Design for circularity and durability: an integrated approach from DFX guidelines

The design of circular products is now a trending topic that involves enabling reuse, repair, refurbishing, remanufacturing, and upgrading parts and products. In this field, using Design For X (DFX) tools appears to be an interesting and helpful way to address requirements and considerations by applying single design rules that can enhance performance in terms of circularity. However, the current DFX approaches are not formally oriented to a circular economy (CE), and there is no clear pathway to apply design rules for circular products. Therefore, this article proposes a classification of DFX rules based on seven CE strategies related to slowing and closing the loop of products, parts, and materials. The proposed approach consisted of a literature review, an analysis of DFX rules related to CE, and the classification of such rules in terms of CE strategies and product design stages. The analysis of DFX rules in product circularity provided insights to generate a specific design guideline of 51 rules for circular products. The guideline was denominated as the Design for Circularity and Durability (DFCD) and is proposed as a design tool for practitioners, designers, and academicians in CE. A case study is also presented to demonstrate the implementation and benefits of the DFCD guideline.

Within the different approaches to the design of circular products, the Design for X (DFX) tools are highly recommended by technical and academic literature (Benabdellah et al. 2019;Chiu and Okudan Kremer 2011;Sassanelli et al. 2020a, b) Such an approach consists of guidelines to improve some aspects (enhancing the technical performance in a specific lifecycle stage or a feature or attribute of the product) in the design process. Some of the most studied DFX approaches in previous research are (i) Design for Assembly (DFA) (Boothroyd 1996;Bouissiere et al. 2019;Cabello Ulloa et al. 2018), which is oriented to minimize the cost of assembly within the constraints imposed by the other design features of the product, this method covers fastening selection, symmetry, and size of the parts, angle of insertion among other assembly variables. (ii) Design for Manufacturing (DFM) (Stoll 1988(Stoll , 1990 consists of a guideline to select manufacturing processes based on physical and geometrical attributes of the part and includes raw material selection, process selection, use of modular design, usage of standardized components, multi-use part development, usage of separate fasteners and assembly direction minimization. (iii) Design for Disassembly (DFD) (Zust and Wagner 1992), which covers the selection of reversible fasteners, the definition of optimal disassembly sequence, modular design to facilitate subassemblies, and disassembly processes, among others. Another approach is the (iv) Design for Recycling (DFR) (Simon 1993), which is also associated with dismantling techniques to facilitate the identification and separation of materials suitable for recycling in terms of entire products and parts once the product reaches its end-of-life stage. Furthermore, (v) Design for Environment (DFE) (J. Fiksel and Wapman 1994) is one of the more comprehensive and relevant design approaches that include topics such as environmental risk management, product safety, occupational health and safety, pollution prevention, ecology, resource conservation, accident prevention, and waste management (J. R. Fiksel 2009;Ross et al. 2022). DFE is commonly aimed at minimizing the use of non-renewable resources, effectively managing renewable resources, and reducing toxic releases to the environment. Similarly, to the DFX approaches, as mentioned earlier, many others have been developed during the last four decades, and now it is possible to find more than 25 approaches with specific applications.
However, despite using different DFX tools or guidelines, there is a lack of approaches formally oriented to circular products. Some guidelines like DFMA and DFE have demonstrated improvement in some aspects of circularity, like the facilitation of disassembly and re-assembly, the use of materials with lower environmental impact, and better recyclability. Furthermore, other approaches like Design for Modularity and Design for Durability contribute to more circular products. Regarding product design, there is a research opportunity in the field of DFX approaches oriented to CE. Specific DFX rules are required to address new product designs and redesigns, especially when companies integrate more circularity into their processes, business models, and markets. Thus, design rules for circularity are required to facilitate the implementation of circularity in the lifecycle of products, parts, and materials. This article aims to contribute to that research direction, proposing a formal DFX approach focused on circular products and considering different CE strategies. Moreover, it provides a proper implementation path of rules to facilitate the transition from linear to circular products, considering the conventional product design process. The research is developed following a literature review and further identifying and classifying specific DFX guidelines contributing to each CE strategy. This research output is a list of guidelines denominated Design for Circularity and Durability (DFCD), which provides different paths of design interventions and considerations to enable the circularity of a physical or tangible product. The circularity aspect considers the ability to generate multiple cycles around different CE strategies, and the durability is considered to last longer without accelerating product replacement. The proposed approach's novelty focuses on two main aspects: (i) defining a set of design rules dedicated to improving the circularity of products. (ii) the classification and organization of such rules according to each stage of the product design process. In practical terms, the DFCD guideline provides a simple and easy-to-follow path to enhance the circularity of existing products or design new products from a more circularity perspective.
The rest of the structure of this article is organized as follows: Sect. 2 consists of the methodology employed to identify, classify, and define specific design guidelines for circular products. Section 3 includes the results after implementing the methodology and summarizes the literature review regarding circularity, DFX approaches, and the identification and classification of guidelines for designing circular products during the product design process. The case study implementation is described in Sect. 4. Findings and discussion are presented in Sect. 5. Lastly, Conclusions and future works are presented in Sect. 6.

