![]() This review article examines the evolution of multimodal sensors in detecting numerous input signals ( Figure 1) from discrimination (Figure 1a) and interference suppression (Figure 1b) to decoupling (Figure 1c–g). Although signal processing sometimes can be helpful to minimize the effect of interference, multimodal sensors with decoupled sensing mechanisms can also reduce the complexity of the signal process. As multimodal sensors are sensitive and respond to various input signals, it is vitally important to exploit them with decoupled sensing mechanisms for detecting the target signal without being affected by the cross-sensitivity. The development of multimode sensors is enabled by a variety of materials and structures, while signal recognition determines availability when numerous stimuli are delivered. On the other side, the force-induced structures of the active layer in resistive sensors such as interlocking, and helix cause changes in conductive pathways and then resistances under mechanical stimuli. For example, graphene oxide (GO) or reduced graphene oxide (rGO) are very sensitive to humidity, chemicals and temperature due to their abundant surface functional groups, including hydroxyl, carboxyl and epoxy groups. The multimodal sensing of the flexible sensors to multiple stimuli has been realized with active materials that respond to multiple physical stimuli or with delicate structures that show various deformations in sensing different mechanical stimuli. To further advance flexible sensors to mimic the human skin that possesses a wide spectrum of mechanical properties and multiple sensing capabilities, multimodal flexible and stretchable sensors are proposed to measure multiple external stimuli by electrical signals (e.g., capacitance, resistance, current, or voltage). Due to their sensing performance to different input signals, these devices find applications in human–machine interfaces, robotics, prostheses, and healthcare devices. Inspired by the functions of human skins, flexible electronic devices are designed with a monomodal function to detect force, temperature, humidity, among others. Numerous sensory receptors underneath the skin (e.g., temperature receptors, pain receptors, and four types of mechanoreceptors ) can help humans perceive the external environment. Finally, current challenges and potential opportunities are discussed in order to motivate future technological breakthroughs. The presence of the various decoupling methods also helps avoid the use of complicated signal processing steps and allows multimodal sensors with high accuracy for applications in bioelectronics, robotics, and human–machine interfaces. The recent insights into the materials' properties, structure effects, and sensing mechanisms in recognition of different input signals are highlighted. Next, this study discusses the methods for the suppression of the interference, signal correction, and various decoupling strategies based on different outputs to simultaneously detect multiple inputs. Early efforts explore different outputs to distinguish the corresponding input signals applied to the sensor in sequence. Hence, this review article introduces varying methods to decouple different input signals for realizing truly multimodal sensors. Therefore, the selection of the multifunctional materials and the design of the sensor structures play a significant role in multimodal sensors with decoupled sensing mechanisms. However, cross-sensitivity prevents accurate measurements of the target input signals when a multiple of them are simultaneously present. Highly sensitive and multimodal sensors have recently emerged for a wide range of applications, including epidermal electronics, robotics, health-monitoring devices and human–machine interfaces.
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