## 2 Relevant Approaches to Digital Twins for Sustainability in the Existing Literature
### The Conceptualization of Sustainability in the Case of Digital Twins  
From a methodological point of view, article [1] addresses the need to implement a concurrent designing, modeling, and simulation as native approaches when creating digital twins, in order to be able to maximize the benefits of this system, which is also concurrent in nature. We believe this conclusion to be suitable for the case of sustainable manufacturing digital twins to an even greater degree, since the digital environment, the factory floor, and the local and global environment act independently and in parallel, and, at the same time, exhibits strong, model-altering interactions. In the same direction, after an extensive analysis of 192 papers, the authors of [2] propose an integrative framework called Function-Structure-Behavior-Control-Intelligence-Performance that aims to enhance the synergies of the technical components, the logical approach, and the human elements involved in the functioning of a digital twin for Industry 4.0 smart manufacturing. This work is relevant for our case too, as the analysis we undertake addresses the connections of these factors as enablers of sustainable results in intelligent production systems, more than individual environmental preoccupations or clean manufacturing technologies. Another important distinction that we observed is the one between traditional simulation approaches and the currently conceptualized bidirectional and proactive digital twins that act upon the process based on intelligent decisions reached by processing sensor data, described by [3] as they observed the limitations of CAD/CAM/CAE/CFD/FEM in achieving the same goals as a fully powered doppelganger. Another review that studied 123 papers, article [4] reinforces the role of modeling, simulation, and VR visualization, but also finds that big data analytics and cloud/edge computing are mandatory components of modern modular digital twins. Based on a large body of literature of over 240 sources, article [5] proposes a complex approach to achieving sustainable intelligent manufacturing systems that seek to implement the virtual-real dichotomy in three directions: manufacturing system, manufacturing equipment, and manufacturing service. The novelty here is the identification of services and processes as a stand-alone component that can undergo digital transformation to answer to challenges in the environment, rather than them being used as the connecting bridge between the real manufacturing systems and its machines and the virtual environmental simulation. We consider this approach valuable for sustainability, as it facilitates its absorption as an integral part of digitalized factories. Another paper [6] proposes five features that can be used to assess the usefulness and impact of using digital twins on the shop floor, namely realness, integration, dynamicity, visibility, and computability. By studying these attributes, we can also gauge the environmental footprint of a manufacturing system in order to support its sustainability-relatedefforts. Additionally, of high importance in this direction is the ability to perform advanced material management in a manufacturing system as presented in [7], because this is the source of most unwanted side-effects of production systems (e.g., waste, resource depletion, emissions, carbon dioxide). Still, in the meta-review performed by [8] using 29 review papers, the authors have not identified an exclusive approach centered upon the contribution of digital twins to environmental sustainability. In a comprehensive understanding of sustainability in terms of economic, social, and environmental benefits, article [9] arrives at the conclusions that the digital transformation in a firm must be accompanied by a shift of focus from the needs of the enterprise to the needs of the user.