Boosting Sensitivity of Cellulose Pressure Sensor via Hierarchically Porous Structure

Abstract

Pressure sensors are essential for a wide range of applications, including health monitoring, industrial diagnostics, etc. However, achieving both high sensitivity and mechanical ability to withstand high pressure in a single material remains a significant challenge. This study introduces a high-performance cellulose hydrogel inspired by the biomimetic layered porous structure of human skin. The hydrogel features a novel design composed of a soft layer with large macropores and a hard layer with small micropores, each of which contribute uniquely to its pressure-sensing capabilities. The macropores in the soft part facilitate significant deformation and charge accumulation, providing exceptional sensitivity to low pressures. In contrast, the microporous structure in the hard part enhances pressure range, ensuring support under high pressures and preventing structural failure. The performance of hydrogel is further optimized through ion introduction, which improves its conductivity, and as well the sensitivity. The sensor demonstrated a high sensitivity of 1622 kPa⁻¹, a detection range up to 160 kPa, excellent conductivity of 4.01 S m⁻¹, rapid response time of 33 ms, and a low detection limit of 1.6 Pa, outperforming most existing cellulose-based sensors. This innovative hierarchically porous architecture not only enhances the pressure-sensing performance but also offers a simple and effective approach for utilizing natural polymers in sensing technologies. The cellulose hydrogel demonstrates significant potential in both health monitoring and industrial applications, providing a sensitive, durable, and versatile solution for pressure sensing.

1. Introduction

In recent years, due to the progress in technology and the growing demand for human health, flexible wearable devices have become increasingly prominent, showing a wide range of applications, such as human–machine interfaces, health monitoring fields and so forth [1–3]. As a kind of flexible wearable device, the pressure sensor can be divided into piezoelectric [4, 5], capacitive [6–8], piezoresistive [9–12], triboelectric [13] and optical [14] types according to the transduction mechanism. Different conduction mechanisms have their own advantages and disadvantages, and in capacitive pressure sensors the more variables affecting capacitance make them more sensitive to pressure changes. Also, they exhibit good repeatability, temperature independence, low power consumption [15, 16], and simple device structure, which has led to broader research.

To further enhance the performance of capacitive pressure sensors, current research primarily focuses on optimizing the material or structure of dielectric layers and electrodes to improve the sensitivity and other important properties [17, 18]. For example, Ha et al. [17] devised a flexible hybrid-response pressure sensor composed of an electrically conductive porous nanocomposite (PNC) laminated with an ultrathin dielectric layer. Using a nickel foam template, the PNC was fabricated with carbon nanotubes doped Ecoflex to be 86% porous and electrically conductive, resulting in significantly enhanced sensitivity over wide pressure ranges, from 3.13 kPa⁻¹ within 0–1 kPa to 0.43 kPa⁻¹ within 30–50 kPa. Gao et al. [19] developed a Ti3C2Tx-derived iontronic pressure sensor (TIPS) by taking the advantages of the high intercalation pseudo-capacitance under high pressure and rationally designed structural configuration. TIPS achieved an ultrahigh sensitivity (over 45,000 kPa⁻¹) in a broad sensing range of over 1.4 MPa and low limit of detection of 20 Pa as well as stable long-term working durability for 10,000 cycles. Liu et al. [20] achieved superhigh maximum sensitivity of 9280 kPa⁻¹ by using polyurethane-ionic liquid foam with a high porosity (95.4%) and a low modulus (3.4 kPa). Wang et al. [21]