Sustainable Material Selection Framework: Taxonomy and Systematisation of Design Approaches to Sustainable Material Selection

: Design can play a fundamental role in addressing the climate crisis and preserving the planet’s ﬁnite resources. Through design, it is possible to reduce the environmental impact of products and services right from concept stage. The elements that concur within a project are diverse and often have an impact on each other. The material is one of them, being able to inﬂuence the product, but also the business model, company relations, etc. To help the designer keep all these aspects under control, various methodologies and tools have been developed, among them design strategies and guidelines. To date, several authors have dealt with the topic, offering different perspectives and generating a critical mass of information, which differs in the level of depth and operability of the suggestions, often differing only in terminology rather than content. This inhomogeneity can confuse both professionals and students. This study proposes an ordered taxonomy of the different levels of detail and a uniﬁed terminology of the strategies and guidelines in the literature. To test taxonomy and systematisation, this article focuses on guidelines for material choice, resulting in a framework to guide the selection of materials with a view to sustainability.


Introduction
Millions of products are sent to landfill every year due to the throwaway culture permeating society.Because of the increasing global population, this issue will continue to grow and strain the environment in terms of energy and material resources [1].In fact, the greater the number of people consuming, the greater the rate at which consumption will occur, and, in turn, the demand for materials will rise [2].As some experts point out [2,3], reserves of some resources, particularly some materials, are reaching a critical threshold, bringing to light the negative impacts that depletion could have on the environment and human activities [4].Conscious use of resources must be seriously considered and implemented if we wish to succeed in achieving the 17 sustainable development goals-11 of which have direct implications for resource use- [5] and realise the vision of sustainability presented by the Brundtland Commission [6], and other organisations, such as IUCN, UNEP, and WWF [7].
Design and the designer play a fundamental role in achieving this vision and solving these problems since it is during design phase that it is possible to determine more than 80% of a product's environmental impact [8][9][10][11][12].A design artefact focusing on sustainability must pay attention to several factors, one of which is certainly the material.The latter represents an initial response, a fundamental gateway to creating a sustainable product.Therefore, designers and companies have contributed to introducing and applying various green materials in the last decade.Usually, during the new product development (NPD) process, it is difficult to assess one material as more or less sustainable than another since this depends on the stakeholders, the contexts in which the product will be introduced and used, and the meanings that the user attributes to the material or the whole object [13][14][15][16].Moreover, the selection of a material for a new product will also influence production processes, relationships that the manufacturing company has, the proximity of suppliers and primary resources, local recycling facilities, and the possibilities of recovering energy at the end of its life cycle.Therefore, restricting selection to properties such as embodied energy or carbon footprint may be simplistic and reductive [2], as well as evaluating a product as sustainable just by looking at the material used to manufacture it [17].However, the factors listed above must work in synergy as a starting point to develop a product that brings environmental improvement or innovation, guiding the design process, engineering, and, ultimately, also the material selection [18].The importance of having a holistic vision emerges from the first design phases, acquired by considering the product's whole life cycle and an overall picture of the context and the company.
Although this represents a considerable effort for the designer, different methodologies and design approaches have been developed to support sustainable design over time.Since the early years of the new millennium, three main currents of thought have become more widespread in the literature: ecodesign, design for environment (DfE)/design for sustainability (DfS), and circular design.Although with different characteristics, all the approaches listed above aim to create products and services with the least possible environmental impact throughout their life cycle.To achieve this and to simplify understanding and practical application for designers, several authors have addressed this topic by translating these concepts into strategies and guidelines.In this article, the indications given in the form of strategies and guidelines are interpreted as generalised problem-solution combinations [19].These can be compared to patterns as described by Alexander and colleagues [20], i.e., "a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice".Like patterns, the indications identified in this article by the various authors are derived from experience and empirical observation and are necessary tools for encoding tacit knowledge into explicit knowledge [19].The strategies and guidelines are specifically dedicated to the product design field.Although they exist in other knowledge domains, the purpose of the design ones is to guide the designer in simplifying the decision-making process.Furthermore, it is necessary to specify that these, although they may have a high degree of specificity, are qualitative and not quantitative indications.This, especially in material selection, entails parallel work to provide supporting data from LCA or reports to have a complete overview and avoid rebound effects or reductionist choice.
Even though they are not always treated individually within the different approaches, material-related indications are an integral part.By analysing the different perspectives and guidelines, it is possible to find some recurring indications which reach very different levels of specificity and often indicate very similar concepts but with different terms.To clarify and examine this issue, the previously listed approaches and other schools of thought-that cannot be attributed to any of them but are, nevertheless, of equal importance-will be dealt with below, giving some examples of the relevant strategies and guidelines of the different authors and visualised in Figure 1.

