Name: creation of patient-specific silicone cardiac models with applications in pre-surgical plans and hands-on training
Uploaded: 2022-02-10T13:00:00
Duration: 9 min 15 s
Description: Watch this Scientific Journal Video about Creation of Patient-Specific Silicone Cardiac Models with Applications in Pre-surgical Plans and Hands-on Training at JoVE.com

The authors would like to acknowledge OSF HealthCare for making this study possible, as well as Dr. Mark Plunkett&#160;for his procedural knowledge and application of skills to our final product.

<ol>
	<li>Hoffman, J. I. E., Kaplan, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+incidence+of+congenital+heart+disease.">The incidence of congenital heart disease.</a> Journal of the American College of Cardiology. 39 (12), 1890-1900 (2002).</li><li>Hussein, N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hands-on+surgical+simulation+in+congenital+heart+surgery:+Literature+review+and+future+perspective.">Hands-on surgical simulation in congenital heart surgery: Literature review and future perspective.</a> Seminars in Thoracic and Cardiovascular Surgery. 32 (1), 98-105 (2020).</li><li>Wilson, H. K., Feins, R. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Simulation in cardiothoracic surgery. Comprehensive Healthcare Simulation: Surgery and Surgical Subspecialties. Comprehensive Healthcare Simulation. , 263-274 (2019).</li><li>Badash, I., Burtt, K., Solorzano, C. A., Carey, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Innovations+in+surgery+simulation:+A+review+of+past,+current+and+future+techniques.">Innovations in surgery simulation: A review of past, current and future techniques.</a> Annals of Translational Medicine. 4 (23), 1-10 (2016).</li><li>Yoo, S. J., Spray, T., Austin, E. H., Yun, T. J., Van Arsdell, G. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hands-on+surgical+training+of+congenital+heart+surgery+using+3-dimensional+print+models.">Hands-on surgical training of congenital heart surgery using 3-dimensional print models.</a> Journal of Thoracic and Cardiovascular Surgery. 153, 1530-1540 (2017).</li><li>Hadeed, K., Acar, P., Karsenty, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cardiac+3D+printing+for+better+understanding+of+congenital+heart+disease.">Cardiac 3D printing for better understanding of congenital heart disease.</a> Archives of Cardiovascular Disease. 111 (1), 1-4 (2018).</li><li>Velasco Forte, M. N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Living+the+heart+in+three+dimensions:+applications+of+3D+printing+in+CHD.">Living the heart in three dimensions: applications of 3D printing in CHD.</a> Cardiology in the Young. 29, 733-743 (2019).</li><li>Illmann, C. F., Ghadiry-Tavi, R., Hosking, M., Harris, K. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Utility+of+3D+printed+cardiac+models+in+congenital+heart+disease:+a+scoping+review.">Utility of 3D printed cardiac models in congenital heart disease: a scoping review.</a> Heart. 106, 1631-1637 (2020).</li><li>Su, W., Xiao, Y., He, S., Huang, P., Deng, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Three-dimensional+printing+models+in+congenital+heart+disease+education+for+medical+students:+a+controlled+comparative+study.">Three-dimensional printing models in congenital heart disease education for medical students: a controlled comparative study.</a> BMC Medical Education. 18 (178), (2018).</li><li>Farooqi, K. M., Mahmood, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Innovations+in+preoperative+planning:+insights+into+another+dimension+using+3D+printing+for+cardiac+disease.">Innovations in preoperative planning: insights into another dimension using 3D printing for cardiac disease.</a> Journal of Cardiothoracic and Vascular Anesthesia. 32, 1937-1945 (2018).</li><li>Illmann, C. F., Hosking, M., Harris, K. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Utility+and+access+to+3-dimensional+printing+in+the+context+of+congenital+heart+disease:+an+international+physician+survey+study.">Utility and access to 3-dimensional printing in the context of congenital heart disease: an international physician survey study.</a> Canadian Cardiovascular Society. 2, 207-213 (2020).</li><li>Lau, I., Gupta, A., Sun, Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Clinical+value+of+virtual+reality+versus+3D+printing+in+congenital+heart+disease.">Clinical value of virtual reality versus 3D printing in congenital heart disease.</a> Biomolecules. 11 (884), (2021).