Surface tension biomimetics

Last updated January 08, 2022

Surface tension is one of the areas of interest in biomimetics research. Surface tension forces will only begin to dominate gravitational forces below length scales on the order of the fluid's capillary length, which for water is about 2 millimeters. Because of this scaling, biomimetic devices that utilize surface tension will generally be very small, however there are many ways in which such devices could be used.

Applications

Coatings

A lotus leaf is well known for its ability to repel water and self-clean. Yuan ^[1] and his colleagues fabricated a negative mold of alotus leaf from polydimethylsiloxane (PDMS) to capture the tiny hierarchical structures integral for the leaf's ability to repel water, known as the lotus effect. The lotus leaf's surface was then replicated by allowing a copper sheet to flow into the negative mold with the assistance of ferric chloride and pressure. The result was a lotus leaf-like surface inherent on the copper sheet. Static water contact angle measurements of the biomimetic surface were taken to be 132° after etching the copper and 153° after a stearic acid surface treatment to mimic the lotus leaf's waxy coating. A surface that mimics the lotus leaf could have numerous applications by providing water repellent outdoor gear.

Various species of floating fern are able to sustain a liquid-solid barrier of air between the fern and the surrounding water when they are submerged. Like the lotus leaf, floating fern species have tiny hierarchical structures that prevent water from wetting the plant surface. Mayser and Barthlott^[2] demonstrated this ability by submerging different species of the floating fern salvinia in water inside a pressure vessel to study how the air barrier between the leaf and surrounding water react to changes in pressure that would be similar to those experienced by the hull of a ship. Much other research is ongoing using these hierarchical structures in coatings on ship hulls to reduce viscous drag effects.

Biomedical

A lung is composed of many small sacks called alveoli that allow oxygen and carbon dioxide to diffuse in and out of the blood respectively as the blood is passed through small capillaries that surround these alveoli. Surface tension is exploited by alveoli by means of a surfactant that is produced by one of the cells and released to lower the surface tension of the fluid coating the inside of the alveoli to prevent these sacks from collapsing. Huh ^[3] and his fellow researchers created a lung mimic that replicated the function of native alveolar cells. An extracellular matrix of gel, human alveolar epithelial cells, and human pulmonary microvascular endothelial cells were cultured on a polydimethylsiloxane membrane that was bound in a flexible vacuum diaphragm. Pressurization cycles of the vacuum diaphragm, which simulated breathing, showed similar form and function to an actual lung. The type II cells were also shown to emit the same surfactant that lowered the surface tension of the fluid coating the lung mimic. This research will hopefully some day lead to the creation of lungs that could be grown for patients that need to have a transplant or repair performed.

Locomotion

Microvelia exploit surface tension by creating a surface tension gradient that propels them forward by releasing a surfactant behind them through a tongue-like protrusion. Biomimetic engineering was used in a creative and fun way to make and edible cocktail boat that mimicked the ability of microvelia to propel themselves on the surface of water by means of a phenomenon called the marangoni effect. Burton ^[4] and her colleagues used 3D printing to make small plastic boats that released different types of alcohols behind the boat to lower the surface tension and create a surface tension gradient that propelled each boat. This type of propulsion could one day be used to make sea vessels more efficient.

Actuators

Fern sporangia consist of hygroscopic ribs that protrude from a spine on the part of the plant that encapsulate spores in a sack (diagram). A capillary bridge is formed when water condenses on to the surface of these spines. When this water evaporates, surface tension forces between each rib cause the spine to retract and rip open the sack, spilling the spores. Borno ^[5] and her fellow researchers fabricated a biomimetic device from polydimethylsiloxane using standard photolithography techniques. The devices used the same hygroscopic ribs and spine that resemble fern sporangia. The researchers varied the dimensions and spacing of the features of the device and were able to fine-tune and predict movements of the device as a whole in hopes of using a similar device as a microactuator that can perform functions using free energy from a humid atmosphere.

A leaf beetle has an incredible ability to adhere to dry surfaces by using numerous capillary bridges between the tiny hair-like setae on its feet. Vogel and Steen ^[6] noted this and designed and constructed a switchable wet adhesion mechanism that mimics this ability. They used standard photolithography techniques to fabricate a switchable adhesion gripper that used a pump driven by electro-osmosis to create many capillary bridges that would hold on to just about any surface. The leaf beetle can also reverse this effect by trapping air bubbles between its setae to walk on wet surfaces or under water. This effect was demonstrated by Hosoda and Gorb ^[7] when they constructed a biomimetic surface that could adhere objects to surfaces under water. Using this technology could help to create autonomous robots that would be able to explore treacherous terrain that is otherwise too dangerous to explore.

