Abstract:
A thin film piezoelectric polymer acoustic sensor for the passive detection of heart and blood flow sounds is described. Sensors may have initially slack film components which are tented by the mass of a housing when a sensor is positioned on acoustic medium. Enhanced sensor performance is provided by accommodation of the sensor spring constant to the spring constant of human flesh. Sensor performance may be enhanced by a combination of physical parameter ranges. A plurality of sensors may compose a linear array or an array aperture.

Description:
FIELD OF THE INVENTION 
     This invention relates to a flexible, thin-film sensor for the passive acoustic detection of heart and blood-flow sounds. 
     BACKGROUND OF THE INVENTION 
     Certain polymers, copolymers and blends demonstrate piezoelectric behavior due to dipolar ferroelectricity residing in specific crystal phases. The most common of the semicrystalline ferroelectric polymers are poly(vinylidene fluoride) (PVDF) and its copolymer with trifluoroethylene, P(VDF-TrFE). As with any piezoelectrically active material, if electrodes are deposited on both of the major surfaces of a sample of piezoelectric film, the element functions as an electromechanical transducer and thus can be used as a sensor or actuator. If the film, operating as a sensor, is subjected to stress either in the direction corresponding to the film thickness, or in either of the transverse directions, an electric potential proportional to the applied stress is developed across the thickness of the film. The electrodes enable connection to external electronic circuitry, making it possible to process the information provided by the sensor. 
     Sensors having piezoelectric film transducers for passive detection of body sounds are known. See, e.g., U.S. Pat. Nos. 5,365,937 and 5,595,188 and published applications WO/92-08407 and WO/90-08506. However, known sensors may be characterized by undesirable performance constraints, including low signal to noise ratio, cross-talk, and signal contamination from power line harmonics or ambient room noise. These constraints may in part be attributable to transducer non-conformity to human body surfaces. 
     An improved thin-film sensor for the efficient passive detection of heart and blood-flow sounds is needed. 
     SUMMARY OF THE INVENTION 
     Integral components of the flexible thin-film sensor of this invention may comprise a piezoelectric film which may have a rectangular active area, a housing including film support means onto which the film is mounted, and a foot. The foot is the only sensor component intended to contact the skin. Hereafter these components are referred to as the “film”, “housing”, and “foot”. 
     The invention may include sensors in which an initially slack piezoelectric film having fixed opposite edges is tented by the sensor mass to produce a stress upon vertical displacement of the acoustic medium, e.g., human flesh, upon which the sensor may be positioned. 
     The invention may also include sensors wherein the piezoelectric film is tensioned initially. 
     An important feature of the invention comprises sensors wherein the sensor spring constant is of the same order of magnitude and preferably matched to the spring constant of human flesh. It is preferred that the sensor be configured such that the film segment operates with a spring constant of from about 2 kN/m to about 4 kN/m. 
     The invention may comprise a single sensor or a plurality of individual sensors. A plurality of individual sensors may be assembled in a linear array or snake. A linear array may also comprise a plurality of links, each comprised of two or more individual sensors. Linear arrays of sensors may be positioned within a patient intercostal space (ICS). Undesired movement of a positioned array may be avoided by a laterally adjacent strip or laterally adjacent strips of adhesive tape. 
     A plurality of linear sensor arrays or of individual sensors may be positioned to accommodate a patient acoustic window. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematic of a tented film (foot and housing not shown). 
     FIG. 2 is a section of a single sensor which may be in a link comprising a plurality of sensors. Links typically comprise three sensors. The housing, the upper and lower components of the film support structure within the housing, an arch in the lower film support component to accommodate a tented film, rails at the outer bottom edges of the housing and details illustrating the clamping of the film by the support structure are shown. 
     FIG. 3 is an assembly drawing. On the left, the film (with attached foot) and the two piece film support structure are shown disassembled. On the right the same components are shown assembled. 
