Abstract
Conventional pressure sensors rely on solid sensing elements. Instead, inspired by the air entrapment phenomenon on the surfaces of submerged lotus leaves, we designed a pressure sensor that uses the solid–liquid–liquid–gas multiphasic interfaces and the trapped elastic air layer to modulate capacitance changes with pressure at the interfaces. By creating an ultraslippery interface and structuring the electrodes at the nanoscale and microscale, we achieve near-friction-free contact line motion and thus near-ideal pressure-sensing performance. Using a closed-cell pillar array structure in synergy with the ultraslippery electrode surface, our sensor achieved outstanding linearity (R2 = 0.99944 ± 0.00015; nonlinearity, 1.49 ± 0.17%) while simultaneously possessing ultralow hysteresis (1.34 ± 0.20%) and very high sensitivity (79.1 ± 4.3 pF kPa−1). The sensor can operate under turbulent flow, in in vivo biological environments and during laparoscopic procedures. We anticipate that such a strategy will enable ultrasensitive and ultraprecise pressure monitoring in complex fluid environments with performance beyond the reach of the current state-of-the-art.
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Data availability
All data generated or analysed during this study are included in the Article and its Supplementary Information, and are available from the corresponding author upon reasonable request.
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Acknowledgements
This work is supported by the National Foundation Research Fellowship (NRFF-2017-08 to B.C.K.T.), National Robotics Programme - Robotics Enabling Capabilities and Technologies (RECT) grant (W2025d0244 to B.C.K.T.) and the Agency for Science Technology and Research A*STAR (A20H8a0241 to B.C.K.T.). X.W. is supported by the National University of Singapore Research Scholarship. We thank Z. W. Lim and J. T. Teo for assistance with the pressure characterization tests.
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Contributions
B.C.K.T., J.S.H., W.C., X.W. and Z.X. conceived the project. B.C.K.T. supervised the project. W.C. initiated the idea of using conductive hydrophobic 3D structures, the elasticity of air and EDL to construct a pressure sensor. W.C., X.W., B.C.K.T. and J.S.H., designed the experiments. W.C. and X.W. performed the experiments, and collected and analysed the data. Z.X. designed and carried out experiments for wireless sensing. Z.X. and Y.J. performed the in vivo animal experiments. J.L. and Z.L. conducted the computational fluid dynamics simulation. X.W. and W.C. introduced the use of slippery surfaces. X.W., W.C. and H.Y. performed device fabrication and surface characterization. Z.X., W.C. and X.W. contributed to designing and conducting the confocal imaging. W.C., X.W., B.C.K.T., Z.X., J.S.H. and T.-S.W. co-wrote the paper. All authors discussed the results and commented on the paper.
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Competing interests
B.C.K.T., C.W. and W.X.Y. are inventors on a patent filing on the pressure sensor devices described in the paper. T.S.W. is an inventor on a number of issued patents and patent applications on liquid-infused surface coating technologies including slippery liquid-infused porous surfaces and liquid-entrenched smooth surfaces that have been licensed to Adaptive Surface Technologies (AST) and spotLESS Materials Inc., respectively. In addition, T.-S.W. is a co-founder of spotLESS Materials Inc.
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Extended data
Extended Data Fig. 1 Devices with different types of structures and their performance under varying liquid pressure over multiple cycles.
a, Photo of a “lotus leaf sensor” and SEM image of the lotus leaf surface. b, c, Capacitance response of the “lotus leaf sensor” under varying liquid pressure over multiple cycles. The sensor showed unstable sensing performance with large thresholds and hysteresis. d, SEM image of a pillar-array structure. e, f, Capacitance response of a hydrophilic pillar-array device under varying pressure over multiple cycles. A saturation at small pressure occurred (< 5 kPa), and no unloading response was shown due to the irreversible wetting on the hydrophilic surface of the pillars after fully wetted. Contact angle of hydrophilic gold surface is measured as 60.9 ± 2.4°. This suggests that hydrophobic surfaces are essential to establish a pressure-sensitive air layer. g, Optical image of water-air interface in an ODT treated hydrophobic pillar-array structure, showing the non-uniform underwater wetting condition. h, i, Capacitance response of ODT treated hydrophobic pillar-array device under varying liquid pressure over multiple cycles. j, SEM image of a hexagon-array structure. k, l, Capacitance response of ODT treated hydrophobic hexagon-array structure sensor under varying liquid pressure over multiple cycles. The results showed that the repeatability between different cycles was greatly improved compared to the pillar-array structure sensor. Contact angle of hydrophobic gold/ODT surface is measured as 107.3 ± 0.5°.
Extended Data Fig. 2 Surface roughness characterization.
a, Atomic force microscope (AFM) image of surface IV (without oil) and surface I (with oil). The root mean square (r.m.s) roughness decreased from 34.2 nm to 1.1 nm after oil coating, showing a more slippery and smoother surface. b-d, SEM image of surface II, III and IV. e, The relationship between total layer thickness of the surface treatment (PAni/Cr/Au/ODT) and PAni growth time. The data points represent the mean values, and the error bars represent s.d. (5 samples).
