Electronics 3D Printing Part 4: Research Toward the Future

What gets developed in university and corporate labs often defines the next generation of a given technology. While we have covered two of the most established methods for 3D printing electronics, direct writing and inkjetting, researchers are currently paving the future for fabricating 3D-printed electronic parts.

One of the areas with the greatest interest is flexible circuits, given the potential to incorporate electronic devices into clothing and other non-flat objects. A team from Tsinghua University researchers and the Technical Institute of Physics and Chemistry at the Chinese Academy of Sciences (TIPCCAS) has used a material extrusion approach to 3D print hollow, elastomeric channels before injecting those channels with liquid metal to create electrical traces. The authors of the study were able to create a probe signal circuit on a flexible substrate.

Graphical abstract of the research paper, visually describing the injection of liquid metal into hollow channels. Image courtesy of Science China.

Researchers from the UC San Diego Jacobs School of Engineering demonstrated the extent stretchable electronics can reach by designing stretchable biofuel cells that extract energy from human sweat. The cells featured an enzyme that oxidizes lactic acid in sweat to generate an electric current.

Researchers showed that the flexible biofuel cell could power LEDs and a Bluetooth radio. Image courtesy of Energy & Environmental Science.

To demonstrate the applications, the team used 3D printing, along with lithography and screen printing, to create carbon nanotube-based cathode and anode arrays. A 3D carbon nanotube structure was screen printed atop the biofuel cell’s anodes and cathodes, allowing for the addition of more of the sweat-reactive enzyme, increasing the overall energy capacity and transfer of the printed object. The biofuel cell was then connected to a circuit board, which was worn by four test subjects, who were able to power a blue LED for four minutes by riding a stationary exercise bike.

Developing new inks is important for diversifying the existing portfolio of materials available with electronics 3D printing. Most conductive inks are naturally limited to metal materials; however, one team has concocted a conductive polymer bioink for use with direct writing technologies. Made up of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), the ink was used to create a series of polymer meshes that displayed high electrical conductivity. The team was able to produce over 100 circuit patterns in less than 30 minutes. One demonstrator application involved the fabrication of a neural probe that was inserted into the skull of a lab rat.

a Sequential snapshots for 3D printing of high-density flexible electronic circuit patterns by the conducting polymer ink. b Lighting up of LED on the 3D-printed conducting polymer circuit. PETE indicates polyethylene terephthalate. c Bending of the 3D-printed conducting polymer circuit without failure. d Image of the 3D-printed soft neural probe with 9-channels by the conducting polymer ink and the PDMS ink. e Image of the 3D-printed soft neural probe in magnified view. f Images of the implanted 3D-printed soft neural probe (top) and a freely moving mouse with the implanted probe (bottom). g, h Representative electrophysiological recordings in the mouse dHPC by the 3D-printed soft neural probe. Local field potential (LFP) traces (0.5 to 250 Hz) under freely moving conditions (g). Continuous extracellular action potential (AP) traces (300 to 40 kHz) recorded under freely moving conditions (h). i Principal component analysis of the recorded single-unit potentials from (h). j Average two units spike waveforms recorded over time corresponding to clusters in (i). Scale bars, 5 mm (a–c); 1 mm (d, e); 2 mm (f). Image courtesy of Nature Communications.

MIT researchers developed multimaterial filament for use on a simple desktop 3D printer to produce optoelectronics. The study’s authors created a range of multi-material filaments using polycarbonate (PC) and cyclic olefin copolymer (COC) as a thermoplastic matrix to connect with conductive materials.

In some cases, conducting polyethylene (CPE) and arsenic-selenide (As2Se5) was blended with the plastics during the initial drawing of the filaments, while in other cases bismuth-tin (BiSn) spheres with tungsten (W) wire and Zinc-sulphide (ZnS) were applied in post-processing or during the printing process itself. The team was able to create a light emitting filament made up of a BiSn core, electrically-conductive W, electroluminescent ZnS, and insulated with PC, with COC used to create bonds between layers.

From the study: “Three-dimensional printed displays and sensors. a Photograph of a filament dotted with 90 pixelated light emitters. The inset shows the cylinder printed from this filament, which is capable of b, c displaying electrically-activated stripe patterns all around its body. The inset of b shows the desired light design. d A patterned vase, designed as stacked layers of serpentines, is printed from the light-detecting filament and is capable of e detecting light and producing photocurrent from its entire structure when impinged with light. The design file for the patterned vase is from Hakalan at Scripted Vases (https://www.thingiverse.com/thing:104694) under the CC BY 3.0 license: https://creativecommons.org/licenses/by/3.0/. f A printed sphere with the ability for omnidirectional localized-sensing anywhere on its surface. To test its detecting accuracy, a low-power laser pointer shines at g different points (1 and 2) on the sphere, producing distinct current ratios (i1/i2) which allows for h exact imaging and reconstruction of the laser spots. All connections for the light-detecting macrostructures are made across opposite CPE electrodes.” Image courtesy of Nature Communications.

The process for creating the filament was an involved one, but, in the end, the study’s authors were able to print the material on a low-cost RoVa 3D fused filament fabrication system. In addition to creating light emitting filament, which was used to create 3D-printed objects with surfaces that could display pixelated light, the team also made light detecting filament.

The Singapore Centre for 3D Printing at Nanyang Technological University has described some of the developments occurring with regard to inks used in electronics 3D printing, pointing out that electrically conductive composite inks are becoming increasingly popular. These include a material made up of “carboxyl-terminated silver nanoparticles, silver flakes, amine-functionalized carbon nanotubes, and thermoplastic polystyrene–polyisoprene–polystyrene.” These composite inks make it possible to build electrically conductive 3D structures, while overcoming some of the difficulties with the metallic nanoparticle inks commonly used in electronics 3D printing. These difficulties include the high cost of silver nanoparticle inks, the high sintering temperatures needed, problems with stretchability, and geometric repeatability.

Carbon nanotubes are of particular interest for their combination of low weight, high strength and good electrical and thermal conductivity, as well as optical anisotropy. The same Singapore Centre for 3D Printing has described in detail how carbon nanotubes are currently explored in 3D printing research, including the various 3D printing technologies and applications. Among the applications are the shrinking of electronics and creation of stretchable wearables. Researchers, however, are working to overcome the issue of properly aligning this anisotropic material while controlling its diameter and post-processing techniques that don’t damage the conductivity of the carbon nanotubes.

In upcoming installments, we’ll be taking a look at the materials involved in electronics 3D printing, as well as low-cost methods for printing electronics on the desktop.