A practical microfluidic pump enabled by acoustofluidics and 3D printing

Recent Advances in Acoustofluidics for Point-of-Care Testing

Point-of-care testing (POCT) has played important role in clinical diagnostics, environmental assessment, chemical and biological analyses, and food and chemical processing due to its faster turnaround compared to laboratory testing. Dedicated manipulations of solutions or particles are generally required to develop POCT technologies that achieve a "sample-in-answer-out" operation. With the development of micro- and nanotechnology, many tools have been developed for sample preparation, on-site analysis and solution manipulations (mixing, pumping, valving, etc.). Among these approaches, the use of acoustic waves to manipulate fluids and particles (named acoustofluidics) has been applied in many researches. This review focuses on the recent developments in acoustofluidics for POCT. It starts with the fundamentals of different acoustic manipulation techniques and then lists some of representative examples to highlight each method in practical POC applications. Looking toward the future, a compact, portable, highly integrated, low power, and biocompatible technique is anticipated to simultaneously achieve precise manipulation of small targets and multimodal manipulation in POC applications.

A Miniaturized Wireless Micropump Enabled by Confined Acoustic Streaming

Miniaturization of health care, biomedical, and chemical systems is highly desirable for developing point-of-care testing (POCT) technologies. In system miniaturization, micropumps represent one of the major bottlenecks due to their undesirable pumping performance at such small sizes. Here, we developed a microelectromechanical system fabricated acoustic micropump based on an ultrahigh-frequency bulk acoustic wave resonator. The concept of an inner-boundary-confined acoustic jet was introduced to facilitate unidirectional flow. Benefitting from the high resonant frequency and confined acoustic streaming, the micropump reaches 32.620 kPa/cm3 (pressure/size) and 11.800 ml/min∙cm3 (flow rate/size), showing a 2-order-of-magnitude improvement in the energy transduction efficiency compared with the existing acoustic micropumps. As a proof of concept, the micropump was constructed as a wearable and wirelessly powered integrated drug delivery system with a size of only 9×9×9 mm3 and a weight of 1.16 g. It was demonstrated for ocular disease treatment through animal experimentation and a human pilot test. With superior pumping performance, miniaturized pump size, ultralow power consumption, and complementary metal-oxide-semiconductor compatibility, we expect it to be readily applied to various POCT applications including clinical diagnosis, prognosis, and drug delivery systems.

Acoustic Atomization-Induced Pumping Based on a Vibrating Sharp-Tip Capillary

Pumping is an essential component in many microfluidic applications. Developing simple, small-footprint, and flexible pumping methods is of great importance to achieve truly lab-on-a-chip systems. Here, we report a novel acoustic pump based on the atomization effect induced by a vibrating sharp-tip capillary. As the liquid is atomized by the vibrating capillary, negative pressure is generated to drive the movement of fluid without the need to fabricate special microstructures or use special channel materials. We studied the influence of the frequency, input power, internal diameter (ID) of the capillary tip, and liquid viscosity on the pumping flow rate. By adjusting the ID of the capillary from 30 µm to 80 µm and the power input from 1 V_pp to 5 V_pp, a flow rate range of 3 to 520 µL/min can be achieved. We also demonstrated the simultaneous operation of two pumps to generate parallel flow with a tunable flow rate ratio. Finally, the capability of performing complex pumping sequences was demonstrated by performing a bead-based ELISA in a 3D-printed microdevice.

Manipulation with sound and vibration: A review on the micromanipulation system based on sub-MHz acoustic waves

Manipulation of micro-objects have been playing an essential role in biochemical analysis or clinical diagnostics. Among the diverse technologies for micromanipulation, acoustic methods show the advantages of good biocompatibility, wide tunability, a label-free and contactless manner. Thus, acoustic micromanipulations have been widely exploited in micro-analysis systems. In this article, we reviewed the acoustic micromanipulation systems that were actuated by sub-MHz acoustic waves. In contrast to the high-frequency range, the acoustic microsystems operating at sub-MHz acoustic frequency are more accessible, whose acoustic sources are at low cost and even available from daily acoustic devices (e.g. buzzers, speakers, piezoelectric plates). The broad availability, with the addition of the advantages of acoustic micromanipulation, make sub-MHz microsystems promising for a variety of biomedical applications. Here, we review recent progresses in sub-MHz acoustic micromanipulation technologies, focusing on their applications in biomedical fields. These technologies are based on the basic acoustic phenomenon, such as cavitation, acoustic radiation force, and acoustic streaming. And categorized by their applications, we introduce these systems for mixing, pumping and droplet generation, separation and enrichment, patterning, rotation, propulsion and actuation. The diverse applications of these systems hold great promise for a wide range of enhancements in biomedicines and attract increasing interest for further investigation.

