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Page 18 of 26 Jin et al. Soft Sci 2023;3:8 https://dx.doi.org/10.20517/ss.2022.34

Close-looped duplex interaction
Ideal HMIs have capacities for multiple objects decision-making and duplex information exchanging
between human manipulators and robotics, which can realize high-level interactive tasks with sophisticated
manipulations. The robotics receive and process human instructions while making decisions autonomously,
and the on-site situation can be conveyed to human beings through different tactile feedback mechanisms
[182]
such as mechanical, thermal, or electrical stimulation . Humans can get a vivid perception of the
operation site, which serves as a practical reference for revising the input instructions. Cooperative missions
[36]
[183]
between humans and robotics have long been reported , such as elderly attendant care and intuitive
teaching . Tactile feedback also helps delicate control in the grasping manipulator. Yu et al. proposed a
[22]
[54]
close-looped controlling task of the remote robotic hand to grasp a softball, as in Figure 9C . Human
manipulators utilize the EMG signal to actuate the robotic hand, and the multi-modal sensing information
between the grasping hand and the object is conveyed through electrical stimulation on human arms.
Similar works applied underwater or in other extreme environments have also been reported. Recently,
[23]
virtual reality (VR) and augmented reality (AR) technology have been reported to enable high-level HMI
applications [8,184] . Sun et al. have reported a digital-twin-based virtual shop by leveraging the loT and AI
analytics . Human beings equipped with VR devices can achieve real-time feedback about the grasped
[185]
product, which shows great potential for advanced HMI in virtual space, as shown in Figure 9D.

SUMMARY AND FUTURE PROSPECTS
In summary, tactile sensors and sensing technology can significantly improve the intellectual perception
capacity of robotics, which have been applied in various robotic applications such as dexterous
manipulation and human-machine interactions. We summarized tactile sensing principles, promising
tactile sensors for robotics, and mainstream robotic application scenarios. Diverse structure designs of the
pressure sensors, tri-axis sensors, large-scale sensing arrays, and multi-modal sensors were summarized in
detail, which provided a helpful reference for tactile sensing devices structure design in robotics. On the
other hand, different robotic applications, including object properties recognition, grasping and
manipulation, and human-machine interactions, were thoroughly discussed, and it may be heuristic for the
development of the next generation of intelligent robotics in the future.

Although tactile sensing technology for robotic applications has achieved remarkable progress, there still
remain some challenges to be addressed. For object properties recognition, it is sometimes time-consuming
to extract feature information, and researchers are always pursuing advanced algorithms and multi-modal
sensors for fast and accurate recognition. Besides, the grasping and manipulation task calls for advanced
motion control algorithms to acquire high-efficient operation, and both force feedback and slippage
detection should be conducted instantaneously. Finally, for the ideal human-machine interactions, the data
processing should be conducted on site, and multiple feedback information is more desired. However,
recent works show limited data processing speed and less realistic feedback. Therefore, we have summarized
some improvements that can be achieved in the future:

(1) Large-scale tactile sensor array with high spatial resolution for robotics still needs to be investigated. To
achieve haptic perception capacity similar to human beings, the tactile sensing array is not only required to
cover all over the robotic body but also owes high spatial resolution for subtle stimuli measurement.
Although high spatial resolution sensor arrays realized by ML algorithms and full-body electronic skin
[30]
[22]
have been reported, a large-scale sensing array with high or scalable spatial resolution needs to be further
investigated. Besides, numerous signals generated by a large amount of sensing units also cause a significant
challenge to the ancillary data-collecting devices, which calls for great efforts to solve problems such as
wireless transmission and crosstalk elimination.