Electrochemical 3D Printing of Copper

Additive manufacturing (AM), also referred attractively as 3D printing, is a well-established process of creating complex 3D geometries through a layer-by-layer building approach. 3D printing has received considerable attention in automotive, aerospace, and biomedical sectors. Direct metal laser sintering (DMLS) is the most commonly used method for 3D printing of metals which involves selective laser sintering of layers of metal powders. However, the high capital cost, defects in the manufactured components and inability to work with multiple materials, limit 3D printing of metals by DMLS and warrant the development of novel non-laser based 3D printing techniques.

Electrochemical additive manufacturing (ECAM) (electrochemical 3D printing) involves deposition of thin and highly adherent layers of metal with a layer-by-layer approach onto a conducting surface through reduction of metal ions from the electrolyte. It combines the basic principles of electrodeposition and 3D printing. Two different approaches, viz., localized electrochemical deposition (LCD) and meniscus confined electrode (MCE), have been used for electrochemical 3D printing of metals. These methods, however, require the use of expensive piezo-based movement stages and nanopippettes/ultrafine electrodes. The slow rate deposition rate of metals (Cu: from ≈100 nm/s to ≈0.18 μm/s), inability to develop components of complex geometries, porosity and roughness are some of the major limitations. Moreover, majority of the structures fabricated using these methods are simple wire-based architectures. Methods for 3D electrochemical printing of metals with improved deposition rates and capable of printing complex structures are warranted. In this perspective, researchers at Dyson School of Design Engineering and Department of Earth Science and Engineering, Imperial College London, UK, have reported a novel strategy for ECAM using a MCE approach.

The electrochemical 3D printer assembly comprised of a plastic syringe and nozzle (diameter 400 μm) with a porous sponge filled with 1 M CuSO₄ was mounted on a carriage and its movement in the x-, y- and z- directions was precisely controlled using computer controlled stepper motors. Two copper rods suspended in the CuSO₄ electrolyte served as the counter and reference electrodes while a copper plate served as the working electrode. The print head was moved to contact the copper plate and retract back by a small amount. The sufficient back pressure provided by the porous sponge to the hydraulic head enabled the formation of a stable meniscus (localized electrolyte). When a positive potential was applied between the working and counter electrodes, deposition of Cu occurs on the working electrode through reduction of Cu²⁺ ions with a simultaneous replenishment of Cu²⁺ ions from the counter electrode. To create 3D printed structures, the print head was moved in x-, y- and z-directions with varying traverse speed as directed by the 3D model. The various components of the electrochemical 3D printer assembly is schematically represented in Figs. 1 (a), 1(b) and 1(c). Optical image of the 3D printed Cu with the shape of the letters “I”, “C”, and “L” using 1 M CuSO₄ at 4 V at a print head speed of 0.4 mm/s is shown in Fig. 1(d).

Fig. 1 Schematic illustration of the electrochemical 3D printer assembly: (a) Print head set-up; (b) electrode arrangement; (c) print nozzle and sponge in the tip, highlighting how they act during the deposition of Cu; and (d) optical images of the printed Cu structures featuring the letters “I”, “C”, and “L” printed using 1 M CuSO₄ at 4 V at a print head speed of 0.4 mm/s.

Electrochemical 3D printing of Cu dots and lines (lateral print head velocity: 0.4 mm/s) were made using 1 M CuSO₄ as the electrolyte at varying voltages from 1 to 6 V for 1 h. The Cu dots printed at 1 V exhibited a dense structure with a high degree of concentricity. In spite of a faster rate of growth along with a dense structure, those printed at 2 V exhibited a convex shape due to preferential deposition at the center. Formation of Cu dendrites becomes apparent at 3V due to the limitations imposed by the mass transport and the a continuous increase in porosity is observed with a further increase in deposition potential (Fig. 2(a)). Hence, achieving dimensionally accurate structures would be difficult using electrochemical printing of Cu dots at potentials beyond 2 V. Irrespective of the deposition potentials in the rage of 1-6 V, electrochemically printed Cu lines fails to exhibit any Cu dendrites since the relative position of the print head aids mass transport of Cu²⁺ ions or might have assisted mechanical removal of the dendrites (Fig. 2(b)). The thickness of the Cu lines is increased from 3 μm at 1 V to 15 μm at 4 V, beyond which it decreased due to the decrease in deposition efficiency. For a given lateral speed of 0.4 mm/s for 3600 s, Cu lines can be printed for 144 passes over a distance of 10 mm.

Fig. 2 SEM images of (a) single Cu dot; and (b) Cu lines (lateral print head speed: 0.4 mm/s) electrochemically printed using 1 M CuSO₄ at 3-6 V for 1 h

The electrochemical 3D printed Cu dots and Cu lines exhibit a higher hardness than cold worked cast Cu. The Vickers hardness of Cu dots varies from 184 to 196 MPa, Cu lines ranging from 211 to 228 MPa when compared to the hardness of cold worked cast Cu, which varies from 50 to 176 MPa; the smaller the grain size, the higher is the hardness. The electrical conductivity of Cu lines is ranging between 1.31×10⁶ and 6.86×10⁶ S/m, which agrees well with that of nanocrystalline Cu (5.41×10⁶ S/m) but relatively lower than coarse grained Cu.

