Capacitive Sensing for Monitoring of Microfluidic Protocols Using Nanoliter Dispensing and Acoustic Mixing.

The development of protocols for bio/chemical reaction requires alternate dispensing and mixing steps. While most microfluidic systems use the opening of additional parts of the channel to allow the ingress of fixed volumes of fluid, this requires knowledge of the protocol before the design of the chip. Our approach of using a microfluidic valve to regulate the flow into an initially empty cavity allows for on-chip protocol development and refinement. Mixing is provided by way of surface acoustic wave excitation; this high-frequency vibration causes steady-state streaming flows. We show that capacitive sensing can be used to measure fluid levels, even if multiple fluid types are used, such that nanoliter dispensing accuracy is achieved. Also, the capacitive readout can be used to establish mixing quality and to monitor temperature fluctuations. These capabilities allow for protocols to be conducted without optical assessment and thus will allow for multiplexing, such that reactions could be conducted, simultaneously, in multiple chambers across a chip.

Correction: Comparison of bulk and microfluidic methods to monitor the phase behaviour of nanoparticles during digestion of lipid-based drug formulations using in situ X-ray scattering.

Correction for 'Comparison of bulk and microfluidic methods to monitor the phase behaviour of nanoparticles during digestion of lipid-based drug formulations using in situ X-ray scattering' by Ben J. Boyd et al., Soft Matter, 2019, 15, 9565-9578.

Coalescence of Surfactant-Stabilized Adjacent Droplets Using Surface Acoustic Waves.

A novel, on-demand microfluidic droplet merging mechanism is presented in this paper. We demonstrate that a narrow beam surface acoustic wave, targeted at the oil buffer, causes nearby surfactant-stabilized droplets to coalesce. The lack of direct exposure of the droplet to the excitation stimulus makes this method ideal for sensitive samples as harm will not occur. This powerful technique works on a straight channel with no special design, is not affected by surfactant concentration and droplet volume hence promises seamless integration into existing microfluidic systems. It offers high-throughput, biologically safe, on-demand droplet merging for applications ranging from fast reaction kinetics to microfluidic high throughput screening. We thoroughly characterize the physical mechanism triggering droplet-droplet coalescence and observe a cutoff distance from the center of the acoustic beam to the droplet-droplet interface after which the merging mechanism does not work anymore. We establish that the most likely mechanism for merging is acoustic streaming induced droplet deformation.

Comparison of bulk and microfluidic methods to monitor the phase behaviour of nanoparticles during digestion of lipid-based drug formulations using in situ X-ray scattering.

The performance of orally administered lipid-based drug formulations is crucially dependent on digestion, and understanding the colloidal structures formed during digestion is necessary for rational formulation design. Previous studies using the established bulk pH-stat approach (Hong et al. 2015), coupled to synchrotron small angle X-ray scattering (SAXS), have begun to shed light on this subject. Such studies of digestion using in situ SAXS measurements are complex and have limitations regarding the resolution of intermediate structures. Using a microfluidic device, the digestion of lipid systems may be monitored with far better control over the mixing of the components and the application of enzyme, thereby elucidating a finer understanding of the structural progression of these lipid systems. This work compares a simple T-junction microcapillary device and a custom-built microfluidic chip featuring hydrodynamic flow focusing, with an equivalent experiment with the full scale pH-stat approach. Both microfluidic devices were found to be suitable for in situ SAXS measurements in tracking the kinetics with improved time and signal sensitivity compared to other microfluidic devices studying similar lipid-based systems, and producing more consistent and controllable structural transformations. Particle sizing of the nanoparticles produced in the microfluidic devices were more consistent than the pH-stat approach.

Thermally-actuated microfluidic membrane valve for point-of-care applications.

Microfluidics has enabled low volume biochemistry reactions to be carried out at the point-of-care. A key component in microfluidics is the microfluidic valve. Microfluidic valves are not only useful for directing flow at intersections but also allow mixtures/dilutions to be tuned real-time and even provide peristaltic pumping capabilities. In the transition from chip-in-a-lab to lab-on-a-chip, it is essential to ensure that microfluidic valves are designed to require less peripheral equipment and that they are transportable. In this paper, a thermally-actuated microfluidic valve is presented. The valve itself is fabricated with off-the-shelf components without the need for sophisticated cleanroom techniques. It is shown that multiple valves can be controlled and operated via a power supply and an Arduino microcontroller; an important step towards transportable microfluidic devices capable of carrying out analytical assays at the point-of-care. It is been calculated that a single actuator costs less than $1, this highlights the potential of the presented valve for scaling out. The valve operation is demonstrated by adjusting the ratio of a water/dye mixture in a continuous flow microfluidic chip with Y-junction channel geometry. The power required to operate one microfluidic valve has been characterised both theoretically and experimentally. Cyclical operation of the valve has been demonstrated for 65 h with 585 actuations. The presented valve is capable of actuating rectangular microfluidic channels of 500 µm × 50 µm with an expected temperature increase of up to 5 °C. The fastest actuation times achieved were 2 s for valve closing (heating) and 9 s for valve opening (cooling).

