Estimation of disruption of animal cells by laminar shear stress

Mesenchymal Stem Cell and Hematopoietic Stem and Progenitor Cell Co-Culture in a Bone-Marrow-on-a-Chip Device toward the Generation and Maintenance of the Hematopoietic Niche

Bone marrow has raised a great deal of scientific interest, since it is responsible for the vital process of hematopoiesis and is affiliated with many normal and pathological conditions of the human body. In recent years, organs-on-chips (OoCs) have emerged as the epitome of biomimetic systems, combining the advantages of microfluidic technology with cellular biology to surpass conventional 2D/3D cell culture techniques and animal testing. Bone-marrow-on-a-chip (BMoC) devices are usually focused only on the maintenance of the hematopoietic niche; otherwise, they incorporate at least three types of cells for on-chip generation. We, thereby, introduce a BMoC device that aspires to the purely in vitro generation and maintenance of the hematopoietic niche, using solely mesenchymal stem cells (MSCs) and hematopoietic stem and progenitor cells (HSPCs), and relying on the spontaneous formation of the niche without the inclusion of gels or scaffolds. The fabrication process of this poly(dimethylsiloxane) (PDMS)-based device, based on replica molding, is presented, and two membranes, a perforated, in-house-fabricated PDMS membrane and a commercial poly(ethylene terephthalate) (PET) one, were tested and their performances were compared. The device was submerged in a culture dish filled with medium for passive perfusion via diffusion in order to prevent on-chip bubble accumulation. The passively perfused BMoC device, having incorporated a commercial poly(ethylene terephthalate) (PET) membrane, allows for a sustainable MSC and HSPC co-culture and proliferation for three days, a promising indication for the future creation of a hematopoietic bone marrow organoid.

The tardigrade-derived mitochondrial abundant heat soluble protein improves adipose-derived stem cell survival against representative stressors

Human adipose-derived stem cell (ASC) grafts have emerged as a powerful tool in regenerative medicine. However, ASC therapeutic potential is hindered by stressors throughout their use. Here we demonstrate the transgenic expression of the tardigrade-derived mitochondrial abundant heat soluble (MAHS) protein for improved ASC resistance to metabolic, mitochondrial, and injection shear stress. In vitro, MAHS-expressing ASCs demonstrate up to 61% increased cell survival following 72 h of incubation in phosphate buffered saline containing 20% media. Following up to 3.5% DMSO exposure for up to 72 h, a 14-49% increase in MAHS-expressing ASC survival was observed. Further, MAHS expression in ASCs is associated with up to 39% improved cell viability following injection through clinically relevant 27-, 32-, and 34-gauge needles. Our results reveal that MAHS expression in ASCs supports survival in response to a variety of common stressors associated with regenerative therapies, thereby motivating further investigation into MAHS as an agent for stem cell stress resistance. However, differentiation capacity in MAHS-expressing ASCs appears to be skewed in favor of osteogenesis over adipogenesis. Specifically, activity of the early bone formation marker alkaline phosphatase is increased by 74% in MAHS-expressing ASCs following 14 days in osteogenic media. Conversely, positive area of the neutral lipid droplet marker BODIPY is decreased by up to 10% in MAHS-transgenic ASCs following 14 days in adipogenic media. Interestingly, media supplementation with up to 40 mM glucose is sufficient to restore adipogenic differentiation within 14 days, prompting further analysis of mechanisms underlying interference between MAHS and differentiation processes.

Improving product quality and productivity of an antibody-based biotherapeutic using inverted frustoconical shaking bioreactors

The Chinese hamster ovarian (CHO) cells serve as a common choice in biopharmaceutical production, traditionally cultivated in stirred tank bioreactors (STRs). Nevertheless, the pursuit of improved protein quality and production output for commercial purposes demand exploration into new bioreactor types. In this context, inverted frustoconical shaking bioreactors (IFSB) present unique physical properties distinct from STRs. This study aims to compare the production processes of an antibody-based biotherapeutic in both bioreactor types, to enhance production flexibility. The findings indicate that, when compared to STRs, IFSB demonstrates the capability to produce an antibody-based biotherapeutic with either comparable or enhanced bioprocess performance and product quality. IFSB reduces shear damage to cells, enhances viable cell density (VCD), and improves cell state at a 5-L scale. Consequently, this leads to increased protein expression (3.70 g/L vs 2.56 g/L) and improved protein quality, as evidenced by a reduction in acidic variants from 27.0% to 21.5%. Scaling up the culture utilizing the Froude constant and superficial gas velocity ensures stable operation, effective mixing, and gas transfer. The IFSB maintains a high VCD and cell viability at both 50-L and 500-L scales. Product expression levels range from 3.0 to 3.6 g/L, accompanied by an improved acidic variants attribute of 20.6%-22.7%. The IFSB exhibits superior productivity and product quality, underscoring its potential for incorporation into the manufacturing process for antibody-based biotherapeutics. These results establish the foundation for IFSB to become a viable option in producing antibody-based biotherapeutics for clinical and manufacturing applications.

