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Exploring Organ-on-a-Chip Technology: A comprehensive review

Introduction

Organ-on-a-Chip (OoC) technology represents a groundbreaking advancement in biomedical research, offering a transformative approach to mimic the complex microenvironments and physiological functions of human organs in vitro. These microfluidic devices incorporate small structures designed for cell culture, creating precise biochemical and mechanical stimuli to recreate physiologically relevant conditions. By integrating microfluidic and bioengineering techniques, OoC platforms aim to recapitulate tissue architecture and simulate key organ functions, making them invaluable tools for preclinical drug testing and disease modeling[1].

Organs on chip represented like a collection of realistic human organs cultured in a lab dish, interconnected by a complex wiring system.
Figure 1-Organs-on-Chip (OoC) generated by OpenAI.

Since its inception in the early 2010s [2], OoC technology has witnessed rapid evolution and garnered increasing interest from the biomedical community. Traditional 2D petri dish cultures and animal models have inherent limitations in replicating human physiology accurately. In contrast, OoC systems offer a more representative in vitro model, bridging the gap between preclinical and clinical outcomes in drug development and disease studies.

Why do we need ooc?

Limitations of 3D Cell Culture

3D cell culture definitely offers great advantages over 2D cell culture. It facilitates cell differentiation and tissue organization getting closer to physiological conditions of living organisms.

Need for Microfluidics for physiologically relevant models

However, 3D cells organization brings new challenges to overcome:

  • Heterogeneity: the variability in cell distribution and organization within 3D cultures can lead to inconsistent cell behavior and responses.
  • Diffusion Constraints: the nutrient and oxygen diffusion limitations in dense 3D cultures can result in cellular stress and compromised viability, particularly in inner regions. Also, the lack of transparency of scaffolds makes them challenging to image accurately using conventional microscopy techniques, adding another layer of difficulty in assessing cellular behavior and interactions.
  • Mechanical Influence: the variations in scaffold mechanical properties may affect cellular behavior and phenotype, impacting tissue mimicry.
3D cell culture
Figure 2- 3D cell culture generated by openAI.

Microfluidic systems are essential for relevant 3D cell culture models due to their ability to mimic physiological conditions and provide precise control over the microenvironment [3]. Unlike traditional static cultures, microfluidic systems allow for dynamic flow of culture medium, ensuring uniform nutrient distribution and waste removal throughout the cellular construct [4]. This continuous perfusion maintains cell viability and function over extended periods, closely resembling the nutrient exchange observed in vivo. Additionally, microfluidic platforms enable the establishment of spatial gradients of signaling molecules, such as growth factors and oxygen, which play crucial roles in cellular behavior and differentiation [5]. By accurately replicating these complex microenvironments, microfluidics enhances the fidelity of 3D cell culture models, making them more physiologically relevant for biomedical research and drug development.

WHAT IS AN Ooc?

OOC Fundamentals

OoC platforms are micro-engineered chips designed to emulate the essential functions of organs and tissues in vitro. Unlike traditional cell culture plates or animal models, the primary goal of OoC is not to replicate entire organs but to mimic specific organ functions for targeted studies. These platforms offer a balance between complexity and controllability, providing researchers with well-defined, physiologically relevant environments to study organ-specific phenomena.

At the core of OoC platforms are three fundamental characteristics [2]:

  • Arrangement of Cells in In-Vivo-Like Layouts: OoC systems mimic the spatial organization of cells found in native tissues, allowing for the recreation of tissue-specific architectures and cellular interactions. This spatial arrangement is crucial for modeling organ function and response to stimuli accurately.
  • Culturing Multiple Cell Types: OoC platforms enable the co-culture of multiple cell types, reflecting the heterogeneity and complexity of native tissues. By incorporating different cell populations, researchers can recreate the microenvironmental cues and cellular crosstalk observed in vivo, enhancing the physiological relevance of OoC models.
  • Biochemical and Biophysical Stimulations: OoC platforms integrate biochemical and biophysical stimuli to mimic the dynamic nature of tissue function. These stimuli, such as growth factors, electrical signals, mechanical forces, and thermal gradients, play vital roles in regulating cellular behavior and tissue homeostasis in vivo.

To realize these characteristics, OoC platforms leverage a combination of engineering principles and cell biology techniques. These chips are linked to microfluidic systems to enable effective cell perfusion and regulate diverse stimulus modalities. By carefully manipulating these factors, researchers can tailor OoC platforms to model specific organ functions and pathophysiological conditions accurately.

How to fabricate OoC?

