1 Introduction

Circular Economy (CE) is a trending topic that aims to minimize resource use and emissions by slowing, closing, and narrowing products, parts, materials, and energy flow loops (Bocken et al. 2016). In recent years, research efforts have concentrated on overall lifecycle interventions that include selecting low-impact materials, reducing material use, optimizing production techniques or technologies, distribution systems, and lifetime (Mestre and Cooper 2017). Circularity strategies can be divided into two main groups. The first one includes strategies for recirculating parts and products, such as upgrade, repair and maintenance, reuse, refurbishing, remanufacturing, and repurposing. The second group, dedicated to material recirculation, involves recycling, cascade, and recovery (Blomsma et al. 2019).

The design phase system considers all possible strategies to increase the circularity of products, components, and materials (Ellen MacArthur Foundation 2015). Nevertheless, design approaches and guidelines to identify or determine which product attributes or characteristics need to be modified to increase circularity are still under development (Bocken et al. 2016), especially when implementing more than one strategy. On the other hand, most research efforts focus on material recirculation strategies, especially around recycling, which is commonly included in methodological approaches, design guidelines, and indicators for measuring product circularity (Allwood 2014). Therefore, there is a need for more research to cover product and component circularity to enhance product lifespans. There is limited literature on detailed design interventions to improve circularity for products and components. Instead, the vast majority of approaches are based on measuring circularity potential for material recycling like the Material Circularity Indicator—MCI (Ellen MacArthur Foundation—EMAF 2015), the Circular Economy Index—CEI (di Maio and Rem 2015), or the Recycling Rates—RR (Haupt et al. 2016)), while other approaches are focused on specific CE strategies (i.e., reuse, refurbish, remanufacture, and repair) like Reuse Potential Indicator—RPI (Park and Chertow 2014), or the Resource Duration Indicator—RDI (Franklin-Johnson et al. 2016). As a common denominator of previous research, there is no formal approach to identifying product attributes that need to be addressed during the design phase to enhance their performance in products and components.

This paper proposes a diagnostic tool to measure product circularity and identify potential improvement interventions for redesign scenarios. This proposed tool is meant to help designers, manufacturers, and practitioners identify product attributes and features that affect product circularity and to encourage the generation of more circular redesign alternatives during the conceptual design stage. The proposed tool includes eight product attributes that provide a quantitative analysis of circularity performance in terms of upgrade, repair and maintenance, reuse, refurbish, remanufacture, and repurposing. A case study revealed single and combined interventions that positively impact the circularity performance of the product.

The remainder of this paper is organized as follows. Section 2 presents the research background around the design of circular products, attributes, and circularity metrics. Section 3 describes the proposed tool, with emphasis on the methodological steps. Section 4 evaluates the proposed tool through a case study, while results are discussed in Sect. 5. Finally, in Sect. 6, conclusions are presented.

2 Background

2.1 Circular economy strategies

CE emphasizes the design—or, perhaps, more accurately—the redesign of products and services to facilitate restorative mechanisms (Tam et al. 2019). Figure 1 shows the classification of nine CE strategies based on recirculation of parts and products and the recirculation of materials. Commonly, a better circularity performance is often achieved through the top eight strategies, since the last three imply additional energy and resource consumption. For example, recycling and cascade typically require energy and water for cleaning, handling, and homogenizing materials. Here, virgin material helps to ensure an acceptable quality of the resulting raw material. In addition, the inefficiencies of all equipment and handling systems need to be considered.

Fig. 1
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Degree of circularity for 9R's strategies. Comparison of desirability and consumption of resources. Based on Blomsma et al. (2019)

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In the case of recovery, thermal energy generation from materials is the less-preferable strategy, especially for non-biological materials, because of the potentially harmful emissions. Nevertheless, in some cases, recycling, cascade, and recovering are more suitable due to risk elimination (e.g., medical disposable products). However, as mentioned earlier, all those strategies involve a certain degree of circularity. In the case of product design, strategies related to the recirculation of parts and products can be employed to extend the existing lifecycle of products or to promote their incorporation into new ones. Each one of these strategies is described below.

