Gear-based 3D-printed Micromachines Actuated by Optical Tweezers

TL;DR

The paper tackles the challenge of powering microscale gear transmissions without electrical contacts by introducing optically actuated, 3D-printed gear-based micromachines. It combines two-photon polymerization fabrication of spur-gear trains for in-plane motion and bevel gears for converting in-plane actuation into out-of-plane rotation, with mechanical independence between components. Using a Transition Matrix-based optical-force framework, the authors demonstrate continuous rotation, torque amplification, and out-of-plane motion, validating the concept experimentally with careful control of meshing and alignment. The work expands the toolkit of light-controlled micromachines, with potential impact on lab-on-a-chip and biomedical platforms requiring precise, minimally invasive microscale actuation.

Abstract

The miniaturization of mechanical mechanisms is crucial to enable the development of compact, high-performance micromachines. However, the downscaling actuation of conventional gears and micromotors has remained limited by the inherent challenges of implementing mechanical/electrical powering. Here, we present the design, fabrication, and characterization of an optomechanical, gear-driven micromachine realized through two-photon polymerization 3D printing. The actuation is achieved using optical tweezers. The device integrates a microgear transmission system with an optically actuated part, enabling light-controlled micromachines. When illuminated by a highly focused laser source, the first gear generates rotational torque within the gear assembly, converting optical energy into directional mechanical work that can be transmitted to the coupled gear. We demonstrate the fabrication of micromachines using two-photon polymerization (2PP) laser writing, enabling the fabrication of spur gear trains and bevel gears that can produce out-of-plane rotations, which is not achievable with traditional micromachining fabrication techniques. The micromachines are composed of a single gear or a train of two or three gears without any unwanted adhesion between the components, leading to functioning systems. Experimentally, the fabricated micromachines were actuated using optical tweezers, demonstrating continuous gear rotation, effective motion transmission in gear trains, out-of-plane rotations, and the ability to amplify velocity or torque. Optical-tweezer actuation broadens the potential applications of these micromachines, particularly in biomedical and lab-on-a-chip systems, where precise, minimally invasive control at the microscale is essential.

Figures (6)

• Figure 1: Principle of the gear-based micromachines actuated by optical tweezers for controlled rotational motion transmission. (a) A train of spur gears to give in-plane rotation. A large spur gear ($S_1$) is actuated by multiplexed optical traps that move in a controlled circular motion of velocity $\omega_1$, and the motion is transferred to the small gears ($S_2$ and $S_3$), which rotate with an amplified velocity of magnitude $\omega_2$. (b) A system of bevel gears to give out-of-plane rotations; the larger bevel horizontal gear ($B_1$) is rotated in-plane using the optical tweezers, and accordingly the smaller one above it ($B_2$) is rotated in out-of-plane direction with higher velocity $\omega_4$. (c) The radial optical trapping efficiency (the normalised optical force) on the spherical optical handles of $5 \mu$m diameter is shown versus the position of the trap (calculated using the T-matrix method). (d) The calculated optical torque for the two-gear system for different laser power, for the spur gears, the calculations are done assuming one of the smaller gears is actuated by optical traps and the bigger gear is driven ($S_1$); while in the bevel gears, the larger gear ($B_1$) is the driving one, and accordingly $B_2$ is driven.
• Figure 2: SEM images of the fabricated micromachines by 3D-printing using 2PP. (a) a single microgear with a central stator and a surrounding rotor gear. The stator is a rod anchored to the substrate with a wide base and a cap. The rotor is a gear with two rings surrounding the stator, connected by four rods between them. The gear also has four spherical optical handles to direct the optical traps on, in addition to four pillars connecting the gears to the substrate for support during the fabrication and to be mechanically broken after to get a free-rotating gear. (b) a micromachine consisting of a train of two microgears. (c-d) Two designs of a three-microgear train. (e-f) Two different orientations of a micromachine composed of a horizontal bevel gear coupled to a smaller gear positioned above it, enabling out-of-plane rotational motion. All the images were taken while the substrate is tilted by an angle $45\deg$, shown from different orientations to show that the micromachines are efficiently fabricated without adhesion or fusion between their components.
• Figure 3: Rotation of single gears. (a,b) Time-lapse optical microscopy snapshots of the (a) small gear and (b) large gear rotating at their maximum angular velocity, obtained at a laser power of $P=50\%$. The rotation is followed by tracking a red marker placed on one of the optical handles of the gear, which serves as a visual reference for the angular displacement over time. The gears rotate in a clockwise direction. Consecutive frames are separated by approximately 60 ms. The rotational motion is induced by steering multiplexed optical traps. Scale bars: $20\mu m$. (c) Illustration of angular displacement as a function of time for the small and large microgears at maximum used laser power, showing continuous rotation. (d) maximum obtainable angular velocity $\omega$ as a function of laser power for the small and large gears. Each data point corresponds to the mean value obtained from three measurements, while the error bars represent the standard deviation. Dashed lines represent a linear fit. Some of the measured velocities here are shown in video S1.
• Figure 4: Rotation of optically actuated microgear trains. (a) Time-lapse optical microscopy images of a two-gear train showing counter-rotating motion of the large (driver) and small (driven) microgears under multiplexed optical trapping at maximum velocity which is also shown in in part 4 of Video S2. (b) A three-microgear train actuated at maximum velocity illustrating sequential transfer of rotation through the meshed gears. Screenshots are taken from Video S3 part 4. The direction of rotation is indicated by curved arrows, while coloured dots mark selected optical handles used to track angular displacement over time. The driver gears are indicated by red dots and arrows, while the driven gear is indicated by blue ones. Here, we show the anti-clockwise rotation of the driver microgear in the two-gear train and its clockwise rotation in the three-gear train to indicate the freedom in choice of the rotation directions. The screenshots show half of a complete rotation of the driver gear. Scale bars: 20 µ m. (c) Maximum angular velocity $\omega_{\max}$ of individual gears as a function of the number of gears in the train. The decrease in $\omega_{\max}$ with increasing train length reflects the cumulative mechanical load, giving an exponential drop. Error bars denote the standard deviation. (d) The angular velocities of the driven and as a function of the driver angular velocity for two- and three-gear trains. The dashed line indicates the theoretical value. The measured velocities are taken from three rotations, giving the same values. The close agreement between experimental data and the theoretical value over the investigated speed range evidences stable meshing and efficient motion transfer with negligible slip.
• Figure 5: Time-lapse optical microscopy images showing the out-of-plane rotation of the driven bevel gear actuated by the in-plane rotation of the driver gear. The blue marker indicates a reference point on the driven gear arm, while red arrows highlight the rotation direction. The screenshots are taken from part 3 of video S4. Scale bar is $10\mu$m