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Regarding the signal clinical worth, it must be noted that, although respecting Einthoven’s triangle, a large number of the direct-contact implementations use only 3 electrodes instead of the 12-lead ECG of most importance to a comprehensive clinical evaluation. Necessarily, an unobtrusive implementation of capacitive coupled ECG cannot guarantee enough electrode positioning accuracy, namely the ones embedded inconspicuously in the environment. The devices developed often use the minimum number of electrodes [10-12], [59-66] because of the low positioning accuracy, and their typical architecture is depicted in Fig. 3. The latest studies state that incorporating voltage followers of high input impedance, i.e. operational amplifiers with FET input, on the electrodes is a good decision as it minimizes the pickup of external signals. Furthermore, providing a path to bias currents, Rpigs in the previous Fig. 3, and the addition of a shield to these active electrodes are also common and well-advised actions with the same purpose [10-12], [59-70].

Figure 3 Regarding the signal clinical worth, it must be noted that, although respecting Einthoven’s triangle, a large number of the direct-contact implementations use only 3 electrodes instead of the 12-lead ECG of most importance to a comprehensive clinical evaluation. Necessarily, an unobtrusive implementation of capacitive coupled ECG cannot guarantee enough electrode positioning accuracy, namely the ones embedded inconspicuously in the environment. The devices developed often use the minimum number of electrodes [10-12], [59-66] because of the low positioning accuracy, and their typical architecture is depicted in Fig. 3. The latest studies state that incorporating voltage followers of high input impedance, i.e. operational amplifiers with FET input, on the electrodes is a good decision as it minimizes the pickup of external signals. Furthermore, providing a path to bias currents, Rpigs in the previous Fig. 3, and the addition of a shield to these active electrodes are also common and well-advised actions with the same purpose [10-12], [59-70].

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Fig. 1. Ballistocardiograms acquired from the backrest (top), and from the seat (bottom) of a healthy young subject seated on a wheelchair. In the chair implementations, the BCG signal may be obtained from the chair’s backrest, seat, or both [3-5], [35-38], as exemplified with the results of Fig. 1.
Fig. 1. Ballistocardiograms acquired from the backrest (top), and from the seat (bottom) of a healthy young subject seated on a wheelchair. In the chair implementations, the BCG signal may be obtained from the chair’s backrest, seat, or both [3-5], [35-38], as exemplified with the results of Fig. 1.
noise removal of two electromechanical film sensors and interactive patient counseling competences embedded on mobile units, such as a wheelchair, was developed in 2009, [37], and it is shown in Fig. 2. This recent device provided an important advance to the physically disabled requiring 24 hours a day, 7 days a week, cardiovascular monitoring, e.g. recovering stroke victims and diabetes patients. Furthermore, patient recognition features were implemented, which in addition to wireless communication abilities extend the counseling competences to other subjects whose access to the data is found useful by the patients.
noise removal of two electromechanical film sensors and interactive patient counseling competences embedded on mobile units, such as a wheelchair, was developed in 2009, [37], and it is shown in Fig. 2. This recent device provided an important advance to the physically disabled requiring 24 hours a day, 7 days a week, cardiovascular monitoring, e.g. recovering stroke victims and diabetes patients. Furthermore, patient recognition features were implemented, which in addition to wireless communication abilities extend the counseling competences to other subjects whose access to the data is found useful by the patients.
Fig. 4. Diagram of the arterial tonometry method. Even if the pressure transducer follows the artery perfectly, the pressure pulse transfer to the sensor through the skin is not a perfect bond as it is affected by the elastances of the different layers of muscle, bone, and skin in-between, Fig. 5, so the registered pressure variations exhibit some differences from the actual pressure profile. Generally, any pressure recording at the skin is necessarily different from the pressure effectively registered in the artery, with the related uncertainties introducing distortions in the resulting waveforms.
Fig. 4. Diagram of the arterial tonometry method. Even if the pressure transducer follows the artery perfectly, the pressure pulse transfer to the sensor through the skin is not a perfect bond as it is affected by the elastances of the different layers of muscle, bone, and skin in-between, Fig. 5, so the registered pressure variations exhibit some differences from the actual pressure profile. Generally, any pressure recording at the skin is necessarily different from the pressure effectively registered in the artery, with the related uncertainties introducing distortions in the resulting waveforms.
