Accessing 3D microtissue metabolism: Lactate and oxygen monitoring in hepatocyte spheroids

Organ-on-a-Chip: Ubi sumus? Fundamentals and Design Aspects

This review outlines the evolutionary journey from traditional two-dimensional (2D) cell culture to the revolutionary field of organ-on-a-chip technology. Organ-on-a-chip technology integrates microfluidic systems to mimic the complex physiological environments of human organs, surpassing the limitations of conventional 2D cultures. This evolution has opened new possibilities for understanding cell-cell interactions, cellular responses, drug screening, and disease modeling. However, the design and manufacture of microchips significantly influence their functionality, reliability, and applicability to different biomedical applications. Therefore, it is important to carefully consider design parameters, including the number of channels (single, double, or multi-channels), the channel shape, and the biological context. Simultaneously, the selection of appropriate materials compatible with the cells and fabrication methods optimize the chips' capabilities for specific applications, mitigating some disadvantages associated with these systems. Furthermore, the success of organ-on-a-chip platforms greatly depends on the careful selection and utilization of cell resources. Advances in stem cell technology and tissue engineering have contributed to the availability of diverse cell sources, facilitating the development of more accurate and reliable organ-on-a-chip models. In conclusion, a holistic perspective of in vitro cellular modeling is provided, highlighting the integration of microfluidic technology and meticulous chip design, which play a pivotal role in replicating organ-specific microenvironments. At the same time, the sensible use of cell resources ensures the fidelity and applicability of these innovative platforms in several biomedical applications.

Sensor-integrated brain-on-a-chip platforms: Improving the predictive validity in neurodegenerative research

Affecting millions of individuals worldwide, neurodegenerative diseases (NDDs) pose a significant and growing health concern in people over the age of 60 years. Contributing to this trend are the steady increase in the aging population coupled with a persistent lack of disease-altering treatment strategies targeting NDDs. The absence of efficient therapeutics can be attributed to high failure rates in clinical trials and the ineptness of animal models in preceding preclinical studies. To that end, in recent years, significant research effort has been dedicated to the development of human cell-based preclinical disease models characterized by a higher degree of predictive validity. However, a key requirement of any in vitro model constitutes the precise knowledge and replication of the target tissues' (patho-)physiological microenvironment. Herein, microphysiological systems have demonstrated superiority over conventional static 2D/3D in vitro cell culture systems, as they allow for the emulation and continuous monitoring of the onset, progression, and remission of disease-associated phenotypes. This review provides an overview of recent advances in the field of NDD research using organ-on-a-chip platforms. Specific focus is directed toward non-invasive sensing strategies encompassing electrical, electrochemical, and optical sensors. Additionally, promising on- and integrable off-chip sensing strategies targeting key analytes in NDDs will be presented and discussed in detail.

High sensitivity and automatic chemiluminescence detection of glucose and lactate using a spin-disc paper-based device

This paper reports a spin-disc paper-based device with 10 individual detection units containing electromagnetic modules controlling the sample incubation time before chemiluminescence (CL) signal detection. After the sample was added to the top paper chip and incubated with the enzyme, the electromagnet was turned off to allow contact between the top and bottom paper. The H2O2 generated by the sample flowed vertically to the bottom paper and initiated the oxidase of the luminol to generate the CL signal. After one detection the disc was automatically rotated to the next position to repeat the above detection. The advantage of using the device over the lateral flow and the in situ detection was firstly proved using the detection of H2O2 and the glucose/lactate sample with 5 minute incubation. The CL intensity was increased 300 times/1000 times as the glucose/lactate was incubated for 5 minutes compared to the non-incubated samples. Afterward, the device was employed to separately detect glucose and lactate diluted in PBS, artificial sweat, artificial saliva, and fresh cell culture media. Finally, the device was employed to detect the glucose and lactate in the media collected over the 24 hour culture of PC3 cells. The uptake and production rates of glucose and lactate were correspondingly determined as 0.328 ± 0.015 pmol h-1 per cell and 1.254 ± 0.053 pmol h-1 per cell, respectively. The reported device has wide application potential due to its capabilities in automatic detection of multiple samples with very high sensitivity and small sample volume (down to 0.5 μL).

Microphysiological Constructs and Systems: Biofabrication Tactics, Biomimetic Evaluation Approaches, and Biomedical Applications

In recent decades, microphysiological constructs and systems (MPCs and MPSs) have undergone significant development, ranging from self-organized organoids to high-throughput organ-on-a-chip platforms. Advances in biomaterials, bioinks, 3D bioprinting, micro/nanofabrication, and sensor technologies have contributed to diverse and innovative biofabrication tactics. MPCs and MPSs, particularly tissue chips relevant to absorption, distribution, metabolism, excretion, and toxicity, have demonstrated potential as precise, efficient, and economical alternatives to animal models for drug discovery and personalized medicine. However, current approaches mainly focus on the in vitro recapitulation of the human anatomical structure and physiological-biochemical indices at a single or a few simple levels. This review highlights the recent remarkable progress in MPC and MPS models and their applications. The challenges that must be addressed to assess the reliability, quantify the techniques, and utilize the fidelity of the models are also discussed.

Stretchable Sponge-Based Electrochemical Biosensor for Real-Time Sensing of Cells in Three-Dimensional Culture

For the study of cell biology, real-time information on cell physiological processes will be more accurate and closer to the in vivo condition in a three-dimensional (3D) culture system. Although most reported 3D cell culture scaffolds can better mimic the in vivo dynamic microenvironment, the real-time analysis technique is deficient or lacking. Herein, a stretchable and conductive 3D scaffold is developed to construct an electrochemical biosensor for real-time monitoring of cell release in 3D culture under stimulation of drug stimulant and mechanical force. In our design, the polyurethane sponge (PU) dipped with conductive carbon ink (CC/PU) was used as a conductive scaffold, and gold nanoparticles (nano-Au) were electrodeposited on the CC/PU (nano-Au CC/PU) to improve the electrochemical sensing performance. The prepared nano-Au CC/PU scaffold exhibits a good electrocatalytic ability to H2O2 with a linear range from 20 nM to 43 μM. Due to the great biocompatibility, HeLa cells can be cultured directly on the nano-Au CC/PU and the in situ and real-time tracking of H2O2 secretion from cells was achieved. The results demonstrate that both the drug stimulant and mechanical force can rapidly activate the release of reactive oxygen species. This study indicates that the stretchable 3D sensing scaffold has good potential for cell biology research in an in vivo-like microenvironment and can be extensively used in the fields of tissue engineering, drug screening, and pathological research.

