Review on Recent Developments in Bioinspired-Materials for Sustainable Energy and Environmental Applications

Abstract

Nature has always inspired innovative minds for development of new designs. Animals and plants provide various structures with lower density, more strength and high energy sorption abilities that can incite the development of new designs with significant properties. By observing the important functions of biological structures found in nature, scientists have fabricated structures by bio-inspiration that have been proved to exhibit a significant improvement over traditional structures for their applications in the environmental and energy sector. Bio-fabricated materials have shown many advantages due to their easy synthesis, flexible nature, high performance and multiple functions as these can be used in light harvesting systems, batteries, biofuels, catalysis, purification of water, air and environmental monitoring. However, there is an urgent need for sensitive fabrication instruments that can synthesize bio-inspired structures and convert laboratory scale synthesis into large scale production. The present review highlights recent advances in synthesis of bio-inspired materials and use of hierarchical nanomaterials generated through biomolecular self-assembly for their use in removal of environmental contaminants and sustainable development.

1. Introduction

Nature is a supreme artificer which has been refining structures for many years. The evolution has generated various structures with their specific functions and properties in living organisms to cope with the harsh environmental conditions. The surfaces of biological organisms have evolved through continuous evolution and natural selection for better functionality. The mechanism behind the useful biological materials with fascinating properties have inclined scientists to the development of novel artificial surfaces and materials which possess the same or better properties as compared to their counterparts. The bio-inspiration term is linked with efforts to identify, produce and emulate natural entities which gives us a better understanding of nature [1]. Bio-inspiration, biomimetics, inspiration, and biomimicry are synonyms. Inspiration refers to a primary phase of observation of design or function that provides ideas for developing similar objects. The mimicry is an advanced form of inspiration which applies various technological tools for construction of materials that are the same as natural objects, with main aim to obtain sustainability. Bio-inspiration has a significant potential for promoting sustainable development [2]. Bio-inspiration and biomimetics are fast flourishing applied areas where knowledge of biological sciences can be applied to resolve design challenges. The term biomimetics was first proposed by Otto Herbert Schmitt in 1957, which was associated with knowledge transfer from biological systems to design or engineering. Research in bio-inspiration or biomimetics has grown over the last twenty years. Biomimetics changed to the related term, bio-inspiration, in which new designs are incited from natural objects [3]. Both the terms are similar but have separate approaches and it has been observed that transmission of knowledge is not required. An example is the flapping of wings or extension of proboscis in butterfly which can be duplicated by soft robotics [4]. However, bio-informed approaches exhibit an understanding of the detailed processes or mechanism of biological system that is motivating for the innovative design. A bullet train has bullet shaped nose which is the same as kingfisher’s beak. Animals can obtain aquatic prey with the help of the beak as it moves fast from air to water with high speed.

The timescale for natural and engineering objects are different but their objectives such as optimization, multi-functionality and cost effectiveness and design constraints are the same [5]. The natural materials are superior due to their structural hierarchical integrative material system whereas engineering materials depend on their features to develop design. Natural materials production is a slow, bottom-up strategy where materials develop, self-assemble and change as per the environmental conditions. Bottom-up approaches permit natural objects to be hierarchical at all scales and they are sustainable because they are developed by nature-triggered protocol. Biological materials such as bone, tooth and wood have a hierarchical structure which provides more strength with less material [6]. It is difficult to duplicate the production of properly arranged biomaterials such as bone and nacre, which is regulated with living system via large scale production methods [7]. Bio-inspired synthesis gives an alternative green solution for the production of functional bio-nanomaterials by applying specific nanostructure properties and biological molecules function where biomolecular identification and aggregation procedures assist in shaping structure as well as defining role of bio-fabricated substances [8]. Compared to the conventional chemical synthesis techniques, the bio-inspired or biomimetic syntheses have many advantages. The reaction conditions are mild at ambient temperature and size and shape of the desired bio-inspired nanomaterials can be regulated easily in comparison to traditional material processing methods. Biomolecules show distinctive features of aggregation and molecular identification and exhibit significant functions in the modification of structure and the role of bio-fabricated devices [9,10]. Scientists are developing multi-functional mimetics such as self-cleaning building in which compartments of building are influenced by crystal structure or termite mound to gain more strength [11]. It can be further modified by changing outer wall surface inspired by the lotus effect, photosynthesis promoted by a tree, color change by wings or feathers of birds etc. For achieving multiple sustainable functionalities in bio-inspired materials, a holistic approach is needed; however, work in this area is still in its inception.

Earlier studies have revealed that various bio-inspired structures have been generated with some novel properties or activities such as the wings of butterfly, photonic materials developed from inspiration of opal [12], desert beetle and water harvesting devices incited from cactus [13], damage resistant substances inspired from nacre, artificial armor influenced from fish scale, self-cleaning surfaces motivated by lotus leaf, insect compound eye inspired antifogging coating [14] and mussels inspired adhesive materials [15], nepenthes inspired rapid water transport system [16] and oil-water separation system developed by inspiration of fish-gill and cactus-needle [17] etc. Chameleons can change their body color from the state of camouflaged to an excited condition and the change in colour is reversible [18]. The change in color is achieved by modulating the lattice of guanine nanocrystals inside the iridophore cells. Rasouli et al. designed mechanochromic elastomers that imitate the photonic structure of chameleon iridophore cells [19]. The rigid silica nanocrystals are present inside the elastomers matrix to form packed crystals. The colour change in sensors is reversible as they exhibit change in red to blue colour under stretching whereas red to green colour under condensed state. Sensors can be applied to signboards, wallpaper and optical recording.

Inspiration from nature opens new avenues for development of sustainable technologies. The knowledge of structure, property and their relationship is needed for the fabrication of bio-inspired materials. In this review, we have compiled the research findings of the last nine years (2014–2022) in synthesis of novel biomimetic/bio-inspired products and applications of bio-inspired nanomaterials in nanoscience, analytical and materials sciences, environmental monitoring. The present review also targets various challenges and their possible solution. The present study will be useful for researchers to understand the biomolecules self-assembly and procedures of bio-fabrication for designing new materials which can be used for wide applications. Thus, focus has been laid on all those aspects which have a scalable probability for large-scale production and utilization for imparting benefits to environment and society.

2. Need for Synergistic Approaches for Bio-Inspiration

Some biological sources for the development of bio-inspired materials are shown in Figure 1. As per the International Union for Conservation of Nature [20], the total number of species on the earth is approximately two million, in which eukaryotic and microbial species are more than five and one trillion, respectively [21]. This shows that biodiversity can provide the solution of environmental problems, but the major question is whether research in bio-inspiration are benefitting from biodiversity. The designs developed by engineers can perform only one function, however, designs formed by biological traits may show various functions. Schroeder et al. reported that wings of butterfly have stimulated for the generation of structurally colored substances [22]. The wings must be thermally efficient, flexible, hydrophobic and durable for flight besides light manipulation function [23,24].

