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A Biomimetic Approach to Protective Glove Design: Inspirations from Nature and the Structural Limitations of Living Organisms

Publicado en línea: 08 Apr 2022
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Revista
eISSN
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19 Oct 2012
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4 veces al año
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Abstract

Drawing inspiration from nature for the design of new technological solutions and smart materials constitutes an important development area for engineers and researchers in many disciplines. Biomimetic materials design brings numerous benefits, especially the possibility of implementing promising interdisciplinary projects based on effective existing solutions that have emerged in the course of natural evolution.

A major aspect of biomimetic materials design, especially relevant to protective gloves, is the identification of an optimum combination of the physicochemical properties and microstructural characteristics of a surface with a view to its potential applications. Properties such as wetting and adhesion can be adjusted by modifications of the surface morphology both on micro- and nanoscales. From the standpoint of the occupational safety performance of polymeric protective gloves, biomimetic materials should exhibit two crucial properties: reversible adhesion (via a large number of contact points) and hydrophobicity (water repellence).

This review analyzes the superhydrophobic and reversible adhesion patterns found in nature that can be used to improve the properties of polymeric protective gloves with major commercial implications.

Keywords

Introduction

From the standpoint of the safety performance and comfort of polymeric protective gloves, it is important to improve the wearer’s manual dexterity under unfavorable workplace conditions, especially in contact with wet or contaminated objects. This can be done, among other ways, by the implementation of functional superhydrophobic polymeric materials characterized by high surface area and reversible adhesion.

Polymeric protective gloves provide a barrier protecting the palmar and dorsal parts of the hand when people work with harmful substances or in hazardous workplace environments. Their main objective is to isolate the hand from noxious substances or prevent mechanical injury. Currently, chemical-resistant gloves are mostly made of polymers, such as natural and synthetic rubbers and plastics [1, 2]. Glove manufacturers may also combine several layers of such materials to enhance the protective properties of their products. Glove modification may also involve the application of grip dots of different structures in the palmar region to increase adhesion and facilitate the handling of objects. Grip dots differ in terms of geometry and thickness. The mean thickness of gloves with grip dots ranges from 0.70 to 1.70 mm (our own measurements). These parameters are particularly important for gloves protecting against chemical factors.

Gloves should combine three properties: reversible adhesion (via a large number of contact points), hydrophobicity (water repellence), and impermeability. The issue of reversible adhesion is important from the viewpoint of holding and handling objects so that they would not slip out of the hand. In the course of work with hazardous chemical factors, this aspect is crucial to worker safety. In turn, problems with the facile handling of objects and small laboratory equipment may lead to undesirable and dangerous situations.

Thus, from the standpoint of occupational safety, a very important area of research involves protective glove materials and design, especially in the grip region. However, in the case of exposure to chemical substances, equally important is hydrophobicity, which imparts self-cleaning properties to the material and enables the effective removal of harmful substances from the glove area. However, to date, no research has been devoted to this particular issue.

Solutions found in the natural environment may be reviewed to identify those that could be used to modify the structure of protective gloves to improving their safety performance. With technological advances, we are increasingly able to imitate nature and adapt its solutions for our practical needs, including occupational safety. Naturally inspired materials, dedicated to the manufacturing of protective gloves, should feature two characteristics, i.e., reversible adhesion and hydrophobicity (resistance to moisture).

To that end, it is necessary to analyze ways to modify protective glove materials to increase their adhesion and hydrophobicity based on biological analogies. The biomimetic functionalization of protective gloves may be an important aspect in the development of material technologies, enhancing the safety performance of those products, and providing substantial benefits to the user.

Based on observations of hydrophobic and adhesion patterns found in nature as well as in analysis of the literature, the present paper indicates directions for the development of polymer surfaces to enhance the safety performance of protective gloves involving improved hydrophobic and reversible adhesion properties.

Biomimetics: area of interest and scientific foundations

Over the past decades, there has been increased research interest in the observation of nature as a source of inspirations for developing innovative technical, technological, and design solutions. The application of laws and principles of nature and the analysis of biological patterns and systems are some of the pathways taken by scientists seeking patterns of outstanding perfection that could be converted into new solutions [3]. The search for innovative solutions is primarily motivated by the rapid development of new technologies. Furthermore, it should be noted that the design of materials, devices, and technological solutions is an increasingly complex and time-consuming process characterized by decreasing product life-cycles. Thus, novel interdisciplinary fields of science are needed to keep pace with the inevitable changes and to meet the market requirements in terms of supplying innovative, cutting-edge products [4].

The scientific discipline that combines the aforementioned characteristics is known as biomimetics (also termed “bionics”), bringing together biology and technology. It is a relatively recent field of study: the first indications of research efforts in this area emerged at the turn of the 20th century, while the first attempts at defining the research domain did not appear until the 1950s. The idea of design based on natural models was set forth by the German scientist Otto Schmitt. The term “biomimetics” is derived from two words of Greek origin: the roots “bio,” meaning “life,” and “mimesis,” meaning “imitation.”