In Vitro Tumor Mimetic Spheroid Model: Void Space within a Self-Detachable Cross-Linked Hydrogel

The three-dimensional (3D) spheroid cell culture model is crucial in screening anticancer drugs in vitro and understanding tumor cell behavior. However, the current in vitro models require highly skilled techniques. Here, we present an in vitro, tumor-mimetic, self-detachable, cancer cell spheroid model that provides the confined space of a tumor microenvironment, convenient spheroid retrieval, immunostaining, treatment, and imaging. We formed a void space within alginate macrobeads by ionic disintegration at a specific region inside. The macrobeads were further destabilized with bovine serum albumin to retrieve the spheroid cultured within the void space. Quantitative analysis of the immunofluorescence images of the cultured spheroids showed enhanced expressions of the hypoxia-inducible factor-1α (HIF-1α) and carbonic anhydrase-9 (CA-9), like monolayer cultures of cancer cells under hypoxic conditions (0.2% oxygen). Furthermore, adding CoCl2 to the cell culture media induces even higher amounts of HIF-1α and CA-9 in the cultured spheroids. In conclusion, the present work highlighted the in vitro spheroid model, which is closer to the tumor microenvironment and has user-friendly cell seeding, spheroid retrieval, and immunostaining steps.

Cancer Models on Chip: Paving the Way to Large-Scale Trial Applications

Cancer kills millions of individuals every year all over the world (Global Cancer Observatory). The physiological and biomechanical processes underlying the tumor are still poorly understood, hindering researchers from creating new, effective therapies. Inconsistent results of preclinical research, in vivo testing, and clinical trials decrease drug approval rates. 3D tumor-on-a-chip (ToC) models integrate biomaterials, tissue engineering, fabrication of microarchitectures, and sensory and actuation systems in a single device, enabling reliable studies in fundamental oncology and pharmacology. This review includes a critical discussion about their ability to reproduce the tumor microenvironment (TME), the advantages and drawbacks of existing tumor models and architectures, major components and fabrication techniques. The focus is on current materials and micro/nanofabrication techniques used to manufacture reliable and reproducible microfluidic ToC models for large-scale trial applications.

In situbiosensing technologies for an organ-on-a-chip

Thein vitrosimulation of organs resolves the accuracy, ethical, and cost challenges accompanyingin vivoexperiments. Organoids and organs-on-chips have been developed to model thein vitro, real-time biological and physiological features of organs. Numerous studies have deployed these systems to assess thein vitro, real-time responses of an organ to external stimuli. Particularly, organs-on-chips can be most efficiently employed in pharmaceutical drug development to predict the responses of organs before approving such drugs. Furthermore, multi-organ-on-a-chip systems facilitate the close representations of thein vivoenvironment. In this review, we discuss the biosensing technology that facilitates thein situ, real-time measurements of organ responses as readouts on organ-on-a-chip systems, including multi-organ models. Notably, a human-on-a-chip system integrated with automated multi-sensing will be established by further advancing the development of chips, as well as their assessment techniques.

Emerging strategies of engineering retinal organoids and organoid-on-a-chip in modeling intraocular drug delivery: current progress and future perspectives

Retinal diseases are a rising concern as major causes of blindness in an aging society; therapeutic options are limited, and the precise pathogenesis of these diseases remains largely unknown. Intraocular drug delivery and nanomedicines offering targeted, sustained, and controllable delivery are the most challenging and popular topics in ocular drug development and toxicological evaluation. Retinal organoids (ROs) and organoid-on-a-chip (ROoC) are both emerging as promising in-vitro models to faithfully recapitulate human eyes for retinal research in the replacement of experimental animals and primary cells. In this study, we review the generation and application of ROs resembling the human retina in cell subtypes and laminated structures and introduce the emerging engineered ROoC as a technological opportunity to address critical issues. On-chip vascularization, perfusion, and close inter-tissue interactions recreate physiological environments in vitro, whilst integrating with biosensors facilitates real-time analysis and monitoring during organogenesis of the retina representing engineering efforts in ROoC models. We also emphasize that ROs and ROoCs hold the potential for applications in modeling intraocular drug delivery in vitro and developing next-generation retinal drug delivery strategies.

Biosensor integrated brain-on-a-chip platforms: Progress and prospects in clinical translation

Because of the brain's complexity, developing effective treatments for neurological disorders is a formidable challenge. Research efforts to this end are advancing as in vitro systems have reached the point that they can imitate critical components of the brain's structure and function. Brain-on-a-chip (BoC) was first used for microfluidics-based systems with small synthetic tissues but has expanded recently to include in vitro simulation of the central nervous system (CNS). Defining the system's qualifying parameters may improve the BoC for the next generation of in vitro platforms. These parameters show how well a given platform solves the problems unique to in vitro CNS modeling (like recreating the brain's microenvironment and including essential parts like the blood-brain barrier (BBB)) and how much more value it offers than traditional cell culture systems. This review provides an overview of the practical concerns of creating and deploying BoC systems and elaborates on how these technologies might be used. Not only how advanced biosensing technologies could be integrated with BoC system but also how novel approaches will automate assays and improve point-of-care (PoC) diagnostics and accurate quantitative analyses are discussed. Key challenges providing opportunities for clinical translation of BoC in neurodegenerative disorders are also addressed.

Integrated technologies for continuous monitoring of organs-on-chips: Current challenges and potential solutions

Organs-on-chips (OoCs) are biomimetic in vitro systems based on microfluidic cell cultures that recapitulate the in vivo physicochemical microenvironments and the physiologies and key functional units of specific human organs. These systems are versatile and can be customized to investigate organ-specific physiology, pathology, or pharmacology. They are more physiologically relevant than traditional two-dimensional cultures, can potentially replace the animal models or reduce the use of these models, and represent a unique opportunity for the development of personalized medicine when combined with human induced pluripotent stem cells. Continuous monitoring of important quality parameters of OoCs via a label-free, non-destructive, reliable, high-throughput, and multiplex method is critical for assessing the conditions of these systems and generating relevant analytical data; moreover, elaboration of quality predictive models is required for clinical trials of OoCs. Presently, these analytical data are obtained by manual or automatic sampling and analyzed using single-point, off-chip traditional methods. In this review, we describe recent efforts to integrate biosensing technologies into OoCs for monitoring the physiologies, functions, and physicochemical microenvironments of OoCs. Furthermore, we present potential alternative solutions to current challenges and future directions for the application of artificial intelligence in the development of OoCs and cyber-physical systems. These "smart" OoCs can learn and make autonomous decisions for process optimization, self-regulation, and data analysis.

Conformational-switch biosensors as novel tools to support continuous, real-time molecular monitoring in lab-on-a-chip devices

Recent years have seen continued expansion of the functionality of lab on a chip (LOC) devices. Indeed LOCs now provide scientists and developers with useful and versatile platforms across a myriad of chemical and biological applications. The field still fails, however, to integrate an often important element of bench-top analytics: real-time molecular measurements that can be used to "guide" a chemical response. Here we describe the analytical techniques that could provide LOCs with such real-time molecular monitoring capabilities. It appears to us that, among the approaches that are general (i.e., that are independent of the reactive or optical properties of their targets), sensing strategies relying on binding-induced conformational change of bioreceptors are most likely to succeed in such applications.