Image-Based Single Cell Sorting Automation in Droplet Microfluidics.

The recent boom in single-cell omics has brought researchers one step closer to understanding the biological mechanisms associated with cell heterogeneity. Rare cells that have historically been obscured by bulk measurement techniques are being studied by single cell analysis and providing valuable insight into cell function. To support this progress, novel upstream capabilities are required for single cell preparation for analysis. Presented here is a droplet microfluidic, image-based single-cell sorting technique that is flexible and programmable. The automated system performs real-time dual-camera imaging (brightfield & fluorescent), processing, decision making and sorting verification. To demonstrate capabilities, the system was used to overcome the Poisson loading problem by sorting for droplets containing a single red blood cell with 85% purity. Furthermore, fluorescent imaging and machine learning was used to load single K562 cells amongst clusters based on their instantaneous size and circularity. The presented system aspires to replace manual cell handling techniques by translating expert knowledge into cell sorting automation via machine learning algorithms. This powerful technique finds application in the enrichment of single cells based on their micrographs for further downstream processing and analysis.

Droplet control technologies for microfluidic high throughput screening (µHTS).

The transition from micro well plate and robotics based high throughput screening (HTS) to chip based screening has already started. This transition promises reduced droplet volumes thereby decreasing the amount of fluids used in these studies. Moreover, it significantly boosts throughput allowing screening to keep pace with the overwhelming number of molecular targets being discovered. In this review, we analyse state-of-the-art droplet control technologies that exhibit potential to be used in this new generation of screening devices. Since these systems are enclosed and usually planar, even some of the straightforward methods used in traditional HTS such as pipetting and reading can prove challenging to replicate in microfluidic high throughput screening (µHTS). We critically review the technologies developed for this purpose in depth, describing the underlying physics and discussing the future outlooks.

Surface acoustic wave enabled pipette on a chip.

Mono-disperse droplet formation in microfluidic devices allows the rapid production of thousands of identical droplets and has enabled a wide range of chemical and biological studies through repeat tests performed at pico-to-nanoliter volume samples. However, it is exactly this efficiency of production which has hindered the ability to carefully control the location and quantity of the distribution of various samples on a chip - the key requirement for replicating micro well plate based high throughput screening in vastly reduced volumetric scales. To address this need, here, we present a programmable microfluidic chip capable of pipetting samples from mobile droplets with high accuracy using a non-contact approach. Pipette on a chip (PoaCH) system selectively ejects (pipettes) part of a droplet into a customizable reaction chamber using surface acoustic waves (SAWs). Droplet pipetting is shown to range from as low as 150 pL up to 850 pL with precision down to tens of picoliters. PoaCH offers ease of integration with existing lab on a chip systems as well as a robust and contamination-free droplet manipulation technique in closed microchannels enabling potential implementation in screening and other studies.

Microfluidic plug steering using surface acoustic waves.

Digital microfluidic systems, in which isolated droplets are dispersed in a carrier medium, offer a method to study biological assays and chemical reactions highly efficiently. However, it's challenging to manipulate these droplets in closed microchannel devices. Here, we present a method to selectively steer plugs (droplets with diameters larger than the channel's width) at a specially designed Y-junction within a microfluidic chip. The method makes use of surface acoustic waves (SAWs) impinging on a multiphase interface in which an acoustic contrast is present. As a result, the liquid-liquid interface is subjected to acoustic radiation forces. These forces are exploited to steer plugs into selected branches of the Y-junction. Furthermore, the input power can be finely tuned to split a plug into two uneven plugs. The steering of plugs as a whole, based on plug volume and velocity is thoroughly characterized. The results indicate that there is a threshold plug volume after which the steering requires elevated electrical energy input. This plug steering method can easily be integrated to existing lab-on-a-chip devices and it offers a robust and active plug manipulation technique in closed microchannels.

Vibrating membrane with discontinuities for rapid and efficient microfluidic mixing.

This study presents a novel acoustic mixer comprising of a microfabricated silicon nitride membrane with a hole etched through it. We show that the introduction of the through hole leads to extremely fast and homogeneous mixing. When the membrane is immersed in fluid and subjected to acoustic excitation, a strong streaming field in the form of vortices is generated. The vortices are always observed to centre at the hole, pointing to the critical role it has on the streaming field. We hypothesise that the hole introduces a discontinuity to the boundary conditions of the membrane, leading to strong streaming vortices. With numerical simulations, we show that the hole's presence can increase the volume force responsible for driving the streaming field by 2 orders of magnitude, thus supporting our hypothesis. We investigate the mixing performance at different Peclet numbers by varying the flow rates for various devices containing circular, square and rectangular shaped holes of different dimensions. We demonstrate rapid mixing within 3 ms mixing time (90% mixing efficiency at 60 µl min(-1) total flow rate, Peclet number equals 8333 ± 3.5%) is possible with the current designs. Finally, we examine the membrane with two circular holes which are covered by air bubbles and compare it to when the membrane is fully immersed. We find that coupling between the holes' vortices occurs only when membrane is immersed; while with the bubble membrane, the upstream hole's vortices can act as a blockage to fluid flow passing it.

