# Development and Assessment of a 4D Printing Technique for Space Applications
## 2 Materials and Methods
### Design of Experiments  
In this study, our primary aims were two-fold. First, we aimed to develop a device that enables the programming and hence the 4D printing of foldable, self opening structures. Second, we aimed to 4D print hinges with the previously mentioned device using PEEK, with the potential application as self-deploying hinges in space environments. We sought to demonstrate that SMEs in PEEK were sufficient to enable a fully self-deploying system. Notably, prior research ([17]) cast doubt on this hypothesis. Should the SMEs prove insufficient for the desired application, we will explore methods for optimizing the achievable SMEs. Our experimental design involves the following steps:  
1. **Hinge Fabrication:** Hinges made from PEEK will be 3D printed on Orion's A150 3D printer to ensure high density and isotropy. Because of confidentiality agreements established with Orion, specific parameters employed for 3D printing the hinges on Orion's printers cannot be disclosed at this point. This is integral to protect proprietary details, preserving Orion's competitive edge, and protecting their techniques from unauthorized replication or exploitation. 2. **Programming Process:** After printing, the hinges will be programmed using the build chamber of Orion's 3D printer. The programming process involves heating the hinges and utilizing a custom-made programming device. This device will be constructed and optimized to efficiently support the programming of hinges. 3. **Device Iteration:** Following an initial set of experiments, the programming device will be altered and optimized, and all experiments will be repeated using this updated device. This device will also be 3D printed using PEEK to match the high-temperature environment required for programming. The device was printed using a rectangular infill pattern with an infill density of 30%. 4. **Experiments:** The experiments are designed to assess the shape memory characteristics of the hinges, with a focus on programming and recovery: (a) **Programming Assessment:** The hinges will be programmed at different temperatures inside the print chamber (150 degC, 175 degC, 200 degC, and 250 degC) and for varying time durations (5 min, 10 min, and 20 min). The success of the programming process will be measured using the achieved fixed angle, denoted as \(\alpha\). This angle, as illustrated in Figure 5d), represents the angle between the inner two surfaces of the hinge after programming, with 0 deg being the smallest possible angle (perfect programming) and 180 deg being the largest possible angle (no programming at all). The fixity ratio (\(R_{\text{f}}\)) is calculated using the formula: \[R_{\text{f}}=\frac{180^{\circ}-\alpha}{180^{\circ}}\] (1) Subsequently, the hinges will recover their deformation at the same temperature and time duration as the initial programming. The recovered angle \(\beta\) will be measured to calculate the recovery ratio (\(R_{\text{r}}\)). \(\beta\) is the angle between the inner two surfaces of the hinge, with 180 deg being the largest possible angle (perfect recovery) and \(\alpha\) being the smallest possible angle (no recovery at all). The recovery ratio (\(R_{\text{r}}\)) is calculated using the formula: \[R_{\text{r}}=\frac{\beta-\alpha}{180^{\circ}-\alpha}\] (2) This experiment aims to determine the optimal temperature and minimum time required for effective programming. (b) **Recovery Assessment:** Once the optimal programming conditions are determined, the hinges will be tested to measure their recovery potential. After successful programming at the identified optimal conditions, the hinges will be subjected to recovery tests at various lower temperatures (\(-10\) degC, \(-30\) degC,\(-50\) \({}^{\circ}\)C, and \(150\) \({}^{\circ}\)C), with the recovery time measured. The programming and recovery ratios will be calculated. This experiment aims to establish the lower limit for efficient recovery.