# Concepts and Terminologies in 4D Printing
## 3 Properties of 4D Printed Parts
### Material Properties and Materials Associated with 4D Printing  
The earlier section described stimuli-responsive materials that respond by producing a useful effect, such as actuation mechanisms or a shape change behaviour. In general, most stimuli-responsive materials used in 4 DP are \"shape memory materials\" that are a subdivision of multi-phase smart materials. They can recover from a deformed state to their initial shape, known as the \"shape memory effect\". Shape memory materials exhibit a plastic deformation (temporary shape) when an external stimulus is applied, and they are able to recover to their original shape [12; 26]. \"Shape changing material\" also refers to the reversible material properties that can spontaneously change shape on the application of stimuli and have the ability to regain their original shape upon the removal of stimuli [27]. Other classes of matter include \"smart materials\", which are substances that are able to couple or convert energy between various physical domains, such as converting thermal energy from heat into mechanical work [28]. In particular, \"thermo-responsive materials\" are those that are sensitive to temperatures. The temperature is transferred to the material through a medium, such as water, UV light, or other sources. \"Metamaterials\" refer to a category of multi-scale structures that widely exhibit thermomechanical characteristics not usually found in nature, and instead utilize properties from the design of the structure and geometry [29]. \"Tunable materials\" are substances that efficiently adapt to a dynamic environment. For example, [PERSON] et al. [30] demonstrated mechanically tunable lightweight metamaterials utilizing 4D printing produced using photo-crosslinkable and temperature-responsive SMPs. [PERSON] and [PERSON] [31] also introduced tunable metamaterials with reversible thermo-mechanical memory operations. The term \"functionally graded materials\" (FGMs) is used to describe substances that are designed and fabricated for specific functions and applications, by means of varying the composition and the structure gradually over the volume, resulting in changes in the material properties. FGMs fabricated by additive manufacturing are known as Functionally Graded Additive Manufacturing (FGAM), and examples include the varied densification of concrete [32]. The term \"digital materials\" is also cited in literature, often making reference to the material jetting process from Stratasys Objet Connex machines that work by mixing two base materials, made up of specific ratios of rigid plastic VeroWhite as an \"inactive material\" that remains inert, and a rubbery TangoBlack as the \"active material\" that responds to stimuli to achieve the desired shape memory behavior [33]. These customized materials are also known as \"programmable materials\" whereby the properties can be modified using a software such as GrabCAD to specify the desired properties of a material [34]. Voxel-based modeling editors such as Autodesk Monolith can be used to represent a three-dimensional bitmap, represented as a \"voxel\" that contains a singular material. When an array of different voxels made up of distinct materials in a design part are combined, a multi-material structure can be achieved. The layer wise deposition of material enables engineers to control the density and directionality of material deposition as well as to combine various materials to produce a seamless monolithic component. According to [PERSON] et al. [35], complex matter can be produced through (1) variable densification within a homogeneous composition; (2) heterogeneous composition through simultaneously combining two or more materials through a gradual transition; or (3) using a combination of variable densification within a heterogeneous composition. A \"homogeneous\" composition consists of a single material in a single component; a \"heterogeneous\" composition consists of a mixture of two or more materials in a single component.