A review on bioinspired strategies for an energy-efficient built environment

Energy efficiency is an emerging challenge for the built environment due to rapid urbanisation. The built environment impacts human health and comfort, along with generating greenhouse gas emissions that deteriorate the climate. The development of bioinspired strategies is an evolving topic and is recently gaining popularity for effectuation in the built environment. At present, the existing reviews are conducted primarily to cover niche themes, which leads to a lack of holistic and multidisciplinary overview. Therefore, a detailed analysis is presented in this study to understand the current trends and research gaps in achieving an energy-efficient built environment. An organised assessment of the performance and effectuation of the bioinspired strategies is carried out using systematic analysis (in four steps). Bibliometric analysis is also performed based on the output generated from multiple searches, which are divided into two scales: Building and Urban, resulting in 906 and 146 documents, respectively. It is observed that the existing research focuses on material-related (structural analysis and development) topics primarily, and limited emphasis is devoted to thermal analysis. Furthermore, studies on achieving high solar reflectance envelope surfaces and integrated thermophysical properties are found to be limited. This study also observed that the form and ecosystem level of bioinspiration needs additional focus. Furthermore, the energy efficiency of strategies in heating dominant climates needs to be adequately covered. The technology readiness level of the bioinspired strategies is also analysed.


Introduction
Climate change is causing enormous risks to the environment and, subsequently, to human health.The entrapment and re-radiation of the solar heat by the urban structure result in significantly higher temperatures in densely populated urban areas.The rapid expansion of the built environment (BE), which generates anthropogenic heat, significantly increases the urban air temperature (5 • -15 • C) in comparison to its adjacent rural areas, resulting in the urban heat island (UHI) effect [1].Around 60% of urban surfaces are covered with humanmade massive heat absorbent surfaces (building envelopes, roads, and pavements), a major cause of the UHI effect along with low evapotranspiration, poor wind circulation, and air pollution [2].Even in locations with high latitudes, the UHI effect has an adverse effect on outdoor thermal comfort, resulting in many heat-related health conditions [3].Furthermore, hundreds of people die each year due to extremely cold weather, with some becoming increasingly linked to heat waves [4].The adverse impact also extends to degraded air quality, additional cooling energy consumption, along with collateral social and economic costs [5].Global carbon emissions have been rising again after a record decline in 2020 (due to the COVID-19) to a 4.8% increase in 2021 (34.9 GtCO 2 ) and a 1.5% increase in 2022 (36.1 GtCO 2 ) [6,7].In contrast, the carbon emission in the year 2000 was 24.69 GtCO 2 [8].All these are accentuated further by high energy demand and carbon emissions in the BE.The BE is a material, architectural, and cultural product of human endeavour that incorporates physical components and energy into forms for dwelling and conducting various activities.The term 'built environment' generally refers to a larger physical setting that covers buildings and other surrounding spaces such as pavements, roads, hard and soft landscapes, parks, and transportation networks [9].Buildings, which form a majority of BE, consume around 40% of the overall primary energy consumption [10] and contribute to an enormous amount (~39%) of carbon emissions [8].This percentage is even higher (~57%) for the cooling-intensive tropical climate [11].Overall, the BE is solely responsible for around 50% of total carbon emissions across the United Kingdom (UK) [12].Furthermore, the urban footprint is expected to grow by 0.6 to 1.3 million km 2 by 2050 as compared to 2015, putting half of the future urban dwellers at risk of heat [13].
Multiple factors, such as weather conditions, geographical location, microclimate, building design, urban planning, and construction materials, regulate energy efficiency in the BE [14][15][16].Construction materials, which comprise thermo-physical (thermal conductivity, specific heat capacity, and density) and surface radiative (solar reflectance, thermal emittance) properties, are important in this context [17].For instance, the choice of envelope materials can impact changes in building energy efficiency [18,19] as well as microclimate [20].Heat transfer occurs at different levels within the BE, and to achieve energy efficiency, it is important to optimise the same depending on the weather conditions.The main components of heat transfer mechanisms are radiation (solar heat gain and thermal emission), conduction (heat exchange through materials), and convection (heat exchange with surrounding air) [21].Numerous biological systems effectively manage their thermal system through the heat exchange phenomenon and maintain their internal body/core temperature for their vital functioning, which can be a source of inspiration for developing energyefficient BE.
Biological systems have learned to be efficient, innovative, and sustainable over 3.8 billion years of evolution by identifying what works, what is acceptable, and what lasts [22].Bioinspiration is a trend popular in many fields, such as chemistry, robotic navigation, aerodynamics, medicine, and material science, and it is slowly getting attention in the BE sector [23].A few bioinspired solutions have already been effectively integrated into the BE [24,25], even though their widespread and practical implementation in architectural designs remains largely unreached [26,27].Over the last two decades, there has been a steady increase in research based on bioinspired strategies (Bio-S) [28].There have been previous literature reviews of published documents based on various classifications and themes [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45].However, these reviews lack a holistic and multidisciplinary approach and cover individual niche topics such as cooling strategies [28], energy efficient materials and structures [29][30][31][32], materials [33][34][35][36], envelopes [37][38][39], energy absorption application [40,41], material application in civil structure [42], simulation framework [43], algorithms [44] bioarchitecture [45], and transportation systems [46].In the context of BE, a holistic and multidisciplinary approach is crucial for achieving overall energy efficiency.Furthermore, the existing literature reviews mostly lack in-depth analysis of Bio-S for BE that is centred on their performance outcomes and presents an overview of the research focus.Therefore, the present review aims at examining the available literature conducted in the past three decades on Bio-S for energy-efficient BE.

Methodology
This review is conducted in two parts: systematic and bibliometric analysis.The SCOPUS database is used for the search as it contains a large number of works relating to technological topics as compared to databases such as the Web of Science [47].Table 1 presents the inclusion and exclusion criteria of the papers summated for this study.The systematic bibliographic search query is carefully selected to include all common words attributed to finding possible literature for the identified topic.Initially, two search queries were chosen to reflect the reviews' main topic-BE.All possible searches and synonyms of "Bioinspired" were considered for the search queries along with "Building" and "Urban" using Boolean operators -AND / OR / AND NOT."Building" here is in reference to individual structures and all the systems (façade system, heating, cooling, and ventilation systems) used within them.Similarly, "Urban" is in reference to the spatial scale of the microscale (neighbourhood scale) or mesoscale (city scale) [48,49].Table 2 consists of the search queries for Building Scale and Urban Scale.Fig. 1 shows the distribution of search results by each subject area; and it can be observed that the results are from a wide range of subject matter.
The search resulted in 906 and 146 publications for Building and Urban, respectively.However, a separate search was also conducted with AND Boolean operators between the words "Bioinspired" (and its synonyms), "Building", and "Urban" to search specifically for those documents which included both Building as well as Urban.These would have been missed by the search query due to the use of AND NOT Boolean operators.The publications were screened to 484 and 111 on the Building Scale and Urban Scale.An additional 96 publications were segregated after reading the title into the Building and Urban Scale list of documents.The publications were further filtered into 136 and 86 numbers after reading the title and abstract and further applying the inclusion and exclusion criteria.An additional 47 publications were also added through the supplementary hand.An additional search conducted to support the systematic analysis is presented in Section 3. The

Table 1
Inclusion and Exclusion criteria and its step wise application.