Toward Personalized Nanomedicine: The Critical Evaluation of Micro and Nanodevices for Cancer Biomarker Analysis in Liquid Biopsy

Liquid biopsy for the analysis of circulating cancer biomarkers (CBs) is a major advancement toward the early detection of cancer. In comparison to tissue biopsy techniques, liquid biopsy is relatively painless, offering multiple sampling opportunities across easily accessible bodily fluids such as blood, urine, and saliva. Liquid biopsy is also relatively inexpensive and simple, avoiding the requirement for specialized laboratory equipment or trained medical staff. Major advances in the field of liquid biopsy are attributed largely to developments in nanotechnology and microfabrication that enables the creation of highly precise chip-based platforms. These devices can overcome detection limitations of an individual biomarker by detecting multiple markers simultaneously on the same chip, or by featuring integrated and combined target separation techniques. In this review, the major advances in the field of portable and semi-portable micro, nano, and multiplexed platforms for CB detection for the early diagnosis of cancer are highlighted. A comparative discussion is also provided, noting merits and drawbacks of the platforms, especially in terms of portability. Finally, key challenges toward device portability and possible solutions, as well as discussing the future direction of the field are highlighted.

Investigating the Performance of the Multi-Lobed Leaf-Shaped Oscillatory Obstacles in Micromixers Using Bulk Acoustic Waves (BAW): Mixing and Chemical Reaction

Proper mixing in microfluidic devices has been a concern since the early development stages. Acoustic micromixers (active micromixers) attract significant attention due to their high efficiency and ease of implementation. Finding the optimal geometries, structures, and characteristics of acoustic micromixers is still a challenging issue. In this study, we considered leaf-shaped obstacle(s) having a multi-lobed structure as the oscillatory part(s) of acoustic micromixers in a Y-junction microchannel. Four different types of leaf-shaped oscillatory obstacles, including 1, 2, 3, and 4-lobed structures, were defined, and their mixing performance for two fluid streams was evaluated numerically. The geometrical parameters of the leaf-shaped obstacle(s), including the number of lobes, lobes' length, lobes' inside angle, and lobes' pitch angle, were analyzed, and their optimum operational values were discovered. Additionally, the effects of the placement of oscillatory obstacles in three configurations, i.e., at the junction center, on the side walls, and both, on the mixing performance were evaluated. It was found that by increasing the number and length of lobes, the mixing efficiency improved. Furthermore, the effect of the operational parameters, such as inlet velocity, frequency, and intensity of acoustic waves, was examined on mixing efficiency. Meanwhile, the occurrence of a bimolecular reaction in the microchannel was analyzed at different reaction rates. It was proven that the reaction rate has a prominent effect at higher inlet velocities.

Novel Pumping Methods for Microfluidic Devices: A Comprehensive Review

This review is an account of methods that use various strategies to control microfluidic flow control with high accuracy. The reviewed systems are divided into two large groups based on the way they create flow: passive systems (non-mechanical systems) and active (mechanical) systems. Each group is presented by a number of device fabrications. We try to explain the main principles of operation, and we list advantages and disadvantages of the presented systems. Mechanical systems are considered in more detail, as they are currently an area of increased interest due to their unique precision flow control and "multitasking". These systems are often applied as mini-laboratories, working autonomously without any additional operations, provided by humans, which is very important under complicated conditions. We also reviewed the integration of autonomous microfluidic systems with a smartphone or single-board computer when all data are retrieved and processed without using a personal computer. In addition, we discuss future trends and possible solutions for further development of this area of technology.

A robot-assisted acoustofluidic end effector

Liquid manipulation is the foundation of most laboratory processes. For macroscale liquid handling, both do-it-yourself and commercial robotic systems are available; however, for microscale, reagents are expensive and sample preparation is difficult. Over the last decade, lab-on-a-chip (LOC) systems have come to serve for microscale liquid manipulation; however, lacking automation and multi-functionality. Despite their potential synergies, each has grown separately and no suitable interface yet exists to link macro-level robotics with micro-level LOC or microfluidic devices. Here, we present a robot-assisted acoustofluidic end effector (RAEE) system, comprising a robotic arm and an acoustofluidic end effector, that combines robotics and microfluidic functionalities. We further carried out fluid pumping, particle and zebrafish embryo trapping, and mobile mixing of complex viscous liquids. Finally, we pre-programmed the RAEE to perform automated mixing of viscous liquids in well plates, illustrating its versatility for the automatic execution of chemical processes.

