DESCRIPTIVE ARTICLE
Dual-Extrusion 3D Printing of Anatomical Models for Education Michelle L. Smith, James F.X. Jones* Anatomy Unit, Biomedical Section, School of Medicine, University College Dublin, Dublin, Ireland
Two material 3D printing is becoming increasingly popular, inexpensive and accessible. In this paper, freely available printable files and dual extrusion fused deposition modelling were combined to create a number of functional anatomical models. To represent muscle and bone FilaFlex3D flexible filament and polylactic acid (PLA) filament were extruded respectively via a single 0.4 mm nozzle using a Big Builder printer. For each filament, cubes (5 mm3) were printed and analyzed for X, Y, and Z accuracy. The PLA printed cubes resulted in errors averaging just 1.2% across all directions but for FilaFlex3D printed cubes the errors were statistically significantly greater (average of 3.2%). As an exemplar, a focus was placed on the muscles, bones and cartilage of upper airway and neck. The resulting single prints combined flexible and hard structures. A single print model of the vocal cords was constructed which permitted movement of the arytenoids on the cricoid cartilage and served to illustrate the action of intrinsic laryngeal muscles. As University libraries become increasingly engaged in offering inexpensive 3D printing services it may soon become common place for both student and educator to access websites, download free models or 3D body parts and only pay the costs of print consumables. Novel models can be manufactured as dissectible, functional multi-layered units and offer rich possibilities for sectional and/or reduced anatomy. This approach can liberate the anatomist from constraints of inflexible hard models or plastinated specimens C and engage in the design of class specific models of the future. Anat Sci Educ 00: 000–000. V 2017 American Association of Anatomists.
Key words: gross anatomy education; medical education; anatomical models; 3D printing; dual extrusion printing
INTRODUCTION The application of 3D printing to anatomy is recently undergoing a transformative and explosive exponential rise in the published literature (McMenamin et al., 2014; Sander et al., 2017). This technical advance is of interest to anatomists because it moves independent control of model making for anatomical instruction back into their direct control (Chia and Wu, 2015; O’Reilly et al., 2016). The technology is also proving to be a valuable resource in patient specific preoperative planning (Ventola, 2014; Naftulin et al., 2015; Valverde *Correspondence to: Prof. James F.X. Jones, Anatomy, School of Medicine, University College Dublin, Dublin 4, Ireland. E-mail: [email protected] Received 11 May 2017; Revised 19 July 2017; Accepted 22 August 2017. Published online 00 Month 2017 in Wiley (wileyonlinelibrary.com). DOI 10.1002/ase.1730 C 2017 American Association of Anatomists V
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et al., 2015; DeFerm et al., 2016; Marro et al., 2016; Smith et al., 2017), comparative physiology (Lauridsen et al., 2016) and anatomy education (Torres et al., 2011; Abouhashem et al., 2015; Vaccarezza and Papa, 2015). The latter has been confirmed in randomized control trials (Lim et al., 2016). The free availability of software which can segment X-ray computed tomography data sets and create printable files has made the technology very accessible (Doney et al., 2013; G€ ur, 2014; Shui et al., 2017). The addition of color to single material prints can produce more realistic representations of anatomy prosections (Adams et al., 2015) and corrosion casts (Li et al., 2012). However, the major limitation of these models is the lack of dissectability of the single stiff material. Cadaver dissection still remains a very valuable method of learning Anatomy, but new occupational exposure limit values for formaldehyde published by the Health Safety Authority of Ireland have been reduced tenfold from 2.0 to 0.2 parts per million (HSA, 2016). The updated values pose a challenge to anatomy departments using cadaveric materials which are preserved in formaldehyde. Multi-material 3D Anat Sci Educ 00:00–00 (2017)
printing produces representations of anatomical structures which can supplement or replace cadaveric specimens for teaching anatomy. In this regard, the most significant recent advance has involved the application of Connex technology utilized in large Stratasys printers (Mogali et al., 2017). For example, the Stratasys J750 3D printer (Stratasys Ltd., Eden Prairie, MN) permits simultaneous jetting of six materials and 350,000 colors. It has a relatively large build volume of 49 cm 3 39 cm 3 20 cm (X,Y,Z). This has been employed relatively rarely in anatomy (Mogali et al., 2017) but more commonly in surgery probably due to the prohibitive cost of the machine and photopolymers. The machine costs approximately US$328,000 and US$2,500 for six cartridges (3.6 kg per cartridge). Less expensive Stratasys PolyJet technology has been used to teach heart anatomy (Luo et al., 2017) and is the most common 3D printing method for the planning of liver surgery (Shafiee and Atala, 2016; Witowski et al., 2017). As an increasing number of libraries incorporate 3D printing services into their portfolio it is becoming increasingly important to estimate the economic impact on students of the consumable costs. Therefore a less expensive alternative to PolyJet technology was found which involves dual extrusion fused deposition modelling. This approach has the advantage of having a multitude of filaments as raw materials. These filaments are diverse in mechanical, electrical and chemical properties and are constantly evolving due to advances in material science. One major drawback of adopting the fused deposition modelling over PolyJet technology is that adding materials has a major lengthening effect on printing times. Therefore, a focus was placed on just two material printing but involving filaments with very divergent mechanical properties. Following preliminary experimentation with a number of filaments including PLA, PLA Flex, NinjaFlexV, FilaFlex3D, acrylonitrile butadiene styrene, polyethylene terephthalate, thermoplastic elastomer, high impact polystyrene, polyvinyl alcohol and the Poro-Lay series (Poro-Lay-Fomm 60, Poro-Lay-Fomm 40, and Poro-Lay_Gel-Lay) a choice was made for the combination of PLA and FilaFlex3D. The reason for this choice related to the unique soft elastic and adhesive properties of FilaFlex3D combined with the ease of printability of PLA. Although soft compressive filaments are difficult to print due to their predilection for blocking printing channels, the Big Builder design proved surprisingly effective for FilaFlex3D. This may relate to the very short distance from extruder motor to hot end and the common channel for two filaments cut into the extrusion nozzle. In this paper, a method is described for producing a multitude of model combinations of laryngeal and neck anatomy using a rich resource of 3D anatomical parts (Mitsuhashi et al., 2009; Center for Life Science, 2013). The combination of elastic and rigid elements is exploited to model the workings of the vocal apparatus. Some of the models are inspired by the ancient work of Italian physiologist and natural scientist Felice Fontana (1730–1805) whose full and reduced-scale anatomical wax models are on display in the Museum for Physics and Natural History (Museo Della Specola) in Florence (Von During et al., 2001). Although the combination of PLA and FilaFlex3D materials for 3D printing is not novel (Stoelen et al., 2016; Recreus, 2017) and FilaFlex3D has been applied to creating a neonatal ribcage (Thielen and Delbressine, 2016) and aortic arch (Valverde et al., 2015) to the authors’ knowledge this is the first description of this particular dual material application to anatomy. R
