This work was supported by the NSF (CMMI-1351370).

1. Design Masks for Fabrication of Donor Substrate
<ol>
	<li>Design a mask with desired geometry. To fabricate 100 &#956;m x 100 &#956;m square silicon individual units, draw an array of 100 &#956;m x 100 &#956;m squares.</li>
	<li>Design a second mask with an identical geometry, with each side extending out an additional 15 &#956;m. For the array of 100 &#956;m x 100 &#956;m squares, draw an array of 130 &#956;m x 130 &#956;m squares that can cover the squares in step 1.1.</li>
	<li>Design the anchor geometry. Draw four 20 &#956;m x 40 &#956;m rectangles, each centered along one edge of a square. Place the structures so that the first 15 &#956;m covers the original 100 &#956;m x 100 &#956;m square in step 1.1 and the remaining 25 &#956;m extends outward (as shown in Figure 2). 
	NOTE: Any shape and dimensions can be used as long as the anchor contacts both the patterned material and the substrate. One end of this anchor covers the original geometry in step 1.1 and the other end should extend out the geometry in step 1.2.</li>
</ol>
2. Prepare Retrievable Donor Substrate
<ol>
	<li>Prepare a p-type doped silicon on insulator (SOI) wafer with 3 &#956;m device layer thickness, with sheet resistance of 1-20 &#937;&#8226;cm and box oxide layer thickness of 1 &#956;m. NOTE: For various applications these parameters can be altered.</li>
	<li>Spin coat photoresist (AZ5214, 3,000 rpm for 30 sec, 1.5 &#956;m thick) and attach the mask designed in step 1.1.</li>
	<li>Using a reactive ion etching (RIE) instrument, pattern the device layer of the SOI wafer and remove photoresist mask. After this step, the RIE etched region has exposed the box oxide layer (Figure 2A).</li>
	<li>Spin coat photoresist (AZ5214, 3,000 rpm for 30 sec, 1.5 &#956;m thick) and pattern with mask designed in step 1.2.</li>
	<li>Heat the wafer at 125 &#176;C for 90 sec on a hot plate.</li>
	<li>Immerse the wafer into 49% HF for 50 sec to etch the exposed box oxide layer from step 2.3. After completely drying, remove the masking photoresist (Figure 2B).</li>
	<li>Spin coat (AZ5214, 3,000 rpm for 30 sec, 1.5 &#956;m thick) and pattern the anchoring design from step 1.3.</li>
	<li>Heat the wafer at 125 &#176;C for 90 sec on a hot plate.</li>
	<li>Immerse into 49% HF for 50 min. This step etches the box oxide layer remaining underneath the remaining patterned device layer silicon, resulting in suspended silicon individual units on the photoresist (Figure 2C).</li>
</ol>
3. Design Masks for a Microtip Stamp
<ol>
	<li>Design a mask with a single 100 &#956;m x 100 &#956;m square.</li>
	<li>Design a mask with multiple 12 &#956;m x 12 &#956;m squares inside a 100 &#956;m x 100 &#956;m area.</li>
</ol>
4. Make the Mold for a Microtip Stamp
<ol>
	<li>Clean a silicon wafer with crystalline orientation of &#60;1-0-0&#62;, deposit 100 nm of silicon nitride using Plasma Enhanced Chemical Vapor Deposition (PECVD) equipment.</li>
	<li>Spin coat photoresist (AZ5214, 3,000 rpm for 30 sec, 1.5 &#956;m thick) and pattern with mask designed in step 3.2.</li>
	<li>Pattern the silicon nitride layer using 10:1 Buffered Oxide Etchant (BOE).</li>
	<li>Dissolve 80 g of potassium hydroxide (KOH) in 170 ml of deionized water and 40 ml of isopropyl alcohol (IPA) mixture a beaker.</li>
	<li>Heat the KOH, IPA, and water mixture at 80 &#176;C on a hot plate.</li>
	<li>Vertically place the prepared wafer in the beaker with KOH mixture to etch the exposed silicon in crystalline structure (etching rate is around 1 &#956;m/min).</li>
	<li>After the exposed silicon is fully etched, remove the wafer from KOH mixture, etch away the silicon nitride using HF, and perform RCA 1 and RCA 2 cleaning (Figure 3A).</li>
	<li>Spin coat with SU-8 100 and pattern with the prepared mask from step 3.1 with following recipe: 3,000 rpm for 1 min, soft bake at 65 &#176;C for 10 min and 95 &#176;C for 30 min, expose with 550 mJ/cm2, and post bake at 65 &#176;C for 1 min and 95 &#176;C for 10 min (Figure 3B).</li>
	<li>After the SU-8 100 is fully cured, apply a monolayer of (tridecafluoro-1,1,2,3-tetrahydro octyl)-1-trichlorosilane by dropping 3-5 drops of (tridecafluoro-1,1,2,3-tetrahydro octyl)-1-trichlorosilane into a vacuum jar and placing the wafer in the jar and applying the vacuum.</li>
</ol>
5. Duplicate a Microtip Stamp Using a Mold
<ol>
	<li>Mix polydimethylsiloxane (PDMS) base and curing agent with the ratio of 5:1.</li>
	<li>Degas the mixture by placing it in a vacuum jar.</li>
	<li>Pour a small portion of the degassed PDMS mixture on the mold and let the PDMS reflow to achieve a flat top surface (Figure 3C).</li>
	<li>Place the mold with PDMS in the oven at 70 &#176;C for 2 hr to fully cure the PDMS.</li>
	<li>Remove the mold from the oven and peel the PDMS off (Figure 3D).</li>
</ol>
6. Retrieve Ink from the Donor Substrate and Print on the Target Area
<ol>
	<li>Place the donor substrate onto motorized rotational and x,y-translation stages equipped with a microscope.</li>
	<li>Attach the microtip stamp to an independent vertical translational stage.</li>
	<li>Under the microscope, align the microtip stamp with the Si ink on the donor substrate using translational and rotational stages. Furthermore, do the tilting alignment between the microtip surface and the Si ink by adjusting a tilting stage. Afterwards, bring the microtip stamp down to make contact.</li>
	<li>Slowly bring the microtip stamp down further after initial contact, so that small tips are fully collapsed and the whole surface is in contact with the Si ink on donor substrate.</li>
	<li>Quickly raise the z stage, breaking the anchors due to the large contact area between the microtip stamp and the Si ink, to retrieve the Si ink from the donor substrate and attach it to the microtip stamp. 
	NOTE: When the microtip stamp is free of any stress, the compressed microtip restores to its original pyramidal shape, making minimal contact with the retrieved Si ink.</li>
	<li>Place the receiver substrate onto an x,y-translation stage and align the retrieved Si ink under the microtip stamp at the desired location.</li>
	<li>Descend the z stage until the retrieved Si ink barely makes contact with the receiver substrate.</li>
	<li>After making contact, slowly raise the z stage to release the Si ink, printing it on the desired location.</li>
</ol>
7. Bonding Process
<ol>
	<li>Program a rapid thermal annealing furnace to cycle from RT up to 950 &#176;C in 90 sec, remain at 950 &#176;C for 10 min and cool down to RT (by removing any heat supply in the furnace).</li>
	<li>Place the printed receiver substrate in the furnace in an ambient air environment and anneal at 950 &#176;C for 10 min for Si-Si bonding or at 360 &#176;C for 30 min for Si-Au bonding.</li>
</ol>

