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Page 4 of 26 Jin et al. Soft Sci 2023;3:8 https://dx.doi.org/10.20517/ss.2022.34

of the ambient environment, and using data fusion technology can extract additional features, which can
[29]
improve the accuracy of interactive tasks, especially object recognition . Except for the sensing modality,
the amount and arrangement of tactile sensors should also be carefully considered. Tactile sensors with one
sensing unit for single-point sensing may no longer be desirable for the increasing demand of intelligent
robotics, which calls for significant requirements of the distributed tactile sensor array. The high spatial
[30]
resolution arrays would be suitable for subtle stimuli perception similar to human somatosensory , and a
large area tactile sensor array can cover almost the entire body of robotics to act as electronic skin .
[31]
To obtain accurate information from objects and the ambient environment, the sensing performance of
tactile sensors needs to be carefully designed according to specific applications. Since different robotic
operation tasks may have different performance requirements, it is not very meaningful to summarize a
[9]
general performance target . For instance, tactile sensors mainly exploring environment conditions and
object properties are generally required to have a wide sensing range, high stability, and quick dynamic
[32]
response, which would be more suitable for diverse applications . For daily manipulation, the grasping
force is generally less than 10 N and no higher than 30 N, and the response time is preferably less than
[20]
[16]
1 ms to keep up with the controlling and actuating systems. Further, tactile sensors used for robotic HMIs
are required to have sensing performance similar to that of human somatosensory , such as the force
[33]
[18]
detection range (0.01-100 kPa), temperature detection range (20-50 °C) , and space resolution (1 mm for
fingertips, 5 mm for palms, etc.) , which assists the development of human-machine cooperative systems.
[16]
With advanced signal processing algorithms and data training models, tactile sensors could well conduct
complicated interaction tasks without high-level performance . Table 1 compares the performances and
[34]
robotic applications of several novel tactile sensors and commercially available products. We can see that
parts of the performance indexes of some tactile sensors have reached the ideal or even far beyond the
human perception level. However, the sensors with excellent performances that are suitable for industrial
production are still under research.

PROGRESS OF FLEXIBLE TACTILE SENSORS FOR ROBOTICS
Tactile sensing principles
Tactile sensors can transmit mechanical stimuli into electrical signals, which can be further analyzed by the
microprocessor in robotics for feature extraction. Contact force and pressure are generally considered the
most intuitive tactile information, which needs to be accurately detected during robotic grasping and
[25]
manipulation applications . Typically, there are mainly six sensing principles for the commonly designed
tactile force sensors, as shown in Figure 2. Among numerous tactile force sensors, piezoresistive, capacitive,
piezoelectric, and triboelectric sensors have been intensively studied and utilized [55-57] . The piezoresistive-
based tactile sensor can measure the force changes based on the resistance variation caused by sensing
elements’ deformation , tunneling effects , or contact principle . The capacitive-based tactile sensor can
[58]
[59]
[60]
be simplified as a parallel-plate capacitor, which detects forces by the changes of parallel-plate distance or
[61]
overlapping area . Both piezoresistive-based and capacitive-based sensors have simple structural designs
[62]
and readout circuits, and the scalability of sensitivity and detection range makes them suitable for multiple
robotic scenarios, from dexterous manipulation to collision detection [14,63] . Piezoelectric and triboelectric-
based tactile sensors have been receiving widespread attention due to their outstanding ability of self-
powering and dynamic force sensing . The piezoelectric sensor generates mutually exclusive charges when
[64]
piezoelectric sensing elements are subjected to external force , while the triboelectric sensor can measure
[65]
voltage induced by surface contact of materials with different triboelectric polarities .
[50]
Several other types of tactile sensors have been reported in the past decade. The magnetic tactile sensor can
measure the force and strain based on Hall effects , giant magneto-resistance (GMR) effects , or
[67]
[66]