Unfolding the human–material interaction of material flows in societies: DNA as a conceptual metaphor

The management of material flows in societies is complex yet crucial for the sustainable coexistence of humans and materials. While industrial ecology (IE) has long examined material flows, studies acknowledging their sociomaterial nature are scarce. Consequently, the existing IE research has not yet answered why materials flow in societies as they do. This study therefore examines human–material interaction (HMI) in material flows. We build on the IE and sociomateriality literature and empirical findings from a qualitative multiple‐case study of two material flows (recycled concrete aggregate; biogas and recycled nutrients) where humans interact with materials to advance material flows in society more sustainably. We identify and conceptualize 11 HMI elements (adaptability, general acceptance, public interest, regulation, compatibility, consistency, degradability, availability and continuity, intensity, proximity, and re‐utilizability) that further divide into three categories (human‐driven, material‐driven, and equally driven HMI elements) to explain in detail the manifestations of HMI in societal material flows. Together, these HMI elements explain material flows as the physical movement of materials motivated by goal‐oriented humans who engage with materials, a process that leads to humans and materials becoming constitutively intertwined in spatiotemporal practice. To visualize our findings on this complex yet pivotal HMI phenomenon, we employ DNA as a conceptual metaphor. The study contributes to IE by uncovering the dynamic HMI in material flows and guiding practitioners on how to manage material flows in societies, acknowledging both human and material perspectives.


INTRODUCTION
The purposeful management of material flows is crucial for the functioning of modern economies and societies (cf.Chen, 2009).For example, waste management (Lehtokunnas & Pyyhtinen, 2022), the security of supply during pandemics (O'Sullivan et al., 2022), and the just utilization of materials between generations (cf.sustainable development; World Commission on Environment & Development (WCED), 1987) are all fundamentally concerned with the balanced management of material flows in human societies.The human-material interaction (HMI) has gained importance with the rise of the circular economy (CE) and its aim of achieving harmony between the human economy and nature ecology by more efficiently managing material flows (e.g., Ellen McArthur Foundation (EMF), 2015; Geissdoerfer et al., 2018;Ghisellini et al., 2016).Accordingly, this study examines the co-existence of humans and materials at the heart of societal material flows and contributes by closely examining HMI and identifying its nuances to further explain why materials flow in societies as they do.
HMI-related concerns are of particular interest in industrial ecology (IE), which studies the organization of industrial material flows and compares them to the flows seen in material-efficient natural ecosystems (e.g., Chertow, 2000;Korhonen, 2004).In practice, IE is interested in industrial ecosystems in which the "consumption of energy and materials is optimized, waste generation is minimized and the effluents of one process (. . . ) serve as the raw material for another" (Frosch & Gallopoulos, 1989, p. 114).The understanding of HMI in these ecosystems has since evolved: From being considered exchanges of static materials between collocated organizations, HMI is now seen as a dynamic and ever-evolving process (cf.Boons et al., 2014;Felicio et al., 2016;Lombardi & Laybourn, 2012) taking place in all arrangements in which organizations exchange outputs (Chertow & Ehrenfeld, 2012).
As IE has acknowledged the dynamism of HMI in industrial material flows, the general comprehension of the complexity of HMI has broadened.
Namely, humans are affected by social institutions, such as regulations and norms, that serve as institutional drivers and barriers shaping what humans and organizations do with and for materials (Ranta et al., 2018), and humans are obliged to operate on the terms set by the material world (i.e., nature), not the other way around (Heikkurinen et al., 2021).Yet, social parameters related to HMI are rarely examined in IE (Bruel et al., 2019), and IE still lacks social perspectives on the approaches, tools, and frameworks used in the field (Covarrubias, 2019;Hobbes et al., 2007).Common IE tools, such as material flow analysis (MFA) and lifecycle assessment (LCA), tend to represent a technical perspective on HMI as a physical movement of materials that can be analyzed, quantified, and optimized with predetermined tools.These technical, material-focused perspectives in IE can nevertheless still be seen as very anthropocentric in that they see materials as subordinate to humans (cf.Frosch and Gallopoulos' (1989) industrial ecosystem concept where materials "serve" humans' industrial processes).This anthropocentric view of materials and lack of social perspectives have prevented the IE literature from answering why materials flow in societies as they do.
To complement the human and material perspectives of HMI, Baumann and Lindkvist (2022) have recently presented a sociomaterial (cf., Orlikowski, 2007;Orlikowski & Scott, 2008) theoretical perspective on material flows in IE suggesting that material flows are the results of humans interacting with materials.Building on this perspective, we address HMI in industrial material flows where: (a) the human refers to humans and any human-related (or social) events, actions, themes, etc., connected with resources' provisioning processes (Covarrubias, 2019); (b) the material (as an adjective) is anything "insofar as it has consequences we value in a particular context" (Pentland & Singh, 2013, p. 292), though we particularly focus on concrete and tangible materials in this study; (c) the interaction reflects the constitutive intertwinement between humans and materials whereby they are interactively stabilized over time (Martini et al., 2013;Pickering, 1995); and (d) the material flow is "the continuous stream of objects, materials, resource units, ideas or information, or any other form that moves along, at least, between two points" (Covarrubias, 2019, p. 279).In this study, however, "material flow" refers especially to industrial material flows consisting of tangible and concrete materials.
The purpose of this study is to examine HMI at the heart of societal material flows.Here, we continue along the path laid down by Baumann and Lindkvist (2022, p. 658), who examined material flows in the nexus of IE and sociomateriality as "human-material interactions that create and shape material flows and of human networks surrounding and connecting them, enabling a linkage between material flows and the social realm."It is the HMI that ultimately constitutes material flows as processes of refinement and production in which materials gain their relevance (as resources or products) for humans (Rusthollkarhu & Uusikartano, 2021).Consequently, for the most efficient management and organization of material flows, it is crucial to understand HMI.Thus, our aim is to thoroughly understand the HMI and its nuances in a societal context.For this purpose, we use DNA (see Watson & Crick, 1953) as a conceptual, figurative metaphor for an appropriate simplification of complex HMI and a suitable communicative means to highlight the pivotality of HMI for societal material flows.
Our study proceeds as follows.We first conceptualize HMI with the help of the existing IE and sociomateriality literature and highlight the missing knowledge on HMI (Section 2).We then explain the qualitative multiple-case study we conducted to unfold HMI (Section 3), namely, of two societally critical material flows in Finnish industrial ecosystems: recycled concrete aggregate and biogas and recycled nutrients.As a result, we identify and conceptualize HMI and its nuances through 11 HMI elements (Section 4).Finally, we present our findings as a comprehensive conceptual DNA metaphor and further discuss the related theoretical contributions, practical implications, limitations, and future research avenues (Section 5).