Various life forms found in nature exploit surface tension in different ways. Hu ^[8] and his colleagues looked at a few examples to create devices that mimic the abilities of their natural counterparts to walk on water, jump off the liquid interface, and climb menisci. Two such devices were a rendition of the water strider. Both devices mimicked the form and function of a water strider by incorporating a rowing motion of one pair of legs to propel the device, however one was powered with elastic energy and the other was powered by electrical energy. This research compared the various biomimetic devices to their natural counterparts by showing the difference between many physical and dimensionless parameters. This research could one day lead to small, energy efficient water walking robots that could be used to clean up spills in waterways.

Environment

The Stenocara beetle, a native of the Namib Desert has a unique structure on its body that allows it to capture water from a humid atmosphere. In the Namib Desert, rain is not a very common occurrence, but on some mornings a dense fog will roll over the desert. The stenocara beetle uses tiny raised hydrophilic spots on its hydrophobic body to collect water droplets from the fog. Once these droplets are large enough, they can detach from these spots and roll down the beetle's back and into its mouth. Garrod et al.^[9] has demonstrated a biomimetic surface that was created using standard photolithography and plasma etching to create hydrophilic spots on a hydrophobic substrate for water collection. The optimal sizing and spacing of these spots that allowed the most water to be collected was similar to the spacing of the spots on the body of the stenocara beetle. Currently, this surface technology is being studied to implement as a coating on the inside of a water bottle the will allow the water bottle to self fill if left open in a humid environment, and could help to provide aid where water is scarce.

Related Research Articles

A photoresist is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronic industry.

Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems. The terms "biomimetics" and "biomimicry" are derived from Ancient Greek: βίος (bios), life, and μίμησις (mīmēsis), imitation, from μιμεῖσθαι (mīmeisthai), to imitate, from μῖμος (mimos), actor. A closely related field is bionics.

The respiratory system is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In land animals the respiratory surface is internalized as linings of the lungs. Gas exchange in the lungs occurs in millions of small air sacs; in mammals and reptiles these are called alveoli, and in birds they are known as atria. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood. These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the trachea, which branches in the middle of the chest into the two main bronchi. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the bronchioles. In birds the bronchioles are termed parabronchi. It is the bronchioles, or parabronchi that generally open into the microscopic alveoli in mammals and atria in birds. Air has to be pumped from the environment into the alveoli or atria by the process of breathing which involves the muscles of respiration.

A pulmonary alveolus also known as an air sac or air space is one of millions of hollow, distensible cup-shaped cavities in the lungs where oxygen is exchanged for carbon dioxide. Alveoli make up the functional tissue of the lungs known as the lung parenchyma, which takes up 90 percent of the total lung volume.

Surfactant Substance that lowers the surface tension between a liquid and another material

Surfactants are compounds that lower the surface tension between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, or dispersants. The word "surfactant" is a blend of surface-active agent, coined c. 1950.

Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a biological membrane that forms the boundary between an organism and its extracellular environment.

Polydimethylsiloxane (PDMS), also known as dimethylpolysiloxane or dimethicone, belongs to a group of polymeric organosilicon compounds that are commonly referred to as silicones. PDMS is the most widely used silicon-based organic polymer, as its versatility and properties lead to many applications.

Atelectasis Collapse or closure of a lung resulting in reduced or absent gas exchange

Atelectasis is the collapse or closure of a lung resulting in reduced or absent gas exchange. It is usually unilateral, affecting part or all of one lung. It is a condition where the alveoli are deflated down to little or no volume, as distinct from pulmonary consolidation, in which they are filled with liquid. It is often called a collapsed lung, although that term may also refer to pneumothorax.

Pulmonary surfactant is a surface-active complex of phospholipids and proteins formed by type II alveolar cells. The proteins and lipids that make up the surfactant have both hydrophilic and hydrophobic regions. By adsorbing to the air-water interface of alveoli, with hydrophilic head groups in the water and the hydrophobic tails facing towards the air, the main lipid component of surfactant, dipalmitoylphosphatidylcholine (DPPC), reduces surface tension.

The lotus effect refers to self-cleaning properties that are a result of ultrahydrophobicity as exhibited by the leaves of Nelumbo, the lotus flower. Dirt particles are picked up by water droplets due to the micro- and nanoscopic architecture on the surface, which minimizes the droplet's adhesion to that surface. Ultrahydrophobicity and self-cleaning properties are also found in other plants, such as Tropaeolum (nasturtium), Opuntia, Alchemilla, cane, and also on the wings of certain insects.

Pulmonary hemorrhage is an acute bleeding from the lung, from the upper respiratory tract and the trachea, and the pulmonary alveoli. When evident clinically, the condition is usually massive. The onset of pulmonary hemorrhage is characterized by a cough productive of blood (hemoptysis) and worsening of oxygenation leading to cyanosis. Treatment should be immediate and should include tracheal suction, oxygen, positive pressure ventilation, and correction of underlying abnormalities such as disorders of coagulation. A blood transfusion may be necessary.

