WO2009129351A2

WO2009129351A2 - Stiffness sensor

Info

Publication number: WO2009129351A2
Application number: PCT/US2009/040728
Authority: WO
Inventors: Shyam Sivaramakrishnan; Rajesh Rajamani
Original assignee: Regents Of The University Of Minnesota
Priority date: 2008-04-15
Filing date: 2009-04-15
Publication date: 2009-10-22
Also published as: WO2009129351A3

Abstract

In general, this disclosure is directed to a stiffness sensor that uses capacitance measurements to determine a stiffness of a film applied to a pre-charged membrane. A capacitance is measured between the pre-charged membrane and a back-plate. When applied to the pre-charged membrane, the film may alter the stiffness of the pre-charged membrane. A change in the stiffness of the pre-charged membrane may cause a movement or deflection of the pre-charged membrane, which in turn may cause a change in the capacitance between the pre-charged membrane and the back-plate. In one example, the disclosure is directed to an apparatus that includes a pre-charged membrane, a metallic plate, and a film applied to the pre-charged membrane. The apparatus further includes a capacitance measurement block configured to measure a capacitance between the pre-charged membrane and the metallic plate, and to determine the stiffness of the pre-charged membrane based on the measured capacitance.

Description

STIFFNESS SENSOR

TECHNICAL FIELD

[0001] This disclosure relates to sensors, and more particularly, to sensors for detecting a stiffness of a film.

BACKGROUND

[0002] Various techniques have been proposed for stiffness sensing including resonant techniques and surface acoustic wave techniques. Resonant techniques involve determining a resonant frequency, and are therefore limited by the frequency sweeping time of the sensor. In addition, the post-processing required for determining a resonant frequency can increase the cost and complexity of the sensor. Surface acoustic wave devices, while useful, tend to be highly sensitive to other types of changes in film properties. In particular, they cannot be used with thin films which change other properties like mass and conductivity along with stiffness in the presence of analytes.

SUMMARY

[0003] In general, this disclosure is directed to a stiffness sensor that uses capacitance measurements to determine a stiffness of a film applied to a pre-charged membrane. A capacitance is measured between the pre-charged membrane and a back-plate. When applied to the pre-charged membrane, the film may alter the stiffness of the pre-charged membrane. A change in the stiffness of the pre-charged membrane may cause a movement or deflection of the pre-charged membrane, which in turn may cause a change in the capacitance between the pre-charged membrane and the back-plate. Thus, one more capacitance measurements may be used to determine the stiffness of the film. [0004] In one embodiment, the invention is directed to an apparatus that includes a pre- charged membrane and a metallic plate. The apparatus further includes a film applied to the pre-charged membrane. The film alters a stiffness of the pre-charged membrane. The apparatus further includes a capacitance measurement block configured to measure a capacitance between the pre-charged membrane and a metallic plate, and to determine the stiffness of the pre-charged membrane based on the measured capacitance. The stiffness of the pre-charged membrane is indicative of the stiffness of the film. [0005] In another embodiment, the invention is directed to a method that includes altering a stiffness of a pre-charged membrane with a film that is applied to the pre-charged membrane. The method further includes measuring a capacitance between the pre- charged membrane and a metallic plate. The method further includes determining the stiffness of the pre-charged membrane based on the measured capacitance. The stiffness of the pre-charged membrane is indicative of the stiffness of the film. [0006] In another embodiment, the invention is directed to an apparatus that includes film means for altering a stiffness of a pre-charged membrane, wherein the film means is applied to the pre-charged membrane. The apparatus further includes means for measuring a capacitance between the pre-charged membrane and a metallic plate. The apparatus further includes means for determining the stiffness of the pre-charged membrane based on the measured capacitance. The stiffness of the pre-charged membrane is indicative of the stiffness of the film.

[0007] In another embodiment, the invention is directed to an apparatus that includes a pre-charged membrane comprising a stiffness-sensitive film. The stiffness-sensitive film is configured to alter a stiffness of the pre-charged membrane when an analyte is proximate to the stiffness-sensitive film. The apparatus further includes a stiffness measuring unit configured to measure the stiffness of the pre-charged membrane when the analyte is proximate to the stiffness-sensitive film.

[0008] In another embodiment, the invention is directed to a method that includes altering a stiffness of a pre-charged membrane in a sensing apparatus, the pre-charged membrane comprising a stiffness-sensitive film, when an analyte is proximate to the stiffness- sensitive film. The method further includes measuring the stiffness of the pre-charged membrane when the analyte is proximate to the stiffness-sensitive film. [0009] In another embodiment, the invention is directed to an apparatus that includes means for altering a stiffness of a pre-charged membrane in a sensing apparatus, the pre- charged membrane comprising a stiffness-sensitive film, when an analyte is proximate to the stiffness-sensitive film. The apparatus further includes means for measuring the stiffness of the pre-charged membrane when the analyte is proximate to the stiffness- sensitive film.

[0010] The use of a pre-charged membrane may augment the sensitivity and resolution of the developed sensor. For example, in some embodiments, a charge in the pre-charged membrane may control the sensitivity of the sensing apparatus with respect to the analyte concentration. In additional embodiments, a DC bias voltage may control the sensitivity of the sensing apparatus with respect to the analyte concentration. [0011] In some examples, the film may comprise a stiffness-sensitive film. In such examples, the stiffness of the pre-charged membrane may be indicative of the presence, absence, or concentration of an analyte proximate to the sensor. The stiffness-sensitive film may be a thin film that is deposited on the membrane. A self generated or externally generated electric field may pull the membrane inwards which is resisted by the mechanical stiffness of the membrane. The change in stiffness of the stiffness-sensitive film in the presence of an analyte may affect the mechanical stiffness of the composite membrane which results in small vertical deflections of the membrane. In some embodiments, a change in analyte concentration may also cause a change in stiffness of the stiffness-sensitive film. These small motions are recorded as a change in capacitance of the sensor which is subsequently used to determine the stiffness, and hence the presence or absence of an analyte as well as an analyte concentration. [0012] Some analytes (such as gases) are known to selectively change the stiffness of stiffness-sensitive thin films. The developed sensing technique is a novel portable method of measuring such changes and thereby detecting the presence or absence of an analyte as well as an analyte concentration. The capacitive sensor element, along with the readout electronics, are highly compact and designed for portable use. The entire apparatus may be designed to operate in a stand-alone fashion or in wired/wireless communication with a monitoring unit like a laptop for example. [0013] Stiffness sensing is currently the only solid state measurement technique for developing selective gas sensors (unlike mass and conductivity sensing, which have proven to be cross-sensitive to other gases). However, the lack of a simultaneously sensitive, inexpensive and portable measurement system has prevented commercialization of gas sensors that utilize stiffness sensing. The developed sensing technique is expected to bridge this gap and herald a new class of inexpensive gas sensors. [0014] The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF DRAWINGS

[0015] FIG. IA is a perspective diagram illustrating a stiffness sensor, according to an example embodiment of the invention.

