Hydrodynamic shear stress' impact on mammalian cell properties and its applications in 3D bioprinting

As an effective cell assembly method, three-dimensional bioprinting has been widely used in building organ models and tissue repair over the past decade. However, different shear stresses induced throughout the entire printing process can cause complex impacts on cell integrity, including reducing cell viability, provoking morphological changes and altering cellular functionalities. The potential effects that may occur and the conditions under which these effects manifest are not clearly understood. Here, we review systematically how different mammalian cells respond under shear stress. We enumerate available experimental apparatus, and we categorise properties that can be affected under disparate stress patterns. We also summarise cell damaging mathematical models as a predicting reference for the design of bioprinting systems. We concluded that it is essential to quantify specific cell resistance to shear stress for the optimisation of bioprinting systems. Besides, as substantial positive impacts, including inducing cell alignment and promoting cell motility, can be generated by shear stress, we suggest that we find the proper range of shear stress and actively utilise its positive influences in the development of future systems.

Development of vessel mimicking microfluidic device for studying mechano-response of endothelial cells

The objective of this study is to develop a device to mimic a microfluidic system of human arterial blood vessels. The device combines fluid shear stress (FSS) and cyclic stretch (CS), which are resulting from blood flow and blood pressure, respectively. The device can reveal real-time observation of dynamic morphological change of cells in different flow fields (continuous flow, reciprocating flow and pulsatile flow) and stretch. We observe the effects of FSS and CS on endothelial cells (ECs), including ECs align their cytoskeleton proteins with the fluid flow direction and paxillin redistribution to the cell periphery or the end of stress fibers. Thus, understanding the morphological and functional changes of endothelial cells on physical stimuli can help us to prevent and improve the treatment of cardiovascular diseases.

Development and characterization of matrix-derived microcarriers from decellularized tissues using electrospraying techniques

Stirred bioreactor systems integrating microcarriers represent a promising approach for therapeutic cell manufacturing. While a variety of microcarriers are commercially available, current options do not integrate the tissue-specific composition of the extracellular matrix (ECM), which can play critical roles in directing cell function. The current study sought to generate microcarriers comprised exclusively of ECM from multiple tissue sources. More specifically, porcine decellularized dermis, porcine decellularized myocardium, and human decellularized adipose tissue were digested with α-amylase to obtain ECM suspensions that could be electrosprayed into liquid nitrogen to generate 3D microcarriers that were stable over a range of ECM concentrations without the need for chemical crosslinking or other additives. Characterization studies confirmed that all three microcarrier types had similar soft and compliant mechanical properties and were of a similar size range, but that their composition varied depending on the native tissue source. In vivo testing in immunocompetent mice revealed that the microcarriers integrated into the host tissues, supporting the infiltration of host cells including macrophages and endothelial cells at 2 weeks post-implantation. In vitro cell culture studies validated that the novel microcarriers supported the attachment of tissue-specific stromal cell populations under dynamic culture conditions within spinner flasks, with a significant increase in live cell numbers observed over 1 week on the dermal- and adipose-derived microcarriers. Overall, the findings demonstrate the versatility of the electrospraying methods and support the further development of the microcarriers as cell culture and delivery platforms.

A multi-domain shear-stress dependent diffusive model of cell transport-aided dialysis: analysis and simulation

Kidney dialysis is the most widespread treatment method for end-stage renal disease, a debilitating health condition common in industrialized societies. While ubiquitous, kidney dialysis suffers from an inability to remove larger toxins, resulting in a gradual buildup of these toxins in dialysis patients, ultimately leading to further health complications. To improve dialysis, hollow fibers incorporating a cell-monolayer with cultured kidney cells have been proposed; however, the design of such a fiber is nontrivial. In particular, the effects of fluid wall-shear stress have an important influence on the ability of the cell layer to transport toxins. In the present work, we introduce a model for cell-transport aided dialysis, incorporating the effects of the shear stress. We analyze the model mathematically and establish its well-posedness. We then present a series of numerical results, which suggest that a hollow-fiber design with a wavy profile may increase the efficiency of the dialysis treatment. We investigate numerically the shape of the wavy channel to maximize the toxin clearance. These results demonstrate the potential for the use of computational models in the study and advancement of renal therapies.

Microfluidic In Vitro Platform for (Nano)Safety and (Nano)Drug Efficiency Screening

Microfluidic technology is a valuable tool for realizing more in vitro models capturing cellular and organ level responses for rapid and animal-free risk assessment of new chemicals and drugs. Microfluidic cell-based devices allow high-throughput screening and flexible automation while lowering costs and reagent consumption due to their miniaturization. There is a growing need for faster and animal-free approaches for drug development and safety assessment of chemicals (Registration, Evaluation, Authorisation and Restriction of Chemical Substances, REACH). The work presented describes a microfluidic platform for in vivo-like in vitro cell cultivation. It is equipped with a wafer-based silicon chip including integrated electrodes and a microcavity. A proof-of-concept using different relevant cell models shows its suitability for label-free assessment of cytotoxic effects. A miniaturized microscope within each module monitors cell morphology and proliferation. Electrodes integrated in the microfluidic channels allow the noninvasive monitoring of barrier integrity followed by a label-free assessment of cytotoxic effects. Each microfluidic cell cultivation module can be operated individually or be interconnected in a flexible way. The interconnection of the different modules aims at simulation of the whole-body exposure and response and can contribute to the replacement of animal testing in risk assessment studies in compliance with the 3Rs to replace, reduce, and refine animal experiments.

