Biomechanics of Breastfeeding: Fluid-Structure Interaction Simulation of Milk Expression
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Breastfeeding is a highly dynamic and complex mechanism. There are two theories for the dynamics of milk expression by the infant. One hypothesis is that milk expression is due to negative pressure applied by infant sucking; the alternative hypothesis is that the tongue movement and squeezing of nipple/areola due to mouthing is responsible for the extraction of milk from the nipple. In this study, a 3-D Fluid-Structure Interaction (FSI) simulation is conducted to investigate the factors that play the primary role in milk expression from the nipple. The models include solid deformation and periodic motion of the tongue and jaw movement. To obtain boundary conditions, ultrasound images of the oral cavity, and motion of tongue movement during breastfeeding are extracted in parallel to the intra-oral vacuum pressure. The numerical results are cross-validated with clinical data. The results show that, while vacuum pressure plays an important role in the volume of milk removal, the tongue/jaw movement is essential for facilitating this procedure by decreasing the shear stress within the main duct. The developed model can contribute to a better understanding of breastfeeding complications that are due to physical infant and/or breast abnormalities and for the design of medical devices such as artificial teats and breast pumps. The second part of this study focuses on investigation of the non-Newtonian behavior of the milk during breastfeeding. The accumulated milk from the nipple varies depending on the milk properties and transient flow rate during the suckling cycle. The rheological studies on raw human milk have indicated non-Newtonian shear-thinning flow behavior. There exists no prior numerical simulation in the area of breastfeeding to study the non-Newtonian flow behavior inside the milk ducts. The novelty of this study is to investigate the nonNewtonian milk flow through the breast ductal system using fluid-structure interaction (FSI) simulation. The geometry of an infant’s mouth and breast is used in the 3-D FSI simulations. The analyses are performed using Newtonian and non-Newtonian models to quantify the effects of non-Newtonian behavior of milk through the milk duct. The results of the nonNewtonian effects on the surface of milk duct, velocity profiles, and volume of expressed milk are presented. The non-Newtonian Carreau model provides a promising model to simulate human milk flow during suckling. The Newtonian model is also acceptable for the numerical simulation but choosing an adequate Newtonian viscosity from experimental data is challenging. The final part discusses the contact pressure distribution on the surface of the breast and Von-Mises stress as the result of the infant’s suckling. The values of contact pressure on the nipple/areola complex from simulation are in good agreement with the observation in clinical data. The finite element analysis (FEA) is performed to study the areas that are exposed to tension or compression. The study of stresses on the nipple provides a better understanding of the potential physical sources of nipple pain after natural breastfeeding. The physical factors of nipple soreness are discussed by studying mouthing only and suckling only along with natural suckling.