Additive Manufacturing: a Value-Add in More Ways Than One
Whether it’s your cell phone, the chair you’re sitting on, the insoles in your shoes, or the window through which you can see the sky, everything around you is manufactured. But just like how Michelangelo sculpted David for 3 years five hundred years ago, this classical form of manufacturing called subtractive manufacturing is getting old. Not to mention, it’s time-consuming, costly, and wasteful.
That’s why a relatively new method called additive manufacturing is being popularized by companies all around the world, closing the loop between concept, design and manufacturing without a loss of information in the digital designs that typically precede fabrication.
More specifically, additive manufacturing (commonly known as 3D printing) is the capability to print an object through layer-by-layer deposition of material based on a computer aided design. The CAD model may be created using software like Autodesk’s Fusion360, Inventor, Blender, or SOLIDWORKS depending on the application, and once sent to the printer, there are several methods that may be used to fabricate the object.
These methods are usually one of the following:
- SLS (selective laser sintering)
- SLA (stereolithography)
- FDM (fused deposition modeling) in plastics
- DMLS (direct metal laser sintering)
- LMD (laser metal deposition)
So let’s dive into what each of these methods entail.
Selective laser sintering uses a high-powered laser to sinter (make a powdered material coalesce into a solid or porous mass through heating) small particles of polymer powder into a solid structure. First, a thin layer of powder is coated onto the build platform and heated to just under the melting point of the material (known as the build temperature). Then, a laser traces the cross-section of the object’s geometry of the first layer, providing enough energy to locally melt the material. This melt must have low viscosity and surface to coalesce correctly and create a uniform melt pool.
The surrounding powder remains solid and maintains the shape of the molten geometry, eliminating the need for a support structure (thereby reducing waste). The build platform is then lowered by a layer to make room for the next, when a sweeper roller then moves across the surface to pick up extra material from the reservoir and deposit new/cold powder for the next layer.
Recent advancements in SLS have allowed for nearly one million laser diodes to accelerate production.
Stereolithography uses UV light to selectively polymerize (in other words, cure or solidify) photosensitive resin that is poured into a tank. The resin is cured layer by layer until the final object is complete, and each layer is typically around 50µm — about as thin as a human hair! — but can be as small as 10µm.
Lastly, fused deposition modelling, or FDM, is one of the most common and inexpensive methods on the market. It is an extrusion based method that works by liquefying the build material, which are typically thermoplastic polymers like polylactic acid, then depositing the melted materials layer by layer in a path defined by the CAD model. This method is also highly accurate, however can be a bit more wasteful since support structures are typically used to ensure structural soundness.
When it comes to metals, methods range from direct metal laser sintering to laser metal deposition. They are somewhat similar to their plastic-based counterparts.
Metal sintering involves a bed of metal powder and a high-powered lasere used to selectively fuse the powder together on a molecular level until the final part is complete. The printer hopper is first filled with the metal powder, then heaters bring it to a temperature near its sintering range, and finally the printer pushes the powder into a print bed where a recoater blade or roller spreads it into a thin layer across the build plate, similar to the motion of a windshield wiper.The lasers then trace the layer onto the powder and the build platforms lowers for the next layer to be applied and fused to the last until the object is fabricated.
Finally, in laser metal deposition, a laser beam forms a melt pool on a metallic substrate, into which powder is fed. The powder melts to form a deposit which is then fusion-bonded to the substrate, building up the required 3D structure layer by layer. The laser and nozzle are both operated by a gantry system or robotic arm.
Benefits of additive manufacturing to industry and beyond
The broadest 3D printing applications range from rapidly prototyping new products, to replacing parts in existing products, to designing flexible and customizable finished products.
The powerful thing about 3D printing is its ability to bring the production line to our fingertips, essentially cutting out the dozens of middlemen that typically block production from consumption.
In terms of design and engineering, the time to market can significantly improve and it grants greater customization and product enhancement, paving the way for mass-scale customization. For manufacturing, the processes can be faster/more flexible and make better use of materials, simplifying supply chains, potentially localizing production and making the sales process more efficient as well. Lastly, AM can simplify maintenance and product support — like Mercedes, who started 3D printing metal spare parts for its classic truck and Unimog models, now 15 years out of production.
The practical implications range from energy to healthcare.
First, let’s talk solar.
Currently, the efficiency of a solar cell has fundamental limits like a maximum efficiency using a single p–n junction of 32% for c-Si solar cells. Thus, 3D printing can be used to deposit solar cells directly or to create exterior light-trapping structures.
The reason that 3D printing offers such a beneficial alternative is that screen printing, the most mature fabrication technique of cells, has lengthy processing steps that cause high production costs. On the other hand, spin coating is unable to meet market demands as it cannot work with large substrates.
