ENHANCED PRESSURE SENSING PERFORMANCE FOR PRESSURE

SENSORS

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Application No. 62/454,452, filed February 3, 2017, the contents of which are incorporated herein by reference in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT

[0002] This invention was made with Government support under Grant Number N00014-15-1-2368, awarded by the Navy Office of Naval Research and under Grant Number 1508692, awarded by the National Science Foundation. The Government has certain rights in the invention.

TECHNICAL FIELD

[0003] This disclosure generally relates to pressure sensors.

BACKGROUND

[0004] Electronic skins (E-skins) with sensory capabilities mimicking human-skin or beyond are desirable for wearable healthcare monitoring devices, robotic technologies, human-machine interface (HMI) and artificial intelligence (AI). The ability to detect local pressure variation represents an important function of E-skins.

[0005] It is against this background that a need arose to develop the embodiments described herein.

SUMMARY

[0006] In some embodiments, a pressure sensor includes: (1) a first electrode layer; (2) a second electrode layer including multiple conductive microstructures extending toward the first electrode layer; and (3) a dielectric layer between the first electrode layer and the second electrode layer. [0007] In additional embodiments, a pressure sensor includes: (1) a source electrode; (2) a drain electrode; (3) a channel extending between the source electrode and the drain electrode; (4) an electrode layer including multiple conductive microstructures extending toward the channel; and (5) a dielectric layer between the electrode layer and the channel.

[0008] In additional embodiments, a pressure sensor includes: (1) M elongated strips; and (2) N elongated strips. The M elongated strips extend crosswise relative to the N elongated strips to define M ^χ N intersections. Each of the M elongated strips includes a first electrode layer and a dielectric layer, and each of the N elongated strips includes a second electrode layer including multiple conductive microstructures extending toward the first electrode layer, with the dielectric layer between the first electrode layer and the second electrode layer.

[0009] In further embodiments, a pressure sensor includes: (1) M elongated strips; and (2) N elongated strips. The M elongated strips extend crosswise relative to the N elongated strips to define M ^χ N intersections. At each of the M ^χ N intersections, a corresponding one of the M elongated strips includes a gate-absent transistor and a dielectric layer, and a corresponding one of the N elongated strips includes an electrode layer including multiple conductive microstructures extending toward the gate-absent transistor, with the dielectric layer between the gate-absent transistor and the electrode layer.

[0010] Other aspects and embodiments of this disclosure are also contemplated. The foregoing summary and the following detailed description are not meant to restrict this disclosure to any particular embodiment but are merely meant to describe some embodiments of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] For a better understanding of the nature and objects of some embodiments of this disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.

[0012] Figure 1. Capacitance- and transistor-based E-skin with conductive microstructured air gaps (CMAGs). (a) Schematic illustration of a 5 ^χ 5 capacitive pressure sensor array with the CMAGs. (b) Zoomed-in view of the device structure from (a). The E- skin is made by laminating a top stack (the CMAGs on a polyethylene terephthalate (PET) substrate) and a bottom stack (dielectric/bottom electrode/PET substrate) together, (c) Schematic illustration of a CMAG-M0S ₂ E-skin including two assemblies. A top stack includes the CMAGs on a PET substrate and a bottom stack is composed of a dielectric layer, source and drain electrodes, and M0S ₂ as a semiconducting channel on a silicon dioxide substrate, (d) Scanning electron microscope (SEM) image of a top view of conductive microstructured pyramids with an inset showing a zoomed-in image. The height, width, and periodicity of the conductive pyramids are about 3.3 μιτι, about 7.5 μιτι, and about 13.5 μιτι, respectively. The scale bar is denoted as 20 μιη.

[0013] Figure 2. Flexible CMAG capacitance-based E-skin. Schematic illustrations of (a) a comparative capacitive pressure sensor with an elastic microstructured dielectric (a conductive electrode is deposited on a flat backside of a microstructured polydimethylsiloxane (PDMS)) and its approximately electrical equivalent circuit (left bottom) and (b) a proposed capacitance-based E-skin with CMAGs (a conductive electrode is coated on a side of microstructured pyramids) and its approximately electrical equivalent circuit (right bottom), (c) The normalized capacitance change and (d) the corresponding sensitivity as a function of varying pressure up to about 1.5 kPa. (e) The normalized capacitance change and (f) the sensitivity for the CMAG device in the ultralow pressure regime < about 8 Pa. The height, width and periodicity of the conductive pyramids are about 4.1 μιη, about 7.5 μιτι, and about 15.0 μιτι, respectively.

[0014] Figure 3. Tunability of CMAG capacitance-based E-skin. (a, b) Top-view SEM images of two different sizes of conductive pyramids: Pyramid #1 in (a) with a height of about 4.1 μιη, width of about 7.5 μιη and periodicity of about 15.0 μιτι, and Pyramid #2 in (b) with a height of about 12 μιτι, width of about 20 μιτι, and periodicity of about 100 μιη. The scale bars represent 50 μιη. (c) The normalized capacitance change and (d) the sensitivity of E-skins with different sizes and spatial arrangements of the conductive pyramids. The normalized capacitance change, sensitivity, and pressure sensing range can be readily tuned by changing the pyramid size and the periodicity, (e, f) Normalized capacitance change and the sensitivity of an E-skin made of the CMAGs with and without a PET supporting layer. The CMAGs without PET are much softer and more readily deformed, leading to a higher sensitivity.

[0015] Figure 4. Pressure sensing performance of a CMAG-M0S ₂ E-skin. (a) Transfer curves of the E-skin at constant V _sc i = about 1 V under different pressures, (b) The normalized channel resistance change and (c) the corresponding sensitivity at constant V _s a = about 1 V and different V _g . (d) Transfer curves of the CMAG-M0S ₂ E-skin with an optimized bottom dielectric and a lower threshold voltage at constant V _sc i = about 1 V under varying applied pressures, (e) The normalized channel resistance change and (f) the corresponding sensitivity at constant V _s a = about 1 V.

[0016] Figure 5. Flexible CMAG capacitance-based E-skin for static pressure mapping and real-time pulse wave monitoring of the radial artery, (a) Top view of a 5 ^χ 5 pixel array upon placing grains of rice, soybean, and red bean with a weight of about 20, about 158, and about 219 mg, respectively and (b) the corresponding distribution of the normalized capacitance change on the sensor array, (c) Top view of a 5 ^χ 5 pixel array upon placing a penny with a weight of about 3.11 g and (d) the corresponding distribution of the normalized capacitance change, (e) Photograph of an E-skin for detecting the wrist pulse wave, (f) The real-time pulse wave monitoring of human subjects A and B before and after 5- minute exercise, (g) Comparisons of zoomed-in waveforms of human subject A before and after 5-minute exercise extracted from the dashed square boxes in (f) showing the important health monitoring information such as Reflection Index R.I.) = P ₂ IP ₁ ) ^x 100 % and Arterial Stiffness Index (S.I.) = subject e[g t/AT _D y _P (in unit of m/s).

[0017] Figure 6. CMAG-M0S ₂ E-skin for acoustic wave detection and speech pattern recognition, (a) Electrical signal response of the device to the same music for three times at about 94 dB. (b) Zoomed-in waveforms within a time duration from about 22.67 to about 22.82 s extracted from (a) showing the clearly distinct peaks and valleys, (c) Zoomed- in waveforms extracted from (b) within a time duration from about 22.72 to about 22.74 s showing high fidelity recognition of the music signal, (d) The source-drain current response to the specific acoustic wave (about 100 dB) at a fixed frequency of about 7 kHz, highlighting the rapid response time of the E-skin device (about 0.15 ms that is set by a threshold of the measurement setup), (e) Comparisons of the acoustic waves measured by a standing microphone (top figure) and the E-skin (two figures at the bottom), respectively. The source-drain current response of the device was measured twice at constant V _sc i = about 1 V, while a speaker played a non-native female speaker saying "U-C-L-A" at about 85 dB. All the measurements were carried out in a normal conversation environment (about 62 dB). (f) Auditory spectrogram changes of the device corresponding to two bottom figures in (e), highlighting high fidelity speech pattern recognition.