Biosensor integrated tissue chips and their applications on Earth and in space

The development of space exploration technologies has positively impacted everyday life on Earth in terms of communication, environmental, social, and economic perspectives. The human body constantly fluctuates during spaceflight, even for a short-term mission. Unfortunately, technology is evolving faster than humans' ability to adapt, and many therapeutics entering clinical trials fail even after being subjected to vigorous in vivo testing due to toxicity and lack of efficacy. Therefore, tissue chips (also mentioned as organ-on-a-chip) with biosensors are being developed to compensate for the lack of relevant models to help improve the drug development process. There has been a push to monitor cell and tissue functions, based on their biological signals and utilize the integration of biosensors into tissue chips in space to monitor and assess cell microenvironment in real-time. With the collaboration between the Center for the Advancement of Science in Space (CASIS), the National Aeronautics and Space Administration (NASA) and other partners, they are providing the opportunities to study the effects of microgravity environment has on the human body. Institutions such as the National Institute of Health (NIH) and National Science Foundation (NSF) are partnering with CASIS and NASA to utilize tissue chips onboard the International Space Station (ISS). This article reviews the endless benefits of space technology, the development of integrated biosensors in tissue chips and their applications to better understand human biology, physiology, and diseases in space and on Earth, followed by future perspectives of tissue chip applications on Earth and in space.

Cytocentric measurement for regenerative medicine

Any Regenerative Medicine (RM) business requires reliably predictable cell and tissue products. Regulatory agencies expect control and documentation. However, laboratory tissue production is currently not predictable or well-controlled. Before conditions can be controlled to meet the needs of cells and tissues in culture for RM, we have to know what those needs are and be able to quantify them. Therefore, identification and measurement of critical cell quality attributes at a cellular or pericellular level is essential to generating reproducible cell and tissue products. Here, we identify some of the critical cell and process parameters for cell and tissue products as well as technologies available for sensing them. We also discuss available and needed technologies for monitoring both 2D and 3D cultures to manufacture reliable cell and tissue products for clinical and non-clinical use. As any industry matures, it improves and standardizes the quality of its products. Cytocentric measurement of cell and tissue quality attributes are needed for RM.

The Challenges of O2 Detection in Biological Fluids: Classical Methods and Translation to Clinical Applications

Dissolved oxygen (DO) is deeply involved in preserving the life of cellular tissues and human beings due to its key role in cellular metabolism: its alterations may reflect important pathophysiological conditions. DO levels are measured to identify pathological conditions, explain pathophysiological mechanisms, and monitor the efficacy of therapeutic approaches. This is particularly relevant when the measurements are performed in vivo but also in contexts where a variety of biological and synthetic media are used, such as ex vivo organ perfusion. A reliable measurement of medium oxygenation ensures a high-quality process. It is crucial to provide a high-accuracy, real-time method for DO quantification, which could be robust towards different medium compositions and temperatures. In fact, biological fluids and synthetic clinical fluids represent a challenging environment where DO interacts with various compounds and can change continuously and dynamically, and further precaution is needed to obtain reliable results. This study aims to present and discuss the main oxygen detection and quantification methods, focusing on the technical needs for their translation to clinical practice. Firstly, we resumed all the main methodologies and advancements concerning dissolved oxygen determination. After identifying the main groups of all the available techniques for DO sensing based on their mechanisms and applicability, we focused on transferring the most promising approaches to a clinical in vivo/ex vivo setting.

Liver-on-a-chip: Considerations, advances, and beyond

The liver is the largest internal organ in the human body with largest mass of glandular tissue. Modeling the liver has been challenging due to its variety of major functions, including processing nutrients and vitamins, detoxification, and regulating body metabolism. The intrinsic shortfalls of conventional two-dimensional (2D) cell culture methods for studying pharmacokinetics in parenchymal cells (hepatocytes) have contributed to suboptimal outcomes in clinical trials and drug development. This prompts the development of highly automated, biomimetic liver-on-a-chip (LOC) devices to simulate native liver structure and function, with the aid of recent progress in microfluidics. LOC offers a cost-effective and accurate model for pharmacokinetics, pharmacodynamics, and toxicity studies. This review provides a critical update on recent developments in designing LOCs and fabrication strategies. We highlight biomimetic design approaches for LOCs, including mimicking liver structure and function, and their diverse applications in areas such as drug screening, toxicity assessment, and real-time biosensing. We capture the newest ideas in the field to advance the field of LOCs and address current challenges.

Bioprinting-based automated deposition of single cancer cell spheroids into oxygen sensor microelectrode wells

Three-dimensional (3D) cell agglomerates, such as microtissues, organoids, and spheroids, become increasingly relevant in biomedicine. They can provide in vitro models that recapitulate functions of the original tissue in the body and have applications in cancer research. For example, they are widely used in organ-on-chip systems. Microsensors can provide essential real-time information on cell metabolism as well as the reliability and quality of culture conditions. The combination of sensors and 3D cell cultures, especially single spheroids, is challenging in terms of reproducible formation, manipulation, and access to spheroids, precise positioning near sensors, and high cell-to-volume ratios to obtain meaningful biosignals in the most parallel approach possible. To overcome this challenge, we combined state-of-the-art bioprinting techniques to automatically print tumour spheroids directly into microwells of a chip-based electrochemical oxygen sensor array. We demonstrated highly accurate and reproducible spheroid formation (diameter of approx. 200 μm) and a spheroid deposition precision of 25 μm within a volume of 22 nl per droplet. Microstructures and hydrogel-coated microwells allowed the placement of single MCF-7 breast cancer spheroids close to the sensor electrodes. The microelectrode wells were sealed for oxygen measurements within a 55 nl volume for fast concentration changes. Accurate and stable amperometric oxygen sensor performance was demonstrated from atmospheric to anoxic regions. Cellular respiration rates from single tumour spheroids in the range of 450-850 fmol min^-1 were determined, and alterations of cell metabolism upon drug exposure were shown. Our results uniquely combine bioprinting with 3D cell culture monitoring and demonstrate the much-needed effort for facilitation, parallelization, sensor integration, and drug delivery in 3D cell culture and organ-on-chip experiments. The workflow has a high degree of automation and potential for scalability. In order to achieve greater flexibility in the automation of spheroid formation and trapping, we employed a method based on drop-on-demand liquid handling systems, instead of the typical on-chip approach commonly used in microfluidics. Its relevance ranges from fundamental metabolic research over standardization of cell culture experiments and toxicological studies, to personalized medicine, e.g. patient-specific chemotherapy.

