Recent exploration of bio-mimetic nanomaterial for potential biomedical applications

1. Introduction

The most promising areas of research within the array of biological materials science and engineering nowadays are bioinspired, biomimetic and nanobiomaterials. The technological significance of this area is immensely used for applications such as drug delivery systems to bio-mimicked sensors, tissue engineering and optical devices [1]. The research and refinement of basics activities found in biological systems are involved in the study of bio-inspiration and biomimetic which has evolved through development. Applying the knowledge involves production of unique and exciting basic technologies and also this has modern methods for resolving various scientific issues [2].

For example, particles of polymer and lipid are used as synthetic carriers, which often struggle to meet clinical expectations. Therefore natural particulates ranging from pathogens to mammalian cells are worth examining in further depth, due to their highly optimized in vivo activity and convincing features that are often desired in drug delivery carriers [1,3]. Having clear understanding of these biological systems, researchers attempt to use natural particulates for multiple applications in the delivery of proteins, small interfering RNA (siRNA) and other therapeutic agents. The natural drug delivery carrier boosts up the thirst for new drug delivery systems, which has been reviewed here.

1.1. Interpretation, concepts and fields of biomimetics

1.1.1. Interpretation of biomimetics

The term “biomimetics” originates from the Greek words “bios” (life) and “mimesis” (to imitate), yet its definition is not as simple as just those two words. Biomimetics is the understanding of nature and natural phenomena to study the principles of underlying mechanisms, to obtain ideas from nature, and to apply concepts that may benefit science, engineering, and medicine [1]. In terms of Chemistry it is defined as ‘a field that creates synthetic chemicals which act like biological molecules’. In the field Molecular biology it is defined as ‘the development of synthetic systems based on information from biological systems’. In Technology it is referred as ‘the formal study of biological processes and systems as a model for creating synthetic structures similar to those produced in nature’ [1].

1.1.2. Concepts of biomimetics

The study of biomimetics is not a recent trend, but for a long time it was a practice for researcher to use the idea of looking into nature and take the inspiration from it and make something new and conceptual. The idea mostly focused on the fact there is no model better than nature for developing something innovative, which has excellent productivity and function. It has been called by different names such as “intellectual structure” in Japan and “smart material” in the USA. This idea reduced research expenses and as well as gaining realistic result by eliminating waste [1,2].

1.1.3. Field of biomimetics

Biomimetics can help to avoid the extensive industrialization and resource extraction done by human. On the basis of innovating new products, the biomimetics can go beyond just applying natural properties. Such products can be designed to play a part in general industry as well as to provide human convenience in the fields of chemistry, biology, architecture, engineering, medicine, and biomedical engineering (Fig. 1a) [2,3].

Many human creations and innovations were inspired by biological systems. This program focuses on the intrinsic functions of biological molecules to direct 1) nanomaterial synthesis, 2) molecular recognition and 3) self-assembly processes to develop next generation bioinspired materials and devices that are multifunctional and sustainable [1,3]. Collaboration with engineers, biologists and clinical investigators have solved interdisciplinary biological problems and translate these technologies into commercial products for biomedical applications in sensing, imaging, drug screening, delivery and theranostics.

1.2. History of biomimetics and research methods

1.2.1. The history of biomimetics

Biomimetics is a broad field with a long history which will be found easily in everyday life and no knowledge will be needed to use this. From knives and axes inspired by the dental structures of currently extinct animals to the strongest cutting-edge carbon nanomaterials, bioengineering has always evolved along with human history [4,5]. Furthermore, the natural beauty of plant origin like Mimosa pudica plant inspired us to learn more lessons from nature (Fig. 1b). These type brand new biomimetics lesson can possible to use to solve different critical question in biomedical sciences.

Leonardo da Vinci's (1452–1519) work is a fundamental example of biomimicry was a “flying machine” designed by him that was inspired by a bird [6]. In the Far East; the turtleship was built by General Yi Sun-sin, which was modelled after a turtle, to fight Japanese raiders during invasions [7]. The Wright brothers (1867–1948) took note of the wings of eagles and made a powered airplane that succeeded in human flight for the first time in 1903. Over the next century, the airplane became faster, more stable, and more aerodynamic [8,9]. Schmitt was the first to coin the term biomimetics in 1957, and he announced a turning point for biology and technology [10]. Jack E Steele of NASA, who coined the word bionics in 1960, was also the first to use the word biomimetics in a paper in 1969, which led to the addition of the term to the dictionary in 1974. In 1997, Janine M Benyus published her book Biomimicry, which emphasizes that biomimicry is leading the path to a new age of technological development by taking lessons from nature as the groundwork for products, rather than just using it for raw materials [1]. Janine Benyus and others stepped further to organize a social enterprise called Biomimicry to share ideas and concepts of biomimicry and biomimetics as well as to connect interdisciplinary researchers, scientists, artists, engineers, business leaders, and stakeholders [5,10].

