Starting any medical design project with a traditional design-for-manufacturability (DFM) checklist can handicap innovation. When the limits of traditional manufacturing technology— casting, milling or turning—are factored in—the form, function, and quality of the finished product can be affected.
Engineers designing with laser-sintering in mind, however, are freed from such constraints and instead can focus on such matters as how to achieve the best flow rate and how to achieve desired design results with the least amount of material. Product designers still need to prove out the fundamentals of load, durability, and performance using finite element analysis (FEA), but now they also can envision geometries that satisfy those requirements while consolidating and integrating parts into elegant, simple designs that serve the entire system’s functionality, not just its manufacturability.
For high-volume production runs, cost-effectiveness is still best served in many cases by applying design-for-manufacture strategies and using traditional manufacturing methods. But for physical prototyping, manufacturing limited production lots, or engineering patient-specific products, additive manufacturing technologies such as laser-sintering are increasingly providing better answers.
Less is more
Form may follow function, but with laser-sintering, form also can enhance function and other attributes, as illustrated by the washing rotor example (above) from Hettich (America), Alpharetta, GA, a maker of centrifuges.
Using laser-sintering as the driver for a functional integration project, the company was able to reduce the number of parts in this component from 32 to three without tooling. How? By thinking way outside the box: design engineers realized that the multiple parts of the centrifuge’s core could actually be thought of as a single system that could be sintered out of plastic in just two pieces (the third part was a metal ring). By employing additive manufacturing they were able to do away with the need for several tools and a special steel-injection pipe requiring costly deburring. The finished product had better integrated functions, reduced assembly costs, and improved performance.
Realizing the economies of production to be gained with laser sintering, Hettich is now using it on other products. The company can respond more quickly to customers’ individual design requirements, make variations in materials at minimal costs, and produce limited production runs “on demand” as their markets dictate.
Designing as nature intended
Much as embryonic cell layers grow and differentiate into a complete human body, laser-sintered materials are built into a finished product in a way that seems almost organic.
A human parallel can help illustrate the vision-expanding freedom of design that comes with additive manufacturing: think about how the body lays out its network of blood vessels to nourish every organ. Picture the heart, embraced by arteries and veins (see image above) that closely follow its exterior shape to furnish it with a continuously circulating supply of freshly oxygenated blood.
Now envision the injection- or blow-mold designer trying to draw heat away from a newly formed component using cooling channels. Traditional manufacturing would call for circular channels drilled straight into the mold block from several directions at right angles (see images at right). But designers with additive manufacturing in their arsenal can build oval conformal cooling channels right into a laser-sintered mold. Precisely enveloping the product’s shape from 360 degrees—in much the same way as the exterior blood vessels of the heart—these will bring cooling liquid close to the mold surface quickly and evenly.
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Taking technology to the body
When designing a titanium hip replacement for an elderly man, using a traditional manufacturing mindset, the designer might start with a block of metal and mill away the excess to get down to the shape you need. But by using this process, as much as 90% of that material would be left lying on the machine shop floor. Now think about taking an MRI of the patient’s hip, translating that into 3D CAD, and using the resulting data to guide a lasersintering system that fuses layers of titanium powder to build only those dimensions of the hip part that are needed. Now, instead of wasting 90% of the material, it is possible to save 90%.
This hip example is a reminder that while every human body is unique, current medical technology can already deliver structural geometry data that is as personalized as a fingerprint. Additive manufacturing works directly from that technology to offer design flexibility—like no other manufacturing method—that can take that uniqueness into account and precisely tailor a wide variety of medical products to the individual.
For example, CAD data based on dental scans can now be used to laser-sinter hundreds of individual cobalt-chrome bridges and copings on a single build platform of a direct metal laser-sintering (DMLS) system, within 24 hours. Dentists get consistent fit and margin lines that are superior to simple cast parts: the 0.1 mm spot-sized fiber laser provides a typical accuracy of +/- 20 microns. Plastic laser-sintering also has a role in this industry: data for a tooth restoration can be used to create plastic dental models that enable the proper postprocessing of the restoration.
The concept of customization also extends to the tools that surgeons use on individual patients, particularly in spinal surgery. Laser-sintering has cut delivery times from several months to less than a week for metal prototypes of surgical tools such as benders, extractors, screws, clamps, and reduction devices at DePuy Spine, Raynham, MA. Producing multiple iterations of a tool prototype in a matter of days is a big advantage when working with surgeons requiring exacting specifications.
The variety of bone and joint applications for which additive manufacturing is playing a role in the customization of medicine continues to expand. Witness the DEKA R&D Corporation’s fully integrated prosthetic arm. The humeral mount was produced in an EOS M 270 DMLS machine quickly, cost-effectively, and accurately. The image below shows the holes designed into this elbow part to reduce weight. On the subject of lighter weight, see image on the next page of professional racing cyclist Michael Teuber, gold medalist at the Beijing Paralympics, and his laser-sintered leg orthesis. Replacing his previous leg brace, this tailor-made polyamide support matches the shape of his leg and foot precisely and is not only lighter, but more robust and durable as well.
Materials are key
The design freedom provided by additive manufacturing is clearly rising to meet many new challenges in medical manufacturing. Key to these successes has been the development of plastics and metals that lend themselves to powdering and sintering while retaining strength, either stiffness or flexibility, corrosion resistance and, in the case of anything coming into contact with human flesh, biocompatibility.
Available metals include cobalt chrome, titanium, and stainless steel. In the dental industry, EOS’ CobaltChrome SP2 alloy is biocompatible and CE certified for bridges and copings. Both metal and plastic are seen as playing a future role in dental veneers. And titanium jaw screw implants are already being laser-sintered with a porous surface that helps promote new bone growth.
In orthopedics, cobalt-chrome is currently the material of choice for areas subject to large loads, like knee joints and hip stems. But pick up a knee sample and you’ll notice how heavy it is. Designers are already “thinking laser-sintering” with the goal of producing “hollow” metal knees with internal support structuring that allows for a lighter-weight prosthesis.
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While surgical instrument makers are currently prototyping with plastic and metals, stainless steel PH1 (precipitation hardening), is now available. This material is characterized by high hardness, strength, and corrosion resistance and can be machined, sparkeroded, welded, micro shot-peened, polished, and coated.
Of great interest to the medical implant community is the devlopment of PEEK (polyether ether ketone) plastic for additive manufacturing. Both biocompatible and neutral in terms of interaction with the body, PEEK is less prone to cellular adhesions as some metals can be and has the added advantage of being permeable to X-rays.
Research on custom-made laser-sintered PEEK, as well as titanium, spine implants is currently underway. Future efforts may lead to the development of PEEK “bones” designed with an internal honeycomb structure to be lightweight, strong, and as bionic in behavior as real bones. Then, the stress shielding and bone deterioration caused by some of the current implants could be eliminated.
How laser-sintering works
Laser-sintering builds a plastic or metal product layer-by-layer, in three dimensions, turning the designer’s 3D CAD model into streamlined reality. The system cuts the CAD data into thin cross-sections (.004 to .006 inch for plastic, for example) and projects a plot file for each layer onto a build platform covered with a powdered form of the material to be sintered.
A laser traces the plot file, melting the material, which then solidifies. The platform is lowered by one layer, a new cross-section is sintered onto the previous one and the process repeats automatically to produce whatever geometry the designer has envisioned.
Applications for laser-sintering range from prototyping to manufactured series products and end parts. Particularly well-suited for smaller volume, custom-designed parts, laser-sintering’s precision and design flexibility are increasingly popular for use in the medical device, dental, surgical instrument and orthopedics industries—as well as aerospace and toolmaking.