Methodology
The proposed methodology for the development of the DFCD guideline consisted of three steps: (a) a literature review of attributes or features of circular products and DFX guidelines related to such attributes, and (b) the identification and analysis of DFX guidelines that contribute to the circularity of products, and (c) the identification of individual guidelines or rules for circularity and their classification for each CE strategy during the product design process. The three-phase methodology implemented in this research combines a literature review, a descriptive analysis, and a classification of design rules focused on circular products. Figure 1 shows the methodology to generate the DFCD guideline. Each methodological stage is defined in detail as follows; the results of each stage are later described in Sect. 3.
The first step includes revising previous literature around the DFX concept and the most relevant approaches around the CE concept. This literature search was performed using a systematic approach in the SCOPUS database and combining the results of different search queries. Articles that included rules or guidelines related to resource optimization, complexity reduction, robust design, material selection, and lifecycle impacts were considered in this search. After collecting and analyzing the existing literature, design rules related to circularity were identified using a manual keyword search into the selected articles. Later, different rules of DFA, DFM, DFMA, DFE, and DFR were analyzed and selected regarding their contribution to product circularity (e.g., Implementation of easy-to-use joints in the product facilitates the disassembly and re-assembly of products, therefore contributes to strategies such as repair, upgrade, remanufacture and refurbish; Select materials with high mechanical and chemical durability, especially those which comprise the enclosure or external layer of the product enables the reuse of the product or components).
Once the DFX guidelines were analyzed, the next step was to classify them according to each CE strategy and their application during the design process. At this point, three main phases were proposed for classifying the DFCD rules: The conceptual design stage includes functional analysis, the definition of preliminary product architecture, and the generation and evaluation of concepts. The embodiment design comprises the arrangement of physical functions (product architecture), the preliminary selection of materials, modeling and size of parts, robust design, and the selection of final dimensions/parameters and tolerances. Finally, the detailed design covers the make-buy decisions, the final selection and sizing of components, the generation of engineering drawings, the bill of materials, and prototyping. Design rules were organized in an orderly manner according to the design process steps. The rules were also classified in relevance for each CE strategy.

Literature review
As the first step proposed in the methodology, a nonexhaustive literature review is performed using the SCOPUS database. The results from nine searches based on title, abstract, and keywords were analyzed to obtain a list of selected works related to CE and DFX approaches. As inclusion criteria, articles, books, and conference proceedings published under peer review processes were selected considering the fulfillment of at least one of the following conditions: (i) Combination of CE and product design, (ii) Analysis or implementation of DFX tools, (iii) Combination of CE and DFX approaches. It is important to clarify that other design approaches not formally denominated DFX were considered in the literature search. For example, eco-design conventionally provides design rules or recommendations to reduce the environmental impact of products across their lifecycle. This review process did not include gray literature, technical reports, or secondary information from governments or companies.
Moreover, the approach is limited to academic literature solely. Search queries were generated using the most common and simple words related to the topic of interest. Therefore, 909 entries were obtained from the literature search using the Scopus database. The search had a broad scope since it was necessary to analyze the whole picture regarding DFX approaches and the CE concept. Table 1 summarizes the search topics, the query employed, and the total number of entries and works for each query. From the literature revision, 141 articles were classified as selected works, which after an in-deep revision and duplicate elimination, resulted in 110 highly related articles. These articles were analyzed in detail, providing valuable insights about circular product attributes and CE DFX approaches.