According to Pialot and Millet (2014), upgrading is a functional enrichment process that applies when a product reaches one of two conditions: (i) physical lifetime, which involves a product failure, and (ii) value lifetime, which occurs when the product is disposed of because its performance, functionality, or appearance cannot satisfy the customer needs anymore. Upgradeability is commonly associated with remanufacturing, which implies the reconditioning of existing products to satisfy a new use cycle. Upgradeability is one of the key criteria for design based on lifecycle aimed at service life extension. A modular structure of products is required to facilitate the transition between different product generations by adding or replacing the modules for the next generation of the product life cycle (Aziz et al. 2016).

Repair and maintenance imply keeping a product usable and in a good state, as long as possible to prolong its lifetime (Ackermann 2018). Therefore, this is a key strategy toward more sustainable consumption based on a circular economy (Aguiar et al. 2022), Hebrok 2014), Mestre and Cooper 2017). Research has been oriented toward facilitating repair and maintenance tasks (van Nes and Cramer 2005), Vezzoli and Manzini 2008). Some of the most relevant design approaches include Design for Disassembly. Furthermore, repair and maintenance are recently gaining relevance in product-service systems (Baines et al. 2007), which implies alternative ways of product use, like renting or sharing instead of purchasing. Reuse is one of the most studied strategies and entails extending the product’s lifetime to the maximum possible. This strategy implies environmental, social, and economic benefits when considered from the early design phases (Zacho et al. 2018), since it avoids manufacturing new products and prolongs the product's use time, even in multiple lifecycles among different users in distinct generations. It facilitates purchasing second-hand products and value recovery for the initial consumer. Additionally, reuse provides higher advantages than other CE processes that imply additional transformation processes (i.e., recycling) and, therefore, a higher implicit environmental desirability (Zink et al. 2014).

Refurbishment is another strategy covered by the circular economy concept, considered the most profitable and environmentally beneficial process, demonstrated by a growing trend of manufacturers with product collection and refurbishment programs (Chen et al. 2018). Refurbishment is defined as “return a used product to a satisfactory working condition by rebuilding or repairing major components that are close to failure, even where there are no reported or apparent faults in those components” [British Standard BS8887-2–2009 Design for Manufacture, Assembly, Disassembly and End-of-Life Processing (MADE), 2009]. Recently, this strategy is also related to circular business models based on leasing, where refurbishment is considered a value recovery strategy (Sumter et al. 2018).

Another crucial strategy is Remanufacturing, which has experienced a gradual rise with solid interest from researchers and government bodies alike to face resource scarcity in the field of product manufacturing (Cao et al. 2021). Remanufacturing consists of restoring used products to a like-new or better state, offering the same warranty as a new product (Burggräf et al. 2021). Remanufacturing includes several industrial processes, such as collection, dismantling, repair, and testing (Krystofik et al. 2018). The most relevant savings of remanufacturing lie in energy consumption (60%), metallic materials (70%), and emission of air pollutants (Kelly et al. 2017). Finally, Repurposing is one of the less-known circular economy strategies. This strategy is defined as the reuse of a product for other applications, not necessarily equal to those that the product had in origin (Cooper and Gutowski 2017). Repurposed products have an environmental advantage compared to market products. In the case of electronic and electric products, it is possible to reach an environmental impact of between 25 and 80% less than in a scenario without repurposing (Pérez-Martínez et al. 2021).

2.2 Attributes of circular products

This subsection expands on the identification and characterization of circular products to define which product attributes need to be considered for each CE strategy. From such attributes, it is possible to identify the interventions regarding materials, geometry, and joints during the early design stages for the six strategies related to the recirculation of parts and products. A literature review in SCOPUS enabled the identification of the main attributes of circular products considered in previous research. The detailed search queries are summarized in Table 1, including the total number of articles identified and selected for this study. Table 2 summarizes eight attributes associated with CE strategies for recirculating parts and products proposed by Blomsma et al. (2019) and identified in the literature.

