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<h1 id="vat-photopolymerization">Vat Photopolymerization</h1>
<p>The Birth of 3D Printing </p>
<p><a href="https://www.dropbox.com/s/e63up8u48ktqs08/US4575330.pdf?dl=0">https://www.dropbox.com/s/e63up8u48ktqs08/US4575330.pdf?dl=0</a></p>
<p>Photopolymerization printing technology is composed of several different processes that rely on the same basic strategy: a liquid photopolymer contained in a vat/tank that is selectively cured by a heat and/or light source - a part is completed layer by layer. </p>
<h2 id="free-radical-photopolymerization">Free Radical Photopolymerization</h2>
<p>Before diving into the specific types of photoresing printing, it is important to understand the polymer chemistry powering them. While there are many types of polymerization, in this section we will be focusing on Free Radical Polymerization. Free Radical Polymerization is when a initiator break down and attacks the pi-bond of a monomer. This initiates a chain reaction where monomers link up (propagation) until a free radical bonds on the other end (Termination). For 3D printing, the initiators are usually photoinitators, which break down to free radicals in response to certain wavelengths of light. Photolysis is great for 3D printing because it only occurs where and when there is light. <a href="http://libgen.rs/book/index.php?md5=C10977BD7AC094535941FDC33D55BB81">This textbook</a> has a great chapter that goes into all the details. </p>
<p><img src="https://www.researchgate.net/profile/Mek-Zah-Salleh/publication/261013512/figure/fig2/AS:318033676521474@1452836545482/UV-free-radical-polymerization-process.png" alt=""></p>
<p>The speed of polymerization (Rp) is determined by Io, the light intensity; Kp, is how quickly monomers add themselves onto the chains; Kt, the rate at which free radicals are consumed; M, the amount of monomer present; I, the amount of photoinitiator present; f, the quantum yield/initiator efficiency; and e, the absorptivity of initiator (which is different at different wavelengths). </p>
<p><img src="https://paper-attachments.dropbox.com/s_01AC21F708A7445610B0DE943C6C7B86D6D9B43DCE2B7B1E54659153C16A4340_1619066935818_Screen+Shot+2021-04-22+at+12.48.49+AM.png" alt=""></p>
<p>Essentially, the RP of polymerization increases with 1) more monomers 2) more photoinitiator added 3) more light 4) good matching of the absorptivity profile of the initiator and the light wavelength 5) a more efficiency photo initiator and 6) monomers that propogate faster. In practice 1), 3), 4), and 2) are the most important to keep in mind. </p>
<h2 id="stereolithography-sla-">StereoLithogrAphy (SLA)</h2>
<p>SLA / Optical Fabrication / Photo-Solidification / Resin Printing
During the SLA manufacturing process, a concentrated beam of UV light or laser is focused onto the surface of a tank filled with a liquid photopolymer. SLA 3D printers use light-reactive thermoset materials called “resin.” When SLA resins are exposed to certain wavelengths of light, short molecular chains join together, polymerizing monomers and oligomers into solidified rigid or flexible geometries.The beam or laser is focused, creating each layer of the desired 3D object by means of cross-linking or degrading a poylmer. </p>
<p><a href="https://pubs.acs.org/doi/10.1021/acsapm.8b00165">https://pubs.acs.org/doi/10.1021/acsapm.8b00165</a></p>
<p><a href="https://formlabs.com/blog/ultimate-guide-to-stereolithography-sla-3d-printing/">https://formlabs.com/blog/ultimate-guide-to-stereolithography-sla-3d-printing/</a></p>
<p><strong>Classic SLA</strong>
