In this paper, we review novel techniques in the emerging field of spatiotemporal 4D PET imaging. We will discuss existing limitations in conventional dynamic PET imaging which involves independent reconstruction of dynamic PET datasets. Various approaches that seek to attempt some or all of these limitations are reviewed in this work, including techniques that utilize iterative temporal smoothing, advanced temporal basis functions, principal components transformation of the dynamic data, wavelet-based techniques as well as direct kinetic parameter estimation methods. Extension of 4D PET to 5D PET in which the additional dimension of (respiratory or cardiac) gating is considered has also been discussed.

3D-printing or additive manufacturing (AM) attempts to building objects by using computer-controlled layer-by-layer manner. 3D printing plays a essential role in the fabrication of systems utilized in the biomedical field, such as the creation of complex tissue structures and scaffolds, customized drug delivery systems (DDS), and soft robots. 4D-printing is an extended concept of 3D-printing by adding time as a fourth dimension. In other words, this technology makes it possible to change the shape and function of the printed object over time after triggered by stimuli such as temperature, moisture, electrical and magnetic fields, light and pH. 4D printing has shown interesting promise in biotechnology with a great shape control after printing by using stimuli-responsive materials. For example, in tissue engineering the repairing process of tissue was known as a dynamic process and 3D printed structures are ineffective in this dynamic environment, while 4D printing technology, due to its unique features, makes it possible to use scaffolds printed by this method for planting in the intended environment. In this study, the use of stimuli-responsive materials in 4D-printed structures and their attractive applications in tissue engineering have been reviewed.

4D printing is an innovative digital manufacturing technology that originated by adding a fourth dimension, i.e., time, to pre-existing 3D technology or additive manufacturing (AM). AM is a fast-growing technology used in many fields, which develops accurate 3D objects based on models designed by computers. Dentistry is one such field in which 3D technology is used for manufacturing objects in periodontics (scaffolds, local drug-delivering agents, augmentation of ridges), implants, prosthodontics (partial and complete dentures, obturators), oral surgery for reconstructing jaw, and orthodontics. Dynamism is a vital property needed for the survival of materials used in the oral cavity since the oral cavity is constantly subjected to various insults. 4D printing technology has overcome the disadvantages of 3D printing technology, i.e., it cannot create dynamic objects. Therefore, constant knowledge of 4D technology is required. 3D printing technology has shortcomings, which are discussed in this review. This review summaries various printing technologies, materials used, stimuli, and potential applications of 4D technology in dentistry.

With the development and expansion of 3D printing and upgrading to 4D printing, it became possible for static 3D printed structures to turn them into a dynamic structure over time by applying one or more stimuli simultaneously. 4D printing was able to transform a simple geometry into a complex geometry without the need for printing the entire geometry from the beginning. Also, 4D printing reduced production costs, wasted fewer materials, increased production speed, and added features such as self-assembly, self-compatibility, and self-healing to the features related to 3D printed structures. In this study, first, different types of smart materials (such as shape memory polymers, shape memory alloys, hydrogels, etc. ) and activation mechanisms (such as water, heat, pH, etc. ) used in 4D printing were evaluated. Then, focusing on polymers and polymer composites, the properties and characteristics of 4D printing were described. In the following, the types of 3D printing methods utilized in the 4D printing of polymers were explained, and in a separate section, the effect of the printing process parameters using an FDM printer on the response of the materials used to the applied stimulus is discussed. Furthermore, the expansion of 4D printing in various industries such as medicine, aerospace, sensors, robotics, and other fields of application in which 4D printing entered was discussed and at the end of this study, the existing challenges and future opportunities were profoundly discussed.