Ecodesign Strategies and guidelines
Ecodesign is the first school of thought that links the field of sustainability to design.Vezzoli and Manzini [17] embrace it, encouraging the designer to develop not the single product but its entire life cycle to reduce the environmental impact in each of its five phases (pre-production, production, distribution, use, and disposal), thus, defining life cycle design (LCD).This perspective leads the authors to specify a plethora of strategies and subsequent increasingly detailed guidelines, ranging from extending the lifespan of materials to more specific indications such as "avoiding additives that emit toxic fumes during incineration".Similarly, Allione et al. [21] also adopt the perspective of ecodesign and LCD, reaching a lower degree of detail.Even these authors, for example, encourage the designer to think about material lifetime extension by simply indicating the possibilities of the material at the end of its life, such as incineration, recycling, biodegradability, or compostability.Giudice [22] does not explicitly state the design approach adopted in his guidelines; however, he also discusses useful life extension, indicating that it is necessary to increase the use of low-impact and biodegradable materials to achieve a sustainable product.

Design for Environment/Design for Sustainability Strategies and Guidelines
The indications cited in the paragraph above are also echoed by those adopting a DfE/DfS approach.They are both included in this article because, although they have slightly different theoretical aspects, they are often used interchangeably in the literature [31], and both can be linked to design for X (DfX), a design technique that emerged in the early 1980s [32,33].Among those relying on DfE, it is possible to find Bevilacqua et al. [23].They, linking back to LCD, aim at life cycle optimisation by including guidelines such as material recycling within the different phases.Go et al. [24] also refer to DfX and other strategies of this technique related to sustainability, such as DfE, but also design for recycling (DfR) or design for disassembly (DfD).Go and his colleagues schematise the different strategies into areas containing the various guidelines.This segmentation allows the authors to achieve a higher level of specificity in their guidelines, even suggesting the use

Ecodesign Strategies and Guidelines
Ecodesign is the first school of thought that links the field of sustainability to design.Vezzoli and Manzini [17] embrace it, encouraging the designer to develop not the single product but its entire life cycle to reduce the environmental impact in each of its five phases (pre-production, production, distribution, use, and disposal), thus, defining life cycle design (LCD).This perspective leads the authors to specify a plethora of strategies and subsequent increasingly detailed guidelines, ranging from extending the lifespan of materials to more specific indications such as "avoiding additives that emit toxic fumes during incineration".Similarly, Allione et al. [21] also adopt the perspective of ecodesign and LCD, reaching a lower degree of detail.Even these authors, for example, encourage the designer to think about material lifetime extension by simply indicating the possibilities of the material at the end of its life, such as incineration, recycling, biodegradability, or compostability.Giudice [22] does not explicitly state the design approach adopted in his guidelines; however, he also discusses useful life extension, indicating that it is necessary to increase the use of low-impact and biodegradable materials to achieve a sustainable product.

Design for Environment/Design for Sustainability Strategies and Guidelines
The indications cited in the paragraph above are also echoed by those adopting a DfE/DfS approach.They are both included in this article because, although they have slightly different theoretical aspects, they are often used interchangeably in the literature [31], and both can be linked to design for X (DfX), a design technique that emerged in the early 1980s [32,33].Among those relying on DfE, it is possible to find Bevilacqua et al. [23].They, linking back to LCD, aim at life cycle optimisation by including guidelines such as material recycling within the different phases.Go et al. [24] also refer to DfX and other strategies of this technique related to sustainability, such as DfE, but also design for recycling (DfR) or design for disassembly (DfD).Go and his colleagues schematise the different strategies into areas containing the various guidelines.This segmentation allows the authors to achieve a higher level of specificity in their guidelines, even suggesting the use of screws made of materials compatible with the connected parts.

Circular Design Strategies and Guidelines
The circular economy concept has emerged as the scientific debate on sustainability has evolved and spread in the last decade [34].Some authors have tried to link strategies and design guidelines to these concepts.It is the case of den Hollander et al. [25], who use circular product design, explicitly distancing themselves from ecodesign (for the authors, a methodology rooted in the linear economy).They developed two design approaches: design for product integrity and design for recycling.However, these have not been explored in depth by the authors, and no guidelines or indications emerge when using the lens of materials.Moreno et al. [26] also adopt circular design, relying extensively on techniques developed within DfX.The framework and the review led Moreno and his team to develop a taxonomy of strategies and guidelines for circular design [35].This breaks with the previously discussed and analysed approaches, as it starts from the circular design aspect (such as resource conservation), then employs the DfX approach (such as design for material conservation and elimination of waste), and, finally, concludes with strategies and design guidelines of various natures and depth (such as selecting the best materials (non-toxic and pure if possible)).Finally, Bocken et al. [27] also adopt a circular design perspective, focusing on the life cycle of resources.In particular, the authors suggest that slowing down or closing the resource loop is necessary in a circular perspective.In the second case, closing the resources loop, a strategy list concerning the technological cycle is suggested, whereby using materials that have upcycling at the recycling stage.

