3D printable tough silicone double networks

The study addresses a core limitation in stereolithography (SLA) and other additive manufacturing methods for soft devices: the scarcity of printable elastomers that are both soft and tough, and the difficulty of bonding printed elastomers to dissimilar materials. The authors propose silicone double networks (SiDNs) that form two orthogonal networks: a fast photocured thiol–ene network that provides shape fixity during printing (“green body”), and a slower room-temperature condensation network that subsequently forms to dictate final mechanical properties and enable interfacial bonding. The hypothesis is that decoupling processing (photocure) from final mechanics (condensation) enables independently tunable printability and performance, yielding soft, tough, tear-resistant silicones that can cohesively bond to a wide range of substrates for multi-material assemblies relevant to medical simulators, wearables, and soft robotics.

Prior 3D-printable silicones and elastomers typically rely on single-network chemistries, where increasing crosslink density to gain strength and toughness also raises modulus, sacrificing softness. Existing two-stage resins usually form a single polymer network whose second-stage polymerization stiffens and embrittles the material. Double-network (DN) strategies in hydrogels have achieved large toughness by combining networks with complementary mechanics. In SLA silicones and commercial SLA urethanes, there is a trade-off between softness and toughness; similarly soft SLA materials (E ~100–670 kPa) generally have much lower toughness (~0.1 MJ·m−3) than desired. The authors build on click-chemistry stereolithography and DN concepts by creating orthogonal silicone networks to maintain low effective crosslink density while boosting toughness and tear resistance. The work situates itself among prior advances in soft robotic printing, medical simulators, and bonding strategies that introduce condensable groups or surface treatments to bond dissimilar polymers.

Materials and resin preparation: The SiDN resin combines a photocurable thiol–ene silicone network with a tin-catalyzed condensation-cured silicone (MM10T, MM14NV, MM29NV, or MM40). The thiol–ene resin comprises 61.7 wt% vinyl-terminated PDMS (Mw ~6000) and 38.3 wt% (4–6% mercaptopropyl)methylsiloxane–dimethylsiloxane copolymer at 1:1 vinyl:thiol stoichiometry, with 1% w/v photoinitiator (Speedcure 2022). Two-part condensation silicones are mixed per manufacturer instructions, then blended with the thiol–ene resin at defined mass fractions using a centrifugal mixer.

Printing process: SLA on a modified bottom-up desktop printer (Autodesk Ember) with a 405 nm LED projector (J = 20 mW cm−2) and PMP window; a wiper blade improves layer reliability. Optional dye (Sudan I) can improve z-resolution. Green bodies are UV-cured, rinsed briefly in isopropyl alcohol, then allowed to cure at room temperature for the condensation reaction, with an additional 2 h at 65 °C to complete curing when specified.

Photorheology and kinetics: Rheology (DHR3) with photo-curing accessory monitors G′, G″, and viscosity under oscillatory shear (2 Hz, γ=1%) with illumination (400–500 nm, J=10 mW cm−2). FTIR tracks condensation conversion via alkoxy peak decrease (2855–2825 cm−1). Photo-DSC quantifies thiol–ene conversion by integrating photopolymerization exotherms during 1 min illumination pulses (400–500 nm) and normalizing by heat of reaction.

Mechanical testing: Tensile specimens are cast (3 mm sheets) then punched (ASTM die C). Green body and fully cured samples are tested (Instron 5943) at 75 mm min−1, E calculated over 5–100% strain. Tear testing uses ASTM Die B at 500 mm min−1. Print anisotropy is assessed by varying build orientation and comparing to molded controls.

Bonding and peel tests: For 90° peel tests (ASTM D3135), SiDN substrates are photocured to green bodies, aligned, and bonded via the latent condensation reaction (>12 h RT plus 2 h at 65 °C). Diverse substrates (glass, aluminum, PP, PE, PET, TPU, polyamide textiles, hydrogels, platinum-cured silicones, rigid polyurethane, nylon, urethane acrylates, silicone urethane, other SiDNs) are prepared via innate condensable groups or by surface modification (oxidative plasma and silane coatings such as poly-vinyl methoxy siloxane; or specific silanes for aluminum/glass). Peel at 300 mm min−1; bond strength is maximum force normalized by width (25 mm).

Applications and durability: Printed organ phantom (infant-scale heart) from 64%MM10T demonstrates surgical manipulation (injection, incision, suturing). Multi-material mechanical gradients are assembled from different SiDNs and rigid segments (RPU) to study strain localization and failure loads. Wearable integration bonds actuators to a polyamide/elastane textile; laundering durability tested following AATC TM 124 (hot wash, detergent, defined agitation), then peel-tested. A bellows actuator is bonded to a flexible PCB (Kapton) with LEDs to demonstrate low-temperature integration and actuation durability.

Data analysis: Rheological gelation dose, viscosity vs composition; FTIR and photo-DSC conversions; tensile/tear properties (N≥7 per material); peel strength and failure modes across substrates (N≥7).

