Rapid prototyping (RP), also referred to as layer-based additive fabrication, is in its third decade of commercial technological development. Since its introduction there have been a number of significant changes, including improvements in accuracy and material strength, increases in the range of applications and reductions in the cost of machines and parts.
Almost from the outset, RP models have been used in medical applications. Both RP and Computerized Tomography (CT) developed alongside 3D representation techniques. While CT data was originally used only for imaging and diagnostic purposes, it soon was commonly used in CAD/CAM systems for model design. Due to the complex, organic nature of the product, though, RP technology is turning out to be a more effective means of realizing these models.
Medical data generated from patients is unique to the individual. RP is an excellent choice for generating products based on patient data, and has been used to develop surgical and diagnostic aids, prosthetics and medical products, medical device manufacturing, and is used in tissue engineering.
Surgical and Diagnostic Aids: Surgeons perform much of their work in the operating room using sight and touch. Consequently, body and organ models, which can be seen and touched from any angle, are useful because they help surgeons in complex surgical procedures. RP developed models also help improve communication between the surgery team members as well as between surgeon and patient. RP models help reduce surgery time in complex cases by allowing the surgeons to plan ahead. In such cases, sterilized models are brought into the operating room.
The model materials resemble bone and respond to cutting in a similar manner. Thus, most models are of bony tissue rather than soft tissue constructs. While CT data provides the source for the majority of medical models, MRI data can also be used. For example, cases of complex vascular models based on MRI data have been reported. RP models of soft tissue are useful for certain aspects of visualization but little can be learnt from practicing surgery on them.
Models can be multi-colored to highlight important features, such as tumors, cavities, and vascular tracks. And, when these features are buried inside bone or other tissue, an opaque material encased in a transparent material can be used.
Parts made from rapid prototyping proceses can function as communication aids from physician to patient and between members of surgical teams.
Prosthetics and Medical Product Development: Initially, CT-generated data were combined with low resolution RP to create models that looked anatomically correct but were not accurate when compared with the patient. With improvements in the RP technology, it is now possible to fabricate close-fitting prosthetic devices.
CAD software supports the process with fixtures for orientation and tooling guidance when screwing into bone. It is quite common for surgeons to use flexible titanium mesh for bone replacement or for joining pieces of broken bone prior to osteointegration. Models can be used as templates for these meshes, allowing the surgeon’s technical staff to bend the mesh into the
correct shape prior to surgery, so that minimal rework is required during surgery. Alternatively, RP processes can create parts for casting or reference patterns during machining processes.
By tailoring some of the components, prosthetics can offer greater comfort and performance. A socket fixture for a total hip joint replacement built using a standard process often returns joint function to the patient.
However, if it doesn’t fit precisely, the socket can cause discomfort, requiring extensive physiotherapy. A custom fixture reduces the discomfort because it more precisely matches the original or preferred geometry and kinematics action.
Manufacturing: The move to develop RP into rapid manufacturing (RM) could extend the scope of the industry by an order of magnitude. RP technology lets you incorporate custom features into mass-produced products, an ability well suited to products that require anatomical information to perform effectively.
The two most well known examples are in-the-ear hearing aids and orthodontic aligners. Both applications involve taking precise data from an individual and applying them to the design of a manufactured product. RP technology may make the production process more expensive but the product will perform more effectively and can, therefore, sell at a price that recovers development costs.
Tissue Engineering: The ultimate in medical implants would be to fabricate replacement body parts. Potentially, this can be feasible with RP technology. The deposited materials would include cells, proteins, and other materials that assist in the generation of integrated tissue structures. An indirect process would be to create a scaffold from a biocompatible material that represents the shape of the final tissue construct and then add cells later. Such processes generally use bioreactors to incubate the cells prior to implantation. While creating soft tissue structures or load-bearing bone is still not technologically possible, some non-load bearing bone constructs have already been commercially proven.
Objet 3D printers have an excellent throughput rate in terms of build speed and post-processing. Additional improvements in throughput would allow these machines to be used in outpatient clinics. However, this application is contingent on improvements in the supporting software for 3D model generation.
Limitations of RP for medical applications
There are several deficiencies with RP technology and medical models. For example, RP equipment was not specifically developed to solve medical problems. Technical developments have focused on improvements to help manufacturers rather than surgeons. However, recent improvements will open the way to wider application in the medical industry. Key issues that will make RP technologies more useful in the future include:
Speed: RP models can take a day or more to create. Because medical data must be segmented and processed according to anatomical features, data preparation can take longer than the RP build time. This limits medical models to surgical procedures that involve long-term planning rather than emergency operations.
Cost: For the medical products mentioned above, machine cost is not as important as some other factors. However, when medical models are used for diagnosis, surgical planning and prosthetic development, cost-effectiveness is difficult to quantify since the results are shorter planning times and improved quality, effectiveness, and efficiency. Consequently, only complex cases can currently justify the expense of the models.
