Bridging barriers: advances and challenges in modeling biological barriers and measuring barrier integrity in organ-on-chip systems

Overview of Biological Barriers and Organ-on-Chip Systems

  • Biological barriers, such as the blood-brain barrier, skin, and intestinal mucosal barrier, are crucial for maintaining homeostasis by regulating the internal environment and protecting against external threats. These barriers play significant roles in disease physiology, as their dysfunction can lead to various medical conditions and complications. Moreover, they are integral to drug delivery, influencing the absorption, distribution, metabolism, and excretion of pharmaceuticals.

  • The development of in vitro models, such as organ-on-chip systems, is paramount for enhancing our understanding of these biological barriers and advancing therapeutic development. These models provide a platform for simulating human physiological conditions more accurately than traditional methods.

Advances in Organ-on-Chip Technology

  • Microfluidic cell culture systems, known as organ-on-chip (OOC) technologies, represent a significant advancement in the field of biomedical research. These systems allow for greater physiological fidelity by mimicking various environmental aspects, including fluid shear stress, microscale dimensions, and mechanical cues that cells experience in vivo.

  • OOC platforms enable enhanced modeling of barrier formation and disruption, providing insights into cellular responses to different stimuli. They have shown promise for applications such as:

    • Fundamental biology understanding: Investigating cell behavior and interactions in a controlled environment.

    • Pharmaceutical development: Screening drug efficacy and toxicity more accurately, reducing reliance on animal models.

    • Safety evaluations before animal trials: Identifying potential adverse effects in human-relevant models before moving to in vivo testing.

Assessment of Barrier Integrity

  • Assessing barrier integrity is vital to understanding and validating the functionality of biological barriers in vitro. Tools must be available that do not disrupt the microenvironment, ensuring accurate and reliable results.

  1. Molecular Permeability:

    • Monitoring tracer diffusion across biological barriers is essential. This is accomplished through methods such as using fluorescent tracer dyes (e.g., FITC-dextran), which allow for quantifying permeability while minimizing direct interference with the tissue.

  2. Electrical Sensing:

    • Non-invasive techniques, such as Trans-Epithelial/Endothelial Electrical Resistance (TEER), provide real-time data on barrier integrity. This method supports continuous monitoring, allowing for the detection of functional changes in the barrier under various conditions.

Biological Barrier Structures and Functions

  • Key Functions:

    • Biological barriers regulate selective transport, ensuring that essential nutrients and signals can enter while blocking harmful substances, pathogens, and toxins.

  • Structural Features:

    • These barriers are characterized by tight junctions made up of proteins such as occludins, claudins, and Junctional Adhesion Molecules (JAMs), which are critical for maintaining barrier integrity.

    • Adherens junctions further facilitate cell adhesion and are essential for maintaining the physical and functional aspects of barriers.

    • The interplay of these junctional complexes is crucial for maintaining concentration gradients and preventing pathogen access, thus serving protective functions.

Microenvironmental Cues

  • Four key microenvironmental cues critical for maintaining barrier function include:

    1. Cell-Cell Interactions:

    • The influence of neighboring cells, including microbial interactions (e.g., gut microbiota), significantly impacts barrier integrity and function.

    1. 3D Architecture:

    • The spatial organization of various cell types and their extracellular matrix (ECM) contributes to the overall functionality of biological barriers. Three-dimensional structures are more representative of in vivo conditions compared to two-dimensional cultures.

    1. Fluid Flow:

    • The effect of shear stress from fluid flow in vascular systems directly impacts barrier properties, influencing how cells respond to mechanical stimuli.

    1. Mechanical Stretch:

    • Forces acting on cells, such as those experienced in the gut and lung, can alter cellular behaviors and the properties of barriers, making mechanical cues essential for understanding barrier dynamics.

In Vitro Modeling Approaches

  • Conventional Models:

    • Traditional in vitro models such as well-plate and Transwell systems have been widely used for co-culture studies but often lack physiological relevance.

  • OOC Platforms:

    • Organ-on-chip systems address the limitations of these conventional models by incorporating key physiological cues such as shear stress and ECM interactions. They utilize innovative designs including membrane-based, parallel channel, and patterned ECM constructs to more accurately replicate native tissue environments.

Measuring Barrier Integrity: Techniques and Challenges

  • Technical Methods:

    1. Molecular Permeability: Fluid sampling is necessary to quantify barrier tightness and allows researchers to evaluate how substances traverse the barrier.

    2. Electrical Sensing:

    • TEER measurements provide insights into paracellular transport mechanisms, critical for understanding drug permeability and barrier functions.

  • Challenges Arise From:

    • The material properties of the OOC systems (e.g., PDMS absorption of small molecules) can complicate results.

    • Heterogeneity in barrier properties between different cell types and cultures may affect measurements and interpretations, necessitating careful experimental design.

Future Directions

  • Advancements in Fabrication:

    • The field is moving toward lab-on-PCB (printed circuit board) technologies and developing flexible, stretchable designs integrated with a variety of sensors to monitor biological activity in real time.

  • Enhancing Sensor Design:

    • A focus on non-invasive, real-time monitoring capabilities will be crucial to gather data continuously while minimizing disruption to the biological systems being studied.

  • Standardization Needs:

    • It is imperative to establish common standards to ensure comparability across various models and techniques. This includes the development of specific evaluation benchmarks and protocols.

  • Collaboration across Disciplines:

    • Enhanced collaboration among biologists, engineers, and industry stakeholders will bridge existing gaps and promote the development of user-friendly and clinically relevant platforms.

Conclusion

  • Organ-on-chip models are pivotal for a comprehensive understanding of biological barriers and their implications for health and disease. The integration of advanced technology alongside solid standardization approaches is essential for driving significant progress in this field, leading to more effective therapeutic strategies and drug development.