Polyaniline-based neuromorphic devices towards interfacing sensing

Abstract

Memristors are electronic elements that belong to a new generation of computational systems with a great potential to contribute to the promising revival of scientific research dedicated to the hardware realization of Artificial Neural Networks (ANNs) towards Artificial Intelligence (AI) and computer-brain interfaces. A distinctive property of these devices is the dependence of their internal resistance on the electrical charge that passed through them. In other words, there is a correlation of the output characteristics of the device and the history of its use, in which it resembles biological synapses and can be considered as their artificial analogue, displaying spike time dependent plasticity (STDP). Furthermore, the ability of controlled switching between different internal resistive states makes memristors some of the most promising candidates for the implementation in memories. Contrary to the conventional Von Neumann architecture, hardware-realized ANNs have the potential of combining the storage and processing of information, carried out by the same kind of elements, mimicking biological neurons in the brain. Parallel information processing would allow to simultaneously work with a whole array of inputs, rather than carrying out one single operation at a time. These properties pave the way towards more energy- and time-efficient computing by avoiding the need to exchange information between the processor and a passive memory. The similarity to biological synapses suggests an excellent biocompatibility and the possibility to emulate some functionalities of biological systems, enabling computer-brain interfaces with a seamless transformation and processing of bioelectronic signals. The Organic Memristive Device (OMD) is a polymer-based representative of such elements. It is a two-terminal device featuring a conductive channel of polyaniline (PANI) whose resistance is modulated through electrochemically controlled transitions between the polymer’s insulating and conductive state. These transitions take place in a heterojunction of PANI and a polyethylene oxide (PEO)-based solid polyelectrolyte (SPE) doped with a source of chloride among other stability enhancing additives, promoting the reaction with a silver counter electrode. Its advantages with respect to other memristors are its low-cost fabrication and the ability of fine-tuning of the channel resistance, granted by accessing intermediary resistive states. The main goal of the present PhD thesis is to develop OMDs suitable for neuromorphic applications such as the interfacing sensing and signal processing that can be realized by means of multilayer perceptron structures. The combination of a large array of elements in one network as well as working with complex biological systems implies the occurrence of electrical noise, which poses the question how it interferes with the functioning of our device. Preliminary research has shown that the current-state OMD does not possess the necessary level of stability to reliably to carry out such sensitive experiments. Hence, a significant part of our research was devoted to the optimization of the materials employed in OMD fabrication. The goal is to advance towards devices with higher endurance, i. e., reproducible output characteristics over longer periods of time, and to examine how to design the fabrication techniques to improve biocompatibility. A significant part of our work was focused on the stabilization of the labile, polyethylene oxide-based SPE system by optimizing its composition with particular attention to the dopant salts. This work follows and builds upon an extensive theoretical study of the role of the single dopant ion species on the operation mechanism of the OMD. It was demonstrated that the former concept which attributes a significant role in the switching process to lithium ion doping is inaccurate. Instead, it is shown that the anions are the most critical dopant species in terms of device operation. Furthermore, through a combination of theoretical and extensive practical work, novel recipes of PEO-based SPEs have been developed, providing unprecedented short-term and long-term stability with a remarkable reproducibility of I-V-characteristics, improving the endurance of the OMD by up to two orders of magnitude. A significant part of this success is due to the implementation of aluminium chloride to the SPE system, providing intrinsic acidity and enabling to avoid doping by strong, volatile acids such as HCl. This discovery was followed up by intensive research of routes to stabilize the PEO-based gels and impart them with favourable properties for long-term endurance by preventing aggregation. During this research, the formerly unheeded concept of lyotropicity was introduced to OMDs, recognizing it as a major aspect for the stability of the polyelectrolyte system. Furthermore, silk fibroin solutions have been successfully employed for the first time as an alternative to PEO-based SPEs, further advancing prior research towards the introduction of biocompatible materials into OMD fabrication. Another important aspect of the implementation of OMDs in complex systems for the interfacing of sensing and signal processing is the integrability with other electronic systems such as OECT-based sensors. This can be most effectively achieved by shifting the paradigm of OMD manufacturing from largely manual towards high-precision, automated fabrication. To this end, we developed materials that can be applied by means of direct-writing techniques such as Aerosol Jet Printing (AJP). Particular attention was paid to the conductive polymeric component of the device. Successful attempts have been made by developing AJP-compatible inks based on pristine PANI in a mixture of organic solvents such as NMP, xylene and isopropanol, that were successfully applied on quartz and Si/SiO2 substrates. Furthermore, a water dispersible composite material of PANI and chitosan was synthesized, further improving the quality of the printed features and the coverage of the substrates. Both printed materials have been implemented to successfully fabricate the first functioning PANI-based OMDs featuring a printed conductive channel, displaying the characteristic properties of a memristor. The combination of all the experimental results and intensive theoretical work lays the foundation and paves the way towards the implementation of OMDs in advanced, complex, biocompatible, (bio)integrable systems for bio-interfacing purposes.