Giulia Petrucci

Thermal characterization of a thin film heater on glass substrate for lab-on-chip applications

This paper presents for the first time an accurate thermal characterization of thin film heaters manufactured on glass substrates. The characterization has been performed on Cr/Al/Cr meandered heaters. Techniques commonly adopted for measuring the temperature coefficient of resistance, the thermal resistance and thermal capacitance in the case of Si-based microheaters have been conveniently modified to take into account the fundamentally different thermal parameters of a heater manufactured on glass. In order to reduce power consumption, 250 μm wide trenches were manufactured on the back side of the heaters, obtaining an increase of the thermal resistance of about 50% when the glass is in good thermal contact with a heat sink. The measured values of thermal resistance and time constants on a heat sink and in air have been justified with simple physical considerations. Guidelines are also given for a further increase of the thermal resistance of the thin film heaters.

Multifunctional System-on-Glass for Lab-on-Chip applications.

Lab-on-Chip are miniaturized systems able to perform biomolecular analysis in shorter time and with lower reagent consumption than a standard laboratory. Their miniaturization interferes with the multiple functions that the biochemical procedures require. In order to address this issue, our paper presents, for the first time, the integration on a single glass substrate of different thin film technologies in order to develop a multifunctional platform suitable for on-chip thermal treatments and on-chip detection of biomolecules. The proposed System on-Glass hosts thin metal films acting as heating sources; hydrogenated amorphous silicon diodes acting both as temperature sensors to monitor the temperature distribution and photosensors for the on-chip detection and a ground plane ensuring that the heater operation does not affect the photodiode currents. The sequence of the technological steps, the deposition temperatures of the thin films and the parameters of the photolithographic processes have been optimized in order to overcome all the issues of the technological integration. The device has been designed, fabricated and tested for the implementation of DNA amplification through the Polymerase Chain Reaction (PCR) with thermal cycling among three different temperatures on a single site. The glass has been connected to an electronic system that drives the heaters and controls the temperature and light sensors. It has been optically and thermally coupled with another glass hosting a microfluidic network made in polydimethylsiloxane that includes thermally actuated microvalves and a PCR process chamber. The successful DNA amplification has been verified off-chip by using a standard fluorometer.

Thermal characterization of thin film heater for lab-on-chip application

This paper presents the design, fabrication and characterization of a thin film heater functional for thermal treatments in lab-on-chip system. The spatial temperature distribution determined by different heater geometries has been studied through electro-thermal simulations by using COMSOL Multiphysics. The heater showing the more uniform temperature distribution has been subsequently fabricated and characterized. A very good agreement between modeled and measured data has been attained. Results show a spatial temperature distribution of about ±1°C over an area comparable to the heater area and a directly USB powered heater, demonstrating the suitability of the proposed device for lab-on-chip thermal applications.

Thermal control system based on thin film heaters and amorphous silicon diodes

In this paper we present a system able to perform thermal treatments on lab-on-chip devices fabricated on glass substrates. The system includes a thin film resistor acting as heater and thin film hydrogenated amorphous silicon diodes acting as temperature sensors. An electronic system controls the lab-on-chip temperature through a Proportional-IntegralDerivative algorithm. In particular, an electronic board infers the system temperature measuring the voltage across the amorphous silicon diodes and drives the heater to achieve the set-point temperature. Taking into account the 16-bit ADC resolution and the sensors sensitivity, which is around 3.6 mV/oC, we estimate that our system is able to detect temperature variation as low as 3.5·10-3oC. Furthermore, the experimental results show that the system is able to stabilize the system temperature with a precision better than 0.1 oC.

Rapid prototyping of glass microfluidic chips based on autonomous capillary networks for physiological solutions

In this work, a technological solution is proposed for the fast and low cost realization of autonomous capillary systems that can operate with saline solutions typically used for the implementation of bioanalytical protocols. In order to develop and test the technological process, a simple microfluidic network has been designed. The network includes elementary structures as mixers, T-junctions and capillary pumps for studying the behavior of the fluid inside the test chips. The proposed microfluidic network is made in SU-8 deposited on glass substrate and the microchannels are sealed using a glass cover. A novel procedure has been implemented for the fabrication of devices using low-pressure and low-temperature bonding avoiding the risk of breaking the glass slides while ensuring a very good sealing of the microchannels. The fabricated devices showed strong capillary force and successful mixing of colored physiological solutions. Thanks to the chemical inertia of the used materials and to their mechanical properties, the chip can be easily reused after a chemical rinse, high pressure air blow and thermal treatment.

