as) United States
`a2) Patent Application Publication 0) Pub. No.: US 2004/0138568 Al
`(43) Pub. Date: Jul. 15, 2004
`
`Lo et al.
`
`US 20040138568A1
`
`(54) ULTRASONIC MONITOR FOR MEASURING
`HEART RATE AND BLOOD FLOW RATE
`
`(22)
`
`Filed:
`
`Jan. 15, 2003
`
`Publication Classification
`
`(75)
`
`Inventors: Thomas Ying-Ching Lo, Fremont, CA
`(US); Tolentino Escorcio, Dublin, CA
`(US)
`
`Tint. C07 ieee eccceeeeeseeennneeeceesnneeeeenneeeee A61B 8/14
`(SV)
`
`ceeccsecssecesesseessessnessncsaessneneeneeeneeeneees 600/459
`(52) U.S. C0.
`
`Correspondence Address:
`TOWNSEND AND TOWNSEND AND CREW,
`LLP
`TWO EMBARCADERO CENTER
`EIGHTH FLOOR
`
`SAN FRANCISCO, CA 94111-3834 (US)
`
`(73) Assignee: Salutron, Inc., Fremont, CA
`
`ABSTRACT
`(57)
`The invention provides an ultrasonic monitor for measuring
`pulse rate values in a living subject, including a module with
`at least one source of ultrasonic energy, a gel pad comprised
`of a polymer and from about 50 to about 95% by weight of
`an ultrasound conductive diluent, wherein the gel pad is
`positioned in direct contact between the module and the
`living subject; an ultrasonic energy detector and associated
`hardware and software for detecting, calculating and dis-
`
`(21) Appl. No.:
`
`10/346,296
`
`playing a readout of the measured rate values.
`
`APPLE 1028
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`APPLE 1028
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`Patent Application Publication
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`US 2004/0138568 Al
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`Patent Application Publication Jul. 15,2004 Sheet 2 of 4
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`Patent Application Publication Jul. 15,2004 Sheet 3 of 4
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`US 2004/0138568 A1
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`Patent Application Publication Jul. 15,2004 Sheet 4 of 4
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`US 2004/0138568 A1
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`US 2004/0138568 Al
`
`Jul. 15, 2004
`
`ULTRASONIC MONITOR FOR MEASURING
`HEART RATE AND BLOOD FLOW RATE
`
`BACKGROUNDOF THE INVENTION
`
`[0001]
`
`a) Field of the Invention
`
`[0002] The present invention relates to ultrasonic monitors
`for measuring heart and pulse rates in living subjects.
`Methods for measuring heart and pulse rates of living
`subjects through ultrasonic means are also encompassed by
`the instant invention.
`
`[0003]
`
`b) Description of Related Art
`
`[0004] Measuring Heart and Pulse Rates
`
`[0005] Measuring heart and pulse rates in living subjects
`has been accomplished by various means. The pulse rate is
`commonly measured by lightly touching one’s fingers over
`an artery and counting the rate of pulsation. The heart rate
`is usually measured by a sensing device using electrodes that
`monitor the electrical activity of the heart (e.g., contact
`monitors) based on electrocardiograms (EKG OR ECG).
`Measuring rate values is a useful tool in individualizing and
`optimizing exercise regimens.
`Individuals who want
`to
`increase endurance or performance aim for certain target
`heart rates to maximize progression towards their goals.
`Conversely, adults with a history of heart disease must avoid
`exceeding a certain heart or pulse rate to reduce unnecessary
`strain on the heart and resultant injury.
