I. Interpretation of the Core Summary

Direct Ink Writing (DIW) 3D printing of silicone rubber holds tremendous application potential in flexible electronics, medical devices, and soft robotics. Addressing the current challenges associated with silicone rubber printing materials, equipment, and processes, this paper establishes a comprehensive, standardized workflow for printing highly stretchable silicone rubber:

• Test the rheological properties of Dow Corning DC737 silicone rubber, compare the fluid‑flow simulation results for conical and straight‑cylinder print nozzles, and select the optimal nozzle design.
• Orthogonal experiments were conducted on the in-house DIW printing platform to optimize single-filament and thin-wall stacking parameters, identifying the optimal combination of air pressure, printing speed, and layer thickness.
• Based on optimized-parameter, integrated-printing, dual-material bionic artificial muscles and complex internal‑cavity flexible grippers, we validate the feasibility of directly fabricating high‑precision soft actuators.

The entire study provides a comprehensive practical reference for direct-write 3D printing of soft materials across various application scenarios.

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

This study employs direct-writing extrusion 3D printing as the core approach for the integrated fabrication of soft actuators, replacing conventional methods such as mold-based casting and discrete assembly, and fulfilling four key functions:

• Achieving integrated co-printing of multiple silicone rubber materials

Leveraging a dual‑material DIW platform, it is possible to simultaneously extrude two types of silicone rubber—high‑elasticity Ecoflex‑0030 and high‑strength DC737—eliminating the need for secondary bonding. This enables direct fabrication of soft–hard composite artificial muscles, with axial contraction upon inflation achieved through the differential stretchability of the two materials, thereby addressing the challenges of delamination and poor air tightness inherent in separate‑component bonding.

• Integral molded structure with a sealed internal air chamber

Traditional molds are limited to simple single‑layer cavities. By employing a well‑designed bridging strategy and appropriate tilt angles, 3D printing can directly fabricate trapezoidal, multi‑layer sealed pneumatic internal cavities without supports. This enables the production of flexible grippers that, under positive and negative air pressure, can perform forward grasping and reverse opening, significantly expanding their range of adaptability.

• Quantify process parameters and establish universal printing standards.

Through a three‑stage experimental approach—fluid dynamics simulation of the nozzle, single‑filament extrusion, and interlayer stacking—we have identified the optimal matching thresholds for nozzle geometry, air pressure, printing speed, and layer thickness. This has enabled us to address four major printing defects—extrudate swell, filament breakage, interlayer voids, and nozzle clogging—and establish a reusable library of process parameters.

• Rapid Iterative Validation of Biomimetic Structures

No custom molds are required; by simply modifying the model, artificial muscles, grippers, and complex flexible structures can be rapidly printed. Combined with finite-element simulations for concurrent validation of inflation‑induced deformation patterns, this approach significantly shortens the R&D and iteration cycle for soft actuators.

III. Four Major Innovative Breakthroughs of the Article

Breakthrough 1: Pre‑selection of needle‑tip fluid simulation to address material clogging at the source.

Innovative experiments conducted a comparative fluid‑flow simulation of conical and straight‑cylinder nozzles for high‑viscosity silicone rubber: under identical air pressure, the straight‑cylinder nozzle exhibits lower flow rate and greater pressure loss, making it prone to nozzle clogging; in contrast, the conical nozzle features a more gradual velocity gradient and smoother material discharge, enabling stable extrusion even at low pressures. Consequently, a 0.41 mm conical nozzle was selected as the standard printing consumable, significantly enhancing print continuity.

Breakthrough 2: A hierarchical, stepwise parameter optimization framework that pinpoints the optimal forming window.

Establish a three-tier experimental framework: “Nozzle Simulation → Filament Printing → Thin-Wall Layer Stacking”:

• Monofilament test: At an air pressure of 40 psi and a printing speed of 20 mm/s, the line width was 411.6 μm, perfectly matched to the nozzle diameter, with no waviness or filament breakage.
• Thin-wall stacking: a layer thickness of 0.30 mm ensures seamless interlayer bonding; too thin a layer leads to material overflow, while too thick a layer results in internal voids.
• A precise and stable set of direct-write printing parameters for silicone rubber has been established.

Breakthrough 3: A support-free bridge‑forming process for printing complex, enclosed internal cavities.