[0016] FIG. IB is a perspective diagram illustrating the stiffness sensor of FIG. IA in the presence of an analyte.

[0017] FIG. 2Ais a perspective diagram illustrating a stiffness sensor having an electret membrane in accordance with an example embodiment of the invention.

[0018] FIG. 2B is a perspective diagram illustrating a stiffness sensor where the pre- charged membrane is charged using a direct current (DC) bias voltage, according to another example embodiment of the invention.

[0019] FIG. 3 A is a schematic diagram illustrating the stiffness sensor without an analyte proximate to the sensor, according to an example embodiment of the invention.

[0020] FIG. 3B is a schematic diagram illustrating the stiffness sensor of FIG. 3 A with an analyte proximate to the sensor.

[0021] FIG. 4 is a perspective diagram illustrating an array of stiffness sensors according to an example embodiment of the invention.

[0022] FIG. 5 is a block diagram illustrating a stiffness sensing apparatus in accordance with an example embodiment of the invention.

[0023] FIG. 6 is a conceptual diagram illustrating various capacitance measurement circuits for use within the capacitance measurement circuit illustrated in FIG. 5.

[0024] FIG. 7 is a conceptual diagram illustrating various techniques for wireless transmission for use within the wireless transmission circuit illustrated in FIG. 5.

[0025] FIG. 8 A is a chart illustrating the change in capacitance with respect to carbon dioxide (CO2) concentration in an example sensing apparatus.

[0026] FIG. 8B is a chart illustrating the humidity in the surrounding environment with respect to the measurements illustrated in FIG. 8A.

[0027] FIG. 9A is a chart illustrating the change in capacitance with respect to carbon dioxide (CO2) concentration in an example sensing apparatus.

[0028] FIG. 9B is a chart illustrating the humidity in the surrounding environment with respect to the measurements illustrated in FIG. 9A.

[0029] FIG. 10 is a flow diagram illustrating an example technique for measuring a stiffness of a pre-charged membrane when a film is applied to the membrane. DETAILED DESCRIPTION

[0030] In general, this disclosure is directed to a stiffness sensor that uses capacitance measurements to determine a stiffness of a film applied to a pre-charged membrane. A capacitance is measured between the pre-charged membrane and a back-plate. When applied to the pre-charged membrane, the film may alter the stiffness of the pre-charged membrane. A change in the stiffness of the pre-charged membrane may cause a movement or deflection of the pre-charged membrane, which in turn may cause a change in the capacitance between the pre-charged membrane and the back-plate. In some examples, the stiffness of the pre-charged membrane is indicative of the stiffness of the film. Thus, one more capacitance measurements may be used to determine the stiffness of the film. In additional examples, the stiffness of the pre-charged membrane may be indicative of the presence, absence, or concentration of an analyte proximate to the sensor.

[0031] The techniques described in this disclosure may be useful for monitoring stiffness of film, including thin film samples. For example, the film may include a tissue culture, and a stiffness of the tissue culture may be indicative of the health of the tissue culture. The health of the tissue culture may be related to one or more parameters that can be measured using a stiffness sensor described in this disclosure. For example, the stiffness of the tissue culture may be indicative of an amount of tumor cells within the tissue culture. In this manner, the stiffness sensor of the present disclosure may be able to detect cancerous cells within a tissue culture. In addition, the stiffness sensor may be utilized in invitro cell and tissue culture monitoring applications as well as in stem cell growth applications.

[0032] As another example, a film capable of adsorbing hydrogen may be used for storing hydrogen at low temperatures. The stiffness of the film and/or the pre-charged membrane may be related to an amount of hydrogen adsorbed into the film. Thus, the stiffness-sensing apparatus of this disclosure may be used to determine the amount of hydrogen that a film can effectively hold at a low temperature. In order to conduct these experiments, vacuum chambers are often utilized. The stiffness sensor described in this disclosure provides a significant advantage over prior systems of stiffness testing due to the compact nature and potential wireless operation of the components in the present sensor. [0033] As another example, the stiffness sensors described in this disclosure may be incorporated into one or more biosensors. Example biosensors include environmental monitoring biosensors as well as security and/or biodefense biosensors. A biosensor uses a biological element that creates a recognition event in response to a liquid, gas or solid (i.e., target substance) proximate to the sensor. For example, a film containing antibodies of a target substance can be applied to a membrane within the sensor. As the target substance enters the biosensor, the antibodies release, which causes a change in stiffness of the film and membrane. A recognition event may occur once the stiffness of the film and/or membrane reaches a particular threshold. Thus, the change in stiffness of the film may be indicative of the presence of a target substance or a particular amount of the target substance proximate to the sensor.

[0034] In another example, a stiffness-sensitive film that is sensitive to a particular type of released antibodies may be applied to a membrane within the biosensor. In this example, the antibodies need not be applied to the membrane, but may be attached or disposed anywhere within the biosensor. As the target substance enters the bio-cell, antibodies are released, which are adsorbed by the stiffness-sensitive film causing a change in the stiffness of the film. Examples of target substances include, but are not limited to, nitro-glycerin, Trinitrotoluene (TNT), opiates, cocaine, Severe Acute Respiratory Syndrome (SARS) coronavirus, Escherichia coli bacteria (E. CoIi), West Nile vile, etc.

[0035] As another example, the stiffness sensing techniques in this disclosure may be used to measure thin films in vaccum chambers. For example, a quartz crystal microbalance (QCM) is commonly used to measure the thickness of thin films within thin-film deposition chambers. Since QCMs work at radio frequencies, QCM performance is limited in sputtering (i.e., thin-film deposition) due to interference caused by the RF plasma. A stiffness sensor designed in accordance with the techniques described in this disclosure may be used to replace and/or supplement the QCM used for thin- film deposition. For example, a stiffness sensor according to this disclosure could be placed within the vacuum chamber such that a portion of the film is deposited onto the membrane within the sensor. As the thickness or amount of film deposited on the membrane of the sensor changes, the stiffness of the membrane may also change. Thus, the stiffness of the pre-charged membrane may be indicative of a total thickness and/or an amount of thin- film deposited during a thin-film deposition process. [0036] In additional examples, the film applied to the pre-charged membrane within the stiffness sensor may comprise a stiffness-sensitive film that is sensitive to one or more analytes. In other words, the stiffness-sensitive film may adsorb the analyte when the analyte is proximate to the sensor. In such examples, the stiffness of the film and/or pre- charged membrane changes as the amount of analyte adsorbed changes. [0037] For example, when the analyte is a gas, the stiffness of the pre-charged membrane may be indicative of a concentration of the gas proximate to the stiffness-sensitive film. The gas may include carbon dioxide, hydrogen, nitric oxide, water vapor as well as other volatile organic compounds. In this manner, the stiffness sensing apparatus may be utilized as a gas sensor, and more particularly as a carbon dioxide sensor. [0038] When the analyte is a gas, the stiffness sensing apparatus may be used for respiratory monitoring. For example, diagnostic measurement of blood gas (oxygen, carbon dioxide) levels can be obtained by monitoring the exhaled gas concentration. Such measurements may be useful in emergency care, anesthetic monitoring, exercise monitoring and sleep diagnostics. Modern optical analyzers are known to suffer from blocks and flow distortion in sampling tubes. Though optical sensors are accurate by design, the sampling method through tubes is a well-known hindrance to reliable respiratory monitoring. The sensors described in this disclosure are not only attractive due to their cost and size, but also may not require the use of sampling tubes. While the sensors described in this disclosure may not require sensing tubes, the sensors may nevertheless, in some cases, be designed to be compatible with standard tubing used for intubated patients.