</li><li>Birbara, N. S., Otton, J. M., Pather, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=3D+modelling+and+printing+technology+to+produce+patient-specific+3D+models.">3D modelling and printing technology to produce patient-specific 3D models.</a> Heart Lung and Circulation. 28, 302-313 (2019).</li><li>Yoo, S. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=3D+printing+in+medicine+of+congenital+heart+disease.">3D printing in medicine of congenital heart disease.</a> 3D Printing in Medicine. 2 (3), (2016).</li><li>G&#243;mez-Ciriza, G., G&#243;mez-C&#237;a, T., Rivas-Gonz&#225;lez, J. A., Velasco Forte, M. N., Valverde, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Affordable+three-dimensional+printed+heart+models.">Affordable three-dimensional printed heart models.</a> Frontiers in Cardiovascular Medicine. 8, 498 (2021).</li><li>Lau, I., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantitative+and+qualitative+comparison+of+low-+and+high-cost+3D-printed+heart+models.">Quantitative and qualitative comparison of low- and high-cost 3D-printed heart models.</a> Quantitative Imaging in Medicine and Surgery. 9 (1), 107-114 (2019).</li><li>Scanlan, A. B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparison+of+3D+echocardiogram+derived+3D+printed+valve+models+to+molded+models+for+simulated+repair+of+pediatric+atrioventricular+valves.">Comparison of 3D echocardiogram derived 3D printed valve models to molded models for simulated repair of pediatric atrioventricular valves.</a> Pediatric Cardiology. 39 (3), 538-547 (2019).</li><li>. Dragon skin fx-pro, Smooth-On Available from: <a target="_blank" href="https://www.smooth-on.com/products/dragon-skin-fx-pro/">https://www.smooth-on.com/products/dragon-skin-fx-pro/</a> (2021)</li><li>Tejo-Otero, A., Fenollosa-Art&#233;s, F., Buj-Corral, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mimicking+soft+living+tissues+for+3D+printed+surgical+planning+prototypes+using+different+materials.">Mimicking soft living tissues for 3D printed surgical planning prototypes using different materials.</a> Congreso Anual de la Sociedad Espa&#241;ola de Ingenier&#237;a Biom&#233;dica. , 307-310 (2019).</li><li>Lezhnev, A. A., Ryabtsev, D. V., Hamanturov, D. B., Barskiy, V. I., Yatsyk, A. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Silicone+models+of+the+aortic+root+to+plan+and+simulate+interventions.">Silicone models of the aortic root to plan and simulate interventions.</a> Interactive CardioVascular and Thoracic Surgery. 31 (2), 204-209 (2020).</li><li>Laing, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+patient-specific+cardiac+phantom+for+training+and+pre-procedure+surgical+planning.">A patient-specific cardiac phantom for training and pre-procedure surgical planning.</a> Electronic Thesis and Dissertation Repository. , 4964 (2017).</li><li>Hoashi, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Utility+of+a+super-flexible+three-dimensional+printed+heart+model+in+congenital+heart+surgery.">Utility of a super-flexible three-dimensional printed heart model in congenital heart surgery.</a> Interactive CardioVascular and Thoracic Surgery. 27, 749-755 (2018).</li><li>Mena, K. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Exploration+of+time-sequential,+patient-specific+3D+heart+unlocks+clinical+understanding.">Exploration of time-sequential, patient-specific 3D heart unlocks clinical understanding.</a> 3D Printing in Medicine. 4 (1), 15 (2018).</li><li>Russo, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Advanced+three-dimensionally+engineered+simulation+model+for+aortic+valve+and+proximal+aorta+procedures.">Advanced three-dimensionally engineered simulation model for aortic valve and proximal aorta procedures.</a> Interactive CardioVascular and Thoracic Surgery. 30, 887-895 (2020).</li><li>Al Ali, A. B., Griffin, M. F., Butler, P. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Three-dimensional+printing+surgical+applications.">Three-dimensional printing surgical applications.</a> ePlasty. 15, 37 (2015).</li><li>. Rubber glass water-clear silicone rubber compound, Smooth-On at Available from: <a target="_blank" href="https://www.smooth-on.com/product-line/rubber-glass/">https://www.smooth-on.com/product-line/rubber-glass/</a> (2021)</li><li>Riedle, H., Molz, P., Franke, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+the+mechanical+properties+of+cardiac+tissue+for+3D+printed+surgical+models.">Determination of the mechanical properties of cardiac tissue for 3D printed surgical models.</a> IEEE-EMBS Conference on Biomedical Engineering and Science. , 171-176 (2018).</li></ol>

The authors have nothing to disclose.

