bio-inspired – Softbites

2022-12-07

Bioinspired e-skins: camouflaging with the flip of a switch

Original paper: Bioinspired MXene-Based User Interactive Electronic Skin for Digital and Visual Dual Channel Sensing

Content review: Heather Hamilton
Style review: Arthur Michaut

Human skin has many functions beyond ensuring that all of our insides stay, well, inside. Skin also acts as a giant sensor that feels sensations like pressure, temperature, or vibration, and converts them into electrical signals to be processed by the brain. In the animal kingdom, some species like chameleons can even use their skin to selectively blend into their environments. Scientists have set out to create electronic skins, or e-skins, that can mimic or even outperform the typical functions of the human skin by taking on color-changing abilities like chameleon skin. Polymer materials are excellent for mimicking skin due to their soft and elastic nature that allows them to bend and stretch without tearing. However, most polymers are not conductive, a necessary property for transporting electrical signals. To create e-skin, conductive materials are often integrated into polymer materials to add electronic properties to a flexible skin-like matrix. By also integrating a color-changing pigment, researchers from Tongji University created a unique e-skin that could digitally via measurement of current flow and visibly detect mechanical movements and change color for effective camouflaging.

Conductive, metal-based nanosheets, called MXenes, were used because of their hydrophilic quality, which means they like water similar to our skin cells. When encapsulated in the polymer matrix, the nanosheets are connected and can conduct electrical current. However, when the e-skin stretches, the nanosheets become disconnected and stop conducting the current. To remedy this, smaller conductive materials called carbon nanotubes were mixed with cellulose nanofibers and added to the e-skin. The cellulose nanofibers (CNF) help disperse the carbon nanotubes (CNT) similar to how coating fruit with flour before adding to cake batter helps the fruit disperse in the cake during baking rather than sinking to the bottom. The carbon nanotubes act as a bridge between nanosheets in stretched states as seen in Figure 1.

**Figure 1**. Schematic of conductive e-skin components in response to stretching.

The researchers measured the resistance to electrical current flow, or resistivity, to determine the e-skin electrical response to stretching. The difference in electrical current between an unstretched and stretched e-skin can be used to detect large movements, like a finger bending at different angles, and subtle movements, like throat movements while swallowing. As shown in Figure 2, the e-skin exhibits different relative resistance to the flow of the current at different finger bending angles.

**Figure 2**. Signal responses in the form of relative electrical resistance, (R-R₀)/R₀, showing mechanical movements can be detected by e-skin.

From their experiments, the researchers realized that the e-skin may have Joule heating capabilities meaning the e-skin would heat it up when current is flowing through it and quickly cool down as soon as the current is turned off. Inspired by color-changing abilities found in some animal skins, the researchers also mixed thermochromic pigment into the polymer matrix before casting the e-skin. These pigments change color as they are heated above 31°C. (Figure 3a).

**Figure 3**. E-skin containing thermochromic pigment in response to (a) Joule heating and (b) stretching.

As soon as the current is turned off, the e-skin begins to cool, returning back to its initial color. Since the current flow can be altered by stretching the e-skin, the temperature and subsequent color can also be changed by stretching the skin. When current is flowing through the unstretched skin, the e-skin temperature increases, causing it to turn white. When stretched, an increase in electrical resistance causes the e-skin to return to its initial color. Instead of waiting for an electrical current reading, strain and temperature are visibly detected by the color of the e-skin. This strain color-changing ability was used to detect finger movements when the e-skin was applied on a hand (Figure 3b). To demonstrate the camouflage phenomenon similar to a chameleon changing its colors to mimic the environment, an e-skin was attached to a green plant (Video).

Video. E-skin changing color to match the color of the green plant.

The e-skin changes color from a dark green to a light green similar to the plant in about 90 seconds simply by turning on a current. This e-skin camouflaging ability goes well beyond the natural abilities of human skin and would be useful for military applications for quick camouflaging. The creation of this multifunctional e-skin is an exciting innovation for the wearable technology field and brings us one step closer to becoming chameleons.

