Incorporating Nanoparticles in Porous Foam Templating for Enhanced Sensitivity of Capacitive Pressure Sensors

Capacitive pressure sensors based on porous foams have been demonstrated for various biomedical applications (0–10 kPa). Many different methods for fabricating porous foams have been reported. In this work, for the first time, the incorporation of silica nanoparticles are reported into the templating process of porous foams fabricated through a combination of particle and emulsion templating, in order to enhance the formation of smaller microstructures in polydimethylsiloxane foams. The foams are coated with graphene, and pressure sensors developed using these foams showed increased sensitivity, up to 4.08 kPa−1. The incorporation of nanoparticles also improves the linearity of the sensitivity, giving a linear sensitivity for the pressure sensors over a pressure range of 0–6 kPa. Further, these pressure sensors have a low limit of detection of ≈13 Pa. These results indicate that incorporation of suitable nanoparticles in the templating of foams is a promising strategy for developing foam‐based pressure sensors with high and linear sensitivity.


Introduction
Flexible pressure sensors are an attractive area of research due to their wide range of applications in wearable devices. [1,2]][11] Among pressure sensors, capacitive pressure sensors, which work on the simple principle of parallel plate capacitance, have been widely DOI: 10.1002/adsr.202300149investigated owing to their simple design, [5] low-cost of fabrication, [6] and reproducibility. [12]chieving high sensitivity is one of the main goals of research on pressure sensors, as it is important for accuracy and efficiency in detecting the applied pressure.Some of the most common strategies to enhance the sensitivity are to introduce highly deformable materials, [5,13,14] incorporate structural modifications, [6,15,16,17] and add conductive fillers. [13,18,19]There are also many studies that have demonstrated a combination of structural modifications and additive fillers in the dielectric matrix [20] to enhance the sensitivity.Among options for the dielectric matrix, polydimethylsiloxane (PDMS) is the most commonly used because of its flexibility, biocompatibility, and low cost. [18,21]n terms of additive fillers, graphene is a good candidate because it has high electrical conductivity, elasticity, and flexibility. [22]It has been shown that the addition of graphene to PDMS increases its dielectric constant from 2.69 to 3.21 at 1 kHz, and subsequently increased sensitivity of the pressure sensors. [23]n our previous investigations, we found that pressure sensors using PDMS foams coated with graphene are more sensitive than PDMS pressure sensors with graphene incorporated into the structure of the foam. [6]We also showed that sensitivity Scanning Electron Microscopy (SEM) images of cross-sections of PDMS foams over a larger a-e) and smaller f-j) smaller field of view.Scalebars equal 400 μm a-e), and 100 μm f-j).The foams had a, f) 5 mg mL −1 , b, g) 10 mg mL −1 , c, h) 15mg mL −1 , d, i) 20mg mL −1 , and (25mg mL −1 ) of SNPs during fabrication of the foams.
depends on porosity, pore size, and pore distribution range. [24]dditionally, sensitivity was highest for the graphene coated PDMS foams with the highest variation in pore sizes over a wide pore size range, with a sensitivity of 3.69 kPa −1 . [24]However, we were able to increase sensitivity only within a particular range of pressure (2-6 kPa), which resulted in highly nonlinear sensitivity of over the range tested.Nonlinearity in sensitivity is seen in most reports on capacitive pressure sensors. [24]There are reports that linearity of sensitivity can be increased by increasing the permittivity of the dielectric material through the addition of carbon nanoparticles, [26] spiky Ni-based percolative composites, [27] or barium titanate fillers. [28]These methods aim to increase the linearity through addition of fillers that increase the dielectric constant and permittivity.On the other hand, our previous results indicated that smaller pore sizes would increase sensitivity at lower pressures (<2 kPa), which would improve the linearity of sensitivity. [24]We achieved small pore sizes of ≈50 μm through emulsion templating. [25]However, achieving smaller pore sizes below 50 μm is a significant challenge.