THEORETICAL BACKGROUND
This study approaches HMI in the nexus of IE and sociomateriality.IE addresses HMI through the idea of mimicking the dynamics of natural ecosystems' material flows in human-made industrial ecosystems (cf. Frosch & Gallopoulos, 1989).To open up the nuances of dynamics between industrial and natural ecosystems (i.e., humans and materials), we apply a sociomaterial lens to HMI (cf.Baumann & Lindkvist, 2022).Next, we present the current knowledge on humans, materials, and their interaction in HMI separately; thereafter, we combine them into the conceptual metaphor of DNA.
Seen from the lens of sociomateriality, material flows as human-made flows (Baumann & Lindkvist, 2022) that purposively move in societies and economies (cf.Chen, 2009) denote humans engaging with materials to use them for a specific goal (cf.Pickering's (1993) "dance of agency").
Thus, the related HMI is motivated by humans' perceptions of the involved materials' abilities to promote or limit the pursuit of specific goals (Leonardi et al., 2013;Rusthollkarhu & Uusikartano, 2021).These perceptions, in turn, are affected by social institutions (i.e., regulative [laws, taxes], cultural-cognitive [perceptions], and normative [norms, preferences] institutions (Scott, 1995)) that motivate, drive, encourage, inhibit, and otherwise affect-but do not necessarily dictate-humans' engagement with materials (Aarikka-Stenroos et al., 2023;Ranta et al., 2018).Hence, "human" in HMI refers not only to humans but to any human-related events, actions, values, perceptions, norms, societal structures, etc., in the social realm that are connected with materials' provisioning processes (cf.Covarrubias, 2019) and embedded in and influenced by the material world (Borgmann, 2013;Leonardi et al., 2013).

