A microfluidic platform for studying the effects of small temperature gradients in an incubator environment

An integrative temperature-controlled microfluidic system for budding yeast heat shock response analysis at the single-cell level

Cells can respond and adapt to complex forms of environmental change. Budding yeast is widely used as a model system for these stress response studies. In these studies, the precise control of the environment with high temporal resolution is most important. However, there is a lack of single-cell research platforms that enable precise control of the temperature and form of cell growth. This has hindered our understanding of cellular coping strategies in the face of diverse forms of temperature change. Here, we developed a novel temperature-controlled microfluidic platform that integrates a microheater (using liquid metal) and a thermocouple (liquid metal vs. conductive PDMS) on a chip. Three forms of temperature changes (step, gradient, and periodical oscillations) were realized by automated equipment. The platform has the advantages of low cost and a simple fabrication process. Moreover, we investigated the nuclear entry and exit behaviors of the transcription factor Msn2 in yeast in response to heat stress (37 °C) with different heating modes. The feasibility of this temperature-controlled platform for studying the protein dynamic behavior of yeast cells was demonstrated.

Numerical evaluation and experimental validation of fluid flow behavior within an organ-on-a-chip model

BACKGROUND AND OBJECTIVE

By combining biomaterials, cell culture, and microfluidic technology, organ-on-a-chip (OoC) platforms have the ability to reproduce the physiological microenvironment of human organs. For this reason, these advanced microfluidic devices have been used to resemble various diseases and investigate novel treatments. In addition to the experimental assessment, numerical studies of biodevices have been performed aiming at their improvement and optimization. Despite considerable progress in numerical modeling of biodevices, the validation of these computational models through comparison with experimental assays remains a significant gap in the current literature. This step is critical to ensure the accuracy and reliability of numerical models, and consequently enhance confidence in their predictive results. The aim of the present work is to develop a numerical model capable of reproducing the fluid flow behavior within an OoC, for future investigations, encompassing the geometry optimization.

METHODS

In this study, the validation of a numerical model for an OoC microfluidic device was undertaken. This comprised both quantitative and qualitative assessments of trace microparticles flowing through a physical OoC model. High-speed microscopy images of the flow, using a blood analog fluid, were analyzed and compared with the numerical simulations run using the Ansys Fluent software. For a qualitative analysis, the particles' paths through the inlet and bifurcations were observed whereas, for a quantitative analysis, the particle velocities were measured. Furthermore, oxygen transport was simulated and evaluated for different Reynolds numbers.

RESULTS

In both qualitative and quantitative analyses, the results predicted by the numerical model and the ones outputted by the experimental model were in good agreement. These findings underscore the capability and potential of the developed numerical model. The examination of oxygen transport at various vertical positions within the organoid has revealed that for lower positions, oxygen transport predominantly occurs through diffusion, leading to a symmetric distribution of oxygen. Contrastingly, the convection phenomenon becomes more evident in the upper region of the organoid.

CONCLUSIONS

The successful validation of the numerical model against experimental data shows its accuracy and reliability in simulating the fluid flow within the OoC, which consequently can expedite the OoC design process by reducing the need for prototypes' fabrication and costly laboratory experiments.

Temperature-sensitive migration dynamics in neutrophil-differentiated HL-60 cells

Cell migration plays an essential role in wound healing and inflammatory processes inside the human body. Peripheral blood neutrophils, a type of polymorphonuclear leukocyte (PMN), are the first cells to be activated during inflammation and subsequently migrate toward an injured tissue or infection site. This response is dependent on both biochemical signaling and the extracellular environment, one aspect of which includes increased temperature in the tissues surrounding the inflammation site. In our study, we analyzed temperature-dependent neutrophil migration using differentiated HL-60 cells. The migration speed of differentiated HL-60 cells was found to correlate positively with temperature from 30 to 42 °C, with higher temperatures inducing a concomitant increase in cell detachment. The migration persistence time of differentiated HL-60 cells was higher at lower temperatures (30-33 °C), while the migration persistence length stayed constant throughout the temperature range. Coupled with the increased speed observed at high temperatures, this suggests that neutrophils are primed to migrate more effectively at the elevated temperatures characteristic of inflammation. Temperature gradients exist on both cell and tissue scales. Taking this into consideration, we also investigated the ability of differentiated HL-60 cells to sense and react to the presence of temperature gradients, a process known as thermotaxis. Using a two-dimensional temperature gradient chamber with a range of 27-43 °C, we observed a migration bias parallel to the gradient, resulting in both positive and negative thermotaxis. To better mimic the extracellular matrix (ECM) environment in vivo, a three-dimensional collagen temperature gradient chamber was constructed, allowing observation of biased neutrophil-like differentiated HL-60 migration toward the heat source.

