This study develops a novel mutual capacitive tactile sensor for a touch panel with shifted finger-type electrodes with high sensitivity and other benefits. This tactile sensor detects normal, shear, and pulling forces with good simultaneous visible transmittance and flexibility for the next generation of 3-D flexible portable displays. The tactile sensor is composed of polyethylene terephthalate, gold, indium tin oxide, and polydimethylsiloxane. It exhibits 75% transmittance on average in the visible region, 0.04-N force resolution, and 1.5-pF/N responsiveness. The finger-type electrode design improves sensitivity by 112%-4100%, depending on the number of fingers and shifted direction. This study conducts theoretical calculations, simulations, fabrication, and measurements.

In this study, stoppers were designed and allocated in a planar four-capacitor (P4C) sensing unit for modulating capacitive tactile sensor (CTS) characteristics. The stoppers deterred the structural deformation of the CTS when applied with normal forces, realizing a three-stage operation: high detection sensitivity, wide dynamic range, and saturation. Comprehensive studies including theoretical design, numerical evaluation, demonstrator fabrication, experimental analysis, and a technical discussion were conducted. The proposed CTS achieved 0.901 pF/N sensitivity, enhanced dynamic range by 235.3%, and supported 2.1 N detection in the first, second, and third stages under normal force applications, respectively. With the CTS characteristics resulting from varying stopper locations, a relationship between the capacitive response and applied force is established to support typical P4C CTSs in arbitrary dimensions. In addition, systematic tolerance is studied to explain detection accuracy for the first time. The proposed CTS provides operation flexibility in normal force detections, and unaltered supportiveness on shear force and its angle detections, compared with typical CTSs.

In this paper, five sensing mechanisms including capacitive, piezoresistive, inductive, piezoelectric, and optical are reviewed in details with representative literature. Basic functions of flexible force sensors (such as normal force, shear force, shear angle, torsional angle, and rotational angle) with additional physical quantities (such as environmental temperature, gesture, vibration, and bending) are discussed and categorized clearly in this article. In addition, materials used in the flexible force sensors as electrodes, substrates, and deformable dielectrics are also highlighted and summarized. Furthermore, structural design, specifications of sensitivity, spatial resolution, dynamic window and cyclic life are unambiguously outlined. Meanwhile, key design considerations are listed and evaluated in this review to indicate major contributions to the characteristics of the flexible force sensors. Notable challenges, potential business, and examples of commercial players in the market are also listed. This article offers a stepping stone for those who plan to enter this fields by revisiting the outcomes from the past decades.

This paper proposes a novel friction-assisted capacitance tactile sensing mechanism to simultaneously detect pulling, normal, and shear forces for 3D display image control applications with integrated transparency and flexibility. The sensing mechanism supports fingertip sensing ranges with ergonomic considerations, which is an improvement on previous studies. The mechanism produces demonstration sensitivities of 0.38, 0.28, 0.24, and 0.33 pF/N and sensing ranges 0-1 N, 0-1.6 N, 0-1.4 N, and 0-2.0 N for pulling (θ = 90°), friction-assisted pulling (θ = 30°), normal, and shear forces, respectively. In this paper, we proposed friction-assisted pulling force under θ = 30°, which fit a human fingertip and allowed it to control 3D virtual image. On average, the demonstration tactile sensor transparency is over 80% in the visible region. This study conducts and examines the theoretical design, simulation, fabrication, and measurement of the mechanism.

Here, in comparison to indirect and nonvisualizable detections that usually convert strains from electrical signals, the strain sensors that show visible signals with operation simplicity and intuitive perception for practical applications in flexible, printed, and hybrid microelectronics are summarized. The sensors are categorized into four mechanisms of optical phenomena (diffraction, reflection, interference, and photonic crystal), fluorescent mapping (mechanochromism and mechanoluminescence), Moiré effect, and thermal imaging. In addition to their operating principles, three characteristics of sensitivity, dynamic window, and spatial resolution are examined. Furthermore, three types of strains (uniaxial, biaxial, and multiaxial) with subcategories in strain mode (tensile and compressive) and uniformity (isotropic and anisotropic) that are supported by corresponding sensors are also summarized here. With three potential applications and markets of healthcare and biomedical engineering, human motion detection (sports science), and roll‐to‐roll manufacturing listed at the end, a steppingstone is offered here for those who works or intends to work in this field by revisiting the outcomes from key literature published in recent years.