Cell adhesion and proliferation on common 3D printing materials used in stereolithography of microfluidic devices

Three-dimensional (3D) printing has recently emerged as a cost-effective alternative for rapid prototyping of microfluidic devices. The feature resolution of stereolithography-based 3D printing is particularly well suited for manufacturing of continuous flow cell culture platforms. Poor cell adhesion or material-induced cell death may, however, limit the introduction of new materials to microfluidic cell culture. In this work, we characterized four commercially available materials commonly used in stereolithography-based 3D printing with respect to long-term (2 month) cell survival on native 3D printed surfaces. Cell proliferation rates, along with material-induced effects on apoptosis and cell survival, were examined in mouse embryonic fibroblasts. Additionally, the feasibility of Dental SG (material with the most favored properties) for culturing of human hepatocytes and human-induced pluripotent stem cells was evaluated. The strength of cell adhesion to Dental SG was further examined over a shear force gradient of 1-89 dyne per cm2 by using a custom-designed microfluidic shear force assay incorporating a 3D printed, tilted and tapered microchannel sealed with a polydimethylsiloxane lid. According to our results, autoclavation of the devices prior to cell seeding played the most important role in facilitating long-term cell survival on the native 3D printed surfaces with the shear force threshold in the range of 3-8 dyne per cm2.

Biological Considerations in Scaling Up Therapeutic Cell Manufacturing

Cell therapeutics - using cells as living drugs - have made advances in many areas of medicine. One of the most clinically studied cell-based therapy products is mesenchymal stromal cells (MSCs), which have shown promising results in promoting tissue regeneration and modulating inflammation. However, MSC therapy requires large numbers of cells, the generation of which is not feasible via conventional planar tissue culture methods. Scale-up manufacturing methods (e.g., propagation on microcarriers in stirred-tank bioreactors), however, are not specifically tailored for MSC expansion. These processes may, in principle, alter the cell secretome, a vital component underlying the immunosuppressive properties and clinical effectiveness of MSCs. This review outlines our current understanding of MSC properties and immunomodulatory function, expansion in commercial manufacturing systems, and gaps in our knowledge that need to be addressed for effective up-scaling commercialization of MSC therapy.

Investigation of Leukocyte Viability and Damage in Spiral Microchannel and Contraction-Expansion Array

Inertial separation techniques in a microfluidic system have been widely employed in the field of medical diagnosis for a long time. Despite no requirement of external forces, it requires strong hydrodynamic forces that could potentially cause cell damage or loss during the separation process. This might lead to the wrong interpretation of laboratory results since the change of structures and functional characteristics of cells due to the hydrodynamic forces that occur are not taken into account. Therefore, it is important to investigate the cell viability and damage along with the separation efficacy of the device in the design process. In this study, two inertial separation techniques-spiral microchannel and contraction-expansion array (CEA)-were examined to evaluate cell viability, morphology and intracellular structures using a trypan blue assay (TB), Scanning Electron Microscopy (SEM) and Wright-Giemsa stain. We discovered that cell loss was not significantly found in a feeding system, i.e., syringe, needle and tube, but mostly occurred in the inertial separation devices while the change of cell morphology and intracellular structures were found in the feeding system and inertial separation devices. Furthermore, percentage of cell loss was not significant in both devices (7-10%). However, the change of cell morphology was considerably increased (30%) in spiral microchannel (shear stress dominated) rather than in CEA (12%). In contrast, the disruption of intracellular structures was increased (14%) in CEA (extensional and shear stress dominated equally) rather than spiral microchannel (2%). In these experiments, leukocytes of canine were used as samples because their sizes are varied in a range between 7-12 µm, and they are commonly used as a biomarker in many clinical and medical applications.

Expansion Culture of Human Pluripotent Stem Cells and Production of Cardiomyocytes

Transplantation of human pluripotent stem cell (hPSCs)-derived cardiomyocytes for the treatment of heart failure is a promising therapy. In order to implement this therapy requiring numerous cardiomyocytes, substantial production of hPSCs followed by cardiac differentiation seems practical. Conventional methods of culturing hPSCs involve using a 2D culture monolayer that hinders the expansion of hPSCs, thereby limiting their productivity. Advanced culture of hPSCs in 3D aggregates in the suspension overcomes the limitations of 2D culture and attracts immense attention. Although the hPSC production needs to be suitable for subsequent cardiac differentiation, many studies have independently focused on either expansion of hPSCs or cardiac differentiation protocols. In this review, we summarize the recent approaches to expand hPSCs in combination with cardiomyocyte differentiation. A comparison of various suspension culture methods and future prospects for dynamic culture of hPSCs are discussed in this study. Understanding hPSC characteristics in different models of dynamic culture helps to produce numerous cells that are useful for further clinical applications.

Effect of fluid shear stress on in vitro cultured ureteric bud cells

Most kidney cells are continuously exposed to fluid shear stress (FSS) from either blood flow or urine flow. Recent studies suggest that changes in FSS could contribute to the function and injury of these kidney cells. However, it is unclear whether FSS influences kidney development when urinary flow starts in the embryonic kidneys. In this study, we evaluated the influence of FSS on in vitro cultured ureteric bud (UB) cells by using a pumpless microfluidic device, which offers the convenience of conducting parallel cell culture experiments while also eliminating the need for cumbersome electronic driven equipment and intricate techniques. We first validated the function of the device by both mathematical model and experimental measurements. UB cells dissected from E15.5 mouse embryonic kidneys were cultured in the pumpless microfluidic device and subjected to FSS in the range of 0.4-0.6 dyn mm-2 for 48 h (dynamic). Control UB cells were similarly cultured in the device and maintained under a no-flow condition (static). We found from our present study that the exposure to FSS for up to 48 h led to an increase in mRNA expression levels of UB tip cell marker genes (Wnt11, Ret, Etv4) with a decrease in stalk cell marker genes (Wnt7b, Tacstd2). In further support of the enrichment of UB tip cell population in response to FSS, we also found that exposure to FSS led to a remarkable reduction in the binding of lectin Dolichos Biflorus Agglutinin. In conclusion, results of our present study show that exposure to FSS led to an enrichment in UB tip cell populations, which could contribute to the development and function of the embryonic kidney when urine flow starts at around embryonic age E15.5 in mouse. Since UB tip cells are known to be the proliferative progenitor cells that contribute to the branching morphogenesis of the collecting system in the kidney, our finding could imply an important link between the FSS from the initiation of urine flow and the development and function of the kidney.

