Microfluidics technology is one of the central elements of organ-on-chip (OoC) devices. In order to ultimately recapitulate a human organ, this technology needs to seamlessly integrate with several other disciplines, including tissue engineering, cell biology, and biochemical engineering.
The next model-level above OoC devices are multi-organ systems: frequently called body-on-a-chip (BoC) systems. These higher-order systems promise to replicate physiological interactions between different organ types, enabling a systems-level view of compound action and tissue reaction, currently achievable only in in vivo models. The technical implementation of BoC devices adds another level of complexity with several challenges that need to be addressed to enable reproducible research and routine use.
In my first Understanding Microfluidics blog post, I provided an introduction to the technology behind emerging #body-on-a-chip and #organs-on-chips systems. I emphasized that the robustness of microfluidic systems for cell biology is largely dependent on the design quality of three critical features: 1) bubble-free priming of the microchannel system, 2) simple, harmless introduction of cells and/or 3D tissues at the proper location in the microfluidic system, and 3) reliable bubble-free operation over time.
Today, I'll focus on factors that play an important role in multi-organ system engineering and discuss:
Intuitive, multi-organ systems can be assembled by the simple connection of several individual OoC devices (Type A) using tubing and external pumps. Each device is self-contained and prepared separately prior to connection and can vary in design and fabrication method, tissue morphology, and maturation type. The flow routing is highly flexible and can be readily extended with pumps, bubble traps, biosensors, and other modules.
In a monolithic design (Type B), the channels for the fluidic interconnection of individual organ models are directly integrated into a single multi-organ platform fabricated from a single material at the same time. The microchannels and organ compartments are hard-coded, creating precise and reproducible flow paths and defined sizes and geometries of organ compartments (including their volume ratio). This precision, however, comes with the cost of less flexibility.
Plug-and-play systems (Type C) combine elements of both Type A and B systems. Transferable organ models are placed “just-in-time” into a microfluidic culturing platform. The concept leverages miniaturization and production technologies from Type B monolithic designs and individual organ model bioproduction, maturation, and quality control. Subsequently, fluidics paths are short, dead volumes are small, and liquid routing is precise. Limitations, however, lie in the choice of cell and tissue model, which need to be compatible with transfer into the microfluidic system. Examples of plug-and-play systems include transwells, inserts, and scaffold-free spheroid models, such as InSphero 3D InSight™ microtissues.
Type A multi-organ systems connected by tubing have a very intuitive design. The system setup, however, often involves several handling steps, which can be cumbersome, especially under sterile conditions, restricting experiments to a few replicates. Tubing and connection ports also may lead to leakage, non-specific binding, and are prone to contamination.
Type B Monolithic devices, on the other hand, are typically robustly fabricated and may be used more than one time. The dead volumes are small, resulting in rapid organ-organ communication, and the number of connections is reduced to only a single link between the device and external pumps. Alternatively, pumps can be fully integrated, resulting in a completely tubeless system but with the drawback of higher device and fabrication complexity.
Interconnected and plug-in models permit the production and maturation of tissues “off-chip” and allow for the introduction of a quality control step prior to system assembly. This ensures only models that meet quality criteria are used. Models that don't make the QC cut are discarded. This yields more robust and reproducible results from multi-tissue experiments and significantly increases the overall success of the assembled system. The larger the number of organ models combined, the more important pre-assembly QC is to system success (Rogal et al., 2017).
Individual organ models may vary significantly in production complexity, maturation time, and longevity. Thus, model bioproduction tasks must be well-orchestrated to have all models ready prior to starting the experiment. For example, if you plan to procure plug-in models, you must factor in ordering and delivery times. Long-lived organ models that offer an extended functional assay window can provide more flexibility in an experimental setup. Equally important, the culturing system, system priming, and functional tests, as well as downstream bioanalytics, all need to be decided upon and ready before the multi-tissue experiment begins.
Akura™ Flow represents a Type C plug-and-play concept, where 3D spheroid models or microtissues (MTs) are loaded into specially designed compartments connected by microfluidics. Ten identical interconnected microtissue compartments form a single channel. Each Akura™ Flow plate includes eight independent channels to enable parallel testing on a minimal footprint.
InSphero has more than a decade of experience developing highly standardized, morphologically uniform microtissue models. We have well-defined bioproduction processes that enable us to quickly engineer organ models that recapitulate healthy and diseased human organs and media that facilitate the flow of biological information within the organ network. We currently offer 3D InSight™ microtissue models for several organs, including the liver, brain, heart, skin, pancreas, and a wide range of tumor types.
Both the off-chip production plates and the Akura™ Flow system utilize ANSI/SLAS plate standards, and microtissues in these plates are untethered and scaffold-free. It is also each to transfer organ models from one culturing platform to another using automated robotic handling systems.
Our unique combination of physiological microtissues and microfluidic technology enables us to quickly develop flexible, modular organ-organ configurations that incorporate the type and number of organs most appropriate for your study. This approach enables us to engineer a wide range of applications while carefully controlling complexity. A controlled increase in complexity minimizes variability and simplifies the characterization and optimization of multi-organ systems. Inversely, modular systems enable deconvolution into single parts so that we can better isolate and track the origin of an observed behavior.
Akura™ Flow is engineered for scalability, from the tissue model and culturing platform to operations and compatibility with automated handling. By relying on standardized microtissue models, we ensure continuity of endpoints between several different culturing platforms (e.g. Akura™ 96 and 384 plates as well as microphysiological systems).
We have defined rigorous bioproduction and QC processes to enable just-in-time microtissue production, which we consider a prerequisite for successful multi-tissue experiments. Equally important, our models have well-defined maturation times combined with well-characterized longevity and extended assay windows for study flexibility. The result? Unmatched flexibility to support a wide range of preclinical, multi-tissue applications in drug discovery and risk assessment.
I hope you've found this blog useful. If you'd like to learn more, I encourage you to read Organ-on-a-Chip: Engineered Microenvironments for Safety and Efficacy Testing, a book that gives a comprehensive overview of the field, with sections covering the major organ systems and currently available technologies, platforms, and methods. My colleague Kasper Renggli from ETH Zürich and I contributed to Chapter 12: Design and Engineering of multi-organ Systems.
You may also be interested in attending my webinar Predicting Metabolism-Related Drug-Drug Interactions with Organ-on-a-Chip Technology, in which Dr. Christian Lohasz, also from the Bio Engineering Laboratory at ETH Zürich, and I present a scalable, gravity-driven microfluidic system for studying DDIs in a multi-tissue network comprised of 3D human liver and tumor models, using drug combinations known to cause DDIs in vivo. Additional resources include:
If there are other topics related to microfluidics and microphysiological systems you'd like me to cover in future blogs, please share your ideas in the comments section.