This material is based on work supported by the U.S. Department of Energy, Materials Sciences and Engineering Division (Award No. DEFG-02-07ER46471) and the DOE Energy Research Center on Light-Materials Interactions in Energy Conversion (Award No. DE-SC0001293), and benefitted from access to the Center for Microanalysis of Materials within the Frederick Seitz Materials Research Laboratory (FSMRL).

1. Introduction
<ol>
	<li>This paper demonstrates the 1D, 2D, and 3D printing of conductive microelectrodes via direct-write assembly.</li>
	<li>Direct-write assembly is a method of building up 1D-to-3D printed structures by the deposition of concentrated inks through fine nozzles.</li>
	<li>Our system consists of a computer-controlled 3-axis translation stage, an ink reservoir and nozzle, and 10x telescopic lens for imaging (Figure 1).</li>
	<li>Direct-write assembly is a filamentary printing approach in which concentrated inks are extruded through cylindrical nozzles, whose diameters range from 0.1 to 250 μm (Figure 2). Notably, due to the viscoelastic ink characteristics, direct-write assembly enables self-supporting spanning features (Figure 3). To date, a wide range of inks, including those composed of ceramic6,7, organic8-10, metallic11-15, polymeric16,17, and sol-gel18,19 materials have been developed for this printing approach (Figure 4).</li>
</ol>
2. Preparation of highly concentrated silver nanoparticle inks
<ol>
	<li>Silver nanoparticle inks are prepared by first dissolving a blend of 5,000 and 50,000 molecular weight of poly(acrylic acid) in a mixture of 50 g water and 40 g diethanolamine (Video 2.1).</li>
	<li>The polymer acts as a capping agent to control the size of the silver nanoparticles.</li>
	<li>Next, an aqueous solution of silver nitrate is injected into the polymer solution. After addition, a light yellow transparent solution is obtained (Video 2.2).</li>
	<li>After stirring for 24 hrs at room temperature, the solution develops a reddish-brown color (Video 2.3), that coincides with the formation of silver nanoparticles of 5 nm diameter, as determined by transmission electron microscopy.</li>
	<li>Next, the solution is sonicated at 65 °C in a water bath for 2 hrs for further particle growth (Video 2.4).</li>
	<li>After sonication, the solution is transferred to a 500 ml beaker and cooled to room temperature. Then, 300 ml of ethanol is titrated at a rate of 30 ml / min. Since ethanol is a poor solvent for the poly(acrylic acid) capping agent, the particles rapidly coagulate and precipitate from solution (Video 2.5).</li>
	<li>After decanting the supernatant, the precipitate is collected into a centrifuge tube and centrifuged at 9000 rpm for 20 minutes (Video 2.6).</li>
	<li>After this step, a highly concentrated silver nanoparticle ink with a solids loading of ~85 wt% is obtained (Video 2.7).</li>
	<li>Further control over ink viscosity and elastic modulus can be achieved by dilution, followed by homogenization. For an example, a humectant solution, such as ethylene glycol, can be added to the ink and then homogenized at 2000 rpm for 3 minutes using a Thinky homogenization mixer. After this process, a uniform ink of a bluish to magenta color is obtained (Video 2.8).</li>
	<li>The TEM image shows silver nanoparticles obtained by this synthesis procedure (Figure 5_left). The particles have a mean diameter of 20 nm with a size distribution of 5-50 nm. Printed structures require post-annealing to enhance their conductivity. After annealing at 250°C for less than 30 min, the silver nanoparticles form conductive microelectrodes with an electrical resistivity approaching to 10-5 Ω•cm (Figure 5_bottom right). The microstructural evolution of the printed silver microelectrodes as a function of annealing temperature is shown in Figure 5_top right. As the temperature increases from 150° to 550°C, the microelectrodes undergo densification with a total volumetric shrinkage of ~30%.11</li>
	<li>The ink rheology, which strongly depends on its solids loading, determines its printability. The ink viscosity increases with increasing solids loading (Figure 6). Because dilute inks with low viscosity result in a significant lateral spreading, concentrated inks with a solids loading ranging from 70 to 85 wt% are required for printing of planar and spanning ink filaments.</li>
	<li>The ink elastic modulus increases with increasing solids loading (Figure 7). In the linear viscoelastic region, the elastic modulus rises nearly three orders of magnitude as the solids loading increases 60 to 75 wt%. A minimum elastic modulus of 2000 Pa is required to produce self-supporting or spanning features.</li>
</ol>
3. Direct-write assembly
<ol>
	<li>Direct-write assembly is carried out by first loading the ink into a syringe barrel. After attaching a deposition nozzle, the ink-loaded syringe barrel is mounted onto the 3-axis printing stage (Video 3.1).</li>
	<li>Using a computer program, arbitrary designs, including linear, planar, and complex three-dimensional structures can be easily generated (Video 3.2).</li>
	<li>Next, the nozzle height is adjusted with the aid of telescope lens with a 10x zoom (Video 3.3).</li>
	<li>After applying pressure using an air-powered fluid dispensing system, the ink is deposited onto the substrate with a controlled printing speed (Video 3.4). The required pressure depends upon ink rheology, nozzle diameter, and printing speed, but typical values range from 10-100 psi at 20-500 μm/s. This printing is performed in air at room temperature. Using this printing procedure, the printing of silver microelectrodes in different layouts and size scales is demonstrated.</li>
	<li>For an example, the printing of conductive silver grids with a center-to-center line spacing of 100 μm, patterned by a 5-μm nozzle on a silicon wafer substrate is demonstrated (Video 3.5).</li>
	<li>In addition, this video shows how to create a high aspect ratio cylindrical structure by a 30-μm nozzle using a layer-by-layer printing method (Video 3.6).</li>
	<li>Furthermore, the omnidirectional printing of silver microelectrodes between two glass substrates offset by a 1-mm height difference is demonstrated using a 30-μm nozzle (Video 3.7).</li>
	<li>Completely free-standing, vertically printed silver microspikes can be created by a 30-μm nozzle on a Si wafer substrate (Video 3.8).</li>
	<li>Finally, this video shows direct writing of a spanning silver microelectrode using a 10-μm nozzle (Video 3.9). The printed feature can span distances up to one centimeter with minimal drooping or buckling.</li>
</ol>
4. Representative results:
