Not All 3D Printed Prosthetics Are Created Equal

TrueLimb is a 3D printed prosthetic arm that is manufactured using Multi Jet Fusion (MJF) 3D printers

3D printing allows for customized manufacturing of an endless array of previously difficult-to-make products and parts at a fraction of the cost. Prosthetics are incredibly personal, expensive devices that need to be created in a large variety of sizes and configurations. With these definitions in mind, it’s easy to see why 3D printing is ripe for adoption by the prosthetics industry. And within the last few years, a vast array of 3D printed prosthetics have emerged, particularly upper limb prosthetics. While it is exciting to watch 3D printed prosthetics become more widespread, not all 3D printed prosthetics are created equal. In fact, there is a vast range of quality and capabilities when it comes to this technology. 

Even though 3D printing is 40-year-old technology, it remains largely misunderstood. How big are 3D printers? What materials do they print with? Can you 3D print a car? A house? A body part? At its core, 3D printing is a lot like 2D printing, simply with objects instead of images. You might have an inkjet printer at home for printing documents. It might print nice photos as well. But it’s nowhere near the quality of professional printers creating giant, Times Square-sized billboards. Likewise, there are 3D printers available for under $200 that create little models and figurines. Advanced 3D printers that can create extremely durable prosthetic devices are also available. 

 

Basic 3D Printing

We originally developed TrueLimb using one of these at-home 3D printers. They use a method known as Fused Deposition Modeling (FDM). FDM is the process of building a part by laying down layers of a continuous melted filament of thermoplastic. This is a uniquely affordable process, with the lower end of printers costing as little as $100. Lower end printers often print in plastics, while higher end printers can work with nylon and polycarbonate. 

One of the drawbacks of FDM printing is that the end products are brittle and lack durability. They can’t withstand the wear and use that a human arm or leg regularly endures. So while they are excellent for prototyping, and fun for at-home printing projects, they aren’t an ideal method for crafting long-lasting prosthetics. 

 

Advanced 3D Printing

On the other end of the spectrum is Multi Jet Fusion (MJF). In MJF printing, thin, uniform layers of powder are spread and then chemically fused together. The processes can provide high strength, tight tolerance, isotropic parts in a number of materials, most commonly nylon. It offers the ability to print more elaborate shapes without the need for support materials or a high degree of geometrical considerations. It has the added advantage of printing fast, and in full color. 

Multi Jet Fusion is the method that we now use to create each personalized TrueLimb, our advanced upper limb prosthetic. We chose MJF for its amazing durability, light weight, and levels of customization it unlocks — we can offer TrueLimb in 450 skin tones because of MJF. To get an idea of how strong MJF printing is, Hewlett Packard put it to the test in this video. A quarter-pound chain link printed in under 30 minutes holding up a 10,000-pound car? That’s the kind of strength that advanced 3D printed prosthetics demand. 

Of course, there are more than two methods of 3D printing. Read on for a deep dive all of the current methods of 3D printing in use.

 

Fused Filament Fabrication (FFF) or Fused Deposition Modeling (FDM)

Fused Filament Fabrication (FFF), more commonly known as Fused Deposition Modeling (FDM), is the highly popular process of laying down a continuous melted filament of thermoplastic in layers to build a part. This is a uniquely affordable process, with the lower end of 3D printers costing as little as $100. Lower end printers often print in ABS and PLA plastics, while higher end printers can work with nylon and polycarbonate. 

FDM always produces non-isotropic parts due to poor interlaminar filament adhesion, meaning that parts always have strong and weak orientation, varying by upwards of 50 percent. A branch of FDM is the application of fiber reinforcement to the filament. Materials such as carbon fiber and Kevlar can be embedded in the plastic matrix to produce stiffer and stronger parts. While this technology is promising, a high degree of variance and uncertainty remains, especially due to high sensitivity to fiber orientation.

 

Stereolithography or Direct Light Processing (DLP)

Stereolithography (SLA) technology and similar Direct Light Processing (DLP) work by passing light onto a photopolymer resin, causing hardening and adhesion to the rest of the part. This process can often produce very strong, tight tolerance parts that are isotropic (same strength in all directions). Some photopolymers do, however, often face degradation and brittleness issues over time, especially when exposed to strong light. Cost is often a concern, as SLA machines are costly and require a high degree of post processing.

 

Continuous Light Interface

Continuous Light Interface Production (CLIP) technology is an advancement on SLA and DLP methods, using a proprietary process to print parts continuously, at a rate one to two orders of magnitude faster than SLA. CLIP produces parts similar to SLA, but at some of the highest speeds possible in the current 3D printing landscape.

 

Material Jetting

Material jetting, often branded as other names such as PolyJet, is a process in which materials are deposited using traditional printheads (such as on a 2D printer) and cured using light. It can produce highly accurate, isotropic parts of different materials and even multiple colors. As with any photopolymer process, cost, brittleness, and degradation due to light can be major issues.

 

Selective Laser Sintering (SLS) and Multi Jet Fusion (MJF)

Powder bed technologies, such as Selective Laser Sintering (SLS) and Multi Jet Fusion (MJF) offer the ability to print more elaborate shapes without the need for support materials or a high degree of geometrical considerations. Powder is spread in thin, uniform layers, and either melted (SLS) or chemically fused (MJF) together. Both of these processes can provide high strength, tight tolerance, isotropic parts in a number of materials, most commonly nylon. MJF has the added advantage of printing quicker and in full color. A subvariant of SLS, Direct Metal Laser Sintering (DMLS) has seen the most success in metallic materials and is seeing increased use in aerospace and in the medical implant space.

A major downside of all powder bed technologies is excess powder removal — a large amount of post-processing must be dedicated to cleaning the parts.