Integrated biosensors for monitoring microphysiological systems

Microphysiological systems (MPSs), also known as organ-on-a-chip models, aim to recapitulate the functional components of human tissues or organs in vitro. Over the last decade, with the advances in biomaterials, 3D bioprinting, and microfluidics, numerous MPSs have emerged with applications to study diseased and healthy tissue models. Various organs have been modeled using MPS technology, such as the heart, liver, lung, and blood-brain barrier. An important aspect of in vitro modeling is the accurate phenotypical and functional characterization of the modeled organ. However, most conventional characterization methods are invasive and destructive and do not allow continuous monitoring of the cells in culture. On the other hand, microfluidic biosensors enable in-line, real-time sensing of target molecules with an excellent limit of detection and in a non-invasive manner, thereby effectively overcoming the limitation of the traditional techniques. Consequently, microfluidic biosensors have been increasingly integrated into MPSs and used for in-line target detection. This review discusses the state-of-the-art microfluidic biosensors by providing specific examples, detailing their main advantages in monitoring MPSs, and highlighting current developments in this field. Finally, we describe the remaining challenges and potential future developments to advance the current state-of-the-art in integrated microfluidic biosensors.

Electrochemical microwell sensor with Fe-N co-doped carbon catalyst to monitor nitric oxide release from endothelial cell spheroids

Endothelial cells have been widely used for vascular biology studies; recent progress in tissue engineering have offered three-dimensional (3D) culture systems for vascular endothelial cells which can be considered as physiologically relevant models. To facilitate the studies, we developed an electrochemical device to detect nitric oxide (NO), a key molecule in the vasculature, for the evaluation of 3D cultured endothelial cells. Using an NO-sensitive catalyst composed of Fe-N co-doped reduced graphene oxide, the real-time monitoring of NO release from the endothelial cell spheroids was demonstrated.

Nanosafety: An Evolving Concept to Bring the Safest Possible Nanomaterials to Society and Environment

The use of nanomaterials has been increasing in recent times, and they are widely used in industries such as cosmetics, drugs, food, water treatment, and agriculture. The rapid development of new nanomaterials demands a set of approaches to evaluate the potential toxicity and risks related to them. In this regard, nanosafety has been using and adapting already existing methods (toxicological approach), but the unique characteristics of nanomaterials demand new approaches (nanotoxicology) to fully understand the potential toxicity, immunotoxicity, and (epi)genotoxicity. In addition, new technologies, such as organs-on-chips and sophisticated sensors, are under development and/or adaptation. All the information generated is used to develop new in silico approaches trying to predict the potential effects of newly developed materials. The overall evaluation of nanomaterials from their production to their final disposal chain is completed using the life cycle assessment (LCA), which is becoming an important element of nanosafety considering sustainability and environmental impact. In this review, we give an overview of all these elements of nanosafety.

Advanced Materials and Sensors for Microphysiological Systems: Focus on Electronic and Electrooptical Interfaces

Advanced in vitro cell culture systems or microphysiological systems (MPSs), including microfluidic organ-on-a-chip (OoC), are breakthrough technologies in biomedicine. These systems recapitulate features of human tissues outside of the body. They are increasingly being used to study the functionality of different organs for applications such as drug evolutions, disease modeling, and precision medicine. Currently, developers and endpoint users of these in vitro models promote how they can replace animal models or even be a better ethically neutral and humanized alternative to study pathology, physiology, and pharmacology. Although reported models show a remarkable physiological structure and function compared to the conventional 2D cell culture, they are almost exclusively based on standard passive polymers or glass with none or minimal real-time stimuli and readout capacity. The next technology leap in reproducing in vivo-like functionality and real-time monitoring of tissue function could be realized with advanced functional materials and devices. This review describes the currently reported electronic and optical advanced materials for sensing and stimulation of MPS models. In addition, an overview of multi-sensing for Body-on-Chip platforms is given. Finally, one gives the perspective on how advanced functional materials could be integrated into in vitro systems to precisely mimic human physiology.

Generation and evaluation of input values for computational analysis of transport processes within tissue cultures

Techniques for tissue culture have seen significant advances during the last decades and novel 3D cell culture systems have become available. To control their high complexity, experimental techniques and their Digital Twins (modelling and computational tools) are combined to link different variables to process conditions and critical process parameters. This allows a rapid evaluation of the expected product quality. However, the use of mathematical simulation and Digital Twins is critically dependent on the precise description of the problem and correct input parameters. Errors here can lead to dramatically wrong conclusions. The intention of this review is to provide an overview of the state‐of‐the‐art and remaining challenges with respect to generating input values for computational analysis of mass and momentum transport processes within tissue cultures. It gives an overview on relevant aspects of transport processes in tissue cultures as well as modelling and computational tools to tackle these problems. Further focus is on techniques used for the determination of cell‐specific parameters and characterization of culture systems, including sensors for on‐line determination of relevant parameters. In conclusion, tissue culture techniques are well‐established, and modelling tools are technically mature. New sensor technologies are on the way, especially for organ chips. The greatest remaining challenge seems to be the proper addressing and handling of input parameters required for mathematical models. Following Good Modelling Practice approaches when setting up and validating computational models is, therefore, essential to get to better estimations of the interesting complex processes inside organotypic tissue cultures in the future.

Biosensors to Monitor Cell Activity in 3D Hydrogel-Based Tissue Models

Three-dimensional (3D) culture models have gained relevant interest in tissue engineering and drug discovery owing to their suitability to reproduce in vitro some key aspects of human tissues and to provide predictive information for in vivo tests. In this context, the use of hydrogels as artificial extracellular matrices is of paramount relevance, since they allow closer recapitulation of (patho)physiological features of human tissues. However, most of the analyses aimed at characterizing these models are based on time-consuming and endpoint assays, which can provide only static and limited data on cellular behavior. On the other hand, biosensing systems could be adopted to measure on-line cellular activity, as currently performed in bi-dimensional, i.e., monolayer, cell culture systems; however, their translation and integration within 3D hydrogel-based systems is not straight forward, due to the geometry and materials properties of these advanced cell culturing approaches. Therefore, researchers have adopted different strategies, through the development of biochemical, electrochemical and optical sensors, but challenges still remain in employing these devices. In this review, after examining recent advances in adapting existing biosensors from traditional cell monolayers to polymeric 3D cells cultures, we will focus on novel designs and outcomes of a range of biosensors specifically developed to provide real-time analysis of hydrogel-based cultures.

Applied Hepatic Bioengineering: Modeling the Human Liver Using Organoid and Liver-on-a-Chip Technologies

The liver is the most important metabolic hub of endo and xenobiotic compounds. Pre-clinical studies using rodents to evaluate the toxicity of new drugs and cosmetics may produce inconclusive results for predicting clinical outcomes in humans, moreover being banned in the European Union. Human liver modeling using primary hepatocytes presents low reproducibility due to batch-to-batch variability, while iPSC-derived hepatocytes in monolayer cultures (2D) show reduced cellular functionality. Here we review the current status of the two most robust in vitro approaches in improving hepatocyte phenotype and metabolism while mimicking the hepatic physiological microenvironment: organoids and liver-on-chip. Both technologies are reviewed in design and manufacturing techniques, following cellular composition and functionality. Furthermore, drug screening and liver diseases modeling efficiencies are summarized. Finally, organoid and liver-on-chip technologies are compared regarding advantages and limitations, aiming to guide the selection of appropriate models for translational research and the development of such technologies.

