# Sustainability Requirements of Digital Twin-Based Systems: A Meta Systematic Literature Review
## 4 Discussion  
The study of DTs is recent, mainly after 2015 [33; 40], and there is an unclear definition of DT. However, regardless of the growing complexity of their applications, it is agreed that there are several benefits to using DTs, such as optimization of Industry 4.0, and the sustainability of the product design process. In addition, the lack of studies with technical details can be a difficulty when adopting this technology. A closer look into possible environmental sustainability benefits and at the product design level can help a clear understanding of this reality. The analysis of the available literature allows us to identify several aspects of the relationship between DTs, product design and sustainability, and hence, to answer the original research question, we preliminary discuss the involved sub-questions (SQi):  
SQ1: What is the relationship between DTs and product design? Concerning SQ1, from the set of selected papers we verify that there are primarily two relationships between DTs and product design: (i) DTs are digital models of physical products fed with real-time data, having an important role in understanding real behaviors and needed adaptations, and (ii) tests using DTs are less expensive and easier than building new physical prototypes. SQ2: What are the environmental sustainability requirements of DTs? Concerning SQ2, and based on the selected literature, the main environmental classes of sustainability requirements are: (i) control of energy consumption and (ii) use of environmentally friendly materials. When CE at the I4.0 level is considered, there is a clear need for a tradeoff between complexity and energy consumption versus the results of the new technology implementation. Possibly, making DT tests and creating less complex products is a main topic of research. SQ3: What are the open issues and challenges in future research paths for DTs and sustainability? Concerning SQ3, at a first look and as already mentioned, the complexity reduction to build and set up DTs is only one future research path. The impact of complexity and energy consumption is paramount when the decision to introduce DTs at the factory level is considered. This has an overall impact on sustainability in its dimensions of security, environmental and financial sustainability. The discussed meta-SLR allowed us to identify several possible open issues and challenges for research. First, methods and processes to design and implement DTs are needed. Since there are several specific application domains with their characteristics, this should be considered. Although there are detailed descriptions of DTs [38; 56], it is unclear how to realize them, especially if there is no previous experience on how to do it. Second, SLR studies point out gaps such as: (i) perceived benefits have not been identified; (ii) DT across the product life cycle or the DT life cycle is not sufficiently studied (whole life cycle, evolving digital profile, historical data); (iii) DTs have not been created, thus, it is not clear how DTs contribute to reducing cost or improving service or supporting decision making; (iv) technical implementations must be improved and detailed in the context of IoT(v) the level of fidelity is not evaluated in terms of the number of parameters, their accuracy and levels of abstraction; (vi) data ownership of data stored within the DT must be determined; and (vii) integration between virtual entities must be improved, because better methods are needed for communication [1]. As already mentioned, DTs are a recent technology, and this fact partially explains these gaps or diversified future research paths. At the same time, there is an attempt to clearly define the DT concept, for example, to classify existing standards such as the \"Plattform Industry 4.0\", which describes a standardized DT in 14.0 [47]. If there are several approaches and contexts to define DTs, how can we identify the most important gaps in the study of sustainability in DT usage?