Leakage Dynamics of Glass Bottles on Container Closure Integrity Testing: Influence of Different Laser-Drilled Microhole Geometries

Container closure integrity testing (CCIT) is a critical step in ensuring package integrity and providing feedback on package designs. In practical applications, CCIT methods, namely physical and probabilistic methods, must be appropriately selected and validated to ensure their suitability for the intended use. However, the industry still lacks practical recommendations regarding the choice of CCIT methods and artificial leaks to set the acceptance criteria. The main reason is the lack of correlation between testing methods. Artificially introduced leak microholes are the only way to determine the sensitivity of a CCIT method and to implement the method correlation. However, the type of artificial leakage is a key factor because in most studies, leakage is described and valued using a single parameter, such as size. This can significantly affect the credibility of the relevant test results, especially in the case of microbial invasion, where the difference in test conditions and samples will severely affect the probability of microbial invasion. Therefore, it is vital to conduct a systematic study on the influence of leakage conditions on CCIT methods. In this study, the influence of the shapes of artificial leaks on the two kinds of testing methods was systematically studied based on a laser-drilled microhole-a highly potential and non-exogenous artificial leak manufacturing method that can fabricate different leakage geometries. The reason for the influence of the shape of an artificial leak on the CCIT is that the deterministic method takes defects as an idealized model and ignores the influence of the leak shape, wall thickness, and other factors on leakage and pollution risks. However, these factors seriously affect the dynamic process of leakage and microbial invasion. The pressure decay method is used to test the leakage flow rate of conical and straight holes. Microbial challenge tests are then used to verify the impact of leakage shapes on the pollution risk. The results of the tests indicated that the probability of microbial invasion in the conical holes is much higher than that in straight holes with the same flow test results and that the wall thickness can also affect microbial invasion. Thus, it can be proven that the risk of leakage and invasion or the sensitivity of different methods cannot only be compared through the leak diameter. Numerous influencing factors, including leakage geometry (e.g., shape and thickness), must be considered in practical applications.

Cytoprotection of Human Progenitor and Stem Cells through Encapsulation in Alginate Templated, Dual Crosslinked Silk and Silk-Gelatin Composite Hydrogel Microbeads

Susceptibility of mammalian cells against harsh processing conditions limit their use in cell transplantation and tissue engineering applications. Besides modulation of the cell microenvironment, encapsulation of mammalian cells within hydrogel microbeads attract attention for cytoprotection through physical isolation of the encapsulated cells. The hydrogel formulations used for cell microencapsulation are largely dominated by ionically crosslinked alginate (Alg), which suffer from low structural stability under physiological culture conditions and poor cell-matrix interactions. Here the fabrication of Alg templated silk and silk/gelatin composite hydrogel microspheres with permanent or on-demand cleavable enzymatic crosslinks using simple and cost-effective centrifugation-based droplet processing are demonstrated. The composite microbeads display structural stability under ion exchange conditions with improved mechanical properties compared to ionically crosslinked Alg microspheres. Human mesenchymal stem and neural progenitor cells are successfully encapsulated in the composite beads and protected against environmental factors, including exposure to polycations, extracellular acidosis, apoptotic cytokines, ultraviolet (UV) irradiation, anoikis, immune recognition, and particularly mechanical stress. The microbeads preserve viability, growth, and differentiation of encapsulated stem and progenitor cells after extrusion in viscous polyethylene oxide solution through a 27-gauge fine needle, suggesting potential applications in injection-based delivery and three-dimensional bioprinting of mammalian cells with higher success rates.

Prediction of vascular injury by cavitation microbubbles in a focused ultrasound field

Many studies have shown that microbubble cavitation is one mechanism for vascular injury under ultrasonic excitation. Previous work has attributed vascular damage to vessel expansions and invaginations due to the expansion and contraction of microbubbles. However, the mechanisms of vascular damage are not fully understood. In this paper, we investigate, theoretically and experimentally, the vessel injury due to stress induced by ultrasound-induced cavitation (UIC). A bubble-fluid-vessel coupling model is constructed to investigate the interactions of the coupling system. The dynamics process of vessel damage due to UIC is theoretically simulated with a finite element method, and a focused ultrasound (FU) setup is carried out and used to assess the vessel damage. The results show that shear stress contributes to vessel injury by cell detachment while normal stress mainly causes distention injury. Similar changes in cell detachment in a vessel over time can be observed with the experimental setup. The severity of vascular injury is correlated to acoustic parameters, bubble-wall distance, and microbubble sizes, and the duration of insonation..

Air bubble removal: Wettability contrast enabled microfluidic interconnects

The presence of air bubbles boosts the shear resistance and causes pressure fluctuation within fluid-perfused microchannels, resulting in possible cell damage and even malfunction of microfluidic devices. Eliminating air bubbles is especially challenging in microscale where the adhesive surface tension force is often dominant over other forces. Here, we present an air bubble removal strategy from a novel surface engineering perspective. A microfluidic port-to-port interconnect was fabricated by modifying the peripheral of the microfluidic ports superhydrophobic, while maintaining the inner polymer microchannels hydrophilic. Such a sharp wettability contrast enabled a preferential fluidic entrance into the easy-wetting microchannels over the non-wetting boundaries of the microfluidic ports, while simultaneously filtering out any incoming air bubbles owing to the existence of port-to-port gaps. This bubble-eliminating capability was consistently demonstrated at varying flow rates and liquid analytes. Compared to equipment-intensive techniques and porous membrane-venting strategies, our wettability contrast-governed strategy provides a simple yet effective route for eliminating air bubbles and simultaneously sealing microfluidic interconnects.

