Magnetically Responsive Microprintable Soft Nanocomposites with Tunable Nanoparticle Loading

TL;DR

The paper tackles the challenge of creating microscale magnetically responsive materials by integrating two-photon lithography with in situ iron oxide nanoparticle coprecipitation to yield 3D-printed features as small as ~8 µm while enabling tunable NP loading via printing dose. The authors demonstrate a core-shell diffusion mechanism where crosslink density controls NP distribution, and they comprehensively characterize magnetic and mechanical properties across doses, achieving stiffness up to ~53 MPa and measurable magnetization. Functional demonstrations include a magnetically actuated microscale gripper and a bistable bit that can encode encrypted messages or sense magnetic field gradients, illustrating programmable structure-function coupling at the microscale. This approach enables precise control over geometry and magnetic loading, paving the way for microscale metamaterials and soft robotic components with tailorable actuation and sensing capabilities.

Abstract

Magnetic remote actuation of soft materials has been demonstrated at the macroscale using hard-magnetic particles for applications such as transforming materials and medical robots. However, due to manufacturing limitations, few microscale magnetically responsive devices exist -- light-based additive manufacturing methods, which are ideal for realizing microscale features, struggle with light scattering induced by the magnetic particles. Moreover, large hard-magnetic microparticles prevent high-resolution features from being manufactured altogether, and soft-magnetic nanoparticles require impractically high loading and high magnetic gradients, incompatible with existing printing techniques. Among successfully fabricated microscale soft-magnetic composites, limited control over magnetic-particle loading, distribution, and matrix-phase stiffness has hindered their functionality. Here, we combine two-photon lithography with iron-oxide nanoparticle co-precipitation to fabricate 3D-printed microscale nanocomposites having features down to 8 um with spatially tunable nanoparticle distribution. Using uniaxial compression experiments and vibrating sample magnetometry, we characterize the mechanical and magnetic properties of the composite, achieving millimeter-scale elastic deformations. We control nanoparticle content by modulating laser power of the print to imbue complex parts with magnetic functionality, demonstrated by a soft robotic gripper and a bistable bit register and sensor. This approach enables precise control of structure and functionality, advancing the development of microscale metamaterials and robots with tunable mechanical and magnetic properties.

Figures (12)

• Figure 1: Fabricating magnetically responsive nanoparticle composites. (A) The fabrication process involves printing a hydrogel via two-photon polymerization (TPP), infusing with iron ions in an iron salt bath, then immersing the sample in ammonium hydroxide to coprecipitate IONPs within them. (B) Images of printed samples as a function of laser power (or dose) after each stage of the fabrication process. Scale bars, 5 mm. (C) Increasing the two-photon dose during printing creates higher polymer crosslink density, resulting in lower amounts of iron ion diffusion and subsequent nanoparticle content, corresponding to a change in the quantity and spatial distribution of magnetic nanoparticles.
• Figure 2: Energy dispersive x-ray spectroscopy (EDS) characterization of the magnetic composite. (A) Scanning electron microscope images (left) and corresponding EDS area scans of iron (Fe) counts (right) as a function of the dynamic range. (B) EDS line scans of cross-sections of an IONP composite block printed at (i) 16%, (ii) 36%, (iii) 64%, and (iv) 100% dynamic range. Normalized distance $x/r$ represents the line scan location $x$ along the radial distance $r$ of the white lines in (A). Scale bars, 200 µm.
• Figure 3: Magnetic and mechanical properties of the IONP composite material. (A) Magnetization curves of monolithic 5 $\times$ 5 $\times$ 0.5 mm3 blocks printed with different laser powers: 2% (light pink), 18%, 51%, and 100% (red) dynamic range. Shaded regions indicate one standard deviation from the mean (solid lines) across three sample replicates. (B) Stiffness (red triangles) and effective yield strength (black circles) are plotted against percent of dynamic range. An example stress-strain curve with stiffness obtained from the linear-loading-regime slope (red line) and yield strength obtained from its 0.2% offset (blue line) are shown in the inset.
• Figure 4: Functional microstructures via IONP nanocomposite. (A) Schematic (i) and experimental demonstration (ii)-(iii) of the deflection of an array of spheres printed with constant 2% dynamic range, attached to cylinders printed at 18% dynamic range and a base printed at 51% dynamic range. Images show the array from a side view before and after a permanent magnet is placed near enough to deflect the array. Scale bars, 400 µm. (B) Schematic (i) and experimental demonstration (ii)-(iii) of the deflection of an array of spheres of varying laser powers, attached to cylinders printed at 18% dynamic range and a base printed at 51% dynamic range. Images show the array from a side view before and after a permanent magnet is placed near enough to deflect the array. Scale bars, 1 mm. (C) Microscale gripper fabricated with varying two-photon doses, resulting in a magnetically-active state with varying degrees of deflection of different arms. (D) Experimental microscope images show a gripper holding onto a payload attached to the permanent magnet (green). Scale bars, 400 µm.
• Figure 5: Functional microstructures via IONP nanocomposite. (A) Bistable structure toggled between stable states using magnetic actuation. Simulated results of strain energy versus rigid body displacement (rigid body in bright red) show a bistable energy landscape with two energy minima. (B) Experimental microscope images show a bistable "bit" switching back and forth between its multiple stable states. All scale bars 1 mm.