Computational Simulations in Advanced Microfluidic Devices: A Review

Numerical simulations have revolutionized research in several engineering areas by contributing to the understanding and improvement of several processes, being biomedical engineering one of them. Due to their potential, computational tools have gained visibility and have been increasingly used by several research groups as a supporting tool for the development of preclinical platforms as they allow studying, in a more detailed and faster way, phenomena that are difficult to study experimentally due to the complexity of biological processes present in these models—namely, heat transfer, shear stresses, diffusion processes, velocity fields, etc. There are several contributions already in the literature, and significant advances have been made in this field of research. This review provides the most recent progress in numerical studies on advanced microfluidic devices, such as organ-on-a-chip (OoC) devices, and how these studies can be helpful in enhancing our insight into the physical processes involved and in developing more effective OoC platforms. In general, it has been noticed that in some cases, the numerical studies performed have limitations that need to be improved, and in the majority of the studies, it is extremely difficult to replicate the data due to the lack of detail around the simulations carried out.

A practical microfluidic pump enabled by acoustofluidics and 3D printing

Simple and low-cost solutions are becoming extremely important for the evolving necessities of biomedical applications. Even though, on-chip sample processing and analysis has been rapidly developing for a wide range of screening and diagnostic protocols, efficient and reliable fluid manipulation in microfluidic platforms still require further developments to be considered portable and accessible for low-resource settings. In this work, we present an extremely simple microfluidic pumping device based on three-dimensional (3D) printing and acoustofluidics. The fabrication of the device only requires 3D-printed adaptors, rectangular glass capillaries, epoxy and a piezoelectric transducer. The pumping mechanism relies on the flexibility and complexity of the acoustic streaming patterns generated inside the capillary. Characterization of the device yields controllable and continuous flow rates suitable for on-chip sample processing and analysis. Overall, a maximum flow rate of ~ 12 μL/min and the control of pumping direction by frequency tuning is achieved. With its versatility and simplicity, this microfluidic pumping device offers a promising solution for portable, affordable and reliable fluid manipulation for on-chip applications.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10404-020-02411-w.

Transportable system enabling multiple irradiation studies under simultaneous hypoxia in vitro

Background

Cells in solid tumours are variably hypoxic and hence resistant to radiotherapy - the essential role of oxygen in the efficiency of irradiation has been acknowledged for decades. However, the currently available methods for performing hypoxic experiments in vitro have several limitations, such as a limited amount of parallel experiments, incapability of keeping stable growth conditions and dependence on CO₂ incubator or a hypoxia workstation. The purpose of this study was to evaluate the usability of a novel portable system (Minihypoxy) in performing in vitro irradiation studies under hypoxia, and present supporting biological data.

Materials and methods

This study was conducted on cancer cell cultures in vitro. The cells were cultured in normoxic (~ 21% O₂) or in hypoxic (1% O₂) conditions either in conventional hypoxia workstation or in the Minihypoxy system and irradiated at dose rate 1.28 Gy/min ± 2.9%. The control samples were sham irradiated. To study the effects of hypoxia and irradiation on cell viability and DNA damage, western blotting, immunostainings and clonogenic assay were used. The oxygen level, pH, evaporation rate and osmolarity of the culturing media on cell cultures in different conditions were followed.

