Untethered flight of an insect-sized flapping-wing microscale aerial vehicle.

Heavier-than-air flight at any scale is energetically expensive. This is greatly exacerbated at small scales and has so far presented an insurmountable obstacle for untethered flight in insect-sized (mass less than 500 milligrams and wingspan less than 5 centimetres) robots. These vehicles1-4 thus need to fly tethered to an offboard power supply and signal generator owing to the challenges associated with integrating onboard electronics within a limited payload capacity. Here we address these challenges to demonstrate sustained untethered flight of an insect-sized flapping-wing microscale aerial vehicle. The 90-milligram vehicle uses four wings driven by two alumina-reinforced piezoelectric actuators to increase aerodynamic efficiency (by up to 29 per cent relative to similar two-wing vehicles5) and achieve a peak lift-to-weight ratio of 4.1 to 1, demonstrating greater thrust per muscle mass than typical biological counterparts6. The integrated system of the vehicle together with the electronics required for untethered flight (a photovoltaic array and a signal generator) weighs 259 milligrams, with an additional payload capacity allowing for additional onboard devices. Consuming only 110-120 milliwatts of power, the system matches the thrust efficiency of similarly sized insects such as bees7. This insect-scale aerial vehicle is the lightest thus far to achieve sustained untethered flight (as opposed to impulsive jumping8 or liftoff9).

Biomechanical properties of non-flight vibrations produced by bees.

Bees use thoracic vibrations produced by their indirect flight muscles for powering wingbeats in flight, but also during mating, pollination, defence and nest building. Previous work on non-flight vibrations has mostly focused on acoustic (airborne vibrations) and spectral properties (frequency domain). However, mechanical properties such as the vibration's acceleration amplitude are important in some behaviours, e.g. during buzz pollination, where higher amplitude vibrations remove more pollen from flowers. Bee vibrations have been studied in only a handful of species and we know very little about how they vary among species. In this study, we conducted the largest survey to date of the biomechanical properties of non-flight bee buzzes. We focused on defence buzzes as they can be induced experimentally and provide a common currency to compare among taxa. We analysed 15,000 buzzes produced by 306 individuals in 65 species and six families from Mexico, Scotland and Australia. We found a strong association between body size and the acceleration amplitude of bee buzzes. Comparison of genera that buzz-pollinate and those that do not suggests that buzz-pollinating bees produce vibrations with higher acceleration amplitude. We found no relationship between bee size and the fundamental frequency of defence buzzes. Although our results suggest that body size is a major determinant of the amplitude of non-flight vibrations, we also observed considerable variation in vibration properties among bees of equivalent size and even within individuals. Both morphology and behaviour thus affect the biomechanical properties of non-flight buzzes.

Buzz-pollinating bees deliver thoracic vibrations to flowers through periodic biting.

Pollinator behavior is vital to plant-pollinator interactions, affecting the acquisition of floral rewards, patterns of pollen transfer, and plant reproductive success. During buzz pollination, bees produce vibrations with their indirect flight muscles to extract pollen from tube-like flowers. Vibrations can be transmitted to the flower via the mandibles, abdomen, legs, or thorax directly. Vibration amplitude at the flower determines the rate of pollen release and should vary with the coupling of bee and flower. This coupling often occurs through anther biting, but no studies have quantified how biting affects flower vibration. Here, we used high-speed filmography to investigate how flower vibration amplitude changes during biting in Bombus terrestris visiting two species of buzz-pollinated flowering plants: Solanum dulcamara and Solanum rostratum (Solanaceae). We found that floral buzzing drives head vibrations up to 3 times greater than those of the thorax, which doubles the vibration amplitude of the anther during biting compared with indirect vibration transmission when not biting. However, the efficiency of this vibration transmission depends on the angle at which the bee bites the anther. Variation in transmission mechanisms, combined with the diversity of vibrations across bee species, yields a rich assortment of potential strategies that bees could employ to access rewards from buzz-pollinated flowers.

Piezoelectric ribbons printed onto rubber for flexible energy conversion.

The development of a method for integrating highly efficient energy conversion materials onto stretchable, biocompatible rubbers could yield breakthroughs in implantable or wearable energy harvesting systems. Being electromechanically coupled, piezoelectric crystals represent a particularly interesting subset of smart materials that function as sensors/actuators, bioMEMS devices, and energy converters. Yet, the crystallization of these materials generally requires high temperatures for maximally efficient performance, rendering them incompatible with temperature-sensitive plastics and rubbers. Here, we overcome these limitations by presenting a scalable and parallel process for transferring crystalline piezoelectric nanothick ribbons of lead zirconate titanate from host substrates onto flexible rubbers over macroscopic areas. Fundamental characterization of the ribbons by piezo-force microscopy indicates that their electromechanical energy conversion metrics are among the highest reported on a flexible medium. The excellent performance of the piezo-ribbon assemblies coupled with stretchable, biocompatible rubber may enable a host of exciting avenues in fundamental research and novel applications.

A biologically inspired, flapping-wing, hybrid aerial-aquatic microrobot.

From millimeter-scale insects to meter-scale vertebrates, several animal species exhibit multimodal locomotive capabilities in aerial and aquatic environments. To develop robots capable of hybrid aerial and aquatic locomotion, we require versatile propulsive strategies that reconcile the different physical constraints of airborne and aquatic environments. Furthermore, transitioning between aerial and aquatic environments poses substantial challenges at the scale of microrobots, where interfacial surface tension can be substantial relative to the weight and forces produced by the animal/robot. We report the design and operation of an insect-scale robot capable of flying, swimming, and transitioning between air and water. This 175-milligram robot uses a multimodal flapping strategy to efficiently locomote in both fluids. Once the robot swims to the water surface, lightweight electrolytic plates produce oxyhydrogen from the surrounding water that is collected by a buoyancy chamber. Increased buoyancy force from this electrochemical reaction gradually pushes the wings out of the water while the robot maintains upright stability by exploiting surface tension. A sparker ignites the oxyhydrogen, and the robot impulsively takes off from the water surface. This work analyzes the dynamics of flapping locomotion in an aquatic environment, identifies the challenges and benefits of surface tension effects on microrobots, and further develops a suite of new mesoscale devices that culminate in a hybrid, aerial-aquatic microrobot.