As discussed in Chapter I, Organic Electrochemical Transistor are the most used electronic devices for biosensing and for interfacing biological systems, because of their low operating voltage (< 1 V), with the unique feature of ion to electron amplifying transduction. Several applications have been reported for sensing and biosensing of a variety of biomarkers and biomolecules, in vitro and in vivo
264,263. A rising field is organic bio-electronic where these devices are playing a major role for bio signal monitoring. 9,273
Monitoring the electrical activity of live organs is a consolidated method to monitor their integrity and to understand potential diseases. In clinical applications, Bio-signal monitoring of organs and cells are fundamentals to predict diseases and possibly resolve them in time before major damages occur. Most of such signals are at low frequency (<100 Hz) in a quite low voltage range typically from ten microvolt to millivolts. 274 The most simple and studied bio-signal is the cardiac electrical activity, that is a very good indication of correct operation or type of malfunctioning of the hearth, that is characterized by a voltage peak to peak in the range of mV when measured on the skin. 9 The quality of the extracted signals depends on the monitoring set up and on the material composing the electrodes. The typical commercial electrodes for electrocardiogram (ECG) monitoring are made of Au or Ag/AgCl with the need of interfacing to the skin with a conductive paste (gel type) that has the function of ion reservoir playing the role of reducing the electrode impedance at the interface, which is the principal physics parameters that determines the quality of the recording.
A low impedance is critical for ECG recording and hence there is the need to increase the electrode area. There is a growing interest in making wearable devices that could monitor in real time the heart that would require microelectrodes to be as embedded as possible. Large electrodes represent hence a serious limitation to this type of application that respond to a growing interest.
To overcome this problem and achieve, at the same time, a low impedance, several strategies have been developed such as the use of polymeric materials as electrodes (PEDOT:PSS) over flexible and possibly stretchable substrates to achieve a good conformability with skin.275,12
OECT devices represent a potentially ideal solution as a prominent electrical probe for bio signal monitoring directly on skin, because of the mentioned their marked ion-to-electron transducing capability combined with the development of innovative fabrication processes such as printing technologies on several kinds of flexible and conformable substrates.
With these applications in mind, we interfaced the OECTs fabricated and studied as described in the previous paragraphs with the body for in vivo ECG monitoring. ECGs have been measured upon standard configuration used in clinic, mounted the set up on human body in the standard Einthoven’s tringle configuration 250 where three electrodes (working, reference and noise) have been placed in the specific body positions to form a tringle.
In our set up the printed OECT, acting as working electrode, has been wired on the right arm and two commercial Ag/AgCl electrodes, acting as reference and noise subtraction, have been wired on the left arm and behind the left ear, as reported in Fig. 5.11
Figure 5.11: Schematics of the printed OECT connections for ECG monitoring
Monitoring bio signals, in particular ECG, through the use of an OECT presents two fundamental issues. First of all, the background noise. Noise could be either of biological origin or can be generated from the electronic devices and electronic systems involved. For these reasons, a filter system is needed cut down all the different noise signals. In this experiment an acquisition board has been used to filter the signal coming from the OECT.
Another question arises from the fact that the majority of commercially available acquisition systems for biological signals are fabricated to measure difference in voltage while the output of an OECT is current and hence is not directly compatible with classic acquisition systems. At this stage of the work
we were willing to use standard electronics to compare our results to the available standard electrodes hence we decided to convert the current output in a voltage signal by applying resistor (Rload) on the drain in a common source (amplifying) configuration.273
In this study, the OECT has been used under a common source configuration, by connecting it directly to the board together with the Ag/AgCl reference and noise electrodes. The electrical potential of the body drives hence the gate current and a Vds = - 0,1 V has been applied between source and drain electrodes to generate the current in the channel. This Vds value corresponds to the maximum transconductance calculated for our devices at a very low gate voltage bias (i.e. the potential generated by the heart activity).
The typical ECG recorded with one of our OECT device (Fig.5.12a) well gives to the expected electrical activity of the human heart.
Figure 5.12: In a) the real time ECG signal recording with an all-Printed-OECT on a Kapton substrate. In b) the magnification of the ECG signal with a 1 mVpp amplitude of the main peak QRS.
All the fundamental feature has been nicely recorded and shown by our OECT.
All the fundamental features of the ECG can be easily recognized in the acquired signal. The main peak QRS, that corresponds to the depolarization of the ventricles, shows a peak-to-peak voltage around 1 mVpp (Fig.5.12b) that is equal to the value registered with passive electrode such as Ag/AgCl. 32
Here we demonstrate that our device presents a performance competitive with best to that of some OECTs on flexible substrate fabricated by photolithography reported in literature. 252
Further experiments based on the optimization of the common source configuration aimed at showing an amplified signal, are still in progress.