Thus, all the tonometers patented have tried someway to address these issues, improving the detection algorithms, the sensor’s sensibility and characteristics, correction of measured waveform and profile, and device’s physical aspects and applanation control. To alleviate all these dependencies, inherent to single sensor devices, some tonometers in the past have been composed of an array of miniature pressure transducers [74], [75], a solution which was recovered recently [30], [76]. All the transducers of the array directly sense the mechanical forces in the tissue, but each only covers a fraction of the artery, so some of them are supposed to sense the pressure pulses, regardless of array position on the subject. However, a significant setback is inherent to this approach, the lack of anatomic compatibility with the contour of the subject's tissue overlying the artery being sensed.
Thus, all the tonometers patented have tried someway to address these issues, improving the detection algorithms, the sensor’s sensibility and characteristics, correction of measured waveform and profile, and device’s physical aspects and applanation control. To alleviate all these dependencies, inherent to single sensor devices, some tonometers in the past have been composed of an array of miniature pressure transducers [74], [75], a solution which was recovered recently [30], [76]. All the transducers of the array directly sense the mechanical forces in the tissue, but each only covers a fraction of the artery, so some of them are supposed to sense the pressure pulses, regardless of array position on the subject. However, a significant setback is inherent to this approach, the lack of anatomic compatibility with the contour of the subject's tissue overlying the artery being sensed.
Fig. 7. Evolution of the BCG, ECG and PPG signals during 2 seconds (using the device referenced in [4]). he PAT is composed of the pre-ejection period and the pulse transit time, as Fig. 7 illustrates
Fig. 7. Evolution of the BCG, ECG and PPG signals during 2 seconds (using the device referenced in [4]). he PAT is composed of the pre-ejection period and the pulse transit time, as Fig. 7 illustrates
Fig. 8. Results for BCG implemented in a wheelchair’s backrest. During a subject’s unconstrained use of the wheelchair for 125 seconds (left). 10 seconds of recording of a subject with a coronary stent (top), and of a healthy young subject (bottom). The implementation of a ballistocardiograph in a wheelchair has been achieved in recent years [3 [20], [100-102]. Typically, a film-type force transducer is embedded in the wheelchair’s backrest an seat. Both force oscillations due to the heart activity and the person’s movement are registered, Fig. 8.
Fig. 8. Results for BCG implemented in a wheelchair’s backrest. During a subject’s unconstrained use of the wheelchair for 125 seconds (left). 10 seconds of recording of a subject with a coronary stent (top), and of a healthy young subject (bottom). The implementation of a ballistocardiograph in a wheelchair has been achieved in recent years [3 [20], [100-102]. Typically, a film-type force transducer is embedded in the wheelchair’s backrest an seat. Both force oscillations due to the heart activity and the person’s movement are registered, Fig. 8.
A prototype implementing the circuitry previously described may be incorporated in a w having an area of 210 cm” were implemented in manual wheelchair with polyester linin g p poly g heelchair’s backrest. Electrodes with a copper surface with a surface area of 20 cm’, with the plane electrode 101]. The signal conditioning circuitry may be based on any operational amplifier with very low bias current anc high input impedance. In the case, INA116 operational amplifiers (bias current 3 fA, input impedance >10'° Q || 0.2 pF, and common mode rejection ratio 84 dB) were used. The most problema he design of the printed circuit board and the sensing probes. These tasks are vital to common mode rejection and to minimize susceptibility to interferences. Fig. 9 presents tests in a subject without relevant clinical history dressed with a nylon shirt and denim jeans, sh he other methods. ic issue is maximize conductec owing tha his type of embedded sensing is feasible, although suffering from motion artifacts, the same way as
A prototype implementing the circuitry previously described may be incorporated in a w having an area of 210 cm” were implemented in manual wheelchair with polyester linin g p poly g heelchair’s backrest. Electrodes with a copper surface with a surface area of 20 cm’, with the plane electrode 101]. The signal conditioning circuitry may be based on any operational amplifier with very low bias current anc high input impedance. In the case, INA116 operational amplifiers (bias current 3 fA, input impedance >10'° Q || 0.2 pF, and common mode rejection ratio 84 dB) were used. The most problema he design of the printed circuit board and the sensing probes. These tasks are vital to common mode rejection and to minimize susceptibility to interferences. Fig. 9 presents tests in a subject without relevant clinical history dressed with a nylon shirt and denim jeans, sh he other methods. ic issue is maximize conductec owing tha his type of embedded sensing is feasible, although suffering from motion artifacts, the same way as
Fig. 10. Simultaneous BCG, IPG and PPG device diagram (left). IPG during a subject’s unconstrained use of the wheelchair for 125 seconds (top), and 10 seconds of PPG of a healthy subject (bottom).
Fig. 10. Simultaneous BCG, IPG and PPG device diagram (left). IPG during a subject’s unconstrained use of the wheelchair for 125 seconds (top), and 10 seconds of PPG of a healthy subject (bottom).
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Sensors and transducers
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