Understanding the evolutionary process can inform us about the role and need for various traits in bio-fabricated designs [25]. However, a tool named AskNature and bionics system database have been developed which can transform biological information for non-biologists for their application [26]. They have databases of living organisms and their functions which may be useful for the development of new designs in preliminary research work. These databases do not provide the potential solution available in nature, hence, they cannot replace biologists who have in-depth knowledge of biological mechanisms and strategies in the fabrication of bio-inspired designs [27,28]. Scientists can study the evolution and functional structures of extinct species by replication of their structures by biomimetic tools [29]. However, natural fabrication processes are different from engineering techniques, which restricts design flexibility. Natures’ inspiration is used in the development of structure, function and procedure. The biologists can take help from engineers for further exploration of their queries about biological organisms in tangible references [30]. A move towards a bioinformation approach is synergistic, which permits biologists and engineers to generate useful designs. The current status of bio-inspired design requires meticulous knowledge of biology and nature. Thus, the application of a combined approach, i.e., knowledge of the biological system with its design and engineering inspiration, can provide a sustainable solution for environmental problems.

3. Synthesis of Bio-Fabricated Materials

A summary of bio-inspired synthesis of smart structures is presented in Figure 2. The natural biomaterials have a properly arranged architecture and a complicated morphology with chemical structures which can perform various applications in biological system. Different materials such as inorganic crystals or hybrid materials with specific sizes, shapes and forms have been produced as bio-inspired materials [31]. The bio-inspired materials can be generated through direct or indirect synthesis.

Direct synthesis of bio-inspired materials is based on traditional production methods such as solvo, hydro or iono thermal methods and precipitation. In this method, multi-functional self-assembled biomolecules can be generated with specific properties against targets with more structural symmetry, and different interfaces between their subunits. Bhattacharya et al. reported ferritins were the first protein cages applied as biomaterials for the generation of inorganic nanoparticles [32]. This protein family utilized iron for the sequestration of carbon dioxide. The indirect synthesis is dependent on the association between biomolecules and inorganic and organic materials, polymers, nanomaterials etc. via biomimetic mineralization [33]. Therefore, knowledge of the self-assembly of biomolecules for nanoparticles generation can enhance the production of useful nanomaterials. Different biomolecular inspired self-assembled nanomaterials have been constructed and used as sensors, optical materials, nano-devices, storage of energy and environmental monitoring [34]. Several limitations have been identified with the direct synthesis of bio-inspired materials such as lower production of biomolecules, requirement of specialized facilities, improper condition for growth and a lower modification rate. However, indirect synthesis is advanced process which is not restricted to biomolecules but is also used for the synthesis of polymer and inorganic/organic nanocomposite materials. Self-assembly and bio-inspired synthesis of different nanostructures from various biological sources are presented in Figure 3.

Bio-inspired design can be taken as structure or surface design. The design of the surface includes change in the surface such as leaves of hydrophobic plants, including Phanera pupurea and Pistia stratiotes. The hydrophilic nature of Cathranthus roseus altered wettability after coating with copper film. The nanostructures present on the leaf surfaces decreased reflectance and promoted increased absorbance. The use of nanostructured surfaces can be applied in solar absorber coatings [35]. Similarly to surface design, the bio-inspired structural design also provides enhanced properties.

4. Biomolecular Self-Assembly towards Hierarchical Nanomaterials

The self-assembly of biomolecules gives a promising solution for the development of arranged biomaterials of different dimensions such as zero-, one-, two- and three-dimensional structures [36]. The development of hierarchical nanomaterials by self-assembly of biomolecules is shown in Figure 4. Bio-inspired materials showed various applications because of their distinct characteristics such as regulated self-assembly, excellent bio-compatibility, durability against high temperature, plasticity, easy synthesis and operation. Protein, peptide and nucleic acids were self-assembled into different structures such as zero-dimensional nanoclusters, one-dimensional nanofibers or nanotubes, two-dimensional nanosheets or films and three-dimensional hydrogels [9,36]. The electrostatic, hydrophobic, ligand-receptor interactions, hydrogen bonds, nucleic acid base pairing exhibit significant function for biomolecules aggregation may lead to nanomaterials synthesis. Biological molecules self-assembly are susceptible to different factors such as pH, temperature, surfactants, ethanol, nanoparticles or enzymes.

Different self-congregated peptide nanostructures such as nano tubes, fibers, spheres or sheets and hydrogels have been used [37]. Lee et al. synthesized water levitating nanosheets at the interface of air and water by applying folding of peptides into helical structure [38]. Motamed et al. constructed peptide hydrogel by N-acetylated-β3- tripeptides helices with 7.5 mg/mL concentration operated with hydrophobic acyl chain and produced hydrogel by self-assembly into nanofibers [39]. Ling et al. fabricated two-dimensional peptide nanosheets by the aggregation of peptide amphiphile F6C11 [40]. Xing et al. produced insertable peptide hydrogels via aggregation of peptides of opposite charges [41]. The three-dimensional hydrogels through peptide self-assembly was utilized for drug delivery, tissue engineering and development of biosensors [42].

Proteins self-assembly function of play a key role in gene delivery, regeneration of tissues, molecular identification etc. Many self-assembled protein nanostructures from zero- to three-dimensional have been designed [43]. Amyloid protein fibrils were utilized for the construction of nanofilms [44]. Liljestrom et al. constructed three-dimensional protein crystals via electrostatic interactions in which biotin avidin compounds with a positive charge was formed and after mixing with Cowpea chlorotic mottle virus (contained negative charge), it was gathered into three-dimensional crystals via electrostatic interaction [45]. Kong and Li synthesized a protein hydrogel formed from reconstitution of protein fragments [46]. The silk fibroin was used for the construction of protein nanofibers [47]. Qiao et al. designed two-dimensional wavy structures by Vshape SMAC protein self-assembly by metal coordination [48].

Enzymes can be immobilized on the surface of electrodes by self-assembly for biosensor application. The enzymes can be linked on materials surface by covalent and non-covalent attachment [49]. Lin et al. fabricated enzyme-inorganic hybrid nanoflowers by horseradish peroxidase and copper phosphate self-assembly by metal coordination to enhance nucleation of horseradish peroxidase and copper crystals [50]. Liang et al. designed Zn²⁺/adenosine monophosphate hydrogel for glucose oxidase and horseradish peroxidase loading simultaneously by coordination between enzymes and metal ions [51].

DNA-based nanostructures act as promising constituents which can be used because of their significant applications in biosensors, biomedicine and nano-sciences [52]. He et al. designed DNA nanoparticles by novel methods in which the use of reaction of hairpin assembly, self-assembly of sticky end, target DNA was self-assembled onto nanoparticles [52]. Zhang et al. developed DNA hydrogel activated by aptamer for the identification of protein [53]. DNA nanotubes were synthesized by DNA origami seeds [54]. Ribonucleic acid was used to design nanostructures by bottom-up strategy as ribonucleic acid can produce different base pairs with two hydrogen bonds and can be used as an important component in the development of valuable nanomaterials. The knowledge of self-assembly mechanism behind aggregation of ribonucleic acid molecules are linked with inter- and intra-ribonucleic acid interactions, which are important for the synthesis of useful RNA nanostructures [55].

A virus can aggregate on structures of various dimensions in a substrate, solution, or liquid/liquid interface. The tobacco mosaic virus was used to aggregate on the nanorods, nanofibers and nanowires [56]. The virus-based structures have been used in sensing, diagnosis, tissue engineering and synthesis of hybrid nanomaterials.

The biopolymers such as sodium alginate can be obtained from algae, chitosan from animals and plants such as soybean are rich source of biopolymers. Viswanathan et al. constructed biopolymer nanofibers with cellulose heparin complexes aggregation in non-volatile ionic liquid solvent by electrospinning [57]. The biopolymers can aggregate to form hydrogels, nanofibers and films and exhibited various uses [58]. Guan et al. fabricated biopolymer hydrogels with chitosan covered with silver ions for their antibacterial applications [59].

5. Strategies for Synthesis of Bio-Inspired Materials

The biomolecules exhibited immense potential for fabrication of hybrid bionanomaterials with enhanced properties via different approaches such as the interaction between molecules, the identification of molecules, molecule medicated oxidization or reduction or nucleation [60,61]. With the help of bio-inspiration strategies, various hybrid bionanomaterials by combination of nanoparticles, carbon nanotubes, graphene can be designed to develop zero-dimensional [62,63], one-dimensional [63,64], two-dimensional [65,66] and three-dimensional structures [67,68,69]. The knowledge about bio-fabrication and its application in development of sustainable materials can promote innovation [70,71]. Bio-inspired polymeric structures and with special wettability and their applications are shown in Figure 5.

5.1. Molecule-Molecule Interaction

The development of functional nanomaterials through molecule-molecule interactions provides many benefits such as regulated size, nanostructure, properties and functions. The interactions between molecules such as peptide-peptide, protein-protein, and protein-peptide and between two DNA molecules have been used in the synthesis of hybrid nanomaterials [60,73].

5.2. Molecule-Material Identification for Bio-Inspired Synthesis

Biomolecules interaction and materials such as metal nanoparticles, solid substrate, graphene and carbon nanotube can promote the development of hybrid nanomaterials [74]. Song et al. constructed hybrid structure of virus and graphene oxide for identification and purification since under room temperature, graphene oxide can identify and grab the virus and then sterilize it under rising temperature [75]. Li et al. produced copper-based protein nanoflowers on polyvinyl alcohol-co-ethylene/PET nanofiber membrane [76]. It was soaked in a solution of sodium hydroxide and dioxane for copper ions sorption. At the end, nanofiber membrane with copper ions was immersed in protein solution and copper based inorganic protein nanoflowers were synthesized.

5.3. Molecule Mediated Nucleation

It is a sustainable method for production of bio-fabricated hybrid nanomaterials in which biomolecules or bio-nanomaterials can promote nanoclusters generation when reducing agents were absent [77,78].

5.4. Molecule Mediated Oxidization or Reduction

Molecule moderated reduction or oxidization for the development of hybrid nanomaterials depends on reactions with oxidizing or reducing agents.

Biomolecular structures assist in the production and binding of different nanoparticles for production of hybrid materials. Nanofibrils of fibrinogen by regulating fibrinogen aggregation in ethanol and nanofibrils were used to adsorb tetrachloroaurate and formed gold nanoparticles by reduction [79]. They reported the production of fibrinogen, globulin hemoglobin and the fabrication of gold nanoparticles by adsorption and chemical reduction method [63]. The molecular self-assembly of amyloid-β (Aβ) peptide was carried out for the production of nanoribbons, nanofibrils and nanoflowers which were used for bio-fabrication of gold nanostructures.

6. Development of Bio-Inspired Functional Materials from Biological Resources

The biological materials exhibit extra-ordinary mechanical and functional properties, however, they have weak constituents and a mild synthesis processes. The efficiency of the performance of biological materials exists in the hierarchical structure i.e., how the structural components are arranged at multiple length scales. The development of bio-inspired materials requires fundamental information of biological organisms that may inspire the fabrication of hierarchically structured substances with enhanced characteristics which can provide a solution for environmental problems. Biomaterials are developed by nature so that they can survive under challenging environmental conditions [80]. Yan et al. revealed that they show intelligent strategy which fulfils a variety of mechanical as well as functional requirements [81]. These are distinct from the engineering materials which are based on complicated chemicals or costly production processes. Due to their robust nature, solidity and non-hazardous nature, such substances provide inspiration for designing modern advanced materials. Therefore, biological materials are the inspiration source for the development of new designs. The hierarchical structure helps natural materials to perform better, for example spider silk has tensile strength of 1.1 GPa and high strength steel has 1.5 GPa whereas in density, spider silk is four times stronger per unit mass. The spider silk structure was used for collection of fog water harvesting substances [82]. Chen et al. found Sarracenia trichome mimicked as hierarchical microchannel substance for fog water harvesting [16]. Spider web and cactus-like fog collectors were developed with laser structuring and origami methods [6]. Materials were developed from the inspiration of nacre-like structure by a combination of optical transparency and toughness [83].

Similarly, the nacre-inspired development of tough substances and crustaceans fabricated fracture resistant materials [17,84]. Synthesis of micro- or nano-scale bio-fabricated hybrid materials from bone or nacre inspired structural substances have been reported in earlier workers [85]. The biological structures show various functions such as lotus leaves exhibiting topographies of a surface which permits purification [86] and butterfly colors are reflected due to the interaction of microstructure with light [87]. The ecstatic role was observed by biomaterials that are stable, authentic, non-hazardous thus have been inspiring for the fabrication of functional materials with wide practical applications such as bio-inspired self-shaping composites [88], bio-inspired anticorrosion coatings [89], gecko-inspired adhesion pads and nature inspired reversible underwater adhesives [90]. Libonati et al. designed osteonal secondary structure of mammalian bone inclined structure on fiber-reinforced composites [91]. The PSeD-U elastomers were used with unique physical and covalent hybrid crosslinking structure for synthesis of skin-like substances [92]. Kimura et al. developed sticking devices stimulated by insect footpads which contain multiple hairs and produce liquid; they generate capillary force in which the footpad attaches to the surface [93].

Bio-inspired materials are of two types i.e., functional and structural. Bio-fabricated materials will emphasize structures that show super wettability, bioactivity, and response against stimuli whereas bio-fabricated structures will incorporate such designs which provide better durability with less mass. A classification of nature inspired materials by virtue of the gain they provide in shown in Figure 6. Many structures utilize super wettability to conduct important role which can provide constraint at outer surface and transport of liquid. The surface super wettability can be obtained by physical structure or chemical components present on surface. This provides inspiration for the development of bio-inspired structures with specific wettability. The structures with super wettability can be observed in nature such as rose petals [94], cactus spines [95], leaves of lotus [90], and exoskeletons of desert beetles and. Shark skin [96]. The surfaces with super wettability show many properties such as hydrodynamic drag, antireflection and self-cleaning [97]. Mollusk shells and teeth are highly mineralized biological materials as they provide high strength and toughness [98]. The mineral constituents are hard and stiff whereas organic components play pivotal role in increasing the toughness.