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MATERIALS AND METHODS Construction of Airway Models The neck and airway models were based on freely available anatomically accurate 3D model files from the BodyWorks3D website (Center for Life Science, 2013) as (.obj) (Mitsuhashi et al., 2009). Files were downloaded for each iteration of the model. Table 1 contains the anatomical components employed and divides them into hard (cartilage and bone) and soft (muscle and ligament) elements. These files were opened in the free mesh processing software MeshMixer, version 11.05 (Autodesk Inc., San Francisco, CA). Individual structures were imported and combined in groups based on whether they were “soft” or “hard”. Once the collections of structures were assembled, they were prepared for print by repairing the mesh and making slices where required. The files were exported as stereolithographic (.stl) files, ready for printing. The (.stl) files were imported separately into the Cura LulzBotV Edition software, version 14.12.1 (Aleph Objects, Inc., Loveland, CO), which generated the G-code instructions for the dual-extrusion printer. R
Dual Extrusion 3D Printing A single common extrusion channel (nozzle diameter 0.4 mm) was used for two material printing. This approach offers the advantage of obviating the calibration step necessary when two independent extruders and nozzles are used. This is not only time-efficient, but also allows for accurate automatic alignment of soft and hard structures as they are produced, hence, avoiding the need for manually combining the components post-printing. The Builder Premium Large (red) printer (Builder 3D Printers B.V., Noordwijkerhout, The Netherlands) used for this study was a fused-deposition modelling (FDM) printer, which retails at a price of US$4,077 (or e3,750). It has a build volume of 370 3 380 3 890 mm. The manufacturer supports (.stl), (.obj) and (.amf) files and can utilize a wide range of filaments of 1.75 mm diameter. For this study, FilaFlex3D flexible filament with the diameter of 1.75 mm (Recreus Industries S.L., Elda, Alicante, Spain) and PLA filament with the same diameter of 1.75 mm (MatterHackers Inc., Foothill Ranch, CA) were used at a cost of approximately US$26 for 250 grams and US$40 for 1,000 grams, respectively.
Printing Parameters Calibration for printer setting optimization. Each material was printed using a standardized 3D printer calibration matrix test plate as shown on Figure 1 (Brastaviceanu, 2013) which was designed for use with digital light processing stereolithography printers. For FDM printing, the scale of the test plate was increased by 200%. When printing the test plates for both materials, the fill was set to 100% and the Bottom/Top thickness was set at 0.8 mm. The layer height and the shell thickness were adjusted and recorded for each material. The settings required to produce the highest quality results in the briefest time were found by examining the test plates. The models were then printed using the settings which offered the best balance between fine resolution and time efficiency. Smith and Jones
Table 1. Anatomical Components for Models Model
Hard structures
Soft structures
Posterior triangle of neck model
Mandible
Trapezius
Occipital bone
Sternocleidomastoid
Temporal bone
Posterior belly of digastric
Clavicle Dissectible laryngeal model
Thyroid cartilage
Median thyrohyoid ligament
Cricoid cartilage
Mylohyoid muscle
Epiglottis
Hyoglossus muscle
Arytenoid cartilage
Inferior pharyngeal constrictor
Corniculate cartilage
Middle pharyngeal constrictor
Trachea
Superior pharyngeal constrictor Lateral cricoarytenoid Transverse arytenoid Oblique arytenoid Posterior cricoarytenoid Thyroarytenoid Vocalis Vocal ligament Conus elasticus Thyroepliglottic ligament
Intrinsic muscles of the larynx
Thyroid cartilage
Conus elasticus
Cricoid cartilage
Inferior pharyngeal constrictor
Arytenoid cartilage
Middle pharyngeal constrictor
Corniculate cartilage
Geniohyoid
Upper trachea
Thyrohyoid membrane Lateral cricoarytenoid Transverse arytenoid Oblique arytenoid Posterior cricoarytenoid Thyroarytenoid ligament Vocal ligament Vocalis
Functional vocal cord model
Cricoid cartilage (partial)
Vocal cord
Arytenoid cartilage
Oblique arytenoid
Corniculate cartilage
Transverse arytenoid
These individual structures were collated from the Body Parts 3D website (Center for Life Science, 2013) and then combined to form separate hard and soft (.stl) files.
XYZ Accuracy Each material was used to print five cubes which were digitally rendered to measure 5 3 5 3 5 mm. The following printing parameters were used: layer height 0.1 mm, shell Anatomical Sciences Education
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thickness 0.4 mm, bottom/top thickness 0.8 mm, fill 100%, X,Y print speed 35 mm/s and nozzle temperature 2408 C. The X-, Y- and Z-axis of each cube was measured with a digital calipers (Absolute Digimatic; Mitutoyo Corp, Kawasaki, Kanagawa, Japan) and recorded. 3
RESULTS Calibration Test Plates The spatial resolution of the two materials were assessed and compared. Table 2 outlines the level of spatial detail achieved with different combinations of print parameters. A layer height of 0.1 mm with a shell thickness of 0.4 mm resulted in the highest quality print for both materials. However, these parameters also resulted in the longest print duration. For this study, the more time-efficient settings were used for large models, with the higher quality setting reserved for small models with fine structures. The time saved is significant when considered in the context of multiple productions of large dual-material models.
XYZ Accuracy
Figure 1. Calibration test plates for polylactic acid (white) and FilaFlex3D (pink). This test plate was originally designed for calibrating liquid stereolithography printers. It provides information on the limits of spatial resolution. Axes in cm.
