This study introduces an innovative method for creating 4D metal structures using multi-metal Electrochemical Additive Manufacturing (ECAM). Building upon the foundation of low-cost desktop electrochemical 3D printing pioneered by Chen et al., our research delves into the dynamic motion of printed copper-nickel bimetallic strips, influenced by factors such as layer thickness, temperature, and deposition positions. We present a comprehensive analysis of the metallic properties of these structures, including electrical conductivity and surface morphology, utilizing Scanning Electron Microscopy (SEM) and X-ray Computed Tomography (XCT).
Rig Design for Advanced 4D Printer Capabilities
Figure 1 illustrates the design of our cost-effective electrochemical multi-metal 3D printer, adapted from a commercial FDM 3D printer to enable sophisticated 4D printing. The modified setup features a stage equipped with two in-line plastic syringes, spaced 80 mm apart. One syringe dispenses a copper sulfate electrolyte (visualized as blue), while the other contains a nickel sulfate electrolyte (green).
Figure 1: Illustration of the low-cost electrochemical multi-metal 3D printer designed for 4D printing applications, showing the front view, print head setup, and detailed deposition nozzles.
Within the copper sulfate syringe assembly, two copper wires are inserted into the electrolyte, serving as a counter electrode and a reference electrode. To ensure precise electrode spacing, the reference electrode is attached to an insulated holder. The counter electrode wire is coiled around the insulated rod to maximize surface area. Sense and working electrode probes from a potentiostat are connected to a copper substrate platform, the site of metal deposition. The nickel solution syringe setup mirrors the copper setup, but employs nickel foams and wires as reference and counter electrodes respectively. The deposition unit allows for movement along the X and Y axes, while the deposition platform moves along the Z axis, all controlled by computer-driven stepper motors for accurate 4d Printer operation.
Fabrication and Characterization of Bimetallic Strips for 4D Structures
The process for fabricating a copper-nickel bimetallic strip, crucial for achieving 4D functionalities, is outlined in Figure 2. In this process, one syringe is active for deposition, while the other remains empty to prevent electrolyte cross-contamination. Initially, a copper layer is deposited using an aqueous copper sulfate electrolyte (Figure 2a). A stable electrolyte meniscus is maintained between the nozzle and the substrate by carefully balancing the electrolyte’s hydraulic head with its surface tension and the back pressure from a porous medium within the print nozzle.
Figure 2: Schematic illustration detailing the multi-material 3D printing process for 4D metal structures, including meniscus confined copper and nickel electrodeposition.
A potentiostat applies a constant potential, reducing Cu2+ ions in the electrolyte to metallic copper on the substrate. Simultaneously, the consumed Cu2+ ions are replenished via oxidation at the counter electrode, maintaining concentration but creating a gradient in the solution. This gradient influences current density and the morphology of the printed copper. While previous research utilized sponges for back pressure to stabilize the meniscus, our approach uses an electrospun nanofiber nib. This offers superior back pressure and minimizes diffusion resistance, evidenced by a 34% increase in current density during copper deposition (from 480 mA.cm−2 to 640 mA.cm−2 at 5 V vs Cu with 1 M copper sulfate solution). The print head’s movement precisely controls the deposition location in this 4D printer.
Following copper deposition, the copper electrolyte is replaced with a nickel electrolyte for depositing a nickel layer on top (Figure 2b). The setup remains consistent, but with nickel electrodes and electrolyte. The potentiostat again applies a constant potential to reduce Ni2+ ions to metallic nickel onto the copper layer. The print head follows a defined path, depositing nickel where the meniscus and potential are applied, with repeated passes increasing layer thickness. The nanofiber nib again proves superior, yielding an 85% current density increase for nickel deposition (from 100 mA.cm−2 to 190 mA.cm−2 at 2 V vs Ni). SEM micrographs and optical images of the nanofibers are available in Supplementary Materials (Figure S1).
Figure 3 presents optical images and SEM micrographs of printed copper-nickel bimetallic strips at varying nickel deposition times, alongside Energy Dispersive X-ray Spectroscopy (EDS) line scans. A consistent lateral print head velocity of 0.4 mm.s−1 over 20 mm, and deposition potentials of 5 V vs Cu and 2 V vs Ni for copper and nickel respectively, were used. A 3-hour copper base layer deposition (~45 μm thickness after 216 passes, ~200 nm resolution) was used in all samples. Morphological changes are observed with nickel deposition times of 1, 3, and 5 hours (72, 216, and 360 passes, ~600 nm, 1 μm, and 2 μm nickel layer thickness, ~5–8 nm resolution). Nickel’s finer layer resolution is attributed to its lower reaction current density and slower deposition kinetics compared to copper. SEM and EDS scans confirm a tight, clear interface between layers, both exhibiting polycrystalline, often nanocrystalline, morphology. Lower magnification images reveal a convex shape in printed strips, caused by higher current density at the deposition nozzle center, a characteristic of this 4D printer’s output.
