DESC:TECHNICAL FIELD
[1] The present disclosure relates, in general, to pressure sensing devices. In particular, the present disclosure relates to an optofluidic low-pressure sensor.

BACKGROUND
[2] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[3] A pressure sensor is a device which detects a pressure signal and converts it into an output electric signal according to certain mechanisms. Precision low-pressure monitoring has gained tremendous importance due to its application in many critical healthcare and engineering fields. The application of low-pressure measurement sensor can be found in many fields including, biomedical engineering, microfluidic lab on a chip, heating, ventilation, and air conditioning (HVAC), and space applications, etc. The application of the sensor in biomedical engineering includes incorporation of low-pressure measurement sensors in respiratory signal detection devices, Deep Vein Thrombosis, Infusion Pumps, inflatable mattresses, and blood pressure measurement etc. These machines need accurate pressure sensors to maintain constant pressure or controlled flow.
[4] Particularly, on chip precision low-pressure monitoring has gained tremendous importance due to the recent increase of interest in lab-on-a-chip device and its role in many critically important healthcare and engineering applications. Accurate monitoring of fluidic pressure inside the microfluidic system is essential for the success of many miniatured lab-on-a-chip devices.
[5] An important bio-medical application for a low-pressure sensor can be in diagnosing respiratory disorders by detecting a low-pressure respiratory signal from a subject.
[6] Although there are many low-pressure detection sensors in the market, however, most of the of the sensor are either microelectromechanical system (MEMS) or optical fibre-based sensors. In the case of MEMS based low-pressure sensors, fabrication is complicated and requires sophisticated microfabrication techniques which includes Electron Beam Lithography (EBL), Optical photolithography, Deep Reactive Ion Etching, LIGA, etc., all of which require cleanroom facilities. As a result, MEMS based sensor are expensive. On the other hand, optical fibre-based sensor requires sophisticated analysing devices such as UV-Vis reflection spectrometer, precise aligning aids, image capturing module etc., making them expensive and bulky.
[7] There is, therefore, a requirement in the art for an accurate and portable pressure sensor that is easy to fabricate and economical. Additionally, it is preferable that the pressure sensor can be adapted for integration with a lab-on-chip device.

OBJECTS OF THE PRESENT DISCLOSURE
[8] An object of the present disclosure relates in general, to pressure sensing devices. In particular, the present disclosure relates to an optofluidic low-pressure sensor.
[9] Another object of the present disclosure is to provide a device that easily fabricates LDP based optofluidic low-pressure monitoring sensor that works on the principle of total internal reflection (TIR) of light.
[10] Another object of the present disclosure is to provide a device that measures the change in intensity of light caused by the applied pressure through a detector without using sophisticated analyzing devices such as microscope, camera, spectrometer and the likes.
[11] Another object of the present disclosure is to provide a device that is fabricated using a well-developed mould-based layer-by-layer soft lithographic (LSL) and 3D printing technique without involving sophisticated cleanroom microfabrication.
[12] Another object of the present disclosure is to provide a device that is cost effective, compact and highly sensitive.
[13] Another object of the present disclosure is to provide a device that is deployed in in critical healthcare and measures the respiratory pressure signal of normal human at various posture and conditions.
[14] Another object of the present disclosure is to provide a device that enables early detection of fatal biomedical ailment such as chronic respiratory disease (CRDs), severe acute respiratory syndrome (SARS), Chronic obstructive pulmonary disease (COPD) and sleep apnea and the likes.
[15] Yet another object of the present disclosure provides a device that is robust and portable.

SUMMARY
[16] The present disclosure relates in general, to pressure sensing devices. In particular, the present disclosure relates to an optofluidic low-pressure sensor.
[17] In an aspect, the present disclosure provides a device for low-pressure measurement, the device includes a liquid-Dove prism (LDP) adapted to be placed in a housing, which is filled with a liquid having a higher refractive index (RI), wherein a top surface of the LDP is covered by a flexible membrane of suitable thickness, a light source configured at a first side of sloped edges of the LDP, the light source adapted to supply light rays to the LDP, a microchannel having an inlet and an outlet configured above the flexible membrane, wherein a finite pressure applied at the inlet of the microchannel causes deflection of the flexible membrane of the LDP, the deflection causes deviation of the path of total internally reflected light, resulting in variation of the intensity of light at the output and a detector configured at a second side of sloped edges of the LDP, the detector adapted to detect changes in intensity of light travelling through the LDP, wherein based on the change in the intensity of light a corresponding voltage signal is collected from the detector to determine pressure variations.