[0039] The stiffness sensing apparatus may also be used for indoor air quality monitoring. For example, quantitative measurement of room gas levels to aid in controlled ventilation may be obtained from the sensor. Controlled ventilation is known to translate to large energy savings during air exchanges with the ambient air. However, application of indoor gas monitoring is restricted by the cost of reliable sensors. The low cost of the sensors described in this disclosure make them attractive for indoor air quality monitoring.

[0040] Also, the stiffness sensing apparatus may be used for food quality monitoring, such as the remote measurement of gas exhalation by foods for food-quality control. Food quality monitoring requires gas sensors to be small and wireless while responding reliably to specific gases. The sensors described in this disclosure can assist in fulfilling these requirements. Aside from the above gas sensing applications, the stiffness sensor of the present disclosure may be able to detect particular patterns within an analyte. As an example, the analyte may include deoxyribonucleic acid (DNA) having one or more patterns. The stiffness-sensitive film may be particularly sensitive to only a single pattern of DNA molecules. Thus, the stiffness of the pre-charged membrane may be indicative of whether or not a particular pattern of DNA molecules is proximate to the stiffness-sensitive film. In this manner, one or more stiffness sensors may be able to accurately and efficiently determine the molecular composition of a strand of DNA.

[0041] FIG. IA is a perspective diagram illustrating a stiffness sensor 10 according to an example embodiment of the invention. Stiffness sensor 10 includes a pre-charged membrane 12, a back-plate 14, and capacitance leads 16A, 16B. Stiffness sensor 10 measures the stiffness of pre-charged membrane 12. In some embodiments, the stiffness of pre-charged membrane 12 may be indicative of the presence or absence of an analyte proximate to stiffness sensor 10. In other embodiments, the stiffness of pre-charged membrane 12 may be indicative of an amount or concentration of an analyte proximate to stiffness sensor 10. In yet other embodiments, the stiffness of pre-charged membrane 12 may be indicative of the stiffness of an analyte that is applied to or coated onto stiffness sensor 10.

[0042] Pre-charged membrane 12 flexes or deflects in response to an analyte proximate to pre-charged membrane 12. Pre-charged membrane 12 may be composed of a stretched capacitive membrane. Pre-charged membrane 12 includes a stiffness-sensitive film 18 that may be applied to an outer surface of pre-charged membrane 12. In some embodiments, the pre-charged membrane 12 may be a part of a microelectromechanical (MEM) capacitor. The stiffness of pre-charged membrane 12 as used herein may refer to the resistance of pre-charged membrane 12 to deflection or deformation when a force is applied to membrane 12.

[0043] In some embodiments, pre-charged membrane 12 may include a metal coating or electrode. In such embodiments, stiffness-sensitive film 18 may be applied onto a first face of the metal coating, and pre-charged membrane 12 may be adjacent to a second face of the metal coating. Stiffness-sensitive film 18 may be applied to the pre-charged membrane using techniques such as e-beam evaporation, sputtering, chemical vapor deposition, Langmuir-blodgett coating and layer-by-layer self-assembly, for example. [0044] Stiffness-sensitive film 18 may be configured to alter the stiffness of pre-charged membrane 12 when an analyte is proximate to stiffness-sensitive film 18. In some cases, the change in stiffness of pre-charged membrane 12 may be related to the amount or concentration of an analyte proximate to stiffness-sensitive film 18. Stiffness-sensitive film 18 may alter the stiffness of pre-charged membrane 12 at least in part by altering the stiffness of pre-charged membrane 12 upon adsorption of the analyte by stiffness- sensitive film 18. That is, when an analyte is adsorbed into stiffness-sensitive film 18, the stiffness-sensitive film may cause the stiffness of pre-charged membrane 12 to change. The amount of change in the stiffness of pre-charged membrane 12 may be related to the amount of analyte adsorbed by the stiffness-sensitive film 18. In some embodiments, the amount of charge in pre-charged membrane 12 controls the sensitivity of the sensing apparatus with respect to the analyte concentration.

[0045] Stiffness-sensitive film 18 may be deposited onto pre-charged membrane 12 in a particular pattern. In some cases, the pattern may be specifically chosen to enhance the selectivity of the film for a particular analyte. Stiffness-sensitive film 18 may be composed of one or more porous molecules such as, for example, carbon nanotubes, boron nanotubes, silicon nanotubes, and zeolites. In other embodiments, stiffness- sensitive film 18 may be composed of one or more polymers such aspolyimides, polyvinyl alcohol, Teflon® and Nafion®. Stiffness-sensitive film 18 may comprise elongated molecules having a central axis. The elongated molecules may be bonded to pre-charged membrane 12 at opposite ends of the central axis.

[0046] In one embodiment, stiffness-sensitive film 18 may comprise single walled carbon nanotubes (SWNT) for carbon dioxide sensing. The stiffness-sensitive SWNT film may be self-assembled on pre-charged membrane 12 to enhance the adhesion of the film to the membrane and prevent aggregation of stiffness-sensitive film 18 due to Van der Waal's attraction. When stiffness-sensitive film 18 is composed of single walled carbon nanotubes (SWNT), this procedure may involve acidification of stiffness-sensitive film 18 to introduce negatively charged carboxylic groups (COO-) at the ends of the group. Hydrophilic positively charged polymers may then used to introduce positive charges on membrane 12. Subsequent self-assembly of the negatively charged acidified SWNTs on the positively charged membrane results in uniform film formation. [0047] Metallic back-plate 14 may be a metallic plate that serves as a second conductor for capacitance measurements. Back-plate 14 may include one or more holes or apertures to decrease the damping. Pre-charged membrane 12 may induce a charge onto metallic back-plate 14 that is opposite in polarity to the charge of pre-charged membrane 12. The opposite polarity of charges present on pre-charged membrane 12 and back-plate 14 may generate an electric field and corresponding electric force between pre-charged membrane 12 and back-plate 14, which in turn causes the pre-charged membrane to be attracted to the back-plate. This attraction may cause pre-charged membrane 12 to move, deflect, or "pull in" towards back-plate 14. The mechanical stiffness of membrane 12 causes the membrane to resist the attractive force of the electric field, and in turn causes membrane 12 to move or retract away from back-plate 14. An equilibrium position may be established for pre-charged membrane 12. The equilibrium position may be the position where the force caused by the electric field and force caused by the mechanical stiffness of pre-charged membrane 12 are balanced. In some embodiments the equilibrium position may be a pre-stressed position. In other words, the initial charge already present on the pre-charged membrane may cause the pre-charged membrane to be slightly deflected or bent toward the back-plate 14 even when an analyte is not proximate to the sensor 10.