Microfluidic organ-on-chip system for multi-analyte monitoring of metabolites in 3D cell cultures

Three-dimensional cell cultures using patient-derived stem cells are essential in vitro models for a more efficient and individualized cancer therapy. Currently, culture conditions and metabolite concentrations, especially hypoxia, are often not accessible continuously and in situ within microphysiological systems. However, understanding and standardizing the cellular microenvironment are the key to successful in vitro models. We developed a microfluidic organ-on-chip platform for matrix-based, heterogeneous 3D cultures with fully integrated electrochemical chemo- and biosensor arrays for the energy metabolites oxygen, lactate, and glucose. Advanced microstructures allow straightforward cell matrix integration with standard laboratory equipment, compartmentalization, and microfluidic access. Single, patient-derived, triple-negative breast cancer stem cells develop into tumour organoids in a heterogeneous spheroid culture on-chip. Our system allows unprecedented control of culture conditions, including hypoxia, and simultaneous verification by integrated sensors. Beyond previous works, our results demonstrate precise and reproducible on-chip multi-analyte metabolite monitoring under dynamic conditions from a matrix-based culture over more than one week. Responses to alterations in culture conditions and cancer drug exposure, such as metabolite consumption and production rates, could be accessed quantitatively and in real-time, in contrast to endpoint analyses. Our approach highlights the importance of continuous, in situ metabolite monitoring in 3D cell cultures regarding the standardization and control of culture conditions, and drug screening in cancer research. Overall, the results underline the potential of microsensors in organ-on-chip systems for successful application, e.g. in personalized medicine.

Continous, non-invasive monitoring of oxygen consumption in a parallelized microfluidic in vitro system provides novel insight into the response to nutrients and drugs of primary human hepatocytes

Oxygen plays a fundamental role in cellular energy metabolism, differentiation and cell biology in general. Consequently, in vitro oxygen sensing can be used to assess cell vitality and detect specific mechanisms of toxicity. In 2D in vitro models currently used, the oxygen supply provided by diffusion is generally too low, especially for cells having a high oxygen demand. In organ-on-chip systems, a more physiologic oxygen supply can be generated by establishing unidirectional perfusion. We established oxygen sensors in an easy-to-use and parallelized organ-on-chip system. We demonstrated the applicability of this system by analyzing the influence of fructose (40 mM, 80 mM), ammonium chloride (100 mM) and Na-diclofenac (50 µM, 150 µM, 450 µM, 1500 µM) on primary human hepatocytes (PHH). Fructose treatment for two hours showed an immediate drop of oxygen consumption (OC) with subsequent increase to nearly initial levels. Treatment with 80 mM glucose, 20 mM lactate or 20 mM glycerol did not result in any changes in OC which demonstrates a specific effect of fructose. Application of ammonium chloride for two hours did not show any immediate effects on OC, but qualitatively changed the cellular response to FCCP treatment. Na-diclofenac treatment for 24 hours led to a decrease of the maximal respiration and reserve capacity. We also demonstrated the stability of our system by repeatedly treating cells with 40 mM fructose, which led to similar cell responses on the same day as well as on subsequent days. In conclusion, our system enables in depth analysis of cellular respiration after substrate treatment in an unidirectional perfused organ-on-chip system.

Microfluidic-Based Oxygen (O2) Sensors for On-Chip Monitoring of Cell, Tissue and Organ Metabolism

Oxygen (O₂) quantification is essential for assessing cell metabolism, and its consumption in cell culture is an important indicator of cell viability. Recent advances in microfluidics have made O₂ sensing a crucial feature for organ-on-chip (OOC) devices for various biomedical applications. OOC O₂ sensors can be categorized, based on their transducer type, into two main groups, optical and electrochemical. In this review, we provide an overview of on-chip O₂ sensors integrated with the OOC devices and evaluate their advantages and disadvantages. Recent innovations in optical O₂ sensors integrated with OOCs are discussed in four main categories: (i) basic luminescence-based sensors; (ii) microparticle-based sensors; (iii) nano-enabled sensors; and (iv) commercial probes and portable devices. Furthermore, we discuss recent advancements in electrochemical sensors in five main categories: (i) novel configurations in Clark-type sensors; (ii) novel materials (e.g., polymers, O₂ scavenging and passivation materials); (iii) nano-enabled electrochemical sensors; (iv) novel designs and fabrication techniques; and (v) commercial and portable electrochemical readouts. Together, this review provides a comprehensive overview of the current advances in the design, fabrication and application of optical and electrochemical O₂ sensors.

Engineered l-Lactate Responding Promoter System Operating in Glucose-Rich and Anoxic Environments

Bacteria equipped with genetically encoded lactate biosensors are promising tools for biopharmaceutical production, diagnostics, and cellular therapies. However, many applications involve glucose-rich and anoxic environments, in which current whole-cell lactate biosensors show low performance. Here we engineer an optimized, synthetic lactate biosensor system by repurposing the natural LldPRD promoter regulated by the LldR transcriptional regulator. We removed glucose catabolite and anoxic repression by designing a hybrid promoter, containing LldR operators and tuned both regulator and reporter gene expressions to optimize biosensor signal-to-noise ratio. The resulting lactate biosensor, termed ALPaGA (A Lactate Promoter Operating in Glucose and Anoxia), can operate in glucose-rich, aerobic and anoxic conditions. We show that ALPaGA works reliably in the probiotic chassisEscherichia coliNissle 1917 and can detect endogenous l-lactate produced by 3D tumor spheroids with an improved dynamic range. In the future, the ALPaGA system could be used to monitor bioproduction processes and improve the specificity of engineered bacterial cancer therapies by restricting their activity to the lactate-rich microenvironment of solid tumors.

Human Organs-on-Chips: A Review of the State-of-the-Art, Current Prospects, and Future Challenges

New emerging technologies, remarkably miniaturized 3D organ models and microfluidics, enable simulation of the real in vitro microenvironment ex vivo more closely. There are many fascinating features of innovative organ-on-a-chip (OOC) technology, including the possibility of integrating semipermeable and/or stretchable membranes, creating continuous perfusion of fluids into microchannels and chambers (while maintaining laminar flow regime), embedding microdevices like microsensors, microstimulators, micro heaters, or different cell lines, along with other 3D cell culture technologies. OOC systems are designed to imitate the structure and function of human organs, ranging from breathing lungs to beating hearts. This technology is expected to be able to revolutionize cell biology studies, personalized precision medicine, drug development process, and cancer diagnosis/treatment. OOC systems can significantly reduce the cost associated with tedious drug development processes and the risk of adverse drug reactions in the body, which makes drug screening more effective. The review mainly focus on presenting an overview of the several previously developed OOC systems accompanied by subjects relevant to pharmacy-, cancer-, and placenta-on-a-chip. The challenging issues and opportunities related to these systems are discussed, along with a future perspective for this technology.