     FIG. 4 depicts one side of a piezoelectric film strip. Nine conductive sensing areas, and details of the conductive ink printing on the film are shown. Because the film is transparent, some printing on the opposite side is illustrated as well. 
     FIG. 5 is a top view of a single linear array or snake comprising three links, each comprising three sensors (nine sensors). Each of the three links may comprise one of the sensing areas of a film strip as shown by FIG. 4. A tape is shown adjacent each side of the three link array to eliminate undesirable sensor housing motion that may lead to mechanical cross-talk and to accommodate appropriate PVDF film tenting on a curved chest. Linear arrays comprising any desired number of links and sensors may be assembled in like manner. 
     FIG. 6 illustrates a three dimensional view without the tape of a linear three link (nine sensor) array as shown by FIG.  5 . 
     FIG. 7 is a bottom view of the snake of FIGS. 5 and 6 showing circular feet attached to the film with plastic bars (hashed lines) rectangular in cross-section. The feet may be other than circular, e.g., polygonal and the attachment means may be other than bars, e.g., discs. 
     FIG. 8 is a side view of the three link (nine sensor) snake shown by FIGS. 5 and 6. 
     FIG. 9 is a three dimensional view of the underside of a three link (nine sensor) snake as shown by FIGS. 5 and 6. 
     FIG. 10 illustrates five, three link snakes positioned one within each of the five intercostal spaces (ICS&#39;s) of a patient. The snakes are positioned to accommodate a patient acoustic window. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One preferred embodiment of the sensor is shown by FIGS. 1 to  3 . The film is slightly longer than the distance between the points at which the film is fixed to the housing, so that the film is initially slack. Thus, when the sensor foot is put in contact with the surface of the intervening acoustic medium, the film is tented by the weight of the housing. For a given acoustic medium, e.g., a patient&#39;s chest, the sensitivity and frequency response of the sensor are dependent upon the housing mass, the surface area of the bottom of the foot, and the degree to which the film is tented (i.e., the “offset angle” θ). Referring to FIG. 1, if the foot is displaced vertically (i.e., the direction perpendicular to the upper surface of the sensor housing) just until the film is relieved of its slack, then the difference in vertical position between the resultant apex and a point at either of the fixed edges of the film is referred to as the “initial static displacement” (X 1  in the figure). The term “static” is used to differentiate the gross film displacement described here from the small transient displacements the device is designed to detect. When the sensor is placed in contact with the surface of the acoustic medium, the film is stretched by the load of the housing mass, providing an additional incremental static displacement (X 2 ). The sum of the initial and incremental static displacements comprise the total static displacement (X 1 +X 2 ), which, for a given distance (L) determines the static offset angle (θ=θ 1 +θ 2 ) of the film. 
     It is apparent from FIG. 2 that a small transient displacement of the surface of the acoustic medium trapped beneath the sensor foot results in a corresponding vertical displacement of the foot, which produces a stress, T 1 , along the length of the film. Since the film is piezoelectric, a voltage, v, proportional to the induced stress is generated between the film electrodes according to Equation 1              v   =         -     d   31       ɛ          T   1        t             (   1   )                                
     where d 31  and ε are the appropriate piezoelectric stress constant and dielectric permittivity of the film, respectively, and t is the film thickness. 
