Micro-electromechanical systems (MEMS) and lab-on-a-chip (LOC) technologies have integrated various functions onto very small chips. The physical and chemical signals received are transformed into electrical signals and biochemical reactions can be generated and processed on a chip. Beyond on-chip sensing or feedback, micromachines have potential as an intermediary tool for various interactions. A humanoid robot requires various sensors and actuators to mimic human functions. Specialized and differentiated industrial robots are equipped with sensors and actuators to fulfill their mission. Reduced-size microsensors are suitable for functionalizing robots without disturbing their fundamental functions. Additionally, micromachines have potential as an intermediary tool for various interactions with a small world. Micro robots are able to interact with an environment in this small world, while humanoid robots are designed for human-robot interaction in a macro world. A combination of haptic interfaces with microrobots could even allow interaction between the little world and us.
Microsensors have been used to measure the strength of small living things such as insects. The flight force of flying insects, as a typical insect force, has been measured by various means1,2,3,4,5,6. Direct measurement by microsensors and image processing for motion capture were used for force measurement. The deformation, movement, and force generated of a moth’s wings were measured optically using fringe pattern projection1. The aerodynamic vertical force was about 7 mN. It is about 5 times stronger than the gravitational force acting on the butterfly (about 1.3 mN).
Drosophila flight forces were measured using a MEMS capacitive force sensor (3.6 mm × 2.1 mm × 0.5 mm) to understand the biomechanics of Drosophila flight (3 mm long)2. The capacitive sensor was developed using a silicon-on-insulator (SOI) substrate to capture instantaneous flight force in real time. Drosophila samples were attached to a sensor probe (3 mm × 50 μm × 50 μm). The total flight force was estimated at a few tens of micronewtons. The collision avoidance behavior of the locust (40 mm body length) was studied with simultaneous force measurements and high-speed video recording. Interesting results on the relationship between wing flapping, lift and thrust have been reported3. The strength and timing of fruit fly take-off flight were analyzed using high-speed video techniques4. The contribution of jumping leg and wing flapping strength to weightlifting was examined. The vertical force of the jumping legs (order μN) is sufficiently greater than the corresponding aerodynamic force. Social forces in the interaction by laboratory swarms of flying midges Chironomus riparius have been studied using multi-camera stereoimaging and particle tracking techniques to understand collective animal behavior5. The acceleration of each insect towards its nearest neighbor was measured to estimate the repulsive and attractive forces in this study. The tensile force of male Strepsiptera (Insecta) was measured to estimate the dependence of the surface state of the substrates6. The force was measured using a force sensor based on strain gauges, which were attached to the insects by a thin polymer thread. The average values of the force measured were less than 0.5 mN.
In addition to flight force measurement, the leg forces of various insects have been measured7,8,9,10. Leg strength measurement of a walking stick insect has been reported to investigate the mechanism controlling joint positioning in the legsseven. A platform with a dynamometer has been prepared in the path where the stick insects walk. The median of the difference between the force value at the beginning and at the end of the stimulation ramp was −3.0 mN (flexion) and 6.0 mN (extension). A multi-axis piezoresistive sensor with micronewton force resolution has been reported to measure the foot force of insects such as ants smaller than cockroaches8. The sensor demonstrated had a minimum force resolution of the order of 0.5 mN. Plant-insect interactions on leaflets have been studied by measuring the attachment (pulling) forces generated by beetles on various plant substrates.9. The dorsal surface of the beetle’s thorax was attached to a load cell force sensor by means of hair. The peak tensile forces measured on plant surfaces differed from the force generated on glass, with the control ranging from 0.5 to 11.8 mN. A force microplate array using strain gauges for the measurement of insect leg ground reaction forces has been reported.ten. The force resolution was 1 μN.
Not only animals but also plants generate forces to deform their shape and change their physical characteristics. The movement of the upper leaf of the Venus flytrap is an example of a well-known movement generated by a plant. The measurement of the forces generated by the Venus flytrap, which strikes, holds and compresses the prey, has been reported11. A piezoelectric sensor was used for direct measurements of the average trap impact force with a video camera for the determination of time constants. The average impact force between the rims of two lobes in the Venus flytrap was found to be 149 mN, for example.
Most previous work has focused on measuring insect behavior, such as flight forces and leg forces. This paper, for the first time, presents microrobot-insect interactions by a flexible microfinger integrated with an artificial muscle actuator and a tactile strain sensor, as shown in Fig. 1a. A micro finger can apply force to an objective insect and stimulate the insect. The microfinger artificial muscle actuator, which is a pneumatic balloon actuator (PBA) made of polymer, is soft and safe enough to gently interact with insects12.13. Manipulative robot systems with micro fingers have been developed using PBA for object grabbing motion12. A tiny fish egg was successfully manipulated. Additionally, micro-fingers for manipulation of cell aggregates have been developed13. A spherical aggregate of human mesenchymal stem cells (hMSCs) (φ200 μm) was pinched and released onto a microwell plate. Our study, in addition to artificial muscle microactuators, integrates the integration of tactile sensors in a microfinger. A flexible temperature sensor has been integrated into a microfinger for temperature sensing functionality14. Several types of strain sensors have been studied for motion detection of a micro-finger. Recently, a strain sensor using a microchannel filled with liquid metal (Galinstan) has been developed for the PBA15,16,17. The liquid metal strain sensor is of the resistive type and its gauge factor is reported to be approximately 115. The strain sensor can be fabricated by filling microchannels with liquid metal. This study takes advantage of the common channel structures for the sensor and the pneumatic balloon actuator. This study presents that the liquid metal strain sensor embedded in the microfinger can detect the reaction force of an insect. Therefore, the microfinger enables active force sensing against living insects. Previously, a bilateral mechanical scaling instrument was developed and applied to insect-to-insect interactions by Mohand Ousaid et al.18. The instrument system consisted of an active probe and hand interfaces for bilateral interaction. They used an insect leg to probe the water droplets. We also developed and reported a haptic teleoperation robotic system composed of a slave microfinger and a master interface device for an operator.19.20. This paper introduces a microfinger as a microscopic end-effector for the active sensing of an insect’s reaction force, which has the potential for microfinger-insect interactions in the small world in combination with the interaction system, such that a bilateral control system18 and a haptic teleoperation system19.20.