I. Interpretation of the Paper’s Core Abstract

Molded extrusion (MEX) is the mainstream manufacturing method for complex silicone rubber structures, widely used in soft robotics, aerospace equipment, and biomedical applications. However, during the fabrication of ultra-thin layers, the substantial printing forces can severely constrain the fabrication of fine, thin-walled structures.

This study innovatively introduces an electromagnetic-assisted approach: magnetic Fe₃O₄ nanoparticles are incorporated into Ecoflex silicone rubber to fabricate a magnetically responsive printing ink, which is paired with a custom‑designed print cartridge containing a 325‑turn copper coil. When energized, the coil generates a downward electromagnetic force that counteracts extrusion resistance; under ultra‑thin layer‑thickness conditions of 0.1 mm, the overall printing force is reduced by 21.08%.

The study employs piezoresistive sensors to perform full‑process force measurements and develops a multiphysics numerical simulation model, achieving a prediction accuracy of 95.63% under low‑height, low‑flow‑rate conditions. Leveraging this process, magnetic soft silicone and rigid thermoplastic materials can be co‑extruded in a single step, enabling the integrated fabrication of self‑deformable micro‑soft robots and bioinspired composite structures, thereby thoroughly validating the engineering application potential of this electromagnetically assisted 3D‑printing approach.

II. The Core Carrier Role of Silicone Rubber 3D Printing in This Study

This study uses modified silicone rubber extrusion-based 3D printing as its experimental platform, with all electromagnetic‑assisted technologies implemented within the MEX printing system, fulfilling four core functions:

• Ultra-thin thin-walled structure forming verification carrier

Traditional silicone rubber printing typically features layer thicknesses of ≥0.5 mm, making it impossible to fabricate thin-walled structures with wall thicknesses as low as 0.8 mm or intricate curved surfaces. By leveraging an enhanced DIW printing platform coupled with an electromagnetic assistance system, we have achieved stable, continuous deposition at a layer thickness of 0.1 mm, enabling the fabrication of snowflake‑like geometries, helical thin walls, and high‑precision biomimetic contours while effectively mitigating issues such as soft‑material collapse and wave‑like deformation.

• Integrated Forming Platform for Multi-Material Soft and Hard Components

A dual-extruder printer has been modified: one extrusion head dispenses a magnetic silicone-based soft ink, while the other extrudes ABS hard filament. During a single print cycle, no secondary bonding or assembly is required, enabling the direct fabrication of bioinspired composite components featuring a “hard skeleton + soft joints,” such as bionic fingers, knee joints, and spider‑like robots.

• Print Strength Quantitative Testing and Simulation Verification Platform

A high-precision in‑situ force sensor was integrated into the 3D‑printed piston to acquire, in real time, the total extrusion force and interfacial reaction forces for pure silicone and various magnetically filled inks under different layer heights and extrusion speeds. The experimentally measured 3D‑printing data were used to calibrate a COMSOL multiphysics model, thereby establishing a predictive simulation framework for the extrusion mechanics of silicone rubber.

• Fundamentals of Magnetically Responsive Soft Actuators

Fe₃O₄ nanoparticles are dispersed within a silicone rubber matrix, endowing the printed structure with intrinsic magnetic anisotropy. After fabrication, no additional actuator components need to be implanted; an external magnetic field can directly induce bending and switching between stable states, enabling the one-step fabrication of magnetically controlled, self‑deforming soft actuators.

III. Research on Four Major Core Innovation Breakthroughs

Breakthrough 1: Electromagnetic force counteracts drag, overcoming the bottleneck of ultra-thin-layer printing. The total printing force is composed of the material barrel‑pushing force and the interlayer reverse pressure.

This scheme passes a current of 4.5 A, generating a downward electromagnetic pulling force \(F_{EM}\) that counteracts part of the total resistance, with the formula updated to \(F_T = F_S + F_{C-P} - F_{EM}\). Using a 30 wt% Fe₃O₄–silicone ink at a layer height of 0.1 mm and a flow rate of 0.1 mL/min, the printing force is reduced by as much as 21.08%, effectively addressing issues such as material overflow, contour distortion, and wrinkling in thin walls caused by high resistance in thin layers.

