Ultrasound-activated ciliary bands for microrobotic systems inspired by starfish

The study addresses how to design and actuate synthetic ciliary bands to generate controllable flows and propulsion at microscales where viscosity dominates. Natural cilia are widespread in organisms (e.g., algae, invertebrate larvae, mammalian airways, and Fallopian tubes), performing tasks such as transport and filtration in low-Reynolds-number regimes. Inspired by these systems, artificial cilia have been explored for propulsion, pumping, mixing, and particle manipulation, but most engineered ciliary arrays produce simple unidirectional flows and lack the diversity and complex hydrodynamics observed in biology. Marine invertebrate larvae (e.g., starfish) can reorient dense ciliary bands to create local fluid sources and sinks for propulsion and feeding. This work proposes ultrasound-activated synthetic ciliary bands, arranged in + (angled toward) and − (angled away) configurations, to reproduce source/sink flow fields, leverage nonlinear acoustics to bypass the scallop theorem, achieve microrobot propulsion, and implement a starfish-inspired particle trapping mechanism.

Artificial cilia have been actuated using magnetic, electric, optical, and pneumatic methods. Examples include handheld magnets driving magnetic–polymer nanorod arrays; self-assembled superparamagnetic particle chains actuated by rotating magnetic fields; electrostatically activated metal-coated polymer films for mixing; light-driven azo-benzene liquid crystal cilia; and pneumatically controlled cilia. Collective behavior in ciliary carpets (e.g., metachronal waves) has transported fluid, but many studies are numerical and most fabricated arrays yield simple unidirectional flows. Starfish larvae can switch cilia orientation to form flow sources and sinks for propulsion/feeding, inspiring the present designs. Despite ultrasound’s widespread use in microfluidics and biomedicine, acoustically activated cilia and ciliary bands have been little explored beyond single-cilium liquid pumping. Ultrasound is noninvasive, biocompatible, and body-penetrating, making it attractive for microrobotics. This work fills the gap by engineering ultrasound-driven ciliary bands that orchestrate complex hydrodynamics reminiscent of biological counterparts.

Design and fabrication: + and − ciliary bands were fabricated via projection UV photopolymerization on an inverted microscope. A high-resolution photomask at the field stop was illuminated with a UV lamp through a 20× objective to polymerize a PEG-based photoresist (downsizing factor ≈ −16.3). Exposure time: 500–3000 ms at 12.5–100% UV intensity, controlled by an electronic shutter. Geometry: bands comprised two to eight cilia per side; individual cilium length L ≈ 100 µm, base thickness W = 15–35 µm, height H = 50 µm; cilia arranged in series with 20–40 µm separation. + bands: two arrays angled toward each other; − bands: two arrays angled away. Materials: Photosensitive mixture of 50% (v/v) PEG diacrylate MW 700, 30% (v/v) PEG diacrylate MW 258, 15% (v/v) TE buffer, and 5% (v/v) 2-hydroxy-2-methyl-1-phenyl-propan-1-one (Darocur 1173). Small additions of Rhodamine B and food dye aided focusing and contrast. Acoustic setup: Structures fabricated on a 25×75×1 mm glass slide with a bonded piezo transducer (Murata 7BB-27-4L.0) using epoxy. A droplet (~150 µl) of DI water mixed 10:1 (v/v) with 5.7–6 µm tracer particles was confined under a 22×22 mm coverslip to form a manipulation chamber. The piezo was driven by a function generator (Tektronix AFG 3011C) via an amplifier (Thurlby Thandar WA301). Operating frequencies: 20–100 kHz; drive amplitudes: 1–25 Vpp. The system produces transverse waves in the glass and longitudinal waves in liquid, causing small-amplitude oscillations of cilia and inducing acoustic streaming. Estimated viscous boundary layer thickness δ = √(ν/πf) ≈ 1.8–4.0 µm for water (ν ≈ 10⁻⁶ m²/s) and f = 20–100 kHz. Imaging and analysis: Experiments on Zeiss Axiovert 200 M with a fluorescence camera (Coolsnap EZ) and a high-speed camera (Chronos 1.4). Particle tracking and PIV performed in ImageJ and MATLAB (PIVlab). PIV regions of interest were defined near the bands (magenta boxes in figures) and velocities averaged over 150–400 measurements per data point to evaluate scaling versus Vpp. Microrobot preparation and tests: Starfish-inspired microrobots incorporated a + band on one side and a − band on the other. Devices were released by lifting the top coverslip and nudging the bulk body while avoiding ciliary regions. Under ultrasound (typically ~68.5–68.8 kHz, ~20 Vpp), translational motion along the short axis was characterized by time-lapse imaging and PIV to map surrounding vortical flow patterns. Numerical simulation: COMSOL Multiphysics finite element simulations using a perturbation approach. First-order (time-harmonic) acoustic fields (v₁, p₁) were solved at the actuation frequency; second-order (steady) streaming fields (v₂, p₂) were then computed with forcing terms from nonlinear interactions of first-order fields. 2D simulations on a sufficiently large rectangular domain with oscillating cilia prescribed nonzero first-order velocity (Dirichlet) and zero second-order Lagrangian velocity (Dirichlet). Triangular P1–P2 elements for pressure/velocity and direct solvers were used. Simulations reproduced experimental qualitative flow features, including counter-rotating vortices at innermost tips for both + and − bands. Theoretical framework: The frequency Reynolds number Re_f = 2πf Δs L / ν characterizes the relative importance of inertial to viscous effects in the first-order response and the strength of forcing terms in the second-order streaming equations. With ultrasound-actuated small-amplitude reciprocal motions, sufficiently large Re_f and separated time/length scales enable nonzero time-averaged flows that can generate propulsion, circumventing the scallop theorem.

