A Guide to Organ-on-a-Chip

In the vast realm of biomedical research, the quest for innovative, efficient and ethically sound experimentation methods has led scientists to explore groundbreaking technologies. Among these, organ-on-a-chip (OOAC) technology stands out as a transformative approach, revolutionizing the way researchers study biological systems. By replicating the microenvironment of human organs within microscale devices, OOAC technology has emerged as a powerful tool for understanding human physiology and disease mechanisms with unparalleled precision.

At its core, OOAC technology entails the development of microfluidic cell culture devices that mimic the microenvironment of human organs. These microscale models provide a physiologically relevant platform for studying cellular behavior, interactions and responses to stimuli, bridging the gap between traditional in vitro cell cultures and in vivo animal models. Unlike conventional cell cultures, OOAC devices offer a three-dimensional context that closely resembles the human body, allowing researchers to observe cellular processes in a realistic setting.

The inception of OOAC technology stemmed from the limitations of conventional experimentation methods. While traditional cell cultures lack the complexity of in vivo environments, animal models present ethical concerns, high costs and challenges in scalability. OOAC technology emerged as a solution to these challenges, offering a middle ground where researchers can explore the intricacies of human biology without compromising ethical standards or scientific accuracy.

While early attempts were being made to develop more organized cell cultures towards the end of the 1980s¹, the development of optically transparent poly(dimethylsiloxane) (PDMS) gave rise to the field of biomicrofluidics^2,3,4 in the late 1990s. The goal of replicating the functions of organs inside microfluidic chips first appeared in 2004 as a “cell-culture analog” of lung–liver interactions.⁵

As various early simple microfluidic models were developed, researchers attempted to reconstruct 3D cell cultures from disparate tissues such as the brain,⁶ blood vessels,⁷ liver,⁸ airways,⁹ gut,¹⁰ bones,¹¹ muscles¹² and kidney.¹³ As the field began to expand, the term “organ-on-a-chip” was coined in 2010 with the development of a microfluidic chip mimicking the functions of the human lung¹⁴. In the decade since, the study of organ-chips has exploded, with tissue engineering and microfluidics, organoids and 3D bioprinting technologies converging with the potential to transform the drug discovery industry.¹⁵

Understanding the nuances of OOAC technology requires insight into the key apparatus and experimental setups involved. Central to every OOAC device are microfluidic channels, cell chambers and porous membranes. These components work in harmony to create a microscale replica of human organs, enabling scientists to investigate cellular behavior within a controlled environment.

Figure 1: A model organ-on-a-chip (OOAC) system. This system studies two cell substrates – a cell layer and an organoid. The OOAC is encased within an incubator where temperature and atmospheric conditions are carefully controlled. Mechanical actuators built into the system provide necessary mechanical stimulation. The system features two pumps, where different types of metabolite-containing media are perfused through the system. An important consideration in an OOAC system containing multiple cell types is that different media concentrations and ingredients may be required to optimally support individual cell types.  A transepithelial electrical resistance (TEER) sensor allows a quantitative measure of the integrity of the cell layer arrangement and a microscope permits optical analysis of the system. Credit: Technology Networks. Adapted from: Leung et al. https://www.nature.com/articles/s43586-022-00118-6

Microfluidic channels form the backbone of OOAC devices, allowing precise control over the flow of fluids. These intricately designed channels replicate the vascular networks found in human organs, ensuring a constant supply of nutrients, oxygen and biochemical cues to the cells. The design of these channels is fundamental, as it directly influences the physiological relevance of the organ model under study. Additionally, there is increasing awareness of the need to recapitulate the mechanical microenvironment of cells in order to maintain or guide their behavior. Indeed, mechanical stimulation is now seen as a crucial aspect to consider to fully mimic the in vivo physiology.¹⁶

Within OOAC devices, cell chambers provide a confined space for cells to grow and interact. These chambers come in various shapes and sizes, tailored to accommodate different cell types and mimic the three-dimensional arrangement of cells within human organs. The integration of porous membranes enhances the realism of OOAC models, enabling cell adhesion, facilitating cell–cell interactions and enabling the formation of tissue-like structures.

