Millifluidic

Millifluidics provides a controlled microenvironment for the culture and analysis of cells, tissues, and organoids.

Millifluidic platforms, like organ on chips, are used for resembling in vitro cell behavior, tissue development, drug responses, and disease mechanisms, offering insights into various biomedical processes. These devices enable precise manipulation of culture conditions such as nutrient supply, oxygenation, and mechanical stimulation, allowing researchers to mimic physiological conditions more accurately than static conditions.

Millifluidic systems are employed in drug discovery and development processes, including high-throughput screening, pharmacokinetic studies, and drug formulation optimization.

These devices enable researchers to miniaturize and automate various experimental procedures, accelerating the identification of lead compounds, elucidation of drug mechanisms, and optimization of drug formulations. Millifluidic-based approaches offer advantages such as reduced reagent consumption, faster experimental turnaround times, and enhanced experimental reproducibility.

Millifluidic organ-on-chip models replicate the structure and function of human organs in vitro, providing platforms for studying organ physiology, disease mechanisms, and drug responses.

These devices integrate fluidic channels, cell culture chambers, and functional components to recreate the complexity of human organs in a controlled environment. Millifluidic organ-on-chip models have applications in drug screening, toxicity testing, disease modeling, and personalized medicine, offering insights into organ-level responses that cannot be captured using traditional cell culture or animal models.

This larger size allows for the maintenance and observation of cellular and tissue constructs under precisely controlled flow conditions.
Studies such as those conducted by Bhargava, Thompson, & Malmstadt (2014) highlight the importance of fluid properties, including inertial force and viscous force, in governing nutrient transport within millifluidic systems. The study of these parameters allows researchers to design and optimize millifluidic devices for efficient nutrient diffusion, thereby supporting the long-term viability of tissue cultures.

The forces experienced within millifluidic devices are primarily governed by fluid properties such as inertial force, viscous force, and fluid interfacial properties (Bhargava, Thompson, & Malmstadt, 2014). These devices offer highly controlled shear flow conditions, which have been extensively studied and documented over the past three decades (Dahl et al., 2015). As a result, they are often utilized for shear and viscosity measurements. However, they also find applications in particle focusing, separation and transport, as well as phase transitions of polymeric materials.

The MIVO^® Millifluidic platform used in this application, significantly improved the delivery of oxygen and nutrients to cells while effectively removing waste products, closely replicating in vivo conditions. Marrella et al. (2020) also demonstrated that dynamic cultures using millifluidics maintain cellular viability and function more effectively than static cultures. Notably, these figures show that the structure of villi and microvilli is preserved under dynamic conditions, whereas these structures deteriorate in static environments.

Marrella, A. (2020) “In vitro demonstration of intestinal absorption mechanisms of different sugars using 3D organotypic tissues in a fluidic device”, ALTEX – Alternatives to animal experimentation.

Millifluidic systems offer a more physiological environment for drug diffusion within 3D tissues. Studies, such as those by Sung and Shuler (2009) and Marrella et al (2021), have shown that drug diffusion in these platforms is highly predictive of in vivo scenarios, providing more reliable data than static culture methods.

Marrella, A. (2021) “3D fluid-dynamic ovarian cancer model resembling systemic drug administration for efficacy assay”

Sung and Shuler (2009) “A micro Cell Culture Analog (micro CCA) with 3-D hydrogel culture of multiple cell lines to assess metabolism-dependent cytotoxicity of anticancer drugs”

The dynamic flow conditions in millifluidic devices promote faster and more efficient cell differentiation. Chandorkar P et al. (2017) found that epithelial junctions are tighter under flow conditions, indicating enhanced tissue maturation and functional integrity compared to static conditions.

Chandorkar P et al. (2017) “Fast-track development of an in vitro 3D lung/immune cell model to study Aspergillus infections.”

