I. Research Background: The Challenges of Printing on Flexible Electronic Substrates

Liquid silicone rubber (LSR), with its high flexibility, stretchability, and biocompatibility, is a core material for the substrates of flexible circuit boards in wearable devices. However, conventional molding processes such as injection molding and compression molding suffer from two major drawbacks: limited forming accuracy and difficulty in fabricating complex, integrated structures.

Conventional LSR direct‑extrusion 3D printing still suffers from poor rheological properties, process instability, insufficient mechanical strength, nozzle clogging, and severe deformation of printed layers, all of which hinder the practical fabrication of customized components for flexible electronics and soft robotics.

II. The Core Role of 3D Printing Technology in This Study

This study employs extrusion-based 3D printing as the core process for the integrated fabrication of LSR flexible devices. Leveraging the advantages of layer-by-layer deposition and freeform shaping, it addresses the key limitation of conventional mold-making—namely, the inability to fabricate complex hollow, mesh‑structured, or curved flexible circuit substrates.

• Free-form structural design: No mold is required, enabling direct printing of diverse structures such as 0°/90° cross‑laminated lattice supports, dense thin films, and thin‑walled impact specimens, thereby accommodating the varied application needs of flexible circuits and sensor substrates.
• Process‑coordinated control platform: Leveraging a 3D printing system as the experimental platform, this approach enables simultaneous, precise control of multiple process parameters—including barrel temperature, heating bed thermal history, extrusion pressure, and interlayer waiting time—while quantitatively investigating the influence of the temperature field on silicone rubber curing and print‑path continuity.
• Material performance verification platform: A comprehensive printing test system has been established to quantitatively compare the forming quality of three types of printing inks—pure LSR, PEG‑modified LSR, and SiO₂/PEG composite‑modified LSR—based on metrics such as continuous printing duration, filament width accuracy, extrusion pressure fluctuations, and tensile/impact strength.
• Process–Material Integrated Optimization Platform: Combining orthogonal experiments with the entropy‑weight method for multi‑criteria comprehensive evaluation, it leverages 3D printing process data to identify the optimal filler formulation and elucidates, from a rheological perspective, the fundamental principles underlying the compatibility of modified inks with extrusion‑based printing.

III. Four Major Innovative Breakthroughs in Research

Breakthrough 1: Precise control of the thermal process, locking in the optimal printing temperature window.

The effects of barrel temperature (20–30°C), bed heating, and interlayer curing interval on printing time were systematically investigated, and 25°C was identified as the optimal printing temperature.

• When the temperature is below 20°C, the material exhibits poor flowability, and the lines are severely widened.
• Silicone rubber cures rapidly above 30°C, causing the nozzle to clog prematurely and significantly reducing continuous printing time.
• Comes with a 55°C heated build platform and an 8-second interlayer wait time, enabling layer-by-layer controlled curing and significantly enhancing print stability.

Breakthrough 2: Binary synergistic modification with PEG and SiO₂ simultaneously addresses both molding precision and mechanical performance limitations.

PEG alone only modulates thixotropic recovery, and the interfacial adhesion between pure SiO₂ and silicone rubber is poor. In this study, we innovatively employed PEG as a surface‑activating agent to pre‑coat silica, thereby constructing a composite system featuring hydrogen bonding coupled with a filler network:

• Leap in Forming Accuracy: With composite-modified ink, line-width forming accuracy reaches 98.5%, and the line-width variation rate is only 7.296%, resulting in uniform lines with no spreading or shrinkage; by contrast, pure LSR‑printed lines exhibit significant widening and severe deformation.
• Tensile performance is significantly enhanced: the tensile strength of the composite-modified material is 72.96% higher than that of pure LSR, while the impact strength remains at approximately 6.1 MPa, without compromising the inherent toughness of silicone rubber.
• Enhanced print stability: Extrusion pressure fluctuations are reduced over time, continuous printing duration is significantly extended, and the likelihood of nozzle clogging is greatly diminished.

Breakthrough 3: Orthogonal experiment combined with the entropy weight method for objective multi-objective optimization Than optimization

An L9(3⁴) orthogonal experiment was designed with three factors—LSR, PEG, and SiO₂—each at three levels. The three key evaluation metrics selected were continuous printing duration, tensile strength, and nozzle clogging. The entropy weight method was employed to assign objective weights (thus mitigating subjective biases associated with manual scoring), and a comprehensive evaluation function was constructed to determine the optimal mass ratio of LSR:PEG:SiO₂ = 300:30:1.

