Often, the activity of the heart can be represented by the electrocardiogram (ECG) signal, which provides general information about the cardiovascular system. Since the ECG is periodic, the heart rate can be obtained from the R wave-to-R wave (RR) interval of the ECG signal [8]. Traditional monitoring methods generally use gel-assisted Ag/AgCl, but this method is still relatively cumbersome to use [57]. The skin-contactable ECG sensor allows users to easily identify their cardiac condition and diagnose critical cardiac problems early, such as cardiomyopathy and arrhythmia, and high blood pressure [58,59]. The ECG electrodes are the key to monitoring ECG signals in flexible wearable devices. Traditional monitoring electrodes generally use gel-assisted Ag/AgCl electrodes, but there are still some troubles in using this method, such as causing skin problems [57,60]. With further research, flexible dry electrodes have gradually attracted extensive attention of researchers due to their soft and non-invasive characteristics. A flexible and wearable electrode was reported for ECG signal monitoring [61]. Through plasma sputtering technology, silver with excellent electrical conductivity was made on the substrate made of cowhide as a flexible material. In this work, the researchers performed ECG tests on six subjects and demonstrated that the monitored ECG signal quality was comparable to that of conventional Ag/AgCl electrodes. It is worth mentioning that this kind of cowhide material is easier to make clothes with integrated flexible sensors, and it is more easily accepted by people.

Plethysmography and ultrasound are commonly used methods for monitoring pulse and heart rate [62]. However, the bulky monitoring equipment and the accuracy of long-term monitoring prevent these two methods from becoming the best choice for wearable pulse sensors [63,64]. Recently, a flexible strain sensor for real-time pulse monitoring has been proposed, which can be easily attached to the skin [65]. The sensing element of this patch-type sensor adopted polyaniline with high sensitivity and excellent flexibility. Through the pressure changes caused by blood flow, the signal characteristics monitored by the pulse sensor were analyzed for clinical analysis. With more in-depth research, a flexible pressure sensor that can monitor three pulse positions simultaneously was designed [66]. This design combined traditional Chinese pulse theory with modern sensor technology to achieve innovation. Ionogel-based pressure sensor arrays were fabricated on polyethylene terephthalate (PET) flexible substrates to convert pressure changes brought about by arterial flow into resistance changes. The special feature of this research is that three-dimensional pulse mapping can be formed. The key information, including the strength and waveform of the pulse, is displayed on maps, perfectly simulating the sensation of a doctor touching the pulse on the skin. Another successful example of flexible sensors for monitoring pulse rate is based on poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) [67]. The device integrated analog amplification circuitry and detector elements on the flexible material that was capable of detecting weak pressures (10 kPa) and amplified electrical signals by a factor of 10. Therefore, the performance of the sensor is excellent for monitoring the pulse of the human body.

Correct exercise and fitness, medical rehabilitation training, and daily healthy life are inseparable from accurate and timely monitoring of human body movement signals. In daily life, detecting physical activity and exercise habits can provide useful information for fitness and maintaining correct posture. In the field of medicine, continuous monitoring and regular analysis of body movements will help doctors timely detect abnormalities in the patient’s body, thereby improving the efficiency of treatment. For example, abnormal posture and tremors in the hands are symptoms of some deadly diseases such as diabetes [68], Alzheimer’s [69], and Parkinson’s [70,71]. Wearable medical devices are essential in detecting sudden tremors and abnormal movements. Therefore, designing continuous monitoring sensors will help detect symptoms in time and treat them accurately and efficiently [72]. The normal movement of hands and limbs can ensure a high quality of life and efficient work, among which gestures are also vital and often serve as a range of signs to transmit information [73]. Given that the feet of the body bear most of the pressure of the body, especially for manual workers or athletes, monitoring the pressure changes on the soles of the feet can effectively prevent injuries [74,75,76].

To date, many efforts and achievements have been reported on flexible sensors for continuous monitoring of human motion. Wearable motion detection is mostly based on strain sensors, whereby the change in the base resistance is correlated with motion-related activities [77]. Strain sensors were developed using metal embedded into an elastomer and placed onto the hip, knee, and ankle joints to monitor their bending angles. A flexible strain sensor integrating sensing elements into textiles was proposed, which can effectively monitor the movement of the knee joint [78]. This study analyzed and guided sensor design based on the biomechanics of knee motion deformation. The sensing elements that monitor movement were encapsulated to avoid errors in measurement results. In addition, another special feature of this work was that they designed the sensor to be personalized, designed in different sizes and shapes to meet the comfort needs of different wearers. Long-term use of metals as electrodes may cause skin problems. A human motion detection device was developed using triboelectric nanogenerator (TENG) yarn, which has good skin compatibility and can be washed [79]. The device monitored body movement by attaching highly flexible and stretchable TENG fabric to different parts of the body, such as arms and knees. The TENG fabric was designed as a five-layer structure, of which the innermost layer was a coil spring. Since both ends of the coil spring are hook-shaped, they can be connected to each other to form an 11 × 11 array sensing fabric for multi-channel sensing. Surface-parallel TENG fabricated polytetrafluoroethylene yarns for large-scale energy harvesting. These works provide strong support for continuous monitoring of body movement, which should be widely used in the field of medical monitoring in the future. Recently, a magnetic tactile sensor was developed that distinguishes stretching and bending stimuli accordingly by opposite electrical signals [80]. The sensor used PDMS as the flexible substrate and mask-patterning and spin-coating technology to design a sandwich structure thin film sensor. The sandwich structure sensor attached to the surface of the skin was converted into the electrical signal output by feeling the pull force when the joint was bent. Since the sandwich structure sensor contained magnetic nanoparticles and the ability of self-sensing, it can be used as a non-contact magnetic sensor to effectively prevent the spread of bacteria in medical environments.

2.4. Respiratory Rate