Microscale hydrodynamic confinements: shaping liquids across length scales as a toolbox in life sciences

Hydrodynamic phenomena can be leveraged to confine a range of biological and chemical species without needing physical walls. In this review, we list methods for the generation and manipulation of microfluidic hydrodynamic confinements in free-flowing liquids and near surfaces, and elucidate the associated underlying theory and discuss their utility in the emerging area of open space microfluidics applied to life-sciences. Microscale hydrodynamic confinements are already starting to transform approaches in fundamental and applied life-sciences research from precise separation and sorting of individual cells, allowing localized bio-printing to multiplexing for clinical diagnosis. Through the choice of specific flow regimes and geometrical boundary conditions, hydrodynamic confinements can confine species across different length scales from small molecules to large cells, and thus be applied to a wide range of functionalities. We here provide practical examples and implementations for the formation of these confinements in different boundary conditions - within closed channels, in between parallel plates and in an open liquid volume. Further, to enable non-microfluidics researchers to apply hydrodynamic flow confinements in their work, we provide simplified instructions pertaining to their design and modelling, as well as to the formation of hydrodynamic flow confinements in the form of step-by-step tutorials and analytical toolbox software. This review is written with the idea to lower the barrier towards the use of hydrodynamic flow confinements in life sciences research.

Designing and validation of an automated ex-vivo bioreactor system for long term culture of bone

Several different bioreactors have been developed to study bone biology. Keeping a bone viable for long-term studies is still a challenge. We have developed an ex-vivo bone bioreactor that can keep the ex-vivo live bone viable for more than 4 weeks. Keeping a bone viable for over a month can be used as an alternative model for in-vivo experiments in animals. We hypothesize that the perfusion flow and mechanical load on the bone provide a real-time environment for the bone to survive. Cancellous bones were harvested from the bovine metatarsals and were placed in the dynamic culture with cyclic loading at regular intervals. After a period of week 4, the bone cores were retrieved from the bioreactor and tested for viability using calcein-AM and ethidium homodimer -1 fluorescent dyes and were compared with the cores that were placed in static culture with and without any loads on them and Day 0 bone core that acted as a positive control. The bone blocks were then fixed in 10% formalin, and bone mineral density was evaluated using a DXA scanner before staining them for H&E to study the morphological changes. Results revealed that the bone cultured in the bioreactor was viable as compared to the one in the static culture with and without constant load. Bone cores cultured in our ex-vivo bioreactor system also maintained their morphology and no statistical difference was found in the bone mineral density compared to positive controls and the statistical difference was found when compared with the cores cultured in static culture. This tool can be used to study bone biology for various applications such as bone ingrowth studies, to study the effect of drugs, hormones, or any growth factors, and much more.

Miniature auto-perfusion bioreactor system with spiral microfluidic cell retention device

Medium perfusion is critical in maintaining high cell concentration in cultures. The conventional membrane filtration method for medium exchange has been challenged by the fouling and clogging of the membrane filters in long-term cultures. In this study, we present a miniature auto-perfusion system that can be operated inside a common-size laboratory incubator. The system is equipped with a spiral microfluidic chip for cell retention to replace conventional membrane filters, which fundamentally overcomes the clogging and fouling problem. We showed that the system supported continuous perfusion culture of Chinese hamster ovary (CHO) cells in suspension up to 14 days without cell retention chip replacement. Compared to daily manual medium change, 25% higher CHO cell concentration can be maintained at an average auto-perfusion rate of 196 ml/day in spinner flask at 70 ml working volume (2.8 VVD). The auto-perfusion system also resulted in better cell quality at high concentrations, in terms of higher viability, more uniform and regular morphology, and fewer aggregates. We also demonstrated the potential application of the system for culturing mesenchymal stem cells on microcarriers. This miniature auto-perfusion system provides an excellent solution to maintain cell-favorable conditions and high cell concentration in small-scale cultures for research and clinical uses.

Protection of human induced pluripotent stem cells against shear stress in suspension culture by Bingham plastic fluid

Suspension culture is an important method used in the industrial preparation of pluripotent stem cells (PSCs), for regenerative therapy and drug screening. Generally, a suspension culture requires agitation to keep PSC aggregates suspended and to promote mass transfer, but agitation also causes cell damage. In this study, we investigated the use of a Bingham plastic fluid, supplemented with a polysaccharide‐based polymer, to preserve PSCs from cell damage in suspension culture. Rheometric analysis showed that the culture medium gained yield stress and became a Bingham plastic fluid, after supplementation with the polymer FP003. A growth/death analysis revealed that 2 days of aggregate formation and 2 days of suspension in the Bingham plastic medium improved cell growth and prevented cell death. After the initial aggregation step, whereas strong agitation (120 rpm) of a conventional culture medium resulted in massive cell death, in the Bingham plastic fluid we obtained the same growth as the normal culture with optimal agitation (90 rpm). This indicates that Bingham plastic fluid protected cells from shear stress in suspension culture and could be used to enhance their robustness when developing a large‐scale.

Hybrid microchannel-solid state micropore device for fast and optical cell detection

This paper presents a methodology for cell detection and counting using a device that combines PDMS (polydimethylsiloxane) microfluidic multilayer channels with a single solid state micropore. Optimal conditions of solid-state micropore fabrication from crystalline silicon wafers are presented. Micropores of varying size can be obtained by directly etching using an etchant agent concentration of 50 wt% KOH, at varying temperatures (40, 60, 80 °C) and voltages (100, 500, 1000 mV). Scanning Electron Microscopy (SEM), and profilometry techniques have been used for the micropore characterization. In order to find optimal conditions for cell detection a COMSOL Multiphysics simulation was performed. Pressure drop, shear stress, fluid viscosities and flow rates parameters were evaluated. The potential viability of the device for cell detection and counting, avoiding cellular damage, is demonstrated.

This paper presents a methodology for cell detection and counting using a device that combines PDMS (polydimethylsiloxane) microfluidic multilayer channels with a single solid state micropore.