1.2.2. Research methods for biomimetics

There are six steps for basic research method for biomimetics, which can be used to apply biomimetics to design, product, service, and agriculture [4]. Like the sticky substance found in mussel byssal and spider silk (Fig. 2) [5,11,12].

Step 1: The functional possibilities of biologically should not be just applied as it is used by the organism, it must be researched to make an inspired design. Although the discovery or fusion is crucial for an increased profit was to make innovative technology, the idea was to make a simple creative design which can provide greater convenience for human life.

Step 2: The relationship between the function of the organism and the principles under which that function is achieved must be established. Through research and database compilation knowledge and application of various materials need to be accumulated. The relationship between structure and function can be observed by a scanning through electron microscopy technique which usually comes from the surface structure. These fine structures play an important role in the organism and are said to be the first step for biomimetics. The US researchers are using the Biomimicry Taxonomy as a practical database.

Step 3: The greatest challenge faced by biomimetics is to find their relationship with the organism and the environment to determine how nano- and microstructures function, especially if these have not been fully explored yet [4]. Finding substantial examples through the integration of biology, natural history, and materials science is the next step in biomimetic research.

Step 4: The next research frontier is to identify various functional and environmental adaptation mechanisms of organisms and their energy-minimizing design [4]. A successful example of this is the antireflective coating that was inspired by the 200 nm structures reflecting visible light rays from a moth's eye [4,13,14]. The nature of new biomimetic materials is to remodel hierarchical structures and their corresponding functions, and utilize them into something important.

Step 5: The combination of newly discovered materials with biomimetics research will be a key to understanding their applications and limitations [4]. Along with the pros and cons of biomimetics, the morphological and functional uses of the new material must first be understood as well, and the results from their combination have to be unravelled. Active research is going on these fronts, but making progress in these areas is realistically a difficult approach.

Step 6: The determined biological material's structure and the function is the source of innovation for the development of other materials as well as for a new material [4]. The tests and assessments that took place for the structure and function were for known materials, which then helps them to morph and evolve into new materials. By combining them with current advancements in medicine, chemistry, and nanotechnology, we may find novel utilities that may benefit human life.

2. Moving smaller and going natural in medical equipment and drug discovery

After billions of years of evolution, creatures in nature still acquire almost perfect frameworks and functions [15]. Biological materials which are characterized for multiscale structures ranging from nano, micro to macro have performed important role in achieving structural and functional integrity. A huge number of natural structures have been scrutinized by scientists and engineers in the last few decades for their multidisciplinary fields. The natural multiscale framework of biological structures was found to be multifunctional not unifunctional, i.e., it possess functions more than one. Therefore the practical coalescence, such as lotus leaves, butterfly wings, red rose petals, mussel byssal and spider silks was found to design useful materials. Apparently, the multifunctional properties are presented to their corresponding biomimetic materials. This may be lead to inspiration of collaborating professionals of material science, chemistry, physics, biology, engineering, etc., which is required for the further exploration of additional function of biological materials and judicious outline and the coherent edifice of bio-inspired multi objective materials [15].

2.1. Biomimetic materials of lotus leaves

It was seen for over millions of years that there are many plants in nature whose leaves demonstrate special wet-ability. Out of which the lotus (Nelumbo nucifera) leaf, which commonly known as water lily, is one of the most inspiring. For over 2000 years, Asian religious people took lotus as sacred flower that was referred as symbol of purity. It also has dirt resistant properties which was always praised and investigated, for a long time, by the scientists [[25], [26], [27]]. Lotus leaf surfaces are made up of micropapillae randomly distributed (5–9 mm in diameter) which is enveloped by magnificent branch-like nanostructures (Fig. 3b). The concurrence confers high water contact angle and small sliding angle due to multiscale surface and hydrophobic epicuticle wax, inducing super-hydrophobic and low-adhesion functions. They also have self-cleaning effect which is again known as lotus effect, because dirt particles get easily picked up by the spherical water droplets which roll across the lotus leaf easily cleaning its surface (Fig. 3) [[25], [26], [27]].