Table 1 Summary of search queries for attributes of circular products
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Table 2 Product attributes required for circular products according to existing literature
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2.3 Design tools for circular products

A growing trend of product development methodologies related to sustainability resulted in different “Design-for-X” approaches during the last 2 decades. Those approaches consist of design methods, guidelines, and rules to incorporate an “X” attribute, such as environment, durability, and recyclability, among others, during the product development process. At this point, it is possible to identify noteworthy relationships between Design-for-X approaches and CE strategies. Table 3 summarizes 36 Design-for-X approaches and their associations with CE strategies.

Table 3 Design-for-X approaches and their relationship to CE strategies
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After reviewing the most relevant product attributes regarding CE in product design research, it is possible to relate them to the six CE strategies. Figure 2 shows the relationships and matches between CE strategies to extend product lifespan and required attributes according to existing literature. The analysis showed that Disassemblability, Reassemblability, Durability, and Modularity are transversal attributes required to support all strategies related to the recirculation of parts and products. Standardization plays a key role as well, involved in five out of six strategies. Finally, other attributes like simplicity, commonality, and affordability of spare parts play a relevant role, but only in four strategies. Another product attribute is the possibility of secondary functions, thus enabling product repurposability.

Fig. 2
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Product attributes required for circular products according to the existing literature

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2.4 Metrics for product circularity

In terms of indicators or metrics related to the circularity of products or parts, just a few approaches cover the analysis of attributes involved in the design process. Most indicators or metrics associated with circularity and found in the literature, like RR (Haupt et al. 2016), MCI (Ellen MacArthur Foundation—EMAF, 2015), and CEI (di Maio and Rem 2015), are focused on recycling, while other approaches are concentrated only in a unique CE strategy, like RPI (Park and Chertow 2014) or RDI (Franklin-Johnson et al. 2016). However, it is possible to find more comprehensive approaches to measuring overall circularity (including all CE strategies). This literature review identified five indicators oriented to the product level and identified current research gaps. Table 4 summarizes such approaches and includes an overall description and calculation parameters for each indicator or metric.

Table 4 Summary of metrics or indicators for recirculation of parts and products
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After analyzing the existing circularity indicators, a lack of coverage of CE strategies for recirculating parts and products is noticeable. The following research gaps were identified:

  • Approaches listed in Table 4 are mainly based on questionnaires and relative weights, which implies a degree of subjectivity in the decision-making process related to improvement actions.

  • Some indicators are based on the product's overall performance as a black box, skipping the detailed circularity from components. Therefore, the design interventions can be limited to general geometry and material selection.

  • Existing approaches do not consider the impact on all CE strategies when improving a single strategy. Thus, enhancing durability benefits remanufacture and upgrade, but the strategic plan can be oriented to a single strategy.

  • None of the indicators provides guidelines on design tasks or interventions to improve product circularity with detailed, measurable parameters.

3 Developing a diagnostic tool for product circularity

This section describes the four main steps that comprise the proposed approach. The first step provides valuable diagnostic data on the current circularity performance of the product. Later, the second one includes the definition of target values oriented to improve product performance in circularity. Then, the third step consists of analyzing CE attributes and selecting those that represent major interests or improvement opportunities for the design team or company. Finally, the last step is dedicated to generating specific design modifications that must be implemented during the product redesign to reach the previously defined circularity goals. Figure 3 shows the sequence and outputs of each step proposed in this approach.

