Establishment of surface functionalization methods for spore-based biosensors and implementation into sensor technologies for aseptic food processing

Aseptic processing has become a popular technology to increase the shelf-life of packaged products and to provide non-contaminated goods to the consumers. In 2017, the global aseptic market was evaluated to be about 39.5 billion USD. Many liquid food products, like juice or milk, are delivered to customers every day by employing aseptic filling machines. They can operate around 12,000 ready-packaged products per hour (e.g., Pure-Pak® Aseptic Filling Line E-PS120A). However, they need to be routinely validated to guarantee contamination-free goods. The state-of-the-art methods to validate such machines are by means of microbiological analyses, where bacterial spores are used as test organisms because of their high resistance against several sterilants (e.g., gaseous hydrogen peroxide). The main disadvantage of the aforementioned tests is time: it takes at least 36-48 hours to get the results, i.e., the products cannot be delivered to customers without the validation certificate. Just in this example, in 36 hours, 432,000 products would be on hold for dispatchment; if more machines are evaluated, this number would linearly grow and at the end, the costs (only for waiting for the results) would be considerably high. For this reason, it is very valuable to develop new sensor technologies to overcome this issue. Therefore, the main focus of this thesis is on the further development of a spore-based biosensor; this sensor can determine the viability of spores after being sterilized with hydrogen peroxide. However, the immobilization strategy as well as its implementation on sensing elements and a more detailed investigation regarding its operating principle are missing. In this thesis, an immobilization strategy is developed to withstand harsh conditions (high temperatures, oxidizing environment) for spore-based biosensors applied in aseptic processing. A systematic investigation of the surface functionalization’s effect (e.g., hydroxylation) on sensors (e.g., electrolyte-insulator semiconductor (EIS) chips) is presented. Later on, organosilanes are analyzed for the immobilization of bacterial spores on different sensor surfaces. The electrical properties of the immobilization layer are studied as well as its resistance to a sterilization process with gaseous hydrogen peroxide. In addition, a sensor array consisting of a calorimetric gas sensor and a spore-based biosensor to measure hydrogen peroxide concentrations and the spores’ viability at the same time is proposed to evaluate the efficacy of sterilization processes

Real-Time Impedance Monitoring of Epithelial Cultures with Inkjet-Printed Interdigitated-Electrode Sensors

From electronic devices to large-area electronics, from individual cells to skin substitutes, printing techniques are providing compelling applications in wide-ranging fields. Research has thus fueled the vision of a hybrid, printing platform to fabricate sensors/electronics and living engineered tissues simultaneously. Following this interest, we have fabricated interdigitated-electrode sensors (IDEs) by inkjet printing to monitor epithelial cell cultures. We have fabricated IDEs using flexible substrates with silver nanoparticles as a conductive element and SU-8 as the passivation layer. Our sensors are cytocompatible, have a topography that simulates microgrooves of 300 µm width and ~4 µm depth, and can be reused for cellular studies without detrimental in the electrical performance. To test the inkjet-printed sensors and demonstrate their potential use for monitoring laboratory-growth skin tissues, we have developed a real-time system and monitored label-free proliferation, migration, and detachment of keratinocytes by impedance spectroscopy. We have found that variations in the impedance correlate linearly to cell densities initially seeded and that the main component influencing the total impedance is the isolated effect of the cell membranes. Results obtained show that impedance can track cellular migration over the surface of the sensors, exhibiting a linear relationship with the standard method of image processing. Our results provide a useful approach for non-destructive in-situ monitoring of processes related to both in vitro epidermal models and wound healing with low-cost ink-jetted sensors. This type of flexible sensor as well as the impedance method are promising for the envisioned hybrid technology of 3D-bioprinted smart skin substitutes with built-in electronics.The work by D.M.-M. has been performed in the frame of an FPU Program, FPU015/06208, and a Mobility Fellows Program, both granted by the Spanish Ministry of Education, Culture and Sports. This work has been funded by the Comunidad de Madrid under the grant BIOPIELTEC-CM (P2018/BAA-4480) and the Ministerio de Ciencia e Innovación under the grant PARAQUA (TEC2017-86271-R)