<ol><li>Stix, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Toward+%E2%80%9CPoint+one%5BTitle%5D)+AND+%22Sci+Am.+Feb%22%5BJournal%5D)">Toward “Point one.</a> Sci Am. Feb. , 90-95 (1995).</li>
<li>Appenzeler, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((The+Man+Who+Dared+to+Think+Small%5BTitle%5D)+AND+%22Science%22%5BJournal%5D)">The Man Who Dared to Think Small.</a> Science. 254, 1300-1301 (1991).</li>
<li>Madou, M. J. Fundamentals of Microfabrications The Science of Miniaturization. , CRC Press. Boca Raton, FL. (2002).</li>
<li>Xia, Y., Whitesides, G. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Soft+Lithography%5BTitle%5D)+AND+%22Angew+Chem+Int+Ed%22%5BJournal%5D)">Soft Lithography.</a> Angew Chem Int Ed. 38, 551-575 (1998).</li>
<li>Judy, J. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Microelectromechanical+systems+%28MEMS%29+fabrication%2C+design+and+applications%5BTitle%5D)+AND+%22Smart+Mater+Struct%22%5BJournal%5D)">Microelectromechanical systems (MEMS) fabrication, design and applications.</a> Smart Mater Struct. 10, 1134-1154 (2001).</li>
<li>Jain, V. K. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aJain%2C+VK+%22Micromanufacturing+Process%22">Micromanufacturing Process</a>. , CRC Press. (2012).</li>
<li>Keum, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Silicon+micro-masonry+using+elastomeric+stamps+for+three-dimensional+microfabrication%5BTitle%5D)+AND+%22J+Micromech+Microeng%22%5BJournal%5D)">Silicon micro-masonry using elastomeric stamps for three-dimensional microfabrication.</a> J Micromech Microeng. 22, 55018(2012).</li>
<li>Keum, H., Chung, H., Kim, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Electrical+Contact+at+The+Interface+between+Silicon+and+Transfer-Printed+Gold+Films+by+Eutectic+Joining%5BTitle%5D)+AND+%22ACS+Appl+Mater+Interfaces%22%5BJournal%5D)">Electrical Contact at The Interface between Silicon and Transfer-Printed Gold Films by Eutectic Joining.</a> ACS Appl Mater Interfaces. 5, 6061(2013).</li>
<li>Keum, H., Seong, M., Sinha, S., Kim, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Electrostatically+Driven+Collapsible+Au+Thin+Films+Assembled+Using+Transfer+Printing+for+Thermal+Switching%5BTitle%5D)+AND+%22Appl+Phys+Lett%22%5BJournal%5D)">Electrostatically Driven Collapsible Au Thin Films Assembled Using Transfer Printing for Thermal Switching.</a> Appl Phys Lett. 100, 211904(2012).</li>
<li>Klaassen, E. H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Silicon+fusion+bonding+and+deep+reactive+ion+etching%3A+a+new+technology+for+microstructures%5BTitle%5D)+AND+%22Sens+Actuators+A%22%5BJournal%5D)">Silicon fusion bonding and deep reactive ion etching: a new technology for microstructures.</a> Sens Actuators A. 52, 132-139 (1996).</li>
<li>Barth, P. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Silicon+fusion+bonding+for+fabrication+of+sensors+actuators+and+microstructures%5BTitle%5D)+AND+%22Sens+Actuators%22%5BJournal%5D)">Silicon fusion bonding for fabrication of sensors actuators and microstructures.</a> Sens Actuators. A21 - A23, 919-926 (1990).</li>
</ol>

