## 1 Introduction  
Strain sensors for the detection of applied stress have attracted considerable attention for wearable electronics in the fields of sports, personal health monitoring, soft robots, and prosthetic devices [1; 2; 3; 4; 5]. The key factors in determining the performance of a strain sensor are high sensitivity, stability, and low manufacturing cost with simple processability [6]. Although conventional strain sensors with metal foils have been commercialized, they still have the limitations of low sensitivity, high material costs, and complexity of the manufacturing process [7]. In recent years, considerable efforts have been made to develop strain sensors with a high performance and low cost using functional materials. Carbon-based materials, such as graphene and carbon nanotubes, are widely used in strain sensors because of their excellent electrical conductivity and mechanical reliability. [PERSON] et al. reported a highly sensitive humidity sensor consisting of silver interdigital electrodes and a graphene/methyl-red composite layer, which is fabricated on a low-cost transparent polyethylene terephthalate (PET) substrate using a commercialized Dimatix Materials printer [8]. [PERSON] et al. also investigated a humidity sensor film fabricated by loading large amounts of multi-walled carbon nanotubes (MWCNTs) in a poly(acrylic acid) polymer matrix containing poly(4-styrenesulfonic acid) to enhance MWCNT dispersion [9]. However, the relatively high manufacturing costs of graphene and carbon nanotubes have limited the mass production of such sensors. Alternatively, carbon black (CB) has been employed for manufacturing strain sensors, owing to its high conductivity and low price [10]. However, there are only limited reports on the use of CB for manufacturing strain sensors. The most widely used method to prepare wearable strain sensors with a low cost is to utilize resistive sensors with simple structures. Resistive sensors can be fabricated via printing technology, which enables the mass production of wearable devices with a high flexibility. Among existing printing methods, the screen-printing process has been widely used in the field of electronics and has been successfully employed for various functional pastes and substrates [6; 11; 12; 13; 14; 15; 16]. Previous studies have proposed various screen-printing methods. The research trend in these studies has evolved toward the development of low-cost, diversified, simple, or low-waste manufacturing processes [6; 17; 18; 19; 20; 21]. Recently, [PERSON] et al. have reported a screen-printed strain sensor with CBs [22]. [PERSON] et al. have demonstrated a flexible strain sensor with a composite of CB and silver nanoparticles (AgNPs) as the sensing material and thermoplastic polyurethane as the matrix [23]. However, the sensitivities of the reported sensors were low and the effect of printability on their sensitivity was not investigated. In fact, the characteristics of screen-printed strain sensors are highly related to printability, which is affected by the rheological properties of the paste inks and the dimensions of the printed patterns. Several reports on using spherical and/or flake-shaped silver particles/nanowires and carbon-based materials, such as graphite, single-walled carbon nanotubes, and graphene, for resistive-type printed sensors have been published so far [6; 17; 21; 24; 25]. However, the relationship between the performance of a sensitive strain sensor and printability in the screen-printing process has not been investigated so far. In this article, we propose a simple and low-cost method to manufacture a flexible and high-performance sensitive strain sensor using CB/AgNP composites through a screen-printing process. During the fabrication process, the effects of the rheological property of the paste and the dimensions of the printed pattern are systematically studied to maximize the sensing properties. Consequently, the gauge factor (GF) of the sensor shows a high value of 444.6 with a linearity of R\({}^{2}\) = 0.99.