Bovine myoblast cell production in a microcarriers-based system

For several tissue engineering applications, in particular food products, scaling up culture of mammalian cells is a necessary task. The prevailing method for large scale cell culture is the stirred tank bioreactor where anchor dependent cells are grown on microcarriers suspended in medium. We use a spinner flask system with cells grown on microcarriers to optimize the growth of bovine myoblasts. Freshly isolated primary cells were seeded on microcarriers (Synthemax®, CellBIND® and Cytodex® 1 MCs). In this study, we provide proof of principle that bovine myoblasts can be cultured on microcarriers. No major differences were observed between the three tested microcarriers, except that sparsely populated beads were more common with CellBIND® and Synthemax® II beads suggesting a slower initiation of exponential growth than on Cytodex®. We also provide direct evidence that bovine myoblasts display bead-to-bead transfer. A remarkable pick up of growth was observed by adding new MCs. Bovine myoblasts seem to behave like human mesenchymal stem cells. Thus, our results provide valuable data to further develop and scale-up the production of bovine myoblasts as a prerequisite for efficient and cost-effective development of cultured meat. Applicability to other anchorage dependent cells can extend the importance of these results to cell culture for medical tissue engineering or cell therapy.

Miniaturization of aqueous two-phase extraction for biological applications: From micro-tubes to microchannels

Aqueous two-phase extraction (ATPE) is a biocompatible liquid-liquid (L-L) separation technique that has been under research for several decades towards the purification of biomolecules, ranging from small metabolites to large animal cells. More recently, with the emergence of rapid-prototyping techniques for fabrication of microfluidic structures with intricate designs, ATPE gained an expanded range of applications utilizing physical phenomena occurring exclusively at the microscale. Today, research is being carried simultaneously in two different volume ranges, mL-scale (microtubes) and nL-scale (microchannels). The objective of this review is to give insight into the state of the art at both microtube and microchannel-scale and to analyze whether miniaturization is currently a competing or divergent technology in a field of applications including bioseparation, bioanalytics, enhanced fermentation processes, catalysis, high-throughput screening and physical/chemical compartmentalization. From our perspective, both approaches are worthy of investigation and, depending on the application, it is likely that either (i) one of the approaches will eventually become obsolete in particular research areas such as purification at the preparative scale or high-throughput screening applications; or (ii) both approaches will function as complementing techniques within the bioanalytics field.

Defined Essential 8™ Medium and Vitronectin Efficiently Support Scalable Xeno-Free Expansion of Human Induced Pluripotent Stem Cells in Stirred Microcarrier Culture Systems

Human induced pluripotent stem (hiPS) cell culture using Essential 8™ xeno-free medium and the defined xeno-free matrix vitronectin was successfully implemented under adherent conditions. This matrix was able to support hiPS cell expansion either in coated plates or on polystyrene-coated microcarriers, while maintaining hiPS cell functionality and pluripotency. Importantly, scale-up of the microcarrier-based system was accomplished using a 50 mL spinner flask, under dynamic conditions. A three-level factorial design experiment was performed to identify optimal conditions in terms of a) initial cell density b) agitation speed, and c) to maximize cell yield in spinner flask cultures. A maximum cell yield of 3.5 is achieved by inoculating 55,000 cells/cm2 of microcarrier surface area and using 44 rpm, which generates a cell density of 1.4x106 cells/mL after 10 days of culture. After dynamic culture, hiPS cells maintained their typical morphology upon re-plating, exhibited pluripotency-associated marker expression as well as tri-lineage differentiation capability, which was verified by inducing their spontaneous differentiation through embryoid body formation, and subsequent downstream differentiation to specific lineages such as neural and cardiac fates was successfully accomplished. In conclusion, a scalable, robust and cost-effective xeno-free culture system was successfully developed and implemented for the scale-up production of hiPS cells.

Advances in cell culture: anchorage dependence

Anchorage-dependent cells are of great interest for various biotechnological applications. (i) They represent a formidable production means of viruses for vaccination purposes at very large scales (in 1000-6000 l reactors) using microcarriers, and in the last decade many more novel viral vaccines have been developed using this production technology. (ii) With the advent of stem cells and their use/potential use in clinics for cell therapy and regenerative medicine purposes, the development of novel culture devices and technologies for adherent cells has accelerated greatly with a view to the large-scale expansion of these cells. Presently, the really scalable systems--microcarrier/microcarrier-clump cultures using stirred-tank reactors--for the expansion of stem cells are still in their infancy. Only laboratory scale reactors of maximally 2.5 l working volume have been evaluated because thorough knowledge and basic understanding of critical issues with respect to cell expansion while retaining pluripotency and differentiation potential, and the impact of the culture environment on stem cell fate, etc., are still lacking and require further studies. This article gives an overview on critical issues common to all cell culture systems for adherent cells as well as specifics for different types of stem cells in view of small- and large-scale cell expansion and production processes.

Biomimetics of fetal alveolar flow phenomena using microfluidics

At the onset of life in utero, the respiratory system begins as a liquid-filled tubular organ and undergoes significant morphological changes during fetal development towards establishing a respiratory organ optimized for gas exchange. As airspace morphology evolves, respiratory alveolar flows have been hypothesized to exhibit evolving flow patterns. In the present study, we have investigated flow topologies during increasing phases of embryonic life within an anatomically inspired microfluidic device, reproducing real-scale features of fetal airways representative of three distinct phases of in utero gestation. Micro-particle image velocimetry measurements, supported by computational fluid dynamics simulations, reveal distinct respiratory alveolar flow patterns throughout different stages of fetal life. While attached, streamlined flows characterize the shallow structures of premature alveoli indicative of the onset of saccular stage, separated recirculating vortex flows become the signature of developed and extruded alveoli characteristic of the advanced stages of fetal development. To further mimic physiological aspects of the cellular environment of developing airways, our biomimetic devices integrate an alveolar epithelium using the A549 cell line, recreating a confluent monolayer that produces pulmonary surfactant. Overall, our in vitro biomimetic fetal airways model delivers a robust and reliable platform combining key features of alveolar morphology, flow patterns, and physiological aspects of fetal lungs developing in utero.