Synthetic setae emulate the setae found on the toes of a gecko and scientific research in this area is driven towards the development of dry adhesives. Geckos have no difficulty mastering vertical walls and are apparently capable of adhering themselves to just about any surface. The five-toed feet of a gecko are covered with elastic hairs called setae and the ends of these hairs are split into nanoscale structures called spatulae. The sheer abundance and proximity to the surface of these spatulae make it sufficient for van der Waals forces alone to provide the required adhesive strength. Following the discovery of the gecko's adhesion mechanism in 2002, which is based on van der Waals forces, biomimetic adhesives have become the topic of a major research effort. These developments are poised to yield families of novel adhesive materials with superior properties which are likely to find uses in industries ranging from defense and nanotechnology to healthcare and sport.

Ultrahydrophobic surfaces are highly hydrophobic, i.e., extremely difficult to wet. The contact angles of a water droplet on an ultrahydrophobic material exceed 150°. This is also referred to as the lotus effect, after the superhydrophobic leaves of the lotus plant. A droplet striking these kinds of surfaces can fully rebound like an elastic ball. Interactions of bouncing drops can be further reduced using special superhydrophobic surfaces that promote symmetry breaking, pancake bouncing or waterbowl bouncing.

In cell biology, lamellar bodies are secretory organelles found in type II alveolar cells in the lungs, and in keratinocytes in the skin. They are oblong structures, appearing about 300-400 nm in width and 100-150 nm in length in transmission electron microscopy images. Lamellar bodies in the alveoli of the lungs fuse with the cell membrane and release pulmonary surfactant into the extracellular space.

Surfactant protein B is an essential lipid-associated protein found in pulmonary surfactant. Without it, the lung would not be able to inflate after a deep breath out. It rearranges lipid molecules in the fluid lining the lung so that tiny air sacs in the lung, called alveoli, can more easily inflate.

Poractant alfa is a pulmonary surfactant sold under the brand name Curosurf by Chiesi Farmaceutici. Poractant alfa is an extract of natural porcine lung surfactant. As with other surfactants, marked improvement on oxygenation may occur within minutes of the administration of poractant alfa. The new generic form of surfactant is SLS developed in PersisGen Co. and commercialized by ArnaGen Pharmad. It has fully comparable quality profile with Curosurf.

Surfactant metabolism dysfunction is a condition where pulmonary surfactant is insufficient for adequate respiration. Surface tension at the liquid-air interphase in the alveoli makes the air sacs prone to collapsing post expiration. This is due to the fact that water molecules in the liquid-air surface of alveoli are more attracted to one another than they are to molecules in the air. For sphere-like structures like alveoli, water molecules line the inner walls of the air sacs and stick tightly together through hydrogen bonds. These intermolecular forces put great restraint on the inner walls of the air sac, tighten the surface all together, and unyielding to stretch for inhalation. Thus, without something to alleviate this surface tension, alveoli can collapse and cannot be filled up again. Surfactant is essential mixture that is released into the air-facing surface of inner walls of air sacs to lessen the strength of surface tension. This mixture inserts itself among water molecules and breaks up hydrogen bonds that hold the tension. Multiple lung diseases, like ISD or RDS, in newborns and late-onsets cases have been linked to dysfunction of surfactant metabolism.

The Salvinia effect describes the permanent stabilization of an air layer upon a hierarchically structured surface submerged in water. Based on biological models, biomimetic Salvinia-surfaces are used as drag reducing coatings (up to 30% reduction were previously measured on the first prototypes. When applied to a ship hull, the coating would allow the boat to float on an air-layer; reducing energy consumption and emissions. Such surfaces require an extremely water repellent super-hydrophobic surface and an elastic hairy structure in the millimeter range to entrap air while submerged. The Salvinia effect was discovered by the biologist and botanist Wilhelm Barthlott and his colleagues and has been investigated on several plants and animals since 2002. Publications and patents were published between 2006 and 2016. The best biological models are the floating ferns with highly sophisticated hierarchically structured hairy surfaces, and the back swimmers with a complex double structure of hairs and microvilli. Three of the ten known Salvinia species show a paradoxical chemical heterogeneity: hydrophilic hair tips, in addition to the super-hydrophobic plant surface, further stabilizing the air layer.

Self-cleaning surfaces are a class of materials with the inherent ability to remove any debris or bacteria from their surfaces in a variety of ways. The self-cleaning functionality of these surfaces are commonly inspired by natural phenomena observed in lotus leaves, gecko feet, and water striders to name a few. The majority of self-cleaning surfaces can be placed into three categories: 1) superhydrophobic, 2) superhydrophilic, and 3) photocatalytic.

