As the line between rapid prototyping and additive manufacturing blurs, new medical applications are coming into sharper focus.
To maximise productivity and profit in a sluggish economic climate, the medical technology industry is under increasing pressure to reduce time to market. However, regulatory requirements and the clinical trial process place great restrictions on the ability to bring products to the marketplace rapidly. Fortunately, recent advances in prototyping can significantly accelerate product development and design.
|Image courtesy: Objet Ltd.|
Prototyping is used to evaluate and test the design, ergonomics, safety, functionality and other aspects of a device. Choosing the right prototyping process can enable companies to notice design errors and other issues that could later cause significant problems, thereby saving both money and time.
Conventional prototyping methods such as machining, injection moulding and soft tooling can provide high-quality and highly accurate prototypes. However, these processes can be time-consuming, expensive and complicated. Rapid prototyping methods have dramatically changed the landscape, allowing for a prototype to be made in as little as one day.
In some circumstances, the same methods can be used to make the final product, further speeding up the production process by means of additive manufacturing. These technologies have also opened the door for rapid 3-D printing of anatomical models of patients and tissue engineering applications.
Rapid prototyping methods
The most common types of rapid prototyping methods are additive technologies, meaning that the model is built by adding material layer by layer. By contrast, subtractive prototyping methods create a model by removing material, typically by means of standard machining methods such as milling, grinding and drilling.
A rapid prototype originates with a computer model. Typically, this model is fabricated using computer-aided design (CAD). In some cases, where the final product will be custom-made for the patient, the computer model is created from the patient’s CT scan. A prototyping machine reads the computer data and slices it into different layers. The machine then builds the prototype by adding material layer by layer.
Below is a list of common rapid prototyping/additive manufacturing methods.
The most appropriate method for a product will depend on a variety of factors, including the model’s function and the material used. Most prototyping methods only accommodate a few types of materials. If the prototype needs to be in the same material as the final product, which can be important for testing and other purposes, the available methods will be limited.
In these cases, rapid injection moulding can offer an advantage compared with other rapid prototyping methods, because it can allow the prototypes to be made in the same material as the final products. This is not always the case with other methods, says Damian Hennessey, Commercial Manager for Europe at Protolabs (Maple Plain, MN, USA).
Protolabs offers two services for prototyping and rapid manufacturing: Protomold, used for rapid injection moulding, and Firstcut, which combines rapid CNC machining and rapid prototyping technologies for low-volume production of parts and prototypes.
“Firstcut combines the speed of traditional rapid prototyping and the functionality of a CNC-machined component. This makes it possible for engineers to have functional prototypes made from a selection of many different engineering-grade resins, aluminium and brass in lead times typically only associated with traditional rapid prototyping technologies,” says Hennessey.
Producing prototypes in a day rather than weeks can have its disadvantages. When speed is prioritised, quality can suffer as a result. This may or may not be a concern, says Damian Muldoon, Senior Design Engineer at the medical design and manufacturing company Creganna-Tactx Medical (Galway, Ireland).
“Accuracy is not really an issue when you are creating proof-of-concept models; you simply want to show that this mechanism or assembly will work mechanically,” says Muldoon. “The closer you move to design freeze, the more important it becomes to have your prototypes as representative of the part and as close to the design tolerances as possible. With rapid prototyped parts there is always some form of clean up, however minimal, and one wayward sweep of a little sandblaster can change features enough to make a difference.
“It really is all about choosing the right method of prototyping to meet the requirements of testing, lead time or cost,” Muldoon says. “You need to understand the limitations of the prototype processes you employ and take these limitations into account.”
Rapid prototyping is most appropriate for products that incorporate complex geometries but don’t require extremely tight tolerances, such as reconstructed bones and orthopaedic implants, according to Daniel Anderson, Manager, Prototype Technology & Design at Greatbatch Medical (Clarence, NY, USA).
