**1. Introduction**

The technological and cultural development of mankind includes rapid-growing interconnectivity of markets as a result, culminating in a globalized economy is the main driver for the tremendous improvements that mankind is profiting from in manifold respects. It enabled certain autonomy from the dependence on natural cycles and dealt as one of the main drivers towards what is known as civilization [1]. The Neolithic revolution can be identified as the first step of this emancipation process, offering a way to decouple availability and demand for food in time. Coming along with innovative transportation methods and economic development trade was established which reduced the spatial dependency. Although the resulting human activities already had strong impacts on their environments, these remained mainly local and did not affect the global ecosystem as such [2]. With the discovery of fossil energy, the availability of energy underwent a similar process, enabling shifting energy in time and space and thus overcoming spatial dependency. This final step of emancipation from the restrictions of natural cycles facilitated the globalized economy of the modern society. Although providing indisputable advantages in almost every sphere of life for humans, this development comes at a cost of both the alienation of humans from nature itself and of the current global depletion and destabilization of natural systems [1,3]. Regarding the results of men's worldwide activities; a point is reached where potentially irreversible impacts on systems of global relevance are likely to be threatening human life [4,5]. Thus, solutions have to be created to address these challenges in a way that allows ongoing societal prosperity.

Over the history of sustainability living nature has played a major role in its understanding and application of frameworks to strive for and oftentimes was referred to as source of inspiration [6–9]. To facilitate the transfer of advantageous aspects from biology to technology, the functions of biological systems deal as fundamental basis for the assessment system structure. Therefore, a methodological approach to assess innovations has been developed through conflating bio-inspiration and sustainability based on the reintegration of basic bio-inspired principles into material systems of humankind. Its goal is to effectively develop sustainable construction products, which requires an adaptive assessment system to accompany product development. This is achieved through the abstraction of basic principles of biological systems on artificial systems and the deriving of a set of indicators and according weighting based on these transposed rules. While the conceptual framework has already been described, its concretization and exemplary application is subject to this work [10,11]. The framework has been put into practice by applying a structured approach to design and specify the elements of the sustainability assessment including their interrelation on a quantitative basis. This assessment system and its development are the core of the publication.

#### **2. Development of the Assessment System**

The proposed assessment system has been developed following a structured approach. It grounds on the bio-inspired sustainability framework developed and is applied in the Collaborative Research Centre "TRR141: Biological Design and Integrative Structures—Analysis, Simulation and Implementation in Architecture", funded by the German Research Foundation DFG [10]. The framework is inspired by one of the basic principles of living systems, namely the autopoietic model describing self-maintaining through the fulfilling of elementary functions using available resources [12,13]. As the area of application is restricted to the built environment and mainly deals with the development of innovative, bio-derived products, the assessment has been designed for this field and its applicability is restricted to it.

The development of an assessment system usually consists of a consolidation of existing and specifically developed fragments through a scientific approach including a validation of the assessment framework and its underlying calculation scheme. The central underlying existing frameworks applied in this context are bio-inspiration and life cycle based sustainability assessment, coming together through the scientific process of biomimetics. Within the Collaborative Research Center (TRR 141) using an interdisciplinary team of experts is jointly working on the development of bio-inspired innovations for the construction sector aiming among others at sustainability of the solutions. As this requirement is an integral part of the TRR 141, it proves an ideal development and application environment for a Bio-inspired Sustainability Assessment (BiSA) model. The method development is conducted based on the following steps:

	- a. Sustainability
	- b. Bio-inspiration

The requirements (I) to the accompanying assessment of bio-inspired product development are defined in Section 2.1. This includes both general requirements for assessment systems and specific requirements concerning sustainability, bio-inspiration and decision support in product development. In Section 2.2 (IIa, sustainability) and Section 2.3 (IIb, biology) the fundamental frameworks are presented focusing on the adaptions to the state of the art that are required and the specifically developed schemes. The BiSA system derived as synthesis from the underlying basic concepts is described in Section 2.4 (III) and applied to a case study in Section 3. Based on this application (IV) the compliance to the requirements are discussed in Section 4 (V) and recommendations (VI) are presented in Section 5, giving an insight in the planned adaption and improvement of the assessment.

