Ultra conformable and multimodal tactile sensors based on organic field-effect transistors

Cognitive psychology is the branch of psychology related to all the processes by which sensory input is transformed, processed and used. Academic and industrial research has always invested time and resources to develop devices capable to simulate the behavior of the organs where the perceptions are located. In recent years, in fact, there have been numerous discoveries related to new materials, and new devices, capable of reproducing, in a reliable manner, the sensory behavior of humans. Particular interest in scientific research has been aimed at understanding and reproducing of man's tactile sensations. It is known that, through the receptors of the skin, it is possible to detect sensations such as pain, changes in pressure and/or temperature. The development of tactile sensor technology had a significant increase in the last years of 1970s, thanks to the important surveys of Stojiljkovic, Harmon and Lumelsky who presented the firsts prototype of sensors for artificial skin applications, and summarized the main characteristics and requirements of tactile sensors. Recently, organic electronics has been deeply investigated as technology for the fabrication of tactile sensors using biocompatible materials, which can be deposited and processed on ultra flexible and ultra conformable substrates. In general, the most attractive property of these materials is mainly related to their high mechanical flexibility, which is mandatory for artificial skin applications. The main object of this PhD research activity was the development and optimization of an innovative technology for the realization of physical sensors able to detect pressure and temperature variations, which can be applied in the field of biomedical engineering and biorobotics. By exploiting the particular characteristics of the employed materials, such as mechanical flexibility, the proposed sensors are very suitable to be integrated with flexible structures (for example plastics) as a pressure and temperature sensor, and therefore, ideal for the realization of an artificial skin like. In Chapter 1, the basics of humans somatosensory system will be introduced: after a brief description of tactile thermoreceptors, mechanoreceptors and nociceptors, a definition of electronic skin and its characteristics will be provided. In Chapter 2, a wide analysis of the state of the art will be reported. Several and different examples of tactile sensor (in inorganic and organic technology) will be presented, underlining advantages and disadvantages for each approach. In Chapter 3, the firsts experimental results, obtained in the first part of my PhD program, will be presented. All the steps of the fabrication process of the devices will be described, as well as the measurement setup used for the electrical characterization of the sensors. In Chapter 4, the sensor structure optimization will be presented. It will be demonstrated how the presented devices are able to sense simultaneously thermal and mechanical stimuli. Moreover, it will be demonstrated that, thanks to an alternative and innovative fabrication process, the sensors can be transferred directly on skin, thus proving the suitability of the proposed sensor architecture for tactile applications

Charge-Modulated Field-Effect Transistor: technologies and applications for biochemical sensing

The research activity described in the attached dissertation focused on the development, fabrication and characterization of field-effect transistor-based biochemical sensor (bioFET) developed in different technologies. Such a research field has been attracting a significant interest in the last decades, as electronic sensors can represent as valuable, portable and low cost alternative to the bulk, expensive laboratory instrumentation. Among the biochemical reactions, genetic processes have been thoroughly investigated in literature: in particular, DNA hybridization detection represents a basic biological reaction for several, more sophisticated analysis in medical, pharmaceutical and forensic fields. The development of the research activity was centered on a specific biosensor, namely Charge-Modulated Field-Effect Transistor (CMFET), originally proposed in 2005 by the Electronic Department at the University of Cagliari. In particular, the aim of the activity was to make a significant step forward with respect to the results already presented in literature for DNA hybridization detection, employing two different technologies: CMOS process and organic electronics. As regards CMOS process, the activity mainly focused on the testing of a Lab-on-a-Chip (LoC), hosting several CMFET structures, developed and fabricated before but never tested. The activity carried out allowed to develop a precise electrical model of the device, validated by actual measurements, by which the basic performances of the device were derived. Subsequently, the application of the LoC for DNA hybridization detection was demonstrated: a reliable biochemical protocol for the modification of the chip surface with DNA strands was developed, as well as a precise measurement procedure. A complete evaluation of the sensitivity and selectivity of the device with respect to DNA hybridization was obtained; from the obtained results, several consideration about the relationship between the chip layout and the performances of the device were inferred. In conclusion, a road-map for the development of a new chip, customized for the application as DNA hybridization sensor, was developed. As regards the Organic CMFET (OCMFET), the activity comprised design, fabrication and testing of devices particularly conceived as disposable DNA hybridization sensors for field-measurement kits. Such a task required the development of innovative technological processes for the fabrication of high-performances organic transistors, i.e. transistors capable to be operated at low voltages (about 1 V) with quasi-ideal electrical performances. In particular, a highly reliable fabrication process, compatible with plastic electronics and easily up-scalable to an industrial size, was determined. Consequently, novel OCMFET were fabricated and tested. World record results in terms of sensitivity and selectivity among the organic transistor-based DNA sensors were reproducibly obtained. Thanks to the reliability of the results, the performances of the OCMFET were carefully studied, and design rules for the optimization of the device were inferred; an optimized, low voltage OCMFET allowed to further enhance the result, determining final performances even better than the one of silicon-based sensors. Finally, thanks to an innovative analysis on the influence of the device polarization to the characteristics of the bioreceptor layer at a micro-nanometrical size, a physical effect related to a tilting of the DNA molecules with respect to the surface was observed. This feature, possibly related to the CMFET working principle, can allow to overcome a general limitation of the bioFET technologies that have limited so far the application of these devices in vivo, thus opening novel possible applications for the CMFET working principle beyond the measurements in vitr

