Nature has evolved over the 3.8 billion years, since life is estimated to have appeared on the Earth. Evolution has produced objects with the high performance of biomaterials and surfaces formed from the complex interactions between surface morphology and physicochemical properties, and the emerging field of biomimicry allows the development of nanomaterials and nanodevices by mimicking biology or nature (Bhushan, 2009). Learning from nature has inspired scientists to design novel materials with unique functionalities, such as artificial stimulus-response systems inspired by the ability of chameleons and zebrafish to change their appearance (Isapour and Lattuada, 2018), based on beetle-inspired gradient slant structures (GSS) to endow capacitive pressure sensors with extensive linearity range and excellent sensor-to-sensor uniformity (Wu et al., 2024), stable and economical catalysts inspired by the active sites of enzymes in bioenergy processes (Artero, 2017), and optoelectronic devices with ultra-high optical conductivity inspired by colorful seashells (Sun et al., 2019). Experiments have proved that bio-inspiration is very effective for material innovation, through rational material selection and structural design to achieve better performance by imitating the structure and function of biological entities (Zhang et al., 2016; Wegst et al., 2015).

Recently, shells with a solid surface, such as conch, pearl oyster, and snail, have attracted increasing interest due to their unique structure and remarkable properties (Mao et al., 2016). For example, the pearly structure of shellfish shells, which consists of thin aragonite slabs alternating with biopolymers, has inspired the design of tough materials from fragile, raw materials, such as in the case of mother-of-pearl composites, which are much stronger and tougher than single materials (Jiang and Park, 2014). At the microscale, the ripples of the tablet will gradually lock, harden, and undergo nonlinear deformation dif usion around cracks and defects, providing the pearl layer with extremely high toughness. Unfortunately, the preparation of pearl-like nanomaterials usually requires high-temperature or high-pressure mineralization processes, which causes difficulties in the design and production of such materials. Although molecular dynamics modeling and experimental measurements have confirmed the excellent mechanical properties of shell-like structures, they are still far from being available for use in practical applications.

Recent developments in electronics have focused on flexible devices, which have been widely applied in various fields, including wireless health monitoring, electronic skin, flexible sensor networks, artificial muscles, flexible human–computer interfaces, and engineered tissue constructs (Huang et al., 2014). A number of flexible pressure sensors have been reported recently, including microstructured layered rubber dielectric capacitors (Mannsfeld et al., 2013), organic field-effect transistors (Takei et al., 2010; Schwartz et al., 2013), organic microstructure piezoresistors (Pan et al., 2014), carbon nanotube-based resistive strain sensors (Michelis et al., 2015), reduced–graphene oxide flakes (Park et al., 2016; Chun et al., 2015), and nanowire arrays (Ha et al., 2015), which can divided into three categories, capacitance, piezoresistivity, and piezoelectricity. Embedded resistive elements demonstrate excellent monitoring performance and failure warning capabilities, but the inclusion of foreign sensing elements may degrade the mechanical properties of the composite and potentially increase the risk of damage and failure (Chen et al., 2023a). Traditional inorganic piezoelectric ceramics are of poor toughness, high stiffness and low durability, and therefore have limited applications in conformal wearable electronics. In this context, piezoelectric nanocomposites are introduced in the piezoelectric sensors to meet the requirements of lightweight, flexibility, processability and biocompatibility. However, agglomeration of high concentration of nanofillers and high viscosity of the resulting mixtures lead to sacrificed mechanical strengths and poor capability of stress transfer from low stiffness polymer matrix to piezoelectric nanofillers. This in turn, produces much lower piezoelectric response than pure inorganic piezoelectric materials. Furthermore, high stiffness nanocomposites caused by high loadings of inorganic nanofillers suppress their processability and restrict their applications in lightweight, portable, and conformal sensors (Zhang et al., 2020). Compared with piezoresistive and piezoelectric sensors, capacitive pressure sensors have the advantages in terms of simple structure, excellent stability, high sensitivity, and low power consumption (Lee et al., 2015). In addition, it is more valuable to develop capacitive sensors than resistive sensors considering that they are more compatible with wireless detection systems, and thus the detection based on change in capacitance has been widely applied in the field of pressure sensing (Yang et al., 2019; Qin et al., 2021). Existing flexible capacitive sensors and devices have difficulties in achieving large strain range and high sensitivity simutaneously (Nesser and Lubineau, 2021; Gao et al., 2022). The effective relative dielectric constant of the sensor is critical to the sensitivity, as it varies with the applied stress. Therefore, it is a viable way to develop new dielectric materials with suitable structures to improve the sensitivity (Li et al., 2020).