#### *2.1. Requirements to a Bio-Inspired Sustainability Assessment*

The development of a comprehensive sustainability assessment model for a targeted development of sustainable products underlies certain requirements in terms of methodology and applicability. The general requirements refer to basic principles for scientific methods such as consistency, comparability, reproducibility and falsifiability [14]. Besides the general requirements that apply for all assessment systems, specific context-related requirements have to be considered for evaluating the assessment system. These are related to decision support systems, sustainability assessment systems and bio-inspired systems [11].

Although mainly focused on the managemen<sup>t</sup> of decision making processes, there have been several approaches to specify requirements for decision support systems (DSS) in decision theory that provide universal requirements [15–17]. Their common denominator is the emphasis of an adaptive and flexible applicability of DSS. A DSS should therefore also be capable of supporting semi-structured and unstructured decisions, for all levels of decision makers, regardless of their proficiency and throughout all phases of the decision making process [17]. For sustainability assessment systems, the systemic framework shown in Figure 1 is applied [18,19]. It provides a semi-quantitative scale of seven criteria covering the most relevant aspects that are prevailing in scientific literature [20–23]. All criteria are staggered in three levels and providing a scorecard of the assessment system.

The classification of an assessment system as bio-inspired can be described based on the intrinsic system properties of effectivity, adaptability and resilience [10]. Effectivity is defined with regard to the required effort by the practitioner required to generate the desired information yield. As this is an aspect that can only be investigated through practitioner monitoring through application, it requires a minimum number of applied studies with integrated BiSA. The system is classified as adaptive when it offers flexibility and expandability in an indicator and weighting scheme and realizes this through ongoing self-evaluation and adaption. If the assessment model is able to absorb changes in input in terms of reasonable system deflections, it is regarded as being resilient [24].

#### *2.2. Properties of Sustainability Assessment Methods*

Sustainability can foremost be understood as societal paradigm and its perception as such has potentially a high influence on almost every decision that is taken, starting from everyday decisions up to global politics. Its constructive ambiguity together with its level of abstraction as well as the complexity of cause-action-relation, interconnectivity and multidimensionality are leading to the point that the meaning of the term is commonly changed and shaped due to subjective perceptions [25]. Although sustainability nowadays shows ubiquitous appearance there is still a lack of consensus when it comes to defining detailed concepts going beyond the overall agreemen<sup>t</sup> shaped in "Our common future" [26]. The sustainability development goals (SDGs) can be seen as a milestone in the effort of consensus finding but still does not offer a comprehensive and quantifiable catalogue of indicators capable of assessing the sustainability especially when it comes to dedicatedly developing sustainable products. Overall the multitude of concepts, interpretations and respective methods and models to assess sustainability gives a hint that the paradigm of sustainability is still evolving and its shape is still to be found [27].

**Figure 1.** Requirements depicted as spider chart to assess the capability of sustainability assessment methods to address sustainability (adapted from Sala et al. (2015), figure licensed under CC BY NC ND) [19].

In the following, concepts and assessment methods are presented focusing on their consistency and suitability to a quantified assessment of products over their life cycle. While there are numerous concepts available, most are originally restricted to a schematic level and have to be transferred and differentiated to fully apply quantified life cycle thinking and thus provide comparable and specific results on a level that facilitates detailed decision support [28]. The concept of cradle to cradle, for example, is presented as a design framework for sustainable products but offers several inconsistencies when combined with quantified life cycle thinking [29,30]. The same does apply to the concept of natural capitalism, which only monetarizes all environmental resources and is therefore a method for single point creation in Life Cycle Assessment (LCA) than a sustainability concept. If enhanced by human and man-made capital as applied in the triple-bottom-line LCA, all the pillars can be addressed, while still their interpretation is restricted to monetary quantities [31,32].

There are numerous approaches available which are neither consistent nor comparable among each other on a quantitative basis. As the investigation of existing sustainability assessments has been extensively conducted by several recent publications, these are chosen as a basis for the assessment of the research situation [33–35]. Guinée has provided an extensive meta-assessment of Life Cycle

Sustainability Assessment (LCSA) studies in scientific literature including a comprehensive list of general recommendations and potential improvements that are lacking for existing studies and should be tackled in the LCSA assessment development [35]. Among others, the following key points have been stated: general need for data and methods, especially for social indicators; communication of results; integration of beneficial aspects; avoiding of double counting and inconsistent application. As these points are still unsolved, there is clear evidence for a demand of further methodological development for improved sustainability assessments, dealing with these issues and thus improving both broadness and depth as well as communication [35].