Most industrial SLA printers fire a laser down into a vat of liquid polymer. This means that the vat must be deep enough to contain the entire part. As an example, an industrial SLA printer might require a 30-gallon vat of resin!</p>
<p><a href="https://www.3dhubs.com/knowledge-base/introduction-sla-3d-printing/">https://www.3dhubs.com/knowledge-base/introduction-sla-3d-printing/</a></p>
<p><a href="https://www.protolabs.com/services/3d-printing/stereolithography/">https://www.protolabs.com/services/3d-printing/stereolithography/</a></p>
<p><img src="https://paper-attachments.dropbox.com/s_EBEDDDA7335D5677E7DED4E9148177E13D105EC5FDB069D11C1B5D5E0909A8A7_1618872666381_F_ProcessAdditive_Photopolymerization2x.png" alt=""></p>
<p><strong>Inverted SLA</strong>
This configuration presents unique challenges, but it allows the machine design to be much more compact and resource-efficient than industrial SLA printers.</p>
<p><a href="https://www.youtube.com/watch?v=8a2xNaAkvLo&">https://www.youtube.com/watch?v=8a2xNaAkvLo&</a></p>
<p><a href="https://youtu.be/8a2xNaAkvLo">https://youtu.be/8a2xNaAkvLo</a></p>
<p><img src="https://paper-attachments.dropbox.com/s_EBEDDDA7335D5677E7DED4E9148177E13D105EC5FDB069D11C1B5D5E0909A8A7_1618873424655_sla_graphic.png__1354x0_q85_subsampling-2.png" alt=""></p>
<ul>
<li><a href="https://formlabs.com/blog/ultimate-guide-to-stereolithography-sla-3d-printing/">Guide To SLA 3D Printing Formlabs</a></li>
</ul>
<hr>
<h2 id="digital-light-processing-dlp-">Digital Light Processing (DLP)</h2>
<p>SLA and DLP are the two most common processes for resin 3D printing. Resin 3D printers are known for producing high-accuracy, isotropic, and watertight prototypes and parts in a range of advanced materials with fine features and smooth surface finish. Resin 3D printers like SLA and DLP offer some of the finest Z resolutions—thinnest layers—of all 3D printing processes and users can normally choose from a range of layer height options between <strong>25-300 microns</strong>.</p>
<p>The difference is the light source. DLP 3D printers use a digital projector screen to flash an image of a layer across the entire platform, curing all points simultaneously. Since the projector is a digital screen, the image of each layer is composed of square pixels, resulting in a layer formed from small rectangular bricks called voxels. </p>
<p><img src="https://paper-attachments.dropbox.com/s_EBEDDDA7335D5677E7DED4E9148177E13D105EC5FDB069D11C1B5D5E0909A8A7_1618873879447_dlp_graphic.png__1354x0_q85_subsampling-2.png" alt=""></p>
<p>SLA vs DLP</p>
<p><img src="https://paper-attachments.dropbox.com/s_EBEDDDA7335D5677E7DED4E9148177E13D105EC5FDB069D11C1B5D5E0909A8A7_1618873944763_sla-v-dlp_hero-3png__1354x0_q85_subsampling-2.png__1354x0_q85_subsampling-2.png" alt=""></p>
<p><strong>Resolution:</strong> </p>
<p><img src="https://paper-attachments.dropbox.com/s_EBEDDDA7335D5677E7DED4E9148177E13D105EC5FDB069D11C1B5D5E0909A8A7_1618874065832_sla-dlp-lfs-04_1.jpg__1354x0_q85_subsampling-2.jpg" alt=""></p>
<p><img src="https://paper-attachments.dropbox.com/s_EBEDDDA7335D5677E7DED4E9148177E13D105EC5FDB069D11C1B5D5E0909A8A7_1618874410696_sla-v-dlp-graphic-edgesjpg__1354x0_q85_subsampling-2.jpg__1354x0_q85_subsampling-2.jpg" alt=""></p>
<p>DLP: There’s a direct trade-off between resolution and build volume. The resolution depends on the projector, which defines the number of pixels/voxels available. If one moves the projector closer to the optical window, the pixels get smaller, which increases the resolution, but limits the available build area.