Circulating tumor cell isolation, culture, and downstream molecular analysis

Circulating tumor cells (CTCs) are a major contributor of cancer metastases and hold a promising prognostic significance in cancer detection. Performing functional and molecular characterization of CTCs provides an in-depth knowledge about this lethal disease. Researchers are making efforts to design devices and develop assays for enumeration of CTCs with a high capture and detection efficiency from whole blood of cancer patients. The existing and on-going research on CTC isolation methods has revealed cell characteristics which are helpful in cancer monitoring and designing of targeted cancer treatments. In this review paper, a brief summary of existing CTC isolation methods is presented. We also discuss methods of detaching CTC from functionalized surfaces (functional assays/devices) and their further use for ex-vivo culturing that aid in studies regarding molecular properties that encourage metastatic seeding. In the clinical applications section, we discuss a number of cases that CTCs can play a key role for monitoring metastases, drug treatment response, and heterogeneity profiling regarding biomarkers and gene expression studies that bring treatment design further towards personalized medicine.

Hydrodynamics in Cell Studies

Hydrodynamic phenomena are ubiquitous in living organisms and can be used to manipulate cells or emulate physiological microenvironments experienced in vivo. Hydrodynamic effects influence multiple cellular properties and processes, including cell morphology, intracellular processes, cell–cell signaling cascades and reaction kinetics, and play an important role at the single-cell, multicellular, and organ level. Selected hydrodynamic effects can also be leveraged to control mechanical stresses, analyte transport, as well as local temperature within cellular microenvironments. With a better understanding of fluid mechanics at the micrometer-length scale and the advent of microfluidic technologies, a new generation of experimental tools that provide control over cellular microenvironments and emulate physiological conditions with exquisite accuracy is now emerging. Accordingly, we believe that it is timely to assess the concepts underlying hydrodynamic control of cellular microenvironments and their applications and provide some perspective on the future of such tools in in vitro cell-culture models. Generally, we describe the interplay between living cells, hydrodynamic stressors, and fluid flow-induced effects imposed on the cells. This interplay results in a broad range of chemical, biological, and physical phenomena in and around cells. More specifically, we describe and formulate the underlying physics of hydrodynamic phenomena affecting both adhered and suspended cells. Moreover, we provide an overview of representative studies that leverage hydrodynamic effects in the context of single-cell studies within microfluidic systems.

Capturing Circulating Tumor Cells through a Combination of Hierarchical Nanotopography and Surface Chemistry

Circulating tumor cells (CTCs) have become known as minimally invasive multifunctional biomarkers for earlier diagnosis, prognosis, recurrence risk assessment, and therapeutic monitoring in recent years. However, effectively capturing these CTCs is still difficult because of the extremely low abundance of CTCs and the diverse phenotypes of cancer cells. In this study, we present a novel necklace-like polydopamine nanosphere (PDA NS)/alginate composite nanofiber with a hierarchical nanotopographical structure and a surface chemical signal for capturing the CTCs. The height of the nanotopography, which is formed by connecting PDA NSs with nanofibers via electrospinning, can be easily adjusted by changing the size of the PDA NSs. Four types of cancer cells are employed to investigate the capture efficiency of the fiber. More importantly, in a blood environment containing rare cancer cells, the fiber still has a great ability to capture these cells. Therefore, this nanofiber is identified as a potential device for the diagnosis of cancer.

Bioprinting: uncovering the utility layer-by-layer

Bioprinting is becoming a must have capability in tissue engineering research. Key to the growth of the field is the inherent flexibility, which can be used to answer basic scientific questions that can only be addressed under 3D culture conditions, or organ-on-chip systems that could quickly replace underperforming animal models. Almost certainly the most challenging application of bioprinting will be for bottom-up tissue construction, which faces many of the same challenges as scaffold-based tissue engineering. In this review, the current state-of-the-art approaches to 3D bioprinting are discussed in terms of performance and suitability. This is complemented by an overview of hydrogel-based bioinks, with a special emphasis on composite biomaterial systems.

Cell Isolation and Recovery Using Hollow Glass Microspheres Coated with Nanolayered Films for Applications in Resource-Limited Settings

Established cell isolation and purification techniques such as fluorescence-activated cell sorting (FACS), isolation through magnetic micro/nanoparticles, and recovery via microfluidic devices have limited application as disposable technologies appropriate for point-of-care use in remote areas where lab equipment as well as electrical, magnetic, and optical sources are restricted. We report a simple yet effective method for cell isolation and recovery that requires neither specialized lab equipment nor any form of power source. Specifically, self-floating hollow glass microspheres were coated with an enzymatically degradable nanolayered film and conjugated with antibodies to allow both fast capture and release of subpopulations of cells from a cell mixture. Targeted cells were captured by the microspheres and allowed to float to the top of the hosting liquid, thereby isolating targeted cells. To minimize nonspecific adhesion of untargeted cells and to enhance the purity of the isolated cell population, an antifouling polymer brush layer was grafted onto the nanolayered film. Using the EpCAM-expressing cancer cell line PC-3 in blood as a model system, we have demonstrated the isolation and recovery of cancer cells without compromising cell viability or proliferative potential. The whole process takes less than 1 h. To support the rational extension of this platform technology, we introduce extensive characterization of the critical design parameters: film formation and degradation, grafting with a poly(ethylene glycol) (PEG) sheath, and introducing functional antibodies. Our approach is expected to overcome practical hurdles and provide viable targeted cells for downstream analyses in resource-limited settings.

