# Concepts and Terminologies in 4D Printing
## 3 Properties of 4D Printed Parts
### Material Properties and Materials Associated with 4D Printing
They can be a combination of smart or conventional materials or using various smart materials that react differently under various stimuli [36]. \"Hydrophilic materials\" are sometimes used in 4 DP applications. These substances are easily wetted and have more thermodynamically favorable interactions with water. Certain hydrophilic materials, when exposed to water, will lead to absorption that can increase in its volume substantially, evoking a shape change effect through linear expansion. \"Hydrogels\" belong to the group of hydrophilic polymers that can swell in water and hold a large amount of liquid while retaining the structure. 4 DP hydrogels are responsive to hydration and temperature in which the hydrogel swells, and a transformation is initiated. Examples of natural polymer-based hydrogels include Dextran, Chitosan, and Collagen; synthetic polymer-based hydrogels include Poly (vinyl alcohol). The \"swelling ratio\" is defined as the fractional increase in the weight of the hydrogel (or a similar hydrophilic material) due to water absorption. The \"porosity\" of the material, which is the measure of void spaces can also significantly improve the response speed of the shape change effect [37]. Another characteristic in 4 DP applications is \"anisotropy\", which refers to the physical properties that are directionally dependent. Within the AM process, there is a certain degree of anisotropy in the mechanical properties, due to the layer wise method of fabrication. In a 4 DP context, the mechanical properties in terms of the tensile strength in a glassy state and its elongation in a rubbery state are important factors to consider when programming shape memory materials. These two properties dictate the capability of shape fixity and the recovery, which are consequential for the function of SMPs [21]. Other research studied the use of \"printed active composites\", which are typically soft composites made up of glassy SMP fibers and the stacking of laminas to reinforce the elastomeric matrix. These laminates undergo thermomechanical programming to achieve shape change behaviours. Other forms of \"elastomeric matrices\" have been described in [PERSON] et al. [38] where they described elastomers as having more free volume than other polymers, which accelerates the rate of swelling. The swelling induces internal stress, in which in the recovery step converts the stored potential energy into elastic energy that acts as an actuation device. A paper by [PERSON] et al. [39] discussed the use of \"Joule heating\", which is a controlled process in which passing an electric current through an electrical conductor produces localized thermal energy in the form of heat. This is also known as resistive, resistance, or Ohmic heating. Localized Joule heating can be used to initiate active hinges in 4 DP parts and constant, a copper-nickel alloy, has been used for such purposes. In terms of the materials commonly used for 4 DP, shape memory alloys and shape memory polymers are the most widely studied. \"Shape memory alloys\" (SMAs) are popular smart materials that possess two phases: A martensite phase (low temperature) and austenite phase (high temperature). These two phases allow SMAs to alter their shapes and return to the original shape when exposed to high temperatures [40; 41]. SMAs are thermomechanical alloys, in which, once treated to acquire a specific shape, they have the ability to indefinitely recover from large strains without permanent deformation and to remember their original geometry. For example, after undergoing a physical deformation, an SMA wire can be heated through resistive heating to its final transformation temperature (Af) and regain its original shape [42]. This is mostly applicable to NiTi as others, such as Cu based wires, have low electrical resistance and cannot be easily joule heated. \"Nickel-titanium\" (NiTi) is a class of SMA that exhibits excellent shape memory behaviour including a high percentage of shape recovery, a large super-elastic strain, and recovery stress [43].