• Orthogonal double-network formation: Rapid thiol–ene photocure yields a printable green body; a latent condensation reaction proceeds in the dark at room temperature to form a second network. In situ measurements show ~94% thiol group conversion at H ≈ 600 mJ cm−2 during green-body formation, and subsequent -OR consumption correlates with a second plateau in G′/G″.
• Printability window and viscosity: Blends remain homogeneous due to shared PDMS backbones. Viscosity decreases with increasing thiol–ene fraction; even at ~85 wt% condensation network, combined viscosity stays <5 Pa·s, suitable for SLA. Above ~85 wt% condensation, resins become unprintable due to inability of the thiol–ene network to percolate (gelation dose >1600 mJ cm−2).
• Mechanical performance gains: Increasing condensation fraction weakens the green body but strongly strengthens the final part. For 82%MM10T, ultimate strength increases from ~0.008 MPa (green body) to ~0.92 MPa (final). Ultimate elongation increases 12-fold compared to pure thiol–ene: from ~0.33 to ~4.11 strain. Across toughest printable blends from each condensation base (MM10T, MM14NV, MM29NV, MM40), toughness U ≈ 0.92–1.37 MJ·m−3 and strength σ ≈ 0.92–1.54 MPa, with low modulus (E100% in the 100–670 kPa range depending on composition).
• Tear resistance and isotropy: Normalized tear strengths are high, exceeding those of commercial SLA polyurethane elastomers in some cases, while maintaining low modulus. Mechanical properties show little dependence on print orientation or layer height, suggesting interlayer condensation crosslinking.
• Medical simulator: A 64%MM10T printed hollow heart (250 µm layers) closely matches target geometry and has E100% ~100 kPa (cardiac-like). Thin walls withstand suturing; estimated tear resistance exceeds 2.5 kN·m−1 based on ~5 N reactive force in 2 mm walls.
• Robust bonding to diverse substrates: The latent condensation enables cohesive bonding to metals, glass, thermoplastics (PP, PE, PET, TPU), hydrogels (polyacrylamide), platinum-cured silicones (Ecoflex 0020, Sylgard 184), rigid polymers (RPU, nylon, urethane acrylates), silicone urethane, and other SiDNs. Peel tests predominantly show cohesive failure; bond strengths are limited by SiDN strength (≈800 N·m−1). Bonding persists after 2 months aging at ambient conditions.
• Textile integration and laundering: SiDN-to-polyamide textile bonds show Γ ≥ 800 N·m−1 initially; after repeated home laundering (10 cycles, hot wash), bond strength decreases to ~550 N·m−1 but remains cohesive.
• Electronics integration: Low-temperature bonding to a flexible PCB (Kapton) with LEDs preserves function; the PCB serves as a strain-limiting layer directing actuator bending. Bonds endure large actuation strains and repeated cycles; overpressurization ruptures the membrane before the interface, and destructive tests show cohesive failure in SiDN.
• Mechanical gradients: Assemblies with steep modulus gradients localize strain and fail before 16 N, while shallow gradients distribute strain and sustain >30 N, enabling programmed mechanical response.

The results validate that orthogonal double-network silicone chemistry decouples processing from performance: the thiol–ene photocure provides rapid shape formation and green strength for SLA, while the slow condensation reaction determines final mechanical properties and creates interfacial bonds. This strategy circumvents the traditional trade-off in single-network SLA elastomers, achieving low modulus with high toughness, elongation, and tear resistance. The intrinsic compatibility of the condensation reaction with diverse condensable groups (native or surface-induced) enables cohesive bonding to a wide variety of materials spanning >7 orders of magnitude in modulus, facilitating multi-material assemblies with controlled mechanical gradients. Demonstrations in surgical simulation, textiles, and PCB integration show practical relevance for medical training models, wearable haptics/assistive devices, and soft robotic systems. The negligible anisotropy suggests effective crosslinking across layers, supporting reliable mechanical performance in printed parts.

This work introduces SLA-printable silicone double networks that independently control printability and final properties via orthogonal thiol–ene and condensation reactions. The materials achieve low modulus (E100% < 700 kPa) alongside high toughness (~1.4 MJ·m−3), strength (~1 MPa), and extensibility (up to ~400% strain), with high tear resistance and minimal print-induced anisotropy. A key contribution is latent, room-temperature reactive bonding that enables cohesive adhesion to diverse substrates (metals, ceramics, thermoplastics, hydrogels, silicones, textiles, PCBs), allowing multi-material assemblies and mechanical gradients. Application demonstrations include cardiac phantoms, wearable actuators bonded to textiles surviving laundering, and actuator–PCB integrations. Future directions include: accelerating or externally controlling the condensation reaction using photo-latent catalysts for on-demand bonding or concurrent DN formation during printing; exploring stiffer photocurable networks combined with softer condensation networks to further boost toughness and green strength; identifying alternative photocurable silicone chemistries with suitable photorheology for DN printing; and employing advanced condensation networks (e.g., self-healing) to add functionality.

• Manufacturing time is extended by the slow, room-temperature condensation reaction (hours), impacting throughput and assembly windows; faster condensation would reduce pot life and bonding window.
• Printability constraints: above ~85 wt% condensation network, the thiol–ene phase cannot percolate sufficiently for gelation at reasonable doses, limiting composition space.
• Green body strength decreases with increasing condensation fraction, potentially affecting handling during printing and post-processing.
• Some substrates require surface modification or elevated thermal post-treatments to introduce condensable groups, which may complicate workflows; high-temperature requirements for certain third-party materials can weaken SiDNs.
• While tissue-like modulus can be matched, complex tissue features (anisotropy, fiber orientation) are not replicated.
• Textile bond strength decreases after laundering (though remains cohesive), indicating some durability loss under harsh consumer use conditions.