Aptamer-functionalized nano/micro-materials for clinical diagnosis: isolation, release and bioanalysis of circulating tumor cells

Detection of rare circulating tumor cells (CTCs) in peripheral blood is a challenging, but necessary, task in order to diagnose early onset of metastatic cancer and to monitor treatment efficacy. Over the last decade, step-up produced aptamers have attracted great attention in clinical diagnosis. They have offered great promise for a broader range of cell-specific recognition and isolation. In particular, aptamer-functionalized magnetic particles for selective extraction of target CTCs have shown reduced damage to cells and relatively simple operation. Also, efforts to develop aptamer-functionalized microchannel/microstructures able to efficiently isolate target CTCs are continuing, and these efforts have brought more advanced geometrically designed substrates. Various aptamer-mediated cell release techniques are being developed to enable subsequent biological studies. This article reviews some of these advances in aptamer-functionalized nano/micro-materials for CTCs isolation and methods for releasing captured CTCs from aptamer-functionalized surfaces. Biological studies of CTCs after release are also discussed.

Fabrication of hASCs-laden structures using extrusion-based cell printing supplemented with an electric field

In this study, we proposed a hybrid cell-printing technique that combines a conventional extrusion-based cell-printing process with an electrohydrodynamic jet. The electric field stabilized the extruded struts of cell-embedding-hydrogel and reduced the damage to dispensed cells caused by the high wall shear stress in the dispensing nozzle. The new cell-printing process was optimized in terms of various processing parameters, applied electric field strength, nozzle movement speed, and distance between the nozzle tip and working stage. Using the optimal cell-embedding hydrogel composition (1×10(6)cellsmL(-1) in 4wt% alginate) and cell-printing process parameters (applied voltage, 1kV; nozzle movement speed, 12mms(-1); distance, 0.7mm; current, 10.67±1.1nA), we achieved rapid and stable fabrication of a cell-laden structure without loss of cell viability or proliferation, the values of which were similar to those of the process without an electric field. Furthermore, by applying the same pneumatic pressure to fabricate cell-laden structures, considerably higher volume flow rate and cell viability at the same volume flow rate were achieved by the modified process compared with conventional extrusion-based cell-printing processes. To assess the feasibility of the method, the hydrogel containing human adipose stem cells (hASCs) and alginate (4wt%) was fabricated into a cell-laden porous structure in a layer-by-layer manner. The cell-laden structure exhibited reasonable initial hASC viability (87%), which was similar to that prior to processing of the cell-embedding-hydrogel.

STATEMENT OF SIGNIFICANCE

The extrusion-based cell-printing process has shortcomings, such as unstable flow and potential loss of cell viability. The unsteady flow can occur due to the high cell concentration, viscosity, and surface tension of bioinks. Also, cell viability post extrusion can be significantly reduced by damage of the cells due to the high wall shear stress in the extrusion nozzle. To overcome these limitations, we suggested an innovative cell-printing process that combines a conventional extrusion-based cellprinting process with an electric field. The electric field in the cell-printing process stabilized the extruded struts of bioink and dramatically reduced the damage to dispensed cells caused by the high wall shear stress in the dispensing nozzle.

Deformation and rupture of Dunaliella salina at high shear rates without the use of thickeners

BACKGROUND

High-density cultures require operating below the critical threshold of shear stress, in order to avoid reducing the specific growth rate of the cells. When determining this threshold, direct inspection of the cells in flow provides insight into the conditions of shearing.

OBJECTIVE

Aim of this study was using a novel rheo-optical setup for the observation of cells in laminar shear flow and the determination of the critical shear stress required to damage them in their natural environment.

METHODS

Dunaliella salina cells were sheared and observed in flow for shear stresses of up to 90 Pa, at ambient temperature, without adding thickeners. The critical shear stress was determined by fitting a hydrodynamics-based criterion to the experimental data on the percentage of deformed cells after shearing.

RESULTS

Single cells, clusters and strings of cells were visible in shear flow. The strings formed at maximum shear stresses of 10 Pa or higher. Cells lost motility for maximum shear stresses higher than 15 Pa, and more than 80% of the cells were deformed at maximum shear stresses higher than 60 Pa. The estimated critical shear stress was 18 Pa.

CONCLUSIONS

Shear stresses higher than 18 Pa should be avoided when cultivating D. salina.

Biodegradable nano-films for capture and non-invasive release of circulating tumor cells

Selective isolation and purification of circulating tumor cells (CTCs) from whole blood is an important capability for both clinical medicine and biological research. Current techniques to perform this task place the isolated cells under excessive stresses that reduce cell viability, and potentially induce phenotype change, therefore losing valuable information about the isolated cells. We present a biodegradable nano-film coating on the surface of a microfluidic chip, which can be used to effectively capture as well as non-invasively release cancer cell lines such as PC-3, LNCaP, DU 145, H1650 and H1975. We have applied layer-by-layer (LbL) assembly to create a library of ultrathin coatings using a broad range of materials through complementary interactions. By developing an LbL nano-film coating with an affinity-based cell-capture surface that is capable of selectively isolating cancer cells from whole blood, and that can be rapidly degraded on command, we are able to gently isolate cancer cells and recover them without compromising cell viability or proliferative potential. Our approach has the capability to overcome practical hurdles and provide viable cancer cells for downstream analyses, such as live cell imaging, single cell genomics, and in vitro cell culture of recovered cells. Furthermore, CTCs from cancer patients were also captured, identified, and successfully released using the LbL-modified microchips.

