Can biology-inspired complexity be obtained without biochemical components? Can we replicate ubiquitous biological processes using only model physical building blocks like DNA-coated colloids that have simple but programmable interactions? The last decades have seen tremendous progress in understanding the self-assembly mechanisms that enable the formation of complex, sub-micron scale structures, but embedding these structures with bio-inspired functional behaviors remains a considerable challenge. Here, we demonstrate a scheme for transferring energy between two colloidal clusters, in analogy to ATP hydrolysis. By coupling the two clusters, we show how the one acting as a machine catalyzes a structural transition in the one acting as a fuel source, releasing energy that drives the machine into a higher energy structural state. The coupled system shows a significantly reduced mean-first passage time.
This work demonstrates that energy consumption, a fundamental and enabling biological process, can be replicated without complex biochemical reactions. In contrast, theories of active matter often focus on the effect of energy consumption, not on the mechanism itself. However, the mechanism is intimately connected to the type of physical phenomena that can result. In a next step, we extend the scheme by a third structural state in the machine. This allows us to convert energy into work by driving a net flux in the machine, which is not possible in equilibrium and requires a fuel source.

How thousands of microtubules and molecular motors self-organize in a dynamic fashion in spindles remains a very complex and largely unexplained phenomenon. However, recent data from large-scale electron tomography (EM) has enabled quantification of the positions, lengths and configurations of individual microtubules (MTs) in metaphase spindle in HeLa cells. We leverage the static ultrastructural data from the EM to quantitatively extract the density and orientational fluctuations of the individual microtubules around a well-defined coarse grained steady state. We then compare the equal-time correlations computed from the data with those derived from a coarse-grained active liquid crystal theory. Our preliminary analysis reveal that director-director correlations decay as ∼q^-2 consistent with the theory in which MTs are locally aligned through passive cross-linkers, molecular motors, and steric effects. The density-density correlations display ∼q^-4 scaling for large q indicating that MT sliding interactions alongside with diffusive-like motions of microtubules dominate at short wavelengths. In addition, from the EM data we can quantitatively extract the transverse fluctuation of an individual filament about its average tangent. The static structure factor associated with the transverse fluctuations decay as ∼q^-2 and their amplitude as ∼q^-4 revealing a thermal-like Fourier bending spectrum. However, the apparent persistance lengths inferred from the data is orders of magnitudes smaller than expected from thermal fluctuations alone, hinting at large nonthermal forces bending the microtubules. In summary, our work highlights how microtubule resolution data from EM in combination with simple coarse-grained theories can be used to describe mechanical behavior of spindles with measurable and interpretable parameters.

The piconewton forces generated by molecular motors carrying cargo along microtubules, or by microtubules polymerizing against the cell cortex or artificial boundaries, are sufficient to deform long microtubules. When microtubules are sparse in the cytoplasm, their deformations are disordered, characterized by high-frequency buckling and inducing only localized cytoplasmic flows. When the microtubules are instead arranged in a dense forest, the nature of the microtubule deformations and induced cytoplasmic flows can change dramatically, giving rise to long-range order and coherent flows. Using a combination of experiments, large-scale simulations of microtubules interacting hydrodynamically through a viscous fluid, and a coarse-grained theory for dense beds of filaments, we elucidate the mechanisms that underlie the self-organization of microtubule ensembles and their subsequent generation of cell-spanning rotation in two examples: cytoplasmic streaming in the Drosophila Melanogaster oocyte, and spontaneous rotation of artificially confined asters in Xenopus Laevis extract.