Characterization of printable electrical sensors applied to cellular cultures

Impedance biosensors have turned in a special interest as label-free and low cost platforms for real time detection of biological phenomena. The objective of this study was to characterize a given model of an interdigitated electrode sensor (model 1) manufactured by inkjet printing technology over a flexible substrate (PET). This characterization was applied to HaCaT cellular cultures at different confluences in order to distinguish an impedance response first associated with the cellular presence and then proportional to the cellular confluence of such presence. With this aim in mind, impedance spectroscopy principle was employed as the main analysis method. From the empirical impedance response, electrical equivalent circuits were obtained by fitting the empirical values with theoretical circuit’s elements. In addition, inverted and scanning electron microscopes were used in order to visualize the whole process, as a way of ensuring that the electrical changes recorded had a real and relevant biological meaning. Three different conditions were tested over the sensor: culture medium without cells, HaCaT epithelial cells seeded over the sensor at 40% of confluence and at 80% of confluence. Impedance responses were recorded each 2 h during 36 h. Results obtained showed that at some point the cellular presence changed the equivalent electrical circuit when compared with the control measurements performed without cells. This change in the circuit has been associated with the cellular attachment of the cells on the IDE, which, as it was later confirmed by the visualization of the cellular culture, has been identified to take place from 22 h on and coincides with the presence of an additional time constant in the electrical circuit. Moreover, the constant phase element of such equivalent circuits was compared for the three conditions, obtaining that its variation is inversely proportional to the area covered by the cells on the sensor. The main drawback encountered during the process was the noise coming at low frequencies that compromise the measurement from 0,1 Hz to 10 Hz. In conclusion, this work has been useful to prove that IDEs can provide an impedance response associated to cellular presence. Based on the main findings, another setup to reduce the noise was proposed (solution design: model 2) and tested for the following experiments, reporting good results.Ingeniería Biomédica (Plan 2010

Optimal interdigitated electrode sensor design for biosensors using multi-objective particle-swarm optimization

Interdigitated electrodes (IDEs) are commonly employed in biological cellular characterization techniques such as electrical cell-substrate impedance sensing (ECIS). Because of its simple production technique and low cost, interdigitated electrode sensor design is critical for practical impedance spectroscopy in the medical and pharmaceutical domains. The equivalent circuit of an IDE was modeled in this paper, it consisted of three primary components: double layer capacitance, Cdl, solution capacitance, CSol, and solution resistance, RSol. One of the challenging optimization challenges is the geometric optimization of the interdigital electrode structure of a sensor. We employ metaheuristic techniques to identify the best answer to problems of this kind. multi-objective optimization of the IDE using multi-objective particle swarm optimization (MOPSO) was achieved to maximize the sensitivity of the electrode and minimize the Cut-off frequency. The optimal geometrical parameters determined during optimization are used to build the electrical equivalent circuit. The amplitude and phase of the impedance versus frequency analysis were calculated using EC-LAB® software, and the corresponding conductivity was determined

Design, characterization and validation of integrated bioelectronics for cellular studies: from inkjet-printed sensors to organic actuators