We prepared a highly concentrated silver nanoparticle inks and demonstrated printed conductive features in planar and 3D motifs for electronic and optoelectronic applications with printing resolution ~ 2 - 30 μm. For an example, Figure 8 exhibits the printing resolution of this technique. Printed features with minimum electrode width of ~ 2 μm (1.4 μm thick) are obtained in a single pass using a 1-μm nozzle11.
Figure 9 shows transparent conductive silver grids, patterned by a 5 μm nozzle on a flexible polyimide film12. The texts underneath the printed grids are clearly visible. These transparent silver grids could be attractive alternatives for transparent conducting oxide (TCO) materials.
Conformal printing onto a non-planar substrates is also enabled by this method. Figure 10 demonstrates the conformal printing of a 3D electrically small antenna. A 100 μm metal nozzle is used to print meander-line patterns on the surface of a glass hemisphere13. This approach may find several applications including implantable and wearable antennas, electronics, and sensors.
Applications of spanning silver microelectrodes in three-dimensional photovoltaics and light-emitting diodes are demonstrated (Figure 11-14).
First, Figure 11 is an example of silicon spherical shell. This tenuous film with a 2-μm thickness can be wire-bonded to an external circuit by omnidirectional printing14. This method uses minimal contact pressure, which is highly advantageous for delicate devices.
Next, Figure 12 demonstrates an example of printing a spanning interconnect for a silicon solar microcell array in which silicon microribbon elements are separated by 33-μm gap15.
Next, Figure 13 shows silver interconnects for the gallium arsenide-based LED array with 4-by-4 pixels, where each pixel (500 x 500 x 2.5 μm3) is spaced 200 μm apart11. The bottom image displays the LED array, emitting uniform red light under an applied bias of 6 V from a single pixel. The ability to print spanning electrodes enables multilayer interconnection without the use of supporting or sacrificial layers (top images).
As a final demonstration, Figure 14 shows SEM images for the complex 3D microperiodic silver lattice printed by a 5μm nozzle.
<img alt="Optical microscope setup with 3-axis motion stage for precise material analysis and inspection." src="/files/ftp_upload/3189/3189fig1.jpg" data-altupdatedat="1747911167000" data-alt="Figure 1"> 
Figure 1. Optical image of direct ink writing apparatus.
<img alt="3D printing process diagram, nozzle extrudes filamentary feature onto surface for material deposition." src="/files/ftp_upload/3189/3189fig2.jpg" data-altupdatedat="1747911167000" data-alt="Figure 2"> 
Figure 2. Direct ink writing of a filamentary feature.
<img alt="3D printing process diagram with ink nozzle and spanning features for additive manufacturing study." src="/files/ftp_upload/3189/3189fig3.jpg" data-altupdatedat="1747911167000" data-alt="Figure 3"> 
Figure 3. Direct ink writing of self-supporting spanning features.
<img alt="Ceramic, organic, metal, polymer, sol-gel inks; microstructural analysis; feature size gradient diagram." src="/files/ftp_upload/3189/3189fig4.jpg" data-altupdatedat="1747911167000" data-alt="Figure 4"> 
Figure 4. Ink designs for direct ink writing. A wide range of concentrated viscoelastic inks have been developed for direct writing of planar and complex 3D structures with microscale features.
<img alt="Nanoparticle morphology and conductivity, TEM and annealing temperature vs. time graph analysis." src="/files/ftp_upload/3189/3189fig5.jpg" data-altupdatedat="1747911167000" data-alt="Figure 5"> 
Figure 5. (Left) Transmission electron microscopy (TEM) image of silver nanoparticles. (Top right) SEM images of silver microelectrodes patterned with a 15-μm nozzle as a function of annealing temperature. (Bottom right) Electrical resistivity of silver microelectrodes as a function of annealing temperature and time.
<img alt="Graph showing ink viscosity vs. silver wt% in 3D printing; shear rate 10 s⁻¹, lateral spreading." src="/files/ftp_upload/3189/3189fig6.jpg" data-altupdatedat="1747911167000" data-alt="Figure 6"> 
Figure 6. Apparent viscosity (η) of the silver nanoparticle inks as a function of solids loading.
<img alt="Elastic modulus vs shear stress, chart; rheology, material properties, stress-strain analysis." src="/files/ftp_upload/3189/3189fig7.jpg" data-altupdatedat="1747911167000" data-alt="Figure 7"> 
Figure 7. Shear elastic modulus (G') as a function of shear stress for silver nanoparticle inks of varying solids loading.
<img alt="Microfabrication process, grating pattern analysis, SEM image showing nanoscale structures, 50 µm scale." src="/files/ftp_upload/3189/3189fig8.jpg" data-altupdatedat="1747911167000" data-alt="Figure 8"> 
Figure 8. SEM images of planar arrays of silver microelectrodes patterned on a Si wafer with a 1-μm nozzle.
<img alt="Flexible electronic sensor, grid pattern detail, material development, 3 mm scale, advanced technology." src="/files/ftp_upload/3189/3189fig9.jpg" data-altupdatedat="1747911167000" data-alt="Figure 9"> 
Figure 9. Optical image of transparent conductive silver grids (left) and SEM images of the grids printed as a function of line pitch (right).
<img alt="3D printing, silver microstructure on curved surface; nozzle, meanderline pattern; conductive material." src="/files/ftp_upload/3189/3189fig10.jpg" data-altupdatedat="1747911167000" data-alt="Figure 10"> 
Figure 10. Optical image captured during conformal printing of electrically small antennas on a hemispherical glass substrate.
<img alt="Spanning Ag microelectrode setup with 10 μm nozzle for microfluidic flow analysis, microscope image." src="/files/ftp_upload/3189/3189fig11.jpg" data-altupdatedat="1747911167000" data-alt="Figure 11"> 
Figure 11. Optical image obtained during printing of spanning silver microelectrodes onto a thin (2-μm) silicon spherical shell.
<img alt="Silicon micro-cells, spanning Ag microelectrode, microscopy image of photovoltaic setup, 25µm scale." src="/files/ftp_upload/3189/3189fig12.jpg" data-altupdatedat="1747911167000" data-alt="Figure 12"> 
Figure 12. SEM image of a spanning silver microelectrode printed onto a silicon solar microcell array.
<img alt="Micro-LEDs on silver electrodes; diagram of emission and interconnects in optoelectronic circuits." src="/files/ftp_upload/3189/3189fig13.jpg" data-altupdatedat="1747911167000" data-alt="Figure 13"> 
Figure 13. SEM images (top) and optical image (bottom) of a 4-by-4 LED chip array interconnected by silver microelectrodes.
<img alt="3D-printed microstructure, SEM image, showing lattice arrangement at microscale, 10 μm, 50 μm scales." src="/files/ftp_upload/3189/3189fig14.jpg" data-altupdatedat="1747911167000" data-alt="Figure 14"> 
Figure 14. SEM image of 3D microperiodic silver lattice.