In terms of materials, OOAC devices are primarily constructed using biocompatible polymers such as PDMS. PDMS offers several advantages, including transparency, flexibility and ease of fabrication. Its biocompatibility ensures optimal cell growth and viability within the device. Additionally, PDMS-based chips enable real-time visualization of cellular processes using advanced microscopy techniques, providing valuable insights into the behavior of cells within the microscale environment.

The versatility of OOAC technology is evident in the diverse array of organ models that have been developed. One notable example is the lung-on-a-chip, a microfluidic device that replicates the alveolar–capillary interface of the human lung.¹⁴ This model serves as a platform for studying lung diseases such as asthma, chronic obstructive pulmonary disease (COPD) and pulmonary edema, offering valuable insights into disease progression and potential therapeutic interventions.

Liver-on-a-chip models represent another significant advancement in the field. These devices mimic the hepatic microenvironment, allowing researchers to study drug metabolism, toxicity and liver diseases. By observing the behavior of hepatocytes within the chip, scientists can assess the effects of pharmaceutical compounds and identify drug candidates with enhanced safety profiles. Indeed, a momentous recent advancement for the OOAC field occurred in the form of a landmark liver toxicology study.¹⁷ Running a series of in vitro trials using 870 advanced liver-chips, this study showed that OOAC technology can correctly identify 27 known toxic or non-toxic drugs, with a sensitivity of 87% and a specificity of 100%.¹⁷ This significant achievement demonstrated the ability of the field to select better drug candidates for clinical trials, with the potential to save an estimated $3 billion in annual drug development costs.¹⁷

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Advances in Organ-on-a-Chip Technology

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Gut-on-a-chip models offer a comprehensive view of the human intestinal tract. These microfluidic devices incorporate multiple cell types, including epithelial cells, immune cells and microbiota, enabling researchers to explore the complex interactions between the gut microbiome and the host.¹⁸ Given the explosion of interest in our microbiome and its effects on our health, gut-on-a-chip models provide valuable insights into gastrointestinal diseases, inflammatory responses and the impact of dietary factors on human health.

Demonstrating the versatility of the OOAC field, chips are now being used to explore treatments in physiological systems that have traditionally been understudied or neglected by animal models or molecular bioscience. An excellent example is female reproductive tissue, such as the uterus and vaginal canal, which suffer from a large range of pathologies (e.g. endometriosis) that are poorly understood, and for which few treatments are available. Therefore, a welcome recent development came in the form of a vagina-on-a-chip to model host microbiome interactions¹⁹, providing a unique test-bed for developing new treatments for a range of common infections, many of which currently rely on the use of antibiotics.

Beyond individual organ models, scientists are actively working on creating interconnected systems known as body-on-a-chip platforms. These advanced setups replicate multiple organs and their interactions within a single device, offering a holistic view of human physiology.²⁰ Body-on-a-chip models hold immense potential for drug testing, disease modeling and personalized medicine, as they allow researchers to study the systemic effects of drugs and therapies on various organs simultaneously.

Lastly, with the realization that tumors are composed of an inherently heterogeneous population of rapidly mutating cells that respond and adapt to their environment, there are now increasing efforts to use 3D models such as organoids, tumoroids and 3D printing to better mimic these arrangements of cells within a tumor. OOAC platforms provide the ideal systems in which to do this, as the interactions with the microenvironment can be precisely controlled to model both primary tumors²¹ and events in the metastatic cascade as cancer spreads from, for example, the breast or prostate to bone tissue.^22,23 This has led to exponential increases in the development of chip models for oncology, aimed at both fundamental discovery science and therapeutic testing.²⁴

Despite the remarkable progress in OOAC technology, it is not without its limitations.

Here, we will discuss three key limitations of OOAC technology: 

• Fidelity
• Scalability
• Technical challenge

One significant challenge lies in replicating the entire physiological environment of human organs within a microscale device. While OOAC models provide valuable insights, they may not fully capture all aspects of organ function and interaction. Achieving a high degree of fidelity remains a formidable task, especially for organs with intricate structures and functions.²⁵

Another limitation of OOAC technology is its scalability. Current devices are primarily designed for small-scale experiments, limiting their applicability in high-throughput screening and large-scale drug testing. Researchers are actively working on optimizing fabrication techniques and experimental protocols to address this limitation. Scalable OOAC platforms would significantly enhance their utility in drug discovery and development, allowing researchers to test a broader range of compounds efficiently.