A self-contained acoustofluidic platform for biomarker detection

Self-contained microfluidic platforms with on-chip integration of flow control units, microreactors, (bio)sensors, etc. are ideal systems for point-of-care (POC) testing. However, current approaches such as micropumps and microvalves, increase the cost and the control system, and it is rather difficult to integrate into a single chip. Herein, we demonstrated a versatile acoustofluidic platform actuated by a Lamb wave resonator (LWR) array, in which pumping, mixing, fluidic switching, and particle trapping are all achieved on a single chip. The high-speed microscale acoustic streaming triggered by the LWR in the confined microchannel can be utilized to realize a flow resistor and switch. Variable unidirectional pumping was realized by regulating the relative position of the LWR in various custom-designed microfluidic structures and adoption of different geometric parameters for the microchannel. In addition, to realize quantitative biomarker detection, the on-chip flow resistor, micropump, micromixer and particle trapper were also integrated with a CMOS photo sensor and electronic driver circuit, resulting in an automated handheld microfluidic system with no moving parts. Finally, the acoustofluidic platform was tested for prostate-specific antigen (PSA) sensing, which demonstrates the biocompatibility and applied potency of this proposed self-contained system in POC biomedical applications.

A 3D Miniaturized Glass Magnetic-Active Centrifugal Micropump Fabricated by SLE Process and Laser Welding

A miniaturized pump to manipulate liquid flow in microchannels is the key component of microfluidic devices. Many researchers have demonstrated active microfluidic pumps, but most of them still required additional large peripherals to operate their micropumps. In addition, those micropumps were made of polymer materials so that their application may be limited to a variety of fields that require harsh conditions at high pressures and temperatures or organic solvents and acid/base. In this work, we present a 3D miniaturized magnetic-driven glass centrifugal pump for microfluidic devices. The pump consists of a volute structure and a 3D impeller integrated with two magnet disks of Φ1 mm. The 3D pump structure was 13 mm × 10.5 mm × 3 mm, and it was monolithically fabricated in a fused silica sheet by selective laser-induced etching (SLE) technology using a femtosecond laser. The pump operation requires only one motor rotating two magnets. It was Φ42 mm × 54 mm and powered by a battery. To align the shaft of the motor to the center of the 3D glass pump chip, a housing containing the motor and the chip was fabricated, and the overall size of the proposed micropump device was 95 mm × 70 mm × 75 mm. Compared with other miniaturized pumps, ours was more compact and portable. The output pressure of the fabricated micropump was between 215 Pa and 3104 Pa, and the volumetric flow rate range was 0.55 mL/min and 7.88 mL/min. The relationship between the motor RPM and the impeller RPM was analyzed, and the flow rate was able to be controlled by the RPM. With its portability, the proposed pump can be applied to produce an integrated and portable microfluidic device for point-of-care analysis.

A Battery-Powered Fluid Manipulation System Actuated by Mechanical Vibrations

Miniaturized fluid manipulation systems are an important component of lab-on-a-chip platforms implemented in resourced-limited environments and point-of-care applications. This work aims to design, fabricate, and test a low-cost and battery-operated microfluidic diffuser/nozzle type pump to enable an alternative fluid manipulation solution for field applications. For this, CNC laser cutting and 3D printing are used to fabricate the fluidic unit and casing of the driving module of the system, respectively. This system only required 3.5-V input power and can generate flow rates up to 58 µL/min for water. In addition, this portable pump can manipulate higher viscosity fluids with kinematic viscosities up to 24 mPa·s resembling biological fluids such as sputum and saliva. The demonstrated system is a low-cost, battery-powered, and highly versatile fluid pump that can be adopted in various lab-on-a-chip applications for field deployment and remote applications.