Microfluidic on-demand droplet merging using surface acoustic waves.

Individual droplets can be isolated within microfluidic systems by use of an immiscible carrier layer. This type of two phase systems, often termed "digital microfluidics", find wide ranging applications in chemical synthesis and analysis. To conduct on-chip biochemical analysis, a key step is to be able to merge droplets selectively in order to initiate the required reactions. In this paper, a novel microfluidic chip integrating interdigital transducers is designed to merge multiple droplets on-demand. The approach uses surface acoustic wave induced acoustic radiation forces to immobilize droplets as they pass from a channel into a small expansion chamber, there they can be held until successive droplets arrive. Hence, no requirement is placed on the initial spacing between droplets. When the merged volume reaches a critical size, drag forces exerted by the flowing oil phase act to overcome the retaining acoustic radiation forces, causing the merged volume to exit the chamber. This will occur after a predetermined number of droplets have merged depending on the initial droplet size and selected actuation power.

Hydrodynamic cavitation kills prostate cells and ablates benign prostatic hyperplasia tissue.

Hydrodynamic cavitation is a physical phenomenon characterized by vaporization and bubble formation in liquids under low local pressures, and their implosion following their release to a higher pressure environment. Collapse of the bubbles releases high energy and may cause damage to exposed surfaces. We recently designed a set-up to exploit the destructive nature of hydrodynamic cavitation for biomedical purposes. We have previously shown that hydrodynamic cavitation could kill leukemia cells and erode kidney stones. In this study, we analyzed the effects of cavitation on prostate cells and benign prostatic hyperplasia (BPH) tissue. We showed that hydrodynamic cavitation could kill prostate cells in a pressure- and time-dependent manner. Cavitation did not lead to programmed cell death, i.e. classical apoptosis or autophagy activation. Following the application of cavitation, we observed no prominent DNA damage and cells did not arrest in the cell cycle. Hence, we concluded that cavitation forces directly damaged the cells, leading to their pulverization. Upon application to BPH tissues from patients, cavitation could lead to a significant level of tissue destruction. Therefore similar to ultrasonic cavitation, we propose that hydrodynamic cavitation has the potential to be exploited and developed as an approach for the ablation of aberrant pathological tissues, including BPH.

Kidney stone erosion by micro scale hydrodynamic cavitation and consequent kidney stone treatment.

The objective of this study is to reveal the potential of micro scale hydrodynamic bubbly cavitation for the use of kidney stone treatment. Hydrodynamically generated cavitating bubbles were targeted to the surfaces of 18 kidney stone samples made of calcium oxalate, and their destructive effects were exploited in order to remove kidney stones in in vitro experiments. Phosphate buffered saline (PBS) solution was used as the working fluid under bubbly cavitating conditions in a 0.75 cm long micro probe of 147 µm inner diameter at 9790 kPa pressure. The surface of calcium oxalate type kidney stones were exposed to bubbly cavitation at room temperature for 5 to 30 min. The eroded kidney stones were visually analyzed with a high speed CCD camera and using SEM (scanning electron microscopy) techniques. The experiments showed that at a cavitation number of 0.017, hydrodynamic bubbly cavitation device could successfully erode stones with an erosion rate of 0.31 mg/min. It was also observed that the targeted application of the erosion with micro scale hydrodynamic cavitation may even cause the fracture of the kidney stones within a short time of 30 min. The proposed treatment method has proven to be an efficient instrument for destroying kidney stones.

Bubbly cavitating flow generation and investigation of its erosional nature for biomedical applications.

This paper presents a study that investigates the destructive energy output resulting from hydrodynamic bubbly cavitation in microchannels and its potential use in biomedical applications. The research performed in this study includes results from bubbly cavitation experiments and findings showing the destructive effects of bubbly cavitating flow on selected solid specimens and live cells. The bubbles generated by hydrodynamic cavitation are highly destructive at the surfaces of the target medium on which they are carefully focused. The resulting destructive energy output could be effectively used for biomedical treatments, such as destroying kidney stones (renal calculi) or killing cancer cells. Motivated by this potential, the cavitation damage to cancerous cells and material removal from chalk pieces (which possess similar material properties as some kidney stones) was investigated. Our results showed that cavitation could induce damage both on chalk pieces and leukemia/lymphoma cells. We discovered that hydrodynamic cavitation exposure had early and delayed effects on cancer cell survival. Hence, the potential of hydrodynamic bubbly cavitation generated at the microscale for biomedical treatments was revealed using the microchannel configuration as a microorifice (with an inner diameter of 147 µm and a length of 1.52 cm), which acts as the source of bubbly cavitating flows.