Stable chemical bonding of porous membranes and poly(dimethylsiloxane) devices for long-term cell culture

Extracellular Matrix Stiffness: Mechanotransduction and Mechanobiological Response-Driven Strategies for Biomedical Applications Targeting Fibroblast Inflammation

The extracellular matrix (ECM) is a dynamic network providing mechanical and biochemical cues that regulate cellular behavior. ECM stiffness critically influences fibroblasts, the primary ECM producers, particularly in inflammation and fibrosis. This review explores the role of ECM stiffness in fibroblast-driven inflammation and tissue remodeling, focusing on the physicochemical and biological mechanisms involved. Engineered materials, hydrogels, and polydimethylsiloxane (PDMS) are highlighted for replicating tissue-specific stiffness, enabling precise control over cell-matrix interactions. The surface functionalization of substrate materials, including collagen, polydopamine, and fibronectin, enhances bioactivity and fibroblast adhesion. Key mechanotransduction pathways, such as integrin signaling and YAP/TAZ activation, are related to regulating fibroblast behaviors and inflammatory responses. The role of fibroblasts in driving chronic inflammatory diseases emphasizes their therapeutic potentials. Advances in ECM-modifying strategies, including tunable biomaterials and hydrogel-based therapies, are explored for applications in tissue engineering, drug delivery, anti-inflammatory treatments, and diagnostic tools for the accurate diagnosis and prognosis of ECM stiffness-related inflammatory diseases. This review integrates mechanobiology with biomedical innovations, providing a comprehensive prognosis of fibroblast responses to ECM stiffness and outlining future directions for targeted therapies.

Heart-on-a-chip systems: disease modeling and drug screening applications

Cardiovascular disease (CVD) is the leading cause of death worldwide, casting a substantial economic footprint and burdening the global healthcare system. Historically, pre-clinical CVD modeling and therapeutic screening have been performed using animal models. Unfortunately, animal models oftentimes fail to adequately mimic human physiology, leading to a poor translation of therapeutics from pre-clinical trials to consumers. Even those that make it to market can be removed due to unforeseen side effects. As such, there exists a clinical, technological, and economical need for systems that faithfully capture human (patho)physiology for modeling CVD, assessing cardiotoxicity, and evaluating drug efficacy. Heart-on-a-chip (HoC) systems are a part of the broader organ-on-a-chip paradigm that leverages microfluidics, tissue engineering, microfabrication, electronics, and gene editing to create human-relevant models for studying disease, drug-induced side effects, and therapeutic efficacy. These compact systems can be capable of real-time measurements and on-demand characterization of tissue behavior and could revolutionize the drug development process. In this review, we highlight the key components that comprise a HoC system followed by a review of contemporary reports of their use in disease modeling, drug toxicity and efficacy assessment, and as part of multi-organ-on-a-chip platforms. We also discuss future perspectives and challenges facing the field, including a discussion on the role that standardization is expected to play in accelerating the widespread adoption of these platforms.

Regulation of Corneal Stromal Cell Behavior by Modulating Curvature Using a Hydraulically Controlled Organ Chip Array

Curvature is a critical factor in cornea mechanobiology, but its impact on phenotypic alterations and extracellular matrix remodeling of cornea stroma remains unclear. In this work, we investigated how curvature influences the corneal stroma using a hydraulically controlled curvature array chip. The responses of stromal cells to low, medium, and high curvatures were observed by preparing three phenotypes of corneal stromal cells: corneal keratocytes, fibroblasts, and myofibroblasts. Keratocytes exhibited phenotypic alterations in response to curvature changes, notably including a decrease in ALDH3 expression and an increase in α-SMA expression. For focal adhesion, corneal fibroblast and myofibroblasts showed enhanced vinculin localization in response to curvature, while corneal keratocytes presented reduced vinculin expression. For cell alignment and ECM expression, most stromal cells under all curvatures showed a radially organized f-actin and collagen fibrils. Interestingly, for corneal fibroblast under medium curvature, we observed orthogonal cell alignment, which is linked to the unique hoop and meridional stress profiles of the curved surface. Furthermore, lumican expression was upregulated in corneal keratocytes, and keratocan expression was increased in corneal fibroblasts and myofibroblasts due to curvature. These results demonstrate that curvature influences both the phenotype of corneal stromal cells and the structural organization of corneal stroma tissue without any external stimuli. This curvature-dependent behavior of corneal stromal cells presents potential opportunities for creating therapeutic strategies for corneal shape dysfunctions.

