A hybrid living/organic electrochemical transistor based on the Physarum polycephalum cell endowed with both sensing and memristive properties
(iii) There are changes in the oxidation potential which are quite relevant, in particular for the Pt-gate electrode.
On the basis of these observations, we have significant indication that the OECTs based on a living cell/PEDOT:PSS interface sensitively monitor intracellular processes. This aspect will be a subject of future studies.
An important aspect regards the fact that the PPC membrane in contact with the PEDOT:PSS is a wet surface composed of a complex environment, formed by ions, but also by bacteria and other species (different substances present in the body of the slime mould). It is not known a priori whether the doping/de-doping of PEDOT:PSS is mainly caused by cations coming from the wet surface of the membrane instead of the inside of the cell. To corroborate the idea that cations involved in the doping/de-doping of PEDOT:PSS come from the inside of the cell, we isolated the Ag gate-wire body with a Teflon film, except its apex. Then, we recorded the kinetic curves again in order to rule out the possible role of the membrane in terms of the device operation, since in this way the exposed tip of the metallic gate is in contact exclusively with the inside of the cell and the membrane does not experience the applied bias (Fig. 3A).
As reported in Fig. 3B, a single step of the resulting kinetic curve, recorded by applying V gs = +0.4V at t = 240 s, shows that the current modulation is mainly due to the ionic intracellular content of the cell and does not depend on its wet external environment, which is in direct contact with the device channel. This is because the rising time of both the unexposed (Fig. 1B) and isolated electrodes (Fig. 3B) are comparable and no ionic transmembrane delay has been found in the latter case. This is in agreement with previous results dealing with the effect in the time domain of artificial lipid membranes on the device response.45,46 Moreover, this corroborates the idea of using such a device configuration to directly study the cell membrane response to pathologies and/or external agents,47 such as pore forming toxins.48 It is important to note that the repeated application of voltages to the PPC, in the range explored here with the gate immersed into the membrane, does not induce any stress or affect the viability of the cell.
To further investigate the role of the gate electrode/PPC interface and to demonstrate the memristive properties of the device, we performed a conventional electrochemical study. To this aim, the same device was used in a 2-terminal configuration, with the PEDOT:PSS stripe as the reference electrode and a bias voltage applied between the gate, acting as working electrode, and one of the channel electrodes. Fig. 4A–C show the I–V curves for Au, Pt and Ag working electrodes, respectively.
The I–V curve in the case of the Pt-electrode shows a reduction peak at about –3 V and a broad oxidation peak, between +0.1 and +2.1 V. The reduction peaks observed for Au and Ag electrodes are located at about –2.8 and –1.9 V, respectively, while narrower oxidation peaks are located at +1.6 V and +0.7 V, respectively. The trend of the redox peaks is consistent and explains the behaviour shown in Fig. 2D–F. In fact, the saturation observed in the transfer curves in the positive gate voltage branch corresponds to the oxidation of the related gate electrode (+0.6 V for Ag gate, Fig. 2F), so that in the analysed gate voltage range, a Faradaic reaction is occurring at the Ag-gate electrode. Similarly, the onset of the channel current saturation in the transfer curve for the Au electrode is at about +1.6 V (Fig. 2D), while for Pt no onset of saturation is observed in the voltage range studied, confirming that oxidation of the electrode is not complete below +2.2 V (Fig. 2E).
It is worth noting that the oxidation peak for the Pt-electrode (Fig. 4B) seems to be the convolution of those obtained using Ag and Au electrodes. Pt electrodes, although substantially inert, are in fact also able to sustain Faradaic reactions in the presence of biomolecules,49 and the cytoplasm of the PCC is surely an environment rich in complex molecular species.
