3D Printing Technologies



Fig. 2.1
DICOM images cannot be directly communicated to 3D printers for printing. 3D printers currently accept digital 3D models, typically defined by surfaces stored in the STL file format. A CT (left panel) from which the humerus is segmented (second panel from the left) for 3D printing must be converted into an STL file (two right-most panels) for sending to the 3D printer. Although STL files are usually presented by a rendering (third panel from the right), the underlying surface is in fact composed of simple triangles (far right panel) that fit together precisely and exactly as a jigsaw puzzle, with no gaps between any triangles (inset)



In any case, the operator generating these STL files must not only ensure that the tissues described in the files accurately represent the anatomy, but also that the two models touch along a single side of each of the two surfaces described by the STL files, without leaving any space between them, otherwise the printed model would neither reflect physiology nor remain in one piece after printing. This approach does not scale well; for example, there is no simple way to use STL files to print this vessel if it contains a mixed plaque, with several small calcifications within a lipid-rich core. For this example, a digital description of the plaque model would ideally describe a single anatomic model (plaque) and differentiate only specific locations within that model that are calcified versus lipid-rich so that they can be printed with different materials of, e.g., different colors to reflect their different tissue properties, rather than requiring independent STL files for each small calcification. Furthermore, STL files offer no opportunity to manufacture an object with a graded transition between two or more 3D printing materials, which could be used to 3D print a model that also conveys tissue “texture.” For example, it is not readily possible to print cancellous bone with inhomogeneous material properties (e.g., hardness) that could represent information regarding trabeculae and marrow or the gradual transition to healthy tissue in the case of an infiltrating tumor.

Approaches to achieve 3D printing of organs with inhomogeneous material properties are an active area of research to enable medical models to convey not only tissue biomechanical properties but also radiographic properties. For example, we are actively exploring the use of inhomogeneous 3D printing material mixtures when printing a single organ to be able to generate a printed model that replicates the image signal characteristics of the organ under computed tomography (CT) and magnetic resonance (MR) imaging (George et al. 2017b; Mitsouras et al. 2017; Guenette et al. 2016; Mayer et al. 2015). Such radiographically “biomimicking” models (Fig. 2.2) could enable the use of 3D printing for interventional radiology procedures such as thermal and nonthermal ablations, ultrasound-guided biopsies, and invasive catheter angiography-based procedures that are an important field in which 3D printing currently has only limited applications.

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Fig. 2.2
3D-printed model of a patient with L1 left lamina osteoblastoma that replicates radiographic signal intensities similarly to in vivo patient imaging, including the tumor (red arrows), adipose tissue including foraminal fat (brown arrows), and spinal nerves (green arrows). At present there is no way to readily communicate such models to 3D printers

A second limitation of STL files is that there is no standard that is portable across softwares to store the intended color and material properties for a tissue model. At present, 3D printer-specific software is used to assign these properties to each STL file loaded for printing, which can be a tedious process and error-prone if there is a disconnect between the needs of the clinician producing the model and the technician running the printer.

The Additive Manufacturing File Format (AMF) and 3D Manufacturing Format (3MF) are newer file formats designed to overcome many of the limitations of the simple STL format, including the ability to incorporate features such as surface texture, color, and material properties into each part (Hiller and Lipson 2009). The AMF format standard was approved by the American Society of Testing and Materials (ASTM) in June 2011 (ISO/ASTM 2016), but with a few exceptions, it is not yet available in most softwares used to convert DICOM images into 3D-printable models. We expect it will become more commonplace in the next few years as the medical applications of 3D printing are expanded to better fit the richness of tissues differentiated by present-day imaging, for example, producing elastic vascular models with embedded hard plastics to represent stents or calcifications.

It is likely however that these newer formats will also be insufficient for emerging specialized medical applications, for example, the interventional radiology paradigm described above, where each location in the interior of a digital organ model would ideally need to be assigned different material properties (e.g., to achieve a model that possesses different CT numbers or MR signal intensities within the 3D-printed volume). We expect such complex medical 3D applications will lead to the development of additional file formats that are less reliant on the concept of a set of solid “parts” (e.g., organs) each of which has a single set of color and material properties. Such future file formats will likely enable one to specify, radiologic and/or mechanical material properties within the volume occupied by the tissue to be printed, corresponding more directly to the concept of an organ composed of multiple tissues rather than a “part” commonly considered in engineering 3D printing applications.