Yang et al. developed superhydrophobic eggbeater-like structure by three-dimensional printing which was stimulated Salvinia molesta structure [99]. The eggbeater structure developed from a photocurable substance exhibited hydrophobic and water sticking properties. A Salvinia stimulated structure can be used to separate oil from water within six seconds. Shark skin shows many small riblets that helps in motion during swimming. The riblets can significantly decrease the resistance of water flow over skin surface. Liu et al. synthesized polydimethylsiloxane hydrophobic films which imitated shark skin structure by the chemical alteration or duplication method [100]. PDMS films exhibited large water contact angles in comparison to non-grooved structures. Thus, PDMS films provide superhydrophobicity and self-cleaning functions. PDMS films can trap air and liquid can freely move on surface, which shows slippage and decreases flow resistance. The spines of cacti collect water by their hierarchical surface structure under arid environmental conditions. Ju et al. designed bio-fabricated water collector by gradient chemical alteration and electrochemical corrosion [101]. Artificial cactus spine stores water drops at a fast rate and transports them rapidly. The stems of many plants are strong parts by which they can survive under harsh environmental conditions. Thalia dealbata, perennial plant can survive under strong winds due to its magnificent height/diameter ratio. Its porous stem reflected lamellar layers with joint bridges [102]. Hierarchical structure was imitated in graphene aerogels by using bidirectional freezing methods.

Lignin, present in wood is an indeterminate matrix substance which attaches cellulose fibrils and indicate pivotal functional in enhancing robustness [103]. Yu et al. reported development of polymeric woods by unformed polyphenol matrix substances such as phenol and melamine formaldehyde resins by aggregation and a thermocuring procedure of conventional resin [104]. The polymeric woods exhibited better compressive properties (strength up to 45 MPa) compared to natural woods. The axial compressive production and density of bio-fabricated wood was better in comparison to the other available engineering materials. The spider silk shows excellent mechanical properties due to their properly arranged and hydrogen bonded β-sheet crystals within protein matrix. Song et al. developed bio-inspired composite film by fabrication of graphene quantum dots on polyvinyl alcohol which acts as protein matrix [105]. The bio-inspired graphene quantum dots-reinforced PVA composite films revealed that by regulating graphene quantum dots content mechanical properties can be improved. It was observed that with an increase in the graphene quantum dots content up to 5% wt, productivity and elastic modulus of polyvinyl alcohol was increased by 66 and 88%, respectively.

7. Bio-Inspired 2D and 3D Nanomaterials for Sustainable Application

The bio-inspired materials exhibit mechanical, optical and electrical characteristics [106,107]. Mei et al. constructed a honeycomb-inspired CoMoOx nano-structures with 2D nanosheets made up of inter-connected pores and channels which showed better stoutness to structures and capacity for lithium storage in comparison to electrode materials with the same chemical composition but without any cellular nanostructure [108]. Electrophorus electricus, an aquatic animal also known as electric eel, can produce current and high voltage from ion gradient via membranes containing compact ion channels. The ion gradient power generator developed by inspiration of electric eel and novel triboelectric generators constructed from 2D materials have also been fabricated by the wings of hummingbird [109], human skin [110] and other natural structures. The polyacrylamide hydrogel mimicked by electric eel changes chemical to electrical energy which helps in synthesis of self-powered body implant [111]. Ravi et al. reported storage of electrical charge in different layers of photo-proteins separated from Rhodobacter spheroids [112]. These proteins can be used as charge storage media and light harvesting and developed self-charging biophotonic device. The alumina compact with bilayer structures with shape change facility during sintering were constructed which were based on plant seed dispersal units and can fold during differential swelling [113].

Gur et al. reported the optical features in male sapphirinid copepods that can be used for the development of latest optical or light-harvesting appliances [114]. The research revealed that biological substances produced from natural species showed better functions as electrocatalysis, photocatalysis and transformation of biomass [61].

The traditional fabrication technologies cannot exactly imitate the complex bio-fabricated structures which limits utilization of biomimetic approach. Bio-fabricated self-shaping is a strategy applied to generate three-dimensional structures with modification in internal structure of flat substance by application of external factors such as temperature and moisture which twists flat structure onto three-dimensional configurations. The bio-inspired self-shaping can produce substances that respond to external stimulant and there is no requirement of electrical power as an input [115,116]. With the advent of 3D printing methods, better bio-fabricated structures can be developed. Due to more demand of high-performance materials, devices with specific properties are being developed [117,118]. The three-dimensional printing procedures can be applied to develop structures with capricious geometry due to its flexible nature [119,120,121]. Three-dimensional printing technologies utilize various methods such as ink writing, laser sintering, fused deposition modelling, MultiJet printing, laser melting which can remove constraints in the development of bio-fabricated composition [81,122,123]. However, the price and rate of three-dimensional printing are major constraints. Three-dimensional printing is receiving attention because of its flexibility in shape. It can be used for imitating natural structures such as scaffolds developed from spinal cord [124], very light biomimetic form developed by cellular structure [125], and damage resistant building units constructed from crystal forms. With the advent of the multinozzle three-dimensional printing technique, materials variety can be extended with accuracy.

7.1. Environmental Applications

The applications of bio-inspired materials in different environmental and energy fields are shown in Figure 7. The industrialization, urbanization, overpopulation and exploitation of natural resources are responsible for the elevation of hazardous pollutants in the environment such as heavy metals, pesticides, cosmeceuticals, dye, pharmaceutically active compounds [126,127,128,129]. A huge amount of wastewater is released during the industrial operations which are considered as the most contaminated due to the presence of complex constituents [130]. Dyes show negative impacts on both the surface water and groundwater quality by blocking light transmission, affecting the photosynthesis of aquatic flora and by reducing dissolved oxygen in water [131]. Dyes are toxic to aquatic ecosystem and human beings due to its mutagenic, carcinogenic and genotoxic nature [132,133]. Many technologies have been applied for the treatment of wastewater such as electrochemical methods, reverse osmosis, ozone oxidation, chemical precipitation, membrane separation, adsorption and photocatalysis etc. [134]. The above-mentioned conventional remediation methods are not efficient for managing contaminants due to the long processing time, high energy requirement, expensive procedures and the production of sludge which obstructs their applicability at a large scale [135]. Hence, scientists are working toward the construction of new bio-nanomaterials to promote green methods [136].

Nowadays, more focus has been given to the nanomaterials for environmental clean-up due to their chemical properties, quantum size, flexible functions, adaptability, high ratio between surface area to volume. Bio-inspired materials have more surface to volume ratio, and they show target specific characteristics with locations for the binding of functional groups which can be utilized for the holding of pollutants [137,138]. Bio-inspired nanomaterials showed significant potential for the environmental uses such as wastewater treatment, removal of heavy metals, dye, purification of air and pollutants monitoring in the environment.

7.1.1. Removal of Toxic Metal Ions

Silica nanopowder calcium alginate hybrid was prepared for lead sorption [139]. Chen et al. reported a signal amplification process dependent on nicking endonuclease and methylene blue dye for the identification of mercury [140]. Li et al. developed electro-chemical biosensor with deoxyribo-nucleic acid hybridization chain reaction with the amplification of silver and gold nanoparticles for mercury identification [76]. Zhang et al. reported an amplified identification process for the detection of lead ions in aqueous medium by application of gold nanoparticles [85]. The iron supported on green nanosilica substances has been used for arsenate ions removal from aqueous solution. They reflected significant sorption ability i.e., 69 mg arsenic per g of Fe@GN [141].