Upon completion of the protocol, a patient-specific silicone cardiac model for surgical preparation should result. However, there are several critical steps that must be completed correctly for this to be achieved. A summary of the critical steps in the protocol can be seen in Supplementary File 2,&#160;as well as potential outcomes if the steps are not performed correctly. The first critical step involves the segmentation of the patient&#39;s radiologic imaging data. This step requires the acquisition of a diagnostic 3D imaging dataset. Model utility in pre-surgical planning or education is dependent on the quality of the 3D dataset. It is recommended to use an image set collected with a slice size between 0.625 mm and 2.6 mm to ensure the data set will be of adequate resolution for model production. However, all imaging parameters should be set by a clinician expert in radiology, with patient care being the priority. It should be noted that it may be possible to produce a model from an image set collected with a slice size outside the recommended values, but model resolution and quality will be negatively impacted. After images are obtained, if the segmentation is not performed correctly, it is commonly not realized until the final model is produced and cut into, resulting in a loss of time and materials. To prevent this negative outcome, it is recommended that a subject matter expert review the segmented files prior to creating the digital molds for quality control. The next critical step occurs during the creation of the digital molds. It is important to ensure that the myocardial case will be able to be assembled around the blood pool model. If the case does not close around the blood pool, it cannot be used to create the silicone model as unset silicone will continually leak out, and anatomy could be distorted. A handheld rotary sanding tool can be used to lightly remove pieces of the myocardial mold only if small adjustments are needed. If large adjustments are needed, the digital mold will need to be altered and an updated case printed. The final critical step is the pouring of the silicone. Strict adherence to material instructions is necessary when using the silicone, as failure to do so may result in silicone that cures with a tacky surface. If the surface is deemed too tacky for use by the SME, the blood pool may have to be reprinted if it cannot successfully be removed from the silicone. The silicone will have to be re-poured, resulting in a loss of time and materials. If insufficient silicone is used or the silicone leaks out of the myocardial case mold during the setting process, the resulting model will be incomplete. This failure can be remedied by mixing and pouring additional silicone into the mold. A material such as hot glue or clay can be used to seal the seams of the myocardial case mold if a small amount of silicone appears to be leaking through during the curing process.
This method of creating patient-specific silicone cardiac models can be modified to allow the creation of a model of any soft anatomical structure with patient-specific or complex inner and outer geometry. Assuming the target anatomy is segmented correctly, the remaining steps of the protocol could be followed with minimal change. While not the focus of the current work, the protocol has been applied to liver parenchyma with similar success. The utilized 3D print material can also be modified. ABS and PLA are recommended for use due to their low cost, but any dissolvable 3D print material can take the place of ABS, and any desirable 3D print material can take the place of PLA with minimal or no change to the protocol. All filament manufacturer-specified printing parameters should be followed when using other print materials. This method can further be modified by the use of a different silicone. The silicone recommended for use in this protocol has a shore hardness of 2 A, but if another shore hardness value is desirable, a different silicone can be substituted with minimal or no change to the protocol. Be sure to adhere to all manufacturing specifications and procedures when using a different silicone product.
While this protocol outlines an improved cardiac modeling procedure, it is not without limitations. The major limitation of this protocol is that while the utilized platinum cure silicone is closer to the hardness of cardiac tissue than other available materials, hardness is not the only property that plays a role in the fine-motor skill of surgical training. In particular, real cardiac tissue will demonstrate friability or tearing under force. The utilized silicone is very elastic, with an elongation at break of 763% and tensile strength of 1,986 kPa19. Porcine cardiac tissue, which is believed to be an accurate representation of human cardiac tissue, has an elongation at break of 28-66%&#160;and tensile strength of 40-59 kPa26. This difference presents a problem, as cardiac surgical fellows may perform a practice operation on a silicone model heart and gain a false sense of confidence because the model can withstand forces that real cardiac tissue cannot. This methodology also has the potential to be limited by a cardiac model with very complex geometry. As the anatomical complexity of the model increases, the protocol can compensate by increasing the number of pieces in the myocardial mold. Essentially, increasingly complex models will require increasingly complex mold designs and increased design time.