SLA: Inherently more scalable, SLA 3D printer’s build volume is completely independent of the resolution of the print. A single print can be any size and any resolution at any location within the build area.</p>
<ul>
<li>Software: <a href="https://formlabs.com/software/">https://formlabs.com/software/</a></li>
<li>SLA vs DLP <a href="https://formlabs.com/blog/resin-3d-printer-comparison-sla-vs-dlp/">https://formlabs.com/blog/resin-3d-printer-comparison-sla-vs-dlp/</a></li>
</ul>
<hr>
<h2 id="continuous-liquid-interface-production-clip-">Continuous Liquid Interface Production (CLIP)</h2>
<p><a href="https://www.thomasnet.com/articles/custom-manufacturing-fabricating/continuous-liquid-interface-production-3d-printing/">https://www.thomasnet.com/articles/custom-manufacturing-fabricating/continuous-liquid-interface-production-3d-printing/</a></p>
<p><a href="https://www.youtube.com/watch?v=ihR9SX7dgRo&&t=1s">https://www.youtube.com/watch?v=ihR9SX7dgRo&&t=1s</a></p>
<p>Continuous liquid interface production (CLIP) is a proprietary 3D printing method patented in 2014 by <a href="https://www.carbon3d.com/">Carbon3D</a> (formerly EiPi Systems). It is a 3D printing technology that falls under the general process of vat polymerization and shares many similarities with the older stereolithography (SLA) and digital light processing (DLP) printing methods. CLIP is unique from SLA and DLP as it is a truly continuous process that “grows” parts, removing the discrete steps of previous printing methods. CLIP’s innovation lies in its oxygen-permeable membrane that creates a dead zone underneath the part, allowing for continuous curing as the part is drawn out of the resin. Instead of using a layer-by-layer approach</p>
<p><img src="https://paper-attachments.dropbox.com/s_EBEDDDA7335D5677E7DED4E9148177E13D105EC5FDB069D11C1B5D5E0909A8A7_1618881711005_fig1_combined.jpg" alt="SLA/DLP (LEFT) - CLIP (RIGHT)"></p>
<p>The difference between CLIP and DLP is that, though they have comparable resolutions, the CLIP process smoothly blurs the layers together and eliminates the so-called voxelated effect that is typically seen in DLP prints > leading to less processing work and faster build times. </p>
<p><img src="https://paper-attachments.dropbox.com/s_EBEDDDA7335D5677E7DED4E9148177E13D105EC5FDB069D11C1B5D5E0909A8A7_1618880071955_Capture2.JPG" alt=""></p>
<p><img src="https://paper-attachments.dropbox.com/s_EBEDDDA7335D5677E7DED4E9148177E13D105EC5FDB069D11C1B5D5E0909A8A7_1618880066742_Capture.JPG" alt=""></p>
<p><img src="https://paper-attachments.dropbox.com/s_EBEDDDA7335D5677E7DED4E9148177E13D105EC5FDB069D11C1B5D5E0909A8A7_1618881856327_fig3.jpg" alt=""></p>
<p><a href="https://youtu.be/ihR9SX7dgRo?t=1s">https://youtu.be/ihR9SX7dgRo?t=1s</a></p>
<h2 id="high-area-rapid-printing-harp-">High Area Rapid Printing (HARP)</h2>
<p>Printing speeds within the previous processes are limited by the heat buildup from the exothermic polymerization process, thus limiting the ultimate size of the printed object. HARP use a pumped, nonreactive fluorinated oil to act as the dead layer that removes heat during polymerization. This approach allows for both speedup and scale-up of the printing process. </p>
<ul>
<li><p>The oil is also filtered to remove fragments of plastic generated during the process, which can scatter light and decrease the resolution of the printing. As with other continuous printing systems, UV light enters the tank through its transparent base.</p>
</li>
<li><p>Because HARP does not require an oxygen dead layer, it is compatible with both oxygen-sensitive and -insensitive ink chemistries, increasing the scope of applicable resins and resulting materials.</p>
</li>
</ul>
<p>38 cm x 61 cm x 76 cm lattice structure in 105minutes - the highest throughput achieved by any stereolithography system</p>
<p><a href="https://www.youtube.com/watch?v=mSkvJizACfc&">https://www.youtube.com/watch?v=mSkvJizACfc&</a></p>
<p><a href="https://youtu.be/mSkvJizACfc">https://youtu.be/mSkvJizACfc</a></p>
<p><a href="https://science.sciencemag.org/content/366/6463/360">https://science.sciencemag.org/content/366/6463/360</a></p>
<p><img src="https://paper-attachments.dropbox.com/s_EBEDDDA7335D5677E7DED4E9148177E13D105EC5FDB069D11C1B5D5E0909A8A7_1618882817992_600f34b4966ca500d6a4ecef_AZUL_2.gif" alt=""></p>
<h2 id="https-www-azul3d-com-"><a href="https://www.azul3d.com/">https://www.azul3d.com/</a></h2>
<h2 id="two-photon-polymerization">Two-Photon Polymerization</h2>