     The relative stiffness, K eff , (reciprocal of compliance) of the tented film element and that for the acoustic medium, K med , as seen by the sensor foot are important to the operation of the sensor. Specifically, broadband sensitivity is maximized for given housing mass and film dimensions when K eff =K med . For a film element of given dimensions, two parameters may be manipulated in order to match K eff =K med ; total static displacement and sensor foot radius, respectively. Addressing the former, under sufficient mass loading, the film behaves like a nonlinear spring, for which the stiffness increases with increasing static displacement according to Equation 2:                  K   eff     =       4        K        (     1   -       L   O           L   2     +     4        X   2               )         +       16        KX   2          L   O           (       L   2     +     4        X   2         )       3   2             ,           (   2   )                                
     where L o  is the length of the film, X is the total static displacement, L is the distance between housing rails, and K is the stiffness of the film given by Equation 3:              K   =       YA     L   O       .             (   3   )                                
     In Equation 3, Y is Young&#39;s modulus for the piezoelectric polymer, and A is the cross-sectional area through the width of the film. It should be noted that for practical housing mass the dominant component of X is the initial static displacement due to the slack in the film. With regard to the foot size, the apparent stiffness of the acoustic medium, as seen by the sensor, increases with increasing foot radius, a, according to Equation 4:                 K   med     =     6      π                   a        (       μ   1     -     a                 ω                   G   ′            ρ   /   2           )           ,           (   4   )                                
     where ρ is the density of the medium, and μ 1  and μ 2  are its Lamé constants, where G′={square root over (|G|−μ 1 +L )}, where G=μ 1 +jωμ 2  is the shear modulus of the medium. Thus the flesh beneath the sensor foot looks stiffer for a foot which contacts the skin over a larger area. 
     As mentioned previously, the housing mass has a significant effect on the performance of the sensor. First, assuming all components of the sensor are allowed to move only in the vertical direction (the purpose of the tape of FIG. 5 is to reduce unwanted housing motion in the transverse directions), the response of the sensor to displacement of the surface with which it is in contact can be described as an under-damped, second-order, high-pass filter. The resonance frequency, f n , of the filter is a function of the housing mass, m h , the number of sensors in a single link, n s , and the equivalent stiffness, K eq  (the series combination of K eff  and K med  for a single sensor). The relationship is                f   n     =         K   eq         m   h     /     n   s                   (   5   )                                
     In Equation 5 the mass of the sensor foot and the entrained mass of the acoustic medium trapped beneath the foot are neglected since they are each much smaller than the housing mass. Secondly, the broadband sensitivity of the device increases with increasing housing mass, but practical considerations impose an upper limit on this parameter. These considerations include limits on patient tolerance of mass on his or her cheat and the threat of severely degrading sensor performance by having the housing rails come in contact with the patient&#39;s skin. 
     FIGS. 2 and 3 illustrate the design of a single, slack film, sensor having an initial static displacement angle θ 1  of about 9°, a single sensor (link) housing mass of about 42 grams, and a spring constant of about 3 kN/m. 
     FIG. 2 shows a section of a single link  100  within a preferably metal housing  101  for a link of three sensors. The link housing comprises integral top and bottom parts  102 ,  103 . The perimeter of housing part  102  is defined by a generally trapezoidal end wall  104 , by sidewalls  105 ,  106 , top  107  and bottom  108 . Because the area of the top  107  is smaller than the area of the bottom  108 , the end and sidewalls  104 ,  105  and  106  slope inwardly to render the top part  102  of the link housing trapezoidal in cross-section and so impart flexibility to a linear sensor array (see FIGS.  5  and  6 ). The bottom housing part  103  defines a chamber in which top and bottom film support components  109  and  110  are positioned. The outer bottom edges of the bottom housing part  103  provide sensor side rails  111  and  112 . 
     A film  200  is shown pinched at the spaced apart points A, A′ between the top and bottom film support components  109 ,  110 . The film segment which spans the space between the pinch points is slack. An arch  113  in the lower film support element  110  accommodates tenting of the slack film  200 . 
     The sensitivity of the sensors may increase as a function of the housing mass (film mass loading). In the preferred practice of the invention, the mass of a single housing is from about 40 grams to about 45 grams. However, the housing mass may be less, e.g., 20 grams, or greater, e.g., 200 grams. 
     Means  114 , preferably plastic bars, attach circular foot  115  to the exposed lower surface of the film  200 . In preferred embodiments of the invention, the circular foot has a diameter of 0.2 to 0.5 inch, preferably 0.3 to 0.4 inch. A polygonal foot of like area may be used instead of a circular foot. This foot and the support bars may be fabricated from any desired plastic, e.g., polystyrene, polypropylene, polycarbonate or the like. 
