During development. the formation of biological networks (such as organs and neuronal networks) is controlled by multicellular transportation phenomena based on cell migration. In multi-cellular systems. cellular locomotion is restricted by physical interactions with other cells in a crowded space. https://www.roneverhart.com/Motivo-Tour-Rolling-Upright-Walker-Folding-4-Wheel-Rollator-with-Tray-Cup-Holders-Seat-Storage/