Optical imaging techniques in microfluidics and their applications

Microfluidic devices have undergone rapid development in recent years and provide a lab-on-a-chip solution for many biomedical and chemical applications. Optical imaging techniques are essential in microfluidics for observing and extracting information from biological or chemical samples. Traditionally, imaging in microfluidics is achieved by bench-top conventional microscopes or other bulky imaging systems. More recently, many novel compact microscopic techniques have been developed to provide a low-cost and portable solution. In this review, we provide an overview of optical imaging techniques used in microfluidics followed with their applications. We first discuss bulky imaging systems including microscopes and interferometer-based techniques, then we focus on compact imaging systems that can be better integrated with microfluidic devices, including digital in-line holography and scanning-based imaging techniques. The applications in biomedicine or chemistry are also discussed along with the specific imaging techniques

Optical microsensor for counting food substance particles in lab-on-a-chips

Integrated optical detection is considered to be an important operation in lab-on-a-chips. This paper presents an optical fiber-based micro-sensor that is capable of detecting food substance particles in a lab-on-a-chip. The system consists of a microcontroller and associated circuitry, a laser emitter, a laser receiver, fiber optic cables, a microfluidics chip, and the food substance samples to be tested. When the particles flow through the microfluidic channel in the chip, the receiver’s output voltage varies due to the particles blocking the passage of the laser ray. The changes in the collected signals are analyzed to count the number of particles. Experiments are conducted on several food substance samples including talcum powder, ground ginger, and soy sauce. The experimental results are presented and discussed

Study of light driven phenomena in integrated opto-microfluidic lithium niobate platforms

In last decades microfluidics has gained an increasing interest by the scientific community due to its capability of control liquids on the microscale. Especially, droplet microfluidics technology provides a high manipulation level on very small volumes of fluids. This feature makes it a promising candidate for biological, medical and chemistry applications. With a compact and simple microfluidic device, droplets can be coalesced, mixed, sorted and employed either as micro chemical reactors or as carriers of biological sample. From the experimental point of view there are many ways to produce and see micro droplets. The device used for this thesis consists of an opto-microfluidic chip made of lithium niobate, in which are integrated a microfluidic circuit and an optical waveguide, normal to the channel and produced by titanium diffused in lithium niobate. Droplets flowing in the channel are detected using a laser light transmitted in the optical waveguide. After having illuminated the microfluidic channel, where a ”train” of water droplets in oil pass through, the light is collected by another waveguide facing on the other side of the channel: the intensity transmitted is recorded by way of a tailored electronic circuit. The water droplet in oil makes the signal of the light transmitted beam change in intensity, from a fixed value, relative to oil, to another basically due to the different refractive index of the droplet respect the surrounding flux. The signal variation is not sharp, but is preceded by a peak of intensity. Thus, the acquisition profile of the transmitted light intensity identifies the phenomenon of droplet passage, but up to now no studies were reported from a theoretical point of view. In this thesis, we aimed to clarify this aspect and provide a fine comprehension of it. Therefore, we correlated the curves obtained to some physical parameters of the droplets and to give a theoretical explanation to the shape of such a profile. Since the length and the velocity of the droplets are proportional to the ratio of the liquids’ fluxes flowing in the microfluidic channel, it is possible to measure and analyse the effect of the droplet passage and find new physical observables to identify the droplet, its shape and relative velocity. In particular, since the optical transmitted light across the droplet presents intensity modulation, great care was devoted to identifying their origin and their systematic behaviour. The aim was that of relating the modulation intensity peaks to the meniscus of the droplet and then to investigate all the other zones in its middle with an accurate phenomenological analysis. The idea that features in the light transmitted intensity signal could be related with the meniscus of the droplet, as well as intuitive, has never proved in literature and has confirmed by a simulation carried out in the frame of this thesis. In fact, a theoretical analysis of this phenomena is made: assuming ray optic, two-dimensional version of the problem and after having chosen the best shape for the source, a program that simulate the passage of a droplet in the microfluidic channel was written. After having simulates the physical interaction between the droplet that advances in the channel and the light exiting the waveguide, the software computes the transmitted intensity in order to reproduce the acquisition profile. The numerical profiles thus obtained are reported and commented, also by varying the parameters of the simulation. Finally, comparisons between the results of the simulations and the experimental profiles enable us to understand better the shape of the droplet. Based on these results we analysed also the shape that the interface water-oil should be in order to reproduce the low transmitted intensity typical of the droplet's centre. Performing an accurate analysis over a wide range of the fluxes ratio ϕ=Q_oil/Q_water also the role of the so called "secondary peaks" is investigated and fingerprints for a detailed description of the transmitted intensity profile are provided. This, up to our knowledge, has not been investigated yet. Finally, we can identify the droplet passage, so that to estimate its length with perspectives of setting up an automatized and fast data analysis procedure. This feature exploits an integrated opto-microfluidic device, independent from the standard imagining processes and with the possibility to perform a feedback loop with manipulation stages. Moreover, the same measures, with a one-to-one comparison with synchronous acquisitions of microscope images of the droplets, let us investigate also the sensibility of the optical transmitted light from the overall geometrical shape of the droplet. New physical observables in the acquisition profile allow us to describe and identify the transition from different production regimes of the droplets, also in different geometrical configuration of the microchannels, in a more accurate way than other imaging technique presented in literature

