Ultrasound in the Food, Drug and Device Industries
WELFARE PUBLIC HEALTH SERVICE
FOOD AND DRUG ADMINISTRATION
Related Program Areas:
Food, Drugs, and Medical Devices
ITG SUBJECT: ULTRASOUND IN THE FOOD, DRUG, AND DEVICE INDUSTRIES
Ultrasound technology is an area of increasing application in the food, drug, and device industries. Ultrasonic techniques are presently utilized for applications ranging from bonding and sealing of thermoplastic packaging to medical diagnosis. This ITG serves as an introductory issue for future ITGs that will describe specific applications of ultrasound in the food, drug, and medical device industries. The limited space involved allows only a brief look at the uses of ultrasound but it is hoped that these ITGs will stimulate further reading and research on the subject.
Ultrasound is a mechanical vibration and is produced by converting electrical energy into mechanical energy. Conversion from electrical to mechanical energy is accomplished by applying electrical energy to a transducer. In general terms, a transducer is any device that converts one form of energy into another. Our discussion of the generation of ultrasound will be restricted to piezoelectric and magnetostrictive effect transducers which are generally used for producing sound in the ultrasonic region. Piezoelectric transducers utilize the phenomenon that an asymmetrical crystalline material will produce electrical excitation when mechanical stress or strain is applied along a particular crystalline axis. Conversely, applying an alternating electrical field across the same crystalline axis will alternately compress and release compression producing vibration and acoustical energy. Piezoelectric transducers are normally made of natural or cultured quartz or one of the ceramics, usually lead zirconate titinate. Magnetostriction occurs in certain ferromagnetic materials and certain non-metals called ferrites. A change in dimension occurs when a rod or bar of this material is subjected to a magnetic field. When an alternating magnetic field is applied vibration and acoustical energy is produced.
Although some ultrasonic devices use air as a transfer medium at lower frequencies, ultrasound energy does not pass through air effectively at the higher MHz frequencies. Therefore, a conduction medium is normally used in conjunction with the transducer to eliminate air between the transducer and the specimen or medium. The transducer may be directly bonded to the specimen, immersed in water with the object, or a paste or liquid may be used between the transducer and the object.
Ultrasound features a wide range of power and intensities. Ultrasound power (the rate at which work is done) is measured in watts. Intensity (power per unit area) is measured in watts/cm 2.
Other than the use of ultrasound vibration directly as for cleaning, mixing, etc., two of the most common ultrasonic techniques used are pulse-echo ultrasound and the Doppler effect.
The pulse-echo technique was developed in the 1940's as a metals flaw detector. In this technique, the transducer is coupled to the specimen and a series of focused ultrasonic pulses are generated and beamed into the specimen. If the pulses encounter a change in density or elasticity (interface) during travel through the specimen, a portion of the ultrasonic energy will be reflected back to the transducer. The reflected signal arrives at the transducer between transmitted bursts and is converted into an electrical signal, amplified, conditioned, and displayed in various ways depending upon the specific application. In this method the transducer acts as both a receiving and transmitting element. If more than one interface is encountered, a series of echoes, with each echo representing an interface, will be reflected back to the transducer. Therefore, if the normal construction of the specimen is known (i.e., the number of internal interfaces or changes in density), abnormalities can be detected. The time delay between pulses is directly proportional to the distance between interfaces. With proper electronic circuitry, echoes may be displayed on a cathode ray tube in various configurations, as a meter reading, an audible signal, counter reading or graphic recording. The pulse-echo method is commonly used to control fluid level, measure animal tissue thickness, material thickness, ultrasonic medical diagnostic devices, etc.
Ultrasound techniques can be used to detect motion, and with proper signal conditioning, direction of motion. This is accomplished by utilizing the Doppler effect. For a stationary target the relationship existing between the ultrasound frequency transmitted and the resulting echoes is the same for successive pulses. In contrast, the effective frequency of the echoes received from a moving target continually changes with respect to the frequency of the transmitted pulses because the distance to the target is changing. As the target moves toward or away from the transducer or receiver, the frequency detected is larger or smaller than that emitted. A familiar example is the apparent change to the ear of the frequency of a train whistle as the train passes. This is known as the Doppler effect. With appropriate processing the reflected frequencies can be used to indicate direction of movement or flow, away or toward the transducer (increase or decrease in frequency). The Doppler method may use either pulsed or continuous wave ultrasound. Two transducers, a transmitter and receiver, must be employed when continuous wave ultrasound is utilized. The Doppler technique is used extensively in clinical diagnostic ultrasound devices such as blood flow measurements and cardiological diagnosis.
Other industrial applications of ultrasound include cleaning equipment which utilizes ultrasound to form bubbles in the cleaning or sterilizing liquid. The formation and collapse of the gas or vapor filled bubbles (cavitation) generates powerful local forces which cause the desired cleaning action. Ultrasonically agitated baths containing abrasive particles are used to cut metals and for erosive cleaning. Ultrasonics is also used to form foam in beverage bottles and cans to expel air before closing occurs. Ultrasound may be used to generate frictional heat for sealing thermoplastic packaging. Airborne ultrasound is utilized in rodent repellent systems. Ultrasonic energy is used in clinical therapy as a deep heating agent. These are only a few of the many uses of ultrasound.