<ol>
	<li>Chrisey, D. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+power+of+direct+writing.">The power of direct writing.</a> Science. 289, 879-881 (2000).</li><li>Sirringhaus, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-resolution+inkjet+printing+of+all-polymer+transistor+circuits.">High-resolution inkjet printing of all-polymer transistor circuits.</a> Science. 290, 2123-2126 (2000).</li><li>Kim, R. -. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Waterproof+AllnGaP+optoelectronics+on+stretchable+substrates+with+applications+in+biomedicine+and+robotics.">Waterproof AllnGaP optoelectronics on stretchable substrates with applications in biomedicine and robotics.</a> Nat. Mater. 9, 929-937 (2010).</li><li>Lewis, J. A., Gratson, G. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+writing+in+three+dimensions.">Direct writing in three dimensions.</a> Mater. Today. 7, 32-39 (2004).</li><li>Lewis, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+ink+writing+of+3D+functional+materials.">Direct ink writing of 3D functional materials.</a> Adv. Funct. Mater. 16, 2193-2204 (2006).</li><li>Lewis, J. A., Smay, J. E., Stuecker, J., Cesarano, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+ink+writing+of+three-dimensional+ceramic+structures.">Direct ink writing of three-dimensional ceramic structures.</a> J. Am. Ceram. Soc. 89, 3599-3609 (2006).</li><li>Smay, J. E., Gratson, G. M., Shepherd, R. F., Sesarano, J., Lewis, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Directed+colloidal+assembly+of+3D+periodic+structures.">Directed colloidal assembly of 3D periodic structures.</a> Adv. Mater. 14, 1279-1283 (2002).</li><li>Therriault, D., White, S. R., Lewis, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chaotic+mixing+in+three-dimensional+microvascular+networks+fabricated+by+direct-write+assembly.">Chaotic mixing in three-dimensional microvascular networks fabricated by direct-write assembly.</a> Nat. Mater. 2, 265-271 (2003).</li><li>Hansen, C. J., Wu, W., Toohey, K. S., Sottos, N. R., White, S. R., Lewis, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Self-healing+materials+with+interpenetrating+microvascular+networks.">Self-healing materials with interpenetrating microvascular networks.</a> Adv. Mater. 21, 4143-4147 (2009).</li><li>Therriault, D., Shepherd, R. F., White, S. R., Lewis, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fugitive+ink+for+direct-write+assembly+of+three-dimensional+microvascular+networks.">Fugitive ink for direct-write assembly of three-dimensional microvascular networks.</a> Adv. Mater. 17, 395-399 (2005).</li><li>Ahn, B. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Omnidirectional+printing+of+flexible,+stretchable,+and+spanning+silver+microelectrodes.">Omnidirectional printing of flexible, stretchable, and spanning silver microelectrodes.</a> Science. 323, 1590-1593 (2009).</li><li>Ahn, B. Y., Lorang, D. J., Lewis, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transparent+conductive+grids+via+direct+writing+of+silver+nanoparticle+inks.">Transparent conductive grids via direct writing of silver nanoparticle inks.</a> Nanoscale. 3, 2700-2702 (2011).</li><li>Adams, J. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Conformal+printing+of+electrically+small+antennas+on+three-dimensional+surfaces.">Conformal printing of electrically small antennas on three-dimensional surfaces.</a> Adv. Mater. 23, 1335-1340 (2011).</li><li>Guo, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Two-+and+three-dimensional+folding+of+thin+film+single-crystalline+silicon+for+photovoltaic+power+applications.">Two- and three-dimensional folding of thin film single-crystalline silicon for photovoltaic power applications.</a> PNAS. 106, 20149-20154 (2009).</li><li>Yoon, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultrathin+silicon+solar+microcells+for+semitransparent,+mechanically+flexible+and+microconcentrator+module+designs.">Ultrathin silicon solar microcells for semitransparent, mechanically flexible and microconcentrator module designs.