As far as the electrode reduction is concerned, the strong separation between reduction peaks with respect to the overlapped oxidation peaks and their own separation lead to a memristive-like behavior for our device. The memristive behavior can be attributed to a competition between the capacitive coupling at the PPC/PEDOT:PSS interface and the choice of the working electrode that promotes a flux of ionic species towards the underlying polymer. This flux is sustained to a greater or a smaller extent, depending on whether or not a Faradaic reaction takes place at the working electrode. In detail, since PPC is a macroscopic multinucleate single cell, two opposite features of the cell membrane characterize this system. First, in their simplest form cell membranes are phospholipid bilayers, showing a selective permeability with respect to ions and neutral molecules through the formation of ionic channels; in particular, leakage channels and voltage-sensitive channels that can be opened and closed in response to the applied voltage across the membrane, as happens for example in the case of the electroporation technique.50 Secondly, the membrane promotes the formation of an electric double layer at its edges,51 thus a capacitive coupling between the PPC and the PEDOT:PSS thin film is expected to be promoted. The strength of the external driving force (i.e. the applied voltage), responsible for the ionic flux through the cell membrane, is strictly connected to the redox reactivity of the electrode.17,49 On the other hand, the restoration of the initial state (i.e. the electrode reduction) in its early stage, that is when the external voltage decreases but is still positive, is assisted by the concentration gradient generated during the electrode oxidation between the inner and outer part of the cell.
To better corroborate this idea, I–V curves have been acquired using a conventional electrolyte (i.e. a buffer solution, PBS) in place of PPC (reported in Fig. 4D). These curves, if compared with those obtained using the PPC cell, do not show a convincing memristive response: there is no evidence of stable switching between two well-defined and stable conductive states, as the oxidation and reduction peaks strongly overlap. In fact, when PEDOT:PSS/liquid electrolyte interfaces are promoted, during the restoration of the initial electrochemical state (reduction reaction) ions are evidently free to repopulate the liquid electrolyte. In this case, within a simplified picture, the concentration gradient effectively assists the electrolyte repopulation, favoring the reversibility of the electrochemical process and inducing, consequently, the overlapping between oxidation and reduction peaks. In addition, the residual negative current showed by I–V curves at V = 0 during the electrode reduction reaction, is a fingerprint of the electrolyte repopulation by the ions injected into the polymer during the electrode oxidation. This residual current in the case of the PPC is lower than that of the saline buffer PBS and shows a dependence on the chosen electrode. A salient aspect emerging from the above analysis concerns the classification of our memristor in terms of the relevant features characterizing an ideal memristor. In this respect, as stated by Chua,4,52 some fingerprints should be exhibited by an ideal memristor. The first fingerprint is constituted by the pinching of the I–V curve at the axis origin. In our case, this fingerprint is not completely fulfilled, but it is clear that the choice of the electrode is crucial for controlling this memristive feature.
Therefore, the choice of an appropriate working electrode can induce a larger separation among the reduction peaks and, consequently, a more ideal behavior in terms of the first Chua's fingerprint. As the second fingerprint according to Chua's classification, a memristive device should exhibit a dependence of the hysteresis lobe area on the frequency of the applied periodic external signal, and the device output should tend to a single-valued function through the origin, when the frequency of this signal tends to infinity. In our case, the applied staircase voltage with different durations of the scan sweep represents a triangular waveform voltage sweep with varying frequency. A preliminary study (not shown) has indicated that the area of hysteresis loops decreases by a factor 2 if the frequency of the biasing signal is increased from 2.5 × 10–3 Hz to 1.25 × 10–2 Hz. Currently, we are systematically studying the features of our memristive device and the strategies aimed at optimizing its response.