[0018] Figure 7. Fabrication processes of CMAGs. (a) Silicon (Si) mold with inverse microstructured pyramids was patterned using photolithography, silicon dioxide (Si0 ₂ ) uncovered by a photoresist was stripped using buffered hydrofluoric acid (BOE), and the remaining Si0 ₂ was used as a mask for potassium hydroxide (KOH) etching, (b) A dilute solution of PDMS mixture was drop cast onto an octadecyltrichlorosilane (OTS)-treated Si mold, and then a PET supporting layer was placed on top of the PDMS followed by curing at about 120 °C for about 4 hours, (c) After curing, the flexible PDMS/PET was peeled from the mold, (d) Au/Cr layers were deposited on microstructure pyramids by electron-beam (e- beam) evaporation. A rotating sample holder was used during the e-beam process to ensure a continuous conductive layer across the entire microstructured surface.

[0019] Figure 8. Comparisons of the normalized capacitance change for comparative and conductive air-gap devices to different thickness change of air gaps (Ad _Ail .) by using a parallel plate capacitor model. Δ<¾ _;Γ increases with increasing applied pressure. Here, ¾ _;r , e _PDM s, d _PDM s are substituted as about 1, about 3, about 4.1 μ ^η ι, and about 20 μηι, respectively, into the equations (3) and (5) in Supplementary Note.

[0020] Figure 9. Pressure response of a flexible CMAG capacitance-based E-skin. The height, width, and periodicity of conductive pyramids are about 12 μηι, about 20 μηι, and about 100 μηι, respectively. The device shows linear response as pressure increases. The sensitivity is about 44.3 kPa ^"1 over the pressure range from about 0-5 kPa.

[0021] Figure 10. Bending stability test of a flexible CMAG capacitance-based E- skin responding to a load of about 86 Pa on a curved surface with a bending radius of about 32.5 mm.

[0022] Figure 1 1. Stability test of a CMAG-M0S ₂ E-skin responding to a load of about 86 Pa over about 6,000 loading-unloading cycles on a flat surface at V _sc i = about 1 V.

[0023] Figure 12. Real-time wrist pulse wave monitoring over about 9.5-second period at low operation voltage of about 0.1 V. The pulse wave indicates the resting heart rate is about 63 b.p.m.

[0024] Figure 13. Speech pattern recognition. Comparisons of acoustic waveforms measured by a standing microphone (top figure) and a CMAG-MoS ₂ E-skin (two figures at the bottom), respectively. Source-drain current response of the device was measured twice at constant V _sd = about 1 V, while a speaker played a non-native female speaker saying (a) "The important thing is not to stop questioning" and (b) "Electronic skin" at about 85 dB. All measurements were carried out in a normal conversation environment (about 62 dB).

[0025] Figure 14. Remote pressure monitoring. Integration of a CMAG-M0S ₂ E- skin with system on chip (SoC) allows portable real-time remote pressure monitoring, (a) Source-drain current response to an applied pressure when a finger touched the device, (b) The real-time remote pressure monitoring can be assessed through a user's cell phone by using an open on-line website. The results can be sent to a social media network.

DETAILED DESCRIPTION

[0026] Overview:

[0027] Here some embodiments are directed to a design of pressure-sensing E-skin by integrating a conductive microstructured air-gap gate with two-dimensional (2D) molybdenum disulfide (M0S ₂ ) transistors to achieve an unprecedented combination of high sensitivity, rapid response, low power consumption and long-term stability. It is shown that the design of conductive microstructured air gaps (CMAGs) can be used to create capacitance-based E-skins with an ultrahigh sensitivity of up to about 770.4 kPa ^"1 (or more) at a low operation voltage of about 1.5 V, which is more than about 170 times better than a comparative capacitance-based device. By employing the CMAGs as pressure sensitive gate for 2D M0S ₂ transistors, it is shown that the pressure sensitivity can be further amplified to achieve an unprecedented value of about 2.61 ^χ 10 ⁷ kPa ^"1 (or more), more than 5 orders of magnitude better than that of other reported transistor-based pressure sensor. Along with ultrafast response time (< about 0.15 ms), lower power consumption (about 9 pW-270 nW), low minimum detectable pressure (< about 0.4 Pa), and excellent stability, the device delivers an overall performance far exceeding that of other pressure-sensing E-skins, and allows realtime human pulse wave measurement, static pressure mapping, sound wave detection, speech pattern recognition, and remote pressure monitoring.

[0028] Introduction:

[0029] E-skin typically refers to an artificial skin with human-like sensory capabilities, especially for pressure sensing that transduces an applied force into an electrical signal, which has attracted significant attention due to the increasing demand for flexible electronic devices such as wearable healthcare monitoring, HMI, robotic technologies, microelectromechanical systems (MEMS), microphones, hearing aids, and so forth. E-skins should not just mimic the real human skin, but provide functions or performance beyond the natural skin to satisfy many application specifications, such as higher sensitivity especially in low pressure regime (< about 100 Pa, suitable for lower vibrational force; < about 10 kPa, approximate to gentle touch), along with other desirable features including fast response time in the millisecond range, lower power consumption, long-term air stability and flexibility for wearable electronic devices. Both capacitance- and transistor-based E-skins are extensively investigated. The capacitance-based E-skins feature streamlined device design, ready readout, excellent stability, and low power consumption, while the transistor based E-skins offer signal amplification, higher sensitivity, and lower power consumption. Promising routes to both types include using compressible dielectrics or air gaps to overcome the less compressible and viscoelastic behavior of rubber, thus enhancing the sensitivity and response time of capacitance-based E-skins. Integrating carbon nanotube or organic transistors with pressure sensitive rubber (PSR) or microstructured dielectrics also can be used.

[0030] Mechanisms of pressure sensors are usually governed by the compressibility of a material contacted directly with an applied force. In general, the greater the compressibility the material exhibits, the higher the sensitivity and the signal-to-noise ratio (SNR) of the pressure sensors can be achieved. The compressibility is also related to the Young's modulus of elastomers. For example, under the same stress, elastomers with a low Young's modulus tend to deform more, leading to a higher sensitivity compared to that with a higher Young's modulus. However, the elastomers with a low Young's modulus exhibit greater viscoelastic behavior that constrains a response time. As a result, there are considerable challenges to achieve both a higher sensitivity and a fast response time at the same time. Using silicone elastomer as a dielectric in compressible capacitors for pressure sensing has achieved a sensitivity of about 2.3 ^χ 10 ^"4 kPa ^"1 ; however, the pressure sensing range is from about 50 kPa to about 1 MPa, which is too large for mimicking human-like sensing capabilities such as gentle manipulation of items. The development of capacitive pressure sensors by using elastic microstructured dielectric has shown dramatic improvements of the sensitivity (about 0.55 kPa ^"1 ) and response/relaxation times (« about 1 s) compared to the one with an unstructured elastic dielectric (sensitivity of about 0.02 kPa ^"1 and response/relaxation times > about 10 s) or a polymer foam (sensitivity of about 5 ^χ 10 ^"4 kPa ^"1 and response/relaxation times » about 20 s). Nevertheless, the sensitivity is still constrained by (i) the less compressible dielectrics and (ii) the large thickness of the elastomer (about 10-500 μπι) that contributes to the denominator in the normalized capacitance change, leading to a lower sensitivity and a lower SNR. In addition, the response time is restricted by the natural viscoelastic behavior of rubber. Therefore, the main challenge is to form air-gap devices that are not constrained by thick and less compressible elastomers. In order to overcome these challenges, a flexible aluminum/polyimide composite electrode can be used as an air-gap capacitive pressure sensor with a high sensitivity of about 4.5 kPa ^" when the pressure is in the range of about 0-3 kPa.

