An array of silver-TPU ink sensors printed on a flexible TPU base. (Image courtesy of the Wyss Institute for Biologically Inspired Engineering)

New method combines precision printing of stretchable conductive inks with pick-and-place of electronic components to make flexible, wearable sensors

Low-cost wearables manufactured by hybrid 3D printing

Nature has produced exquisite composite materials&mdash;wood, bone, teeth, and shells, for example&mdash;that combine light weight and density with desirable mechanical properties such as stiffness, strength and damage tolerance.Since ancient civilizations first combined straw and mud to form bricks, people have fabricated engineered composites of increasing performance and complexity. But reproducing the exceptional mechanical properties and complex microstructures found in nature has been challenging.Now, a team of researchers at the&nbsp;<a href="http://seas.harvard.edu/" target="_blank">Harvard John A. Paulson School of Engineering and Applied Sciences</a> (SEAS) has demonstrated a novel 3D printing method that yields unprecedented control of the arrangement of short fibers embedded in polymer matrices. They used this additive manufacturing technique to program fiber orientation within epoxy composites in specified locations, enabling the creation of structural materials that are optimized for strength, stiffness, and damage tolerance.Their method, referred to as &ldquo;rotational 3D printing,&rdquo; could have broad ranging applications. Given the modular nature of their ink designs, many different filler and matrix combinations can be implemented to tailor electrical, optical, or thermal properties of the printed objects.&ldquo;Being able to locally control fiber orientation within engineered composites has been a grand challenge,&rdquo; said the study&rsquo;s senior author, <a href="https://www.seas.harvard.edu/directory/jalewis" target="_blank">Jennifer A. Lewis</a>, Hansjorg Wyss Professor of Biologically Inspired Engineering at Harvard SEAS. &ldquo;We can now pattern materials in a hierarchical manner, akin to the way that nature builds.&rdquo; Lewis is also a Core Faculty Member of the <a href="http://wyss.harvard.edu/" target="_blank">Wyss Institute for Biologically Inspired Engineering at Harvard.</a><img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1569469293043-LewisNozzle.gif" style="width: 766px;" class="fr-fic fr-dib">The work, described in the journal&nbsp;<a href="http://www.pnas.org/" target="_blank">PNAS</a>, was carried out in the Lewis lab at Harvard. Collaborators included then-postdoctoral fellows Brett Compton (now Assistant Professor in Mechanical Engineering at the University of Tennessee, Knoxville), and Jordan Raney (now Assistant Professor of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania); and visiting PhD student Jochen Mueller from Prof. Kristina Shea&rsquo;s lab at ETH Zurich.&nbsp;&ldquo;Rotational 3D printing can be used to achieve optimal, or near optimal, fiber arrangements at every location in the printed part, resulting in higher strength and stiffness with less material,&rdquo; Compton said. &ldquo;Rather than using magnetic or electric fields to orient fibers, we control the flow of the viscous ink itself to impart the desired fiber orientation.&rdquo;Compton noted that the team&rsquo;s nozzle concept could be used on any material extrusion printing method, from fused filament fabrication, to direct ink writing, to large-scale thermoplastic additive manufacturing, and with any filler material, from carbon and glass fibers to metallic or ceramic whiskers and platelets.The technique allows for the 3D printing of engineered materials that can be spatially programmed to achieve specific performance goals. For example, the orientation of the fibers can be locally optimized to increase the damage tolerance at locations that would be expected to undergo the highest stress during loading, hardening potential failure points.&ldquo;One of the exciting things about this work is that it offers a new avenue to produce complex microstructures, and to controllably vary the microstructure from region to region,&rdquo; Raney said. &ldquo;More control over structure means more control over the resulting properties, which vastly expands the design space that can be exploited to optimize properties further.&quot;&nbsp;&quot;Biological composite materials often have remarkable mechanical properties: high stiffness and strength per unit weight and high toughness. One of the outstanding challenges of designing engineering materials inspired by biological composites is control of fiber orientation at small length scales and at the local level,&rdquo; said Lorna J. Gibson, Professor of Materials Science and Engineering at MIT, who was not involved in the research. &ldquo;This remarkable paper from the Lewis group demonstrates a way of doing just that. This represents a huge leap forward in the design of bio-inspired composites.&quot;The <a href="http://otd.harvard.edu/" target="_blank">Harvard Office of Technology Development</a> has protected the intellectual property relating to this project.Previously, Lewis has conducted groundbreaking research in the 3D printing of&nbsp;<a href="http://www.seas.harvard.edu/news/2014/02/essential-step-toward-printing-living-tissues" target="_blank">tissue constructs with vasculature</a>,&nbsp;<a href="http://www.seas.harvard.edu/news/2013/06/printing-tiny-batteries" target="_blank">lithium-ion microbatteries</a>,&nbsp;and the first&nbsp;<a href="https://www.seas.harvard.edu/news/2016/08/first-autonomous-entirely-soft-robot" target="_blank">autonomous, entirely soft robot</a>.

