In this paper, five sensing mechanisms including capacitive, piezoresistive, inductive, piezoelectric, and optical are reviewed in details with representative literature. Basic functions of normal force, shear force, shear angle, torsional angle, rotational angle; and characteristics in sensitivity, dynamic window, and spatial resolution are discussed and categorized clearly in this article. In addition, materials used in the flexible force sensors for electrodes, substrates, and deformable dielectrics are highlighted and summarized. Furthermore, structural design, specifications of sensitivity, spatial resolution, dynamic range and cyclic life are also 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 commercial players in the market are listed at the end of this manuscript. This article offers a stepping stone for those who plan to enter the flexible force sensor fields by revisiting the outcomes from the past decades.

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.

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