Active matter is matter driven out of equilibrium by its microscopic constituents. Common examples include the cytoskeleton of the cell, bacteria swimming in a fluid, or herds of wildebeest on the African planes. In these systems, the individual parts create forces and motion driving the collective system out of equilibrium. But now imagine cells dividing or a tumor growing -- a growing material is also active matter. However, activity does not enter the dynamics via the forces, but by material conservation. The material generates itself. This source of activity has severe consequences and leads to fascinating new physics [1]. In this presentation, I will address which fundamental mechanical aspects are expected, and which consequences they yield. Growth implies a change in volume. In physical terms, the conjugate force to a change in volume is pressure. Thus, in order to grow, cells must exert mechanical pressure on the neighboring tissue. In turn, mechanical stress influences growth and may play a role in cell competition. This insight has led to the notion of homeostatic pressure – the pressure exerted by a tissue in homeostasis [2,3]. This suggests that the tissue with higher homeostatic pressure overwhelms the weaker one [4–6]. We use particle based computer simulations [7] to model growing tissues. Surprisingly, we find that when cross interactions are taken into account tissues with different homeostatic pressure can coexist [6] and the evolution can favor the weaker tissue, or even results in tumor heterogeneity [8]. On a substrate, cells can also exert motility forces, further enhancing the rich phenomenology, like division alignment [9], interfacial instability [10], or glassy arrest [11,12]
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[5] N. Podewitz, et al., New J. Phys., 18, 083020 (2016). [6] N. Ganai, et al., New J. Phys., 21, 063017 (2019).
[7] M. Basan, et al., Phys. Biol., 8, 026014 (2011). [8] T. Büscher, et al., New J. Phys., 22, 033048 (2020).
[9] A.-K. Marel, et al., New J. Phys., 16, 115005 (2014). [10] T. Büscher, et al., New J. Phys., 22, 083005 (2020).
[11] S. Garcia, et al., Proc. Natl. Acad. Sci., 112, 15314–15319 (2015). [12] D. Sarkar, et al., Commun. Phys., 4, 36 (2021).