</a> Nat. Mater. 7, 907-915 (2008).</li><li>Gratson, G. M., Xu, M., Lewis, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+writing+of+three-dimensional+webs.">Direct writing of three-dimensional webs.</a> Nature. 428, 386-386 (2004).</li><li>Lebel, L. L., Aissa, B., Khakani, M. A. E., Therriault, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultraviolet-assisted+direct-write+fabrication+of+carbon+nanotube/polymer+nanocomposite+microcoils.">Ultraviolet-assisted direct-write fabrication of carbon nanotube/polymer nanocomposite microcoils.</a> Adv. Mater. 22, 592-596 (2010).</li><li>Ahn, B. Y., Lorang, D. J., Duoss, E. B., Lewis, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct-write+assembly+of+microperiodic+planar+and+spanning+ITO+microelectrodes.">Direct-write assembly of microperiodic planar and spanning ITO microelectrodes.</a> Chem. Commun. 46, 7118-7120 (2010).</li><li>Duoss, E. B., Twardowski, M., Lewis, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sol-gel+inks+for+direct-write+assembly+of+functional+oxides.">Sol-gel inks for direct-write assembly of functional oxides.</a> Adv. Mater. 19, 3485-3489 (2007).</li><li>Salaita, K., Wang, Y. H., Mirkin, C. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Application+of+dip-pen+nanotechnology.">Application of dip-pen nanotechnology.</a> Nat. Nanotech. 2, 145-155 (2007).</li><li>Zhang, H., Lee, K. -. B., Li, Z., Mirkin, C. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Biofunctionalized+nanoarrays+of+inorganic+structures+prepared+by+dip-pen+nanolithography.">Biofunctionalized nanoarrays of inorganic structures prepared by dip-pen nanolithography.</a> Nanotechnology. 14, 1113-1117 (2003).</li><li>Hung, S. -. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dip-pen+nanolithography+of+conductive+silver+traces.">Dip-pen nanolithography of conductive silver traces.</a> J. Phys. Chem. C. 114, 9672-9677 (2010).</li><li>Park, J. -. U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-resolution+electrohydrodynamic+jet+printing.">High-resolution electrohydrodynamic jet printing.</a> Nat. Mater. 6, 782-789 (2007).</li><li>Schirmer, N. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+ejecting+colloids+against+capillarity+from+sub-micrometer+openings:+On-demand+dielectrophoretic+nanoprinting.">On ejecting colloids against capillarity from sub-micrometer openings: On-demand dielectrophoretic nanoprinting.</a> Adv. Mater. 22, 4701-4705 (2010).</li><li>Park, J. -. U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=electrofield+liquid+jets+for+high-resolution+printing+of+charge.">electrofield liquid jets for high-resolution printing of charge.</a> Nano. Lett. 10, 584-591 (2010).</li><li>Ghosh, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct-write+assembly+of+micro-periodic+silk+fibroin+scaffolds+for+tissue+engineering+applications.">Direct-write assembly of micro-periodic silk fibroin scaffolds for tissue engineering applications.</a> Adv. Funct. Mater. 18, 1883-1889 (2008).</li><li>Barry, R. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct-write+assembly+of+3D+hydrogel+scaffolds+for+guided+cell+growth.">Direct-write assembly of 3D hydrogel scaffolds for guided cell growth.</a> Adv. Mater. 21, 2407-2410 (2009).</li><li>Shepherd, J. N. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=3D+microperiodic+hydrogel+scaffolds+for+robust+neuronal+cultures.">3D microperiodic hydrogel scaffolds for robust neuronal cultures.</a> Adv. Mater. 21, 47-54 (2011).</li><li>Wu, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct-write+assembly+of+biomimetic+microvascular+networks+for+efficient+fluid+transport.">Direct-write assembly of biomimetic microvascular networks for efficient fluid transport.</a> Soft. Matter. 6, 739-742 (2010).</li><li>Wu, W., DeConinck, A., Lewis, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Omnidirectional+Printing+of+3D+Microvascular+Networks.">Omnidirectional Printing of 3D Microvascular Networks.</a> Adv. Mater. 23, H178-H183 (2011).</li></ol>