#### *2.3. Biological Idea Generators for Sustainability Assessment*

Particularly because the described sustainability assessment is inspired by biology and tailored to the construction sector, different concepts of learning from nature are presented and illustrated by means of selected examples from the building sector. Learning from nature is linked with the hope of learning from biological solutions that seem to be optimized in the evolutionary process over the last 3.8 billion years. In principle, three levels of learning from nature can be distinguished: (i) learning from the results, (ii) the processes and (iii) the principles of biological evolution [36]. These three levels have a common systematic approach of knowledge transfer but differ in the type of the transferred knowledge.

The first level of learning from living nature is the study of the form-function relationships of biological role models. Taking into account that even the transfer of an inspiring idea is a conscious process, the transferred inspiration leads to a bio-inspired product. A famous example is the plant-inspired reinforced concrete developed by the French gardener Joseph Monier in 1867 [37]. Based on an inspiration, additional knowledge transfer is possible, such as the transfer of morphology leading to a biomorphic product such as the Crystal Palace, a cast-iron construction built by the gardener Sir Joseph Paxton being inspired by the ribbed leaves of water lilies [38] and the transfer of a functional principle resulting in a biomimetic product [11]. Special attention should be paid to the transfer of a function or in other words the statement that the biological role model and the technical product possess the same function as for example the self-cleaning surfaces of lotus leaves and the façade paint Lotusan® or different functions such as the façade shading system Flectofin® inspired by the pollination mechanisms of the bird-of-paradise flower [39,40]. The meaning of function is thereby different whether used in the field of biology or technology. Biological functions are understood in the sense of traits evolved to increase the organism's fitness and contribute to the evolutionary success [41]. In contrast, technical functions are defined in the sense of a specific process, action or task [42]. Examples for the second level of learning from evolutionary processes are the optimization algorithms based on growth rules of trees (Computer Aided Optimization) and bones (Soft Kill Option) and the evolutionary algorithms, which lead to biomimetically optimized products [11]. The third level of learning from nature is based on the principles of biological evolution such as multifunctionality, hierarchy, robustness (fault tolerance), resilience (failure tolerance), redundancy, self-X-functions, adaptation, consistency, modularity, sudden transitions (i.e., leaf drop), gradual transitions, growth, opportunism, metabolism under mild environmental conditions (enzymes) [43] and resource efficiency [8]. In ecology, the term "resources" refers to essential environmental factors that can be subdivided into biotic (e.g., food, host, reproductive partners) and abiotic factors (e.g., space, light, water) [44].

In summary, it can be said that despite the inspiratory flow and knowledge transfer from biology to technology, bio-inspired products are not necessarily sustainable as a side effect. The challenge is that there is no biological model and no method for a straightforward transfer into any model of the paradigm of sustainability. This is due to the fact that living nature itself as a result of biological evolution cannot be comprehensively described through the concept of sustainability. It is a man-made teleological and anthropocentric paradigm with the goal of preserving the status quo for the next generations [45,46]. The paradigm of sustainability is of teleological nature and therefore

to be distinguished from biological systems, where teleology is seen as an insufficient concept to describe reproduction and evolution [47]. In contrast, biological evolution is seen a blind process characterized by the dynamics of evolutionary adaptations on basis of mutation, recombination and selection in an ever-changing environment with the result of multifunctional and optimized structures or processes after several generations [41]. On the one hand, the concept of teleology is a useful element of explaining adaption, when using the goal-directedness to explain the composition and processes of systems [47]. On the other hand, a teleological approach can put us on the wrong foot as explained in the review "If bone is the answer, then what is the question?" describing the increasing understanding of adaptive bone architecture over time [48]. Thus, the principles that facilitate adaption, especially the principles of biological evolution may serve as idea generators and may have grea<sup>t</sup> potential to contribute to sustainable solutions, precisely because the challenging situation that the stable preservation of certain ecological systems requires constant changes. However, this proposed transferability cannot be seen as an automatic transfer and has to be investigated thoroughly. Furthermore, what is called the social pillar of sustainability has no counterpart in biological systems and no direct conclusions concerning social aspects may be drawn from nature.

