Over the last decade, flexible pressure sensors for a variety of wearable applications, such as electronic skin [1], human health monitoring [2, 3], and human–machine interfaces [4, 5], have received significant attention. Many researchers have concentrated on the development of various types of flexible and wearable pressure sensors, including piezoresistive [6, 7], capacitive [8,9,10], piezoelectric [11, 12], and triboelectric types [13, 14]. In particular, capacitive-type pressure sensors show significant advantages given their high sensitivity, low power consumption, simple design, and low hysteresis [15, 16]. A parallel-plate capacitive sensor is composed of a dielectric layer sandwiched between two conductive electrode plates for the detection of capacitance changes. With these sensors, the capacitance parameters change when external pressure is applied to the dielectric layer, with the result ultimately reflected in the change of the capacitance of the sensor. Specifically, the relative permittivity \(\varepsilon_{r}\) and the distance between the electrodes d are the key factors affecting the performance capabilities of sensors such as those designed to measure pressure responsive sensitivity levels [17].
To ensure the fabrication of high-performance capacitive pressure sensors, recent studies have reported that microstructured or patterned polydimethylsiloxane (PDMS) dielectric layers represent the most effective means of improving the sensitivity of the capacitive-type sensors due to their excellent elasticity and biocompatibility [18, 19]. Pyramidal [20, 21], micro-pillar [22, 23], micro-wrinkle [24, 25], and microdome [26, 27] shapes created with elastic materials reportedly offer high sensitivity for use as electronic skin. However, the fabrication of a Si microstructured mold is complicated, with significant dependence on the equipment, multiple necessary steps, and high-cost manufacturing processes. Additionally, these sensors operate mainly at a low measurement level (< 10 kPa), which is insufficient for wearable systems in the medium–high pressure regime (10–100 kPa, suitable for object manipulation), as such systems must be capable of detecting changes [8].
For these reasons, the 3D monolithic porous PDMS dielectric layer has attracted much attention for use in capacitive-type pressure sensors [9]. Chhetry et al. reported a pressure sensor based on a dielectric hybrid sponge consisting of calcium copper titanate (CCTO), a material with very high dielectric permittivity, encased in polyurethane (PU) for detection in low-pressure regime (< 1.6 kPa) [28]. Pruvost et al. also suggested a PDMS foam decorated with carbon black particles for use in the fabrication of highly sensitive capacitive pressure sensors exceeding 35 kPa−1 for pressures of < 0.2 kPa [29]. These studies focused on improving performance capabilities via the use of conductive nanomaterials while maintaining the porosity of the sponge. However, aggregation and agglomeration cause low production reliability of composites of nanomaterials and elastomers, and the methods above cannot regulate or tune sensor sensitivity levels. The easiest and fastest way to tune a sensor without conductive nanomaterials is to adjust its porosity. Thus, many studies have focused on high sensitivity and wide sensing ranges when applying a porous elastomer to a capacitive pressure sensor. Kang et al. suggested a capacitive pressure sensor with a porous structure consisting of a polydimethylsiloxane (PDMS) thin film dielectic layer. The morphology of the porous structured dielectric layer could be controlled by changing the pore sizes [30]. Jung et al. showed that a PDMS and microsphere composite could be applied to a capacitive pressure sensor by maximizing the porosity of microspheres via the effect of high compressibility and an enhanced piezocapacitive effect [9]. Li et al. also reported a pressure sensor based on a dielectric layer of PDMS with uniformly distributed micropores which displayed high elasticity, a wide pressure-sensing range (> 200 kPa), and high sensitivity (0.023 kPa−1) [31].
In this report, we propose a facile approach by which to adjust the porosity of PDMS foam by controlling the viscosity of the PDMS immersion solution. Due to the synergy effect of the high elasticity of the PDMS elastomer and the highly porous 3D micro-hollow network structure, the as-prepared highly porous PDMS dielectric layer showed excellent mechanical resilience, extremely high compressibility, and stable cyclic performance. These flexible pressure sensors are also capable of not only detecting low pressure levels but also enabling real-time human motion sensing applications.
Preparation of the hollow PDMS foam and the pressure sensor
Figure 1 shows a schematic of the fabrication process of the PDMS foam for the capacitive pressure sensor. Commercial copper foam (supplied by ITASCO) with porosity of approximately 95% was used as template. First, a PDMS precursor (Sylgard 184 Silicone Elastomer, Dow Corning) consisting of a mixture of a resin and a hardener at a 5:1 ratio was prepared and placed in a vacuum chamber for 20 min to remove bubbles. In this case, a PDMS diluent (OS-10, Sylgard 184, Dow Corning Corp., USA) is used to adjust the porosity of the PDMS foam. PDMS solution samples of 30 and 50 wt% in terms of the PDMS diluent were created by adding set amounts of the PDMS diluent to the PDMS precursor mixture, followed by ultrasonication for 30 min. Here, PDMS foam samples with different porosity levels are denoted as PDMS-x, where x denotes the weight percentage (wt%) of PDMS in the immersion solution. The porosity of the PDMS dielectric layer in the fabricated pressure sensors was controlled by changing the weight percentage of PDMS in the immersion solution.
In order to remove impurities remaining on the copper surface, the copper foam is rinsed with acetone, ethanol, and deionized water. After drying for 30 min in an oven at 60 °C, the copper foam was then soaked in a diluted PDMS solution (30, 50 wt%) for an hour to ensure a conformal coating, after which it was dried at 90 °C for 1 h. For the bare PDMS foam, the copper foam was then immersed into the PDMS precursor mixture for 1 h and cured at 90 °C for 3 h. Finally, the sacrificial copper foam template was removed by a wet etching method. The PDMS-coated copper foam was then positioned such that it floated on the solutions of acetic acid, hydrogen peroxide, and deionized (DI) water at a ratio of 1:1:5 for 12 h to etch away the copper foam. After completely etching the copper, the PDMS foam was soaked in deionized (DI) water for 6 h to remove the acid residue. Subsequently, the PDMS foam samples were dried at room temperature. The fabrication process finished with the formation of the top and bottom electrodes. Polyethylene terephthalate (PET) films (Fine chemical Industry, Korea) attachable with double-sided bonding copper conductive tape (DTS-272, Ducksunghitech Corp., Korea) were attached onto the top and bottom surfaces of the fabricated PDMS foam.
Characterization
To characterize the morphology of the porous PDMS foam, field emission scanning electron microscopy (FE-SEM, SUPRA 25, Zeiss, Germany) was used. To measure the compressive stress–strain curves at various compressive strain levels, a universal testing machine (JSV-H1000, Japan) was employed to characterize the performance with a load cell (HF-1, maximum load 10 N, load resolution 0.001 N). The fabricated PDMS foam samples were compressed at a speed of 10% strain min−1 for a mechanical evaluation. The capacitance of the pressure sensors was measured using an LCR meter (Hioki-3536, Hioki, Japan) at 200 kHz with 1 V of bias. A universal testing machine (JSV-H1000, Japan) was used to apply the pressure and a load cell (HF-1, maximum load 10 N, load resolution 0.001 N) measured the applied pressure values. All sensor evaluations were conducted by connecting the LCR meter to a computer for real-time measurements.