[0048] FIG. IB is a perspective diagram illustrating the sensing apparatus of FIG. IA in the presence of an analyte 20. As shown in FIG. IB, when analyte 20 is proximate to stiffness-sensitive film 18 or when analyte 20 increases in concentration proximate to stiffness-sensitive film 18, stiffness-sensitive film 18 may adsorb the analyte molecules 20 (shown as spheres). The adsorbing of analyte 20 may cause a reduction in stiffness of stiffness-sensitive film 18, which in turn causes a reduction in the stiffness of pre-charged membrane 12. In some embodiments, the reduction in stiffness of stiffness-sensitive film 18 may occur due to an expansion of the molecules in stiffness-sensitive film 18. Likewise, when analyte 20 is no longer present near stiffness sensor 10 or when the concentration of analyte 20 is reduced proximate to stiffness-sensitive film 18, stiffness- sensitive film 18 may desorb analyte 20. The desorbing of analyte 20 may cause an increase in the stiffness of pre-charged membrane 12.

[0049] FIG. 2A is a perspective diagram illustrating a stiffness sensor 20 having an electret membrane 22 in accordance with another example embodiment of the invention. FIG. 2B is a perspective diagram illustrating a stiffness sensor 40 where pre-charged membrane 42 is charged using a direct current (DC) bias voltage 46 according to another example embodiment of the invention. Similar to stiffness sensor 10 described above with respect to FIG. IA, stiffness sensors 20 and 40 in FIGS. 2 A and 2B each include a pre-charged membrane 22, 42 and a metallic back-plate 24, 44. A pre-charged membrane as used herein may refer to a membrane having an intrinsically stored charge as well as to a membrane that is externally charged. An externally charged membrane may include, for example, a membrane that is charged by a DC bias voltage source with respect to a metallic plate. Each of the pre-charged membranes 22, 42 may include a metal coating 28, 48 applied to a surface of the membrane 22, 42 and a stiffness-sensitive film (not shown).

[0050] In FIG. 2A, pre-charged membrane 22 may comprise an electret membrane having a permanent or quasi-permanent electric charge or dipole polarization. In this embodiment, stiffness sensor 20 may also include a first capacitance lead 30 electrically coupled to the pre-charged membrane 22 and a second capacitance lead 32 electrically coupled to metallic back-plate 24. Capacitance leads 30, 32 facilitate capacitance measurements between pre-charged membrane 22 and the metallic back-plate 24. [0051] In FIG. 2B, stiffness sensor 40 may also include a direct current (DC) bias voltage source 46 electrically coupled between pre-charged membrane 42 and metallic back-plate 44. DC bias voltage source 46 is configured to pre-charge membrane 42 to a DC bias voltage with respect to metallic plate 44. The DC bias voltage may control the sensitivity of the sensing apparatus with respect to an analyte concentration. In some embodiments, the entire pre-charged membrane 42 may be composed of conductive material. In other embodiments, membrane 42 may be composed of insulating materials, and have a metallic coating applied to one face of the membrane. As shown in FIG. 2B, stiffness sensor 40 may also include a DC blocking capacitor 50 having a first end electrically coupled to the pre-charged membrane 42 . The second end of the DC blocking capacitor may be electrically coupled to a first capacitance lead 52. A second capacitance lead 54 may be electrically coupled to metallic back-plate 44.

[0052] FIG. 3 A is a schematic diagram illustrating a stiffness sensor 60 without an analyte proximate to the sensor, according to an example embodiment of the invention. FIG. 3B is a schematic diagram illustrating stiffness sensor 60 of FIG. 3 A with an analyte 72 proximate to the sensor. An electret membrane 62 may be employed in stiffness sensor 60, in order to measure the elastic property changes of films 64. In the embodiment described below, electret membrane 62 and back-plate 66 may be part of an electret microphone. An electret microphone is a capacitive device with a permanently charged electret membrane and a metallic back-plate. The electret microphone may also include an amplifier, which may be removed when incorporating the microphone into the sensor. [0053] The presence of the charge on membrane 62 causes the membrane to be attracted to back-plate 66, which is resisted by the mechanical stiffness of the membrane resulting in an equilibrium vertical position for membrane 62 given by the following equation:

where ε is the dielectric constant of the medium, A is the capacitive area, V_DC is the voltage between membrane 62 and back-plate 66, d₀ is the original separation between membrane 62 and back plate 66, k is the stiffness of membrane 62, and x_e is the equilibrium vertical position of membrane 62.

[0054] Any change in the stiffness of membrane 62 results in an imbalance between the mechanical restoring force and the attractive electrical force. There is a subsequent vertical deflection of membrane 62 to appropriately compensate for the change in membrane stiffness altering the vertical equilibrium position of membrane 62. The shift in the membrane's vertical position is monitored as a change in capacitance between membrane 62 and back-plate 66. Thus, in contrast to resonant sensors, the intrinsic electric field between electret membrane 62 and metallic back-plate 66 may be used to cause a static deflection of the membrane. Then a capacitance measurement may be used for monitoring stiffness change in stiffness-sensitive films 64 as well as the pre-charged membrane 62. This design eliminates cumbersome instrumentation for independent excitation and measurement and the need for frequency sweeping, which results in a small sensor package and decreased response time respectively. [0055] Pre-charged membrane 62 may be coated with SWNTs to obtain a CO2 gas sensor. SWNTs are known to selectively change their stiffness upon adsorption of CO2 gas molecules. Consequently, adsorption of CO2 causes a change in capacitance of the electret microphone. This principle is used to develop a sensitive CO2 gas sensor. [0056] As shown in FIGS. 3 A & 3B, pre-charged membrane 62 may include a first portion 68 defining a first distance (e.g., d₀ ) between first portion 68 and the metallic plate 66. The first distance remains substantially fixed during operation of the sensing apparatus. Pre-charged membrane 62 may also include a second portion 70 defining a second distance (e.g., x_eq) between second portion 70 and metallic plate 66. The second distance may change or vary based on an amount of analyte 72 proximate to stiffness- sensitive film 64. Because the capacitance between two plates is related to the distance between the plates, the capacitance between pre-charged membrane 62 and back-plate 66 may change as the distance between pre-charged membrane 62 and metallic back-plate 66 changes. More particularly, as the second distance decreases from x_eq to x_eq-new, the capacitance between pre-charged membrane 62 and metallic back-plate 66 may increase. Conversely, as the second distance increases from x_eq-_new to x_eq, the capacitance between pre-charged membrane 62 and metallic back-plate 66 may decrease. In this manner, the capacitance between pre-charged membrane 62 and back-plate 66 may be measured without applying an external excitation force to pre-charged membrane 62 and without causing pre-charged membrane 62 to vibrate.