Figure 3: Visual and microscopic analysis of copper-nickel bimetallic strips created with the 4D printer, showing optical and SEM cross-section micrographs at different nickel deposition times with EDS analysis.
Thermo-Mechanical Properties Enabling 4D Motion
To explore the thermo-mechanical properties of these printed bimetallic strips, crucial for 4D functionality, samples were placed on a heated bed, fixed at one end, and free to move at the other. An optical camera recorded displacement as temperature increased from room temperature to 300 °C in 50 °C increments. Measurements were repeated thrice to ensure reliability. Figure 4 shows sample deformation at different temperatures and copper-nickel layer configurations.
Figure 4: Thermo-mechanical response of 4D printed copper-nickel structures, illustrating deformation under different heating conditions and configurations, including programmed shapes.
In all cases, temperature increase induced mechanical deformation due to the differing thermal expansion coefficients of copper (16 × 10−6 °C−1) and nickel (13 × 10−6 °C−1). The tight bond between layers creates a zero displacement boundary at the interface, generating internal stresses. Copper’s higher thermal expansion coefficient results in curvature, with tensile stress on the copper side and compressive stress on the nickel side. A bimetallic strip with a 3-hour copper and 5-hour nickel deposition (~45 µm and ~2 µm respectively) exhibited a maximum deformation of 40° at 300 °C when heated perpendicularly. Deformation angle was measured by image binarization and circle fitting in MATLAB.
Limiting nickel deposition to a central 6 mm region (Figure 4b) resulted in curvature only in the copper-nickel region when heated nickel-side down. Pure copper ends remained parallel to the heating bed due to absent bending stresses and pressure from the nickel-copper region.
A trilayer (Cu-Ni-Cu) configuration (Figure 4c) balanced thermal stresses in the Cu-Ni bilayer, preventing bending and enabling “L-shaped” geometries, allowing for programmable combinations of linear and curved structures. Using these, the letters “ICL” were formed upon heating a linear bimetallic sample to 300 °C (Figure 4d), showcasing the 4D printer’s ability to create complex, shape-changing structures.
Key design variables influencing bimetallic strip curvature (κ) include layer thicknesses, Young’s modulus, and thermal expansion coefficients, as described by Clyne and Gill’s equation:
$$kappa =frac{6{E}_{1}{E}_{2}({h}_{1}+{h}_{2}){h}_{1}{h}_{2}({{rm{alpha }}}_{1}-{{rm{alpha }}}_{2}){rm{Delta }}T}{{E}_{1}^{2}{h}_{1}^{4}+4{E}_{1}{E}_{2}{h}_{1}^{3}{h}_{2}+6{E}_{1}{E}_{2}{h}_{1}^{2}{h}_{2}^{2}+4{E}_{1}{E}_{2}{h}_{2}^{3}{h}_{1}+{E}_{2}^{2}{h}_{2}^{4}}$$
(1)
Figure 5a shows bending angles of 20 mm Cu-Ni strips with a 3-hour copper base and varying nickel layer thicknesses across temperatures up to 300 °C. Bending angle increases non-linearly with nickel layer thickness and temperature. Theoretical deflections based on Equation 1 (Figure 5b) show a similar linear trend but with lower absolute deflection angles, likely due to thickness variations from uneven current density during printing, causing the convex cross-section.
Figure 5: Performance characteristics of 4D printed structures, including bending angles, XCT reconstructions, interface analysis, displacement measurements, electrical conductivity, and a demonstration of circuit actuation.
XCT was used to investigate the 3D morphology (Figure 5c). It reveals a maintained convex cross-section along the copper phase, with edge plating. Increased nickel plating duration results in nickel nodules and uneven coating, likely contributing to the non-linear temperature-deflection behavior (Figure 5a).
SEM and EDS mapping confirm tight adhesion in the trilayer (Cu-Ni-Cu) configuration (Figure 5d). “L-shaped” deflection is achievable (Figure 4c), and Figure 5e demonstrates deflection control by varying the gap width in Cu-Ni-Cu trilayers, with deflection increasing with gap distance. Finite Element Analysis (FEA) of the trilayer configuration (Figure 5e) validates the observed deformation.
Actuation is also achievable through ohmic heating due to the metallic nature of the strips. Figure 5f shows electrical conductivity, ranging from 1.4 × 106 S.m−1 to 5.5 × 106 S.m−1, lower than single-line copper (6.86 × 106 S.m−1) but comparable to nanocrystalline copper (5.4 × 106 S.m−1). This is attributed to nanocrystalline nickel’s lower conductivity (8.2 × 105 S.m−1). Conductivity decreases with increased nickel deposition time, converging towards nickel’s conductivity.
Finally, a simple circuit demonstration (Figure 5g) showcases the application in high-temperature environments. At 300 °C, the Cu-Ni bimetallic strip bent, closing the circuit and powering an LED. This highlights the potential of this 4D printing technique for creating environment-sensing structures and enabling smarter 3D printed devices.