Co-Culture of Glomerular Endothelial Cells and Podocytes in a Custom-Designed Glomerulus-on-a-Chip Model Improves the Filtration Barrier Integrity and Affects the Glomerular Cell Phenotype

Crosstalk between glomerular endothelial cells and glomerular epithelial cells (podocytes) is increasingly becoming apparent as a crucial mechanism to maintain the integrity of the glomerular filtration barrier. However, in vitro studies directly investigating the effect of this crosstalk on the glomerular filtration barrier are scarce because of the lack of suitable experimental models. Therefore, we developed a custom-made glomerulus-on-a-chip model recapitulating the glomerular filtration barrier, in which we investigated the effects of co-culture of glomerular endothelial cells and podocytes on filtration barrier function and the phenotype of these respective cell types. The custom-made glomerulus-on-a-chip model was designed using soft lithography. The chip consisted of two parallel microfluidic channels separated by a semi-permeable polycarbonate membrane. The glycocalyx was visualized by wheat germ agglutinin staining and the barrier integrity of the glomerulus-on-a-chip model was determined by measuring the transport rate of fluorescently labelled dextran from the top to the bottom channel. The effect of crosstalk on the transcriptome of glomerular endothelial cells and podocytes was investigated via RNA-sequencing. Glomerular endothelial cells and podocytes were successfully cultured on opposite sides of the membrane in our glomerulus-on-a-chip model using a polydopamine and collagen A double coating. Barrier integrity of the chip model was significantly improved when glomerular endothelial cells were co-cultured with podocytes compared to monocultures of either glomerular endothelial cells or podocytes. Co-culture enlarged the surface area of podocyte foot processes and increased the thickness of the glycocalyx. RNA-sequencing analysis revealed the regulation of cellular pathways involved in cellular differentiation and cellular adhesion as a result of the interaction between glomerular endothelial cells and podocytes. We present a novel custom-made glomerulus-on-a-chip co-culture model and demonstrated for the first time using a glomerulus-on-a-chip model that co-culture affects the morphology and transcriptional phenotype of glomerular endothelial cells and podocytes. Moreover, we showed that co-culture improves barrier function as a relevant functional readout for clinical translation. This model can be used in future studies to investigate specific glomerular paracrine pathways and unravel the role of glomerular crosstalk in glomerular (patho) physiology.

Microphysiological Drug-Testing Platform for Identifying Responses to Prodrug Treatment in Primary Leukemia

Despite increasing survival rates of pediatric leukemia patients over the past decades, the outcome of some leukemia subtypes has remained dismal. Drug sensitivity and resistance testing on patient-derived leukemia samples provide important information to tailor treatments for high-risk patients. However, currently used well-based drug screening platforms have limitations in predicting the effects of prodrugs, a class of therapeutics that require metabolic activation to become effective. To address this issue, a microphysiological drug-testing platform is developed that enables co-culturing of patient-derived leukemia cells, human bone marrow mesenchymal stromal cells, and human liver microtissues within the same microfluidic platform. This platform also enables to control the physical interaction between the diverse cell types. Herein, it is made possible to recapitulate hepatic prodrug activation of ifosfamide in their platform, which is very difficult in traditional well-based assays. By testing the susceptibility of primary patient-derived leukemia samples to the prodrug ifosfamide, sample-specific sensitivities to ifosfamide in primary leukemia samples are identified. The microfluidic platform is found to enable the recapitulation of physiologically relevant conditions and the testing of prodrugs including short-lived and unstable metabolites. The platform holds great potential for clinical translation and precision chemotherapy selection.