In this work, we show that incorporating nanoparticles in the templating process of PDMS porous foams improves the sensitivity of the pressure sensors by changing the microstructures and porosity of the foams.This increases the overall sensitivity of the pressure sensors over the entire pressure range (0-12 kPa) and significantly for pressures below 2 kPa.The increased sensitivity in the very low-pressure range improved the linearity in sensitivity and achieved a linear sensitivity in the pressure range of 0-6 kPa.Our findings are significant and pave ways to create low-cost pressure sensors with high sensitivity and improved linearity of sensitivity in the range <10 kPa, which are suitable for various biomedical applications.

Results and Discussions
In our previous work, [24] we fabricated graphene coated PDMS porous foams through a combination of sugar and emulsion templating followed by coating in graphene.Here, we have used the same templating process but with a different starting solution: sulphonated mesoporous silica nanoparticles dispersed in deionized water instead of just deionized water, to generate smaller microstructures than possible with our previous method.Figure S1 (Supporting Information) shows a schematic of the fabrication steps of graphene coated PDMS foams.Sulphonated mesoporous silica nanoparticles, hereafter referred simply as silica nanoparticles (SNPs), were chosen for templating as the sulphonated group makes the nanoparticle hydrophilic and is therefore more easily washable with water after curing the foam.
Five different concentrations of SNPs (5, 10, 15, 20, and 25 mg mL −1 ) were used to fabricate these foams to study the effect of SNP density in the foam fabrication process on sensor performance.Before coating with graphene, we looked at the scanning electron microscopy (SEM) profiles of the PDMS foams to see the impact of the SNPs on the structure of the foams.Figure 1 shows SEM images of cross-sections of the PDMS foams, where a, f), b, g), c, h), d, i), and e, j) are images of foams that had 5, 10, 15, 20, and 25 mg mL −1 of SNPs, respectively, in the fabrication process.Figure 1a-e are images with lower magnification, while Figure 1f-j are the corresponding images with higher magnification that are focused on the middle section of images in Figure 1a-e.SEM images of entire cross-sections of the foams (that are ≈2 mm thick) are shown in Figure S2 (Supporting Information).From Figure 1a-e, all the foams have similar macroporous structures, but with significant differences in microstructures.The microstructures in the foams are significantly smaller for the foams that had 15 and 20 mg mL −1 of SNPs during fabrication (Figure 1c,d).The difference is seen more clearly in the images with higher magnification (Figure 1f-j).From the images, we can infer that at lower concentrations of SNPs, there is no significant impact on the structure of the foams, and they are very similar to the foam that had no SNPs during fabrication (Figure S3a,b, Supporting Information).As the concentration of SNPs increases (15 and 20 mg mL −1 ), it is likely that SNPs will form microbeads.The formation of microbeads or particle clusters in SNPs have been reported earlier. [29,30]These clusters when washed away will leave voids causing the significantly smaller microstructures in the foam.It has been shown earlier that removal of microbeads of SNP clusters result in micrometer and nanometer size voids. [29,30]However, when the concentration of SNP was further increased to 25 mg mL −1 , the microstructures of the foam become like those of the foams that had 5 and 10 mg mL −1 of SNPs during fabrication.This might be an indication that the concentration of the SNPs is too high to form microbeads during fabrication of foams.In all the foams, there would still be some SNPs embedded in the PDMS bulk/matrix that are not reachable by water and not removed by the ultrasonication step during fabrication (Figure S1, Supporting Information).The ultrasonication step is to aid in removal of any SNPs that are loosely attached to the foam.The ultrasonication by itself does not induce any significant change to the PDMS foams, as shown in Figure S3 (Supporting Information) that compares SEM images of PDMS foams before and after ultrasonication.
The PDMS foams were coated with graphene by dip coating in graphene solution to obtain the graphene-coated foams.Clusters of graphene flakes are attached on the surface of the PDMS foam as seen from the SEM image of graphene coated foams in Figure S4 (Supporting Information).Figure 2a shows the stressstrain curves of different graphene coated PDMS foams (individual stress-strain curves shown in Figure S5, Supporting Information).The stress-strain curves can be roughly divided into two regions based on the rate of change of strain: region 1 (<≈6 kPa) where the strain increases rapidly with stress and region 2 (> ≈6 kPa) where strain increases at a considerably lower rate as compared to region 1.This is due to the complete closure of the smaller pores in the low-pressure region followed by the gradual closing of larger pores at high-pressures replacing air with solid PDMS.
