# A Force Measurement Method Based on Flexible PDMS Grating
## 1 Introduction  
With the development of industrial technology, various sensors have been invented to tackle various measurement problems. The majority of the traditional sensors are based on metal and semiconductor materials, of which tensile and sensitivity are limited and not suitable for detecting human activities. Compared with the traditional sensors, the flexible sensors overcome the shortcoming of brittleness and possess advantages in biocompatibility, stretchability, wearability and continuous detection [1]. Based on these unique characters, flexible sensors have been well adopted in the areas of electronic skin, robots, the bionic implants, man-machine interaction systems, as well as other artificial intelligence equipment [2, 3]. As the basis of flexible sensors, flexible materials have also attracted attention. Generally speaking, flexible materials include nanoparticles, nanowires, carbon nanotubes, graphene, organic materials, and so on [4]. At present, the most widely used flexible materials are polymers, which belong to the category of organic materials. The hardness of polymers is between the solid materials (e.g., metal, ceramics) and the liquid materials (e.g., hydragyrum), resulting in a good comparability to human skin [5]. The most common types of polymer include PDMS (polydimethylsiloxane), PEN (polyethylene glycol terephthalate), PI (polyimide), PEI (polyetherimide), and PET (polyester). PI has high tensile strength and can be continuously used under high temperature. The elongation of PET can reach up to 150%, and it is therefore suitable for particular applications such as artificial organs [6]. Compared with the above materials, PDMS has better flexibility and it is usually used as a polymer substrate. Combined with the MOEMS (micro optical electronic mechanical system)technique, PDMS has been used in various flexible electronic products, including nodes in the internet of things, medical equipment, solar cells, smart watches, and other devices [7]. One of the most important applications of PDMS is the flexible force sensor. In recent years, a variety of works on electronic sensors have been published, where the force signal is transferred to an electronic signal (e.g., capacitance, resistance, voltage). [PERSON] et al. [8] designed a flexible three-dimensional tactile sensor based on PDMS to simulate natural touch. The author measured the value of force by collecting the change of capacitance's value. [PERSON] et al. [9] proposed a soft force sensor using micro-fluidic channels filled with conductive liquid. The normal and in-plane forces can be detected by measuring the electrical resistance changes of the channels when compressed. [PERSON] et al. [10] presented a flexible piezoelectric sensor to measure the shear and normal forces by adapting polyvinylidene fluoride (PVDF) and three-dimensionally structured PDMS. Meanwhile, optical sensors are also developed for force measurement in specific cases. [PERSON] et al. [11] presented a PDMS sensor for measuring fingertip force based on optical waveguide. When the sensor is deformed under applied force, the transmission modes in the PDMS waveguide change, so that the force can be derived by measuring the light intensity loss of the waveguide. In addition, [PERSON] et al. [12] developed a strain measurement method based on PDMS grating. When the sample is stretched, the strain can be calculated by measuring the position shift of light spot due to the grating diffraction. However, the shape of the grating pattern made by oxygen treatment is not easy to precisely control. In this study, a force measurement method based on PDMS grating is presented. The PDMS grating is fabricated using the soft casting method, which is precise and easy to follow. On the basis of previous studies [12], this paper not only introduced the fabrication and force sensing principle of the PDMS grating, but also investigates the property control of the force sensor by adjusting the curing temperature and the mixture ratio of the PDMS.