Chemo and biodetection in liquid with organic transistor

In the past, quality and quantity of the substances could be evaluated only in analytical chemistry laboratories, well-equipped with centralized and powerful data centres. The rising of miniaturized technology has made possible to conceive the realization of portable devices, i.e. chemical sensors capable to analyse small quantities in situ. Nowadays, the availability of portable devices allows to test those samples which are di�cult or impossible to transport for long distances without degradation. This work concerns the realization of two kinds of devices for detection of chemical and biological species in liquids by using organic transistor technology. One device, which is named Organic Charge Modulated Field E�ect Transistor (OCMFET), is a charge sensor which was tested as pH and DNA sensor. The other device is an Organic Electrochemical Transistor (OECT) entirely made of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) which was realized by ink-jet printing. As these devices can be fabricated with low-cost processes, they can be considered portable and disposable. A brief introduction on sensors, organic technology and the aim of this work is de- scribed in chapter one. In order to explain the reasons and the speci�c choices behind the development of these sensors, an overview of the most relevant sensing applications realized with Organic Thin Film Transistors and Organic Electrochemical Transistors is reported in chapter two. The working principle, the materials and methods and the recorded experimental results related to the OCMFET are treated in chapter three. A brief introduction on OECTs with a PEDOT:PSS channel and the study carried out to elucidate the operating regime of the OECT all made of PEDOT:PSS is treated in chapter four. The conclusions about this work are brie y summarized in chapter �ve. An overview on organic semiconductors and Organic Thin Film Transistors (OTFTs), the electrochemical techniques used in this work are treated in the appendixes

Chemo and biodetection in liquid with organic transistor

In the past, quality and quantity of the substances could be evaluated only in analytical chemistry laboratories, well-equipped with centralized and powerful data centres. The rising of miniaturized technology has made possible to conceive the realization of portable devices, i.e. chemical sensors capable to analyse small quantities in situ. Nowadays, the availability of portable devices allows to test those samples which are di�cult or impossible to transport for long distances without degradation. This work concerns the realization of two kinds of devices for detection of chemical and biological species in liquids by using organic transistor technology. One device, which is named Organic Charge Modulated Field E�ect Transistor (OCMFET), is a charge sensor which was tested as pH and DNA sensor. The other device is an Organic Electrochemical Transistor (OECT) entirely made of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) which was realized by ink-jet printing. As these devices can be fabricated with low-cost processes, they can be considered portable and disposable. A brief introduction on sensors, organic technology and the aim of this work is de- scribed in chapter one. In order to explain the reasons and the speci�c choices behind the development of these sensors, an overview of the most relevant sensing applications realized with Organic Thin Film Transistors and Organic Electrochemical Transistors is reported in chapter two. The working principle, the materials and methods and the recorded experimental results related to the OCMFET are treated in chapter three. A brief introduction on OECTs with a PEDOT:PSS channel and the study carried out to elucidate the operating regime of the OECT all made of PEDOT:PSS is treated in chapter four. The conclusions about this work are brie y summarized in chapter �ve. An overview on organic semiconductors and Organic Thin Film Transistors (OTFTs), the electrochemical techniques used in this work are treated in the appendixes

Carbon based materials for electronic bio-sensing

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Charge-Modulated Extended Gate Organic Field Effect Transistor for Biosensing Applications

The interest in organic field effect transistors (OFETs) employed as a biosensing platform has grown in recent years, driven largely by the potential to create inexpensive, sensitive analytical devices with a wide range of chemical and biological sensing applications. A particularly promising architecture for these type of devices is the Charge-Modulated Organic Field-Effect Transistor (CM-OFET). In the CM-OFET, a control gate electrode is capacitively coupled to a floating gate and used to bias the OFET, eliminating the need for an additional, often macroscale, reference electrode. In addition, charge accumulated in a designated sensing region of the floating gate modulates the output source drain current, ISD, of the transistor, providing sensing activity that is spatially separated from the organic semiconductor layer. Here, a CM-OFET based on solution processed Tips-pentacene as the organic semi-conductor that is both low cost and very simple to fabricate is reported. The CM-OFET biosensors fabricated here were predominantly based on the widely used Si/SiO2 substrates, where the degenerately doped Si acted as the gate electrode with a SiO2 dielectric layer. A limited number of Al/Al2O3 based CM-FETS are also presented. This thesis includes a detailed description of fabrication of these CM-OFET devices alongside a detailed discussion of the principle of operation, both as organic transistors and as analytical for monitoring pH and protein detection. The thesis focusses primarily on the characteristics of CM-OFET devices based on the Si/SiO2 substrate. The fabrication of Si/SiO2 CM-OFETs was very simple, requiring only a single lithography or shadow evaporation stage. Despite the simplicity, the CM-OFETs reliably displayed electrical characteristics typical of organic field effect transistors. The electrical characteristics were reproducible with over 90% yield. However, the responses of the devices when tested for pH sensing and protein detection, were inconsistent and with large error. Further analysis of the CM-OFET architecture revealed limitations associated with the geometrical layout of the Si/SiO2 CM-OFET device may have caused this deficiency in sensing response. A modified CM-OFET employing Al/Al2O3 as gate and gate dielectric layers was designed in which the geometry was optimized to maximise sensitivity to changes in charge within the sensing region. A process for the fabrication of the Al/Al2O3 CM-OFET was developed and the Al-based CM-OFETs were found to exhibit behaviour typical of an organic transistor, albeit with relatively lower source drain current compared to Si/SiO2 CM-OFET devices. Due to limited time, the sensitivity of the Al-based CM-OFET was not fully characterized. Further work regarding the enhancement of the device’s charge carrier mobility of the device and particularly, experimental investigation of the Al/Al2O3 CM-OFET for sensing applications is needed

