**1. Introduction**

Since the discoveries of carbon nanotubes (CNT) by Iijima [1] and graphene nanoplatelets (GNP) by Novoselov et al. [2], they have received a great attention as raw materials for the development of nanomaterials due to their excellent thermal, electrical and mechanical properties, low density and high specific surface area [3]. Nowadays, enormous efforts have been devoted to the use of CNT and graphene in various applications, such as energy storage [4,5], field effect transistors (FETs) [6], electrodes [7,8] and sensors [9,10]. Thin films or paper-like materials consisting of GNP or CNT have drawn extensive attention and they are being widely employed for supercapacitors [11], pressure sensors [12], monitoring cure behavior of polymer composite [13], flexible temperature sensors [14], and as reinforcing fillers in polymers [15–17]. These free-standing thin films are cohesively bound by van der Waals interactions among entangled CNTs and GNPs. The main idea behind the fabrication of thin film is to utilize the excellent properties of individual GNP and CNT in macroscopic form. This thin film is advantageous to facilitate easier handling of GNPs and CNTs and to improve the capability of using GNPs and CNTs in industry [18]. These thin films are suitable for both lightweight structural and functional applications.

Flexible strain sensors have been highly desirable in applications such as electronic skin, structural health monitoring, and robot sensors in recent years. CNT and GNP are applicable for piezoresistive strain sensors and have been of great interest among researchers. Lu et al. [19] employed a flexible GNP/epoxy strain sensor to monitor the deformation and damage in structural composites. Liu et al. [20] reported a highly reliable strain sensor based on graphene composite film with layered structure. Moriche et al. [21] studied the strain monitoring mechanism of GNPs incorporated into epoxy matrix. Sanli et al. [22] investigated the piezoresistive performance of strain-sensitive MWCNT/epoxy nanocomposites. Wang et al. [23] utilized a CNT composite film as a strain sensor to monitor biaxial strain under tensile tests. Wang et al. [24] developed a new processing technique of MWCNT strain sensors with tunable strain gauge factors. Natarajan et al. [25] examined the efficiency and effectiveness in terms of piezoresistive properties of natural rubber composites based on MWCNT, carbon black and their mixtures (hybrid). The change of relative resistance, was found to be as much as ∼1300 at around 120% elongation. Boland et al. [26] incorporated graphene into a lightly cross-linked polysilicone, resulting in a change of its electromechanical properties substantially. These nanocomposites were sensitive electromechanical sensors with gauge factors >500 that can measure pulse, blood pressure, and even the impact associated with the footsteps of a small spider. Li. et al. [27] fabricated flexible and electrical conductive carbon cotton/polydimethylsiloxane composites by vacuum-assisted infusion for highly sensitive pressure sensor. The flexible pressure sensor exhibited a maximum sensitivity of 6.04 *KPa*−<sup>1</sup> in a wide working pressure up to 700 kPa. Samad et al. [28] developed a graphene foam/polydimethylsiloxane flexible sensor to sense both compressive and bending strains in the form of change in electrical resistance. They found that resistances can be increased to 120% and 52% of its original value by applying a 30% compressive strain and bending a sample to a radius of 1 mm, respectively. Samad et al. [29] fabricated freestanding, mechanically stable, and highly electrically conductive graphene foam with two-step facile, adaptable and scalable techniques. They demonstrated the capability of graphene foam as strain/pressure sensor for both high and low strains and pressures with tunable densities.

The potential applications of CNTs and GNPs are limited because CNTs are easy to entangle and agglomerate due to the large aspect ratio and GNPs also tend to restack due to van der Waals and strong interactions. One of the most efficient ways to avoid the agglomeration is to incorporate CNTs with GNPs to produce a nanocomposite material or a hybrid. CNTs can bridge adjacent graphene layers and retard graphene interlayer stacking, resulting in an increased contact surface area between GNPs. Hybrid CNT/graphene films are typically bonded by π–π interaction, which can induce functionalization due to the difference in geometry between the GNP and the CNT. Apart from the non-covalent interaction, covalent bonds and hydrogen bonds have also been used to construct hybrid graphene and CNT nanomaterials, which are confirmed to be of ultrahigh strength, modulus, electrical conductivity and thermal performance [17]. Most of the existing literature, as mentioned above, studied the sensing capability of the carbon nanomaterials such as MWCNT and GNP individually. Relatively few studies have been reported on electrical and mechanical properties of hybrid films. This work seeks to explore the piezoresistive behavior and strain-sensing ability of the MWCNT/GNP hybrid film. The synergistic effect of MWCNT and GNP on the electrical conductivity is presented. These are the novelty and originality of present work. In this study, MWCNT/GNP hybrid films were fabricated by vacuum filtration of mixed dispersion with varied MWCNT-to-GNP weight ratios. A series of hybrid films with different amounts of MWCNTs and GNPs were prepared. The loading of GNPs varied from 0 wt.% to 50 wt.% and that for MWCNTs was 100 wt.% to 50 wt.%, respectively. The effect of GNP content on mechanical properties, electrical conductivity and strain-sensing performance of hybrid films are investigated. A controllable strain sensitivity of the hybrid film can be achieved by varying the GNP content. It is important in understanding the MWCNT/GNP hybrid films so as to further improve their properties for end applications.