The model creation process described in this protocol is superior to many of the other available alternatives due to its ability to re-create low-cost exact anatomic replicas of surgically encountered anatomy. Cadaveric and animal tissue does allow for high fidelity simulations, but they have a much higher cost and require specific laboratory set-ups to be utilized and maintained2,6. Further, cadaveric and animal tissue models have ethical concerns, are not patient-specific, and complex CHD often must be manually manufactured by a surgeon or instructor, often leading to inaccuracies or damage to the surrounding tissues and organs. Another potential modeling technique involves the use of virtual reality. Virtual reality allows for the digital replication of patient-specific cardiac models, which is an effective tool for establishing accurate mental representations of patient anatomy and surgical plans. Additionally, some VR systems have allowed for basic simulations with the incorporation of haptic feedback. However, the available haptic feedback lacks the realism necessary to replicate necessary fine-motor skills for congenital heart surgical procedures4. 3D printing is another available method to produce patient-specific cardiac models2,24. However, the widespread implementation of high-fidelity 3D printers capable of producing multi-material, soft models are inhibited by their extremely high cost11,14,15. Low-cost 3D printers are available but can only print in materials that are much firmer than real myocardium. When one of the softest available materials for a 3D printer was used to create a model by Scanlan et al., the model was found to be firmer than real cardiac tissue17. The described material had a shore hardness between 26 A and 28 A, giving it a texture similar to a rubber band.&#160;The platinum cured silicone used in this protocol has a shore hardness of 2 A, giving it a texture similar to a gel shoe insert and much closer to the hardness of real cardiac tissues, which is 43 0020 or ~0 A. Hoashi et al. also utilized a similar method to the one described in this protocol to develop a flexible 3D printed heart model. Two molds, representing the inner and outer myocardial geometry, were 3D printed utilizing an SLA printer followed by vacuum casting a rubber-like polyurethane resin. While this method did produce a soft cardiac model, the proposed production cost of this method per model was 2,000 to 3,000 USD22. Comparatively, the total material cost of the method described in the presented protocol is less than 10 USD. Finally, a similar method was also used by Russo et al. to create silicone models of the aortic valve and proximal aorta for procedural practice. While the Russo et al. method is focused on a similar goal, their presented process aimed at replicating far simpler anatomies of the aorta or aortic valves. The protocol presented herein differentiates itself by focusing on intra-cardiac and myocardial anatomies that are smaller, more complex, and would be extremely difficult to replicate given historical methodologies. Despite this difference, the models created by Russo et al. were highly useful for simulation and training in cardiac surgery by surveyed cardiac surgeons23. Essentially, the method described in this protocol allows for the low-cost creation of complex, patient-specific congenital cardiac models with accurately represented defects and material properties more similar to real cardiac tissue than other modeling methods1,16, allowing models to be operated on with a realistic haptic fidelity.
Moving forward, this methodology could be applied to the formation of a model of any patient anatomy with complex internal and external features. Developing an alternative blood pool material that could be removed from within the silicone model in a less destructive manner or produced using a less time-consuming method would make the process more time- and cost-efficient. As a result, a new blood pool would not have to be reproduced for each subsequent molding process, leading to the scalability of the associated training. The physical properties of the silicone used to create the model could also be improved. Silicone with less elongation at break would increase the realism of the model and help to improve its value as an educational tool for cardiac surgical fellows trying to learn the necessary fine motor skills for performing these complex procedures. A group of materials currently on the market worthy of consideration to aid in this solution are silicone simulated glass materials25. These silicone materials demonstrate much less elongation at break leading to a distinct &#34;shattering&#34; upon force application in a manner similar to glass. Modulating the platinum cure silicone used in this protocol with additions of this silicone simulated glass material may allow for control of the friability characteristics of the model while still maintaining the appropriate shore hardness, improving the overall haptic fidelity. Finally, the resolution of anatomy this protocol can produce is limited by the resolution of the 3D printer utilized to generate the molds. As technology continues to improve, the resolution of anatomy that can be created with this protocol should also improve.

