DNA topology dictates strength and flocculation in DNA-microtubule composites

Polymer composites are ubiquitous in biology and industry alike, owing to their emergent desirable mechanical properties not attainable in single-species systems. At the same time, polymer topology has been shown to play a key role in tuning the rheology of polymeric fluids. However, how topology impacts the rheology of composites remains poorly understood. Here, we create composites of rigid rods (microtubules) polymerized within entangled solutions of flexible linear and ring polymers (DNA). We couple linear and nonlinear optical tweezers microrheology with confocal microscopy and scaled particle theory to show that composites of linear DNA and microtubules exhibit a strongly non-monotonic dependence of elasticity and stiffness on microtubule concentration due to depletion-driven polymerization and flocculation of microtubules. In contrast, composites of ring DNA and microtubules show a much more modest monotonic increase in elastic strength with microtubule concentration, which we demonstrate arises from the increased ability of rings to mix with microtubules.