DFX approaches
After analyzing the existing literature, 15 DFX approaches were directly related to the previously mentioned CE Fig. 1 Methodology to develop the DFCD guideline strategies after manually revising highly related articles using a word search into each document. DFX approaches that contribute to CE strategies are enlisted and presented as enablers of product circularity. Table 2 shows the relevance (measured on a three-level scale) of each DFX approach versus the seven CE strategies according to different authors. The most relevant approaches identified in the literature were Design for Assembly/Disassembly, Design for Durability, Design for Modularity, and Design for Upgradability. Nevertheless, each DFX approach is related to at least one CE strategy.

Circular products
Circular products are defined as those that operate within the circular economy model. Thus, they are designed to be reused, repaired, refurbished, remanufactured, upgraded, or recycled. However, there are many categories of circular products; some are designed for extended lifespans (i.e., products with high durability like military appliances), and others for short lifespans (i.e., biodegradable packaging). To define circularity in more detail, it is possible to identify which attributes or features define or enable the circularity of products and parts. The literature analysis identified eight attributes as key circular features after a manual and in-deep revision of selected works in which characteristics or features need to be addressed to enable circularity and extended lifespan. Such attributes are assemblability, disassemblability, durability, modularity, simplicity, standardization, commonality, and affordability of spare parts.
Seven strategies are presented in this article following the frameworks proposed by Potting et al. (2017) and Blomsma et al. (2019). Six of them are strategies oriented to extend the lifespan of products and their parts (R1-R6): Upgrade (R1), which involves extending the existing use cycle by adding value or improving the function of a product in comparison to previous versions, this can be esthetic or functional. Repair (R2) covers the repair and maintenance of defective products so they can be used with their original function. Reuse (R3) means the reuse by another consumer of a discarded product that is still in good condition and fulfills its original function. Refurbish (R4) is associated with restoring an old product and bringing it up to date. Remanufacture (R5) involves using parts of a discarded product in a new product with the same function. The Repurpose (R6) strategy covers using a discarded product or its parts in a new product with a different function. Recycling (R7) is also presented as a seventh strategy, but it is related to recirculating material to obtain new raw material for transformation processes. R1-R7 strategy taxonomy followed in this research is proposed considering two primary sources: (i) the circular strategies framework proposed by (Blomsma et al. 2019) and (ii) the multiple life cycle design approach developed by (Mestre and Cooper 2017). Therefore, the definition of CE strategies and design perspectives correspond to those specific approaches. Table 3 summarizes previous research's attributes and their relation to CE strategies.
As a guideline for assessing the product attributes aforementioned in Table 3, a scoreboard summarized in the Table 4 is proposed to assess the product attributes related to circular product design. Similarly, Table 5 is presented as a generic measurement of the relationship between the product attributes and the CE strategies. Scores presented in Table 5 as a generic measurement of circularity but can be modified according to the type of product if necessary. Table 1 Detail of literature review topics and queries proposed to identify primary entries and selected works Once selected works have been identified, a final revision helps to filter and obtain the highly related articles. Date of search 12th October 2022

Identification and analysis of DFX rules related to circular products
From the analysis of the 15 DFX approaches, it was possible to identify and propose 51 rules that can be performed during the design phase of any product or system for enabling CE in general terms (See Table 4). The classification of design rules was developed following three main stages of product design: (i) conceptual design, (ii) embodiment design, and (iii) detailed design. Rules for conceptual design were oriented to identify potential CE strategies that can be added to the product lifecycle, the generation of more circular conceptual alternatives, and the hierarchization of alternatives considering CE in addition to conventional selection parameters such as functionality, cost, and esthetics, among others. Rules related to embodiment design were focused on two main aspects: the definition of product architecture and geometry and the definition of materials. Such processes consider different CE strategies oriented to extend the product lifespan and enable repairing, refurbishing, remanufacturing, upgrading, and recycling. Lastly, several rules were classified into the detailed design phase, providing relevant considerations to facilitate the circularity of products and parts across their whole lifecycle.

