Growth of Physarum polycephalum under confinement

The slime mold Physarum polycephalum is a unicellular organism that can grow its network-like body to adapt to its environment and solve mazes. Due to its simple geometry and fascinating abilities, fluid dynamicists used it as a model organism to investigate morphogenesis, network theory, and physically encoded memory. Central to our investigation is Physarum polycephalum's discerning approach to path selection. We used microfluidic channels of various lengths and diameters to vary their hydraulic resistance and study Physarum growth in a confined environment. We found that instead of taking the shortest path to connect two food sources, as commonly stated in the literature, it consistently opts for the path with minimal hydraulic resistance, suggesting an optimization for efficient nutrient transportation and growth purely dictated by fluid mechanics. Additionally, Physarum’s growth pattern under high confinement shifts to a "stick-slip” motion reminiscent of hydraulic cylinders, accentuating its intrinsic pulsative flow nature, suggesting that confinement might affect cytoplasmic calcium concentration. Our findings pivot from the “shortest path” paradigm and offer a new framework to understand how Physarum polycephalum optimizes nutrient transport and illustrates how simple fluid dynamic laws can control a morphogenetic response. This novel understanding of Physarum polycephalum growth has potential applications in optimization algorithms and transportation network designs.