Fig. 3
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Schematic representation of the four-step approach proposed for the circularity diagnostic

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3.1 Circularity diagnostic

This first stage aims to obtain data from the product, its circularity performance, and even gaps compared to competitors’ products. Thus, two main tasks encompass the circularity diagnostic: product inventory, which summarizes all parts that comprise the product, and the calculation of product attributes related to the CE strategies. Product inventory demands disassembling the whole product and quantifying the number of parts and pieces that comprise the product. The inventory can be arranged using a table. Then, product attributes are calculated to measure the product's circularity. Such calculation is performed using eight parameters: Dis/Reassemblability, Durability, Modularity, Simplicity, Standardization, Commonality, Relative cost of spare parts, and Secondary Functions. Table 5 summarizes the attributes and their corresponding equation and description. The same attributes should be calculated when competitors are present in the market to consider improvement goals based on competitiveness. Appendix A summarizes the definition of parameters for C1, C2, and C8.

Table 5 Product attribute description for circularity diagnostic
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3.2 Definition of target values

Once the circularity diagnostic is done, it is time to select which CE strategy will be improved, considering that any circularity intervention can positively impact others. Definition of target values must be developed considering the following aspects:

  1. a.

    Interests of the company based on its product development planning in the mid term and long term, in addition to the previously settled corporate sustainability goals.

  2. b.

    The differential added value offered by the company or competitors. Thus, the company does not necessarily require to surpass competitors’ performance in product circularity if there are other attributes highly desired by the customer. The improvement could be solely oriented to reducing the circularity gap existing between the company and competitors.

  3. c.

    Availability of resources to develop redesign process iterations, including highly qualified personnel, manufacturing technologies, and prototyping testing resources.

  4. d.

    Customer consciousness on product circularity and market acceptance regarding more circular products.

The definition of target values is, therefore, a particular task that depends on the company's capabilities and goals, market competitiveness, and cultural context. There is a specific set of target values depending on the decision-making scenario.

3.3 Circularity improvement

One or more modifications can be proposed to improve the strategy's performance. In this step, the attributes involved in each CE strategy need to be revised and analyzed to select those that can be modified to increase the desired circularity performance. The modifications should be presented as improvement scenarios oriented to a specific CE strategy. The number of improvement scenarios also depends on the company’s interest in analyzing several circular approaches. This step also involves recalculating attributes employed during the circularity diagnostic and comparing such values as related to the initial values. This process can be iterative until reaching the desired value for the strategy of interest.

3.4 Generation of potential redesign modifications

This last step includes the potential design interventions that can be performed to reach the target values previously established. Following the conventional structure of design requirements, these specifications must consist of the design parameter to improve, the current value, and the desired or ideal value. Note that circularity improvements can be achieved by applying simultaneous modifications in geometry and materials), and therefore, it is possible to reach higher values than the ideal ones.

4 Case study

The proposed approach is now applied to the circularity redesign of a conventional skateboard. The case study covers the use of supporting data and knowledge and methods and tools proposed in the framework to generate the design specifications for a circular product redesign. Figure 4 shows the exploded view of the skateboard.

Fig. 4
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Skateboard disassembly

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4.1 Circularity diagnostic

All calculation parameters and values for product attributes were obtained and calculated for the skateboard. Table B-1 in Appendix B corresponds to the part inventory, while Table 6 summarizes the values for calculating parameters after analyzing the materials, geometry, component inventory, functionalities, and cost of spare parts.

Table 6 Values of parameters for calculating product attributes
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The calculation of each design attribute is described in detail in Appendix B. After that calculation, a relationship between product attribute values and CE strategies can be obtained using a matrix to include the influence of attributes in each strategy. The relationship is measured in three levels: not related (0), related (1), and necessary (2). Once the product attribute value and the relationship value are well defined, they are multiplied and weighted for each CE strategy. Finally, scores of CE strategies were converted to relative values using the maximum possible value for each strategy. Table 7 shows the results of the circularity diagnostic for R1 to R6 for the skateboard.

Table 7 Circularity diagnostic for the skateboard
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4.2 Definition of target values

For the case study, all CE strategies are susceptible to improvement. Figure 5 illustrates the graphical representation of the circularity performance through a radial chart. Four improvement scenarios for specific CE strategies are proposed to demonstrate the definition of target values: Repurpose (R6), Repair and Maintenance (R2), Refurbish (R4), and Remanufacture (R5). Each improvement scenario is described in detail in the following subsection.