For the material PLA five cubes were analyzed for XYZ accuracy and these displayed the following % errors in their respective XY and Z directions 1.84(1.28), 1.64(1.49), and 0.24(0.26). For FilaFlex3D the % errors were a little greater 2.6(2.03), 4.84(3.26), and 2.2(1.34). There was a statistically significant difference in the accuracy of the 2 materials for the X and Z directions (P 5 0.01 and P 5 0.02 respectively).
Model Production The models were printed at a speed of 35 mm/s and a temperature 2408C. The layer height, shell thickness and infill percentage was varied depending on the model. PLA was used for the supports, with a 9 mm FilaFlex3D brim and a priming tower.
Data Analysis The data is presented as mean (6SD) and analyzed with a two tailed unpaired Student’s t-test using Microsoft Excel, version 15.33 (Microsoft Corp., Redmond, WA). This test was chosen because the data were continuous variables and collected in small samples without an a priori expectation that either FilaFlex or PLA would have a greater accuracy.
Anatomical Models Solid structures such as bones and cartilage are represented by the hard material PLA (Shore hardness: 75D) and the muscles and soft tissues are represented by the soft flexible material FilaFlex3D (Shore hardness: 82A; Recreus, 2017). The combination of two materials in a single model offers the possibility for greater user interaction. Thus the posterior triangle of neck can be explored by retracting the trapezius and sternocleidomastoid (Figure 2). The laryngopharynx can be incised to display the posterior aspects of selected laryngeal anatomy (Figure 3). The larynx can be printed to a greater than normal scale and to display selected structures (Figure 4). Functional models can be designed and printed that allow the educator to show the action of individual
Table 2. Printer Settings and Resultant Spatial Resolution for Two Materials Layer height (mm)
Shell thickness (mm)
Retraction distance (mm)
Narrowest lumen diameter (mm)
Minimal wall thickness (mm)
Narrowest space (mm)
Narrowest angle (degrees)
Duration of print (hours:minutes)
PLA
0.2
0.6
3.0
0.7
0.70
0.5
40
0:38
PLA
0.1
0.6
3.0
1.0
0.65
0.5
40
1:04
PLA
0.1
0.4
4.5
0.5
0.78
0.3
25
1:38
FilaFlex3D
0.2
0.6
3.0
1.5
0.68
0.6
40
0:38
FilaFlex3D
0.1
0.6
3.0
1.5
0.85
0.4
25
1:04
FilaFlex3D
0.1
0.4
4.5
1.6
0.80
0.3
25
1:38
Material
This table provides the printer settings for polylactic acid (PLA) and FilaFlex3D filaments and the resulting print times and spatial resolution.
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Smith and Jones
Figure 4. Enlarged laryngeal models to show detail of intrinsic muscles of the larynx. The models were constructed in a single print using white polylactic acid (PLA) and two different types of flesh colored FilaFlex3D. They are twice normal size.
Figure 2. Posterior triangle of neck. The trapezius and sternocleidomastoid muscle can be easily retracted. Each square on the background grid is 12 mm 3 12 mm.
muscles, for instance the dilator action of the posterior cricoarytenoid muscle (Figure 5). Table 3 shows that these models can be inexpensive to manufacture (ranging from $1.44 to $13.87).
DISCUSSION The selection of two compatible but mechanically diverse filaments (PLA and FilaFlex3D) was key to the success of dual material printing described in this paper. The models captured the essential features of gross anatomy however, the resolution and accuracy of the two materials sets a limit on
the scale of anatomical structures than are printable (e.g., the diameter of a hollow blood vessel). FilaFlex 3D is a softer material than NinjaFlex across a wide range of percentage infill patterns (Yarwindran et al., 2016). It exhibited superb adhesiveness to the printers tape and did not require a heated platform. In addition, the choice of printer for dual extrusion proved to be important. The single extrusion nozzle of the Big Builder obviated the requirement for spatial calibration that a two nozzle machine would normally require. The proximity of the extrusion stepper motor to the hot end minimized soft filament blockages. Although one hundred times less expensive than the Stratasys J750 it has over four times the Z build height which means it is capable of constructing complete full size limbs.
Figure 5. Figure 3. Dissectible laryngeal model (posterior view). A single dual material print created this 21 component model of cartilage and muscle.
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Functional model of the vocal cords (superior view). The models were constructed in a single print using white polylactic acid (PLA) and flesh colored FilaFlex3D. Note the cords are composed of segments of 1.75 mm FilaFlex3D filament which were added after the print. This was necessary because that particular component of the model could not be adequately supported and was therefore unprintable.
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Table 3. Estimates of Cost and Printing Time for Each Model
Model Description Posterior triangle of neck (75% scale)
Dissectible laryngeal model (100% scale)
Intrinsic muscles of the larynx (200% scale)
Functional vocal cord model (100% scale)
Duration of print (hours:minutes) 12:28
2:10
3:43
2:36
Material
Cost per weight (US$/gram)
Weight of material used (gram)
Cost of material (US$)
Total cost of materials for model (US$)
FilaFlex3D
0.11
87
9.57
13.87
PLA
0.05
86
4.3
FilaFlex3D
0.11
15
1.65
PLA
0.05
12
0.6
FilaFlex3D
0.11
27
2.97
PLA
0.05
29
1.45
FilaFlex3D
0.11
9
0.99
PLA
0.05
9
0.45
2.25
4.42
1.44
The filaments are sold in kilogram quantities but the Cura software provides an estimate of how much material is used and how long the print will take: PLA, polylactic acid.