[0031] To further enhance the sensitivity, transistor-based E-skins with an impressive sensitivity of about 8.4 kPa ^"1 and fast response time of < about 10 ms can integrate organic thin film transistors (OTFTs) with elastic microstructured dielectrics. Other transistor-based pressure sensors can use a flexible suspended gate on gate-absent OTFTs to create unstructured air gaps, delivering an ultrahigh sensitivity of about 192 kPa ^"1 and rapid response time of < about 10 ms. However, the sensitivity, S R, and response time of the pressure sensors are still constrained by the lack of actual microstructured air gaps and inherent viscoelastic behavior of rubber. Moreover, the performance of the OTFTs usually degrade in air and their relatively low mobility often specifies large operation voltage (e.g., up to about 200 V), which is not desirable for practical applications in E-skins.

[0032] To improve the sensitivity, response time, air stability and reduce the power consumption of pressure sensing E-skins, some embodiments are directed to a design of conductive microstructured air gaps (CMAGs) to replace a microstructured dielectric, in which the overall capacitance is solely or primarily determined by the highly compressible microstructured air gaps with little contribution from a less compressible thick elastomer, realizing a "true" microstructured air-gap device with an ultrahigh sensitivity and fast response time. Taking a step further, the CMAGs are integrated with 2D semiconductor MoS ₂ transistors to create a CMAG-M0S ₂ E-skin with further enhanced pressure sensing performance. The capacitance-based E-skins deliver an impressive sensitivity of up to about 770.4 kPa ^"1 (or more) at a very low operation voltage of about 1.5 V, far exceeding the highest sensitivity (about 4.5 kPa ^"1 ) achieved in some other capacitive pressure sensors. Significantly, the CMAG-M0S ₂ E-skins exhibit an unprecedented sensitivity of up to about 2.61 x 10 ⁷ kPa ^"1 (or more), more than 5 orders of magnitude better than other reported transistor-based pressure sensors (about 192 kPa ^"1 ). Additionally, the CMAG-MoS ₂ E-skin exhibits many other favorable features, including ultrafast response time (< about 0.15 ms vs. about 10 ms), lower power consumption (about 9 pW - about 270 nW), minimum pressure detection (< about 0.4 Pa), excellent air stability and robustness. With these outstanding pressure sensing capabilities, the E-skins allow real-time acoustic wave detection, remote pressure monitoring, speech pattern recognition, human pulse wave measurement, and static pressure mapping, which lay the foundations for the applications in the next generation of flexible and wearable electronics. [0033] Results:

[0034] Device fabrication. The detailed fabrication processes are provided in the Method section and Figure 7. The CMAGs are composed of a regular array of microstructured pyramids fabricated by casting an elastomer (polydimethylsiloxane (PDMS)) in a silicon mold to form a top electrode layer including a base layer 100 composed of the elastomer and the microstructured pyramids as protrusions composed of the elastomer, followed by direct deposition of a conductive coating on the microstructured pyramids to form conductive microstructures 102. The sizes (heights and widths) and periodicity of pyramids can be readily tailored for specific pressure sensing specifications. The CMAG- based flexible capacitive E-skin was assembled by laminating the conductive microstructured PDMS pyramid array (supported on a polyethylene terephthalate (PET) substrate 104) and a flexible PET substrate 106 with a bottom electrode layer 108 composed of a continuous layer of Cr/Au thin film (about 20 nm/about 80 nm) and an A1 ₂ 0 ₃ dielectric layer 110 (about 30 nm) (Figure la-b), which allows large-area, low-cost, scalable fabrication of a matrix array of pressure sensing devices. To further enhance the pressure sensing performance, the transistor- based E-skins are fabricated by integrating the top electrode layer including the CMAGs with a 2D MoS ₂ transistor, with the dielectric layer 110 disposed in between (Figure lc). As shown in Figure lc, the 2D MoS ₂ transistor (supported on a silicon dioxide (Si0 ₂ ) substrate 118)) includes a source electrode 112, a drain electrode 114, and a channel 116 composed of MoS ₂ and extending between the source electrode 112 and the drain electrode 114. Figure Id shows a scanning electron microscope (SEM) image of the conductive microstructured PDMS, which shows a highly uniform array of micro-pyramids.

[0035] The design of the CMAGs can be used to create higher pressure sensitivity, fast response time, and pressure sensing tunability of capacitance-based, transistor-based, or related pressure sensors (Figure la-d). Unlike other microstructured air-gap devices, which usually exhibit relatively lower pressure sensitivity and slower response time due to (i) less compressible dielectrics, (ii) a large thickness of an elastomer (about 10-500 μπι) that contributes to the denominator in the normalized capacitance change, and (iii) the natural viscoelastic behavior of elastomers, the design of the CMAGs provides a "real" microstructured air-gap device (Figure 2b) which can enhance the pressure sensitivity and improve the response time without the unwanted effect caused by a less compressible, thick microstructured elastomer. The integration of the CMAGs as a pressure sensitive dielectric with gate-absent transistors can amplify the sensing signal, dramatically enhancing the pressure sensitivity and SNR.

[0036] The design and evaluation of the CMAGs. A simplified parallel plate capacitor model is used to evaluate and compare the normalized capacitance change to physical deformation of the CMAG device versus a comparative air-gap device. Comparative microstructured air-gap devices have a conductive layer on a flat backside of a microstructured PDMS (Figure 2a). With this design, the total capacitance is approximately contributed by two capacitors in series: the elastomer-based capacitor (which has relatively small change upon compression) and the air-gap capacitor that responds sensitively to the mechanical deformation. The normalized capacitance change can thus be expressed as:

Conventional _ ^£ PDMS^Air ~*~ ^£ Air^PDMS ^£ PDMS^Air ί

C ^o d A'ir £ PDMS +d P'DMS ε Air d A'ir ε PDMS +d P'DMS ε Air where e _Air (= 1.0) and e _PDM s (about 3.0) are the dielectric constants of air and PDMS, respectively; M _Air and Ad _PDM s are the thickness change of air and PDMS, respectively; d' _Air and d'p _D Ms are denoted as the final thickness of air and PDMS under applied pressure. The stiffness of PDMS is usually about 0.57-3.7 MPa. Under an applied pressure of less than about 10 kPa, d' _PDM s approximately equals to d _PDM s, resulting in e _Air Ad _PD MS « £PDMS ^Air- Therefore, one can obtain the equation of the normalized capacitance change for the comparative E-skin as shown in the last term in equation (1).

