Biomimetic bio-inspired biomorph sustainable? An attempt to classify and clarify biology-derived technical developments

Over the past few years numerous and diverse terms have appeared, all of which describe the inspiratory flow of ideas from nature to technical solutions: biomimetics, bionics, bio-inspiration, biomimicry, biomimetic promise, nature-based solutions and many more. Given the quantity of academic and popular literature on this subject and with respect to the huge number of authors very different and sometimes contrary explanations can be found. The situation is further complicated because the terms have appeared from various scientific disciplines, whereby each discipline has its own language, way of thinking and methodological approaches. Another source of confusion is the translation of the terms into other languages. A closer look, however, reveals that differences can indeed be found with regard to, for example, the selected role model, the quality of knowledge transfer, the respective function and the developmental status. Here, we have to take into consideration that besides the descriptive content each term also possesses normative and emotional aspects. Especially noteworthy is the assumption that living nature may serve as concept generator to meet the challenges of sustainable development in terms of social, environmental and economic goals. Thus, to rationalize and refine the discussion, we need to define these key terms precisely. Clarifying the meaning of terms, especially in an interdisciplinary field of research and development, might help to eliminate misunderstandings in daily work.

In 1957, the American engineer and physicist Otto Herbert Schmitt coined the term 'biomimetics' as a biological approach to engineering in contrast to 'biophysics', which describes the engineering/physical approach to biology [39, 56, 66]. The term 'biomimetics' first appeared in 1974 in Webster's Dictionary defined as the study of the formation, structure, or function of biological materials, mechanisms and processes in order to synthesize artificial products that mimic natural ones [66]. This description can be found unchanged today in Merriem-Webster's online Dictionary [82]. In 1958, the American medical doctor Jack E. Steele introduced the word 'bionics' in terms of copying functions from nature during his time at the Aerospace Medical Research Lab. Officially, 'bionics' was used in 1960 as the title of a meeting at the Wright-Patterson Air Force Base in Dayton [66]. After his retirement, Steele studied engineering and, in 1977, wrote a master's thesis with the title 'Bionic Design of Intelligent Systems' [83]. In contrast to 'biomimetics' and 'bionics', the term 'bio-inspiration' is more encompassing. The chemist Whitesides [81] defined 'bio-inspiration' as 'using phenomena in biology to stimulate research in non-biological science and technology'. He further stated that abstracting simplified versions of a living creature means taking inspiration from its capabilities and mimicking some of its functionality by using simplified and probably different mechanisms [81]. The American forestry scientist Janine M. Benyus popularized the term 'biomimicry' by publishing the book 'Biomimicry—Innovation Inspired by Nature' [11] and by founding the 'Biomimicry Institute'. In general 'biomimicry' means learning from natural world by imitating or taking inspiration from nature's designs and processes to solve human problems. Furthermore, there is a particular emphasis on sustainable design ideas emulated from nature. 'Biomimicry uses an ecological standard to judge the 'rightness' of our innovations. After 3.8 billion years of evolution, nature has learned: What works. What is appropriate. What lasts.' [11]. The German biologist Arnim von Gleich has coined the term 'biomimetic promise', which indicates that, precisely because of the inspiratory flow from biology to technical products, biomimetic solutions have the specific potential to contribute to sustainable technological development [76–79]. The European Commission, the International Union for Conservation of Nature  and other political organisations have claimed the so-called 'nature-based solutions'. An explanation is written on the official EU-Website [84]: 'In this context, we define nature-based solutions to societal challenges as solutions that are inspired or supported by nature, which are cost-effective, simultaneously provide environmental, social and economic benefits and help build resilience...' This definition is in the context of the EU R&I agenda for Nature-Based Solutions and Re-Naturing Cities and therefore covers a broad thematic spectrum of societal challenges to be solved: 'These nature-based solutions provide sustainable, cost-effective, multi-purpose and flexible alternatives for various objectives. Working with nature, rather than against it, can further pave the way towards a more resource efficient, competitive and greener economy. It can also help to create new jobs and economic growth, through the manufacture and delivery of new products and services, which enhance the natural capital rather than deplete it' [84].