The authors have nothing to disclose.

Micro-masonry, presented in Figure 4, involves silicon fusion bonding in a material bonding step. Silicon fusion bonding is achieved by placing the sample in a rapid thermal annealing furnace (RTA furnace) and heating the sample at 950 &#176;C for 10 min. This annealing condition is both adoptable between Si &#8211; Si and Si &#8211; SiO2 bonding10,11. Alternatively, the Au bonded with a Si strip as found in Figure 5C&#160;adopts eutectic bonding, and therefore, the bonding temperature is around the Si-Au eutectic temperature (363 &#176;C) for 30 min&#160;8. To ensure eutectic bonding, prior to printing Au inks on a Si strip, the Si strip needs to be thoroughly cleansed with 49% HF in order to prevent any impurity such as native oxide at the interface of the Au and Si. Being able to assemble Au films with micro-masonry hugely improves wide adaptation of the micro-masonry manufacturing scheme since it introduces a metal type of material. Due to its low electrical contact resistance with silicon, it can be used as an electrode in the finalized MEMS devices as well as a suspended membrane flexure as presented in Keum et al.9
Transferrable inks developed are currently limited to Si and Au and the materials of their corresponding receiver substrates are Si and SiO2 for Si, and Au and Si for Au. Overall, larger contact area between a receiver substrate and an ink results in ease in the printing step. However, printing an ink while partially contacting the surface is also feasible, resulting in suspended structure, as demonstrated in Figure 4E.
While micro-masonry is a novel approach of microfabrication, there still are limitations to overcome in the process. First and far most is manufacturing scalability since the current deterministic assembly of solid inks is conducted individually rather than at the same time. Also, since the silicon fusion bonding is exercised in high temperature, differences in Si and SiO2 thermal expansion coefficient may result in buckling/delamination in the interface. These limitations need to be further investigated for more broad adaptation of the micro-masonry technique.
As presented in Figure 4, micro-masonry has an immense influence on conventional MEMS processes, which mainly rely on monolithic microfabrication, through its unique additive and flexible manufacturing capability of three dimensional microscale structures that is previously unseen. Furthermore, micro-masonry has the ability to manipulate delicate features in the microscale without damaging the surface because it uses soft elastomeric stamps. Future work includes parallel transfer printing to reduce assembly time, localized bonding processes enabled by laser-assisted annealing, and extending this process to diverse MEMS materials such as SiO2, SixNy, Al, etc.

Microelectromechanical systems (MEMS), such as the miniaturization of large scale ordinary 3D machines, are indispensable for advancing modern technologies by providing performance enhancements and manufacturing cost reduction1,2. However, the current rate of technological advancement in MEMS cannot be maintained without continuous innovations in manufacturing technologies3-6. Common monolithic microfabrication primarily relies on layer-by-layer processes developed for the manufacture of integrated circuits (IC). This method has been quite successful at enabling mass production of high performance MEMS devices. However, owing to its complex layer-by-layer and electrochemically subtractive nature, manufacturing of diversely-shaped 3D MEMS structures and devices, while easy in the macroworld, is very challenging to achieve using this monolithic microfabrication. To enable more flexible 3D microfabrication with less process complexity, we developed a 3D additive micromanufacturing strategy (termed &#8216;micro/nano-masonry&#8217;) which involves a transfer printing-based assembly of micro/nanoscale materials in conjunction with rapid thermal annealing-enabled material bonding techniques.
Transfer printing is a method to transfer solid microscale materials (i.e.,&#160;&#8216;solid inks&#8217;) from a substrate where they are generated or grown to a different substrate by using controlled dry adhesion of elastomeric stamps. The typical procedure of micro-masonry starts with transfer printing. Prefabricated solid inks are transfer printed using a microtip stamp that is an advanced form of elastomeric stamps and the printed structures are subsequently annealed using rapid thermal annealing (RTA) to enhance ink-ink and ink-substrate adhesion. This manufacturing approach enables the construction of unusual microscale structures and devices that cannot be accommodated using other existing methods7.
Micro-masonry provides several attractive features not present in other methods: (a) the ability to integrate functional and structural solid inks of dissimilar materials to assemble MEMS sensors and actuators all integrated within the 3D structure; (b) the interfaces of assembled solid inks can function as electrical and thermal contacts9,10; (c) the assembly spatial resolution can be high (~1 &#956;m) by utilizing highly-scalable and well-understood lithographic processes for generating solid inks and highly-precise mechanical stages for transfer printing7; and (d) functional and structural solid inks can be integrated on both rigid and flexible substrates in planar or curvilinear geometries.