From the observed response, our system can be classified quite well within the generalized Chua model, even though it cannot fully be considered as an ideal memristor. Actually, it is worth noting that none of the memristive systems reported in literature can be considered as an “ideal memristor”, since several processes are always responsible for their properties.53 However, the properties shown by the characteristics reported in Fig. 4A–C, that is the presence of the hysteresis loop and the rectification, allow us to consider the system under analysis as a memristive device in a wide sense. Regarding the device performance, the current values (I gm) for positive and negative biases measured in correspondence to the redox peaks, upon biasing lasting 10 seconds, allow for calculation of a rectification parameter (defined as IOxgm/IRedgm) of 2.9, 2.2 and 7.4 for the Ag, Au and Pt-based structures, respectively.49 Of course, the difference in the working principles determines the difference in the observed characteristics, comparing them with those of titanium oxide systems, polyaniline-based devices, and even memristive devices based on a pure PPC.54 However, at least with respect to the pure Physarum-based device, the memory effect in our case is much more pronounced due to the modulation of the organic semiconductor layer conductivity. Making a comparison with the polyaniline-based devices, the suggested system has an advantage in the switching velocity: 1 s in the present case, compared with about a minute in the case of polyaniline memristive devices. Finally, comparing with oxide memristors, the organic nature of our system allows better biocompatibility and, therefore, easier integration into living organisms.
In addition, the advantage of using a PPC-OECT compared with previously reported memristor devices is that the transition to the conductive state takes place as soon as a V gm > 0 V is applied. At present PPC-OECT works efficiently and reliably as a memristor element for more than ten cycles before the natural degradation of the organic polymer starts depleting the device performance.
Finally, a very important point we would like to make here is that the PPC-OECT is a good candidate for an innovative memristive element since it satisfies the requirements sought for memristors, defined as electronic elements with memory properties. Recent literature considers the memristor as a viable circuital element for the manufacturing of bio-inspired information processing systems, of adaptive bio-inspired electronic networks (neuromorphic systems) and for mimicking learning capability.55
In conclusion, we have demonstrated a fully working hybrid bio-organic OECT device, where the electrolyte is efficiently replaced by a living cell, the Physarum polycephalum cell (PPC), that can be operated as a memristor device. The semiconducting polymer PEDOT:PSS is used both as the transistor channel and as the reference electrode of a memristive device. The PPC-OECT is stable and reliable and has been characterized with 3 different electrodes (Pt, Au and Ag) used as the gate under the transistor-mode of operation and for monitoring the cell activity. The PPC-OECT device response is quite sensitive to the presence of the PPC on the surface of PEDOT:PSS and envisages the potential that the response curves in transistor mode could yield information about the PPC state and properties. Moreover, I–V measurements give insight into the memory capabilities of the mould, showing a memristive response ascribable to the cell membrane properties.
In a broader perspective, we believe that the device proposed here can be classified as a Bio-Organic Sensing/Memristive Device (BOSMD) that enables new and unexplored opportunities in bioelectronics. In particular, for the first time the unique properties of cellular systems are interfaced with the electronic/ionic responses that characterize OECTs. Such a combination is uniquely suitable to explore and efficiently produce bioelectronic actions combining the bifunctional transistor/memristor response and the sensing properties. In fact, the integration of PPC with OECTs could in principle allow the direct monitoring of the internal cellular bioactivities, including cell metabolism and the reactions/interactions with environmental changes. The electrical transduction of such processes and the control (both ionically and electronically) of the mentioned “smart” functions that PPC is capable of (if possible, extended to other cells or bioactive systems too), pave the way to devices enabling remarkable novel activities such as responding to external stimuli and changing their structural/chemical properties, as living beings do (examples in the case of PPC are the development of a protoplasmatic network assembly in a well-ordered manner that can be exploited as an electrical array, or the ability of PPC to change its 3-dimensional shape in order to catch food). Furthermore, the BOSMD being a device simultaneously sensitive and capable of memorizing previous activities, it offers jointly the unprecedented ability to mimic the behaviour of living organisms, to study their interaction with the environment and, ultimately, to implement new neuromorphic systems.
Thus, we consider the PPC-OECT proposed here as a prototype of BOSMDs that are a family of hybrid living–artificial man made devices.40 The present work is a starting point for the development of such kinds of devices. We have demonstrated here only the feasibility of PEDOT:PSS transistors and memristive devices with living beings. Further efforts will be directed to the particular realization of multi-sensitive elements, providing an integral response to Physarum metabolism as a result of variations in the environmental conditions. In parallel, we plan to explore the memory effects, when the growth of the PPC will vary the properties of individual devices and their mutual connections in related networks.