Quantitative characterization of endothelial cell morphologies depending on shear stress in different blood vessels of domestic pigs using a focused ion beam and high resolution scanning electron microscopy (FIB-SEM)

Microstructured surfaces mimicking the endothelial cell (EC) morphology is a new approach to improve the blood compatibility of synthetic vascular grafts. The ECs are capable of changing their shapes depending on different shear conditions. However, the quantitative correlation between EC morphology and shear stress has not yet been investigated statistically. The aim of this study was to quantitatively investigate the morphology of ECs in dependence on the shear stress. Blood flow rates in different types of natural blood vessels (carotid, renal, hepatic and iliac arteries) originated from domestic pigs were first measured in vivo to calculate the shear stresses. The EC morphologies were quantitatively characterized ex vivo by imaging with high resolution scanning electron microscopy (SEM) and cross-sectioning of the cells using a state-of-the-art focused ion beam (FIB). The relationships between EC geometrical parameters and shear stress were statistically analyzed and found to be exponential. ECs under high shear stress conditions had a longer length and narrower width, i.e. a higher aspect ratio, while the cell height was smaller compared to low shear conditions. Based on these results, suitable and valid geometrical parameters of microstructures mimicking EC can be derived for various shear conditions in synthetic vascular grafts to optimize blood compatibility.

Cationic surface charge combined with either vitronectin or laminin dictates the evolution of human embryonic stem cells/microcarrier aggregates and cell growth in agitated cultures

The expansion of human pluripotent stem cells (hPSC) for biomedical applications generally compels a defined, reliable, and scalable platform. Bioreactors offer a three-dimensional culture environment that relies on the implementation of microcarriers (MC), as supports for cell anchorage and their subsequent growth. Polystyrene microspheres/MC coated with adhesion-promoting extracellular matrix (ECM) protein, vitronectin (VN), or laminin (LN) have been shown to support hPSC expansion in a static environment. However, they are insufficient to promote human embryonic stem cells (hESC) seeding and their expansion in an agitated environment. The present study describes an innovative technology, consisting of a cationic charge that underlies the ECM coatings. By combining poly-L-lysine (PLL) with a coating of ECM protein, cell attachment efficiency and cell spreading are improved, thus enabling seeding under agitation in a serum-free medium. This coating combination also critically enables the subsequent formation and evolution of hPSC/MC aggregates, which ensure cell viability and generate high yields. Aggregate dimensions of at least 300 μm during early cell growth give rise to ≈15-fold expansion at 7 days' culture. Increasing aggregate numbers at a quasi-constant size of ≈300 μm indicates hESC growth within a self-regulating microenvironment. PLL+LN enables cell seeding and aggregate evolution under constant agitation, whereas PLL+VN requires an intermediate 2-day static pause to attain comparable aggregate sizes and correspondingly high expansion yields. The cells' highly reproducible bioresponse to these defined and characterized MC surface properties is universal across multiple cell lines, thus confirming the robustness of this scalable expansion process in a defined environment.

Microfluidic kit-on-a-lid: a versatile platform for neutrophil chemotaxis assays

Improvements in neutrophil chemotaxis assays have advanced our understanding of the mechanisms of neutrophil recruitment; however, traditional methods limit biologic inquiry in important areas. We report a microfluidic technology that enables neutrophil purification and chemotaxis on-chip within minutes, using nanoliters of whole blood, and only requires a micropipette to operate. The low sample volume requirements and novel lid-based method for initiating the gradient of chemoattractant enabled the measurement of human neutrophil migration on a cell monolayer to probe the adherent and migratory states of neutrophils under inflammatory conditions; mouse neutrophil chemotaxis without sacrificing the animal; and both 2D and 3D neutrophil chemotaxis. First, the neutrophil chemotaxis on endothelial cells revealed 2 distinct neutrophil phenotypes, showing that endothelial cell-neutrophil interactions influence neutrophil chemotactic behavior. Second, we validated the mouse neutrophil chemotaxis assay by comparing the adhesion and chemotaxis of neutrophils from chronically inflamed and wild-type mice; we observed significantly higher neutrophil adhesion in blood obtained from chronically inflamed mice. Third, we show that 2D and 3D neutrophil chemotaxis can be directly compared using our technique. These methods allow for new avenues of research while reducing the complexity, time, and sample volume requirements to perform neutrophil chemotaxis assays.

Agitation and aeration effects in suspension mammalian cell cultures

Three different hybridoma cell lines, grown in serum-free media with different levels of Pluronic F-68, were subjected to a shear force of 0.6 N m(-2). Some protective effect due to the polymer was found, indicating it to be a potentially useful adjuvant in serum-free media. Other observations of liquid and gas effects at the reactor level have been included here. A discussion of the difference between suspension and microcarrier cultures, in relation to hydrodynamic effects, is included.