Results

The oxygen concentration in interest (5, 1 or 0% O₂) was maintained inside the individual culturing chambers of the Minihypoxy system also during the irradiation. The radiosensitivity of the cells cultured in Minihypoxy chambers was declined measured as lower phosphorylation rate of H2A.X and increased clonogenic capacity compared to controls (OER~ 3).

Conclusions

The Minihypoxy system allows continuous control of hypoxic environment in multiple wells and is transportable. Furthermore, the system maintains the low oxygen environment inside the individual culturing chambers during the transportation and irradiation in experiments which are typically conducted in separate facilities.

Electronic supplementary material

The online version of this article (10.1186/s13014-018-1169-9) contains supplementary material, which is available to authorized users.

Thermometry of photosensitive and optically induced electrokinetics chips

Optically induced electrokinetics (OEK)-based technologies, which integrate the high-resolution dynamic addressability of optical tweezers and the high-throughput capability of electrokinetic forces, have been widely used to manipulate, assemble, and separate biological and non-biological entities in parallel on scales ranging from micrometers to nanometers. However, simultaneously introducing optical and electrical energy into an OEK chip may induce a problematic temperature increase, which poses the potential risk of exceeding physiological conditions and thus inducing variations in cell behavior or activity or even irreversible cell damage during bio-manipulation. Here, we systematically measure the temperature distribution and changes in an OEK chip arising from the projected images and applied alternating current (AC) voltage using an infrared camera. We have found that the average temperature of a projected area is influenced by the light color, total illumination area, ratio of lighted regions to the total controlled areas, and amplitude of the AC voltage. As an example, optically induced thermocapillary flow is triggered by the light image-induced temperature gradient on a photosensitive substrate to realize fluidic hydrogel patterning. Our studies show that the projected light pattern needs to be properly designed to satisfy specific application requirements, especially for applications related to cell manipulation and assembly.

Nano-Architectural Approaches for Improved Intracortical Interface Technologies

Intracortical microelectrodes (IME) are neural devices that initially were designed to function as neuroscience tools to enable researchers to understand the nervous system. Over the years, technology that aids interfacing with the nervous system has allowed the ability to treat patients with a wide range of neurological injuries and diseases. Despite the substantial success that has been demonstrated using IME in neural interface applications, these implants eventually fail due to loss of quality recording signals. Recent strategies to improve interfacing with the nervous system have been inspired by methods that mimic the native tissue. This review focusses on one strategy in particular, nano-architecture, a term we introduce that encompasses the approach of roughening the surface of the implant. Various nano-architecture approaches have been hypothesized to improve the biocompatibility of IMEs, enhance the recording quality, and increase the longevity of the implant. This review will begin by introducing IME technology and discuss the challenges facing the clinical deployment of IME technology. The biological inspiration of nano-architecture approaches will be explained as well as leading fabrication methods used to create nano-architecture and their limitations. A review of the effects of nano-architecture surfaces on neural cells will be examined, depicting the various cellular responses to these modified surfaces in both in vitro and pre-clinical models. The proposed mechanism elucidating the ability of nano-architectures to influence cellular phenotype will be considered. Finally, the frontiers of next generation nano-architecture IMEs will be identified, with perspective given on the future impact of this interfacing approach.