Phenomena such as bubble formation, evaporation and nutrient depletion, are far more difficult to solve at the microscale volumes (i.e. lab on chip), since resistance to flow through a fine channel is inversely proportional to the fourth power of the tube radius r⁴ (Poiseuille’s law) so a small capillary tube of diameter 200μm will have a resistance per unit length 625 times higher than a tube of 1 mm diameter (Wilkinson, 2023).
Fluid flow in microfluidic channels is particularly difficult to optimize and leads to failure of many organ on chip (OOC) devices. It was Michael Shuler, currently President of Hesperos, who first pointed out that air bubbles are a serious obstacle to a successful operation of a long-term microfluidic systems using cell culture (Sung and Shuler (2009)).

Shear stress plays a crucial role in cellular biology as it significantly influences cell behavior, differentiation, and function. In physiological environments, cells are naturally exposed to shear stress due to blood flow and interstitial fluid movement, which affects processes such as gene expression, protein synthesis, and cellular morphology.
Replicating these dynamic conditions in vitro is essential for creating accurate biological models, particularly for applications like drug testing, disease modeling, and tissue engineering. Therefore, the choice of pumping systems in Organ-on-Chip (OOC) devices is fundamental.
Effective pumping systems ensure consistent and precise fluid flow, providing the necessary shear stress to mimic in vivo conditions. This not only enhances the reliability of experimental results but also enables long-term cell culture and complex tissue formation, making it a cornerstone for advancing biomedical research and applications. If higher pressures are used then there is a risk of rupturing the cell walls of the tissue under culture

Another difficulty for microphysiological systems is the lack of medium flow and low oxygen delivered to the cells. There are two simple approaches to solving this problem: the first is to simply increase the radius r of the flow channels, and the second is to increase the pressure. Several commercial OOC developers have taken the latter approach (Emulate, CNBio and TissUse). Unfortunately, this dramatically increases the capital cost and complexity of the control system for the OOC device.
A different strategy has been adopted by Kirkstall and React4life. The chamber and tubing diameters have been increased to millifluidic scale so that low pressures are enough to create adequate flow. Simple peristaltic pumps can then be used. These pumps are low cost and can fit inside a standard cell culture incubator, which is not the case for high pressure pneumatic pumps. The result is a technology which is affordable and yet flexible enough to build multi-organ models.

So far, several modes have been adopted for the feeding of these millifluidic stystems, including peristaltic pumps that are mechanically operated and syringe pumps.

For Organ on a Chip devices, the goal should be to limit the pressures that the cells experience to those found in the human body and certainly below those found in large Arteries (diastolic 80 mm of Hg to systolic 120 mm of Hg). These pressures should then guide the selection of pumping mechanisms for use in Organ on a Chip devices. If higher pressures are used then there is a risk of rupturing the cell walls of the tissue under culture.

Syringe pumps use a motorized screw mechanism to move a syringe piston that ensures flows of liquid precisely, in a manner very similar to that of a syringe used for injecting drugs. Peristaltic pumps move liquids using a rotor that gently compresses a tube containing the liquid, producing a regular “pulse” of liquid flow that varies based on the internal volume of the tube, the number of rollers in the rotor and the speed of rotation of the rotor. Peristaltic pumps are more limited in the pressures they can support at around 0.1bar to 0.2 bar (100mm–200 mm of Hg) and these are well within the normal physiological range.

In early Organ on a Chip Research it was hoped that silicon chip based micropumps manufactured by photolithography and using the piezoelectric effect would be effective way to pump medium round flow circuits. However, micropumps were ineffective because they could not overcome the high resistance of microfluidic channels. Many of the OOC developers who originally planned to use micropumps have now migrated to externally driven pneumatic pumps using compressed air or laboratory vacuum lines switched through miniature valves. These valves can be on the chip (for example, TissUse) or on the cartridge that holds the chip (for example, CNBio). Pneumatic pumps can also deliver dangerously high pressure (2 x atmospheric pressure or 2000 mm of Hg) unless there is a pressure monitor in the flow circuit. Although not strictly a pump, Rocker Plates are also used to induce flow under gravity and the equivalent pressure is 0.001–0.01 atmospheric (1–10 mm of Hg). If this type of gravity driven flow device is connected to microfluidic channels with high resistance the levels of flow will be very limited.

Wilkinson, 2023

If you have any questions or need further information about millifluidics and its applications, please don’t hesitate to contact us. We are here to help you explore the potential of this innovative technology. Feel free to reach out to us at info@millifluidic.com.