Several parts of the solar cell are ripe for additive manufacturing, like parts of the frame and the sealing. On that note, Bernardi, Ferralis, Wan, Villalon, and Grossman found that mounting commerical solar cells onto 3D-printed plastic frames with optimized geometries resulted in higher energy densities compared to flat stationary panels, with an enhancement factor of 1.5 to 4 times and energy densities per projected area (kWh/m2) by a factor between 2 and 20.
The counter electrode (CE) in dye sensitized solar cells (DSSCs) collects electrons from the external circuit and catalyzes the redox reduction in the electrolyte, which has a significant influence on the photovoltaic performance, long-term stability and cost of the devices. They are typically composed of a conductive substrate layer on which the catalyst material is deposited. Most use glass coated with a thin layer of indium tin oxide ****(ITO) or fluorine-doped tin oxide (FTO) material.
One study fabricated 6 unique CEs, some inspired by the fractal pattern of an Asian leaf. The new fractal-based design CEs overcame the limitations of conventional planar designs by significantly increasing the number of active reaction sites which enhanced the catalytic activity and improved the performance.
In addition to high catalytic activity, optimal CEs should possess qualities high electrical conductivity, large surface area, optimal thickness, excellent stability and low fabrication cost.
The fractal design for engineering structures provide better stability, optimal surface coverage, uniform ion transportation, and facilitate the efficient collection of thermal and electrical energy.
Four designs were fabricated, including planar design, cross design, two-dimensional (2D) fractal design and 3D fractal design. AM of the designs was followed by a selective electrochemical deposition process where a thin layer of metal along with catalyst particles are deposited on the 3D printed structure.
The a base substrate used a non-conductive Acrylonitrile Butadiene Styrene (ABS) plastic filament, while the CE structures were printed using conductive black Polylactic Acid (PLA) filaments. The 3D printed CE is then coated with a conductive material along with the catalyst (Cu/graphite) particles.
Results were extremely promising, and the 3D fractal design performed best. The increased number of active sites for electron collection and faster transportation improved electro-catalytic activity, conductivity and photocurrent efficiency. The design possessed the lowest average sheet resistance of 0.03 kΩ compared to other CEs after deposition, owing to its fractal structure facilitating faster and uniform ion transport.
The 3D fractal design was the best performing one. It provides more active sites for electron collection and faster transportation which in turn improves electro-catalytic activity and conductivity of CE. It has the lowest average sheet resistance of 0.03 kΩ compared to other CEs after deposition. The considerable decrease in sheet resistance for the 3D fractal design based CEs can be attributed its fractal structure helping faster and uniform ion transport.
Next, metal-halide perovskites hold a lot of promise for thin film applications because of their high charge carrier mobilities, low exciton binding energies, long charge carrier diffusion lengths, broad light absorption spectra, large absorption coefficient, and low-cost solution processability.
However, most perovskite layers are being fabricated using the nonscalable spin-coating method with low material utilization ratio. Inkjet printing is a viable solution to this problem, though, control of the perovskite crystallization process during inkjet printing remains a challenge.
One study demonstrated production of high quality large area (>22cm2) perovskite films with inkjet printing and an innovative vacuum-assisted thermal annealing (VTA) post-treatment and optimized solvent composition.
The perovskite precursor solution, consisting of methylammonium iodide and PbI2 () in a mixture solvent of DMSO:GBL, was first printed on top of a low-temperature processed (<100C) TiO2 substrate. After printing, the liquid film was placed in a vacuum heating chamber for 2 min to boost rapid crystallization of the perovskite through the simultaneous process of vacuum flashing and thermal annealing. This process enables the formation of the intermediate phase. Subsequently, a layer of spiro-OMeTAD was spin-coated onto the perovskite film to function as the hole extraction layer, while a layer of Au was deposited with thermal evaporation to function as the cathode.
Typically, solvent evaporation and perovskite crystallization are extremely slow in inkjet printing. However, with the innovative heat-intensive post-treatment, the solid, uniform and dense perovskite film could be rapidly obtained. The obtained perovskites showed outstanding performance with short circuit current (JSC) of 22.40mAcm2, open circuit voltage (OCV) of 1.03V, fill factor (FF) of 65.87% and power conversion efficiency (PCE) of 15.27%, which is the reported highest efficiency for PSCs based on inkjet-printed perovskties. As comparison, the optimum PSCs based on spin-coated MAPbI3 shows OCV of 1.01V, JSC of 19.79mAcm2, FF of 63.76%, and PCE of 12.69%.
