# Development and Assessment of a 4D Printing Technique for Space Applications
## 3 Results
### Recovery Assessment  
After exploring the programming capabilities of the hinges, next, the recovery behavior was investigated. But for these experiments, the results from the previous investigations were used. Obviously, the hinges had to be programmed before they could recover the induced deformation. The temperature \(T_{\text{set}}\) and the exposure time \(t_{\text{e}}\) chosen for this purpose were based on the results of the first experiment. The best programming behavior for the first iteration of the programming device was achieved at \(T_{\text{set}}=250\,^{\circ}\mathrm{C}\) and \(t_{\text{e}}=10\,\mathrm{min}\). In contrast, a similar fixity ratio was achieved using the second iteration programming device with a shorter exposure time of only \(t_{\text{e}}=5\,\mathrm{min}\). Notably, the shorter exposure time  
Figure 11: The calculated recovery ratio \(R_{\text{r}}\) in relation to the calculated fixity ratio with a linear regression and a 95% confidence interval.  
Figure 10: The calculated recovery ratio \(R_{\text{r}}\) in relation to the measured surface temperature before programming \(T_{\text{S}}\) with a linear regression and a 95% confidence interval.  
contributed positively to the (energy) efficiency of the whole 4D printing process, which is why the shorter \(t_{\text{e}}\) was used for the second version of the programming device.  
The hinges were programmed at their respective temperatures and times, always achieving a fixity ratio of close to 100%, with the lowest fixity ratio being 94%.  
After exploring the programming of the hinges, we shifted our attention to the recovery process, the main purpose of this experiment. As explained earlier, the hinges were allowed to recover the deformation at different lowered temperatures. Figure 12 shows how the achieved recovery ratio changed with the decreasing temperatures set on the printer's interface for recovery \(T_{\text{set(r)}}\).  
Surprisingly, we can see in Figure 12 that the lowered recovery temperature had almost no influence on the recovery ratio of the hinges, with the hinges even achieving full recovery up to 150 \({}^{\circ}\)C. However, the 100% recovery was only achieved at 150 \({}^{\circ}\)C using the second iteration programming device, thus proving again the value of upgrading the programming device.  
As previously noted, the recovery time \(t_{\text{r}}\) was documented in these experiments. However, the recovery time was not a primary focus for us. In the context of space structures, it is essential that the recovery did not occur abruptly within an exceedingly brief time frame, but instead over a longer period to minimize vibrations. Nonetheless, this was the case across all observed hinges, with the minimum recovery time being 30 s. Although the hinges exhibited significantly faster recovery at elevated temperatures, the recovery duration was approximately twice as long at the lowest achievable temperature of 150 \({}^{\circ}\)C.  
We also tested the second iteration of the programming device at \(T_{\text{set(r)}}=120\) \({}^{\circ}\)C, but no recovery was achieved. The hinges were left inside the printer for longer than 5 min, but it quickly became clear that no recovery could be achieved at this temperature.