Against the background of a wide range of terms with partly unclear, contradictory or synonymous meanings and the request for methodological approaches how to learn from biology for technology, the Association of German Engineers (VDI) and International Organization of Standardization (ISO) have developed technical rules and standards with the intention of raising the profile of biomimetics. Among others, the guidelines set target on the description of state of the art, harmonisation of terminology and technical language, and assistance in the practical realisation of technical applications. Therefore, they can be seen as a valuable contribution to the definition and developmental approach of bio-derived solutions. Recently, a total of nine VDI technical rules, available in German and English, have been worked out by experts focusing on various areas of application: concept and strategy [67], biomimetic surfaces [68], biomimetic robots [69], biomimetic materials, structures and components [70], biomimetic optimization [71–73], biomimetic information processing [74] and basic principles of architecture, civil engineering and industrial design in the framework of biomimetics [75]. In this context, various definitions are available, such as that for 'biomimetics' as being a combination of biology and technology with the goal of solving technical problems through the abstraction, transfer and application of knowledge gained from biological models [67]. Since 2015, ISO standards in English and French provide several definitions and practical realisation concepts [33, 34]. In the ISO standard [33] 'biomimetics' is defined as an '...interdisciplinary cooperation of biology and technology or other fields of innovation with the goal of solving practical problems through the function analysis of biological systems, their abstraction into models, and the transfer into and application of these models to the solution.', whereas 'bionics' is describes as a '...technical discipline that seeks to replicate, increase, or replace biological functions by their electronic and/or mechanical equivalents.' Moreover, the terms 'biomimicry' and 'biomimetism' are both defined as '...philosophy and interdisciplinary design approaches taking nature as a model to meet the challenges of sustainable development (social, environmental, and economic)' [33]. Further technical rules, guidelines and standards addressing additional fields of development are planned, if experts from interested circles are willing to work together in an honorary capacity. A commenting procedure guarantees the opportunity to exert influence by interested public. Because the guidelines are drawn up by numerous international experts working in the respective field, they are of upmost importance for the description of the status quo.

Many excellent reviews are available on bio-inspiration and biomimetics focusing on specific topics such as publications and topics [39], the analysis system TRIZ [66], selected bio-inspired solutions in robotics and prosthetics [7] or bio-inspired soft robotics [55], and biological and biomimetic materials [1, 80]. In addition, over recent years important contributions have to be mentioned especially in the field of biologically inspired design (BID) with focus on computational methods and tools [22, 45], healthcare and medicine [38, 65] and the building sector together with architecture [27, 35, 48, 51]. An upcoming topic is the focus on future biomimetic contributions to global challenges [20] and the systematically generation of sustainable products using biomimetics [76–79], or biomimicry as a sustainable design tool [11, 54] and in the framework of bio-inspired design using biological analogies [47]. Moreover, biomimicry's potential role in addressing sustainable building engineering [14, 49, 50] and sustainable architecture [44] has to be mentioned. However, this enumeration of topics and publications could be further complemented because inspiration from living nature has influence on basic and applied sciences in a variety of topics.

The intension of this study, carried out in the framework of the currently running project 'The biomimetic promise: natural solutions as concept generators for sustainable technology development in the construction sector' as a subproject of the CRC-Transregio 141 'Biological Design and Integrative Structures—Analysis, Simulation and Implementation in Architecture', is a contribution to rationalize and refine the discussion about bio-derived and technology-derived developments by taking into account that each of them possesses descriptive, normative and emotional aspects. In this context specific attention has been paid to the role of sustainability.

The first objective of this study is the focus on the descriptive aspect by the provision of a straightforward classification tool to clearly describe, distinguish and identify biology-derived and technology-derived developments. This was achieved by the compilation of a dataset of relevant developments characterized by descriptive attributes with several values. The second objective is a critical reflection of the normative and emotional aspects of bio-derived products as to whether they promise an additional extraordinary quality in terms of sustainability potential or emotional perception. Third, a 'bone like' ceiling is chosen as a case study from the building sector to critically discus the descriptive, normative and emotional aspects.