Bolisetty and Mezzenga reported the development of bio-inspired hybrid membranes from aggregated amyloid protein fibrils and activated porous carbon for heavy metal removal from water [142]. Nandi et al. developed a hydrogel by peptide-based molecule at pH 7.5 which may be applied for nickel and cobalt elimination [143]. Yu et al. fabricated magnetic microspheres with biochar containing calcium alginate and Fe₃O₄ for copper ions removal from wastewater [144]. Glycine activated magnetic nanoparticles were introduced in alginate beads for lead ions adsorption from aqueous solution [145]. Chitosan, alginate and magnetic iron oxide@silica nanocomposites were developed for the elimination of lead [146]. Elimination of copper and uranium ions from wastewater by sodium alginate, polyvinyl alcohol and graphene oxide hydrogel microspheres was observed by Yi et al. [147]. The removal of copper, lead and cadmium ions from effluent by magnetic alginate microspheres was observed, based on Fe₃O₄/magnesium aluminum layered double hydroxide with encapsulation of Fe₃O₄ double layered hydroxide by calcium alginate [148].

Yu et al. stated development of AuNC-embedded protein nanofibers graphene hybrid membranes for detection and separation of mercury in water [104]. Dinari and Tabatabaeian reported biosynthesis of magnetic Fe₃O₄@layered double hydroxide@guargum bio-nanocomposites for purification of water [149]. The guargum bio-nanocomposites showed cadmium removal ability within 5 min from aqueous solution with 258 mg/g sorption ability. Lou et al. designed a sodium alginate composite modified by zirconium chitosan for elimination of arsenite and arsenate from wastewater [150]. It showed highest adsorption abilities i.e., 43 and 77 mg/g for arsenite and arsenate, respectively. Hu et al. reported the formation of magnetic carbon and aerogel composite made up of calcium alginate for removal of cadmium from soil [151]. The development of calcium crosslinked alginate-encapsulated bacteria biosorbent for elimination of cadmium from effluent [152]. Application of electrospun nano-fibrous hydrogel for the elimination of heavy metals from aqueous medium [153,154]. The synthesis of magnetic polymer diatomite composite for palladium removal from wastewater was reported by Rasoulzadeh et al. [155]. Zhang et al. reported protein amyloid fibrils can enhance zirconium dioxide particles production by the charged amino acids components and significantly removed fluoride [17]. The fluoride has shown its adverse effects on the plants, animals and entire ecosystem [156]. A high level of fluoride has been reported in surface and groundwater of more than nineteen states of India.

Many approaches have been used to eliminate fluoride from potable water [157,158]. Chitosan-zirconium nanocomposite was synthesized to analyze defluoridation process and it removed 99% fluoride with 97 mg/g sorption ability at pH 7 [159]. The aluminum impregnated coconut fiber showed 98% fluoride elimination ability with 0.05 g/l adsorbent amount for one hour exposure period at the pH 12 [160]. Peng et al. reported that tea waste contained aluminum oxide showed a 53% fluoride elimination ability [161].

The fluoride elimination capacity of titanium hydride treated Musa paradisica was 99% compared to the untreated plant material which showed 90% deletion at 40 °C with 2 mg/L initial concentration [162]. Chang et al. found that oyster shell coated with aluminum hydroxide exhibited significant ability of defluoridation [163].

7.1.2. Removal of Dyes

Adhikari et al. developed hydrogels by self-assembly of N-terminally Boc-protected tripeptides which can be used for wastewater treatment and hazardous dyes [164]. They reported that hydrogel can be synthesized by peptide dependent molecule at PBS with 7.5 pH [143]. The hydrogel showed network of nanofibrous structure which can removal cationic and anionic dyes. It was observed that less hydrogel content (<1 mg/mL) is required which can be reutilized thrice for dye adsorption. The synthesis of nanotube and organogel was reported by aggregation of tripeptides [165]. Soni et al. found malachite green removal by magnetic alginate microspheres which act as an adsorbent material [166].

Geetha et al. reported 99% malachite green elimination from aqueous medium by calcium alginate nanoparticles at 60 °C with 5 pH within ten minutes [167]. Nouri et al. reported photocatalytic degradation by sunlight for photo bio-composite beads production dependent on TiO₂/calcium alginate for adsorption of basic blue 41 dye [168]. Reddy et al. reported on the development of hydrogels and organogels by the co-assembly of many fluorenyl methoxycarbonyl- functionalized amino acids [169]. The synthesized hydrogels reflected 60% removal of methyl red from wastewater within five minutes. The spores of Lycopodium clavatum were used as a biotemplate for synthesis of micron-sized buckyball shaped TiO₂. Its photocatalytic function was evaluated by photocatalytic degradation of Rhodamine B dye [170]. The removal of methylene blue was observed by hydrogel nanocomposite which was made of sodium alginate and silica nanoparticles [171]. Methylene blue deletion from aqueous solution by carboxyl functionalized multiwalled carbon nanotubes embedded in calcium-alginate beads [153]. Magnetic beads of alginate inculcated with maghemite nanoparticles were treated with citrate and multilayered carbon nanotubes for the elimination of methylene blue from aqueous medium [172]. Fadillah et al. observed dye adsorption by graphene oxide/alginate nanocomposites. In the alginate matrix, graphene oxide was inserted and it showed (≥90%) removal through the implementation of a combined approach [173]. Qian et al. reported composite hydrogels formed by alginate and polyacrylamide for photocatalytic deterioration, dye elimination from solution [174].

Li and Yin reported Direct Green 1 and Acid Blue 113 removal from effluent by lanthanum-sodium alginate hydrogel developed by cross-linking reaction [175]. The production of hydrogel by sodium alginate and its activation by polyethyleneimine for elimination of methylene blue dye from effluent was observed by Godiya et al. [176]. The free radical polymerization method was used in production of sodium alginate/poly acrylic acid/ multi-walled carbon hydrogel nanocomposite which showed 2265% methylene blue sorption ability observed by nanohybrid construct at pH 8 [177]. The development of a nanocomposite by magnetic multi0walled carbon nanotubes-loaded alginate for dyes removal from the textile effluent was reported by Azari et al. [178]. Eltaweil et al. prepared metal organic composite beads adsorbent with inclusion of UiO-66-MOF and carboxylated graphene oxide onto sodium alginate [179]. The results indicated that construct showed high adsorption abilities 491 and 343 mg/g for methylene blue and copper ions. Boukoussa et al. synthesized composite beads based on calcium alginate infused with mesoporous silica for elimination of methylene blue and it showed 333 mg/g adsorption ability [180]. A limestone chitosan alginate nanocomposite was produced with mixture of limestone chitosan and alginate powder [181]. Nanocomposite significantly removed congo red and brilliant green from waste effluent. The brilliant green showed 2020 mg/g adsorption ability whereas congo red reflected maximum sorption 2250 mg/g.

7.1.3. Removal of Organic Pollutants

Graphene oxide was used for the adsorption of various organic as well as inorganic pollutants [182]. The rape pollen was used to develop and enlarge a specific area of copper-doped titanium dioxide hollow microspheres by sol-gel method. In this method, titanium (IV)-isopropoxide based sol was enveloped on rape pollen surface. The photocatalytic degradation of chlortetracycline was reported by microspheres [183]. Deng et al. designed helical fiber actuators that may analyze solvent or vapor [184].

