Teacher-Informed Tactile Sensing Upgrade for Child–Robot Physical Interaction: A Comparison of Velostat and FBG Sensors
Social robots designed to engage with children require additional co-designed features that reflect authentic interactions. Physical human robot interaction (pHRI) in this application requires tactile sensors that are reliable and safe for natural children’s touch. This work reports on the design and assessment of two tactile sensing systems—one using Velostat and the other based on optical sensing—developed in line with teacher feedback to enhance the integration of the CASTOR robot into educational activities. Both sensors were characterized under controlled forces representative of human touch. The Velostat sensor exhibited a nonlinear force response, higher spatial resolution, and lower hysteresis, while the optical sensor demonstrated a highly linear response and greater sensitivity. The sensing systems were subsequently evaluated with 5 participants (25.73 ± 8.32 years) in a hand-shaking pHRI task. A heatmap-based analysis revealed that both sensors effectively captured the temporal and spatial distribution of contact forces, while also highlighting qualitative differences in localized forces among participants, with a maximum exerted force of 1.30 N. Overall, the results indicate that the Velostat-based sensor is advantageous for the educational scenario, as it provides detailed contact localization, exhibits low hysteresis, is more robust, and is replicable.
Highly Stretchable Eu3+ – Doped PEBAX Elastomers With Complementary Optical Readouts of Strain and Temperature
Soft photonic materials capable of maintaining optical functionality under large mechanical deformation are highly desirable for wearable photonics, flexible sensors, optical and mechanically adaptive photonic systems. Here, we report highly stretchable luminescent elastomers based on the thermoplastic copolymer PEBAX 2533 incorporating a europium β-diketonate complex. The transparent films combine intense europium emission with exceptional mechanical flexibility, displaying elongation at break exceeding 1200%, while maintaining high optical transparency in the visible range. The elastomers exhibit intense red luminescence with absolute quantum yields of 0.61. Under mechanical deformation, the 5D0 → 7F0 transition shows a strain-dependent spectral shift and intensity modulation, enabling optical strain readout with sensitivities of –1.6 ± 0.2 cm−1·ε−1 and −0.44 ± 0.04 ε−1, respectively. The luminescence spectra also display a thermal dependence, characterized by a blue shift and intensity decrease of the Eu3+ emission with a temperature sensitivity of 0.08 ± 0.01 cm−1·°C−1 over the investigated temperature range. The coexistence of large deformability, high transparency, efficient Eu3+ emission, and two independently calibrated optical responses establishes Eu-doped elastomers as a versatile soft photonic platform for multimodal optical transduction, highlighting their potential for flexible sensing technologies in which mechanical and thermal perturbations are optically encoded within the same material platform.