In Figure 2a, as the concentration of SNPs increases, there is a shift in the stress-strain curves toward higher strain percentages up to 20 mg mL −1 , which is more clearly seen in the expanded view in Figure 2b.The foams fabricated using 20 mg mL −1 concentration achieved the highest strain percent of 90.13%.Also, in Figure 2b, the change in maximum strain percent of the foams is higher as the concentration increases (Table S1, Supporting Information), column Avg.Maximum strain (%)).Upon further increasing the concentration to 25 mg mL −1 , the stress-strain curve of the foam shifts back significantly toward lower strain percent.This clearly depicts that with the increase in concentration of SNPs up to 20 mg mL −1 , the stiffness of the foams decreases.But as the concentration of SNPs is further increased, the trend reverses.This pattern of stress-strain curves with variations concentration of SNPs is explained by the porosity and density of the foams.The porosity of the foams increases, and hence, density decreases, till up to 20 mg mL −1 of SNPs, and the trend reverses when concentration is 25 mg mL −1 (Figure 2c; Table S1, Supporting Information).The foams reached a highest porosity of ≈87.9% for the case of 20 mg mL −1 concentration of SNPs.
With increase in concentration of SNPs in the templating process, it is expected that it will create more microstructures and hence, increase the porosity of the foams.From the calculated porosity data, this is exactly what we see for up to a concentration of 20 mg mL −1 of SNPs.But 25 mg mL −1 of SNPs led to denser films.To investigate this, microcomputed tomography (micro-CT) images of corresponding PDMS foams (not graphene coated) were taken, which are shown in Figure 3 (Figure S6, Supporting Information).The micro-CT image of foam with no SNPs (Figure 3a) has more cube-shaped voids, which disappear with the addition of SNPs (Figure 3b-d).This might be due to SNPs sticking to the sugar granules and thus modifying the surfaces of the PDMS foams.From the images, the white regions, which indicate solid PDMS, and fade with increase in SNPs concentration up to 20 mg mL −1 (Table S2, Supporting Information).For SNPs concentration of 25 mg mL −1 , the white region is increased, indicating that for this concentration the foam is less porous.The reflection of porosity on micro-CT images agrees with our calculated values of porosity and density (Table S2, Supporting Information).It is unclear why higher concentrations of SNPs lead to denser films beyond 20 mg mL −1 .One possibility is that at high concentrations, SNPs aggregate and there are more SNPs trapped in the bulk PDMS.This would also make the foam stiffer and may likely be the reason why the stress-strain relationship of 25 mg mL −1 foam shows that it is stiffer as compared to the others (Figure 2a).Further investigation of this aspect is necessary to uncover the full picture and is the subject of ongoing studies.
Using the graphene coated foams, capacitive pressure sensors were fabricated to test the performance of the foams using a structure (Figure S7, Supporting Information) described in our earlier works. [6,28]The initial capacitance of the pressure sensors before any pressure was applied (no load) is shown in Figure 4a.The initial capacitance was highest for pressure sensors using foams that used 5 mg mL −1 of SNPs (2.01 pF) during fab- rication.initial capacitance gradually decreased with the increase in concentration of SNPs in fabrication up to 20 mg mL −1 , but this trend reversed when the concentration was increased to 25 mg mL −1 (Table S3, Supporting Information).As expected, this trend in initial capacitance is inversely related to porosity (see Table S1, Supporting Information), because the less porous foams have more solid PDMS, resulting in higher permittivity and hence, a higher capacitance.