Multiplexed microfluidic platform for stem-cell derived pancreatic islet β cells

Stem cell-derived β cells offer an alternative to primary islets for biomedical discoveries as well as a potential surrogate for islet transplantation. The expense and challenge of obtaining and maintaining functional stem cell-derived β cells calls for a need to develop better high-content and high-throughput culture systems. Microphysiological systems (MPS) are promising high-content in vitro platforms, but scaling for high-throughput screening and discoveries remain a challenge. Traditionally, simultaneous multiplexing of liquid handling and cell loading poses a challenge in the design of high-throughput MPS. Furthermore, although MPS for islet β culture/testing have been developed, studies on multi-day culture of stem-cell derived β cells in MPS have been limited. We present a scalable, multiplexed islet β MPS device that incorporates microfluidic gradient generators to parallelize fluid handling for culture and test conditions. We demonstrated the viability and functionality of the stem cell-derived enriched β clusters (eBCs) for a week, as assessed by the ∼2 fold insulin release by the clusters to glucose challenge. To show the scalable multiplexing for drug testing, we demonstrated the loss of stimulation index after long-term exposure to logarithmic concentration range of glybenclamide. The MPS cultured eBCs also confirmed a glycolytic bottleneck as inferred by insulin secretion responses to metabolites methyl succinate and glyceric acid. Thus, we present an innovative culture platform for eBCs with a balance of high-content and high-throughput characteristics.

Design and Fabrication of Organ-on-Chips: Promises and Challenges

The advent of the miniaturization approach has influenced the research trends in almost all disciplines. Bioengineering is one of the fields benefiting from the new possibilities of microfabrication techniques, especially in cell and tissue culture, disease modeling, and drug discovery. The limitations of existing 2D cell culture techniques, the high time and cost requirements, and the considerable failure rates have led to the idea of 3D cell culture environments capable of providing physiologically relevant tissue functions in vitro. Organ-on-chips are microfluidic devices used in this context as a potential alternative to in vivo animal testing to reduce the cost and time required for drug evaluation. This emerging technology contributes significantly to the development of various research areas, including, but not limited to, tissue engineering and drug discovery. However, it also brings many challenges. Further development of the technology requires interdisciplinary studies as some problems are associated with the materials and their manufacturing techniques. Therefore, in this paper, organ-on-chip technologies are presented, focusing on the design and fabrication requirements. Then, state-of-the-art materials and microfabrication techniques are described in detail to show their advantages and also their limitations. A comparison and identification of gaps for current use and further studies are therefore the subject of the final discussion.

Facile Route for 3D Printing of Transparent PETg-Based Hybrid Biomicrofluidic Devices Promoting Cell Adhesion

3D printing has emerged as a promising fabrication technique for microfluidic devices, overcoming some of the challenges associated with conventional soft lithography. Filament-based polymer extrusion (popularly known as fused deposition modeling (FDM)) is one of the most accessible 3D printing techniques available, offering a wide range of low-cost thermoplastic polymer materials for microfluidic device fabrication. However, low optical transparency is one of the significant limitations of extrusion-based microfluidic devices, rendering them unsuitable for cell culture-related biological applications. Moreover, previously reported extrusion-based devices were largely dependent on fluorescent dyes for cell imaging because of their poor transparency. First, we aim to improve the optical transparency of FDM-based microfluidic devices to enable bright-field microscopy of cells. This is achieved using (1) transparent polymer filament materials such as poly(ethylene terephthalate) glycol (PETg), (2) optimized 3D printing process parameters, and (3) a hybrid approach by integrating 3D printed microfluidic devices with cast poly(dimethylsiloxane) (PDMS) blocks. We begin by optimizing four essential 3D printing process parameters (layer height, printing speed, cooling fan speed, and extrusion flow), affecting the overall transparency of 3D printed devices. Optimized parameters produce exceptional optical transparency close to 80% in 3D printed PETg devices. Next, we demonstrate the potential of FDM-based 3D printing to fabricate transparent micromixing devices with complex planar and nonplanar channel networks. Most importantly, cells cultured on native 3D printed PETg surfaces show excellent cell attachment, spreading, and proliferation during 3 days of culture without extracellular matrix coating or surface treatment. Next, we introduce L929 cells inside hybrid PETg-PDMS biomicrofluidic devices as a proof of concept. We demonstrate that 3D printed hybrid biomicrofluidic devices promote cell adhesion, allow bright-field microscopy, and maintain high cell viability for 3 days. Finally, we demonstrate the applicability of the proposed fabrication approach for developing 3D printed microfluidic devices from other FDM-compatible transparent polymers such as polylactic acid (PLA) and poly(methyl methacrylate) (PMMA).