<table><tbody><tr><td>Az 5214</td><td>Clariant</td><td></td><td>1.5 mm thick Photoresist</td></tr><tr><td>Su8-100</td><td>Microchem</td><td></td><td>100 mm Photoresist used in mold</td></tr><tr><td>Sylgard 184</td><td>Dow Corning</td><td></td><td>PDMS mixed to fabricate stamp</td></tr><tr><td>Hydrofluoric acid</td><td>Honeywell</td><td></td><td>Acid to etch silicon oxide layer</td></tr><tr><td>Silicon on insulator</td><td>Ultrasil</td><td></td><td>Donor substrate was fabricated</td></tr><tr><td>Trichlorosilane</td><td>Sigma-Aldrich</td><td></td><td>Chemical used to help pealing of PDMS from mold</td></tr></tbody></table>

micro-masonry for 3d additive micromanufacturing

Transfer printing is a method to transfer solid micro/nanoscale materials (herein called &#8216;inks&#8217;) from a substrate where they are generated to a different substrate by utilizing elastomeric stamps. Transfer printing enables the integration of heterogeneous materials to fabricate unexampled structures or functional systems that are found in recent advanced devices such as flexible and stretchable solar cells and LED arrays. While transfer printing exhibits unique features in material assembly capability, the use of adhesive layers or the surface modification such as deposition of self-assembled monolayer (SAM) on substrates for enhancing printing processes hinders its wide adaptation in microassembly of microelectromechanical system (MEMS) structures and devices. To overcome this shortcoming, we developed an advanced mode of transfer printing which deterministically assembles individual microscale objects solely through controlling surface contact area without any surface alteration. The absence of an adhesive layer or other modification and the subsequent material bonding processes ensure not only mechanical bonding, but also thermal and electrical connection between assembled materials, which further opens various applications in adaptation in building unusual MEMS devices.

