Emphasis On Piezo Film The Piezo Solution for Vital Signs ... · but the adhesive is not rated for...
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www.mdtmag.com 36 Medical Design Technology ® March 2008 By Richard H. Brown W hen a strip of piezoelectric PVDF polymer film (piezo film) is stretched, it gener- ates an electrical signal (charge or voltage) between upper and lower electrode surfaces, propor- tional to the amount of elongation. Piezoelectric materials are generally thought of as responding to pressure, but in the case of piezo film, the geometry of the element means that very high stress levels can be achieved in the cross-section of the film when very low force is applied in the longitudinal direction, while the same level of force applied over a larger area to the surface of the film would generate far lower stress. Therefore piezo film demonstrates phenomenal sensitivity to dynamic strain, typically in the region of 10 to 15 mV per microstrain (ppm change in length) for 28 μm thickness PVDF. The phrase “dynamic” strain is used because electri- cal charge generated by a change in strain leaks away into the electrical circuit connected to the film, and so static conditions of strain cannot actually be detected. This can prove to be a positive advantage in cases where the sensor may be placed under varying levels of pre-load. The film sees only time-varying changes in strain, with a frequency response that may start from as low as 0.1 Hz. Piezo film is also lightweight, thin, and highly flexible, and requires no external power in order to function. This unique combination of proper- ties enables a wide range of medical applications to be addressed where very low-level mechanical signals must be detected. It is obviously a technology of great interest where available power is limited (it can even generate tiny quantities of power in some configurations). It is also extremely durable, capable of with- standing hundreds of millions of flex- ing cycles, and shock resistant. Direct Skin Contact Exploiting the properties of piezo film as a “dynamic strain gauge,” an element can be easily bonded directly onto the skin (e.g., on the inside of the wrist). Part number 1001777 from Measurement Specialties is a general- purpose sensor that already has pres- sure-sensitive adhesive on one side, but the adhesive is not rated for bio- compatibility, so for short-term trials, a patch of 3M 9842 (thin polyurethane tape with adhesive coat- ing) was fixed on the skin and the piezo film sensor applied on top. Figure 1 shows a pulse signal, Emphasis On Piezo Film The unique properties of piezo film as a dynamic strain sensor make it particularly well-suited to the detection of vital signs, whether mounted in direct contact with the skin or mechanically coupled through intervening layers. Some film elements are sensitive enough to detect a human pulse in a device which is carried in the pocket of outer clothing. This article will present examples of sensors and detection methods in production or in advanced development today. The Piezo Solution for Vital Signs Monitoring Richard H. Brown is EMEA manager for the piezo film sensors business unit of Measurement Specialties. He offers many years experience in both sales and applications engineering. Brown can be reached at +49-6074-862822 or [email protected]. Figure 1: A pulse signal that was obtained using a charge amplifier with low frequency limit set to 1 Hz and a sensitivity of 1 mV/pC. Figure 2: The response of the piezo sensor while an object was gripped then released repeatedly. Piezo film is also lightweight, thin, and highly flexible, and requires no external power in order to function. A piezo film sensor
Transcript of Emphasis On Piezo Film The Piezo Solution for Vital Signs ... · but the adhesive is not rated for...
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Medical Design Technology® � March 2008
By Richard H. Brown
When a strip of piezoelectricPVDF polymer film (piezofilm) is stretched, it gener-ates an electrical signal
(charge or voltage) between upperand lower electrode surfaces, propor-tional to the amount of elongation.Piezoelectric materials are generallythought of as responding to pressure,but in the case of piezo film, thegeometry of the element means that
very high stress levels can beachieved in the cross-section of thefilm when very low force is applied inthe longitudinal direction, while thesame level of force applied over alarger area to the surface of the filmwould generate far lower stress.Therefore piezo film demonstratesphenomenal sensitivity to dynamicstrain, typically in the region of 10 to15 mV per microstrain (ppm change
in length) for 28 µmthickness PVDF.
The phrase“dynamic” strain isused because electri-cal charge generatedby a change in strain leaks away intothe electrical circuit connected to thefilm, and so static conditions of straincannot actually be detected. This canprove to be a positive advantage incases where the sensor may be placedunder varying levels of pre-load. Thefilm sees only time-varying changesin strain, with a frequency responsethat may start from as low as 0.1 Hz.
Piezo film is also lightweight, thin,and highly flexible, and requires noexternal power in order to function.This unique combination of proper-ties enables a wide range of medicalapplications to be addressed wherevery low-level mechanical signalsmust be detected. It is obviously atechnology of great interest whereavailable power is limited (it can evengenerate tiny quantities of power insome configurations). It is alsoextremely durable, capable of with-standing hundreds of millions of flex-ing cycles, and shock resistant.