A Numerical Investigation of the Mixing Performance in a Y-Junction Microchannel Induced by Acoustic Streaming

In this study, the mixing performance in a Y-junction microchannel with acoustic streaming was investigated through numerical simulation. The acoustic streaming is created by inducing triangular structures at the junction and sidewalls regions. The numerical model utilizes Navier-Stokes equations in conjunction with the convection-diffusion equations. The parameters investigated were inlet velocities ranging from 4.46 to 55.6 µm/s, triangular structure's vertex angles ranging from 22° to 90° oscillation amplitude ranging from 3 to 6 µm, and an oscillation frequency set to 13 kHz. The results show that at the junction region, a pair of counter-rotating streaming vortices were formed, and unsymmetrical or one-sided vortices were formed when additional triangles were added along the sidewalls. These streaming flows significantly increase the vorticity compared with the case without the acoustic stream. Mixing performances were found to have improved with the generation of the acoustic stream. The mixing performance was evaluated at various inlet velocities, the vertex angles of the triangular structure, and oscillation amplitudes. The numerical results show that adding the triangular structure at the junction region considerably improved the mixing efficiency due to the generation of acoustic streaming, and further improvements can be achieved at lower inlet velocity, sharper vertex angle, and higher oscillation amplitude. Integrating with more triangular structures at the sidewall regions also improves the mixing performance within the laminar flow regime in the Y-microchannel. At Y = 2.30 mm, oscillation amplitude of 6 µm, and flow inlet velocity of 55.6 µm/s, with all three triangles integrated and the triangles' vertex angles fixed to 30°, the mixing index can achieve the best results of 0.9981, which is better than 0.8355 in the case of using only the triangle at the junction, and 0.6642 in the case without acoustic streaming. This is equal to an improvement of 50.27% in the case of using both the junction and the two sidewall triangles, and 25.79% in the case of simply using a junction triangle.

Investigating the mechanotransduction of transient shear stress mediated by Piezo1 ion channel using a 3D printed dynamic gravity pump

Microfluidic systems are widely used for studying the mechanotransduction of flow-induced shear stress in mechanosensitive cells. However, these studies are generally performed under constant flow rates, mainly, due to the deficiency of existing pumps for generating transient flows. To address this limitation, we have developed a unique dynamic gravity pump to generate transient flows in microfluidics. The pump utilises a motorised 3D-printed cam-lever mechanism to change the inlet pressure of the system in repeated cycles. 3D printing technology facilitates the rapid and low-cost prototyping of the pump. Customised transient flow patterns can be generated by modulating the profile, size, and rotational speed of the cam, location of the hinge along the lever, and the height of the syringe. Using this unique dynamic gravity pump, we investigated the mechanotransduction of shear stress in HEK293 cells stably expressing Piezo1 mechanosensitive ion channel under transient flows. The controllable, simple, low-cost, compact, and modular design of the pump makes it suitable for studying the mechanobiology of shear sensitive cells under transient flows.

A simple acoustofluidic device for on-chip fabrication of PLGA nanoparticles

Miniaturization of systems and processes provides numerous benefits in terms of cost, reproducibility, precision, minimized consumption of chemical reagents, and prevention of contamination. The field of microfluidics successfully finds a place in a plethora of applications, including on-chip nanoparticle synthesis. Compared with the bulk approaches, on-chip methods that are enabled by microfluidic devices offer better control of size and uniformity of fabricated nanoparticles. However, these microfluidic devices generally require complex and expensive fabrication facilities that are not readily available in low-resourced laboratories. Here, a low-cost and simple acoustic device is demonstrated by generating acoustic streaming flows inside glass capillaries through exciting different flexural modes. At distinct frequencies, the flexural modes of the capillary result in different oscillation profiles that can insert harmonic forcing into the fluid. We explored these flexural modes and identified the modes that can generate strong acoustic streaming vortices along the glass capillary. Then, we applied these modes for fluid mixing using an easy-to-fabricate acoustofluidic device architecture. This device is applied in the fabrication of poly(d,l-lactide-co-glycolide) (PLGA) nanoparticles. The acoustic device consists of a thin glass capillary and two polydimethylsiloxane adaptors that are formed using three-dimensional printed molds. By controlling the flow rates of the polymer and water solutions, PLGA nanoparticles with diameters between 65 and 96 nm are achieved with polydispersity index values ranging between 0.08 and 0.18. Owing to its simple design and minimal fabrication requirements, the proposed acoustofluidic mixer can be applied for microfluidic fluid mixing applications in limited resource settings.