Fig. 5
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Graphical performance of circularity for the Skateboard

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4.3 Circularity improvement

Once the target values are defined, it is necessary to establish the potential design modifications to enhance the skateboard’s circularity performance. Each one of the improvement scenarios is described as follows, indicating the combination of modifications to increase the circularity performance of each CE strategy. It is necessary to enhance the specific performance of product attributes related to each CE strategy in the relationship matrix of the skateboard to define the potential modifications. Table 8 summarizes the list of potential modifications and the involved product attributes. The list of potential modifications can be obtained by selecting the product attributes and modifying calculation parameters to raise the specific performance for such attributes. Table 9 shows the proposal of modifications for product attributes in each scenario. Finally, the design team must discuss and define the proposal of new values for product attributes. Values in bold in Tables 8 and 9 indicate scenarios with more than one product attribute modification.

Table 8 Circularity improvement based on design modifications
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Table 9 Circularity improvement based on combined potential modifications for the four CE strategies
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With the proposal of new values for product attributes, it is possible to calculate the circularity performance for each CE strategy. Figure 6 summarizes the graphical representation of circularity improvement for the eight CE strategies for each improvement scenario. Table 9 summarizes the modifications, including new values and the corresponding result for each CE strategy (R1–R6). It is possible to observe that improvement scenarios affect the targeted CE strategy and other related strategies based on mutual product attributes.

Fig. 6
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Graphical performance of circularity for the skateboard

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4.4 Generation of potential redesign modifications

As the final output, it is possible to define a list of redesign modifications to improve the circularity performance of the skateboard. Table 10 lists the parameters to be modified and the ideal value to increase circularity in the R2, R4, R5, and R6 strategies. Such a list of ideal values depends on the product attributes selected in the previous step. Parameters and ideal values are examples of concrete actions by the designer or design team to increase the product’s circularity.

Table 10 List of circularity specifications for R2, R4, R5, and R6—example
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5 Findings and discussion

The proposed approach provides valuable information regarding the overall redesign of products toward a higher circularity performance, considering the eight product attributes identified in the literature. Four attributes were identified as the most influencing factors in the circularity performance of a product: disassemblability, reassemblability, durability, and modularity. Disassemblability and reassemblability were defined in terms of the ease of separating a product into components, which enables repair, refurbishing, remanufacturing, and upgrading of the product. Therefore, these parameters ensure a more circular path of products and components. On the other hand, durability enables the long-lasting path of products based on manufacturing materials and geometric features that ensure the product can be reused. In addition, durability enables repair, refurbishment, remanufacturing, and upgrading, since most durable components serve as core parts with a high-reliability performance that can provide several useful lifetimes. Finally, modularity enables the use of components as modules that can be removed, added, or exchanged to modify the functionalities of any product.

Six CE strategies were considered in the proposed tool: R1 Upgrade, R2 Repair and Maintenance, R3 Reuse, R4 Refurbish, R5 Remanufacture, and R6 Repurpose. However, such strategies are related and co-dependent on product attributes like disassemblability and reassemblability in upgrading, repairing, remanufacturing, and refurbishing. Therefore, any modification of materials or geometry oriented to improve one strategy will indirectly affect the performance of other strategies. This study proposes product circularity as an overall attribute that can be achieved simultaneously through one or more strategies. Circularity performance largely depends on the type of product, the availability of design and manufacturing resources, the sustainability strategy adopted by the designer or manufacturer, the product business model, and the long-term product development plan. Such plans facilitate the identification of which product parameters are vital in guaranteeing a successful enhancement of circularity for multiple lifecycles.