Analysis of variance confirmed that the LSR loading has the most significant impact on printability and mechanical properties, followed by SiO₂, while PEG exerts a relatively weaker effect, thereby providing a quantitative basis for standardizing silicone rubber printing ink formulations.

Breakthrough 4: The rheological mechanism has been fully elucidated, revealing the underlying principles governing the printability of modified inks.

The sources of molding advantages are revealed through steady-state shear and three‑region thixotropy tests:

• Pure LSR behaves like a Newtonian fluid, exhibiting no shear thinning and readily flattening upon extrusion.
• PEG/LSR exhibits only weak shear-thinning;
• SiO₂/PEG/LSR is a typical pseudoplastic fluid: its viscosity drops sharply under high shear (during nozzle extrusion), enabling smooth dispensing, while it rapidly recovers at low shear (after deposition), allowing the deposited lines to self‑support without collapsing.

The material exhibits reversible thixotropic recovery: at high strains, the filler‑induced hydrogen‑bonded physical network enables flow; upon unloading, the storage modulus G′ rapidly recovers. This behavior relies on physical interactions rather than irreversible crosslinking, ensuring long‑term printing stability.

IV. Prospects for Application of the Findings and Future Research Directions

(1) Current Real-World Application Scenarios

• Wearable flexible electronics: flexible circuit‑board substrates, stretchable sensing films, and body‑conforming electronic skins; biocompatible modified LSRs designed for long‑term wear on the human body.
• Soft robotics: high‑toughness, high‑precision integrated silicone actuators and bioinspired soft structures; 3D printing enables direct fabrication of complex hollow or lattice‑based actuation architectures.
• Biomedical devices: Biocompatible liquid silicone rubber can be used for customized soft-tissue prostheses and flexible implantable interconnect substrates.
• High-precision sealing / Microstructured components: microfluidic silicone channels and ultra-thin sealing gaskets, enabling stable fabrication of minute features with 98.5% linewidth accuracy.

(II) Future Directions for Research

• In-situ photopolymerization and real-time online monitoring system integration further enhance print‑size stability and part‑quality reliability.
• By integrating machine learning and computational modeling to predict printing parameters, we can reduce forming defects and achieve intelligent process control.
• Expand the multi-material co‑printing system to simultaneously print conductive ink onto a modified LSR substrate, enabling the one‑step fabrication of a complete flexible circuit.
• Deepen the synergistic modification strategy of temperature field and filler, develop a novel silicone rubber ink tailored for high-speed industrial-grade extrusion printing, and advance the industrialization of flexible intelligent manufacturing.

Image and Text Guide

Figure 1. Preparation process of modified materials and 3D printing process

Table 1. Orthogonal Design Table for the Improved LSR

Figure 2. Study on internal and external temperature–assisted curing. (a) Schematic diagram of an extrusion-based 3D printing process employing external temperature–assisted curing. (b) Effect of drum temperature and vulcanization time on printing duration.

Figure 3. Effects of PEG and silica on material modification. Shown are optical schematic diagrams of linear printing outcomes after curing for 40 minutes using silica/PEG/LSR, PEG/LSR, and LSR as printing inks (a–c); (d) SEM images of the cross-section of LSR modified with PEG and silica; (e) extrusion pressure curves as a function of extrusion time for the printed materials; (f) variations in line width of the printed samples; and (g) results of mechanical property measurements.

Figure 4. Printing time (a) and tensile modulus (b) as functions of different factors. (i), (ii), and (iii) correspond to LSR, PEG, and silica, respectively.

Figure 5. Visual Assessment Affected by Different Materials

Table 2. Orthogonal Analysis of Variance

Figure 6. Rheological properties. (a) Representative apparent viscosities of the printing ink, silica/PEG/LSR, PEG/LSR, and LSR; (b) Variation of the storage modulus (G′) and loss modulus (G″) of the silica/PEG/LSR printing ink with shear rate under a three‑step thixotropic cycle (low–high–low: 5%–800%–5%), at a fixed test frequency of 1 Hz, illustrating the ink’s thixotropic behavior.