Nearly 1 in 100 children in the United States are born with congenital heart defects (CHD). Due to the propensity of mothers with CHDs to have children with CHDs, there is an expectation that the rate may more than double over the next seven generations1. While not each CHD is considered complex or severe, the general growth expectation indicates that there is motivation to improve the technology and procedures capable of addressing CHD treatment. As technology improves, cardiac surgeons often express a willingness to tackle more complex procedures. This willingness has led to an increased number of complex cardiac procedures, driving the need for more advanced techniques of surgical planning and education. In turn, this leaves cardiac surgeons in need of highly accurate, patient-specific models and cardiac surgical fellows in need of highly effective training methods.
Congenital cardiac surgery is one of the most technically demanding surgical disciplines due to the small size of the patients, the complexity of the cardiac abnormalities, and the rarity of some abnormalities2. In the most extreme cases, a child may be born with a single ventricle. It is not uncommon for the surgeon to take a vessel 2.0 mm in diameter and patch it with fixed pericardium to create a 1.0 cm vessel allowing a newborn to grow in this life-saving procedure - all while under the clock, as the newborn is in complete circulatory arrest. Between the normal four-chamber heart and these extreme examples are innumerable possibilities of chamber size and valve positions that constitute highly complex 3D puzzles. The role of the congenital cardiac team is to clearly delineate the unique anatomy and develop a plan to reconfigure the organic tissue into a functional heart that will allow a child to grow with the best chance at a normal life. Accurate models allow for deliberate surgical practice and repetition in an environment where errors can be forgiven and will not result in patient harm3,4. This training leads to the development of improved surgical expertise, as well as technical and judgment skills. However, limited resources and the rarity of certain cardiac conditions can make achieving the desired level of repetition and visualization nearly impossible. To help account for this resource deficiency, there has been an increase in the utilization of simulations for education2,3. Commonly utilized simulation or modeling techniques include human cadavers, animal tissues, virtual reality models (VR), and 3D printed models.
Cadaveric tissue has historically been regarded as the gold standard for surgical simulation, with animal tissue a close second. Cadavers and animal tissues can produce high fidelity simulations because they contain the anatomical structure of interest, all surrounding tissues, and allow for perfusion techniques to simulate blood flow4. Despite the benefits of tissue models, there are downsides. Embalmed tissue experiences reduced mechanical compliance, making some operations unrealistic and difficult to perform. Tissues require constant maintenance, specific facilities, are not reusable2, can be costly to obtain3, and have historically been the subject of ethical concerns. Most significantly, congenital cardiac conditions are simply not available in cadaveric samples.
VR and 3D printed models5,6,7,8,9,10 provide another option for cardiac education, simulation, and modeling to aid in the creation of pre-operative plans. These models reduce ambiguity associated with a user&#39;s varied visuo-spatial ability to interpolate 2D images as a 3D structure10,11. The virtual environment can contain surgical tools that can be manipulated and interact with models,&#160;allowing surgeons and fellows to develop hand-eye coordination, fine motor skills, and familiarity with some procedures4. Current popular 3D printing technologies, including fused deposition modeling (FDM), stereolithography (SLA), selective laser sintering (SLS), and polyjet have been found to produce models with submillimeter precision13. Both VR and 3D printed models are reusable and can be extremely detailed; models can even be generated from patient radiologic imaging data, resulting in replicas of patient anatomy. Despite the many benefits of a VR or 3D printed models, they fall short when the cost and haptic fidelity requirements of congenital heart surgery are considered. The setup of a VR environment has a high cost, and VR environments cannot provide real-world haptic feedback. While haptic fidelity technology is improving, the current gap inhibits a student&#39;s ability to become familiar with the fine-motor skills necessary to perform procedures4. Similarly, depending on the type of 3D printing technology used, the cost of 3D printing can be quite high, as the printer purchase price and print material cost must be considered11,14. A single high-fidelity cardiac model with realistic haptic feedback can be produced using a high-end printer but will cost hundreds of dollars in material alone with a printer purchase price over 100,000 USD15. A cardiac model produced using a filament with a shore hardness of 26-28 A was found to cost approximately 220 USD&#160;per model16. Alternatively, many low-cost 3D printers and technologies are available that have a printer purchase price of less than 5,000 USD. Average material prices for a cardiac model generated on a low-cost FDM printer was found to be about 3.80 USD using a material with a shore hardness of 82 A and 35 USD&#160;using a material with a shore hardness of 95 A15,16.&#160;While these machines do offer a low-cost solution, it comes at the cost of haptic fidelity.