[0057] As shown in FIG. 3A, even when an analyte is not proximate to stiffness-sensitive film 64, pre-charged membrane 62 may be pre-stressed or pre-deflected. In other words, the second distance may be less than the first distance when the analyte is not proximate to stiffness-sensitive film 64. Pre-stressing pre-charged membrane 62 may provide improved granularity or resolution in the capacitance measurement performed by the sensing device. The sensitivity of the capacitance measurement to the stiffness change may increase upon increasing the charge within the pre-charge in membrane 62. [0058] In addition, because pre-charged membrane 62 is not vibrated during the capacitance measurement, the capacitance between pre-charged membrane 62 and metallic back-plate 66 can be calculated without the use of resonant frequencies. Thus, in some embodiments, stiffness sensing apparatus 60 may be able to perform a single capacitance measurement between pre-charged membrane 62 and metallic plate 66 when analyte 72 is proximate to stiffness-sensitive film 64. Based on this single capacitance measurement, stiffness sensing apparatus 60 may be able to determine the stiffness of pre- charged membrane 62. Stiffness sensing apparatus 60 may determine the stiffness of pre- charged membrane 62 at least in part by performing one or more reference capacitance measurements between pre-charged membrane 62 and metallic plate 66 when the analyte is not proximate to stiffness-sensitive film 64, and by determining the stiffness of pre- charged membrane 62 based on the single capacitance measurement and the one or more reference capacitance measurements.

[0059] In one embodiment, pre-charged membrane 62 comprises a second portion 70. A first distance (e.g., x_eq) may be defined between second portion 70 of pre-charged membrane 62 and metallic plate 66 when the analyte is not proximate to the stiffness- sensitive membrane. A second distance (e.g., x_eq-new) may be defined between second portion 70 of pre-charged membrane 62 and metallic plate 66 when analyte 72 is proximate to the stiffness-sensitive membrane. The second distance may be less than the first distance. The second distance may change based on an amount of analyte 72 proximate to stiffness-sensitive film 64.

[0060] FIG. 4 is a perspective diagram illustrating an array of stiffness sensors 80 according to an example embodiment of the invention. Each of the stiffness sensors 82 A, 82B, 82C, 82D (collectively, "stiffness sensors 82" or "sensing elements 82") in the array may be constructed according to any the techniques described in this disclosure. In addition, each of the sensing elements 82 may be operatively coupled to a capacitance measurement block 84 or circuit. For example, electrical leads may connect each of sensing elements 82 to capacitance measurement block 84 and transmit capacitance, voltage, or other electrical information from the respective one of sensing elements 82 to measurement block 84. Capacitance measurement block 84 may be a part of a stiffness measuring unit within a stiffness sensing apparatus.

[0061] In one embodiment, the array of sensors 80 may include sensing elements 82 that have different stiffness sensitive films. The different stiffness-sensitive films may have different selectivities for a particular combination of analytes and non-analytes. The selectivity of a stiffness-sensitive film may refer to the ability of the stiffness-sensitive film to distinguish the analyte from other non-analyte components. For example, when the analyte is a gas, other ambient gases may also surround the sensor. The ambient gases may cause alterations to the stiffness of a pre-charged membrane in addition to the alterations caused by the analyte. Certain stiffness-sensitive films may be able to distinguish between the analyte and the surrounding non-analyte gases better than others for a particular combination of gases. Thus, an array of sensing elements 80 may provide a robust sensor capable of effectively detecting when an analyte is proximate to the sensor as well as the amount of analyte even in the presence of other non-analyte components that can vary or change.

[0062] Such a sensing apparatus may measure the stiffness of a first pre-charged membrane and a second pre-charged membrane, and then determine an amount of the analyte proximate to the sensing apparatus based on the measured stiffness of both the first pre-charged membrane and the measured stiffness of the second pre-charged membrane. The sensing apparatus may determine the amount of analyte proximate to the sensing apparatus by using principal component analysis (PCA) techniques. [0063] In another embodiment, the array of sensors 80 may include sensing elements 82 that have different stiffness sensitive films that are responsive to different analytes. For example, the array of sensors may include a first stiffness-sensitive film that is configured to alter a stiffness of a first pre-charged membrane when the analyte is proximate to the first stiffness-sensitive film, and a second stiffness-sensitive film that is configured to alter a stiffness of the second pre-charged membrane when the analyte is proximate to the second stiffness-sensitive film. Thus, the first stiffness-sensitive film may be more selective with respect to a first analyte and the second stiffness-sensitive film may be more selective with respect to a second analyte. In this manner, an array of sensing elements 80 may provide a robust sensor capable of effectively detecting when multiple analytes are proximate to the sensor as well as the amount of each analyte proximate to the sensor. Stiffness sensing apparatus 80 may determine an amount of the first analyte and an amount of the second analyte proximate to the sensing apparatus at least in part by using principal component analysis (PCA) techniques.

[0064] FIG. 5 is a block diagram illustrating a stiffness sensing apparatus 90 in accordance with an example embodiment of the invention. Stiffness sensing apparatus 90 may include an analyte sensor 92, a capacitance measurement circuit 94, a wireless transmission circuit 96, and a microcontroller 98. Analyte sensor 92 may be operatively coupled to capacitance measurement circuit 94 via one or more capacitance leads 100, 102. Capacitance leads 100, 102 may transmit capacitance, voltage, or other electrical information from the sensing element 92 to measurement block 94. A first capacitance lead 100 may be coupled to a pre-charged membrane within analyte sensor 92, and a second capacitance lead 102 may be coupled to a metallic back-plate within analyte sensor 92. In some cases, a DC blocking capacitor (not shown) may be used to filter out any DC voltage components applied to the pre-charged membrane. In such a case, a first terminal of a DC blocking capacitor may be coupled to the pre-charged membrane and a second terminal of the DC blocking capacitor may be coupled to the first capacitance lead.

[0065] Analyte sensor 92 measures the stiffness of a pre-charged membrane within sensor 92 when an analyte is proximate to sensor 92. Analyte sensor 92 may be a stiffness sensor designed in accordance with any of the stiffness sensors described in this disclosure. For example, analyte sensor 92 may correspond to stiffness sensors 10, 20, 40, 60 and 80 illustrated any of FIGS. 1-4.

[0066] A stiffness measuring unit may be defined to include all or a subset of the capacitance measuring circuit 94, wireless transmission circuit 96, and microcontroller 98. Each of the components of the stiffness measuring unit may be operatively coupled to each of the other components. For example, capacitance measurement circuit 96 may be operatively coupled to wireless transmission circuit 96 and microcontroller 98, and microcontroller 98 may also be operatively coupled to wireless transmission circuit 96. Microcontroller 98 provides control and synchronization between capacitance measurement circuit 94 and wireless transmission circuit 96. [0067] Capacitance measurement block 94 may be configured to receive electrical information from analyte sensor 92 via capacitance leads 100, 102 and to measure the capacitance between a pre-charged membrane and a metallic plate within the analyte sensor 92 based on the received electrical information. The electrical information may include voltage information related to a voltage between the pre-charged membrane and the back-plate of analyte sensor 92. The electrical information may also include distance information relating to one or more distances between the pre-charged membrane and the back-plate. In some embodiments, capacitance measurement block 94 may measure the capacitance between the pre-charged membrane and the metallic plate without causing the pre-charged membrane to vibrate.