Other Perspectives
Finally, it is important to emphasise that the literature contains a range of materialspecific recommendations, strategies, and approaches not directly related to the schools of thought mentioned above.These include the material efficiency approach of Allwood et al. [28], which aims to provide the same object functionality but with the lowest possible use of material and manufacturing processes.Or the strategies based on emotional durability developed by Hainess-Gadd et al. [29] and Karana et al. [30], which aim to extend the product's life by stimulating the creation of an emotional bond or transformation over time through the material.

Research Gap and Objective
At the end of this overview of the different strategies and guidelines for material selection within the different design approaches for sustainability, a lack of an integrated vision emerges.It is noted that although the declared design approaches are different, the selection guidelines often overlap and differ only due to a purely terminological issue.Moreover, the taxonomy's terminology is highly fragmented, feeding confusion about the different levels of detail of the strategies and guidelines proposed by the various authors, which are very different from each other.Therefore, this research aims to propose a precise taxonomy to unify terminology to let practitioners navigate into the different levels of in-depth strategies and guidelines.According to the authors, this procedure will help apply them to an integrated and sustainable materials selection activity.The research proposes a systematisation of the literature strategies and guidelines, positioning them within the presented framework.

Methodology
The research takes an exploratory approach, using qualitative methods, to categorise and interpret the current state of the art.The starting point was an extensive analysis of design approaches, strategies, and guidelines in the field of sustainable material selection through an integrative literature review to critically assess and recontextualise the foundations of the topic under analysis [36].A first set of articles has been analysed using texts derived from the authors' previous research and knowledge to collect referring literature for the work.From that, specific keywords (i.e., sustainability*, material*, selection*, design for*, guidelines*, and circular economy) in different combinations were used by querying on Scopus.Journal articles, conference proceedings, books, and book chapters written in English were analysed.Among these, only the ones without a focus on specific materials families or sectors (e.g., buildings, vehicles, and packaging) have been selected.From these results, other sources were obtained through snowballing sampling analysis [37] of previous and subsequent academic publications discussing sustainable selection of materials in product design.
Of these, some already offered a clear visualisation through schematisation of the strategies and guidelines for designers.In contrast, others discussed them in a more textual way.Of the texts analysed, 11 were considered suitable and, subsequently, used for the mapping activity (Table 1).Since the main purpose of the article is not to provide an extensive overview of all sustainability-oriented strategies and guidelines for material selection, but to explore, propose, and validate a taxonomy for design strategies and guidelines applied in the field of sustainable material selection, the number of contributions selected, according to the authors, reached the quality, diversity, and saturation of the information mapped.The first step (Figure 2) in constructing the sustainable materials selection framework (SMaS framework) was to schematise each contribution extensively.The schematisation was first carried out using a collaborative online platform (Miro boards) to highlight the gap and encourage a more visual and fluid approach in the association of similar strategies and guidelines, and then transferred to Microsoft Excel to construct a structured database that would allow the clustering of indications in systematic taxonomy and allow tracking of aggregation in the following steps.It was also chosen to use Microsoft Excel to enable the subsequent use of an online software for data visualisation (RAWGraphs 2.0) [38].b, D. Abd., in, H.
Multiple Generation Life-Cycles for Product Sustainability: The Way Forward

Journal article. Journal of Cleaner Production 2015
The first step (Figure 2) in constructing the sustainable materials selection framework (SMaS framework) was to schematise each contribution extensively.The schematisation was first carried out using a collaborative online platform (Miro boards) to highlight the gap and encourage a more visual and fluid approach in the association of similar strategies and guidelines, and then transferred to Microsoft Excel to construct a structured database that would allow the clustering of indications in systematic taxonomy and allow tracking of aggregation in the following steps.It was also chosen to use Microsoft Excel to enable the subsequent use of an online software for data visualisation (RAWGraphs 2.0) [38].The second step consisted of a selection, within the mapped sources, of information concerning the selection and use of materials in the sustainability project.This activity highlighted about one third of the information derived in step 1 (n = 222 lines out of the initial 675).This activity incontrovertibly confirmed the disuniformity in the terminology used by the authors analysed.
The third step aimed to put in order the information for the designer regarding sustainable materials selection, proposing an orderly and defined systematisation (Figure 3).This led to the definition of the following taxonomy:

•
Author's perspective: An umbrella term for the many design approaches for sustainability; • Objective: The purpose toward which the design effort is directed and which it seeks to achieve: the aim, goal, or end of action; • Strategy: The act of devising plans which represent the best possible way to deal with a design challenge and have the best possible sustainability benefit; • Tactic: The particular methods, actions, and themes used to achieve a sustainable design objective; • Guidelines: A rule, instruction, or information that guides the designer on how something should be carried out or how something should be.In this case, indication or outline to apply in design activities to foster materials selection; • Parameters and criteria: Standard or sharp information (unambiguous, testable, or measurable) that indicates fixed limits on how something should be, which guide the decision-making process.In this case, limit, indication, or physical properties necessary for materials selection.The second step consisted of a selection, within the mapped sources, of information concerning the selection and use of materials in the sustainability project.This activity highlighted about one third of the information derived in step 1 (n = 222 lines out of the initial 675).This activity incontrovertibly confirmed the disuniformity in the terminology used by the authors analysed.
The third step aimed to put in order the information for the designer regarding sustainable materials selection, proposing an orderly and defined systematisation (Figure 3).This led to the definition of the following taxonomy:

•
Author's perspective: An umbrella term for the many design approaches for sustainability; • Objective: The purpose toward which the design effort is directed and which it seeks to achieve: the aim, goal, or end of action; • Strategy: The act of devising plans which represent the best possible way to deal with a design challenge and have the best possible sustainability benefit; • Tactic: The particular methods, actions, and themes used to achieve a sustainable design objective; • Guidelines: A rule, instruction, or information that guides the designer on how something should be carried out or how something should be.In this case, indication or outline to apply in design activities to foster materials selection; • Parameters and criteria: Standard or sharp information (unambiguous, testable, or measurable) that indicates fixed limits on how something should be, which guide the decision-making process.In this case, limit, indication, or physical properties necessary for materials selection.
These definitions arise by comparing the terminology used by the various authors analysed with different online dictionaries such as the Merriam-Webster or the Oxford Learners (see Appendix A) and gradually developed and implemented during the literature review work, up to reach a reasonable definition of them and to be properly adapted to the design field.This operation on the mapped data made it possible to allocate the information by standardising the degree of specificity, from the most aleatory to the most concrete for design practice (from left to right in Figure 3).Therefore, the listed terms formed a framework to map the strategies and guidelines, allowing a precise data distribution.
The fourth step was aimed to eliminate repetition and redundancy: the same concepts expressed by different authors have been grouped by codes considering the most subtle differences, re-examining the articles, and keeping track of the "history" of each mapped contribution.This operation made it possible to provide a lightened mapping and readable to the last level of detail.
OR PEER REVIEW 7 of 23 These definitions arise by comparing the terminology used by the various authors analysed with different online dictionaries such as the Merriam-Webster or the Oxford Learners (see Appendix A) and gradually developed and implemented during the literature review work, up to reach a reasonable definition of them and to be properly adapted to the design field.This operation on the mapped data made it possible to allocate the information by standardising the degree of specificity, from the most aleatory to the most concrete for design practice (from left to right in Figure 3).Therefore, the listed terms formed a framework to map the strategies and guidelines, allowing a precise data distribution.
The fourth step was aimed to eliminate repetition and redundancy: the same concepts expressed by different authors have been grouped by codes considering the most subtle differences, re-examining the articles, and keeping track of the "history" of each mapped contribution.This operation made it possible to provide a lightened mapping and readable to the last level of detail.Therefore, the flow followed to obtain the mapping was a top-down approach, from the grouping of macro-data to their selection and systematisation down to the smallest detail.The mappings shown below in the results section were obtained using RAWGraph [38], a data visualisation tool developed by the DensityDesign research group of the Department of Design at Politecnico di Milano, and subsequently re-elaborated using Adobe Illustrator.
The overall work has been carried on with a constructivist approach, embracing the possibility that there is no vision of objective external reality independent of individuals [39] and incrementing the notions and the clusters proportionally with information retrieved by the readings.The choice of the constructivist approach is also reflected in the purpose of the strategies and guidelines.The aim of the indications is to be interpreted by Therefore, the flow followed to obtain the mapping was a top-down approach, from the grouping of macro-data to their selection and systematisation down to the smallest detail.The mappings shown below in the results section were obtained using RAWGraph [38], a data visualisation tool developed by the DensityDesign research group of the Department of Design at Politecnico di Milano, and subsequently re-elaborated using Adobe Illustrator.
The overall work has been carried on with a constructivist approach, embracing the possibility that there is no vision of objective external reality independent of individuals [39] and incrementing the notions and the clusters proportionally with information retrieved by the readings.The choice of the constructivist approach is also reflected in the purpose of the strategies and guidelines.The aim of the indications is to be interpreted by the individual researcher or designer (usually in the concept design phase or product development), allowing them to adapt the guidelines to the contextual conditions.