Definition of design rules for circular and durable products (DFCD)
A progressive route to implement the proposed Design for Circularity and Durability rules is proposed in this subsection after analyzing each rule and its relationship to the seven CE strategies. Thus, the rules were organized following the three-phase design process to provide design criteria and tasks to enable circularity in any of the abovementioned strategies (see Table 6). Design rules are not co-dependent or mandatory, and their application depends on the type   1 3 of product, the selected CE strategy or strategies, and the flexibility of the development process since manufacturing must provide enough flexibility to perform geometry and material modifications. Table 7 summarizes the design rules according to each CE strategy. The concept of eco-design is relevant to the proposed approach, especially in those rules related to materials; however, it is conventionally limited to environmental issues, and some rules related to biodegradable materials were not included in the DFCD guideline.

Case study
The tricycle redesign is presented as a case study for implementing the DFCD guideline (see Fig. 2). The detailed implementation of the DFCD is described as follows in Sect. 4.1, which includes the conceptual design, meanwhile sub Sect. 4.2 describes the implementation of rules for embodiment and detail design.

Conceptual redesign
In this phase, the first step is determining which CE strategies are the most suitable for the tricycle. Here, rules 101, 102, and 103 are applied to diagnose the tricycle circularity and to select the most suitable strategies that can be applied to it. Figure 3 details the part inventory for the tricycle, while Table 8 summarizes the diagnostic of circularity of the tricycle based on the manufacturing materials, geometric attributes, and joints. Circularity diagnostic is based on a numerical valuation (high-medium-low) of design attributes and their relationship with CE strategies (R1 to R7). Appendix A shows the scores and description for each level. After inventorying parts of the tricycle and identifying materials, geometries and joints it is possible to determine the CE strategies for the tricycle. The valuation of each product attribute is determined following Table 4, and then is multiplied by the relevance of each CE strategy according to Table 5. Then, the score of each CE strategy is obtained adding up the result of all multiplications between attributes valuation's and CE strategy relevance. Table 9 summarizes the score of CE strategies for the case study. In this case, Repair (R2) and Remanufacture (R5) can be suitable for implementation in the tricycle (with scores equal to 38). No secondary functionalities or second-life functions were identified in the tricycle, therefore R6 offers the lower score. Reviewing the commercial solutions around circularity of similar products, it is found that remanufacturing is one of the most relevant strategies implemented, especially in the EU, where companies like Roetz-Bikes (https:// roetz-bikes. com/ circu lar) offer remanufacturable bikes. Therefore, remanufacture is selected as the target strategy. However, other CE strategies can be enhanced indirectly since several rules are shared among them.

