome isolation along with their working principles.

Enrichment of Skeletal Stem Cells from Human Bone Marrow Using Spherical Nucleic Acids

Human bone marrow (BM)-derived stromal cells contain a population of skeletal stem cells (SSCs), with the capacity to differentiate along the osteogenic, adipogenic, and chondrogenic lineages, enabling their application to clinical therapies. However, current methods to isolate and enrich SSCs from human tissues remain, at best, challenging in the absence of a specific SSC marker. Unfortunately, none of the current proposed markers alone can isolate a homogeneous cell population with the ability to form bone, cartilage, and adipose tissue in humans. Here, we have designed DNA-gold nanoparticles able to identify and sort SSCs displaying specific mRNA signatures. The current approach demonstrates the significant enrichment attained in the isolation of SSCs, with potential therein to enhance our understanding of bone cell biology and translational applications.

Correlative fluorescence and atomic force microscopy to advance the bio-physical characterisation of co-culture of living cells

An understanding of the cell mechanical properties involved in numerous cellular processes including cell division, cell migration/invasion, and cell morphology, is crucial in developing and informing cell physiology and function. Atomic force microscopy (AFM) offers a powerful biophysical technique that facilitates the imaging of living cells under physiological buffer conditions. However, AFM in isolation cannot discriminate between different cell types within heterogeneous samples for example in a solid biopsy. The current studies demonstrate the potential of AFM in combination with correlative fluorescence optical sectioning microscopy for live cell imaging. Furthermore, this work establishes the advantage of fluorescence-AFM imaging to distinguish and analyse single-cell bio-physical properties in mixed human cell populations, in real-time. Critically, our results show that correlative fluorescence-AFM imaging allows the simultaneous co-localised detection of fluorescence coupled with nano-mechanical mapping. The findings from this work contribute to the promotion and dissemination of correlative multimodal imaging in life sciences, providing a platform for further investigations in biological and pre-clinical research.

The Fabrication and Application Mechanism of Microfluidic Systems for High Throughput Biomedical Screening: A Review

Microfluidic systems have been widely explored based on microfluidic technology, and it has been widely used for biomedical screening. The key parts are the fabrication of the base scaffold, the construction of the matrix environment in the 3D system, and the application mechanism. In recent years, a variety of new materials have emerged, meanwhile, some new technologies have been developed. In this review, we highlight the properties of high throughput and the biomedical application of the microfluidic chip and focus on the recent progress of the fabrication and application mechanism. The emergence of various biocompatible materials has provided more available raw materials for microfluidic chips. The material is not confined to polydimethylsiloxane (PDMS) and the extracellular microenvironment is not limited by a natural matrix. The mechanism is also developed in diverse ways, including its special physical structure and external field effects, such as dielectrophoresis, magnetophoresis, and acoustophoresis. Furthermore, the cell/organ-based microfluidic system provides a new platform for drug screening due to imitating the anatomic and physiologic properties in vivo. Although microfluidic technology is currently mostly in the laboratory stage, it has great potential for commercial applications in the future.

Emerging Trends in Microfluidics Based Devices

One of the major challenges for scientists and engineers today is to develop technologies for the improvement of human health in both developed and developing countries. However, the need for cost-effective, high-performance diagnostic techniques is very crucial for providing accessible, affordable, and high-quality healthcare devices. In this context, microfluidic-based devices (MFDs) offer powerful platforms for automation and integration of complex tasks onto a single chip. The distinct advantage of MFDs lies in precise control of the sample quantities and flow rate of samples and reagents that enable quantification and detection of analytes with high resolution and sensitivity. With these excellent properties, microfluidics (MFs) have been used for various applications in healthcare, along with other biological and medical areas. This review focuses on the emerging demands of MFs in different fields such as biomedical diagnostics, environmental analysis, food and agriculture research, etc., in the last three or so years. It also aims to reveal new opportunities in these areas and future prospects of commercial MFDs.