A novel 3D printing method, called “rotational 3D printing,” yields unprecedented control of the arrangement of short fibers embedded in polymer matrices. Researchers used additive manufacturing technique to program fiber orientation within epoxy composites in specified locations, enabling the creation of structural materials that are optimized for strength, stiffness, and damage tolerance. (Video courtesy of the Lewis Lab/Harvard SEAS)

Arranging fibers just like nature does it

Novel 3D printing technique yields high-performance composites

Harvard researchers have developed a platform for creating soft robots with embedded sensors that can sense movement, pressure, touch, and temperature. (Image courtesy of Ryan L. Truby/Harvard University)

Soft robots that can sense touch, pressure, movement and temperature

Novel 3D printing method embeds sensing capabilities within robotic actuators

<h1>Introduction</h1>3D printing technologies based on <a href="https://www.wevolver.com/article/digital.light.processing.3d.printing.explained" rel="noopener noreferrer" target="_blank">DLP printing (Digital Light Processing)</a> are frequently used for fabricating complex objects without tooling and machining. DLP printing involves the light-mediated conversion of a liquid resin containing monomer or oligomer photopolymers to a solid object. This process leverages the versatility of polymer chemistry to make complex objects with programmable optical, chemical or mechanical properties. Versatility underpins the popularity of polymer materials within the 3D printing community, encouraging research in photopolymer chemistry and printing applications. <iframe src="https://www.youtube.com/embed/97ARLiTHjX0?&wmode=opaque&enablejsapi=1&origin=https://www.wevolver.com" allowfullscreen="" class="fr-draggable" style="width: 788px; height: 500px;" frameborder="0" data-lf-form-tracking-inspected-kn9eq4roawkarlvp="true" data-lf-yt-playback-inspected-kn9eq4roawkarlvp="true" data-lf-vimeo-playback-inspected-kn9eq4roawkarlvp="true"></iframe>This video explains the process and working of DLP printers in detail - by Professor Bill Hammack, Department of Chemical Engineering, University of Illinois Despite the growing enthusiasm for 3D printing in rapid prototyping applications, implementation in manufacturing environments to produce end-use parts has lagged. Common problems include setting and maintaining minimum specifications, specifically those related to material durability and weathering. Additive manufacturing methods rely on tight synchronicity between hardware, software and chemistry to achieve reproducibility at scale. This article highlights the role of polymerization in the co-development and simultaneous tuning of each component in the DLP process. By understanding the fundamentals of polymerization, developers can simultaneously tune process components, avoiding common pitfalls contributing to part defects, detachment, post-processing and weathering problems.This article is sponsored by polySpectra, which makes functional materials for advanced additive manufacturing. PolySpectra CEO Raymond Weitekamp explained to Wevolver that they&#39;ve spent thousands of hours and hundreds of thousands of dollars characterizing the thermomechanical properties of dozens of additive manufacturing polymers. For Wevolver readers they are providing a free specification sheet of their newest materials at&nbsp;<a href="http://www.polyspectra.com/wevolver" rel="noopener noreferrer" target="_blank">www.polyspectra.com/wevolver</a> <h1>Photopolymerization: A multi-stage, dynamic process</h1>A photopolymer is a polymer whose properties alter upon exposure to light. Photopolymers are a liquid mixture containing monomers and oligomers, which can be manipulated to achieve specified physical properties. The process of photopolymerization (also known as photocuring or photo crosslinking) describes the crosslinking of monomers / oligomers within the liquid resin to form a network polymer. This process typically occurs via cationic or radical mechanisms and is mediated by the addition of photoinitiators. Photoinitiators are molecules that convert photolytic energy into reactive species to activate polymerization through specific functional groups.<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNTY4Mjk2NjE4NDExLVBob3RvLXBvbHltZXJfc2NoZW1lMSAwMi5wbmciLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjk1MCwiZml0IjoiY292ZXIifX19" style="width: 550px;" class="fr-fic fr-dib">Photopolymerization is an attractive method of curing polymers for 3D printing applications because it can be performed selectively. However, the process of photopolymerization is complex and sensitive to a range of parameters including wavelength, temperature and moisture. The extent and homogeneity of the polymerized cross-linked network is an important determinant in the properties of the final hardened material. Photopolymerization is a multi-stage and dynamic process. <a href="http://rawwerks.com/sla-utility/" rel="noopener noreferrer" target="_blank">Photopolymerization