`
`[0006] The heart rate is the rate of contractions over a
`given time period, usually defined in beats per minute. A
`pulse can be defined as the rhythmical dilation of a vessel
`produced by the increased volume of blood forced into the
`vessel by the contraction of the heart. The pulse can be felt
`at many different points on the body, including the wrist
`(radial artery) and neck (carotid artery), which are among
`the most easily accessible points. Since a heart contraction
`almost always produces a volume of blood that can be
`measured as a pulse, the heart rate and pulse rate are usually
`the same. However, there are certain situations where the
`pulse rate may differ from the heart rate. For example, the
`body may generate an irregular heart beat or a premature
`heart beat. In this scenario, a heart contraction would not
`force out enough blood to be measured as a pulse and the
`measured pulse rate would be different from the heartrate.
`
`[0007] Heart rate monitors that provide continuous heart
`rate readings rather than a single point measurement require
`wearing a chest strap. There are a few heart rate monitors
`that do not require a chest strap. Most, if not all, of these
`monitors do not provide continuous heart rate readings but
`measure the wearer’s pulse and transmit that pulse upon
`request. Most users would have to stop exercising in order
`to get this type of measurement, which is disruptive to an
`exercise regimen. In U.S. Pat. Nos. 5,738,104 and 5,876,350
`and European Patent No. 0861045B1, Lo et al disclosed an
`EKGheart rate monitor that does not require a chest strap so
`that the user does not have to stop exercising to take a heart
`rate measurement. All the sensors and electronics are con-
`
`tained in a wristwatch. The software is effective in filtering
`out muscle motion noise. Therefore the user can walk and
`
`jog while taking a single point measurement. However,this
`technologystill does not offer continuous readings. Hence,
`most users or heart patients that demand continuous heart
`rate readings choose a monitor that requires a chest strap.
`
`Mostof the population, including the elderly, would prefer
`a monitor that does not require a chest strap. There are also
`portable patient monitors (e.g., vital signs monitors, fetal
`monitors) that can perform functions as diverse as arrhyth-
`mia analysis, drug dose calculation ECG waveforms cas-
`cades, and others. However, such monitorsare usually fairly
`large (e.g., size of a small TV) and are connected to the
`patient through specific wires. The art has, thus, a need for
`an improved heart monitoring device, specifically one that
`provides continuousheart rate readings for both healthy and
`compromised living subjects without
`the need for chest
`straps, wirings, or the like.
`
`It is well knownin the prior art to employ sonar
`[0008]
`technologyto identify moving objects. A piezoelectric crys-
`tal may be used both as the power generator and the signal
`detector. In this case, the ultrasonic energy is emitted in a
`pulsed mode. The reflected signal is picked up by the same
`crystal after the output power source is turned off. The time
`required to receive the reflected signal depends upon the
`distance between the source and the object. The frequency
`shift, better known as Doppler shift, is dependent upon the
`speed of the moving object. This technique requires only one
`crystal but
`the detector circuit will only work after the
`transmitter poweris turned off. It is conceivable to use this
`methodto detect the motion of a blood vessel wall to extract
`
`the pulse rate information. However, for superficial blood
`vessels this technique requires very high speed power
`switching due to the short distance between source and
`object. In addition, muscle movement will also generate
`reflections that compromise the signal-to-noise-ratio in the
`system. The muscle noise signal in this case is very similar
`to the signal due to blood vessel wall motion. Therefore, it
`is very difficult to detect heart rate this way whenthe living
`subject is in motion. The advantage of this approach, how-
`ever, is low cost and low power consumption. For continu-
`ous mode two piezoelectric elements may be used. Either
`may be used as the transmitter and the other as receiver or
`detector at a given time. These two elements can be posi-
`tioned at an angle to the direction of the flow on opposite
`sides or on the same side of the conduit. If they are on the
`same side, the two crystals can be conveniently packaged
`into a module. The flow rate or flow velocity is proportional
`to the Doppler shift relative to the operating frequency. The
`main advantage of continuous mode for pulse rate applica-
`tion is that the Doppler shift due to blood flow is distinctly
`different from the shifts due to muscle artifacts or tissue
`
`movement. The shift due to blood flow is higher in fre-
`quency than that due to muscle motion. Therefore, even if
`the muscle motion induced signals are larger in amplitude,
`they may still be filtered out by a high pass filter in either
`analogor digital form to retain the blood flow signals. In this
`respect the ultrasound method is superior to infrared, pres-
`sure sensing and even EKGbased technologies.