The pressure sensors work on the basic principle of a parallel plate capacitor that is described by the equation: where  0 is the permittivity of free space,  r is the permittivity of the dielectric, A is the area of the capacitor (overlapping area between the two electrodes), and d is the distance between the two electrodes or the thickness of the foam.Upon application of pressure, the capacitance of the pressure sensors increases (see Figure S8, Supporting Information) causing a relative change in capacitance as shown in Figure 4b,c.The increase in the capacitance of pressure sensors with applied pressure is due to the a) reduction in thickness, d, b) increase in the relative permittivity of the foam composite by replacement of air cavities with solid PDMS, and c) increase in conductance of the foam due to the formation of multiple conductive pathways, enhancing electron transport. [6,20]n Figure 4b, the shapes of the relative change in capacitance for pressure sensors made with foams that had SNPs (5 mg mL −1 ) and did not have SNPs in fabrication differ significantly.For the pressure sensor that had SNPs in fabrication, there are two distinct regions (0-6 and 6-12 kPa) with linear increase in relative capacitance with pressure.Whereas, for the pressure sensor where SNPs were not used in making the foam, there are three distinct regions (0-2, 2-6, and 6-12 kPa) where relative capacitance increases linearly with pressure.This pressure sensor is very sensitive in the pressure range 2-6 kPa but has much lower sensitivity in 0-2 kPa range.The pore sizes of this foam range from 50 to 700 μm. [28]Upon inclusion of SNPs in the fabrication of the foam, the sensitivity in 0-2 kPa range improves remarkably, bringing it to almost the same sensitivity as in the 2-6 kPa range, and hence achieving a linear sensitivity of 3.17 kPa −1 for the entire range of 0-6 kPa.This sensitivity is less than the sensitivity of pressure sensors without SNPs in fabrication of foam, which have a sensitivity of 3.67 kPa −1 in 2-6 kPa range.However, the sensitivity of pressure sensors with foams that had higher concentrations of SNPs (15 and 20 mg mL −1 ) in fabrication surpasses the sensitivity of the pressure sensors with foams that had no SNPs in fabrication (Figure 4d; Table S3, Supporting Information).
Figure 4C shows the relative change in capacitance with applied pressure for all pressure sensors made with foams that had different concentrations of SNPs during fabrication.For all these pressure sensors, there are two distinct regions, region I (0-6 kPa) and region II (6-12 kPa), with different linear increase in relative capacitance with pressure.These two regions can be correlated to the stress-strain relationship of the foams in Figure 2a.Region I corresponds to the zone where the increase in strain with stress is very large (≈< 6 kPa) due to closing of pores.Region II corresponds to the zone where the increase in strain with stress is significantly smaller (region ≈>6 kPa) due to most pores being fully compressed and contribution to strain is mainly from bulking of PDMS.
In region I, the relative change in capacitance, and hence sensitivity (Figure 4d), increases with increasing SNP concentration up to 20 mg mL −1 , giving the highest sensitivity of ≈4.08 kPa −1 .However, the trend reverses when the concentration is increased to 25 mg mL −1 and the sensitivity is reduced.On the other hand, in region II, the relative change in capacitance, and hence sensitivity (Figure 4d), does not change as significantly as in region I.With increasing concentration of SNPs, the sensitivity increased slightly in the beginning, stayed constant, and then reduced slightly at 25 mg mL −1 (Table S3, Supporting Information).
The trends for sensitivity of pressure sensors seen in Figure 4c,d are directly correlated to the porosity of the foams (Figure 2c), where more porous foams have higher sensitivities.This is expected as a more porous foam would have a more significant change in  r and d (Equation ( 1)), due to the closure of more pores with applied pressure.The sensitivity of region II did not change significantly with porosity.This is because most of the pores of the foams are exhausted by the time the pressure reaches 6 kPa and there is little contribution to sensitivity from the pores and microstructures on the surface of the pores.There is also a contribution to the increase in sensitivity from the drop in resistance of the foams with applied pressure (Figure S9, Supporting Information), as lower resistance will facilitate better charge-transfer.The drop in resistance of our porous foams is also related to porosity.The average drop in resistance is highest for the most porous foam, which is the foam that corresponds to 20 mg mL −1 of SNP.Finally, a lowest detection limit of ≈13 Pa is achieved with the most sensitive pressure sensor, made using foam that had 20 mg mL −1 of SNPs in the fabrication process (Figure S10, Supporting Information).This pressure sensor has a response and recovery time of 0.58 and 0.23 s, respectively (Figure S11, Supporting Information).When subjected to repeated pressure cycles, the pressure sensor gave the same response (Figure S12, Supporting Information).Last, all the pressure sensors have a stable response over a wide range of frequencies (Figure S8, Supporting Information), 20 Hz -10 kHz, demonstrating that these pressure sensors are suitable for low-pressure biomedical applications.