Quality control methods in musculoskeletal tissue engineering: from imaging to biosensors

Tissue engineering is rapidly progressing toward clinical application. In the musculoskeletal field, there has been an increasing necessity for bone and cartilage replacement. Despite the promising translational potential of tissue engineering approaches, careful attention should be given to the quality of developed constructs to increase the real applicability to patients. After a general introduction to musculoskeletal tissue engineering, this narrative review aims to offer an overview of methods, starting from classical techniques, such as gene expression analysis and histology, to less common methods, such as Raman spectroscopy, microcomputed tomography, and biosensors, that can be employed to assess the quality of constructs in terms of viability, morphology, or matrix deposition. A particular emphasis is given to standards and good practices (GXP), which can be applicable in different sectors. Moreover, a classification of the methods into destructive, noninvasive, or conservative based on the possible further development of a preimplant quality monitoring system is proposed. Biosensors in musculoskeletal tissue engineering have not yet been used but have been proposed as a novel technology that can be exploited with numerous advantages, including minimal invasiveness, making them suitable for the development of preimplant quality control systems.

Three-Dimensional Liver Culture Systems to Maintain Primary Hepatic Properties for Toxicological Analysis In Vitro

Drug-induced liver injury (DILI) is the major reason for failures in drug development and withdrawal of approved drugs from the market. Two-dimensional cultures of hepatocytes often fail to reliably predict DILI: hepatoma cell lines such as HepG2 do not reflect important primary-like hepatic properties and primary human hepatocytes (pHHs) dedifferentiate quickly in vitro and are, therefore, not suitable for long-term toxicity studies. More predictive liver in vitro models are urgently required in drug development and compound safety evaluation. This review discusses available human hepatic cell types for in vitro toxicology analysis and their usage in established and emerging three-dimensional (3D) culture systems. Generally, 3D cultures maintain or improve primary hepatic functions (including expression of drug-metabolizing enzymes) of different liver cells for several weeks of culture, thus allowing long-term and repeated-dose toxicity studies. Spheroid cultures of pHHs have been comprehensively tested, but also other cell types such as HepaRG benefit from 3D culture systems. Emerging 3D culture techniques include usage of induced pluripotent stem-cell-derived hepatocytes and primary-like upcyte cells, as well as advanced culture techniques such as microfluidic liver-on-a-chip models. In-depth characterization of existing and emerging 3D hepatocyte technologies is indispensable for successful implementation of such systems in toxicological analysis.

Oxygen Biosensors and Control in 3D Physiomimetic Experimental Models

Traditional cell culture is experiencing a revolution moving toward physiomimetic approaches aiming to reproduce healthy and pathological cell environments as realistically as possible. There is increasing evidence demonstrating that biophysical and biochemical factors determine cell behavior, in some cases considerably. Alongside the explosion of these novel experimental approaches, different bioengineering techniques have been developed and improved. Increased affordability and popularization of 3D bioprinting, fabrication of custom-made lab-on-a chip, development of organoids and the availability of versatile hydrogels are factors facilitating the design of tissue-specific physiomimetic in vitro models. However, lower oxygen diffusion in 3D culture is still a critical limitation in most of these studies, requiring further efforts in the field of physiology and tissue engineering and regenerative medicine. During recent years, novel advanced 3D devices are introducing integrated biosensors capable of monitoring oxygen consumption, pH and cell metabolism. These biosensors seem to be a promising solution to better control the oxygen delivery to cells and to reproduce some disease conditions involving hypoxia. This review discusses the current advances on oxygen biosensors and control in 3D physiomimetic experimental models.

Airway-On-A-Chip: Designs and Applications for Lung Repair and Disease

The lungs are affected by illnesses including asthma, chronic obstructive pulmonary disease, and infections such as influenza and SARS-CoV-2. Physiologically relevant models for respiratory conditions will be essential for new drug development. The composition and structure of the lung extracellular matrix (ECM) plays a major role in the function of the lung tissue and cells. Lung-on-chip models have been developed to address some of the limitations of current two-dimensional in vitro models. In this review, we describe various ECM substitutes utilized for modeling the respiratory system. We explore the application of lung-on-chip models to the study of cigarette smoke and electronic cigarette vapor. We discuss the challenges and opportunities related to model characterization with an emphasis on in situ characterization methods, both established and emerging. We discuss how further advancements in the field, through the incorporation of interstitial cells and ECM, have the potential to provide an effective tool for interrogating lung biology and disease, especially the mechanisms that involve the interstitial elements.

Liver Organoids: Recent Developments, Limitations and Potential

Liver cell types derived from induced pluripotent stem cells (iPSCs) share the potential to investigate development, toxicity, as well as genetic and infectious disease in ways currently limited by the availability of primary tissue. With the added advantage of patient specificity, which can play a role in all of these areas. Many iPSC differentiation protocols focus on 3 dimensional (3D) or organotypic differentiation, as these offer the advantage of more closely mimicking in vivo systems including; the formation of tissue like architecture and interactions/crosstalk between different cell types. Ultimately such models have the potential to be used clinically and either with or more aptly, in place of animal models. Along with the development of organotypic and micro-tissue models, there will be a need to co-develop imaging technologies to enable their visualization. A variety of liver models termed "organoids" have been reported in the literature ranging from simple spheres or cysts of a single cell type, usually hepatocytes, to those containing multiple cell types combined during the differentiation process such as hepatic stellate cells, endothelial cells, and mesenchymal cells, often leading to an improved hepatic phenotype. These allow specific functions or readouts to be examined such as drug metabolism, protein secretion or an improved phenotype, but because of their relative simplicity they lack the flexibility and general applicability of ex vivo tissue culture. In the liver field these are more often constructed rather than developed together organotypically as seen in other organoid models such as brain, kidney, lung and intestine. Having access to organotypic liver like surrogates containing multiple cell types with in vivo like interactions/architecture, would provide vastly improved models for disease, toxicity and drug development, combining disciplines such as microfluidic chip technology with organoids and ultimately paving the way to new therapies.

Automated 3D Microphysiometry Facilitates High-Content and Highly Reproducible Oxygen Measurements within 3D Cell Culture Models

Microphysiometry is a powerful technique to study metabolic parameters and detect changes to external stimuli. However, applying this technique for automated label-free and real-time measurements within cell-laden three-dimensional (3D) cell culture constructs remains a challenge. Herein, we present an entirely automated microphysiometry setup that combines needle-type microsensors with motorized sample and sensor positioning systems inside a standard tissue-culture incubator. The setup records dissolved oxygen as a metabolic parameter along the z-direction within cell-laden 3D constructs in a minimally invasive manner. The microphysiometry setup was applied to characterize the spatial oxygen distribution within thick cell-laden 3D constructs, study the time-dependent changes on the oxygen tension within 3D breast cancer models following a chemotherapeutic treatment, and identify kinetics and recovery effects after drug exposure over 5 weeks. Our data suggest that the microphysiometry setup enables highly reproducible measurements without human intervention, due to the high degree of automation and positional accuracy. The results demonstrate the applicability of the setup to provide valuable long-term insights into oxygenation within 3D models using minimally invasive, label-free, and entirely automated analysis methods.