The first formal use of the term dates back to the 1970s, when biomimetics was defined as “the study of the formation, structure, or function of biologically produced substances and materials (such as enzymes or silk) and biological mechanisms and processes (such as protein synthesis or photosynthesis), especially for the purpose of synthesizing similar products by artificial mechanisms which mimic natural ones.” Other terms applied to this field of study include bionics, biomimicry, and biognosis. A more synthetic definition of bionics coined in the 1950s is “the application of laws of nature in technology.” While over the years a large number of definitions of biomimetics and bionics have been proposed, the field can be succinctly described as “the study of biological systems in order to apply the rules governing them in the design of technical systems. [5, 6]”

Biomimetics is an interdisciplinary field within which biologists, physicists, chemists, and materials scientists strive to understand biological functions and structures as well as the principles governing different natural objects in order to design and manufacture materials and devices for commercial applications [7]. Biomimetics draws on both the animal and plant kingdoms, sometimes combining solutions from both.

Before the emergence of biomimetics, technological development was mostly focused on the engineering design of new machines, devices, manufacturing systems, and organization systems. The main methods used to that end included

the development of theoretical mathematical and physical frameworks enabling the calculations needed for the design of novel and increasingly complex solutions;

the application of computer systems and specialized software;

in-depth analysis and synthetic conclusions;

the use of mathematic and physical models for simulations in engineering design; and

the application of specialized research apparatus for the experimental verification of design inputs.

Due to the rapid development of new technologies, many implementations based on conventional design and construction methods ended up in failure. Consequently, the traditional approach to the development of new materials, devices, and process design was redefined in the spirit of openness to interdisciplinary methods, including biomimetics. Biology has come to be employed as a source of inspiration among others because biological materials exhibit complex interrelationships between their surface morphologies and physicochemical properties. Since nature contains myriad organisms and strategies that have proven effective in addressing a variety of functional problems, bioinspiration in technology seems to have an unlimited potential as a strategy for developing new materials as well as novel design and systemic solutions [8]. While engineers have discovered an exceptional excellence of organisms and a profusion of their forms in nature, biologists have embraced the possibility of using engineering methods in the analysis of living organisms in terms of systems performing certain functions. Both groups of scientists have observed a special analogy between the laws and rules governing the structure and function of living organisms and machines. When we consider this, the fundamental principle of biomimetic research can be given as follows:

Figure 1

Scheme of basic elements of biomimetic design.

Currently, the main benefit from biomimetic studies lies in the fact that this discipline makes it possible to conduct optimum projects taking advantage of solutions that have arisen through evolution in living nature and to effectively utilize natural resources in a more sustainable and environmentally friendly way. Thus, biologically inspired materials and surfaces have attracted considerable interest and continue to contribute to a “greener” science and technology [9].

The structure of biological materials that provide inspirations for new solutions in materials engineering is analyzed both on a macro- and nanoscale to ensure the optimum effects and functional properties from the standpoint of the envisioned applications. Biological materials are highly organized, often in a hierarchical manner, with a complicated nanoarchitecture, which ultimately gives rise to vast numbers of diverse functional elements.

Biomimetic research, which has already enabled many innovative technological solutions, can be divided into several areas, as presented in Table 1.

Areas of biomimetic research.

AREA OBJECTIVES
Biokinematics, movement of land organisms Analysis and improvement of systems and devices moving on solid surfaces, development of new solutions in kinematic systems
Biohydrodynamics Improving the design of floating equipment
Bioaerodynamics Development of active and passive flying systems
Research on animal structures, skeletal systems, and body surfaces Developing new materials and evaluating their morphology, functionality, and unique physical and chemical features
Research on plant structures and surfaces
Biocommunications Research on biological systems receiving and processing signals and stimuli from the environment
Bioenergy Seeking new sources of energy and fuels

In-depth analysis of biological patterns and new knowledge gained by engineers regarding biological structures and characteristics has revealed the most prevalent inspirations and properties of biological structures relevant for commercial applications.

An example of categorization of biological objects, structures, and functions that can provide inspiration for biomimetic researchers is given in Figures 2 and 3 (based on [10]).

Figure 2

Organisms and their functions relevant for biomimetics.

Figure 3

Biological structures/systems and their functions relevant for biomimetics.

Principles of biomimetic design: The top-down approach

Biomimetic design and the identification of natural models is a particularly important aspect of materials engineering as modern materials derived from them may fulfill a variety of functions, similar to those of the natural structures. However, it would be extremely difficult and complicated for biomimetic synthetic materials to reflect all elements responsible for the effective functioning of biological materials, e.g., due to the differences in chemical composition between natural and artificial materials and the limited abilities to synthesize materials at the atomic or particle levels, as is done by living organisms.

Prior to beginning a biomimetic design project, one needs to determine what kind of material is to be developed, what functions it is supposed to fulfill, and what morphological and structural characteristics it should exhibit to meet the designer’s expectations.

Biomimetic design typically departs from the conventional process of material design, in which a given material is subjected to a finishing treatment with a view to imparting some desirable properties to the final product. Instead, the objective is to prepare a final material with the expected shape, structure, and properties. This paradigm corresponds to the demand for new technologies compatible with natural processes [11]. Researchers who concern themselves with the biomimetic design of materials may adopt either of two basic approaches.

In one, the structure of the material is reproduced by means of conventional production methods. The other approach focuses not only on the implementation of a natural structure, but also on transposing the natural mechanism of its production, which can then be used to develop new ways of material synthesis [12].

In addition to technological innovation, the approach presented here to biomimetic design affords an in-depth understanding of the relationship between the form and structure of selected biological objects. Importantly, it is possible to combine several biomimetic models which do not occur together in nature, and some biomimetic technical systems can be successfully deployed in environments in which biological systems would fail or underperform [13].

Biomimetic design techniques can also be divided into two approaches, compared in Table 2 [14].

A comparison of top-down and bottom-up approaches in biomimetic design [15].

Stages Top-down Bottom-up
1: Scientific question An application-oriented question is asked in order to solve a particular technical challenge. A basic research-oriented question is asked in order to gain knowledge about biological systems.
2: Biological concept generator Within the scope of a screening process, initially relevant criteria are defined, and then suitable biological models are selected that are quantitatively analyzed in terms of their morphological and anatomical structure and mechanical properties.
3: Functional principle The results of in-depth investigations of the biological model lead to the underlying functional principle being deciphered.
4: Abstraction The functional principle is translated into a common language understood by natural scientists and engineers, such as functional models, construction plans, circuit diagrams, and numerical and analytical models.
5: Technical application Based on the abstracted description, feasibility studies are carried out and samples at the laboratory scale, prototypes, and pilot series can be produced.
6: Biomimetic product A transition from laboratory and pilot scales to commercial production is made.

Top-down biomimetic design draws on materials engineering methods that enable material fabrication on both macro- and nanometric scales. In the top-down development of a concept for a new material, a technological problem is addressed by methods known from engineering sciences, such as photolithography (reproducing a pattern or photomask on a substrate), microextrusion, direct extrusion, selective laser sintering, 3D printing, and stereolithography.

In the first stage of a top-down process, the main objective is to precisely define the technical problem and boundary conditions. The next step involves searching for solutions in nature that correspond to the desired technical solution and that could potentially be used to achieve it (this step is usually done by biologists). This gives rise to several biological templates leading to a general concept of a solution based on specific technical requirements. Subsequently, the most promising biological solutions are selected for further analysis, experimental studies, and evaluation in terms of their usefulness in solving the challenge at hand. In the next stage, the solutions identified are separated from their natural examples (in the literature this is called abstraction, which often turns out to be the most difficult step in biomimetic design). Following successful knowledge transfer, the engineer’s task involves assessment of the technological possibilities of implementing the proposed solution. This may result in preliminary biomimetically optimized prototypes with their functional parameters verified in laboratory conditions. If these tests lead to satisfactory outcomes, the product is ready for industrial implementation. A schematic of the top-down process is given in Figure 4.

Figure 4

Schematic of top-down biomimetic design.

Top-down biomimetic design can be defined as a biomimetic development process in which an existing, functional technical product is given new, improved functions by transposing and implementing some biological principles. An advantage of this approach is the relatively short time of new technology development, usually ranging from two to four years.

Natural inspirations and models in materials engineering

Biomimetics can take advantage of the vast number of models occurring in nature in terms of surface organization, mechanisms, and systemic solutions. The degree to which an engineering solution imitates its biological model can be graded into five categories:

Full imitation, in which the designed object does not differ from the natural model in structural, functional, or material terms. This is almost impossible to implement in engineering; furthermore, such solutions are often characterized by low effectiveness.

Partial imitation, in which the object designed represents a modification of a natural solution.

Analogy, in which similarities between natural and designed objects are sought in the functional rather than the structural domain; this approach is most often found in transposing natural models into technological solutions.

The application of existing structures; transposing them into abstract solutions.

Creative inspiration by natural solutions associated mostly with their esthetic qualities; often found in construction and architecture.

The process of identifying a suitable model is difficult due to the fact that it is necessary to find organisms displaying a sufficient degree of analogy to the technological challenge at hand, which must be characterized in great detail. If the engineering object is a machine or technological device, one should consider the following properties of natural models in seeking analogies [16]:

the diversity of forms/taxa in nature;

the complexity of each organism (its hierarchical structure);

differences in the basic properties even between individuals belonging to the same species (a deterministic description of parameters is unlikely);

the complexity of functions (a given structure often performs several different functions).

Of great importance in seeking natural models for biomimetic design is the fundamental difference between human products and living organisms, i.e., the fact that the latter are not deliberately designed. Their outstanding adaptations have arisen through millions of years of evolution through trial and error whereby organisms with minimally superior adaptations prevailed by producing proportionately more offspring. However, an adaptation does not have to be perfect; it just needs to be better than the variants present in competing organisms.

Also, the way in which natural structures develop is completely different from human production methods as the former arise from a program encoded in the genome, which is itself derived from programs present in the ancestors of a given organism. The implications of this fact are of great significance as each structure in nature is but a modification of another structure, which may have fulfilled quite a different function. It is impossible to create an adaptation entirely de novo, without some precursor structure. Therefore, the solutions found in nature are not always optimal from the technical point of view, always being encumbered by their evolutionary history.

While transposing natural models into technical solutions, one should take into account differences in terms of materials—living organisms do not have at their disposal metals or advanced polymers but build their structural elements from proteins, polysaccharides, and simple inorganic compounds. Those substances have lower mechanical strength as compared with engineered materials; they are also produced in a very different way. Furthermore, the physical and chemical conditions in which biological materials are used are not as varied and extreme as those under which many artificial structures are supposed to operate.

Biomimetic engineering faces yet another, perhaps even more important challenge. Although biologists have gained quite comprehensive knowledge about the diversity and structures of living organisms, few of them adopt a biomimetic perspective. In turn, engineers focusing on their respective disciplines often lack sufficient knowledge about living organisms. Thus, it seems that the further development of biomimetics is possible only in interdisciplinary teams.

Surface modifications in organisms

Hundreds of millions of years of evolution have given rise to a great structural diversity of plant and animal surfaces, which has enabled various species to adapt to different, sometimes extreme, environmental conditions. Living organisms primarily owe their crucial adaptations to hierarchical macro- and nanostructures on their surfaces as well as the relationships between the morphological features and physicochemical properties of surfaces. Thus, biological solutions optimized in the course of evolution have inspired researchers to design multi-function surfaces.

The biological materials that are of particular interest from the standpoint of surface modifications are insect cuticles and plant surfaces. The macro- and nanostructures on the surface of cuticles impart to them a number of outstanding properties, such as superhydrophobicity, directional wetting, self-cleaning, and controllable attachment. For instance, the shape, size, and distribution of structures across cicada wings modifies their properties, such as wettability, self-cleaning, anti-reflexivity, etc. In turn, the microstructures present on the surface of plant cuticles are crucial for their numerous functions, including those shown in Figure 5.

Figure 5

Schematic of the most important functions of leaf surfaces resulting from microstructuring: (A) transport barrier: limiting uncontrolled water loss or leaching; (B) surface wettability; (C) anti-adhesive and self-cleaning properties: reduction of contamination and pathogen attack and control of attachment and locomotion of insects; (D) signaling: cues for host-pathogens/insect recognition and epidermal cell development; (E) spectral properties: protection against harmful radiation; (F) mechanical properties: resistance against mechanical stress and maintenance of physiological integrity; (G) reduction of surface temperature by increasing turbulent air flow over the leaf (based on [17]).

Superhydrophobic properties
Model of superhydrophobic surfaces

Ever since the lotus leaf effect was discovered and patented by the botanists W. Barthlott and C. Neinhuis in 1998, numerous attempts have been made to imitate this phenomenon by developing superhydrophobic surfaces. Such surfaces are characterized by static water contact angles (θw) greater than 150° and sliding angles lower than 10° [18].

Figure 6

Droplets on hydrophilic and hydrophobic surfaces (based on [19])

The natural surface of lotus leaves, which is universally cited as an example of self-cleaning and superhydrophobic properties, is covered with quite regularly arranged 20–50 μm papillae, which in turn feature 0.5–3 μm tubules additionally covered with a layer of organic substance, i.e., wax. Other plants with similar leaf surfaces include nasturtium, azalea, and cabbage. The mechanism of surface wettability can be explained by, among other things, a theoretical model based on a modified Cassie–Baxter equation, as given below: cosθ=fcosθ(1f) \cos \;\theta^\prime = f\cos \theta - \left( {1 - f} \right) where f is part of the solid/water phase boundary and (1 – f) is part of the air/water phase boundary. This model shows that when a rough surface comes in contact with water, “air traps” may appear in the rough area, considerably increasing the hydrophobic characteristics of a given surface [20].

Energy equilibrium determines whether a given system is in a state of heterogeneous or homogeneous wetting; the latter case is associated with a Wenzel contact angle, which is observed for structures with substantial pore fractions (large distances between protuberances). In the case of a large quantity of protuberances (a small pore fraction), water droplets do not penetrate into the grooves in between, which leads to heterogeneous wetting at a Cassie contact angle.

According to Wenzel’s model, there is a relationship between surface area and the roughness coefficient r, which can be computed from the following formula: r=roughness=actualsurfacegeometricsurface r = {\rm{roughness}} = {{{\rm{actual}}\;{\rm{surface}}} \over {{\rm{geometric}}\;{\rm{surface}}}}

Thus, an increase in roughness leads to decreased θw for θ < 90° and increased θ for θ > 90°. The contact angle in Wenzel’s model is θw, in contrast to Young’s assumptions for a smooth surface, where it equals θ.

However, if a droplet cannot penetrate the grooves in a rough surface due to the effect of surface tension, this model is not effective. Therefore, in 1944 Cassie and Baxter proposed a new approach. The complementarity of Cassie’s and Wenzel’s approaches becomes obvious in the case of imperfect superhydrophobic surfaces. In the case of perfect surfaces, one should take into account structural parameters and inclinations in overcoming energy barriers [21].

These considerations have stimulated intensive research on the development of superhydrophobic surfaces by modifying outer layer roughness. It has been found that the topographic structures present on a surface substantially affect its dynamic wettability properties. It has also been noted that the relations between selected physicochemical properties, chemical composition, and surface morphology are of crucial importance to attaining the desired degree of wettability as well as good antiadhesive properties and anisotropic liquid removal [22].

Superhydrophobic surfaces reveal self-cleaning properties because water droplets can freely roll off them while picking up dust particles [23]. Under the circumstances, the low adhesion of water to the surface (< 9.6 mJ·m−2) is associated with reduced friction between the resulting droplets and the surface, which facilitates droplet movement and the removal of particles that are not firmly attached to it. While considering the self-cleaning effect, one should take into account the key interactions occurring between dust particles and the substrate in the case of topographies with and without nanotexturing:

interactions arising from surface charges on the substrate and particles when they are immersed in an aqueous solution;

van der Waals forces;

capillary forces characteristic of wet environments;

surface tension forces.

Surface self-cleaning mechanisms are presented in Figure 7.

Figure 7

Self-cleaning mechanisms (reproduced by permission [24]).

All the above forces and interactions are affected by changes in surface microstructuring or the chemical composition of the surface, which translates into the ability of water droplets to move on it. These effects can be primarily observed in terms of how the water contact angle and droplet geometry change in the course of covering and uncovering of particles found on the surface analyzed. The process of wetting surfaces with the same chemical composition but different morphologies are presented in Figure 8.

Figure 8

Effects of surface structure on wettability (based on [25]).

The presence of a hierarchical micro- and nanostructure has a crucial effect on the self-cleaning properties of materials [26]. It has been noted that in contrast to smooth surfaces without any specific topography, water droplets on superhydrophobic surfaces are quasi-spherical. In the case of surfaces inclined at a certain angle, denoted as θ, the droplet simply passes over dust particles on a smooth surface, although on a superhydrophobic surface, it readily rolls off while picking up such particles (Figure 9) [27].

Figure 9

Schematic illustration of the self-cleaning mechanism for smooth and superhydrophobic surfaces.

The development of superhydrophobic surfaces inspired by the lotus leaf structure is not an easy process as it requires technologies that would enable the fabrication of a matrix of nanostructures with controllable morphology and physicochemical properties on a modified surface. Typically, such technologies can be divided into those modifying the porosity of materials with low surface energy and those depositing low surface energy layers on materials with high surface energy [28].

Hierarchic micro- and nanostructures are by no means the only ways in which superhydrophobic surfaces can be produced: such surfaces can also be obtained by imitating other structures, e.g., surfaces with irregularly arranged hairs as in the case of Benincasa hispida leaves (Figure 10) [29].

Figure 10

Photographs of the surface of Benincasa hispida leaves showing their superhydrophobic effect (Reproduced by permission [29]).

Superhydrophobic surfaces with special adhesive properties

The superhydrophobic properties of the lotus leaf surface have also inspired the development of surfaces exhibiting low friction and/or adhesion coefficients due to the fact that friction depends on the interface area and interfacial free energy. Improvements in tribological properties resulting from the lotus leaf model have contributed to the design of nanomaterials and electronic components.

Another interesting source of inspiration for biomimetic design is provided by rose petals, which exhibit an outstanding feature: their entire area is characterized by very low wettability, while their individual fragments differ in terms of adhesive properties (Figure 11) [30].

Figure 11

Rose petals and their microstructure (reproduced by permission [30]).

It has been found that the key parameter for modifying the adhesion coefficient while maintaining very low surface wettability is the density of nanostructure arrangement. In addition, the combination of specific microstructures and nanostructures may significantly affect the static contact angle of the substrate. Ideas for the combined organization of nanostructures and microstructures are presented in Figure 12, which also shows the shape of the droplet surface deposited on the selected nanostructures (exhibiting good and poor adhesive properties, respectively).

Figure 12

Ideas for the distribution of micro- and nanostructures to modify surface adhesion without changing the water contact angle.

Superamphiphobic surfaces

The ability to produce surfaces that are both water- and oil-repellent is a major challenge in many practical applications [31, 32]. A theoretical model of the solid/oil/water interface is shown in Figure 13.

Figure 13

Effect of surface structure on the wetting behaviors of solid substrates in solid/oil/water three-phase systems: (a) diagram of Young’s equation for one liquid droplet on a smooth surface at a contact angle of θ in a liquid phase; (b) an oil droplet on a microstructured substrate in water, in which 1 represents oil and 2 represents water; (c) the same system on a micro/nanostructured substrate (based on [33]).

Materials with special adhesive properties

Of great interest to researchers concerned with biomimetic design is the development of production methods for synthetic counterparts to natural materials and elements with specific morphologies and sizes on the micro and/or nano scales. In the area of materials engineering, hierarchical nanostructures and nano-objects associated with reversible and controllable adhesion have attracted increasing attention over the years.

The adhesive properties of solid surfaces are determined by a range of factors, such as their chemical composition, density, true contact area, surface energy, roughness, as well as structuring and geometry. The strong adhesion between some surfaces and water (in addition to capillary forces and the vacuum produced by air bubbles) is attributed to van der Waals forces [34].

The ability to attach to inanimate objects, living organisms, and other parts of the same organism is of crucial importance in nature. Adhesion enables organisms, among others things, to

maintain position on a substrate (e.g., by developing root systems, attaching immobile life stages such as eggs and pupae, occupying a territory by sessile organisms, etc.);

catch and hold food;

have locomotion (via sequential attachment to and detachment from the substrate);

maintain contact during copulation;

have interactions between different parts of the same organism;

clean and filter (e.g., maintaining pollen attached to insects, capturing dust particles by the respiratory system).

Sample elements used for locking or attaching insect body parts are presented in Figure 14.

Figure 14

Attachment devices in insects: SEM images of two different systems. Thoracic (a) and elytral (b) counterparts of the elytra-to-body locking device in the tenebrionid beetle (Tenebrio molitor); neck (c) and head (d) counterparts of the head-arresting system in the cordulegastrid dragonfly Anotogaster sieboldii (reproduced by permission [35]).

For an animal to remain attached to a substrate while being able to move on it, the movement system must enable the application and maintenance of substantial contact forces (using friction, adhesion, or mechanical locking). Some living organisms exhibit specialized surfaces for minimizing or maximizing contact forces with the substrate by friction and anti-friction systems (see Figure 15).

Figure 15

Functional significance and working principles of contacting surfaces in biology (based on [36]).

In some cases, organisms need to use not only their structural features (claws, hairs, hooks, etc.), but also various liquids or secretions (wet adhesion). Long-term attachment to the substrate usually involves mechanical locking or the application of some “glue”; usually both mechanisms are present [37].

Figure 16 shows a schematic summary of the various mechanisms of adhesion in biological structures.

Figure 16

Principles of action of adhesion systems known in biology (based on [38])

Animals greatly differing in terms of their body mass, such as arthropods (e.g., flies and spiders) and geckos can easily climb and remain on vertical walls and ceilings as they have developed effective mechanisms of adhesion. That ability is attributable to attachment systems employing specialized external structures that interact in specific ways with various substrate profiles [39]. Under laboratory conditions many researchers have attempted to recreate adhesive mechanisms found in living organisms. One of the most important findings is that contact efficiency depends not only on the shape and size of specific micro- and/or nanostructures, but also on the elastic properties of their constituent fibers and fiber endings [40]. Figure 17 presents photomicrographs of setose adhesive devices in different animal species.

Figure 17

Terminal elements in animals with a hairy design of attachment pads (heavier animals exhibit finer adhesion structures) (reproduced by permission [41]).

A major advantage of such adhesive structures is the reliability of their contact with various surface profiles as well as greater tolerance to surface defects. The presence of dust particles or mechanical damage to a single seta could affect contact adhesion and potentially lead to immediate contact breakage, especially on a rough surface. In living organisms, this eventuality is prevented by the development of structures covered by a multiplicity of hairs [42].

The most popular example of controlled reversible adhesion in the world of animals is the gecko’s foot, which has a surface area of approximately 200 mm2 and can generate an adhesive capacity of approximately 20 N. The gecko’s foot features 1–2 mm long lamellae covered with setae with an overall length of 100 µm and a diameter of 5 µm. Those setae exhibit a hierarchical branched morphology—each of their branches out into hundreds of subunits known as spatulas (200–300 nm wide, 500 nm long, and 10 nm thick). Each seta interacts with the surface via a van der Waals force of 10−7 N, which translates into an adhesive force of approx. 10 N/cm2 g for a pad covered with a million setae [37].

Researchers trying to recreate the adhesive structures present on gecko feet have formulated several principles which have significantly contributed to the development of new technologies involving (mostly polymeric) materials exhibiting controllable adhesion [43, 44]:

of great importance is the angle between the setae and the substrate (with the maximum adhesion strength recorded at 30.6°);

adhesion strength increases from the moment the seta approaches the substrate to the moment when the seta begins to slide on the substrate;

van der Waals interactions are responsible for gecko foot adhesion as confirmed by calculating the adhesive force that seta endings exert on the substrate.

The structures of the feet of Geckos and other animals exhibiting controllable, reversible adhesion to the substrate have inspired researchers to seek new materials with strong adhesive forces. In the literature there are a large number of studies describing the effects of ordered micro- and nanostructures on adhesive processes [45, 46]. However, the design and production of a densely packed hierarchical structure of nano-hairs terminating with nano-sized contact elements remain the greatest challenge.

Adhesion in a wet environment

Attachment strategies based on mechanical locking or molecular attraction between surfaces may not be effective in the presence of liquids. Technical solutions developed to date in the area of wet adhesion involve, among other things, hierarchical porous or “mushroom-like” structures promoting capillary forces or “suction,” as well as supramolecular structures containing nanoparticles. A major challenge to the research community is the development of adhesive structures that would be effective both under wet and dry conditions while resisting contamination [47]. Over the past several years, a variety of technologies and adaptation principles have been developed to mimic the adhesive structures present on the toe pads of tree frogs characterized by excellent reversible adhesion in a wet environment [48].

The adhesion of tree frog toe pads has provided an inspiration for the studies of Meng et al. [49]. The mechanism used by tree frogs is different from that used by geckos. It involves the phenomenon of wet adhesion with a thin layer of liquid between the surface of the pads and the substrate, which gives rise not only to van der Waals forces, but also to capillary forces and hydrodynamic forces based on viscosity. A model explaining the operation of these forces is presented in 18.

Another example of strong controllable adhesion under wet conditions found in nature is the operation of octopus and squid tentacles, used not only for attachment and locomotion, but also for holding and manipulating objects.

Summary

The outstanding hydrophobic properties of natural surfaces, including lotus leaves, rose petals, water strider legs, and cicada wings are attributed to their hierarchical micro- or nanostructures. Moreover, many reptiles and insects exhibit attachment elements with microrelief, which may provide an inspiration for smart hydrophobic surfaces. It is therefore critical to develop and produce surfaces featuring micro-or nanostructures, especially with structural memory and restoration capacities. In recent years solutions inspired by natural structures have been widely researched in terms of potential applications for controllable water and oil separation, self-cleaning of various surfaces, capture and release of biological contaminants, and anti-bioadhesion [51].

Figure 18

Model of a sphere on a plane surface connected by a drop of fluid (based on [50]).

Ordered surface topographies, and especially superhydrophobic surfaces, exhibit considerable potential in terms of promoting poor attachment or repulsion of chemical and biological contaminants due to the self-cleaning properties of such surfaces. In designing superhydrophobic surfaces, two parameters are of the essence: (1) low surface energy and (2) micro- and nano-hierarchical substrate roughness. Thus, the problem of surface contamination can be resolved by decreasing surface energy and generating micro- and nano-hierarchical roughness. This not only significantly reduces adhesive forces between the surface and water molecules analyzed, but also increases adhesion between water droplets and contaminant particles.

Of particular note is the presence of surface structures (e.g., on a nano scale), especially due to the high ratio of their specific area to volume, which is conducive to hydrophobic properties [52].

The dynamic technological development of polymeric materials has heightened market expectations and fueled the drive to develop new, increasingly reliable personal protective equipment, including safety gloves. The adaptations found in living organisms have provided an inspiration for designing innovative solutions and resolving technological problems, both in the past (Leonardo da Vinci) and today. Technological progress has enabled increasingly advanced imitation of nature and the application of the developed solutions to practical problems, including the safe performance of work. A bridge between natural solutions and technology has been established by a new scientific discipline known as bionics or biomimetics, understood as a multidisciplinary field drawing inspirations from nature for the development and production of new materials. Its objectives include not only the recreation of biological structures, but also an understanding, description, and practical application of natural process and optimized solutions.

The identification of natural solutions for the purpose of improving existing technologies and materials is a major contributor to the development of engineering sciences, including materials technology and the utilitarian applications of its products. The biomimetic design of nature-inspired materials enables the development of increasingly advanced solutions and the modification of their products.

Figure 1

Scheme of basic elements of biomimetic design.
Scheme of basic elements of biomimetic design.

Figure 2

Organisms and their functions relevant for biomimetics.
Organisms and their functions relevant for biomimetics.

Figure 3

Biological structures/systems and their functions relevant for biomimetics.
Biological structures/systems and their functions relevant for biomimetics.

Figure 4

Schematic of top-down biomimetic design.
Schematic of top-down biomimetic design.

Figure 5

Schematic of the most important functions of leaf surfaces resulting from microstructuring: (A) transport barrier: limiting uncontrolled water loss or leaching; (B) surface wettability; (C) anti-adhesive and self-cleaning properties: reduction of contamination and pathogen attack and control of attachment and locomotion of insects; (D) signaling: cues for host-pathogens/insect recognition and epidermal cell development; (E) spectral properties: protection against harmful radiation; (F) mechanical properties: resistance against mechanical stress and maintenance of physiological integrity; (G) reduction of surface temperature by increasing turbulent air flow over the leaf (based on [17]).
Schematic of the most important functions of leaf surfaces resulting from microstructuring: (A) transport barrier: limiting uncontrolled water loss or leaching; (B) surface wettability; (C) anti-adhesive and self-cleaning properties: reduction of contamination and pathogen attack and control of attachment and locomotion of insects; (D) signaling: cues for host-pathogens/insect recognition and epidermal cell development; (E) spectral properties: protection against harmful radiation; (F) mechanical properties: resistance against mechanical stress and maintenance of physiological integrity; (G) reduction of surface temperature by increasing turbulent air flow over the leaf (based on [17]).

Figure 6

Droplets on hydrophilic and hydrophobic surfaces (based on [19])
Droplets on hydrophilic and hydrophobic surfaces (based on [19])

Figure 7

Self-cleaning mechanisms (reproduced by permission [24]).
Self-cleaning mechanisms (reproduced by permission [24]).

Figure 8

Effects of surface structure on wettability (based on [25]).
Effects of surface structure on wettability (based on [25]).

Figure 9

Schematic illustration of the self-cleaning mechanism for smooth and superhydrophobic surfaces.
Schematic illustration of the self-cleaning mechanism for smooth and superhydrophobic surfaces.

Figure 10

Photographs of the surface of Benincasa hispida leaves showing their superhydrophobic effect (Reproduced by permission [29]).
Photographs of the surface of Benincasa hispida leaves showing their superhydrophobic effect (Reproduced by permission [29]).

Figure 11

Rose petals and their microstructure (reproduced by permission [30]).
Rose petals and their microstructure (reproduced by permission [30]).

Figure 12

Ideas for the distribution of micro- and nanostructures to modify surface adhesion without changing the water contact angle.
Ideas for the distribution of micro- and nanostructures to modify surface adhesion without changing the water contact angle.

Figure 13

Effect of surface structure on the wetting behaviors of solid substrates in solid/oil/water three-phase systems: (a) diagram of Young’s equation for one liquid droplet on a smooth surface at a contact angle of θ in a liquid phase; (b) an oil droplet on a microstructured substrate in water, in which 1 represents oil and 2 represents water; (c) the same system on a micro/nanostructured substrate (based on [33]).
Effect of surface structure on the wetting behaviors of solid substrates in solid/oil/water three-phase systems: (a) diagram of Young’s equation for one liquid droplet on a smooth surface at a contact angle of θ in a liquid phase; (b) an oil droplet on a microstructured substrate in water, in which 1 represents oil and 2 represents water; (c) the same system on a micro/nanostructured substrate (based on [33]).

Figure 14

Attachment devices in insects: SEM images of two different systems. Thoracic (b) and elytral (c) counterparts of the elytra-to-body locking device in the tenebrionid beetle (Tenebrio molitor); neck (d) and head (e) counterparts of the head-arresting system in the cordulegastrid dragonfly Anotogaster sieboldii (reproduced by permission [35]).
Attachment devices in insects: SEM images of two different systems. Thoracic (b) and elytral (c) counterparts of the elytra-to-body locking device in the tenebrionid beetle (Tenebrio molitor); neck (d) and head (e) counterparts of the head-arresting system in the cordulegastrid dragonfly Anotogaster sieboldii (reproduced by permission [35]).

Figure 15

Functional significance and working principles of contacting surfaces in biology (based on [36]).
Functional significance and working principles of contacting surfaces in biology (based on [36]).

Figure 16

Principles of action of adhesion systems known in biology (based on [38])
Principles of action of adhesion systems known in biology (based on [38])

Figure 17

Terminal elements in animals with a hairy design of attachment pads (heavier animals exhibit finer adhesion structures) (reproduced by permission [41]).
Terminal elements in animals with a hairy design of attachment pads (heavier animals exhibit finer adhesion structures) (reproduced by permission [41]).

Figure 18

Model of a sphere on a plane surface connected by a drop of fluid (based on [50]).
Model of a sphere on a plane surface connected by a drop of fluid (based on [50]).

A comparison of top-down and bottom-up approaches in biomimetic design [15].

Stages Top-down Bottom-up
1: Scientific question An application-oriented question is asked in order to solve a particular technical challenge. A basic research-oriented question is asked in order to gain knowledge about biological systems.
2: Biological concept generator Within the scope of a screening process, initially relevant criteria are defined, and then suitable biological models are selected that are quantitatively analyzed in terms of their morphological and anatomical structure and mechanical properties.
3: Functional principle The results of in-depth investigations of the biological model lead to the underlying functional principle being deciphered.
4: Abstraction The functional principle is translated into a common language understood by natural scientists and engineers, such as functional models, construction plans, circuit diagrams, and numerical and analytical models.
5: Technical application Based on the abstracted description, feasibility studies are carried out and samples at the laboratory scale, prototypes, and pilot series can be produced.
6: Biomimetic product A transition from laboratory and pilot scales to commercial production is made.

Areas of biomimetic research.

AREA OBJECTIVES
Biokinematics, movement of land organisms Analysis and improvement of systems and devices moving on solid surfaces, development of new solutions in kinematic systems
Biohydrodynamics Improving the design of floating equipment
Bioaerodynamics Development of active and passive flying systems
Research on animal structures, skeletal systems, and body surfaces Developing new materials and evaluating their morphology, functionality, and unique physical and chemical features
Research on plant structures and surfaces
Biocommunications Research on biological systems receiving and processing signals and stimuli from the environment
Bioenergy Seeking new sources of energy and fuels

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