Embodiment and detailed redesign
Seven rules for embodiment and four for detailed redesign will be implemented in the tricycle after selecting Remanufacturing as CE target strategy. Table 10 shows the rules selected from the DFCD guideline. Seven rules related to embodiment design and four rules in detailed redesign None of the parts follow a modular architecture Simplicity High The assembly has an intuitive structure, no complex geometries or specialized tool requirements Medium The assembly has an intuitive structure, several parts present geometrical complexity Low The assembly does not have an intuitive structure, the majority of parts present geometrical complexity Standardization High All joints are standardized. It is possible to find spare parts in the market Medium Most joints are standardized. It is possible to find several spare parts in the market Low No standardized parts. Difficult to find spare parts Affordability spare parts High Overrun cost is less than 25% Medium Overrun cost vary from 26 to 50% Low Overrun cost is higher than 50% Recyclability High All materials can be easily recycled Medium Most materials can be recycled Low Just a few materials can be recycled Table 5 Generic relevance of CE strategies with respect to the product attributes Scale: low (1), medium (2), high (3) Generic relevances for each CE strategy were proposed from product attributes aforementioned in Tables 2  and 3 Product Attributes Generic Relevance respect to each CE strategy R1  R2  R3  R4  R5  R6  R7   Assemblability  2  3  0  2  3  1  2  Disassemblability  2  3  0  2  3  1  2  Durability  2  3  3  3  3  3  0  Modularity  2  2  2  2  2  2  0  Simplicity  3  2  0  1  1  1  2  Standardization  2  2  2  2  2  0  2  Commonality  ----  103-Analyze state-of-the-art and existing approaches to the CE strategy in the type of product Developing alternative design concepts 104-Generate alternative design concepts, preferably using modular structures a Evaluation of concepts and selection 105-Include evaluation parameters associated with CE attributes (assembly, disassembly, durability, standardization, simplicity, etc.) a 106-Hierarchize concepts based on the most interesting CE strategies scenarios a Embodiment design Definition of Product architecture 201-Divide product into modules or subcomponents to facilitate assembly, disassembly, repair, and failure identification 202-Include, if possible, a secondary function for repurposing the product in its end of life 203-Include indicators or modules for self-diagnostic Preliminary selection of materials, modeling, and size of parts 204-Select materials with low environmental impact 205-Select materials suitable for burning with a minimum of toxic, harmful emissions and avoid toxic substances 206-Select materials with high mechanical and chemical durability, especially those which comprise the enclosure or external layer of the product 207-Reduce the number of materials in the product. Use one kind of material 208-Include a high content of recycled material. Different materials should not be mixed 209-Use by-products or waste from other companies as raw material 210-Reduce the number of parts 211-Avoid components with large and complex shapes 212-Utilization of light components to facilitate disassembly and extraction of failed parts 213-Implementation of easy-to-use joints in the product 214-Avoid sharp edges and corners, and reduce, if possible, stress concentrators 215-Assures that non-recyclable parts or materials can be ecologically disposed 216-Standardize components and parts across a range of brands and products. Design the product as a member of a product family Robust design, final dimensions/parameters, and tolerances, DFMA 217-The design of the product should enable easy accessibility to components during disassembly and maintenance activities 218-Reduce the number and type of special tools and equipment required for disassembling. Use only hands if possible 219-Use, if possible, one type of joint method (i.e., threaded joints) 220-Select reversible and separable joints 221-Design core components with higher safety factors and reliability to enable future use cycles 223-Design joints and surfaces between joints using materials with high mechanical and chemical durability to face multiple disassembly and re-assembly cycles were selected to demonstrate how them can be implemented in the tricycle. It is important to clarify that more than those 11 rules can be implemented to obtain more circularity in a product; however, in this case study, it is considered a moderate redesign instead a radical one. Figure 4 shows a graphical description of the 11 DFCD rules selected. A brief description of each rule or set of rules is also included to explain the modification performed on the tricycle. For this case study, materials, joints, thicknesses of structural components, identification marks and modularity comprise the redesign modification.
Implementation of rules 206 and 207 enable higher durability of pedals, which are commonly exposed to wear, impact and dynamic loads. As a drawback, the use of a metallic material increases the mass of the tricycle. Rules 213, 216, 218 and 220 are directly related to modifications in the joints employed in the tricycle, the use of standardized butterfly nuts facilitates the manual assembly and disassembly, which is a key issue to support repairing, remanufacturing, refurbishing and upgrading. The modification related to rule 221 involves an increase in the mass of the tricycle; however, it enables the future remanufacturing (polishing, cleaning) and guarantee an increase in reliability. Rules 306, 208, and 312 facilitate the rapid identification of parts that can be remanufactured in the case of extended lifespan or recycled in the case of Prepare design project report 319-Include section describing the design guidelines implemented and the detail of parts designed under such guidelines 320-Include detailed information to get spare parts from the manufacturer and repair shops 321-Include the process to get warranty extensions and compensation 322-Include detail of secondary functionalities in the case of repurposing a Recommended for new design processes material recirculation. Finally, implementation of rule 310 increases the functional performance of the tricycle, which can have different accessories (hopper or additional chair).