Furthermore, the integration of multiple organ systems into a single platform, as seen in body-on-a-chip models, presents technical challenges. Coordinating the functions of different organs and replicating their intricate interconnections within a microfluidic device requires advanced engineering and biological expertise.²⁰ While progress has been made in this area, there is still much to explore to achieve a fully functional and reliable body-on-a-chip system that accurately reflects the complexity of the human body.

Despite these challenges, the field of OOAC technology continues to advance at a rapid pace, driven by interdisciplinary collaborations and innovative solutions. Scientists are exploring novel biomaterials, microfabrication techniques and organ-specific primary human cell sources to enhance the fidelity of OOAC models. Advances in microfluidic automation and integration with analytical techniques enable real-time monitoring of cellular responses, providing a wealth of data for research and drug discovery.

Moreover, the integration of artificial intelligence (AI) and machine learning (ML) algorithms has the potential to revolutionize the analysis of data generated from OOAC experiments. AI and ML technologies could enhance the predictive capabilities of OOAC models, allowing researchers to extract valuable insights from complex datasets. These advanced analytical tools facilitate data analysis, pattern recognition and the identification of subtle changes in cellular behavior, and a flurry of interesting projects and discussions on this topic have begun in the last couple of years.^26,27,28,29  By harnessing the power of AI and ML, researchers can accelerate the discovery of new drugs and therapies, paving the way for personalized medicine and targeted interventions.

Furthermore, microphysiological systems, which combine OOAC technology with advanced sensors and microelectronics, hold immense promise. These integrated systems enable real-time monitoring of physiological parameters, such as electrical activity, contractility and metabolic rates, providing a comprehensive view of organ function. By stepping away from endpoint monitoring, with destructive imaging methods, towards this real-time sensing and monitoring, these devices will be taken ever closer to the clinic.³⁰ Microphysiological systems have the potential to revolutionize drug testing and disease modeling, allowing researchers to study the effects of drugs and therapies on specific organs with unparalleled precision.

In addition to technological advancements, global research collaborations are driving the standardization of OOAC platforms. Standardization is crucial for ensuring reproducibility and comparability across different studies. Establishing standardized protocols and benchmarks will enhance the reliability of OOAC experiments, enabling researchers to build upon each other's work and accelerate scientific progress. This process is being actively aided by regulatory agencies, with the European Medicines Agency committed to reducing animal testing since 2010 (Directive 2010/63/EU), ³¹ and in particular by the recent passing of the US FDA Modernization Act 2.0 (USS.5002)³² which is changing the regulatory landscape. Drug sponsors will now have the capacity to use alternative complex in vitro or in silico models in place of animal testing, where suitable.

Perhaps most importantly, future efforts must be concentrated on pushing this technology closer to industry and the clinic. Recently, the UK Organ-on-a-Chip Technology Network surveyed a broad range of organ-chip developers and end-users.³³ Despite the wide variety of responders, there was a high level of agreement on the technological bottlenecks faced by the community, the requirements for new technologies and services and the need for more detailed validation of individual and interconnected models in order to reach broader adoption. This translational roadmap will ease the transition of these new technologies through the start-up and scale-up phases.³³

In conclusion, OOAC technology is an exciting paradigm in biomedical research. By replicating the microenvironment of human organs within microfluidic devices, scientists can unravel the complexities of diseases, study cellular interactions and develop targeted therapies with precision. While challenges remain, the continuous efforts of researchers and the integration of cutting-edge technologies are propelling the field forward, opening new horizons for drug discovery, disease modeling and personalized medicine.

With each advancement, we move closer to a future where diseases are understood in unprecedented detail, treatments are customized to individual patients, and the boundaries of medical science are pushed. The potential of OOAC technology is a beacon illuminating the path toward a healthier, more informed and interconnected world.