[0037] The normalized capacitance change, sensitivity, and SNR of this structure are governed by the thickness change of the air gap {Ad _Air ) and the final thickness of the elastomer and air dielectrics (d' _PDM s and d' _Air ). Typically, the thickness of air gaps is about 2-3 μπι and the thickness of the elastomer is about 10-500 μπι. As a result, d' _PDM s (~ d _PDM s) in equation (1) dominates the pressure sensing performance. Therefore, in this design, the thinner the elastomer dielectric, the higher the sensitivity and SNR of the E-skin. However, it is of considerable challenge to shrink the thickness of the elastomer while maintaining the microstructured air gaps, which has thus constrained the normalized capacitance change and sensitivity of such devices. In addition, although the comparative microstructured air-gap devices have shown that the response/relaxation times is about 10-100 times faster than those without the microstructured elastomer dielectrics, the devices still inherit the viscoelastic behavior of elastomers, which constrains their response/relaxation times (typically about 10-1,000 ms regime). [0038] To address these challenges, it is proposed to use the CMAGs (Figure 2b) as a pressure sensitive dielectric. With this design, the capacitance is solely or primarily approximately contributed by the air-gap component, so the change of the capacitance under an applied pressure is determined by the deformation of the microstructured air gaps without considering the effects of the elastomer dielectric, which leads to greatly enhanced sensitivity, normalized capacitance change, and SNR. The normalized capacitance change for the CMAG devices can be expresses as:

^Conductive _ ^ Air

C d A'ir

[0039] In the compression state, the higher the compression pressure applied, the less the final thickness of air (i¾ _r ). As a result, AC conductive/ C ₀ is increasingly larger than AC conventional/ ₀ as the applied pressure increases (Figure 8). Under a specific applied pressure, when <¾ _;r approaches 0, resulting in:

Conductive ^^Conventional

[0040] Moreover, this "true" microstructured air-gap design reduces the impact by the unwanted viscoelastic behavior of elastomers, thus leading to a fast response time. With this simplified model, the analysis does indicate that CMAGs have significant advantages over the comparative design (a detailed discussion is provided in Supplementary Note).

[0041] Pressure sensing performance of the flexible CMAG capacitance-based E- skin. The sensitivity of capacitive pressure sensors is specified as S = S(AC/C ₀ )/SP, where AC (= C - C ₀ ) is the relative change in capacitance; C and C ₀ are the capacitance of the sensor with and without pressure load, respectively; P is the applied pressure. To evaluate the pressure sensitivity of the device, measurement is made of the capacitance change under different pressures with a relatively low operation voltage of about 1.5 V. The normalized capacitance change (AC/C ₀ ) of the device can reach up to about 217 at P of about 1.5 kPa (Figure 2c), which is about 35 times higher than that of another capacitance-based E-skin. Importantly, the device can exhibit an extraordinary sensitivity up to about 330 kPa ^"1 (Figure 2d), which is considerably higher than those achieved in other devices: 0.005 kPa ^"1 ; 0.21 kPa ^"1 ; 0.55 kPa ^"1 ; 0.55-0.58 kPa ^"1 ; 1.5 kPa ^"1 . Moreover, a moderately high sensitivity of about 44.3 kPa ^"1 was achieved in a broader pressure sensing range (about 0-5 kPa) (Figure 9). With such high sensitivity, the device can respond to lightweight substances such as tiny pieces of paper (the pressure of a single piece of paper corresponds to about 0.76 Pa). The measured capacitance changes with increasing number of pieces of paper show highly reproducible results even at ultralow pressure regime (< about 8 Pa when 10 pieces of paper are loaded) (Figure 2e). The corresponding sensitivity at ultralow pressure regime can reach up to about 36.6 kPa ^"1 (Figure 2f), along with a minimum detectable pressure of about 0.76 Pa, which far outperforms other types of pressure sensors. The flexible E-skins constructed on PET are also highly stable against repeated bending cycles. The bending stability test shows that the device exhibits stable performance up to about 1,000 bending cycles while it was bent on a curved surface with a bending radius of about 32.5 mm, which is similar to the radius of a human wrist (Figure 10).

[0042] Tunability of pressure sensing performance. Generally, varied sensitivities in different pressure regimes are specified for specific sensing applications, so tunable sensing performance is desired but challenging. There are several factors in the CMAG design that could affect the capacitance response to an applied pressure, such as the geometry and the spatial arrangement of conductive pyramids (e.g., the size of pyramids and periodicity, respectively), Young's modulus of a substrate as a supporting layer, volume fraction of air within the CMAGs, and so forth. Indeed, the normalized capacitance change, sensitivity, and the pressure sensing range can be readily tailored for specific applications by adjusting these factors. First, it is demonstrated that the geometry and the spatial arrangement of the conductive pyramids can considerably affect the sensing performance. Pyramid #1 (height of about 4.1 μπι, width of about 7.5 μπι, periodicity of about 15.0 μπι) with smaller conductive pyramids and smaller periodicity exhibits higher normalized capacitance change and higher sensitivity but narrower pressure sensing range compared to Pyramid #2 (height of about 12 μπι, width of about 20 μπι, periodicity of about 100 μπι) with larger conductive pyramids and wider periodicity (Figure 3a-d). Pyramid #2 has more space to deform before the conductive microstructured pyramids touch a bottom surface, resulting in a wider pressure sensing range.

[0043] Moreover, investigation is made of the effect of the Young's modulus of the supporting layer on the sensing performance. From the classical stress-strain curve, the lower Young's modulus a material exhibits, the larger deformation will be under the same stress. Therefore, the supporting layer with a lower Young's modulus leads to higher normalized capacitance change and sensitivity compared to that with a higher Young's modulus. To highlight the effect of Young's modulus, investigation is made of the sensitivity of the CMAG devices with and without a PET supporting layer. As observed, the one without PET (exhibiting a lower Young's modulus) is much easier to deform, leading to a record-high sensitivity of up to about 770.4 kPa ^"1 (average sensitivity of about 205.4 kPa ^"1 ) (Figure 3e-f), which is more than about 170 times higher than that of another capacitance-based E-skin. In these ways, one can readily change the pressure sensing range, sensitivity and the normalized capacitance change by applying different Young's modulus of supporting layers or substrates (e.g., PDMS/PET, PET, and so forth), volume fraction of air gaps, geometry and spatial arrangement of the conductive pyramids to satisfy various pressure sensing applications.

[0044] E-skins from CMAG-MoS? transistors. Among different E-skins for pressure sensing, transistor-based E-skins are particularly attractive for several reasons, such as signal amplification by directly converting a capacitance signal to source-drain current, the capability of integrating tactile sensors by constructing active matrix arrays that can provide faster scan rates and less cross-talk between each pixel, and so forth. Comparative transistor- based pressure sensors are usually fabricated using an organic polymer, with relatively low electrical performance, poor air and thermal stability, and large operation voltage of up to about 200 V, which are difficult to implement for practical applications. Here, some embodiments are directed to a design of E-skin integrating MoS ₂ transistors with CMAG gates (Figure lc). In the design, the CMAGs offer "true" microstructured air gaps without unwanted effects caused by a less compressible thick dielectric, and few-layer MoS ₂ is chosen as a semiconducting channel for its higher electron mobility (about 10-100 cm ² /V-s), higher current on/off ratio (about 1 x 10 ⁸ ), excellent mechanical flexibility and air stability. Together, the design can lead to unprecedented sensitivity, along with ultrafast response time, lower power consumption, and long-term air stability and robustness. The pressure sensing performance of the devices can be determined by:

I _sd = j- M - C (V _g - V _t ) - V _sd (4) where I _s d, V _s d, V _t , and V _g are the source-drain current and voltage, threshold voltage, and gate voltage, respectively; W, L, μ, and C, are denoted as the channel width and length, mobility, and specific gate capacitance, correspondingly. It is noted that the source-drain current is proportional to the specific gate capacitance that allows I _s d to respond quickly to the change in an applied pressure. By using the CMAGs as the gate for a MoS ₂ transistor, the applied pressure can readily vary the air-gap thickness within the CMAGs, and thus the gate capacitance can be sensitively read out through the source-drain current.