Various functional materials have been used in strain and motion sensors, such as [P(VDF-TrFE)] sponge (Parida et al., 2017), carbon nanofibers (Pang et al., 2012) and MXene-coated glass fiber (Chen et al., 2023b). As a new type of carbon material with a flexible 2D structure, graphene has excellent stability in lattice structure and mechanical flexibility, which facilitates the application of graphene-based materials in various flexible sensors (Zhu et al., 2017). Graphene oxide (GO) foam, as a novel building block for future wearable electronic devices, exhibits both excellent elastic properties and high relative dielectric permittivity. It is reported that GO-based sensors can detect a subtle pressure of ∼0.24 Pa with a fast response time of 100 m and high sensitivity of 0.8 kPa⁻¹ (Wan et al., 2017). However, the detection range and sensitivity of GO pressure sensors in strain environment greatly restrict their applications in flexible sensing (Xu et al., 2018; Cao et al., 2021), and how to integrate the advantages of high sensitivity, wide detection range, excellent durability, and lower limit of detection is still a challenge in developing wearable sensors (Troullier and Martins, 1991; Kresse and Hafner, 1993).

Inspired by nature, two-dimensional heterostructures with shellfish-like structures were prepared by GO and MoS₂ dispersions with the expectation of improving the sensitivity of flexible pressure sensors based on the excellent mechanical properties of the bionic structures. Unlike the traditional high-temperature or high-pressure calcination methods, the vacuum suction filtration method can easily obtain different material combination structures at room temperature, so 2D GO and MoS₂ dispersions can be poured into vacuum suction filters sequentially to obtain 2D materials with GO/MoS₂/GO (G/M/G) heterogeneous structures. Based on the strategy of strain engineering, sensing devices on flexible PDMS plate were constructed by dielectric layering of G/M/G and GO filter films and silver electrodes that were connected with an LCR digital bridge and a push–pull gauge to measure changes in capacitance under different external loads. The superiority of the bionic structures was verified by comparing the pressure capacitance response sensitivity of G/M/G and GO sensors, and the sensitivity variation mechanisms under different strain were studied based on band theory, parallel plate capacitor model, and crack growth theory.

After a vertical downward external load of the push–pull force meter was applied to the GO/PDMS and G/M/G/PDMS sensors, the capacitance pressure response curves were easily recorded by the LCR digital bridge (Figure 3C). The blue square and red circle denote relative capacitance change △C/C₀ under different external loads, and the slope of the curves, calculated by Δ C / C 0 / P and labeled with dashed lines, represents the sensitivity of the GO/PDMS sensor at 0.74 and 0.13 kPa⁻¹ and G/M/G/PDMS sensor at 1.60 and 0.28 kPa⁻¹, respectively. The sensitivity of the G/M/G/PDMS sensor at the entire pressure range is obviously higher than that of the GO/PDMS sensor, indicating enhanced sensitivity as a result of the shellfish heterostructure. The change in relative capacitance increases with increasing pressure, and sensitivity is higher in the low pressure range of 0–2 kPa than the high pressure range of 2–7 kPa, which is in agreement with the reported results. To understand the mechanism of enhanced sensitivity caused by shellfish heterostructure, supercells of GO and GO/MoS₂/GO were constructed for DFT modeling, and the initial energy band structures under zero strain were calculated, as shown in Figures 3E, F.

The energy band gap of the former (2.05 eV) is obviously larger than that of the latter (1.66 eV). Because the low band gap leads to high conductivity and small capacitance (Du et al., 2019), the sensitivity of the G/M/G structure was enhanced compared with the GO sensor. This result is in good agreement with a previous report (Lin et al., 2019).

The test device for two-point bending flexible pressure sensing is shown in Figure 4A, and the strain loading device is based on the formula ε = dsin θ / 2 a × 100 % , where d, θ and a refer to the thickness, angle of bending and end-to-end length of the sensor, respectively (Chen et al., 2021). Tensile strains in the range of 1.4%–3.5% are achieved by adjusting the intersection angles of the PDMS plate and horizontal plane in the range of 25°–65°.

The pressure capacitance response curves of GO/PDMS and G/M/G/PDMS sensors are given in Figures 4B, C, which show reduced sensitivity from 0.2 to 0.14 kPa⁻¹ and 0.63 to 0.31 kPa⁻¹, respectively. It is clear that the sensitivity of not only GO sensors but also GMG sensors decreases with increasing tensile strain less than 3.5%, which can be well explained by the parallel plate capacitance model (Gong et al., 2017). Notably, the G/M/G heterostructured sensors resisted strains better than the conventional monolayered structures, with a significant increase in sensitivity of 44%–112% over the same strain range.

As shown in Figure 5A, strain greater than 5% was achieved through the cross-feed of the fixture table, in which the middle part of GO and G/M/G sensors on PDMS plates was narrowed. The strain received by the sensor at this point can be calculated according to the traditional parallel plate stretching formula, i.e., ε = Δ L / L × 100 % . The pressure capacitance response curves under larger tensile strain in the range of 5%–50% are given in Figures 5B,C for the GO/PDMS and G/M/G/PDMS sensors. By comparison, the sensitivity of GO/PDMS and G/M/G/PDMS sensors was 0.34 and 1.06 kPa⁻¹ at 5% strain, and they dropped to 0.19 kPa⁻¹ at 40% strain and 0.28 kPa⁻¹ at 50% strain. The sensitivity of G/M/G/PDMS sensors at different tensile strain levels is relatively well known in the field of flexible sensors based on PDMS (Wan et al., 2017). It can be concluded that the sensitivity of the G/M/G/PDMS sensor was 75%–102% higher than that of the GO sensor at strain greater than 5%, and the work scope of strain sensing of the G/M/G/PDMS sensor increased to 50%, far more than the 40% for the single-layer GO/PDMS sensor.