[0068] In some cases, capacitance measuring block 94 may be configured to perform a single capacitance measurement between the pre-charged membrane and the metallic plate when the analyte is proximate to a stiffness-sensitive film within analyte sensor 92, and to determine the stiffness of the pre-charged membrane based on the single capacitance measurement. Capacitance measuring block 94 may be further configured to determine the stiffness of the pre-charged membrane based on the single capacitance measurement at least in part by performing one or more reference capacitance measurements between the pre-charged membrane and the metallic plate when the analyte is not proximate to the stiffness-sensitive film within analyte sensor 92, and determining the stiffness of the pre-charged membrane based on the single capacitance measurement and the one or more reference capacitance measurements. [0069] FIG. 6 is a conceptual diagram illustrating various capacitance measurement circuits for use within the capacitance measurement circuit 94 illustrated in FIG. 5. For example, capacitance measurement circuit 110 may include one or more of an LC oscillator circuit 112, a time-constant circuit 114, or a relaxation oscillator circuit 116. [0070] FIG. 7 is a conceptual diagram illustrating various techniques for wireless transmission for use within the wireless transmission circuit 96 illustrated in FIG. 5. The wireless transmission circuit 120 may include a wireless transmitter configured to transmit a parameter indicative of the measured stiffness of the pre-charged membrane to a remote apparatus. Wireless transmission circuit 120 may use various techniques including, for example, standard modulation 122, zigbee communication protocols 124, or magnetic coupling 126.

[0071] FIG. 8A is a chart 130 illustrating the change in capacitance with respect to carbon dioxide (CO2) concentration in an example sensing apparatus. The horizontal axis defines time in units of seconds increasing from left to right. The vertical axis defines the measured capacitance in terms of a chip output voltage. As shown in the FIG. 8A, an increase in output voltage from the capacitance chip is indicative of an increase in CO2 concentration. In addition, FIG. 8A shows an experimentally observed change in the capacitance with different concentrations of CO2. It is observed that the capacitance increases with increasing CO2 concentration indicating a decrease in tensile stiffness of the membrane consistent with the reported introduction of compressive stresses in SWNT films by adsorbed CO2 molecules. The sensitivity of individual sensors to CO2 was found to vary between 2-12mV/%CO2 with a resolution of 0.5%. [0072] FIG. 8B is a chart 132 illustrating the humidity in the surrounding environment with respect to the measurements illustrated in FIG. 8A. The horizontal axis defines time in units of seconds increasing from left to right. The vertical axis defines the humidity in terms of a percentage. As shown in FIG. 8B, the response of the example stiffness sensitive film to humidity is minimal. In some cases, the sensing device may be able to measure the humidity surrounding the sensor by coating with an appropriate film that has a stiffness that is sensitive to humidity.

[0073] FIG. 9A is another chart 134 illustrating the change in capacitance with respect to carbon dioxide (CO2) concentration in an example sensing apparatus. The horizontal axis defines time in units of seconds increasing from left to right. The vertical axis defines the measured capacitance in terms of a chip output voltage. FIG. 9A illustrates an increase in output voltage corresponding to an increase in CO2 concentration and a decrease in output voltage corresponding to a decrease in CO2 concentration. [0074] FIG. 9B is a chart 136 illustrating the humidity in the surrounding environment with respect to the measurements illustrated in FIG. 8A. The horizontal axis defines time in units of seconds increasing from left to right. The vertical axis defines the humidity in terms of a percentage. FIG. 9B shows that the response of the example stiffness sensitive film to humidity is minimal.

[0075] FIG. 10 is a flow diagram illustrating an example technique measuring a stiffness of a pre-charged membrane when a film is applied to the membrane. The technique illustrated in the flow diagram of FIG. 10 may be implemented with any of the sensing apparatuses described in this disclosure including sensing apparatuses 10, 20, 40, 60, 80 and 90 described above with respect to FIGS. 1-7. According to the technique illustrated in FIG. 10, a film alters a stiffness of a pre-charged membrane (160). In some embodiments, the film may be applied to or coated onto the pre-charged membrane. Capacitance measurement block 94 measures a capacitance between the pre-charged membrane and a metallic plate (162). Capacitance measurement block 94 determines the stiffness of the pre-charged membrane based on the measured capacitance (164). The stiffness of the pre-charged membrane may be indicative of the stiffness of the film. In some embodiments, capacitance measurement block may contain a processor, firmware, or software to determine the stiffness of the pre-charged membrane. In other embodiments, the stiffness of the pre-charged membrane may be determined by another component, such as microcontroller 98. In such embodiments, capacitance measurement block 94 may transmit the capacitance measurements to microcontroller 94 for further processing.

[0076] Anon-limiting example of a sensing apparatus that utilizes the techniques described in this disclosure is now provided. The sensing apparatus may be incorporated into any of the embodiments described above with respect to FIGS. 1-7 of this disclosure. The sensing apparatus may include a gas sensor based on an electret microphone. The gas sensor may be used for carbon dioxide (CO2) gas sensing by coating the electret membrane with single walled carbon nanotubes (SWNT). The elastic modulus of the SWNT film is altered due to adsorption of CO2 molecules thereby modifying the stiffness of the SWNT-electret composite membrane, which is measured as a variation in capacitance of the microphone. The measured capacitance of the microphone may be more sensitive to stiffness changes than to gas-induced gravimetric and external sound pressure changes. The developed sensor package may be highly amenable to portable gas sensing.

[0077] Several methodologies including optical, electrical, and gravimetric techniques have been investigated for carbon dioxide (CO2) gas sensing. Optical sensors currently provide the best selectivity during gas sensing but suffer from limitations of cost, speed of response and portability. Other CO2 sensors that may be less expensive and/or more portable suffer from lack of selectivity in the sensing mechanisms, which has made such sensors responsive to unwanted gas species. [0078] In an example embodiment of the invention, a method is provided for the application of a stiffness monitoring technique using an electret microphone for portable CO2 gas sensing. Compared to earlier resonant frequency techniques, the presented method greatly simplifies measurement, reduces size and cost and improves the speed of response. Though the sensor may be designed to respond to CO2 gas by coating the electret microphone with SWNTs, the developed technique could be equally used for other sensing applications by appropriately choosing different stiffness-sensitive films. [0079] Resonant techniques developed for stiffness measurement are impractical for sensor development because of the need for independent instrumentation in order to produce mechanical excitation and extract measurements. For example, such instrumentation may include piezoresistive components, capacitive components, laser vibrometry components, and more. The amount of independent instrumentation required for implementing resonant techniques can result in a complex system. Moreover, many resonant sensors utilize cantilever resonators which can suffer from non-specific analyte adsorption below the cantilever thus lowering the signal to noise ratio of the sensor. High-resolution optical techniques for sensitive detection of cantilever resonance are also bulky and expensive. Other techniques, such as using surface acoustic wave (SAW) sensors, are not suitable for monitoring elastic property changes of SWNT coatings because of their extreme sensitivity to conductivity changes in SWNT films. [0080] MEO96PD-00-604-NF (field-effect transistor) FET-less omnidirectional electret condenser microphones obtained from ICC Intervox may be used to construct the pre- charged membrane and back-plate. The capacitance measurement circuit may comprise a MS3110 universal capacitive readout integrated circuit (IC) from Irvine Sensors Corporation. The capacitance readout IC may provide a resolution of 4aF/VHz and a voltage output proportional to the capacitance change, which can be directly acquired into a computer. A custom fabricated printed circuit board (PCB) may be used to achieve a small sensor package with the IC and electret microphone along with reference and decoupling capacitors.