Popular currently available manufactured anatomical models, most of which are primarily composed of hard plastics, are expensive and relatively limited in functionality. For example, commercially available models: “Half head with musculature” (item: C14 [1000221]; 3B Scientific, 2017) retails at US$425.00 and a 7-part larynx model two-times full size (item: G21 [1000272]; 3B Scientific, 2017) costs approximately US$305.00. Although this is within the budget of most anatomy units, most medical students, who already bear the financial burden of fees, would not be able to afford these models for private study. The most sophisticated multimaterial printers (Stratasys printers with Connex technology) produce models which are expensive as the photopolymers are two orders of magnitude more expensive than filaments. In the future, if libraries become part printing houses, students will be able to print the digital files selected by their instructors. This present research shows that some functional models can be less expensive than a cup of coffee. As the body parts are freely downloadable there is very little time required for model building apart from selecting which combination of body parts are required. In some US and European universities the libraries have already become custodians of 3D printers and tend to charge students only fees to defray the cost of consumables. For instance the University of Arizona library charges US$0.10 per gram of filament. This is also the case at the J. Willard Marriott Library at the University of Utah where there is no cost to use the printers (also at University of Alabama and University of Florida). The Center for Life Science website (Mitsuhashi et al., 2009; Center for Life Science, 2013) contains data on almost 4,000 anatomical structures and the number of websites which offer free downloadable models is increasing. For example US Department of Health and Human Services—National Institutes of Health (NIH) has an active 3D print exchange website (NIH, 2017) which provides models in formats that are readily compatible with 3D printers as well as tools to 6
create and share 3D-printable models related to biomedical science. There are few structures that are not currently available and the variety is continuously expanding whilst the quality of the files is also improving. Some authors have suggested that digital 3D model libraries should be established following peer review and revision of the model (Fredieu et al., 2015). The transition from single to dual material printing represents a fresh opportunity to create novel models to illustrate form and function. In the traditional anatomical model ligamentous structures are pinned on e.g., the US$230 functional model of the knee joint (NS 50; Marcus Sommer, SOMSO Modelle GmbH., Coburg, Germany) and can look rather crude. Using the method described in this paper it is possible to fuse and embed tendons directly into the bony parts at regulated depths (Figure 2). Pulling the muscle does not break the bond between PLA and FilaFlex3D and in fact the “muscle” fails first. FilaFlex3D is a thermoplastic elastomer and although it does not print as easily as PLA (or as demonstrated here as accurately) it does possess some desirable qualities. It is dissectible, compatible with PLA at 2408C printing temperatures and useful in mimicking muscles, tendons and ligaments. It also has great elasticity and can stretch to 700% of original length before failure (Recreus, 2017). Recent advances in bioprinting (Hong et al., 2015, Guvendiren et al., 2016) may be expanded in the future to not only include functional organ printing (e.g., artificial trachea Park et al., 2015) but also nonfunctional organ printing for education and surgical training. When future multi-material prints obtain mechanical properties closer to living tissue than formaldehyde fixed cadaveric tissue then this technology will rival body donor programs. As illustrated in Figure 5 the arytenoid cartilages were manufactured to articulate by displacing them superiorly by a single layer height from the cricoid cartilage in the digital file before printing. The FilaFlex3D posterior cricoarytenoid muscles helped to anchor the arytenoids in position so that after a single print the cartilages moved whenever the muscles were deformed (Figure 5). Smith and Jones
During the summer period the best anatomists of the UCD School of Medicine class are selected to compete in a dissection medal competition. The resulting prosections are then used throughout the new academic year. Following the success of this dual material printing research a parallel 3D printing competition will be launched so that future students benefit from the imaginative functional designs of their peers. The educational impact of these models will be assessed in future studies. One major advantage of rapid prototyping involves the continuous generation of model diversity; the competitive arena of student education will ensure that only the fittest models will endure.
Limitations As the printed models are derived from radiological data sets they are necessarily limited by the care and accuracy of the segmentation process which was used to isolate the anatomical structures. The limitations of dual extrusion include increased printing time, increased technical difficulty and potentially uneven print quality according to filament selection. The stability of long vertical hollow tubes is limited with flexible filament printing and it is not possible to incorporate dissolvable support without a third extruder.
CONCLUSIONS Dual extrusion printing can produce inexpensive functional anatomical models which offer a diverse range of user interaction. This technology can radically change how anatomists choose to teach via models. The combination of hard/stiff and soft/elastic materials permits a much greater diversity in manufacture of customized models which can illustrate Anatomy, biomechanics or principles of function.
ACKNOWLEDGMENTS The authors wish to acknowledge the support of the School of Medicine, University College Dublin. The authors declare no conflict of interest.
NOTES ON CONTRIBUTORS MICHELLE L. SMITH, M.B.B.Ch., B.A.O., M.Sc., M.R.C.P.I., is an anatomy lecturer in the School of Medicine, University College Dublin, Dublin, Ireland. She is teaching anatomy to medical students and her research interest is in 3D printing applied to radiological data sets. JAMES F. X. JONES, M.D., Ph.D., is a full professor of anatomy and chair and Head of Anatomy in the School of Medicine, University College Dublin, Dublin, Ireland. He has been teaching medical students for 25 years and his research interests include autonomic neuroscience and 3D printing for anatomy and surgery. LITERATURE CITED 3B Scientific. 2017. Anatomical models. American 3B Scientific, Tucker, GA. URL: https://www.a3bs.com/anatomical-models,pg_65.html [accessed 11 May 2017]. AbouHashem Y, Dayal M, Savanah S, Strkalj G. 2015. The application of 3D printing in anatomy education. Med Educ Online 20:29847. Adams JW, Paxton L, Dawes K, Burlak K, Quayle M, McMenamin PG. 2015. 3D Printed reproductions of orbital dissections: A novel mode of visualising anatomy for trainees in opthamology or optometry. Br J Opthamol 99: 1162–1167.
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Stoelen MF, Bonsignorio F, Cangelosi A. 2016. Co-exploring actuator antagonism and bio-inspired control in a printable robot arm. In: Tuci E, Giagkos A, Wilson M, Hallam J (Editors). Proceedings of the 14th International Conference on the Simulation of Adaptive Behavior (SAB 2016): From Animals to Animats 14; Aberystwyth, Wales, UK, 2016 August 23–26. 1st Ed. Berlin, Germany: Springer Verlag. p 244–255. Thielen MW, Delbressine FL. 2016. Rib cage recreation: Towards realistic neonatal manikin construction using MRI scanning and 3D printing. In: Polthier K, Bærentzen JA, Akleman E (Editors). Proceedings of Shape Modeling International (SMI 2016): Fabrication and Sculpting Event (FASE); Berlin, Germany, 2016 June 20–24. Paper 5. International Society of Arts, Mathematics and Architecture, Albany New York. URL: http://www.geometrysummit.org/ proceedings/fase2016/papers/5.pdf [accessed 17 July 2017]. Torres K, Staskiewicz G, Snie_zy nski M, Drop A, Maciejewski R. 2011. Application of rapid prototyping techniques for modelling of anatomical structures in medical training and education. Folia Morphol (Warsz) 70:1–4.
