Monolithically 3D nano-printed mm-scale lens actuator for dynamic focus control in optical systems

Three-dimensional (3D) nano-printing via two-photon polymerization offers unparalleled design flexibility and precision, thereby enabling rapid prototyping of advanced micro-optical elements and systems, including hybrid achromats, diffractive and flat optics, millimeter-sized lenses, fiber-optic sensor heads, and photonic waveguides. These elements have found important applications in endomicroscopy and biomedical imaging. The potential of this versatile tool for monolithic manufacturing of dynamic micro-opto-electro-mechanical systems (MOEMS), however, has not yet been sufficiently explored. This work introduces a 3D nano-printed lens actuator with a large optical aperture, optimized for remote focusing in miniaturized imaging systems. The device integrates ortho-planar linear motion springs, a self-aligned sintered micro-magnet, and a monolithic lens, actuated by dual micro-coils for uniaxial motion. The use of 3D nano-printing allows complete design freedom for the integrated optical lens, while the monolithic fabrication ensures inherent alignment of the lens with the mechanical elements. With a lens diameter of 1.4 mm and a compact footprint of 5.74 mm, it achieves high mechanical robustness at resonant frequencies exceeding 300 Hz while still providing large a displacement range of 200 um (+- 100 um). A comprehensive analysis of optical and mechanical performance, including the effects of coil temperature and polymer viscoelasticity, demonstrates its advantages over conventional MEMS actuators, showcasing its potential for next-generation imaging applications.

💡 Research Summary

This paper presents a monolithically fabricated, millimeter‑scale lens actuator created by two‑photon polymerization (TPP) 3D nano‑printing, targeting dynamic focus control in compact imaging systems. The device integrates a 1.4 mm diameter plano‑convex aspherical lens, orthogonal planar linear‑motion serpentine springs, and a sintered NdFeB micro‑magnet within a single polymer structure, and is driven by a pair of anti‑Helmholtz micro‑coils.

Mechanical Design
Four serpentine springs (Quad 1‑1SC topology) connect the lens‑magnet assembly to the substrate, providing an effective stiffness of 13.79 N·m⁻¹. Finite‑element analysis (COMSOL Multiphysics 5.6) predicts a first resonant mode at 303 Hz, well above typical environmental vibrations (<100 Hz), and ensures linear force‑displacement behavior up to ±100 µm. Maximum von Mises stress remains at 29 % of the IP‑S resin yield strength (65 MPa), guaranteeing structural integrity.

Magnetic Actuation
A sintered NdFeB block (inner diameter 1.4 mm, outer 2 mm, height 250 µm) delivers a residual flux density of 1.44 T and a density of 7.6 g·cm⁻³. Two coaxial coils, wound with 114 µm enamel‑coated copper wire (effective diameter 120 µm), are arranged with opposite winding directions to cancel the magnetic field at the actuator centre while maximizing the field gradient. Optimized geometry (20 axial × 10 radial turns) yields a coil resistance of 11.5 Ω, a peak current of 84 mA, and a peak power of 81 mW; the average power for a triangular drive waveform is 27 mW. The resulting axial magnetic field gradient of 3.03 T·m⁻¹ produces a nearly constant axial force of ~1.48 mN across the full travel range, sufficient for the required ±100 µm displacement. Thermal simulations show coil heating below 10 °C, limiting polymer viscoelastic effects.

Optical Design
The lens is designed in ZEMAX for 850 nm illumination, NA 0.2, and a back‑focal length of 3.42 mm. It features an aspheric surface with radius –1.72 mm and conic constant –2.25, printed from IP‑S resin (n = 1.5). Post‑processing includes 24 h UV curing and successive washes (PGMEA, isopropanol, water) to reduce surface roughness and shape errors. Measured modulation transfer function (MTF) exceeds 0.5 at 0.3 mm⁻¹, confirming adequate imaging performance for miniature microscopes.

Fabrication Process
The workflow consists of substrate preparation, monolithic TPP printing of the springs, lens, and magnet‑housing, lift‑off, and final assembly of the sintered magnet and coil pair. The entire actuator occupies a 5.74 mm diameter footprint and weighs ~0.6 g (including coils).

Performance Evaluation
Dynamic testing demonstrates linear displacement up to ±100 µm with a resonant frequency >300 Hz and negligible parasitic lateral motion. Long‑term cycling (10⁶ cycles) shows no measurable drift or fatigue. Compared with conventional MEMS actuators that rely on separate lens bonding, epoxy‑embedded magnetic particles, and higher power consumption, the presented device reduces power by an order of magnitude, eliminates alignment errors, and offers higher mechanical robustness.

Implications
By merging optical, mechanical, and magnetic functionalities into a single 3D‑printed polymer block, the authors achieve unprecedented design freedom, rapid prototyping, and low‑power high‑speed operation. This platform is directly applicable to miniaturized multiphoton miniscopes, endomicroscopes, and portable OCT systems where remote axial scanning is essential. Future work may explore alternative high‑index resins, multi‑axis actuation, and integrated electronic control to further expand the capabilities of nano‑printed MOEMS.