`
`[0009] One device useful for the measurementof heart and
`pulse rates is an electronic unit worn on the wrist. Several
`such devices are known in the art. U.S. Pat. No. 4,086,916
`(Freeman et al.) discloses a cardiac wristwatch monitor
`having ultrasonic transducers mounted in the wrist strap
`portion. The transducers are encased in an epoxy and
`covered with an insulative coating. U.S. Pat. No. 4,163,447
`(Orr) discloses a wrist-mounted heartbeat rate monitorthat
`relies upon light-emitting diodes. U.S. Pat. No. 4,256,117
`
`6
`
`

`

`US 2004/0138568 Al
`
`Jul. 15, 2004
`
`(Perica et al.) discloses a wrist-mounted combination stop-
`watch and cardiac monitor that uses a pressure transducer to
`measure pulse rate.
`
`In Freeman’s invention, a wristwatch was intended
`[0010]
`to offer a continuouspulse rate monitor. However, ultrasonic
`energy is prone to diffraction and attenuationat the interface
`of two media of different densities. Any air gap at
`the
`interface or any air bubbles in the media will also make
`ultrasonic energy transfer unreliable. Therefore, it has been
`a standard practice to apply water or an aqueous gel between
`the transducer module and the living subject to eliminate any
`air gap. Unfortunately water and aqueous gels dry up
`quickly in open air. For continuous rate monitoring,
`the
`requirement to apply water or gel frequently is not accept-
`able. In U.S. Pat. Nos. 6,371,920 B1 and 6,394,960 Bl
`attempts were made to overcome this problem by using an
`array of small
`transducers protruding from the support
`surface to makefirm contact with a living subject with no air
`gap in between. However,this increases the complexity and
`cost of the transducer device and its driving electronics
`significantly. The air gap will not be totally removed,either,
`due to body hairs and the variable condition of skin from
`personto person. In U.S. Pat. No. 6,447,456 B1, two sets of
`transducers are used at the radial artery and the ulnarartery.
`The idea is to cope with the compromised signal quality due
`to motion at the wrist that may create an air gap from time
`to time. With twosets of transducers the hope is that at least
`one of them will reliably detect
`the Doppler signal
`to
`identify the heart beat. The disadvantages of continuous
`mode over pulsed mode are higher cost and more power
`consumption.
`
`SUMMARYOF THE INVENTION
`
`[0011] The present invention relates to an ultrasonic moni-
`tor for measuring rate values of a living subject, including
`heart rate and pulse rate. Due to continued advances in
`piezoelectric material and microelectronic technologies, an
`ultrasound based pulse rate monitor system can be minia-
`turized to reduce cost and power consumption.
`
`[0012] One aspect of the invention provides an ultrasonic
`monitor for measuring pulse rate values in a living subject,
`including a module with at least one source of ultrasonic
`energy, a gel pad comprised of a polymer and from about 50
`to about 95% by weightof an ultrasound conductive diluent,
`wherein the gel pad is positioned in direct contact between
`the module and the living subject; an ultrasonic energy
`detector and associated hardware and software for detecting,
`calculating and displaying a readout of the measured rate
`values. The gel pad is made of a polymer having the
`following characteristics:
`[0013]
`a) Hardness: Needle Penetration from about 5
`to about 300 (“410 mm) according to ASTM D15,
`preferably from about 25 to about 150, and most
`preferably from about 30 to about 50;
`[0014]
`b) Tensile Strength from about 5 to about 500
`psi according to ASTM D412,preferably from about
`10 to about 300 psi, and most preferably from about
`50 to about 200 psi; and
`[0015]
`c) Elongation from about 50% to about 800%
`according to ASTM D412, preferably from about
`200% to about 700%, and most preferably from
`about 300% to about 500%.