[0081] The measured change in the microphone's capacitance is almost entirely due to the stiffness change of the SWNT coated electret membrane. The effect of mass change due to CO2 adsorption is found to be negligible upon estimating the force exerted on the membrane due to mass change. The approximate force is computed by assuming an average diameter of 453pm for a CO2 molecule. Given that the diameter of the microphone's electret membrane is less than 1 cm, it is possible to calculate the maximum number of CO2 molecules that can be adsorbed on the membrane. Since surface adsorption is predominant, only a monolayer of CO2 molecules is assumed for the calculations. However, the values obtained below show that even 100s of CO2 layers would not appreciably contribute to membrane deflection.

[0082] Upon computing the maximum number of CO2 molecules that can be adsorbed on the membrane surface and taking the molecular weight of CO2 molecules as 44 g/mol, the maximum force due to CO2 adsorption is calculated to be approximately 8.9x10-1 IN. However, an applied sound pressure level of 6OdB, corresponding to a force of 4xlO-7N, much larger than the weight of an adsorbed monolayer of CO2 molecules may only negligibly affect the membrane capacitance (indeed, a no-FET electret microphone is used to deliberately decrease sensitivity to sound). Thus, the microphone capacitance shows orders of magnitude higher sensitivity to the gas induced stiffness variations in the SWNT film than to the weight of adsorbed CO2 molecules.

[0083] In contrast to earlier work, heating was found unnecessary since the desorption of CO2 was nearly complete in most experiments. This is because the self-assembled films were not covered by surfactants and directly interacted with the CO2 gas molecules. Hence, the adsorbed molecules were also free to diffuse out of the exposed nanotube walls. This is in contrast to the films used earlier where the carbon nanotubes have been assembled using surfactant molecules which cover the nanotube walls and slow down desorption of adsorbed molecules.

[0084] In summary, a technique for stiffness measurement using electret microphones has been established. A SWNT-coated CO2 sensor developed through this technique is the first stiffness-controlled sensor prototype amenable for portable and inexpensive CO2 sensing.

[0085] The various components illustrated herein may be realized by any suitable combination of hardware, firmware, and/or software. Although various components may be depicted as separate units or modules, all or several of these components may be integrated into combined units or modules within common hardware and/or software. Accordingly, the representation of features as components, units or modules is intended to highlight particular functional features for ease of illustration, and does not necessarily require realization of such features by separate hardware or software components. In some cases, various units may be implemented as programmable processes performed by one or more processors. [0086] For example, various aspects of the techniques described in this disclosure may be implemented within one or more general purpose microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or other equivalent logic devices. Accordingly, the terms "processor" or "controller," as used herein, may refer to any of the foregoing structures or any other structure suitable for implementation of the techniques described herein. [0087] If implemented in software, the techniques may be realized at least in part by a computer-readable storage medium comprising instructions or code that, when executed by one or more processors, performs one or more of the methods described above. The computer-readable medium may form part of a computer program product, which may include packaging materials. The computer-readable medium may comprise random access memory (RAM) such as synchronous dynamic random access memory (SDRAM), read-only memory (ROM), non-volatile random access memory (NVRAM), electrically erasable programmable read-only memory (EEPROM), eDRAM (embedded Dynamic Random Access Memory), static random access memory (SRAM), FLASH memory, magnetic or optical data storage media.

Claims

CLAIMS:

1. An apparatus comprising: a pre-charged membrane; a metallic plate; a film applied to the pre-charged membrane, wherein the film alters a stiffness of the pre-charged membrane; and a capacitance measurement block configured to measure a capacitance between the pre-charged membrane and the metallic plate, and to determine the stiffness of the pre-charged membrane based on the measured capacitance, wherein the stiffness of the pre-charged membrane is indicative of the stiffness of the film.

2. The apparatus of claim 1 , wherein the pre-charged membrane comprises a conductive membrane, and wherein the apparatus further comprises: a direct current (DC) voltage source configured to pre-charge the pre-charged membrane to a DC bias voltage with respect to the metallic plate.

3. The apparatus of claim 1, wherein the pre-charged membrane comprises an electret membrane.

4. The apparatus of claim 1 , wherein the pre-charged membrane is part of a micro electromechanical (MEM) capacitor.

5. The apparatus of claim 1, wherein the capacitance measurement block is further configured to measure the capacitance between the pre-charged membrane and the metallic plate without causing the pre-charged membrane to vibrate.

6. The apparatus of claim 1, wherein the capacitance measurement block determines the stiffness of the pre-charged membrane based on a capacitance measurement and one or more reference capacitance measurements between the pre-charged membrane and the metallic plate.

7. The apparatus of claim 1 , wherein the film comprises a pattern of deoxyribonucleic acid (DNA).

8. The apparatus of claim 1, wherein the film comprises a tissue culture, and a stiffness of the tissue culture is indicative of a health of the tissue culture.

9. The apparatus of claim 1, wherein the film comprises a tissue culture, and a stiffness of the tissue culture is indicative of an amount of tumor cells within the tissue culture.

10. The apparatus of claim 1, wherein the film is a stiffness-sensitive film configured to alter the stiffness of the pre-charged membrane when an analyte is proximate to the stiffness-sensitive film, and wherein the capacitance measurement block is further configured to measure the stiffness of the pre-charged membrane when the analyte is proximate to the stiffness-sensitive film.

11. The apparatus of claim 10, wherein the film comprises one or more porous molecules.

12. The apparatus of claim 11, wherein the one or more porous molecules are selected from the group consisting of carbon nanotubes, boron nanotubes, silicon nanotubes, and zeolites.

13. The apparatus of claim 10, wherein the film comprises one or more polymers.

14. The apparatus of claim 10, wherein the stiffness-sensitive film is configured to alter the stiffness of the pre-charged membrane at least in part by altering the stiffness of the pre-charged membrane upon adsorption of the analyte by the stiffness-sensitive film.

15. The apparatus of claim 10, wherein the pre-charged membrane is pre-stressed when the analyte is not proximate to the stiffness-sensitive film.

