Mikael Evander

Review of cell and particle trapping in microfluidic systems

The ability to obtain ideal conditions for well-defined chemical microenvironments and controlled temporal chemical and/or thermal variations holds promise of high-resolution cell response studies, cell-cell interactions or e.g. proliferation conditions for stem cells. It is a major motivation for the rapid increase of lab-on-a-chip based cell biology research. In view of this, new chip-integrated technologies are at an increasing rate being presented to the research community as potential tools to offer spatial control and manipulation of cells in microfluidic systems. This is becoming a key area of interest in the emerging lab-on-a-chip based cell biology research field. This review focuses on the different technical approaches presented to enable trapping of particles and cells in microfluidic system.

Acoustofluidics 20: Applications in acoustic trapping

This part of the Acoustofluidics tutorial series reviews applications in acoustic trapping of micron-sized particles and cells in microfluidic systems. Acoustic trapping enables non-invasive and non-contact immobilisation of cells and particles in microfluidic systems. Acoustic trapping has been used for reducing the time needed to create 3D cell clusters, enhance particle-based bioassays and facilitated interaction studies of both cells and particles. An area that is increasingly interesting is the use of acoustic trapping for enriching low concentration samples and the washing or fractioning of cell populations prior to sensitive detection methods (MALDI-MS, PCR etc.) The main focus of the review is systems where particles can be retained against a flow while applications in which particles are positioned in a stationary fluid will be addressed in part 21 of the Acoustofluidics tutorial series (M. Wiklund, S. Radel and J. J. Hawkes, Lab Chip, 2012, 12, ).

Acoustofluidics 5: Building microfluidic acoustic resonators.

Acoustophoresis is getting more attention as an effective and gentle non-contact method of manipulating cells and particles in microfluidic systems. A key to a successful assembly of an acoustophoresis system is a proper design of the acoustic resonator where aspects of fabrication techniques, material choice, thickness matching of involved components, as well as strategies of actuation, all have to be considered. This tutorial covers some of the basics in designing and building microfluidic acoustic resonators and will hopefully be a comprehensive and advisory document to assist the interested reader in creating a successful acoustophoretic device.

Non-contact acoustic cell trapping in disposable glass capillaries

Non-contact trapping using acoustic standing waves has shown promising results in cell-based research lately. However, the devices demonstrated are normally fabricated using microfabrication or precision machining methods leading to a high unit cost. In e.g. clinical or forensic applications avoiding cross-contamination, carryover or infection is of outmost importance. In these applications disposable devices are key elements, thus making the cost per unit a critical factor. A solution is presented here where low-cost off-the-shelf glass capillaries are used as resonators for standing wave trapping. Single-mode as well as multi-node trapping is demonstrated with an excellent agreement between simulated and experimentally found operation frequencies. Single particle trapping is verified at 7.53 MHz with a trapping force on a 10 microm particle of up to 1.27 nN. The non-contact trapping is proved using confocal microscopy. Finally, an application is presented where the capillary is used as a pipette for aspirating, trapping and dispensing red blood cells.

Acoustophoresis in wet-etched glass chips

Acoustophoresis in microfluidic structures has primarily been reported in silicon microfabricated devices. This paper demonstrates, for the first time, acoustophoresis performed in isotropically etched glass chips providing a performance that matches that of the corresponding silicon microdevices. The resonance mode characteristics of the glass chip were equal to those of the silicon chip at its fundamental resonance. At higher order resonance modes the glass chip displays resonances at lower frequencies than the silicon chip. The cross-sectional profiles of acoustically focused particle streams are also reported for the first time, displaying particles confined in a vertical band in the channel center for both glass and silicon chips. A particle extraction efficiency of 98% at flow rates up to 200 microL/min (2% particle concentration) is reported for the glass chip at the fundamental resonance. The glass and silicon chips displayed equal particle extraction performance when tested for increasing particle concentrations of 2-15%, at a flow velocity of 12.9 cm/s for the glass chip and 14.8 cm/s for the silicon chip.

Acoustic differential extraction for forensic analysis of sexual assault evidence.

Forensic DNA analysis of samples obtained from sexual assault evidence relies on separation of male and female components of the recovered genetic material. The conventional separation method used by crime laboratories, differential extraction (DE), is one of the most time-consuming sample preparation steps, requires extensive sample handling, is difficult to automate, and often results in inefficient separation of female DNA from the male sample components. To circumvent conventional DE, acoustic differential extraction (ADE) analysis was developed on a microfluidic device. The ADE method relies on acoustic trapping of sperm cells in the presence of epithelial cell lysate (which is unretained), and laminar flow valving to direct the male and female fractions to separate outlets. Following the separation of sperm from epithelial cell lysate, DNA extraction, quantitation, amplification, and separation were performed using conventional laboratory methods. The results show that highly purified male and female fractions can be obtained with the ADE microdevice from mock sexual assault samples in 14 min. ADE analysis provides the potential to significantly alter the means by which sexual assault evidence is processed in crime laboratories.