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Vaccarezza M, Papa V. 2015. 3D printing: A valuable resource in human anatomy education. Anat Sci Int 90:64–65. Valverde I, Gomez G, Coserria JF, Suarez-Mejias C, Uribe S, Sotelo J, Velasco MN, Santos De Soto J, Hosseinpour AR, Gomez-Cia T. 2015. 3D printed models for planning endovascular stenting in transverse aortic arch hypoplasia. Cathet Cardiovasc Interv 85:1006–1012. Ventola CL. 2014. Medical applications for 3D printing: Current and projected uses. Pharm Therapeut 39:704–711. Von During M, Poggesi M, Didi-Huberman G. 2001. Encyclopedia Anatomica: Museo La Specola Florence. 1st Ed. K€ oln, Germany: Taschen GmbH. 192 p. Witowski JS, Coles-Black J, Zuzak TZ, Pe R dziwiatr M, Chuen J, Major P, Budzy nski A. 2017. 3D printing in liver surgery: A systematic review. Telemed J E Health (in press; doi: 10.1089/tmj.2017.0049). Yarwindran M, Azwani Sa’aban N, Ibrahim M, Periyasamy R. 2016. Thermoplastic elastomer infill pattern impact on mechanical properties 3D printed customized orthotic insole. ARPN J Eng Appl Sci 11:6519–6524.
Smith and Jones
Dual-Extrusion 3D Printing of Anatomical Models for Education Michelle L. Smith, James F.X. Jones* Anatomy Unit, Biomedical Section, School of Medicine, University College Dublin, Dublin, Ireland
Two material 3D printing is becoming increasingly popular, inexpensive and accessible. In this paper, freely available printable files and dual extrusion fused deposition modelling were combined to create a number of functional anatomical models. To represent muscle and bone FilaFlex3D flexible filament and polylactic acid (PLA) filament were extruded respectively via a single 0.4 mm nozzle using a Big Builder printer. For each filament, cubes (5 mm3) were printed and analyzed for X, Y, and Z accuracy. The PLA printed cubes resulted in errors averaging just 1.2% across all directions but for FilaFlex3D printed cubes the errors were statistically significantly greater (average of 3.2%). As an exemplar, a focus was placed on the muscles, bones and cartilage of upper airway and neck. The resulting single prints combined flexible and hard structures. A single print model of the vocal cords was constructed which permitted movement of the arytenoids on the cricoid cartilage and served to illustrate the action of intrinsic laryngeal muscles. As University libraries become increasingly engaged in offering inexpensive 3D printing services it may soon become common place for both student and educator to access websites, download free models or 3D body parts and only pay the costs of print consumables. Novel models can be manufactured as dissectible, functional multi-layered units and offer rich possibilities for sectional and/or reduced anatomy. This approach can liberate the anatomist from constraints of inflexible hard models or plastinated specimens C and engage in the design of class specific models of the future. Anat Sci Educ 00: 000–000. V 2017 American Association of Anatomists.
Key words: gross anatomy education; medical education; anatomical models; 3D printing; dual extrusion printing
INTRODUCTION The application of 3D printing to anatomy is recently undergoing a transformative and explosive exponential rise in the published literature (McMenamin et al., 2014; Sander et al., 2017). This technical advance is of interest to anatomists because it moves independent control of model making for anatomical instruction back into their direct control (Chia and Wu, 2015; O’Reilly et al., 2016). The technology is also proving to be a valuable resource in patient specific preoperative planning (Ventola, 2014; Naftulin et al., 2015; Valverde *Correspondence to: Prof. James F.X. Jones, Anatomy, School of Medicine, University College Dublin, Dublin 4, Ireland. E-mail: [email protected] Received 11 May 2017; Revised 19 July 2017; Accepted 22 August 2017. Published online 00 Month 2017 in Wiley (wileyonlinelibrary.com). DOI 10.1002/ase.1730 C 2017 American Association of Anatomists V
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et al., 2015; DeFerm et al., 2016; Marro et al., 2016; Smith et al., 2017), comparative physiology (Lauridsen et al., 2016) and anatomy education (Torres et al., 2011; Abouhashem et al., 2015; Vaccarezza and Papa, 2015). The latter has been confirmed in randomized control trials (Lim et al., 2016). The free availability of software which can segment X-ray computed tomography data sets and create printable files has made the technology very accessible (Doney et al., 2013; G€ ur, 2014; Shui et al., 2017). The addition of color to single material prints can produce more realistic representations of anatomy prosections (Adams et al., 2015) and corrosion casts (Li et al., 2012). However, the major limitation of these models is the lack of dissectability of the single stiff material. Cadaver dissection still remains a very valuable method of learning Anatomy, but new occupational exposure limit values for formaldehyde published by the Health Safety Authority of Ireland have been reduced tenfold from 2.0 to 0.2 parts per million (HSA, 2016). The updated values pose a challenge to anatomy departments using cadaveric materials which are preserved in formaldehyde. Multi-material 3D Anat Sci Educ 00:00–00 (2017)
printing produces representations of anatomical structures which can supplement or replace cadaveric specimens for teaching anatomy. In this regard, the most significant recent advance has involved the application of Connex technology utilized in large Stratasys printers (Mogali et al., 2017). For example, the Stratasys J750 3D printer (Stratasys Ltd., Eden Prairie, MN) permits simultaneous jetting of six materials and 350,000 colors. It has a relatively large build volume of 49 cm 3 39 cm 3 20 cm (X,Y,Z). This has been employed relatively rarely in anatomy (Mogali et al., 2017) but more commonly in surgery probably due to the prohibitive cost of the machine and photopolymers. The machine costs approximately US$328,000 and US$2,500 for six cartridges (3.6 kg per cartridge). Less expensive Stratasys PolyJet technology has been used to teach heart anatomy (Luo et al., 2017) and is the most common 3D printing method for the planning of liver surgery (Shafiee and Atala, 2016; Witowski et al., 2017). As an increasing number of libraries incorporate 3D printing services into their portfolio it is becoming increasingly important to estimate the economic impact on students of the consumable costs. Therefore a less expensive alternative to PolyJet technology was found which involves dual extrusion fused deposition modelling. This approach has the advantage of having a multitude of filaments as raw materials. These filaments are diverse