kinetics</a> is described as a &ldquo;working curve&rdquo;, mapping cure depth (depth to which light induces polymer crosslinking) as a function of light exposure at different wavelengths. Critical exposure (Y-intercept on the working curve) is defined as the minimum light exposure required for the onset of curing. The behaviour of the photopolymer under specified light exposure conditions (wavelength, power, exposure time and velocity) affect the printing process. There are a wide range of photopolymers available, responding to wavelengths in the UV (190-400 nm), visible (400-700 nm) and IR ranges (700-1000 nm). Light of different wavelengths penetrate materials to different cure depths. <img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNTY4Mjk4MzAwNTYzLVBSLTQ4LVdDLnBuZyIsImVkaXRzIjp7InJlc2l6ZSI6eyJ3aWR0aCI6OTUwLCJmaXQiOiJjb3ZlciJ9fX0=">An example of a Working Curved that shows the relation between UV light dose and curing depth of the resin. Image courtesy Polyspectra. <iframe src="https://www.youtube.com/embed/Lw3BY2_yGVU?&wmode=opaque&enablejsapi=1&origin=https://www.wevolver.com" allowfullscreen="" class="fr-draggable" style="width: 788px; height: 470px;" frameborder="0" data-lf-form-tracking-inspected-kn9eq4roawkarlvp="true" data-lf-yt-playback-inspected-kn9eq4roawkarlvp="true" data-lf-vimeo-playback-inspected-kn9eq4roawkarlvp="true"></iframe>This video explains how the working curve impacts the 3D print and how to find the correct settings - by Autodesk Ember <h1>Photopolymerization: A fundamental process in DLP printing</h1>The dynamic nature of photopolymerization suggests that it is a non-binary process. In DLP printing, light is projected into the polymer resin forming a 2D image. This simultaneously exposes all the designated regions within a polymer layer. The overall printing time is reduced through compared to rastered stereolithography, due to the parallelization of the photocuring process. The projected 2D image is composed of pixels, which become voxels when translated into three dimensions. This has led to the misconception that DLP is an essentially digital binary processtechnology and that print resolution is purely a function of printer hardwarethe DLP optics. In reality, voxels contain photopolymers that are fluxing between stages of the photopolymerization process: going from liquid to their gel point and then on towards their full cross-linking density.<iframe src="https://www.youtube.com/embed/5qTAmPrHLow?&wmode=opaque&enablejsapi=1&origin=https://www.wevolver.com" allowfullscreen="" class="fr-draggable" style="width: 788px; height: 470px;" frameborder="0" data-lf-form-tracking-inspected-kn9eq4roawkarlvp="true" data-lf-yt-playback-inspected-kn9eq4roawkarlvp="true" data-lf-vimeo-playback-inspected-kn9eq4roawkarlvp="true"></iframe>This video shows the impact of voxels on the 3D print and how printers can actually achieve resolutions smaller than the nominal resolution of the DLP projector - by Autodesk Ember. This can influence the 3D printing process in several ways. Firstly, printing speed should be calibrated to account for photopolymerization kinetics. If not, voxels may expand beyond their digitally defined physical space, reducing feature resolution of the finished part. Further, photopolymerization may cause shrinkage in the hardened polymer. Shrinkage coupled with cycles of thermal expansion and contraction experienced during the printing process may induce stressinduces stresses on the part, . These stresseswhich can manifest in warping or layer splitting or separation in the finished object. Strategies to mitigate these effects include tuning both software and hardware to select appropriate exposure protocols (velocity and speed) and control environmental influences (temperature and humidity). Finally, the cure depth is an important parameter to consider when implementing a DLP printing protocol. Cure depth directly impacts the z- axis settings of the printer. It may also influence the selection of printing parameters such as print velocity.Recommended reading:&nbsp;<a href="https://www.wevolver.com/article/digital.light.processing.3d.printing.explained" rel="noopener noreferrer" target="_blank">Digital Light Processing 3D printing explained</a><h1>Detachment: Building and breaking interactions between materials and hardware</h1>3D objects are rendered through the sequential hardening of photopolymer layers onto a substrate (known as a build plate). An essential step in any 3D printing process is the detachment of the finished part from the substrate. This step can prove problematic, as the printing process can induce suction forces, securing the part to the substrate. Numerous separation mechanics have been described to overcome these suction forces. Examples include tilting the resin tray and peeling the part off or using a sliding mechanism. Building the part on a flexible window, to which pressure may be applied to break suction forces is also a common method.