Page 13 - Fall2013

tem of optical tweezers is complex and expensive.
Acoustic tweezers may alleviate these problems. On one hand, it has comparable time efficiency and control accuracy to optical tweezers, and on the other hand, it has lower cost and stronger trapping force than optical tweezers. Moreover, it has deeper penetration depth in the light opaque objects such as tissues and less biological damages, which brings a wide variety of biomedical applications for this technology. It has been applied to study the deformability of red blood cells
with two different approaches.
In a preliminary experiment, fresh blood samples were
obtained from a healthy adult male. Red blood cells (RBC) were diluted in phosphate-buffered saline (PBS) and then washed three times by centrifuging. All preparations were made at room temperature. The experiment process was captured by a CMOS (complementary metal-oxide-semi- conductor) camera (ORCA-Flash2.8, Hamamatsu, Japan) connected to the microscope. RBCs were suspended in a specially designed chamber filled with PBS. The chamber was open at the top and had a thin mylar membrane as its bottom. After 2 hours, the suspended RBCs would sink down to the bottom. A cell stuck to the chamber was chosen as the object of study. A selected RBC was stretched directly or indirectly by an acoustic beam. Two 200 MHz focused transducers were employed in this study. One is a 200 MHz lithium niobate (LiNbO3) single crystal pressed focus (PF) transducer and the other is a 200 MHz Zinc oxide lens focus (LF) transducer.
Method #1: The PF transducer was employed to trap a 5μm polystyrene microsphere. The trapped polystyrene micros- phere was attached to a selected RBC and the RBC could be stretched by an acoustic beam through the trapped microbead. As displayed in the Fig.4 (PF), the RBC was stretched to left and right and deformed to different degrees.
Method #2: The LF transducer was used to deform a RBC (red blood cell) directly. A RBC near the acoustic beam was selected. As the voltage input to the transducer was increased from 200 mVpp to 800 mVpp followed by 50 dB amplification, the RBC was observed to start to elongate after the input voltage was increased to above 200mVpp. The degree of elongation was observed to increase with the increase of input voltage.
Fig.3. Different types of ultrasonic transducers for the acoustic tweezers application.
In addition, needle-type14, ring-type15 and phased-array transducers16 were also fabricated for acoustic tweezers appli- cations. Those transducers may have advantages in certain applications compared to traditional acoustic tweezers. For example, the size of the transducer is crucial in a number of applications, and there is a need to miniaturize the transduc- er size. Phased array transducers could focus and steer the acoustic beam; therefore no mechanical movement of trans- ducer is required. However, phased array transducers are still limited to the frequency range less than 50 MHz and further improvement is needed. Meanwhile, the present authors are also exploring new types of transducers, which may generate the acoustic beam with better cylindrical symmetry and higher intensity, consequently yielding better trapping per- formance and deeper penetration depth.
Biomedical applications of acoustic tweezers
The most common technology for micro-particulate or cellular manipulation is micropipettes, but it has intrinsic shortcomings, such as time consuming and low control accuracy. Optical tweezers which could overcome those problems, has become a powerful tool with broad applica- tions in biology, medicine, and physics. It has been used for noninvasive dynamic control of parti-
cles in the size ranged from tens to
hundreds of nanometers, including
bacteria, viruses and living cells.
There are however a few disadvan-
tages of optical tweezers as well. First
of all, its application has been limited
to optically purified samples or media.
Second, the high energy of focused
lasers may induce local heating and
photo-damage. Third, the maximum
trapping force of optical tweezers is
limited to a few hundred
picoNewtons. Furthermore, the sys-
12 Acoustics Today, October 2013
Fig. 4. Video sequence showing that a RBC was elongated by a 200 MHz PF transducer and by a 200MHz LF transducer