• Ultrasound-actuated ciliary bands generate controlled acoustic streaming that depends on band geometry: + bands (arrays angled toward each other) act as fluid sources with outward flow at the center; − bands (arrays angled away) act as sinks with inward flow at the center.
• Particle transport along angled arrays: tracer particles hop tip-to-tip with peak speeds up to ~10 mm/s near cilium tips; flow direction follows tip orientation, forming localized CW/CCW vortices in each half of the band.
• Control experiment with straight arrays showed negligible inter-cilium transport; tracers were trapped in small vortices near tips, confirming the necessity of angulation for tangential streaming.
• PIV measurements show average streaming velocities scale approximately quadratically with applied voltage (velocity ∝ Vpp²), consistent with nonlinear streaming theory; log–log fits yielded slopes m ≈ 2.1 (+ band) and m ≈ 2.2 (− band). At identical conditions, vertical streaming for + bands exceeded that of − bands due to closer innermost tip spacing.
• Frequency dependence: strongest streaming observed near ~68.5 kHz, attributed to piezo/glass coupling resonance; engineered ciliary bands themselves did not exhibit resonance-dominated behavior, and flow fields were largely independent of swimmer orientation relative to background field.
• Numerical simulations (perturbation approach) reproduced experimental flow fields, including counter-rotating vortices at innermost tips of both + and − configurations.
• Microrobot propulsion: a swimmer with + and − bands on opposite sides translated along its short axis at ~2.6 mm/s (~10 body lengths/s; body width D ≈ 280 µm) under 68.8 kHz and 20 Vpp. The surrounding flow comprised symmetric arrays of CW/CCW vortices, yielding net force along the short axis; control devices without cilia showed no motion in nearly uniform acoustic fields.
• Reynolds number based on swimming speed Re ≈ 0.7 indicates viscous dominance; however, the frequency Reynolds number for cilium oscillation was estimated Re_f ≈ 31.4–62.8 (f ~ 100 kHz, Δs ≈ 0.5–1.0 µm, L characteristic), enabling nonlinear time-averaged forcing to bypass the scallop theorem with reciprocal actuation.
• Particle trapping and transport: an adjacent +/− band arrangement produced starfish-like feeding flows, transporting microparticles from the + source region toward and into the − sink. Particle speeds decelerated leaving the + band and accelerated approaching the − band; peak outward speeds from + band reached ~2 mm/s. Transport efficiency was tunable by acoustic power: optimal transfer at 12 and 18 Vpp; at 24 Vpp trapping dominated with no transfer to the − band.
• The work outlines ultrasound-enabled strategies for propulsion, pumping, mixing, and label-free trapping/separation at low Reynolds numbers, with robustness advantages over bubble-based swimmers.

The findings demonstrate that geometric arrangement of ultrasound-driven synthetic ciliary bands can encode complex hydrodynamics—sources and sinks—analogous to starfish larvae, enabling both propulsion and particle manipulation at microscales. Nonlinear acoustics and the frequency Reynolds number framework explain how reciprocal, small-amplitude oscillations generate steady streaming flows that introduce effective inertia at the second-order level, rendering the scallop theorem inapplicable and enabling directed swimming in low-Re environments. The microrobot’s symmetric vortex arrangement around its long axis cancels forces in one direction while producing net thrust along the short axis, providing a design principle for directional control. Numerical simulations validated the experimental observations and offer a predictive tool for tailoring band geometry and actuation. The combined +/− band configuration mimics larval feeding by transporting and trapping particles, with performance tunable by drive voltage. These results broaden the design space for externally actuated microrobots, highlighting ultrasound’s safety, deep penetration, and noninvasiveness, and point to applications in lab-on-chip pumping/mixing, targeted delivery, and label-free particle capture/separation.

This work introduces ultrasound-activated synthetic ciliary bands that, through + (source-like) and − (sink-like) arrangements, generate controllable streaming flows for microrobotic propulsion and particle manipulation. A starfish-inspired microrobot achieved ~2.6 mm/s swimming (≈10 body lengths/s) via reciprocal cilia oscillations that exploit nonlinear acoustics to circumvent the scallop theorem. Adjacent +/− bands implemented a biomimetic trapping/transport mechanism with performance tunable by acoustic voltage. Simulations based on a perturbation approach reproduced experimental flow patterns, supporting design generalization. Future directions include: integrating superparamagnetic particles for combined ultrasound propulsion and magnetic navigation; hybrid designs with gas-filled microbubbles for frequency-selective band activation; dynamic reorientation using light-responsive liquid crystal polymers to switch between + and − modes; characterization across varied cilium geometries, stiffnesses, and curved substrates; operation and tracking in biological and non-Newtonian media; and studies of multi-robot interactions under ultrasound.

• The study fixed the ciliary band angle; dynamic reorientation was not implemented.
• Direct measurement of cilia oscillation amplitudes was challenging; propulsion force decomposition and full characterization remain difficult experimentally and numerically.
• Optimal performance depended on piezo/glass coupling near ~68.5 kHz; while the cilia themselves did not show resonance, the system exhibited frequency-dependent behavior due to the transducer–substrate coupling.
• Excessively soft microarchitectures deformed under their own weight, degrading tangential streaming and performance.
• Experiments were conducted in water with tracer particles; performance in complex, non-Newtonian, or biologically relevant fluids was not assessed.
• Control and steering were limited to fixed geometry and acoustic parameters; closed-loop navigation was not demonstrated.