Flexible strain sensors can convert mechanical deformation into electrical signals and play a central role in emerging applications such as medical monitoring, electronic skin, and soft robotics. In recent years, breakthroughs in sensing layer engineering have led to the development of stretchable sensors capable of withstanding ultra-large tensile strains (>400%) and compressive sensors functional under extreme compression (>80%). However, these advancements have largely evolved along separate trajectories, with tensile and compressive sensing functions typically optimized independently rather than integrated into a single sensing layer. Fundamentally, tensile and compressive deformations regulate the electrical transport process through entirely distinct and often competing mechanisms. Under tensile strain, the separation of conductive fillers disrupts percolation pathways, thereby weakening electrical connectivity. In contrast, compressive deformation in porous or biomimetic structures increases contact density and area, thereby strengthening the conductive network and amplifying resistance changes. This mechanistic asymmetry makes electrical decoupling of tensile and compressive signals highly challenging, often resulting in nonlinear, asymmetric, or ambiguous sensing responses. Consequently, constructing a highly sensitive sensing layer capable of simultaneously and accurately discerning the direction and magnitude of coaxial bidirectional strain (i.e., tensile and compressive deformations along the same axis) remains an unresolved challenge.
Recent research has attempted to integrate tension and compression sensing functions into flexible devices. For example, Capasso and his team doped zero dimensional (0D) carbon nano onions (CNOs) and one-dimensional (1D) carbon nanotubes (CNTs) into SEBS (styrene ethylene butene styrene) elastomer matrix to prepare conductive ink, and successfully fabricated a device with both tensile (120%) and compressive (80%) sensing capabilities by impregnating polyurethane (PU) sponge. However, the system has almost no linear working range. When the tensile strain exceeds about 8%, the fracture of the conductive network can cause severe signal fluctuations, thereby seriously damaging the stability and reliability of the device. Gao and his team used 3D printing technology to assemble graphene and carbon nanotubes (CNTs) into a tensile compression carbon aerogel with a "Paper Cuttings" (Kirigami) structure, realizing the sensing function in a wide strain range of − 14% to 100%. However, this Paper Cuttings geometry only slightly perturbs the conductive path when it is stretched, resulting in a low tensile sensitivity even under large strain (GF=0.1). These studies highlight the urgent need for a new type of nanomaterial that not only needs to provide a stable and reversible high-sensitivity response window, but also supports coaxial bidirectional strain detection function.
Natural cotton fibers are composed of intertwined cellulose filaments, with a hollow structure inside; This structure endows cotton fibers with a high aspect ratio and good elasticity, making them an ideal three-dimensional scaffold material for constructing multi-level sensing architectures. However, there are still many challenges in introducing secondary nanostructures in a controllable manner on carbonized cotton substrates. Traditional transition metal catalysts (such as Fe/Co/Ni) are often prone to sintering or carbon coating during the growth process. This not only leads to catalyst deactivation, but also causes unevenness in the hierarchical structure, which hinders precise control of contact conductivity and the realization of linear electromechanical response. Therefore, for multimodal sensing layers, it is particularly necessary to develop a new preparation method that can achieve controllable growth and multi-level structural design.
Source: Sensor Expert Network