No conflicts of interest declared.

Conventional droplet-based printing approaches, such as inkjet printing, are limited to the fabrication of planar electrodes with low aspect ratio due to the dilute nature and low viscosity of the inks used. Recently, dip-pen nanolithography (DPN)20-22 and e-jet printing23-25 have been used to pattern conductive features. These routes also employ dilute, low viscosity inks. Pearton and co-workers used DPN to deposit a commercially available silver nanoparticle ink at write speeds of up to 1600 &mu;m s-1 and line widths of approximately 0.5 &mu;m22. However, fabrication of reproducible patterns over large areas has yet to be demonstrated by this approach. Silver nanoparticle inks have also been deposited by e-jet printing to form conductive traces with line widths of ~ 1.5 &mu;m25. However, as with inkjet printing, inhomogeneous printed features may arise due to satellite drop formation and non-uniform drop drying24,25.
As demonstrated above, direct-write assembly of concentrated silver nanoparticle inks overcomes these limitations through a filamentary-based printing approach. This technique enables the fabrication of conductive microelectrodes with a high aspect ratios (h/w &asymp; 1.0) in a single passes allowing the creation of 1D, 2D, and 3D architectures. The size of the printed features depends upon nozzle diameter, ink solids loading, applied pressure, and printing speed. To date, conductive traces as small as ~ 2 &mu;m have been patterned using a 1 &mu;m nozzle at modest speeds (&lt; 2 mm s-1). By tailoring the ink composition and nozzle geometry, maximum printing speeds in excess of 10 cm s-1 are possible. However, high speed printing of using fine nozzles (&lt; 5 &mu;m) remains a significant challenge.
To demonstrate applications of direct-write assembly, we fabricated conductive grids, electrically small antennas, solar cells, and light-emitting diodes with planar and spanning printed electrodes (Figure 8-14). Notably, our approach is not restricted to the creation of metallic structures. Using other ink designs, such as those based on silk fibroin, hydrogel and fugitive organic inks, we have constructed 3D scaffolds and microvascular networks for tissue engineering and cell culture via direct-write assembly26-30.
Looking towards the future, there are many opportunities and challenges. Further advances require new ink designs, better modeling of ink flow dynamics, and enhanced robotic and control systems. Large-area fabrication of 1D to 3D structures with high throughput and nanoscale resolution (&lt; 100 nm) remains a significant challenge.

<table><tbody><tr><td>Poly(acrylic acid)</td><td>Polysciences, Inc.</td><td>06519</td><td>m.w. 5,000 g/mol</td></tr><tr><td>Poly(acrylic acid)</td><td>Polysciences, Inc.</td><td>00627</td><td>m.w. 50,000 g/mol</td></tr><tr><td>Silver nitrate</td><td>Sigma-Aldrich</td><td>209139</td><td>Silver source</td></tr><tr><td>Diethanolamine</td><td>Sigma-Aldrich</td><td>D8885</td><td>Solvent/Reducing agent</td></tr><tr><td>Ethylene glycol</td><td>Sigma-Aldrich</td><td>102466</td><td>Humectant</td></tr><tr><td>Sonicater</td><td>Fisher Scientific</td><td>FS30H</td><td>-</td></tr><tr><td>Centrifuge</td><td>Beckman Coulter Inc.</td><td>Avanti&lt;sup&gt;TM&lt;/sup&gt; J-25 I</td><td>-</td></tr><tr><td>Robotic stage</td><td>Aerotech Inc.</td><td>ABL 900010</td><td>3-axis motion</td></tr><tr><td>Syringe barrel</td><td>EFD Inc.</td><td>5109LBP-B</td><td>3 ml</td></tr><tr><td>Nozzle</td><td>EFD Inc.</td><td>-</td><td>i.d. = 0.1 - 250 &amp;#x03BC;m</td></tr><tr><td>Dispenser</td><td>EFD Inc.</td><td>800</td><td>Air-powered</td></tr><tr><td>Design software</td><td>Custom Made</td><td>-</td><td>Mingjie Xu</td></tr></tbody></table>

planar and three-dimensional printing of conductive inks

Printed electronics rely on low-cost, large-area fabrication routes to create flexible or multidimensional electronic, optoelectronic, and biomedical devices1-3. In this paper, we focus on one- (1D), two- (2D), and three-dimensional (3D) printing of conductive metallic inks in the form of flexible, stretchable, and spanning microelectrodes.
Direct-write assembly4,5 is a 1-to-3D printing technique that enables the fabrication of features ranging from simple lines to complex structures by the deposition of concentrated inks through fine nozzles (~0.1 - 250 &mu;m). This printing method consists of a computer-controlled 3-axis translation stage, an ink reservoir and nozzle, and 10x telescopic lens for visualization. Unlike inkjet printing, a droplet-based process, direct-write assembly involves the extrusion of ink filaments either in- or out-of-plane. The printed filaments typically conform to the nozzle size. Hence, microscale features (&lt; 1 &mu;m) can be patterned and assembled into larger arrays and multidimensional architectures.
In this paper, we first synthesize a highly concentrated silver nanoparticle ink for planar and 3D printing via direct-write assembly. Next, a standard protocol for printing microelectrodes in multidimensional motifs is demonstrated. Finally, applications of printed microelectrodes for electrically small antennas, solar cells, and light-emitting diodes are highlighted.