     The length of foot attachment means  114  is dimensioned to avoid patient skin contact of the sensor rails  111 ,  112  provided by the bottom part  103  of the housing as shown in FIG.  2 . 
     A through hole  115  for a set screw to maintain the housing  101  and film support parts  109 ,  110  in assembly is shown (hashed lines). 
     FIG. 3 shows a sensor disassembled (left portion). Upper and lower film support parts  109  and  110  are shown with pins  116  and pin receiving holes  116   a  (hatched lines). The upper film support component is held in contact with the bottom component by friction between the pins  116 . 
     FIG. 4 shows one side of a film  200  having nine rectangular spaced apart conductive sensing areas  1000  and associated printed conductive ink lines  1001  which connect the sensing areas to pins within a male component  1002  of a plastic connector. The sensing areas  1000  are equally spaced apart and separated by holes  1003  in the film  200 . Stiffeners  1004  have holes  1005  for assembly pins (not shown). Connector  1002  houses conducting pins  1006 . Polarity ridges  1007  are provided on the connector  1002  to facilitate proper orientation between the male  1002  and female (not shown) components of the connector. 
     The invention includes a linear array of sensors positioned on a continuous piezoelectric film strip having a plurality of separate sensing elements spaced longitudinally apart on a surface thereof. A plurality of similarly longitudinally spaced sensors is positioned along the strip, wherein each of said sensors comprises at least one of the plurality of sensing elements on the surface of film strip. 
     FIGS. 5 to  8  illustrate one embodiment of such a three-link (nine sensor) linear array  300  of links  301 . Each link comprises three sensors  100  (not shown). 
     As shown in FIG. 5, the top of each link  301  has two holes  117  and  118  for set screws or like means to secure the link housing  101  to the film support components  109 ,  110  (see FIG.  2 ). A rectangular reflective tape piece  119  is affixed to the top of each sensor  100  to accommodate photogrammetric sensor location procedures. 
     Means for securing the three link linear sensor array against unwanted movement may be provided by adhesive tape  120  positioned at each side of the array. 
     As FIG. 5 shows, the portions of the film strip between each of the links included in the link  301  functions as a hinge to impart array flexibility to the array  300 . 
     Housings are preferably metal. Zinc castings are appropriate to provide weight and shielding from electromagnetic interference. 
     In a flexible linear array  300  as illustrated by FIGS. 5 and 6, individual links  300  are appropriately spaced apart (center to center). 
     Each flexible linear array is preferably capable of at least 10 degrees torsional rotation link-to-link from flat (by weight of the sensors alone) and a vertical hinge angle of at least +45 degrees and −30 degrees relative to the adjacent link. The spacing between each of the linear arrays included in a group or assembly of linear arrays may be as little as 2 cm or as wide as 4 cm (center-to-center). The back of each linear array is preferably flat to accommodate visual imaging. Links and sensor elements preferably do not rub or clang on each other in order that mechanical cross-talk is reduced. 
     Sensors included in an assembly or array preferably have interchannel phase differences of less than 1 degree standard deviation over the 100 Hz to 2 kHz frequency band when applied over the angles given. The sensor housing is preferably metal, e.g. zinc castings to provide weight and stability. 
     Sensor temperatures during data acquisition may lie between 20° C. and 38° C. Sensor-to-sensor variations may be as high as 10° C. External temperature differences and dynamics between array elements preferably do not affect interchannel phase specifications within the performance band between 100 Hz and 2 kHz. 
     The amplifier impedance to which each sensor may be attached (excluding cabling) is typically at 199 megohms in parallel with 25 pf or more. 
     Cross-talk between any two sensors, other than through the intended propagation path is preferably less than −30 dB. 
     Conventional individual sensor connectors and cabling are preferably mated with the array connector. Connector/cable design is selected to minimize external noise propagation down the cable into the sensors, and preferably adapt to the array layout on human contours so that array performance is not compromised.