The calcium alginate beads were synthesized by Iron and nanosized magnetite immobilization to decrease nitrate level in groundwater [185]. Sun et al. found MOF (UiO-66) sodium alginate dependent sorbent for elimination of pharmaceutical compounds from wastewater [186]. Soltani et al. reported peroxymonosulfate activation by carbon nanosphere encapsulated on hydrogel of calcium alginate for deterioration of acetaminophen [187]. Verma et al. observed organic contaminants sorption by sodium alginate or graphite dependent hybrid hydrogel [188]. Eltaweil et al. used UiO-66/GOCOOH@SA beads for the elimination of various organic and inorganic pollutants [179].

Younis et al. synthesized CTAB silicon dioxide @alginate, urea silicon dioxide @alginate by encapsulation of cetyltrimethyl ammonium bromide and urea activated silicon dioxide nanoparticles in alginate hydrogel [189]. They found that the adsorption of chlorophenol from petrochemical wastewater was more with CTAB silicon dioxide@alginate (7.9 L/g) in comparison to urea silicon dioxide@alginate microbeads (3.8 L/g). da Costa et al. synthesized composite hydrogel made up of alginate grafted mesoporous silica and poly vinyl alcohol for toluene and benzene sorption from wastewater [190]. A lower amount (10 mg) of composite reflected maximum sorption capacity was observed: 1482.8 mg/g benzene and 596.6 mg/g toluene at pH 7 and 25 °C.

7.1.4. Monitoring of Pesticide Residues

Sun et al. constructed an amperometric biosensor by the changes in carbon electrode with gold nanospheres and chitosan from chitosan sorption [191]. Residues of pesticide were attached on modified electrode encapsulated by acetylcholinesterase. Inspired acetylcholinesterase dependent biosensors have been used for the identification of pesticides in vegetables and fruits. Their detection range were 0.1–150 and 0.1–200 μg/L for chlorpyrifos and carbofuran, respectively. Chen et al. constructed a biosensor by acetylcholinesterase aggregation on acoustic resonator surfaces; t promoted organophosphorous pesticides binding [192].

7.1.5. Removal of Pathogenic Micro-Organisms

The pathogenic micro-organisms such as bacteria, viruses and parasites are present in the surface and groundwater, which are responsible for various ailments such as fever, diarrhea, hepatitis etc. In comparison with traditional nanomaterials, bio-fabricated nanomaterials have higher activity due to good bio-compatibility and large specific surface area and they can easily remove infectious microbes from water. Song et al. constructed biosensor by applying nicking enzymes aggregation on carbon nanotubes for the detection of Salmonella enteritidis [193]. The biosensor showed a linear association with Salmonella enteritidis amount from 10² to 3 × 10³ and 1.5 × 10² to 3 × 10³ CFU/mL in water and milk, respectively.

Miller et al. synthesized colistin functionalized gold nanoparticles for capturing of Acinetobacter baumannii [194]. Gold nanoparticles were linked with colistin by applying PEG as linker by self-assembly and colistin-PEG gold nanoparticles hybrid nanomaterials reflected Acinetobacter baumannii sorption within seven minutes. Thin membrane for removal of Escherichia coli from waste effluent by cellulose nanofibers, palm fruit stalk and activated carbon was synthesized by Hassan et al. [195]. The hybrid membrane exhibited significant capacity for removal of Escherichia coli and Staphylococcus aureus.

The antibody conjugated Fe₃O₄-Ag@APTES hetero-nanocomposites were analyzed Salmonella enteritidis at a much lower concentration [196]. Magnetic materials revealed 91% efficiency for elimination of Salmonella enteritidis from liquid medium within 20 min.

7.1.6. Air Purification

Self-assembly of biomolecules have been utilized for the development of carbon-dioxide capturing materials for air purification. Li et al. reported sequence of hexapeptide from Tau protein which aggregate on amyloid fibers for carbon storage. Amyloid fibers with alkylamine groups can attach to carbon dioxide via carbamate production [197]. Radhakrishnan et al. developed natural absorbents by crosslinking and accumulation of Moringa oleifera pods and calcium alginate, it showed significant reduction in the emission of gas exhaust from vehicles [198]. The sorption ability was 15 and 124 mg/g for hydrocarbons and nitrogen oxides, respectively.

By aggregation of 1,3,5-benzenetrisamide polymer, supramolecular nanofibers were generated which were present in polyester microfiber nonwovens [199]. Constructed nanofiber-based filter showed a significant filtering ability for elimination of more than 95% particles and offer promising potential for air filtration application. The carbonic anhydrase-based bio-inspired materials are considered as one of the significant approaches for sequestration of carbon dioxide. The silica-carbonic anhydrase nanocomposite bio-inspired materials were constructed by R5 mediated auto-encapsulation for the generation of sturdy biocatalyst for capturing of carbon dioxide [200]. A summary of a variety of biomaterials utilized in environmental applications is presented in

Table 1. Development of bio-inspired materials from biomolecules for environmental applications.

Biomolecules	Bio-Inspired Materials	Applications	References
Cellulose	Carbon fiber film	Detection of Escherichia coli	Hassan et al. [195]
Chitosan	Chitosan Au-dependent Acetylcholinesterase sensor	Chlorpyrifos and carbofuran sensing	Sun et al. [191]
Peptide	Hydrogels	Removal of anionic and cationic dyes	Nandi et al. [143]
Peptide	Hydrogels	Removal of cobalt and nickle ions	Nandi et al. [143]
Peptide or hexapeptide	Nanofibers	Removal of CO2	Li et al. [197]
Peptide	Nanofibrils on Au electrode	Identification of Cu2+ ions	Viguier et al. [201]
Tripeptide	Hydrogel and organogel	Removal of toxic organic dyes	Koley and A. Pramanik [165]
Polyamide	Hydrogel and xerogel	Removal of crystal violet dye	Kar et al. [202]
Glutamic acid	Biomolecule -metal two-dimensional framework	Anionic dye	Pu et al. [203]
Protein	Nanofiber-carbon	Removal of metal ions	Bolisetty and Mezzenga [142]
Bovine serum albumin	BSA-AuNPs/polymer NFs	Identification of Pb2+ ions	Li et al. [204]
Bovine serum albumin	Nanofiber-AuNC	Identification of Hg2+ ions	Yu et al. [104]
Polydopamine	Graphene aerogel	Organic dyes	Cheng et al. [205]
Deoxyribozymes	Hemin/quadruplex/DNAzyme	Removal of Pb2+ ions	Pelossof et al. [206]
Deoxyribozymes	Nanoporous AuNPs	Removal of Pb2+ ions	Zhang et al. [85]
Deoxyribozymes	Hemin/quadruplex/DNAzyme	Removal of Cu2+ ions	Ge et al. [207]
Laccase	3D Graphene oxide/carbon nanotubes	Elimination of water soluble dyes	Li et al. [208]
Acetylcholinesterase Nicking enzyme	Acetylcholinesterase-FBARs	Removal of ethyl paranitro-phenyl	Chen et al. [192]
Acetylcholinesterase Nicking enzyme	Enzyme carbon nanotubes	Identification of Salmonella enteritidis	Song et al. [193]
DNA	Aptamer-Au nanoparticles	Removal of E. coli	Jin et al. [209]
DNA	Ag@Au nanoparticles	Removal of Hg2+ ions	Li et al. [67]
DNA	Aptamer- carbon nanotubes	Removal of Hg2+ ions	Guo et al. [210]
DNA	Photonic crystals	Removal of Hg2+ ions	Zhang et al. [53]
Oligo DNA	DNA–Au nanoparticles	Removal of methylene blue dye	Mashhadizadeh and Talemi [211]
Oligo DNA	DNA–Au nanoparticles	Removal of Hg2+ ions	Memon et al. [212]

.