Membrane integration into PDMS-free microfluidic platforms for organ-on-chip and analytical chemistry applications

Membranes play a crucial role in many microfluidic systems, enabling versatile applications in highly diverse research fields. However, the tight and robust integration of membranes into microfluidic systems requires complex fabrication processes. Most integration approaches, so far, rely on polydimethylsiloxane (PDMS) as base material for the microfluidic chips. Several limitations of PDMS have resulted in the transition of many microfluidic approaches to PDMS-free systems using alternative materials such as thermoplastics. To integrate membranes in those PDMS-free systems, novel alternative approaches are required. This review provides an introduction into microfluidic systems applying membrane technology for analytical systems and organ-on-chip as well as a comprehensive overview of methods for the integration of membranes into PDMS-free systems. The overview and examples will provide a valuable resource and starting point for any researcher that is aiming at implementing membranes in microfluidic systems without using PDMS.

Facile Patterning of Thermoplastic Elastomers and Robust Bonding to Glass and Thermoplastics for Microfluidic Cell Culture and Organ-on-Chip

The emergence and spread of microfluidics over the last decades relied almost exclusively on the elastomer polydimethylsiloxane (PDMS). The main reason for the success of PDMS in the field of microfluidic research is its suitability for rapid prototyping and simple bonding methods. PDMS allows for precise microstructuring by replica molding and bonding to different substrates through various established strategies. However, large-scale production and commercialization efforts are hindered by the low scalability of PDMS-based chip fabrication and high material costs. Furthermore, fundamental limitations of PDMS, such as small molecule absorption and high water evaporation, have resulted in a shift toward PDMS-free systems. Thermoplastic elastomers (TPE) are a promising alternative, combining properties from both thermoplastic materials and elastomers. Here, we present a rapid and scalable fabrication method for microfluidic systems based on a polycarbonate (PC) and TPE hybrid material. Microstructured PC/TPE-hybrid modules are generated by hot embossing precise features into the TPE while simultaneously fusing the flexible TPE to a rigid thermoplastic layer through thermal fusion bonding. Compared to TPE alone, the resulting, more rigid composite material improves device handling while maintaining the key advantages of TPE. In a fast and simple process, the PC/TPE-hybrid can be bonded to several types of thermoplastics as well as glass substrates. The resulting bond strength withstands at least 7.5 bar of applied pressure, even after seven days of exposure to a high-temperature and humid environment, which makes the PC/TPE-hybrid suitable for most microfluidic applications. Furthermore, we demonstrate that the PC/TPE-hybrid features low absorption of small molecules while being biocompatible, making it a suitable material for microfluidic biotechnological applications.

Integrated Isogenic Human Induced Pluripotent Stem Cell-Based Liver and Heart Microphysiological Systems Predict Unsafe Drug-Drug Interaction

Three-dimensional (3D) microphysiological systems (MPSs) mimicking human organ function in vitro are an emerging alternative to conventional monolayer cell culture and animal models for drug development. Human induced pluripotent stem cells (hiPSCs) have the potential to capture the diversity of human genetics and provide an unlimited supply of cells. Combining hiPSCs with microfluidics technology in MPSs offers new perspectives for drug development. Here, the integration of a newly developed liver MPS with a cardiac MPS—both created with the same hiPSC line—to study drug–drug interaction (DDI) is reported. As a prominent example of clinically relevant DDI, the interaction of the arrhythmogenic gastroprokinetic cisapride with the fungicide ketoconazole was investigated. As seen in patients, metabolic conversion of cisapride to non-arrhythmogenic norcisapride in the liver MPS by the cytochrome P450 enzyme CYP3A4 was inhibited by ketoconazole, leading to arrhythmia in the cardiac MPS. These results establish integration of hiPSC-based liver and cardiac MPSs to facilitate screening for DDI, and thus drug efficacy and toxicity, isogenic in the same genetic background.

Review of Design Considerations for Brain-on-a-Chip Models

In recent years, the need for sophisticated human in vitro models for integrative biology has motivated the development of organ-on-a-chip platforms. Organ-on-a-chip devices are engineered to mimic the mechanical, biochemical and physiological properties of human organs; however, there are many important considerations when selecting or designing an appropriate device for investigating a specific scientific question. Building microfluidic Brain-on-a-Chip (BoC) models from the ground-up will allow for research questions to be answered more thoroughly in the brain research field, but the design of these devices requires several choices to be made throughout the design development phase. These considerations include the cell types, extracellular matrix (ECM) material(s), and perfusion/flow considerations. Choices made early in the design cycle will dictate the limitations of the device and influence the end-point results such as the permeability of the endothelial cell monolayer, and the expression of cell type-specific markers. To better understand why the engineering aspects of a microfluidic BoC need to be influenced by the desired biological environment, recent progress in microfluidic BoC technology is compared. This review focuses on perfusable blood-brain barrier (BBB) and neurovascular unit (NVU) models with discussions about the chip architecture, the ECM used, and how they relate to the in vivo human brain. With increased knowledge on how to make informed choices when selecting or designing BoC models, the scientific community will benefit from shorter development phases and platforms curated for their application.