[18] According to an embodiment, the device configured to monitor respiratory distress of subjects by measuring the respiratory pressure signal at various posture and conditions, wherein the device can be used as a force sensor.
[19] According to an embodiment, at zero applied pressure, the simulated light rays undergo total internal reflection (TIR) at the top surface of the LDP and are received at the detector, wherein at the application of finite pressure, the simulated light rays enter the LDP undergo TIR from the deflected membrane, the deviation of the light rays undergoing TIR increased with increase in deflection of the flexible membrane results in drop in intensity of light detected by the detector.
[20] According to an embodiment, the device can include a pressure release chamber (PRC) that is fluidically coupled to the LDP through a channel, wherein upon application of finite pressure at the inlet, a depression occurs at the flexible membrane above the LDP and the liquid displaced in the housing of LDP due to the deflection of the flexible membrane flows into the PRC, wherein a membrane above the PRC bulges out to compensate the depression of the flexible membrane of LDP.
[21] According to an embodiment, the liquid of high RI can be any or a combination of immersion oil (IO) and di-iodomethane (DI).
[22] According to an embodiment, the device includes a manometer coupled to the outlet of the microchannel to calibrate the pressure of the LDP, wherein values obtained from the detector voltage and from the manometer are compared for calibration.
[23] According to an embodiment, the voltage signals from the detector are collected using a data acquisition system (DAQ) with a sampling frequency of about 1000 samples per second, wherein the DAQ operatively coupled to the detector.
[24] According to an embodiment, the light source and the detector are aligned accurately to provide optimal sensitivity to the LDP, wherein the light source and detector, once aligned, are secured in position by 3D printed cages, wherein a power supply is adapted to supply power to the light source.
[25] According to an embodiment, the device is fabricated using a well-developed mould-based layer-by-layer soft lithographic (LSL) and 3D printing technique.
[26] In an aspect, the present disclosure provides a method for low-pressure measurement, the method includes placing a liquid-Dove prism (LDP) in a housing, which is filled with a liquid having a higher refractive index (RI), wherein a top surface of the LDP is covered by a flexible membrane of suitable thickness, supplying, from a light source, light rays to the LDP, the light source configured at a first side of sloped edges of the LDP, configuring, a microchannel having an inlet and an outlet above the flexible membrane, wherein a finite pressure applied at the inlet of the microchannel causes deflection of the flexible membrane of the LDP, the deflection causes deviation of the path of total internally reflected light, resulting in variation of the intensity of light and detecting, at a detector configured at a second side of sloped edges of the LDP, the change in intensity of light travelling through the LDP, wherein based on the change in the intensity of light a corresponding voltage signal is collected from the detector to determine pressure variations.
[27] Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF DRAWINGS
[28] The accompanying drawings are included to provide further understanding of the present disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure. The diagrams are for illustration only, which thus is not a limitation of the present disclosure.
[29] FIGs. 1A and 1B illustrate exemplary representations of a liquid-Dove prism (LDP) - based low pressure measurement sensor, in accordance with an embodiment of the present disclosure.
[30] FIG. 1C illustrate exemplary representations of two liquid-Dove prism (LDP) - based low pressure measurement sensor, in accordance with an embodiment of the present disclosure.
[31] FIG. 1D illustrate exemplary representations of using the device as a force sensor, in accordance with an embodiment of the present disclosure.
[32] FIG. 2 illustrates an exemplary representation of working principle of the proposed sensor, in accordance with an embodiment of the present disclosure.
[33] FIGs. 3A – 3C illustrate exemplary representation of a process to fabricate the proposed low-pressure sensor, in accordance with an embodiment of the present disclosure.
[34] FIGs. 4A and 4B illustrate exemplary representations of ray tracing simulations in the proposed sensor to determine path of light rays through the LDP, in accordance with an embodiment of the present disclosure.
[35] FIG. 5A illustrates an exemplary experimental set-up for calibrating the proposed LDP sensor, in accordance with an embodiment of the present disclosure.
[36] FIG. 5B illustrates an exemplary plot of calibration of the LDP sensor in terms of pressure and light sensitivity, for differing thicknesses of the flexible membrane, in accordance with an embodiment of the present disclosure.
[37] FIG. 5C illustrates an exemplary plot of calibration of the LDP sensor in terms of time-domain response for a membrane thickness of 200 µm, in accordance with an embodiment of the present disclosure.
[38] FIG. 5D illustrates an exemplary plot comparing calibration of an LDP sensor filled with IO and one filled with DI, in accordance with an embodiment of the present disclosure.