Inclusion criteria
Peer reviewed journal ✓ Selection of relevant subject areas ✓ Written in English language ✓ Title and abstract relevant to Bio-S in BE ✓

Exclusion criteria
Any journal published after March 2023 ✓ Conference publication and documents other than Journals

✓
Journal on temporary structure such as exhibitions, water bodies

Table 2
Query used for searching documents related to BE.
S. Shashwat et al. systematic analysis was conducted based on the heat transfer classification, their effectuation and performance.Secondly, a detailed bibliometric review (shown in Section 4) was conducted using network links and assessing the keywords.A large number of published documents on the Building Scale were available from the initial search.Refinement was required to ensure these documents belonged to BE, which was conducted in many steps.In the SCOPUS database, specific subject areas such as Biochemistry, Genetics and Molecular biology, Physics and Astronomy, chemical engineering and others are excluded using the available filters option.After evaluating the relevancy of most journal titles belonging to BE, the finalised selection of subject areas was made.The subject areas which are finally selected are Engineering, Material Science, Environmental Science, Energy, Social Science, and Multidisciplinary.However, an extended subdivision was also carried out to comprehensively analyse specifically the Building Scales due to a large number of published documents (refer to Appendix A).Building Scales are subdivided into 1) Active systems, 2) Passive systems, and 3) Renewable energy systems and three respective queries were conducted in separate SCOPUS searches.In this review, Active System refers to the systems that use energy for operation, whereas Passive system refers to the strategies that do not need energy for operations.The Heating, Ventilation, and Air Conditioning (HVAC) system is one example of Active systems in the building.Likewise, the building envelope and building integrated photovoltaic system (BIPV) are examples of Passive and Renewable systems, respectively.
A detailed description of the entire review process is presented in Fig. 2. The bibliometric mapping tool "VOSviewer" was used to interpret the bibliometric data obtained from the documents collected and reviewed under Section 4. VOSviewer allows for the presentation of bibliometric analysis using informative visualisation [50].SCOPUS database has two kinds of keywords, one is the 'Author keyword', and the other is the 'Indexed keyword'.Author keywords were those that were specified by the author for their document, whereas indexed keywords were chosen by SCOPUS, considering their synonyms, variable spelling, and plurals.This analysis was conducted using 'All Keywords', which comprises both author and indexed keywords.The keywords with equal significance were grouped together under one keyword for a better analysis.Table 3 shows all such groupings of keywords.For instance, some of the keywords, such as "biologically inspired", "nature inspired", and "bio-inspiration", were grouped under the keyword "bioinspired".

Bio-S based on heat transfer mechanisms
In order to understand the impact of the material's thermal and surface properties, Bio-S was classified based on its heat transfer Fig. 2. Methodological process of the comprehensive literature review presented in this study.

S. Shashwat et al.
mechanism.A classification based on heat transfer mechanisms is needed for the study of thermal management systems [51].Solar reflectance and thermal emittance are two surface radiation properties that have a significant role in achieving the energy efficiency of BE [21].Additionally, properties such as thermal storage and evaporation are also augmented to achieve energy efficiency in BE.These thermal properties play an important role in maintaining the desired thermal comfort.Therefore, it is important to understand Bio-S based on these thermophysical properties.Convection was used primarily in Bio-S for naturally ventilated building structure designs such as Eastgate Centre, Harare, Zimbabwe [52], and HVAC systems such as heat exchanger designs [53].Many designs of heat-exchanging devices are also inspired by Bio-S that use phase change mechanisms, such as conversion from solid to liquid and vice-versa [53][54][55].Table 4 presents a list of Bio-S showing the performance evaluation conducted based on heat transfer mechanisms and climate conditions.The table also list the technology readiness level of the Bio-S based on the maturity state of the Bio-S following the standard assessment method [56].Structural components such as walls, windows, and roofs primarily leverage the conduction, radiation, and thermal storage mechanisms of heat transfer.Conduction uses various organisms' skin or outer surface heat resistivity concepts for developing Bio-S.Different thermal properties such as thermal emittance, solar transmittance, and thermal reflectance have also been focused on for developing Bio-S.These mechanisms are primarily used for developing Bio-S for envelopes (walls, roofs, and windows), including opaque and transparent materials [57][58][59][60][61][62][63][64][65][66].
Many Bio-S are specific to climatic conditions; therefore, it is important to see these strategies in the context of climatic zones.Based on the Koppen-Geiger climate classification [101], the performance of Bio-S was analysed under five different climatic zones (refer to Table 4) -Tropical (A), Dry (B), Temperate (C), Continental (D), and Polar (E).
The Bio-S research was majorly conducted for dry and temperate climate conditions and relatively less for the other climates.The use of computational simulation provides an effective platform for conducting performance analysis for numerous locations, as shown in Table 4.However, at the same time, it shows the need for experimentally validated Bio-S and their climate-wise performance analyses.It can also be noted from Table 4 that in dry climates, the emphasis of implementing Bio-S is on the development of shading systems, façade systems, or   radiative cooling systems.However, in the temperate climate, the implementation is diverse, such as window glazing, radiative and evaporative cooling.The studies presented show a positive trend in terms of the energy saving potential of Bio-S (9% to 90% depending on climate conditions).The Bio-S simultaneously aids in reducing the building's exterior surface temperature (up to 20 • C) and the indoor air temperature (up to 18 • C).The Bio-S for shading device also assists in improving the daylighting of indoor space [67,68,72].A detailed description of Bio-S in presented in the following sub-sections based on the identified themes.

Solar shading
Solar control through building orientation, facade design, and shading devices reduces unwanted heat.However, the requirement varies based on the altitude and azimuth of the location.It also becomes crucial as these strategies also affect visual and thermal comfort.Plants and trees mostly depend on solar power for their existence, so multiple botanical strategies have been studied for performance improvement.Varying from the structure of the leaf and flower to the tree morphology has been a source of inspiration in the BE.
Stomata in plant leaves are small portals that control the gas exchange for photosynthesis [102].This behaviour of stomata opening has inspired to develop an interactive kinetic façade (based on external factors such as sun-timing position, climate and position of multiple occupants) which was proven to be efficient in enhancing daylight performance (Useful Daylight Illuminance (UDI) of 83% on east orientation and 87% on west orientation) and providing visual comfort for the hot desert climate of Yazd [67].Tree morphology of multi-layered configuration and movement have also been mimicked for the development of adaptive kinetic façade for the hot desert climates, providing a maximum of 86% of UDI and acceptable spatial Daylight Autonomy (sDA) of 50.6% along with a 90% reduction in heat gain for over the base case of plain glass window [68].Sometimes, inspiration is taken from several bioinspired solutions by combining ideas.For instance, the rotational motion mechanism from the giant white ipomoea, ribs shapes from cacti, solar energy harvesting strategy from sunflower and overall design from mangrove flower has led to the scheming of adaptive façade resulting in 50% of solar heat gain reduction and 39% of cooling energy savings along with improving the daylight penetration and visual comfort as well [69].Inspiration from the leaf Oxalis Oregana was drawn to design a façade which can fold both way along the horizontal and vertical axes.It helps in blocking the high as well as low-angle solar radiation resulting in a projection of 27%-32% (depending upon the orientation) of energy load reduction without compromising the daylighting factor hugely [72].The potential of bioinspired design in reducing the heat load and saving energy while improving daylighting is evident, which was mainly drawn from the simulation studies.However, increasing research is being focused on the implementation design of these Bio-Ss [27,70].There are many successful applications of bioinspired adaptive façade systems, such as the Theme Pavilion Expo, and Yeosu Al Bahr Towers in Dubai, UAE [103][104][105].However, applying this idea in real life is still complicated.Similarities in implementation can be drawn with other types of adaptive façade design, such as openable, responsive and switchable façades, which are growing in interest [106,107].

Radiation
Radiation is another heat transfer mechanism commonly occurring from any object through either emittance or reflectance.White poplar leaf hairs that make up the bottom surface of the leaf effectively keep the leaf cool; based on the same, a highly reflecting superhydrophobic white coating is developed that uses a similar structure to the leaf hairs [61].The white fabricated material incurs a low cost to produce and has high reflectance of about 60%, which is lightweight and exhibits superhydrophobic properties.Organisms such as Saharan silver ants maintain comfortable body temperatures through increased solar reflection due to  Technology readiness levels [56].
their uniquely shaped triangular hair [83,108].Inspired by the Saharan silver ants, geometrically modified polymer is used to create a radiative cooler which can help in reducing around 6 • C as compared to outdoor air temperature in subtropical climate zone.The material also exhibited excellent emissivity of about 95% resulting in a cooling capacity of 144 W/m 2 .Another recent research was conducted in the same climatic zone that developed a novel transparent thermochromic hydrogel wood inspired by jellyfish, which resulted in a low thermal conductivity of 0.37 W/mK [76].The experiment demonstrated that luminous transmittance close to 83% is achieved during a lower outdoor temperature resulting in a reduction of about 4.3 • C in indoor temperature and up to 10% savings (simulation using EnergyPlus) in energy consumption.
Similarly, cephalopod skin inspired to develop a thermomechanochromic film layer for windows which provides privacy and energy saving dynamically as per requirement [64].Experimental and simulation demonstrated that the film could achieve solar energy modulation of about 11.5%, and visible transmittance can be controlled between 17% and 60% with the potential of scalability.

Thermal conduction
Organisms that dwell in a cold environment have adapted their skin to restrict heat loss from the body's core.Similarly, in the building, the envelope acts as a barrier by reducing heat loss where the outdoor temperature is high and conserves the heat from flowing out when the outdoor temperature is low.Along with maintaining the occupant's thermal comfort, it also helps reduce energy usage and CO2 emission [109].A polar bear which dwells in a cold environment has fur, which consists of a hollow hair structure on the outside and a foam-like structure on the inside with black-coloured skin and a thick fat layer, which acts as a thermal barrier for its body [94,96].This unique design of the hollow-haired fur helps to entrap air, and black-coloured skin absorbs the sunlight preventing the loss of heat from the body to maintain the internal temperature of about 35 • C even in frigid environments of Artic and Antarctic [28,110].This mechanism has inspired the development of lightweight carbon tube aerogel with thermal insulation capabilities [94].The thermal conductivity achieved by the carbon tube aerogel was 0.023 W/mK [111].This hydrophobic hollow fibre structure entraps and restricts the free movement of air, which improves thermal conductivity considerably.Similarly, the natural protein fibre of silkworm cocoons is a lightweight construct which provides thermal insulation to silkworm pupae against the harsh cold environment [112].A hollow biomorphic alumina fibre was developed by taking inspiration from the structure of silk fibre as a bio-template, which shows a low thermal conductivity of 0.04 W/mK.It also exhibited an increase in thermal conductivity with the increase in its temperature [110].Both above-mentioned bioinspired materials have shown good thermal conductivity and have the potential of applicability in the BE for their thermal insulation characteristics.However, the development of these bioinspired materials is still in the initial phase and ready for BE applications is awaited.Another research which was also inspired by animal fur (one of the many cases) is a viable option for deployment at the current state of development.A parametric study which investigates using computational simulation and experiments approach to develop the sculpted façade tile inspired by different natural sources such as animal fur and cacti along with other shapes and designs [87].Animal fur-inspired sculpted tile shows the best improvement of heat transfer rates of nearly 25% (thermal resistance achieved 2.09 m2K/W) compared to plain façade tile.Another improvised version of the same reached up to 29% of improvement in heat transfer rate.A façade insulation names Plyskin was also developed based on the polar bear fur and skin structure using recyclable polyamide, which has white colour transparent hair-like external layer [113].

Multi-mode heat transfer mechanisms
In nature, many organisms use fluid flow control to manage the thermal exchange of heat, such as human vascular blood flow across the body.It dynamically restricts or increases the blood flow movement to regulate the skin temperature.For instance, during heat stress, the blood flow helps to transfer the heat from the core of the skin to its surface [114].This pattern has been mimicked to design (prototype demonstrated) by making the vascular network running along the window containing fluid between two glass surfaces using steady-state energy balance principles [115].A similar concept is also a source of inspiration for controlling the colour and transparency of the window panel by utilising a transparent fluid network (using dye or colloidal suspension) for adaptive shading [80].Water has also been tested as the medium for the development of building integrated solar thermal collector windows by running between two glass panels controlled through an inexpensive programmable mini controller [81].Leaf also has interconnected heat transfer vascular formation, which regulates the absorption of solar radiation.A proof of concept under a laboratory condition is also developed using this leaf-like model, which uses water flow (at 200 ml/min) to increase the thermal efficiency of the internal spaces [116].Issues such as freezing at low temperature is a major challenge which is resolved by introducing an antifreeze element in the fluid.This alternative is developed as a cost-efficient measure for the glazed façade and windows application in comparison to currently used technologies such as electrochromic glass panels.However, full-scale building applications and large-scale deployment are still not conducted.