Results
The in-depth study of the presented literature (Table 1) has been compulsory to confirm the assumptions expressed in Paragraph 1.5.The indications developed over time to design in the perspective of sustainability, and, in particular, focusing on the material selection, are homogeneous, even if authors refer to different research approaches (e.g., ecodesign, DfX, or others mentioned above).Since some strategies and guidelines are commonly shared by them, independently from the author's perspective, the authors of this article have highlighted and systematised the common elements rising across the diverse research.Consequently, the authors believed it could be worth collecting them according to their purpose (meaning to guide a sustainability-oriented material selection activity) rather than the perspective from which they were conceived.
The literature review and subsequent mapping activity also confirmed how similar concepts, strategies, and guidelines usually may appear as different due to language divergences.The immediate consequence of this language ambiguity results in the diffi-culty of finding a homogeneous panorama of material selection guidelines for sustainable product design.
Therefore, through the presented methodology and subsequent homogenisation of the clusters definition (see Section 2), it has been possible to depict an overview of sustainable material selection strategies and guidelines, according to different authors, to be implemented in design activity and research.The realisation of an Excel file blindly collecting all the directives emerging from the literature reading is presented in Figure 4, and a total of 222 rows have been collected from the 11 different contributions analysed.Subsequently, some codes have been attributed to each row to group similar elements under the same tag (in Figure 4 represented in the grey columns with E code, G code, I code, K code, and M code; the coding names are derived from the Excel file columns).
The literature review and subsequent mapping activity also confirmed how similar concepts, strategies, and guidelines usually may appear as different due to language divergences.The immediate consequence of this language ambiguity results in the difficulty of finding a homogeneous panorama of material selection guidelines for sustainable product design.
Therefore, through the presented methodology and subsequent homogenisation of the clusters definition (see Section 2), it has been possible to depict an overview of sustainable material selection strategies and guidelines, according to different authors, to be implemented in design activity and research.The realisation of an Excel file blindly collecting all the directives emerging from the literature reading is presented in Figure 4, and a total of 222 rows have been collected from the 11 different contributions analysed.Subsequently, some codes have been attributed to each row to group similar elements under the same tag (in Figure 4 represented in the grey columns with E code, G code, I code, K code, and M code; the coding names are derived from the Excel file columns).After grouping certain elements under the same tag, authors were able to realise some terminology homogenisation, realising a systematised group of information divided into approaches, strategies, tactics, guidelines, and parameters and criteria.
In Figure 5, the systematised information emerging from the adopted methodological path is presented graphically and subsequently discussed.
Literature contributions pertaining to a similar author's perspective have been represented with shades of the same colour.
As it can be immediately noticed, some blank spaces in the fluxes emerged.This graphical expedient has been useful for the authors to convey a precise message: due to divergences in terminology and the definition of guidelines according to different operability levels, authors needed to "allocate" in the new taxonomy certain contributions coming for literature.This means that sometimes authors defined "guidelines" extremely operative notions, creating ambiguity with other contributions that, under the term "guidelines", inserted more generic indications.Therefore, it could happen that certain steps defined for the new taxonomy were not completely fulfilled by the literature.
Hence, these blank spaces represent this terminological ambiguity.In this first instance, authors decided to maintain this feature to avoid losing fidelity with the literature analysed that, since no taxonomy was defined before, rightly adopted terminology according to their precise studies.After grouping certain elements under the same tag, authors were able to realise some terminology homogenisation, realising a systematised group of information divided into approaches, strategies, tactics, guidelines, and parameters and criteria.
In Figure 5, the systematised information emerging from the adopted methodological path is presented graphically and subsequently discussed.
Literature contributions pertaining to a similar author's perspective have been represented with shades of the same colour.
As it can be immediately noticed, some blank spaces in the fluxes emerged.This graphical expedient has been useful for the authors to convey a precise message: due to divergences in terminology and the definition of guidelines according to different operability levels, authors needed to "allocate" in the new taxonomy certain contributions coming for literature.This means that sometimes authors defined "guidelines" extremely operative notions, creating ambiguity with other contributions that, under the term "guidelines", inserted more generic indications.Therefore, it could happen that certain steps defined for the new taxonomy were not completely fulfilled by the literature.
Hence, these blank spaces represent this terminological ambiguity.In this first instance, authors decided to maintain this feature to avoid losing fidelity with the literature analysed that, since no taxonomy was defined before, rightly adopted terminology according to their precise studies.

Objectives
From the 11 sources (Table 1) analysis, five different authors' perspectives have been highlighted (respectively, ecodesign, circular design, material efficiency, emotionally durable design, and DfX).Each of these perspectives led singular authors to define different ways to pursue a sustainability-oriented material selection activity, providing an objective to be pursued.On a wider level, it has been possible to group these objectives as follows: 1.
Resource conservation: reflects upon how to manage natural resources without compromising the ecosystem; 2.
Low-impact resources and processes: focuses on reducing emissions and compromising effect on production processes and energy consumption; 3.
Material life cycle optimisation and/or extension: characterised by improved LCA efficiency directly in the definition of sustainable material selection activity strategies and guidelines; 4.
Material manufacturer ethics and politics: focusing on the importance and relevance of creating awareness of the consequences of material manufacturing; 5.
Circular design: guidelines and strategies explicitly referring to circular economy principles.

Strategies
This first clustering activity (Figure 6) was quite affected by the author's research field and perspective manifested in the specific resource analysed.However, the subsequent development of these approaches and objectives into strategies, tactics, and guidelines directly drove a series of concepts overlapping, repetitions, and convergences among the different sources analysed, creating mismatched accordance of nomenclature.Using the proposed taxonomy, it already emerges from the first steps how the depth of the indications found in the literature is variable.For emotionally durable design, DfX, and part of ecodesign, the indications found in the literature are too specific to be clustered as objectives or strategies.