In the following, we will briefly point out the essential facts of selected biology-derived and technology-derived developments to describe them in the best possible manner and to make them distinguishable from the others. This is supported by the description of well-known and strong examples. The described examples and many more can be found in the dataset (supplementary appendix 1, table S1).

In total 17 classes are unambiguously defined, whereas four classes belong to the technology-derived developments (technical product, technical product developed in parallel to biology, technical product with natural appearance, and algorithm) and 13 classes belong to the bio-derived developments (biotechnological product, bio-inspired product, biomimetic algorithm, biomimetically optimised product, biomorphic product, functional biomimetic product, structural biomimetic product, functional biomimetic and biomorphic product, structural biomimetic and biomorphic product, functional biomimetic and biotechnological product, structural biomimetic and biotechnological product, functional biomimetic, biomorphic and biotechnological product, structural biomimetic, biomorphic and biotechnological product). There are nine descriptive attributes and their respective values (model, direct use of living organisms, bio-inspiration, application of biomimetic algorithm, transfer of morphology, transfer of principle, natural appearance, same function, and status (supplementary appendix 1)).

In the framework of supervised learning a binary ('dichotomous') decision tree (figure 2) was generated by the ID3 algorithm [53] using the whole dataset as training set (supplementary appendix 1 and 2). The decision tree is a 'flowchart-like' classifier which originates in a root node proceeding down to its leaf nodes (=class names) via internal nodes (=attribute-based tests) and branches (=possible outcomes). The underlying ID3 algorithm starts with the original data set and calculates the entropy and the information gain of all attributes (equations (1) and (2) in supplementary appendix 2). The attribute with the smallest entropy or largest information gain (=splitting function) is the root node and splits the original dataset into subsets. In terms of a recurse approach ID3 splits the respective subsets using remaining attributes.

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In order to evaluate the learning procedure, 5 different decision trees (classifiers) were evaluated using the 5-fold cross validation method [42]. Overall, an accuracy of 90.0% was achieved to predict the classes of the test examples correctly. For example, compared to a baseline algorithm which always predicts the most common class of the training set (17.1% accuracy in the 5-fold validation) the used learning procedure performs well. Thus, we can conclude that the provided decision tree using the whole dataset as training set is an accurate model to predict new (unknown) instances. In this context, the provided decision tree is a handy and accurate tool that allows a rapid identification which class a given development belongs to. In biology and medicine such printed or computer-aided identification keys are very common to identify biological entities and diseases, respectively.

3.1.1. Bio-inspired developments

3.1.2. Biomimetic (bionic) developments

In 2014, experts published VDI-Guideline 6220 (2012) [67] dealing with the concept and strategy of 'biomimetics' and differences between biomimetics and conventional methods/products and provided the following definition: 'Biomimetics combines the disciplines of biology and technology with the goal of solving technical problems through the abstraction, transfer, and application of knowledge gained from biological models. Biological models in the sense of this definition are biological processes, materials, structures, functions, organisms, and principles of success as well as the process of evolution itself'. This definition may also hold true for 'biotechnology' and therefore has to be completed by the statement that, in biomimetics, living organisms are exclusively indirectly (i.e. as concept generator) involved in the production of biomimetic products and the development of biomimetic process engineering.

Two biomimetic approaches are well established, which differ from each other at the starting point for development: starting with a question from biology leads to the biology push process = bottom-up approach (see also figure 3), whereas starting with a question from engineering sciences leads to the technology pull process = top-down approach (see also figure 4). Both approaches result in a 'biomimetic development', i.e. a technical application developed on the basis of the transfer of knowledge (operating principles, manufacturing processes) gained from one or several biological concept generators [60, 67].

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During the implementation of functional principles and manufacturing processes found in biological solutions into technical developments ('biomimetics'), new findings arise that, in turn, contribute to a deeper insight into the functioning of the biological concept generators ('reverse biomimetics'). This requires a re-investigation of the biological samples to achieve a continuous improvement of the technical models of the engineers. In the sense of the heuristic spiral consisting of 'technical biology' (e.g. functional morphology and biomechanics of the biological concept generator), 'biomimetics' and 'reverse biomimetics', the process leads to a considerable gain in understanding of both the biological and the technical systems and to a refinement of the developed technical applications.