Micro-masonry enables heterogeneous material integration to generate MEMS structures that are very challenging or impossible to achieve by monolithic microfabrication processes. In order to demonstrate its capability, a structure (called a &#8216;micro teapot&#8217;) is fabricated solely through micro-masonry. Figure 4A&#160;is an optical microscope image of fabricated Si inks on a donor substrate. The designed inks are discs with different dimensions made of single crystalline silicon, which are the building blocks of the micro teapot. Once a donor substrate is independently prepared, discs are transfer printed onto a receiver substrate and annealed layer by layer utilizing a microtip stamp as shown in Figure 4B. The inner region of the micro teapot is hollow as can be seen from each assembled disc. Furthermore, the delicateness of micro-masonry processes is also tested by transfer printing and annealing a rather exquisite photonic crystal platelet (Figures 4C-E). Photonic surfaces are first patterned with nanoimprint lithography and made as transferrable inks on a donor substrate as outlined in the protocol. Once the ink is fully prepared, the photonic crystal platelet is transferred onto four Si rings with 50 &#956;m thickness, forming a table like configuration shown in Figure 4E.
Apart from micro-masonry for silicon inks, images in Figure 5 show examples of micro-masonry adopted to assemble thin Au films. Figure 5A&#160;is an optical microscope image of prepared 400 nm thick Au films on a donor substrate. These inks are further processed and tested to transfer print onto a Au surface (Figure 5B), as well as on a Si surface (Figure 5C).
Of key importance with this micro-masonry for Au thin film assembly is that in the absence of any adhesive layer, the transfer printed Au films exhibit electrical conductance with the receiver substrate. While it is difficult to achieve strong mechanical bonding between transfer printed Au films and a receiver Au surface, components are held in place through van der Waal&#8217;s force and exhibit great electrical conductance without any further processing (Figure 5B)9.
Conversely, heterogeneous integration of Au thin films with a Si surface is also achieved through transfer printing and rapid thermal annealing at approximately Si-Au eutectic temperature. Through the annealing process, contact resistance at the Si-Au interface is significantly reduced similar to that of sputter deposited sample due to Si-Au eutectic bonding. By transmission line measurement (TLM) experiments, this claim has been substantiated (Figure 5C)10.
<img alt="Figure 1" fo:content-width="5in" src="/files/ftp_upload/51974/51974fig1highres.jpg" width="500" /> 
Figure 1.&#160;General process flow of micro-masonry7. As a preparation step, a donor substrate, a stamp, and a receiver substrate are independently prepared. (A) Once all of the different components are arranged, first a microtip stamp is attached to a transparent glass slide placed upside down such that the microtips in the stamp are pointing downwards. After the stamp is securely placed, the donor substrate is located on an x,y-axis stage and the stamp is aligned with inks on the donor substrate through a microscope. (B) Subsequently, the stamp is brought down to the donor substrate and a preload is applied on the stamp, such that all microtips in the stamp are fully collapsed. (C) Afterwards, the stamp is quickly raised and the ink is retrieved and attached to a stamp. (D) In order to print the retrieved ink, the stamp with the ink is cautiously aligned to target area and lowered in such that the ink makes contact with the receiver substrate gently while the tips are partially collapsed. (E) While the ink is in contact with the receiver substrate, the stamp is raised slowly. Due to larger van der Waals interaction at ink-receiver interface than at stamp-ink interface, the ink remains on the receiver substrate. (F) The receiver substrate with the assembled inks is moved to a rapid thermal annealing furnace and annealed at 950 &#176;C for 10 min for Si-Si bonding or at 360 &#176;C for Si-Au bonding for 30 min. The annealing step following the transfer printing step completes the micro-masonry procedure. Reproduced with permission from Keum&#160;et al.7 <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/51974/51974fig1highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" fo:content-width="5in" src="/files/ftp_upload/51974/51974fig2highres.jpg" width="500" /> 
Figure 2.&#160;Schematic of donor substrate preparation. (A) On a SOI wafer, the device layer is patterned into desired dimensions and geometry. (B) A following HF wet etching process removes the exposed SiO2 box layer except for the regions beneath the patterned Si. (C) The photoresist is spun and patterned to form anchors. (D) Afterwards, the substrate is submerged into HF to etch away the remaining SiO2. After sufficient time in HF, the array of Si square inks is suspended and free standing with only photoresist anchors on the donor substrate. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/51974/51974fig2highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" fo:content-width="5in" src="/files/ftp_upload/51974/51974fig3highres.jpg" width="500" /> 
Figure 3.&#160;Schematic of stamp fabrication7. (A) In order to make a mold for microtip stamps, a Si wafer is cleaned and small pyramid shaped micropits are created on the wafer through KOH etching. (B) After the etching is done, the surface of the wafer is cleaned through RCA 2 cleaning process followed by applying and patterning SU8 to form a cavity over the microtips. Subsequently, a monolayer of trichlorosilane is coated on the mold to promote the following PDMS molding/demolding process by dropping 3-5 drops of trichlorosilane into a vacuum jar and placing the wafer and vacuuming the vacuum jar. (C) Once the coating is done, the PDMS precursor is poured and cured in the oven. (D) The cured PDMS is simply peeled off from the mold to complete the mold preparation process for a microtip stamp. Reproduced with permission from Keum&#160;et al.7 <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/51974/51974fig3highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" fo:content-width="5in" src="/files/ftp_upload/51974/51974fig4highres.jpg" width="500" /> 
Figure 4.&#160;Representative work in Si micro-masonry7. (A) Optical microscope images of ring shape Si inks on a donor substrate, (B) Scanning electron microscope (SEM) image of a micro teapot structure formed by micro-masonry, (C) Illustration of micro-masonry of a silicon photonic surface on four Si rings, (D, E) SEM images of the nanostructures on the photonic surface (D) and the assembled silicon photonic surface on four silicon rings (E). Reproduced with permission from Keum&#160;et al.7 <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/51974/51974fig4highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" fo:content-width="5in" src="/files/ftp_upload/51974/51974fig5highres.jpg" width="500" /> 
Figure 5.&#160;Representative work in Au micro-masonry8. (A) Optical microscopic view of the prepared donor substrate with Au inks retrieved in top row and ready to be retrieved in bottom row, (B) SEM image of a transfer printed Au film on a patterned Au surface, (C) SEM image of transfer printed Au films on a patterned Si strip. Reproduced with permission from Keum&#160;et al.8 <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/51974/51974fig5highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Micro-masonry for 3D Additive Micromanufacturing at JoVE.com

Micro-masonry for 3D Additive Micromanufacturing

This paper introduces a 3D additive micromanufacturing strategy (termed &#8216;micro-masonry&#8217;) for the flexible fabrication of microelectromechanical system (MEMS) structures and devices. This approach involves transfer printing-based assembly of micro/nanoscale materials in conjunction with rapid thermal annealing-enabled material bonding techniques.

Transfer printing is a method to transfer solid micro/nanoscale materials (herein called &#8216;inks&#8217;) from a substrate where they are generated to ...

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10.3K Views.  University of Illinois at Urbana-Champaign.  Dit artikel introduceert een 3D additief micromanufacturing strategie (zogenaamde "micro-metselwerk ') voor de flexibele productie van micro-elektromechanische systeem (MEMS) structuren en apparaten. Deze benadering houdt in transfer-printing gebaseerd assemblage van micro / nanoschaal materialen in combinatie met een snelle thermische-gloeien ingeschakeld materiaal verbindingstechnieken. 

