In the last few years the interest in Lab-On-a-Chip devices has been increasing. These low cost devices are able to diagnose a number of diseases, such as malaria, tuberculosis, neoplasias, HIV and to get fast genetic tests without using a specific laboratory. It makes them suitable for Point-Of-Care applications and screening tests of early diagnosis. In fact, their use would entail a reduction of the human and economic costs that characterize these deseases and affect our society. The use of Lab-On-a-Chip to diagnose an increasing number of deseases has required an improvement of molecular and cellular detection strategies. A particularly useful system in terms of simplicity and cost of implementation in this field of application is based on the measurement of electrical parameters. Among them, the capacitive measurements of a liquid sample can be used to detect the presence of biomolecules and analytes in a biological sample, exploiting the bioreceptors and the chemical reactions that are only available in water-based solutions. Comparing them with other methods (e.g. optical, gravimetric, mechanical, magnetic methods), these techniques entail a cost reduction of the analysis by a major simplicity in terms of production and by a lower usage of chemical reagents. Nevertheless, they are often affected by problems in stability, whose dynamics have not been deeply inspected. The aim of the project, which is reported in the following essay, is the analysis of the possible factors that limits the stability of the capacitive measurements into a liquid environment. The instability of the measurements has been associated with the evaporation and the appearence of temperature fluctuations. On the basis of this working hypothesis, a device has been designed. It consists of two capacitive sensors, a microfluidic circuit and a temperature sensor. These sensors have been realized on a glass substrate using a planar technology with only one photolithographic step, in order to guarantee the reproducibility and the low cost of the fabbrication process. The temperature sensor has been located near the capacitive sensors in order to stabilize the temperature of the detection area. The solution under test is injected in the sensor using a microfluidic channel with the purpose of blocking evaporation. This device has been created using the Cleanroom in the PoliFAB laboratory; it has been inserted in a specifically developed experimental setup, then it has been tested to evaluate the method of capacitive and temperature measurement, as displayed in this essay. The evaporation effect has been evaluated by monitoring the capacitive signal in presence and in absence of the microfluidic circuit. The temperature effect has been quantified by measuring the capacitance at different temperatures. The system stability has been analyzed with overnight recordings. The improvement of the stability offered by a local control of the temperature has been evaluated. In addition, the usage of a differential measurement in order to compensate temperature fluctuations has been evaluated as well. The experimental results confirms the importance of temperature control and the importance of limiting the evaporation of the solution during the experiment. In the course of a number of experiments, the initial assumptions about the origin of instability have been verified. The proposed method has led to an enhancement in stability up to 154 times, lowering the normalized root mean square deviation from 1253ppm to 8.1ppm as far as a capacitive measurement is concerned. The device has been successfully used to detect the sedientation of microspheres into the microfluidic channel. Although the first prototype has been realized with spacing between the electrodes of 25um, a dimension that is not compatible with molecular detection issues, this method can be fabbricated using submicrometric patterns, paving the way to an improving of the capacitive sensors for molecular detection.
I recenti anni sono caratterizzati da un forte interesse per i Lab-On-a-Chip. Questi dispositivi a basso costo possono diagnosticare un gran numero di patologie (come per esempio malaria, tubercolosi, neoplasie o HIV) o effettuare test genetici senza l'utilizzo di un laboratorio specializzato in tempi molto rapidi. Ciò li rende particolarmente adatti per applicazioni Point-Of-Care e per test di screening finalizzati alla diagnosi precoce, infatti il loro utilizzo permetterebbe di abbattere il costo umano ed economico che tali patologie hanno a livello sociale. L'uso dei Lab-On-a-Chip per diagnosticare un crescente numero di patologie ha reso necessario il miglioramento delle strategie di detezione molecolari e cellulari. Un insieme di tecniche particolarmente utile per semplicità ed economicità di implementazione in tale settore applicativo è costituito dai metodi basati sulla misura di parametri elettrici. Tra questi figurano le misure capacitive in liquido, che possono essere anche utilizzate per individuare la presenza di biomolecole ed analiti in un campione biologico sfruttando dei biorecettori e delle reazioni chimiche disponibili solo in soluzioni acquose. Rispetto ad altri metodi (per esempio ottici, gravimetrici, meccanici, magnetici), tali misure permettono di ridurre i costi dell'analisi grazie ad una maggiore semplicità di produzione e ad una minore necessità di reagenti, ma sono generalmente affette da problemi di stabilità, le cui dinamiche non sono ancora del tutto note. L'obiettivo del progetto esposto nella seguente tesi è quello di identificare ed analizzare le possibili cause delle limitazioni di stabilità delle misure capacitive in liquido. Esse sono state ricondotte all'evaporazione del liquido e alla presenza di fluttuazioni termiche e dunque, sulla base di tale ipotesi, è stato progettato un dispositivo composto da due sensori capacitivi, da un circuito microfluidico e da un sensore di temperatura. I tre sensori sono realizzati su un substrato di vetro in tecnologia planare con un unico passo litografico garantendo un processo riproducibile e a basso costo. Il sensore di temperatura è realizzato nelle immediate vicinanze dei sensori capacitivi per permettere una stabilizzazione della temperatura nell'area di detezione, mentre il