**1. Introduction**

Flexible wearable pressure sensors have received extensive attention in recent years because of their benefits such as integratability, their lightweight nature, and their portability [1–7]. Compared to the capacitive, piezoelectric, and triboelectric sensors, piezoresistive sensors were widely applied in health management, humanoid robots, human machinery, and artificial intelligence due to their simple structure and easily collectable signal [8–14]. However, the low sensitivity still restricts the practical applications of the piezoresistive sensors. Recently, many structures were designed and constructed to improve the device performance including sensitivity and the working pressure range. Among them, fiber assemblies were considered as ideal substrates that can help improve the sensitivities due to their remarkable deformation ability [15–21]. When the external pressure was loaded, the porous structures constructed by the stacking fibers present larger deformation when compared to solid materials, which resulted in greater growth of the contacting areas of the conductive components. The increasing interconnection of the conductive components formed more effective conductive networks, which improves the sensitivity of the piezoresistive sensors. In addition, research indicates that a higher porosity of the porous substrates will further strengthen the sensitivity of the piezoresistive sensors [13,22].

Nanofiber aerogels, which were obtained by removing the solvent component from the nanofiber suspension in a supercritical state, possess the highest porosity (>80%) among the fiber assemblies. The randomly distributed POE nanofibers with a high length-diameter ratio would lead to hierarchical self-entanglement and would help form a three-dimensional nanofiber-based network [23,24]. Moreover, the ultra-high specific surface area (>500 m<sup>2</sup> /g) of the nanofiber aerogel provides the structural basis for the interconnection of conductive components under compression. Therefore, a highly sensitive piezoresistive sensor can be expected by using the nanofiber aerogels as the flexible substrate. However, traditional aerogels always show narrow weak compression strength and low structural stability under revised external pressure [25–27]. Their internal structure will be easily destroyed under excessive external pressure. As a result, the working pressure range and the operational stability of the nanofiber aerogels-based piezoresistive sensor are generally difficult to meet the requirements of practical use. Literature [28] indicates that the aero carbon materials can enhance the mechanical property by forming the hierarchical three-dimensional structure. Therefore, designing and constructing a three-dimensional carbon material/nanofiber composite aerogel might be an effective solution to achieve ultra-high sensitivity, a wide working pressure range, and excellent cycle stability simultaneously for flexible piezo-resistive sensors.

In the present research, the three-dimensional RGO/POE nanofiber composite aerogels were prepared by chemically reducing the GO/POE nanofiber composite aerogels, which were obtained by freeze drying the mixture of the GO aqueous solution and the POE nanofiber suspension. The RGO/POE nanofiber proportions were adjusted to improve the sensing performance of the composite aerogels. It was found that the volumetric shrinkage of thermoplastic POE nanofibers during the reduction process enhances the compression strength of the composite aerogels while decreasing the sensitivity slightly. Therefore, the sensitivity and working pressure range of the composite aerogel were balanced by adjusting the additional amount of the POE nanofibers. The results indicate that the composite aerogel with POE nanofiber/RGO proportion of 3:3 was the optimal sample, which exhibits high sensitivity (ca. 223 kPa−<sup>1</sup> ), wide working pressure range (0–17.7 kPa), and strong stability either compressed with different frequencies or reversibly compressed and released for 5000 times.

#### **2. Materials and Methods**

Polyolefin elastomer (POE) and cellulose acetate butyrate (CAB. Butyrate content 35–39%) were purchased from Sigma-Aldrich (Saint Louis, MO, USA), Dow Chemical Company (Midland, MI, USA) and Acros Chemical Co. Ltd., Geel, Belgium, respectively. Tertiary butanol, acetone, concentrated sulfuric acid, potassium permanganate, and hydrochloric acid were from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The dispersing agent was supplied by Lubrizol (Lake County, OH, USA). Deionized water is self-made in the laboratory. All the chemicals are used without further purification. The micro morphologies were observed by JSM-6510LV (JEOL, Tokyo, Japan). The chemical structures were measured via FTIR mechine of Tensor 27 (Bruck, Karlsruhe, Germany) The resistances of the aerogels were measured via the 15b+ multimeter (Fluke, Washington, USA). The I-t characteristics of the pressure sensors were collected through ST600L motorized dynamic resistance station (Shente, Shanghai, China)

The POE nanofibers were prepared by extraction removal of the CAB from the POE/CAB composite fiber obtained sea island method. Previous studies reported the detailed method [7,29–32]. Then, the POE nanofibers were dispersed in tertiary butanol-water and the dispersing agent under high speed (10,000 r/m) shearing. The dispersing agent marked with Lubrizol 27,000 was used to help the uniform dispersion process of the POE nanofiber with a 10% mass proportion. The suspension was purified via a filter with a diameter of 150 µm to remove the aggregations. The photographs of the obtained POE nanofiber were presented in Figure S1, which indicates the good uniformity.

The GO aqueous solution was prepared using a modified Hummer's method. The concentration using GO aqueous solution was 5 mg/mL. As shown in Figure 1, the prepared POE nanofiber suspension and the GO aqueous solution were directly mixed together by continuously stirring, according to a GO/POE nanofiber proportion of 6:0, 5:1, 4:2, 3:3, 2:4, and 0:6, respectively. After 5 min of ultrasonic treatment, the mixtures were transferred to a low temperature freezer (−38 ◦C) for 8 h. The GO/POE nanofiber aerogels were obtained after the mixtures were freeze dried for more than 24 h.

**Figure 1.** The illustration schematic of the preparation of the RGO/POE nanofiber composite aerogels.

A total of 10 mL of hydrazine hydrate aqueous solution was added to the bottom of the beaker. The obtained GO/POE nanofiber aerogels were placed on a suspension bracket hanging in the baker. Then the beaker was placed in an oven at 90 ◦C for 100 min after the beaker was sealed. The RGO/POE nanofiber aerogels were obtained after they were placed in a fume hood for more than 6 h.