2022-03-03

Don’t French kiss the frog prince : how to make better adhesives by mimicking the frog tongue and saliva

Original paper: Frogs use a viscoelastic tongue and non-Newtonian saliva to catch prey

Content review: Olga Shishkov
Style review: Heather S. C. Hamilton

What do a frog’s tongue and a piece of Scotch tape have in common? Not much at first glance. However, if you press your finger on either one of them, you will certainly feel a sensation of stickiness. Taking a closer look at the schematic in Figure 1, you will realize that these materials have a similar structure, simply a layer of glue on a substrate. For the sticky tape, the substrate is a plastic strip and the glue is made of polymers and additives which increase the stickiness of the glue. For the frog tongue, the substrate is the tongue and the glue is the saliva.

**Figure 1.** Schematics highlighting the structural resemblances between Scotch tape and frog tongue, both composed of a substrate on which a glue lays on

These two adhesives work quite in the same way. Let’s have a look at your classic sticky tape: when you press some scotch tape on a wall to hang your favorite poster, the glue will flow and wet all the crevasses of the wall and the poster. This process is not instantaneous though. Indeed, in order to have an efficient wetting of the surface, you have to press on your Scotch tape for several seconds, and maybe press several times. Eventually, this will generate many anchoring points between the glue and the texture of the surface. When you stop applying pressure to the tape, the glue does not flow any more, it becomes stiffer and remains adhered to your wall. However, when you peel the scotch tape, or when the object you taped is too heavy, stress is reapplied to the tape adhesive as illustrated in Figure 2. These forces will make the glue flow, the anchor points will fail, and the adhesive will be removed from the wall. No chemical bonding is involved in these adhesives: only applied stress, or the lack thereof, makes the glue flowable or not, which is why these adhesives are called pressure-sensitive. The longer you press on your sticky tape, the better the final adhesion will be as you will increase the time for the glue to flow into every detail of the surface you would like to stick your tape to.

**Figure 2.** Principle of a pressure-sensitive adhesive : (a) Application of the adhesive by fluidizing the glue due to the pressure applied on the tape, thus wetting the wall. (b) Removal of the adhesive by fluidizing the glue due to the force applied on the glue by peeling the tape

But a frog catches insects or bigger animals such as snails or slugs on its tongue in less time than the blink of an eye (under 0.07 seconds), way faster than applying your Scotch tape to a wall. A team of researchers from Georgia Tech tried to understand how a frog’s tongue can achieve this feature, because understanding the biomechanics involved can inspire synthetic high-performance adhesives with fast application times. Alexis Noel and colleagues first measured just how powerful a frog’s tongue strike is by taking a high-speed video of a leopard frog, Rana pipiens, capturing an insect. They discovered that the captured insect can feel a force equal to up to 12 times its own weight when the tongue impacts it. To find out how the frog uses these high forces to stick the insect to its tongue, Noel and colleagues characterized the saliva and the tongue separately.

By testing the mechanical response of the saliva under deformation in an instrument called a rheometer, researchers showed that the saliva was shear-thinning. This means that when a small rate of deformation is applied, the saliva is very viscous, 50,000 times more viscous than human saliva. However, at high rates of deformation, the viscosity of the saliva decreases significantly causing it to flow like a fluid. When the frog strikes an insect with its tongue, the shock from the impact deforms the saliva making it less viscous so it flows easily and coats the insect as shown in Figure 3. After the initial deformation caused by the impact, the viscosity of the saliva increases and the insect is trapped in a very thick fluid.

**Figure 3.** (a) to (c): Three-step mechanism of prey capture of an insect by a frog. The tongue stretches (a) until it impacts and wraps around the insect, covering it with saliva (b). The insect is glued to the tongue which retracts into the mouth of the frog (c). Details of the saliva coating of the insect are shown in (e) before the impact of the tongue when the saliva stands at rest and in (f) after the impact when the saliva has lower viscosity and spreads on the insect. Courtesy of the original article.