Goel et al [23] present comparable approaches in BID. The designer's starting point of the solutions-based analogical process is a biological source of interest, followed by the extraction of principles and the subsequent search for human problems which can be solved by the application of the principle. The problem-driven analogical process starts with a problem identified by a designer, goes on with a phase of biological solution search, and the extraction and application of a principle.

A well-known biomimetic product is the façade paint Lotusan® equipped with the self-cleaning Lotus-Effect® developed on the basis of the self-cleaning function found e.g. in the leaves of Nelumbo nucifera [9]. Because the biomimetic application has the same function as the biological model, it is called a 'functional biomimetic product'. The development of self-cleaning surfaces bearing the Lotus-Effect® trademark is a typical example of a biology push process (bottom-up approach) in biomimetics [68]. Figure 3 shows a simplified flow chart starting with basic biological research on plant surfaces showing self-cleaning effects. The transferred principle is the minimization of the contact area between adhesives (e.g. dirt particles) and substrate (e.g. surface) based on a hierarchically micro- and nanostructured and water-repellent (superhydrophobic) surface in combination with the surface tension of water [68]. Superhydrophobicity of a surface can be measured by the contact angle between the liquid and the solid surface, which in this case, by definition, has to be larger than 150°. The contact angle of a water drop on a lotus leaf surface is approximately 147°. Lotusan® paint has been on the market since 1999 and has been used for millions of square meters of façade throughout the world [85]. However, the self-cleaning effect in biology and technology occurs under the restriction that water droplets are available to wash off the dirt.

If the biological model and the biomimetic application do not have the same function, as for example in the case of the façade-shading system Flectofin®, they are called 'structural biomimetic products'. Flectofin® is a hinge-less flapping shading system for architectural constructions derived from the deformation principle found in the bird-of-paradise (Strelitzia reginae) flower during pollination by birds [41]. Figure 4 shows the stepwise development of Flectofin® in terms of a top-down approach (technology pull process). State-of-technology constructions often consist of separate stiff elements connected with hinges, whereas local hinges are error-prone and require regular and cost-intensive maintenance. Consequently, the starting point of the Flectofin® development was the challenge to find a hinge-less kinematics solution of deployable systems for architectural purposes. The concept generator was the flapping mechanism of the perch of the bird-of-paradise flower which is triggered by the weight of the pollinating bird in order to reach the nectar at the base of the flower. Analyses showed that the model's operating principle is a lateral-torsional buckling, which—in engineering—is well-known as an undesirable failure mode. After several levels of abstraction, the kinetic structure could be simulated with finite element modelling. After optimization of both material and shape of Flectofin®'s laminae and backbone, solutions with a single or double wing have been made available [41]. At EXPO 2012 in Yeosu (South-Korea), the kinematic façade-shading system of the Thematic Pavilion was achieved with modified Flectofin® lamellae [61].

'Biomimetic algorithms' are characterized by biological evolution and processes being available as evolutionary algorithms and complete mathematical formulations in order to optimize technical products and process engineering [71]. Furthermore, computer-based biomimetic optimization tools, such as computer-aided optimization (CAO) derived from the growth rules of trees and the soft kill option (SKO) following the growth rules of bones, modify the shape and topology of components and homogenize the stresses at the component's surface. The subsequent application of these optimization algorithms from the plant kingdom and animal kingdom is a completely unique combination that does not even exist in living nature [72]. The biomimetic design tool ELiSE 3D draws on the shell or skeleton design of marine organisms (e.g. diatoms and radiolarian) to improve large scale technical lightweight structures [73].

The increasing availability of biomimetic algorithms leads to numerous 'biomimetically optimised products'. Famous example are the SKO-optimised frame of the Mercedes-Benz bionic car [89] and the CAO-optimised orthopaedic screw which can withstand 5 million load cycles in contrast to the non-optimised one which fatigue cracks after only 220 000 load cycles [67].