7.1.7. Fluorescent and Colorimetric Monitoring

The fluorescence probe detection by fluorescence analysis has been utilized for the identification of heavy metals with bio-fabricated nanomaterials. Guo et al. constructed fluorescent sensor for identification of mercury by applying Sybr Green I as signal reporter and aptamer single walled carbon nanotubes as a quencher [210]. Zhao et al. synthesized fluorescence sensor dependent on xylenol orange activated cadmium sulfide quantum dots for the identification of lead ions [213].

Li et al. synthesized colorimetric lead ion sensor via applying nanofiber nitrocellulose membrane as scaffold and BSA gold nanoparticles composite as probe [204]. Mashhadizadeh and Talemi provided details of DNA spectrophotometric biosensor for checking mercury ions by utilizing oligonucleotides self-assembly on the surface of gold nanoparticles [211]. Memon et al. fabricated colorimetric mercury sensor by utilizing the DNA oligonucleotides and gold nanoparticles as indicators [212].

7.2. Bio-Inspired Materials for Energy Application

More than eighty five percent energy requirements are fulfilled by fossil fuels [214]. The utilization of fossil fuels has led to the release of carbon dioxide, methane, nitrogen dioxide and chlorofluorocarbons in the atmosphere. The bio-fabricated sorbent materials such as activated carbon, zeolite and hybrid nanomaterials have been considered as energy efficient approach compared to traditional adsorption materials [215].

Table 2. Fabrication of bio-inspired materials from biomolecules for energy applications.

Applications	Biomolecules	Bio-Inspired Materials	Performance	References
Microbial and enzymatic fuel cells	Laccase	Carbon nanotubes	High power density	Zhao et al. [216]
	Peptide nanotubes	Riboflavin as redox mediator on carbon cloth	Enhanced efficiency for power production	Xu and Quan [217]
	Nanotube of peptides	Redox mediator riboflavin on carbon sheet	Increased removal of azo dyes	Xu et al. [218]
	Protein	Ferrooxin	More efficiency of electron transfer	Roger et al. [219]
Solar cells	Peptide	Titanium dioxide	High efficiency	Orf et al. [220]
	DNA	Titanium dioxide	Long life cycle	Nithiyanantham et al. [221]
	Engineered M13	Single walled nanotube/Titanium dioxide	High electron mobility	Dang et al. [222]
Photocatalyst	Peptide	CdS/Pt nanoparticle	Light stability and resistance	Slocik et al. [223]
	Protein	Titanium dioxide	Useful for regeneration after the treatment	Garderes et al. [224]
Electrocatalyst	Peptide	Nickle nanocomposites	Better catalytic function	Reback et al. [225]
	Peptide	AuPd	More catalytic efficiency	Li et al. [65]
	Protein	FePt	Improved catalytic efficiency	Wei et al. [226]
Hydrogenation catalyst	Peptide	Metal/palladium nanoparticles	Catalyst can be reutilized	Pacardo et al. [227]
Supercapacitors	DNA	Carbon nanotubes	Better biocompatibility and circulation stability	Hur et al. [228]
	DNA	Titanium dioxide	Increase in capacitance	Nithiyanantham et al. [221]
	Peptide	Carbon	Increase in capacitance	Beker et al. [229]
Light harvesting materials	Protein nanowires	Micelles	Templates are flexible	Sun et al. [230]
	Dipeptide	Porphyrin	Enhanced photocatalytic ability and durability	Liu et al. [231]
	SP1	Quantam dots	High energy transfer ability	Miao et al. [232]
	DNA	Porphyrin	More light harvesting and transmission ability	Woller et al. [233]
	DNA origami	Cy3 and Cy5	Enhanced capacity of energy transfer and light harvesting	Hemmig et al. [234]
	Tobacco mosaic virus	Cysteine residues with fluorescent chromophores	Increased light accumulation and energy transfer ability	Miller et al. [235]
Sodium-ion battery	M13	Single walled carbon nanotubes	Enhanced power and capacity	Moradi et al. [236]
Lithium ion battery	Tobacco mosaic virus	Silicon	Excellent discharge cycle stability	Chen et al. [237]
Lithium-S ion battery	Peptide	Sulphur	Promoting recyclability	Jewel et al. [238]
Microcell	Virus	Cobalt oxide	Stable and flexible	Nam et al. [239]
Battery of nickle-zinc	Tobacco mosaic virus	Metal	Enhanced durability and surface area of battery	Chu et al. [240]

highlights the bio-inspired materials produced from biomolecules for energy fields.

7.2.1. Microbial Biofuel Cells

They can transform biochemical metabolic into electric energy and show significant application in energy and environment. Xu et al. reported on a new anodic modification procedure by applying peptide nanotubes as they stabilized riboflavin oxidation and reduction mediators on the carbon fiber [241]. The electron transfer ability was increased by RF/PNT anode between bacteria, electrodes and electrochemical activity and can improve power production ability of microbial fuel cells. Electronic conductivity and microbial fuel cells capacity was enhanced by PNT/RMs/CP cathode and removed azo dyes effectively. Sun et al. utilized protein nanowires in development of scaffolds for light harvesting system [230]. They observed that a protein self-assembly mainly depends on electrostatic interactions. Henry et al. found that a protein self-assembly was incited by DNA to obtain innovative light harvesting material [242].

7.2.2. Solar Cells

The bio-fabrication can help in the construction of solar cells. For light storage in the solar cells, peptide nanomaterials can be applied. Dang et al. applied genetically engineered M13 virus for production of single walled carbon nanotube TiO₂ core-shell nanocomposites [222]. This system showed excellent movement of electrons which can be utilized to increase power conversion capacity in dye sensitized solar cells. The peptide nanostructures act as scaffold which permit energy transfer and show absorption ability in ultraviolet region, by enhancing their capacity they were used for dye sensitized solar cells [243]. Titanium dioxide can be merged with aggregated protein fibers to produce a template for titanium dioxide photocatalyst [224]. Wang et al. fabricated copper phosphate nanosheets in flower shape in which titanium dioxide nanoparticles were integrated over the flower petals which act as photocatalyst for solar light harvesting device [244]. It acts as an antenna for absorption of solar energy and converts water molecules into oxygen and hydrogen gases. This phenomenon is similar to the photosynthesis in plants. The photocatalysts generated from self-assembly of biomolecules and bio-fabrication showed high catalytic ability and can be applied in mimicking photosynthesis.

