# The Role of Industry 4.0 and Circular Economy for Sustainable Operations: The Case of Bike Industry
## 1 Introduction
[29] considered the sequential nature of ramp-up and proposes a Cyber-Physical Systems approach based on data capturing, learning mechanisms, and knowledge extraction, leading to an Industry 4.0-compliant Decision Support System (DSS) for human operators. [PERSON] et al. [30] incorporates the concept of I4.0 in an imperfect EPQ model with predictive maintenance and reworking. Figure 1 illustrates the continuous flow of technical and biological materials through a circular economy. A sustainable CE system consists of some phases, with designing, reused, repaired, and remanufactured products. This retains the functional value of products, rather than just recovering the energy or materials they contain and continuously making products anew. The system consists of 10 items (ex: farming/collection, regeneration/biogas/extraction of biochemical feedstock/recycle, refurbish/remanufacture, reuse/redistribute, maintain/prolong) and is divided into two parts (ex: industrial cycle/biogeochemical cycle). An increasing number of recent publications have reassessed the positive contribution that I4.0 can make to circular economy in the studies of [31; 32; 33; 34; 35; 36; 37; 38; 39]. The bike-sharing system (BBS) is becoming an increasingly popular item of the transport system in urban spaces in many cities around the world. The goal of BBS is to deepen our understanding of global environmental challenges and energy-saving solutions based on the sharing economy and circular economy. [PERSON] and [PERSON] [40] addressed the development of a public bike-sharing system, considering random factors, based on selected external environmental analysis methods. [PERSON] and [PERSON] [41] identified factors that influence the occurrence and severity of bicyclist injury in bicyclist-vehicle crashes. With the COVID-19 pandemic, we witnessed an increasing awareness of bikes as an alternative means of transport, as many people either avoid using mass transit or encounter reduced mass transit services. In times of crisis, bikes can provide resilience in transport systems, satisfying our mobility needs when mass transit systems are inaccessible. [PERSON] et al. [42] investigated a CE policy in an imperfect production system with an ETO component and general component. Reviewing the applications of CE issues in a sustainable supply chain includes green logistics, waste recovered, and virgin resources use. In discussions of anxiety in CE issues, the current issues are in regard to environmental economics. The purpose of this paper is to prove that the CE is the effective utilization of resources, and I4.0 has been established to shorten the design lead-time and CNC machining time, while improving the production rate of ETO components. In light of these concerns, the following three issues are addressed:  
**RQ1. **: _When is the optimal design time of general components in stage 1?_
**RQ2. **: _What is the optimal production rate of ETO components in stage 1?_
**RQ3. **: _What is the optimal sales promotions time in stage 2?_  
It would appear the study of I4.0 and CE might pose a fruitful and important issue in understanding the goals that Taiwan's SMEs seek to achieve through digital transformation, and the current status of and future SME needs for digital-related tools (I4.0 technology). The results are of great interest both for application and scientific research. For convenience, Table 1 indicates a brief comparison of the results of the studies mentioned above. As shown in Table 1, [PERSON] et al. 's [22] proposed problem is formulated as a joint economic order quantity (EQQ) and economic production quantity (EPQ) model with CE and GM aimed at cost minimization. [PERSON] et al. [30] developed imperfect economic production quantity (EPQ) models that consider predictive maintenance (PM) and reworking of defective products by I4.0 to improve PM. This paper may be critically important in laying the groundwork for understanding how bike manufacturers use I4.0 strategies.