By poking the tongues of six different frogs and two different toads with a metal cylinder, a test called indentation, the researchers measured the ratio between the applied force on the tongue and its resulting deformation. This ratio, called Young’s modulus, describes the softness of the tongue. On average, the frog tongues have Young’s modulus ten times lower than that of a human tongue, making them ten times softer than a human tongue. Frog tongues are able to deform around the insect to increase the prey-tongue contact surface area. Soft frog tongues also absorb the massive shock from impacting an insect much like a car’s shock absorber. The internal damping of the soft tissue allows the tongue to wrap around the insect during the capture and keep it in contact with the prey for good adhesion with the saliva. Now that the insect is trapped, how does the frog bring back its prey to its mouth? It is crucial to keep the insect coated in the sticky saliva as the tongue is retracted back into the mouth of the frog because this happens as fast as the initial stretching for insect capture. Remember the sticky tape example? Applying too much stress on the glue by peeling off the tape makes the glue flow, resulting in a loss of adhesion. The soft tongue dampens the stress of retraction on the saliva to prevent it from flowing. Once the insect is in the frog’s mouth, the only way to release the prey from its tongue and eat the insect is to retract its eyeballs, thereby squeezing the tongue hard enough so the saliva can flow again and free the prey.

Overall, this mechanism using a soft tongue and shear-thinning saliva shows great adhesive performances at speeds unmatched with synthetic materials. This article shows once again that nature can produce amazing materials and concepts which could be used for bio-inspired designs of synthetic analogs. Thanks to the deeper understanding of the frog prey-catching phenomenon, one can of course think about more efficient pressure-sensitive adhesives or glue guns, but also about robots trained to catch dangerous insects as fast as a real frog.

2021-03-242021-03-26

Real soft bites made by a model tongue to better assess food texture

Original paper: Compression Test of Food Gels on Artificial Tongue and Its Comparison with Human Test

Content review: Arthur Michaut
Style review: Heather S. C. Hamilton

Food texture, or how soft or hard your food is, is usually assessed with a machine that compresses food between two metallic plates. This instrument oversimplifies what really happens in our mouth, because the stiffness of the metallic parts does not mimic the natural deformability of the tongue. This leads to a strong deviation of the results of these experiments from sensory evaluations of food texture by humans. Practically, this can lead to hard-to-swallow food being considered soft and safe for people with dental or swallowing problems. A team of Japanese scientists tried to tackle that problem by developing a synthetic tongue which would lead to better food texture assessment with a test machine. But what are the important properties that characterize a successful model tongue?

To answer this question, the researchers compressed the tongues of seven brave human subjects using a clamp equipped with a force sensor like the schematic in Figure 1a. The human tongue is considered an elastic material, meaning that it can be deformed (by compression for example) and will return to its original size and shape after the force causing the deformation is removed. Deformation is often measured as percent strain, or the ratio of deformed size to original size of an object. Using this compression technique, which seems a bit medieval, and with the help of the human subjects, the researchers determined how the human tongue responds to an applied strain of up to 20%. Understanding the mechanical properties of the tongue means it can be modeled by an equivalent soft material and tested in a similar manner, as shown in Figure 1b.

**Figure 1:** Schematic of the steps to design an in vitro system for tongue-palate compression tests. Tongue elasticity is first measured with a clamp (a). A silicone rubber tongue with a similar elasticity is then put on the bottom plate of a compression test machine to mimic the tongue (b). The metallic top plate will act as the palate, or the hard roof of the mouth.