Conclusion
In this work, we have shown that the incorporation of SNPs in the fabrication of foams has a synergistic effect on the perfor-mance of capacitive pressure sensors based on graphene-coated PDMS foams.The incorporation of SNPs in the fabrication process impacts the stress-strain relationship and porosity of the foams, which in turn resulted in a significant increase in the sensitivity of pressure sensors at very low-pressures (0-2 kPa).It also resulted in a linear sensitivity in the range of 0-6 kPa.Our results indicate that inclusion of different nanomaterials in the fabrication of porous foams may lead to pressure sensors with superior characteristics.Therefore, our results are significant for future research in the development of highly sensitive porous pressure sensors for low-pressures.

Experimental Section
Synthesis of SNPs: The synthesis for SNPs was taken from literature. [31]The surfactant cetyltrimethylammonium bromide (0.2 g, CTAB, and Merck), 1.4 mL 1 m of NaOH solution (Grüssing) and 95.5 mL of deionized water were mixed and heated to 80 °C.After obtaining a clear solution, 0.8 mL of tetraethyl orthosilicate (TEOS, Sigma-Aldrich) was added to the solution.The mixture was stirred for 15 min for prehydrolysis, and subsequently 0.2 mL MPTMS ((3mercaptopropyl) trimethoxysilane, Alfa Aesar) were added.Stirring at 80 °C was continued for another 105 min.The reaction dispersion was cooled down in an ice bath and subsequently centrifuged and washed with water and ethanol (Grüssing).Template extraction of the powder was performed in acidified ethanol solution (97 mL ethanol : 3 mL 37% hydrochloric acid (VWR chemicals)), while refluxing for 24 h.Subsequent oxidation of the powder was done in 10 mL of 30% hydrogen peroxide solution and 2 mL 65% nitric acid (both Fisher Chemicals) at room temperature and stirring for 24 h.Afterward, the powder was washed several times with deionized water and dried.
Graphene-Coated PDMS Foams: The foams were fabricated by a combination of particle templating and emulsion templating.For particle templating, sugar and SNPs were used as particles.For emulsion templating, water-in-oil emulsion was used.
Initially, the SNPs were ground to a fine powder using a mortar and pestle.5, 10, 15, 20, and 25 mg of SNPs were mixed separately with 1 mL of deionized water in individual vials to create various concentrations.The solutions were left stirring using magnetic stirrers for 48 h.Arabic gum was added in the SNPs solutions in the ratio 1:10 (one part Arabic gum to 10 parts SNPs solution) to make the water phase for the emulsion templating part.
The oil phase for emulsion templating was prepared by mixing PDMS precursor (1.4 g) and curing agent (RTV Momentive 615) at a ratio 10:1 along with a surfactant, lauryl PEG-8 dimethicone (Silube J208-812, Siltech).The surfactant constitutes 5.0 wt.% of the final mixture.To make the water-in-oil emulsion, the water phase and oil phase were mixed in a ratio 2:3 and were agitated vigorously in a vortex mixer for 5 min.Sugar granules were then added to the water-in-oil emulsion in a 4:1 ratio.Equal quantities of sugar granules of two size ranges (100-400 and 400-700 μm) were used.The mixture of sugar granules was mixed vigorously in a dry state using a vortex mixer to ensure even distribution of the particles.After adding sugar, the solution was mixed again using a vortex for 1 min and manually stirred for 5 min to ensure uniform mixing and granule distribution.Thereafter, the whole mixture was transferred into a carbon-fiber reinforced 3D printed mold with multiple slots of 20 × 20 × 2 mm.This was kept in a vacuum oven at room temperature at a pressure of 1 × 10 −1 Torr for 15 min to remove trapped air bubbles.After this step, the temperature of the oven was increased to 100 °C and the sample was kept for 30 min for curing.The mold was then transferred into a large beaker with water and kept submerged for 24 h for dissolution of sugar and removal of SNPs.The foams were removed from the slots carefully and cut to dimensions of 10 × 10 × 2 mm, using a template to ensure consistency.The re-sized foams were ultrasonicated in water for 30 min followed by manual rinsing in water 10 times, each time shaking for a duration of 1 min to further re-move SNPs.The wet porous foams were then dried on hotplate at 60 °C for 20 min.
