## 4 Discussion
### Limitations and Future Recommendations  
Some limitations need to be acknowledged in this review. Firstly, the studies included have small sample sizes and lack longitudinal data. The results of these studies are also based on the participants' subjective evaluations, and there could be bias in evaluating the simulator using questionnaires [24]. Secondly, there is a lack of comparison with conventional clinical training and other models. Only two studies compared the 3D-printed model with previous simulators [22; 37]. Thirdly, these studies lack follow-ups of the participants' clinical practice after their model experience. The impact of 3D-printed simulators on patient outcomes remains unknown [37]. Finally, little effort has been made to compare and evaluate the mechanical properties of different materials. Some studies did not provide details of their 3D printing, making it difficult to compare the methods. Another limitation of current studies is a lack of comparisons of 3D-printed models with virtual reality (VR) or mixed reality (MR) visualization tools. Recent studies have shown that VR and MR are the best modalities in presurgical planning, demonstrating a spatial relationship between cardiac structures and enhancing depth perception in congenital heart disease. Furthermore, 3D-printed models have been found to be the preferred modality in facilitating communication with patients [9; 45; 46]. Therefore, it is worth comparing 3D-printed gastrointestinal tract models with VR and MR.  
Future research will focus on performance improvements, focusing on less-explored areas, in addition to currently reported 3D-printed models in education, clinical training, and procedure simulation. The optimization of CT scanning protocols deserves investigation given the widespread use of virtual colonoscopy in screening colonic polys. A recent study showed the feasibility of using 3D-printed personalized chest models to stimulate lung nodules to determine low-dose or ultra-low-dose CT protocols in lung cancer screening [47]. The use of 3D printing technology for drug delivery has been widely reported in the literature, and with respect to gastrointestinal tract diseases, great potential lies in combining 3D printing with nanotechnology to achieve dose personalization for colon-specific  
Figure 7: Endoscopic views of the rectum. The photos in the upper panels are from the NKS colonoscopy simulator (novel silicone rectal 3D-printed models), the middle panels from the patient, and the lower panels from the CM15 model. _1 st HV_: first [PERSON]’s valve; _2 nd HV_: second [PERSON]’s valve; _3 rd HV_: third [PERSON]’s valve. Reprinted with permission under open access from [PERSON] et al. [22].  
diseases with improved treatment outcomes [48]. Three-dimensional bioprinting shows promise in revolutionizing medicine by printing functional tissues, organs, or even living cells. However, very limited research is available on the gastrointestinal tract, as most of the current reports are mainly related to cardiovascular or liver disease [49; 50; 51; 52]. Figure 8 shows the mechanisms for performance improvement strategies utilizing 3D printing technology in gastrointestinal diseases.