Findings and discussion
Five major findings can be remarked on from the literature analysis and the consolidation of the DFCD rules based on DFX approaches. Such findings are described in detail as follows: From the literature review-no DFX approaches are massively linked to the development of circular products. Thus, the existing rules focus on improving product design, manufacturing, use, and final disposal stages. Nevertheless, circular design involves two considerations that are not commonly considered in the conventional design process: the extension of product lifespan through durable components, the ability to be dismantled for enabling repair, refurbishing, and remanufacturing, which is denominated "To extend loops" and the second one denominated "To close loops" (Bocken et al. 2016) which implies that the product can be reintegrated to new use cycles and not necessarily after a useful life, the case of products easy to recycle or easy to biodegrade are examples of this. Therefore, DFCD guideline proposed in this study is limited to specific resource consumption and improvements from a conventional design perspective based on attributes of circular products and not from a broader point of view from CE, with all the environmental, economic, and social implications derived from that concept. It is important to clarify that circular attributes mentioned in this research (assemblability, disassemblability, durability, modularity, simplicity, standardization, commonality, and affordability of spare parts) are complimentary and do not compete with conventional ones (functionality, cost, etc.). Thus, circular products involve more attributes of features compared to conventional products, therefore their design process involve more complexity and rigor since the lifecycle performance is largely settled during the design phase.
The DFCD rules were based on conventional DFX approaches such as Design for Manufacturing and Assembly (Boothroyd 1996;Bouissiere et al. 2019;British Standard BS8887-2-2009 Design for Manufacture, Assembly, Disassembly and End-of-Life Processing (MADE) 2009; Hsu and Lin 2002;Urrutia et al. 2014), Ecodesign ((Luttropp andLagerstedt 2006); Design for Environment (J. R. Fiksel 1996;J. Fiksel and Wapman 1994); Design for Sustainability (Arnette et al. 2014;Ko 2020;Ljungberg 2007;Page 2014); Design for lifecycle and EOL (Cappelletti et al. 2022;Hapuwatte et al. 2022;Rogkas et al. 2021;Zikopoulos 2022) among others. Such rules lie in the reduction of materials, optimization of geometry, increase of reliability and functional/performance enhancing. Therefore, some rules are directly associated with reduction of energy and mass consumption during extraction of raw materials and processing as well. In the case of extended lifespan strategies like repair, refurbish, and remanufacture, DFX rules (from DFA, DFM, Design for modularity) enable a more flexible lifecycle to reuse parts and components. However, more rules can be     Implementation of easy-to-use joints 216 Standardize components 218 Reduce the number and type of tools 220 Use reversible and separable joints 221 Design core components with higher safety factors Detailed redesign 306 Facilitate the identification of parts 308 Label parts with the date of manufacturing 310 Design additional modules that can serve as upgrades 312 Include identification of parts designed to be circularized obtained depending on focusing issues like: (i) the type of product (i.e., electronical, mechanical); (ii) the industry (i.e., energy related products, automotive, aerospacial, mining, construction) and, (iii) the business model strategy of the company (i.e., leasing, refurbishing, repairing, remanufacturing, product as a service platform among others). Regarding CE strategies, reuse (R3) and repurpose (R6) have an important research gap compared to other strategies. In the case of reuse, more research is required related to the selection of durable materials, robust design, and product lifecycle management, but it also demands other interventions in consumer behavior to ensure that products can last as long as they can. Concerning repurposing, the challenge is complex since secondary or tertiary functionalities need to be included from the early design stages (product architecture) and the user is responsible for using such functionalities in a repurpose scenario. Some approaches were oriented to specific CE strategies like upgrading (Nurhasyimah et al. 2016;Umemori et al. 2002;Xing and Belusko 2008), repairing (Sabbaghi and Behdad 2017), remanufacturing (Ijomah et al. 2007), recycling (Ferro and Bonollo 2019;Leal et al. 2020). However, there is no connection between design rules and the product design process. Conventionally, design contributions are presented without formally considering the stage of design that applies for their implementation or are presented for a specific stage, which is the conceptual design.
The proposed design rules for CE involve the combination of different DFX approaches and tasks to ensure or improve Reversible and separable joint 221 -Increase of safety factor of core components (tubular chassis). Tube thickness is increased to compensate wear, corrosion and surface finishing processes during remanufacture 306, 308, 312 -Label for material and manufacturing date is included as well as identification of parts to be remanufactured and other CE strategies that can be implemented (example on handlebars) circularity during the design process. However, such rules must be carefully addressed to avoid constraints and contradictions during the selection of materials and the definition of geometrical parameters. Thus, several rules cannot be fully applied simultaneously without establishing a trade-off or pros and cons analysis. For example, a robust design can involve fewer components. Meanwhile, modularization involves the separation of functionalities into individual components or modules. This situation can be studied from the optimization perspective and depends on the product type and the CE strategy target. As a challenging barrier, the integration of geometry and material rules for more circular products is evident, especially for complex products comprised of several subassemblies. In terms of applicability, the DFCD guideline is proposed initially for tangible products, services and product-service systems were not included in the scope of the research. Nevertheless, it can be interesting for future works to create a framework for software and services similar to DFCD following CE strategies.
While the DFCD rules were identified and classified, it was identified that there is a need for more research regarding specific CE strategies. Some general approaches, such as DFMA, DFA, and DFM, can be applied to several CE strategies like repair, remanufacturing, refurbishing, and upgrading, but the main objective of such approaches is not product circularity. Thus, some DFX rules can be used with demonstrated success in the product design process, but it is necessary to develop specific rules for circular products not only for redesign approaches but also for new product developments. As an interesting topic for future research, the development of DFC rules for different types of products (electronical appliances, furniture, plastic products, and building components, among others) appears as a vast research field with potential for academic studies and industrial applications. In addition, industry 4.0 tools can significantly enhance product circularity from a broader perspective. Thus, the analysis of design modifications throughout the whole lifecycle (manufacturing, use, final disposal) can generate resource savings and predict product functionality issues and potential failures.
Regarding the case study implementation, it is clear that DFCD rules in the conceptual design phase require more design efforts and resources, since they involve diagnostic, ideation, and conceptualization of preliminary alternatives. The tricycle case study was developed as a redesign process; therefore, it is possible that new designs demand more rules and a specific approach to avoid design conflicts and confusion during their implementation. It is possible to implement more than the 11 rules included in the case study. Nevertheless, the case study was used as a demonstrative example of how rules can be implemented. Thus, more radical redesigns can be generated and therefore a better circularity performance of the tricycle.
Rules concerning embodiment and detailed design have a more technical implementation and can be proved without complexity in products. However, the rules need to be addressed step by step in more complex products where small modifications involve drastic performance results in product functionality (i.e., automotive industry). As was demonstrated in the tricycle case study, many rules can be applied; however, the designer or design team need to prioritize which ones are suitable and relevant in terms of circularity or added value. The proposed approach can be replicated for future works related to new design for circularity rules following the methodology proposed in this article: literature review (not exhaustive) about DFX + analysis of circularity in product design, identifying DFX rules applicable to circular design and proposing specific rules for circularity. However, design rules for circularity can be obtained from other sources following scientific approaches like surveys, interviews, focus groups and discussions among academy, industry, and policymakers.