2.2
The "material" in human-material interaction In industrial ecosystems, materials usually manifest as resource exchanges between organizations (Aarikka-Stenroos et al., 2021;Korhonen & Snäkin, 2005;Lowe & Evans, 1995;Tsujimoto et al., 2018) that aim for "competitive advantage involving physical exchange of materials, energy, water, and/or by-products" (Chertow, 2000, p. 314).In the exchanges, organizations need to deal with the varying properties of materials (e.g., the quality, volume, and availability of by-products) and their environmental impacts to turn one's waste into another's resource.For this purpose, IE mainly uses MFA to quantify and analyze the flow of materials and their properties and LCA to control the environmental impacts of materials (Neves et al., 2020).To simplify, materials in IE are often treated as data about the inputs (materials, energy, water) and outputs (emissions, waste) that are to be optimized by various calculations.
Yet, in the pursuit of their goals, humans provoke materials, which, in turn, makes the materials act (Leonardi et al., 2013) and causes the need to actively manipulate materials to become humans' allies cf.(Fatimah & Arora, 2016).This view highlights that other-than-humans can also have considerable consequences for HMI and are not necessarily easily controllable by (e.g., Latour, 2005;Malafouris, 2008;Pickering, 1995) or attributable to humans (Pickering, 1993).Hence, materials have their active ways of shaping the social world, for example, by reshaping human practices (Fatimah & Arora, 2016), resisting humans' actions toward them (cf. the concepts of accommodation and resistance (Pickering, 1995) or backtalk (Styhre et al., 2012)), causing surprising outcomes (e.g., aspects related to weather during bushfires (Pierides & Woodman, 2012)), or facilitating and affording humans' behavior and discovery (cf.Gibson, 1979).Thus, the material view of HMI denotes the physical implications of materials, their characteristics, and their (inter)actions with humans that manifest in a material way (cf.Borgmann's (2013) concepts of "matter" and "materiality"), that is, as the concrete flows of materials.Here, "material" is anything (e.g., artifacts, texts, physical matter) "insofar as it has consequences we value in a particular context" (Pentland & Singh, 2013, p. 292).
F I G U R E 1 Initial visualization of the conceptual DNA metaphor for human-material interaction (HMI).Here, humans (A) and materials (B) represent two strands intertwined into a double-helix structure through different ways of interacting, namely, HMI elements (C).
Sociomateriality explains the dynamism in HMI by stating that the social world of humans and the material world affect and are affected by each other (see Malafouris' (2008) take on material engagement).Thus, "there is no social that is not also material, and no material that is not also social" (Orlikowski, 2007), (p.1437).In practice, mutual adjustments occur between humans and materials (see Pickering's (1993Pickering's ( , 1995) ) "mangle of practice" and "dance of agency", respectively), and Jokinen et al.'s (2021) take on interagency) as humans try to control resisting materials (see Section 2.2) in an attempt to move toward their goals (Section 2.1).Thus, in HMI, humans and materials are constitutively intertwined (Pickering, 1993).Here, "interaction" means that when humans and materials interact with each other, in practice they become inseparably intertwined 1 in the flows of materials (Baumann & Lindkvist, 2022) and cannot be further divided into distinct classes of "humans" and "materials."

Human-material interaction through the conceptual DNA metaphor
Let us summarize our theoretical take on HMI by simultaneously considering humans, materials, and their interaction through the conceptual DNA metaphor.DNA is the vessel of hereditary information made of a double-helix structure of two strands connected by chemical bonds between base pairs (Watson & Crick, 1953).Similarly, in HMI, humans and materials represent two strands intertwined into a double-helix structure through interaction.Yet, the "base pairs" explaining why human and material strands intertwine in particular manners in HMI remain unknown (cf., e.g., Baumann & Lindkvist, 2022).Thus, the rest of the study specifically focuses on exploring these base pairs of HMI-referred to hereafter as HMI elementsthat ultimately help explain why materials flow in societies as they do.Our take on HMI through the conceptual DNA metaphor is presented in Figure 1.
We see DNA as a particularly fitting metaphor for the present study as mimicking nature concepts and utilizing metaphors from ecology and biology in industrial ecosystems are central means of sense-making and problem-solving in IE (e.g., Bey, 2001;Ehrenfeld, 2002Ehrenfeld, , 2004;;Graedel & Allenby, 1995).Yet, it is worth highlighting that we use DNA as a conceptual, figurative metaphor here; further developing the ontological analogy between HMI and DNA remains a subject for future studies.