In addition to biomimetics, one often finds the term 'bionics'. The latter is frequently used as a synonym for biomimetics in terms of learning from biology for technical innovations. Specific uses of 'bionics' in medicine or robotics might have their origins in the original focus on electronics. An additional role may have been played by the famous American television series 'The Six Million Dollar Man' and its spin-off 'The Bionic Woman' in which human beings possess advanced bionic prosthetics and implants to confer artificially enhanced skills. Strong examples for the diverse use of the word 'bionics' are the following societies, networks and journals. The 'International Society of Bionic Engineering' and its 'Journal of Bionic Engineering', both are located in China. 'BIOKON international—The Biomimetics Association' is a European network and the 'Competence Network Biomimetics' is located in Germany. Over the last few years, bio-inspired and biomimetic research has shown a large increase and, consequently, journals such as 'Bioinspiration & Biomimetics' or book series such as 'Biologically-Inspired Systems' [5, 16, 24, 29, 64] or 'Biomimetics: Nature-Based Innovation' [8] are much needed.

3.1.3. Biomorphic developments

3.1.4. Biotechnological developments

3.1.5. Technical developments with natural appearance

3.1.6. Technical developments in parallel to biology

Knowledge transfer from biology to technology takes place in a variety of precisely describable ways leading to several classes of bio-derived solutions, in general, and bio-inspired developments, in particular (see chapter 3.1). In addition, learning from living nature is linked with the desire and hope of learning from biological solutions that have been optimized over the course of 3.8 billion years of evolution. Thus, bio-inspired solutions ought to come together with the promise of exceptional quality in terms of a contribution to sustainable development by fitting into natural cycles, lower levels of risk, fault tolerance, environmental compatibility, energy and material efficiency, and many others [11, 47, 49, 50, 76–79]. Time and again, in popular and scientific literature the reader comes across with general statements about bio-inspired developments from a normative and/or emotional perspective. Here, only some selected statements should be mentioned. Benyus [11] summarize at the beginning of her book 'Innovation Inspired by Nature': 'Nature as measure. Biomimicry uses an ecological standard to judge the 'rightness' of our innovations...'. Mazzoleni [44] writes in the introduction of her book 'Architecture follows Nature': 'Sustainable design is a way for us to begin to harmonize man-made structures with the natural environment. Biomimicry can help us change our perception by looking to nature as a source of functional and aesthetic solutions rather than as a source of obstacles to overcome'. In summary, to rationalize and refine the discussion about bio-derived developments, descriptive, normative and emotional aspects have to be taken into consideration (table 1).

During human cultural history, the vision of sustainability has been redefined continuously in order to give orientation and to act as a guidance and motivation [26]. Despite of this long history, it is still an elusive term, questions of 'sustainability' do not have a simple 'yes/no' answer [6], and assessments of 'sustainability' cannot fall to zero. Many attempts have been made to measure sustainability and therefore numerous assessment methods are recently available, but the choice is left to the practitioner. Most frameworks suggest an assessment of the three pillars separately, enhancing the method previously well established for ecological assessment—the Life Cycle Assessment (LCA)—to all pillars [18, 32, 36, 86]. Hence, Life Cycle Sustainability Assessment  is defined as a combined assessment of Life Cycle Costs, Social Life Cycle Assessment and Environmental Life Cycle Assessment. Meanwhile several methods such as 'pro Suite' [86], 'PROSA' [25, 87] and 'Calcas' [88] have been developed including these aspects. In the context of sustainable products inspired by biology, the method of sustainability assessment is not specified. Rather, it is of upmost interest to clarify whether the contribution to sustainability stands on its own or only applies in relation to a (conventional) reference system. In case of using a comparative approach, the sustainability assessment striven for ought to be identical.

Recently, the term 'sustainable development' is preferred, because it refers to growth and progress. Nevertheless, 'sustainability' and 'sustainable development' formulate an anthropocentric target state of inter- and intra-generational equity by accomplishing environmental (or ecological), economic and social aims. Bearing in mind that, in living nature, both anthropocentric perspective and teleological thinking and acting are unknown [3, 58], we need to clarify in what sense natural concept generators can serve as templates for sustainable developments. Particularly interesting in the nature versus culture context is the question as to whether the exceptional quality of 'bio-derived solutions' is an implicit and/or explicit contribution. The implicit contribution by means of a by-catch through various types of knowledge transfer from biological models to biomimetic developments was coined by von Gleich as 'biomimetic promise' [76–79]. This questionable effect might be intensified by a conscious transfer of knowledge in the context of environmental, economic and social sustainability. In the framework of BID the generation of sustainable design is explicitly envisaged [22, 47]. In the context of ecosystem biomimicry the potential of sustainable building engineering are explicitly addressed [11, 49, 50].