The synthetic tongue produced for this study was a cylinder made of silicone rubber with an elasticity that is similar to the elasticity of the real tongues of seven human subjects. Remarkably, the tongues of seven subjects were considered a large enough sample size to determine the elasticity of an average human tongue, with resulting measurements close to values reported in the literature. In the food texture assessment device, the top metallic plate is left unaltered to mimic the palate. Pieces of agar gel, a jelly-like material made from seaweed, are used as a test food product. Several agar gel samples were prepared, each requiring some force to fracture, breaking into smaller pieces. Mechanical tests were performed with the in vitro tongue-palate system, while in vivo testing was determined by the same seven subjects who had their tongues clamped in the compression test. The in vivo results determined if the subjects were able to fracture the gel by tongue-palate compression or if they needed to chew the gel. These results were compared to the in vitro observations.

The most interesting result highlighted by the study, shown in Figure 2, is the threshold above which mastication, the action of chewing with your teeth, was needed to break the agar gel into edible pieces. At a strain of 10%, if the agar gel sample was more deformed than the silicone rubber of the artificial tongue, the agar gel would fracture to the consistency of mashed potatoes. Otherwise, if the silicone rubber was more deformed than the agar gel sample, the gel sample would remain intact, meaning that a force higher than the one provided by tongue-palate compression would be needed for fracture, such as the force provided by mastication. Indeed, during tongue-palate compression, both the tongue and the model food are deformed under the action of reciprocal forces.

**Figure 2:** Strain profiles of the agar gel and the silicone rubber over time under compression by the tongue-palate model. At 10 % strain, if the agar gel curve is below the silicone rubber curve (a, top), which means that it is less deformed, then it will be more resistant to compression and will not collapse (a, bottom). If the opposite scenario takes place (b, top), then the gel will fail (b, bottom).

This is exactly how the human tongue acts like a mechanical sensor, thanks to the difference of stiffness between the organ’s tissues and the material probed. The resulting feeling will then influence the strategy used by our mouth to break down food in small ingestible (soft)bites. Food with a soft texture will only need this tongue-palate compression to be edible. Alternatively, foods with a tougher texture will need fragmentation through mastication. It is to be noted that the strategy used by the subjects eating the same agar gels matched the results of the in vitro compression tests: if the agar gel collapsed during the test, only tongue-palate compression was needed.

Therefore, food products that would fracture during this compression test can be considered soft enough to be edible without chewing. Even if this system is not a perfect model since the synthetic tongue does not replicate the complex arrangements of different tissue layers present in the human tongue and a model saliva is not included, it can be of great help to design safe food for people with mastication problems.

2018-06-062018-06-05

Are squid the key to invisibility?

Original paper: Adaptive infrared-reflecting systems inspired by cephalopods

While many today would associate a “cloak of invisibility” with Harry Potter, the idea of a magical item that renders the wearer invisible is not a new one. In Ancient Greek, Hades was gifted a cap of invisibility in order to overthrow the Titans, whereas, in Japanese folklore, Momotar? loots a straw-cloak of invisibility from an ogre, a story which is strangely similar to the English fairytale Jack the Giant-Slayer. Looking to the future in Star Trek, Gene Roddenberry imagined a terrible foe known as the Klingons, a war-driven race that could appear at any moment from behind their cloaking devices – indeed, any modern military would bite your arm off to get hold of this kind of device. Clearly, invisibility is a concept that has captured minds across many cultures, genres, and eras, so it should be no wonder that scientists are working on making it a reality.

As is often the case in materials science, a good starting point for inspiration is to look to biology, after all, life has had billions of years of competition-driven evolution to craft its tools. To that end, Gorodetsky and his team at of the University of California, Irvine, have been attempting to replicate the master of disguise: the cephalopod. From this class of mollusk, squid and octopuses in particular excel at adaptively altering the color, texture, and patterning of their skin to camouflage against a wide variety of oceanic backdrops (see Figure 1). They accomplish this primarily by stretching and contracting pigment-containing skin cells. More importantly to this research, some species of squid have additional skin cells called ‘iridocytes’, which are structured reflective cells that resemble a microscopic comb or folded pleats. These folds reflect light at specific wavelengths that correspond to the fold size. By actively stretching and contracting these cells on-demand, the squid effectively becomes a self-modifying bioelectronic display.