While VR and 3D printing can allow for detailed visual and conceptual evaluation of a cardiac condition, the high price associated with producing a model for hands-on surgical simulation is often a significant barrier. One solution is the use of silicone to create a physically and texturally accurate cardiac model. Patient-specific silicone models can facilitate a deeper understanding of unique anatomy by allowing surgeons to see, feel, and even practice a procedure while experiencing realistic haptic feedback in an environment that involves minimal risk to a patient and has no consequences if the procedure is unsuccessful9. Silicone molding has been shown to be an effective method to model human anatomy that produces models with physical properties that are significantly closer to real tissue than models generated from low-cost 3D printing17.&#160;Scanlan et al., compared the properties of low-cost 3D printed to silicone molded cardiac valves to evaluate similarity to real tissue; the study found that while the physical properties of the silicone valves were not an exact replica of real tissue, the properties were far superior to the 3D printed valves17. The 3D printing material used in the study is among the softest materials available for low-cost 3D printers and possesses a shore hardness between 26 and 28 A18. The platinum cure silicone recommended for use in the protocol below has a shore hardness of 2 A&#160;which is far closer to the shore hardness of cardiac tissue, 43 on the 00 scale, or approximately 0 A19,20. This difference is significant because the silicone models allow for high-fidelity fine- motor skill training that the directly 3D printed materials do not achieve. The total material cost for the model proposed in this protocol is less than 10 USD. The proposed silicone models combine the soft tissue properties necessary for realistic haptic feedback with the versatility and precision of low-cost 3D printed models.
While the benefits of silicone may appear to make it the obvious choice for model creation, the use of silicone has been restricted by the anatomy that can be molded. Freshly mixed silicone is a liquid that requires a mold to hold it in the desired shape as it cures. Historically, silicone cardiac molds could only contain details of the outer surface of the model. Intra-cardiac details, including the entire blood pool region, would be filled with silicone and lost. Previous studies have achieved silicone models of specific areas of interest within the heart (e.g., aortic root21) or have used an extrapolatory method to simulate myocardial tissue22. This protocol is novel as it seeks to combine the use of silicone material with high-resolution anatomical, full myocardial simulation- specifically avoiding any method of extrapolation. To our knowledge, no descriptive manuscript has provided a methodology combining these aspects. The method described in this protocol introduces a technique to achieve a patient-specific cardiac model with intra-cardiac anatomic replication accurate enough for surgical preoperative practice. The method involves the creation of a myocardial mold to hold the silicone in the proper shape as it cures and an inner mold to preserve the internal, intra-cardiac details of the model and prevent the silicone from filling the blood pool region of the heart. The inner mold must then be dissolved away, leaving an entire silicone cardiac model with patient-specific anatomy on the outer and inner surfaces. Without the proposed protocol of cardiac model creation herein, no low-cost solution exists to simulate the surgical procedure with a material that mimics the actual tissue characteristics of the myocardium.