<img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNTY4MzU3NDgwMzU4LUNhcmJvbiBCZWQuanBnIiwiZWRpdHMiOnsicmVzaXplIjp7IndpZHRoIjo5NTAsImZpdCI6ImNvdmVyIn19fQ==">A print (with support structure) attached to a print bed after printing. Source: Carbon 3DBuilding an oxygen permeable window within the substrate allows oxygen to access the photopolymer directly in contact with the substrate. This creates a depletion zone of inactive resin which does not adhere to the substrate, facilitating part removal. However, this method is only suitable for photopolymers which are quenched by oxygen, such as acrylates and methacrylates. An alternative strategy is to modify the viscosity of the liquid photopolymer resin. Low viscosity solutions enable higher printing speeds and concomitant reduction in the suction forces adhering the part to the substrate. This highlights an important aspect of 3D printing; the relationship between the photopolymer materials and the design of the printer hardware. It is widely recognised that surface adhesion is important to provide a foundation for the object to build upon. Consequently, the materials within the hardware and the finished printed object should be compatible.<h1>Post-processing: The significance of photopolymerization in scaling production</h1>DLP printing found initial application as an instrument for rapid prototyping in research settings. Introducing DLP printing into manufacturing settings has been slower due to problems with scaling production, reproducibility and part quality. The co-development of printing processes and materials is not only critical during the initial build process. Photopolymerization kinetics also affect post-processing of end-use parts. The acceptance of 3D printing into manufacturing settings has been restricted by extensive post-processing requirements including:<ul><li>Removal of the support structure (onto which the part is fabricated).</li><li>Part cleaning to remove debris and residual materials.</li><li>Post-curing (usually thermal or optical) to completely harden certain materials.</li><li>Part finishing (such as sanding, painting or varnishing) to remove the &ldquo;boxy&rdquo; surface finish characteristic of DLP printing.</li></ul><img src="https://images.wevolver.com/eyJidWNrZXQiOiJ3ZXZvbHZlci1wcm9qZWN0LWltYWdlcyIsImtleSI6ImZyb2FsYS8xNTY4MzU3NTQ3NzY2LXBvc3QgcHJvY2Vzc2luZy5qcGciLCJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjk1MCwiZml0IjoiY292ZXIifX19" style="width: 766px;" class="fr-fic fr-dib">Rinsing off resing, removing support, and curing with UV light. Source: FormlabsPost-processing steps are often performed in small batches under carefully controlled conditions. These processes can be difficult to automate, adding substantial time and cost to the production process. Advances in automation are ongoing, with some companies utilising robotics to automate assembly lines integrating 3D printers with post-processing equipment. Advances in printing hardware and software enabling automated monitoring could enable DLP printing at scale. However, the widespread implementation of automated processing lines has yet to be realised. Post-processing requirements are heavily dependent on photopolymer characteristics, with some materials requiring more extensive modification than others. Strategies to streamline operations should include selecting materials with limited post-processing requirements yet withstand weathering processes to maintain durability.<h1>Conclusion</h1>The combination of pivotal developments in 3D printing hardware, software and materials has paved the way towards the implementation in prototyping and manufacturing. With an increased awareness of the importance of co-developing each component simultaneously, the development of new applications for 3D printing of end-use parts is slowly becoming a reality. The relevance of DLP printing in the manufacturing sector will depend heavily on the ability to cost-effectively produce end-parts that are durable. The dynamic nature of photopolymerization continues throughout the lifetime of a part. The weatherability of hardened polymers is an important determinant of part durability, specifically the ability to maintain toughness and impact strength over time. By understanding the kinetics of photopolymerization and thermodynamic properties of the resulting polymer networks, developers can select materials to mitigate common problems, yet maintain desired properties throughout the lifetime of a part. <hr>This article was sponsored by&nbsp;<a href="http://polyspectra.com/wevolver" rel="noopener noreferrer" target="_blank">polySpectra</a>. polySpectra makes functional materials for advanced additive manufacturing in Berkeley, California, USA. They use light-activated catalysts to 3D print advanced functional materials. Polyspectra&#39;s platform enables them to deliver materials with a broad spectrum of tailored properties from a single chemical system.For Wevolver readers polySpectra is providing a free specification sheet of their newest materials at&nbsp;<a href="http://www.polyspectra.com/wevolver" rel="noopener noreferrer" target="_blank">www.polyspectra.com/wevolver</a>