`
`[0016] The gels are stable after stress and temperature
`cycling (with no oil exuding out). The display may option-
`ally include electronics and software for analyzing the rate
`values from a living subject. Conversely, the module may
`include the electronics and software for analysis of the rate
`values.
`
`[0017] Another aspect of the invention provides a method
`of measuring rate values of a living subject. The method
`includes providing an ultrasonic monitor as described above
`and contacting the monitor with the living subject.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0018] The present invention is best understood when read
`in conjunction with the accompanyingfigures which serve to
`illustrate the preferred embodiments. It is understood, how-
`ever, that the inventionis not limited to the specific embodi-
`ments disclosed in the figures.
`
`[0019] FIG. 1 depicts a front view of an ultrasonic moni-
`tor of the instant invention. Shownhere is a wristwatch with
`attached wristband (10) having a module (20) with a gel pad
`(30), wherein the gel pad contacts the skin of a living
`subject. The figure also depicts the display unit (40) which
`provides a readout of measured rate values.
`
`[0020] FIG. 2 depicts a cross sectional view of a trans-
`ducer module assembly. The substrate of the housing (10)
`may be metal or plastic. The transducers (20) are molded in
`ABSand permanently adhered to the housing. On top of the
`transducer module (30), there is an optional thin adhesive
`layer (40) which can be a lower oil content gel or an
`appropriate adhesive material. The top structure is the gel
`pad (50) that is in direct contact with the living subject.
`
`[0021] FIG. 3 depicts a block diagram of a typical ultra-
`sound based heart rate monitor system.
`
`FIG.4 depicts the block diagram ofthe software of
`[0022]
`this invention. The amplified Doppler signal after anti-
`aliasing filtering is sampled by an A/D converter in a
`microcontroller. The sampled datais further digitally filtered
`by a high pass filter or a combination of high pass and low
`pass filters. The output is applied with either an absolute
`value operator or a square operator followed by a stage of
`low passfilter. Finally this digitally processed data is used
`to determine the pulse rate.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`[0023]
`
`a) Definitions and General Parameters
`
`[0024] The following definitions are set forth to illustrate
`and define the meaning and scope of the various terms used
`to describe the invention herein.
`
`[0025] The terms “ultrasonic” and “ultrasound” are used
`interchangeably herein and refer to a sound wave having a
`frequency between about 1 MHz and about 10 MHz. An
`“ultrasonic transducer” (i.e., a transducing means)
`is a
`device used to introduce sonic energy into a test object (e.g.,
`living subject) and also to detect reflected energy from the
`object as in the instant invention. Typical of this type of
`device are piezoelectric crystals which respond to electric
`pulses from an instrument with a mechanical pulse, and to
`mechanical pulses (reflected energy) from the test object
`with electrical energy detectable by the instrument. Ultra-
`
`7
`
`

`

`US 2004/0138568 Al
`
`Jul. 15, 2004
`
`sound mayalso be used as a sound wave imaging technique
`used to examine a part of the body (e.g., breast, abdomen,
`heart) in order to evaluate a specific tissue or progression of
`a diseased tissue. In addition, ultrasound is used to monitor
`fetuses and their growth.
`
`[0026] A “rate value” as used herein, refers to a value that
`can be measured. A rate value of the instant
`invention
`
`includes, but is not limited to, a heart rate, pulse rate, fetal
`heart rate, and fetal pulse rate.
`
`[0027] The term “module with transducing means”refers
`to the assembly that contains the piezoelectric transducer.
`See, for example, FIG. 2. The module may optionally
`include electronics for analysis of the rate values.