Thermal convection and evaporation
Nature's significant heat exchange phenomena have inspired researchers to look for Bio-S for heat transfer enhancement in BE [117].The heat exchanger design is crucial in BE, given the dependencies on these systems for human comfort and energy consumption in various climatic zones.The efficiency of the heat exchange mechanism impacts the performance of both passive and active systems in BE.Evaporation and convection are two major processes of heat exchange.Aspirations of Bio-S for improving heat exchange are enhancement of heat transfer rate across and pressure drop reduction.Human skin starts perspiring as the body gets warmed up to remove access heat.The same concept was used to develop a thermoresponsive hydrogel base roof surface, which goes under a phase change from the swollen hydrated state, releasing water to a hydrophobic state when the outdoor temperature goes beyond 32 • C because of high solar radiation.This corresponds to roughly 220 kWh (~140 kg CO 2 ) for a single-house structure [118] in tropical conditions.Similarly, in another study, a façade tile was created after studying the morphology of elephant skin textures [85], highlighting its contribution to aiding cooling through evaporation.Elephant also has fractal geometry in their large network of blood vessels, which help in the heat exchange through convective heat transfer and mass exchange mechanism [53].It has been found that a fractal heat exchanger helps in reducing the pressure drop and increasing the heat transfer rate [28].Some of the other Bio-S which helps in heat exchange are cactus and beetle-inspired two-phase micro-piler heat sink [119], fish-gill-inspired plate heat exchanger [120], and termite mould inspired natural convection in building [32,121].

Bio-S based optimisation techniques
Smart control and building management systems are common in the BE.Automation is operated through logic and algorithms.However, the lag in changing the set-point temperature result in energy inefficiency, an increase in the cost of operation, and occupant discomfort.One of the solutions to the problems is to use thermal management strategies such as an artificial neural network (ANN), which is based on neural systems in the human brain to predict the condition based on historical data and send commands to change the input parameters for maintaining designed conditions [122].The network is trained with the goal of lowering the squared difference between the measured output and those predicted by the ANN model, which helps to reduce the error.This method is also used to manage energy consumption patterns and operation costs and maintain indoor air quality [123,124].Similarly, the S. Shashwat et al. process of natural evolution has been used to develop the genetic algorithm (GA) based computation model, which can assist in maintaining thermal comfort and energy efficiency through multi-objective optimisation [125].Genetic algorithm is popularly used for other optimisation as well, such as daylight performance [90], building design [126], building integrated photovoltaics [127] and even transportation system at Urban Scale [46].These bioinspired techniques have been used for over two decades and are easily applicable and scalable as well in BE, given the newly constructed buildings have the necessary infrastructure, such as automation and a smart building management system [128].

Bio-S based on level of effectuation
In order to understand the current status of the effectuation, Bio-S have been classified based on three levels of bioinspiration [26,108,129].The three levels are "Form" based, "Process" based, and "Ecosystem" based (refer to Table 5).In the Form based Bio-S, inspiration is taken from an organism's form, whereas in the Process level Bio-S, inspiration is taken from the behaviour of the organism or the interaction with its surroundings.Lastly, the Ecosystem level based Bio-S takes inspiration from an entire ecosystem which covers forms, functions, materials, overall process and methods of development [87,130].There are five different dimensions proposed for each of these levels that are form, material, construction, method, and function [26,94].The Process level of effectuation has been the most common among the three levels of effectuation, especially in scientific literature.The primary reason can be attributed to the fact that the process-based Bio-S aims at the components which can provide a solution for the relevant building or its components and can be easily adopted.The impact of the Process level of effectuation on energy efficiency will not vary much between buildings as the performance of building components will be comparable irrespective of its effectuation.However, the Form level of effectuation is commonly adopted by architecture designers, in which each structure is conceptualised as unique in terms of aesthetics.Similarly, the Ecosystem level of effectuation is also conducted on a large scale, where the purpose of adoption is commonly not repeated, keeping the ecology at the central focus.It is observed that the Ecosystem level of effectuation is commonly done at the urban level (which may include the Form and Process) [130][131][132][133][134] whereas the Form and Process level of effectuation is done at the building or component level (refer to Table 5).It is also important to highlight that research focus is required at the Form level to study the natural patterns and designs.

Bio-S based framework and tools
There are numerous databases and framework-based tools developed to facilitate the synergy between biology and engineering for the design process, of which the majority (~65%) are in the scientific literature   S. Shashwat et al. [138].These tools enable different steps of the design process, such as transfer, application, comparative analysis, and abstraction of Bio-S.Some of these tools are 'Ontology explorer' for application [139], 'Functional modelling' for abstraction [140], automatically populating bioinspired taxonomy for scalable systematic Bio-S application [141], and 'biomimicry for sustainability' specifically addressing the need of sustainable change in BE [142].
In terms of simulation tools for performance evaluation of Bio-S, EnergyPlus and TRaNsient System Simulation (TRNSYS) are some of the most popular tools for conducting computational modelling [88,143].These have limitations for conducting dynamic simulation behaviour for Bio-S.However, Eppy, a Python toolkit developed using EnergyPlus Runtime Language, does provide dynamic features [77].Three-dimensional (3D) graphic software Rhinoceros and Grasshopper plugin is another popular software package which has been used for Bio-S due to its interoperability and advanced programming flexibility.It allows the evaluation of complex multi-objective optimisation using GA [90].This tool can also be used for designing interwoven lattice structures and eventually printing them using 3D printing techniques [144,145].ANSYS is Fluent is a well-established and validated computational fluid dynamics tool which are used for the prediction of fluidflow phenomena such as analysing fractal tube heat exchangers [146].Outdoor comfort for Bio-S has been investigated using ENVI-met microclimate software [147].However, there is still a huge gap in the availability of appropriate frameworks and dynamic simulation tools, especially for a non-technical individual, for conducting an evaluation of Bio-S [148,149].

Bioinspired strategies (Bio-S)-Bibliometric analysis
Based on the query statement in Table 2, the research documents (including journals, conferences, and reports) published at both Building Scale and Urban Scale over the past 3 decades are plotted in Fig. 3a.The research conducted on Building Scales is considerably higher than the research conducted on the Urban Scale.The United States and China lead in both Building Scale as well as Urban Scale in terms of the number of documents related to bioinspiration.It is followed by countries such as the UK, Germany, Australia, Canada, and Italy, where the number of published documents on the Building Scale is slightly higher as compared to the Urban Scale.However, the difference is distinctively higher for the Building Scale as compared to the Urban Scale (as shown in Fig. 3b and 3c).

