# 4D Printing of Origami Structures for Minimally Invasive Surgeries Using Functional Scaffold
## 3 Results and Discussion
### Deformation and Shape Recovery of the Origami Tessellations  
The two herringbone and waterbomb tessellation origami structures after twisting and recovery are shown in Figure 4. The most significant find from the measurement of the deformation and recovery of the two herringbone and waterbomb tessellation origami structures was that the expected recovery of approximately 65.1% of each angular fold, suggested by the results of the PLA filament result, was not exhibited. The angle recovery was far greater than that shown by the filament test samples in the first experiment. Recovery rates had a mean average of 96% for the two test samples. All independent variables have been kept constant, except for the geometry of the test samples. Therefore, it can be deduced that the geometry of the origami induced internal stresses within the two models, which aided the SME and explain why greater recovery rates were observed. The twisted waterbomb tessellation origami structures did not deform in the intended direction. The recovery of the tubular models still achieved a result that was 96% of the initial print geometry; however, because the deformation of the waterbomb tessellation origami structure was unpredictable, it did not meet the requirements of the biomedical scaffold, and so it was discounted as a potential biomedical scaffold to be taken forward. Only the herringbone models were now left as the tessellation type to be taken forward. Figure 4d shows the herringbone models after being deformed and recovered. To better evaluate how herringbone tessellation origami could deform and be recovered, the experiment was repeated. The difference was that this time the direction of deformation was changed from twisting to compression along the central axis. The  
Figure 4: (**a**) The 70 \({}^{\circ}\)C water bath for heating waterbomb (up) and herringbone (down) tessellation origami structures. (**b**) After deformation, (**c**) after recovery, (**d**) herringbone tessellation after being deformed and recovered for the fourth time. amount of deformation observed without structural failure was far more significant. The deformation took the height from 100 to 25 mm. The recovery rate for this test was again 96%, though several cracks were observed. This direction of deformation and recovery was repeated on the same herringbone sample several times, and the rate of recovery remained at 96% each time, causing the tube to decrease in recovered height each time the test was repeated. Edges became softer each time the test was repeated, as shown in Figure 5.  
The scaffold interior for substituting cancellous bone was embedded to the herringbone tessellation origami to replicate cancellous bone and to validate its deformation and recovery behaviour. Each thin strand of the scaffold crossing the scaffold core replicates an individual trabecula. trabeculae have a similar cross-sectional microstructure to osteons; however, Haversian canals do not pass through their centre. Therefore, the central core does not require blood vessels to pass through the replica trabeculae. The requirements of the central core's macrostructure were near enough the same as the outer compact bone substitute section of the scaffold. The only real difference was that the density of the central core needed to be less to create the large pores visible in cancellous bone. The herringbone tessellation origami was combined with the central core to create the final CAD model, shown in Figure 6a. Figure 6b shows the 3D printed sample with the porous central core. The figure shows how light can pass through the cancellous scaffold substitute, proving that the pores are interconnected. The same SME procedure was applied to the process using herringbone tessellation origami and substituting the cancellous core, and the results are shown in Figure 7. Unsurprisingly, the amount that the model deformed was considerably less than that of the tubular prototypes shown in Figure 5 due to the substantial increase in stiffness of the central core.