Traditional silicone rubber products rely on mold-forming techniques, which are characterized by long development cycles and high costs, making it difficult to achieve personalization or complex internal structures. The rise of 3D printing (additive manufacturing) technology has brought revolutionary possibilities to fields such as flexible electronics, soft robotics, and customized medical devices. However, due to its high viscosity, inability to thermally melt, and requirement for chemical cross-linking, silicone rubber has long been considered an "unprintable" material. Recent breakthroughs in technologies like photocurable silicones, direct ink writing (DIW), and embedded printing have finally brought silicone rubber into the digital manufacturing era, opening a new chapter of "on-demand customization of flexible functional bodies."

I. Why is 3D Printing Silicone Rubber So Challenging?

Common thermoplastic materials (such as PLA, TPU) can be printed through heating and extrusion, but silicone rubber is a thermosetting elastomer:

Non-melting: Heating above 200°C leads to degradation rather than flow;

Cross-linking Required: It must undergo platinum-catalyzed addition or peroxide-initiated reactions to cure;

High Surface Energy: Before curing, it easily adheres to nozzles; after curing, it is difficult to demold;

Shrinkage Control: Volume changes during cross-linking affect precision.

These characteristics make traditional FDM (Fused Deposition Modeling) and SLS (Selective Laser Sintering) processes unsuitable.

II. Mainstream 3D Printing Technologies

Photocurable Silicone (Vat Photopolymerization)

Principle: Methyl acrylate-modified siloxane oligomers dissolved in reactive diluents with photoinitiators.

Process: UV light (365–405 nm) cures each layer through free radical polymerization to form a cross-linked network.

Representative Materials: ACEO® Printable Silicones by Wacker Chemie AG, Spectroplast's medical-grade photocurable silicones.

Advantages:

High resolution (up to 50 μm);

Smooth surfaces requiring no post-processing;

Capable of printing complex structures like overhangs and enclosed cavities.

Limitations: Slightly lower mechanical properties compared to traditional LSR (tensile strength ≈ 5–7 MPa), requiring post-curing to enhance cross-link density.

Direct Ink Writing (DIW)

Principle: High-viscosity silicone rubber pastes (10⁴–10⁶ mPa·s) are extruded through micro-nozzles.

Curing Methods:

Room Temperature Vulcanization (RTV): Slowly cures upon contact with moisture;

Thermal Curing: Overall heating post-printing for cross-linking.

Advantages:

Can use commercial LSR materials;

Easy incorporation of conductive fillers (silver, carbon nanotubes), magnetic particles, etc.;

Challenges: Precise control of rheology (shear thinning + rapid recovery) to prevent collapse.

Embedded 3D Printing

Principle: Silicone rubber "ink" is injected into supportive gel baths (e.g., Carbopol).

Features: The gel provides isotropic support, allowing printing of arbitrary free forms (e.g., helices, knots).

Demolding: Post-printing, heat or pH adjustment liquefies the gel, extracting the silicone structure.

Applications: Biomimetic vascular networks, flexible microfluidic chips.

III. Typical Application Examples

Personalized Medicine: Custom 3D-printed silicone ear scaffolds and tracheal models based on patient CT data for preoperative planning or implantation.

Flexible Electronics: Wearable bracelets integrating antennas and strain sensors without assembly.

Soft Robotics: Single-step printing of octopus-like appendages with multi-chamber pneumatic networks, eliminating molds and adhesion steps.

Microfluidic Chips: Directly fabricated integrated chips with valves and mixers using photocurable silicones for point-of-care testing (POCT).

IV. Performance Comparison with Traditional Processes

表格

Metric    Mold-formed LSR      Photocured 3D Print   DIW Print

Tensile Strength  7–9 MPa 5–7 MPa 6–8 MPa

Elongation at Break    600–800%     300–500%     400–700%

Minimum Feature Size       0.3 mm  0.05 mm 0.2 mm

Production Efficiency  High (mass production)     Medium (single piece) Low (prototyping)

Material Cost Low High (special resins)    Medium

Currently, 3D printed silicones are mainly used in high-value-added, small-batch scenarios.

V. Future Directions

Multi-material Co-printing: Simultaneous deposition of conductive silicones, insulating silicones, hydrogels, etc., to build multifunctional systems.

4D Printing: Pre-stressed structures that self-fold into target shapes upon exposure to heat/humidity.

Bio-printing Integration: Embedding living cells within silicone scaffolds for tissue engineering.

Closed-loop Recycling: Development of de-crosslinkable silicones for recycling print waste.

Industry standards are also being established, with ASTM initiating work on "Material Specifications for Silicone Rubbers Used in Additive Manufacturing."

Conclusion

The breakthrough in 3D printing silicone rubber represents not only a revolution in manufacturing processes but also a liberation of design thinking. It frees engineers from constraints imposed by draft angles and parting lines, enables doctors to customize flexible implants for patients, and allows researchers to rapidly iterate prototypes of soft robots. From liquid precursors to elastic finished products, digital instructions impart shape and function to silicone rubber through layer-by-layer stacking. This marks the official entry of flexible materials into the "what you imagine is what you get" era—because the future of softness should not be defined by molds but shaped by creativity.



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