<p>Two-Photon polymerization uses the constructive interference of two-photons of light to initiate polymerization. With this scheme, each photo has half the energy needed to initiate. This is the opposite of single-photon exciting, which mostly hardens where the laser is focused but has some bleed because the photon leaving the focus still harden the resin. With two-photon, the probability of initiation depends on the probability of two separate photons absorbing into the photo initiator. This absorbance nonlinearity means it is essentially only the exact spot where the light if focused that the resin hardens. While extremely precise, this method is extremely slow, the printers cost hundreds of thousands of dollars, and only small objects can be printed. </p>
<p><img src="https://physicsworld.com/wp-content/uploads/2018/04/PW-Optics-Steen.jpg" alt=""></p>
<p><a href="https://www.youtube.com/watch?v=CZifB2aQDDM&&t=827s">https://www.youtube.com/watch?v=CZifB2aQDDM&&t=827s</a></p>
<p><a href="https://www.nature.com/articles/s41467-019-12360-w">https://www.nature.com/articles/s41467-019-12360-w</a></p>
<h2 id="computed-axial-lithography-cal-">Computed Axial Lithography (CAL)</h2>
<p>Distinguishing from previous stereolithography processes that fabricate structures layer by layer, volumetric stereolithography produces 3D objects with the formation of 3D volumes as a unit operation. This system works by taking a 3D object, breaking that into slices of it viewed at different angles. That image is projected into the rotating resin vat. A key element they are using here is oxygen inhibition. When the photo initiator is broken down to free radicals, oxygen will quickly quench the radicals until the oxygen is depleted. Once depleted, then the cross linking occurs. The researchers use this “cross linking threshold” as a way to selectively build up energy in the vat, allowing for area specific cross linking. </p>
<p><a href="https://www.3dprintingmedia.network/more-details-emerge-on-uc-berkeley-llnl-new-cal-volumetric-3d-printing-method/">https://www.3dprintingmedia.network/more-details-emerge-on-uc-berkeley-llnl-new-cal-volumetric-3d-printing-method/</a></p>
<p><a href="https://science.sciencemag.org/content/363/6431/1075">https://science.sciencemag.org/content/363/6431/1075</a></p>
<h2 id="xolography">Xolography</h2>
<p>This work is the advanced version of DLP. It can generate prints 10x more detailed than CAL and with 4-5 times higher print speeds that Two-Photon polymerization. To do this, it uses a photo initiator that requires two colors of light to initiate polymerization. The first beam of light is generated by a laser “sheet” that goes through the cross section of the print. The second team is generated by a projector (similar to DLP). Each layer is build by moving the laser sheet through the tank. </p>
<p><a href="https://www.nature.com/articles/s41586-020-3029-7">https://www.nature.com/articles/s41586-020-3029-7</a></p>
<hr>
<h2 id="stereolithography-apparatus-for-tissue-engineering-slate-">Stereolithography Apparatus for Tissue Engineering (SLATE)</h2>
<p>In general, the difficulty with resin technique for bioprinting is that 1) once cross linked, it is hard to get cells into the matrix evenly and often the matrix can be quite weak 2) the resin precursor can often be toxic to the cells. </p>
<p><a href="https://n-e-r-v-o-u-s.com/blog/?p=8433">https://n-e-r-v-o-u-s.com/blog/?p=8433</a></p>
<p><a href="https://news.rice.edu/2019/05/02/organ-bioprinting-gets-a-breath-of-fresh-air-2/#:~:text=Bioprinted%20human%20organs%20might%20someday,one%20layer%20at%20a%20time">https://news.rice.edu/2019/05/02/organ-bioprinting-gets-a-breath-of-fresh-air-2/#:~:text=Bioprinted%20human%20organs%20might%20someday,one%20layer%20at%20a%20time</a>.</p>
<ul>
<li> The SLATE printer can embed live cells into soft gels containing very small, intricate blood vessels down to 300 microns in diameter. Hydrogels printed in only minutes by SLATE can function as lung-like networks with entangled air / blood networks</li>
<li> SLATE creates individual layers of soft hydrogels using a liquid pre-hydrogel solution. When exposed to blue light the layers become solid enough to maintain their shape, yet sturdy enough to act as internal human organs.</li>
<li> A digital light projector illuminates the gel from below, with a resolution of 10 to 50µm per pixel. Solidified layers are raised by an overhead arm to allow more fluid to flow in underneath and be exposed as the next layer.</li>
</ul>
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