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	<tbody>
		<tr>
			<td>1.75 mm ABS filament</td>
			<td>Matter Hacker</td>
			<td>matterhackers.com/store/l/175mm-abs-filament-white-1-kg/sk/MFJ1U2CG-</td>
			<td>Anecdotally consistent quality, budget-conscious price</td>
		</tr>
		<tr>
			<td>1.75 mm PLA filament</td>
			<td>Matter Hacker</td>
			<td>https://www.matterhackers.com/store/l/175mm-pla-filament-white-1-kg/sk/MEEDKTKU</td>
			<td>Anecdotally consistent quality, budget-conscious price</td>
		</tr>
		<tr>
			<td>8220 12VMax High-Performance Cordless</td>
			<td>Dremel</td>
			<td>https://us.dremel.com/en_US/products/-/show-product/tools/8220-12vmax-high-performance-cordless</td>
			<td>Cordless for easier access to small features in model</td>
		</tr>
		<tr>
			<td>Acetone</td>
			<td>Sunnyside</td>
			<td>https://www.sunnysidecorp.com/product.php?p=t&#38;b=s&#38;n=840G5</td>
			<td>Bulk</td>
		</tr>
		<tr>
			<td>Dragon Skin Fx-Pro</td>
			<td>Smooth-On</td>
			<td>https://shop.smooth-on.com/dragon-skin-fx-pro</td>
			<td>Industry-standard, characterized skin-safe</td>
		</tr>
		<tr>
			<td>Ease Release 200</td>
			<td>Smooth-On</td>
			<td>https://shop.smooth-on.com/ease-release-200</td>
			<td>Coating to ensure easy removal of silicone from mold</td>
		</tr>
		<tr>
			<td>GORE- TEX patch</td>
			<td>GORE</td>
			<td>https://www.goremedical.com/products/cardiovascularpatch</td>
			<td>Cardiovascular patch</td>
		</tr>
		<tr>
			<td>ideaMaker</td>
			<td>Raise 3D</td>
			<td>https://www.raise3d.com/download/</td>
			<td>&#160;Included G-code CAD software for Raise 3D printers</td>
		</tr>
		<tr>
			<td>Magics</td>
			<td>Materilise</td>
			<td>https://www.materialise.com/en/software/magics</td>
			<td>Feature-rich CAD software capeable of manipulating organic surfaces</td>
		</tr>
		<tr>
			<td>Mimics</td>
			<td>Materilise</td>
			<td>https://www.materialise.com/en/medical/mimics-innovation-suite/mimics</td>
			<td>Feature-rich segmentation software</td>
		</tr>
		<tr>
			<td>Patient DICOM data</td>
			<td>-</td>
			<td>-</td>
			<td>DICOM data will typically come from a patient CT or MRI</td>
		</tr>
		<tr>
			<td>Pro2 Plus</td>
			<td>Raise 3D</td>
			<td>https://www.raise3d.com/products/pro2-plus-3d-printer/</td>
			<td>Anecdotallay reliable, dual extrusion FDM 3D printer</td>
		</tr>
		<tr>
			<td>PRO2-100 Industrial Glue Gun</td>
			<td>Surebonder</td>
			<td>https://surebonder.com/collections/industrial-glue-guns/products/pro2-100-100-watt-high-temperature-professional-heavy-duty-hot-glue-gun-uses-full-size-7-16-glue-sticks</td>
			<td>Industrial-quality hot glue gun</td>
		</tr>
		<tr>
			<td>Silc Pig</td>
			<td>Smooth-On</td>
			<td>https://shop.smooth-on.com/silc-pig-pigments</td>
			<td>Pigment for adding color to silicone</td>
		</tr>
		<tr>
			<td>Vacuum Chamber</td>
			<td>Smooth-On</td>
			<td>https://shop.smooth-on.com/vacuum-chamber</td>
			<td>Anecdotally reliable vacumm chamber for removing air bubbles from mixed silicone</td>
		</tr>
	</tbody>
</table>

creation of patient-specific silicone cardiac models with applications in pre-surgical plans and hands-on training

Three dimensional&#160;models can be a valuable tool for surgeons as they develop surgical plans and medical fellows as they learn about complex cases. In particular, 3D models can play an important role in the field of cardiology, where complex congenital heart diseases occur. While many 3D printers can provide anatomically correct and detailed models, existing 3D printing materials fail to replicate myocardial tissue properties and can be extremely costly. This protocol aims to develop a process for the creation of patient-specific models of complex congenital heart defects using a low-cost silicone that more closely matches cardiac muscle properties. With improved model fidelity, actual surgical procedural training could occur in advance of the procedure. Successful creation of cardiac models begins with the segmentation of radiologic images to generate a virtual blood pool (blood that fills the chambers of the heart) and myocardial tissue mold. The blood pool and myocardial mold are 3D printed in acrylonitrile butadiene styrene (ABS), a plastic dissolvable in acetone. The mold is assembled around the blood pool, creating a negative space simulating the myocardium. Silicone with a shore hardness of 2A is poured into the negative space and allowed to cure. The myocardial mold is removed, and the remaining silicone/blood pool model is submerged in acetone. The described process results in a physical model in which all cardiac features, including intra-cardiac defects, are represented with more realistic tissue properties and are more closely approximated than a direct 3D printing approach. The successful surgical correction of a model with a ventricular septal defect (VSD) using a GORE-TEX patch (standard surgical intervention for defect) demonstrates the utility of the method.