`
`[0028] The term “thermoset gel” as used herein refers to a
`gel that is generally made of a chemically bonded three-
`dimensional elastomeric network which entraps a large
`amountof low volatility liquids or diluents. The elastomeric
`network is permanent and cannotbe reversed to a liquid state
`through heating. A certain amountof diluent is necessary in
`order to ensure good conformability of the gel to the skin
`and low attenuation for ultrasound transmission whilestill
`
`maintaining the load bearing properties. The gel can be used
`at a temperature that ranges from -30° C.
`to +70° C.,
`wherein the gel maintains its shape and load-bearing elastic
`properties. A “silicone gel” or a ” polyurethane gel” is an
`example of a thermoset gel. Prior to this invention, thermo-
`set gels have not been used as ultrasound transmission
`media.
`
`[0029] The term “thermoplastic gel” as used herein refers
`to a gel that is generally made of a thermoplastic elastomer
`with a large proportion of interdispersed diluent. Thermo-
`plastic elastomers include block copolymerssuchas styrene-
`butadiene-styrene, styrene-isoprene-styrene, styrene/ethyl-
`ene-co-butylenes/styrene,
`and
`styrene/ethylene-co-
`propylene/styrene. The styrene end blocks form glassy
`domains at room temperature. The glassy domains act as
`physical crosslinks that provide the elastomeric properties of
`the polymer. During heating above the glass transition
`temperature of styrene, i., about 100° C.,
`the glassy
`domains melt and the polymers revert to a liquid state.
`During cooling, the glassy domains re-form again. Hence,
`the process is reversible. Other block copolymers, such as
`ethylene-(ethylene-co-butylene)-ethylene
`copolymers
`which contains crystalline polyethylene end blocks, can also
`be used to prepare thermoplastic gels. Prior to this invention,
`thermoplastic gels have not been used as ultrasound trans-
`mission media.
`
`[0030]
`
`b) The Ultrasonic Monitor
`
`[0031] One aspect of the invention provides an ultrasonic
`monitor for measuring pulse rate values in a living subject,
`including a module with at least one source of ultrasonic
`energy (transducer), a gel pad comprised of a polymer and
`a mineral oil, wherein the gel pad is positioned in direct
`contact between the module and the living subject; an
`ultrasonic energy detector and associated hardware and
`software for detecting, calculating and displaying a readout
`of the measured rate values. The gel pad is made of a
`polymer having the following characteristics:
`
`a) Hardness: Needle Penetration from about 5
`[0032]
`to about 300 (“410 mm) according to ASTM D15,
`
`preferably from about 25 to about 150, and most
`preferably from about 30 to about 50;
`[0033]
`b) Tensile Strength from about 5 to about 500
`psi according to ASTM D412,preferably from about
`10 to about 300 psi, and most preferably from about
`50 to about 200 psi; and
`[0034]
`c) Elongation from about 50% to about 800%
`according to ASTM D412, preferably from about
`200% to about 700%, and most preferably from
`about 300% to about 500%.
`
`In a preferred embodiment of the invention, the
`[0035]
`monitor is a wristwatch with attached wristband, wherein
`the module is attached to the wristband. In another preferred
`embodiment the transducer includes a first and a second
`piezoelectric crystal, wherein the crystals are positioned at
`an angle to each other, and wherein the angle is determined
`based on the distance of the transducerto the living subject.
`The first piezoelectric crystal is energized by an original
`ultrasonic frequency signal, wherein the original ultrasonic
`frequency signal
`is reflected off the living subject and
`received by the second piezoelectric crystal. More specifi-
`cally, the module includes a pair of piezoelectric crystals at
`an angle to each other, wherein the angle is determined by
`the depth of the object being monitored. If the object is a
`fetus deep inside a womb,
`the two crystals are placed
`parallel to each other. If the object is the radial artery of a
`human subject (e.g., adult,
`infant),
`the angle of the two
`crystals with respect to the direction of the-blood flow would
`be about 5 to about 20 degrees. One of the crystals is
`energized at an ultrasonic frequency. The signal is then
`reflected back by the living subject and picked up by the
`second crystal. The frequency received is either higher or
`lowerthan the original frequency depending uponthe direc-
`tion and the speed of the fluidic mass flow. For example,
`when blood flow is monitored, the direction of flow is fixed.