Mención Internacional en el título de doctorAdvances in bioinspired and biomimetic electronics have enabled coupling engineering devices to biological systems with unprecedented integration levels. Major efforts, however, have been devoted to interface malleable electronic devices externally to the organs and tissues. A promising alternative is embedding electronics into living tissues/organs or, turning the concept inside out, lading electronic devices with soft living matters which may accomplish remote monitoring and control of tissue’s functions from within. This endeavor may unleash the ability to engineer “living electronics” for regenerative medicine and biomedical applications. In this context, it remains a challenge to insert electronic devices efficiently with living cells in a way that there are minimal adverse reactions in the biological host while the electronics maintaining the engineered functionalities. In addition, investigating in real-time and with minimal invasion the long-term responses of biological systems that are brought in contact with such bioelectronic devices is desirable. In this work we introduce the development (design, fabrication and characterization) and validation of sensors and actuators mechanically soft and compliant to cells able to properly operate embedded into a cell culture environment, specifically of a cell line of human epithelial keratinocytes. For the development of the sensors we propose moving from conventional microtechnology approaches to techniques compatible with bioprinting in a way to support the eventual fabrication of tissues and electronic sensors in a single hybrid plataform simultaneously. For the actuators we explore the use of electroactive, organic, printing-compatible polymers to induce cellular responses as a drug-free alternative to the classic chemical route in a way to gain eventual control of biological behaviors electronically. In particular, the presented work introduces inkjet-printed interdigitated electrodes to monitor label-freely and non-invasively cellular migration, proliferation and cell-sensor adhesions of epidermal cells (HaCaT cells) using impedance spectroscopy and the effects of (dynamic) mechanical stimulation on proliferation, migration and morphology of keratinocytes by varying the magnitude, frequency and duration of mechanical stimuli exploiting the developed biocompatible actuator. The results of this thesis contribute to the envision of three-dimensional laboratory-growth tissues with built-in electronics, paving exciting avenues towards the idea of living smart cyborg-skin substitutes.En los útimos años los avances en el desarrollo de dispositivos electrónicos diseñados imitando las propiedades de sistemas vivos han logrado acoplar sistemas electrónicos y órganos/tejidos biológicos con un nivel de integración sin precedentes. Convencionalmente, la forma en que estos sistemas bioelectrónicos son integrados con órganos o tejidos ha sido a través del contacto superficial entre ambos sistemas, es decir acoplando la electrónica externamente al tejido. Lamentablemente estas aproximaciones no contemplan escenarios donde ha habido una pérdida o daño del tejido con el cual interactuar, como es el caso de daños en la piel debido a quemaduras, úlceras u otras lesiones genéticas o producidas. Una alternativa prometedora para ingeniería de tejidos y medicina regenerativa, y en particular para implantes de piel, es embeber la electrónica dentro del tejido, o presentado de otra manera, cargar el sistema electrónico con células vivas y tejidos fabricados por ingeniería de tejidos como parte innata del propio dispositivo. Este concepto permitiría no solo una monitorización remota y un control basado en señalizaciones eléctricas (sin químicos) de tejidos biológicos fabricados mediante técnicas de bioingeniería desde dentro del propio tejido, sino también la fabricación de una “electrónica viva”, biológica y eléctricamente funcional. En este contexto, es un desafío insertar de manera eficiente dispositivos electrónicos con células vivas sin desencadenar reacciones adversas en el sistema biológico receptor ni en el sistema electrónico diseñado. Además, es deseable monitorizar en tiempo real y de manera mínimamente invasiva las respuestas de dichos sistemas biológicos que se han añadido a tales dispositivos bioelectrónicos. En este trabajo presentamos el desarrollo (diseño, fabricación y caracterización) y validación de sensores y actuadores mecánicamente suaves y compatibles con células capaces de funcionar correctamente dentro de un entorno de cultivo celular, específicamente de una línea celular de células epiteliales humanas. Para el desarrollo de los sensores hemos propuesto utilizar técnicas compatibles con la bioimpresión, alejándonos de la micro fabricación tradicionalmente usada para la manufactura de sensores electrónicos, con el objetivo a largo plazo de promover la fabricación de los tejidos y los sensores electrónicos simultáneamente en un mismo sistema de impresión híbrido. Para el desarrollo de los actuadores hemos explorado el uso de polímeros electroactivos y compatibles con impresión y hemos investigado el efecto de estímulos mecánicos dinámicos en respuestas celulares con el objetivo a largo plazo de autoinducir comportamientos biológicos controlados de forma electrónica. En concreto, este trabajo presenta sensores basados en electrodos interdigitados impresos por inyección de tinta para monitorear la migración celular, proliferación y adhesiones célula-sustrato de una línea celular de células epiteliales humanas (HaCaT) en tiempo real y de manera no invasiva mediante espectroscopía de impedancia. Por otro lado, este trabajo presenta actuadores biocompatibles basados en el polímero piezoeléctrico fluoruro de poli vinilideno y ha investigado los efectos de estimular mecánicamente células epiteliales en relación con la proliferación, migración y morfología celular mediante variaciones dinámicas de la magnitud, frecuencia y duración de estímulos mecánicos explotando el actuador biocompatible propuesto. Ambos sistemas presentados como resultado de esta tesis doctoral contribuyen al desarrollo de tejidos 3D con electrónica incorporada, promoviendo una investigación hacia la fabricación de sustitutos equivalentes de piel mitad orgánica mitad electrónica como tejidos funcionales biónicos inteligentes.The main works presented in this thesis have been conducted in the facilities of the Universidad Carlos III de Madrid with support from the program Formación del Profesorado Universitario FPU015/06208 granted by Spanish Ministry of Education, Culture and Sports. Some of the work has been also developed in the facilities of the Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration (IZM) and University of Applied Sciences (HTW) in Berlin, under the supervision of Prof. Dr. Ing. H-D. Ngo during a research visit funded by the Mobility Fellows Program by the Spanish Ministry of Education, Culture, and Sports. This work has been developed in the framework of the projects BIOPIELTEC-CM (P2018/BAA-4480), funded by Comunidad de Madrid, and PARAQUA (TEC2017-86271-R) funded by Ministerio de Ciencia e Innovación.Programa de Doctorado en Ingeniería Eléctrica, Electrónica y Automática por la Universidad Carlos III de MadridPresidente: José Antonio García Souto.- Secretario: Carlos Elvira Pujalte.- Vocal: María Dimak

Monitoring the efficacy of aseptic sterilization processes by means of calorimetric and impedimetric sensing principles