[0045] The sensitivity and SNR of the devices are related to highly sensitive pressure-dependent CMAGs and affected by the electric performance of the MoS ₂ transistors, such as the transconductance and the current on/off ratio. To achieve higher sensitivity and SNR of the devices, the CMAG-MoS ₂ E-skin is operated near the regime with the highest transconductance in the MoS ₂ transistor, where the source-drain current can sensitively change upon a small pressure load, leading to an ultrahigh sensitivity. A back gate also can be applied to tune the transconductance of the transistor itself. The CMAG-MoS ₂ E-skin exhibits I _sd of about 1.45 ^χ 10 ^"6 A without applied pressure (Figure 4a). As the applied pressure increases to about 1.63 kPa, I _s d dramatically decreases in off-state regime by more than six orders of magnitude, whereas I _s d in on-state regime slightly increases. The normalized channel resistance change (AR/R ₀ , where AR = R - R ₀ ; R and R ₀ are the channel resistance with and without pressure load, respectively) can reach more than about 4.63 x 10 ⁶ and vary with the exact gate voltage applied (Figure 4b). The pressure sensitivity of the E-skin, specified as S = 0(AR/R ₀ )/0P, can reach an unprecedented value of up to about 2.61 ^χ 10 ⁷ kPa ^"1 (average sensitivity of about 3.64 ^χ 10 ⁶ kPa ^"1 ) in the pressure range of about 0-1.63 kPa (Figure 4c). The sensitivity and SNR are the highest among various types of E-skins for pressure detection (about 5.8 ^χ 10 ⁶ , about 1.36 ^χ 10 ⁵ , and about 4 times greater than the sensitivity of some other capacitance-, transistor-, and resistance-based pressure sensors, respectively). Additionally, dielectrics on MoS ₂ transistors can also be tailored to further lower the operation voltage and broaden the pressure sensing range while maintaining the high sensitivity and SNR. The E-skin exhibits higher AR/R ₀ and sensitivity within the pressure range of about 0-3.2 kPa (Figure 4d-f), along with lower power consumption from about 9 pW to about 270 nW (operation voltages, V _s d = about 1 V and V _g = about -3 V), which is significantly lower than that of organic transistor-based pressure sensors (up to 200 V). The stability test of the device responding to a load of about 86 Pa demonstrates that the E-skin maintains highly stable performance over about 6,000 loading cycles (Figure 11).

[0046] Static pressure mapping. In order to meet application specifications for healthcare and FDVII, it is desired to scale up E-skin devices and create sensory arrays for monitoring the spatial distribution of pressure information. As a demonstration, fabrication is carried out of a flexible 5 x 5 pixel array of a CMAG capacitance-based E-skin with the total device area of about 2.5 ^χ about 2.5 cm ² and the area of each pixel of about 6 mm ² (Figure 5 a-d). The E-skin array was fabricated by orthogonally integrating 5 stripe-patterned CMAGs as top electrodes (about 3 -mm wide) with 5 parallel bottom electrodes (about 2-mm wide Au strips with 30 nm-thick A1 ₂ 0 ₃ dielectric) on a PET substrate. For pressure mapping test, grains of rice, soybean, and red bean (with a weight of about 20, about 158, and about 219 mg, respectively) were placed on different pixels in the arrays (Figure 5a). Normalized capacitance change distribution of the E-skin (Figure 5b) clearly shows the response of the device to the placement of different grains, where the brighter color pixel indicates a higher capacitance sensed. Three brighter spots corresponding to the greater compression indicate the locations of the grains and the corresponding pressure variation. A penny (with a weight of about 3.11 g) is placed on the E-skin arrays (Figure 5c) and the normalized capacitance change clearly maps the pressure distribution corresponding to the location of the penny (Figure 5d).

[0047] Human pulse wave measurement. Wearable healthcare devices are of considerable interest for telemedicine. For example, cardiovascular diseases are the leading causes of disability and death globally. A wearable E-skin as a smart device provides a noninvasive way to continually monitor the pulse wave of the radial artery for early detection and prevention of cardiovascular diseases. To demonstrate the ability to monitor the human pulse, the E-skin is attached on a position above the radial artery of an adult human wrist (Figure 5e). At rest, the pulse wave signals of two experimental subjects A (about 169-cm-tall female) and B (about 180-cm-tall male) show the heart rates of about 77 b.p.m. and about 67 b.p.m, respectively (Figure 5f); both are within the normal range between about 60-100 b.p.m. After 5-minute of exercise, both heart rates increase (A: about 120 b.p.m. and B: about 81 b.p.m.) and the pulse waveforms show clear differences compared to the ones before exercising. At rest, the characteristic pulse wave exhibits two clearly distinguishable peaks-percussion peak (Pi) and diastolic peak (P ₃ ), along with a late augmentation shoulder (P ₂ ) (Figure 5g); the shape of the pulse wave is caused by the contraction of the left ventricle and a reflected wave from the lower body, allowing diagnosis of the arterial stiffness from parameters such as Reflection Index (R.I.) = (P ₂ IP ₁ ) ^x 100 % and Arterial Stiffness Index (S.I.) = human subject height (in unit of meter)/transit time (AT _DVP , in unit of second). The transit time is specified as the time delay between first and second peaks. At rest, R.I. and S.I. were calculated to be about 64.6 % and about 4.86, respectively, indicating a healthy state of an adult female. After the 5-minute exercise, the late systolic augmentation (P ₂ ) disappears (Figure 5g), which can be attributed to several reasons such as large artery stiffness/PWV, altered heart rate/ventricular ejection characteristics and so on. Notably, the real-time pulse wave signals can be obtained at a very low operation voltage of about 0.1 V (Figure 12), in stark contrast to other devices that specify operation voltage of up to about 100 V.

[0048] Sound wave detection, speech pattern recognition, and remote pressure monitoring. In order to evaluate the response time and sensitivity performance of the CMAG- MoS ₂ E-skin in detecting the subtle and extremely fast vibration caused by acoustic waves, the device is placed near a computer-controlled speaker. The sound pressure level from the speaker was measured by a sound meter in a normal conversation environment (at about 62 dB). The repeatable, real-time, time-dependent source-drain current variations were recorded while music was played (at about 94 dB, the corresponding pressure of about 1 Pa), clearly demonstrating that the device with high sensitivity and fast response time is able to distinguish different types of music sounds (Figure 6a-c). The measured response time of the E-skin is less than about 0.15 ms when a specific acoustic wave was applied (about 7 kHz, about 100 dB, the corresponding pressure of about 2 Pa), which is the threshold of the measurement setup instead of the intrinsic threshold of the device (Figure 6d).

[0049] Moreover, the E-skin can be used as a speech pattern recognition system in human-robot communication, authentication system, aids of speech visualization in teaching/learning languages, wearable speech training aids for the deaf, and so forth. Comparisons of the acoustic waveform measured by a standing microphone and the device show highly similar characteristic peaks and valleys of different words and sentences such as "UCLA", "Electronic skin", and "The important thing is not to stop questioning" at about 85 dB (the corresponding pressure of about 0.356 Pa), which also indicates the minimum pressure detection < about 0.4 Pa (Figure 6e, Figure 13a-b). In order to give a more visual representation of an acoustic signal, the auditory spectrograms of a computer-controlled speaker played "U-C-L-A" twice were analysed by using the short-time Fourier transform (STFT) (Figure 6f). The repeatable characteristic peaks and patterns of the acoustic waveforms and the auditory spectrograms demonstrate the E-skin is very reliable and stable for recognizing the specific words and sentences.

