Studies during PhD

My PhD research interests focus on the intersection of microfluidics and surface acoustic waves. My research contributions have been to discover and design an efficient and reliable microfluidic device for three-dimensional patterning and manipulating of microparticles and cells in fluid using surface acoustic waves, experimentally and numerically.

What is surface acoustic wave?

surface acoustic wave device with 4 interdigital transducers and coupling liquid (red color) for transmitting the wave to an attachable chamber

Surface acoustic wave (SAW) is a deformation of surface of elastic materials in form of mechanical waves that propagate on a free surface of an infinite homogeneous isotropic elastic solid. SAWs (or Rayleigh SAWs) are created of both longitudinal and vertical shear components which result in the elliptical motion. The amplitudes of surface acoustic waves decay exponentially with the depth of the substrate where most of their energy confine at the surface. In a SAW device, surface acoustic waves are generally generated from the interdigital transducers (IDTs) which are the thin electrodes bonded on a piezoelectric substrate. The IDTs are commonly fabricated using MEMS techniques to deposit a thin layer of metal on the substrate which has been well developed in fabrication of semiconductor industry.

Patterning and manipulating microparticles into 3D matrix pattern (go to article)

In this earliest study, a method based on standing surface acoustic waves (SSAWs) is proposed to pattern and manipulate microparticles into a three-dimensional (3D) matrix inside a microchamber. An optical prism is used to observe the 3D alignment and patterning of the microparticles in the vertical and horizontal planes simultaneously. The acoustic radiation force effectively patterns the microparticles into 3D lines pattern when using one pair of interdigital transducers or crystal-lattice-like matrix patterns when using two pairs of interdigital transducers. A microparticle can be positioned precisely at a specified vertical location by balancing the forces of acoustic radiation, drag, buoyancy, and gravity acting on the microparticle. This method has great potential for acoustofluidic applications, building the large-scale structures associated with biological objects and artificial neuron networks.

Microparticles formed a 3D lines pattern
Microparticles formed a 3D lines pattern
Microparticles formed a 3D matrix pattern

Acoustofluidic closed-loop control of microparticles and cells (go to article)

In the second study, an acoustofluidic closed-loop control system which is combined with computer vision techniques and surface acoustic wave device o implement selective, automatic and precise position control of an object, such as a single cell or microparticle in a microfluidic chamber. Position of the object is in situ extracted from living images that are captured from a video camera. By utilizing the closed-loop control strategy, the object is precisely moved to the desired location in 3D patterns or along designed trajectories by manipulating the phase angle and power signal of the SSAWs. Controlling of breast cancer cells has been conducted to verify the principle and biocompatibility of the control system. This system could be employed to build an automatic system for cell analysis, cell isolation, self-assembling of materials into complex microstructures, or lab-on-chip and organ-on-chip applications.

Above video is about using acoustofluidic closed-loop system to precisely control a single micro-particle in fluid, it is drawing the letter “NTU” which is name of the postgraduate university.

stacked images of the microparticle at different moments to show the word of “NTU”



Above video is about controlling a single microparticle into a rectangular trajectory as viewed from the side to demonstrate 3D controlling capability

Fabrication of three-dimensional scaffold-based patterns (go to article)


In the third study, a reusable acoustofluidic device is developed using surface acoustic waves for manipulating microparticles/cells to form a 3D matrix pattern inside a scaffold‐based hydrogel contained in a millimetric chamber. The 3D patterned and polymerized hydrogel construct can be easily and safely removed from the chamber using a proposed lifting technique, which prevent any physical damages or contaminations and promote the reusability of the chamber. The generated 3D patterns of microparticles and breast cancer cells are numerically studied using a finite‐element method, which is well validated by the experimental results. The proposed acoustofluidic device is a useful tool for in vitro engineering 3D scaffold‐based artificial tissues for drug and toxicity testing and building organs‐on‐chip applications.

3D matrix pattern of fluorescent particles inside hydrogel
3D matrix pattern of fluorescent particles inside hydrogel

3D patterning/manipulating microparticles and yeast cells using ZnO/Si thin film surface acoustic waves (go to article)

SAW thin film device

In the forth study, a thin film ZnO/Si SAW device is developed to realize the patterning and 3D manipulation of microparticle/yeast cells inside a chamber with a height of 1 mm. Effects of SAW frequency, channel width and thickness on alignment of microparticles were firstly investigated, and positions of the microparticles in the direction of SAW propagation can be controlled precisely by changing the phase angle of the acoustic waves from the ZnO/Si SAW device. A numerical model has been developed to investigate the SAW acoustic field and the resulted 3D motions of microparticles under the acoustic radiation forces within the microchamber. This device shows a great potential for acoustofluidic, neural network research and biomedical applications using the ZnO/Si SAW devices.

3D pattern of microparticles and yeast cells using SAW thin film device


Manipulation of self-assembled three-dimensional architecture in reusable acoustofluidic device (article will be published soon)

In the fifth study, a method to manipulate the orientation and curvature of 3D matrix pattern by redesigning the top-wall of microfluidic chamber and the technique to create a 3D longitudinal pattern along pre-inserted polydimethylsiloxane (PDMS) rods. Experimental results showed a good agreement with model predictions. This research can actively contribute to the development of better organs-on-chips platforms with capability of controlling cell architecture and density. Meanwhile, the 3D longitudinal pattern is suitable for self-assembling of microvasculatures.

(A) Illustration of the experimental setup. The device consists of a piezoelectric substrate (i.e. LiNbO3 material), four sets of interdigital transducers (IDTs), a chamber that comprises of an open-top PDMS chamber bonded on a cover glass, a PDMS spacer for containing coupling liquid, geometric top-walls and a PDMS rod. When the PDMS top-wall is placed on top of the chamber and the electrical signal to IDTs is turned on, the microspheres in the chamber are rearranged in different three-dimensional (3D) patterns corresponding to different top-wall geometries, such as: (B) 3D curved line pattern. (C) 3D cross-linked pattern. (D) When the PDMS rod is inserted inside the chamber and the electrical signal applied to IDTs is turned on, microspheres are arranged in a 3D longitudinal pattern.

Inspired to reconstruct the structure of microvasculature by creating a 3D longitudinal pattern along a rod and then remove the rod to create a void for the vessels, the effect of a pre-inserted structure (i.e., a rod) on the 3D patterns in both the DI water and the hydrogel solution was investigated.

fluorescent images of side and top views of a rod with diameter of 1 mm in hydrogel collagen (scale bar: 1 mm)