Imaging Local Acoustic Pressure in Microchannels

A method for determining the spatially resolved acoustic field inside a water-filled microchannel is presented. The acoustic field, both amplitude and phase, is determined by measuring the change of the index of refraction of the water due to local pressure using stroboscopic illumination. Pressure distributions are measured for the fundamental pressure resonance in the water and two higher harmonic modes. By combining measurement at a range of excitation frequencies, a frequency map of modes is made, from which the spectral line width an

Dissolved oxygen sensing using an optical fibre long period grating coated with hemoglobin

A long period grating fiber optic sensor coated with hemoglobin is used to detect dissolved oxygen. The sensitivity of this sensor to the ratio of dissolved carbon dioxide to dissolved oxygen is demonstrated via the conversion of carboxyhemoglobin to oxyhemoglobin on the sensor surface. The sensor shows good repeatability with a %CV of less than 1% for carboxyhemoglobin and oxyhemoglobin states with no measurable drift or hysteresis

Review: Biosensors for the detection of Escherichia coli

The supply of safe potable water, free from pathogens and chemicals, requires routine  analyses and the application of several diagnostic techniques. Apart from being  expensive, many of the detection methods require trained personnel and are often time-consuming. With drastic climate changes, severe droughts, increases in  population and pollution of natural water systems, the need to develop ultrasensitive, low-cost and hand-held, point-of-use detection kits to monitor water quality is critical. Although Escherichia coli is still considered the best indicator of water quality, cell numbers may be below detection limits, or the cells may be non-culturable and thus only detected by DNA amplification. A number of different biosensors have been developed to detect viable, dead or non-culturable microbial cells and chemicals in water. This review discusses the differences in these biosensors and evaluates the application of microfluidics in the design of ultra-sensitive nano-biosensors.Keywords: Biosensors, microfluidics, nano-biosensors, E. coli detectio

Real-time sweat pH monitoring based on a wearable chemical barcode micro-fluidic platform incorporating ionic liquids

This work presents the fabrication, characterisation and the performance of a wearable, robust, flexible and disposable chemical barcode device based on a micro-fluidic platform that incorporates ionic liquid polymer gels (ionogels). The device has been applied to the monitoring of the pH of sweat in real time during an exercise period. The device is an ideal wearable sensor for measuring the pH of sweat since it does not contents any electronic part for fluidic handle or pH detection and because it can be directly incorporated into clothing, head- or wristbands, which are in continuous contact with the skin. In addition, due to the micro-fluidic structure, fresh sweat is continuously passing through the sensing area providing the capability to perform continuous real time analysis. The approach presented here ensures immediate feedback regarding sweat composition. Sweat analysis is attractive for monitoring purposes as it can provide physiological information directly relevant to the health and performance of the wearer without the need for an invasive sampling approac

Low loss high index contrast nanoimprinted polysiloxane waveguides

Nanoimprint lithography is gaining rapid acceptance in fields as diverse as microelectronics and microfluidics due to its simplicity high resolution and low cost. These properties are critically important for the fabrication of photonic devices, where cost is often the major inhibiting deployment factor for high volume applications. We report here on the use of nanoimprint technology to fabricate low loss broadband high index contrast waveguides in a Polysiloxane polymer system for the first time

Dual beam swept source optical coherence tomography for microfluidic velocity measurements

Microfluidic flows are an increasing area of interest used for “lab-on-a-chip” bioanalytical techniques, drug discovery, and chemical processing. This requires optical, non-invasive flow-visualization techniques for characterising microfluidic flows. Optical Coherence Tomography (OCT) systems can provide three-dimensional imaging through reasonably-opaque materials with micrometre resolution, coupled to a single optical axis point using optical fibre cables. Developed for imaging the human eye, OCT has been used for the detection of skin cancers and endoscopically in the human body. Industrial applications are growing in popularity including for the monitoring of bond-curing in aerospace, for production-line non-destructive-testing, and for medical device manufacturing and drug encapsulation monitoring. A dual beam Optical Coherence Tomography system has been developed capable of simultaneously imaging microfluidic channel structures, and tracking particles seeded into the flow to measure high velocity flows, using only a single optical access point. This is achieved via a dual optical fibre bundle for light delivery to the sample and a custom high-speed dual channel OCT instrument using an akinetic sweep wavelength laser. The system has 10 μm resolution in air and a sweeping rate of 96 kHz. This OCT system was used to monitor microfluidic flows in 800 μm deep test chips and Poiseuille flows were observed