Building Scale
Fig. 4 shows the network and links for the keyword of Building Scale related research.The illustration contains multiple clusters (each colour represents an individual cluster), indicating that bioinspired Building Scale research is spread across multiple themes.Table 6 presents the most frequently occurring keywords and their co-occurrence counts.As mentioned previously (refer to Section 2), the keywords (refer to Table 3) are grouped so that the illustration depicts a more realistic network and links between them.The keyword "Bioinspired" shows the highest occurrence among all the keywords and is connected to all the other clusters.The keyword "materials" is the dominant keyword in the occurrence count even after grouping under three different keywords, which are: "materials", "bioinspired materials," and "construction materials."This shows that the research on materials or bioinspired materials was significant and conducted under different subject areas.The publication on the structure and mechanical properties of biological materials is the most relevant study on bioinspired materials and has  attracted the highest citation from the research community [76].
Another relevant study is conducted on the development of high performing structural materials from natural wood to increase their strength, stability, and toughness [54].Fig. 5a presents details of the journals with respective number of papers published.It can be observed that the journals that are popular in material-related research have the most documents published, followed by building-related journals such as "Energy and Building".Furthermore, Fig. 5b shows the authors who have published the most papers on material-related bio-S.The focus on the keyword "bioinspired materials" shows that it has relations with almost all the other keywords in the cluster, as illustrated in Fig. 6a, which implies that the research is distributed under various themes.The research on bioinspired materials in the context of buildings is primarily for structure, as evidenced by the keywords such as "structural characteristic", "structural design", "toughening mechanism", and "self assembly".The bioinspired materials have a strong relationship to the keyword "energy efficiency," indicating the research community's interest in developing materials for energy conservation.However, the links to the keyword "construction materials" (refer to Fig. 6b) show that the research has been focused on building structures due to their relationship with words such as "stiffness" or "structural characteristic."The keyword "construction materials" has also been co-occurred with "energy efficiency" and "architecture", which shows the inclination of the research community towards energy conservation in the BE.Both the keywords "construction materials" and "energy efficiency" in Fig. 6 show a similar trend, especially in a Building Scale context.
Fig. 7a shows a detailed focus on the keywords "energy efficiency" for its obvious relation with material-related research, as evidenced by the links to keywords such as "materials" and "bioinspired material".It is important to mention that the keyword "energy efficiency" has a relation to other keywords such as "biomaterial", and "smart material" but since it is grouped under the word "materials" it does not show in the figure .Bioinspired is an important research topic to achieve energy efficiency, which is evident by its connection with keywords such as "design", "bioinspired processes", and "design process".These keywords highlight that in order to achieve "energy efficiency", research has been conducted on "environment", "sustainability", "climate change", and "energy management" (relationship seen between highlighted keywords).The keyword "Renewable energy" co-occurs with energy efficiency, which shows that the research direction is also towards net zero energy building.However, it can be observed that there is no relation between "energy efficiency" and keywords such as "thermal comfort", and "well-being" showing the lack of study related to Bio-S for human   thermal comfort.John [150] has published a review on building solutions for achieving energy efficiency, which has attracted a high number of citations.Another important study presents the theoretical framework for the various levels of Bio-S, highlighting the advantages and disadvantages within the design process [26].
A detailed co-occurrence and links for the keyword "buildings" highlights its relationship with materials and others (refer to Fig. 7b), which is obvious as the co-occurrence of material-related keywords ("chemistry", "self-assembly", and "structural characteristics") are high in number.However, the cluster (red colour nodes in Fig. 4) shows the overall wider trend, specifically in Building Scale.The detailed view highlights multiple other keywords that highlight the direction of building-related Bio-S.Some of the keywords are "heat storage", "multifunctionality", "ergonomics", "fabrication method", "ventilation", and "optimization".It is interesting to note that the energy-related keywords ("energy efficiency," "energy management," and "renewable energy") have strong links to the "buildings" keyword, indicating that building energy research is prevalent.One of the most cited documents related to building energy is the review of energy absorption applications of bioinspired materials and designing lightweight structures [40].In the red cluster, the co-occurrence with "energy efficiency" and "environmental" keywords depicts the ongoing research for achieving energy conservation using Bio-S.The areas that attract vast attention in the Building Scale are highlighted by the links to keywords such as "building envelope", "structural design" and "ventilation".Alongside, the process that is commonly followed for studying is highlighted by keywords such as "optimization", "experimental method", and "detection method".
The VOSviewer software also allows you to plot the keywords on an overlay visualisation that shows the keywords' timescale.Fig. 8 is plotted using this visualisation style to analyse trends in the last decade specific to one cluster (the red colour cluster in Fig. 4), because it represents the broad trends of the building's topic (excluding materialrelated trends).The time scale shows an evaluation of the research community's interest in the past decade and various changes in the methods of study.The keywords picked are presented in three groups, which are blue (before the year 2014), green (between the years 2014 and 2018), and yellow (after the year 2018).The areas of research before 2014 are highlighted by keywords such as "automation", "building automation", "benchmarking", "deployable structure", "ergonomics", and "actuators."The methods used, on the other hand, are highlighted by the keywords "pattern recognition" and "artificial intelligence".The keywords "bioinspired technologies," "living organism," and "plant movement" are among the most frequently used Bio-S.Similarly, keywords such as "climate change", "solar energy", Fig. 10.Top five authors with the highest number of publications on the Bioinspired Urban Scale.Fig. 9. Networks illustration for keywords on bioinspired Urban Scale strategies."performance", "building construction", "control system", "planning" and "energy efficiency" show the trend between the years 2014 and 2018.During this period, methods used are highlighted by the keywords "parametric design", and "neural network".Moreover, it is interesting that these studies are performed in various types of buildings, such as office, commercial, and even religious buildings (keywords: "office building," "commercial building," and "religious building").Recently (after the year 2018), the popular trend has been highlighted by the terms "heat transfer", "thermal performance", "esthetics", "greenhouse gases", "well-being", "air conditioning", "cooling", "renewable energy", "solar absorber".Keywords such as "building performance simulation", "computer software", "machine learning", "experimental study", and "CFD" highlight the techniques that have recently been used in research.
The presence of keywords such as "wearable technologies" and "virtual reality" shows the use of advanced digital technology to conduct research on a building scale.However, it is worth noting that the cooccurrence count of these words is low, indicating that research in Bio-S in the building is still in its early stages.However, it is noticeable that the research is conducted on various themes such as "materials", "design", "structural characteristics", and "architecture" (evident from the presence of links to these keywords), whereas other areas such as heating, ventilation and air conditioning (HVAC), lighting, renewable energy are not so evident from these maps.Therefore, a detailed investigation is needed to know the state of research for different systems of a building.Research on Bio-S can be further categorised into distinct system types based on the usage and/or generation of energy.Building Scale is classified into different categories to get an insightful analysis of different building systems.As mentioned in the methodology (section 2), Building Scale is categorised under three systems: Passive, Active, and Renewable energy systems.For example, façades, windows, and daylighting designs are positioned under passive system whereas HVAC, which needs energy to operate, is categorised under active system.The renewable sources are placed in a separate section, which also includes systems such as building integrated photovoltaic (BIPV).In order to understand the trends in various systemlevel literature, a separate search is conducted and analysed for each sub-section (Passive systems, Active systems, and Renewable systems).The search queries, networks illustration and major findings for each sub-section are illustrated in Appendix A.