A frugal microfluidic pump

Manipulation of fluid flow is paramount for microfluidic device operation. Conventional microfluidic pumps are often expensive, bulky, complicated, and not amenable in limited resource settings. Here, we introduce a Fully self-sufficient, RobUst, Gravity-Assisted, Low-cost (FRUGAL) microfluidic pump. The pump consists of a syringe, a syringe holder and loading masses. The system is easy to assemble, inexpensive, portable, and electrical power-free. Inside the syringe, the fluid is driven by the pressure from the weight of the loading masses. During operation, the exerted pressure is dynamically controllable and stable for hours. These features are useful for optimization of microfluidics assays and dynamic temporal studies. We demonstrate the application of this system to control the formation of water-in-oil droplet emulsion. Benefitting from its simplicity and versatility, the frugal microfluidic pump will enable global adoption of microfluidic technology in chemistry and biomedical applications, especially in limited resource environments.

Potential of the acoustic micromanipulation technologies for biomedical research

Acoustic micromanipulation technologies are a set of versatile tools enabling unparalleled micromanipulation capabilities. Several characteristics put the acoustic micromanipulation technologies ahead of most of the other tweezing methods. For example, acoustic tweezers can be adapted as non-invasive platforms to handle single cells gently or as probes to stimulate or damage tissues. Besides, the nature of the interactions of acoustic waves with solids and liquids eliminates labeling requirements. Considering the importance of highly functional tools in biomedical research for empowering important discoveries, acoustic micromanipulation can be valuable for researchers in biology and medicine. Herein, we discuss the potential of acoustic micromanipulation technologies from technical and application points of view in biomedical research.

Generation of programmable dynamic flow patterns in microfluidics using audio signals

Customised audio signals, such as musical notes, can be readily generated by audio software on smartphones and played over audio speakers. Audio speakers translate electrical signals into the mechanical motion of the speaker cone. Coupling the inlet tube to the speaker cone causes the harmonic oscillation of the tube, which in turn changes the velocity profile and flow rate. We employ this strategy for generating programmable dynamic flow patterns in microfluidics. We show the generation of customised rib and vortex patterns through the application of multi-tone audio signals in water-based and whole blood samples. We demonstrate the precise capability to control the number and extent of the ribs and vortices by simply setting the frequency ratio of two- and three-tone audio signals. We exemplify potential applications of tube oscillation for studying the functional responses of circulating immune cells under pathophysiological shear rates. The system is programmable, compact, low-cost, biocompatible, and durable. These features make it suitable for a variety of applications across chemistry, biology, and physics.

Dynamic measurement of the acoustic streaming time constant utilizing an optical tweezer

The combination of a bulk acoustic wave device and an optical trap allows for studying the buildup time of the respective acoustic forces. In particular, we are interested in the time it takes to build up the acoustic radiation force and acoustic streaming. For that, we measure the trajectory of a spherical particle in an acoustic field over time. The shape of the trajectory is determined by the acoustic radiation force and by acoustic streaming, both acting on different time scales. For that, we utilize the high temporal resolution (Δt=0.8μs) of an optical trapping setup. With our experimental parameters the acoustic radiation force on the particle and the acoustic streaming field theoretically have characteristic buildup times of 1.4μs and 1.44ms, respectively. By choosing a resonance mode and a measurement position where the acoustic radiation force and acoustic streaming induced viscous drag force act in orthogonal directions, we can measure the evolution of these effects separately. Our results show that the particle is accelerated nearly instantaneously by the acoustic radiation force to a constant velocity, whereas the acceleration phase to a constant velocity by the acoustic streaming field takes significantly longer. We find that the acceleration to a constant velocity induced by streaming takes in average about 17 500 excitation periods (≈4.4ms) longer to develop than the one induced by the acoustic radiation force. This duration is about four times larger than the so-called momentum diffusion time which is used to estimate the streaming buildup. In addition, this rather large difference in time can explain why a pulsed acoustic excitation can indeed prevent acoustic streaming as it has been shown in some previous experiments.

Flow pumping by external periodic shear applied to a soft interface

Flow pumping in viscous fluids is of prime importance in micro-fluidic applications. Here we show that a single colloidal particle in front of a soft wall, manipulated by external means like an optical tweezer, can pump the ambient viscous fluid. The particle, moving back and forth parallel to the soft wall, can produce an averaged net flow in a direction perpendicular to the wall. Using a perturbative scheme, we present the results. Analysis show that this flow in terms of capillary number, scales as [Formula: see text].