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# Fabrication of low cost soft tissue prostheses with the desktop 3D printer

Soft tissue prostheses such as artificial ear, eye and nose are widely used in the maxillofacial rehabilitation. In this report we demonstrate how to fabricate soft prostheses mold with a low cost desktop 3D printer. The fabrication method used is referred to as Scanning Printing Polishing Casting (SPPC). Firstly the anatomy is scanned with a 3D scanner, then a tissue casting mold is designed on computer and printed with a desktop 3D printer. Subsequently, a chemical polishing method is used to polish the casting mold by removing the staircase effect and acquiring a smooth surface. Finally, the last step is to cast medical grade silicone into the mold. After the silicone is cured, the fine soft prostheses can be removed from the mold. Utilizing the SPPC method, soft prostheses with smooth surface and complicated structure can be fabricated at a low cost. Accordingly, the total cost of fabricating ear prosthesis is about $30, which is much lower than the current soft prostheses fabrication methods. ## Introduction Soft tissue prostheses are widely used in maxillofacial rehabilitation. Patient numbers is ever increasing due to the advancements in medicine, surgical techniques and in particular cancer survival rates1. However, the high fabrication cost of traditional prostheses prompted the researchers to explore alternatives to reduce the cost and time of fabrication. Three-dimensional (3D) printing or rapid prototyping (RP) is a layered manufacturing method that can be used to fabricate complex structures. It is an additive manufacturing (AM) process, fundamentally different from the conventional subtractive manufacturing processes. Using 3D printing a physical object can be fabricated from a 3D computer-aided design (CAD) model layer by layer, more conveniently and more rapidly than other manufacturing means. Additionally, it is easy to customize a product according to personal requirement, consequently a wide variety of applications of 3D printing technology in soft matter fabrication have emerged recently2,3,4,5,6,7,8. The use of 3D printing can help in the development of omnibearing biodevices4. Such as described in the report by Wang et al.,5, who used hydrogel and cells to fabricate models that mimic complex soft organs in vitro with the help of 3D printing. Also, Landers et al., printed a hydrogel scaffolds using 3D plotter to attain a designed external shape and a well-defined internal pore structure6. Many reports also discussed the fabrication of soft tissues for implant and scaffolds with cell-seeded hydrogel or collagen matrix for clinical application were developed9,10,11,12. High-fidelity ears with collagen type I hydrogel scaffolds were fabricated by injection. Even following an extended period after implantation, they continue to effectively mimic the native auricle both biomechanically and histologically10. To improve the mechanical properties of the fibrous collagen matrix, composite ear scaffolds with wire frameworks were used to ensure a 3D shape after implantation11. Mannoor, M. S., et al12 printed bionic ears able to gather inductively-coupled signals from cochlea-shaped electrodes, demonstrating the merging capabilities of biological and nanoelectronic functionalities. However, because scaffolds or soft tissues based on tissue engineering are designed to acquire real tissues after implanting, further clinical study and a thorough ethics evaluation are still needed. Furthermore, in 2003 Cheah et al.,13,14 integrated CAD/CAM techniques for developing facial prostheses and described a complete fabrication process using the selective laser sintering (SLS) 3D printing technology. It is worth noting, they and Eggbeer et al.15 also discussed the difference between indirect mold production via silicone rubber molding and direct mold production via RP fabrication. In addition, a template-based framework was proposed to rebuild the nasal 3D model with the data from 3D and computed tomography (CT) scans16. The results showed that this method had advantages over other rebuilding methods in terms of accuracy, speed, safety and cost. E. Bassoli et al.,17 reported a combined 3D printing/indirect replication method to fabricate 3D vascular-like structures of soft tissue17; using the ProJet® HD 3000 3D printer from 3D Systems Inc. to print the negative mold of the vascular structure. A team from Italy led by De Crescenzio and Ciocca made a significant contribution to the facial rehabilitation field18,19,20 when they reported a comprehensive solution to the design and fabrication of prosthesis. They created a real anatomic reference model library, namely the “Ear & Nose Digital Library”, which can be very useful for the reconstruction of the facial defects. In addition, they described how the definitive acrylic ear cast mold was printed by ®Z Printer 310 (Z Corp, taken over by 3D system Inc. in 2012), which was then transformed into a wax ear with the vinyl polysiloxane material. A team from UK led by Eggbeer and Bibb performed a long term and remarkable research to study how to apply digital technologies in maxillofacial prosthetics1,15,21,22,23,24,25. Eggbeer et al., compared the prostheses fabricated by 3D printing against a prosthesis made by conventional techniques15. In their nasal prosthesis case study, 3D printing enabled the prosthetics to work in a more flexible manner. However, they determined that the directly printed prosthesis has poor mechanical properties and untested biological responses and suggested that the best way is to print the prosthesis mold and cure the prosthesis with silicone rubber. Worth mentioning, skin features such as wrinkles and texture in auricular and nasal prostheses have been successfully fabricated by 3D printing1. The time required for prosthesis fabrication by 3D printing was found to be considerable shorter than that required by the conventional methods. As the prosthesis is personally customized, mass fabrication methods such as metallic mold casting or injection molding are typically not suitable. A conventional process for the construction of prostheses is particularly lengthy, as it includes many steps: impression, plaster replica, base plate fabrication, mold fabrication, mold packing, curing and extrinsic finishing. The process is laborious and expensive in addition to time-consuming. The maxillofacial prosthodontist determines the correct size, shape and position of the prosthesis for each individual patient, hand-sculpts an impression in wax, then casts the shape in silicone. A handmade silicone ear or nose costs up to$4000 and requires 5–7 hospital visits for customization. The process takes 5–10 weeks in total26.