Research design
As a case-study strategy enables empirical investigations of contemporary phenomena within their real-life contexts (Robson, 2002;Yin, 2009), we conducted a qualitative empirical multiple-case study on HMI.Following purposive, theoretical case sampling (see Eisenhardt, 1989), we chose two societally critical material flows in industrial ecosystems as our cases, namely, recycled concrete aggregate and biogas and recycled nutrients.Our cases display diverse humans (public and private organizations, consumer-citizens, unions, etc.) interacting (processing, using, transporting, regulating, etc.) with materials (concrete, nutrients) to advance the flows of materials in society and focal regional industrial settings due to their critical societal relevance and impact.
We argue that these cases are a relevant fit for our purposes for several reasons.First, in terms of their pragmatic relevance, both cases follow CE principles where the purposeful management of material flows is highly important in realizing the closed material cycles that the CE approach seeks.Moreover, recycled materials are often more complex than virgin materials since their quality and quantity may be irregular; hence, different nuances of HMI are likely to be seen in the material flows of recycled materials.Second, in terms of generalizability, both cases represent societally significant material flows as they are used and produced in large volumes around the world, have high environmental impacts (e.g., greenhouse gas emissions), and are very material-intensive.Third, in terms of maximum variation, the cases represent material flows from two different material cycles within the technosphere and biosphere (see EMF, 2015): recycled concrete aggregate represents the technological cycle while biogas and recycled nutrients present the biological cycle.Both cases are located in the Turku region of Finland.Visualizations of the material flows are presented in Appendices A (for recycled concrete aggregate) and B (for biogas and recycled nutrients).

Case A-Recycled concrete aggregate
The humans in case A denote those involved in the production, transport, testing, regulation, and use of recycled concrete aggregate.The feedstock for recycled concrete comes from demolition and construction companies, households, industrial manufacturers, and end-product users.In turn, users of recycled concrete include construction companies, infrastructure constructors, and manufacturers of concrete products.During the processing, consulting companies test the recycled concrete and perform quality assurance whereas public authorities decide the suitability, set legal thresholds, and grant product approvals for recycled concrete products.
The materials in case A refer to recycled concrete aggregate and its various forms throughout the material flow.The feedstock for recycled concrete aggregate includes surplus hardened concrete, demolition and construction concrete waste, failed concrete products, and other concrete waste.In turn, the recycled concrete aggregate can be utilized as part of infrastructure (as a base for roads and streets, etc.), as a substructure (the base for landfills, sports fields, industrial buildings, etc.), and as a partial substitute for so-called new concrete (i.e., in the production of new concrete products such as concrete slabs) depending, for example, on its particle size and contaminant concentrations and humans' capabilities.
The interaction in case A manifests in the diverse and multiple processes required for making end-products out of recycled concrete feedstock.
These include harvesting, transporting, cleaning, crushing into preferred particle sizes, sorting, qualifying the crushed concrete, and producing it as recycled concrete aggregate and related end-products.The processes performed by humans have to follow certain procedures and requirements and sometimes be adjusted to ensure the recycled concrete fulfills certain pre-set quality and safety standards related to its intended use.

Case B-Biogas and recycled nutrients
The humans in case B denote those involved in the feedstock providing, production, utilization, and regulation of biogas and recycled nutrients.The feedstock comes from-and related end-products are ultimately used by-farmers, industries (e.g., food, chemical, forest industry), logistics companies, municipalities, households, and consumer-citizens.Further, the feedstock is processed into biogas and recycled nutrients by congruous organizations (biogas producers, fertilizer manufacturers, municipal waste, and wastewater processors).During the processing, public authorities and policymakers set legal requirements for the safe and effective utilization of the feedstock and end-products, while unions, research organizations, and media constitute new knowledge of the bio-based side streams.
The materials in case B refer to biomasses and their different forms through the material flow where biogas and recycled nutrients are produced.
The feedstock for biogas includes, for example, household and commercial biowaste, municipal sewage sludge, and agricultural biomasses (manure, plant waste).When processed, biomasses result in biogas and digestate.Biogas can be utilized as a fuel or energy source for heating and electricity whereas the digestate containing high levels of nutrients (e.g., phosphorus, nitrogen) is used as recycled nutrients, fertilizers, and part of soil amendment.
The interaction in case B manifests in the diverse and multiple processes through which biomasses become biogas and recycled nutrients and fertilizers.These include harvesting biomasses, transporting them, pre-treating them for the anaerobic digestion process (including process steps, such as the separation of non-utilizable fractions, pre-digestion, hygienization, and heating of the biomass), digesting them, post-processing the biogas and the digestate (e.g., removing impurities, such as carbon dioxide), and utilizing the end-products.The processes performed by humans affect and are affected by the properties of different feedstocks and the effects of different post-processing methods.

Role of data types in the analysis
Enabled identification of the specific interaction of each organization with the material (e.g., how organizations handle and operate the flow of material; what kind of challenges and possibilities they see in the material flow).
Enabled analysis of the flows of materials between different organizations (e.g., organization structures around certain materials, the nature of the interaction between organizations).
Deepened our analysis by validating and deepening the knowledge acquired by other data types (e.g., how material flow management, perceptions of material flows, and the roles of humans and materials were represented in the media).

