Even active systems that appear simple at first sight, show a plethora of intriguing phenomena and often we find complexity were we would have expected simplicity. In many such systems it is neither possible to predict the dynamic properties nor to draw a comprehensive picture of the underlying mechanisms and the emerging properties. This becomes even worse if active processes are coupled to other generic processes like aggregation and growth, as it is the case for cytoskeletal systems. Here the mechanical properties of the emergent structures nontrivially interfere with the active processes like motor-mediated transport. To shed light on the dynamic properties and the aggregation mechanisms in active systems, we investigate reconstituted systems consisting of highly concentrated actin filaments and associated motor proteins in quasi 2D and 3D geometries. The bulk material properties of these systems like the connectivity and the elasticity are adjusted by actin binding and crosslinking proteins. We show that the interplay of active force generation and passive crosslinking leads to a heterogenization of the active material, with structures crucially depending on the morphology of the specific crosslinker. The reconstituted approach consisting of a minimal set of purified components allows us to address the microscopic processes underlying the pattern formation in these materials.
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