Microfabrication Techniques for OoC:

Multiple microfabrication strategies are employed to create OoC systems, with soft lithography being the predominant approach [2].
  • Soft lithography, pioneered by the Whitesides group , involves replicating micro-scale features from photolithographic molds into polydimethylsiloxane (PDMS) , which are then assembled with glass slides via oxygen plasma-assisted bonding. This technique offers advantages such as ease of fabrication, handling, integration, long-term cell culture, and real-time imaging and monitoring of OoC cultures, but has its limitations due to small molecule adsorption[6].
  • Injection molding is another widely used fabrication method, particularly in commercialized OoC systems, due to its ability for mass production. Thermoplastics and elastomers are commonly used materials, offering optical transparency, rigidity, and compatibility with microfluidic applications. Injection-molded OoC devices can be designed to mimic conventional well-plates, facilitating compatibility with existing liquid handling and readout systems [7].
  • 3D printing techniques have emerged as a versatile tool for fabricating OoC devices with precise control over microchannel geometry and have a true potential for large-scale production [8]. Printing ink, composed of natural or synthetic materials, can be deposited layer-by-layer to create complex microfluidic architectures. 3D printing enables the replication of intricate tissue and organ-level structures, enhancing the physiological relevance of OoC models. Various methods can be employed (micro-extrusion, inkjet or laser-assisted printing). The best spatial resolution is attributed to the laser-assisted technics but inkjet allows simultaneous cell printing.
Those various microfabrication techniques play a critical role in the development of OoC platforms, offering diverse capabilities for creating physiologically relevant microenvironments.

What are OoC used for?

Applications of Organ-On-a-Chip Technology

Disease modeling

Preclinical animal models have been essential for drug development but struggle to accurately predict human responses due to limitations in mimicking human physiology and genetics. Also, recent studies have questioned their predictive ability and highlighted ethical concerns, high costs, and low yields. Disease-specific OoC models provide unique insights into disease progression, pathophysiology, and potential therapeutic interventions. By faithfully mimicking the physical, behavioral, and structural characteristics of tissues and organs, OoC platforms offer a robust approach for studying disease mechanisms and testing novel treatments [9]. 

Drug Screening

Drug screening frequently faces challenges in finding cost-effective and physiologically relevant solutions, as traditional animal models continue to dominate preclinical evaluation. The limited scalability associated with animal testing means that only a fraction of potential drug candidates from extensive molecular libraries can be thoroughly assessed. OoC technology represents a paradigm shift in drug screening, offering superior physiological relevance compared to traditional cell cultures (Fig. 3). Microfluidic chips embedded in OoC platforms facilitate the generation of biochemical gradients, concentration, allowing researchers to study cellular responses to drug compounds with unprecedented detail. OoC platforms serve as invaluable tools for evaluating drug pharmacokinetics, toxicity profiles, and potency, thereby streamlining the drug development process and reducing reliance on animal models [10], [11].

Drug screening
Figure 3- Standard roadmap for drug development, from in sillico models to patients. Comparing the balance between reproducibility and physiological relevance. Adapted from [12]

Toxicity Testing

Compared to traditional preclinical models, OoC platforms offer enhanced predictability and accuracy in evaluating drug toxicity Integrated sensors within OoC devices enable real-time monitoring of cellular responses to drug compounds, facilitating the identification of adverse effects and potential safety issues. By reconstructing complex tissue microenvironments and simulating organ functions, OoC technology enhances the assessment of drug-induced toxicity and improves the efficiency of drug candidate selection processes [13], [14].

What organs are modeled with OOC?

Organ Models in Organ-on-a-Chip Technology

OoC technology has revolutionized the field of biomedical research by providing advanced platforms for modeling complex organ systems in vitro. These organ models, fabricated using microfluidic devices, faithfully replicate the structural and functional characteristics of human organs, offering unprecedented insights into disease mechanisms, drug responses, and toxicity profiles. Here, we delve into some of the key organ models developed in OoC platforms, highlighting the associated cell types and disease models:

Lung Model (LOC)

The lung-on-chip model was one of the first milestone of OoC technologies with the “breathing lung” developed in 2010 [15] by theWyss institute of Harvard University. As air quality continues to decline globally, resulting in an increase in respiratory diseases, there is a growing need for in vitro lung models like OoC technologies [16]. The lung-on-chip model comprises epithelial cells, endothelial cells, and immune cells, mimicking the alveolar-capillary interface. This model is instrumental in studying respiratory diseases such as asthma, chronic obstructive pulmonary disease (COPD), and pulmonary fibrosis.  By exposing the lung model to environmental toxins, pathogens, and drug compounds, researchers can elucidate disease pathogenesis and evaluate therapeutic interventions.

Liver Model (LiOC)

The liver-on-chip model incorporates hepatocytes, Kupffer cells, stellate cells, and endothelial cells, or human pluripotent stem cells derivatives (Fig.4) recapitulating hepatic functions and interactions. It is a valuable tool for studying liver diseases including hepatitis, fatty liver disease, and drug-induced liver injury. The liver model facilitates the assessment of drug metabolism, hepatotoxicity, and drug-drug interactions, aiding in the development of safer and more effective medications. [3], [17].

Liver on chip
Figure 4- Liver-on-chip montage from human pluripotent stem cells derivatives (hPSC). Adapted from [18]

Kidney Model (KOC)

The kidney-on-a-chip model consists of renal epithelial cells, endothelial cells, and immune cells, resembling the nephron structure and function. Researchers utilize this model to investigate kidney diseases such as acute kidney injury, polycystic kidney disease, and diabetic nephropathy. By simulating glomerular filtration, tubular reabsorption, and interstitial inflammation, the kidney model enables the study of disease mechanisms and the screening of potential therapeutics.