Video: Micro-masonry for 3D Additive Micromanufacturing

Manufacturing of Three-dimensionally Microstructured Nanocomposites through Microfluidic Infiltration

Microstructured composite beams reinforced with complex three-dimensionally (3D) patterned nanocomposite microfilaments are fabricated via nanocomposite in&#64257;ltration of 3D interconnected microfluidic networks. The manufacturing of the reinforced beams begins with the fabrication of microfluidic networks, which involves layer-by-layer deposition of fugitive ink filaments using a dispensing robot, filling the empty space between filaments using a low viscosity resin, curing the resin and finally removing the ink.&#160;Self-supported 3D structures with other geometries and many layers (e.g.&#160;a few hundreds layers) could be built using this method. The resulting tubular micro&#64258;uidic networks are then infiltrated with thermosetting nanocomposite suspensions containing nanofillers (e.g.&#160;single-walled carbon nanotubes), and subsequently cured. The infiltration is done by applying a pressure gradient between two ends of the empty network (either by applying a vacuum or vacuum-assisted microinjection). Prior to the infiltration, the nanocomposite suspensions are prepared by dispersing nanofillers into polymer matrices using ultrasonication and three-roll mixing methods. The nanocomposites (i.e.&#160;materials infiltrated) are then solidified under UV exposure/heat cure, resulting in a 3D-reinforced composite structure. The technique presented here enables the design of functional nanocomposite macroscopic products for microengineering applications such as actuators and sensors.

Three-dimensional (3D) microstructured composite beams are fabricated through the directed and localized infiltration of nanocomposites into 3D porous microfluidic networks. The flexibility of this manufacturing method enables the utilization of different thermosetting materials and nanofillers in order to achieve a variety of functional 3D reinforced nanocomposite macroscopic products.

Microstructured composite beams reinforced with complex three-dimensionally (3D) patterned nanocomposite microfilaments are fabricated via nanocomposite ...

Chemistry

Microfabrication of Chip-sized Scaffolds for Three-dimensional Cell cultivation

Using microfabrication technologies is a prerequisite to create scaffolds of reproducible geometry and constant quality for three-dimensional cell cultivation. These technologies offer a wide spectrum of advantages not only for manufacturing but also for different applications. The size and shape of formed cell clusters can be influenced by the exact and reproducible architecture of the microfabricated scaffold and, therefore, the diffusion path length of nutrients and gases can be controlled.1 This is unquestionably a useful tool to prevent apoptosis and necrosis of cells due to an insufficient nutrient and gas supply or removal of cellular metabolites.
Our polymer chip, called CellChip, has the outer dimensions of 2 x 2 cm with a central microstructured area. This area is subdivided into an array of up to 1156 microcontainers with a typical dimension of 300 m edge length for the cubic design (cp- or cf-chip) or of 300 m diameter and depth for the round design (r-chip).2
So far, hot embossing or micro injection moulding (in combination with subsequent laborious machining of the parts) was used for the fabrication of the microstructured chips. Basically, micro injection moulding is one of the only polymer based replication techniques that, up to now, is capable for mass production of polymer microstructures.3 However, both techniques have certain unwanted limitations due to the processing of a viscous polymer melt with the generation of very thin walls or integrated through holes. In case of the CellChip, thin bottom layers are necessary to perforate the polymer and provide small pores of defined size to supply cells with culture medium e.g. by microfluidic perfusion of the containers.
In order to overcome these limitations and to reduce the manufacturing costs we have developed a new microtechnical approach on the basis of a down-scaled thermoforming process. For the manufacturing of highly porous and thin walled polymer chips, we use a combination of heavy ion irradiation, microthermoforming and track etching. In this so called "SMART" process (Substrate Modification And Replication by Thermoforming) thin polymer films are irradiated with energetic heavy projectiles of several hundred MeV introducing so-called "latent tracks" Subsequently, the film in a rubber elastic state is formed into three dimensional parts without modifying or annealing the tracks. After the forming process, selective chemical etching finally converts the tracks into cylindrical pores of adjustable diameter.

We present two processes for the microfabrication of porous polymer chips for three-dimensional cell cultivation. The first one is hot embossing combined with a solvent vapour welding process. The second one uses a recently developed microthermoforming process combined with ion track technology leading to a significant simplification of manufacture.

Using microfabrication technologies is a prerequisite to create scaffolds of reproducible geometry and constant quality for three-dimensional cell ...

Biology

A Soft Tooling Process Chain for Injection Molding of a 3D Component with Micro Pillars

The purpose of this paper is to present the method of a soft tooling process chain employing Additive Manufacturing (AM) for fabrication of injection molding inserts with micro surface features. The Soft Tooling inserts are manufactured by Digital Light Processing (vat photo polymerization) using a photopolymer that can withstand relatively high temperaturea. The part manufactured here has four tines with an angle of 60&#176;. Micro pillars (&#216;200 &#181;m, aspect ratio of 1) are arranged on the surfaces by two rows. Polyethylene (PE) injection molding with the soft tooling inserts is used to fabricate the final parts. This method demonstrates that it is feasible to obtain injection-molded parts with microstructures on complex geometry by additive manufactured inserts. The machining time and cost is reduced significantly compared to conventional tooling processes based on computer numerical control (CNC) machining. The dimensions of the micro features are influenced by the applied additive manufacturing process. The lifetime of the inserts determines that this process is more suitable for pilot production. The precision of the inserts production is limited by the additive manufacturing process as well.

A protocol for fabricating injection molding inserts for complex geometry with micro features on surfaces employing Additive Manufacturing (AM) is presented.

The purpose of this paper is to present the method of a soft tooling process chain employing Additive Manufacturing (AM) for fabrication of injection ...