Watch this Scientific Journal Video about Planar and Three-Dimensional Printing of Conductive Inks at JoVE.com

Planar and Three-Dimensional Printing of Conductive Inks

Planar and three-dimensional printing of conductive metallic inks is described. Our approach provides new avenues for fabricating printed electronic, optoelectronic, and biomedical devices in unusual layouts at the microscale.

Printed electronics rely on low-cost, large-area fabrication routes to create flexible or multidimensional electronic, optoelectronic, and biomedical ...

planar-and-three-dimensional-printing-of-conductive-inks

Research

JoVE Journal

Bioengineering

37.0K Views.  University of Illinois at Urbana-Champaign. Planar and three-dimensional printing of conductive metallic inks is described. Our approach provides new avenues for fabricating printed electronic, optoelectronic, and biomedical devices in unusual layouts at the microscale.

Video: Planar and Three-Dimensional Printing of Conductive Inks

Creating Transient Cell Membrane Pores Using a Standard Inkjet Printer

Bioprinting has a wide range of applications and significance, including tissue engineering, direct cell application therapies, and biosensor microfabrication.1-10 Recently, thermal inkjet printing has also been used for gene transfection.8,9 The thermal inkjet printing process was shown to temporarily disrupt the cell membranes without affecting cell viability. The transient pores in the membrane can be used to introduce molecules, which would otherwise be too large to pass through the membrane, into the cell cytoplasm.8,9,11
The application being demonstrated here is the use of thermal inkjet printing for the incorporation of fluorescently labeled g-actin monomers into cells. The advantage of using thermal ink-jet printing to inject molecules into cells is that the technique is relatively benign to cells.8, 12 Cell viability after printing has been shown to be similar to standard cell plating methods1,8. In addition, inkjet printing can process thousands of cells in minutes, which is much faster than manual microinjection. The pores created by printing have been shown to close within about two hours. However, there is a limit to the size of the pore created (~10 nm) with this printing technique, which limits the technique to injecting cells with small proteins and/or particles. 8,9,11
A standard HP DeskJet 500 printer was modified to allow for cell printing.3, 5, 8 The cover of the printer was removed and the paper feed mechanism was bypassed using a mechanical lever. A stage was created to allow for placement of microscope slides and coverslips directly under the print head. Ink cartridges were opened, the ink was removed and they were cleaned prior to use with cells. The printing pattern was created using standard drawing software, which then controlled the printer through a simple print command. 3T3 fibroblasts were grown to confluence, trypsinized, and then resuspended into phosphate buffered saline with soluble fluorescently labeled g-actin monomers. The cell suspension was pipetted into the ink cartridge and lines of cells were printed onto glass microscope cover slips. The live cells were imaged using fluorescence microscopy and actin was found throughout the cytoplasm. Incorporation of fluorescent actin into the cell allows for imaging of short-time cytoskeletal dynamics and is useful for a wide range of applications.13-15

A description of the methods used to convert an HP DeskJet 500 printer into a bioprinter. The printer is capable of processing living cells, which causes transient pores in the membrane. These pores can be utilized to incorporate small molecules, including fluorescent G-actin, into the printed cells.

Bioprinting has a wide range of applications and significance, including tissue engineering, direct cell application therapies, and biosensor ...

Human Cartilage Tissue Fabrication Using Three-dimensional Inkjet Printing Technology

Bioprinting, which is based on thermal inkjet printing, is one of the most attractive enabling technologies in the field of tissue engineering and regenerative medicine. With digital control&#160;cells, scaffolds, and growth factors can be precisely deposited to the desired two-dimensional (2D) and three-dimensional (3D) locations rapidly. Therefore, this technology is an ideal approach to fabricate tissues mimicking their native anatomic structures. In order to engineer cartilage with native zonal organization, extracellular matrix composition (ECM), and mechanical properties, we developed a bioprinting platform using a commercial inkjet printer with simultaneous photopolymerization capable for 3D cartilage tissue engineering. Human chondrocytes suspended in poly(ethylene glycol) diacrylate (PEGDA) were printed for 3D neocartilage construction via layer-by-layer assembly. The printed cells were fixed at their original deposited positions, supported by the surrounding scaffold in simultaneous photopolymerization. The mechanical properties of the printed tissue were similar to the native cartilage. Compared to conventional tissue fabrication, which requires longer UV exposure, the viability of the printed cells with simultaneous photopolymerization was significantly higher. Printed neocartilage demonstrated excellent glycosaminoglycan (GAG) and collagen type II production, which was consistent with gene expression. Therefore, this platform is ideal for accurate cell distribution and arrangement for anatomic tissue engineering.

The methods described in this paper show how to convert a commercial inkjet printer into a bioprinter with simultaneous UV polymerization. The printer is capable of constructing 3D tissue structure with cells and biomaterials. The study demonstrated here constructed a 3D neocartilage.

Bioprinting, which is based on thermal inkjet printing, is one of the most attractive enabling technologies in the field of tissue engineering and ...

Inkjet Printing All Inorganic Halide Perovskite Inks for Photovoltaic Applications

A method for synthesizing photoactive inorganic&#160;perovskite quantum dot inks and an inkjet printer deposition method, using the synthesized inks, are demonstrated. The ink synthesis is based on a simple wet chemical reaction and the inkjet printing protocol is a facile step by step method. The inkjet printed thin films have been characterized by X-ray diffraction, optical absorption spectroscopy, photoluminescent spectroscopy, and electronic transport measurements. X-ray diffraction of the printed quantum dot films indicates a crystal structure consistent with an orthorhombic room temperature phase with (001) orientation. In conjunction with other characterization methods, the X-ray diffraction&#160;measurements show high quality films can be obtained through the inkjet printing method.