Breakthrough 2: A magnetic silicone ink system that balances flexibility with printability. System testing was conducted across four nanoparticle loading levels—0, 10, 20, and 30 wt%. The optimal formulation, at 30 wt%, resulted in only a modest 12.7% reduction in tensile strength, while preserving Ecoflex’s inherent high extensibility.

The filler is uniformly dispersed without generating significant bubbles, exhibits shear-thinning rheological behavior, is compatible with extrusion-based printing, and imparts magnetic-field‑responsive deformation to the final product.

Breakthrough 3: A high-precision multiphysics mechanical prediction and simulation model, based on the Navier–Stokes equations and incorporating shear-thinning rheological data for silicone rubber, establishes a two-dimensional simulation framework that accounts for both internal fluid flow within the nozzle and interlayer deposition. The model distinguishes among three types of loads: propulsion force, interfacial reaction force, and electromagnetic force.

Under ultra-thin-layer, high‑performance operating conditions, the simulation achieves a prediction accuracy of up to 98.44%, enabling early assessment of printing resistance under varying fill materials, flow rates, and layer heights, thereby significantly reducing process‑trial-and‑error costs.

Breakthrough 4: A novel soft–hard co-extrusion manufacturing process enables the integrated fabrication of bionic soft components in a single workflow, without assembly. An ABS rigid skeleton and magnetic silicone soft joints are simultaneously deposited, enabling the monolithic formation of bionic fingers, knee joints, bistable actuators, and miniature spider‑like pipeline robots.

The finished product requires no bonding or assembly, offering enhanced structural integrity and continuity; a external magnetic field can drive reciprocating motion, making it well-suited for operations in confined, sealed microspaces.

IV. Technical Application Scenarios and Future Development Directions

(1) Existing Application Areas

• Microscale Bionic Soft Robot

Millimeter-scale magnetically controlled crawling and bending robots can enter 10-mm‑diameter narrow pipelines and the internal cavities of equipment to perform nondestructive testing, making them suitable for flaw detection in precision instruments and exploration in confined spaces. Magnetically actuated flower‑like structures and bistable switching mechanisms are employed as micro‑actuating components.

• High-Precision Flexible Bionic Device

Biomimetic structures mimicking human muscles and joints, flexible rehabilitation assistive devices, and soft robotic fingers with a humanoid‑inspired design—integrating soft and rigid components to perfectly replicate the flexible motion characteristics of the human body.

• Microfluidics, thin-walled sensing substrates

0.8 mm ultra-thin silicone rubber thin-walled channels and a high-precision biomimetic contour‑sensing substrate, suitable for fabricating microfluidic chips and flexible pressure sensors.

• Customized Bio‑Soft Tissue Model

High-precision, thin-walled silicone human tissue simulants, used for preoperative surgical simulation in dentistry and orthopedics, as well as for medical education models.

(II) Directions for Future Research Development

• Expansion of Electromagnetic Printing with General-Purpose High-Viscosity Ink

The electromagnetic-assisted approach has been extended to a range of difficult-to-extrude soft materials, including carbon aerogels, hydrogels, and high-viscosity ceramics, thereby becoming a versatile, low-resistance extrusion‑based printing technology.

• Development of Low-Fill, High-Magnetic-Response Silicone Ink

Optimize the surface modification of nano-fillers to reduce their loading level, thereby further minimizing the loss of mechanical properties in silicone rubber while enhancing the amplitude of magnetic-field‑induced deformation.

• Integrated Temperature Control + Electromagnetic Composite Printing System

A localized heating module has been integrated into the nozzle, and combined with electromagnetic drag reduction, enabling continuous printing of ultra‑high, large‑size thin‑walled silicone rubber structures while addressing issues of premature curing in high‑altitude applications and poor interlayer adhesion.

• Machine Learning–Coupled Simulation for Intelligent Process Matching

By integrating simulation data with machine learning, the system automatically determines the optimal current, flow rate, and layer height based on the target wall thickness and material parameters, enabling fully autonomous, intelligent printing.

• Research and Development of Medical-Grade Magnetic Silicone Ink

Develop cell‑non‑toxic, sterilizable magnetic silicon materials for use in minimally invasive flexible catheters and implantable micro‑actuated soft medical devices.