A differential pressure laminar flow reactor supports osteogenic differentiation and extracellular matrix formation from adipose mesenchymal stem cells in a macroporous ceramic scaffold

We present a laminar flow reactor for bone tissue engineering that was developed based on a computational fluid dynamics model. The bioreactor design permits a laminar flow field through its specific internal shape. An integrated bypass system that prevents pressure build-up through bypass openings for pressure release allows for a constant pressure environment during the changing of permeability values that are caused by cellular growth within a porous scaffold. A macroporous ceramic scaffold, composed of zirconium dioxide, was used as a test biomaterial that studies adipose stem cell behavior within a controlled three-dimensional (3D) flow and pressure environment. The topographic structure of the material provided a basis for stem cell proliferation and differentiation toward the osteogenic lineage. Dynamic culture conditions in the bioreactor supported cell viability during long-term culture and induced cell cluster formation and extra-cellular matrix deposition within the porous scaffold, though no complete closure of the pores with new-formed tissue was observed. We postulate that our system is suitable for studying fluid shear stress effects on stem cell proliferation and differentiation toward bone formation in tissue-engineered 3D constructs.

Tissue engineered skin substitutes created by laser-assisted bioprinting form skin-like structures in the dorsal skin fold chamber in mice

Tissue engineering plays an important role in the production of skin equivalents for the therapy of chronic and especially burn wounds. Actually, there exists no (cellularized) skin equivalent which might be able to satisfactorily mimic native skin. Here, we utilized a laser-assisted bioprinting (LaBP) technique to create a fully cellularized skin substitute. The unique feature of LaBP is the possibility to position different cell types in an exact three-dimensional (3D) spatial pattern. For the creation of the skin substitutes, we positioned fibroblasts and keratinocytes on top of a stabilizing matrix (Matriderm®). These skin constructs were subsequently tested in vivo, employing the dorsal skin fold chamber in nude mice. The transplants were placed into full-thickness skin wounds and were fully connected to the surrounding tissue when explanted after 11 days. The printed keratinocytes formed a multi-layered epidermis with beginning differentiation and stratum corneum. Proliferation of the keratinocytes was mainly detected in the suprabasal layers. In vitro controls, which were cultivated at the air-liquid-interface, also exhibited proliferative cells, but they were rather located in the whole epidermis. E-cadherin as a hint for adherens junctions and therefore tissue formation could be found in the epidermis in vivo as well as in vitro. In both conditions, the printed fibroblasts partly stayed on top of the underlying Matriderm® where they produced collagen, while part of them migrated into the Matriderm®. In the mice, some blood vessels could be found to grow from the wound bed and the wound edges in direction of the printed cells. In conclusion, we could show the successful 3D printing of a cell construct via LaBP and the subsequent tissue formation in vivo. These findings represent the prerequisite for the creation of a complex tissue like skin, consisting of different cell types in an intricate 3D pattern.

Mosaic hydrogels: one-step formation of multiscale soft materials

The one-step, continuous formation of mosaic hydrogel sheets is presented. A microfluidic device allows controllable incorporation of secondary biopolymers within a flowing biopolymer sheet followed by a cross-linking step that retains the microscale composition. Information is encoded; mosaic stiffness and diffusivity patterns are created; tessellations are populated with biomolecules, microparticles and viable primary cells; and 3D soft material assemblies are demonstrated.

Mechanical disruption of mammalian cells in a microfluidic system and its numerical analysis based on computational fluid dynamics

The lysis of mammalian cells is an essential part of different lab-on-a-chip sample preparation methods, which aim at the release, separation, and subsequent analysis of DNA, proteins, or metabolites. Particularly for the analysis of compartmented in vivo metabolism of mammalian cells, such a method must be very fast compared to the metabolic turnover-rates, it should not affect the native metabolite concentrations, and should ideally leave cell organelles undamaged. So far, no such a method is available. We have developed a microfluidic system for the effective rapid mechanical cell disruption and established a mathematical model to describe the efficiency of the system. Chinese hamster ovary (CHO) cells were disrupted with high efficiency by passing through two consecutive micronozzle arrays. Simultaneous cell compression and shearing led to a disruption rate of ≥90% at a sample flow rate of Q = 120 μL min(-1) per nozzle passage, which corresponds to a mean fluid velocity of 13.3 m s(-1) and a mean Reynolds number of 22.6 in the nozzle gap. We discussed the problem of channel clogging by cellular debris and the resulting flow instability at the micronozzle arrays. The experimental results were compared to predictions from Computational Fluid Dynamics (CFD) simulations and the critical energy dissipation rate for the disruption of the CHO cell population with known size distribution was determined to be 4.7 × 10(8) W m(-3). Our model for the calculation of cell disruption on the basis of CFD-data could be applied to other microgeometries to predict intended disruption or undesired cell damage.

Bubbles no more: in-plane trapping and removal of bubbles in microfluidic devices

Gas bubbles present a frequent challenge to the on-chip investigation and culture of biological cells and small organs. The presence of a single bubble can adversely impair biological function and often viability as it increases the wall shear stress in a liquid-perfused microchannel by at least one order of magnitude. We present a microfluidic strategy for in-plane trapping and removal of gas bubbles with volumes of 0.1-500 nL. The presented bubble trap is compatible with single-layer soft lithography and requires a footprint of less than ten square millimetres. Nitrogen bubbles were consistently removed at a rate of 0.14 μL min(-1). Experiments were complemented with analytical and numerical models to comprehensively characterize bubble removal for liquids with different wetting behaviour. Consistent long-term operation of the bubble trap was demonstrated by removing approximately 4000 bubbles during one day. In a case study, we successfully applied the bubble trap to the on-chip investigation of intact small blood vessels. Scalability of the design was demonstrated by realizing eight parallel traps at a total removal rate of 0.9 μL min(-1) (measured for nitrogen).

