Planar and Three-Dimensional Printing of Conductive Inks

The aim of this experiment is to demonstrate the planar and three dimensional printing of conductive micro electrodes via direct right assembly. Direct right assembly is a deposition technique, which is composed of a computer controlled three axis translation stage, ink, nozzle, and air supply. In this printing method, highly concentrated inks are extruded through a fine cylindrical nozzle whose diameters range from 0.1 to 250 microns due to the viscoelastic ink characteristics.

Direct right assembly enables self-supporting spanning features. To date, a wide range of inks, including those composed of ceramic, organic metal polymer, and soul gel materials have been developed for this printing approach. The main advantage of this technique of the existing vessel, such as is a direct wide assembly of complex release structure, is faster that using, Although this method provides insight into conductive inks for electronic and optical electronic applications, it can also be applied to other material systems that could be used for biomedical scaffolds and sensors.

Visualizing this procedure is critical because the ink radiology and printing expertise strongly influenced the quality of the printed structures demonstrating this procedure is bhan a postdoc and Annalisa Russo, a PhD student from my laboratory, Silver nanoparticle inks, are prepared by first dissolving a blend of 5, 000 and 50, 000 molecular weight acrylic acid in a mixture of water and di ethanol amine. The polymer acts as a capping agent to control the size of the silver nanoparticles. Next, inject an aqueous silver nitrate solution into the polymer solution.

The mixture will become a transparent light yellow, continue stirring for 24 hours at room temperature. After stirring, the solution develops a reddish brown color. This is due to formation of five nanometer diameter silver nanoparticles, which can be confirmed using TEM to further particle growth.

Sonicate the solution in a 65 degree Celsius water bath for two hours after sonication transfer the solution to a 500 milliliter beaker to call it to room temperature, once called titrate 300 milliliters of ethanol into the solution. Because ethanol is a poor solvent for the capping agents, the particles rapidly coagulate and precipitate now decant the supena and collect the precipitate in a centrifuge tube. Centrifuge the precipitate at 9, 000 RPM for 20 minutes.

The result will be a highly concentrated silver nanoparticle ink with about 85%solids by weight. For further control over ink viscosity and elastic modulus dilute the ink into a humectant solution such as ethylene glycol, then homogenize the solution at 2000 RPM for three minutes using a thinky homogenization mixer. This dilution and homogenization process makes a uniform bluish to magenta ink.

The TEM image shows silver nanoparticles obtained by this synthesis procedure. The particles have a mean diameter of 20 nanometers with a size distribution of five to 15 nanometers. Printed structures require poster kneeling to enhance their conductivity.

After an kneeling at 250 degrees Celsius for less than 30 minutes, the silver nanoparticles form conductive micro electrodes with an electrical resistivity approaching 10 to the minus five ohm centimeters. The micro structural evolution of the printed silver micro electrodes as a function of a kneeling temperature is measured by SEM. As the temperature increases from 150 degrees to 550 degrees Celsius, the micro electrodes undergo densification with a total volumetric shrinkage of about 30%The ink ology, which strongly depends on its solid loading, determines its printability.

The ink viscosity increases with increasing solid loading because diluting with low viscosity result in a significant lateral spreading concentrated inks. With the solids loading ranging from 70 to 85, weight percent are required for printing of planar and spanning ink filaments. The ink elastic modulus increases with increasing solids loading in the linear viscoelastic region.

The elastic modulus rises nearly three orders of magnitude. As the solids loading increases 60 to 75 weight percent. A minimum elastic modulus of 2000 pascals is required to produce self-supporting or spanning features.

To begin direct right assembly, load the ink into a syringe barrel. After attaching a deposition, nozzle mount the ink loaded syringe barrel to the three axis printing stage. Use a computer program to generate the design of interest, which could be a linear planar or complex three dimensional structure.

Next, the nossel height is adjusted with the aid of a telescope lens with a 10 times zoom. After applying pressure with an air powered fluid dispensing system, deposit the ink onto the substrate at room temperature using a controlled print in speed and pressure. These variables depend on the ink, radiology, nozzle, diameter, and printing speed.

A five micron nozzle can print conductive silver grids with a center to center line spacing of 100 microns on a silicon wafer substrate. Using a 30 micron nozzle and layer by layer printing can make a high aspect ratio Cylindrical structure. Omnidirectional printing of silver micro electrodes between two glass substrates is possible using a 30 micron nozzle.

The glass substrates are offset by one millimeter complete freestanding vertically printed silver micro spikes can be created using a 30 micron nozzle on a silicone wafer substrate. Direct writing of a spanning silver micro electrodes using a 10 micron nozzle is also possible. The printed feature can span distances up to one centimeter with minimal drooping or buckling.

Highly concentrated silver nanoparticle ink was printed as conductive features in planar and 3D motifs for electronic and optoelectronic applications. The printing resolution was between two and 30 microns. A minimum electrode width of about two microns and a thickness of 1.4 microns was obtained in a single pass using a one micron nozzle, a five micron nozzle pattern.

Transparent conductive silver grids on a flexible polyamide film. The text underneath the grid remained clearly visible. These metallic grids may serve as high conductivity alternatives to transparent conductive oxide materials.

Conformal printing of a 3D electrically small antenna was possible with a 100 micron metal nozzle meander. Line patterns were printed on the surface of a glass hemisphere. This approach may find several applications, including implantable and wearable antennas, electronics and sensors, applications of spanning silver micro electrodes in three dimensional photo volta and light emitting diodes are also possible.

A tenuous spherical silicon shell with a two micron thickness was wire bonded, an external circuit by omnidirectional printing. This method uses minimal contact pressure, which is highly advantageous for delicate devices. Also printed was a spanning interconnect for a silicon solar micro cell array in which the silicon micro ribbon elements are separated by a 33 micron gap.

Silver interconnects for gallium arsenide based LED array with four by four pixels were printed. The LED array emitted uniform red light under an applied bias of six volts from a single pixel. The ability to print spanning electrodes enabled multilayer interconnection without the use of supporting or sacrificial layers.

As a final demonstration, a complex 3D micro periodic silver lattice is printed by a five micron nozzle using layer by layer printing sequence. Once mastered, this technique can be done in a couple of hours that performed properly. Using this procedure, we have demonstrated ink synthesis printing of 1D to 3D structures and applications that include printed electrodes for electronic and optoelectronic devices.

Ongoing work in our laboratory is now focused on extending this approach to biomedical applications using biocompatible inks. We are printing three dimensional scaffolds and microvascular networks for tissue engineering and cell culture. After watching this video, we hope you have a better appreciation for our direct right assembly technique.