Keeping on the theme of energy-related applications, 3D printing shows plenty of promise for electrochemical purposes, such as electrode manufacturing.
The material from which the electrode needs to be made dictates the 3D-printing technique used. For example, for thermoplastic materials including polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), and polyurethane to be used, FDM would be the 3D-printing technique of choice. However, due to the fact that these materials do not impart conductivity to the electrode, post-processing steps need to be taken such as electrodeposition of conductive materials such as Au, electrochemical activation, enzyme activation, thermal activation, or solvent activation.
SLM utilizes only metal powder precursors, and is a popular choice for water splitting electrodes. Unfortunately, as none of the precursor materials available for SLM printing are particularly active for either the OER or the HER (oxygen/hydrogen evolution reaction), materials are routinely deposited onto the SLM electrodes to make active water splitting catalysts.
The ability to tailor geometry with 3D printing is advantageous to electrode manufacturing. One group fabricated electrodes in shapes ranging from a basket, to a mesh, to a ribbon, and more (as seen above). Their findings indicate that a direct comparison of such geometries as a function of OER and HER activity can be made.
For the OER, the basket electrode exhibits a potential of 1.7 V vs RHE at a current of 5 mA, the ribbon-like electrode reaches a potential of 1.55 V vs RHE at 10 mA cm−2, and, at the same current density, the mesh-structured electrodes obtain a potential of 1.6 V vs RHE. Hence, based on these OER metrics, the ribbon is the optimum geometry for the base electrodes, followed by the mesh and the basket structure. The electrode with no holes (Figure 2C) is the optimum electrode, which could show that porous/mesh structures hinder the evolution of oxygen perhaps by blocking bubbles from detaching.
One of my favourite applications of AM is up next: 3D cement for construction.
Typically, concrete shaping necessitates false works, or formworks, made of plywood, steel, aluminum, etc. that serve the purpose of resisting the gravity load and lateral pressure of placed material. Formwork usually comprises between 50–90% of the construction cost. Therefore, powder-based and extrusion-based printing are being explore for concrete printing, with the latter being far more developed. Extrusion-based manufacturing is when a cementitious material is extruded layer by layer from a nozzle in an automated system.
The printed material presents anisotropic properties for consolidation and compaction. To obtain the desired properties like flowability, buildability, extrudability, and open time (the length of time the 3D concrete mix maintains the same viscosity allowing the materials to be pumped, transported, and extruded), chemical admixtures, mineral admixtures, supplementary cementing materials (S.C.M.s), and reinforcing fibres are used. To possess these properties, 3D-printed concrete should be thixotropic, indicating that the fluid material can reduce its viscosity under shear and increasing its viscosity again when the shear stress is removed. This property determines the aforementioned ones.
In its hardened state, the studied properties are compressive strength, density, tensile bond strength, flexural strength, shrinkage, and cracking. Most mixes use a combination of cement, sand, geopolymers, a superplasticiser, a retarder, S.C.M.s, and other additives to maintain a sufficient open time, as well as an accelerator when being pumped.
Select S.C.M.s include fly ash and silica fume, which are used to increase long-term strength and durability and control compressive strength development.
The main types of extrusion 3D concrete printers are Gantry style printers and robotic arm printers. Gantry style printers use a frame which supports a printer head that can move on the X/Y axis. Robotic arm printers are not restricted to a frame and are moved by an arm on the X/Y/Z axis. Gantry style printers are easier to scale up, meaning that they are more applicable to large-scale applications.
When it comes to the size of the printer nozzle, a range of 9 × 6 mm to 38 × 15 mm for rectangular-shaped nozzles is used, and the flow rate of concrete stands at 0.04–0.009L/s; however, with material and printer nozzle developments a target rate of 2 L/s has been set.
Deformation under self-weight is a key roadblock to widespread use. As the wet concrete is deposited, it must solidify to support its shape and the weight of subsequent layers, but the larger surface area exposed to air during printing due to the absence of formwork can lead to cracking due to increased evaporation.
Discovering how to reinforce 3D-printed concrete safely will allow it to be used more widely in large-scale applications.
Moving to an undeniably fascinating application, additive manufacturing has a handful of use cases in healthcare as well. The preparation of organ models, rapid manufacturing of personalized scaffolds, and direct printing at the defect site can all be achieved by 3D printing technology based on a patient’s imaging data such as CT or magnetic resonance imaging.
Presently, research tends to focus on four areas: ① research on manufacturing pathological organ models to aid preoperative planning and surgical treatment analysis ; ② research on personalized manufacturing of permanent non-bioactive implants; ③ research on fabricating local bioactive and biodegradable scaffolds; and ④ research on directly printing tissues and organs with complete life functions.