A Fast Alternative to Soft Lithography for the Fabrication of Organ-on-a-Chip Elastomeric-Based Devices and Microactuators

Organ-on-a-chip technology promises to revolutionize how pre-clinical human trials are conducted. Engineering an in vitro environment that mimics the functionality and architecture of human physiology is essential toward building better platforms for drug development and personalized medicine. However, the complex nature of these devices requires specialized, time consuming, and expensive fabrication methodologies. Alternatives that reduce design-to-prototype time are needed, in order to fulfill the potential of these devices. Here, a streamlined approach is proposed for the fabrication of organ-on-a-chip devices with incorporated microactuators, by using an adaptation of xurography. This method can generate multilayered, membrane-integrated biochips in a matter of hours, using low-cost benchtop equipment. These devices are capable of withstanding considerable pressure without delamination. Furthermore, this method is suitable for the integration of flexible membranes, required for organ-on-a-chip applications, such as mechanical actuation or the establishment of biological barrier function. The devices are compatible with cell culture applications and present no cytotoxic effects or observable alterations on cellular homeostasis. This fabrication method can rapidly generate organ-on-a-chip prototypes for a fraction of cost and time, in comparison to conventional soft lithography, constituting an interesting alternative to the current fabrication methods.

Organ-On-Chip Technology: The Future of Feto-Maternal Interface Research?

The placenta and fetal membrane act as a protective barrier throughout pregnancy while maintaining communication and nutrient exchange between the baby and the mother. Disruption of this barrier leads to various pregnancy complications, including preterm birth, which can have lasting negative consequences. Thus, understanding the role of the feto-maternal interface during pregnancy and parturition is vital to advancing basic and clinical research in the field of obstetrics. However, human subject studies are inherently difficult, and appropriate animal models are lacking. Due to these challenges, in vitro cell culture-based studies are most commonly utilized. However, the structure and functions of conventionally used in vitro 2D and 3D models are vastly different from the in vivo environment, making it difficult to fully understand the various factors affecting pregnancy as well as pathways and mechanisms contributing to term and preterm births. This limitation also makes it difficult to develop new therapeutics. The emergence of in vivo-like in vitro models such as organ-on-chip (OOC) platforms can better recapitulate in vivo functions and responses and has the potential to move this field forward significantly. OOC technology brings together two distinct fields, microfluidic engineering and cell/tissue biology, through which diverse human organ structures and functionalities can be built into a laboratory model that better mimics functions and responses of in vivo tissues and organs. In this review, we first provide an overview of the OOC technology, highlight two major designs commonly used in achieving multi-layer co-cultivation of cells, and introduce recently developed OOC models of the feto-maternal interface. As a vital component of this review, we aim to outline progress on the practicality and effectiveness of feto-maternal interface OOC (FM-OOC) models currently used and the advances they have fostered in obstetrics research. Lastly, we provide a perspective on the future basic research and clinical applications of FM-OOC models, and even those that integrate multiple organ systems into a single OOC system that may recreate intrauterine architecture in its entirety, which will accelerate our understanding of feto-maternal communication, induction of preterm labor, drug or toxicant permeability at this vital interface, and development of new therapeutic strategies.

A dog oviduct-on-a-chip model of serous tubal intraepithelial carcinoma

Ovarian cancer is the fifth cause of cancer-related mortality in women, with an expected 5-year survival rate of only 47%. High-grade serous carcinoma (HGSC), an epithelial cancer phenotype, is the most common malignant ovarian cancer. It is known that the precursors of HGSC originate from secretory epithelial cells within the Fallopian tube, which first develops as serous tubal intraepithelial carcinoma (STIC). Here, we used gene editing by CRISPR-Cas9 to knock out the oncogene p53 in dog oviductal epithelia cultured in a dynamic microfluidic chip to create an in vitro model that recapitulated human STIC. Similar to human STIC, the gene-edited oviduct-on-a-chip, exhibited loss of cell polarization and had reduced ciliation, increased cell atypia and proliferation, with multilayered epithelium, increased Ki67, PAX8 and Myc and decreased PTEN and RB1 mRNA expression. This study provides a biomimetic in vitro model to study STIC progression and to identify potential biomarkers for early detection of HGSC.