Conclusion
This article first reviewed the literature around DFX approaches related to the CE concept to define a guideline or set of design rules to facilitate the implementation of CE strategies within the product design process as complementary attributes to conventional ones (resistance, functionality, or cost). Despite several DFX guidelines covering some circularity, there is a lack of specific guidelines for circular products and their proper implementation during the design or redesign process. As a second contribution of this article, a characterization of existing DFX rules is proposed to define a route of implementation towards the product design process in its primary design phases (conceptual, embodiment, and detailed). As a result, 51 rules were proposed and classified according to the different CE strategies; Upgrade (R1), Repair and Maintenance (R2), Reuse (R3), Refurbish (R4), Remanufacture (R5), Repurpose (R6), and Recycle (R7). Such rules comprise the design for circularity and durability-DFCD, which is proposed as an engineering tool to include or improve circularity during the design of products. The DFCD guideline offers a unique path of rules depending on the selected CE strategy and involves both geometrical and material selection considerations.
In future works, more research efforts are expected to consolidate specific circularity rules for each CE strategy and analyze constraints and potential conflicts in the 1 3 simultaneous implementation of rules. Integrating industry 4.0 technologies into the product design process is necessary to facilitate the lifecycle analysis of parts and products once the design rules are applied and their overall impact on sustainability. Similarly, design rules for non-tangible products like software and services must be generated to cover CE issues related to resource consumption and sustainability performance.