2.1.2 3D Printing Technologies


3D printers use data encoded in the STL, AMF or other file format to successively fuse or deposit thin layers of material. Each layer is circumscribed by a set of closed curves that trace the outer surface(s) of the object being manufactured at that corresponding layer. The printer manufactures each such layer by filling the area enclosed by those curves with a material at a specified thickness (e.g., 0.1 mm). This is similar to the process of segmenting a tissue by successively identifying 2D regions of interest (ROIs) that circumscribe the tissue on consecutive cross-sectional images, each of which was acquired at a given fixed slice thickness. The 2D ROI is considered to fully circumscribe the tissue (and only that tissue) throughout the entire thickness of that cross section.

The taxonomy and terminology of 3D printing, which conveys how each printer’s technology achieves the process of solidifying each layer and/or the fusion of the successive layers, are rapidly evolving. Complicating matters further, to date there has been no standardization of the nomenclature used in the biomedical literature to convey these different processes (Chepelev et al. 2017). However, a thorough understanding of the principles of each technology using a current, commonly accepted classification (Huang and Leu 2013) adopted as ASTM standard F2792 and International Organization for Standardization (ISO) standard 17296-2:2015 (ISO 2015) enables the end user to understand, interpret, and replicate the various techniques published in the literature.

In the current standards classifications, there are seven specific groups of technologies. These are vat photopolymerization, material jetting, binder jetting, material extrusion, powder bed fusion, sheet lamination, and directed energy deposition. The first five technologies are those most commonly encountered in medicine. Sheet lamination and directed energy deposition are less commonly utilized but still may provide a benefit when used for certain applications. Each technology has strengths and weaknesses as it pertains to its uses in clinical 3D printing (Table 2.1), and these are reviewed below.


Table 2.1
Summary of characteristics of 3D printing technologies commonly encountered in medicine, professional equipment (>$5000) only




























































Technology

Other common technology names

Common material(s)

Accuracy

Cost

Advantages (generally)

Disadvantages (generally)

Vat photopolymerization

Stereolithography (SLA)

Digital light processing (DLP)

Continuous digital light processing (CLIP)

Epoxy- and acrylic-based polymers

Infused polymers

+++

$$

Accuracy

Biocompatible (short-term) materials available

Can print small hollow vessels with no intraluminal support

Microprinting possible (e.g., for microfluidics)

Brittle, moderate strength

Models are single material

Limited color options

Material jetting

MultiJet Printing (MJP)

Polyjet

Acrylic-based polymers

+++

$$

Accuracy, variety of materials

Models can be multi-material/multicolor

Biocompatible (short-term) materials available

Moderate strength

Material extrusion

Fused deposition modeling (FDM)

ABS, PLA plastics, composites, metals (rare)

+

$

Low cost

Strong, durable materials

Lower accuracy

Model surfaces have prominent stair stepping ridges

Binder jetting
 
Gypsum, sand, metal (rare)

+

$

Speed

Variety of materials

Color capability in external shell

No attached supports

Low strength

Model infiltration is necessary

Powder bed fusion

Electron beam melting (EBM)

Selective laser sintering (SLS)

Direct metal laser sintering (DMLS)

Selective laser melting (SLM)

MultiJet Fusion (MJF)

Plastics, synthetic polymers, metal

++

$$$

Diverse mechanical properties

Variety of materials

Material strength sufficient for functional parts

Long-term biocompatible (implantable) materials available

Attached supports not usually required (nonmetals)

Various finishes (dependent on the machine)

Models are single material


Note: This table is a generalization of the multiple technologies; some exceptions will exist


2.1.2.1 Vat Photopolymerization


This 3D printing process is more widely known as stereolithography (SLA) or Digital Light Processing (DLP) . It has three basic components: first, a high intensity light source (typically ultraviolet [UV]-A or UV-B); second, a vat or tray that holds an epoxy- or acrylic-based photo-curable liquid resin which contains monomers and oligomers; and third, a controlling system that directs the light source to selectively illuminate the resin (see below). Layers of the resin are sequentially cured by exposing it to the light source in the shape of only that cross section (i.e., ROI) of the model that is being built at that layer (perpendicular to the printer’s z-axis). The light initiates a chemical reaction in the resin which causes the monomers and oligomers to polymerize and become solid. Once a layer of the object becomes structurally stable, the model is lowered (or raised, for bottom-up printers) by one layer thickness away from the active layer so that liquid resin now covers the top (or the bottom for bottom-up printers) of the previously printed layer. Polymerization of each layer is typically not fully completed by the controlled light source in order to allow the next layer to bond to the last one.