Regarding the existing approaches to measure circularity, the proposed tool addresses some issues and limitations in the previous research approaches. First, it provides a whole measurement of six CE strategies related to the recirculation of parts and products. Second, it enables the measurement of circularity and helps identify which product attributes need to be modified and increased. Finally, it is meant to be easily integrated into engineering design processes, since parameters or modifications can be used to define requirements in the early design stages.

The scoring system proposed was validated and adjusted through the case study. Nevertheless, it is necessary to analyze and discuss the reliability and uncertainty related to the indicator, since it depends on several calculations that can involve some subjectivity. Furthermore, as an improvement opportunity for future works, it would be interesting to run an experiment with different users (practitioners, academics, and designers), similar to the study developed by Saidani et al. (2019a, b), to validate the indicator and the subsequent decision-making process properly.

It is essential to clarify that the circularity diagnostic (step 1) involves the total disassembly of the product. Calculating each product attribute proposed in Table 5 considers an ideal scenario in which products can be fully repaired, and the destruction of pieces during the disassembly is not required. Therefore, the proposed approach fits better with products designed to be disassembled and reassembled without damaging parts. Consequently, the A1 value included in the C1 calculation can be done assuming that it is possible to separate pieces without destruction. In the opposite case, a lower value of A1 is expected and will result in a low performance of C1.

Information about the product (name and identification of parts, materials, and joints) is key to implementing the proposed approach. Therefore, applying the method within a multidisciplinary team is highly recommended to cover mechanical, electronic, electrical, and mechatronic issues during the implementation process. Some parts selected from supplier’s catalogs can involve a lack of information (i.e., bearings). Therefore, deciding whether they are considered in calculating CE product attributes is necessary. In the case study, bearing parts were excluded from the durability, considering it as a part that can be replaced and is not suitable to be modified by the design team.

The improvement directions largely depend on manufacturing companies’ interests, business models, and product types. For example, for products designed and manufactured to have long lifespans without repair interventions, the most suitable strategy is reuse, thus implying high-reliability requirements and a robust design oriented to reducing failure possibilities. Other strategies for products susceptible to changes during their whole lifecycle include repair, refurbishing, remanufacturing, and upgrading. However, such changes need to be supported and enabled by designers and manufacturers, providing protocols and circular supply chains to guarantee the recirculation of products and components from users to the former.

6 Conclusion

This approach proposed a holistic diagnostic tool to measure product circularity based on product attributes related to the six CE strategies involved in recirculating parts and products. Eight attributes were identified from the literature analysis of previous research and were later used to obtain a performance rating for each CE strategy. A case study was developed to demonstrate the implementation process and benefits of the proposed tool. The proposed tool provides an overall evaluation of circularity, enabling the performance in those strategies with higher improvement opportunities through the modification of parameters that define the performance for each strategy.

From the proposed approach, it was demonstrated that product circularity is highly related to disassemblability and reassemblability, which facilitate components’ circularity as independent functional units. In the case study, the circularity diagnostic was essential for identifying which of the CE strategies had the highest performance and which ones implied significant improvement opportunities. Moreover, a deeper analysis can be developed for each CE strategy using more specific data and additional attributes not considered in this article. The proposed metric aims to provide an overall measurement of circularity performance using simple and easy-to-calculate indicators.

The proposed tool is expected to be used by designers, practitioners, and academics interested in product circularity, especially in extended-use lifetime approaches. The improvement of circularity performance was carried out through the definition of scenarios based on a single CE strategy. However, due to the nature and structure of the proposed tool, other strategies can be indirectly improved. In future works, it would be interesting to use additional product attributes related to topics such as economic value, reliability, or environmental performance, as well as the definition of a formal method to identify which improvement scenario is most cost-effective and the implications of a lifecycle perspective for the definition of scenarios in terms of economic and environmental impacts.

As future work, the robustness of the proposed approach will need to be validated through broader studies that include academics, practitioners, and designers to analyze the reliability and uncertainty associated with the calculation of the C1–C8 product attributes and the proposal of improvement actions during the redesign phase.