# Concepts and Terminologies in 4D Printing
## 3 Properties of 4D Printed Parts
### Material Properties and Materials Associated with 4D Printing
Studies have shown that NiTi parts can be additively manufactured using selective laser melting (SLM) [44; 45]. \"Copper-aluminum-nickel-manganese\" (Cu-Al-Ni-Mn) is another class of Cu-based SMAs that could be potentially produced using SLM, promising good shape recovery, high transformation temperatures, and suitability for industrial applications [46]. In terms of polymeric materials, \"shape memory polymers\" (SMPs) can be programmed to memorize a less-constrained shape or configuration, and subsequently assume a strained temporary shape, and then revert to the memorized configuration upon being triggered by an external stimulus [47]. Although most polymeric materials have a heat/chemo-responsive SME, the polymeric materials used in additive manufacturing have a creeping and relaxation problem. \"Polylactic acid\" (PLA) is a common SMP that is widely available in a filament form and regarded as suitable for 4 DP. Generally, most PLA materials have a glass transition temperature of 60 \({}^{\circ}\)C and an elastic modulus of 3.5 GPa in the glassy state. The group of \"thermoplastic elastomers\" (TPE) that exhibit properties of SMPs include thermoplastic (TP), styrene block copolymers (TPS), polyolefin (TPO), polyurethane (TPUs), copolyester (TPE), and polyamides [48]. \"Theroplastic polyurethane\" (TPU) contains characteristics of rubbers and plastics with excellent biocompatibility and flexibility. TPU has been widely used in medical devices. TPU has high elongation, moderate tensile strength, a moderate Young's modulus, and strong abrasion and tear resistance [49]. For SMPs, the \"glass transition temperature\" (T\({}_{\text{g}}\)) or \"transition temperature\" (T\({}_{\text{trans}}\)) is used to describe the temperature beyond which a polymer changes from a hard, glass-like state to a rubber-like state. For the ease of comparison with conventional polymers, a single T\({}_{\text{g}}\) value taken from the midpoint of a broader transition is often used in literature. However, since the width of glass transition may have a profound impact on the shape memory performance of SMPs, most studies report the width of the transition along with the midpoint T\({}_{\text{g}}\) value, or the onset glass transition temperature (T\({}_{\text{g onset}}\)) and the ending glass transition temperature (T\({}_{\text{g end}}\)) along with the midpoint T\({}_{\text{g}}\) value [22]. The \"shape deformation temperature\" (T\({}_{\text{d}}\)) is recognized as the working temperature in which the SMP is strained to a temporary shape. The relationship of Td is relative to T\({}_{\text{trans}}\) and it has a significant impact on the shape memory performance of the SMP. The \"temporary shape fixing temperature\" (T\({}_{\text{f}}\)) is the working temperature at which the temporary features of the SMP are fixed. T\({}_{\text{f}}\) is often lower than T\({}_{\text{trans}}\). Lastly, the \"shape recovery temperature\" (T\({}_{\text{r}}\)) is the working temperature that the SMP is triggered at in order to recover from a fixed temporary shape. Usually the T\({}_{\text{r}}\) is higher than (T\({}_{\text{trans}}\)) and is often chosen to be the same as T\({}_{\text{d}}\) (ibid).