Extended Data Fig. 3 Simulation results of hexagon-array devices (inner hexagon chambers side length 40 µm, first loading and unloading cycle).
a, c, e, g, Matching of simulated contact area and measured capacitance of hexagon-array devices treated with surface I - IV, respectively. b, d, f, h, States of water-air interface under different liquid pressures in a hexagon unit chamber treated with surface I - IV, respectively.
Extended Data Fig. 4 Wetting process with receding angle smaller than the initial contact angle in a hexagon chamber (θrec<90°).
a, Illustration of a cross-section of the simplified wetting process in a hexagon chamber under varying liquid pressure when the receding angle is smaller than the initial contact angle (θ0, close to 90°). The background colours represent different regions of the process. A wetting cycle of loading and unloading pressure in a hexagon chamber was considered as four Regions (A-D) with five turning states (S1-S5). CL, contact Line. F, capillary force. The black arrows indicate the moving direction of the liquid. b, c, Relationship between the change of liquid pressure against the contact angle and contact area (or capacitance) during the wetting process. The red solid lines represent the first testing cycle. The blue dash lines represent the second testing cycle which shows a different response behaviour compared to the first cycle.
Extended Data Fig. 5 Effect of dimension variation of hexagon structural devices on forward threshold.
a-d, SEM images of hexagon device structures with different chamber sizes, the inner side lengths of the regular hexagon holes are 20 μm, 40 µm, 70 µm, and 560 µm, respectively. The insets in a and b are the partial enlargements of the corresponding SEM image. All the structures have the same hole depth of 500 μm. The multi-honeycomb structures have a wall thickness of 10 μm, while the single honeycomb structure has a thicker wall of 50 μm. The number of holes of regular hexagon structural devices are 547, 169, 61 and 1, respectively. e, Forward threshold pressure varies with hexagon devices with different hexagon hole sizes. These devices were treated with surface I. The forward threshold is determined by the capillary pressure and the entrapped air pressure at the point where the contact line starts to move. The data are represented as mean values ± s.d. (3 devices) with the corresponding data points overlapped.
Extended Data Fig. 6 SEM images of PAni nanowires with/without sputtered gold at different depths of designed hexagon-walled pillar-array structure.
a, SEM images of PAni nanowires without gold sputtered. b, SEM images of PAni nanowires with gold sputtered. The SEM images show the gold sputtering at different depth positions on a pillar at different magnifications. The nanostructures of PAni nanowires w/wo metal sputtered are evenly distributed on the surface of designed complex 3D microstructures. This uniform nanostructures with further ODT treatments are hydrophobic and oleophilic which provide an excellent oil-locking ability to form ultra-slippery surfaces on pillars.
Extended Data Fig. 7 Confocal images of hexagon-walled pillar-array structure at two cross sections (A and B) during loading and unloading underwater pressure.
The green region of fluorescence signal represents water, the red signal is the reflection from the water-air interface or the water-electrode interface, and the black region is air. This series of images shows water-air interface movement during water wetting process in the hexagon-walled pillar-array structure.
Extended Data Fig. 8 Effect of dimension variation on sensing performances of eAir.
a-c, SEM images of hexagon-walled pillar-array device structures with different pillar densities, the centre-to-centre spacings of the adjacent pillars are 37.5 µm, 50 µm, and 100 µm, respectively. d-f, Sensitivity, hysteresis, and linearity (R2) performance of hexagon-walled pillar-array devices with different pillar densities. These devices were treated with surface I. The data are represented as mean values ± s.d. (3 devices) with the corresponding data points overlapped.
Extended Data Fig. 9 Performance of devices with varying designs and testing orientations, and the encapsulation.
a, Performance differences between devices with single hexagon, pillar array, and hexagon-walled pillar-array structures in the range of 0–15 kPa. All devices were treated with surface I. The performance of eAir is significantly improved with the combination design of single hexagon chamber and pillar array. b, Capacitance response of eAir tested at different facing orientations against gravity. eAir maintains high linearity, low hysteresis, and consistent sensitivity performance with different facing orientations. c, Comparison of sensing performance of a device exposed to bovine serum albumin (BSA) solution, and with encapsulation, respectively. d, Photo of a device encapsulated with Ecoflex/Parylene dome with 1× PBS solution filled inside. e, Performance of the device before and after encapsulation. Note the encapsulated device was tested in BSA to demonstrate the advantage of encapsulation when operating in complex liquid environments. f, Cyclic test in the range of 0–10 kPa before and after encapsulation.
Supplementary information
Supplementary Information
Supplementary Table 1, Notes 1–11 and Figs. 1–23.
Supplementary Video 1
Existence of air layers on underwater lotus leaf surface.
Supplementary Video 2
Structure optimization from hexagon to hexagon-walled pillar-array structures for performance improvements.
Supplementary Video 3
Performance test of eAir under turbulent flow.
Supplementary Video 4
Laparoscopic grasper with eAirs
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Cheng, W., Wang, X., Xiong, Z. et al. Frictionless multiphasic interface for near-ideal aero-elastic pressure sensing. Nat. Mater. 22, 1352–1360 (2023). https://doi.org/10.1038/s41563-023-01628-8
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DOI: https://doi.org/10.1038/s41563-023-01628-8
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