Objectives
From the 11 sources (Table 1) analysis, five different authors' perspectives have been highlighted (respectively, ecodesign, circular design, material efficiency, emotionally du rable design, and DfX).Each of these perspectives led singular authors to define differen ways to pursue a sustainability-oriented material selection activity, providing an objective to be pursued.On a wider level, it has been possible to group these objectives as follows 1. Resource conservation: reflects upon how to manage natural resources without com promising the ecosystem; 2. Low-impact resources and processes: focuses on reducing emissions and compro mising effect on production processes and energy consumption; 3. Material life cycle optimisation and/or extension: characterised by improved LCA efficiency directly in the definition of sustainable material selection activity strategie and guidelines; 4. Material manufacturer ethics and politics: focusing on the importance and rele vance of creating awareness of the consequences of material manufacturing; 5. Circular design: guidelines and strategies explicitly referring to circular economy principles.

Strategies
This first clustering activity (Figure 6) was quite affected by the author's research field and perspective manifested in the specific resource analysed.However, the subsequen development of these approaches and objectives into strategies, tactics, and guideline directly drove a series of concepts overlapping, repetitions, and convergences among the different sources analysed, creating mismatched accordance of nomenclature.Using the proposed taxonomy, it already emerges from the first steps how the depth of the indica tions found in the literature is variable.For emotionally durable design, DfX, and part o ecodesign, the indications found in the literature are too specific to be clustered as objec tives or strategies.From the analysed sources, by deepening the lecture and analysing selected resources, it has been possible to highlight eight main strategies, evolving from previously analysed approaches.

Tactics
In some cases, the literature analysed offered some guidelines that were not as punctual as in other retrieved resources; therefore, tactics have been introduced as a mid-cluster to differentiate elements between strategies and guidelines in terms of information specificity.The analysis highlights 20 different tactics (Figure 7) and can be examined in detail in Appendix B. Since the proposed taxonomy of approaches, strategies, and guidelines (defined by authors in Section 2 of this contribution) did not necessarily meet the ones adopted in the different retrieved sources, it happened that, e.g., as highlighted before, during the clustering activity, some authors were referring to "strategies" when mentioning very detailed drivers, more coherent with the definition of "guideline" (or vice versa).Therefore, the authors critically analysed these insights and clustered them according to the proposed taxonomy to homogenise the language and the insights as well.This implied that sometimes, some information was missing, and some "jumps" between the clusters occurred.As explained, these jumps have been represented in the visualisations to maintain a modus operandi as objective as possible.Therefore, these interruptions should not be interpreted as "missing information" but, instead, as a mismatch with the presented taxonomy.From the analysed sources, by deepening the lecture and analysing selected resources, it has been possible to highlight eight main strategies, evolving from previously analysed approaches.

Tactics
In some cases, the literature analysed offered some guidelines that were not as punctual as in other retrieved resources; therefore, tactics have been introduced as a mid-cluster to differentiate elements between strategies and guidelines in terms of information specificity.The analysis highlights 20 different tactics (Figure 7) and can be examined in detail in Appendix B. Since the proposed taxonomy of approaches, strategies, and guidelines (defined by authors in Section 2 of this contribution) did not necessarily meet the ones adopted in the different retrieved sources, it happened that, e.g., as highlighted before, during the clustering activity, some authors were referring to "strategies" when mentioning very detailed drivers, more coherent with the definition of "guideline" (or vice versa).Therefore, the authors critically analysed these insights and clustered them according to the proposed taxonomy to homogenise the language and the insights as well.This implied that sometimes, some information was missing, and some "jumps" between the clusters occurred.As explained, these jumps have been represented in the visualisations to maintain a modus operandi as objective as possible.Therefore, these interruptions should not be interpreted as "missing information" but, instead, as a mismatch with the presented taxonomy.At the same time, these interruptions do not prove that the taxonomy is wrong.Instead, they represent the actual definition gap that provokes a confusing use of the At the same time, these interruptions do not prove that the taxonomy is wrong.Instead, they represent the actual definition gap that provokes a confusing use of the mentioned lemmas, increasing the blurry panorama of guidelines and strategies emerging in the literature for selecting sustainable materials.
The diverse thicknesses of the coloured streams depend on the cumulative count of elements concerning that specific group and have been visualised using the RAWGraphs platform [38].

Guidelines, Parameters, and Criteria
At this point, the interpolation of the different sources started to be more and more evident: within a collection of 36 different guidelines, it is possible to perceive the detachment from the original, linear division between the analysed authors' works when it comes to specific, operational parameters for a sustainability-oriented material selection, the author's perspective and the research field seems not to affect directly.The guidelines can be analysed in detail in Appendix C. At this level of analysis, research approaches and perspective boundaries become permeable, and it is even difficult to distinguish them if looking at guidelines only (Figure 8).mentioned lemmas, increasing the blurry panorama of guidelines and strategies emerging in the literature for selecting sustainable materials.The diverse thicknesses of the coloured streams depend on the cumulative count of elements concerning that specific group and have been visualised using the RAWGraphs platform [38].