O'Rourke and Seepersad [47] focus on sustainability by means of energy- and materials-efficient designs using biological analogies during ideation. Apart from the fact, that the terms 'efficiency' and 'analogy' mean different things to the researchers (see also [27]), they found further obstacles why not all BIDs are efficient. Even if an efficient-related role model was chosen, the BID is not efficient because, first, it is too difficult to abstract the traits from biology and incorporate them into technical design, second, the environments and constraints of biological and technical systems are too different, and third, over the product's life cycle the efficiency advantage may be countered by unintended effects. Raibeck et al [46] could illustrate this effect within a life cycle inventory study on lotus-inspired self-cleaning surfaces. Comparing the environmental performance of bio-inspired surfaces with standard industrial cleaning techniques shows apparent benefits during use phase, but production burdens can outweigh them.

Coming back to the attributes and values of the dataset and the provided decision tree (supplementary appendices 1, 2, figure 2), it is noticeable that at first glance sustainability-related attributes or values do not show up. A reason could be that this is the consequence of a so-called 'change of categories'. Having a closer look on the environmental impact categories being measured within a sustainability assessment, we find for instance 'global warming potential' (greenhouse gas emission like CO₂, see also chapter 2), 'primary energy demand' (total energy input of all renewable and not demand renewable forms of energy), and 'land use' (amount of land needed to fulfil the function of an investigated product system). These sustainability-related categories can be addressed by the transfer of efficiency-related knowledge transfer (operating principle, morphology, and application of biomimetic algorithms) from biology to technology. A well-known example is the Mercedes-Benz bionic car inspired by several features of the yellow boxfish Ostracion cubicus [47, 89]. Transferring the boxfish's ideal aerodynamic morphology (drag coefficient of the true-to-the-original model cd = 0.06) results in an outstanding aerodynamic efficiency of the concept car (drag coefficient cd = 0.19). In addition, the construction principle of angular, cube-like body with its bony armour was transferred to the car in terms of safety and comfort in order to accommodate four people and their luggage. By applying the biomimetic algorithm of Soft-Skill-Option to the entire body-in-white structure, a biomimetically optimised lightweight construction has been generated. While the weight is reduced by some 30%, the high levels of stability, crashworthiness and driving dynamics remain unchanged. As mentioned by [47] it is important to verify that all features remain efficient in the design context. This is even more astonishing because in the field of aerodynamics it is not easy to scale-up from a 45 cm long fish swimming under water to a 4.24 m long car driving in air. An empirical study carried out by O'Rourke and Seepersad [47] show that compared to conventional car shapes the Mercedes-Benz bionic car shape shows environmental impact advantage. The same refers to the biomimetically optimised frame in comparison to standard car frames. Even more informative as a point of comparison would have been a quantitative analysis. Such a quantitative comparison of biomimetic and conventional solutions from the building sector is presented in detail in chapter 4.

Apart from the descriptive and normative content, our personal and emotional perceptions of biology and technology are important for our emotional attitude to technical and bio-derived applications. The fascination of natural systems and the reverence for life are transferred to bio-inspired products and thus release positive emotions. If the term biomimetics is used in combination with other words, emotions may change because of individual perception, knowledge and experience. An impressive example could be 'nanobiomimetics', which describes the development of biomimetic products at the nano level. Because, in the field of nanotechnology, very little is known about the effects of exposure to nanoparticles on humans and the environment, the general population is anxious and uneasy. These negative thoughts and feelings might then be transferred onto nanobiomimetic applications in particular and/or even onto all biomimetic products.

