A technological advance that may reduce the need for purchasing or maintaining a large library of physical 3D anatomical models is additive manufacturing. Additive manufacturing describes a field of fabrication technologies that build a part by the joining of materials layer by layer to produce a specific object [40]. Additive manufacturing technology (referred to here as 3D printing, a generic term) has advanced tremendously over the past two decades and has become functional in the development and construction of physical 3D anatomical models.
In general, 3D printers use the STL file format as the input for the build geometry. In order to successfully 3D print a model, the STL file needs to be watertight and free of other errors. For example, the model shown in Fig. 2a required preprocessing by propriety software that is vendor and printer specific. In this case, Insight (Stratasys), which is the preprocessing software used to analyze and prepare models for 3D printing on the Stratasys range of 3D printers, was used. Insight provides manipulation of a range of printer build parameters that determine the build quality and resolution of the finished model. This process was followed by a virtual build of the model at the individual layer level to assess the previously set parameters. This is typically an iterative process where some compromises are often made between speed, quality, and cost. For the model in Fig. 2a, this analysis initially indicated that some model features were too small to print appropriately. These errors were mainly a result of data conversion from the native 3D modeling format to a .stl representation, which is the required data format for the preprocessing software. Thus, an otherwise well-constructed model may require several adjustments in order to print using the desired method or material.
Methods of 3D Printing
Several different methods of 3D printing have been developed, and each has specific benefits and limitations in the creation of anatomical models (Table 1). Differences between the methods and the printers themselves include materials available, resolution, accuracy, repeatability, stability, costs, safety, size limitations, speed, and the number of materials per build. The importance of each of these parameters differs depending on the specific application of the printed object. For the creation of anatomical models for education, resolution, stability, and cost are important parameters.
Table 1 Resolution data was obtained from the manufacturer’s specifications at www.stratasys.com, www.3Dsystems.com, and www.Zcorp.com
SLA uses an ultraviolet laser to cure photosensitive resin in sequential, thin, horizontally oriented layers that eventually construct the desired object [41]. Detailed models can be created in hours. However, the disadvantages of this method include costs of the printer ($1000 to >$100,000, although prices are dropping), costs of the materials, postprocessing (removal of support material and curing), and long-term stability of the object.
Selective laser sintering (SLS) uses lasers to fuse powdered materials into a desired shape [42]. This method allows 3D printing in a variety of materials including powders of plastic, metal, glass, and ceramic. SLS does not require support materials since the unsintered material acts as support. Stability depends upon the material used.
Fused deposition modeling (FDM) uses thermoplastics extruded by a heated nozzle in a semiliquid state to produce layers of the desired object that then harden immediately after extrusion [43]. Different types of thermoplastic materials can be used that differ in hardness, flexibility, color, and translucency. Removable support material is required for smaller or protruding parts.
Powder-binder printers lay down a layer of powdered material followed by a binding agent. Colors can be applied selectively in the binder material. Distinct advantages are the printing speed and ability to print an object using multiple colors.
Polyjet technology builds 3D models by laying down photopolymer that is cured by UV. Speed and accuracy are advantages, but support material is required. In addition, the long-term stability of some materials can be a significant issue.
Although much improvement in costs, resolution, object scanning, image segmentation, and printing materials is needed for 3D printing to usurp the business model of commercial anatomical suppliers, these improvements may not be far away. The printer itself has been a costly investment, ranging from a few to hundreds of thousands of dollars. However, the cost is dropping rapidly with the development of desktop 3D printers (<$300) and more inexpensive materials (grass trimmer filament, recycled plastic bottles). The resolution of the printed object is also a consideration. Some higher end printers claim <0.01 mm resolution, but the costs of such a printer can be a substantial burden to most institutions. The resolution of most desktop printers may be insufficient for printing a faithful replica of a structure as delicate as a sphenoid bone or an atrioventricular valve leaflet, especially if scaled to a smaller size. However, a nice articulated hand skeleton can be printed with attached musculature at these resolutions.