Image and Text Guide

Figure 1. Electromagnetically assisted additive manufacturing using magnetic inks for force reduction: (A) Workflow of this study; (B) Comparison between conventional material extrusion and the proposed method in fabricating silicon-based structures, where a new force—electromagnetic force—is introduced into the system.

Figure 2. (A) Variation of the dynamic viscosity of four magnetic nanopowders at different concentrations as a function of shear rate; (B) Storage modulus (G′) and loss modulus (G″) of the four magnetic nanopowders at different concentrations.

Figure 3. A customized material extrusion system capable of simultaneously extruding ink and thermoplastic materials within a single manufacturing cycle. The syringe reservoir houses 325 copper coils, which are connected to a power supply (4.5 A) to reduce the overall printing pressure via electromagnetic force. A force sensor is installed between the syringe plunger and the pushing element to record the applied force and validate the accuracy of the numerical model.

Figure 4. Schematic diagram of force modeling during silicone elastomer extrusion. (A) Schematic of the nozzle geometry, showing the pressure drop distribution along the nozzle axis. (B) Boundary conditions for the numerical simulation of flow inside the nozzle. (C) Schematic of the layer-by-layer deposition process, with key variables involved in the extrusion process labeled. (D) Control volumes used to solve the governing equations for layer-by-layer deposition. (E) Schematic of the extrudate structure and associated variable parameters. (F) Boundary conditions for the numerical simulation of layer-by-layer deposition. (G) Schematic of the syringe plunger diameter.

Figure 5. Reduction of total printing force using the proposed electromagnetic method: (A) At a flow rate of Q = 0.1 mL/min, the variation of the total printing force FT with layer height (h) for different mass fractions of Fe₃O₄, comparing the conventional method and the proposed method. The figure also presents both numerical simulation results and experimental data; (B) For the conventional method, the overall FT of pure Ecoflex 00–10 (without magnetic fillers) as a function of layer height (h), along with the optimal result obtained using the proposed method (silicone + 30 wt% Fe₃O₄); (C) At Q = 0.1 mL/min, when extruding silicone containing 30 wt% Fe₃O₄ using the proposed method (which employs electromagnetic forces), the dependence of three distinct force components on layer height (h).

Figure 6. Schematic diagrams of 3D printing for thin-walled and complex structures: (A) Thin-walled structure; (B) Snowflake-shaped structure; (C) Thin-walled gyroscopic structure; (D) Italian rural terrain. This figure clearly illustrates the high level of detail achievable with the proposed method, as well as the lower-quality details obtained using conventional approaches—both methods employing identical printing parameters.

Figure 7. Enhancement of print quality achieved by the proposed electromagnetic‑assisted technique on the developed soft ink (Ecoflex 00–10 with 30 wt% Fe₃O₄): (A) Effect of varying current inputs (0 A, 3.5 A, and 4.5 A) applied to a copper coil to generate electromagnetic forces on print quality; (B) Advantages of this method in mitigating over‑extrusion when the layer thickness is set to 0.1 mm.

Table 1: Process Parameter Settings Used for Fabricating the Structure Shown in Figure 7

Figure 8. Nozzle scalability. Star-shaped structures were fabricated using nozzles with diameters of 0.2 mm, 1.02 mm, and 2 mm. The figure highlights the improvement in feature resolution and the corresponding increase in fabrication time when using the 0.2‑mm nozzle.

Figure 9. Application examples in the field of soft robotics: (A) Schematic illustration of a magnetic flower actuated by an external magnetic field; (B) The proposed 3D‑printed magnetic soft flower and its actuation mechanism; (C) A knee‑joint‑like structure composed of two rigid ABS struts (red) and flexible silicone (black); (D) A biomimetic 3D‑printed finger fabricated by co‑extruding rigid ABS (red) and magnetic ink (black) during a single 3D‑printing process, driven by a tendon‑based actuation system that replicates human‑like bending motions; (E) A bistable 3D‑printed structure produced by co‑extruding rigid ABS (red) and magnetic ink (black), which can be activated by an external magnetic field; (F) A spider‑inspired soft robot consisting of four legs made of magnetic ink and a central rigid body made of ABS, all formed using the same manufacturing process. When activated by a magnetic field, this robot can move freely in confined spaces.

Table 2 Summary of the Main Findings of This Study