Programmable microactuators phase-lock cilia to local oscillatory flow

Hydrodynamic synchronization of motile cilia is essential for biological functions such as fluid transport, locomotion, and developmental patterning. It comprises the generation and the response to local flows in complex geometries. Besides their central role in physiology, direct experimental tests of ciliary responses to local flows at cellular length and time scales have remained elusive, largely due to the absence of tools capable of applying controlled, and localized flow stimuli. Here, we introduce programmable, nanometer-thin Ti/Pt microactuators that generate well-defined hydrodynamic forcing at biologically relevant frequencies while operating at biocompatible sub-Volt voltages. This platform is pioneering a controlled local hydrodynamic stimulation of individual motile cilia. We quantify the flow fields and forces produced by single microactuators using particle image velocimetry. Applying local oscillatory flows close to motile cilia of the green alga Chlamydomonas reinhardtii, we probe their dynamic response by quantifying phase-locking between cilia and microactuators. This quantification is aided by combining machine-learning-based image segmentation, oscillator phase reconstruction, and circular statistics. During actuation, we observe signatures of phase-locking: those include a reversible modulation of the fluctuations in phase-difference between cilium and actuator and a systematic shift in ciliary beating frequency. Beyond providing a bio-compatible and precise platform for local hydrodynamic stimulation, our approach establishes an experimental framework for directly testing theories of hydrodynamic synchronization and load adaptation in systems of motile cilia.