Direct Skin ContactExploiting the properties of piezofilm as a “dynamic strain gauge,” anelement can be easily bonded directlyonto the skin (e.g., on the inside ofthe wrist). Part number 1001777 fromMeasurement Specialties is a general-purpose sensor that already has pres-
sure-sensitive adhesive on one side,but the adhesive is not rated for bio-compatibility, so for short-term trials,a patch of 3M 9842 (thinpolyurethane tape with adhesive coat-ing) was fixed on the skin and thepiezo film sensor applied on top.
Figure 1 shows a pulse signal,
Emphasis On Piezo Film
� The unique properties of piezo film as a dynamic strain sensor make it particularly well-suited to the detection of
vital signs, whether mounted in direct contact with the skin or mechanically coupled through intervening layers.
Some film elements are sensitive enough to detect a human pulse in a device which is carried in the pocket of
outer clothing. This article will present examples of sensors and detection methods in production or in advanced
development today.
The Piezo Solution for Vital Signs Monitoring
Richard H. Brown is EMEA manager forthe piezo film sensors business unit ofMeasurement Specialties. He offersmany years experience in both sales andapplications engineering. Brown can bereached at +49-6074-862822 [email protected].
Figure 1: A pulse signal that was obtained using acharge amplifier with low frequency limit set to 1 Hzand a sensitivity of 1 mV/pC.
Figure 2: The response of the piezo sensor while anobject was gripped then released repeatedly.
� Piezo film is also lightweight,thin, and highly flexible, andrequires no external powerin order to function.
A piezo film sensor
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obtained using a charge amplifier with lowfrequency limit set to 1 Hz and a sensitivityof 1 mV/pC. The output of approximately130 mV pk-pk corresponds to an open-circuitvoltage of around 100 mV pk-pk, that in turn,can be interpreted as a dynamic strain ofroughly 8 µε.
This signal was recorded while the handwas at rest. Flexing or rotating the wristcould generate much higher amplitude sig-nals, especially when a lower frequency limitwas selected on the preamplifier. As anexample, Figure 2 shows the response of thesensor while an object was gripped thenreleased repeatedly, showing an amplitude ofaround 3 V open-circuit or around 250 µε.
This ability of the film to detect both verytiny physical signals and also gross move-ment arises from the fact that the piezoresponse in PVDF film is linear over a verylarge dynamic range (estimated to be asgreat as 14 orders of magnitude). In manycases, the smaller signals can be extractedby filtering, as long as the bandwidths of thetarget signal and the “noise” are sufficientlywell separated.
Similar self-adhe-sive sensors havebeen used to detectmuscle and skinmovement of thechest, leg, and eyelidduring the study ofsleep disorder. Also,the response of amuscle (for example,between thumb andforefinger) to a delib-erate electrical stimu-lation can be detectedas an indicator of theeffectiveness of anes-thesia (known asneuro-muscular trans-mission).
Acceleration SensorThe Minisense 100 isa standard componentfrom MeasurementSpecialties, thatemploys a mass-loaded cantileverdesign with rigid PCB
material forming the clamp and allows themounting of the connection pins.
The added mass causes the sensor torespond inertially under the influence ofacceleration. The film element bends as themass “stands still,” leading to extremely highvoltage sensitivity (around 1 V/g). A variantof this component detects vital signs when a“smart badge” (equipped with RF telemetry)is worn by a staff member or patient andenables a periodic signal to be transmitted fortracking or location purposes. The badgegoes into sleep mode when the sensor detectsthat the badge has been taken off a live per-son, but is kept “awake” by muscle tremor,gross bodily movement, or even vibrationfrom the pulse.
The standard sensor can reveal detail ofheart “sounds” when held against the chestusing a light elasticated strap. With a verylow frequency limit selected in the electronicinterface, it is possible to see breathing ratealso (Figure 3). In that waveform, the move-ment of the chest wall shows up as slow peri-odic signal with approximately a four secondperiod, while individual heartbeats can be
seen at around 1/s (60 bpm).To eliminate the breathing sig-nal and remove some noise, itis possible to apply filters at 1 Hz (lower) and 10 Hz(upper) to obtain realtime signals (Figure 4).