Microfluidic Devices Developed for and Inspired by Thermotaxis and Chemotaxis

Taxis has been reported in many cells and microorganisms, due to their tendency to migrate toward favorable physical situations and avoid damage and death. Thermotaxis and chemotaxis are two of the major types of taxis that naturally occur on a daily basis. Understanding the details of the thermo- and chemotactic behavioral response of cells and microorganisms is necessary to reveal the body function, diagnosing diseases and developing therapeutic treatments. Considering the length-scale and range of effectiveness of these phenomena, advances in microfluidics have facilitated taxis experiments and enhanced the precision of controlling and capturing microscale samples. Microfabrication of fluidic chips could bridge the gap between in vitro and in situ biological assays, specifically in taxis experiments. Numerous efforts have been made to develop, fabricate and implement novel microchips to conduct taxis experiments and increase the accuracy of the results. The concepts originated from thermo- and chemotaxis, inspired novel ideas applicable to microfluidics as well, more specifically, thermocapillarity and chemocapillarity (or solutocapillarity) for the manipulation of single- and multi-phase fluid flows in microscale and fluidic control elements such as valves, pumps, mixers, traps, etc. This paper starts with a brief biological overview of the concept of thermo- and chemotaxis followed by the most recent developments in microchips used for thermo- and chemotaxis experiments. The last section of this review focuses on the microfluidic devices inspired by the concept of thermo- and chemotaxis. Various microfluidic devices that have either been used for, or inspired by thermo- and chemotaxis are reviewed categorically.

Automated tracking and quantification of angiogenic vessel formation in 3D microfluidic devices

Angiogenesis, the growth of new blood vessels from pre-existing vessels, is a critical step in cancer invasion. Better understanding of the angiogenic mechanisms is required to develop effective antiangiogenic therapies for cancer treatment. We culture angiogenic vessels in 3D microfluidic devices under different Sphingosin-1-phosphate (S1P) conditions and develop an automated vessel formation tracking system (AVFTS) to track the angiogenic vessel formation and extract quantitative vessel information from the experimental time-lapse phase contrast images. The proposed AVFTS first preprocesses the experimental images, then applies a distance transform and an augmented fast marching method in skeletonization, and finally implements the Hungarian method in branch tracking. When applying the AVFTS to our experimental data, we achieve 97.3% precision and 93.9% recall by comparing with the ground truth obtained from manual tracking by visual inspection. This system enables biologists to quantitatively compare the influence of different growth factors. Specifically, we conclude that the positive S1P gradient increases cell migration and vessel elongation, leading to a higher probability for branching to occur. The AVFTS is also applicable to distinguish tip and stalk cells by considering the relative cell locations in a branch. Moreover, we generate a novel type of cell lineage plot, which not only provides cell migration and proliferation histories but also demonstrates cell phenotypic changes and branch information.

Effect of gold nanoparticles on thermal gradient generation and thermotaxis of E. coli cells in microfluidic device

Bacteria responds to changing chemical and thermal environment by moving towards or away from a particular location. In this report, we looked into thermal gradient generation and response of E. coli DH5α cells to thermal gradient in the presence and in the absence of spherical gold nanoparticles (size: 15 to 22 nm) in a static microfluidic environment using a polydimethylsiloxane (PDMS) made microfluidic device. A PDMS-agarose based microfluidic device for generating thermal gradient has been developed and the thermal gradient generation in the device has been validated with the numerical simulation. Our studies revealed that the presence of gold nanoparticles, AuNPs (0.649 μg/mL) has no effect on the thermal gradient generation. The E. coli DH5α cells have been treated with AuNPs of two different concentrations (0.649 μg/mL and 0.008 μg/mL). The thermotaxis behavior of cells in the presence of AuNPs has been studied and compared to the thermotaxis of E.coli DH5α cells in the absence of AuNPs. In case of thermotaxis, in the absence of the AuNPs, the E. coli DH5α cells showed better thermotaxis towards lower temperature range, whereas in the presence of AuNPs (0.649 μg/mL and 0.008 μg/mL) thermotaxis of the E. coli DH5α cells has been inhibited. The results show that the spherical AuNPs intervenes in the themotaxis of E. coli DH5α cells and inhibits the cell migration. The reason for the failure in thermotaxis response mechanism may be due to decreased F-type ATP synthase activity and collapse of membrane potential by AuNPs, which, in turn, leads to decreased ATP levels. This has been hypothesized since both thermotaxis and chemotaxis follows the same response mechanism for migration in which ATP plays critical role.