SquidCamouflage (1) — Figure 1: Squid before (left) and after (right) deploying camouflage. Images reproduced from
a video by H. Steenfeldt under the YouTube Creative Commons Attribution license.

Following this technique of stretch-induced camouflage, the authors devise a technique for replicating some squid-like properties in an artificial material. The procedure consists of using electron-beam evaporation[1] to deposit an aluminum layer onto a stretched polymer film held under strain. When they release the strain, the metallically coated material shrinks and buckles to form microscopic wrinkles that are analogous to the structures in squid iridocyte cells. The polymer film of choice is an excellent proton conductor, so when mounted to electrodes, it can be stretched back to the flat state simply by applying a voltage.

When stretched, the aluminum coated film will reflect infrared light like a perfect mirror, whereas when wrinkled, the incoming light is reflected diffusively in multiple directions, like sunlight hitting the moon. To see this effect, the scientists shine infrared light – a heat source – at the material and position an infrared camera at a specific angle so that when flat, it reflects all the incoming radiation toward the camera and appears hot; when relaxed and wrinkled, much less of the light is scattered towards the camera, making the material appears to take on the thermal properties of the background and disappear from view.

Perhaps as a head-nod to their biological inspiration, the team then recreate this material in the likeness of a squid and watch as its infrared silhouette disappears from the camera’s view (Figure 2).

InfraInvisibleSquidShape_Fixed (1) — Figure 2: The flexible squid-shaped material, viewed under an infrared camera. When relaxed (left), very little infrared is reflected towards the camera, and it appears cold. When stretched (right), it reflects most incoming infrared light towards the camera and appears hot. Image edited from Fig. 5 in the manuscript.

The authors conclude that this ready-for-manufacturing material will have immediate applications in heat-regulating technology, and while it is currently limited to the infrared part of the spectrum, they also note that there is no reason why this technique couldn’t be adapted to the optical range.

Squid haven’t solved our desires for a cloak of invisibility just yet, but these mysterious creatures may hold more secrets than we realize. We would be wise to keep an eye on them … if we can.

[1] Electron beam evaporation is the technique of bombarding a solid metal with energetic electrons, causing it to evaporate. This metal vapor then cools and condenses uniformly on all nearby surfaces, forming a uniform metallic coating.

2017-10-312018-04-14

Color made from structures inspired by bird feathers

Original paper: Biomimetic Isotropic Nanostructures for Structural Coloration

There’s a reason why the word “peacock” has become a verb synonymous with commanding attention. Of course, the size of the peacock tail is enough to turn heads, but it wouldn’t be nearly as beautiful without its signature iridescent, or angle-dependent, color. The brilliant colors of the peacock come from the interaction of light with the nanoscale structure of the feathers, which is much different from the origin of color in regular dyes and pigments. In today’s paper, Jason Forster and his colleagues in the Dufresne group developed a simple way to make colors that is inspired by the structures in certain bird feathers.

Figure 1. An iridescent peacock feather. Source: http://www.publicdomainpictures.net/pictures/100000/velka/peacock-feather.jpg

Colors come from the way our brain interprets different wavelengths of light. Most colors we encounter in dyes and paints are a result of absorption. Certain chemicals absorb specific wavelengths of light, and the other wavelengths are reflected; the colors we see are due to those reflected wavelengths. However, not all colors come from absorption. The color of the sky is perhaps the most widely seen example of this. The molecules that make up air scatter much more light at small wavelengths, which corresponds to blue light.