Electrochemical 3D printing of Cu builds a foundation for future work to achieve high speed deposition as well as printing of multi-metal functional structures. The z-height resolution opens up new avenues for the fabrication of functional electronics such as sensors.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Xiaolong Chen et al., A Low Cost Desktop Electrochemical Metal 3D Printer, Adv. Mater. Technol. 2017, 1700148, DOI: 10.1002/admt.201700148

Meta-biomaterials: Combining rational design and additive manufacturing towards the development of next generation medical devices

Meta-biomaterials are part of the emerging concept of metamaterials that possess a desired combination of mechanical  (i.e. negative Poisson’s ratio), mass transport (e.g. permeability and diffusivity) and biological properties (e.g. tissue regeneration performance).

Total hip replacement (THR) implants often encounter mechanical failure at the implant-bone interface (aseptic loosening), which limits their lifetime. The femoral part of THR is repeatedly loaded under bending for ~2 million cycles per year, which creates tensile loading and compression on either side of the neutral axis of the implant. The implant–bone interface is more susceptible to failure when subjected to tension as compared to compression. Since bone exhibits higher mechanical strength in compression than in tension, the side of the THR that experiences tension (i.e. retracts from the bone) is more susceptible to interface failure. Hence, it is necessary to design THR implants in such as way to create compression on both sides of its neutral axis.

Researchers at Delft University of Technology, The Netherlands, 3D Systems, Leuven, Belgium and University Medical Centre Utrecht, The Netherlands have demonstrated a proof-of-concept of applying a combination of rational design and additive manufacturing in the design of meta-biomaterials to improve longevity of implants. (Reference: Helena M. A. Kolken et al., Rationally designed meta-implants: a combination of auxetic and conventional meta-biomaterials, Mater. Horiz., 2017, DOI: 10.1039/C7MH00699C)

Two types of meta-biomaterials, one with a negative Poisson’s ratio (i.e. auxetic) (‘A’ in Fig. 1) while the other one with a positive Poisson’s ratio (i.e. conventional) (‘B’ in Fig. 1) were designed. Subsequently, both types of meta-biomaterials were combined to create a hybrid meta-biomaterial with different values of the Poisson’s ratio (‘C’ in Fig. 1). The meta-implants were then designed using these combined meta-biomaterials, in which the Poisson’s ratio of the meta-biomaterials changed around the neutral axis to compress the implant against the bone on both sides. Totally, six different combinations were designed and they were manufactured by selective laser melting (SLM) using biomedical-grade titanium alloy Ti6Al4V-ELI powders.

Fig. 1 Schematic drawings showing the topological designs of (A) auxetic and (B) conventional meta-biomaterials, (C) hybrid meta-biomaterials (left); and design of meta-implants (right): (C1) control type 1 with conventional hexagonal honeycombs. (H1) Hybrid type 1 with a 50/50 cell ratio. (C2) Control type 2 with re-entrant hexagonal honeycombs, showing the different parts of the implant: (1) top, (2) porous region and (3) bottom. (H2) Hybrid type 2 with a 50/50 cell ratio and a solid core. (H1) Hybrid type 1 showing the different parts of the implant: (1) top-middle-bottom and (2) porous region. (H3) Hybrid type 3 with a 70/30 cell ratio

Fig. 2 shows the photographs of the selective laser melted Ti6Al4V-ELI THR meta-implants (Fig. 2(a)); the test set-up in which the THR implant was loaded including bone-mimicking materials (Fig. 2(b)); and the horizontal strains in the bone-mimicking materials surrounding the meta-implants at t = 0 and t = 180 s at 1.5 mm displacement for C1, C2, H1, H2 and H3 (Fig. 2(c)).

Fig. 2 (a) Additively manufactured (selective laser melting) Ti6Al4V-ELI THR meta-implants; (b) test set-up in which the THR implant was loaded including bone-mimicking materials; and (c) Horizontal strains in the bone-mimicking materials surrounding the meta-implants at t = 0 and t = 180 s at 1.5 mm displacement for C1, C2, H1, H2 and H3.

The findings of the study clearly reveal that meta-implant with design H2 compress against the bone under repetitive loads that are applied during gait and other daily activities. According to the Hoffman’s failure criterion, this combination of compression and shear is less deleterious than tension and shear.

The current proof-of-concept study demonstrated the feasibility of applying rational design and metamaterials for the development of the next generation of medical devices. Nevertheless, the performance of these materials has to be evaluated using animal models and clinical trials.

T.S.N. Sankara Narayanan

For further information, the reader may kindly refer: Helena M. A. Kolken et al., Mater. Horiz., 2017, DOI: 10.1039/C7MH00699C)