Interplay of chemical and thermal gradient on bacterial migration in a diffusive microfluidic device

Living systems are constantly under different combinations of competing gradients of chemical, thermal, pH, and mechanical stresses allied. The present work is about competing chemical and thermal gradients imposed on E. coli in a diffusive stagnant microfluidic environment. The bacterial cells were exposed to opposing and aligned gradients of an attractant (1 mM sorbitol) or a repellant (1 mM NiSO4) and temperature. The effects of the repellant/attractant and temperature on migration behavior, migration rate, and initiation time for migration have been reported. It has been observed that under competing gradients of an attractant and temperature, the nutrient gradient (gradient generated by cells itself) initiates directed migration, which, in turn, is influenced by temperature through the metabolic rate. Exposure to competing gradients of an inhibitor and temperature leads to the imposed chemical gradient governing the directed cell migration. The cells under opposing gradients of the repellant and temperature have experienced the longest decision time (∼60 min). The conclusion is that in a competing chemical and thermal gradient environment in the range of experimental conditions used in the present work, the migration of E. coli is always initiated and governed by chemical gradients (either generated by the cells in situ or imposed upon externally), but the migration rate and percentage of migration of cells are influenced by temperature, shedding insights into the importance of such gradients in deciding collective dynamics of such cells in physiological conditions.

E. coli DH5α cell response to a sudden change in microfluidic chemical environment

Motile bacteria respond to changing chemical environment by moving towards or away from a particular location. Bacterial migration under chemical gradient is one of the most studied areas in biomedical field. In this work we looked into how bacterial cells respond to sudden change in the microfluidic chemical environment. E. coli DH5α cells were subjected to an attractant gradient (0.1 mM sorbitol--attractant to E. coli cells) and after 120 min the same cells were exposed to an inhibitor (0.1 mM NiSO4) gradient in the same microfluidic device. Our studies revealed that when the E. coli DH5α cells were exposed to 0.1 mM sorbitol, they showed faster chemotaxis towards the attractant (0.1 mM sorbitol) and achieved steady state by 60 min. When we replaced 0.1 mM sorbitol with 0.1 mM NiSO4 in the device we found that that the E. coli DH5α cells started responding to change in chemical environment within 10 min and achieved steady state at the end of 60 min. This shows that the bacterial cells respond to change in local chemical environment is within few minutes.

Nanopatterning effects on astrocyte reactivity

An array of design strategies have been targeted toward minimizing failure of implanted microelectrodes by minimizing the chronic glial scar around the microelectrode under chronic conditions. Current approaches toward inhibiting the initiation of glial scarring range from altering the geometry, roughness, size, shape, and materials of the device. Studies have shown materials which mimic the nanotopography of the natural environment in vivo will consequently result in an improved biocompatible response. Nanofabrication of electrode arrays is being pursued in the field of neuronal electrophysiology to increase sampling capabilities. Literature shows a gap in research of nanotopography influence in the reduction of astrogliosis. The aim of this study was to determine optimal feature sizes for neural electrode fabrication, which was defined as eliciting a nonreactive astrocytic response. Nanopatterned surfaces were fabricated with nanoimprint lithography on poly(methyl methacrylate) surfaces. The rate of protein adsorption, quantity of protein adsorption, cell alignment, morphology, adhesion, proliferation, viability, and gene expression was compared between nanopatterned surfaces of different dimensions and non-nanopatterned control surfaces. Results of this study revealed that 3600 nanopatterned surfaces elicited less of a response when compared with the other patterned and non-nanopatterned surfaces. The surface instigated cell alignment along the nanopattern, less protein adsorption, less cell adhesion, proliferation and viability, inhibition of glial fibrillary acidic protein, and mitogen-activated protein kinase kinase 1 compared with all other substrates tested.