[39] FIG. 6A illustrates an exemplary plot demonstrating the relaxation time of the proposed LDP sensor, in accordance with an embodiment of the present disclosure.
[40] FIG. 6B illustrates an exemplary plot demonstrating the response time of the proposed LDP sensor, in accordance with an embodiment of the present disclosure.
[41] FIGs. 7A and 7B illustrate repeatability of the LDP sensor, in accordance with an embodiment of the present disclosure.
[42] FIG. 8 illustrates an exemplary plot for calibration of flowrate using the LDP sensor, in accordance with an embodiment of the present disclosure.
[43] FIG. 9A illustrates an exemplary representation of set-up for obtaining a respiratory signal from a human subject, in accordance with an embodiment of the present disclosure.
[44] FIG. 9B illustrates exemplary plot of RPS values for a healthy male human subject at different postures, in accordance with an embodiment of the present disclosure.
[45] FIG. 9C illustrates an exemplary plot of time-domain of a single respiratory pressure signal, in accordance with an embodiment of the present disclosure.
[46] FIG. 9D illustrates an exemplary plot of frequency-domain of a single respiratory pressure signal, in accordance with an embodiment of the present disclosure.
[47] FIG. 9E illustrates an exemplary demonstration of portability of the proposed LDP sensor, in accordance with an embodiment of the present disclosure.
[48] FIG. 10 illustrates an exemplary method for low-pressure measurement, in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION
[49] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such details as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[50] The present disclosure relates, in general, to pressure sensing devices. In particular, the present disclosure relates to an optofluidic low-pressure sensor.
[51] FIGs. 1A and 1B illustrate exemplary representations of a liquid-Dove prism (LDP) - based low pressure measurement sensor, in accordance with an embodiment of the present disclosure. A dove prism is derived from a truncated right-angle prism, ABCD, as shown.
[52] In an embodiment, the low-pressure measurement sensor 100 (herein, also referred to as “sensor”) includes a liquid-Dove prism (LDP) 102 having a flexible top membrane 104, which is provided below a microchannel 106. The LDP 102 is placed in a housing 108, which is filled with a liquid having a higher refractive index (RI) than its surrounding components, including the LDP 102. In an exemplary embodiment, the RI of the liquid (?3) can be about 1.5.
[53] In another embodiment, the microchannel 106 is provided with an inlet 110 and an outlet 112.
[54] In another exemplary embodiment, the flexible membrane 104 can be made of polydimethylsiloxane (PDMS), which has an RI (?2) of about 1.4.
[55] In another embodiment, the housing 108 is placed in a medium (such as ambient air), which has an RI (?1) of about 1.
[56] In another embodiment, the sensor 100 includes a light source 114 and a detector 116 placed such that the detector 116 is configured to detect light from the light source 114 after the light has passed through the LDP 102.
[57] In another embodiment, the light source 114 can be coupled with a stopper 118 operable to restrict or limit light emitted from the light source 114.
[58] It may be appreciated that suitable optics such as lenses, collimators, condensers etc. are also provided.
[59] The sensor 100 of the present disclosure operates based on the phenomenon of total internal reflection of light.
[60] FIG. 2 illustrates an exemplary representation of working principle of the proposed sensor, in accordance with an embodiment of the present disclosure.
[61] When light travels from a medium of high RI to a medium of low RI, at an angle of incidence above a critical angle (?C), light undergoes total internal reflection (TIR), where complete reflection of light occurs without partial reflection. When light rays from the light source 114 are incident at a sloping edge of the LDP 102 at an angle greater than (?C), they undergo TIR.
[62] Referring to FIG. 1A, when pressure at inlet 110 is zero, light from the light source 114 emitted in a direction parallel to a longitudinal axis of the LDP 102 is incident on a sloping edge of the LDP 102, light is refracted towards top surface of the LDP 102 to undergo TIR. The reflected light is refracted out of the LDP 102 towards the detector 116. The detector 116 reads intensity of received light and generates a corresponding voltage signal.
[63] Referring to FIG. 1B, when a finite pressure is applied at inlet 110, the membrane 104 undergoes deflection, consequently causing deflection of the top surface of the LDP 102. This results in corresponding deflection of light undergoing TIR, and consequently, some of the deflected light rays deviate away from the detector 116. The detector 116 hence, receives only a portion of the light from the LDP 102, thus receiving light which has an intensity that is typically lower than when the top surface of the LDP 102 is not deflected. The change in intensity results in a change in voltage signal generated by the detector 116, which can be correlated to the amount of deflection of the top surface of the LDP 102, which, in turn, depends on the pressure of the inlet 110 of the microchannel 106.