in mechanical, electrical and chemical properties and are constantly evolving due to advances in material science. One major drawback of adopting the fused deposition modelling over PolyJet technology is that adding materials has a major lengthening effect on printing times. Therefore, a focus was placed on just two material printing but involving filaments with very divergent mechanical properties. Following preliminary experimentation with a number of filaments including PLA, PLA Flex, NinjaFlexV, FilaFlex3D, acrylonitrile butadiene styrene, polyethylene terephthalate, thermoplastic elastomer, high impact polystyrene, polyvinyl alcohol and the Poro-Lay series (Poro-Lay-Fomm 60, Poro-Lay-Fomm 40, and Poro-Lay_Gel-Lay) a choice was made for the combination of PLA and FilaFlex3D. The reason for this choice related to the unique soft elastic and adhesive properties of FilaFlex3D combined with the ease of printability of PLA. Although soft compressive filaments are difficult to print due to their predilection for blocking printing channels, the Big Builder design proved surprisingly effective for FilaFlex3D. This may relate to the very short distance from extruder motor to hot end and the common channel for two filaments cut into the extrusion nozzle. In this paper, a method is described for producing a multitude of model combinations of laryngeal and neck anatomy using a rich resource of 3D anatomical parts (Mitsuhashi et al., 2009; Center for Life Science, 2013). The combination of elastic and rigid elements is exploited to model the workings of the vocal apparatus. Some of the models are inspired by the ancient work of Italian physiologist and natural scientist Felice Fontana (1730–1805) whose full and reduced-scale anatomical wax models are on display in the Museum for Physics and Natural History (Museo Della Specola) in Florence (Von During et al., 2001). Although the combination of PLA and FilaFlex3D materials for 3D printing is not novel (Stoelen et al., 2016; Recreus, 2017) and FilaFlex3D has been applied to creating a neonatal ribcage (Thielen and Delbressine, 2016) and aortic arch (Valverde et al., 2015) to the authors’ knowledge this is the first description of this particular dual material application to anatomy. R
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MATERIALS AND METHODS Construction of Airway Models The neck and airway models were based on freely available anatomically accurate 3D model files from the BodyWorks3D website (Center for Life Science, 2013) as (.obj) (Mitsuhashi et al., 2009). Files were downloaded for each iteration of the model. Table 1 contains the anatomical components employed and divides them into hard (cartilage and bone) and soft (muscle and ligament) elements. These files were opened in the free mesh processing software MeshMixer, version 11.05 (Autodesk Inc., San Francisco, CA). Individual structures were imported and combined in groups based on whether they were “soft” or “hard”. Once the collections of structures were assembled, they were prepared for print by repairing the mesh and making slices where required. The files were exported as stereolithographic (.stl) files, ready for printing. The (.stl) files were imported separately into the Cura LulzBotV Edition software, version 14.12.1 (Aleph Objects, Inc., Loveland, CO), which generated the G-code instructions for the dual-extrusion printer. R
Dual Extrusion 3D Printing A single common extrusion channel (nozzle diameter 0.4 mm) was used for two material printing. This approach offers the advantage of obviating the calibration step necessary when two independent extruders and nozzles are used. This is not only time-efficient, but also allows for accurate automatic alignment of soft and hard structures as they are produced, hence, avoiding the need for manually combining the components post-printing. The Builder Premium Large (red) printer (Builder 3D Printers B.V., Noordwijkerhout, The Netherlands) used for this study was a fused-deposition modelling (FDM) printer, which retails at a price of US$4,077 (or e3,750). It has a build volume of 370 3 380 3 890 mm. The manufacturer supports (.stl), (.obj) and (.amf) files and can utilize a wide range of filaments of 1.75 mm diameter. For this study, FilaFlex3D flexible filament with the diameter of 1.75 mm (Recreus Industries S.L., Elda, Alicante, Spain) and PLA filament with the same diameter of 1.75 mm (MatterHackers Inc., Foothill Ranch, CA) were used at a cost of approximately US$26 for 250 grams and US$40 for 1,000 grams, respectively.
Printing Parameters Calibration for printer setting optimization. Each material was printed using a standardized 3D printer calibration matrix test plate as shown on Figure 1 (Brastaviceanu, 2013) which was designed for use with digital light processing stereolithography printers. For FDM printing, the scale of the test plate was increased by 200%. When printing the test plates for both materials, the fill was set to 100% and the Bottom/Top thickness was set at 0.8 mm. The layer height and the shell thickness were adjusted and recorded for each material. The settings required to produce the highest quality results in the briefest time were found by examining the test plates. The models were then printed using the settings which offered the best balance between fine resolution and time efficiency. Smith and Jones
Table 1. Anatomical Components for Models Model
Hard structures
Soft structures
Posterior triangle of neck model
Mandible
Trapezius
Occipital bone
Sternocleidomastoid
Temporal bone
Posterior belly of digastric
Clavicle Dissectible laryngeal model
Thyroid cartilage
Median thyrohyoid ligament
Cricoid cartilage
Mylohyoid muscle
Epiglottis
Hyoglossus muscle
Arytenoid cartilage
Inferior pharyngeal constrictor
Corniculate cartilage
Middle pharyngeal constrictor
Trachea
Superior pharyngeal constrictor Lateral cricoarytenoid Transverse arytenoid Oblique arytenoid Posterior cricoarytenoid Thyroarytenoid Vocalis Vocal ligament Conus elasticus Thyroepliglottic ligament
Intrinsic muscles of the larynx
Thyroid cartilage
Conus elasticus
Cricoid cartilage
Inferior pharyngeal constrictor
Arytenoid cartilage
Middle pharyngeal constrictor
Corniculate cartilage
Geniohyoid
Upper trachea
Thyrohyoid membrane Lateral cricoarytenoid Transverse arytenoid Oblique arytenoid Posterior cricoarytenoid Thyroarytenoid ligament Vocal ligament Vocalis
Functional vocal cord model
Cricoid cartilage (partial)
Vocal cord
Arytenoid cartilage
Oblique arytenoid
Corniculate cartilage
Transverse arytenoid
These individual structures were collated from the Body Parts 3D website (Center for Life Science, 2013) and then combined to form separate hard and soft (.stl) files.