Heart Model

The heart-on-chip model incorporates cardiomyocytes, fibroblasts, endothelial cells, and cardiac progenitor cells, replicating the myocardial tissue architecture. It serves as a valuable platform for studying cardiovascular diseases including arrhythmias, myocardial infarction, and heart failure [19], [20]. By subjecting the heart model to mechanical strain, electrical stimulation, and drug compounds, researchers can assess cardiac function, drug cardiotoxicity, and the efficacy of cardiac therapies (Fig. 5).

Figure 5- Preparation of Heart-on-chip. (a) Representative macroscopic view of the microfluidic chip. (b) Representative macroscopic view of the HoC. The arrow indicates the human iPSC-derived 3D cardiac microtissues attached to the microfluidic chip. The dotted circle indicates the position of the push bar. Scale bar = 2 mm. (c) (d) Schematic of the structure and working machinery. Adapted from [19]

Brain Model

The brain-on-chip model comprises neurons, astrocytes, microglia, and endothelial cells, mimicking the neural network and blood-brain barrier (Fig. 6). It facilitates the study of neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, and stroke. By recapitulating neuronal connectivity, synaptic function, and neuroinflammatory responses, the brain model offers insights into disease pathogenesis and potential neuroprotective strategies.

BBB on chip
Figure 6- BBB-on-a-chip supported by a living microvascular network. Global view of astrocytes (red) and the microvascular network formed by endothelial cells (green). Adapted from [21]

These organ models in OoC platforms represent sophisticated tools for disease modeling, drug screening, and toxicity testing. By integrating multiple cell types within physiologically relevant microenvironments, OoC technology holds immense promise for advancing our understanding of human biology and accelerating the development of novel therapeutics. Moreover, the advent of multi-organs-on-chip models allows for the integration of multiple organ models (heart-liver[22], liver kidney[23], brain-liver[24], etc.) enabling the study of complex organ interactions and systemic responses to drugs and diseases (Fig. 7). These sophisticated OoC platforms represent powerful tools for disease modeling, drug screening, and toxicity testing, promising to revolutionize biomedical research and therapeutic development.

Multiorgans-on-chip-AI
Figure 7- Multi-organs-on-chip to assess adverse skin drug responses. Adapted from [12]

WHAT ARE THE FUTURE DIRECTIONS OF OOC?

Limitations and perspectives

Complexity of mimicking multicellular interactions

Recreating the intricate multicellular interactions and tissue-organ interfaces found in vivo remains a major challenge in OoC technology. While current models capture certain aspects of organ physiology, they often lack the complexity and heterogeneity of native tissues. Incorporating immune cells, structural cells, lymphatic system and other stromal components into OoC are the key direction to enhance the physiological relevance of these models and improve their predictive value for drug testing and disease modeling.

Reproducibility and standardization efforts

Despite significant advancements, OoC technology still faces challenges related to reproducibility, scalability, and standardization. Variability in cell sources, culture conditions, and microfabrication processes can affect experimental outcomes and hinder comparability between studies. The lack of standardized protocols and regulatory guidelines poses significant barriers to the widespread adoption of OoC technology in preclinical research and drug development. Collaborative initiatives involving academia, industry, and regulatory agencies are needed to establish consensus on experimental methodologies, validation criteria, and ethical considerations. Clear regulatory pathways for OoC-based assays and devices are essential to ensure their acceptance and integration into mainstream biomedical research and clinical practice.

Lack of standardization in organ on chip technology
Figure 8- Standardization challenge for OoC technology. Generated by openAI.

Future Directions

Despite the challenges, OoC technology holds immense promise for advancing our understanding of human biology and disease mechanisms. Future research directions include integrating multiple organ models to simulate systemic physiology, enhancing predictive capabilities through computational modeling and artificial intelligence[12], and advancing personalized medicine by using patient-derived cells and tissues. Continued investment in technological innovation, material study and regulatory support will be crucial to realizing the full potential of OoC technology and translating it into clinical applications for improving human health.

Conclusion

In conclusion, organ-on-a-chip technology represents a groundbreaking approach to biomedical research, offering unprecedented opportunities to model human physiology and disease in vitro. By integrating microfabrication techniques, advanced cell culture methods, and innovative sensing technologies, OOC platforms enable researchers to recapitulate the complexities of human organs and tissues with remarkable fidelity. The burgeoning interest in personalized and precision medicine, coupled with advancements in novel therapeutics, has fueled the research of OoC technology in both academic institutions and pharmaceutical industries. Despite remaining challenges related to reproducibility, scalability, and regulatory approval, ongoing advancements and collaborative efforts hold promise for revolutionizing drug discovery, disease modeling, and personalized medicine in the years to come, leading to a valuable impact on human health[1].

Want to develop your own OoC system?

References

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