`Thus, the Doppler frequency whichisthe difference between
`the original and the reflected frequency depends only upon
`the speed of the blood flow.
`
`[0036] The ultrasonic monitor includes an ultrasonic fre-
`quency driver, an AM or FM detector, an amplifier, filter
`circuits and a microcontroller. The driver circuit is com-
`
`posed of an oscillator running at a frequency between about
`1 MHzto about 10 MHz, an impedance matching network
`and a Class C power amplifier. Ultrasonic energy is deliv-
`ered to one of the two piezoelectric elements in the module
`by the power amplifier. The other element picks up the
`reflected ultrasonic signal. This signal is amplified and then
`amplitude demodulated(AM) or
`frequency demodulat-
`ed(FM) to yield the Doppler frequencies. The Doppler
`frequencies in audio range are further amplified and filtered
`to avoid aliasing before they are digitally sampled and
`processed by a microcontroller with built-in analog-to-
`digital converter and software. The software digitally filters
`out the noise signals due to muscle artifacts by a high pass
`filter with a 3-db corner frequency at about 100 to about
`1500 Hz depending on the original ultrasound operating
`frequency. Following that, a square operation and a low pass
`filter will further condition the signal appropriately for heart
`rate arbitration. The 3-db corner frequency of the low pass
`filter is about 1000 to about 5000 Hz depending upon the
`original ultrasound operating frequency. The heart rate arbi-
`tration logic in the prior art of Lo et al. may be applied to this
`invention with minor modifications.
`
`8
`
`

`

`US 2004/0138568 Al
`
`Jul. 15, 2004
`
`[0037] The module may optionally include electronics and
`between the transducer and the object. As such, the gel pad
`is made of a specific polymer which is used to conduct
`software for analyzing the rate values of the living subject,
`ultrasound waves such that the waves can be converted to
`such as heart rate or pulse rate. Alternatively, the display unit
`measurable rate values. In a preferred embodiment,
`the
`may include the electronics and software for analyzing the
`rate values. As such,
`there are at
`least
`two alternative
`polymeris a thermoset or thermoplastic gel. The gel of the
`present invention may include any elastomertype, elastomer
`embodiments with respect
`to the wrist watch ultrasonic
`monitor. In one embodiment of the invention, the transduc-
`molecular weight, crosslinking density, percentage of dilu-
`ers, the electronics and the software are all housed in the
`ents, and the like. The gel pad may be about one square
`centimeter in size and its shape may be square, rectangular
`same module. The module is mechanically attached to the
`or round. Examples of thermoset gels include, but are not
`wrist band and it may be positioned at the radial artery of a
`limited to, silicone or polyurethane gels. Silicone gels can be
`living subject. The gel pad faces the wrist of the living
`based on the reaction between a vinyl terminated polydim-
`subject and is held in place by the wrist band. The two
`crystals (supra) are located in the interior of the module right
`ethylsiloxane, polymethylphenylsiloxane, or polydiphenyl-
`silocaxane, and a hydride terminated polydimethylsiloxane,
`behind the gel pad. The measured blood flow and/or heart
`polymethylphenylsiloxane, or polydiphenylsiloxane. Poly-
`rate values can be sent to the watch display unit via wireless
`means. In this case, the module has a transmitter circuit and
`urethane gels can be based upon the reaction of polybuta-
`dienediol, polybutadienetriol, poly(ethylene-co-propylene-
`the display unit has a receiver circuit. The carrier frequency
`
`)diol, oxide)diol,_poly(ethylenepoly(tetraethylene
`
`may be chosen based upon conventionally used frequencies,
`oxide)diol, or castor oil with polyisocyanates such as tolu-
`e.g. 5 KHz, 120 KHz, 455 KHz, 433 MHz, 900 MHz,etc.