A protocol for synthesizing inorganic-lead-halide hybrid perovskite quantum dot inks for inkjet printing and the protocol for preparing and printing the quantum dot inks in an inkjet printer with post characterization techniques are presented.

A method for synthesizing photoactive inorganic&#160;perovskite quantum dot inks and an inkjet printer deposition method, using the synthesized inks, are ...

Chemistry

Hybrid Printing for the Fabrication of Smart Sensors

A method to combine additively manufactured substrates or foils and multilayer inkjet printing for the fabrication of sensor devices is presented. First, three substrates (acrylate, ceramics, and copper) are prepared. To determine the resulting material properties of these substrates, profilometer, contact angle, scanning electron microscope (SEM), and focused ion beam (FIB) measurements are done. The achievable printing resolution and suitable drop volume for each substrate are, then, found through the drop size tests. Then, layers of insulating and conductive ink are inkjet printed alternately to fabricate the target sensor structures. After each printing step, the respective layers are individually treated by photonic curing. The parameters used for the curing of each layer are adapted depending on the printed ink, as well as on the surface properties of the respective substrate. To confirm the resulting conductivity and to determine the quality of the printed surface, four-point probe and profilometer measurements are done. Finally, a measurement set-up and results achieved by such an all-printed sensor system are shown to demonstrate the achievable quality.

Here we present a protocol for the fabrication of inkjet-printed multilayer sensor structures on additively manufactured substrates and foil.

A method to combine additively manufactured substrates or foils and multilayer inkjet printing for the fabrication of sensor devices is presented. First, ...

Engineering

Viability of Bioprinted Cellular Constructs Using a Three Dispenser Cartesian Printer

Tissue engineering has centralized its focus on the construction of replacements for non-functional or damaged tissue. The utilization of three-dimensional bioprinting in tissue engineering has generated new methods for the printing of cells and matrix to fabricate biomimetic tissue constructs. The solid freeform fabrication (SFF) method developed for three-dimensional bioprinting uses an additive manufacturing approach by depositing droplets of cells and hydrogels in a layer-by-layer fashion. Bioprinting fabrication is dependent on the specific placement of biological materials into three-dimensional architectures, and the printed constructs should closely mimic the complex organization of cells and extracellular matrices in native tissue. This paper highlights the use of the Palmetto Printer, a Cartesian bioprinter, as well as the process of producing spatially organized, viable constructs while simultaneously allowing control of environmental factors. This methodology utilizes computer-aided design and computer-aided manufacturing to produce these specific and complex geometries. Finally, this approach allows for the reproducible production of fabricated constructs optimized by controllable printing parameters.

A Cartesian bioprinter was designed and fabricated to allow multi-material deposition in precise, reproducible geometries, while also allowing control of environmental factors. Utilizing the three-dimensional bioprinter, complex and viable constructs may be printed and easily reproduced.

Tissue engineering has centralized its focus on the construction of replacements for non-functional or damaged tissue. The utilization of ...

Automated Robotic Dispensing Technique for Surface Guidance and Bioprinting of Cells

This manuscript describes the introduction of cell guidance features followed by the direct delivery of cells to these features in a hydrogel bioink using an automated robotic dispensing system. The particular bioink was selected as it allows cells to sediment towards and sense the features. The dispensing system bioprints viable cells in hydrogel bioinks using a backpressure assisted print head. However, by replacing the print head with a sharpened stylus or scalpel, the dispensing system can also be employed to create topographical cues through surface etching. The stylus movement can be programmed in steps of 10 &#181;m in the X, Y and Z directions. The patterned grooves were able to orientate mesenchymal stem cells, influencing them to adopt an elongated morphology in alignment with the grooves&#39; direction. The patterning could be designed using plotting software in straight lines, concentric circles, and sinusoidal waves. In a subsequent procedure, fibroblasts and mesenchymal stem cells were suspended in a 2% gelatin bioink, for bioprinting in a backpressure driven extrusion printhead. The cell bearing bioink was then printed using the same programmed coordinates used for the etching. The bioprinted cells were able to sense and react to the etched features as demonstrated by their elongated orientation along the direction of the etched grooves.

This protocol describes a bioprinting methodology using an automated robotic depositing system that incorporates etched topographical guidance cues with the precision deposition of a cell bearing hydrogel bioink. The printed cells are directly delivered to the etched features and are able to sense and orientate with them.

This manuscript describes the introduction of cell guidance features followed by the direct delivery of cells to these features in a hydrogel bioink using ...

Elaborate Control of Inkjet Printer for Fabrication of Chip-based Supercapacitors

There are tremendous efforts in various fields to apply the inkjet printing method for the fabrication of wearable devices, displays, and energy storage devices. To get high-quality products, however, sophisticated operation skills are required depending on the physical properties of the ink materials. In this regard, optimizing the inkjet printing parameters is as important as developing the physical properties of the ink materials. In this study, optimization of the inkjet printing software parameters is presented for fabricating a supercapacitor. Supercapacitors are attractive energy storage systems because of their high power density, long lifespan, and various applications as power sources. Supercapacitors can be used in the Internet of Things (IoT), smartphones, wearable devices, electrical vehicles (EVs), large energy storage systems, etc. The wide range of applications demands a new method that can fabricate devices in various scales. The inkjet printing method can break through the conventional fixed-size&#160;fabrication method.

