Optical systems for single cell analyses

Single-molecule fluorescence microscopy review: shedding new light on old problems

Fluorescence microscopy is an invaluable tool in the biosciences, a genuine workhorse technique offering exceptional contrast in conjunction with high specificity of labelling with relatively minimal perturbation to biological samples compared with many competing biophysical techniques. Improvements in detector and dye technologies coupled to advances in image analysis methods have fuelled recent development towards single-molecule fluorescence microscopy, which can utilize light microscopy tools to enable the faithful detection and analysis of single fluorescent molecules used as reporter tags in biological samples. For example, the discovery of GFP, initiating the so-called 'green revolution', has pushed experimental tools in the biosciences to a completely new level of functional imaging of living samples, culminating in single fluorescent protein molecule detection. Today, fluorescence microscopy is an indispensable tool in single-molecule investigations, providing a high signal-to-noise ratio for visualization while still retaining the key features in the physiological context of native biological systems. In this review, we discuss some of the recent discoveries in the life sciences which have been enabled using single-molecule fluorescence microscopy, paying particular attention to the so-called 'super-resolution' fluorescence microscopy techniques in live cells, which are at the cutting-edge of these methods. In particular, how these tools can reveal new insights into long-standing puzzles in biology: old problems, which have been impossible to tackle using other more traditional tools until the emergence of new single-molecule fluorescence microscopy techniques.

Ultrasonic manipulation of single cells

Ultrasonic manipulation has emerged as a simple and powerful tool for trapping, aggregation, and separation of cells. During the last decade, an increasing amount of applications in the microscale format has been demonstrated, of which the most important is acoustophoresis (continuous acoustic cell or particle separation). Traditionally, the technology has proven to be suitable for treatment of high-density cell and particle suspensions, where large cell and particle numbers are handled simultaneously. In this chapter, we describe how ultrasound can be combined with microfluidics and microplates for particle and cell manipulation approaching the single-cell level. We demonstrate different cell handling methods with the purpose to select, trap, aggregate, and position individual cells in microdevices based on multifrequency ultrasonic actuation, and we discuss applications of the technology involving immune cell interaction studies.

Optical manipulation for single-cell studies

In the last decade optical manipulation has evolved from a field of interest for physicists to a versatile tool widely used within life sciences. This has been made possible in particular due to the development of a large variety of imaging techniques that allow detailed information to be gained from investigations of single cells. The use of multiple optical traps has high potential within single-cell analysis since parallel measurements provide good statistics. Multifunctional optical tweezers are, for instance, used to study cell heterogeneity in an ensemble, and force measurements are used to investigate the mechanical properties of individual cells. Investigations of molecular motors and forces on the single-molecule level have led to discoveries that would have been difficult to make with other techniques. Optical manipulation has prospects within the field of cell signalling and tissue engineering. When combined with microfluidic systems the chemical environment of cells can be precisely controlled. Hence the influence of pH, salt concentration, drugs and temperature can be investigated in real time. Fast advancing technical developments of automated and user-friendly optical manipulation tools and cross-disciplinary collaboration will contribute to the routinely use of optical manipulation techniques within the life sciences.

Microrheology with optical tweezers

Microrheology is the study of the flow of materials over small scales. It is of particular interest to those involved with investigations of fluid properties within Lab-on-a-Chip structures or within other micron-scale environments. The article briefly reviews existing active and passive methods used in the study of fluids. It then explores in greater detail the use of optical tweezers as an emerging method to investigate rheological phenomena, including, for example, viscosity and viscoelasticity, as well as the related topic of flow. The article also describes, briefly, potential future applications of this topic, in the fields of biological measurement, in general, and Lab-on-a-Chip, in particular.

Automated focusing of nuclei for time lapse experiments on single cells using holographic optical tweezers

Experiments on single cells are currently gaining more and more interest. Single cell studies often concerns the spatio-temporal distribution of fluorescent proteins inside living cells, visualized using fluorescence microscopy. In order to extract quantitative information from such experiments it is necessary to image the sample with high spatial and temporal resolution while keeping the photobleaching to a minimum. The analysis of the spatial distribution of proteins often requires stacks of images at each time point, which exposes the sample to unnecessary amounts of excitation light. In this paper we show how holographic optical tweezers combined with image analysis can be used to optimize the axial position of trapped cells in an array in order to bring the nuclei into a single imaging plane, thus eliminating the need for stacks of images and consequently reducing photobleaching. This allows more images to be collected, as well as increasing the time span and/or the time resolution in time lapse studies of single cells.