[0050] Furthermore, demonstration of the remote pressure monitoring capability of the E-skin is made by transmitting pressure signals to user's cell phones or computers by integrating a system on chip (SoC) with the E-skin, serving as a platform of the signal transduction from externally applied stimuli to electrical signal, conditioning and processing, which provides the real-time remote pressure monitoring (Figure 14a-b), allowing the pressure sensing information to be conveyed, such as speech patterns or health information (heartbeat pattern and related parameters for diagnosis of arterial stiffness) with remote doctors.

[0051] In summary, by creating a design of CMAGs and integrating them with high performance 2D semiconductor transistors, realization is made of an improved E-skin device with ultrahigh sensitivity, S R, and fast response time. The flexible CMAG capacitance- based E-skin has shown an ultrahigh sensitivity of up to about 770.4 kPa ^"1 (or more), minimum pressure detection of about 0.76 Pa (or less), broader pressure sensing range of about 0-5 kPa (or more), exceptional tunability, ultrafast response time, and excellent bending stability at very low operation voltage (about 1.5 V or less), allowing large-area, scalable, low-cost, and streamlined fabrication. Its potential applications such as static pressure mapping and real-time health monitoring were also demonstrated. The CMAG- MoS ₂ transistor-based E-skin delivers a record-high sensitivity of up to about 2.61 x 10 ⁷ kPa ^"1 , an extremely fast response time of < about 0.15 ms, low power consumption (about 9 pW - about 270 nW), minimum pressure detection < about 0.4 Pa, and outstanding air stability and robustness, allowing the repeatable real-time acoustic waves monitoring, remote pressure monitoring, and human speech pattern recognition. Both E-skins have shown unprecedented sensitivity and excellent performance among different types of E-skins for pressure sensing, which can open up opportunities for ultrasensitive, fast-response, low- power, air-stable and low-cost pressure sensing applications.

[0052] Advantageously, CMAGs provide "real" microstructured air gaps which can overcome issues faced by other microstructured air-gap pressure sensors (e.g., low pressure sensitivity, slow response time, low SNR, low normalized sensing signals (normalized capacitance or normalized current), and so forth). Also, CMAGs can be readily applied to different types of pressure sensors such as capacitance- and transistor-based pressure sensors.

[0053] Although conductive coatings on microstructured elastomers are demonstrated as CMAGs for some embodiments, CMAGs can be fabricated by other types of conductive materials for various applications. For example, CMAGs can be formed of conductive materials such as metals, metal alloys, carbon nanotubes (CNTs), graphene, three- dimensional (3D) graphene foams, conductive polymers, and so forth. Criteria of a conductive material used to form CMAGs include (1) conductivity and (2) relative flexibility. As another example, CMAGs can be formed by directly casting conductive elastomeric microstructured composites (e.g., elastomers mixed with conductive fillers such as CNTs, graphene, 3D graphene foams, and so forth), instead of coating a conductive layer on a microstructured elastomer. In addition, in order to fulfill the specifications for flexible or stretchable applications, other elastomers can be used in place or in combination with PDMS, such as silicone rubber (e.g., available under Ecoflex®), polyurethane (PU) and so on. Supporting layers or substrates can also be removed or omitted if desired. The gate-absent transistors can also be made by various materials (e.g., semiconductor polymers, CNTs, transition metal dichalcogenides (TMD), and so on). Moreover, the CMAG-based pressure sensors exhibit excellent tunability (e.g., pressure ranges, sensitivity, and so on) by changing the Young's modulus of the supporting layers or substrate, volume fraction of air gaps, geometry and spatial arrangement of conductive pyramids to satisfy various pressure sensing applications.

[0054] Although different types of pressure sensors have been proposed, few pressure sensors focus on low to ultralow pressure range (e.g., pressure < about 1-5 kPa) with ultrahigh pressure sensitivity, fast response time, low operation voltage, low-cost, and long-term stability.

[0055] Comparative microstructured air-gap devices have a conductive layer on a flat backside of microstructured PDMS (Figure 2a). With this design, the total capacitance is approximately contributed by two capacitors in series: an elastomer-based capacitor (which has relatively small change upon compression) and an air-gap capacitor that responds sensitively to the mechanical deformation. Therefore, in this design, the thinner the elastomer dielectric is, the higher the sensitivity and SNR of the E-skin. However, it is of considerable challenge to aggressively shrink the thickness of the elastomer while maintaining the microstructured air gaps, which has thus constrained the normalized capacitance change and sensitivity of such devices. Also, the response time is restricted by the natural viscoelastic behavior of elastomers. On the contrary, with the design of the CMAGs, the capacitance is approximately solely or primarily contributed by the air-gap component, so the change of the capacitance under an applied pressure is determined by the deformation of the microstructured air gaps without considering the effects of the elastomer dielectric, which leads to greatly enhanced sensitivity, normalized capacitance change, and SNR. Moreover, this "true" microstructured air-gap design reduces the impact by the unwanted viscoelastic behavior of elastomers, thus leading to a fast response time. [0056] In addition to ultrahigh pressure sensitivity and fast response time, the CMAG-based pressure sensors also have one or more of the following advantages:

[0057] 1. The design of the CMAGs can be applied for different types of pressure sensors. For example, the CMAGs can be integrated with various types of top gate-absent transistors to meet different pressure sensing applications.

[0058] 2. The structure of the CMAG-based pressure sensors can be fabricated by low-cost solution process, allowing fabrication of large-area pressure sensing arrays.

[0059] 3. The CMAG-based pressure sensors can be very robust, stable, and reliable by appropriate choice of materials.

[0060] 4. The operating voltages are relatively low compared to other pressure sensors.

[0061] 5. The CMAG-based pressure sensors can be applied for advanced practical applications due to their enhanced pressure sensing performance.

[0062] Therefore, the CMAG-based pressure sensors can pave a path for advanced pressure sensors and related applications. The CMAG-based pressure sensors can be used for applications such as speech pattern recognition systems, healthcare monitoring (e.g., human pulse wave, heartbeat patterns, breath patterns), acoustic wave detection, remote pressure monitoring, static pressure mapping, and "microphone-like" E-skin.

[0063] Example Embodiments:

[0064] In a first aspect according to some embodiments, a pressure sensor includes: a first electrode layer; a second electrode layer including a plurality of conductive micro structures extending toward the first electrode layer; and a dielectric layer between the first electrode layer and the second electrode layer.

[0065] In some embodiments of the first aspect, the second electrode layer includes a base layer including a plurality of protrusions, and a conductive coating covering the plurality of protrusions to form the plurality of conductive microstructures.

[0066] In some embodiments of the first aspect, the base layer includes an elastomer, and the conductive coating includes a conductive material. Examples of suitable elastomers include silicones (such as PDMS and silicone rubber), PU, acrylic elastomers, and combinations thereof, and examples of suitable conductive materials include metals, metal alloys, conductive polymers, or combinations thereof.

[0067] In some embodiments of the first aspect, the second electrode layer includes an elastomer and conductive fillers dispersed in the elastomer. Examples of suitable conductive fillers include carbonaceous fillers (such as CNTs, graphene, and 3D graphene foams) and other particulate fillers formed of conductive materials.

