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Figure 2. Various measurement mechanisms and engineered designs. (A) Active vibration sensors for tissue stiffness evaluation. Vibration generated by the actuator propagates along the tissue to the sensor; (B) Example of an ultrathin microsystem with active elements consisting of PZT, bottom and top electrodes, and PI for distinguishment of abnormal tissue. Reproduced with permission^[28]. Copyright 2018, Springer Nature; (C) Working principle of ultrasonography for the detection of tissue signals; (D) Optical image of a 12 × 12 stretchable ultrasonic phased array mounted on the human neck and chest. Inset: enlarged image of four transducer (Tx) elements with a pitch (λ) of 0.8 mm. Reproduced with permission^[52]. Copyright 2021, Springer Nature; (E) Stethoscope-based mechanism for monitoring signals from tissue displacement; (F) Optical photograph (left) and microscopic image (right) of an ultrasensitive all-nanofiber mechanoacoustic sensor based on nanofibre electrodes and polyvinylidene fluoride nanofibres. Reproduced with permission^[54]. Copyright 2020, American Chemical Society; (G) Schematic showing the architecture of a strain sensor to measure haptically Young’s modulus; (H) Illustration of a FMS with a self-locking effect composed of an upper cap, a strain sensor, a pair of self-locking elements, and a self-locking frame. Reproduced with permission^[55]. Copyright 2021, John Wiley and Sons; (I) Schematic illustration of light absorption detected through a LED and a PD for photonic diagnostics; (J) Image and exploded-view illustration of thin and stretchable optoelectronic systems including IR LED, a photodetector, and an inductive coil configured to measure cardiac beating signals with clear revelation of the systolic peak and the dicrotic notch. Reproduced with permission^[31]. Copyright 2016, American Association for the Advancement of Science; (K) Techniques based on thermal transport for continuous and precious measurements of blood flow; (L) The geometry of an electronic device combined with thermal analysis techniques consisting of a central thermal actuator (3 mm diameter) with power (typically 25 or 3.5 mW·mm^-2) directionally downstream to vessels and 14 surrounding thermal sensors for measurement of thermal distribution. Reproduced with permission^[63]. Copyright 2015, American Association for the Advancement of Science. PZT: Piezoelectric material lead zirconate titanate; PI: polyimide; FMS: fingertip modulus sensor; PD: photodetector; IR LED: infrared light-emitting diode.