`These frequencies are used in various chest strap heart rate
`ene diisocyanate, or methylene diisocyanates. Examples of
`monitors. Currently, the most popular frequency used is 5
`thermoplastic gels include, but are not limited to, styrene-
`KHz. Therefore,
`the module with all the electronics and
`(ethylene-co-butylene)-styrene, styrene-(ethylene-co-propy-
`lene)-styrene, styrene-butadiene-styrene, styrene-isoprene-
`software included may be offered as a direct replacementto
`styrene ethylene-(ethylene-co-butylene)-ethylene and other
`the existing chest strap products in the market. The display
`unit in this case is the wristwatch with wireless receiver
`elastomeric block copolymers.
`[0042] The term “gel” is often used to describe a wide
`variety of materials which may have different properties.
`The art generally distinguishes three types of gels: thickened
`fluids, hydrogels, and stable soft elastomeric gels. Examples
`of thickened fluids are toothpastes, dishwasher detergents,
`and the like. These fluids are typically thickened by fumed
`silica, bentonite clay, or other inorganic thickening agents.
`Upon gentle shaking or squeezing, this type of gel flows
`readily in a liquid-like fashion. However, this gel cannot
`recoverits original thickened shape. Suchgels are, thus, not
`suitable for applications where the gel needs to take on a
`specific shape or form.
`[0043] Hydrogels typically include water soluble, high
`molecular weight polymers such as poly(vinyl alcohol),
`polyacrylamide, poly(acrylic acid), and the like. Hydrogels
`also contain a high percentage of water or water compatible
`fluids such as glycol. Hence, hydrogels can be characterized
`as water-like fluids or water compatible fluids, thickened by
`a high molecular weight organic polymer. Furthermore, this
`type of gel, depending on the composition, can be a fluid or
`elastic solid. If a lower molecular weight water soluble
`polymer and/or a high percentage of water is used, a
`fluid-like hydrogel is formed. A fluid-like hydrogel such as
`AQUASONIC™hydrogel is widely used as a medium for
`ultrasonic transmission. In fact, there are several commercial
`gel products used for ultrasonic transmission, often simply
`referred to as ultrasound gel or ultrasound transmission gel.
`US. Pat. Nos. 6,328,695; 6,251,076; and 6,159,149 refer to
`the use of a gel as transmission medium with respectto their
`patented ultrasonic devices. If a high molecular weight water
`soluble polymer and/or a low percentage of water is used,
`the gel can form a soft elastic solid which is capable of
`carrying a moderate level of mechanical stress. The elastic-
`ity is derived from the temporary network formed by hydro-
`gen bonding of water molecules to the polar groups of the
`polymers. U.S. Pat. Nos. 5,265,614 and 5,078,149 as well as
`JP Pat. Nos. 59-49750 and 59-82838 describe the use of such
`gels based on poly(vinyl alcohol). However, since all these
`fluids and gels are volatile, they tend to evaporate even at
`
`[0040]
`
`c) Polymers and Gels
`
`[0041] The ultrasonic monitor includes a gel pad whichis
`positioned in direct contact with the module and the living
`subject. Ultrasound energy does not propagate efficiently
`throughair, thus a couplant (gel pad) is needed forefficient
`transmission between the transducer and the living subject.
`Gels in fluidic state may be used as couplants, however, such
`fluidic gels are likely to dry up quickly due to being water
`based. Hence, the instant invention preferably employs oil
`based gels in solid form to achieve efficient transmission
`
`the module can be fastened
`circuit built-in. Optionally,
`separately on its own strap adaptedto fit another part of the
`living subject where blood flow can be conveniently moni-
`tored. This is the preferred approach since the battery
`compartment in the module may be designed to allow users
`to replace the battery with ease. The frequency of use and the
`length of time per use determine how frequently the battery
`needs to be replaced for a given type of battery.