Many researchers have previously demonstrated 3D printing/RP technology as an effective way to accelerate the prostheses fabrication process and reduce its cost. The primary material used in prostheses, either silicon or Polydimethylsiloxane (PDMS), is more suited to casting. The so-called rapid tool, soft tool or bridge tool27 fabricated by 3D printing is commonly used to cast prostheses. It should be noted, however, that the 3D printers used in this previous research were industrial-grade and therefore quite expensive. Indeed, the cost of fine prostheses mold-fabricated by an industrial 3D printer is still high, which has hindered the wider use of 3D printing. According to the quote given by Magicfirm, LLC. Hangzhou, China, the fabrication price is $2.2 per cm3 using the stereolithography apparatus (SLA) of a 3D printer, indicating a cost of$411.80 for an ear mold with a size of 7.8 × 5.0 × 4.8 cm. A prosthetic product fabricated by 3D printing from Fripp Design & Research in the UK costs around $4,000 each, almost as much as a handmade product28. If such prostheses could be manufactured at home, the cost would drop dramatically, greatly benefiting patients. The desktop 3D printer (personal 3D printer), also known as the open source 3D printer, is very popular among the do it yourself (DIY) variants. The advantages of desktop 3D printer include low cost and as a result, many people can afford a small size printer with extremely low to zero toxicity and waste. With the expiration of the key patents of Fused Deposition Modeling (FDM), the technology used in desktop 3D printer, a large open-source development community has emerged and a desktop 3D printer now only cost$200–$1000. Two standard types of desktop 3D printer are the RepRap and the Fab@Home; however, everyone can change the desktop 3D printer design according to their requirements thanks to its open source development. At present the desktop 3D printer is being widely used in education, DIY manufacturing, research and industry. As illustrated by the successful production of micro- and milli-scale reactionware with a desktop 3D printer (3DTouch)29. The main task during the manufacture of soft prostheses is the making of the casting mold, which is also the main component of the cost incurred during the whole prostheses fabrication process. If the casting mold can be fabricated using a desktop 3D printer and as such printers become readily affordable, then doctors could fabricate the prostheses in their offices, or the patients could do it at home. Nevertheless the staircase effect (contour like structure) caused by the layered manufacturing is a challenge which impedes the acquisition of smooth surface in 3D printing. This drawback is especially serious in the desktop 3D printer due to the layer thickness, as shown in Fig. 1. In this study, we focus on how to apply the low cost desktop 3D printer to print the casting mold. The fabrication method used is called Scanning Printing Polishing Casting (SPPC). The key consideration when using SPPC is how to fabricate a negative casting mold with smooth surface inside and will be addressed in this report. To that end, the polishing method, which can ensure that the surface quality of the mold is adequate to be used in the prostheses casting, was systematically investigated. Moreover, it is proven that the soft prostheses can be affordably manufactured, under$30 each, at home or in a personal lab setting without any expensive technical equipment. SPPC also can be widely used in the fabrication of soft muscle, hydrogel structure in tissue engineering, artificial skin, customized jewel, etc.