Package sterilization is an essential step during aseptic packaging of food, pharmaceuticals or medical instruments to prevent microbiological contamination of the product. In food industries, the main objective is to produce consumer-safe and long-term stable food products. In recent years, the favored method to sterilize package material is by use of gaseous hydrogen peroxide (H2O2) at concentrations up to 10% v/v and elevated temperatures up to 300 °C. These process parameters enable a fast and effective, in chain sterilization of packages prior to filling with sterile products. Monitoring of this sensitive process is performed by predefined machine settings and laborious microbiological challenge tests, with earliest results after 72 hours. In previous works different sensors to monitor the packaging sterilization process have been developed, but till now there is no commercial system available to continuously monitor the final gas concentration or the microbial sterilization efficacy online within the package. In the present work, as a first approach the sensing principle of a calorimetric H2O2 gas sensor has been studied in more detail. The sensor is based on a differential set-up of one catalytically activated and one passivated temperature-sensing element. Surface characterizations have been performed to reveal the chemical reaction of H2O2 at the applied catalyst manganese(IV) oxide (MnO2). The surface characterization depicted a transition of the manganese oxidation state. Moreover, the treatment with H2O2 eliminates the polymeric layer on top of the catalyst, which has been applied as polymer matrix to attach the catalyst onto the sensing element. The calorimetric gas sensor has been further described by analytical expressions in order to evaluate the theoretical temperature rise. Thereby, different sensor scenarios (steady-state process, gas diffusion process and convective gas flow) have been described by the sensor's thermochemistry and physical transport mechanisms. These theoretical assumptions have been accompanied by surface and thermal characterizations of polymers applied as passivation materials. The characterizations demonstrate the suitability of the three investigated polymers (SU-8 photoresist, Teflon derivatives PFA and FEP), to act as a passivation against gaseous H2O2. As second approach of this work, a novel biosensor has been developed. This biosensor is based on interdigitated electrodes (IDE) on which a standardized test organism is immobilized. This test microorganism, spores of Bacillus atrophaeus, is commonly applied in industrial microbiological challenge tests to evaluate the efficacy of sterilization processes. Impedance measurements are applied to characterize the microbiological samples at the sensor surface before and after the gaseous H2O2 sterilization process. Thereby, a remaining change in impedance and phase has been observed. Numerical simulation tools have been employed to analyze the sensor signal, and to gather material parameters of the spores. Finally, the impedimetric and calorimetric sensor have been combined to serve as a miniaturized sensor system to analyze the efficacy of the gaseous sterilization process

Development of a low-cost graphene-based impedance biosensor

PhD ThesisThe current applicability and accuracy of point-of-care devices is limited, with the need of future technologies to simultaneously target multiple analytes in complex human samples. Graphene’s discovery has provided a valuable opportunity towards the development of high performance biosensors. The quality and surface properties of graphene devices are critical for biosensing applications with a preferred low contact resistance interface between metal and graphene. However, each graphene production method currently results in inconsistent properties, quality and defects thus limiting its application towards mass production. Also, post-production processing, patterning and conventional lithography-based contact deposition negatively impact graphene properties due to chemical contamination. The work of this thesis focuses on the development of fully-functional, label-free graphene-based biosensors and a proof-of-concept was established for the detection of prostate specific antigen (PSA) in aqueous solution using graphene platforms. Extensive work was carried out to characterize different graphene family nanomaterials in order to understand their potential for biosensing applications. Two graphene materials, obtained via a laser reduction process, were selected for further investigations: reduced graphene oxide (rGO) and laser induced graphene from polyimide (LIG). Electrically conductive, porous and chemically active to an extent, these materials offer the advantage of simultaneous production and patterning as capacitive biosensing structures, i.e. interdigitated electrode arrays (IDE). Aiming to enhance the sensitivity of these biosensors, a novel, radio-frequency (RF) detection method was investigated and compared with conventional electrochemical impedance spectroscopy (EIS) on a well-known biocompatible material: gold (standard). It was shown that the RF detection methods require careful design and testing setup, with conventional EIS performing better in the given conditions. The method was further used on rGO and LIG IDE devices for the electrochemical impedance detection of PSA to assess the feasibility of the graphene based materials as biosensors. The graphene-based materials were successfully functionalized via the available carboxylic groups, using the EDC-NHS chemistry. Despite the difficulty of producing reproducible graphene-based electrodes, highly required for biosensor development, extensive testing was carried out to understand their feasibility. The calibration curves obtained via successive PSA addition showed a moderate-to-high ii sensitivity of both rGO and LIG IDE. However, further adsorption and drift testing underlined some major limitations in the case of LIG, due to its complex morphology and large porosity. To enable low contact resistance to these biosensors, the electroless nickel coating process is shown to be compatible with various graphene-based materials. This was demonstrated by tuning the chemical nickel bath and method conditions for pristine graphene and rGO for nickel contacts deposition