[0068] In some embodiments of the first aspect, heights of the plurality of conductive microstructures are in a range of about 1 μπι to about 500 μπι, about 1 μπι to about 400 μπι, about 1 μπι to about 300 μπι, about 1 μπι to about 200 μπι, about 1 μπι to about 100 μπι, or about 1 μπι to about 50 μπι, widths of the plurality of conductive microstructures are in a range of about 1 μπι to about 500 μπι, about 1 μπι to about 400 μπι, about 1 μπι to about 300 μπι, about 1 μπι to about 200 μπι, about 1 μπι to about 100 μπι, or about 1 μπι to about 50 μπι, and a periodicity of the plurality of conductive microstructures are in a range of about 1 μπι to about 500 μπι, about 1 μπι to about 400 μπι, about 1 μπι to about 300 μπι, about 1 μπι to about 200 μπι, about 1 μπι to about 100 μπι, or about 1 μπι to about 50 μπι.

[0069] In some embodiments of the first aspect, the plurality of conductive microstructures are spaced apart to form gaps therebetween.

[0070] In some embodiments of the first aspect, the dielectric layer includes a ceramic material (such as a metal oxide, a non-metal oxide, a metal nitride, or a non-metal nitride), a polymer, or a combination thereof.

[0071] In some embodiments of the first aspect, the first electrode layer includes a conductive material. Examples of suitable conductive materials include those set forth for the second electrode layer.

[0072] In some embodiments of the first aspect, the pressure sensor further includes a first substrate supporting the first electrode layer and affixed to a side of the first electrode layer facing away from the second electrode layer. In some embodiments, the first substrate, the first electrode layer, and the dielectric layer constitute a first stack.

[0073] In some embodiments of the first aspect, the pressure sensor further includes a second substrate supporting the second electrode layer and affixed to a side of the second electrode layer facing away from the first electrode layer. In some embodiments, the second substrate and the second electrode layer constitute a second stack.

[0074] In a second aspect according to some embodiments, a pressure sensor includes: M elongated strips and N elongated strips. The M elongated strips extend crosswise relative to the N elongated strips to define M ^χ N intersections, and the pressure sensor has a variable capacitance in response to an applied pressure so as to define M ^χ N pressure sensor pixels at locations corresponding to the x N intersections. In some embodiments, is 2 or more, 3 or more, 4 or more, or 5 or more, and N is 2 or more, 3 or more, 4 or more, or 5 or more. In some embodiments, Mis equal to N. In some embodiments, each of the M elongated strips includes the first electrode layer and the dielectric layer as set forth for some embodiments of the first aspect, and each of the N elongated strips includes the second electrode layer as set forth for some embodiments of the first aspect. In some embodiments, each of the M elongated strips includes the first stack (constituted of the first substrate, the first electrode layer, and the dielectric layer) as set forth for some embodiments of the first aspect, and each of the N elongated strips includes the second stack (constituted of the second substrate and the second electrode layer) as set forth for some embodiments of the first aspect.

[0075] In a third aspect according to some embodiments, a pressure sensor includes: a source electrode; a drain electrode; a channel extending between the source electrode and the drain electrode; an electrode layer including a plurality of conductive microstructures extending toward the channel; and a dielectric layer between the electrode layer and the channel.

[0076] In some embodiments of the third aspect, the electrode layer includes a base layer including a plurality of protrusions, and a conductive coating covering the plurality of protrusions to form the plurality of conductive microstructures.

[0077] In some embodiments of the third aspect, the base layer includes an elastomer, and the conductive coating includes a conductive material. Examples of suitable elastomers and suitable conductive materials include those set forth for some embodiments of the first aspect.

[0078] In some embodiments of the third aspect, the electrode layer includes an elastomer and conductive fillers dispersed in the elastomer. Examples of suitable conductive fillers include those set forth for some embodiments of the first aspect.

[0079] In some embodiments of the third aspect, heights, widths, and a periodicity of the plurality of conductive microstructures are in ranges as set forth for some embodiments of the first aspect.

[0080] In some embodiments of the third aspect, the plurality of conductive microstructures are spaced apart to form gaps therebetween.

[0081] In some embodiments of the third aspect, the dielectric layer includes a ceramic material (such as a metal oxide, a non-metal oxide, a metal nitride, or a non-metal nitride), a polymer, or a combination thereof. [0082] In some embodiments of the third aspect, the channel includes a semiconductor material, such as a semiconductor polymer, CNTs, a transition metal dichalcogenide, or a combination thereof.

[0083] In some embodiments of the third aspect, the source electrode, the drain electrode, the channel, and the dielectric layer constitute a gate-absent transistor, and the electrode layer constitutes a pressure sensitive gate of the gate-absent transistor.

[0084] In some embodiments of the third aspect, the pressure sensor further includes a first substrate supporting the source electrode, the drain electrode, the channel, and the dielectric layer and affixed to a side of the channel facing away from the electrode layer. In some embodiments, the first substrate, the source electrode, the drain electrode, the channel, and the dielectric layer constitute a first stack.

[0085] In some embodiments of the third aspect, the pressure sensor further includes a second substrate supporting the electrode layer and affixed to a side of the electrode layer facing away from the channel. In some embodiments, the second substrate and the electrode layer constitute a second stack.

[0086] In a fourth aspect according to some embodiments, a pressure sensor includes: M elongated strips and N elongated strips. The M elongated strips extend crosswise relative to the N elongated strips to define M ^χ N intersections, and the pressure sensor has a variable capacitance in response to an applied pressure so as to define M ^χ N pressure sensor pixels at locations corresponding to the A/ ^χ N intersections. In some embodiments, is 2 or more, 3 or more, 4 or more, or 5 or more, and N is 2 or more, 3 or more, 4 or more, or 5 or more. In some embodiments, M is equal to N. In some embodiments, at each of the M ^χ N intersections, a corresponding one of the M elongated strips includes the source electrode, the drain electrode, the channel, and the dielectric layer as set forth for some embodiments of the third aspect, and a corresponding one of the N elongated strips includes the electrode layer as set forth for some embodiments of the third aspect. In some embodiments, at each of the M ^χ N intersections, a corresponding one of the M elongated strips includes the first stack (constituted of the first substrate, the source electrode, the drain electrode, the channel, and the dielectric layer) as set forth for some embodiments of the third aspect, and a corresponding one of the N elongated strips includes the second stack (constituted of the second substrate and the electrode layer) as set forth for some embodiments of the third aspect. Example

[0087] The following example describes specific aspects of some embodiments of this disclosure to illustrate and provide a description for those of ordinary skill in the art. The example should not be construed as limiting this disclosure, as the example merely provides specific methodology useful in understanding and practicing some embodiments of this disclosure.

[0088] Methods:

[0089] CMAG fabrication. The CMAGs were formed by casting PDMS in a prefabricated silicon (Si) mold (Figure 7). The microstructured Si mold was made from <100> Si wafers with about 300-nm-thick Si0 ₂ . The Si substrate was first patterned using photolithography to produce a square array of open windows with exposed Si0 ₂ . The exposed Si0 ₂ was stripped using buffered hydrofluoric acid (BOE), and the remaining Si0 ₂ was used as a mask for potassium hydroxide (KOH) etching, followed by octadecyltrichlorosilane (OTS) surface treatment to facilitate the subsequent release of PDMS cast from the Si mold. An about 1 :5 mixture of curing agent to PDMS elastomer (Sylgard 184, Dow Corning) was prepared, diluted with hexane (about 1 : 10 PDMS mixture to hexane), and then stirred overnight. The diluted solution was filtered by 0.2^m-PTFE-membrane filter, transferred onto the mold, and then degassed. An about 125^m-thick PET (MELINEX ST505 5.0 mil) was plasma-treated for about 10 min and placed on top of the degassed PDMS with the mold in vacuum, followed by baking at a temperature of about 120 °C for about 4 hours. The microstructured PDMS pyramids supported by the PET substrate were peeled off from the Si mold. A conductive metal thin film (Cr/Au: about 20 nm/about 80 nm) layer was then deposited on the microstructured PDMS by electron-beam (e-beam) evaporation. A rotating sample holder was used during the e-beam process to ensure a continuous conductive layer covering the entire microstructured surface.