Each layer thickness is thus printed until the final layer is complete. After printing, excess resin is drained, and a solvent or alcohol rinse (generally in an industrial parts washer) is used to clean the model. Lattice support structures (Fig. 2.3) that are automatically added by the printer to achieve printing of overhangs also need to be manually removed. A final post-processing step is required, which involves “curing” the model in a UV chamber to complete polymerization of the layer bonds (Fig. 2.4), rendering this as one of the more labor-intensive methods. Finishing may also be required, for example, to smooth step edges (light sanding) and application of a UV-resistant sealant.

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Fig. 2.3
Example of model of a scapula 3D printed using a bottom-up stereolithography vat photopolymerization 3D printer. During printing, the printer also prints a lattice of support rods (red arrow) that allow printing those portions of the model that would otherwise have nothing underneath them to support the printed material


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Fig. 2.4
Models 3D printed using a large, professional top-to-bottom stereolithography vat photopolymerization 3D printer (left panel). Printed models need to undergo UV curing to finish. Lattice supports present must be removed during model post-processing. Materials and machine size can vary

The difference between SLA and DLP is the light source and how it is controlled to selectively illuminate and cure the resin. In SLA, the light source is a laser which is directed by mirrors to different locations on the liquid’s surface (xy plane). The mirrors continuously and progressively cause the laser to trace the entire area of each layer of the object being printed. DLP instead uses a projector, such as those used in movie theaters, which instantly illuminates the entire shape of the layer of the object being printed onto the liquid’s surface. DLP tends to require less time to print an object as each layer doesn’t need to be progressively “raster scanned” but, apart from specific machines, most often lacks the high resolution of SLA afforded by a laser beam. An exciting new bottom-up DLP printer technology has been recently developed that uses an oxygen-inhibiting layer or “dead zone” above a membrane that sits at the bottom of the vat holding the resin. The oxygen layer inhibits polymerization at the interface of the membrane and the printed object. This proprietary technique, termed “continuous liquid interface production” (CLIP) by one 3D printer manufacturer (Carbon 3D, Redwood City, CA), reduces the mechanical steps involved in vat photopolymerization, offering prints at one or two orders of magnitude faster than other 3D printing technologies (Tumbleston et al. 2015), and can be as short as 5–10 min for, e.g., a scapula. Other similar approaches such as the Intelligent Liquid Interface (ILI™, NewPro3D, Vancouver, Canada) can provide larger build platforms, drastically cutting down build speeds and limitations on size. Mechanical steps are otherwise required in bottom-up printers to free the last printed layer from the transparent material (e.g., glass) floor of the vat to which the polymer adheres to as a consequence of the polymerization process. These steps typically involve lowering or shifting the vat by a small amount until the model, held in place by a base at the top, has come fully loose from the vat floor and subsequently returning the vat to just one layer thickness away from the previously printed layer. This process, in conjunction with constraints placed by the resin, e.g., to relieve internal stresses between layers and to allow flow of new resin below the model, accounts for the bulk of printing time with this technology.

Vat photopolymerization is frequently used for medical 3D printing, particularly for bone applications. It is also the only technology with which it is possible (with sufficient care taken in orienting the model in the build tray) to print hollow vessel lumens that are not filled with solid support material (Fig. 2.5) that may pose significant difficulty in removing, particularly for small, long, or tortuous vessels such as the coronaries, cerebrovasculature, and visceral aortic branches. However, materials are relatively expensive ~$210/kg. Top-down SLA printers require the resin to be maintained at a specific level in the vat, which can involve a costly investment for printers with larger build envelopes. Generally, the widely used commercial machine’s build platform sizes range from less than 12.5 × 12.5 × 12.5 cm to as large as 210 × 70 × 80 cm or more. The smaller, desktop devices are often used to fabricate dental models and implant guides and hearing aids. Photopolymer materials are available in many colors and opacities ranging to translucent, as well as with material mechanical properties, such as flexible or rigid (Fig. 2.5). Older stereolithography-printed parts were relatively fragile. Newer acrylonitrile butadiene styrene (ABS) -“like” materials offer improved mechanical properties. Finally, short-term biocompatible material (see Sect. 2.2 below) are available and can be used to print sterilizable surgical tools and guides with appropriate post-processing. It is recommended to follow the manufacturer’s specifications for proper material post-processing, cleaning, and sterilization particularly, but not only for tools and guides.
Nov 14, 2017 | Posted by in NEUROSURGERY | Comments Off on 3D Printing Technologies
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