Urban Scale
The energy usage of urban areas is around 75% of total primary energy usage, even though it covers roughly 2% of land area [151].Therefore, an energy efficiency strategy is an important aspect to consider in urban areas.A bibliometric illustration of the keywords is presented in Fig. 9, which highlights various research themes observed for the Urban Scale Bio-S.Fig. 10 presents a graph with five authors names who have published the highest number of articles in Bio-S for Urban Scale.The presence of multiple clusters shows that the research in the Urban Scale.Bio-S is also distributed similarly to that of the Building Scale (refer to Fig. 4).Table 7 presents the ten most frequently occurring keywords for the bioinspired Urban Scale search (query statement is given in Table 2).The keyword "sustainability" is the most frequently occurring after "bioinspired", which shows that bioinspired urban research has a prominent inclination toward achieving sustainable objectives.The keywords "urban", "urban planning" and urban designing" also have a high occurrence.The presence of keywords such as "energy efficiency", "environmental", and "environmental protection" further supports the arguments.Annual power forecasting for an urban area based on the Ant Lion optimizer is widely cited research, showing that a bioinspired algorithm can improve the forecasting accuracy [152].The high occurrence of keywords such as "urban planning" and "urban design" also suggests that the focus is also to improve traditional planning and design practice.Efficient urban transport planning inspired by ants' behaviour is one of the most relevant studies on urban planning [46].The presence of the keyword "green infrastructure" suggests that the application is not only related to planning or design but also the development of sustainable infrastructure.The keywords such as "optimization", "algorithm", "forecasting", "artificial intelligence", "parametric design", and "survey" show some of the methods employed for conducting research.
The process followed to achieve energy efficiency on Urban Scale is highlighted by the link to keywords such as "parametric design", "regenerative design", "bioinspired design", "bioinspired processes", and "urban planning".Similarly, the presence of keywords such as "urbanization", "population", and "environmental" shows the major problems that have been tackled by this bioinspired urban research.Fig. 11 shows the authors who have published a high number of scientific articles on Urban Scale Bio-S.The network and link for the keyword "energy efficiency" are illustrated in Fig. 11a.It connects to a wide range of keywords showcasing the trend in achieving the sustainable objective.The presence of keywords such as "sustainability", "bioinspired design", "architecture", "buildings", "economic development", and "energy management" shows the overall focus of Bio-S at an Urban Scale for achieving energy efficiency.Some of the methods used to develop the research include connections to keywords such as "optimization", "algorithm", "forecasting", and "artificial intelligence".Fig. 11 (b and C) shows the network and link of the keywords "urban design" and "urban planning" as these are relevant to analyse in detail given the urban settlement.It is significant to highlight that the common processes are highlighted by the keywords such as "computational simulation", "genetic algorithm", and "controlled study".
Fig. 12 presents an illustration of the timeline of all keywords for the Urban Scale.As stated in Building Scale, the timeline-based analysis is presented in three groups which are blue (before the year 2014), green (between the years 2014 and 2018), and yellow (after the year 2018), and should be read along with the timescale colour bar.The prime focus of research before 2014 is highlighted by the keywords "water management", "conservation of nature", "ecology", "population", "bioinspired material", "perception", "physiology", and "survival".During this time, the primary method of conducting studies is highlighted by the keywords "mathematical model" and "computer simulation".During the time period between 2014 and 2018, studies were conducted using methods that are highlighted by the keywords such as "detection making", "controlled study", "surveys", "forecasting", "algorithm", "optimization", "environmental monitoring" and "computer-aided design".The illustration shows that these methods are majorly associated with the keywords "cities", "urban planning", "urban growth", "environmental", "energy", "energy efficiency", and "energy management".Similarly, the methods which are popularly used in recent years (after 2018) are "CFD", "numerical methods", and "parametric design".During this time period, "experimental study" is also used to conduct the research, as evidenced by the presence of the keyword.The research focus can be seen by the presence of keywords "well-being", "temperature", "global warming", and "sea level".The presence of the keywords also shows that the focus is also on conducting studies related to "housing", "tall building", and "urban green space/infrastructure".Overall, it demonstrates the changing trend in the Urban Scale focus and method used over the last decade.