DLP (Digital Light Processing) 3D printing uses photopolymerization to create 3D objects. Understanding the photopolymerization process can help you mitigate common problems that make your prints fail or that reduce print quality.

DLP 3D Printing: Understanding process and materials enables you to prevent production problems

Additive manufacturing, otherwise known as 3D printing, can be used to manufacture porous electrodes for lithium-ion batteries&mdash;but because of the nature of the manufacturing process, the design of these 3D printed electrodes is limited to just a few possible architectures. Until now, the internal geometry that produced the best porous electrodes through additive manufacturing was what&rsquo;s known as an interdigitated geometry&mdash;metal prongs interlocked like the fingers of two clasped hands, with the lithium shuttling between the two sides.Lithium-ion battery capacity can be vastly improved if, on the microscale, their electrodes have pores and channels. An interdigitated geometry, though it does allow lithium to transport through the battery efficiently during charging and discharging, is not optimal.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1568214668191-0730-em-rahul-battery.png" style="width: 766px;" class="fr-fic fr-dib">Lattice architecture can provide channels for effective transportation of electrolyte inside the volume of material, while for the cube electrode, most of the material will not be exposed to the electrolyte.Rahul Panat, an associate professor of mechanical engineering at Carnegie Mellon University, and a team of researchers from Carnegie Mellon in collaboration with Jonghyun Park, assistant professor of mechanical and aerospace engineering at Missouri S&amp;T have developed a revolutionary new method of 3D printing battery electrodes that creates a 3D microlattice structure with controlled porosity. 3D printing this microlattice structure, the researchers show in a paper <a href="https://www.sciencedirect.com/science/article/pii/S2214860418302379#bib0095" rel="noopener" target="_blank">published in the journal Additive Manufacturing</a>, vastly improves the capacity and charge-discharge rates for lithium-ion batteries.&ldquo;In the case of lithium-ion batteries, the electrodes with porous architectures can lead to higher charge capacities,&rdquo; says Panat. &ldquo;This is because such architectures allow the lithium to penetrate through the electrode volume leading to very high electrode utilization, and thereby higher energy storage capacity. In normal batteries, 30-50% of the total electrode volume is unutilized. Our method overcomes this issue by using 3D printing where we create a microlattice electrode architecture that allows the efficient transport of lithium through the entire electrode, which also increases the battery charging rates.&rdquo;<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1568214704238-Screenshot_2019-09-11+3D+printing+the+next+generation+of+batteries.png" style="width: 766px;" class="fr-fic fr-dib"> The additive manufacturing method presented in Panat and Park&rsquo;s paper represents a major advance in printing complex geometries for 3D battery architectures, as well as an important step toward geometrically optimizing 3D configurations for electrochemical energy storage. The researchers estimate that this technology will be ready to translate to industrial applications in about two to three years.The microlattice structure (Ag) used as lithium-ion batteries&rsquo; electrodes was shown to improve battery performance in several ways such as a fourfold increase in specific capacity and a twofold increase in areal capacity when compared to a solid block (Ag) electrode. Furthermore, the electrodes retained their complex 3D lattice structures after forty electrochemical cycles demonstrating their mechanical robustness. The batteries can thus have high capacity for the same weight or alternately, for the same capacity, a vastly reduced weight, which is an important attribute for transportation applications.