# Development and Assessment of a 4D Printing Technique for Space Applications
## 4 Discussion
### Recovery  
The recovery experiments conducted offer profound insight into the shape memory capabilities of 4D printed hinges. The results, particularly from the second experiment, reveal noteworthy characteristics that extend the potential utility of these SMP hinges.  
In the second experiment, a remarkable observation emerged--the hinges exhibited consistent recovery of almost the full previously induced deformation up to a lower temperature barrier of 150 \({}^{\circ}\)C. This effect was notably more pronounced for the second version of the programming device. This finding is intriguing as it surpasses conventional expectations and showcases the remarkable shape memory capabilities of SMP hinges. The observed independence of recovery from the recovery temperature up to the lower barrier of 150 \({}^{\circ}\)C is a notable outcome. This characteristic enhances the potential use-cases of these 4D printed hinges, as it suggests a broader operational range for shape memory activation. The increased flexibility in recovery conditions contributes to the adaptability and versatility of these hinges in various applications.  
The shape memory capabilities exhibited in experiments 1 and 2 surpassed expectations and expand the perceived boundaries of what was previously imagined possible. This remarkable performance stands in contrast with the sobering DMTA results and anticipated SMEs based on prior research [17]. The experimentally observed recovery at lower temperatures challenges existing assumptions and opens new avenues for exploration in the field of shape memory polymers.  
The higher-than-expected shape memory effects observed in our experiments may be attributed to specific design and material properties. The design of the hinges, characterized by their high slenderness, could contribute to the enhanced shape memory effects. Therefore, the heightened slenderness could support the inherent high elasticity of PEEK, allowing for greater deformation and recovery. In addition to slenderness, the material properties, particularly the molecular structure of the specific PEEK variant used in our case, the Evonik Infinam PEEK 9359 F, play a crucial role in the hinge's performance. The unique molecular composition of this PEEK variant likely enhances its shape memory characteristics, contributing to the exceptional recovery observed in our experiments.  
The decision to test at 120 \({}^{\circ}\)C for the second version of the programming device aligns with the potential use case in space structures. However, the lack of recovery at this temperature is unsurprising given the known glass transition temperature (\(T_{g}\)) of PEEK at roughly 150 \({}^{\circ}\)C.  
The outcomes of the recovery experiment not only validate the exceptional shape memory capabilities of the 4D printed hinges, but also introduce new considerations for their deployment in diverse scenarios.  
Figure 14: “DMTA results: Storage modulus (**left**) Tan tan (**right**) of PEEK_NdFeB filaments” by [PERSON] et al. [12], licensed under [[https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)]([https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)), (accessed on 13 September 2023) (CC BY 4.0).