[0090] Device Fabrication. A flexible 5 x 5 capacitance-based E-skin array was assembled by orthogonally laminating 5 conductive microstructured PDMS strips (about 3 mm wide, supported on a PET supporting layer) on 5 parallel electrodes strips (about 2 mm wide, covered by about 30-nm-thick A1 ₂ 0 ₃ dielectric) on a PET substrate. The parallel electrodes were deposited by e-beam evaporation of Cr/Au (about 20 nm/about 80 nm), and A1 ₂ 0 ₃ dielectric was deposited by atomic layer deposition (ALD). The total area of the array is about 2.5 ^χ about 2.5 cm ² and the area of each pixel is about 6 mm ² . The device for human pulse wave measurement assembled by orthogonally laminating the top stack (CMAGs/PET) on the bottom stack (about 30 nm Al ₂ 0 ₃ /about 80 nm Au/about 20 nm Cr/PET) with an overlapping area of about 1 ^χ about 1 cm ² . For the fabrication of CMAG-M0S ₂ E-skin, few- layer M0S ₂ was exfoliated from commercially available crystals of molybdenite onto Si0 ₂ substrate followed by lithography process to form the source-drain electrodes (Ti/Au: about 20 nm/about 80 nm). A polymeric dielectric layer was deposited by spin casting a dielectric solution (poly-4-vinylphenol (PVP) mixed with 4,4'-(Hexafluoroisopropylidene)diphthalic anhydride (HDA) and triethylamine (TEA) in propylene glycol monomethyl ether acetate (PGMEA)), followed by curing at about 150 °C in air for about 30-60 min. An additional about 5- or about 10-nm-thick AI ₂ O ₃ was deposited by ALD. The CMAGs were then laminated on top of gate-absent MoS ₂ transistors to obtain CMAG-M0S ₂ transistor based E- skin devices.

[0091] Device Characterization. SEM images of the conductive microstructured pyramids were taken using TESCAN VEGA3. Capacitance measurements were taken using an Agilent E4980A LCR meter at about 1 MHz frequency with about 1.5 V a.c. signal under ambient conditions. Pressure was applied by loading various weights on the E-skin. Bending stability test of flexible capacitance-based E-skin responding to a load of about 86 Pa was done while the device was placed on a curved surface with a bending radius of about 32.5 mm. Human pulse wave measurement was carried out by attaching the flexible capacitance- based E-skin to the wrist, with the CMAG side facing the skin. Electrical characterization of the CMAG-M0S ₂ E-skin was done under ambient conditions using an Agilent B2902A or built-in data acquisition (DAQ) computer connected with low noise current preamplifier (SR570), and loads were applied by various weights. For the measurements of acoustic wave detection, a computer-controlled speaker served as an acoustic source while continuously recording the I _sc i at constant voltages (V _sc i = about 1 V). A sound meter was used to measure the sound pressure level produced by the speaker (in units of decibels, dB). As a result, the sound pressure (P) can be s ecified by the following equation: where P ₀ = about 20 μPa (the reference sound pressure in air) and L _dB denotes a measured sound pressure level. For remote pressure sensing and reading, a Raspberry Pi microcomputer was integrated with the CMAG-M0S ₂ E-skin for transmitting the signal to a remote computer or cell phone. The variation of source-drain current at constant voltages was recorded by the system and the results can be accessed through the Internet, so the current response upon a pressure applied by a finger can be monitored remotely. All control and application programs were written in Python language.

[0092] Supplementary Note:

[0093] Parallel plate capacitor model. Typically, comparative microstructured air- gap devices include a conductive layer on a flat backside of a microstructured PDMS. With this design, the total capacitance is contributed by two capacitors in series: an elastomer- based capacitor (which has a relatively small change upon compression) and an air-gap capacitor that responds sensitively to mechanical deformation. In order to simply the evaluation of the comparative and the proposed CMAG devices, a parallel plate capacitor model is assumed without the device area change under an applied pressure.

[0094] In this model, the comparative structure includes two parallel capacitors in series, with one highly compressible air-gap capacitor and one less compressible PDMS ca acitor. The capacitance change can thus be expressed as:

(SI)

where A is the device (= about 1.0) and E _PDMS (about 3.0) are the dielectric constants of air and PDMS, respectively; MA* (= - <¾r) and Ad _PDM s (= d _PDM s - d' _PDM s) are the thickness change of air and PDMS, correspondingly; and <¾ _;r , <¾ _;r , d _PDM s and d' _PDM s are denoted as the initial and final thickness of air and PDMS, respectively. Then, the normalized capacitance change can be expressed as:

^ ^■ Conventional _ ^£ PDMS^Air ^~ * ^{~ £} Air^PDMS

^o Air PDMS PDMS Air (g2)

[0095] Typically, the stiffness of PDMS is about 0.5-3.7 MPa; and the thickness of PDMS and air gap are about 10-500 μπι and about 2-3 μπι, respectively. Under an applied pressure of less than about 10 kPa, d' _PDM s approximately equals to d _PDM s, resulting in AdpDMS « S _PDM S AdAir- Therefore, the approximated capacitance change equation for the comparative air-gap device can be obtained as follows: [0096] On the contrary, with the proposed conductive air-gap design, the capacitance is solely or primarily contributed by the air-gap component. As a result, capacitance change can be expressed as:

= C -C = ^ ^£λ ' ^ ^£λ ' ₌ A ^£ Air(d _Air -d _Air ) _ A _Ajr Ad _Ajr

Conductive o

aAir ^a Air ^Q Air ^Q Air " Air ⁶¹ Air (§4)

[0097] Thus, the normalized capacitance change is expressed as following:

Conductive _ ^ Air

C _n d'

^*Air (S5)

[0098] Substitution of ¾>, e _PDM s, d^ and d _PDM s as about 1, about 3, about 4.1 μιτι, and about 20 μιτι, respectively, into the equations (S3) and (S5) yields the plots shown in Figure 8.

[0099] As used herein, the singular terms "a," "an," and "the" may include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to an object may include multiple objects unless the context clearly dictates otherwise.

[00100] As used herein, the term "set" refers to a collection of one or more objects. Thus, for example, a set of objects can include a single object or multiple objects.

[00101] As used herein, the terms "connect," "connected," and "connection" refer to an operational coupling or linking. Connected objects can be directly coupled to one another or can be indirectly coupled to one another, such as via one or more other objects.

[00102] As used herein, the terms "substantially" and "about" are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. For example, when used in conjunction with a numerical value, the terms can refer to a range of variation of less than or equal to ±10% of that numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%), less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.

[00103] Additionally, concentrations, amounts, ratios, and other numerical values are sometimes presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be understood flexibly to include numerical values explicitly specified as limits of a range, but also to include all individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly specified. For example, a range of about 1 to about 200 should be understood to include the explicitly recited limits of about 1 and about 200, but also to include individual values such as about 2, about 3, and about 4, and sub-ranges such as about 10 to about 50, about 20 to about 100, and so forth.

[00104] While the disclosure has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the disclosure as defined by the appended claims. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, method, operation or operations, to the objective, spirit and scope of the disclosure. All such modifications are intended to be within the scope of the claims appended hereto. In particular, while certain methods may have been described with reference to particular operations performed in a particular order, it will be understood that these operations may be combined, sub-divided, or re-ordered to form an equivalent method without departing from the teachings of the disclosure. Accordingly, unless specifically indicated herein, the order and grouping of the operations are not a limitation of the disclosure.