System testing reveals that, among the three bridge‑forming angles—30°, 45°, and 60°—a 30° angle results in filament breakage, a 45° angle leads to structural deformation, while a 60° angle provides uniform bridging transition and optimal internal cavity sealing. Based on this process, a multi‑layer trapezoidal‑cavity flexible gripper can be directly printed without soluble supports or post‑processing for demolding, thereby simplifying the manufacturing workflow.

Breakthrough 4: Forming and Experimental Validation of a Bimaterial Integrated Biomimetic Actuator

An innovative composite silicone rubber with differentiated mechanical properties is used to 3D‑print a biomimetic artificial muscle: when inflated, the highly elastic matrix expands while a high‑strength constraint filament restrains radial deformation, enabling controlled axial contraction. Coupled with a six‑axis robotic arm, this system was tested for grasping flexible grippers, demonstrating adaptability to objects of various shapes and sizes—such as cardboard boxes, drinking cups, and irregularly shaped vases. The simulated deformations closely matched the measured results, validating the practicality of the entire fabrication process.

IV. Prospects for Technological Applications and Future Directions for Expansion

(1) Current Real-World Application Scenarios

• Industrial Flexible Gripping Robot

Gripping food, electronic components, and fragile workpieces without causing damage; replacing rigid grippers; compatible with flexible production lines for 3C, food, and logistics sorting.

• Biomimetic Soft Actuator

Biomimetic muscles and multi-degree-of-freedom continuum soft robots, designed for inspection of narrow pipelines and rescue operations in confined spaces;

• Flexible Sensing and Wearable Devices

An integrated flexible skin with embedded sensing channels and a stretchable sensing substrate, tailored for human motion monitoring and rehabilitation assistive devices.

• Minimally Invasive Medical Devices

Biocompatible silicone rubber catheters and flexible interventional actuators, tailored for minimally invasive surgery and customized production of rehabilitation and orthotic devices.

(II) Future Directions for Research

• Multi-material composite expansion

Conductive ink and hydrogel are simultaneously printed onto a silicone rubber substrate, enabling the monolithic fabrication of an intelligent soft actuator with tactile sensing and real-time feedback on grasping force.

• High-Speed Continuous Printing Process Upgrade

Optimize the thixotropic system of silicone rubber and the temperature-controlled barrel to increase printing speed, thereby meeting the demands of industrial-scale mass production.

• Development of 4D Deformable Soft Structures

By incorporating shape-memory silicone rubber, we enable post‑printing temperature control and pneumatic actuation for autonomous deformation, thereby expanding the range of adaptive robotic applications.

• Fully digital simulation closed loop

By integrating machine learning to predict extrusion swell and interlayer deformation, the system enables one-click model parameter matching, thereby lowering the barrier to process tuning.

• Medical-grade specialized silicone printing system

Develop sterilizable, highly biocompatible medical-grade silicone ink for use in custom prosthetics, soft-tissue repair, and implantable flexible devices.

Image and Text Guide

Figure 1. (a) Soft sensing actuator [19]; (b) Tentacle-like soft robot [20]; (c) Inflatable flat morphology of a three-dimensional origami structure and an LED device fabricated by folding a flat sheet [21]; (d) Soft octopus robot [22].

Figure 2. (a) Gantry‑type printing platform; (b–d) DIW and component modules

Figure 3. (a) Modulus–concentration plot; (b) Shear-thinning behavior.

Figure 4. (a) Schematic diagram of the conical needle and the simulated relationship between flow rate and pressure; (b) Schematic diagram of the straight tubular needle and the simulated relationship between flow rate and pressure.

Figure 5. (a) Schematic diagram of typical DIW 3D printing parameters; (b) Photograph of a single filament measurement; (c) Optical images showing the evolution of fiber morphology with varying extrusion speed; (d) Frequency histogram.

Figure 6. (a) Schematic diagram of the model; (b) Photograph of the physical specimen; (c) Cross-sectional photograph.

Figure 7. (a) Complex two-dimensional structures printed using exemplary silicone; (b) Complex three-dimensional structures printed using exemplary silicone.

Figure 8. (a) Model of the artificial muscle; (b) Printing process for the artificial muscle; (c) Equivalent elastic strain of the artificial muscle versus its actual experimental deformation; (d) Axial contraction under different pressures, as obtained from both finite-element modeling and experiments.

Figure 9. (a) Bridging concept and transition dip angle; (b) Photograph of the bridge

Figure 10. (a) Printing process of the soft gripper; (b) Schematic diagram of the soft gripper’s dimensions; (c) Bending test; (d) Grasping demonstrations with different objects; (e) Experimental demonstration of the grasping process.