Conventional medical devices that record indicators of vital physiological variables such as the electrocardiographic (ECG) signal have portability limitations. Considering the new techniques in the elaboration of electronic components with smaller size and lower cost, and the recently high usage of mobile devices around the world, researchers are currently creating technological innovations that improve the signal-to-noise ratio and increase the system’s portability dedicated to the acquisition of biomedical records [1-3].

Characteristics of conventional electrocardiographic medical devices may create drawbacks in long term monitored patients classified as high risk, or in athletes who perform various activities that require physical effort or mental stress [1,4]. Two of the main aspects that effect the recording of biological signals under the described scenarios are associated with the wiring (lead) that communicates the transducers (electrodes) with the device itself, those are:

Length: Sometimes the accessory may have a length longer than one meter, which may cause poor quality signal because of the semiconductors repetitive movement [5] and make more difficult assistance processes [6].

Problems that have been described before are an incentive regarding to the development of new systems related to analysis of cardiac activity which have portability characteristics and excellent signal to noise ratio [3,8-10].

Currently electrocardiography systems use different types of electrodes, the most common are adhesive ones which are employed with normal vital signs monitors. However, another kind exists which are not that common in hospital areas such as textile electrodes [4,10], contact and no contact (can be used above clothing), they may be considered as active and can be appreciated in Fig. 1 [3,8].

Figure 1 Source: [10]

The use of textiles to record bio potentials is a new trend that includes medical devices using clothes of mainly everyday use to detect the cardiac electrical activity when it is needed to do long term measurements, from the use of micro and Nano electronical components likewise polymeric materials that do not affect the comfort of the person that is using the clothes [4], besides that, fabric should be a conductor material and should possess certain characteristics such as:

It should contain enough conductive material

In addition to the integrated transducers in textiles, different kind of capacitive electrodes has been developed which improve the signal to noise ratio in the skin-electrode medium. This kind of sensor does not require an ohmic connection with the body, which offer numerous advantages such as no skin irritation or the usage of adhesive materials, because it does not need previous skip preparation before the registration [3,5,8,10]. These active electrodes allow signal recording and analogic processing in the same electronic board using only micro electronical components, [3,8] while each one of these electrodes have communication with the other ones by wire [3,8,10], additionally the transmission of the recording is realized wirelessly.

Next in Fig. 2 it is shown an example of an electrocardiograph made using active electrodes.

Figure 2 Source: [1].

The design process of these electronic devices has been based on a reduced size, lower power consumption and good response to the signal to noise ratio [6,8].

Next the development of a system of active electrodes to recording and visualizing a cardiac and bipolar bio potential with performance indicators which guarantee the functionality of the proposed structure is decribed. Initially this document shows the design and construction of each step of the process, from recording, analogic processing, filtering until the digital stage.

To develop the system of active electrodes, a group of stages and software and hardware resources were employed to acquire and process the signal, in addition to all the necessary equipment to collect the data and finally analyze it. Firstly, it references previous research where the latest advances regarding active electrodes have been described. Secondly, the development of the prototype itself. Thirdly the functionality tests and validation have been described. Finally, the results and the conclusions are shown.

The construction of the system was divided into two phases, the first one was made with insertion elements (through hole), while the second was made with SMD surface mount elements, in turn performed functional tests in the amplification of the cardiac signal as shown in Fig. 8.

Figure 8 Source: Authors

- Digital filtering review

A comparative analysis was performed between the spectral components at the output of the analog stage and the signal at the output of the digital stage. Next, in Fig. 9, the ECG signal is presented in the time domain at the output of the non-inverting configuration.

Figure 9 Source: Authors

When comparing the performance of the digital filter in relation of the signal to the output of the non-inverting configuration it was found that the 60 Hz frequency component has a value of -24.75 dB as seen in Fig. 9, after passing this signal through the digital filter this frequency component reaches a value of -103 dB as shown in Fig. 10.

Figure 10 Source: Authors

The first performance evaluation is realized by comparing the correlation index of seven records supplied by the SIMULAIDS electrocardiography simulator (AA-550), which are connected to the active electrode architecture and a calibrated ECG Sensor Cassy.

The SIMULAIDS device has several cardiac conditions. The following conditions were selected for testing:

- A-V second degree block (2ND I).

- A-V third degree block (3RD).

- Nodal bradycardia (JBRADY).

- Normal sinus rhythm (NSR).

- Sinus bradycardia (SBRADY).

- Premature ventricular contraction (SINUS PVC).

Fig. 11 shows the NSR ECG record acquired with the pattern device and the functional prototype of active electrodes.

Figure 11 Source: Authors

For each condition described before, the correlation between the signal resulting from the pattern equipment and the active electrode system is evaluated. The results are shown in Table 1 below.

Table 1

Source: Authors

When comparing the correlation results between the signals acquired by the pattern device and the active electrodes developed, there is a correlation higher than 90%, having a peak with SINUS2 pathology with a maximum of 98.31% On the other hand, when comparing the signal-to-noise ratio (SNR), total harmonic distortion (THD) and distortion (SINAD), calculated with the mathematical expressions represented in (5), (6) and (7). These are merit figures that are determined as quality indicators for the cardiac record resulting from the proposed transducer architecture, the results are presented in Table 2.

Table 2

Source: Authors

The functional prototype in use is presented in Fig. 12 by presenting the spatial distributions of the electrodes on the thorax and the visualization of the ECG signal acquired in real time on a mobile device.

Figure 12 Source: Authors

-A wireless functional model of active electrodes capable of acquiring, analogue and digital processing of an electrocardiographic signal is constructed, making use of an integrated low-cost DsPic and allowing portability, accessibility and connection to multiple platforms that have possess Bluetooth technology.