Complex flow in the gastrointestinal tract

Objectives

Transport and mixing phenomena in the digestive system control nutrient release and absorption, drug delivery, as well as bacterial dynamics and their interaction with the immune system. These transport processes are governed by the spatio-temporal organization of smooth muscle motility, the presence of microstructures—such as the villi covering the digestive mucosa—and the rheological behavior of fluids (digesta and mucus). Our goal is to develop an experimental approach at the interface of complex fluid mechanics and digestive physiology to understand and model transport phenomena within the digestive system at both macro- and microscales.

Results

Secondary flows generated by intestinal villi oscillations

We developed numerical simulations using lattice-Boltzmann methods to study the impact of spontaneous villi movements on fluid flow and microparticle transport in the small intestine. By simulating microstructures that oscillate laterally in a coordinated manner, we show that these motions can induce a rectified flow (“steady streaming flow”) over a timescale of several minutes. Although this flow does not accelerate nutrient absorption, it enhances the transport of microparticles, such as aggregates, bacteria, and particles used for drug delivery. This phenomenon arises because the mechanical forcing occurs at moderate Reynolds numbers (on the order of unity), which favors the emergence of secondary flows. We also demonstrated that both the intensity and spatial structure of this secondary flow depend not only on the Reynolds number but also on the spacing between villi. Since villi spacing varies with the physiological stage of digestion, these numerical results suggest that the intestinal system has a means to regulate mixing.

As part of the ANR TransportGut project, we are developing an experimental system to image villi motility in isolated organs, in order to incorporate more complex and realistic movements into numerical simulations.

Modeling of yield-stress fluid flows for quantitative analysis of rectal function

(collaboration with CHU Grenoble Alpes, TIMC)

Human defecation is a complex process involving the coordination of multiple physiological systems. As a result, it remains challenging for clinicians to clearly distinguish the underlying causes of symptoms. Furthermore, clinical examinations, such as dynamic defecography, are only partially analyzed, even though they contain valuable physical information about rectal function.
We have developed numerical simulations of rectal evacuation based on sequences from dynamic defecographies of different patients. The lattice-Boltzmann code incorporates the rheological properties of the stool substitute used in the examination to simulate flow, pressure, and stress fields. These simulations have enabled us to relate evacuation flow rates to forces applied by the rectal ampulla across various cases and to identify criteria that could assist in clinical diagnosis. These initial results open the way for a quantitative identification of the biomechanical causes of defecation disorders in patients presenting both functional and structural abnormalities.

References

Ahmad, F., Tanguy, S., Dubreuil, A., Magnin, A., Faucheron, J. L., & de Loubens, C. (2022). Flow simulations of rectal evacuation: towards a quantitative evaluation from video defaecography, Interface Focus, 12(6), 20220033.

Puthumana Melepattu, M., & de Loubens, C. (2022). Steady streaming flow induced by active biological microstructures; application to small intestine villi, Physics of Fluids, 34(6).

de Loubens, C., Dubreuil, A., Lentle, R. G., Magnin, A., Kissi, N. E., & Faucheron, J. L. (2020). Rheology of human faeces and pathophysiology of defaecation. Techniques in coloproctology, 24, 323-329.

Ahmad, F., Investigating gastrointestinal function at macro and micro scales : Insights from fluid dynamics models, PhD thesis, Université Grenoble Alpes, 2023,