Summary on research trends and gaps
The earliest research in Building Scale Bio-S was published in 1993, whereas that on Urban Scale was published in 2001 (refer to Table 2 for search queries).The dominant research theme is developing new materials and structural analysis using different Bio-S.397 cooccurrences are there for material-related (45 synonym counts, which are grouped in Table 3) keywords, showing the volume of research conducted till now.Material has been attempted to emulate the hierarchical structure (such as bones and bamboo), porous or layered material by techniques such as additive manufacturing or freeze casting for improving properties such as strength, stiffness, flexibility and lightweight [35,153].Nanomaterials are the most studied bioinspired material, followed by biomaterials.Professionals in the nanotechnology field have the highest awareness of the bioinspired design process [154].
Human comfort is one of the main focuses of Bio-S in BE.Developing material with higher thermal resistivity or insulating properties is conducted to reduce the unwanted heat flow in BE [59].For instance, carbon nanotube aerogel development was inspired by polar bear hair and fur layer, which can act as a thermal insulating layer [95].Research has been conducted by following a variety of heat transfer mechanisms to produce the cooling impact of BE.For instance, evaporation [85,86], radiation [82,83], convection [52], conduction [113], and phase change [155].The improvement of human comfort is also made via different optimisation techniques used for active systems (such as ventilation systems) in BE, such as the use of genetic algorithms [90,126,156] or artificial neural network systems [121,157].These bioinspired optimisation techniques have also been used to generate the enhanced building form [158] and analyse cost savings in BE [159] and improve thermal and daylight performance [90].Biological systems have also inspired many adaptive or responsive passive shading devices.Being responsive and majorly inspired by botany (such as flower, fruit, leaf, and tree morphology), these Bio-S can adapt to outdoor conditions depending upon impacting factors such as solar radiation or outdoor temperature and provide comfort in different climatic conditions [67,68,70,71].Achieving visual comfort targets by mimicking unique features from nature, such as spectrally selective thermochromic coating, helps to enhance performance.Most studies conducted are in tropical, dry, or temperate climates.For daylighting, Useful Daylight Illuminance (UDI) [68] and Spatial Daylight Autonomy (SDA) [67] are mainly investigated, especially for locations in dry climate conditions.
Bio-S is believed to enhance human psychology and well-being by incorporating natural patterns and aesthetic appeal through organic forms as design elements in BE [160].Enhancing thermal and visual comfort makes indoor and outdoor spaces more liveable.At a larger scale (ecosystem level), Bio-S helps to create more resilient and sustainable spaces, which help in promoting well-being and fostering social cohesion in society by creating interconnectedness, resource optimisation, and reducing environmental risk [161].In comparison to Building Scale, Urban Scale research is quite small.Research in urban BE is mainly focused on the interrelation of ecosystem-level bioinspiration with the regenerative design, which includes the diagnostic of the socioecological process [130,132].However, there is still a gap in the implementation of urban ecosystem-level ideas and the operationalisation of regenerative design; nevertheless, there is progress in the right direction [162].Studies also highlighted the need for a comprehensive ecological diagnostic before the design of any project, and a gap is identified between theory and practice.Framework and tools are lacking for the assessment of bioinspired material and Bio-S for sustainable and energy-efficient urban BE [163].
Lack of awareness is one of the main reasons for the slow adoption of Bio-S in BE.The creation of open-source databases containing Bio-S will help people with uni-disciplinary backgrounds to adopt Bio-S easily.The largest open-source depository is AskNature which, even after having over 1700 strategies for emulating, is still not exhaustive enough [164].More establishment of institutes such as the Hub for Biotechnology in the Built Environment at Newcastle, UK and the Centre of Biologically Inspired Design at Georgia Tech for bioinspired is required to advance the focused research and increase awareness.Courses specifically planned to teach bioinspired design in engineering curriculum helps to increase the competencies of next-generation engineers through learning about collaborating across discipline, multidisciplinary openended problem, and critical thinking [165,166].The Bioinspiration awareness programs have been shown to provide the intrinsic motivation for realising the behaviour change towards sustainability initiatives [167].The advancement of programming language and different artificial intelligence-based tools and acquiring knowledge from a wide variety of sources can further help to increase the awareness and adaptation rate.
The effectuation of Bio-S is observed to have efficiency in terms of energy and environment [66].Most research follows the Process level of effectuation, where the functionality of the bioinspired mechanism is emulated.There is limited research available which presents the novel concept related to the form level of effectuation [52].However, the form [73,135] level has real-world adoption by the construction industry for creating aesthetical designs and patterns for BE.In terms of thermal analysis, investigation for thermophysical properties and radiation properties is limited.Research analysing the impact of Bio-S on indoor thermal comfort is another topic which is not adequately covered for all climatic zones in the current scientific literature.The bioinspired system is getting exponentially popular in the recent decade.However, there are not many studies which discuss the whole life-cycle impact or economic analysis in BE context.
The technology readiness level of the Bio-S is important to analyses for a better understanding of their maturity (Refer to Table 4).Some of the strategies, such as bioinspired algorithms (artificial neural network and genetic algorithm), can be adopted as the required infrastructure (such as automation and building management systems) for the same is commonly available in BE.Additionally, some of the Bio-S have been identified to have the potential to be scalable [64].Some measures are highlighted in the future perspective (refer to Section 5.2) to increase the adoption rate of Bio-S further.
The bibliometric review has shown that "energy efficiency", "sustainability", "climate change", and "energy management" are some of the frequently co-occurring keywords, which suggest that there is some inclination towards Bio-S research for the improvement of energy efficiency in the BE.However, these are limited, and further studies are warranted to develop a comprehensive understanding of the Bio-S performance.In building typology, most research is conducted on office/ commercial typologies, followed by religious buildings.However, there is limited research conducted on Bio-S in residential buildings.At Building Scale, the major focus is on the envelope, which entails double skins, wall structures, and kinetic façades.Some of the other components are roofs and windows, but no attention is given to the floor component of a building.Similar observations can also be reaffirmed in Table 4.However, the focus of research in active systems is heavily weighted toward achieving cooling rather than heating.This can also be confirmed concurrently by examining the impact of the climatic condition conducted through systematic analysis (refer to Table 4).the research focus in renewable energy systems is observed to be on optimisation, source, photovoltaic cells, and solar heating.The cooccurrence of the keyword "experiment study" is only seven and three times in Building Scale (906 documents) and Urban Scale (146 documents), respectively.
At the Urban Scale, sustainability is one of the most important focuses, along with climate resilience and the development of green infrastructure.Considerable Bio-S research is also conducted for improvement in the processes of urban planning and urban design, which implies that the research is conducted to establish the planning and design of cities and towns.The research at Urban Scale focused on mitigating issues of urbanisation, the population as well as environmental issues.However, energy efficiency due to Bio-S at Urban Scale has not been covered so far.Some of the topics of research, for instance, at Urban Scale, can be its effect on the urban heat island effect, energy efficiency in different urban densities due to Bio-S, and optimisation of urban microclimates by using Bio-S.
The Bio-S design has numerous advantages in different contexts, such as novel innovation, energy efficiency, thermal and visual comfort, well-being, socioecological Functions sustainability, cost-effectiveness, adaptability, structural robustness, improved functionality, and aesthetics.However, there is no focused study which highlights the disadvantage of Bio-S, which can make people prepared for unforeseen circumstances.Some disadvantages of Bio-S can be a complex electromechanical system, high maintenance in terms of resources and expertise, an additional system for sensing environmental parameters, high initial cost, unclear reaction mechanism, inadequate regularity, and unclear reaction mechanism [168,169].Given that the advantage outperforms the disadvantage by a huge margin, it is beneficial to adopt Bio-S, especially for its innovative problem-solving approach.The efficiency of Bio-S may not be as good as the natural counterpart; however, the progress is in the correct direction.
A holistic and multi-disciplinary conceptual framework (refer to Fig. 13) on the available state of research for Bio-S in energy-efficient BE is presented.This illustration summarises research trends, gaps, and future perspectives on the topic.It is essential to highlight that the trend pattern that has been identified does not imply that the research for the individual subsection has been sufficiently covered but instead that it has been presented as a comparative assessment.

Overcoming uncertainties
Uncertainties in adopting newer strategies are a barrier to the adoption of Bio-S.Based on the studies identified in Table 4, extensive research for various deployable energy-efficient Bio-S should be conducted and validated for different climatic conditions.Also, the bibliometric analysis highlights the material-related topics that attract considerable interest from academia, but the quantification of the thermophysical properties and solar radiation properties for these bioinspired materials have yet to be made available.However, the focus is also required to investigate the impact of Bio-s on energy efficiency in BE for different climatic conditions.In order to capture the adaptive behaviours of Bio-S, it is important to investigate the dynamic behaviour of the strategies.Use of dynamic simulation tools or advanced methodology such as custom-developed algorithms (such as logic for identification of transitory-sensitive areas dynamically) can be utilised to investigate them further to improve accuracy, time-dependent analysis, better optimisation and enhanced occupancy comfort [67,77,170].The effective thermal resistance (R-value) of the materials gives far more information as compared to static due to its dynamic multimodal heat transfer approach [17].The effective R-Value investigation includes the impact of the dynamic variables such as solar radiation, temperature differences and material property variation, which should be prioritised given the adaptive nature of Bio-S.Exploring nanotechnology and electron microscope further can assist in developing new Bio-S for BE through studying the subcellular structure and interaction at a singlecell scale [154].More practical methods, such as the recently developed Python-based biological knowledge acquisition tool, will help to quickly conceive Bio-S for BE applications [171].

