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<h1 id="4d-printing">4D Printing</h1>
<p>“4D Printing” is a term coined by Skylar Tibbits that considers time as the fourth dimension. The basic premise is that something can be printed in one form (usually flat) and triggered to transform to a complex 3D shape. There are a few advantages that come to this technique:</p>
<ol>
<li>Generally faster prints times</li>
<li>No support material needed</li>
<li>Flat packing for space saving </li>
<li>“Active” materials that can change between two preprogrammed states</li>
<li>Features (such as conductive traces) can be screenprinted onto the flat print, then triggered to a 3D shape</li>
</ol>
<p>In fact, Carbon’s Adidas shoes are 4D printed! </p>
<p><img src="https://paper-attachments.dropbox.com/s_CD0553E59A1E05CDA41949761229CE0485F261672AA98D2AFEEB3821E07292D4_1619108525542_PRINT_HERO1.png" alt=""></p>
<p><img src="https://paper-attachments.dropbox.com/s_CD0553E59A1E05CDA41949761229CE0485F261672AA98D2AFEEB3821E07292D4_1619108539634_bd7ae277d7f17ff0ec8952477a2f38ba.jpg" alt=""></p>
<p>There are many different ways to 4D Print with many of the methods covered.</p>
<h2 id="extrusion-4d-printing">Extrusion 4D Printing</h2>
<p>Warped PLA prints are often a point of frustration for 3D printed enthusiasts. A number of works in recent years have looks at exploiting this warpage (which can be controlled by print direction and print speed) to create morphing prints based on arbitrary geometries. <a href="https://www.morphingmatter.cs.cmu.edu/projects/thermorph">Like this one:</a></p>
<p><img src="https://paper-attachments.dropbox.com/s_CD0553E59A1E05CDA41949761229CE0485F261672AA98D2AFEEB3821E07292D4_1619108702118_flower_gif_small.gif" alt=""></p>
<p>Machine learning algorithms have been employed to allow inverse design which takes a desired target shape and computer the flattened print pattern. <a href="https://www.morphingmatter.cs.cmu.edu/projects/simulearn">See here:</a></p>
<p><img src="https://paper-attachments.dropbox.com/s_CD0553E59A1E05CDA41949761229CE0485F261672AA98D2AFEEB3821E07292D4_1619108803875_03-computational-theme.jpg" alt=""></p>
<p>Another paper uses the flat stage as an oppurtunity to screenprint conductive traces before triggering. <a href="https://www.morphingmatter.cs.cmu.edu/projects/morphing-circuit">See here:</a></p>
<p><img src="https://paper-attachments.dropbox.com/s_CD0553E59A1E05CDA41949761229CE0485F261672AA98D2AFEEB3821E07292D4_1619109460262_MorphingCircuit05_1750.jpg" alt=""></p>
<p>This scheme of morphing can also be applied to materials besides PLA. <a href="https://www.nature.com/articles/nmat4544">In a seminal paper</a>, Gladman et al. extruded thermoresponsive gel materials with cellulose. The cellulose particles aligned along the extrusion direction and result in anisotropic swelling. </p>
<p><img src="https://paper-attachments.dropbox.com/s_CD0553E59A1E05CDA41949761229CE0485F261672AA98D2AFEEB3821E07292D4_1619109344019_41563_2016_Article_BFnmat4544_Fig3_HTML.jpg.webp" alt=""></p>
<h2 id="Bilayer">Bilayer</h2>
<p>
By printing with multiple materials, bilayers can be formed. If one bilayer swells or shrinks in response to heat, and the other does not, bending will occur. This has been done with a number of materials for a number of applications, including shape changing pasta!</p>
<iframe src="https://player.vimeo.com/video/199408741" width="640" height="360" frameborder="0" allow="autoplay; fullscreen; picture-in-picture" allowfullscreen></iframe>
<p><a href="https://vimeo.com/199408741">Transformative Appetite</a> from <a href="https://vimeo.com/tangiblemedia">Tangible Media Group</a> on <a href="https://vimeo.com">Vimeo</a>.</p>
<p>Another example is where conductive filament is printed onto paper. When current is put across the filament, the polymer expands causing deformation. When the heat is removed the paper restored the original shape - reversible deformations!</p>
<iframe src="https://player.vimeo.com/video/280417057" width="640" height="360" frameborder="0" allow="autoplay; fullscreen; picture-in-picture" allowfullscreen></iframe>
<p><a href="https://vimeo.com/280417057">Printed Paper Actuator</a> from <a href="https://vimeo.com/user80722544">Morphing Matter Lab</a> on <a href="https://vimeo.com">Vimeo</a>.</p>
<h2 id="Photopolymerization">Photopolymerization</h2>
<p>
Photopolymerization can also be leverage to create these anisotropic structures. By selectively cross linking certain regions of the print more than others (by exposing to light longer) the swelling ratio can be locally decreased. This technique allows fantastically complex 3D shapes to be printed almost instantly. <a href="https://www.nature.com/articles/s41467-021-20934-w">See here:</a></p>
<p><img src="https://paper-attachments.dropbox.com/s_CD0553E59A1E05CDA41949761229CE0485F261672AA98D2AFEEB3821E07292D4_1619109060618_41467_2021_20934_Fig4_HTML.png.webp" alt=""></p>
<p><img src="https://paper-attachments.dropbox.com/s_CD0553E59A1E05CDA41949761229CE0485F261672AA98D2AFEEB3821E07292D4_1619109076812_41467_2021_20934_Fig2_HTML.png.webp" alt=""></p>
<p> This technique has also been shpown to be useful for bioprinting. Since it is hard to get even cell seeded on complex 3D shapes. REsearchers first seeded the flat shape, then the shape change to the desired 3D shape. <a href="https://onlinelibrary.wiley.com/doi/10.1002/adfm.201400176">See here:</a></p>
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