Guidelines, Parameters, and Criteria
At this point, the interpolation of the different sources started to be more and more evident: within a collection of 36 different guidelines, it is possible to perceive the detachment from the original, linear division between the analysed authors' works when it comes to specific, operational parameters for a sustainability-oriented material selection, the author's perspective and the research field seems not to affect directly.The guidelines can be analysed in detail in Appendix C. At this level of analysis, research approaches and perspective boundaries become permeable, and it is even difficult to distinguish them if looking at guidelines only (Figure 8).When authors were extremely precise in providing punctual material selection tips, these insights were clustered as "Parameters and Criteria".A total of 93 different parameters and criteria (Figure 5) for sustainable material selection activity have been collected as follows (the complete legend of the parameters and criteria can be found in Appendix D). similar language for referring to the same concept.These convergences are particularly significant in terms of identifying the possibility of finding a common language concerning material selection oriented to sustainable purposes.
The presented visualisation offers a comprehensive overview of strategies and guidelines for sustainable material selection activity.
Starting from this cumulative visualisation, it is possible to highlight both different guidelines and parameters sets, grouped, e.g., by research approach (Figure 9) and convergences between the different approaches, confirming the necessity for certain cases to adopt similar language for referring to the same concept.These convergences are particularly significant in terms of identifying the possibility of finding a common language concerning material selection oriented to sustainable purposes.

Discussion
Whereas the awareness that the selection of sustainable materials should be framed within a larger and more complex system of design for sustainability, the present work emerged from the shared need to have a holistic view of approaches and strategies for selecting sustainable materials.The multidisciplinary research group has a solid background in materials selection and systemic design.When approaching materials sustainability issues in an academic or industrial context, it has been evident that there is a tendency to follow only a few points of view, missing other possible avenues toward sustainable material selection and application in product design.Moreover, it was noticed that the sector's literature reaches varied levels of comprehensiveness, definition, and criticality, limiting its application in the design practice.Sometimes, on the other hand, authors in the literature were just using different terminology, and, although starting from

Discussion
Whereas the awareness that the selection of sustainable materials should be framed within a larger and more complex system of design for sustainability, the present work emerged from the shared need to have a holistic view of approaches and strategies for selecting sustainable materials.The multidisciplinary research group has a solid background in materials selection and systemic design.When approaching materials sustainability issues in an academic or industrial context, it has been evident that there is a tendency to follow only a few points of view, missing other possible avenues toward sustainable material selection and application in product design.Moreover, it was noticed that the sector's literature reaches varied levels of comprehensiveness, definition, and criticality, limiting its application in the design practice.Sometimes, on the other hand, authors in the literature were just using different terminology, and, although starting from different perspectives, they arrived at the same strategies.Over time, this has produced a critical mass of non-homogenous research, which, consequently, may generate confusion among practitioners or students looking for a clear direction.
In this work, it has been presented an attempt of a structured systematisation of the strategies and guidelines for a sustainable material selection.The overall organisation has been conducted with a systematic mapping activity, described in detail in the previous paragraphs.The objective of such a detailed description of the procedure adopted for this systematisation activity is to provide a repeatable and clear methodology.
Therefore, the SMaS framework attempt to collect contribution for a readable flow guiding practitioners to a comprehensive vision of the academic perspective of sustainable materials selection and application to product design.The SMaS framework deals not only with selection but generally with the use of materials in the design project since the material is a product pillar able to influence the shape, the assembly, the mode of use, and the interaction with the product.
Design practitioners and researchers can use the SMaS framework either starting from parameters and criteria, climbing up to the design perspectives or vice versa, in a top-down direction.The advantage of this double reading leads the user not only to better understand the framework, but also to adapt to specific situations, e.g., where a brief provides specific parameters and criteria, it becomes possible to open up new possibilities and trace further design approaches, implementing creativity, or in the opposite case where a broader approach and perspective is desired, it is possible to ground these aspects.Such visualisation clearly communicates already existing guidelines and directions to pursue a sustainable material selection activity.The SMaS framework, exploiting all the advantages of the graphic visualisation of data collected, allows practitioners at a didactic and professional level to approach material selection with a broad overview of this process's implications, even though their background knowledge is not technical.This helps envision the motivations why the research "pushes" specific directives towards the sustainable selection of materials and helps to ground the basis for an aware management of material selection activity.This aware management of material-related information is a valid activity both in academics and industrial application, responding actively to the practical difficulties encountered in such a transitional moment.
According to the authors, this systematisation represents the first step towards the subsequent realisation of an integrated tool for sustainable design, capable of guiding the designer step by step towards increasingly specific levels of detail.This concept can be valid for material selection guidelines and strategies and other drivers (e.g., sustainable production guidelines and strategies or sustainable supplier selection strategies).Therefore, the authors believe that the proposed methodology could be adopted and followed to map other relevant guidelines and strategies to pursue sustainable production since, as stated in the introduction section, this not only depends on material selection but also passes through it.Indeed, it is important to reiterate that to avoid rebound effects or a reductionist approach, it is always important to read the indications provided by the framework and the various authors with a critical eye, harmonising the needs and requirements of the material with others imposed by the context.
As future developments of this work, the authors aim to resolve the interruptions in the flux visualisation for better reading and use of the proposed tool.This action implies that the author needs to revise the mapping activity, abandoning the "high-fidelity" approach for collecting information used in this contribution, and make some assumptions to harmonise the records registered in the database with the proposed taxonomy.This means that the resulting visualisation would be clearer in terms of reading but little revised through the lens of author knowledge.