Multi-material Ceramic-Based Components &#8211; Additive Manufacturing of Black-and-white Zirconia Components by Thermoplastic 3D-Printing (CerAM - T3DP)

To combine the benefits of Additive Manufacturing (AM) with the benefits of Functionally Graded Materials (FGM) to ceramic-based 4D components (three dimensions for the geometry and one degree of freedom concerning the material properties at each position) the Thermoplastic 3D-Printing (CerAM -&#160;T3DP) was developed. It is a direct AM technology which allows the AM of multi-material components. To demonstrate the advantages of this technology black-and-white zirconia components were additively manufactured and co-sintered defect-free.
Two different pairs of black and white zirconia powders were used to prepare different thermoplastic suspensions. Appropriate dispensing parameters were investigated to manufacture single-material test components and adjusted for the additive manufacturing of multi-color zirconia components.

Here we describe a protocol for additively manufacturing black-and-white zirconia components by Thermoplastic 3D-Printing (CerAM -&#160;T3DP) and co-sintering defect-free.

To combine the benefits of Additive Manufacturing (AM) with the benefits of Functionally Graded Materials (FGM) to ceramic-based 4D components (three ...

Additive Manufacturing of Functionally Graded Ceramic Materials by Stereolithography

An additive manufacturing technology is applied to obtain functionally graded ceramic parts. This technology, based on digital light processing/stereolithography, is developed within the scope of the CerAMfacturing European research project. A three-dimensional (3-D) hemi-maxillary bone-like structure is 3-D printed using custom aluminum oxide polymeric mixtures. The powders and mixtures are fully analyzed in terms of rheological behavior in order to ensure proper material handling during the printing process. The possibility to print functionally graded materials using the Admaflex technology is explained in this document. Field-emission scanning electron microscopy (FESEM) show that the sintered aluminum oxide ceramic part has a porosity lower than 1% and no remainder of the original layered structure is found after analysis.

This manuscript describes the processing of single multifunctional ceramic components (e.g., combinations of dense-porous structures) additively manufactured by stereolithography.

An additive manufacturing technology is applied to obtain functionally graded ceramic parts. This technology, based on digital light ...

Negative Additive Manufacturing of Complex Shaped Boron Carbides

Boron carbide (B4C) is one of the hardest materials in existence. However, this attractive property also limits its machineability into complex shapes for high wear, high hardness, and lightweight material applications such as armors. To overcome this challenge, negative additive manufacturing (AM) is employed to produce complex geometries of boron carbides at various length scales. Negative AM first involves gelcasting a suspension into a 3D-printed plastic mold. The mold is then dissolved away, leaving behind a green body as a negative copy. Resorcinol-formaldehyde (RF) is used as a novel gelling agent because unlike traditional hydrogels, there is little to no shrinkage, which allows for extremely complex molds to be used. Furthermore, this gelling agent can be pyrolyzed to leave behind ~50 wt% carbon, which is a highly effective sintering aid for B4C. Due to this highly homogenous distribution of in situ carbon within the B4C matrix, less than 2% porosity can be achieved after sintering. This protocol highlights in detail the methodology for creating near fully dense boron carbide parts with highly complex geometries.

A method called negative additive manufacturing is used to produce near fully dense complex shaped boron carbide parts of various length scales. This technique is possible via the formulation of a novel suspension involving resorcinol-formaldehyde as a unique gelling agent that leaves behind a homogenous carbon sintering aid after pyrolysis.

Boron carbide (B4C) is one of the hardest materials in existence. However, this attractive property also limits its machineability into complex shapes for ...

Micropatterning and Assembly of 3D Microvessels

In vitro platforms to study endothelial cells and vascular biology are largely limited to 2D endothelial cell culture, flow chambers with polymer or glass based substrates, and hydrogel-based tube formation assays. These assays, while informative, do not recapitulate lumen geometry, proper extracellular matrix, and multi-cellular proximity, which play key roles in modulating vascular function. This manuscript describes an injection molding method to generate engineered vessels with diameters on the order of 100 &#181;m. Microvessels are fabricated by seeding endothelial cells in a microfluidic channel embedded within a native type I collagen hydrogel. By incorporating parenchymal cells within the collagen matrix prior to channel formation, specific tissue microenvironments can be modeled and studied. Additional modulations of hydrodynamic properties and media composition allow for control of complex vascular function within the desired microenvironment. This platform allows for the study of perivascular cell recruitment, blood-endothelium interactions, flow response, and tissue-microvascular interactions. Engineered microvessels offer the ability to isolate the influence from individual components of a vascular niche and precisely control its chemical, mechanical, and biological properties to study vascular biology in both health and disease.

This manuscript presents an injection molding method to engineer microvessels that recapitulate physiological properties of endothelium. The microfluidic-based process creates patent 3D vascular networks with tailorable conditions, such as flow, cellular composition, geometry, and biochemical gradients. The fabrication process and examples of potential applications are described.

In vitro platforms to study endothelial cells and vascular biology are largely limited to 2D endothelial cell culture, flow chambers with polymer or glass ...