[64] In an embodiment, the low-pressure measurement sensor 100 also interchangeably referred to as device 100 that can include LDP 102 adapted to be placed in the housing 108, which is filled with the liquid having a higher RI, where the top surface of the LDP is covered by the flexible membrane 104 of suitable thickness. The light source 114 configured at a first side of sloped edges of the LDP 102, the light source adapted to supply light rays to the LDP 102. The microchannel 106 having the inlet 110 and the outlet 112 configured above the flexible membrane 104, where a finite pressure applied at the inlet 110 of the microchannel 106 causes deflection of the flexible membrane 104 of the LDP, the deflection causes deviation of the path of total internally reflected light, resulting in variation of the intensity of light.
[65] The detector 116 configured at a second side of sloped edges of the LDP 102, the detector 116 adapted to detect changes in intensity of light travelling through the LDP, where based on the change in the intensity of light a corresponding voltage signal is collected from the detector 116 to determine pressure variations. The voltage signals from the detector 116 are collected using a data acquisition system (DAQ) 506 (as illustrated in FIG. 5A) with a sampling frequency of about 1000 samples per second, where the DAQ operatively coupled to the detector 116. The device 100 configured to monitor respiratory distress of subjects by measuring the respiratory pressure signal at various posture and conditions.
[66] FIG. 1C illustrate exemplary representations of two liquid-Dove prism (LDP) - based low pressure measurement sensor, in accordance with an embodiment of the present disclosure. As depicted in FIG. 1C, instead of using one liquid filled Dove prism 102, two oppositely arranged liquid filled Dove prisms can be used, where the two oppositely arranged liquid filled Dove prisms may include light source (114-1, 114-2 (which are collectively referred to as light source 114, hereinafter)), detectors (116-1, 116-2 (which are collectively referred to as detector 116, hereinafter)) and stoppers (118-1, 118-2 (which are collectively referred to as stopper 118, hereinafter)). The two oppositely arranged liquid filled Dove prism can be arranged for enhancing the pressure detection range and sensitivity of the sensor.
[67] FIG. 1D illustrate exemplary representations of using the device as a force sensor, in accordance with an embodiment of the present disclosure. The LDP mounted on a hard material and sloped edges made of hard transparent material e.g., glass slide. The device 100 can also be used as a force sensor.
[68] FIGs. 3A – 3C illustrate exemplary representation of a process to fabricate the proposed low-pressure sensor, in accordance with an embodiment of the present disclosure. In an embodiment, fabrication of the sensor 100 occurs in three main steps: fabrication of a mould for the LDP; fabrication of the LDP; and installation of the optics, including the light source and detector.
[69] Referring to FIG. 3A, in an embodiment, the mould 302 is resin based and is inverted prism shaped to hold the LDP. A material such as polylactic acid (PLA) is used to make the mould 302, which can include a cavity 304 to hold the LDP and can have an open top surface. In an exemplary embodiment, the mould 302 can be fabricated using 3D printing.
[70] In another embodiment, the sides of the cavity 304 can be covered with smooth cellulose acetate sheets to make them optically smooth. The cavity 304 is then covered with a curable resin 306 and covered with a glass plate.
[71] In an exemplary embodiment, the resin may be UV curable.
[72] In another exemplary embodiment, the glass plate may be coated with monolayer of hydrophobic molecule Octadecyl trichlorosilane (OTS) for smooth removal of the cured resin from glass plate.
[73] The resin 306 is cured under UV light to obtain a resin prism 308.
[74] Referring to FIG. 3B, in an embodiment, the prism 308 is taken out of the mould 302 and attached to the glass plate coated with OTS, along with a rectangular pillar 322, which can be fabricated by precisely cutting a sheet of poly (methyl methacrylate) (PMMA) using a cutting means such as laser cutting.
[75] The rectangular pillar 322 works as a mould for making a pressure release chamber (PRC) 324. The pillar 322 is connected to the prism 308 by a small rectangular channel 326.
[76] In another embodiment, polydimethylsiloxane (PDMS) mixed with a cross-linking agent (PDMS: curing agent, 10:1 ratio) is poured on the plate containing the prism 308 and the pillar 322, and is cured at 65 oC for about 10 hrs. On removal of the PDMS, a mould 328 is obtained with a prism shaped cavity 330, a pillar shaped cavity, i.e., the PRC 324, and a rectangular cavity 332 corresponding to the prism 308, the pillar 322, and the rectangular channel 326.