Conclusions
The research work proposed within this text pursues the objective of identifying redundancies and unifying methodologies and tools implemented in the design field for sustainability (the so-called sustainable design methods and tools), in line with the objective proposed by Faludi et al. [40].The effort is aimed at unifying the terminology used in the literature, suggesting to the designer strategies and indications for the realisation of more environmentally aware products.First, this effort led the authors to define a taxonomy of indications, distinguishing between authors' perspectives, objectives, strategies, tactics, guidelines, and parameters and criteria.This systematisation sets out gradually more and more operational indications, thus, moving from generic to more specific information.To simplify the schematisation process, the work proceeded to select indications referring to the material selection, thus, leading to the realisation of the SMaS framework.
As previously mentioned in Section 2, the research used a constructivist epistemological approach.This is because both the schematisation and the realisation of the taxonomy involved, although the proposed methodology tried to avoid this, a work of interpretation by the authors.This could be seen as a limitation to the work carried out; indeed, by adopting either an objectivist or subjectivist epistemological approach, the results derived from both the taxonomy and the subsequent mapping could be different.A further limitation of the research could be seen in the sampling of literature used.The sampling was conditioned not only by the search database initially used (Scopus) but also by the focus identified to simplify and construct the mapping, i.e., the indications regarding material selection.Since the material plays a very important role in sustainability, material selection indications are largely diffused within the design guidelines for sustainability, although they still represent only one of these aspects.
Looking to further development and validation of both the taxonomy and the SMaS framework, one of the future works will be to apply the taxonomy to different clusters of design guidelines.Such an evolution would make the proposed framework an integrated design tool, capable of connecting different aspects of the product and avoiding unforeseen consequences for sustainability.However, this extension work, already envisaged by the authors, requires a previous effort in terms of data accessibility and information visualisation, which is still a critical point to date.
Indeed, this optimisation reveals several future opportunities for the work, such as its transformation into a template or open-source software capable of being navigated and updated easily by operators.In addition, a simplification in usability would allow further expansion and the addition of further information to support the guidelines, such as case studies and best practice examples.Looking in particular at the indications contained within the parameters and criteria, it might be interesting to combine the indications that are qualitative to date with quantitative information wherever possible.
The SMaS framework could have various applications and practical implications within both the academic and professional worlds.Regarding the academic world, the framework could be applied to educational activities.Including sustainability considerations and information from the early academic years would enable students to learn from their first design experiences the most important strategies and main considerations to be made when designing a sustainable artefact.Moreover, the different levels of depth fit well with teaching techniques such as project-based learning (PBL), a widely used teaching approach in design education [41,42].Within the professional world, on the other hand, the SMaS framework positions itself as an exploratory and communicative tool, allowing experienced designers and companies to be inspired and guide design in directions that have yet to be explored.In addition, the tool can be used by designers as a single platform to develop co-design actions, facilitating development work, and identifying possible problems and limitations of certain choices.

Figure 1 .
Figure 1.Example of the indications and different levels of depth for each author analysed by design approach [17,21-30].

Figure 2 .
Figure 2. Schematic representation of the workflow adopted for taxonomy homogenisation and information schematisation.

Figure 2 .
Figure 2. Schematic representation of the workflow adopted for taxonomy homogenisation and information schematisation.

Figure 3 .
Figure 3. Schematisation of the taxonomy and convergence towards operability of the indications.

Figure 3 .
Figure 3. Schematisation of the taxonomy and convergence towards operability of the indications.

Figure 4 .
Figure 4. Screenshot of the data organisation into the Excel file realised by the authors (extract of the first rows to show the construction of the material guidelines and strategies database).

Figure 4 .
Figure 4. Screenshot of the data organisation into the Excel file realised by the authors (extract of the first rows to show the construction of the material guidelines and strategies database).

Figure 5 .
Figure 5. Overview of the SMaS framework showing the different steps within the proposed taxonomy [17,21-30]; the high-quality framework can be found in the Data Availability Statement link.

Figure 5 .
Figure 5. Overview of the SMaS framework showing the different steps within the proposed taxonomy [17,21-30]; the high-quality framework can be found in the Data Availability Statement link.

Table 1 .
Authors and texts selected and used for mapping activity.

Table A1 .
Different definitions provided by online dictionaries.

Table A4 .
Cont.M42Use compatible materials (that can be recycled/composted together) within the product or sub-assemblyM43For joining, use the same or compatible materials as in the components (to be joined, including fasteners, screws, rivets, staples, etc.)