The iridescence of the colors in the peacock feather is caused by constructive interference due to the nanoscale structure of the feather. To explain this, let’s look at a simplified picture. If you have a layered stack of materials, some light will be reflected from each layer in the stack (Figure 2). Since the light reflected from the layers at the bottom stack will have traveled farther, the different sets of reflected waves will be shifted out of phase. When the waves are shifted by exactly one wavelength, they add constructively and give a stronger reflection. This constructive interference happens at a wavelength which depends on the thickness of the layers, their index of refraction, and the angle at which the light is sent and detected. Structural color is a result of the stronger reflectance at a particular wavelength due to this constructive interference of light.

bragg stack — Figure 2. Diagram depicting path length difference from reflection from different layers that gives rise to constructive interference.

Structural color can arise in many different types of structures, from bird feathers and butterfly wings to soap bubbles and opals, but today’s paper is about a type of structural color made from plastic spherical particles. These spheres are only a few hundred nanometers in diameter, on the order of the wavelength of visible light, and they are so small that they can remain suspended in water for long periods of time, forming a colloidal suspension. Jason Forster and his colleagues in the Dufresne group made structurally colored films by starting with a small volume of a colloidal suspension of these particles and allowing it to dry, causing the particles to pack together and self-assemble into structures with color.

The way the particles packed greatly impacted the color of the film. When the researchers used spheres that were all the same size, the particles formed a crystal (an ordered arrangement made of a repeating unit cell) as the suspension dried. In a crystalline structure such as the peacock feather, the structural color is iridescent, or angle-dependent. This angle-dependence of color arises because the angle that light is sent into the sample will affect the distance it travels through the material, therefore changing the wavelength at which the light will constructively interfere. However, the researchers found that when they mixed spheres of two different sizes, the spheres could no longer form a crystal, and instead formed a disordered structure (Figure 3, top). This structure was isotropic, meaning that it looked the same from any angle. The structural color of a crystalline sample is iridescent because light travels different path lengths through it at different angles. Because the isotropic structure is essentially the same at all angles, the color is the same at all angles.

colloid and bird feather — Figure 3. *Top left:* Photo of a structurally colored film. *Top right*: Scanning electron micrograph of particles in a film comparable to the one on the left. *Bottom left*: Photo of bird feathers of *Lipodothrix Coronata*. *Bottom right*: Tunneling electron micrograph of bird feathers on the left.
Adapted from Forster *et al.*

By making a more disordered structure, Forster and his colleagues were able to make a more uniform color! These disordered assemblies of spheres bear a striking resemblance to the nanoscale structures found in bird feathers such as Lipodothrix Coronata (Figure 3, bottom), which are made up air spheres embedded in a disordered array inside a matrix of beta-keratin. These bird feathers have a color similar to the particle films made by the researchers: a blue color that doesn’t change with angle.

Our eyes are a useful tool for observing colors, but they are not the most precise way to measure light. If we want to compare colors precisely and quantitatively, the best way to do that is by looking at a reflectance spectrum. A reflectance spectrum tells you the amount of light reflected from an object at a range of wavelengths. You can measure a reflectance spectrum by shining light at a colored sample and using a spectrometer to detect the reflected light. Combined with a computer, a spectrometer allows you to record an intensity value for a range of wavelengths, giving you a full intensity spectrum. The reflectance spectrum is found by normalizing this data against a perfect reflector such as a mirror or a white material, giving you the percent of light reflected at each wavelength. So if you were to measure the reflectance of a blue material, you would have a spectrum with a peak in the wavelengths that correspond to blue light (~450-495 nm).

One way to infer the reflectance spectrum of a material that has no absorption is to measure transmittance. To measure the transmittance spectrum, you can move the detector to the side opposite to the incident light, so it detects the light that goes through the sample. If you were to measure the transmittance spectrum of this same blue material, you would expect to see a dip corresponding to the blue wavelengths. The blue light would not make it through to the other side because it was reflected.

The researchers measured the transmittance spectra for their structurally colored samples and found that the blue isotropic structural color and the blue crystalline structures both showed a dip in the blue wavelengths (Figure 4). However, the dip in the isotropic structure data was much broader and more shallow, meaning that less light was reflected at that wavelength, making the color less bright and saturated.