To understand the mechanism of enhanced sensitivity and enlarged strain sensing scope, surface profile variation, crack initiation, and propagation were recorded by SEM while continuously increasing strain was applied to the horizontal planes of two sensors, and the typical surface images at 0%, 20%, and 40% tensile strain along the horizontal direction are given in Figures 5D–F for the GO/PDMS sensor and Figures 5G–I for the G/M/G/PDMS sensor. Without strain applied, the two surfaces of the GO/PDMS and G/M/G/PDMS sensors are very dense without any holes (Figures 5D, G). With 20% tensile strain, continuous fractures emerged in the planar regions, while cracks propagated in the textured regions (Figures 5E, H). The number of cracks in the G/M/G/PDMS sensor was obviously smaller than in GO/PDMS sensor. At the same time, the crack initiation and propagation of both sensors was about 90° relative to the horizontal direction, indicating that the cracks were caused by tensile stress. When the tensile strain was increased to 40%, the surface cracks of the GO/PDMS sensor (Figure 5F) became slightly longer compared to the G/M/G/PDMS sensor (Figure 4I), and the continuous propagation along the 90° direction indicates a uniform tensile fracture along across the section. Slower crack propagation can sustain higher uniaxial strain, because the crack propagation brings about the conductive path of the nano-layer to be cut off (Yang et al., 2022). Therefore, the work scope of strain sensing increased to 50% for the G/M/G/PDMS sensor, which is far more than the 40% for the single-layer GO/PDMS sensor, as shown in Figures 5B, C.

The flexing durability of the G/M/G/PDMS sensor was also tested by bending the sensor to an angle of ∼30° between the horizontal and bending directions (Figure 6A) for a thousand cycles, with one end kept fixed with bending in relation to the angle of the other end (Figure 5B). The capacitance change of 1,500 cycles for loading–unloading is given in Figure 5C, and the insets provide enlarged views of the capacitance variations in the ranges of 50–56 s and 2,750–2,756 s. Obvious minimal fluctuations are observed in both the compression and bending results, and the performance loss of the device is less than 1% after 1,500 loading–unloading cycles. This result indicates that there was no fatigue and deterioration of the G/M/G/PDMS sensor with a biological shell structure during 1,500 cycles, which confirms the durability of the biomimetic sensor based on PDMS.

The pressure-sensing properties of the G/M/G/PDMS sensor with shellfish heterostructure were measured by using the LCR meter at an operating frequency 1 kHz (Figures 3A, B), including response time, transient response to the placement and removal of ultra-small weights, and performance of gesture monitoring (Figures 6D–F). There was an instant response to external loading and unloading (Figure 6D), and the response and relaxation time of the G/M/G/PDMS sensor was approximately 100 m, reaching the resolution limit of most testing instruments. As shown in Figure 6E, the sensor could detect the placement and removal of several ultra-small weights. For instance, leaves with different weights (2.5, 4.15, and 6 mg, corresponding to pressure of 0.51, 0.83, and 1.19 Pa, respectively) were placed on and removed from a thin square glass plate covering the entire sensing area. The pressure detection limit of our sensors is 0.51 Pa, which is close to the lowest pressure of 0.24 Pa detected by capacitive pressure sensors as far as we know (Wan et al., 2017). The sensor was placed on the wrist and anchored with tape to monitor gesture performance such as thumb raising, “yeah”, and fist clenching using a pressure sensor, as shown in Figure 6F. After processing the signals, it was found that the sensor attached to the wrist successfully detected various body movements, therefore the G/M/G/PDMS sensor can be used effectively for sensing gesture motion.

G/M/G filtered thin films with shell-like heterostructure were successfully prepared, and a two-point strain sensing test device based on strain engineering was designed to measure the capacitance–pressure response sensitivity of pressure capacitance sensors under different strain levels. Due to the excellent stiffness and Young’s modulus of the bionic shell structure, the sensitivity of G/M/G/PDMS sensors under different levels of strain was much higher compared to the conventional GO sensors, and realized accurate measurement of pressure-sensitive properties with an extended strain measurement range. The changes in sensitivity of G/M/G/PDMS and GO/PDMS flexible pressure sensors at zero strain and tensile strain less than and greater than 5% can be reasonably explained by band theory in DFT simulation, parallel plate capacitor model, and crack growth theory. No fatigue or deterioration were detected in the biological shell structure of the G/M/G/PDMS sensor, and the performance loss of gesture monitoring was less than 1% after 1,500 loading–unloading cycles. The pressure sensor based on G/M/G/PDMS proposed in this article has excellent sensitivity, reliable service life, and a large strain working range; It has broad application potential in fields such as ultra-low pressure monitoring, biological research, and wearable human health monitoring. For example, wrist pulse monitoring and finger joint motion monitoring in smart wearable devices; Alternatively, sensors can be used to monitor finger tapping for early identification of Parkinson’s disease.