Highly Elastic and Conductive Lamellar Wood Sponge via Cell Wall Reconfiguration Toward Smart Multifunctional Applications

Abstract

Three-dimensional porous foams and aerogels with high compressibility and elasticity hold great promise for applications in pressure sensing, electromagnetic interference (EMI) shielding, and thermal insulation. However, their widespread application is often hindered by compromised structural stability and inadequate fatigue resistance under repeated compression. Herein, a sustainable “top-down” cell wall reconfiguration strategy is proposed to fabricate highly elastic, fatigue-resistant, and electrically conductive lamellar wood sponge from natural balsa wood. This strategy involves the conversion of the intrinsic cellular structure of wood into an arch-shaped lamellar architecture reinforced by chemical cross-linking, followed by coating the lamellar scaffold with conductive polypyrrole (PPy) via in situ polymerization. The resulting PPy-coated cross-linked wood sponge (CWS@PPy) demonstrates reversible compressibility, excellent fatigue resistance (∼3.5% plastic deformation after 10,000 cycles at 40% strain). The strain-induced conductivity changes in CWS@PPy enable tunable EMI shielding effectiveness under cyclic compression and also facilities high-sensitivity pressure sensing (0.72 kPa−1). Additionally, CWS@PPy exhibits a low through-plane thermal conductivity of 0.037 W m−1 K−1, which can be dynamically tuned for adaptive thermal management. The proposed mechanically robust and conductive wood sponge provides a versatile and sustainable platform for next-generation smart devices.

1. Introduction

Three-dimensional (3D) compressible and elastic porous materials (CEMs) show great potential for a wide range of applications, including thermal insulation [1–3], oil/water separation [4–6], pressure sensing [7], and electromagnetic interference (EMI) shielding [8, 9]. The versatility of CEMs can be attributed to their ability to undergo reversible microstructural change during compression and subsequent rebound. However, maintaining the structural integrity and mechanical resilience of these materials while minimizing energy dissipation during repeated compression remains a challenge, which significantly impacts the reliability of CEMs in practical applications. Therefore, rational design of the microstructure of CEMs is crucial for achieving high compressibility, superior elasticity, and excellent fatigue resistance [10]. As a typical structure enabling high compressive elasticity, the arch-shaped lamellar structure has attracted great interest in the structural design of high-performance CEMs [11]. From a mechanical perspective, the lamellar structure can accommodate large compression by bending and straightening the arch-shaped lamellas and revert to its original shape immediately upon the release of stress, while preserving the structural integrity [12, 13]. In fact, such arch-shaped spring-like structures are widely used in various vehicles for shock absorption and shaft support [14].

Inspired by the arch-shaped spring-like structures, a variety of CEMs with a similar lamellar architecture have been synthesized from various building blocks including carbon nanotubes (CNTs) [15], SiO2 nanofibers [16–18], graphene oxide (GO) [19–21], MXene nanosheets [22] using diverse bottom-up assembly strategies, such as self-assembly [23–25], directional freezing [26, 27], and 3D printing [28, 29]. Among them, directional freezing stands out as an effective approach for fabricating CEMs with aligned pore channels, in which the ordered ice crystals serve as templates for guiding the assembly of the building blocks into an anisotropic lamellar structure [30, 31]. However, owing to the weak interaction between the pure building blocks, the fabricated CEMs often suffer structural damage and display poor fatigue resistance when subjected to prolonged cyclic compression [11, 32]. To address this issue, polymers and amorphous carbon have been incorporated to reinforce the junctions in the scaffold, thereby enhancing the mechanical stability of the lamellar structure [12, 33]. For example, the arch-shaped chitosan/GO composite aerogel, fabricated using a bidirectional freezing method followed by thermal annealing, can withstand over 25,000 cyclic compressions at a strain of 50% [12]. Despite the versatility of the directional freezing technique to fabricate anisotropic lamellar CEMs, it remains a challenge to achieve a long-range lamellar structure with consistent alignments and pore size.