Electrochemical Sensing in 3D Cell Culture Models: New Tools for Developing Better Cancer Diagnostics and Treatments

Currently, conventional pre-clinical in vitro studies are primarily based on two-dimensional (2D) cell culture models, which are usually limited in mimicking the real three-dimensional (3D) physiological conditions, cell heterogeneity, cell to cell interaction, and extracellular matrix (ECM) present in living tissues. Traditionally, animal models are used to mimic the 3D environment of tissues and organs, but they suffer from high costs, are time consuming, bring up ethical concerns, and still present many differences when compared to the human body. The applications of microfluidic-based 3D cell culture models are advantageous and useful as they include 3D multicellular model systems (MCMS). These models have demonstrated potential to simulate the in vivo 3D microenvironment with relatively low cost and high throughput. The incorporation of monitoring capabilities in the MCMS has also been explored to evaluate in real time biophysical and chemical parameters of the system, for example temperature, oxygen, pH, and metabolites. Electrochemical sensing is considered as one of the most sensitive and commercially adapted technologies for bio-sensing applications. Amalgamation of electrochemical biosensing with cell culture in microfluidic devices with improved sensitivity and performance are the future of 3D systems. Particularly in cancer, such models with integrated sensing capabilities can be crucial to assess the multiple parameters involved in tumour formation, proliferation, and invasion. In this review, we are focusing on existing 3D cell culture systems with integrated electrochemical sensing for potential applications in cancer models to advance diagnosis and treatment. We discuss their design, sensing principle, and application in the biomedical area to understand the potential relevance of miniaturized electrochemical hybrid systems for the next generation of diagnostic platforms for precision medicine.

Advancement of Sensor Integrated Organ-on-Chip Devices

Organ-on-chip devices have provided the pharmaceutical and tissue engineering worlds much hope since they arrived and began to grow in sophistication. However, limitations for their applicability were soon realized as they lacked real-time monitoring and sensing capabilities. The users of these devices relied solely on endpoint analysis for the results of their tests, which created a chasm in the understanding of life between the lab the natural world. However, this gap is being bridged with sensors that are integrated into organ-on-chip devices. This review goes in-depth on different sensing methods, giving examples for various research on mechanical, electrical resistance, and bead-based sensors, and the prospects of each. Furthermore, the review covers works conducted that use specific sensors for oxygen, and various metabolites to characterize cellular behavior and response in real-time. Together, the outline of these works gives a thorough analysis of the design methodology and sophistication of the current sensor integrated organ-on-chips.

Nanotechnology-assisted microfluidic systems: from bench to bedside

With significant advancements in research technologies, and an increasing global population, microfluidic and nanofluidic systems (such as point-of-care, lab-on-a-chip, organ-on-a-chip, etc) have started to revolutionize medicine. Devices that combine micron and nanotechnologies have increased sensitivity, precision and versatility for numerous medical applications. However, while there has been extensive research on microfluidic and nanofluidic systems, very few have experienced wide-spread commercialization which is puzzling and deserves our collective attention. For the above reasons, in this article, we review research advances that combine micro and nanotechnologies to create the next generation of nanomaterial-based microfluidic systems, the latest in their commercialization success and failure and highlight the value of these devices both in industry and in the laboratory.

Spheroids and organoids as humanized 3D scaffold-free engineered tissues for SARS-CoV-2 viral infection and drug screening

The new coronavirus (2019‐nCoV) or the severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) was officially declared by the World Health Organization (WHO) as a pandemic in March 2020. To date, there are no specific antiviral drugs proven to be effective in treating SARS‐CoV‐2, requiring joint efforts from different research fronts to discover the best route of treatment. The first decisions in drug discovery are based on 2D cell culture using high‐throughput screening. In this context, spheroids and organoids emerge as a reliable alternative. Both are scaffold‐free 3D engineered constructs that recapitulate key cellular and molecular events of tissue physiology. Different studies have already shown their advantages as a model for different infectious diseases, including SARS‐CoV‐2 and for drug screening. The use of these 3D engineered tissues as an in vitro model can fill the gap between 2D cell culture and in vivo preclinical assays (animal models) as they could recapitulate the entire viral life cycle. The main objective of this review is to understand spheroid and organoid biology, highlighting their advantages and disadvantages, and how these scaffold‐free engineered tissues can contribute to a better comprehension of viral infection by SARS‐CoV‐2 and to the development of in vitro high‐throughput models for drug screening.

Intestinal Stem Cell-on-Chip to Study Human Host-Microbiota Interaction

The gut is a tubular organ responsible for nutrient absorption and harbors our intestinal microbiome. This organ is composed of a multitude of specialized cell types arranged in complex barrier-forming crypts and villi covered by a mucosal layer controlling nutrient passage and protecting from invading pathogens. The development and self-renewal of the intestinal epithelium are guided by niche signals controlling the differentiation of specific cell types along the crypt-villus axis in the epithelium. The emergence of microphysiological systems, or organ-on-chips, has paved the way to study the intestinal epithelium within a dynamic and controlled environment. In this review, we describe the use of organ-on-chip technology to control and guide these differentiation processes in vitro. We further discuss current applications and forthcoming strategies to investigate the mechanical processes of intestinal stem cell differentiation, tissue formation, and the interaction of the intestine with the microbiota in the context of gastrointestinal diseases.

Engineering organoids

Organoids are in vitro miniaturized and simplified model systems of organs that have gained enormous interest for modelling tissue development and disease, and for personalized medicine, drug screening and cell therapy. Despite considerable success in culturing physiologically relevant organoids, challenges remain to achieve real-life applications. In particular, the high variability of self-organizing growth and restricted experimental and analytical access hamper the translatability of organoid systems. In this Review, we argue that many limitations of traditional organoid culture can be addressed by engineering approaches at all levels of organoid systems. We investigate cell surface and genetic engineering approaches, and discuss stem cell niche engineering based on the design of matrices that allow spatiotemporal control of organoid growth and shape-guided morphogenesis. We examine how microfluidic approaches and lessons learnt from organs-on-a-chip enable the integration of mechano-physiological parameters and increase accessibility of organoids to improve functional readouts. Applying engineering principles to organoids increases reproducibility and provides experimental control, which will, ultimately, be required to enable clinical translation.