This paper provides a technique for manufacturing chip-based supercapacitors using an inkjet printer. Methodologies are described in detail to synthesize inks, adjust software parameters, and analyze the electrochemical results of the manufactured supercapacitor.

There are tremendous efforts in various fields to apply the inkjet printing method for the fabrication of wearable devices, displays, and energy storage ...

Bioprinting of Cartilage and Skin Tissue Analogs Utilizing a Novel Passive Mixing Unit Technique for Bioink Precellularization

Bioprinting is a powerful technique for the rapid and reproducible fabrication of constructs for tissue engineering applications. In this study, both cartilage and skin analogs were fabricated after bioink pre-cellularization utilizing a novel passive mixing unit technique. This technique was developed with the aim to simplify the steps involved in the mixing of a cell suspension into a highly viscous bioink. The resolution of filaments deposited through bioprinting necessitates the assurance of uniformity in cell distribution prior to printing to avoid the deposition of regions without cells or retention of large cell clumps that can clog the needle. We demonstrate the ability to rapidly blend a cell suspension with a bioink prior to bioprinting of both cartilage and skin analogs. Both tissue analogs could be cultured for up to 4 weeks. Histological analysis demonstrated both cell viability and deposition of tissue specific extracellular matrix (ECM) markers such as glycosaminoglycans (GAGs) and collagen I respectively.

Cartilage and skin analogs were bioprinted using a nanocellulose-alginate based bioink. The bioinks were cellularized prior to printing via a single step passive mixing unit. The constructs were demonstrated to be uniformly cellularized, have high viability, and exhibit favorable markers of differentiation.

Bioprinting is a powerful technique for the rapid and reproducible fabrication of constructs for tissue engineering applications. In this study, both ...

High-resolution Patterning Using Two Modes of Electrohydrodynamic Jet: Drop on Demand and Near-field Electrospinning

Electrohydrodynamic (EHD) jet printing has drawn attention in various fields because it can be used as a high-resolution and low-cost direct patterning tool. EHD printing uses a fluidic supplier to maintain the extruded meniscus by pushing the ink out of the nozzle tip. The electric field is then used to pull the meniscus down to the substrate to produce high-resolution patterns. Two modes of EHD printing have been used for fine patterning: continuous near-field electrospinning (NFES) and dot-based drop-on-demand (DOD) EHD printing. According to the printing modes, the requirements for the printing equipment and ink viscosity will differ. Even though two different modes can be implemented with a single EHD printer, the realization methods significantly differ in terms of ink, fluidic system, and driving voltage. Consequently, without a proper understanding of the jetting requirements and limitations, it is difficult to obtain the desired results. The purpose of this paper is to present a guideline so that inexperienced researchers can reduce the trial and error efforts to use the EHD jet for their specific research and development purposes. To demonstrate the fine-patterning implementation, we use Ag nanoparticle ink for the conductive patterning in the protocol. In addition, we also present the generalized printing guidelines that can be used for other types of ink for various fine-patterning applications.

Here, we present a protocol to produce high-resolution conductive patterns using electrohydrodynamic (EHD) jet printing. The protocol includes two modes of EHD jet printing: the continuous near-field electrospinning (NFES) and the dot-based drop-on-demand (DOD) EHD printing.

Electrohydrodynamic (EHD) jet printing has drawn attention in various fields because it can be used as a high-resolution and low-cost direct patterning ...

Conformable Wearable Electrodes: From Fabrication to Electrophysiological Assessment

Wearable electronic devices are becoming key players in monitoring the body signals predominantly altered during physical activity tracking. Considering the growing interest in telemedicine and personalized care driven by the rise of the Internet of Things era, wearable sensors have expanded their field of application into healthcare. To ensure the collection of clinically relevant data, these devices need to establish conformable interfaces with the human body to provide high-signal-quality recordings and long-term operation. To this end, this paper presents a method to easily fabricate conformable thin tattoo- and soft textile-based sensors for their application as wearable organic electronic devices in a broad spectrum of surface electrophysiological recordings. 
The sensors are developed through a cost-effective and scalable process of cutaneous electrode patterning using poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), the most popular conductive polymer in bioelectronics, on off-the-shelf, wearable substrates. This paper presents key steps in electrode characterization through impedance spectroscopy to investigate their performance in signal transduction when coupled with the skin. Comparative studies are required to position the performance of novel sensors with respect to the clinical gold standard. To validate the fabricated sensors' performance, this protocol shows how to perform various biosignal recordings from different configurations through a user-friendly and portable electronic setup in a laboratory environment. This methods paper will allow multiple experimental initiatives to advance the current state of the art in wearable sensors for human body health monitoring.

Two recent technologies-tattoo and textiles-have demonstrated promising results in cutaneous sensing. Here, we present the fabrication and evaluation methods of tattoo and textile electrodes for cutaneous electrophysiological sensing. These electronic interfaces made of conductive polymers outperform the existing standards in terms of comfort and sensitivity.

Wearable electronic devices are becoming key players in monitoring the body signals predominantly altered during physical activity tracking. Considering ...