16. The apparatus of claim 10, wherein the stiffness of the pre-charged membrane is indicative of an amount of the analyte proximate to the stiffness-sensitive film.

17. The apparatus of claim 10, wherein the analyte comprises a gas, and wherein the stiffness of the pre-charged membrane is indicative of a concentration of the gas proximate to the stiffness-sensitive film.

18. The apparatus of claim 17, wherein the gas is selected from a group consisting of carbon dioxide, hydrogen, nitric oxide, water vapor, and one or more volatile organic compounds.

19. The apparatus of claim 10, wherein the pre-charged membrane is a first pre- charged membrane, and the stiffness sensitive film is a first stiffness sensitive film, and the apparatus further comprises: a second pre-charged membrane comprising a second stiffness-sensitive film different from the first stiffness-sensitive film, wherein the second stiffness-sensitive film is configured to alter a stiffness of the second pre-charged membrane when the analyte is proximate to the second stiffness-sensitive film, and wherein the capacitance measurement block is further configured to measure the stiffness of the second pre-charged membrane, and to determine an amount of the analyte proximate to the sensing apparatus based on the measured stiffness of the first pre- charged membrane and the measured stiffness of the second pre-charged membrane.

20. The apparatus of claim 19, wherein the capacitance measurement block determines the amount of the analyte proximate to the sensing apparatus at least in party by using principal component analysis (PCA).

21. The apparatus of claim 10, wherein the pre-charged membrane is a first pre- charged membrane, the stiffness sensitive film is a first stiffness sensitive film, and the analyte is a first analyte, and the apparatus further comprises: a second pre-charged membrane comprising a second stiffness-sensitive film, wherein the second stiffness-sensitive film is configured to alter a stiffness of the second pre-charged membrane when a second analyte, different from the first analyte, is proximate to the second stiffness-sensitive film, wherein the capacitance measurement block is further configured to determine the stiffness of the second pre-charged membrane.

22. The apparatus of claim 21 , wherein the capacitance measurement block is further configured to determine an amount of the first analyte and an amount of the second analyte proximate to the sensing apparatus at least in part by using principal component analysis (PCA).

23. The apparatus of claim 1, further comprising a wireless transmitter configured to transmit a parameter indicative of the measured stiffness of the pre-charged membrane to a remote apparatus.

24. A method comprising: altering a stiffness of a pre-charged membrane with a film that is applied to the pre-charged membrane; measuring a capacitance between the pre-charged membrane and a metallic plate; and determining the stiffness of the pre-charged membrane based on the measured capacitance, wherein the stiffness of the pre-charged membrane is indicative of the stiffness of the film.

25. The method of claim 24, wherein the pre-charged membrane comprises a conductive membrane, and wherein the method further comprises: pre-charging the pre-charged membrane to a DC bias voltage with respect to a metallic plate.

26. The method of claim 24, wherein the pre-charged membrane comprises an electret membrane.

27. The method of claim 24, wherein the pre-charged membrane is part of a micro electromechanical (MEM) capacitor.

28. The method of claim 24, wherein measuring the capacitance between the pre- charged membrane and the metallic plate comprises: measuring the capacitance between the pre-charged membrane and the metallic plate without causing the pre-charged membrane to vibrate.

29. The method of claim 24, wherein measuring the capacitance between the pre- charged membrane and the metallic plate comprises comprises: performing one or more reference capacitance measurements between the pre- charged membrane and the metallic plate.

30. The method of claim 24, wherein the film comprises a pattern of deoxyribonucleic acid (DNA).

31. The method of claim 24, wherein the film comprises a tissue culture, and a stiffness of the tissue culture is indicative of a health of the tissue culture.

32. The method of claim 24, wherein the film comprises a tissue culture, and the stiffness of the tissue culture is indicative of an amount of tumor cells within the tissue culture.

33. The method of claim 24, wherein the film is a stiffness-sensitive film configured to alter the stiffness of the pre-charged membrane when an analyte is proximate to the stiffness-sensitive film, and wherein measuring the stiffness of the pre-charged membrane comprises measuring the stiffness of the pre-charged membrane when the analyte is proximate to the stiffness-sensitive film.

34. The method of claim 33, wherein the film comprises one or more porous molecules.

35. The method of claim 34, wherein the one or more porous molecules is selected from the group consisting of carbon nanotubes, boron nanotubes, silicon nanotubes, and zeolites.

36. The method of claim 33, wherein the film comprises one or more polymers.

37. The method of claim 33, wherein altering the stiffness of the pre-charged membrane comprises altering the stiffness of the pre-charged membrane upon adsorption of the analyte by the film.

38. The method of claim 33, wherein the pre-charged membrane is pre-stressed when the analyte is not proximate to the stiffness-sensitive film.

39. The method of claim 33, wherein the stiffness of the pre-charged membrane is indicative of an amount of the analyte proximate to the stiffness-sensitive film.

40. The method of claim 33, wherein the analyte comprises a gas, and wherein the stiffness of the pre-charged membrane is indicative of a concentration of the gas proximate to the stiffness-sensitive film.

41. The method of claim 40, wherein the gas is selected from a group consisting of carbon dioxide, hydrogen, nitric oxide, water vapor, and one or more volatile organic compounds.

42. The method of claim 33, wherein the pre-charged membrane is a first pre-charged membrane, and the stiffness sensitive film is a first stiffness sensitive film, and the method further comprises: altering a stiffness of a second pre-charged membrane comprising a second stiffness-sensitive film, different from the first stiffness-sensitive film, when the analyte is proximate to the second stiffness-sensitive film; measuring the stiffness of the second pre-charged membrane; and determining an amount of the analyte proximate to the sensing apparatus based on the measured stiffness of the first pre-charged membrane and the measured stiffness of the second pre-charged membrane.

43. The method of claim 42, wherein determining the amount of the analyte proximate to the sensing apparatus comprises performing principal component analysis (PCA).

44. The method of claim 33, wherein the pre-charged membrane is a first pre-charged membrane, the stiffness sensitive film is a first stiffness sensitive film, and the analyte is a first analyte, and the method further comprises: altering a stiffness of a second pre-charged membrane comprising a second stiffness-sensitive film when a second analyte, different from the first analyte, is proximate to the second stiffness-sensitive film; and determining the stiffness of the second pre-charged membrane.

45. The method of claim 44, further comprising determining an amount of the first analyte and an amount of the second analyte proximate to the sensing apparatus at least in part by using principal component analysis (PCA).

46. The method of claim 24, further comprising: transmitting a parameter indicative of the measured stiffness of the pre-charged membrane to a remote apparatus.

47. A sensing apparatus, comprising: film means for altering a stiffness of a pre-charged membrane, wherein the film means is applied to the pre-charged membrane; means for measuring a capacitance between the pre-charged membrane and a metallic plate; and means for determining the stiffness of the pre-charged membrane based on the measured capacitance, wherein the stiffness of the pre-charged membrane is indicative of the stiffness of the film.