Bat tongues are one animal feature with promise for human engineering

Bat erectile tongues are one of many features that inspire robotos

Nectar-eating bats lap up the sweet liquid by engorging their tongues with blood, which, in turn, makes hairlike projections on the tongue stand at attention, new research finds. Together, the erect hairs, called papillae, act like a mop that grabs more liquid than a smooth surface could alone. Using histological techniques, high-speed videography, and anatomical studies, Brown University biologist Cally Harper and colleagues found that the batGlossophaga soricinarelies on hair-like structures known as papillae on its tongue to extract nectar from flowers.

Video of the process shows that as the bat reaches out to grab the nectar, its tongue turns bright red as blood automatically rushes in. The discovery is more than simply weird animal trivia, though; researchers think the tongues could be good models for developing kinder, gentler surgical tools.

Bats are one of the only mammals known to feed on blood. They are also the only mammals capable of flight. There are more than 900 species of bats found all over the world; some experts say there are more than 1,200 species.

Bats are nocturnal creatures that roost during the day in caves, trees or man-made structures. The largest bat is the flying fox with a wingspan of up to 6 feet (1.8 meters). The smallest is the Kitti’s hog-nosed bat with a wingspan of less than 6 inches (15.2 cm).

Many bats use echolocation when they fly. They send out ultrasonic sounds and listen for variations in the echoes that bounce back at them to navigate.

Bats belong to the order Chiroptera and are classified into two main groups, or suborders: Megachiroptera, consisting of the flying fox family, and Microchiroptera. Megabats are medium-size to large bats. Many eat fruit, pollen or nectar; some eat small animals and fish. Microbats use echolocation to find their prey, typically insects.

“These bat tongues are very flexible, and they’re soft,” said study researcher Cally Harper, a doctoral student at Brown University. “They could be really useful [inspiration] in bending around the curves of blood vessels and intestines, but also, they may minimize damage to some of those soft tissue structures.”

Hairy tongues

Scientists have long known that nectar-feeding bats have hairy-looking tongues, as do hummingbirds and other species that rely on flowers for food. These hair projections are called papillae, which are specialized versions of the bumps that dot the tongues of humans and other mammals. Many human papillae host taste buds, but the hairlike papillae on bats show no signs of sensory tissues. (Bats’ taste buds are further back on their tongues.)

Anatomists also have noticed large blood vessels in these bats’ tongues, Harper told LiveScience.

“I thought, ‘Oh, that’s really interesting that there are these enlarged blood vessels and these really specialized papillae,'” Harper said. “There’s the possibility that maybe blood flow was used to move these papillae during feeding.”

Dissections of bat tongues revealed that there were sinuses, or spaces, along the sides of the tongues that extended into the papillae, suggesting blood flowed through the millimeter-long hairs. Harper and her colleagues just had to find out whether the papillae moved during feeding.

To do so, they set up high-speed video cameras around feeding stations and let the nectar-eating bat Glossophaga soricina have at the sweet spots. Nectar-feeding bats have keen spatial memory and return to the same spots to feed again and again, Harper said.

“All I had to do was make sure my feeder full of sugar water was set up in the same location, and then all I had to do was sit and wait,” she said.

Mopping up nectar

At 500 frames per second, the videos showed that as the bats extended their tongues, at first, the papillae were flat against the tongue surface. But then, as the tongue hit its maximum elongation, the hairs became erect. Color video revealed that this change in position occurred as the tongue tip flushed bright red.

“The hairs separate from each other, and that creates a little space between each of the rows of hairs on the tongue,” Harper said. “Each one of those spaces becomes filled with nectar.”

The process is automatic, and likely driven by muscular tension in the tongue, Harper said. She and her colleagues report their findings today (May 6) in the journal Proceedings of the National Academy of Sciences. Mammalian penile erections also use blood to create stiffness, with arteries dilating to fill the penis with blood as contracting muscles prevent that blood from draining back to the body.

Bat tongues are just one of many animal features with promise for human engineering. Scientists have studied snail shells to develop stronger body armor, sticky gecko feet to inspire better adhesives, and insects to engineer miniature flying robots.

A new technique inspired by elegant pop-up books and origami will soon allow clones of robotic insects to be mass-produced by the sheet.

Devised by engineers at Harvard University, the ingenious layering and folding process enables the rapid fabrication of not just microrobots, but a broad range of electromechanical devices. The new technique replaces a tedious and time-consuming manual process for creating such devices.

For prototypes, engineers laminated together layers of carbon fiber, Kapton (a plastic film), titanium, brass, ceramic, and adhesive sheets in a complex, laser-cut design. The structure incorporates flexible hinges that allow the 3-D product — just 0.1 inch (2.4 millimeters) tall — to assemble in one movement, like a pop-up book.

The entire product is approximately the size of a U.S. quarter, and dozens of these microrobots could be fabricated in parallel on a single sheet.

“This takes what is a craft, an artisanal process, and transforms it for automated mass production,” researcher Pratheev Sreetharan, who co-developed the technique with J. Peter Whitney, said. Both are graduate students at the Harvard School of Engineering and Applied Sciences.

Sreetharan, Whitney, and their colleagues in the Harvard Microrobotics Laboratory have been working for years to build bio-inspired, bee-sized robots that can fly and behave autonomously as a colony. Appropriate materials, hardware, control systems, and fabrication techniques did not exist prior to the RoboBees project, so each must be invented, developed, and integrated by a diverse team of researchers.

The RoboBees project is supported by the National Science Foundations’ Expeditions in Computing program, as well as the U.S. Army Research Laboratory and the Wyss Institute for Biologically Inspired Engineering at Harvard.

“The ability to incorporate any type and number of material layers, along with integrated electronics, means that we can generate full systems in any three-dimensional shape,” principal investigator Rob Wood, an Associate Professor of Electrical Engineering at Harvard, said. “We’ve also demonstrated that we can create self-assembling devices by including pre-stressed materials.”

Printed bot boards

A new technique inspired by elegant origami will soon allow clones of robotic insects to be mass-produced by the sheet.
CREDIT: Pratheev Sreetharan, Harvard School of Engineering and Applied Sciences
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Moreover, the layering process builds on the manufacturing process currently used to make printed circuit boards, which means that the tools for creating large sheets of pop-up devices are common and abundant. It also means that the integration of electrical components is a natural extension of the fabrication process — particularly important for projects like RoboBees where the devices are constrained in their size and weight.

“In a larger device, you can take a robot leg, for example, open it up, and just bolt in circuit boards,” explains Sreetharan. Our devices are “so small that we don’t get to do that.” Pointing to the carbon-fiber box truss that constitutes the pop-up bee’s body frame, Sreetharan said, “Now, I can put chips all over that. I can build in sensors and control actuators.”

The implications of this novel fabrication strategy go far beyond the micro-air vehicles. The same mass-production technique could be used for high-power switches, optical systems, and any other tightly integrated electromechanical devices that have parts on the scale of micrometers to centimeters.