For the purpose of this article, we will focus on the last two categories.
To manufacture tissues and organs, there are two routes you can take, varying based on whether cells are directly manipulated during the formation process. The first route is tissue engineering, also called indirect cell assembly, which involves first forming a 3D scaffold, and then seeding cells. Cell seeding is to spread cells to a culture vessel for cell culture activities. Alone or combined with living cells, biocompatible materials, growth factors, and physical factors can be used to create a biomimetic tissue-like microarchitecture scaffold. This will be discussed in further detail below.
The second route, called direct cell assembly, forms cells and materials into a single composite structure. This mixture of cells and gel is encapsulated into 3D scaffolds that are composed of another kind of gel with high mechanical strength, or are printed directly in order to control the spatial distribution of cells and realize in situ repair. Compared with traditional scaffold-fabrication methods, 3D printing offers the ability to fabricate complex structures with both microscopic pores and macroscopic shapes, enabling effective control of the microstructure and physicochemical properties of scaffolds.
To clarify, scaffolds, typically made of polymeric biomaterials, provide the structural support for cell attachment and subsequent tissue development. They must demonstrate good biocompatibility, surface chemistry, and biodegradability, and reactivity with human tissue to promote tissue regeneration after implantation. Hydrogels can further aid cell migration and growth to improve the speed of tissue regeneration and repair, by acting as a new type of functional material with bionic characteristics that resemble those of the extracellular matrix with highly 3D network structures.
Various methods may be selected for producing the scaffolds. But to touch on the previously explained SLS method, Polycaprolactone (PCL), bioresorbable polymer with potential applications for bone and cartilage repair, was computationally designed then fabricated via selective laser sintering (SLS).
SLS constructs scaffolds from 3-D digital data by sequentially fusing regions in a powder bed via a computer controlled scanning laser beam and offers many benefits compared to other scaffold manufacturing methods. It can handle complex internal and external geometries and a wide variety of materials can be supported. Plus, SLS does not require the use of organic solvents or filament. One study applied SLS to engineer bone tissue!
Tissue structures with physiological functions can be formed by printing various materials and “biological ink” containing seed cells, growth factors, and nutritional components layer by layer, followed by culturing the printed tissue or organ.
The biggest technical challenge, however, is replicating the intricate internal vascular network of organs, rather than the manufacturing process itself. Consequently, many researchers have turned their focus to blood vessel printing. In 2009, Ganovo company in the United States was the first one to use 3D printing technology to produce vascular prostheses.
Scientists from the Wyss Institute for Biologically Inspired Engineering, Harvard University, used multiple print heads and special “ink” to fabricate complex living structures with integrated microvessels. To fabricate the tissue integrated with blood vessels , multiple types of cells, and extracellular matrix (ECM) had to be printed precisely and simultaneously with multiple print heads. Gelatin methacrylate (GelMA) was used as a cell and matrix carrier, while poly(dimethyl siloxane) (PDMS) dyed with different fluorophores was used to label different biomaterials.
A heterogeneous 3D architecture, in which each layer is composed of different GelMA, is then co-printed. Green and red fluorescent protein-expressing human neonatal dermal fibroblasts (HNDFs) and human umbilical vein endothelial cells (HUVECs), respectively, could be clearly observed, demonstrating that HUVECs can attach and proliferate in the fabricated channels to produce implantable, fully functional living tissue and even organs!
Mannoor et al. produced a bionic ear by 3D printing a chondrocyte-seeded alginate hydrogel matrix and infused silver nanoparticles in the anatomic geometry of a human ear and cochlea-shaped electrodes. The printed bionic ear possessed better auditory sensing of radio frequencies than the human ear! Lastly, a biologist from Cornell used stem cells and biopolymer materials to print a functioning cardiac valve, and the stem cells gradually differentiated into human cells.
Existing technologies that primarily stack cell-seeded hydrogels are incapable of handling the issues of cellular nutrition and oxygen supply, and we lack a sufficient supply of cells for larger scaffolds.
In addition, other limitations that include cell survival, development, differentiation, and fusion, must be solved for the further development of printed scaffolds, tissues, and organs. Plus, many of the metal materials that are frequently employed for permanent implants have a high elastic modulus, which often leads to an elastic mismatch between the implant and the bone.
As seen, additive manufacturing has wide-stretching effects on nearly every industry, many of which I didn’t get the chance to even mention. I personally love how much closer the consumer gets to be to production with this technology, with 3D design being a really exciting field as well.
Personally, I’ve been fiddling with Fusion 360 for the past few days and recently made my first design of a countertop organizer:
I hope you found this topic as intriguing as I did, and as always, follow for more technical but eyebrow-raising emerging tech content!