Human-interactive and communicative robot has attractive attention from industrial to home-use applications. One of human-friendly robotic structures is pneumatic balloon type soft robots using mainly PDMS operated by compressed air. However, sensor integration to detect contact force and surface temperature has yet to be demonstrated without sacrificing softness and flexibility. This study demonstrates PDMS-based pneumatic balloon actuator integrated with electronic skin (e-skin) to detect tactile pressure and temperature like a human skin. The important advance of this demonstration is to integrate flexible sensors embedded in PDMS films. As a proof-of-concept, contact force and surface temperature of human skin were monitored by moving the PDMS-based robotic finger actuated by air pressure.
For the fabrication, two types of PDMS with different hardness were poured into molds and cured at 80 °C. After removing the PDMS, they were bonded to create pneumatic balloon structure. Tactile pressure sensor formed by conductive cotton and conductive Ag thread was fabricated. After coating PDMS film over the sensor, temperature sensor consisted of mixture of PEDOT:PSS and carbon nanotube was laminated by PDMS thin film. For sensor interconnection, conductive Ag thread was embedded in PDMS layers. Both sensors are resistive change type sensors.
The fabricated soft finger with e-skin with/without air pressure shows that this structure can be actuated by air pressure. First, pressure force corresponding to bending displacement generated by air pressure was measured. At air pressure >20 kPa, the finger structure starts bending, and it is bent drastically at ~36 kPa due to balloon formation of PDMS. For embedded e-skin performance, normalized resistance change of temperature sensor and tactile pressure sensor were characterized. The temperature sensitivity was ~0.73 %/°C. For pressure sensor, at ~1 kPa, the resistance was drastically decreased due to the improvement of contact between conductive cotton and Ag thread. To confirm the mechanical flexibility of both sensors, resistance change as a function of bending was measured, showing that resistance change at 20 mm bending radius is almost negligible compared to the ones caused by each sensor output. It can be concluded that temperature and pressure can be independently distinguished. Finally, e-skin-integrated pneumatic balloon soft finger was demonstrated. When air pressure was supplied to the balloon, the soft finger was actuated and bent, resulting in that soft fingertip contacted onto a human arm. During this contact, tactile pressure to the skin and skin temperature were monitored in real-time. The results indicate that soft finger can detect skin temperature and contacted force successfully.
In summary, we could successfully realize the pneumatic balloon-type soft finger integrated with e-skin functions. This device platform may be able to enhance the development of soft robotic applications.