## Methods

The typical process of SPPC is shown in Fig. 2. Firstly, 3D data for the prostheses are acquired with a 3D scanner. Once acquired, the datasets are used to generate a digital 3D model of the negative mold for casting by using a 3D modeling software such as 3D max, Rhinoceros, or SolidWorks, etc. Subsequently, the 3D model is converted into G code language by a slicing software tool, which can then be read by a desktop 3D printer. A common polymer filament, Acrylonitrile Butadiene Styrene copolymers (ABS), is applied for 3D printing. Since the multi-strip-like structure caused by 3D printing will affect the prostheses surface quality, acetone vapor is used to eliminate the layer thickness effect of the mold, as ABS is soluble in acetone. After polishing, silicone is poured into the mold. The soft prostheses can be easily pulled out/demolded from the mold because the solidified silicone provides extra strength and elasticity.

### Mechanical performance

The mechanical and physical properties of silicone prostheses made by SPPC are shown in Tab. 3. Compared to the properties of a range of other commercially-available silicone rubber maxillofacial materials31, this silicone prosthesis possesses considerable advantages in its tensile strength and tear strength. Its elongation percentage rivals that of other commercial materials, its hardness ranges from 0 to 60 A° and its heat resistance is above 200°C, all of which meet the general requirements of prosthetic products.

### Parting fabrication

Fabricating an ear prosthesis is much simpler than fabricating a hand prosthesis, as only one mold is required. The prosthesis can be easily pulled out of the ear mold, due to the structure of the ear and the high elasticity and low hardness of silicone. Two or more part molds are necessary, however, when an ear prosthesis with low elasticity and high hardness is needed. It is impossible to cast a silicone hand with only one mold. Upper and lower molds are designed and printed, then tightened by bolts, as shown in Fig. 10. A hand prosthesis looks much like a real human hand in shape and color; nevertheless, it requires much further process and refinement for detail, such as hair and varying texture. Technically speaking, any shape of silicone prosthesis can be cast by parting fabrication.

## Conclusion

As a desktop 3D printer is very cheap, it is also referred to as a personal 3D printer and it will become as popular and common as the personal computer, this means that almost everyone can afford and use their desktop 3D printer to print whatever they want. It is every interesting and creative to develop research applications with a desktop 3D printer.

In this study we aimed to fabricate the soft prostheses with the help of a desktop 3D printer and succeeded in attaining this goal. The printing costs of negative mold were negligible when compared to the traditional methods. In this report, a cheap and convenient method (SPPC) of fabricating silicone prosthesis is demonstrated. The staircase effect of the mold caused by the desktop 3D printer is eliminated to produce a prosthesis with smooth surface. Furthermore, prosthesis fabrication is not limited by its shape, which promises a prosperous future for the application of SPPC. Not only for prostheses, but also masks, cosmetic products, sex products, etc., which need to be personally customized, can be fabricated with a desktop 3D printer in the near future.

## References

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## Acknowledgements

This paper is sponsored by the Science Fund for Creative Research Groups of National Natural Science Foundation of China (no. 51221004), National Natural Science Foundation of China(no. 51375440), Fundamental Research Funds for the Central Universities (no. 2013QNA4007) and Zhejiang Provincial Natural Science Foundation of China (no. LY12E05018).

## Author information

Authors

### Contributions

Y.H. proposed the SPPC method, wrote the main manuscript text and G.X. performed experiments, prepared figures 15, 910. J.F. planned the research, prepared figures 68 and edited the paper.

## Ethics declarations

### Competing interests

The authors declare that they have no competing financial interests.

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He, Y., Xue, Gh. & Fu, Jz. Fabrication of low cost soft tissue prostheses with the desktop 3D printer. Sci Rep 4, 6973 (2014). https://doi.org/10.1038/srep06973

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