## 4 Discussions
The Connex printer used here was a three-headed system, and, while there were at least 45 colors available when using a different color on each head, only five were used for the femur and only six colors were used for the lumbar vertebra since a clear material was also included to better visualize the spatial relationship between subsections. However, clear material is not ideal for color mixing, thus limiting the overall palette on the Connex3 Objet350. Additionally, the use of color \"shades\" failed (data not shown), as they were visually too close to each other, making it difficult to differentiate among them. Consequently, only six colors were visibly distinct from each other and available for use on our models. Other practical considerations include durability of printed models, complexity of software processing, pricing, and build area of the 3D printers used. Table 1 details the relative pros and cons of binder-jet printing and poly-jet printing. Firstly, acrylic parts generated with poly-jet technology are widely recognized for better durability compared to sandstone, which is relatively brittle [24]. Secondly, the poly-jet process is limited by hardware and planning. The procedure of splitting the files into multiple parts requires significantly more computer processing power than digitally painting it. Additionally, splitting each model into separate parts requires stepwise planning. For example, the neck and head of the femur must be split from the body before the head can be isolated. The resultant model would comprise multiple files, which can be cumbersome to organize and bring together. Next, poly-jet printing proved to be a lot more expensive. Table 1 (Appendix A) shows that printing via poly-jet is 10-fold that of sandstone binder-jet printing through Shapeways. This price can come down by purchasing an in-house printer; however, this option would require significant initial investment that may be mitigated via access to local fabrication labs. Finally, maximum build size depends on the printer used: \(342\times 342\times 200\) mm for Connex3 Objet350. The maximum build size for sandstone printing by Shapeways is comparable at \(250\times 380\times 200\) mm. Recently, an additional approach combining the file preparation method of binder-jetting and durability of poly-jet printing became available in the Stratasys J750 poly-jet printer. The J750 is a six-head poly-jet printer that allows for printing of 500,000 colors, which was originally announced in 2016 [10]. In July 2019, Shapeways added a software option for full-color poly-jet printing of VRML files on the J750 platform. This announcement is significant for three reasons. Firstly, an increasing prevalence of poly-jet printing technologies enables access via third parties like Shapeways. Secondly, full-color acrylic printing is now practical. Thirdly, VRML files can now be used for poly-jet printing. The capability of J750 in full-color printing will lift the limits associated with Connex machines with fewer than six heads. Additionally, since the J750 accepts VRML files, the process of preparing multi-color digital models is the same as for binder-jet printing. Customers can now upload VRML files, as they did for sandstone printing, and simply select the multi-color poly-jet option. Table 1 (Appendix A) shows prices for the four printed bone models in sandstone and multi-color poly-jet printing through Shapeways, with the latter being over 10 times as expensive.