# Development and Assessment of a 4D Printing Technique for Space Applications
## 1 Introduction  
In recent years, 4D printing has emerged as a new fascinating trend within the 3D printing community [1]. This innovative approach to additive manufacturing represents a significant leap forward in the world of material science and engineering. Four-dimensional printing can be best understood as an extension of 3D printing, with the added dimension of time [2]. At its core, it involves the fabrication of objects that have the ability to transform and adapt their shape and properties over time in response to specific stimuli [2; 3]. One of the key components of 4D printing in our context is the use of shape memory polymers (SMPs). These polymers exhibit a unique property--they can return to a predetermined shape or configuration when exposed to a particular stimulus, typically heat [4]. This ability to \"remember\" and revert to a specific shape is at the heart of 4D printing with SMPs. SMPs, in essence, are materials that have been \"programmed\" to exhibit this shape memory effect (SME). This programming is a critical aspect of their functionality, and it distinguishes them from traditional materials. It is important to note that the shape memory effect is not an inherent property of the material, but a result of this programming process [1; 4]. The allure of 4D printing lies in its potential to design self-deploying objects, offering a wide array of applications. These objects can remain compact and easy to transport before being activated to achieve their intended shape or function [1; 4]. This shape altering capability makes 4D printing particularly attractive, as it opens up new avenues for solving complex engineering challenges. In the medical field, 4D printing has shown promise in the creation of self-assembling medical devices. [PERSON] et al. [4] demonstrated the potential of SMPs in the field of minimal invasive surgery in cardiovascular applications. In the context of space technology, 4D printing takes on a new dimension. The ability to design self-activating and self-deploying mechanisms becomes key. This paper primarily focuses on the application of 4D printing with SMPs in the development of self-activating hinges for space deployment. While shape memory effects have been utilized in spacecrafts in the past, these applications predominantly involved shape memory alloys (SMAs). SMAs are a class of metals known for their shape memory properties (see [5] and [6] p. 609 ff). In this context, shape memory actuated systems, including hold-down and release mechanisms (HDRMs) and actuators for release and deployment, have been developed using SMAs ([6] pp. 610-614). It is worth noting that SMPs have gained attention relatively recently in the context of space structures, especially when compared with the more established use of SMAs ([7] p. 47, [8; 9]). However, SMPs offer several key advantages over SMAs that make them highly attractive for implementation in space structures. * SMPs are significantly more cost-effective compared with SMAs [8; 10]. * SMPs exhibit a significantly greater recoverable deformation compared with SMAs [10]. * Furthermore, SMPs possess a significantly lower density compared with SMAs, which also enhances their attractiveness for space applications [8; 10]. It is worth highlighting that most existing research in the field of SMPs has been centered on their characterization and evaluation using dynamic mechanical analysis (DMA) or dynamic mechanical thermal analysis (DMTA) techniques [4; 11; 12]. The novelty of this research lies in its distinctive approach to the 4D printing process, which is notable through its seamless execution within a singular device, the A150 from Orion Additive Manufacturing GmbH ([[https://orion-am.com/](https://orion-am.com/)]([https://orion-am.com/](https://orion-am.com/)), (accessed on 26 September 2023)) (Orion). Unlike conventional methodologies, both the 3D printing of the SMP and its subsequent programming were achieved with this advanced printer.