Bioprocess forces and their impact on cell behavior: implications for bone regeneration therapy

Bioprocess forces such as shear stress experienced during routine cell culture are considered to be harmful to cells. However, the impact of physical forces on cell behavior is an area of growing interest within the tissue engineering community, and it is widely acknowledged that mechanical stimulation including shear stress can enhance osteogenic differentiation. This paper considers the effects of bioprocess shear stress on cell responses such as survival and proliferation in several contexts, including suspension-adapted cells used for recombinant protein and monoclonal antibody manufacture, adherent cells for therapy in suspension, and adherent cells attached to their growth substrates. The enhanced osteogenic differentiation that fluid flow shear stress is widely found to induce is discussed, along with the tissue engineering of mineralized tissue using perfusion bioreactors. Recent evidence that bioprocess forces produced during capillary transfer or pipetting of cell suspensions can enhance osteogenic responses is also discussed.

Pinched flow coupled shear-modulated inertial microfluidics for high-throughput rare blood cell separation

Blood is a highly complex bio-fluid with cellular components making up >40% of the total volume, thus making its analysis challenging and time-consuming. In this work, we introduce a high-throughput size-based separation method for processing diluted blood using inertial microfluidics. The technique takes advantage of the preferential cell focusing in high aspect-ratio microchannels coupled with pinched flow dynamics for isolating low abundance cells from blood. As an application of the developed technique, we demonstrate the isolation of cancer cells (circulating tumor cells (CTCs)) spiked in blood by exploiting the difference in size between CTCs and hematologic cells. The microchannel dimensions and processing parameters were optimized to enable high throughput and high resolution separation, comparable to existing CTC isolation technologies. Results from experiments conducted with MCF-7 cells spiked into whole blood indicate >80% cell recovery with an impressive 3.25 × 10(5) fold enrichment over red blood cells (RBCs) and 1.2 × 10(4) fold enrichment over peripheral blood leukocytes (PBL). In spite of a 20× sample dilution, the fast operating flow rate allows the processing of ∼10(8) cells min(-1) through a single microfluidic device. The device design can be easily customized for isolating other rare cells from blood including peripheral blood leukocytes and fetal nucleated red blood cells by simply varying the 'pinching' width. The advantage of simple label-free separation, combined with the ability to retrieve viable cells post enrichment and minimal sample pre-processing presents numerous applications for use in clinical diagnosis and conducting fundamental studies.

Substrate stiffness influences high resolution printing of living cells with an ink-jet system

The adaptation of inkjet printing technology for the realisation of controlled micro- and nano-scaled biological structures is of great potential in tissue and biomaterial engineering. In this paper we present the Olivetti BioJet system and its applications in tissue engineering and cell printing. BioJet, which employs a thermal inkjet cartridge, was used to print biomolecules and living cells. It is well known that high stresses and forces are developed during the inkjet printing process. When printing living particles (i.e., cell suspensions) the mechanical loading profile can dramatically damage the processed cells. Therefore computational models were developed to predict the velocity profile and the mechanical load acting on a droplet during the printing process. The model was used to investigate the role of the stiffness of the deposition substrate during droplet impact and compared with experimental investigations on cell viability after printing on different materials. The computational model and the experimental results confirm that impact forces are highly dependent on the deposition substrate and that soft and viscous surfaces can reduce the forces acting on the droplet, preventing cell damage. These results have high relevance for cell bioprinting; substrates should be designed to have a good compromise between substrate stiffness to conserve spatial patterning without droplet coalescence but soft enough to absorb the kinetic energy of droplets in order to maintain cell viability.

Muscle-derived precursor cells isolated on the basis of differential adhesion properties respond differently to capillary flow

Capillary shear stress can improve osteogenic differentiation in muscle-derived precursor cells (MDPCs). This has implications for large-scale bioprocessing of cell therapies where capillary transfer is needed. The recovery, viability, and osteogenic differentiation potential of two subsets of MDPCs, early-adherent pre-plate 1 (PP1) and late-adherent PP3 populations, have been examined: PP1 MDPCs produced a greater degree of osteogenic differentiation than PP3 MDPCs, quantified by Alizarin Red S staining intensity (P < 0.05). For both cell populations, capillary flow-induced significant increases in Alizarin Red S staining (P < 0.05). However, PP1 cells were more susceptible to capillary flow-induced damage than PP3 cells and this was dependent on duration of exposure. Overall, results indicate that different cell subsets, even from within a single tissue, can respond variably to capillary shear stress, necessitating its precise monitoring and control.

Laser printing of skin cells and human stem cells

Laser printing based on laser-induced forward transfer (LIFT) is a new biofabrication technique for the arrangement of biological materials or living cells in well-defined patterns. In the current study, skin cell lines (fibroblasts/keratinocytes) and human mesenchymal stem cells (hMSC) were chosen for laser printing experiments due to their high potential in regeneration of human skin and new application possibilities of stem cell therapy. To evaluate the influence of LIFT on the cells, their survival rate, their proliferation and apoptotic activity, and the DNA damages and modifications of their cell surface markers were assessed and statistically evaluated over several days. The cells survived the transfer procedure with a rate of 98%  +/- 1% standard error of the mean (skin cells) and 90%  +/- 10% (hMSC), respectively. All used cell types maintain their ability to proliferate after LIFT. Further, skin cells and hMSC did not show an increase of apoptosis or DNA fragmentation. In addition, the hMSC keep their phenotype as proven by fluorescence activated cell sorting (FACS) analysis. This study demonstrates LIFT as a suitable technique for unharmed computer-controlled positioning of different cell types and a promising tool for future applications in the ex vivo generation of tissue replacements.