Microfluidics refers to the flow of fluid in channels or networks with at least one dimension on the micron scale. In open microfluidics, also referred to as open surface microfluidics or open-space microfluidics, at least one boundary confining the fluid flow of a system is removed, exposing the fluid to air or another interface such as a second fluid.

References

↑ Yuan, Zhiqing (15 November 2013). "A novel fabrication of a superhydrophobic surface with highly similar hierarchical structure of the lotus leaf on a copper sheet". Applied Surface Science. 285: 205–210. Bibcode:2013ApSS..285..205Y. doi:10.1016/j.apsusc.2013.08.037.
↑ Mayser, Matthias (12 June 2014). "Layers of Air in the Water beneath the Floating Fern Salvinia are Exposed to Fluctuations in Pressure". Integrative and Comparative Biology. 56 (5): 1000–7000. doi: 10.1093/icb/icu072 . PMID 24925548.
↑ Huh, Dongeun (25 June 2010). "Reconstituting Organ-Level Lung Functions on a Chip". Science. 328 (5986): 1662–1668. Bibcode:2010Sci...328.1662H. doi:10.1126/science.1188302. PMC 8335790 . PMID 20576885.
↑ Burton, Lisa (22 May 2014). "The Cocktail Boat". Integrative and Comparative Biology. 54 (6): 969–973. doi: 10.1093/icb/icu052 . PMID 24853727.
↑ Borno, Ruba (21 September 2006). "Transpiration actuation: the design, fabrication and characterization of biomimetic microactuators driven by the surface tension of water". Journal of Micromechanics and Microengineering. 16 (11): 2375–2383. Bibcode:2006JMiMi..16.2375B. doi:10.1088/0960-1317/16/11/018. hdl: 2027.42/49048 .
↑ Vogel, Michael (22 December 2009). "Capillarity-based switchable adhesion". Proceedings of the National Academy of Sciences. 107 (8): 3377–3381. doi: 10.1073/pnas.0914720107 . PMC 2840443 . PMID 20133725.
↑ Hosoda, N. "How a leaf beetle walks underwater". Science Daily.
↑ Hu, David (1 June 2007). "Water-walking devices". Experiments in Fluids. 43 (5): 769–778. Bibcode:2007ExFl...43..769H. doi:10.1007/s00348-007-0339-6. S2CID 12754027.
↑ Garrod, R. (4 October 2006). "Mimicking a Stenocara Beetle's Back for Microcondensation Using Plasmachemical Patterned Superhydrophobic-Superhydrophilic Surfaces". Langmuir. 23 (2): 689–693. doi:10.1021/la0610856. PMID 17209621.

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[1] Yuan, Zhiqing (15 November 2013). "A novel fabrication of a superhydrophobic surface with highly similar hierarchical structure of the lotus leaf on a copper sheet". Applied Surface Science. 285: 205–210. Bibcode:2013ApSS..285..205Y. doi:10.1016/j.apsusc.2013.08.037.

[2] Mayser, Matthias (12 June 2014). "Layers of Air in the Water beneath the Floating Fern Salvinia are Exposed to Fluctuations in Pressure". Integrative and Comparative Biology. 56 (5): 1000–7000. doi: 10.1093/icb/icu072 . PMID 24925548.

[3] Huh, Dongeun (25 June 2010). "Reconstituting Organ-Level Lung Functions on a Chip". Science. 328 (5986): 1662–1668. Bibcode:2010Sci...328.1662H. doi:10.1126/science.1188302. PMC 8335790 . PMID 20576885.

[4] Burton, Lisa (22 May 2014). "The Cocktail Boat". Integrative and Comparative Biology. 54 (6): 969–973. doi: 10.1093/icb/icu052 . PMID 24853727.

[5] Borno, Ruba (21 September 2006). "Transpiration actuation: the design, fabrication and characterization of biomimetic microactuators driven by the surface tension of water". Journal of Micromechanics and Microengineering. 16 (11): 2375–2383. Bibcode:2006JMiMi..16.2375B. doi:10.1088/0960-1317/16/11/018. hdl: 2027.42/49048 .

[6] Vogel, Michael (22 December 2009). "Capillarity-based switchable adhesion". Proceedings of the National Academy of Sciences. 107 (8): 3377–3381. doi: 10.1073/pnas.0914720107 . PMC 2840443 . PMID 20133725.

[7] Hosoda, N. "How a leaf beetle walks underwater". Science Daily.

[8] Hu, David (1 June 2007). "Water-walking devices". Experiments in Fluids. 43 (5): 769–778. Bibcode:2007ExFl...43..769H. doi:10.1007/s00348-007-0339-6. S2CID 12754027.

[9] Garrod, R. (4 October 2006). "Mimicking a Stenocara Beetle's Back for Microcondensation Using Plasmachemical Patterned Superhydrophobic-Superhydrophilic Surfaces". Langmuir. 23 (2): 689–693. doi:10.1021/la0610856. PMID 17209621.

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