Organic Thin-Film Transistor Based Gas Sensors for Putrescine Detection

The application of organic thin film transistors (OTFTs) has been progressing in the area of sensors for decades now. For accomplishing gas sensing in ambient conditions, polymers with good air stability and high enough mobility to detect environmental variations by analytes exposure are required for the OTFT-sensors. In this work, putrescine (PUT) was selected as our primary analyte, as it has high volatility and can be emitted by decayed food products. OTFT-based sensors were made with various polymers as semiconductors to detect PUT vapor. Based on the preliminary study of gas sensing with bottom-gate, bottom-contact (BGBC) OTFTs, a gas sensing system was established with OTFT-sensors, an analyte injection platform and a signal analyzing system, which is presented in Chapter 3. Before and after gas detection, all OTFT devices were characterized by their charge carrier mobility, threshold voltage and current on/off ratio in the glovebox and in air. Additional tests on polymer films were conducted by morphology and crystallinity tests with AFM and XRD. Chapter 4 and Chapter 5 show the screening of polymers as the semiconductor in OTFT sensors. P-type polymers and n-type polymers including commercially available ones and those synthesized by our group were employed in OTFT-sensors. P3HT and N2200 as typical p-type and n-type representatives have succeeded in the detection of PUT vapor as OTFT sensors. In addition, p-type polymers with FTPDO, DPP, indigo and bithiazole cores, n-type polymers with IBDF building block also showed sensing ability towards PUT vapor as OTFT sensors. Chapter 5 especially focuses on a series of 1,4-DPP polymers that exhibited high transistor performance as well as good air stability. As main factors that determine the performance of a sensor, operational temperature, stability, response and recovery time, sensitivity and selectivity of these DPP-polymer sensors showed very promising potential in PUT vapor detection. Food spoilage detection was realized with three types of DPP-sensors to detect vapors emitted by food samples. Mechanism of DPP-sensors to detect PUT vapor was studied by responses of the sensor, IV characteristics of the device, and morphology/crystallinity of the polymer. Combined with chemical properties of DPP-polymers and putrescine molecules, it was proposed that the trapping effect of lone pairs of electrons on putrescine molecules would cause the responses of DPP-OTFT sensors. Mobility dropping, drain-source current decrease and negative shift of VTH were observed with DPP-sensors by this trapping effect with the diffused PUT in the active channel. Future steps are expected based on this study. Better sensing ability can be achieved with the improvement on both transistors quality and polymers properties. More accurate relationships between sensors responses and PUT vapor concentrations can be built by optimizing gas sensing process. Spoilage detection of food products with OTFT sensors can be improved for more precise analysis with assistance of liquid chromatography (LC) and gas chromatography (GC). The ultimate goal of this study is to manufacture smart labels or tags with OTFT-sensors to attach on food packages, by connecting to smartphones or computers, fulfilling a real-time, in-situ detection of PUT vapor with ideal accuracy

An organic transistor-based system for reference-less electrophysiological monitoring of excitable cells

In the last four decades, substantial advances have been done in the understanding of the electrical behavior of excitable cells. From the introduction in the early 70’s of the Ion Sensitive Field Effect Transistor (ISFET), a lot of effort has been put in the development of more and more performing transistor-based devices to reliably interface electrogenic cells such as, for example, cardiac myocytes and neurons. However, depending on the type of application, the electronic devices used to this aim face several problems like the intrinsic rigidity of the materials (associated with foreign body rejection reactions), lack of transparency and the presence of a reference electrode. Here, an innovative system based on a novel kind of organic thin film transistor (OTFT), called organic charge modulated FET (OCMFET), is proposed as a flexible, transparent, reference-less transducer of the electrical activity of electrogenic cells. The exploitation of organic electronics in interfacing the living matters will open up new perspectives in the electrophysiological field allowing us to head toward a modern era of flexible, reference-less, and low cost probes with high-spatial and high-temporal resolution for a new generation of in-vitro and in-vivo monitoring platforms