Effect of capillary shear stress on recovery and osteogenic differentiation of muscle-derived precursor cell populations

Both chemical and physical stimuli can influence the fate of precursor cell populations. Therefore, the impact they have on promoting unwanted differentiation events must be understood to improve the yield and purity of therapeutic cells for regenerative medicine approaches. Capillary shear forces, similar to those encountered during cell processing, can impact upon production of regenerative cell populations. As shear stress can promote osteogenic differentiation in adhered bone marrow-derived stromal cells, we sought to determine whether the same is true for populations of muscle-derived precursor cells (MDPCs) that were isolated from a muscle niche environment. We isolated MDPCs from craniofacial muscle of 5 day-old Royal College of Surgeons rats and subjected them to capillary shear events similar to those encountered during manual bioprocessing of cells. We then assessed whether viability and ectopic osteogenic differentiation of MDPCs was affected. We found that whilst immediate recovery of MDPCs was not significantly affected by shear, viability after 24 h was reduced in comparison to non-sheared MDPCs. By 48 h, sheared MDPCs had all recovered and had similar viability to non-sheared MDPCs. Ostegenic differentiation was enhanced following exposure to capillary shear in both osteogenic and myogenic medium. This indicates that shear forces similar to those encountered during the bioprocessing of cell populations for therapy can have a significant influence on the fate of MDPCs.

Induction of mammalian cell death by simple shear and extensional flows

In this work we investigated whether the type of shear flow, to which cells are exposed, influences the initiation of cell death. It is shown that mammalian cells, indeed, distinguish between discrete types of flow and respond differently. Two flow devices were employed to impose accurate hydrodynamic flow fields: uniform steady simple shear flow and oscillating extensional flow. To distinguish between necrotic and apoptotic cell death, fluorescence activated cell sorting and the release of DNA in the culture supernatant was used. Results show that Chinese Hamster Ovaries and Human Embryonic Kidney cells will enter the apoptotic pathway when subjected to low levels of hydrodynamic stress (around 2.0 Pa) in oscillating, extensional flow. In contrast, necrotic death prevails when the cells are exposed to hydrodynamic stresses around 1.0 Pa in simple shear flow or around 500 Pa in extensional flow. These threshold values at which cells enter the respective death pathway should be avoided when culturing cells for recombinant protein production to enhance culture longevity and productivity.

Special Engineering Aspects

The specific characteristics of mammalian cells discussed in Chap. 2 require adapted solutions for bioreactor design and operation. Especially, cell damage by shear stress and aeration has to be considered. Therefore this chapter starts with a detailed discussion of shear stress effects on mammalian cells (anchorage-dependent and suspendable cells) in model systems and bioreactors, respectively, and consequences for reactor design. Appropriate oxygen supply is another critical issue, as adapted oxygen supply systems are required. Techniques for immobilization of cells, either grown on microcarriers in suspension culture or within macroporous carriers in fixed bed or fluidized bed reactors, are discussed as well. With respect to the operation of bioreactors, the characteristics of different culture modes (batch, fed-batch, chemostat, perfusion) are introduced and practical examples are given. Finally, concepts for monitoring of bioreactors, including offline and online methods as well as control loops (e.g. O₂, pH), are considered.

The impact of process stress on suspended anchorage-dependent mammalian cells as an indicator of likely challenges for regenerative medicines

To quantify the engineering shear constraint on processing, the effect of capillary shear stress (pipe flow) on suspended anchorage-dependent mammalian cells has been investigated. Exposure of cultured rat aortic smooth muscle cells to repeated capillary shear stress (2-120 N m(-2)) causes a decrease in total number of cells, number of intact cells and number of cells able to grow. The optimum wall shear stress for cell survival was found to be 10-50 N m(-2) (flowrate 4-20 mL/min, I.D. 0.45 mm). Cell populations which are able to grow after exposure to shear stress do not exhibit reduced growth rate or altered metabolism.

Excess turbulence as a cause of turbohypobiosis in cultivation of microorganisms

The present review describes the influence of different types of mixing systems under excess turbulence conditions on microorganisms. Turbohypobiosis phenomena were described by applying a method for measurement of the kinetic energy of flow fluctuations based on the piezoeffect. It can be assumed that the shear stress effect (the state of turbohypobiosis) plays a role mainly when alternative mechanisms in cells cannot ensure a normal physiological state under stress conditions. Practically any system (inner construction of a bioreactor, culture and cultivation conditions, including mixing) requires its own optimisation to achieve the final goal, namely, the maximum product and/or biomass yields from substrate (YP/S or/and YX/S), respectively. Data on the biotechnological performance of cultivation as well as power input, kinetic energy (e) of flow fluctuations, air consumption rate, rotational speed, tip speed, etc. do not correlate directly if the mixing systems (impellers-baffles) are dissimilar. Even the widely used specific power consumption cannot be relied upon for scaling up the cultivation performance using dissimilar mixing systems. A biochemical explanation for substrate and product transport via cell walls, carbon pathways, energy generation and utilisation, etc. furnishes insight into cellular interactions with turbulence of different origin for different types of microorganisms (single cells, mycelia forming cells, etc.).