XYZ Accuracy Each material was used to print five cubes which were digitally rendered to measure 5 3 5 3 5 mm. The following printing parameters were used: layer height 0.1 mm, shell Anatomical Sciences Education
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thickness 0.4 mm, bottom/top thickness 0.8 mm, fill 100%, X,Y print speed 35 mm/s and nozzle temperature 2408 C. The X-, Y- and Z-axis of each cube was measured with a digital calipers (Absolute Digimatic; Mitutoyo Corp, Kawasaki, Kanagawa, Japan) and recorded. 3
RESULTS Calibration Test Plates The spatial resolution of the two materials were assessed and compared. Table 2 outlines the level of spatial detail achieved with different combinations of print parameters. A layer height of 0.1 mm with a shell thickness of 0.4 mm resulted in the highest quality print for both materials. However, these parameters also resulted in the longest print duration. For this study, the more time-efficient settings were used for large models, with the higher quality setting reserved for small models with fine structures. The time saved is significant when considered in the context of multiple productions of large dual-material models.
XYZ Accuracy
Figure 1. Calibration test plates for polylactic acid (white) and FilaFlex3D (pink). This test plate was originally designed for calibrating liquid stereolithography printers. It provides information on the limits of spatial resolution. Axes in cm.
For the material PLA five cubes were analyzed for XYZ accuracy and these displayed the following % errors in their respective XY and Z directions 1.84(1.28), 1.64(1.49), and 0.24(0.26). For FilaFlex3D the % errors were a little greater 2.6(2.03), 4.84(3.26), and 2.2(1.34). There was a statistically significant difference in the accuracy of the 2 materials for the X and Z directions (P 5 0.01 and P 5 0.02 respectively).
Model Production The models were printed at a speed of 35 mm/s and a temperature 2408C. The layer height, shell thickness and infill percentage was varied depending on the model. PLA was used for the supports, with a 9 mm FilaFlex3D brim and a priming tower.
Data Analysis The data is presented as mean (6SD) and analyzed with a two tailed unpaired Student’s t-test using Microsoft Excel, version 15.33 (Microsoft Corp., Redmond, WA). This test was chosen because the data were continuous variables and collected in small samples without an a priori expectation that either FilaFlex or PLA would have a greater accuracy.
Anatomical Models Solid structures such as bones and cartilage are represented by the hard material PLA (Shore hardness: 75D) and the muscles and soft tissues are represented by the soft flexible material FilaFlex3D (Shore hardness: 82A; Recreus, 2017). The combination of two materials in a single model offers the possibility for greater user interaction. Thus the posterior triangle of neck can be explored by retracting the trapezius and sternocleidomastoid (Figure 2). The laryngopharynx can be incised to display the posterior aspects of selected laryngeal anatomy (Figure 3). The larynx can be printed to a greater than normal scale and to display selected structures (Figure 4). Functional models can be designed and printed that allow the educator to show the action of individual
Table 2. Printer Settings and Resultant Spatial Resolution for Two Materials Layer height (mm)
Shell thickness (mm)
Retraction distance (mm)
Narrowest lumen diameter (mm)
Minimal wall thickness (mm)
Narrowest space (mm)
Narrowest angle (degrees)
Duration of print (hours:minutes)
PLA
0.2
0.6
3.0
0.7
0.70
0.5
40
0:38
PLA
0.1
0.6
3.0
1.0
0.65
0.5
40
1:04
PLA
0.1
0.4
4.5
0.5
0.78
0.3
25
1:38
FilaFlex3D
0.2
0.6
3.0
1.5
0.68
0.6
40
0:38
FilaFlex3D
0.1
0.6
3.0
1.5
0.85
0.4
25
1:04
FilaFlex3D
0.1
0.4
4.5
1.6
0.80
0.3
25
1:38
Material
This table provides the printer settings for polylactic acid (PLA) and FilaFlex3D filaments and the resulting print times and spatial resolution.
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Smith and Jones
Figure 4. Enlarged laryngeal models to show detail of intrinsic muscles of the larynx. The models were constructed in a single print using white polylactic acid (PLA) and two different types of flesh colored FilaFlex3D. They are twice normal size.
Figure 2. Posterior triangle of neck. The trapezius and sternocleidomastoid muscle can be easily retracted. Each square on the background grid is 12 mm 3 12 mm.
muscles, for instance the dilator action of the posterior cricoarytenoid muscle (Figure 5). Table 3 shows that these models can be inexpensive to manufacture (ranging from $1.44 to $13.87).
DISCUSSION The selection of two compatible but mechanically diverse filaments (PLA and FilaFlex3D) was key to the success of dual material printing described in this paper. The models captured the essential features of gross anatomy however, the resolution and accuracy of the two materials sets a limit on
the scale of anatomical structures than are printable (e.g., the diameter of a hollow blood vessel). FilaFlex 3D is a softer material than NinjaFlex across a wide range of percentage infill patterns (Yarwindran et al., 2016). It exhibited superb adhesiveness to the printers tape and did not require a heated platform. In addition, the choice of printer for dual extrusion proved to be important. The single extrusion nozzle of the Big Builder obviated the requirement for spatial calibration that a two nozzle machine would normally require. The proximity of the extrusion stepper motor to the hot end minimized soft filament blockages. Although one hundred times less expensive than the Stratasys J750 it has over four times the Z build height which means it is capable of constructing complete full size limbs.
Figure 5. Figure 3. Dissectible laryngeal model (posterior view). A single dual material print created this 21 component model of cartilage and muscle.
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Functional model of the vocal cords (superior view). The models were constructed in a single print using white polylactic acid (PLA) and flesh colored FilaFlex3D. Note the cords are composed of segments of 1.75 mm FilaFlex3D filament which were added after the print. This was necessary because that particular component of the model could not be adequately supported and was therefore unprintable.
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Table 3. Estimates of Cost and Printing Time for Each Model
Model Description Posterior triangle of neck (75% scale)
Dissectible laryngeal model (100% scale)
Intrinsic muscles of the larynx (200% scale)
Functional vocal cord model (100% scale)
Duration of print (hours:minutes) 12:28
2:10
3:43
2:36
Material
Cost per weight (US$/gram)
Weight of material used (gram)
Cost of material (US$)
Total cost of materials for model (US$)
FilaFlex3D
0.11
87
9.57
13.87
PLA
0.05
86
4.3
FilaFlex3D
0.11
15
1.65
PLA
0.05
12
0.6
FilaFlex3D
0.11
27
2.97
PLA
0.05
29
1.45
FilaFlex3D
0.11
9
0.99
PLA
0.05
9
0.45
2.25
4.42
1.44
The filaments are sold in kilogram quantities but the Cura software provides an estimate of how much material is used and how long the print will take: PLA, polylactic acid.