Note that the traces in theFigure 4 waveform are alltaken from a patient at rest.Naturally, using an accelerom-eter, bodily movement caneasily dominate signals fromthe heartbeat. In fact, a tinyimplementation (active sensordimensions 1.3 x 3 mm) ofjust such a cantilever beamacceleration sensor is usedwithin a pacemaker to detectthe level of physical activityof the patient, so that the pac-ing rate can be adjustedaccordingly.
Stethoscope ApplicationsSeveral electronic stethoscopeshave used piezo film as theactive sensing element, due to
its robustness, high sensitivity, and broadbandwidth. Here, the sensing element must beheld with some “reaction force” against thebody—normally using a conventional massivebell design as for a direct acoustic stethoscope.Once the dynamic pressure signal has beenconverted into an electrical signal, the infor-mation content can be enhanced by selectivefiltering or amplification, played back as anaudio signal, analyzed using more complexalgorithms to detect specific conditions, trans-ferred over data link to remote base station forfurther analysis and storage, and so on.
Sensor ArrayUsing a compact acoustic sensor means thatmore than one point can be monitored simul-taneously. In fact, the company Deep Breezeuses an array of up to 100 individual sensorsto collect acoustic data for a few seconds asthe patient inhales and exhales. The sensorsare held against the skin using small vacuumcups. The signals are then processed to obtaindynamic “images” of sound, which allowsthe flow of air through the airways and lungsto be visualized as an animation.Obstructions and abnormalities show upclearly in the processed images. This entirelypassive method shows great promise andobviously suggests a safer alternative to X-ray techniques.
In-Bed MonitoringMoving further away from direct contact withthe body, both piezo film and piezo cable havebeen used as part of a mattress to detect heart-beat, breathing, and bodily movement of a
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Emphasis On Piezo Film
Figure 3: The movement of the chest wall showsup during breathing as a slow periodic signal withapproximately a four second period, while individualheartbeats can be seen at around 1/s (60 bpm).
Figure 4: To eliminate the breathing signal andremove some noise from the Figure 3 waveform, it ispossible to apply filters at 1 Hz (lower) and 10 Hz(upper) to obtain realtime signals.
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patient. In the Lifebed patient vigilance systemfrom Hoana Medical Inc., an array of sensorsis integrated into a mattress coverlet that pas-sively measures patient information as thepatient lies or sits upon these collectiondevices, providing accurate measurementseven through clothing, gowns, or sheets. Static“presence” is detected by flexible switches,while piezo film elements convert all dynamicvital signs from the patient into correspondingelectrical signals, that can be displayed on abedside unit with alarm logic. The system cangive early warning of heart or respiratory rateabnormalities as well as unauthorized bed exit,all without directpatient contact.
Infant BreathingMonitorThe extreme sen-sitivity of PVDFto strain has alsobeen exploited ina novel devicecreated byInfantrust andmarketed as aparenting aid thatdetects the breathing rate of an infant from amonitor clipped to the waistband of a diaper.The Respisense contains a piezo film sensorwhich contacts the skin of the abdomen. If nomovement is detected within a programmableperiod of time, a vibrating buzzer is activatedin an effort to gently stir the baby into breath-ing. If no response after a further short delay,a loud audio alarm is sounded.
Pyroelectric Detection of RespirationAs an example of the very high sensitivity ofPVDF to dynamic changes in temperature(on the order of 8 V/°C for 28 µm thickness),
UK company C-Lect Medical has developeda sensor and monitor that is used to detectrespiration rate using a small element ofpiezo film held close to the nose and mouthwithin a mask. Changes in temperature of theinhaled and exhaled air create strong electri-cal signals, so even when warmed air/oxygenis supplied, detection is still effective. ThePIPPA monitor is a small battery-poweredunit with a clear LCD display showing rate inbreaths per minute.
The same principle has been deployed asa contacting sensor using a piezo film ele-ment patterned to fit on the upper lip,
detectingairflowthrough thenose ormouth.Sensitivity isvery highrelative tothermistors,with veryfast responsetime. Thefilm is flexi-ble and can
provide a large detection area.
ConclusionAs can be seen from the variety ofapplications featured, piezo film pres-ents as an excellent alternative to the useof more traditional electronic compo-nents to transmit vital data to patientmonitoring systems. As a result of itsarray of benefits, this technology is cer-tain to be implemented into more lifesaving medical devices in the future.
Emphasis On Piezo Film
ONLINEFor additional information on the products and tech-nologies discussed in this article, see MDT online atwww.mdtmag.com and the following websites:• www.meas-spec.com• www.3m.com• www.hbmeditech.com• www.deepbreeze.com• www.hoana.com• www.infantrust.com
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