Data gathering and analysis
The research data, consisting of primary and secondary data, were collected between March 2020 and March 2022 (see Table 1).First, we used our existing understanding and available secondary sources to create initial maps of the selected material flows (for the finalized maps, see Appendices A and B).Then, we identified the key organizations along the material flows (e.g., permit authorities, customers, producers) and conducted semistructured interviews with key representatives from both material flows.
The interviewees were asked to describe and explain the critical events, factors, and capabilities connecting them to the material flow.The interviews included questions dealing with aspects of HMIs, for example, how interaction with materials and related humans was carried out in practice and the incentives for doing so.Ethnographic observation provided data on how organizations interact and operate with the material flow and what kinds of collaboration between organizations are needed.Thus, ethnographic observation enabled us to form a pre-understanding of and identify meaningful data that could be acquired and analyzed.Secondary data, on the other hand, enabled us to analyze the general environment in which material flows occur through sources such as blog posts, annual reports, and news.
After the data were collected, they were recorded, transcribed, and subjected to qualitative content analysis following an abductive research approach: First, we analyzed the data against the existing literature (i.e., searched for the themes presented in Sections 2.1-2.3) and coded all relevant text excerpts considering HMI (analysis round 1).Second, in a data-driven manner, we further analyzed each recognized HMI data excerpt and coded it based on its most prevalent characteristic.Several rounds of analysis followed in which we compared the coded text excerpts between cases A and B, searched for commonalities, and grouped similar text excerpts (analysis rounds 2−4).Finally, we reached a saturation point resulting in 11 identified HMI elements (analysis round 5).The analysis process is visualized in Figure 2.
It must be noted that commonalities between the cases included aspects occurring both similarly and very differently.Throughout the datagathering process, data triangulation (see, e.g., Flick, 2004) was sought, for example, by using multiple tactics and tools to collect primary and secondary data from diverse sources and by several researchers independently analyzing and coding the same research data and then collectively assessing the results.

F I G U R E 2
The data analysis process.

RESULTS-THE HUMAN-MATERIAL INTERACTION ELEMENTS
Drawing from our empirical data, we identified 11 HMI elements whose distinct characteristics explain the practical manifestations of HMI in societal material flows.These 11 HMI elements, with practical and quotation examples, are summarized in Table 2 and further discussed next.
Adaptability considers the level to which the human-made infrastructure (e.g., machinery, processing equipment, facilities) can deal with and adjust to changes in materials entering the infrastructure.The more changes and variations in material characteristics the infrastructure allows, the more alternative material flows are likely to occur.
General acceptance refers to humans' perceptions (stemming from, e.g., knowledge, beliefs, or prejudices) and choices related to a specific material.Whether humans accept or reject certain materials affects the pace at which materials flow in societies (i.e., it takes more time if humans need to be persuaded to utilize certain materials or end-products).
TA B L E 2 Identified human-material interaction (HMI) elements.

Adaptability
Considers the capacity of human-made infrastructure to adapt to interact with a specific material.
Case A: Existing processing infrastructure (e.g., transport equipment-trucks, etc.; transit routes-roads, etc.; processing equipment-machines, production lines, etc.) is applicable for recycled concrete aggregate.Case B: Natural gas processing infrastructure (e.g., transport equipment-trucks, etc.; transit routes-pipelines, etc.; processing equipment-machines, production lines, etc.) cannot be directly used for biogas production, but the same distribution systems can.
" It [biogas] can utilise the same infrastructure practically completely-all the applications in Finland that have already utilised natural gas for a long time can also utilise biogas, the distribution systems included."(Quality and environment manager of an energy sector company, case B)

General acceptance
Considers societal norms and humans' perceptions and choices related to a specific material that (de)motivate humans' interaction with the material. Case

Intensity
Considers the time it takes for a certain material, in interaction with humans, to re-enter the same point of the material flow (i.e., intensity).
Case A: The lifespans of recycled concrete aggregate's applications are mainly long (decades as a construction material).Case B: Biomasses and nutrients circulate via food systems and loop back as material feeds for the biogas process relatively rapidly (annually).
"For example, we dismantle old concrete pavements when they need to be replaced and take them to a concrete waste recycling point where our dismantled products are crushed and turned into concrete aggregate.In addition, when our production generates waste products, we store them, and then, when the stocks are full enough, we also take them to the concrete waste recycling point to be crushed and turned into concrete aggregate."(Project manager of a company manufacturing concrete products, case A)