[77] In another embodiment, two holes are punched on either side of the mould 328 to introduce the high RI liquid into the prism cavity 330 containing the prism 308; and the PRC 324. A thin PDMS membrane (~200 to 1000 ) is attached by standard oxygen plasma bonding on the top surface of the prism cavity 330 and the PRC 324. The prism 308, high RI liquid and the PDMS membrane together constitute the LDP 102.
[78] In another embodiment, a second rectangular microchannel 334 with equivalent area as that of the top surface of the LDP is prepared using another rectangular master mould 336. Two holes are punched corresponding to the inlet 110 and the outlet 112.
[79] In another embodiment, the LDP and the mould 336 long with holes are joined together by oxygen plasma treatments to complete the fabrication the LDP pressure sensor.
[80] Referring to FIG. 3C, in an embodiment, a 3D printed cage for the light source and a holder for the detector are placed opposite to one another, on either side of the sloped edges of the LDP.
[81] In another embodiment, a micropump is used to apply pressure to the inlet of the LDP sensor. The PRC is fluidically coupled to the LDP housing through the channel. When pressure is applied at the inlet, a depression occurs at the membrane above the prism, and the liquid displaced in the prism housing due to the deflection of the membrane flows into the PRC. As a result, the membrane above the PRC bulges out to compensate the depression of prism membrane.
[82] FIGs. 4A and 4B illustrate exemplary representations of ray tracing simulations in the proposed sensor to determine path of light rays through the LDP, in accordance with an embodiment of the present disclosure.
[83] Referring to FIG. 4A, ray tracing is illustrated when the top membrane of the LDP is not deflected, i.e., there is no pressure acting on the inlet of the microchannel. The simulated light rays parallel to the longitudinal axis of the prism enter the LDP filled with liquid of high RI. At zero applied pressure, the light rays undergo TIR at the top surface of the LDP and are then received at the detector. The simulation shows that, when light rays enter the LDP below a critical height (HC), the light rays do not undergo TIR, and pass through the LDP. Hence the position of the light source and light detector is configured such that the light emitted from the light source is at least at the critical height.
[84] Referring to FIG. 4B, ray tracing is illustrated when the top membrane of the LDP is deflected, i.e., there is a positive pressure acting on the inlet of the microchannel. The simulated light rays enter the LDP at a hight above HC and undergo TIR from the deflected membrane. It can be observed that the deviation of the light rays undergoing TIR increased with increase in deflection of the membrane, which results in drop in intensity of light detected by the detector due to the fact that part of the light exiting the LDP deviates away from the detector.
[85] In an exemplary embodiment, the dimensions of the LDP are top width WT – 13.65mm; bottom width WB – 2mm and height H – 5.7mm.
[86] In an exemplary embodiment, the liquid of high RI can be any such as immersion oil (IO) (RI 1.5) and di-iodomethane (DI) (RI 1.74). it is observed that HC value for DI (3.6mm) is lower than that for IO (4.85mm). The use of a liquid with a lower HC is desirable as experimental results show longer detection range.
[87] However, DI has been shown to be incompatible with PDMS, causing discolouration of the PDMS with extended exposure.
[88] In another embodiment, increasing WT can be shown to decrease HC.
[89] FIG. 5A illustrates an exemplary experimental set-up for calibrating the proposed LDP sensor, in accordance with an embodiment of the present disclosure. The set-up 500 includes a syringe pump 502 to apply pressure at inlet of the microchannel of the sensor. A calibrating sensor 504 such as a U-tube manometer is used to calibrate the pressure of the LDP sensor.
[90] In an exemplary embodiment, the signals from the detector are collected using a DAQ system 506 with a sampling frequency of about 1000 samples per second.
[91] In another exemplary embodiment, a precision power supply with a current resolution of 1mA is used to supply power to the light source such as LED.
[92] The syringe pump is operated to induce pressure in the microchannel. The values obtained from detector voltage and from the manometer are compared, and the LDP sensor is calibrated.
[93] The light source and light detector are aligned accurately in order to provide optimal sensitivity to the LDP sensor. The light source and detector, once aligned, are secured in position by the 3D printed cages.
[94] FIG. 5B illustrates an exemplary plot of calibration of the LDP sensor in terms of pressure and light sensitivity, for differing thicknesses of the flexible membrane, in accordance with an embodiment of the present disclosure. Different membrane thicknesses (tm) are considered, (200, 500 and 100 µm), and it can be observed that all sensors exhibit a linear relationship between applied pressure at the inlet and output voltage collected from the detector.
[95] It is further observed that sensitivity of the sensor decreases with increase in tm, while range of detection increases with increasing tm.