Many different composite approaches have been developed which were inspired by lotus leaves, this included constructing super-hydrophobic self-cleaning surface along with less water adherence by creating multiscale structural surface [25,[28], [29], [30]]. An elementary and cheap mechanism was developed by mimicking the lotus leaf, which leads to the preparation of a super-hydrophobic coating using isotactic polypropylene (i-PP) surface [21]. The resulting coating with a water contact angle of 160° was a gel-like porous which exhibited super-hydrophobicity. Other than glass slides, a wide variety of substrates, including aluminum foil, stainless steel, teflon, high density polyethylene, and polypropylene could be used to give coatings made of super-hydrophobic. For producing micro-to nanoscale fibers or particles, the electro-hydrodynamics (EHD) technique is a versatile and effective method [24]. Making use of the EHD method, a lotus-leaf-like microspheres and nanofibershas been prepared with super-hydrophobic porous composite structure [22]. The porous microsphere increases surface roughness contributing to the superhydrophobicity and a three-dimensional (3D) network which is formed by interweaving nanofibers to reinforce the composite film.

The special wettability of lotus leaves, large area, flexible and super-hydrophobic films inspired to fabricate thermal dispersing of silver nanoparticles on the designed flexible hemispheres arrangement and modifying it with 1-dodecanethiol [23]. The stratification of the obtained biomimic film is quite similar to the natural lotus leaves which consist of hierarchical micro/nano structures (Fig. 3b). A high water contact angle and a low sliding angle gave remarkable super-hydrophobicity and good flexibility of this kind of biomimic polymer film.

2.2. Biomimetic materials of butterfly wings

Since the 17th century Hooke, Newton, Lord Rayleigh everyone got amazed by the dazzling and diversity of colors produced by a wide variety of creatures in nature, which created interest for researching of some scientific giants. Colors are naturally created by complexion, structural color (iridescence), or a combination of both complexion and structural color [17]. The structural color results from the interaction between light with highly rigid and refined architectures, these has different characteristics which are not possible using complexion. Butterfly wings and peacock feathers are taken as a perfect example. An example of a bright insect is the Morpho butterfly (Fig. 4), which can be found in Central and South America, are famous for their dazzling blue iridescent colors. The shiny colors arise from nanometer, micrometer, to millimetre of multiscale photonic structures which reside on their wing scales [[31], [32], [33]]. In addition to the dazzling iridescent blue colors, the butterfly wing scales have super-hydrophobicity, self-cleaning capabilities [[16], [17], [18], [19], [20]]. Furthermore the wing consists of two types of scales. The ground scales are needed for their structural color, while the superhydrophobic and self-cleaning properties are found in the cover scales which are quite similar to the lotus leaves [20]. Further researches showed that the highly ordered photonic and multiscale framework of Morpho butterfly wing show various optical response to different individual vapors (such as water, methanol, ethanol, and isomers of dichloroethylene) [18]. The existing nano-engineered is going on photonic sensors which was inspired by iridescent scales of the butterfly because their selectivity on vapor response. The layout of diverse vapor detection applications was highly acute chemical sensors and promising route for the researcher. For the African swallowtail (Papilio) butterflies the scales are made of pigment-infused 2D photonic crystal which provides fluorescence those are intensely directed and directionally intensified by the distributed Bragg reflectors [19].

The multifunctional properties of the butterfly wing scales are determined by multiscale framework, these inspire to design biomimetic multiscale materials by scientists and engineers for function integration. An inverse opal film construct by mimicking the Morpho butterfly wings that includes self-assembly of polystyrene spheres and silica nanoparticles. As a result, the opal film shows both superhydrophobicity and structural color [22,34,35]. Moreover stratified structures which are photonic can be completely replicated by a coating of alumina with a process of low-temperature atomic layer deposition to form micro and nanometer scale that was also mimic the natural butterfly wings as the templates [22,36]. Furthermore, the alumina replicas with a 3D structure can be served as a beam splitters, optical waveguides, and building blocks of other photonic integrated circuits (Fig. 4) [37].

In recent times, a high-resolution multi-colored arrangement strategy has been developed to resolve fabricated structural color. This biomimic strategy was inspired by the structural color from the special stratified structures found in butterfly wings and peacock feathers, and within seconds this takes place by using only one material and flexible photonic crystal, where the color can be magnetically tune able and lithographically fixable [38].

2.2.1. Butterfly-inspired design enables low-cost thermal imaging

Based on the principles of high gas selectivity of the wing scales of Morpho butterflies, a new gas sensor has been fabricated. These new sensors can detect different type of gases as well as can quantify gases in mixtures. These new class bio-inspired nanostructures would enable broader application of thermal imaging which will improve the manufacturability, image resolution, sensitivity and response time. Next goal of this gas sensors are to be made in a cost-effective manner and will be offered to the market as new attractive sensing solutions [36].

At the same time, the sensors will take over all classical and micro-fabricated instruments based on gas chromatography and mass spectrometry which have limited in field use by power, cost, size, carrier gas or vacuum demands. Applications such as industrial inspection, home health care, wearable units for workplace monitoring, and monitoring in harsh environments could be improved and maintained at low cost [36]. Also gas leak detection can be easier with this highly selective colorimetric sensor. The following are examples where gas sensors can be easily applicable -.

a.
Thermal imaging for advanced medical diagnosis-to better visualize inflammation in the body and understand changes in a patient's health earlier
b.
Advanced thermal vision - to see things at night and during the day in much greater detail than what is possible today
c.
Fire thermal imaging - to aid firefighters with new handheld devices to enhance firefighter safety in operational situations
d.
Thermal security surveillance - to improve public safety and homeland protection
e.
Thermal characterization of wound infections - to facilitate early diagnosis

2.3. Biomimetic materials of spider silks

Spider can construct stable, impenetrable, and incompetent orb-webs for prey grasping, this is one of the biology's best “manufacturing engineers” [[37], [38], [39], [40], [41], [42]]. For different purposes spiders produce different types of silks maintaining ambient temperature and pressure from an aqueous solution, which can resist wind, rain and sunlight. Multiscale structures is present in the spider silk which possess a range from nano to macroscale, thus it includes the density of electrons at the silk fibrils, Angstrom scale, β-sheet nanocrystals, β-strands, etc. (Fig. 5) [37,43]. In carrying out the particular mechanical, physical, and chemical properties of spider silks the stratified assembling of the nano- and microscale plays a pivotal role [[44], [45], [46], [47], [48]]. A huge number of researches have been operated due to the remarkable properties of spider silk to resolve the development mechanism of spider silk [[49], [50], [51]]. Biomaterial science and the synthesis of biomimetic spider silk needed to have deeper perceptive on how the networking of the various spider silk constituents transcribes into the final thread inheritance with multifunctional properties [52].

The most extensively studied biological fibers are the spider dragline silks, show stunning mechanical properties (robustness and toughness) that are preferable to almost all innate and man-made high-performance materials [37,52]. Wetting-induced super contraction of spider dragline silk was shown, their mechanical properties results in an expansion in diameter and reduction in length [[53], [54], [55], [56], [57]]. To modernize the hardness of the silk in the spider's web this process is very important, because by this mechanical properties of fiber materials can provide an avenue to adjust performance, which develops 50 times greater work than the identical mass of human muscle. The torsional shape memory is another remarkable characteristic of spider dragline silk, which show stopping of spider twisting and swinging by an exceptional torsional qualities [58].

Artificial dragline spider silk has been constructed by mimicking the spider, which was made by rotating dissolve recombinant dragline silk proteins (ADF-3 (Araneus diadematus Fibroin-3); 60 kDa) originated in mammalian cells under moderate shear and coagulation circumstances [59]. Correlating to those of natural spider dragline silks, the recovered spun fibers had a fine diameter (10–40 mm) with water insolubility and also showed toughness and modulus values [37]. A variety of potential applications of these strong and lightweight fibers are being compatible for tendons and ligaments, biodegradable ophthalmic sutures, and high-strength fishing line.

Nanotubes made up of carbon are one-dimensional nanomaterials; their strength is extremely high with steep stiffness, low density, and chemical stabile. Due to these preferable resources of carbon nanotubes it is possible to assemble spider silk like super-tough fibers. Single-walled carbon nanotubes (SWNTs)-PVA composite fibers was constructed by spinning to get 100-m-long tougher spider silk and any other innate or fabricated organic fiber proclaimed previously [59]. The preferable toughness of SWNTs-PVA composite fibers shows the combination of high toughness and high elasticity, which is associated due to the existence of PVA between SWNTs and between individual slippage nanotubes within bundles [60,61]. Moreover, at the atomic level the mechanical property of the material can be altered by infusing metals, where spider dragline silks can be strengthened into their protein structure and intensify the durability and toughness [62]. This facility can be a model for a more generic access, such as collagen membranes from eggs.

Capture silk, which is five times as elastic as dragline, is the viscous spiral in the webs of orb-weaving spiders, determining notable elasticity. To consume exceptional amounts of energy orb-weaving, the spiders depend on viscous capture silks and also absorb prey long abundant to be placed and attacked. Thus, spider capture silk can be said to have multi-functional physical maintaining high toughness, elasticity, and stickiness [[63], [64], [65]]. Recently, the ability to collect directional water from moist air was found as another fascinating property of the spider capture silk [66]. Due to the special hierarchical fiber structures the water-collecting ability of the spider capture silk takes place, research indicated that the wet-rebuilt fibers acquire periodic spindle-knots which are made up of irregular nano-fibrils detached by joints made of regulated nano-fibrils. A slant of exterior energy on the fibrils and the axis outline of the knots act to generate forces together which directs the water droplets toward the knots. Multifunctional features of the spider silk have more yet to resolve to the scientific world.