Radiologic imaging data from a patient with a VSD was chosen to generate a representative silicone cardiac model. Patient anatomy was segmented utilizing a CAD segmentation software to generate a digital myocardial model and a digital blood pool model (Figure 1). Manual segmentation of blood pool and myocardium with the presented protocol takes 1-3 h to complete. Upon completion of segmentation, the myocardial model was opened in CAD software for further processing. The model was aligned to a 3D box made within the program and then subtracted away using Boolean operations. This process left a negative of the myocardial model, forming a mold. This myocardial mold was trimmed to a more appropriate size, cut into segments, and modified with props for aligning the pieces (Figure 2). The creation of the case took 2-6 h. All myocardial mold pieces and the blood pool were loaded in a 3D printing slicing software, and G-Code was generated to 3D print in ABS (Figure 3). The 3D printed pieces with support material removed can be seen in Figure 4. The myocardial case pieces were vapor smoothed to enhance the surface finish of the model (Figure 5). Upon the completion of the vapor smooth process, the mold was assembled around the blood pool model, and silicone was poured. The assembly and silicone pour took one hour. After the silicone set, the cardiac model was removed from the myocardial case and submerged in acetone to dissolve the blood pool. After approximately twenty-four hours of soaking, the blood pool had dissolved. A final acetone rinse was performed, and the model was allowed to fully dry. The completed silicone cardiac model can be seen in Figure 6. To evaluate the accuracy and functionality of the silicone model, an incision was made by the CHD (congenital heart defect) expert to allow the inner anatomy to be observed. The expected VSD was present, and a GORE-TEX patch was sewn onto the model by the congenital cardiac surgeon to correct the VSD (Figure 7). In a successfully completed silicone model, all patient anatomy and defects will be present both externally and internally. A summary of the protocol can be seen in Supplementary File 1. 
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/62805/62805fig07.jpg" /> 
Figure 7: GORE-TEX patch sewn in silicone cardiac model with VSD. Photograph of (A) surgeon&#39;s view of a patient-specific silicone cardiac model with a VSD and (B) surgeon&#39;s view of the VSD in the model closed with a GORE-TEX patch. <a href="https://www.jove.com/files/ftp_upload/62805/62805fig07large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Supplementary File 1: Schematic of silicone heart fabrication protocol. Schematic illustration of the protocol in the fabrication of a patient-specific silicone cardiac model. <a href="https://www.jove.com/files/ftp_upload/62805/Schematic_1.psd" target="_blank">Please click here to download this File.</a>
Supplementary File 2: Summary of critical steps and potential negative outcomes. Summary of the steps critical in the development of a patient-specific silicone cardiac model and the potential negative outcomes that can result if the steps are not followed correctly. <a href="https://www.jove.com/files/ftp_upload/62805/Schematic_2.psd" target="_blank">Please click here to download this File.</a>

Watch this Scientific Journal Video about Creation of Patient-Specific Silicone Cardiac Models with Applications in Pre-surgical Plans and Hands-on Training at JoVE.com

Creation of Patient-Specific Silicone Cardiac Models with Applications in Pre-surgical Plans and Hands-on Training

Patient-specific models improve surgeon and fellow confidence when developing or learning surgical plans. Three-dimensional (3D) printers generate adequate detail for surgical preparation, but fail to replicate tissue haptic fidelity. A protocol is presented detailing the creation of patient-specific, silicone cardiac models, combining 3D printing precision with simulated silicone tissue.

Three dimensional&#160;models can be a valuable tool for surgeons as they develop surgical plans and medical fellows as they learn about complex cases. In ...

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