But the quality of the color wasn’t the only thing that changed in the spectra of the isotropic structures. In these samples, the transmittance dip stayed at the same range of wavelengths even when the measurement angle changed, while the dip in the spectrum of the crystalline structure shifted as the measurement angle was changed. By eye, the researchers also saw that the disordered structures made angle-independent color, and the ordered structures made iridescent color. The measurements of the crystalline and isotropic structures show that there is a tradeoff between saturation and angle-independence in structural color.

The thickness of these isotropic structurally colored films also greatly affected the saturation of their color. Films that were just a few micrometers thick had a bright blue color, while much thicker films looked nearly white. The researchers found that adding some carbon black– black nanoparticles that absorb light at all visible wavelengths– made the colors of the thick films more vibrant (Figure 5). The carbon black works by reducing the effective thickness of the samples, absorbing light before it can travel through the entire layer of the sample and causing it to look like a thinner sample.

carbon black colloid films — Figure 5: Isotropic structurally colored films with different amounts of carbon black. The concentrations of carbon black as weight percent are listed beneath the samples.
Adapted from Forster *et al.*

This work showed that structural color, both iridescent and angle-independent, can be made using simple methods that could potentially make the colors in large volumes for real-world applications. Because these colors come from structure and not absorption, they will not fade over time as current dyes do. In addition, one material can be used to make a range of different colors by tuning the structure, so these assemblies could be used as colorimetric sensors that change color in response to environmental changes such as strain or temperature.

2017-10-272018-04-14

Fluids That Flow Themselves

Original paper: Transition from turbulent to coherent flows in confined three-dimensional active fluids (Non-paywall version here.)

Disclosure: The first author of the paper discussed in this post, Kun-Ta Wu, did his Ph.D. at New York University, in the same research group as the present writer (CPK). At NYU, both Wu and CPK worked on topics unrelated to the research discussed here.

*****

When we think about fluid flow, we generally think of motion in response to some external force: rivers run downhill because of gravity, while soda moves through a straw because of the pressure difference created by sucking on one end. Recently, however, scientists have become interested in a class of fluids that have the capacity to move all by themselves — the so-called “active fluids.” Active materials — of which active fluids are a subset — are distinct from regular materials because energy is injected into the system at the level of individual molecules. In today’s paper, Kun-Ta Wu and his co-workers explore how such a material can turn its stored chemical energy into useful work: cargo transport.

Why are active materials so interesting? For one thing, many biological systems are active — for example the actin filaments that drive muscle contraction or bacterial swarms. Although active systems are both common and important in our everyday lives, the physical laws that govern their behavior are not well understood [1]. Studying artificial active systems, which are much simpler than living ones, might give us insight into this difficult problem.

As well as helping us to understand basic physics and biology, Wu and his co-workers hope that their research will move us closer to producing artificial materials that transport cargo without adding energy from an external source — a self–powered fluidic conveyer belt [2]. Such a material would be totally different from those that we currently use, and would greatly expand the possibilities available to engineers in fields such as microfluidics and soft robotics.

Wu’s research focuses on a system made up of protein molecules that assemble into cylindrical rods called microtubules. While microtubules are very important in biology [3], Wu uses these tiny rods, suspended in water, to make an artificial active fluid. As well as microtubules, Wu adds two other critical ingredients: kinesin molecular motors, and ATP (adenosine triphosphate), a chemical that many biological systems use as an energy source [4].

fig1 — A sliding force is generated between microtubules by the action of molecular motors. (Adapted from Figure 1 of the original paper.)

A single kinesin molecule attaches to two parallel microtubules and creates a lateral force that slides or “walks” them along each other. A single “step” of this walk involves a chemical reaction that converts one ATP molecule into ADP (adenosine diphosphate), a lower-energy state, thereby converting chemical potential energy into motion. A collection of millions or billions of microtubules (and a similar number of kinesin and ATP molecules) forms a material that writhes and squirms without any forces acting upon it. In the following video, Wu records the motion of both the microtubules themselves (they’re tagged with a fluorescent red dye), and micrometer-sized green particles, which he uses to trace the flow.