The PDMS foams were dipped in graphene solution for 5 min for coating with graphene.The graphene solution was prepared by mixing 2 wt.% of graphene with respect to the mass of dry PDMS in 3 mL of isopropanol.The solution was mixed vigorously using a vortex mixer for 5 min to disperse the graphene homogeneously.The graphene-coated PDMS foams were dried on a hot plate at 60 °C for 5 min.
Pressure Sensors: Pressure sensors were made using the graphene coated foams using a structure discussed in the previous works. [6,24]The graphene coated foams were sandwiched between two flexible 0.01 mm thick strips of carbon tape (ProSciTech), which served as electrodes.Between the foam and carbon tape on one side, a thin 2 μm PET insulating sheet (Polyk Technologies) was inserted.A 0.05 mm thick piece of Copper tape (ProSciTech) was attached to the carbon electrodes as electrical contacts.Thin wires were soldered to the copper tape to provide a good connection to the test setup.The whole device was encapsulated in a 17 μm thick commercial poly vinylidene chloride (PVC) wrap.
Characterisation: The dimensions of the PDMS foams were measured using digital calipers and the masses were determined by an electronic precision balance (Kern & Sohn).SEM images of the PDMS foams were taken using TESCAN MIRA3 after coating with ≈4 nm of Pt.The stressstrain analysis of the foam was performed by applying a normal uniaxial compressive force using a high-precision universal testing machine (Mecmesin-MultiTest 2.5 dV).The loading-unloading cycles were done at a speed of 10 mm min −1 up to a maximum pressure of 20 kPa.This is a limitation of the measuring system as the maximum load that can be applied is 2 N. Calculations of porosity are discussed in the previous work. [6]Microcomputed tomography scans were done for all samples and scanned three times (μCT50, SCANCO Medical AG, Brüttisellen, Switzerland).Samples were carefully oriented and fixed in a ø9 mm tube using foam inserts.Scanning was done in air at an energy of 45 kVp and a current of 133 μA.A 0.1 mm aluminium filter was used during scanning.The integration time was set at 800 ms, once averaged, resulting in a sample time of 0.8 s.Scans of 0.75 mm height (equivalent to 170 slices) for each sample were taken at an isotropic voxel size of 4.4 μm3.All images were then exported as DICOM files for 3D visualization.The area of white regions in micro-CT images were analyzed using Image J.
Capacitance of the pressure sensors were measured using a benchtop LCR meter (BK Precision 891) set at a frequency of 1 kHz and a voltage of 1 V. Capacitance at different pressures was measured by applying forces in a step-and-hold manner from 0.5 to 12 kPa, with 0.5 kPa increments at a rate of 10 mm min −1 .
Statistical Analysis: For each type of pressure sensor, five samples were characterized for sensor performance determination.At each pressure during the step and hold measurement for a pressure sensor, 500 values of capacitance were collected and averaged to get the capacitance value at that pressure.The error bars in the graphs represent one standard deviation.

Figure 2 .
Figure 2. a) Variation in stress-strain curves of graphene coated PDMS foams when different concentrations of SNPs were used in fabrication.The solid and hollow markers/symbols represent loading and unloading phase of the cycle, respectively.b) Expanded view of boxed sections of a).Variations in c) average porosity, and d) density of graphene coated PDMS foams when different concentrations of SNPs were used in fabrication.The error bars represent one standard deviation from a sample size of N = 5.

Figure 4 .
Figure 4. a) Average capacitance of the pressure sensors with no pressure applied.b) Relative change in capacitance of pressure sensors with SNPs (5 mg mL −1 ) and without SNPs in fabrication process of foam.c) Relative change in capacitance of pressure sensors that had different concentrations of SNPs in fabrication.d) Sensitivity of the various pressure sensors for two regions: region I (0-6 kPa) and region II (6-12 kPa).The error bars represent one standard deviation from a sample size of N = 5.