The lower the machine, materials and operating costs, the more practical RP will be for additional medical applications. There are other RP processes that use less expensive materials. Nevertheless, consumables costs can be reduced with higher volume output.
Accuracy: While many RP processes are undergoing improvements to model accuracy, medical applications do not currently require higher accuracy because the data available from 3D imaging systems is considerably less accurate than what the RP machines can produce.
However, as CT and MRI technologies become more sophisticated, the requirements for RP technology accuracy will increase.
In the manufacture of medical devices, however, the requirements for accuracy are more stringent.
Materials: Only a few RP materials are classified as safe for transport into the operating room and none are currently safe for insertion inside the body. The machines that provide the most suitable material properties are also the most expensive.
Powder-based systems are difficult to implement due to contamination issues. These restrictions limit the range of applications for medical models. Despite the stringent requirements, Objet has a range of materials that are clinically approved for these purposes.
Ease of Use: RP machines generally require a degree of technical expertise to achieve good quality models, especially the larger, more complex and more versatile machines, which are not particularly well suited for medical laboratory environments. Coupled with the software skills required for data preparation, there is a significant training investment required for any medical establishment wishing to use the technology. This is a problem all RP technologies face due to the complexity of the support software for part preparation and design.
RP-based medical models can be used to create prosthetic devices, such as implants precisely fabricated to match a cavity. Engineering features can be easily added, making it possible to correctly fix the implant in place.
Requirements for medical acceptance
It is difficult to say whether a particular RP technology is more or less suited to medical applications because there are numerous applications in this field. Different technologies may find their way into different medical departments based on the specific benefits they provide.
However, for these technologies to be properly accepted in the medical industry, a number of factors must be addressed:
Approvals – While a number of materials have been granted medical approvals (such as Objet’s FullCure720) for use in medical applications, questions remain regarding the fabrication technologies and procedures for generating models. The relatively few surgeons using RP processes have achieved excellent results and are able to present many successful case studies. Nevertheless, the medical industry is understandably cautious about the introduction of new technologies. Surgeons must employ creative approaches to promote the use of PR processes in surgery including having patients sign waivers, using commercial RP service companies, and word of mouth. Additionally, hospitals do not have standard procedures for the purchasing of RP machines in the same way they do for CT machines.
Insurance – Many hospitals treat patients according to their insurance coverage. Insurance companies do not have a protocol for RP in the treatment process. Companies may question the purpose of the RP models, requiring additional paperwork that may deter surgeons.
Engineering Training – Creating RP models requires skills that many surgeons and medical technicians do not possess. While Objet machines do not require significant skills to operate, the preparation of the files and post-processing requirements may require more training.
Location of the Technology – The most likely location would either be in a laboratory where prosthetics are produced or in a specialist medical imaging center. If placed in a laboratory, the necessary operating skills will be present but the accessibility will be low. If placed in an imaging center, accessibility will be high but the applications of the technology will be limited to visualization rather than fabrication. Most hospitals have high speed intranets for access to patient data. It is preferable to locate the RP machine in a separate facility closely linked to the patient data network, with the necessary software and skilled technicians for image processing and for model post-processing and associated downstream activities.
Most of the problems mentioned above are procedural rather than technical issues. A concerted effort to convince the medical industry of the value of RP models offer will advance their usage.
Droplet-based deposition ensures precise detail
Objet’s PolyJet™ Technology employs print heads similar to those used in high-speed inkjet systems, ejecting photopolymer layer by layer. The different print nozzles deposit part material along with support material.
A recent innovation is the introduction of droplet-based deposition, which takes the expertise gained from developments in ink-jet technology and applies it to different substrates and materials. The droplets are extremely small in volume, which allows the high precision needed for fine detail and surface finish of RP parts. In addition, the base material is strong enough for the fine detail to support its own mass.
The parts can be finely detailed due to the droplet size, allowing thin layers and walls in any orientation. Support material is easily removed with water jets. Downward-facing surfaces are very clear. Different material properties can be achieved using different photopolymers, and the newer machines deposit more than one part material to create parts with multiple properties such as two colors or two hardness values.
With multiple nozzles, it is possible to increase deposition speed. In addition, more material can be deposited without compromising accuracy or part quality.
Objet’s machines, which are in effect 3D printers, have up to eight separate jetting heads, with 96 nozzles each, covering a large printing band in a single pass. These 3D printers use a raster scanning system, so each layer takes approximately the same time to build. Thus, build planning is easy to calculate.
In PolyJet Technology the photopolymer materials used do not require lasers for curing. Instead, a low-cost UV lamp follows the print head as each layer is deposited. The technology is concentrated in the inkjet printer heads.
Objet Geometries, Ltd