[96] FIG. 5C illustrates an exemplary plot of calibration of the LDP sensor in terms of time-domain response for a membrane thickness of 200 µm, in accordance with an embodiment of the present disclosure. It can be observed that the range of detection is about 1600 Pa.
[97] FIG. 5D illustrates an exemplary plot comparing calibration of an LDP sensor filled with IO and one filled with DI, in accordance with an embodiment of the present disclosure. It may be observed that there is significant difference in the pressure detection range. For a given thickness of the membrane, the sensor filled with DI has a pressure detection range almost 1.3 times as that of a sensor filled with IO. The value of 1.3 matches with the ratio of the RI values of DI and IO. It may be concluded that, as RI increases, the critical height for LDP decreases, and as a result, the pressure detection range increases.
[98] FIG. 6A illustrates an exemplary plot demonstrating the relaxation time of the proposed LDP sensor, in accordance with an embodiment of the present disclosure, where the relaxation time is ~220ms. FIG. 6B illustrates an exemplary plot demonstrating the response time of the proposed LDP sensor, in accordance with an embodiment of the present disclosure. The plot illustrates three sets of graphs for determining response time (tr) of an LDP sensor having DI and a tm of 1000 µm. tr is estimated by introducing a constant pressure at the inlet and instant pressure release at the outlet of the sensor. The time required to attain 90% of the stable pressure value is considered as the tr of the sensor. A constant pressure is introduced at the inlet by a syringe pump while the outlet is blocked by a binder clip. A constant pressure of ~5000 Pascal is introduced at the inlet to ensure the value of tr covers the complete pressure range of the LDP sensor. The binder clip is attached to the closest position of the inlet of the device to avoid the response due to the expansion/contraction of the tube. The instant release of binder clip gives a step increase of the sensor output which is continuously monitored by the DAQ system with a sampling rate of 1000 samples per second. The measured average tr for the constant pressure is ~ 146 ms with an error of 5 ms. The response time of the sensor can be further reduced by optimizing the distance between the PRC and the prism cavity, increasing the dimension of the channel between prism cavity and the PRC, and increasing the hole diameter of the pressure transduction.
[99] FIGs. 7A and 7B illustrate repeatability of the LDP sensor, in accordance with an embodiment of the present disclosure. The repeatability is measured for an LDP sensor filled with DI and for a membrane thickness of 1000 µm.
[100] Referring to FIG. 7A, the plot illustrates repeatability of the LDP sensor for a single value of light intensity. The syringe pump is programmed to exert periodic pressure of ~ 2500 Pascal with a constant speed of 3 mm per sec at the inlet of the LDP sensor. The result shows ~99.2% a single value repeatability.
[101] Referring to FIG. 7B, the plot illustrates repeatability of the LDP sensor across multiple calibration, which shows an overall repeatability ~ 99.4%, making the sensor reliable for practical application.
[102] FIG. 8 illustrates an exemplary plot for calibration of flowrate using the LDP sensor, in accordance with an embodiment of the present disclosure. The LDP sensor can be used for precise liquid flow rate measurement. The LDP used is filled with IO and has a tm of 1000 µm. The flowrate of water has been measured and the sensor shows a range of ~ 0-950 per min. The flowrate sensitivity of the sensor is 14 mV per 10 per sec. The sensitivity and flowrate range of the sensor can further be increased by manipulating the value of tm as well as top surface area of the prism i.e. either decrease the value of tm or increase the surface area of the prism top surface.
[103] FIG. 9A illustrates an exemplary representation of set-up for obtaining a respiratory signal from a human subject, in accordance with an embodiment of the present disclosure. One exemplary implementation of the proposed LDP sensor can be demonstrated in critical healthcare such respiratory distress by measuring the respiratory pressure signal of normal human at various posture and conditions such as the effect of physical exercise on the respiratory pressure signal (RPS).
[104] In an embodiment, a nasal cannula can be used to access the respiratory signal, where one end of the cannula is connected to the input of the sensor and the other end is closed by a binder clip. An LDP sensor filled with IO and having tm of 200 µm can be used in all the measurements of the respiratory pressure signal.
[105] FIG. 9A also illustrates a typical respiratory pressure signal of a healthy human subject. To realize the capability of the LDP sensor in capturing miniscule change in the respiratory pressure signal, RPS of a healthy male human adult is captured at different posture.
[106] FIG. 9B illustrates exemplary plot of RPS values for a healthy male human subject at different postures, in accordance with an embodiment of the present disclosure. Four postures are demonstrated – namely upward bending (1); standing (2); sitting on a chair (3); and meditation pose (4). It may be observed that RPS for all four postures show distinguished characteristics in terms of amplitude ( , time periods ( ) and features of the signal.