A novel microfluidic platform for high-resolution imaging of a three-dimensional cell culture under a controlled hypoxic environment

Low oxygen tensions experienced in various pathological and physiological conditions are a major stimulus for angiogenesis. Hypoxic conditions play a critical role in regulating cellular behaviour including migration, proliferation and differentiation. This study introduces the use of a microfluidic device that allows for the control of oxygen tension for the study of different three-dimensional (3D) cell cultures for various applications. The device has a central 3D gel region acting as an external cellular matrix, flanked by media channels. On each side, there is a peripheral gas channel through which suitable gas mixtures are supplied to establish a uniform oxygen tension or gradient within the device. The effects of various parameters, such as gas and media flow rates, device thickness, and diffusion coefficients of oxygen were examined using numerical simulations to determine the characteristics of the microfluidic device. A polycarbonate (PC) film with a low oxygen diffusion coefficient was embedded in the device in proximity above the channels to prevent oxygen diffusion from the incubator environment into the polydimethylsiloxane (PDMS) device. The oxygen tension in the device was then validated experimentally using a ruthenium-coated (Ru-coated) oxygen-sensing glass cover slip which confirmed the establishment of low uniform oxygen tensions (<3%) or an oxygen gradient across the gel region. To demonstrate the utility of the microfluidic device for cellular experiments under hypoxic conditions, migratory studies of MDA-MB-231 human breast cancer cells were performed. The microfluidic device allowed for imaging cellular migration with high-resolution, exhibiting an enhanced migration in hypoxia in comparison to normoxia. This microfluidic device presents itself as a promising platform for the investigation of cellular behaviour in a 3D gel scaffold under varying hypoxic conditions.

Mapping of hyperthermic tumor cell death in a microchannel under unidirectional heating

Hyperthermia can be used as an adjunctive method of chemotherapy, radiotherapy, and gene therapy to improve cancer treatment. In this study, we investigate the hyperthermic cell death of cervix cancer CaSki cells in a microchannel integrated with a directional heating scheme. Heat was applied from the inner end to the outer end of the channel and a temperature distribution from 60 °C to 30 °C was established. A three dimensional (3D) numerical model was conducted for the heat transfer simulation, based on which a simple fitting method was proposed to easily estimate the temperature distribution along the channel. Cell death along the channel was mapped 22 h after the heating treatment by dual fluorescent labeling and phase-contrast microscopy imaging. Upstream, where the temperature is higher than 42 °C, we observe necrotic death, late-stage and early stage apoptotic death in sequence along the channel. Downstream and in the middle of the channel, where the temperature is lower than 42 °C, significant cell detachment was noted. Vigorous detachment was observed even in the non-hyperthermic zone (temperature lower than 37 °C), which we believe is due to the direct effect of the hyperthermic zones (higher than 37 °C). The present work not only gives a vivid map of cell responses under a temperature gradient, but also reveals the potential interactions of the heated tumor cells and non-heated tumor cells, which are seldom investigated in conventional petri-dish experiments.

Fabrication of microfluidic devices using polydimethylsiloxane

Polydimethylsiloxane (PDMS) is nearly ubiquitous in microfluidic devices, being easy to work with, economical, and transparent. A detailed protocol is provided here for using PDMS in the fabrication of microfluidic devices to aid those interested in using the material in their work, with information on the many potential ways the material may be used for novel devices.

Fabrication of microfluidic devices using polydimethylsiloxane

Polydimethylsiloxane (PDMS) is nearly ubiquitous in microfluidic devices, being easy to work with, economical, and transparent. A detailed protocol is provided here for using PDMS in the fabrication of microfluidic devices to aid those interested in using the material in their work, with information on the many potential ways the material may be used for novel devices.

Cell culture on MEMS platforms: a review

Microfabricated systems provide an excellent platform for the culture of cells, and are an extremely useful tool for the investigation of cellular responses to various stimuli. Advantages offered over traditional methods include cost-effectiveness, controllability, low volume, high resolution, and sensitivity. Both biocompatible and bio-incompatible materials have been developed for use in these applications. Biocompatible materials such as PMMA or PLGA can be used directly for cell culture. However, for bio-incompatible materials such as silicon or PDMS, additional steps need to be taken to render these materials more suitable for cell adhesion and maintenance. This review describes multiple surface modification strategies to improve the biocompatibility of MEMS materials. Basic concepts of cell-biomaterial interactions, such as protein adsorption and cell adhesion are covered. Finally, the applications of these MEMS materials in Tissue Engineering are presented.