Author: Pierre Lehericey

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.

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.

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.

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. 

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. 

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.

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.