Even technical products with natural appearance are associated with positive or negative feelings [59]. Especially in the context of architecture, '... biomorphism can add further meaning than would be achieved from a purely technical use of biomimicry... Architecture should always have an emotional dimension—it should touch the spirit, it should be uplifting and it should celebrate the age in which it was created' [48]. Moreover, the shape and design of buildings can generate associations finally leading to common used nicknames. The Beijing National Stadium has been nicknamed 'Bird's Nest' [43] and the City Hall of London got the nickname 'The Armadillo' [90]. The emotional perception is usually a crucial aspect for the market potential of innovative products. A positive connotation of bio-inspired products might enable, or at least enhance, their market introduction.

A variety of sustainability assessments also measure the user's perception of a product. In the framework of 'PROSA', the symbolic utility of products can be qualitatively assessed by using a checklist of attributes with respect to, for example, external appearance, specific design, prestige, feelings, moods and hopes [25, 87].

The biological concept generator for the ribbed slab of the old zoology lecture hall at the University of Freiburg was the morphological-anatomical structure of hollow bones and the course of bone trabeculae following the stress trajectories in the direction of the main loading condition. This knowledge transfer resulted in a slab with isostatic ribs made by reinforced concrete. Bones and the ribbed slab are lightweight constructions and 'the cancellous bones trabeculae of the femur have the function as the ribs in the lecture hall slab' ([30], translation from [2]). With respect to the presented decision tree (figure 2), the lecture hall's ribbed slab can be classified as a 'functional biomimetic and biomorphic product'.

The ribbed slab has a diameter of 23.86 m and is mounted on a hollow reinforced concrete pillar and two curved wall supports. This specific structural condition leads to a specific geometrical arrangement of the isostatic ribs (figure 5), which guarantees the most possible stability together with the lowest possible material input [30, 31].

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A first qualitative sustainability assessment on the basis of the integrative sustainability concept of the Helmholtz Association [37] was carried out on the ribbed slab in comparison with a massive limp reinforced slab available back in 1968, when the lecture hall was built. Because a massive slab is definitively not a lightweight construction and additional material and reinforcement would be necessary for the three ceiling supports, a massive slab seems not to be an adequate alternative for analysis in detail. For this very reason, the biomimetic slab gains advantage because of the lower material input in the fields of emission protection, resource efficiency and reduction of environmental impact [4]. Furthermore, a quantitative comparative sustainability assessment on the basis of 'PROSA' [25, 87] was carried out. In this case, two conventional lightweight alternatives in terms of a hollow article slab and a pre-stressed flat slab available in 2010 were chosen. No fundamental differences between the three slab constructions could be found with respect to Social Life Cycle Assessment. Concerning Life Cycle Assessment, the three alternative slabs lay in the same range with the exception of the category 'land use', whereby the impacts of the ribbed slab were 52 times higher than that of the alternatives. In the field of Life Cycle Costing, the category 'overall material costs' was in the similar range, whereas the 'building costs' of the ribbed slab was 2.2 times more expensive than the other options. The differences found in the impact categories 'land use' and 'building costs' were the result of the work-intensive and time-consuming production of the wooden formwork required because of the very complex geometry and the use of wood for the formwork. The chosen land use assessment method is a solely quantitative one, not taking into consideration the way in which the land is utilised or the land use type influences land-use-related ecosystem services [10, 12, 91]. A closer look shows that the ribbed slab has a 3.6-fold and 4.4-fold higher beneficiation because of the combustion of the wooden formwork compared with the two alternatives. In summary, the 'biomimetic promise' can be said to be retained for the bone-like slab, given by a positive validation of the biomimetic check and subsequent sustainability assessment [2]. However, this result makes no assumptions about the conscious knowledge transfer of sustainability parameters from the biological role model to the technological solution. As far as we know from the literature, in the 1960s, sustainability aspects did not explicitly play a role during the planning and building phases of the ribbed slab of the lecture hall. Nowadays, sustainability aspects play a particularly significant role in the construction sector [21].

In the framework of both the integrative sustainability concept of the Helmholtz Association [37] and 'PROSA' [25, 87], the exceptional geometry of the ribbed slab can be considered as an additional architectural benefit from an aesthetic perspective (see [59]). Furthermore, that the architect consciously used the inner bone structure as a biological role model for the construction of the ceiling of the zoology lecture hall in which biologists were trained has a strong symbolic character [2, 4].