Although nature does not bear a fully comprehensible set of role models for a bio-derived understanding of sustainability, it undoubtedly is a grea<sup>t</sup> source of inspiration in terms of multiple aspects. The dynamic adaptation and the efficient utilization of locally and currently available resources but especially the fact that biological systems have been optimized in the course of evolution are fundamentals that qualify biological systems as role models for innovation. This mainly bears the potential for environmentally optimized solutions and offers economic potentials as well as these are related when it comes to efficiency. If one looks at the interaction between sustainability and biology from the perspective of the assessment of sustainable development, the question arises as to what commonalities this can be built on. Although the differences are also reflected in the different definitions of function and resource in biology and technology, the ratio between function and resource seems very promising.

#### *2.4. Bio-Inspired Sustainability Assessment*

One fundamental question when it comes to deriving solutions from biological systems is if living nature actually does provide a fully comprehensive counterpart to what is described as sustainability. The underlying proposition is that if nature is chosen as direct role model, its solutions should have been created considering the same framework conditions that are applied for sustainability assessments. If not, any direct transfer from nature cannot be stated as to create sustainable solutions by itself and the overall concept of sustainability has to be accepted as to be at least partially independent from our understanding of nature and thus artificial. To derive a robust answer to this question, both the concept of sustainability and the fundamental principles of biological systems have to be investigated. However, there are two main inconsistencies when trying to directly derive biological systems to sustainability metrics. First, the prevailing sustainability concept is explicitly anthropocentric and thus does not relate to the nature of biological systems. This becomes apparent by several intrinsic properties of sustainability paradigms such as the explicit focus on mankind in the UN but also when applied as assessment framework [21,26,49]. The second inconsistency arises from the concept of social sustainability that is still under discussion [50,51].

With regard to these considerations sustainability as bio-inspired concept is defined through the interdependence of system functions and the therefore required depletion of resources. A system is defined as sustainable, when a specific set of functions is fulfilled while simultaneously ensuring the ongoing availability of resources in time. The concept depicted in Figure 2 shows the predominant transformation direction of the prevailing economic metabolism, which is to create social functionality by depleting environmental resources driven by economic facilities. The graph is meant to show the dynamics of this metabolism indicating a sustainable system when its shape is kept stable, ensuring an ongoing provision of resources for an ongoing creation of function. Economy is interpreted as means

to an end transforming resources into functions, enabling business models and thus facilitating the application of new products. Besides this main flow direction many processes are motivated otherwise and a general rule cannot be derived. The concept deals as a template to depict mechanisms of actions in terms of the fulfilment of functions including its intended and unintended effects. Nevertheless, it is quite uncommon for processes to dedicatedly create an environmental function or not utilize the environmental resources in a depleting manner. Furthermore, the predominant role of Economy is depicted as connecting element between Society and Environment. To keep this metabolism sustaining it is crucial not to exploit the resources to an extent that prohibits the ongoing fulfilment of societal functions. A sustainable system according to this scheme is achieved if the dynamic societal metabolism is maintained and is kept stable under dynamic conditions. It enhances the existing models through the integration of positive aspects and offers a shell like structure that is able to integrate different assessment methods.

**Figure 2.** Bio-inspired Sustainability Assessment depicted as conceptual structure, showing the three dimensions called society, economy and environment and the two aspects, namely function and resource as integral parts of the assessment. Societal functions are fulfilled through the transformation of environmental, economic and societal resources (own figure).

In the following, a quantifiable BiSA model is presented based on a six-fold structure including the three dimensions of sustainability for both intended and unintended aspects called functions and burdens:


• Social function: the primary design function restricted to the intended building physical function of the assessed system

The aspects are focused on the development of bio-inspired and bio-based products in the construction sector but are not restricted to these. The assessment aspects are chosen in terms of consistency and applicability with the main intension to provide feedback on decisions during product development. Its bio-inspiration lies primarily in its intrinsic setup inspired by biological systems and the overall structure of resource-function-relationships. The underlying physical model is created as a life cycle inventory system based on the GaBi database and supplemented by economic and country related information on process level. This model provides a consistent quantitative basis for 5 of 6 aspects. Moreover, it is applicable for both early and advanced development phases as it provides generic data but can include specific primary data as well. This facilitates a flexible structure and level of detail allowing the specification of the system as precisely as possible while still being able to estimate the coarsely defined aspects. The model has been implemented as semi-automatic tool to provide feedback for specific questions within the embedding project but has not been created as software for automatic application yet. However, an increased level of automation is envisaged for the next project phase.