Proximity
Considers the physical location and closeness of a specific material and related humans, which affects and is influenced by their interaction.
Case A: The manufacturing and utilization of recycled concrete aggregate mainly happen regionally due to logistics costs related to the raw material.Case B: Raw materials for biogas and recycled nutrient production are also gathered regionally, but utilization of the end-products happens within a broader geographic area (e.g., digestate from biogas processing may be utilized to fertilize local fields whereas biogas may be utilized as biofuel nationally).
"Instead of transporting material somewhere far away, we could utilise it regionally and thus also enhance regional employment.After all, if we think about concrete and stone, the distance is usually the main factor for costs.

Re-utilizability
Considers humans' capabilities to (re)interact with alternative versions of a specific material and the material's adaptability to multiple purposes during its lifecycle.
Case A: The refining process for already once-recycled concrete aggregate is more or less the same as for concrete that is to be recycled for the first time.Case B: Biogas per se cannot be re-utilized due to its applications, some of the recycled nutrients eventually re-enter the biogas process as they are re-embedded in the biomasses.
"What happens when the concrete aggregate has been put there in the structures; does it still have some sort of a lifecycle after that?Because that 10,000-year-old stone surely hasn't changed during the 100 years that it has spent, for example, in a road structure."(Researcher of a research institute specialized in infrastructure construction materials and the CE, case A) Public interest is concerned with the societal relevance of a specific material (e.g., in terms of its societal or environmental impacts).The more societal relevance a material has, the more involved public organizations (authorities, governments, etc.) become in the related material flow.Of particular relevance here is the kinds of regulation, permission-granting procedures, and politics public organizations apply to a specific material.
Regulation defines the boundaries of HMI in a specific material flow (e.g., regulation on how a material can be used or the acceptability of a specific material).For some materials, the regulation can even prioritize one alternative utilization option over another.
Compatibility considers the human capability to interact with different alternatives of the same material within the same material flow, depending on which a material flow can have multiple variations or be restrained to a certain trajectory.
Consistency refers to the level of heterogeneity possible for (i.e., the variety of raw materials and their characteristics) and from (i.e., the variety of different end-products) a material when interacting with humans and affects the number and variety of alternative material flows.
Degradability reflects the physical behavior of a material over time, namely, the timespan within which the material breaks down into smaller particles and no longer represents the material that first entered the HMI.The degradation of a material is determined by its unique characteristics and interactions with humans, which, in turn, affects the temporality of a material flow.
Intensity considers the time it takes for a certain material, in interaction with humans, to re-enter the same point of the material flow.Here, human capabilities to facilitate materials and material characteristics affect the frequency of occurrence of a certain material flow.
Proximity concerns the physical location of a specific material and related humans.Here, the natural appearing of a certain material (e.g., minerals) affects the location of human activities (e.g., factories).Where the materials are manufactured and related products are sold is often a human-made choice (e.g., choosing production areas according to customer segments, available infrastructure, production costs, or environmental impacts of logistics).Here, HMI affects the likelihood that material flow will occur in a given space and time.
Re-utilizability concerns humans' capabilities to work with alternative versions of a specific material and a material's adaptability to serve multiple purposes during its lifecycle and affects the longevity of a material flow.

Complementing human-material interaction in the conceptual DNA metaphor
To understand the HMI that is pivotal for material flows, this study has identified 11 HMI elements that together ultimately help explain why materials flow in societies as they do.Furthermore, reflecting the three aspects of HMI-humans, materials, and their interaction (see Section 2)-the HMI elements can be designated into three categories based on their practical manifestations in material flows.The HMI elements, their categorization, and connections to previous literature are shown in Table 3.
Complementing the initial conceptual DNA metaphor introduced in Section 2.4 (see Figure 1), we show in Figure 3  Material-driven elements Denote the physical implications materials and their characteristics have for and direct toward humans as well as humans' capabilities to act accordingly.Thus, HMI elements in this category manifest as concrete characteristics of material flows.

Contribution to theory
To better understand the rationale behind the complex and dynamic material flows in societies, our study has followed Baumann and Lindkvist's (2022) sociomaterial take on IE.While they unveiled that HMI happens in "sociomaterial interaction points" (SMIPs) that ultimately constitute material flows, we go deeper and unfold the nuances of the HMI taking place within these SMIPs (i.e., the HMI elements).The HMI elements ultimately help explain why materials flow in societies as they do, which enriches the traditional IE analyzes and tools such as MFA and LCA in two particular ways: (a) by broadening the social perspectives and understanding of the social realm in IE and (b) by challenging IE's relatively anthropocentric perspectives on materials.
Indeed, the field lacks an understanding of human behavior and economies (Yu & Zhang, 2021), which are the factors that drive companies to collaborate (Domenech & Davies, 2011), and the individuals' motivations and dynamics that lie behind HMI-related decision-making (Walls & Paquin, 2015).The identified HMI elements (especially adaptability, general acceptance, public interest, and regulation) show and describe how IE involves a social realm in which social institutions and constructions (see Scott, 1995) affect humans' actions in HMI and material flows.Alongside previous studies on social factors in IE (see Ashton, 2008;Gibbs, 2003;Hewes & Lyons, 2008;Howard-Grenville & Paquin, 2008;Jacobsen, 2007;Ranta et al., 2018), our HMI elements unveil the inner workings of humans and the social realm that have so far been uncharted within IE.For example, while the commonly used IE tool SNA highlights the structural elements conditioning the operation of an industrial ecosystem and the roles different humans play in it (Domenech & Davies, 2011), the HMI elements illuminate the reasonings and logic behind the industrial ecosystem and its (human and material) actors.
Second, our study widens IE's relatively anthropocentric view of materials in HMI.That is, the HMI elements (especially compatibility, consistency, and degradability) show that materials have distinct characteristics that cannot necessarily be manipulated but need to be dealt with in HMI.
Hence, material flows need to be managed on the terms of both the human and material worlds (cf., Heikkurinen et al., 2021).Accompanying previous IE studies showing material flows to be complex, dynamic, and constantly evolving processes (e.g., Ashton, 2008Ashton, , 2009;;Boons et al., 2014;Genc, 2022;Korhonen & Snäkin, 2005;Parida et al., 2019;Tsujimoto et al., 2018), the HMI elements help us better understand the complexity and dynamism of HMI.This is important, as the most popular IE tools, such as LCA and MFA (see Baldassarre et al., 2019), study materials in a rather reductionistic manner (e.g., as quantities) and may thus be unable to consider occasions when the behavior of materials does not align with calculative models.
Finally, our research returns to the origins of IE and demonstrates the power of its premises.Namely, IE addresses nature as a model for human operations and regards nature-based metaphors as helpful illustrations in pursuing a balanced co-existence between humans and other-thanhumans (Isenmann, 2002).Hence, IE seeks to mimic the dynamics of natural ecosystems (cf., Frosch & Gallopoulos, 1989) and sees that humans cannot be separated from nature (e.g., Graedel & Allenby, 1995, p. 9;Erkman, 1997, p. 1), a perspective that sociomaterial concepts greatly support.

Implications for practice
The identified HMI elements offer 11 detailed intervention points for those who wish to better manage, affect, and analyze material flows.Hence, our research benefits a variety of practitioners dealing with material flows.
The human-driven HMI elements are of particular interest to those who aim to promote more efficient and regular flows of materials, including people working with logistics and managers of development companies.Means to promote material flows include: • locating production facilities in areas that are as accessible as possible (adaptability), • promoting flexible regulation that would allow case-by-case solutions based on the unique characteristics of each material (regulation), • re-branding waste as by-products (general acceptance), and • initiating discussions with public permission authorities and regulators to reassess whether there is a need/possibility to update the permission and safety regulations applied to the material in question (public interest).
The material-driven HMI elements can particularly help those who deal with the physical exchange of materials (by-products, production equipment, facilities, etc.), such as managers of factories.Means to increase the exchange of materials include: • using such materials and processing them in ways that enable compatibility between them and other materials used within the region (compatibility), • reconsidering and reimagining the applications provided by existing knowledge and facilities to process new materials and produce new endproducts (consistency), and • increasing the flexibility of production processes to accommodate different lead times of materials with different degradability levels and paces (degradability).
The equally driven HMI elements offer means for those who create premises and maintain fruitful circumstances for material flows to happen.For example, regional developers and managers of eco-industrial parks can create a supportive setting for material flows by: • ensuring the supply of certain critical materials (availability and continuity), • accelerating or slowing material cycles with the help of, for example, permissions (e.g., standardizing permission application processes) or supporting new business models such as leasing (intensity), • creating a regionally appealing environment for organizations to locate themselves in by, for example, offering a compelling infrastructure and educating a skilled workforce (proximity), and • encouraging collaboration between organizations to create a vast shared pool of resources and competences to process alternative versions of a specific material (re-utilizability).

Limitations and future research avenues
Our study has some limitations due to the research methods chosen.First, to enable a deep empirical take on HMI, we had to concentrate our resources on two easily accessible Finnish cases, which makes our research geographically limited.However, we assume the findings to be relatively generalizable at least to European contexts, which seem to share great similarities, for example, in terms of CE strategies (see, e.g., McDowall et al., 2017).Moreover, although the manifestations of the identified HMI elements may vary greatly between contexts, we regard their existence as universal.Thus, testing the generalizability and repeatability of our results in other countries and continents (particularly Asia and Africa) is an important future research avenue.
Second, the material flows in our study represent recycled materials denoting previously used materials that can act differently than virgin materials.However, this study concentrates on the interaction between humans and materials, and we do not anticipate that the basics of such interaction differ between materials.Moreover, aligning with the sociomateriality approach, we are not interested in a material per se (be it a particle, component, end-product, or other) but in what it becomes and how it changes in the interaction.Yet, further studies can test the applicability of HMI elements to different types of materials.
Third, due to practical limitations, we needed to exclude the mechanisms and dynamism (referring to the question of how) through which the HMI elements ultimately shape material flows in societies over time.Future studies are needed, for example, on how a particular order of appearance of HMI elements affects the shape of a material flow and what the possible inter-hierarchy between the three categories of HMI elements is.Answering these questions would provide us with a much-needed means to better understand HMI and manage material flows more sustainably in the era of sustainability crises.
(...)  It might not be reasonable anymore from the constructor's point of view, if it [concrete aggregate] is transported over long distances."(CEO of a waste management company, case A) that HMI is made of human and material strands (cf. the double-helix structure of DNA) that are intertwined (cf.chemical bonds) by different ways of interacting, namely, HMI elements (cf.base pairs).The identified 11 HMI elements and three categories explain the different ways humans and materials are intertwined in industrial material flows and help explain why materials flow in societies as they do: The physical movement of materials (the material-driven elements) is motivated by goal-oriented humans (the human-driven elements) who engage with the materials, which leads to humans and materials becoming constitutively intertwined in spatiotemporal practices (the equally driven elements) manifesting as 11 different HMI elements, varying combinations of which ultimately make material flow what it is.TA B L E 3The human-material interaction (HMI) elements, categories, and connections to previous literature.Denotes humans, their actions, perceptions, values, norms, societal structures, etc., affecting and influenced by materials.Thus, HMI elements in this category motivate materials to flow in societies. of roundput, diversity, locality, and gradual change(Korhonen, 2001) Institutional drivers and barriers(Ranta et al., 2018) Regulative, cultural-cognitive, and normative institutions(Scott, 1995) Industrial symbiosis antecedents, lubricants, limiters, and consequences(Walls & Paquin, 2015) Abbreviation: IE, industrial ecology.

structured interviews with key organizations (3/2020−3/2022) Ethnographic observation (3/2020−4/2022) Secondary data
TA B L E 1 Data types and sources used in this study.Numbers in parentheses indicate the number of data sources.Semi-

element and its description Example of how the HMI element manifests in the research data Quotation example Consistency
aggregate] functions, and long-term monitoring, that's been done, for example, with roads and earthworks.We've received concrete, crushed it, utilised it, and then monitored the application site to see how the concrete behaves and exists there."(Businessdirector of a company manufacturing stone-based building materials, case A) (Continues)TA B L E 2 (Continued)HMICase A: The material flow is very homogeneous.The consistency and applications of newly cast and recycled concrete are almost the same.Case B: The material flow is very heterogeneous.There are multiple different feedstocks (e.g., biowaste, manure) that can in turn be processed into multiple products (e.g., biogas, recycled nutrients)."They[organicwaste streams directed into biogas process] range all the way from municipal sludge to companies' and grocery stores' biowaste, household biowaste, and grease trap sludges and highly varying biodegradable masses from process industry.They are all suitable for the process.(...)Then, we produce homogeneous, hygienic fertiliser products.Biogas, on the other hand, is directed to refining on multiple plants, meaning that the carbon dioxide is removed, and therefore we get quite pure methane, which can be distributed from various plants to its applications via pipeline or, for example, as compressed gas."(Quality and environment manager of an energy sector company, case B)"Regarding biogas, surely in Finland there're biomasses that can be utilised here as a [biofuel] raw material.If only it were distributed and utilised the right way-in all aspects, that's a reasonable concept."(Sustainability manager of an automotive industry company, case B)