“Companies in the field of orthopaedic implant development have been heavy users of RP technologies. This makes perfect sense, given the complexity of many orthopaedic implant systems. The trend has continued and Greatbatch Medical consistently partners with a wide range of orthopaedic developers to produce prototypes in both plastic and metal,” Anderson says.
Orthopaedics is also a focus at Arcam (Mölndal, Sweden), which has developed an electron beam technology to melt metallic powder. The technology is especially appropriate for titanium alloys. The advantage of the technology is that it can produce both porous and solid sections, imitating the two types of bone structure: cortical (hard) and trabecular (cancellous) bone. According to Patrik Ohldin, Area Sales Manager at Arcam, manufacturing of orthopaedic implants typically requires a two-step process, since the porous metal cannot be made at the same time as the hard metal.
“The traditional method of manufacturing is still the most common way to make implants, but it has a big disadvantage, because you can’t make porous materials,” says Ohldin. “[With electron beam melting], you can make the design completely free of boundaries and you can also make it very inexpensively,” Ohldin says. “At the same time, you save a step in the production process.”
The future of rapid prototyping
The origin of rapid prototyping can be traced back to the 1980s, according to Anderson from Greatbatch Medical.
“It’s pretty widely accepted that the dawn of rapid prototyping was brought on by Chuck Hull’s invention of the Stereolithography Apparatus in the mid-1980s. That was followed by Stratasys’s invention of fused deposition modeling, Dr. Carl Deckard’s invention of selective laser sintering—and then a mass of other contenders,” says Anderson.
Compared with other prototyping technologies, rapid prototyping is still a fairly new development, which some manufacturers see as a disadvantage, notes Ohldin.
“Everything that is new is seen as risky by some. However, every technology that is now established was new at one point,” Ohldin adds.
As rapid prototyping and additive manufacturing methods advance, new application areas become possible. As this happens, the term rapid prototyping is more and more used interchangeably with the term additive manufacturing, since the available technologies often can be used for both processes.
An example of a relatively new use of additive manufacturing is 3-D printing for tissue engineering. The company
envisionTEC GmbH (Gladbeck, Germany) has developed a machine that can print soft tissues, the 3-D Bioplotter.
“The 3D-Bioplotter can process high-temperature polymer melts and ceramic materials for bone regeneration, as well as silicones for surgical restoration and finally very soft hydrogels for soft tissue regeneration, as well as organ printing,” says envisionTEC GmbH Dipl. Chemist Carlos Carvalho. “Using the hydrogels, it is also possible to print cells directly from this machine.”
Another recent development is 3-D anatomical models of human skulls and other body parts. Such models allow surgeons to practice on a 3-D model of a patient’s individual anatomy prior to operating.
One company providing 3-D printers suitable for such models is the Israeli company Objet. In 2002, Objet’s QuadraTempo system was used to plan surgery of conjoined twins. The procedure was extremely complicated because the twins’ blood vessels were crisscrossed. Surgeons at the Mattel Children’s Hospital UCLA in Los Angeles, CA, USA, practised on 3-D models of the two skulls before successfully separating the twins.
The move from rapid prototyping to additive manufacturing is impeded by the fact that not all machines can process all materials and that the accuracy and detail are not always sufficient for final products. Nevertheless, many medical products eventually will be produced using additive manufacturing methods, predicts Avi Cohen, Head of Medical Solutions at Objet. The company recently announced a new biocompatible 3-D printing material, MED610, for dental and medical applications. The material is especially suitable for applications requiring prolonged skin contact of more than 30 days and short term mucosal-membrane contact of up to 24 hours. It can be used on all Objet Eden and Connex 3-D printers.
“The future is moving very fast. It’s not far from the day when we will print most of the final product needed,” says Cohen. “It’s already getting there because the materials are improved and biocompatible. I don’t think it’s going to be called rapid prototyping anymore—it’s going to be rapid manufacturing, for all medical aspects.”
is Associate Editor of EMDT.