Desarrollo del proyecto de investigación

MEMS-Based Ultrasonic Devices

5.1 Introduction ...5-1 5.2 Micro-Ultrasonic-Transducers (MUTs)...5-1 5.3 Acoustic Cavitation ...5-4 5.4 MUTs for Enhancing Antisense Oligonucleotides

Efficacy ...5-5 References ...5-7

Summary

This chapter investigates the use of micro-ultrasonic-transducers (MUTs) for therapeutic applications such as enhancing the transfection efficacy of antisense oligonucleotides (ASOs) for cancer treatment. After a brief description of the two subcategories of MUT and their respective fabrication techniques, some experimental results are discussed. Finally, other potential applications of MUT are outlined and the advantages of MUTs over conventional ultrasonic transducers used in mechatronic applications are discussed.

5.1 Introduction

Ultrasonic transducers are simple yet typical examples of mechantronic devices. Besides, ultrasonic transducers are commonly presented as integral components in mechatronic systems. Acoustic waves, vibration, and movement of molecules in an organized manner are all mechanical phenomena. Yet, to produce these vibrations at high frequency (>20 kHz), one often looks to an electrical source as the driving signal. Furthermore, as MUTs are fabricated using micro-electromechanical systems (MEMS) technology, which evolved from photolithographic techniques of the microelectronic industry, they have inherited a great potential to be integrated with electronic circuits.

5.2 Micro-Ultrasonic-Transducers (MUTs)

Most ultrasonic transducers today use piezoelectric ceramic as their active elements. In recent years, researchers have used micromachining techniques to fabricate ultrasonic transducers that have active acoustic elements with the size of a few microns. Proposed applications for these MUTs include medical ultrasonography [1], acoustic anemometry [2], aerial ranging [3], and so on. In medical ultrasonography in particular, MUTs have many advantages over conventional ultrasonic transducers—for example, greater precision in fabrication and better acoustic matching [4]. In spite of the numerous MUT research around the world, by far no one has looked into the possibility of using MUTs for sonodynamic therapy.

T. Siu M. Chiao

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5-2 Mechatronic Systems: Devices, Design, Control, Operation and Monitoring

There are currently two approaches in fabricating MUTs. The first approach requires the deposition of a piezoelectric thin film on a micromachined diaphragm. The resultant transducers are called piezo-electric micro-ultrasonic transducers (pMUTs). Piezopiezo-electric materials change their physical dimension when an electric field is applied across them. The dimension change occurs in the direction along the electric field and also in the directions perpendicular to the electric field.

In a conventional ultrasonic transducer, only the piezoelectric effect along the electric field is typically used to generate ultrasound. For power ultrasound applications, such as sonochemistry and sonodynamic therapy, the objective is to obtain maximal ultrasonic intensity. In such cases, the transducer is usually operated in a continuous wave fashion at the fundamental thickness-mode resonant frequency. At that frequency, a standing compressive wave is set up within the thickness of the transducer with the largest strain and therefore the least stress on its two surfaces [5]. Figure 5.1 shows a transducer at thickness mode resonant frequency.

Acoustic wave has a constant compressive velocity in a given material, the fundamental thickness-mode resonant frequency is related to its thickness and given by [5]:

(5.1) where f1 is the fundamental thickness-mode resonant frequency, ct is the compressive acoustic wave velocity in the ceramic, and l is the thickness of the ceramic.

In a pMUT, however, the deposited piezoelectric thin film typically has a thickness of a few microns.

The thickness-mode resonant frequency of such a thin film will be on the order of a few GHz. To allow the pMUT to operate at the MHz frequency that is more common in medical applications, the piezo-electric effect perpendicular to the applied piezo-electric field is employed, instead [4]. The micromachined diaphragm on which the thin piezoelectric film rests is made to buckle. When an electric field is applied across the piezoelectric film, the film will experience dimension changes in the directions perpendicular to the electric field. The change in area in the piezoelectric film will force the buckled diaphragm to FIGURE 5.1 A conventional ultrasonic transducer made of piezoelectric ceramic is operated at the fundamental thickness-mode resonant frequency.

Plot of strain along the thickness of Transducer (l)

Plot of stress along the thickness of Transducer (l)

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MEMS-Based Ultrasonic Devices 5-3

flatten or further deform, generating the necessary motions to produce the ultrasound. Therefore, the resonant frequency of the pMUT will depend on the dimension and the material properties of the diaphragm. Figure 5.2 illustrates the operation of a pMUT. For detail fabrication processes of a pMUT, the readers may refer to the published papers of Muralt et al. [6] and Wang et al. [7].

The second way to fabricate MUTs is to utilize the electrostatic force between parallel plates with opposite charges. The resultant transducers are called capacitive micro-ultrasonic transducers (cMUTs).

In general, a bottom electrode is first laid down, and a thin membrane is suspended on the bottom electrode with a cavity in between. On top of the thin membrane is the top electrode. When an alternating voltage is applied across the two electrodes, the electrostatic force between them will attract and then repel them from each other, thereby creating the necessary motions to generate ultrasound. The resonant frequency of a cMUT will also depend on the dimensions and the material properties of the membrane.

The operation of a cMUT is illustrated in Figure 5.3.

FIGURE 5.2 Basic design and operation of a pMUT.

FIGURE 5.3 Basic design and operation of a cMUT.

Substrate

Direction of deformation in piezoelectric thin film

Direction of movement of the whole composite diaphragm Electrode

Piezoelectric thin film Structural diaphragm

Legend

Substrate

Direction of movement of the whole composite diaphragm Electrode

Structural diaphragm

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5-4 Mechatronic Systems: Devices, Design, Control, Operation and Monitoring

A crucial step in the fabrication of cMUT is the incorporation of a precisely controlled cavity between the bottom electrode and the membrane. This cavity needs to be deep enough to allow free movement of the membrane but maintain an adequate electrostatic effect. Methods that are used to incorporate this cavity include: (1) use of sacrificial layer that is etched by chemical [8, 9] and (2) a wafer bonding process [2].

It is understood that cavitation, and therefore sonodynamic phenomenon, occurs only beyond a certain acoustic power density threshold [10]. Therefore, the most important design criterion for a MUT intended for sonodynamic therapy is the output acoustic power density.

The power density of an acoustic wave is defined as “the instantaneous power flowing through a unit area perpendicular to the direction of propagation of the wave as one elemental volume of the fluid acts on a neighboring element” [11]. Given that, one can write:

(5.2)

The acoustic impedance (Z) of a material is defined as the relationship between the pressure and velocity that a particle in the medium feels [11]: