Nature‐Inspired Biomimetic Surfaces for Controlling Bacterial Attachment and Biofilm Development

The use of antibacterial and antifouling materials is widely being investigated to combat the increasing risk associated with bacterial infections and the evolution of drug‐resistant bacteria. Efficient antibacterial materials can be fabricated by mimicking the topography found on the surface of natural antibacterial materials. Natural materials such as the wings of cicadas and dragonflies have evolved to use the structural features on their surface to attain bactericidal properties. The nanopillars/nanospikes present on these natural materials physically damage the bacterial cells that settle on the nanostructures resulting in cell lysis and death. This article reviews the role of nanostructures found on the surface of some of these natural antibacterial and antifouling materials such as lotus leaf, cicadas and dragonflies wings, shark skin, and rose petals. These natural structures provide guidelines for the design of synthetic bio‐inspired materials. This review article also presents some novel fabrication techniques used to produce biomimetic micro‐ and nano‐structures on synthetic material surfaces. The role of size, shape, aspect ratio, and spacing between the micro/nano‐structures on the bactericidal properties is also discussed. Finally, the review is finished with the author's view on the future of the field.


Introduction
The evolution of drugs and antibiotic-resistant bacteria poses significant challenges and threat to human life. [1][2][3][4] It is bio-inspired materials often mimic the surface topography of natural materials. [25][26][27] For example, the surface of lotus leaves is covered with micropapillae that are 3-11 µm in diameter. Each of these micropapillae is covered with nano-tubules that have 100 nm in diameter. This hierarchical structure plays a significant role in imparting self-cleaning and antifouling behavior of lotus leaves. [28][29][30] Jiang et al. fabricated lotus leaf-inspired hierarchical structures and demonstrated that these bio-inspired structures display antifouling properties similar to the natural material. [28] Their study demonstrated that the fabricated hierarchical structures physically rupture the bacterial cells when they come in contact with the surface. Similarly, many other studies have demonstrated the fabrication process of sharp nanoneedles/nanopillars to mimic the structures found on the wings of dragonflies and cicadas for killing bacteria via mechano-bactericidal mechanisms. [7,20,31,32] Brennan et al. employed a microembossing technique to mimic the structures found on the surface of shark skin. [33] The topographical features consisting of rectangular micrometer ribs produced on synthetic material is shown to disrupt Staphylococcus aureus (S. aureus) colonization. In addition, their results show that the rectangular micrometer ribs also enable the surface to display antifouling behavior. This shows that synthetic materials with antifouling properties can be produced by mimicking the molecular-level structures found on the surface of natural materials.
Various natural surfaces have provided inspiration and motivation for researchers to mimic their antibacterial behavior. [34] Combining hierarchical micro/nano-structures and selecting appropriate constituent materials with controlled wettability and surface chemistry can give rise to materials that display superior antimicrobial properties. This review article aims to provide a critical overview of bio-inspired micro/nano-structured surfaces and explain their antibacterial and antifouling mechanisms. In this review article, we explore some of the natural materials that either prevent bacteria from attaching or directly kill bacteria that manage to adhere to the surface. Following this, some fabrication methods used to mimic natural bactericidal surfaces are presented, and the effect of nanostructure parameters on the antibacterial properties is discussed. This article illustrates the various factors that influence the contact-killing bactericidal mechanisms of nanostructures surfaces. A thorough understanding of these mechanisms will guide the design of synthetic mechanobactericidal surfaces.

Bacterial Adhesion and Biofilm Formation
The adhesion of bacteria on the surface of a material is complex and is thought to be influenced by the type of the bacteria and the surface properties of the material. [8,35] The adhered bacteria tend to form organized communities on the surface called biofilms. Biofilm formation consists of four steps, including initial attachment, microcolonization, maturation, and dispersion. [2,3,36] Figure 1 illustrates the stages involved in biofilm formation on a surface.
During the initial attachment step, the bacterial cells are attracted to the surface of a material by interactions, such as gravitational, van der Waals forces, electrostatic charges, and Brownian motion. These physical interactions between bacteria and surfaces are categorized as either long-range or short-range interactions. Long-range interactions between the bacterial cell and material surface occur when the separation distance is greater than 150 nm. The distance and surface free energy influences these long-range interactions, such as van der Waals interactions between the cells and the surface. These longrange physical interactions are responsible for transporting bacterial cells to the surface. On the other hand, short-range interactions dominate when the bacterial cells and the material surface are less than 3 nm apart. Short-range interactions due to hydrogen bonding, ionic or dipole interactions, hydration, or hydrophobic interaction become effective when the bacterial cells come in close contact with the surface. These interactions result in a more stable adhesion of microbes to the surface. Once the bacterial cells touch the material's surface, the appendages strengthen the adhesion between the bacterial cell and the material. At this early stage, the bacterial attachment is assumed to be reversible. Thus, it is possible to prevent biofilm growth during this stage. The initial attachment of the bacteria can be prevented by introducing micro/nano-structure topography on the surface. Following this stage, the bacteria growth and multiplication begin, which leads to the production of EPS. The EPS acts as a layer that protects the bacteria, and the bacteria become more resistant to external environmental factors. Hence, the attachment of bacterial cells is irreversible at this stage.
The process of biofilm maturation begins immediately after the bacteria successfully occupy the surface. During maturation, microcolonies are formed. These structures are resistant to

Naturally Occurring Antibacterial Surfaces
It is known that bacteria cause infections because they attach to surfaces and form multicellular biofilms. One of the methods to tackle this issue is to develop anti-biofouling surfaces that prevent bacterial cells from attaching. There are plenty of examples of surfaces that exist in nature that have evolved to restrict the attachment of bacterial cells. [7,33,34,37] Most natural antibacterial surfaces have evolved to possess high-aspect ratio nano/ micro topographic structures to protect them from pathogenic infestation. Such structures help to reduce the contact area between the cells and the surface and inhibit the formation of biofilms. For instance, the presence of nanopillar structures on the wings of cicadas is demonstrated to impart antimicrobial behavior to the wings. Figure 2 shows the microstructure found on the surface of a cicada wing surface.
The nanostructures of cicada wings have the shape of nanopillars with 82-148 nm diameter and ≈400 nm height. [38,39] These nanostructures are closely packed and are arranged in a near-hexagonal symmetry. [7,40] The presence of these nanostructures is also known to play a significant role in controlling the wettability of cicada wings. Sun et al. [41] investigated the wetting behavior on the forewings of 15 species of cicadas and demonstrated that the wettability is influenced by topographical architecture and their dimensions. Cicada wings are found to display lower hydrophobicity when the nanoprotrusions are disorderly arranged and the protrusions are connected at the apex. On the other hand, wings of cicada species that have high aspect ratio nanoprotrusions with the ordered arrangement were found to display highly hydrophobic behavior. In general, the composition of the wax layer and ordered arrangement of the nanopillars ensure that the wings are superhydrophobic. It is interesting to note that the superhydrophobicity does not contribute to the bactericidal behavior of cicada wings as it is reported that bacteria strongly attach to the nanostructured pillars. [27] The anti-biofouling nature of cicada wings is due to the ability of the nanostructured pillars to kill the bacteria cells upon contact. The attached bacteria undergo a morphological change and are killed within 3-5 min.
The wings of dragonflies have also been shown to display self-cleaning and bactericidal properties. [29] The microstructure on dragonfly wings is found to be similar to the microstructure found on the wings of cicadas. The nanopillars on the wings of dragonflies are irregularly shaped and have a diameter between 83 and 195 nm. [42] Figure 2b shows the microstructure found on the surface of a dragonfly wing. Studies demonstrate that cicada wings are efficient in killing Gram-negative bacteria and inefficient in eliminating Gram-positive bacteria, [20,38] while dragonfly wings can eradicate both Gram-positive and Gram-negative bacteria. The cell wall of Gram-positive bacteria is estimated to be four to five times thicker than cell walls of Gram-negative bacteria. The difference in cell wall rigidity between Gram-positive and Gram-negative explains why Gram-negative bacteria can be easily ruptured by the nanoprotrusions present on cicada wings. However, nanoprotrusions with smaller tip diameters found on the dragonfly and damselfly wings are able to rupture both Gram-negative and Gram-positive bacteria. These small differences in tip diameters are important for surface engineers to design more efficient bactericidal surfaces.
Leaves of plants and some flower petals are good examples of superhydrophobic and antimicrobial surfaces. [28,43] For instance, taro leaves are seen to display anti-biofouling, selfcleaning, and hydrophobic properties due to the presence of well-ordered micro/nano-structures on its surface. [27] It consists of elliptical-shaped bumps 10-30 µm in diameter. These structures are then covered by a layer of waxy polygonal epidermal cells. The combination helps them to display superhydrophobic behavior. The superhydrophobic nature of taro leaves ensures that dirt particles and bacterial cells do not adhere to the leaf surface. [29] Hence, when a water droplet falls on the leaf, it picks up the bacteria and dirt before rolling off the surface.
Similarly, lotus leaves are known to display excellent antifouling and superhydrophobic behavior due to the presence of surface hierarchical structures. [28] Their microstructure consists of microscale elliptical bumps that are between 3 and 11 µm in diameter. These elliptical bumps are covered by nanoscale crystals that are ≈100 nm in diameter. The presence of air pockets between these hierarchical structures ensures that lotus leaves display a high water contact angle (>150°) and a low sliding angle. Like the taro leaves, water droplets from lotus leaves are repelled from the surface and, in the process, remove dirt and any foreign material from the surface. In addition to selfcleaning, the role of air molecules trapped between the hierarchical structures restricts direct contact between bacteria cells and the surface. Hence, the lotus leaf can get rid of bacteria Figure 2. A) Scanning electron microscope (SEM) image of nanostructures found on the surface of cicada wing. The presence of nanopillar structures is clearly evident in the SEM image. Reproduced with permission. [39] Copyright 2017, Elsevier. B) Helium ion microscopy image demonstrating the nanostructures found on the wings of dragonflies. The scale bar in the image corresponds to 200 nm. Reproduced with permission. [42] Copyright 2017, American Chemical Society. such as S. aureus, Staphylococcus epidermidis (S. epidermidis), Pseudomonas aeruginosa (P. aeruginosa), and Planococcus maritimus.
Rose petal is another example of a naturally occurring surface with hierarchical structures. [44] The microstructure of rose petals consists of micron-sized bumps that are ≈20 µm in diameter with multiple nanometer-sized folds that are ≈730 nm in width on top of each of these bumps [28,29] (see Figure 3A). The microscale bumps (called micropapillae) are densely packed on the surface and arranged in a close array pattern. The micropapillae with nanofolds provide surface roughness to the rose petal and are responsible for its superhydrophobicity. Typically, the rose petal surface displays a Cassie wetting state where the water droplets enter the valleys between the micropapillae but not into the grooves of the nanofolds. This also explains why small water drops adhere to the surface of rose petals. Cao et al. duplicated the natural rose petal structures on UV-curable polymer and demonstrated that these synthetic structures prevent the growth of biofilm. [45] The unique structure helps significantly lower the adhesion of S. epidermidis and P. aeruginosa. Compared to neat film, synthetic rose petal mimetic surface is shown to inhibit the formation of biofilms. This is attributed to the nanotopography. In addition to this, the micropapillae arrays are responsible for isolating the bacterial cell clusters. This discourages the formation of a fibrous network.
In addition to plant leaves and wings of insects, the skins of certain animals are also found to display antibacterial and antibiofouling properties. For instance, the skin of sharks offers remarkable resistance to colonization of microbes. This efficient feature of shark skin relates to the unique arrangement of micro topological structures. [33] As shown in Figure 3B, the denticles found on its skin are ribbed and have longitudinal grooves aligned along the direction of water flow. [46] The concave groove surface has nanostructured protuberances. These designs reduce the surface area to prevent microbial adhesion. The structures also help the skin of sharks to self-clean. Pu et al. [47] copied the structure of sharks' skin on polydimethylsiloxane (PDMS) and investigated the role of microstructure in anti-biofouling. Noticeably, shark skin microstructures on PDMS surfaces were found to have a decreased propensity for algae attachment than non-structured PDMS. Furthermore, it has been shown that microalgae can be easily removed from microstructured PDMS by simply washing it water. We have used a similar method in the lab to copy the microstructures found on sharks' skin onto PDMS. Figure 3C shows the scanning electron microscope image of the biomimetic shark skin structures copied onto PDMS. Reproduced with permission. [44] Copyright 2014, Elsevier. B) Microscopy image of denticles found on the surface of shark's skin. The scale bar shown in the image corresponds to 200 mm. Reproduced with permission. [46] Copyright 2018, The Royal Society. C) SEM image of shark skin structures replicated on polymer film.

Antibacterial Mechanism of Natural Materials
Typical strategies adopted by natural surfaces to attain antibacterial properties rely either on prohibiting the attachment of bacteria to the surface or killing the bacteria and removing the surface contamination. [39,48] Roman-Kustas et al. [39] in their study demonstrate that the molecular composition of the nanostructures and the geometry of the nanostructures both play a role in imparting antimicrobial properties to the surface of cicada wings. This section will review some of these surfaces and illustrate their anti-fouling and antibacterial mechanisms in detail.

Nanostructures
The antibacterial mechanism of these surfaces involves membrane dysfunction resulting from the insertion of the nanostructure into the bacterial cell, physically puncturing and damaging the bacterial cell, and non-contact physical tearing of the cells. [49] For example, the nanopillars present on the surface of cicada wings and dragonfly wings are responsible for stretching and rupturing the bacterial cell membrane. [20,29,37,50] In the first step, the bacterial cells contact the nanostructures and adsorb on the surface protrusions. The degree of bacterial attachment is governed by intermolecular forces such as van der Waals, electrostatic, hydrophobic, and steric forces. Once the bacterial cells adhere to the surface protrusions, the bacterial cells experience a stretching force that increases with continued adsorption. [20] Figure 4 illustrates the interaction of P. aeruginosa cells with the structures present on cicada wings. The morphology of the cell is seen to change when it comes in contact with the nanostructures. The nanostructures present on the cicada wings penetrate the bacterial cell. Following this, the cellular components spread in between the nanostructures, ultimately leading to cell death. Physical rupturing and cell lysis occur when the stretching reaches a breaking point, the latter being specific to the physical properties of the cell walls of different bacteria. However, the bacterial cell rupturing is mainly dependent on the surface nanostructures, which can modulate the degree of bacterial stretching. For example, nanopillars found on cicada wings displayed the same antibacterial effect regardless of whether the surfaces are coated with gold or not. This observation showed that the nanostructure is the driving force underpinning the bactericidal mechanism of cicada wings.
Another important factor that can influence the extent of antibacterial performance of natural surfaces is the thickness of the bacterial cell wall. For example, bacterial cell wall of Grampositive bacteria is thicker and stiffer than the cell membrane of Gram-negative bacteria. [20,38] This explains why Gram-negative bacterial cells experience larger stretch on nanopillar structures compared to Gram-positive bacterial cells. It is reported that bacterial cells with thin cell walls require smaller tensile force to tear cell walls, while bacterial cells with thicker cell walls require much larger tensile force to tear the cell. [51] Parameters such as density, nanopillar spacing, and tip diameters of the nanostructures determine the bactericidal efficiency of natural materials. The nanopillars found on the wings of dragonflies are more densely packed than the nanopillars found on the wings of cicadas. [8,27] Bandara et al. [42] demonstrate that the surface of dragonfly wings has two prominent nanopillar populations as opposed to single-height structures commonly fabricated to mimic the bactericidal properties of dragonfly wings. The damage to bacterial cells is attributed to the combination of two forces. Strong adhesion between the bacterial cell and the nanostructures is one of them and the development of shear force when the immobilized bacteria tries to move is the other. This explains why wings of dragonflies can kill both Gram-negative as well as Gram-positive bacteria.

Surface Wettability
It is well known that biofouling takes place when the microorganisms are allowed to stay on the targeted surfaces. The formation of biofilm is initiated by the adhesion of bacterial cells on the surface. Once the biofilms are formed, the microorganisms start to adhere to one another and the surface with the help of EPS. Although there are many factors that influence the attachment of bacterial cells to a surface, the surface properties such as wettability and surface topography are considered to be some of the important factors. Studies have investigated the role of hydrophobicity and hydrophilicity of a surface on bacterial adhesion. Typically, the bacterial cells are found to adhere to a hydrophilic surface when bacterial cells' surface energy is larger than the surface energy of the liquid medium in which they are suspended. [52] On the other hand, bacterial cells adhere to a hydrophobic surface when their surface energy is lower than the surface energy of the liquid medium in which they are suspended. [52] Bacterial cells such as Escherichia coli (E. coli) and S. aureus are found to adhere to hydrophilic surfaces, while bacterial cells such as S. epidermidis and Pseudoxanthomonas taiwanensis adhere to hydrophobic surfaces. [53,54] Tang et al. [55] prepared surfaces with different wettability and investigated the effect of wettability on the adhesion of S. aureus bacteria. They fabricated moderately hydrophilic, hydrophobic, and superhydrophobic surfaces, and studied the bacterial colony formation after 2 and 4 h of culture on these surfaces. Their results show that after 2 h there were more bacteria on the hydrophilic surface compared to the other two surfaces. Although bacteria also adhered to the superhydrophobic surface, the numbers are much lower compared to hydrophobic and hydrophilic surfaces. It is interesting that bacteria on the superhydrophobic surface are seen to be mostly scattered even after 4 h. This shows that the interaction amongst bacteria on a superhydrophobic surface is limited and that bacteria did not easily adhere to the superhydrophobic surface. On the other hand, the bacteria on hydrophilic surface are seen to be in clumps. This suggests that a biofilm can be easily formed on the hydrophilic surface. [55] Similar results are reported in a study by Crick et al. [56] where they show reduced S. aureus and E. coli adhesion on superhydrophobic surfaces compared to moderately hydrophilic and moderately hydrophobic surfaces.
Typically, the hydrophobicity of a material that has low surface energy can be enhanced by introducing micro/nanostructures on its surface. [57][58][59] Low surface energy materials with surface micro/nano-structures display Cassie-Baxter wetting state as they do not allow water to spread on them. Such surfaces are known as superhydrophobic surfaces, and they are known to have low-adhesion properties. This phenomenon prevents microbial attachment and inhibits the initial stage of biofilm development. The presence of nanostructures reduces the binding sites of the microbes. [60] Thus, the colonization of bacteria on these surfaces has a lower chance of occurring. Fadeeva et al. [61] used femtosecond laser ablation to produce hierarchical structures mimicking the surface of a lotus leaf on titanium surfaces and investigated the interaction of S. aureus and P. aeruginosa cells with the fabricated surface. Their results show that the rod-shaped P. aeruginosa cells did not colonize the superhydrophobic surface. However, the spherically shaped S. aureus are shown to colonize the surface. They explain that spherical bacteria require a much lower degree of surface contact to adhere compared to rod-shaped bacteria. In a similar study, Truong et al. fabricated superhydrophobic lotus leaf mimetic structures on titanium and investigated their ability to retain coccoid bacteria. [62] Their results show that surface topography can also modulate bacterial adhesion apart from surface hydrophobicity and charge. Their results show that the bacterial cells preferred to adhere to the crevices between the microscale surface features. On the other hand, the upper region of the microscale surface features are shown to have no attached bacterial cells. The presence of air molecules trapped with the surface features inhibited the contact of the cells with the titanium surface. The cells are able to adhere to the crevices because they 'skate' across the air molecule and rest in the larger crevices. In a study by Lee et al., [63] the effect of surface charges on fouling resistance was determined. The antifouling properties against humic acid were investigated in their study. Compared to neutral and positively charged surfaces, their results show that negatively charged surfaces demonstrated lowest surface fouling. The lowest surface fouling is attributed to the electrostatic repulsive force between the negatively charged surface and the negatively charged humic acid. Surface charge density was suggested to be one of the most critical surface properties determining whether the bacteria adhere to the surface. Bacteria typically possess a net negative charge due to the presence of carboxyl, amino, and phosphate groups on the cell walls of the bacteria. Hence, bacteria find it easier to adhere to positively charged surfaces. [57] Hence, P. aeruginosa adhesion is twofold higher on a positively charged surface than a negatively charged surface. Similarly, Pseudomonas, E. coli, and S. aureus demonstrated greater adhesion on positively charged surfaces. Thus, the surface charge density influences bacterial adhesion and plays a critical role in biofilm formation. It is interesting to note that E. coli adhere to positively charged surfaces initially; the high charge density induces lower cell viability, inhibiting biofilm growth. [57] There are plenty of examples in nature that show low surface energy and superhydrophobicity through the presence of micro/nano-structure. [64,65] For example, the wax layer on lotus leaves enables them to display self-cleaning behavior. [30,66] Similar behavior is exhibited by rice leaves. The surface of rice leaves has stripe structures and is covered with hierarchical micropapillae and waxy nanobumps. [29] The surface of rice leaves displays superhydrophobicity, low adhesion, and anisotropic flow. The anisotropic flow is attributed to the arrangement of the papillae. In one of the directions, they are arranged parallel to the leaf edge and randomly oriented in the other two directions. This architecture effectively resists water droplets. This provides rice leaves with water-repellency and antifouling behavior.
Shark skin is another example of natural material that displays antifouling and self-cleaning behavior. [29,33] The riblets present on their skin can be lifted to pin any vortices that are generated in the solid-liquid interface. Thus, it helps to reduce cross-stream motion of the water and plays a significant role in lowering the drag. The fluid flow and washing of the skin enabled by low drag contributes to antifouling behavior. Another example is the wings of the butterflies. The directiondependent arrangement of flexible nano-tips on ridging nanostripes and microscales overlapped on the wings allow them to easily repulse water droplet. [67,68] These learnings from nature inspire the creation of the novel antifouling materials, which are reviewed in the next section.

Fabrication of Bio-Inspired Surfaces
Researchers have made significant progress in fabricating antimicrobial and antifouling materials by mimicking the design of micro-and nano-structures found on the surface of natural antibacterial and antifouling materials. [27,30,60] Studies have revealed that such synthetic bio-inspired materials can efficiently reduce bacterial adhesion and inhibit biofilm formation. In this section, we aim to highlight some of the common micro/nano fabrication techniques reported in literature for producing antimicrobial micro-and nano-structured materials. Table 1 is presented at the end of this section to summarize the fabrication techniques that can be used to produce antibacterial surfaces.

Soft Lithography or Templating
Soft lithography is one of the facile manufacturing techniques that can be used to transfer micro-and nano-sized structures onto polymer-based substrates. [69][70][71] This polymer replication combines a few fabrication techniques, such as printing, molding, and embossing templates. The first step in this manufacturing technique involves fabricating a negative replica from a master template. For this purpose, the usual approach is to cast an elastomer such as PDMS onto a master template. Figure 5 shows examples of structures fabricated on PDMS in our lab. Briefly, PDMS is poured on a commercial template and allowed to cure. Once cured, it is delaminated to reveal the structures. The negative imprint of the commercial template is obtained onto the PDMS. A similar approach can be followed to copy the structures of natural materials. In this approach, the master template is often the natural material whose surface textures need to be copied. The viscous PDMS is poured on to the master template, and then it is allowed to cure and solidify. This helps in transferring the natural material's patterns on the surface of PDMS. Following this, the PDMS layer is peeled off from the natural material and then used as a template. The microstructures that are copied onto the PDMS are then transferred onto another polymer surface to obtain a positive replica of the natural material. Wang et al. [72] used this soft lithography fabrication approach to produce rose petal mimetic structures on polymer surfaces. The hierarchical rose petal structures are copied on PDMS using a two-step casting technique. In the first step, an aqueous solution of polyvinyl alcohol (PVA) is poured on the rose petal. PVA film with negative rose petal structures are obtained after the water is allowed to evaporate completely. Following this, PDMS is poured on the PVA film and allowed to cure. The cured PDMS revealed the patterned micropapillae with nanofolds on its surface.
Reid et al. [31] used double replica method to copy the structures of cicada wings on to a polymer substrate. In the first step, they used a UV-curable polymer resin and drop-casted the  material onto the wing surface. Once the polymer is UV cured, it is removed from the surface of the wings and then used as primary molds. This primary mold is shown to have a negative replica of the cicada wings. The diameter of the pores found on the primary mold is reported to be similar to the features found on the wings. Following this, the primary mold is inverted, and a UV-curable polymer is drop-casted on to this primary mold. Capillary action is responsible for pulling the low-viscosity UV polymer into the porous structures of the primary mold. Thus, the gravity assisted capillary force is attributed to filling the primary mold cavities. The setup is then exposed to UV source to cure the polymer and obtain the secondary mold. This secondary mold is shown to have features similar to the ones that are found on cicada wings. Their results show that the secondary molds have similar antibacterial activity against the Gram-negative bacteria as the wings of Megapomponia intermedia cicadas.

Nanoimprint Lithography
Nanoimprinting is another form of lithography that is used to copy the features found on natural materials onto other polymer films and substrates. This patterning technique has found tremendous applications in fabricating bio-inspired textures on polymer films due to its ease of fabrication and ability to obtain high-resolution nanoscale textures. [13,38,65,[73][74][75] Nanoimprinting relies on using mechanical pressure along with heat or UV source on a stack of stamp and polymer film or on a polymer resist placed on a stamp/template. Natural material can be directly used as a stamp when nanoimprinting is used for the duplication of natural material's features and structures onto polymer films. [38] Thermoplastic polymer films are typically used in thermal nanoimprint lithography (NIL) or hot embossing lithography where the viscosity of the material can be controlled by varying the temperature. In this process, a hard master stamp with patterns is brought in contact with a thin thermoplastic film. When temperature is applied, the thermoplastic tends to become soft and viscous. After applying the pressure, the softened thermoplastic film sandwiched between the stamp and substrate is forced to flow into the stamp cavities. Once the cavities are filled, the material is allowed to cool down while the pressure is kept constant. This allows the polymer to harden and take the shape of the patterns present on the stamp. Hong et al. [74] in their study used a cicada wing as a template to hot emboss it on an optical grade polyvinyl chloride (PVC) film. The low glass transition temperature of PVC ensured that when the stack of cicada wing and PVC film is heated to 120 °C and 10 bar of pressure, the polymer filled the concave patterns of the cicada wing. Upon demolding, the PVC film is found to have 100 nm hole array patterns. Following this, the PVC film with nanoholes on the surface is brought in contact with a UV-curable polymer. A 10 bar pressure is applied to fill the cavities of the PVC template and exposed to a UV source. The cured resin had cicada wing-like structures on its surface.
In the case of UV-based NIL, a photoresist is coated on a substrate and brought in contact with a transparent stamp (see Figure 6). Moderate pressure is applied to force the liquid photoresin to flow into the cavities of the stamp. Following this, the setup is exposed to UV source to cross-link and harden the resin. This helps to transfer the patterns present on the stamp onto the cured polymer film. A similar approach has been used to fabricate rose petal structures onto polymer films. Briefly, a UV-curable polymer is poured on the rose petal and allowed to cure by exposing it to a UV source. The cured polymer serves as a duplicate of the primary mold. Following this, the primary mold is coated with a monolayer of silane. A second UV-curable polymer is then poured on the primary mold and allowed to cure. This secondary mold is seen to have structures and features like those found on rose petals. Figure 6 shows the rose petal mimetic microstructures fabricated on a polymer film. The structures had the hierarchical design that is commonly found on the surface of rose petals.

Reactive Ion Etching
Reactive ion etching is a micro/nano-fabrication technique that has been used to fabricate high-aspect ratio biomimetic nanostructures on large substrates. [27,76] This technique applies highenergy ions that remove the material at the surface to fabricate nanostructures. In this technique, high-energy ions generated by the plasma source are bombarded onto the surface of the material. The ions diffuse into the material and chemically react with it to remove the material locally (Figure 7). Surface nanostructures are produced by altering the process parameters. Ivanova et al. [76] used reactive ion etching method to fabricate biomimetic nanoscale-sized protrusions on black silicon and demonstrated the bactericidal property of these high aspect ratio structures. The nanoprotrusions fabricated are shown to be 20-80 nm in diameter range and 500 nm in height. These structures displayed good bactericidal property against both Gram-negative and Gram-positive bacteria. Fisher et al. [77] used microwave plasma chemical vapor deposition (MPCVD) followed by a reactive ion etching technique to produce biomimetic nanocones on the surface of a diamond and demonstrated its antibacterial property. In the first step, the MPCVD is used to deposit polycrystalline diamond film on a substrate. In the second step, reactive ion etching is used to fabricate nanocones on the surface of diamond film to mimic the nanopillar structures found on cicada wings. Their study illustrated that surfaces with lower nanocone density and non-uniform structures had higher bactericidal potency than those with high density and uniform nanocones. This can be attributed to the generation of higher stress forces across the bacterial cell wall. The bacterial cells on the surface with non-uniform nanocones are seen to collapse and spread over the surface features which leads to cell death.

Hydrothermal Etched Titanium
Hydrothermal treatment is another simple, cost-effective, and single step method that has been used to produce nanometer length scale structures on the surface of titanium substrates. [78][79][80] Studies have used hydrothermal methods to produce structures such as nanoneedles, nanoleaves, nanoflowers, and nanopillars on the surface of titanium and demonstrated that introducing these structures on titanium enables it to display antibacterial activity properties. [81][82][83][84][85][86] Typically, the hydrothermal technique is performed under controlled pressure and temperature in an autoclave. Briefly, the cleaned titanium substrate is placed in a stainless steel autoclave. It is then treated with an alkali solution such as KOH solution, as shown in Figure 8, and subjected to temperature in the range of 150-200 °C. During this process, the surface of the titanium substrate is etched due to the dissolution of titanium ions from the substrate. This is followed by recrystallization of potassium titanate during the hydrothermal etching process. [87] The next step associated with hydrothermal etching is annealing, which results in the formation of desired structures and crystalline phase of titanium oxide on the titanium substrate. Titanatebased nanostructures are formed on the substrate that can take a wide variety of forms depending on the processing conditions used. Wandiyanto et al. [87] show that the density and height of the nanostructures formed on the titanium substrate can be controlled by varying the time used during the hydrothermal etching process. They show that short and highly dense nanostructures are formed on titanium substrate when hydrothermal etching time was 0.5 h. Upon increasing time, the dense and short nanostructures served as the nucleation site and led to the formation of larger, well-defined nanowires. Finally, they show that these hydrothermally treated titanium surfaces display good antibacterial behavior against P. aeruginosa as well as S. aureus bacteria. In a similar study, Bhadra et al. [81] used hydrothermal method to fabricate nanowires on the surface of titanium such that it mimicked the structures found on dragonfly wings. The fabricated surface is shown to eliminate 50% Gram-negative bacterial cells and 20% Grampositive bacterial cells. A recent paper by Bright et al. examined the effect of etching time and the choice of cation used in the alkaline heat treatment on the resultant nanostructures and their antibacterial properties. [80] NaOH etching produced dense surface nanofeatures, while KOH etching resulted in sparser, disordered surface morphology. The NaOH etched surface was shown to be more effective at killing Gram-negative pathogens, while the KOH surface was shown to be more effective against Gram-positive bacteria.

Photocatalytic Nanostructures
In recent years, nanostructured semiconductor materials have gained much interest as photocatalytic antibacterial materials. These photocatalytic materials have been demonstrated to be effective against both Gram-positive and Gram-negative bacteria. The semiconductor materials such as titanium dioxide (TiO 2 ), zinc oxide (ZnO), and tin dioxide (SnO 2 ), undergo photocatalysis when illuminated with photons that have energy  greater than their respective bandgap widths. The electronic carriers react with water or hydroxyl molecules and produce reactive oxygen species and other free radicals. These reactive species interact with the bacterial cells and provide antibacterial properties to the semiconductor materials. For instance, the reactive species induce the rupture of bacterial cells resulting in the death of the cells as they interact with the outer membranes of the bacterial cells. Most of these semiconductor materials generally have low photocatalytic efficiency under visible light, attributed to their wide bandgap. One way to address this issue is to couple the semiconductor materials with a noble metal. It helps to improve the semiconductor material's light-harvesting efficiency and charge separation, which plays a role in improving the photocatalytic capability of the semiconductor.
Tang et al. [88] combined gold (Au) with ZnO to form nanopillars of ZnO/Au that mimicked the nanopillar arrays of dragonfly wings. These nanopillar arrays of ZnO/Au are grown on PDMS film to attain bactericidal activity and antiadhesive properties. Figure 9 shows a schematic of the fabricated structures on PDMS. ZnO pillars on PDMS are grown using a hydrothermal method. Briefly, the PDMS is first dipped in the seed solution of ZnO, comprising zinc acetate ethanol and sodium hydroxide ethanol. Following this, ZnO seeds-coated PDMS is dipped in a mixture solution of Zn(NO 3 ) 2 ·6H 2 O and hexamethylenetetramine and then heat-treated to obtain PDMS/ZnO. In the final step, PDMS/ZnO is placed in a beaker containing HAuCl 4 solution. Irradiating the ZnO surface with Xe lamp will help in reducing Au 3+ ions into neutral nanoparticles of Au. The results showed that the PDMS/ZnO/Au can kill bacteria in dark conditions mainly due to its surface morphology. [88] When irradiated with visible light, PDMS/ZnO/Au demonstrates improved antibacterial activity, which is attributed to the photocatalytic activity of the semiconductor ZnO/Au under visible light. In a similar study, Pascariu et al. [89] used an electrospinning technique to fabricate photocatalytic ZnO/Au nanostructures. They show that the nanostructures have antimicrobial activity against Gram-positive and Gram-negative bacteria and yeast strains.

Bacteria-Responsive Materials
It is known that bacteria can cause changes to the micro-environment. For instance, the presence of bacteria can increase the amount of exo-enzymes produced by bacteria. In some cases, the presence of bacteria can change the pH in the microenvironment or can increase reactive oxygen species. [90][91][92] Stimulus-responsive antimicrobial materials that release their antibacterial load in response to such changes in their microenvironment have gathered the attention of researchers. As an example, Pavlukhina et al. [93] developed ultrathin hydrogel stimuli-responsive coatings that release antimicrobial agents in response to changes in pH.
Similarly, Zhang et al. [94] developed pH-responsive antibacterial materials by immobilizing pH-responsive moieties on the material's surface. Melittin is used as the antimicrobial peptide. [94] This antimicrobial peptide kills bacteria by making bilayer-spanning pores in the membrane of the bacterial cells. The results of this study show that bactericidal properties are trigged on the surface of the material when the bacterial cells come in contact with the surface. When the acidic environment generated by the bacteria accumulated on the material surface is sensed, peptide is released from the material surface that helps kill the pathogens. This material is shown to be antibacterial against both Gram-positive and Gram-negative bacteria.
Zhou et al. [95] developed antimicrobial wound dressings using UV-photocross-linkable methacrylated gelatin. By incorporating both antimicrobial and fluorescent vesicles in methacrylated gelatin, the authors showed that the material responds to the micro-environment of the wound by changing its color and releasing antimicrobial agents. The material was able to kill and inhibit the growth of S. aureus and P. aeruginosa. The antimicrobial wound dressing material comprised two different layers. The lower layer is composed of vesicles containing the antibiotics, while the upper layer has vesicles containing the self-quenching dye carboxyfluorescein. In the presence of bacteria, the toxins secreted by the bacteria are shown to damage the lipid bilayer shell of the vesicles. This releases the payloads that result in killing of the bacteria and at the same time changes the color the material. [95] In a similar work, Truskewycz et al. [96] developed bandages that display both antimicrobial and pH-dependent fluorometric properties. These bandages are developed by incorporating fluorescent magnesium hydroxide nanosheets into polycaprolactone/poly(ethylene oxide) blend polymer fibers that are obtained using electrospinning. The fluorescence intensity is seen to decrease when the pH is reduced from 10 to 7.5, and no fluorescence is seen when the pH is below 7. The working mechanism of this bandage is illustrated in Figure 10. In the presence of microbial pathogens, the pH of the micro-environment increases, and the fluorescence intensity of the bandage increases. The pathogens then interact with the magnesium hydroxide present in the fibers, which cause their elimination. When the wound starts to heal, the pH in the micro-environment decreases, which results in ionizing the magnesium hydroxide nanosheets. The intensity of fluorescence decreases in response to lower pH, and hence no fluorescence is associated with healing of the wound. Thus, this material is shown to be useful as an antimicrobial material as well as a pH probe that indicates wound healing process.

Shape Change Antimicrobial Materials
The use of stimuli-responsive materials that can change their shape has been receiving much attention due to their potential use in antimicrobial applications. Piktel et al. showed that the size and shape of gold nanoparticles determine their antimicrobial efficiency. [97] Their study showed that the interaction of gold nanoparticles with the bacteria and their ability to penetrate the cytoplasm of the pathogens is determined by the size and shape of the nanoparticles. Similarly, Raza et al. [98] showed that the antimicrobial efficiency of silver nanoparticles depends on their size and shape. Smallest size silver nanoparticles are shown to have better antibacterial activity compared to triangular-shaped silver nanoparticles and larger-diameter silver nanoparticles. These findings demonstrate the potential of developing shape-change antimicrobial nanomaterials. Such shape-changing materials that can alter their shape and/ or internal structure in response to stimuli such as heat, light, or magnetic field will make them potent antimicrobial materials. Pu et al. [99] fabricated nanoparticles as drug carriers that are multifunctional and responsive to both temperature and pH. The multifunctional responsive nanoparticles are composed of poly(N-isopropylacrylamide-co-acrylic acid) (PNA) that provides both temperature and pH-responsive behavior and polydopamine outer layer that acts as a photosensitive agent. Doxorubicin is loaded in the PNA core as a drug, which is released whenever the pH in the environment is reduced. Polydopamine is capable of converting near-infrared laser into heat, which helps to increase the temperature of the nanoparticles. The increase in temperature breaks the hydrogen bonds between PNA and water molecules and causes PNA to shrink. This activates the release of drugs. Elbourne et al. [100] developed antimicrobial materials that can change shape whenever a magnetic field is applied. The antimicrobial materials comprise magnetic-field responsive gallium-based liquid metal. Due to the ability of these particles to change their shape, they are shown to be effective in physically damaging, disintegrating, and killing pathogens within a mature biofilm. As shown in the schematic (Figure 11), the shape of these liquid metal droplets transforms into particles with nanosharp edges. Due to these nanosharp edges, they can physically rupture bacterial cells and can break down dense biofilms.

Colloidal Lithography
Colloidal lithography is a type of micro/nano-fabrication technique with the ability to fabricate micro-and nano-dimensional arrays. [101][102][103] The colloidal lithography technique combined with etching is utilized to create various surface structures. [102] In this approach, colloidal crystals are first obtained via spincoating, dip-coating, or vertical deposition. They are then assembled as a monolayer on the material surface to serve as a mask. Following this, when the vapor deposition of the selected material is initiated, the vapor is blocked by colloidal crystals and is only allowed to reach the substrate through the gap between the crystals. This action obtains a pyramidal structure on the substrate. Once the vapor condensation is complete, the etching process is conducted to remove the colloidal mask (see Figure 12). The size and the shape of the structures formed can be controlled by varying the mask size, the mask morphology, the plasma source, and the etching process. Mo et al. [101] applied colloidal lithography to fabricate cone/pillar-like micro/nanoarrays to mimic the structures found on insect wings. Pillarand cone-like structures were fabricated by adjusting the size of the mask and the operating parameters during the plasma etching process. The findings of this work demonstrated that the final product possessed antibacterial properties due to the exact mechanism observed in cicada wings. Additionally, the results showed that the antibacterial properties are influenced by the surface topography and the dimensions of the surface structures.

Effect of Nanostructure's Physical Characteristics on Its Bactericidal Property
It is evident from the studies that the bactericidal activity of a surface depends on wide variety of parameters such as dimensions, density, and shape of the nanostructures. In this section, we review the effect of these parameters on the antibacterial activity of the nanostructures.

Effect of Spacing between the Nanostructures
Studies show that the topography of a surface influences its ability to prevent bacterial attachment and colonization. Microand nano-structures on a surface influence its physical properties. This, in turn, influences the available contact area as well as the adhesion between the bacterial cells and the material. Researchers determined that surfaces with micrometer-sized structures are more efficient at inhibiting bacterial attachment than surfaces with macro-roughness ( Table 2). Chung et al. [33] fabricated microtopography on PDMS to mimic the structures found on shark skin. They show that this microtopography is efficient in disrupting bacterial biofilm. The biofilm formation on the smooth surface was investigated and compared with the microtopographic surface. Smooth surface promoted biofilm colonization within 7 days. However, the microtopographic surface experienced biofilm colonization only after 21 days. Different strains of bacteria are sensitive to materials that have micron-sized surface topographies. [19,104,105] Pingle et al. [104] revealed that the topographies that are in micron and nanometer size can hinder bacterial attachment. They fabricated binary colloidal crystal (BCC) consisting of micron sized spherical silica and nanometer sized polymethyl methacrylate particles. Their results showed that the bacterial coverage is decreased for BCC samples with larger diameter silica particles. The increase in interstitial spaces between the large and small particles within the BCC selectively traps the bacteria. The separation of bacteria impedes communication among the microbes, hindering biofilm formation. They also reported that the strategy is effective for P. aeruginosa because the size and shape of the bacterial cell is comparable to the BCC pattern size.
Bacterial cells tend to colonize points on a surface they can easily adhere to. For example, surfaces with pits and grooves that have dimensions larger than the bacteria size will be colonized. Whitehead et al. [106] demonstrated the relationship between the surface features and distribution of bacterial cells. In their study, they constructed a range of surfaces with tailored topography. They demonstrated that the size of pits is important with respect to bacterial cell size and its retention on the surface. Their results showed that P. aeruginosa and S. aureus colonize surfaces with regularly spaced pits with dimensions ranging from 1 to 2 µm.
On the other hand, irregularly spaced pits with dimensions smaller than 0.2 microns do not offer points for the bacterial cells to colonize. Valle et al. [19] fabricated periodic structures with micron-and submicron-patterned topography and evaluated the anti-adhesion behavior of these surfaces. Their results showed that the compatible features (1-5 µm) maximize the bacteria-surface contact area. In particular, adhesion is seen for S. aureus bacteria with a size between 0.6 and 1 µm on patterned surfaces. On the other hand, surfaces with smaller topographic feature size than the size of bacteria offer smaller accessible areas for the bacterial cells. Similar results are reported by Chang et al. [21] The authors showed that P. aeruginosa motility is reduced by micron-size topographical features. The displacement of P. aeruginosa is lower when the feature size on surface is 2 µm or larger. Contrarily, the displacement is assisted when the feature size is less than 1 µm.
The available contact area for the bacterial cells can be narrowed down by introducing nanostructured topography on the surface. Dense nanopillars improve antifouling behavior as the bacterial cells find it difficult to fit between the structures. Kelleher et al. [20] investigated the bactericidal properties of cicada wings taken from three different species and determined that bactericidal activity can be related to the density and diameter of the nanostructures present on the wings. Their results show that surfaces covered by tightly packed and smaller diameter nanopillar display improved bactericidal efficiency compared to surfaces with opposite structures. Due to the small size of these structures, the bacterial membrane becomes adsorbed on multiple nanopillars, which results in non-uniform stretching leading to the rupture of the membrane. Rupturing occurs when the bacterial cell membrane is stretched beyond its elastic limit. It is suggested that the degree to which the bacterial cell stretches is directly proportional to the adhesion force between the membrane and surface and inversely related to the rigidity of bacterial cell membrane. The adhesion of the bacterial cell membrane to the nanopillar increases when the nanopillar density is high. However, when the nanopillars are too closely packed, they do not provide sufficient space between the structures for the cell membrane to stretch.

Effect of Nanostructures' Aspect Ratio
Studies have demonstrated that higher aspect ratio structures display enhanced bactericidal activity compared to lower aspect ratio structures, [7,107] which is attributed to the flexibility of the structures. A high aspect ratio increases the mechanical flexibility of the pillar structures, which further stretches the cell membrane. The structures flex and deflect due to the adhesion force. High aspect ratio structures ensure that the bacterial cell membrane rests on top of the nanostructures. On the other hand, low aspect ratio structures allow the bacterial cells to come in contact with and settle on the flat substrate. [107] The bending stiffness of pillar structures, as determined by the mechanics of beams, is directly influenced by their aspect ratio. The pillar deflects and stores elastic energy when a force is applied to the pillar structure. This elastic energy is released when the deflected pillar retracts. High aspect ratio nanopillars experience larger deflection and hence can store higher elastic energy than lower aspect ratio nanopillars. The storage and release of mechanical energy is argued to be responsible for bacterial cell perturbation and death of cells when these bacterial cells are adsorbed on high aspect ratio structures. Linklater et al. [107] built vertically aligned high aspect ratio carbon nanotubes (CNTs) and demonstrated that the structures experience extreme flexibility when in contact with bacterial cells. These flexible CNTs store high elastic energy. Their results demonstrate that this elastic energy is sufficiently high to rupture both Gram-positive and Gram-negative bacterial membranes. Similar results are reported in another study that exploit the bactericidal activity of silicon nanostructures. [108] The authors found that compared to the flat silicon substrate, bacteria that adhered to nanopillars underwent significant morphological changes. The cells were deflated and elongated, and nanopillars emerged from the top surface of the deflated bacterial cells indicating that the nanopillars bend toward the attached bacteria. The deflection of the nanopillars and the storage of mechanical energy within the silicon nanopillars laterally stretch the bacterial cell membrane. In summary, high aspect ratio nanostructures display greater bactericidal potency as they get deflected to a greater degree which can help in pulling the membrane to a greater degree.

Effect of Surface Wettability
The wettability of a surface is influenced not only by its surface chemistry but also by the presence of textures/features present on it. Surfaces with moderate wettability present a conducive surface for bacteria attachment due to the formation of hydrogen bonds and due to hydrophobic interactions between peptidoglycan and surface. Lee et al. [109] demonstrated that bacteria adhered and grew on the hydrophilic surface with a water contact angle between 40° and 70°. Dou et al. [60] showed enhanced adsorption of bacterial peptidoglycan on a moderately wettable surface that displays a water contact angle between 54° and 130°. On the other hand, superhydrophilic and superhydrophobic surfaces resist bacterial attachment. Superhydrophilic surfaces repel bacteria and inhibit the formation of biofilms. A dense layer of water molecules on the surface of a superhydrophilic material helps to reduce the interactions between the bacterial cells and the surface. Ivanova et al. show that hydrophilic [76] black silicon with nanoprotrusion is capable of killing both Gram-positive and Gram-negative bacteria. This study illustrated the bactericidal mechanisms of the nanoprotrusions and did not study in-depth the role of wettability on bacterial attachment and biofilm formation. Ostrikov et al. [110] fabricated silicon nanograss structures and demonstrated that its wettability had a role on S. aureus attachment and colonization. Interestingly, the hydrophilic plasma polymer coating on the silicon nanograss helped to improve the antimicrobial property, while the hydrophobic coating reduced its antimicrobial activity. The water contact angle on the silicon nanograss sample is 11°, and that of smooth silicon wafer is determined to be 31°. This can be explained by the Wenzel theory, which suggests that hydrophilicity of an inherently hydrophilic material increases when the material is made rougher. Following this, the silicon nanograss is made more hydrophobic by depositing a thin layer of the polymer coating. The antimicrobial activity of the samples is then determined by measuring the number of S. aureus colony-forming units present in a solution after the solution is exposed to the samples. The results showed that pristine silicon nanograss untreated samples decreased the number of S. aureus in solution after it was exposed to the sample for 3 h while the hydrophobic silicon nanograss with polymer coating displayed no change in the number of viable colony-forming units in solution. The results further indicated that the presence of nanograss on the surface-enhanced the antimicrobial action of hydrophilic samples. However, the introduction of nanograss topography for hydrophobic samples had the opposite effect.
The physical properties of the bacterial cells determine whether the bacterial cells can adhere to a surface. [111] For example, P. aeruginosa does not adhere to superhydrophobic surfaces. [111] However, S. aureus can colonize hydrophilic as well as superhydrophobic surfaces. [54] One possible explanation for this is the spherical shape of S. aureus cells. Due to their spherical shape, S. aureus requires only a small surface area to adhere and can overcome the antiadhesive property of a superhydrophobic surface.
On the other hand, P. aeruginosa are rod-shaped and requires larger surface area to adhere. Loo et al. [111] reported that the bacterial attachment on the surface of PVC is influenced not only by the wettability behavior of PVC but also by the presence of surface structures. P. aeruginosa colonization occurred on smooth PVC surfaces but did not occur on treated superhydrophobic PVC surfaces.

Summary and Perspectives
Over the last decade, researchers have paid considerable attention to the design of novel topographically engineered antibacterial materials to combat the impending problem of antibiotic-resistant bacteria. Inspired by a wide variety of antibacterial and anti-biofouling surfaces found in nature, material scientists and engineers have constructed synthetic surfaces that can eliminate bacteria based on 'contact killing' mechanisms and can prevent formation of biofilms. The key to achieving efficient mechanobactericidal microstructured topography lies in matching the dimensions of the topographical pattern to the size of the bacterial cells. When the size of topographical features is reduced to the nanometer length scales, the attachment of bacterial cells is minimized by the reduced contact area for the bacterial cells.
This review highlighted how natural antibacterial and bioinspired surfaces interact with bacteria and their underlying antibacterial mechanisms. It is clear that introducing microand nano-structured topographies on a surface offers an interesting and effective strategy to prevent bacterial attachment and biofilm development. However, it is not yet clear how the physical parameters of these nanostructures can be optimized in terms of optimal aspect ratio, diameter, spacing, and stiffness so that these bio-inspired material surfaces can efficiently eradicate both Gram-negative and Gram-positive bacterial cells and avoid the development of biofilms. In this article, we also reviewed some of the contact-killing bactericidal mechanisms of the nanostructured surfaces and discussed emerging developments in the design and fabrication of such bio-inspired nanostructured surfaces. A thorough understanding of optimizing design will help overcome the current limitations associated with mechanobactericidal surfaces and enable the transition to commercial applications. Such transition will also require the development of scalable, cost-effective, and capable of processing large quantities of material.
Krasimir Vasilev is currently a Matthew Flinders professor and a professor in biomedical nanotechnology, and a NHMRC Leadership fellow at Flinders University where he also leads the Biomedical Nanoengineering Laboratory. His research sits at the crossroad between materials, biology, and medicine, focusing on engineering and tailoring at a molecular level the very interface where biological entities interact with biomaterials and devices. He has published more than 270 papers, which have been cited more than 10 000 times. His research is also strongly translational. Several technologies developed in his laboratories are being scaled for production and commercialization by industry partners.
Vi Khanh Truong obtained his Ph.D. in nanobiotechnology. Currently he is a lecturer in medical biotechnology, and co-leads the Biomedical Nanoengineering Laboratory at Flinders University. His research program aims to develop innovative technologies to prevent and detect microbial infections at an early stage to minimize the misuse of antibiotics. He has published more than 100 research articles with citations more than 7500, and his current h-index is 39. His research program has generated effective cutting-edge technologies for combating resistant pathogens that will make significant contributions to patients, clinicians, and society.