Microfluidic impedance cytometer for platelet analysis

We present the design and performance characteristics of a platelet analysis platform based on a microfluidic impedance cytometer. Dielectrophoretic focusing is used to centre cells in a fluid stream, which then forms the core of a two-phase flow (dielectric focusing). This flow then passes between electrodes for analysis by differential impedance spectroscopy at multiple frequencies from 280 kHz to 4 MHz. This approach increases the signal-to-noise ratio relative to a single-phase, unfocused stream, while minimising the shear forces to which the cells are subjected. The percentage of activated platelets before and after passage through the chip was measured using flow cytometry, and no significant change was measured. Measuring the in-phase amplitude at a single frequency is sufficient to distinguish platelets from erythrocytes. Using multi-frequency impedance measurements and discriminant analysis, resting platelets can be discriminated from activated platelets. This multifrequency impedance cytometer therefore allows ready determination of the degree of platelet activation in blood samples.

Acoustic actuated fluorescence activated sorting of microparticles.

In this paper, we present a fluorescence activated sorter realized in a continuous flow microfluidic chip. Sorting is achieved by deflecting a focused particle stream with short acoustic bursts (2.5 ms), in a fluorescence activated configuration. The system utilizes two-dimensional acoustic pre-focusing, using a single actuation frequency, to position all particles in the same fluid velocity regime at flow rates up to 1.7 mL min(-1). Particles were sorted based on their fluorescence intensities at throughputs up to 150 particles s(-1). The highest purity reached was 80% when sorting at an average rate of 50 particles s(-1). The average recovery of a sort was 93.2 ± 2.6%. The presented system enables fluorescence activated cell sorting in a continuous flow microfluidic format that allows aseptic integration of downstream microfluidic functionalities, opening for medical and clinical applications.

Temperature and trapping characterization of an acoustic trap with miniaturized integrated transducers - towards in-trap temperature regulation

An acoustic trap with miniaturized integrated transducers (MITs) for applications in non-contact trapping of cells or particles in a microfluidic channel was characterized by measuring the temperature increase and trapping strength. The fluid temperature was measured by the fluorescent response of Rhodamine B in the microchannel. The trapping strength was measured by the area of a trapped particle cluster counter-balanced by the hydrodynamic force. One of the main objectives was to obtain quantitative values of the temperature in the fluidic channel to ensure safe handling of cells and proteins. Another objective was to evaluate the trapping-to-temperature efficiency for the trap as a function of drive frequency. Thirdly, trapping-to-temperature efficiency data enables identifying frequencies and voltage values to use for in-trap temperature regulation. It is envisioned that operation with only in-trap temperature regulation enables the realization of small, simple and fast temperature-controlled trap systems. The significance of potential gradients at the trap edges due to the finite size of the miniaturized transducers for the operation was emphasized and expressed analytically. The influence of the acoustic near field was evaluated in FEM-simulation and compared with a more ideal 1D standing wave. The working principle of the trap was examined by comparing measurements of impedance, temperature increase and trapping strength with impedance transfer calculations of fluid-reflector resonances and frequencies of high reflectance at the fluid-reflector boundary. The temperature increase was found to be moderate, 7°C for a high trapping strength, at a fluid flow of 0.5mms(-1) for the optimal driving frequency. A fast temperature response with a fall time of 8s and a rise time of 11s was observed. The results emphasize the importance of selecting the proper drive frequency for long term handling of cells, as opposed to the more pragmatic way of selecting the frequency of the highest acoustic output. Trapping was demonstrated in a large interval between 9 and 11.5MHz, while the main trapping peak displayed FWHM of 0.5MHz. A large bandwidth enables a more robust manufacturing and operation while allowing the trapping platform to be used in applications where the fluid wavelength varies due to external variations in fluid temperature, density and pressure.

Frequency tracking in acoustic trapping for improved performance stability and system surveillance

This work proposes and demonstrates an acoustic trapping system where the trapping frequency is automatically determined and can be used to analyse changes in the acoustic trap. Critical for the functionality of this system is the use of a kerfed transducer that removes spurious resonances. This makes it possible to determine the optimal trapping frequency by analysing electrical impedance. It is demonstrated that the novel combination of a kerfed transducer and acoustic trapping in glass capillaries creates a high Q-value resonator. This narrows the frequency bandwidth but allows excellent performance, as confirmed by a ten-fold increase in the flow retention speed when compared to previously reported values. Importantly, the use of automatic frequency tracking allows the use of such a narrow bandwidth resonator without compromising system stability. As changes in temperature, buffer-properties, and the amount of captured particles will affect the properties of the acoustic resonator, corresponding changes in resonance frequency will occur. It is shown that such frequency changes can be accurately tracked using the setup. Therefore, monitoring the frequency over time adds a new feature to acoustic trapping, where experimental progress can be monitored and the amount of trapped material can be quantified.