[107] FIG. 9C illustrates an exemplary plot of time-domain of a single respiratory pressure signal, in accordance with an embodiment of the present disclosure. The difference in amplitude and time period among the respiratory signals are estimated by comparing one full cycle RPS of all the four signals. Upward bending position shows maximum amplitude ( ~ 1.3 kPa) followed by standing position ( ~ 0.8 kPa). The sitting position ( ~ 0.68 kPa) and meditation position ( ~ 0.7 kPa) show nearly same amplitude. On the other hand, the meditation position shows the longest time period ( ~ 4.87 sec), followed by sitting position ( ~3.28 sec), standing position ( ~2.81 sec) and forward bending position ( ~2.26 sec).
[108] FIG. 9D illustrates an exemplary plot of frequency-domain of a single respiratory pressure signal, in accordance with an embodiment of the present disclosure. Fast Fourier Transform (FFT) of the signals is performed to analyse and extract the distinguished features in the signals in frequency domain. The dominant frequencies of the signals 1, 2, 3 and 4 are represented by , , , and , respectively. The values of the dominant frequencies match with the time period values of a complete RPS in the time domain. However, dominant frequency of i.e. sitting position, coincides with one of the sub-dominant features of i.e. forward bending position. This may be due to the involvement of bending posture in both cases.
[109] FIG. 9E illustrates an exemplary demonstration of portability of the proposed LDP sensor, in accordance with an embodiment of the present disclosure. Portability and usefulness of the sensor are conducted by engaged the LDP sensor in field studies. The sensor has been taken to an indoor stadium to capture RPS of badminton players before and after the game. FIG. 9E shows the RPS of two players before and after the game. The lower pair of signals is captured from a male adult and the upper signals from a female player. Interestingly, it has been observed that in both cases, the amplitude of the signal enhanced after playing of the badminton. The enhancement of the RPS signal after playing raise to 1.4 and 1.37 times before the game for the case of males and females, respectively. Therefore, the above results show a strong possibility of our LDP sensor of getting applied to the fields healthcare.
[110] FIG. 10 illustrates an exemplary method for low-pressure measurement, in accordance with an embodiment of the present disclosure.
[111] Referring to FIG. 10, the method 1000, at block 1002, the LDP 102 can be placed in the housing 108, which is filled with a liquid having a higher refractive index (RI), where the top surface of the LDP 102 is covered by the flexible membrane 104 of suitable thickness.
[112] At block 1004, the light source 114 can supply light rays to the LDP 102, the light source 114 configured at a first side of sloped edges of the LDP 102. At block 1006, the microchannel 106 having the inlet 110 and the outlet 112 above the flexible membrane 104, where a finite pressure applied at the inlet 110 of the microchannel 106 causes deflection of the flexible membrane 104 of the LDP 102, the deflection causes deviation of the path of total internally reflected light, resulting in variation of the intensity of light.
[113] At block 1008, the detector 114 configured at a second side of sloped edges of the LDP, detect the change in intensity of light travelling through the LDP 102, where based on the change in the intensity of light a corresponding voltage signal is collected from the detector to determine pressure variations.
[114] The embodiments of the present disclosure described above provide several advantages. The one or more of the embodiments provide the device 100 that easily fabricates LDP based optofluidic low-pressure monitoring sensor that works on the on the principle of Total Internal Reflection (TIR) of light. The device 100 measures the change in intensity of light caused by the applied pressure through a detector without using sophisticated analyzing devices such as microscope, camera, spectrometer and the likes. The device 100 is fabricated using a well-developed mould-based layer-by-layer soft lithographic (LSL) and 3D printing technique without involving sophisticated cleanroom microfabrication. It is cost effective, compact, highly sensitive, robust and portable. The present disclosure provides the device 100 that is deployed in in critical healthcare such as severe acute respiratory syndrome (SARS) caused by recently emerged COVID-19 and measures the respiratory pressure signal of normal human/subjects at various posture and conditions. The device 100 enables early detection of fatal biomedical ailment such as CRDs, SARS, COPD and sleep apnea and the likes.
[115] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES OF THE PRESENT DISCLOSURE
[116] The present disclosure provides a device that easily fabricates LDP based optofluidic low-pressure monitoring sensor that works on the principle of Total Internal Reflection (TIR) of light.
[117] The present disclosure provides a device that measures the change in intensity of light caused by the applied pressure through a detector without using sophisticated analyzing devices such as microscope, camera, spectrometer and the likes.
