Performance Evaluation of Co-Extruded 4D-Printed Wire–Polymer Composites with Programmable Shape and Color Response

This work presents the design, fabrication, and functional evaluation of two types of continuous wire–reinforced thermoplastic composites developed through a customized 4D-printing workflow. Using a multi-material co-extrusion process, the composites were programmed to exhibit controlled, time-dependent responses triggered by internal electrical stimuli.

Two material systems were studied: resistance wire embedded in thermochromic PLA for electrically induced color change, and a thermally active metal wire embedded in thermochromic TPU for coupled shape and color transformations via Joule heating. Color change is driven by embedded thermochromic pigments activated by resistive heating, while shape transformation is enabled by the programmed thermal behavior of the embedded wire, with flexibility of TPU accommodating reversible deformation.

A custom Rhino/Grasshopper (Geometric manipulation) → NanoGcode (Wire Path G-code generator) → Aura (Slicer, Matrix G-code generator) → Sublime (Text editor, G-code merging) workflow enabled precise co-extrusion printing on an Anisoprint A4. To evaluate actuation performance, a camera captured activation sequences at 25 fps. Activation was achieved through Joule heating via Arduino-controlled relays. Custom MATLAB scripts were employed to extract time-resolved shape profiles and color variation from the video data.

To prevent thermal degradation of the polymer due to localized overheating, the wire composites were tested in a water medium, which facilitated faster heat dissipation from the wire. The printed composites demonstrated repeatable and tunable actuation behaviors, with distinct shape recovery and visible color transitions under controlled electrical input.

The thermochromic PLA composites exhibited rapid, repeatable color switching: rectangular test specimens (50 × 10 × 1.5 mm) reached 80% of complete color change in ~78.5 s of Joule heating in air medium. When submerged in water, rapid heat loss limited peak color change at 1.75 A to 28.9%, and over a 0.5–1.75 A current range the color-change index scaled with the square of current (r ≈ 0.99), plateauing at ~30% beyond 1.25 A. A shorter pulse duration at higher currents led to more predictable color transformations compared to constant currents under similar energy conditions.

Similarly, shape recovery performance (bending recovery) of the smart wire–embedded TPU improved progressively with thermal training: from 16% after the first 10 cycles to 20% and 24% after 25 and 40 cycles, respectively. These results suggest that gradual thermal conditioning of composites can significantly improve actuation range and reliability, with the color response offering a valuable diagnostic tool during each stage.

The authors believe that these training sessions and improvements in specimen design could be substantially optimized, further improving the application of the composites. Future work will explore the integration of self-sensing capabilities using the embedded wires to enable real-time feedback and adaptive control. Additionally, upcoming studies aim to assess long-term durability, thermal fatigue, and the effects of cyclic loading on actuation stability. Understanding the relationship between training protocols, thermal cycling conditions, and material fatigue will be essential to translate these composites into reliable, deployable systems.

This approach opens new opportunities for smart actuators, electro-activated displays, and tunable soft robotics.