<img src="https://s3-us-west-1.amazonaws.com/wevolver-project-images/froala%2F1568214728313-0730-em-rahul-battery-microscope.png" style="width: 766px;" class="fr-fic fr-dib">SEM images of 3D printed electrodes for Li-ion batteries used for electrochemical cycling in the researchers&rsquo; study. Image taken from top of microlattice electrodes with height of about 250mm.The Carnegie Mellon researchers developed their own 3D printing method to create the porous microlattice architectures while leveraging the existing capabilities of an Aerosol Jet 3D printing system. The Aerosol Jet system also allows the researchers to print planar sensors and other electronics on a micro-scale, which was deployed at Carnegie Mellon University&rsquo;s College of Engineering earlier this year. This novel 3-D printing method could be applied to battery technology with a collaboration with the Missouri university, where researchers developed 3-D battery design and analysis, cell fabrication technology, and electrochemical analysis via simulation and modeling.Until now, 3D printed battery efforts were limited to extrusion-based printing, where a wire of material is extruded from a nozzle, creating continuous structures. Interdigitated structures were possible using this method. With the method developed in Panat&rsquo;s lab, the researchers are able to 3D print the battery electrodes by rapidly assembling individual droplets one-by-one into three-dimensional structures. The resulting structures have complex geometries impossible to fabricate using typical extrusion methods.&ldquo;Because these droplets are separated from each other, we can create these new complex geometries,&rdquo; says Panat. &ldquo;If this was a single stream of material, as in the case of extrusion printing, we wouldn&rsquo;t be able to make them. This is a new thing. I don&rsquo;t believe anybody until now has used 3D printing to create these kinds of complex structures.&rdquo;This revolutionary method will be very important for consumer electronics, the medical devices industry, as well as aerospace applications. This research will integrate well with the biomedical electronic devices, where miniaturized batteries are required. Non-biological electronic micro-devices will also benefit from this work. And on a bigger scale, electronic devices, small drones, and aerospace applications themselves can use this technology as well, due to the low weight and high capacity of the batteries printed using this method.The team, which also includes mechanical engineering Ph.D. student Mohammad Sadeq Saleh and postdoctoral researcher Jie Li at Missouri S&amp;T, is also working on creating more complex three-dimensional structures, which can simultaneously be used as structural materials and as functional materials. For example, a part of a drone can act as a wing, a structural material, while simultaneously acting as a functional material such as a battery.

A new method of fabricating battery electrodes using Aerosol Jet 3D printing.

3D printing the next generation of batteries

What is DLP is a 3D printing? This article aims to provide an in-depth understanding of DLP 3D printing, its working principles, advantages, limitations, and diverse applications in various sectors.

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