[118] The present disclosure provides a device that is fabricated using a well-developed mould-based layer-by-layer soft lithographic (LSL) and 3D printing technique without involving sophisticated cleanroom microfabrication.
[119] The present disclosure provides a device that is cost effective, compact and highly sensitive.
[120] The present disclosure provides a device that is deployed in critical healthcare and measures the respiratory pressure signal of normal human at various posture and conditions.
[121] The present disclosure provides a device that is robust and portable.
[122] The present disclosure provides a device that enables early detection of fatal biomedical ailment such as CRDs, SARS, COPD and Sleep apnea and the likes.

,CLAIMS:1. A device (100) for low-pressure measurement, said device comprising
a liquid-Dove prism (LDP) (102) adapted to be placed in a housing (108), which is filled with a liquid having a higher refractive index (RI), wherein a top surface of the LDP is covered by a flexible membrane (104) of suitable thickness;
a light source (114) configured at a first side of sloped edges of the LDP (102), the light source adapted to supply light rays to the LDP;
a microchannel (106) having an inlet (110) and an outlet (112) configured above said flexible membrane (104), wherein a finite pressure applied at the inlet (110) of the microchannel (106) causes deflection of the flexible membrane (104) of the LDP, the deflection causes deviation of the path of total internally reflected light, resulting in variation of the intensity of light; and
a detector (116) configured at a second side of sloped edges of the LDP (102), the detector (116) adapted to detect changes in intensity of light travelling through the LDP, wherein based on the change in the intensity of light a corresponding voltage signal is collected from the detector (116) to determine pressure variations.
2. The device as claimed in claim 1, wherein the device (100) configured to monitor respiratory distress of subjects by measuring respiratory pressure signal at various posture and conditions, wherein the device is used as a force sensor.
3. The device as claimed in claim 1, wherein at zero applied pressure, the simulated light rays undergo total internal reflection (TIR) at the top surface of the LDP (102) and are received at the detector (116), wherein at the application of finite pressure, the simulated light rays enter the LDP undergo TIR from the deflected membrane, the deviation of the light rays undergoing TIR increased with increase in deflection of the flexible membrane (104) results in drop in intensity of light detected by the detector.
4. The device as claimed in claim 1, wherein the device (100) comprises a pressure release chamber (PRC) (324) that is fluidically coupled to the LDP (102) through a channel (326), wherein upon application of finite pressure at the inlet (110), a depression occurs at the flexible membrane (104) above the LDP and the liquid displaced in the housing of LDP (102) due to the deflection of the flexible membrane flows into the PRC, wherein a membrane above the PRC (324) bulges out to compensate the depression of the flexible membrane of LDP.
5. The device as claimed in claim 1, wherein the liquid of high RI is any or a combination of immersion oil (IO) and di-iodomethane (DI).
6. The device as claimed in claim 1, wherein the device (100) comprises a manometer (504) coupled to the outlet of the microchannel to calibrate the pressure of the LDP, wherein values obtained from the detector voltage and from the manometer are compared for calibration.
7. The device as claimed in claim 1, wherein the voltage signals from the detector are collected using a data acquisition system (DAQ) (506) with a sampling frequency of about 1000 samples per second, wherein the DAQ operatively coupled to the detector.
8. The device as claimed in claim 1, wherein the light source (114) and the detector (116) are aligned accurately to provide optimal sensitivity to the LDP, wherein the light source and detector, once aligned, are secured in position by 3D printed cages, wherein a power supply is adapted to supply power to the light source.
9. The device as claimed in claim 1, wherein the device (100) is fabricated using a well-developed mould-based layer-by-layer soft lithographic (LSL) and 3D printing technique.
10. A method (1000) for low-pressure measurement, said method comprising
placing (1002) a liquid-Dove prism (LDP) in a housing, which is filled with a liquid having a higher refractive index (RI), wherein a top surface of the LDP is covered by a flexible membrane of suitable thickness;
supplying (1004), from a light source, light rays to the LDP, the light source configured at a first side of sloped edges of the LDP;
configuring (1006), a microchannel having an inlet and an outlet above the flexible membrane, wherein a finite pressure applied at the inlet of the microchannel causes deflection of the flexible membrane of the LDP, the deflection causes deviation of the path of total internally reflected light, resulting in variation of the intensity of light; and
detecting (1008), at a detector configured at a second side of sloped edges of the LDP, the change in intensity of light travelling through the LDP, wherein based on the change in the intensity of light a corresponding voltage signal is collected from the detector to determine pressure variations.