Hydrodynamic effects on BHK cells grown as suspended natural aggregates

Baby hamster kidney (BHK) cell aggregates grown in stirred vessels with different working volumes and impeller sizes were characterized. Using batch cultures, the range of agitation rates studied (25-100 rpm) led to aggregates with maximum sizes of 150 mum. Necrotic centers were not observed and cell specific productivity was independent of aggregate size. High cell viability was found for both single and adherent cells without an increase in cell death when agitation rate was increased. The increase in agitation rate affected aggregates by reducing their size and increasing their concentration and cell concentration in aggregates, while increasing the fraction of free cells in suspension. The experimental relationship between aggregate size and power dissipation rate per unit of mass was close to -1/4, suggesting a correlation with a critical turbulence microscale; this was independent of vessel scale and impeller geometry over the range investigated. Viscous stresses in the viscous dissipation subrange (below Kolmogoroff eddies) appear to be responsible for aggregate breakage. Under intense agitation BHK cells grown in the absence of microcarriers existed as aggregates without cell damage, whereas cells grown on the surface of microcarriers were largely reduced. This is a clear advantage for scaleup purposes if aggregates are used as a natural immobilization system in stirred vessels. (c) 1995 John Wiley & Sons, Inc.

Effects of shear stress and stretch on endothelial function

Vascular endothelial cells (ECs) play a central role in the control of blood vessel function and circulatory system homeostasis. It is well known that that EC functions are regulated by chemical mediators, including hormones, cytokines, and neurotransmitters, but it has recently become apparent that EC functions are also controlled by hemodynamic forces such as shear stress and stretch (cyclic strain). ECs recognize shear stress and cyclic strain as mechanical stimuli, and transmit the signal into the interior of the cells, thereby triggering a variety of cellular responses that involve alterations in cell morphology, cell function, and gene expression. Impaired EC responses to shear stress and cyclic strain lead to vascular diseases, including hypertension, thrombosis, and atherosclerosis. A great deal of research has already been conducted on the mechanotransduction of shear stress and cyclic strain, and its molecular mechanisms are gradually coming to be understood. However, much remains unclear, and further studies of mechanotransduction should increase our understanding of the molecular basis of the hemodynamic-force-mediated control of vascular functions.

Optimization of flowrate for expansion of human embryonic stem cells in perfusion microbioreactors

Microfluidic systems create significant opportunities to establish highly controlled microenvironmental conditions for screening pluripotent stem cell fate. However, since cell fate is crucially dependent on this microenvironment, it remains unclear as to whether continual perfusion of culture medium supports pluripotent stem cell maintenance in feeder-free, chemically defined conditions, and further, whether optimum perfusion conditions exist for subsequent use of human embryonic stem cell (hESCs) in other microfludic systems. To investigate this, we designed microbioreactors based on resistive flow to screen hESCs under a linear range of flowrates. We report that at low rates (conditions where glucose transport is convection-limited with Péclet number <1), cells are affected by apparent nutrient depletion and waste accumulation, evidenced by reduced cell expansion and altered morphology. At higher rates, cells are spontaneously washed out, and display morphological changes which may be indicative of early-stage differentiation. However, between these thresholds exists a narrow range of flowrates in which hESCs expand comparably to the equivalent static culture system, with regular morphology and maintenance of the pluripotency marker TG30 in >95% of cells over 7 days. For MEL1 hESCs the optimum flowrate also coincided with the time-averaged medium exchange rate in static cultures, which may therefore provide a good first estimate of appropriate perfusion rates. Overall, we demonstrate hESCs can be maintained in microbioreactors under continual flow for up to 7 days, a critical outcome for the future development of microbioreactor-based screening systems and assays for hESC culture.

Vitronectin enhances adhesion force and t-PA production of weakly adherent 293 cells exposed to a shear stress

The effects of shear stress on the adhesion andproductivity of 293 cells were studied quantitativelyand compared with those of Vero and human liver cells.These cells were cultured in polystyrene dishes byusing shear stress exposing equipment. 50% of 293cells cultured in 2% FBS supplemented medium detachedfrom the dish after 29 h of exposure to a shear stressof 0.10 Pa. On the other hand, 90% of Vero and humanliver cells remained on the dish under the samecondition. Observations with scanning electronmicroscopy about cell adhesion plaques on the surfaceof the dish showed that the area covered withlamellipodia and the number of microspikes for 293cells were found to be less than those of the othercell lines. Several attachment factors, especiallyvitronectin, were found to enhance the number ofmicrospikes and the adhesion force of 293 cells.Almost 100% of 293 cells remained on thevitronectin-coated dish after 40 h under 0.10 Pa ofshear stress. A higher shear stress (greater than 0.10Pa) caused a decrease in tissue plasminogen activator(t-PA) productivity of 293 cells. But 0.03 Pastimulated the t-PA secretion on the non-coated dish.Vitronectin also enhanced the t-PA secretion evenunder 0.10 Pa. These results indicate that theadhesion force of 293 cells is obviously weaker thanthat of the other cell lines, and vitronectin enhancesthe adhesion force and the productivity of 293 cellsexposed to a shear stress.

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.

Three-dimensional culture and bioreactors for cellular therapies

A bioreactor is defined as a specifically designed vessel to facilitate the growth of organisms and cells through application of physical and/or electrical stimulus. When cells with therapeutic potential were first discovered, they were initially cultured and expanded in two-dimensional (2-D) culture vessels such as plates or T-flasks. However, it was soon discovered that bioreactors could be used to expand and maintain cultures more easily and efficiently. Since then, bioreactors have come to be accepted as an indispensable tool to advance cell and tissue culture further. A wide array of bioreactors has been developed to date, and in recent years businesses have started supplying bioreactors commercially. Bioreactors in the research arena range from stirred tank bioreactors for suspension culture to those with various mechanical actuators that can apply different fluidic and mechanical stresses to tissues and three-dimensional (3-D) scaffolds. As regenerative medicine gains more traction in the clinic, bioreactors for use with cellular therapies are being developed and marketed. While many of the simpler bioreactors are fit for purpose, others fail to satisfy the complex requirements of tissues in culture. We have examined the use of different types of bioreactors in regenerative medicine and evaluated the application of bioreactors in the realization of emerging cellular therapies.