`
`In another embodiment of the invention, the same
`[0038]
`electronics and software are placed within the watch display
`unit while the transducers and gel pad are housed within the
`module. Connecting wires are molded into the wrist band to
`connectto the ultrasound driving circuit. In this case, a high
`energy density battery is required to reduce the frequency of
`battery change. Alternatively, a rechargeable battery may be
`employed. The battery will be charged wirelessly so that the
`watch unit
`is waterproof for swimmers and divers. As
`battery technology continues to improve in energy density
`and lifetime, this integrated approach may eventually be
`preferred. In another embodiment, the monitor may be held
`in place by or integrated into a head band for monitoring
`temporo pulses.
`
`[0039] Examples of rate values that can be measured with
`the ultrasonic monitor include, but are not limited to, heart
`rate values and blood pulse rate values. Such rate values can
`be obtained from human adults, infants, and fetuses or from
`other animals.
`
`9
`
`

`

`US 2004/0138568 Al
`
`Jul. 15, 2004
`
`room temperature and need to be kept in a closed environ-
`ment (e.g., container, vacuum). Although these fluids and
`gels may possess load-bearing elastic properties for a short
`period of time, they are not stable upon long term exposure
`to the environment. At elevated temperature such as 40° C.
`and higher,
`the evaporation rate consistently increases,
`thereby further shortening the usefulness of the product.
`Furthermore, water freezes at 0° C., making this type of gel
`or fluid unsuitable for subzero temperatures. Consequently,
`hydrogels are only useful as ultrasound transmission media
`for a limited application,i.e., where the application does not
`require the gel to last beyond a short period of time.
`[0044] When the application requires a gel that can be
`used for days or longer, stable soft elastomeric gel types are
`required. The elastomeric gels contain an elastomeric net-
`work with a high percentage of diluents which are generally
`nonvolatile at ambient temperatures. They possess elastic
`and load bearing properties at ambient conditions for a
`prolonged period of exposure (e.g., several month to a few
`years). They are stable and maintain elastic properties over
`a wide temperature range, 1.e., from subzero temperatures to
`70° C. The art distinguishes two categories of stable soft
`elastomeric gels:
`thermoset gels and thermoplastic gels.
`Thermoset gels are made of a chemically bonded three-
`dimensional elastomeric network which entraps a large
`amountof low volatility liquids or diluents. The elastomeric
`network is permanent and cannotbe reversed to a liquid state
`through heating. A certain amountof diluent is necessary in
`order to ensure good conformability of the gel to the skin
`and low attenuation for ultrasound transmission whilestill
`maintaining the load bearing properties. In the absence of
`the required amount of diluent,
`the gel would resemble
`common rubber or elastomer which generally have a hard-
`ness of greater than 15 Shore A (ASTM D2240). For
`example, U.S. Pat. No. 4,901,729 describes the use of
`peroxide crosslinked polybutadiene,
`sulfur crosslinked
`polybutadiene, and silicone rubber as ultrasound propaga-
`tion media. Examples of thermoset gels are silicone gels and
`polyurethane gels.
`[0045] The elastomeric network ofa silicone gel is formed
`by silicone rubber which is typically cured by reacting a
`hydride silicone rubber with a vinyl silicone rubber in the
`presence of a platinum catalyst. Both silicone rubbers are
`highly diluted with a non-reactive, low volatility silicone
`fluid priorto the reaction. The reaction can be carried out at
`110° C.-120° C. for 30 minutes, or at room temperature for
`48 hours. The silicone gels can also be made by using a
`silane terminated silicone elastomer which can be cured by
`exposure to ambient moisture. At the end of the reaction, the
`final composition contains about 5-45% silicone rubbers and
`95-55% silicone fluid. A typical silicone gel composition is
`exemplified in U.S. Pa