Phononic-Crystal-Enabled Dynamic Manipulation of Microparticles and Cells in an Acoustofluidic Channel

Contactless manipulation of particles and cells using sound radiation forces that can be tuned and adjusted in real time has become important in various applications. These applications include display technology, biomedical sensors, imaging devices, and diagnostic tools. Phononic crystals have many properties that could be beneficial for tunable manipulation in a microfluidic channel. We use phononic crystals to tune sound fields in a microfluidic channel for controllable manipulation of microparticles and cells. An arbitrary stop-and-go motion of particles and cells along a predefined path in the channel is experimentally demonstrated. Analytical and computational modeling reveals how the resonances of phononic crystals tune the sound fields and radiation forces for the desired manipulations. These concepts and realizations of dynamic manipulation of particles in the microfluidic channel advance the development of the microfluidic channel for dynamic acoustic manipulation technologies, particularly benefiting tunable cell analysis.

Figure 1

Conceptual schematic of stop-and-go manipulation of particles in a microfluidic channel built on a PCP. A lead zirconate titanate (PZT) source drives the PCP into the resonance to excite sound fields transmitted in the fluids of the channel: (i) at a frequency $f_1$ for a “stop” manipulation, where the negative acoustic radiation force in the vertical ($z$) direction enables the particle to adhere to the substrate of the channel; and (ii) at a frequency $f_2$ for a “go” manipulation, where the vertically levitated (in the $z$ direction) and transversely trapped (in the $y$ direction) particle transports along the channel by a gradient force along the $x$ direction. The resonances depend on the spatial periodicity (denoted by $L$) and thickness (denoted by $h$) of the PCP.

Figure 2

Numerical transmission spectrum, dispersion curves, and field distributions of the PCP. (a) Transmission spectrum at normal incidence versus frequency for the PCP with different heights. (b) Dispersion curves (black dots and red dots) for the PCP sample. For comparison, the simply folded dispersion curves for the uniform plate are plotted as lines with the same color; the dashed black line represents water. (c) Pressure-field distribution (left) and velocity-field distribution (right) around the PCP at the resonant frequency of $f_1$. (d) The same as (c) at the resonant frequency of $f_2$. For the velocity field, both the amplitude field (color variations) and the normalized vector field (arrows) are provided. The yellow region indicates the PCP structure in a periodic unit.

Figure 3

Simulated acoustic properties of the channel. (a) Transmission of sound waves from the source to the channel has two maximum peaks, at $f_1=3.72\mathrm{MHz}$ and $f_2=4.4\mathrm{MHz}$, corresponding to resonances of the PCP of periodicity $L=350\mu \mathrm{m}$. The dotted curve is the result for a uniform plate for comparison. (b) Sound pressure field $\mid p\mid$ at $f_1$ in the channel’s $y$-$z$ cross section and the corresponding force potential $U$ along the trapped location at the pressure minimum (solid lines in the left panel), revealing the ability to trap solid elastic particles at the bottom (denoted by stars) for the desired stop manipulation. (c) Same as (b) but for frequency $f_2$, where the potential minimum is around the middle of the channel, resulting in a vertical levitation needed in the go manipulation. (d) Sound pressure field at $f_2$ in the horizontal $x$-$y$ plane ($z=0.57L$) and the force potential $U$ at the pressure minimum (solid lines in the top panel), revealing a gradient force along the channel ($x$ direction) to transport the particle in the go manipulation. (e) Transition between the negative vertical force in the stop manipulation and the positive vertical force in the go manipulation when tuning the frequency; the solid lines are the locations of the pressure minimum in the stop-and-go manipulations at $z=0.1L$ and $0.57L$, respectively.

Figure 4

Experimental setup and a PCP sample. (a) Photograph of the phononic-crystal-based microfluidic device. (b) Illustration of the experimental system configuration. (c) Schematic side view of the experimental setup. (d) Cross section of a PCP sample. PDMS, polydimethylsiloxane.

Figure 5

Measured field distribution. (a) Surface displacement of the PCP measured by a laser Doppler vibrometer at the resonant frequency of $f_1$. (b) Normalized pressure obtained by measurements (solid curve) and simulations (dotted curve) at $y=0$.

Figure 6

Stop, levitation, and go manipulation with side view. (a) Trapping and levitatation of PS particles by switching frequency between the resonant frequency of $f_1$ and $f_2$. (b) The stable height of the levitated PS particles can be controlled by adjusting the input acoustic power, which effectively changes the magnitude of the ARF. For the above process, particles are directly above the source. (c) The process for PS particles switches from stop (trapping) to go (transporting) manipulation toward the source by switching the frequency from $f_1$ to $f_2$, while the initial position of the PS particles is left offset compared with the source center. For videos, see Videos 1–3 in Supplemental Material [38].

Figure 7

Dynamic stop-and-go manipulation by three kinds of PCPs with a top-down view. Photographs of the bottom surface of these PCPs consisting of a uniform stainless-steel plate patterned with a periodic array of rectangular steel gratings with (a) straight lines, (d) ${120}^{\circ }$ polyline lines, and (g) loop lines. The thickness of the plate, the period of the array, and the width and height of the grating in the PCPs are the same. The gray regions indicate the position of the acoustic source of the PZT. (b) The PS particles are transported along designed straight-line pathways in three time frames. (e) The PS particles are transported along designed pathways of ${120}^{\circ }$ polyline lines (1, initial state; 2, transport state; 3, trapping state; 4, transport state). (h) The PS particles are transported along designed pathways of looped lines in four time frames. The blue and red circles indicate that the PZTs are on and off, respectively. (c),(f),(i) The full transportation pathway in (b),(e),(h) demonstrated by superposing different time frames. For videos, see Videos 4–6 in Supplemental Material [38].

Figure 8

Dynamic manipulation of living cells (breast cancer cells). (a) Trapped on the surface of the PCP in fields of frequency $f_1$. (b) Transported along the designed straight pathways in fields of frequency $f_2$. For a video, see Video 7 in Supplemental Material [38].