Image-based sorting and negative dielectrophoresis for high purity cell and particle separation

Microelectrode arrays are used to sort single fluorescently labeled cells and particles as they flow through a microfluidic channel using dielectrophoresis. Negative dielectrophoresis is used to create a "Dielectrophoretic virtual channel" that runs along the center of the microfluidic channel. By switching the polarity of the electrodes, the virtual channel can be dynamically reconfigured to direct particles along a different path. This is demonstrated by sorting particles into two microfluidic outlets, controlled by an automated system that interprets video data from a color camera and makes complex sorting decisions based on color, intensity, size, and shape. This enables the rejection of particle aggregates and other impurities, and the system is optimized to isolate high purity populations from a heterogeneous sample. Green beads are isolated from an excess of red beads with 100% purity at a rate of up to 0.9 particles per second, in addition application to the sorting of osteosarcoma and human bone marrow cells is evidenced. The extension of Dielectrophoretic Virtual Channels to an arbitrary number of sorting outputs is examined, with design, simulation, and experimental verification of two alternate geometries presented and compared.

The Application of Microfluidic Techniques on Tissue Engineering in Orthopaedics

Background:

Tissue engineering (TE) is a promising solution for orthopaedic diseases such as bone or cartilage defects and bone metastasis. Cell culture in vitro and scaffold fabrication are two main parts of TE, but these two methods both have their own limitations. The static cell culture medium is unable to achieve multiple cell incubation or offer an optimal microenvironment for cells, while regularly arranged structures are unavailable in traditional cell-laden scaffolds, which results in low biocompatibility. To solve these problems, microfluidic techniques are combined with TE. By providing 3-D networks and interstitial fluid flows, microfluidic platforms manage to maintain phenotype and viability of osteocytic or chondrocytic cells, and the precise manipulation of liquid, gel and air flows in microfluidic devices leads to the highly organized construction of scaffolds.

Methods:

In this review, we focus on the recent advances of microfluidic techniques applied in the field of tissue engineering, especially in orthropaedics. An extensive literature search was done using PubMed. The introduction describes the properties of microfluidics and how it exploits the advantages to the full in the aspects of TE. Then we discuss the application of microfluidics on the cultivation of osteocytic cells and chondrocytes, and other extended researches carried out on this platform. The following section focuses on the fabrication of highly organized scaffolds and other biomaterials produced by microfluidic devices. Finally, the incubation and studying of bone metastasis models in microfluidic platforms are discussed.

Conclusion:

The combination of microfluidics and tissue engineering shows great potentials in the osteocytic cell culture and scaffold fabrication. Though there are several problems that still require further exploration, the future of microfluidics in TE is promising.

Label-free enrichment of primary human skeletal progenitor cells using deterministic lateral displacement

Skeletal stem cells (SSCs) are present in bone marrow (BM) and offer great potential for bone regenerative therapies. However, in the absence of a unique marker, current sorting approaches remain challenging in the quest for simple strategies to deliver SSCs with consistent regeneration and differentiation capacities. Microfluidics offers the possibility to sort cells marker-free, based on intrinsic biophysical properties. Recent studies indicate that SSCs are stiffer than leukocytes and are contained within the larger cell fraction in BM. This paper describes the use of deterministic lateral displacement (DLD) to sort SSCs based on cell size and stiffness. DLD is a technology that uses arrays of micropillars to sort cells based on their diameter. Cell deformation within the device can change the cell size and affect sorting - here evidenced using human cell lines and by fractionation of expanded SSCs. Following sorting, SSCs remained viable and retained their capacity to form clonogenic cultures (CFU-F), indicative of stem cell potential. Additionally, larger BM cells showed enhanced capacity to form CFU-F. These findings support the theory that SSCs are more abundant within the larger BM cell fraction and that DLD, or other size-based approaches, could be used to provide enriched SSC populations with significant implications for stem cell research and translation to the clinic.