## 2.4.1. Environmental Burden

The depletion of natural resources is the main source of human prosperity and as such of central relevance for the assessment of bio-inspired sustainability. Building upon the treatment of resources in biological systems, similarities can be identified mainly in terms of the dependency from physical sources. As mentioned before, all ecosystems are dependent on biotic (living) and abiotic (nonliving) environmental factors. In the course of the earth's history it has been shown, that especially after mass extinction source–sink dynamics influence the variation in habitat quality affecting biodiversity, population growth and number of organisms [52]. Even though it is repeatedly claimed that nature does not produce any waste, this is not the case on closer inspection. For example, most of the crude oil produced today originates from dead marine organisms, buried underneath sedimentary rocks. The deposits are therefore nothing more than landfills for fossilized organic materials or in other words natural waste. Thus, the use of fossil fuels such as coal, natural gas and crude oil hydrate is associated with respective CO2 emissions.

The environmental burden is calculated by LCA, using a single point value based on a selection of characterization methods. The steps that are to be performed when conducting an LCA according to the pertinent standards cannot be fully applied due to the interactive nature of the BiSA assessment [53–55]. Nevertheless, the functional unit, the system boundaries and consistent specifications of the applied calculation principles such as allocation and cut-off criteria have to be stated. Furthermore, the life cycle inventory models for each of the assessed systems and variants have to be created. This model was created using the Software GaBi 8.2 (thinkstep, Leinfelden-Echterdingen, Germany), which is one of the world's leading LCA software providers and the GaBi SP 34 (thinkstep, Leinfelden-Echterdingen, Germany) database, providing more than 10,000 environmental profiles as a modeling basis [56]. For the assessment of the environmental burden it is possible to directly derive indicators and weighting schemes from the investigation of natural systems. This is, on a quite abstract level, the transfer of biophysical system stability as role model on global scale. A quantifiable concept to address the issue of global biophysical system stability was introduced by Rockström et al. in 2009 and has been since then further refined and continuously updated [5,57]. It identifies the main biophysical systems that are threatened by human activities and provides a framework to quantify planetary boundaries that should not be exceeded by mankind on global scale if the global ecosphere is to be kept intact. The concept provides an approach to address the manifold depletion of nature by man and is subject to the ongoing development to include new insights of scientific discourse. The planetary boundaries have been transferred to deal as life cycle assessment weighting scheme by several authors [58–62]. The approach proposed by Sala et al. is chosen and adapted using the presented values for distance-to-target normalization due to the fact that a single value for the environmental burden is required [59,63]. In addition to this approach, the areas of protection are differentiated in sink and source related categories. Sink related categories are summarized as global biophysical system stability and correlate directly to the biological system they depict. As the ongoing availability of both biotic and abiotic resources is crucial for the metabolism stated above, the depletion of resources is classified in the area of global resource stock. Strictly seen the availability of resources for mankind is mainly underground and does not directly contribute to the stability of the biophysical system but is crucial for the concept of scarce resource utilization prevailing in biological systems. Due to significant methodological improvements since the publication of the distance to target values, the abiotic depletion potential is covered by the anthropogenic stock extended abiotic depletion potential (AADP) model and for land use the biotic production indicator as published in LANCA 2.0 (Fraunhofer IBP, Stuttgart, Germany) is applied, which is freely available in the updated version [64–67]. Nevertheless, the assessment of impacts on the global resource stock still bears strong potentials for improvement, especially in terms of temporal and spatial differentiation. Table 1 shows the considered categories and the according normalization factors as well as the chosen methods to quantify the impacts of each category. The results of each impact category over the whole life cycle are multiplied with the normalization factor based on the planetary boundary concept. The normalized values are then added, creating a single point that can be directly compared to the one of the reference system, which is created similarly.


**Table 1.** Categories and weighting structure for the assessment of the environmental burden derived from [59].