Popular currently available manufactured anatomical models, most of which are primarily composed of hard plastics, are expensive and relatively limited in functionality. For example, commercially available models: “Half head with musculature” (item: C14 [1000221]; 3B Scientific, 2017) retails at US$425.00 and a 7-part larynx model two-times full size (item: G21 [1000272]; 3B Scientific, 2017) costs approximately US$305.00. Although this is within the budget of most anatomy units, most medical students, who already bear the financial burden of fees, would not be able to afford these models for private study. The most sophisticated multimaterial printers (Stratasys printers with Connex technology) produce models which are expensive as the photopolymers are two orders of magnitude more expensive than filaments. In the future, if libraries become part printing houses, students will be able to print the digital files selected by their instructors. This present research shows that some functional models can be less expensive than a cup of coffee. As the body parts are freely downloadable there is very little time required for model building apart from selecting which combination of body parts are required. In some US and European universities the libraries have already become custodians of 3D printers and tend to charge students only fees to defray the cost of consumables. For instance the University of Arizona library charges US$0.10 per gram of filament. This is also the case at the J. Willard Marriott Library at the University of Utah where there is no cost to use the printers (also at University of Alabama and University of Florida). The Center for Life Science website (Mitsuhashi et al., 2009; Center for Life Science, 2013) contains data on almost 4,000 anatomical structures and the number of websites which offer free downloadable models is increasing. For example US Department of Health and Human Services—National Institutes of Health (NIH) has an active 3D print exchange website (NIH, 2017) which provides models in formats that are readily compatible with 3D printers as well as tools to 6
create and share 3D-printable models related to biomedical science. There are few structures that are not currently available and the variety is continuously expanding whilst the quality of the files is also improving. Some authors have suggested that digital 3D model libraries should be established following peer review and revision of the model (Fredieu et al., 2015). The transition from single to dual material printing represents a fresh opportunity to create novel models to illustrate form and function. In the traditional anatomical model ligamentous structures are pinned on e.g., the US$230 functional model of the knee joint (NS 50; Marcus Sommer, SOMSO Modelle GmbH., Coburg, Germany) and can look rather crude. Using the method described in this paper it is possible to fuse and embed tendons directly into the bony parts at regulated depths (Figure 2). Pulling the muscle does not break the bond between PLA and FilaFlex3D and in fact the “muscle” fails first. FilaFlex3D is a thermoplastic elastomer and although it does not print as easily as PLA (or as demonstrated here as accurately) it does possess some desirable qualities. It is dissectible, compatible with PLA at 2408C printing temperatures and useful in mimicking muscles, tendons and ligaments. It also has great elasticity and can stretch to 700% of original length before failure (Recreus, 2017). Recent advances in bioprinting (Hong et al., 2015, Guvendiren et al., 2016) may be expanded in the future to not only include functional organ printing (e.g., artificial trachea Park et al., 2015) but also nonfunctional organ printing for education and surgical training. When future multi-material prints obtain mechanical properties closer to living tissue than formaldehyde fixed cadaveric tissue then this technology will rival body donor programs. As illustrated in Figure 5 the arytenoid cartilages were manufactured to articulate by displacing them superiorly by a single layer height from the cricoid cartilage in the digital file before printing. The FilaFlex3D posterior cricoarytenoid muscles helped to anchor the arytenoids in position so that after a single print the cartilages moved whenever the muscles were deformed (Figure 5). Smith and Jones
During the summer period the best anatomists of the UCD School of Medicine class are selected to compete in a dissection medal competition. The resulting prosections are then used throughout the new academic year. Following the success of this dual material printing research a parallel 3D printing competition will be launched so that future students benefit from the imaginative functional designs of their peers. The educational impact of these models will be assessed in future studies. One major advantage of rapid prototyping involves the continuous generation of model diversity; the competitive arena of student education will ensure that only the fittest models will endure.
Limitations As the printed models are derived from radiological data sets they are necessarily limited by the care and accuracy of the segmentation process which was used to isolate the anatomical structures. The limitations of dual extrusion include increased printing time, increased technical difficulty and potentially uneven print quality according to filament selection. The stability of long vertical hollow tubes is limited with flexible filament printing and it is not possible to incorporate dissolvable support without a third extruder.
CONCLUSIONS Dual extrusion printing can produce inexpensive functional anatomical models which offer a diverse range of user interaction. This technology can radically change how anatomists choose to teach via models. The combination of hard/stiff and soft/elastic materials permits a much greater diversity in manufacture of customized models which can illustrate Anatomy, biomechanics or principles of function.
ACKNOWLEDGMENTS The authors wish to acknowledge the support of the School of Medicine, University College Dublin. The authors declare no conflict of interest.
NOTES ON CONTRIBUTORS MICHELLE L. SMITH, M.B.B.Ch., B.A.O., M.Sc., M.R.C.P.I., is an anatomy lecturer in the School of Medicine, University College Dublin, Dublin, Ireland. She is teaching anatomy to medical students and her research interest is in 3D printing applied to radiological data sets. JAMES F. X. JONES, M.D., Ph.D., is a full professor of anatomy and chair and Head of Anatomy in the School of Medicine, University College Dublin, Dublin, Ireland. He has been teaching medical students for 25 years and his research interests include autonomic neuroscience and 3D printing for anatomy and surgery. LITERATURE CITED 3B Scientific. 2017. Anatomical models. American 3B Scientific, Tucker, GA. URL: https://www.a3bs.com/anatomical-models,pg_65.html [accessed 11 May 2017]. AbouHashem Y, Dayal M, Savanah S, Strkalj G. 2015. The application of 3D printing in anatomy education. Med Educ Online 20:29847. Adams JW, Paxton L, Dawes K, Burlak K, Quayle M, McMenamin PG. 2015. 3D Printed reproductions of orbital dissections: A novel mode of visualising anatomy for trainees in opthamology or optometry. Br J Opthamol 99: 1162–1167.
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