Magnetically Induced Flow Focusing of Non-Magnetic Microparticles in Ferrofluids under Inclined Magnetic Fields

The ability to focus biological particles into a designated position of a microchannel is vital for various biological applications. This paper reports particle focusing under vertical and inclined magnetic fields. We analyzed the effect of the angle of rotation (θ) of the permanent magnets and the critical Reynolds number (Re_c) on the particle focusing in depth. We found that a rotation angle of 10° is preferred; a particle loop has formed when Re < Re_c and Re_c of the inclined magnetic field is larger than that of the vertical magnetic field. We also conducted experiments with polystyrene particles (10.4 μm in diameter) to prove the calculations. Experimental results show that the focusing effectiveness improved with increasing applied magnetic field strength or decreasing inlet flow rate.

New advances in microfluidic flow cytometry

In recent years, researchers are paying the increasing attention to the development of portable microfluidic diagnostic devices including microfluidic flow cytometry for the point-of-care testing. Microfluidic flow cytometry, where microfluidics and flow cytometry work together to realize novel functionalities on the microchip, provides a powerful tool for measuring the multiple characteristics of biological samples. The development of a portable, low-cost, and compact flow cytometer can benefit the health care in underserved areas such as Africa or Asia. In this article, we review recent advancements of microfluidics including sample pumping, focusing and sorting, novel detection approaches, and data analysis in the field of flow cytometry. The challenge of microfluidic flow cytometry is also examined briefly.

Size and dielectric properties of skeletal stem cells change critically after enrichment and expansion from human bone marrow: consequences for microfluidic cell sorting

The capacity of bone and cartilage to regenerate can be attributed to skeletal stem cells (SSCs) that reside within the bone marrow (BM). Given SSCs are rare and lack specific surface markers, antibody-based sorting has failed to deliver the cell purity required for clinical translation. Microfluidics offers new methods of isolating cells based on biophysical features including, but not limited to, size, electrical properties and stiffness. Here we report the characterization of the dielectric properties of unexpanded SSCs using single-cell microfluidic impedance cytometry (MIC). Unexpanded SSCs had a mean size of 9.0 µm; larger than the majority of BM cells. During expansion, often used to purify and increase the number of SSCs, cell size and membrane capacitance increased significantly, highlighting the importance of characterizing unaltered SSCs. In addition, MIC was used to track the osteogenic differentiation of SSCs and showed an increased membrane capacitance with differentiation. The electrical properties of primary SSCs were indistinct from other BM cells precluding its use as an isolation method. However, the current studies indicate that cell size in combination with another biophysical parameter, such as stiffness, could be used to design label-free devices for sorting SSCs with significant clinical impact.

Senescence chips for ultrahigh-throughput isolation and removal of senescent cells

Cellular senescence plays an important role in organismal aging and age-related diseases. However, it is challenging to isolate low numbers of senescent cells from small volumes of biofluids for downstream analysis. Furthermore, there is no technology that could selectively remove senescent cells in a high-throughput manner. In this work, we developed a novel microfluidic chip platform, termed senescence chip, for ultrahigh-throughput isolation and removal of senescent cells. The core component of our senescence chip is a slanted and tunable 3D micropillar array with a variety of shutters in the vertical direction for rapid cell sieving, taking advantage of the characteristic cell size increase during cellular senescence. The 3D configuration achieves high throughput, high recovery rate, and device robustness with minimum clogging. We demonstrated proof-of-principle applications in isolation and enumeration of senescent mesenchymal stem cells (MSCs) from undiluted human whole blood, and senescent cells from mouse bone marrow after total body irradiation, with the single-cell resolution. After scale-up to a multilayer and multichannel structure, our senescence chip achieved ultrahigh-throughput removal of senescent cells from human whole blood with an efficiency of over 70% at a flow rate of 300 ml/hr. Sensitivity and specificity of our senescence chips could be augmented with implementation of multiscale size separation, and identification of background white blood cells using their cell surface markers such as CD45. With the advantages of high throughput, robustness, and simplicity, our senescence chips may find wide applications and contribute to diagnosis and therapeutic targeting of cellular senescence.

A high throughput microfluidic platform for size-selective enrichment of cell populations in tissue and blood samples

We present an integrated platform for highly selective separation and enrichment of cells from blood and tissue samples.