Bioengineering

Design and Development of a Three-Dimensionally Printed Microscope Mask Alignment Adapter for the Fabrication of Multilayer Microfluidic Devices

This project aims to develop an easy-to-use and cost-effective platform for the fabrication of precise, multilayer microfluidic devices, which typically can only be achieved using costly equipment in a clean room setting. The key part of the platform is a three dimensionally (3D) printed microscope mask alignment adapter (MMAA) compatible with regular optical microscopes and ultraviolet (UV) light exposure systems. The overall process of creating the device has been vastly simplified because of the work done to optimize the device design. The process entails finding the proper dimensions for the equipment available in the laboratory and 3D-printing the MMAA with the optimized specifications. Experimental results show that the optimized MMAA designed and manufactured by 3D printing performs well with a common microscope and light exposure system. Using a master mold prepared by the 3D-printed MMAA, the resulting microfluidic devices with multilayered structures contain alignment errors of &lt;10 &#181;m, which is sufficient for common microchips. Although human error through transportation of the device to the UV light exposure system can cause larger fabrication errors, the minimal errors achieved in this study are attainable with practice and care. Furthermore, the MMAA can be customized to fit any microscope and UV exposure system by making changes to the modeling file in the 3D printing system. This project provides smaller laboratories with a useful research tool as it only requires the use of equipment that is typically already available to laboratories that produce and use microfluidic devices. The following detailed protocol outlines the design and 3D printing process for the MMAA. In addition, the steps for procuring a multilayer master mold using the MMAA and producing poly(dimethylsiloxane) (PDMS) microfluidic chips is also described herein.

This project allows small laboratories to develop an easy-to-use platform for the fabrication of precise multilayer microfluidic devices. The platform consists of a three-dimensionally printed microscope mask alignment adapter using which multilayer microfluidic devices with alignment errors of &lt;10 &#181;m were achieved.

This project aims to develop an easy-to-use and cost-effective platform for the fabrication of precise, multilayer microfluidic devices, which typically ...

Computer Numerical Control Micromilling of a Microfluidic Acrylic Device with a Staggered Restriction for Magnetic Nanoparticle-Based Immunoassays

Microfluidic systems have greatly improved immunoassay techniques. However, many microfabrication techniques require specialized, expensive, or complicated equipment, making fabrication costly and incompatible with mass production, which is one of the most important preconditions for point-of-care tests (POCT) to be adopted in low-resource settings. This work describes the fabrication process of an acrylic (polymethylmethacrylate, PMMA) device for nanoparticle-conjugated enzymatic immunoassay testing using the computer numerical control (CNC) micromilling technique. The functioning of the microfluidic device is shown by performing an immunoassay to detect a commercial antibody using lysozyme as a model antigen conjugated to 100 nm magnetic nanoparticles. This device integrates a physical staggered restriction of only 5 &#181;m in height, used to capture magnetic microparticles that make up a magnetic trap by placing an external magnet. In this way, the magnetic force on the immunosupport of conjugated nanoparticles is enough to capture them and resist flow drag. This microfluidic device is particularly suitable for low-cost mass production without the loss of precision for immunoassay performance.

Microfluidics is a powerful tool for the development of diagnostic tests. However, expensive equipment and materials, as well as laborious fabrication and handling techniques, are often required. Here, we detail the fabrication protocol of an acrylic microfluidic device for magnetic micro- and nanoparticle-based immunoassays in a low-cost and simple-to-use setting.

Microfluidic systems have greatly improved immunoassay techniques. However, many microfabrication techniques require specialized, expensive, or ...

An Additive Manufacturing Technique for the Facile and Rapid Fabrication of Hydrogel-based Micromachines with Magnetically Responsive Components

Polyethylene glycol (PEG)-based hydrogels are biocompatible hydrogels that have been approved for use in humans by the FDA. Typical PEG-based hydrogels have simple monolithic architectures and often function as scaffolding materials for tissue engineering applications. More sophisticated structures typically take a long time to fabricate and do not contain moving components. This protocol describes a photolithography method that allows for facile and rapid microfabrication of PEG structures and devices. This strategy involves an in-house developed fabrication stage that allows for the rapid fabrication of 3D structures by building upwards in a layer-by-layer fashion. Independent moving components can also be aligned and assembled onto support structures to form integrated devices. These independent components are doped with superparamagnetic iron oxide nanoparticles that are sensitive to magnetic actuation. In this manner, the fabricated devices can be actuated using external magnets to yield movement of the components within. Hence, this technique allows for the fabrication of sophisticated MEMS-like devices (micromachines) that are composed entirely out of a biocompatible hydrogel, able to function without an onboard power source, and respond to a contact-less method of actuation. This manuscript describes the fabrication of both the fabrication set-up as well as the step-by-step method for the microfabrication of these hydrogels-based MEMS-like devices.

An additive manufacturing strategy for processing UV-crosslinkable hydrogels has been developed. This strategy allows for the layer-by-layer assembly of microfabricated hydrogel structures as well as the assembly of independent components, yielding integrated devices containing moving components that are responsive to magnetic actuation.

Polyethylene glycol (PEG)-based hydrogels are biocompatible hydrogels that have been approved for use in humans by the FDA. Typical PEG-based hydrogels ...

Micro-masonry for 3D Additive Micromanufacturing

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