Video 1 Using fluorescence microscopy, Wu and colleagues can observe the motion of microtubules (red), as well as test cargo — colloidal particles (green) that are carried along in the flow generated by the motion of microtubules. (Movie 1 of the original paper.)

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But converting energy into useful work doesn’t just require motion; it requires motion that is controlled, directed, and uniform over time — coherent motion. This brings us to the main finding of Wu and coworkers: in the microtubules-motors-ATP system, coherent motion can be produced by controlling the shape of the container. Placed in a large rectangular box, the flow in the middle of the box (“in the bulk”) is turbulent but directionless (see panel A of the below figure). However, when placed in a ring with appropriate dimensions, the flow spontaneously organizes into large-scale circular patterns that are capable of transporting cargo — like fluorescent colloidal particles — over lengths of centimeters or even longer (panel B below).

fig2 — Panel A shows the pattern of flow of a bulk sample of active fluid. The arrows represent the velocity field, and colors represent the normalized *vorticity* of the flow: the extent to which it is rotating clockwise or anticlockwise in a local frame of reference. The left half of the panel shows a snapshot of the flow at a single instant in time, while the right half shows the time average. (This convention is also used in the other flow visualizations shown in this post.) In the time-averaged plot, both velocity and vorticity are almost zero: the flow is turbulent but directionless. Panel B-i shows the ring geometry of one of the sample chambers Wu uses to create coherent flow, and B-ii shows the flow pattern in that chamber. Unlike in the bulk sample, a long-lived circular pattern is generated that pushes the cargo around the ring. (Adapted from Figure 1 of the original paper.)

Interestingly, whether or not this happens is controlled only by the aspect ratio of the container: the channel width divided by its height [5]. Coherent flow is observed when the aspect ratio is between ? and 3; in other words, it disappears if the ring is too flat or too tall. Additionally, Wu shows that the direction of the flow– whether it goes clockwise or counterclockwise — can be controlled by decorating the outside of the container with appropriately shaped notches, which Wu calls ratchets.

Finally, the researchers show that the appearance of directed flow coincides with the onset of nematic order: in circulating samples, the rod-like microtubules tend to align with their neighbors, while in the turbulent samples, they are oriented randomly. According to Wu, this alignment allows the fluid to collectively push itself off the walls of the container, thus generating global circulation.

fig3 — Wu and co-workers use ratchets — small asymmetrical notches on the outside of the ring — to control whether the flow is clockwise (CW) or counterclockwise (CCW). The scale bar shows that flow is coherent over lengths of centimeters. (Adapted from Figure 3 of the original paper.)

Of course, this paper only scratches the surface of the technological potential of active materials. Research on this, and similar ideas, continues both at Brandeis University, where this research was done, and in Worcester Polytechnical Institute, where Wu has recently been appointed professor. Here, according to his website, Wu aims to “advance our understanding of self-organization of active matter as well as to create unprecedented bio-inspired materials.”

*****

[1] Physical systems at thermodynamic equilibrium obey the Boltzmann distribution — a formula that (in principle) allows us to calculate macroscopic properties of many-body systems, if we know the interactions between the constituent particles. We don’t know of a similar theory that describes the behavior of out-of-equilibrium systems, and active systems are by definition out of equilibrium.

[2] Of course, the energy ultimately has to come from somewhere. In the case of the material studied by Wu et al, the conveyer belt would have to be “charged” with fresh ATP before use.

[3] In particular, microtubules are the most important structural component of the mitotic spindle – the sub-cellular structure that pulls chromosomes copies apart during cell division.

[4] Wu also adds a chemical known as a depletant, which makes the microtubules bundle together, allowing the kinesin to slide them along each other.

[5] Wu also studies cylinders and shows that a similar geometrical parameter controls the appearance of coherent flow.