Bioelectronic properties of DNA, protein, cells and their applications for diagnostic medical devices

From a couple of centuries ago, understanding physical properties of biological material, their interference with their natural host and their potential manipulation for employment as a conductor in medical devices, has gathered substantial interest in the field of bioelectronics. With the fast-emerging technologies for fabrication of diagnostic modalities, wearable biosensors and implantable devices, which electrical components are of essential importance, a need for developing novel conductors within such devices has evolved over the past decades. As the possibility of electron transport within small biological molecules, such as DNA and proteins, as well as larger elements such as cells was established, several discoveries of the modern charge characterization technologies were evolved. Development of Electrochemical Scanning Tunneling Microscopy and Nuclear Magnetic Resonance among many other techniques were of vital importance, following the discoveries made in sub-micron scales of biological material. This review covers the most recent understandings of electronic properties within different scale of biological material starting from nanometer range to millimeter-sized organs. We also discuss the state-of-the-art technology that's been made taking advantage of electronic properties of biological material for addressing diseases like Parkinson's Disease and Epilepsy.

Engineering liver microtissues for disease modeling and regenerative medicine

The burden of liver diseases is increasing worldwide, accounting for two million deaths annually. In the past decade, tremendous progress has been made in the basic and translational research of liver tissue engineering. Liver microtissues are small, three-dimensional hepatocyte cultures that recapitulate liver physiology and have been used in biomedical research and regenerative medicine. This review summarizes recent advances, challenges, and future directions in liver microtissue research. Cellular engineering approaches are used to sustain primary hepatocytes or produce hepatocytes derived from pluripotent stem cells and other adult tissues. Three-dimensional microtissues are generated by scaffold-free assembly or scaffold-assisted methods such as macroencapsulation, droplet microfluidics, and bioprinting. Optimization of the hepatic microenvironment entails incorporating the appropriate cell composition for enhanced cell-cell interactions and niche-specific signals, and creating scaffolds with desired chemical, mechanical and physical properties. Perfusion-based culture systems such as bioreactors and microfluidic systems are used to achieve efficient exchange of nutrients and soluble factors. Taken together, systematic optimization of liver microtissues is a multidisciplinary effort focused on creating liver cultures and on-chip models with greater structural complexity and physiological relevance for use in liver disease research, therapeutic development, and regenerative medicine.

3D-Printed electrochemical sensor-integrated transwell systems

This work presents a 3D-printed, modular, electrochemical sensor-integrated transwell system for monitoring cellular and molecular events in situ without sample extraction or microfluidics-assisted downstream omics. Simple additive manufacturing techniques such as 3D printing, shadow masking, and molding are used to fabricate this modular system, which is autoclavable, biocompatible, and designed to operate following standard operating protocols (SOPs) of cellular biology. Integral to the platform is a flexible porous membrane, which is used as a cell culture substrate similarly to a commercial transwell insert. Multimodal electrochemical sensors fabricated on the membrane allow direct access to cells and their products. A pair of gold electrodes on the top side of the membrane measures impedance over the course of cell attachment and growth, characterized by an exponential decrease (~160% at 10 Hz) due to an increase in the double layer capacitance from secreted extracellular matrix (ECM) proteins. Cyclic voltammetry (CV) sensor electrodes, fabricated on the bottom side of the membrane, enable sensing of molecular release at the site of cell culture without the need for downstream fluidics. Real-time detection of ferrocene dimethanol injection across the membrane showed a three order-of-magnitude higher signal at the membrane than in the bulk media after reaching equilibrium. This modular sensor-integrated transwell system allows unprecedented direct, real-time, and noninvasive access to physical and biochemical information, which cannot be obtained in a conventional transwell system.

Integrating Biosensors in Organs-on-Chip Devices: A Perspective on Current Strategies to Monitor Microphysiological Systems

Organs-on-chip (OoC), often referred to as microphysiological systems (MPS), are advanced in vitro tools able to replicate essential functions of human organs. Owing to their unprecedented ability to recapitulate key features of the native cellular environments, they represent promising tools for tissue engineering and drug screening applications. The achievement of proper functionalities within OoC is crucial; to this purpose, several parameters (e.g., chemical, physical) need to be assessed. Currently, most approaches rely on off-chip analysis and imaging techniques. However, the urgent demand for continuous, noninvasive, and real-time monitoring of tissue constructs requires the direct integration of biosensors. In this review, we focus on recent strategies to miniaturize and embed biosensing systems into organs-on-chip platforms. Biosensors for monitoring biological models with metabolic activities, models with tissue barrier functions, as well as models with electromechanical properties will be described and critically evaluated. In addition, multisensor integration within multiorgan platforms will be further reviewed and discussed.

Real time monitoring of oxygen uptake of hepatocytes in a microreactor using optical microsensors

Most in vitro test systems for the assessment of toxicity are based on endpoint measurements and cannot contribute much to the establishment of mechanistic models, which are crucially important for further progress in this field. Hence, in recent years, much effort has been put into the development of methods that generate kinetic data. Real time measurements of the metabolic activity of cells based on the use of oxygen sensitive microsensor beads have been shown to provide access to the mode of action of compounds in hepatocytes. However, for fully exploiting this approach a detailed knowledge of the microenvironment of the cells is required. In this work, we investigate the cellular behaviour of three types of hepatocytes, HepG2 cells, HepG2-3A4 cells and primary mouse hepatocytes, towards their exposure to acetaminophen when the availability of oxygen for the cell is systematically varied. We show that the relative emergence of two modes of action, one NAPQI dependent and the other one transient and NAPQI independent, scale with expression level of CYP3A4. The transient cellular response associated to mitochondrial respiration is used to characterise the influence of the initial oxygen concentration in the wells before exposure to acetaminophen on the cell behaviour. A simple model is presented to describe the behaviour of the cells in this scenario. It demonstrates the level of control over the role of oxygen supply in these experiments. This is crucial for establishing this approach into a reliable and powerful method for the assessment of toxicity.

Bioprinting of in vitro tumor models for personalized cancer treatment: a review

Studying biological characteristics of tumors and evaluating the treatment effects require appropriate in vitro tumor models. However, the occurrence, progression, and migration of tumors involve spatiotemporal changes, cell-microenvironment and cell-cell interactions, and signal transmission in cells, which makes the construction of in vitro tumor models extremely challenging. In the past few years, advances in biomaterials and tissue engineering methods, especially development of the bioprinting technology, have paved the way for innovative platform technologies for in vitro cancer research. Bioprinting can accurately control the distribution of cells, active molecules, and biomaterials. Furthermore, this technology recapitulates the key characteristics of the tumor microenvironment and constructs in vitro tumor models with bionic structures and physiological systems. These models can be used as robust platforms to study tumor initiation, interaction with the microenvironment, angiogenesis, motility and invasion, as well as intra- and extravasation. Bioprinted tumor models can also be used for high-throughput drug screening and validation and provide the possibility for personalized cancer treatment research. This review describes the basic characteristics of the tumor and its microenvironment and focuses on the importance and relevance of bioprinting technology in the construction of tumor models. Research progress in the bioprinting of monocellular, multicellular, and personalized tumor models is discussed, and comprehensive application of bioprinting in preclinical drug screening and innovative therapy is reviewed. Finally, we offer our perspective on the shortcomings of the existing models and explore new technologies to outline the direction of future development and application prospects of next-generation tumor models.