Thermally actuated microfluidic system for lab on chip applications

In this paper, we present the design and fabrication of Polydimethylsiloxane based Lab-on-Chip system useful for polymerase chain reaction (PCR) application. The proposed system is fabricated on glass substrate, taking advantage of thermally conductive and electrically insulating substrate. On one side of glass microfluidic system having reaction chamber for PCR, microchannel for fluid handling and two thermally actuated valves for fluid control has been made. On the other side of glass three thin film heaters are integrated, two heaters located under valves actuate the valves membrane and allow the chamber isolation and the middle heater is dedicated to PCR thermal cycle. The design of the system, carried out by using COMSOL Multiphysics, involved the optimization of the system dimensions and the shape of microchannel used to completely close the channel. We have also study the thermal interaction between three heaters and optimize the design to minimize the interference. The system was fabricated and an experiment actuating the valves was performed verifying the closing of the microfluidic channel.

Drop position sensing in digital microfluidics based on capacitance measurement

In this work, we present an electronic circuit able to sense the droplet position in Electro-Wetting On Dielectric (EWOD) systems. The drop position is determined measuring the equivalent capacitance of the EWOD electrode, whose value varies according to the presence of the fluid over the pad. In the presented system, the capacitance measurement is achieved through the “Frequency Shift Oscillator” method: the EWOD electrode is inserted in an oscillator circuit, whose operating frequency is inversely proportional to the electrode capacitance value. A microcontroller, included in the system, counts the number of rising edges at the output of the circuit determining the oscillation frequency. The oscillator has been simulated and subsequently fabricated on a double layer printed circuit board. A very good agreement between simulations and experiments has been achieved. The value of obtained sensitivity is not lower than 1.2 kHz/pF that corresponds to a minimum detectable capacitance variation of 0.167 pF. This value is well below the variation of capacitance due to the presence of the droplet above the EWOD electrode and demonstrates the suitability of our circuit as a successful drop position sensor.

Lab-on-glass system for DNA treatments

This paper presents the fabrication and testing of a lab-on-chip system suitable for treatment of DNA. It includes two main modules: a system-on-glass (SoG) and a disposable microfuidic chip. The SoG integrates, on the same glass substrate, thin film metal heaters and amorphous silicon temperature sensors to achieve a uniform temperature distribution (within ±1°C) in the heated area. Two polydimethylsiloxane microfluidic chips have been developed: a PCR-Chip for DNA amplification and a dsDNA-Chip for separation and selective isolation of a ssDNA from a dsDNA. The proposed system aims therefore to develop compact, low-cost devices that can implement multiple functions in biochemical procedures. In particular, the tested bioanalytical procedures are well suited for carrying-out an on-chip SELEX process, a combinatorial chemistry technique for the selection of aptamers.

Integration of electrowetting technology inside an all-glass microfluidic network

This paper presents a low temperature technological process ahle to integrate an all-glass microfluidic network with an ElectroWetting On Dielectric (EWOD) structure for the digital handling of liquids. The fluidic channels result from the wet-etching of the glass, while the electrodes necessary for the droplet movement are deposited on the bottom and top surfaces of the microfluidic structure. The bottom electrodes are produced by a selective and sequential photolithographic pattern of a stack of metals, insulation layer and hydrophobic film. The top common electrode is made by a continuous transparent conductive oxide, covered by a hydrophobic layer. Compatibility of the technological steps and mechanical robustness of the proposed device have been tested designing and fabricating a microfluidic network integrating a central chamber, with a volume of about 9 μΐ, two reservoirs, two microfluidic channels and 26 EWOD electrodes. The maximum temperature reached during the device fabrication was 330°C, which is two times lower than the one used for the anodic bonding of glass-based microfluidic network.

Autonomous Microfluidic Capillary Network for on Chip Detection of Chemiluminescence

This work reports on the design, simulation and fabrication of an autonomous microfluidic network. It is a part of a highly integrated, new analytical platform for the multiparametric detection of bio-organic molecules in extra-terrestrial environment. The proposed microfluidic system, made in SU-8 3050, allows to obtain an autonomous microfluidic network able to have simultaneous capillary filling and fresh solution into each site of detection avoiding cross-contamination among different sites. Computational Fluid Dynamics (CFD) simulations have been carried in order to verify the proper operation of the designed microfluidic network and to optimize it. Technological processes have been refined and adapted in order to ensure good adhesion, using low-temperature and low-pressure bonding avoiding the risk of breaking the glass slides. Experiments have been conducted to verify the autonomous capillary filling of the entire network and its rinsing with buffer solution. The experimental results are in good agreement with the simulations.