7.2.3. Battery and Supercapacitors

The M13 virus protein shells after modification were used as biological template for the synthesis of Co₃O₄ and Au-Co₃O₄ nanowires via electrostatic interactions [245]. The electrode materials developed from aggregated materials could be utilized as lithium-ion batteries and they may enhance battery power and promote its stability, with more charging potential. The production of three-dimensional high capacity composite silicon anode via a tobacco mosaic virus template was given by Chen et al. [237]. Bio-inspired materials have been utilized as electrode substances in rechargeable lithium batteries which are efficient in promoting electron transfer, offer more electrochemical ability for better function with high specific capacity and recycling time [32]. Lu et al. used recombinant elastin-like polypeptides with metal binding motifs protein hydrogel for production of bio-fabricated N, F co-doped three-dimensional graphitized MnF2 nanocrystal carbon foams [246].

Chu et al. reported the synthesis of nickel-plated tobacco mosaic virus which was assembled on gold coated nanorod and synthesized three-dimensional layered electrodes [240]. The nickel/nickel oxide electrode with tobacco mosaic virus used as template significantly enhanced the accumulation power of battery and electrochemical cycling durability. Fu et al. generated lithium sulfide batteries by applying a polysulfide nanofilter by mixing aggregated proteins and conducting particles for durable conductive layer [247]. Metal oxides such as tin tungstate, niobium pentoxide and strontium titanate can link with biomolecules to enhance capacity of supercapacitors. The hybrid nanomaterials constructed from biomolecular self-assembly and bio-inspired production of transition metals oxides/sulfides/nitrides of iron, nickel, cobalt or manganese can be used for catalytic hydrogen production.

Supercapacitors can store energy between a traditional capacitor and a rechargeable battery, and it shows more power and stability. By a wet-spinning process, DNA with carbon nanotube fibers were linked to form DNA-single walled carbon nanotube-hybrid fibers [248]. The cross-linked DNA acted as a linker on carbon nanotube sidewall via π-π interaction increased capacitance and conductivity. The DNA single-walled carbon nanotube fiber showed better functioning of supercapacitor.

7.2.4. Electrocatalysis

The self-assembled metallo-catalysts can be produced by peptides aggregation by non-covalent interactions [249]. Wei et al. found ferritin-mediated biomimetic production of FePt nanoparticles on graphene nanosheets [226]. The produced graphene nanosheets-FePt nanohybrids used for various functions such as ferromagnetism, fluorescence and increased electrocatalytic power. Graphene nanosheets FePt nanohybrids have various uses in drug delivery, cell imaging and biosensors. The electrocatalyst was prepared from structured peptide outer coordination sphere by Reback et al. [225]. The nickel was incorporated in the peptide-based metal complex provided platform for better efficiency of electrocatalyst, which escalated electrochemical efficiency.

For application in catalysis, peptide sequence-based gold palladium catalyst was used [208]. The bio-fabricated Au33Pd67 showed better catalytic capacity and peptide-enabled method for the fabrication of electro-catalysts. For application in energy sector, M13 virus was used as template to synthesize an electrocatalyst [250]. Due to solderability, cost effective production and more durability of M13, it can be applied as scaffold for construction of bioactive catalysts. The utilization of natural substrates for technological applications is given in

Table 3. Natural substrates used for technological applications.

Template	Morphology	Applications	References
Luffa sponge	Biomorphic hierarchical porous structures	Catalysis	Zampieri et al. [251]
Butterfly wings	Quasi-honeycomb	Photovoltaics	Zhang et al. [252]
Spider’s silk fibers	Periodic lumpy knots on hydrophobic fibers	Fog harvesting	Zheng et al. [253]
Lizard’s surfaces	Microstructured honeycomb-like superhydrophilic patterns	Fog harvesting	Comanns et al. [254]
Cactus	Integrated spines and microgrooved surfaces	Fog harvesting	Ju et al. [255]
Maple leaf	Hierarchical porous structure	Catalysis	Feng et al. [256]
Cotton fiber	Biomorphic microtubule	Sensing	Song et al. [257]
Diatom	Porous architecture	Photocatalysis	Van Eynde et al. [258]
Rape pollen	Hollow microspheres	Photocatalysis	Bu and Zhuang [183]

.

8. Challenges in Development of Bio-Fabricated Materials

The production of bio-inspired materials has enhanced our knowledge on biomaterials which helps in the production of novel bio-inspired substances with various characteristics. However, there are many constraints which should be discussed for better future of bio-fabricated substances. The development of bio-inspired design is a multidisciplinary approach and constraint lies in its function detection and transfer of natural concepts into an engineering aspect and due to this, there is slow progress in this area.

The hierarchical structures of different sizes from atomic, molecular to the macroscales, their proper characterization and in-depth information needs highly advanced methods. The mechanism of molecular toughening for protein fibrils and chains is very difficult due to the sophisticated texture of polymers can be degraded by electron beam, vacuum and desiccation.

It has been observed by scientists that it is a very difficult task to duplicate the hierarchical structures of biological materials as they contain a fabricated architecture which is beyond the production technique of modern nanotechnology. Many bio-inspired materials have been fabricated in laboratory with restricted size and their characteristics have been anaylzed without any defined regulations. The processes of synthesis which can generate structural properties at multiple length scale are tedious and costly which shows constraints for its production at industrial scale. There is need for advanced fabrication equipment that can synthesize structures after bio-fabrication and transfer laboratory synthesis to mass production. However, knowledge needed for the design of materials, association between the production process, structure, characteristics and kinetics is not available. For the enhancement of industrial production, cost analysis, resources availability and down-stream processing are significant but separation and purification of materials are generally ignored at laboratory scale. The natural materials have q self-degradation ability and 5h3y would not generate any harmful effect on environment. Bio-inspired materials must adopt green manufacturing routes for recyclability.

The methods for bio-fabrication utilize additives which may present in the end product as contamination. The downstream purification procedure is energy intensive process which may show adverse impact on environment. Hence, there is requirement of eco-friendly downstream purification processes for production of high quality, functional porous materials. Despite the above-mentioned constraints, bio-fabricated materials have become significant components in enhancing novel innovations for modern industries. There are several opportunities for the development of bio-inspired materials. Further investigations are needed for development of bio-fabrication processes for industrial relevance with better properties, scalability, compatibility and economics.

9. Conclusions

The bio-fabricated materials have shown various applications in the environment and energy sectors. Bio-fabricated materials can be used due to various benefits such as easy construction, flexible nature, better functioning which promoted their applications in development of light-harvesting systems, batteries, biofuel cells, water and air cleansing, monitoring of different environmental parameters. The use of bio-fabricated materials specifically developed from biomolecules should be promoted for environmental application. Development of two- and three-dimensional hybrid bio-fabricated nanomaterials is required for energy application. Biomolecular superstructures such as two-dimensional networks and three-dimensional hydrogels can be utilized to promote porous materials development for metal ions adsorption, binding of molecular superstructures with graphene and for synthesis of aero or hydrogels for removal of coloring materials. Two- and three-dimensional biomolecular superstructure dependent nanomaterials can be used for development of drug delivery and photo-thermal therapy of tumors due to the light-harvesting properties. Bio-inspired materials should be fabricated with digitalized process which can be carried out by predictive modelling and simulations. There is a need to develop recyclable bio-fabricated materials which may be utilized to reduce carbon footprint to deal with the challenges of sustainable development. Investigations are needed for the identification of possible risks of engineered materials on environment and living organisms. However, no reports are available to date about the hazards of bio-inspired materials on animals and human-beings. Though, the environmental concerns may enhance the large-scale applications of bio-inspired materials.