A compression transmission device for the evaluation of bonding strength of biocompatible microfluidic and biochip materials and systems

Bonding of a variety of inorganic and organic polymers as multi-layered structures is one of the main challenges for biochip production even to date, since the chemical nature of these materials often does not allow easy and straight forward bonding and proper sealing. After selection of an appropriate method to bond the chosen materials to form a complex biochip, function and stability of bonding either requires qualitative burst tests or expensive mechanical multi-test stations, that often do not have the right adaptors to clamp biochip slides without destruction. Therefore, we have developed a simple and inexpensive bonding test based on 3D printed transmission elements that translate compressive forces via manual compression, hand press or hydraulic press compression into shear and tensile force. Mechanical stress simulations showed that design of the bonding geometry and size must be considered for bonding tests since the stress distribution thus bonding strength heavily varies with size but also with geometry. We demonstrate the broad applicability of our 3D printed bonding test system by testing the most frequent bonding strategies in combination with the respective most frequently used biochip material in a force-to-failure study. All evaluated materials are biocompatible and used in cell-based biochip devices. This study is evaluating state-of-the-art bonding approaches used for sealing of microfluidic biochips including adhesive bonding, plasma bonding, solvent bonding as well as bonding mediated by amino-silane monolayers or even functional thiol-ene epoxy biochip materials that obviate intermediate adhesive layers.

Robust chemical bonding of PMMA microfluidic devices to porous PETE membranes for reliable cytotoxicity testing of drugs

Here, we report a simple yet reliable method for bonding poly(methyl methacrylate) (PMMA) to polyethylene terephthalate (PETE) track-etched membranes using (3-glycidyloxypropyl)trimethoxysilane (GLYMO), which enables reliable cytotoxicity tests in a microfluidic device impermeable to small molecules, such as anti-cancer drugs. The porous PETE membranes treated with 5% GLYMO were assembled with microfluidic channel-engraved PMMA substrates after air plasma treatment for 1 minute, followed by heating at 100 °C for 2 minutes, which permits irreversible and complete bonding to be achieved within 1 h. The bonding strength between the two substrates (1.97 × 10⁷ kg m^-2) was robust enough to flow culture medium through the device without leakage even at a gauge pressure of above 135 kPa. For validation of its utility in drugs testing, we successfully demonstrated that human lung adenocarcinoma cells cultured in the PMMA devices show more reliable cytotoxicity results for vincristine in comparison to conventional polydimethylsiloxane (PDMS) devices due to the inherent property of PMMA of it being impervious to small molecules. Given that the current organ-on-a-chip fabrication methods mostly rely on PDMS, this bonding strategy will expand simple fabrication capability using various thermoplastics and porous track-etched membranes, and allow us to create 3D-micro-constructs that more precisely mimic organ-level physiological conditions.

pH-Controlled Drug Release by Diffusion through Silica Nanochannel Membranes

We report in this work the fabrication of a flow-through silica nanochannel membrane (SNM) for controlled drug release applications. The ultrathin SNM consists of parallel nanochannels with a uniform diameter of ∼2.3 nm and a density of 4 × 10¹² cm^-2, which provide simultaneously high permeability and size selectivity toward small molecules. The track-etched porous polyethylene terephthalate film premodified with silane on its surface was used to support the ultrathin SNM via irreversible covalent bond formation, thus offering mechanical strength, flexibility, and stability to the ultrathin SNM for continuous and long-term use. Alkylamines were subsequently grafted onto the SNM surface to modulate the "on" and "off" state of nanochannels by medium pH for controlled drug release. Thiamphenicol glycinate hydrochloride (TPG), an intestinal drug, was studied as a model to permeate through an ultrathin SNM in both simulated gastric fluid (pH = 1.2) and simulated intestinal fluid (pH = 7.5). The release in the latter case was 178 times faster than that in the former. Moreover, a nearly zero-order constant release of TPG via single-file diffusion was achieved up to 24 h, demonstrating the feasibility of sustained and continuous release of small-molecule drugs in a pH-controlled manner.

