**2. Protocol of colour image reproduction**

adequate donor tissue that may be required for the surgical repair and reconstruction. Further‐ more, the patient's age and general condition may not permit extensive surgical procedures or allow for the morbidity often associated with protracted courses of reconstructive surgery and recovery [2]. Additionally, the final outcome can often be aesthetically and functionally compromised. In such cases, defects may be replaced artificially with the provision of facial prostheses to provide functional rehabilitation and aesthetic improvements. Often there is an

Traditional methods of prosthesis production are well established and are used even today. These include taking an impression, manufacturing a cast and ultimately hand crafting a prosthesis in a silicone based or similar material. The provision of prostheses in this manner has provided considerable comfort and support to many patients over many years, and allowed them to continue with normal day‐to‐day activities and enhance their social interac‐ tion [4]. Despite the advantages this method can provide, its application has shown some limitations and shortcomings. These are primarily related to the processing strategy, technical expertise required, time, effort, cost and retention problems. Furthermore, there are durability issues due to material degradation and colour fade after a relatively short period of service because of general wear and tear and exposure to ultraviolet radiation. For these reasons, facial prostheses require renewing and periodic replacement, which is a costly and time‐intensive

In the last decade, additive manufacturing technology, including three‐dimensional (3D) printing, has advanced dramatically. Colour 3D printing has also evolved to produce full spectrum coloured solid objects utilising a range of materials [5, 6]. With the evolution of various 3D imaging techniques, accurate acquisition and transformation of target object geometric data into 3D digital models can be achieved. By combining the 3D image capture and printing techniques, there is huge potential to achieve 'What You See Is What You Get' processing. More importantly, it has the ability to directly interconnect with advanced manufacturing techniques, allowing customisation with high accuracy, resulting in savings of both time and costs [7]. It has been extensively utilised in rapid prototyping [7], successfully applied in medical sciences [8, 9], and is gaining popularity in multidisciplinary applications [10–12]. For medicine, captured digital 3D models have great accuracy and have been effec‐ tively used for facial disfigurement diagnosis, surgical planning and assessing treatment outcomes for several years [13, 14]. Additionally, there is the potential to develop this auto‐

Compared with conventional image capturing technology, image processing between 3D image devices has much more complicated working processes. For 3D printing, the quality of 3D printed objects is not only affected by the printing itself including binder/substrate interaction and printer resolution, but also the printing material and any post‐processing or finishing stages [15]. Therefore, without a specific protocol, 3D objects can often be produced with poor reliability, accuracy and quality. Moreover, in terms of 3D image reproduction, image processing methods to transform 3D images from a 3D camera to a 3D printer are far less well developed than existing processes that use 2D technology. Furthermore, accurate colour reproduction for facial prostheses is highly desired and the quality of skin colour

associated improvement in social, emotional status and overall quality of life [3].

90 New Trends in 3D Printing

process, which places a burden on both patients and prosthodontists alike [3].

matic additive manufacture technology for facial prosthetics.

Based on the steps shown in **Figure 1**, a 3D colour image reproduction protocol has been developed for the automated 3D printing of facial prostheses. The first step involves 3D scanning of the face using a 3 pod 3dMD photogrammetry system (3dMD, Atlanta, GA, USA). The system captures both 3D topography and colour information of the scanned surface in controlled illumination and viewing conditions. The resulting mesh data from the scanner often has errors in geometry and colour and therefore requires further editing before it can be sent to the printer. Using a combination of software suites, for example Magics (Materialise, Leuven, Belgium), the raw scanned data can be modified and corrected by removing noisy polygons, along with colour adjustment of areas with specular highlights or other inaccuracies. In order to add realism, the next step involves the addition of fine texture (e.g. pores, wrinkles) over the 3D mesh using high‐field mapping. Finally, thickness is added to the mesh data in order to get a solid printable model.

**Figure 1.** 3D image reproduction protocol.

As with the surface topographic information, the colour images from the 3dMD camera system may also require further processing before they can be overlaid on the 3D mesh prior to final colour printing. This primarily involves colour management of the 2D colour image from the camera RGB to printer RGB for each pixel using specific camera and printer colour profiles respectively. For this specific task, the colour profiles used were developed using both conventional colorimetric and spectral‐based reproduction methods [19, 20]. When the colour management is finalised, surface texture mapping is conducted to map the newly generated colour image onto the manipulated 3D model. The penultimate step involves 3D colour printing to produce the 3D model using the 3D printing system, and then, finally as a post‐ processing step, the strength and flexibility of the printed model are improved by infiltrating it with medical grade silicone.
