In order to meet the requirements of wearable devices, the sensor material needs to have the characteristics of lightness, thinness, softness and corrosion resistance. The main materials currently used in wearable devices include flexible materials, paper-based materials, nanomaterials and organic materials.

Flexible Material

Flexible materials, as commonly used materials for wearable devices, mainly include polydimethylsiloxane (PDMS), polyethylene terephthalate (PET), polyethylene naphthalate, polyurethane, polyimide and latex, etc. Their compatibility and flexibility. The mechanical and temperature stability is relatively good. Among the many flexible materials, PDMS and PET are the most commonly used.

The flexible microfluidic chip that continuously detects sweat for a long time is laminated with three layers of PET film. During detection, the chip on the skin surface absorbs the liquid on the body surface through a filter integrated in the inlet, and then enters the microchannel and the sensing cavity through capillary action for detection.

The ultra-light weight, high mobility, scalability, low cost and compatibility of flexible materials make it possible to integrate many other components. The flexibility of this material can not only alleviate the mechanical strain caused by bending, but also alleviate the mechanical strain caused by other disturbances such as compression, tension and torsion. The flexible and stretchable wearable or device can be firmly combined with the skin, reducing the stress generated at the interface as much as possible, and realizing the monitoring of human activities and personal health under the strong pressure generated during normal body movement and muscle movement .

Paper-based Material

Due to its own unique characteristics, paper base has the advantages of easy recycling and disposal, low cost, strong water absorption and good flexibility. The uniqueness of the paper-based wearable device makes it suitable for breathing monitoring. The wearable paper-based moisture sensor prepared by using the good water absorption of the paper-based paper-based moisture sensor can detect individual respiration by detecting changes in moisture caused by respiration. 

Compared with traditional equipment, its cost is very low. By combining paper with other materials, the strength of paper-based equipment can be improved. Based on microfluidic paper-based chips, cotton threads and filter paper are combined to design and prepare wearable or equipment for glucose detection. The development of wearable devices provides a new way of thinking.

Nanomaterials

Common nanomaterials used in wearable devices include carbon nanotubes. Metal nanowires, metal oxide nanowires, conductive polymer nanowires, etc. As shown in Figure 2(C), a pH sensor based on dielectrophoresis carboxyl functionalized single-walled carbon nanotubes was fabricated. The performance of the carbon nanotubes can be changed in different ways through the arrangement of carbon nanotubes. 

Many technologies have been developed to prepare them. Carbon nanotubes with different morphologies, including vertical carbon nanotubes, curved carbon nanotubes and suspended carbon nanotubes. For metal nanomaterials, there are mainly gold, silver, and copper nanowires. The combination of silver nanowires and PDMS has developed a pressure sensor that can monitor movements such as light touch, swallowing, bending, and twisting.

Organic Material

Organic sensors based on organic semiconductors or conductive materials have become one of the commonly used materials for wearable devices due to their flexibility, stretchability, low cost, and light weight. The unique advantage of organic semiconductors and conductive materials is that they The electrical, mechanical, chemical, and optical properties of the can be optimized through reasonable molecular design. The adjustable optical range of organic semiconductor photosensors can achieve absorption wavelengths from near-infrared to ultraviolet regions through molecular design. Therefore, these sensors are widely used to continuously monitor the physiological state of individuals in real time. 

As shown in Figure 2(D), based on a photoelectric volume sensor based on a high-sensitivity organic photoelectric crystal, an ultra-thin wearable device can continuously monitor changes in heart rate and accurately track changes in pulse pressure in various parts of the body.