Experimental validation and computational modelling
There have been multiple studies which have shown the substantial potential of the Bio-S in the context of BE [28].However, these need experimental validation or real-life demonstration for easier proliferation is required in BE.This performance quantification through prototype testing will help the feasibility study for material selection, scalability and environmental impact based on different parameters.3D printing techniques can provide new possibilities for developing unique bioinspired prototypes, such as microscopically porous structures, complex geometry and organic form, multi-material printed BE systems, and site-specific properties [172].Similarly, the development of software tools for evaluating Bio-S will also help in conducting feasibility studies in a quicker and cost-effective way.

Fabrication and implementation
Natural shapes and patterns are commonly organic in shape, whose mass production for application purposes is relatively time and resourceconsuming over the geometrical shapes.However, with the advancement in both computation and fabrication technologies, BE construction systems (such as façade panels, interior details, and urban furniture) should design and developed at a wider scale [154].Economic benefit analysis of Bio-S and evaluating payback period based on location can convince the construction industry to adopt these strategies [59].A sustainability assessment should be conducted for Bio-S with a comparison to traditional systems or products with the same methodology to allow direct evaluation [173].These future studies will help to persuade the adoption of Bio-S to realise its benefit at a larger scale and frequency.Currently, advanced technologies such as three-dimensional printing and superior computing power make it simpler to implement the complex structure of the biological system.

Incentives and policy
The policymaker initiative to align the different stakeholders to enable the inclusion of Bio-S in their agenda can help to achieve sustainable BE [174].Policy changes and incentivising by government or institution has seen a faster adaptation of technologies for achieving the sustainable, social, and economic goals in BE [175].Definitions, rules and standards need to be set by the policymakers defining clearly.
One of the analyses has shown synergy between the energy category of Building Research Establishment Environmental Assessment Method and Bio-S has shown positive outcomes for energy management of the BE, which is worth exploring further [176].Additional focus is required to include the presence of the natural principle of Bio-S in the green building index to develop conducive living conditions in buildings [177].

Long-term resilience
Creating resilient structures that can withstand extreme weather events can be inspired by Bio-S such as self-healing materials and responsive systems [178,179].It is worthwhile to evaluate the performance of the Bio-S under future weather conditions, given the maturity of these strategies may take some time for wider application [180].Ensuring the Bio-S is functional and effective in BE considering the fastevolving user needs and climate change is an important aspect.

Transcending disciplinary boundaries
Transcending disciplinary boundaries is essential for the development of Bio-S.The design team and engineer need help finding a suitable Bio-S solution for the implementation in BE [181].Biologists and engineers must collaborate by bringing synergy through drawings and communication aids between the two domains [174].However, broadbased biological knowledge is desirable for searching for necessary Bio-S as individual biologists may have a different background given their expertise in one subject's vertical, for instance, organismal biology [149].Additionally, experts from another discipline (such as organic chemistry) sometimes need their representation in these multidisciplinary groups to develop specific Bio-S [182].Knowledge transfer and closer collaboration between industry and academia will assist in the advancement of Bio-S [183].The multidisciplinary groups of academia and industry should also be to focus on research and development for cost-effective and affordable Bio-S by inviting policymakers as one of the stakeholders.Therefore, it is important to develop an interdisciplinary team based on experts who can work together to develop novel Bio-S.This interdisciplinary team should also constitute the end users, as Bio-S is relatively new and untested from the user's perspective [184].An understanding of complex interdependencies of various factors, such as the availability of bioinspired material or strategies for implementation, its benefit assessment analysis and cost implication, is required for easier adoption of Bio-S.It is also important to evaluate the ecological diagnostic [130] before design, life cycle assessment [185] and how it fits in the regenerative design [130] and circular economy [186] road maps.This review also recognises that the identification of novel Bio-S is required as many potential biological systems are yet to be explored, which can assist in improving the energy efficiency of BE.

Conclusions
A holistic and multidisciplinary review of bioinspired strategies for an energy-efficient built environment is carried out.The bioinspired strategies are analysed initially using the systematic method, which is classified based on the heat transfer mechanism, the level of effectuation, and performance in different climatic conditions.The SCOPUS database output from multiple queries is also analysed through the bibliometric method, showing that the focus on Building Scale is significantly higher than Urban Scale.The two methods employed give an in-depth overview of the topic and complement the existing literature reviews that commonly cover only the niche themes.Energy-related articles account for merely 19% of all the studies, and 1% of all the studies focus on renewable energy systems.This study also reviewed the technology readiness level of the current bioinspired strategies.Although current strategies have shown to be effective in terms of energy use and environmental impact, a further detailed investigation is required to examine form-level effectuation, investigate thermophysical and radiation properties, and assess the full life-cycle impact and economic variables.It is observed that the current-state-of-the art on heat transfer mechanisms (evaporation, radiation, convection and conduction) has been mainly explored to develop to achieve cooling technologies and limited studies on heating.The primary reason for the slow adoption of bioinspired strategies has been identified as a need for more awareness, policy support and multidisciplinary collaboration.Six key areas have been identified for future perspectives of bioinspired strategies, such as databases, development of tools and frameworks, experimental validation, and incentives support.Urban bioinspired strategies research focuses primarily on the relationship between ecosystem-level and regenerative design; however, real-world applications are limited.Applying bioinspired strategies to the built environment has various advantages, such as energy efficiency, creative problem solving, comfort, well-being, and sustainability, paving the way for future development and wider adoption.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 5 .
Fig. 5. List of (a) document publication sources, (b) authors with the highest number of documents.

Fig. 8 .
Fig. 8. Networks links and co-occurrence of the keywords on detailed timescale plot for the red color cluster from Fig. 4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 13 .
Fig. 13.Holistic and multi-disciplinary conceptual framework on state of research of Bio-S for an energy-efficient built environment.

Table 3
Related terms (keywords from SCOPUS database) are grouped under stated keyword.

Table 4
Performance evaluation conducted for Bio-S based on heat transfer mechanisms and climate conditions.

System Mechanism Place Climate Heat transfer mechanism Performance Parameters Technology readiness levels References Energy Daylight Temperature reduction Tropical (A) Dry (B) Temperate (C) Continental (D) T Cd T E S T T Cv E T M Cooling Heating UDI Average sDA
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Table 5
Bio-S based on level of effectuation.

Table 6
List of ten most occurred keywords in the bioinspired Building Scale search.

Table 7
List of ten most occurring keywords in the bioinspired Urban Scale search.