A Static Microfluidic Device for Investigating the Chemotaxis Response to Stable, Non-linear Gradients

Microfluidic technology allows fast and precise measurement of chemotaxis responses to both attractant and repellent signals. One of the major drawbacks of current microfluidic chemotaxis assays is the presence of bacterial cells within the concentration gradient flow field, which has the potential for flow effects masking the chemotaxis response. This chapter describes a new microfluidic device for producing stable concentration gradients and measuring the response of cells to the gradient without exposing them to any flow. Unlike other methods described in the literature, this method is capable of producing gradients of any shape, almost instantaneously, allowing the measurement of time-dependent response of cells to a variety of signals.

WAT-on-a-chip: a physiologically relevant microfluidic system incorporating white adipose tissue

Organ-on-a-chip systems possess a promising future as drug screening assays and as testbeds for disease modeling in the context of both single-organ systems and multi-organ-chips. Although it comprises approximately one fourth of the body weight of a healthy human, an organ frequently overlooked in this context is white adipose tissue (WAT). WAT-on-a-chip systems are required to create safety profiles of a large number of drugs due to their interactions with adipose tissue and other organs via paracrine signals, fatty acid release, and drug levels through sequestration. We report a WAT-on-a-chip system with a footprint of less than 1 mm2 consisting of a separate media channel and WAT chamber connected via small micropores. Analogous to the in vivo blood circulation, convective transport is thereby confined to the vasculature-like structures and the tissues protected from shear stresses. Numerical and analytical modeling revealed that the flow rates in the WAT chambers are less than 1/100 of the input flow rate. Using optimized injection parameters, we were able to inject pre-adipocytes, which subsequently formed adipose tissue featuring fully functional lipid metabolism. The physiologically relevant microfluidic environment of the WAT-chip supported long term culture of the functional adipose tissue for more than two weeks. Due to its physiological, highly controlled, and computationally predictable character, the system has the potential to be a powerful tool for the study of adipose tissue associated diseases such as obesity and type 2 diabetes.

Hydrogel-based microfluidic incubator for microorganism cultivation and analyses

This work presents an array of microfluidic chambers for on-chip culturing of microorganisms in static and continuous shear-free operation modes. The unique design comprises an in-situ polymerized hydrogel that forms gas and reagent permeable culture wells in a glass chip. Utilizing a hydrophilic substrate increases usability by autonomous capillary priming. The thin gel barrier enables efficient oxygen supply and facilitates on-chip analysis by chemical access through the gel without introducing a disturbing flow to the culture. Trapping the suspended microorganisms inside a gel well allows for a much simpler fabrication than in conventional trapping devices as the minimal feature size does not depend on cell size. Nutrients and drugs are provided on-chip in the gel for a self-contained and user-friendly handling. Rapid antibiotic testing in static cultures with strains of Enterococcus faecalis and Escherichia coli is presented. Cell seeding and diffusive medium supply is provided by phaseguide technology, enabling simple operation of continuous culturing with a great flexibility. Cells of Saccharomyces cerevisiae are utilized as a model to demonstrate continuous on-chip culturing.

Stable chemical bonding of porous membranes and poly(dimethylsiloxane) devices for long-term cell culture

We have investigated the bonding stability of various silane treatments for the integration of track-etched membranes with poly(dimethylsiloxane) (PDMS) microfluidic devices. We compare various treatments using trialkoxysilanes or dipodal silanes to determine the effect of the organofunctional group, cross-link density, reaction solvent, and catalyst on the bond stability. We find that devices made using existing silane methods delaminated after one day when immersed in cell culture medium at 37 °C. In contrast, the dipodal silane, bis[3-(trimethoxysilyl)propyl]amine, is shown to yield stable and functional integration of membranes with PDMS that is suitable for long-term cell culture. To demonstrate application of the technique, we fabricated an open-surface device in which cells cultured on a track-etched membrane can be stimulated at their basal side via embedded microfluidic channels. C2C12 mouse myoblasts were differentiated into myotubes over the course of two weeks on these devices to demonstrate biocompatibility. Finally, devices were imaged during the basal-side delivery of a fluorescent stain to validate the membrane operation and long-term stability of the bonding technique.