Agitation can induce differentiation of human pluripotent stem cells in microcarrier cultures

One of the factors that can impact human embryonic stem cell expansion in stirred microcarrier culture reactors is mechanical stress caused by agitation. Therefore, we have investigated the effects of agitation on human embryonic stem cell growth and expression of pluripotent markers. Agitation of HES-2 cell line in microcarrier cultures in stirred spinner and agitated six-well plates did not affect expression of pluripotent markers, cell viability, and cell doubling times even after seven passages. However, HES-3 cell line was found to be shear sensitive, showing downregulation of three pluripotent markers Oct-4, mAb 84, and Tra-1-60, and lower cell densities in agitated as compared with static cultures, even after one passage. Cell viability was unaffected. The HES-3-agitated cultures showed increased expression of genes and proteins of the three germ layers. We were unable to prevent loss of pluripotent markers or restore doubling times in agitated HES-3 microcarrier cultures by addition of five different known cell protective polymers. In addition, the human induced pluripotent cell line IMR90 was also shown to differentiate in agitated conditions. These results indicate that the effect of agitation on cell growth and differentiation is cell line specific. We assume that the changes in the growth and differentiation of the agitation-sensitive (HES-3) cell line do not result from the effect of shear stress directly on cell viability, but rather by signaling effects that influence the cells to differentiate resulting in slower growth.

Scalable stirred-suspension bioreactor culture of human pluripotent stem cells

Advances in stem cell biology have afforded promising results for the generation of various cell types for therapies against devastating diseases. However, a prerequisite for realizing the therapeutic potential of stem cells is the development of bioprocesses for the production of stem cell progeny in quantities that satisfy clinical demands. Recent reports on the expansion and directed differentiation of human embryonic stem cells (hESCs) in scalable stirred-suspension bioreactors (SSBs) demonstrated that large-scale production of therapeutically useful hESC progeny is feasible with current state-of-the-art culture technologies. Stem cells have been cultured in SSBs as aggregates, in microcarrier suspension and after encapsulation. The various modes in which SSBs can be employed for the cultivation of hESCs and human induced pluripotent stem cells (hiPSCs) are described. To that end, this is the first account of hiPSC cultivation in a microcarrier stirred-suspension system. Given that cultured stem cells and their differentiated progeny are the actual products used in tissue engineering and cell therapies, the impact of bioreactor's operating conditions on stem cell self-renewal and commitment should be considered. The effects of variables specific to SSB operation on stem cell physiology are discussed. Finally, major challenges are presented which remain to be addressed before the mainstream use of SSBs for the large-scale culture of hESCs and hiPSCs.

Transient shear stresses on a suspension cell in turbulence

A model flow field representative of Kolmogorov eddies in turbulence is proposed, and its two parameters are expressed in terms of the known bioreactor variables epsilon, the rate of turbulent power dissipation, and nu, the fluid kinematic viscosity. The trajectory through this flow field of a small sphere representing a cell is determined, and from that the time-varying local shear rate can be found. This allows calculation of the shear stress at any point on the sphere's surface as it rotates in and moves through the model eddy. The maximum shear stress imposed on the cell by the surrounding turbulence has a range of 0.5-5 dyn/cm(2), and can be estimated by 5.33rho(epsilonnu)(1/2). The shear stress has two major frequency components with ranges of 1-4 and 20-80 s(-1); the higher frequency component is estimated by 0.678(epsilon/nu)(1/2). The rotation of the direction of the shear stress vector at each point on the cell's surface is also discussed. Two ways in which external stresses might affect cell growth are proposed.

Computational modeling of flow-induced shear stresses within 3D salt-leached porous scaffolds imaged via micro-CT

Flow-induced shear stresses have been found to be a stimulatory factor in pre-osteoblastic cells seeded in 3D porous scaffolds and cultured under continuous flow perfusion. However, due to the complex internal structure of porous scaffolds, analytical estimation of the local shear forces is impractical. The primary goal of this work is to investigate the shear stress distributions within Poly(l-lactic acid) scaffolds via computation. Scaffolds used in this study are prepared via salt leeching with various geometric characteristics (80-95% porosity and 215-402.5microm average pore size). High resolution micro-computed tomography is used to obtain their 3D structure. Flow of osteogenic media through the scaffolds is modeled via lattice Boltzmann method. It is found that the surface stress distributions within the scaffolds are characterized by long tails to the right (a positive skewness). Their shape is not strongly dependent on the scaffold manufacturing parameters, but the magnitudes of the stresses are. Correlations are prepared for the estimation of the average surface shear stress experienced by the cells within the scaffolds and of the probability density function of the surface stresses. Though the manufacturing technique does not appear to affect the shape of the shear stress distributions, presence of manufacturing defects is found to be significant: defects create areas of high flow and high stress along their periphery. The results of this study are applicable to other polymer systems provided that they are manufactured by a similar salt leeching technique, while the imaging/modeling approach is applicable to all scaffolds relevant to tissue engineering.

Viscous reduction of turbulent damage in animal cell culture

Animal cells are exposed to turbulent fluid flow in many cell culture processes. If the turbulence in the flow is sufficiently strong, the cells will be damaged or killed by fluid-mechanical forces. Through an increase in viscosity, the turbulence can be damped and the hydro-dynamic damage can be reduced. In this article, new experimental results are presented which illustrate the protective effect of thickening agents. The results follow the prediction of a model based on Kolmogorov's theory of universal equilibrium in turbulent flow fields.

A fiber-bed bioreactor for anchorage-dependent animal cell cultures: part I. Bioreactor design and operations

A concentric-cylinder airlift reactor, in which the annulus is a packed bed of glass fibers, has been developed in order to facilitate the scaleup and enhance the volumetric productivity of anchorage-dependent animal cell cultures. In this bio-reactor, oxygen-containing gas is sparged through the inner draft tube, causing bubble-free medium to flow through the fiber bed in the outer cylinder and providing both oxygenation and convective nutrient transfer to the cells. Several other desirable features for reactor operation are also provided by this design. Cell cultivations in this bioreactor have been successfully carried out and provide data for the feasibility of the large-scale cell cultivation.