Materials
Many different materials can be used to construct a 3D printed object. In addition, several different materials or colors can be used. This ability allows the construction of complex models with hard, soft, opaque, and transparent components. Thermoplastics, including acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA), ferrous and nonferrous metals, ceramic, elastic polymers, and many opaque, colored, and transparent proprietary materials can be used in the construction of an object, along with support materials that can be dissolved or washed away in postproduction processing. The choice of material must be made on esthetic, structural, and practical parameters. Strength, color, flexibility, opacity, stability, and costs must be considered. In addition, the printing process may require postprocessing of the object to ensure strength, durability, or transparency.
Examples of materials are shown in Fig. 4a, b. The heart model described above was printed on a Stratasys Fortus 400 in ABS with PLA as support material (Fig. 4a) and on a Stratasys Connex 350 in TangoPlus (Stratasys) with a separate removable material as support (Fig. 4b). Internal chambers, valve leaflets, chordae tendineae, and coronary artery lumen are demonstrated in both the ABS model and the flexible TangoPlus print (Fig. 4c, d). The TangoPlus model required printing in a split model in order to remove support material (nonpolymerized material) efficiently without damage to the fine structures. The removal of support material should be considered in model design, in particular in printing fine structures or using fragile materials.
Resolution and Accuracy
The resolution of a printed object is dependent upon the resolution of the digital 3D model, the resolution that can be achieved by the printer itself, and the material used in printing. Using a poorly constructed or low-resolution digital 3D model on an expensive, high-resolution printer will produce poorly constructed and low-resolution physical 3D models. Conversely, well-constructed 3D models may not be able to overcome the limitations of a low-resolution less expensive printer. Finally, the choice of material can affect resolution if the material cannot be extruded or cured in a sufficiently small amount or pattern or if the material cannot be supported properly during object construction.
The accuracy of a printed model in the representation of the digital model is dependent on the printing method, the capabilities of the specific printer, and the material used [44]. Accuracy with ±0.0–1.0 % seems to be the range of commercial printers; however, maintaining those tolerances requires calibration and maintenance. Different materials have a different impact on accuracy both at the time of printing and over time after printing. ABS, once cooled, is very stabile in configuration and appearance over time. However, polyjet and SLA resins may show alterations in dimension, opacity, and flexibility. In anatomical model construction for medical education, dimensional accuracy may not be as important as resolution and stability, since there is much variation in anatomical structure.
Desired resolution and accuracy as well as required stability affect the choice of printer and material. These parameters may also affect the speed of the printing process and the amount of materials used. These parameters also affect the cost of the printed object. Trade-offs between resolution and costs must be assessed individually to avoid a negative impact on accuracy or usefulness of the model.
Costs
The costs of a printed object are dependent upon the size and complexity of the 3D model, the choice of material used, and the cost to operate the printer. Commercial models offer some guarantees of quality and accuracy that can be assured through the inspection of the model prior to purchase. The creation of 3D models by faculty for use in local curricula may require several versions to achieve a suitable digital 3D model and printed 3D object. At this time, these costs may indicate that a commercial model is a better choice. Creating and printing a fabricated digital 3D heart model with accurate internal structure may cost thousands of dollars in artist and engineer time, print costs, and assessment of educational impact. Of course, many models can be printed after achieving a good design, which would decrease the costs overall. However, if 3D printing is to be a significant educational technology in medical and dental education, costs need to be reduced drastically.
Safety Concerns
There are safety concerns with the use of 3D printers. Some printers use high voltage and can have extremely hot surfaces, which presents obvious concerns. The results of a recent study suggest that some 3D printers emit ultrafine particles [45]. Particles of similar size have been shown to find their way into pulmonary airways, and even the brain, resulting in respiratory and neurological symptoms [46–49]. In addition to particles, heating some plastics, as in FDM, releases toxic compounds including benzene, carbon monoxide, hydrogen cyanide, and hydrogen chloride, which may be harmful in sufficient concentrations [50, 51]. These are not trivial concerns since the detrimental health effects of ultrafine particle inhalation or toxic compound exposure may not become apparent for many years. Most commercial printers operate in a closed compartment or environment that can contain or filter the emitted particles or fumes. However, unless specific ventilation systems are in place, 3D printing should be performed cautiously. This is especially true when using open DIY printers. In addition, postprocessing may require caustic chemicals, sanding, or coating, which present safety hazards as well.