canale microfluidico, in cui è stata iniettata la soluzione, impedisce l'evaporazione. I due sensori capacitivi permettono di implementare una misura differenziale per ridurre ulteriormente l'effetto delle fluttuazioni indesiderate. Tale dispositivo è stato poi realizzato utilizzando la cleanroom del laboratorio di ateneo PoliFAB, inserito in un setup sperimentale appositamente sviluppato e sottoposto a sperimentazione per valutare il metodo di lettura capacitiva e di temperatura esposto all'interno dell'elaborato. L'effetto dell'evaporazione è stato valutato confrontando l'andamento del segnale capacitivo in presenza ed in assenza del circuito microfluidico. L'effetto della temperatura è stato analizzato valutando le variazioni capacitive con diverse temperature, la capacità del sistema ideato di fornire una lettura capacitiva stabile durante registrazioni notturne, il miglioramento prestazionale introdotto dall'utilizzo di una lettura differenziale per compensare le fluttuazioni termiche e la capacità del sistema di compensare stimoli termici esterni in un possibile scenario di utilizzo. I risultati sperimentali hanno confermato l'importanza del controllo della temperatura e di mantenere limitata l'evaporazione durante l'esperimento. Il metodo proposto ha permesso di migliorare la stabilità del sistema, abbassando la deviazione quadratica media normalizzata della misura capacitiva da 1253ppm a 8.1ppm, e quindi migliorando di 154 volte le prestazioni ottenute. Il sistema realizzato è stato utilizzato con successo per individuare la sedimentazione di microsfere nel canale microfluidico. Benché il primo prototipo sia stato realizzato per semplicità con una spaziatura tra le armature del sensore capacitivo di 25um, non compatibile con applicazioni di detezione molecolare, il metodo proposto può essere applicato su geometrie sub-micrometriche, più adatte a problemi di questo tipo.
Progetto e realizzazione di un sensore di capacità per misure in liquido ad elevata stabilità
LONGO, COSIMO DAMIANO
2015/2016
Abstract
In the last few years the interest in Lab-On-a-Chip devices has been increasing. These low cost devices are able to diagnose a number of diseases, such as malaria, tuberculosis, neoplasias, HIV and to get fast genetic tests without using a specific laboratory. It makes them suitable for Point-Of-Care applications and screening tests of early diagnosis. In fact, their use would entail a reduction of the human and economic costs that characterize these deseases and affect our society. The use of Lab-On-a-Chip to diagnose an increasing number of deseases has required an improvement of molecular and cellular detection strategies. A particularly useful system in terms of simplicity and cost of implementation in this field of application is based on the measurement of electrical parameters. Among them, the capacitive measurements of a liquid sample can be used to detect the presence of biomolecules and analytes in a biological sample, exploiting the bioreceptors and the chemical reactions that are only available in water-based solutions. Comparing them with other methods (e.g. optical, gravimetric, mechanical, magnetic methods), these techniques entail a cost reduction of the analysis by a major simplicity in terms of production and by a lower usage of chemical reagents. Nevertheless, they are often affected by problems in stability, whose dynamics have not been deeply inspected. The aim of the project, which is reported in the following essay, is the analysis of the possible factors that limits the stability of the capacitive measurements into a liquid environment. The instability of the measurements has been associated with the evaporation and the appearence of temperature fluctuations. On the basis of this working hypothesis, a device has been designed. It consists of two capacitive sensors, a microfluidic circuit and a temperature sensor. These sensors have been realized on a glass substrate using a planar technology with only one photolithographic step, in order to guarantee the reproducibility and the low cost of the fabbrication process. The temperature sensor has been located near the capacitive sensors in order to stabilize the temperature of the detection area. The solution under test is injected in the sensor using a microfluidic channel with the purpose of blocking evaporation. This device has been created using the Cleanroom in the PoliFAB laboratory; it has been inserted in a specifically developed experimental setup, then it has been tested to evaluate the method of capacitive and temperature measurement, as displayed in this essay. The evaporation effect has been evaluated by monitoring the capacitive signal in presence and in absence of the microfluidic circuit. The temperature effect has been quantified by measuring the capacitance at different temperatures. The system stability has been analyzed with overnight recordings. The improvement of the stability offered by a local control of the temperature has been evaluated. In addition, the usage of a differential measurement in order to compensate temperature fluctuations has been evaluated as well. The experimental results confirms the importance of temperature control and the importance of limiting the evaporation of the solution during the experiment. In the course of a number of experiments, the initial assumptions about the origin of instability have been verified. The proposed method has led to an enhancement in stability up to 154 times, lowering the normalized root mean square deviation from 1253ppm to 8.1ppm as far as a capacitive measurement is concerned. The device has been successfully used to detect the sedientation of microspheres into the microfluidic channel. Although the first prototype has been realized with spacing between the electrodes of 25um, a dimension that is not compatible with molecular detection issues, this method can be fabbricated using submicrometric patterns, paving the way to an improving of the capacitive sensors for molecular detection.| File | Dimensione | Formato | |
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2017_04_LONGO.pdf
Open Access dal 05/04/2018
Descrizione: Testo della tesi
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https://hdl.handle.net/10589/133334