Vascular mechanobiology: endothelial cell responses to fluid shear stress

Endothelial cells (ECs) lining blood vessel walls respond to shear stress, a fluid mechanical force generated by flowing blood, and the EC responses play an important role in the homeostasis of the circulatory system. Abnormal EC responses to shear stress impair various vascular functions and lead to vascular diseases, including hypertension, thrombosis, and atherosclerosis. Bioengineering approaches in which cultured ECs are subjected to shear stress in fluid-dynamically designed flow-loading devices have been widely used to analyze EC responses at the cellular and molecular levels. Remarkable progress has been made, and the results have shown that ECs alter their morphology, function, and gene expression in response to shear stress. Shear stress affects immature cells, as well as mature ECs, and promotes differentiation of bone-marrow-derived endothelial progenitor cells and embryonic stem cells into ECs. Much research has been done on shear stress sensing and signal transduction, and their molecular mechanisms are gradually coming to be understood. However, much remains uncertain, and many candidates have been proposed for shear stress sensors. More extensive studies of vascular mechanobiology should increase our understanding of the molecular basis of the blood-flow-mediated control of vascular functions.

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.

Cardiac tissue engineering using perfusion bioreactor systems

This protocol describes tissue engineering of synchronously contractile cardiac constructs by culturing cardiac cell populations on porous scaffolds (in some cases with an array of channels) and bioreactors with perfusion of culture medium (in some cases supplemented with an oxygen carrier). The overall approach is 'biomimetic' in nature as it tends to provide in vivo-like oxygen supply to cultured cells and thereby overcome inherent limitations of diffusional transport in conventional culture systems. In order to mimic the capillary network, cells are cultured on channeled elastomer scaffolds that are perfused with culture medium that can contain oxygen carriers. The overall protocol takes 2-4 weeks, including assembly of the perfusion systems, preparation of scaffolds, cell seeding and cultivation, and on-line and end-point assessment methods. This model is well suited for a wide range of cardiac tissue engineering applications, including the use of human stem cells, and high-fidelity models for biological research.

Pulsatile perfusion bioreactor for cardiac tissue engineering

Cardiovascular disease is the number one cause of mortality in North America. Cardiac tissue engineering aims to engineer a contractile patch of physiological thickness to use in surgical repair of diseased heart tissue. We previously reported that perfusion of engineered cardiac constructs resulted in improved tissue assembly. Because heart tissues respond to mechanical stimuli in vitro and experience rhythmic mechanical forces during contraction in vivo, we hypothesized that provision of pulsatile interstitial medium flow to an engineered cardiac patch would result in enhanced tissue assembly by way of mechanical conditioning and improved mass transport. Thus, we constructed a novel perfusion bioreactor capable of providing pulsatile fluid flow at physiologically relevant shear stresses and flow rates. Pulsatile perfusion (PP) was achieved by incorporation of a normally closed solenoid pinch valve into the perfusion loop and was carried out at a frequency of 1 Hz and a flow rate of 1.50 mL/min (PP) or 0.32 mL/min (PP-LF). Nonpulsatile flow at 1.50 mL/min (NP) or 0.32 mL/min (NP-LF) served as controls. Static controls were cultivated in well plates. The main experimental groups were seeded with cells enriched for cardiomyocytes by one preplating step (64% cardiac Troponin I+, 34% prolyl-4-hydroxylase+), whereas pure cardiac fibroblasts and cells enriched for cardiomyocytes by two preplating steps (81% cardiac Troponin I+, 16% prolyl-4-hydroxylase+) served as controls. Cultivation under pulsatile flow had beneficial effects on contractile properties. Specifically, the excitation threshold was significantly lower in the PP condition (pulsatile perfusion at 1.50 mL/min) than in the Static control, and the contraction amplitude was the highest; whereas high maximum capture rate was observed for the PP-LF conditions (pulsatile perfusion at 0.32 mL/min). The enhanced hypertrophy index observed for the PP-LF group was consistent with the highest cellular length and diameter in this group. Within the same cultivation groups (Static, NP-LF, PP-LF, PP, and NP) there were no significant differences in the diameter between fibroblasts and cardiomyocytes, although cardiomyocytes were significantly more elongated than fibroblasts under PP-LF conditions. Cultivation of control cell populations resulted in noncontractile constructs when cardiac fibroblasts were used (as expected) and no overall improvement in functional properties when two steps of preplating were used to enrich for cardiomyocytes in comparison with only one step of preplating.

Differential gene responses in endothelial cells exposed to a combination of shear stress and cyclic stretch

We developed a compliant tube-type flow-loading apparatus that allows simultaneous application of physiological levels of shear stress and cyclic stretch to cultured cells and examined gene responses to a combination of the two forces. Human umbilical vein endothelial cells were exposed to shear stress and/or cyclic stretch for 24h, and changes in the mRNA levels of endothelin-1 (ET-1), a potent vasoconstrictor, and endothelial nitric oxide synthase (eNOS), which catalyzes the production of a potent vasodilator, NO, were determined by reverse transcriptase/PCR. Cyclic stretch (10%, 1 Hz) alone increased ET-1 mRNA levels approximately 1.6-fold, but had no effect on eNOS mRNA levels. A shear stress of 7 dynes/cm(2) and 15 dynes/cm(2) alone decreased ET-1 mRNA levels to around 83% and 61%, respectively, of the basal level, but increased the eNOS mRNA level to around 2.2-fold and 3.2-fold, respectively. When cyclic stretch and shear stress were applied simultaneously, ET-1 mRNA levels did not change significantly, but the eNOS mRNA level increased to a level equivalent to the increase in response to shear stress alone. These results indicate that the response of endothelial genes to shear stress or cyclic stretch depends on whether the two forces are applied separately or together.

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.).

Synthetic oxygen carriers in cardiac tissue engineering

The prominence of cardiovascular diseases has prompted investigations into alternative treatment options, including tissue engineering. Currently, the biggest limitation in cardiac tissue engineering lies in delivering oxygen to all cells within the construct. Synthetic oxygen carriers hold much promise in that they have high affinity for oxygen and can be supplemented to culture medium without adverse effect on the cells. This review highlights two complementary studies by our group that utilized oxygen carriers in cardiac tissue engineering. Experimental and modeling studies were performed to evaluate the effect of a perfluorocarbon (PFC)-based synthetic oxygen carrier, Oxygent, on oxygen supply within tissue engineered cardiac constructs. Porous biorubber scaffolds with an array of parallel channels mimicking the capillary network were seeded with cardiomyocytes and fibroblasts, and cultivated in medium supplemented with PFC. The presence of PFC enhanced the transport of oxygen, increased oxygen concentrations, and yielded constructs that displayed stronger cardiac-like phenotype.

A novel perfusion bioreactor providing a homogenous milieu for tissue regeneration

We developed a novel perfusion bioreactor that is capable of cultivating multiple 3-dimensional (3D) cellular constructs in one flow chamber with a total cross-section area of 20 cm(2). Two unique features integrated into the bioreactor provided a homogenous fluid flow along the bioreactor cross-section and maximal exposure of the cellular constructs to the perfusing medium. Mathematical modeling of the fluid flow regime in the perfusion bioreactor showed that integrating a flow-distributing mesh 1.5 cm upstream from the construct compartment imposed an equal medium flow and shear stress of 0.6 dynes /cm(2) along the entire cell construct cross-section area. The design of 95.8%open-pore-area fixing nets enabled the exposure of 99.88% of the cell construct volume to the perfusing medium. Cardiac cell constructs seeded with physiologically relevant cell density (0.7 x 10(8) cells/cm(3)) in alginate scaffolds developed into homogenous compacted cardiac tissue, as judged using cell staining with fluorescein diacetate and hematoxylin-eosin histology. The cell constructs maintained 80% viability for nearly 2 weeks, whereas in static-cultivated cell constructs, only 50% of the initial cells remained, as determined according to total DNA content and MTT viability assay. Medium perfusion resulted in better cell viability, presumably due to the convective-diffusive transport of oxygen, compared with oxygen diffusion within the static-cultivated cell constructs, as well as due to efficient removal of harmful cell secretions. It is envisioned that this bioreactor would be useful for 3D cultivation of different mammalian cells for purposes of tissue engineering or production of valuable biologicals.

Size-based microfluidic enrichment of neonatal rat cardiac cell populations

Native heart consists of myocytes and non-myocytes. We demonstrate here the feasibility of a size-based microfluidic separation of myocytes and non-myocytes from the neonatal rat myocardium. The device consists of a middle channel (50 microm wide, 200 microm tall, and 4 cm long) connected to adjacent side channels by microsieves (80 microm wide, 5 microm tall and 40 microm in length). The side channels increase in width in a flared shape along the length of the device to ensure constant pressure gradient across all sieves. In the first step, non-myocytes were removed from the myocytes by a conventional pre-plating method for 75 min. Subsequently, the non-myocytes were further enriched in a microfluidic device at 20 microl/min. We demonstrated that the cells in the middle and side channels maintained viability during sorting and the ability to attach and grow in culture. Upon culture for 48 h cardiomyocytes from the reservoir (control) and middle channel stained positive for cardiac Troponin I, exhibited a well developed contractile apparatus and contracted spontaneously and in response to electrical field stimulation. Most of the cells in the side channel expressed a non-myocyte marker vimetin. Fluorescent activated cell sorting indicated significant enrichment in the side channel (p < 0.001) for non-myocytes. Original cell suspension had a bimodal cell size distribution with the peaks in the range from 7-9 microm and 15-17 microm. Upon cell sorting the distribution was Gaussian in both side channel and middle channel with the peaks in the range 7-9 microm and 9-11 microm respectively, indicating that the separation by size occurred.

Bioreactor for mammalian cell culture

Purpose of this article is to review the current status of bioreactor design for mammalian cell culture. Morphological and biochemical features of two major mammalian cell groups, anchorage-dependent and independent cells are proposed as a basis for different behavior at their cultivation. Different bioreactor configurations are systematically discussed through enumerating elementary physical phenomena and through stressing their physiological significance. Special considerations are given to those areas which are inherent to mammalian cell bioreactor.

Ultra scale-down studies of the effect of flow and impact conditions duringE. coli cell processing

The ability to recover cells from a fermentation broth in an intact form can be an important criterion for determining the overall performance of a recovery and purification sequence. Disruption of the cells can lead to undesired contamination of an extracellular product with intracellular components and vice versa loss of intracellular products may occur. In particular, the value of directed location of a product in the periplasmic space of say Escherichia coli (E. coli) would be diminished by such premature non-selective cell disruption. Several options exist for cell recovery/removal; namely centrifugation, in batch or continuous configuration, filtration or membrane operations, and in selected cases expanded beds. The choice of operation is dependant on many variables including the impact on the overall process sequence. In all cases, the cells are exposed to shear stresses of varying levels and times and additionally such environments exist in ancillary operations such as pumping, pipe flow, and control valves. In this study, a small-scale device has been designed to expose cells to controlled levels of shear, time and impact in a way that seeks to mimic those effects that may occur during full-scale processes. The extent of cell breakage was found to be proportional to shear stress. An additional level of breakage occurred due to the jet impacting on the collecting surface. Here it was possible to correlate the additional breakage with the impact velocity, which is a function of the distance that the jet travels before meeting the collection surface and the initial jet velocity.