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CR I T I CAL C L EAN INGU L T R A S O N I C S / M E G A S O N I C S C L E A N I N G

While many ultrasonic applications in the medicaland biological fields are already well established, an inter-

est is growing in new applications in noninvasive medicaltreatment as an alternative for invasive surgery. Dispersion of biolog-ical cells and diagnostic medical imaging are two applications wherethe nature of ultrasonics is substantially different. A low, high pow-ered ultrasonics (20-40 kHz) is used for cell dispersion and a muchsafer, extremely low powered megasonics (3-7 MHz) is used in diag-nostic imaging.

Ultrasonic applications in the pharmaceutical area include the cleaningof certain tools and equipment such as pill punches and dies and general mainte-nance of drug making vessels. Another important application is the use of sonochemistry to enhancedrug manufacturing.

This article will focus on ultrasonic technology and processes in relation to precision cleaning applications.

Critical CleaningCritical cleaning of components or substrates is the complete removal of undesirable contaminants to a desiredpresent level, without introducing new contaminants into the process.1 “Present level” is normally the mini-mum level at which no adverse effects take place in a subsequent operation or final application. To achieve thedesired cleanliness level, it is critical to not introduce new contaminant(s) into the cleaning process. In an aque-ous cleaning process, for example, it is important to have high quality rinse water and a minimum of two rinsesteps. Otherwise, residual detergent and/or ionics from the rinsing water become the new contaminants.

igh level cleaning of various types of medical components or devices

is indispensable for obvious health and performance reasons.

Cleaning with ultrasonics offers several benefits and advan-

tages over other conventional cleaning methods because

complete, consistent, uniform removal of contaminants

using ultrasonics is achievable, regardless of the complexity

and the geometry of the substrates.

SAMI B. AWAD, PH.D.

BY:

hhDevices and Equipment

Improper cleaning

methods can often

be blamed for

rejected parts.

Ultrasonic Cleaning of Medical/Pharmaceutical

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Another source of re-contamination of cleaned parts is out-gassed residues from packaging or storing materials.1,2

Improper cleaning methods can often be blamed forrejected parts, the curse of the assembly line. To meet pro-duction and quality demands, selecting the appropriate clean-ing method, chemistry, and process is essential. Additionalrequirements to consider for high yield processes:

◗Proper ultrasonic frequencies must be selected and tested per application.

◗Cleaning chemistry must be effective and safe on components and the environment.

◗Parts must be properly fixtured and oriented for optimum results.

◗Effective filtration of the cleaning solution and rinses,if needed, is required.

◗Same criteria above are applied if solvent is selected. ◗Automation and process controls are essential in having

a validated high yield consistent process.3

Ultrasonic Cleaning TechnologyTransducers. Ultrasonic cleaning uses high frequencysound waves (higher than 18 kHz and as high as 1 MHz)transmitted into a cleaning solution/liquid. Ultrasonic wavesare mechanical pressure waves formed by actuating ultrason-ic transducers with high frequency, high voltage/current gen-erated by electronic oscillators (power generators). A typical

generator produces ultrasonic frequen-cies greater than 20 kHz (Figure 1).

Transducers commonly used for gen-erating ultrasonic vibrations are piezo-electric, magnetostrictive, electromag-netic, or pneumatic. The piezoelectrictransducer (PZT) is the most widely usedtechnology in cleaning and weldingapplications. It offers wide range of vari-ous frequencies from about 20 kHz tothe megasonic range.

A recent transducer invention disclos-es a greater sound energy transmission atvery low acoustic impedance for varioushigh frequencies. Numerous benefits arerealized from this invention, includinghigh quality surface cleanliness and effi-cient submicron particle removal.

Typically, PZTs are mounted on thebottom and/or the sides of the cleaningtanks. The transducers can be mountedin various designs and sizes of sealedstainless steel containers, and can beimmersed in the cleaning solution/liquid(immersibles).

The push-pull transducer rod is arecently patented immersible titaniumtransducer.4 The immersible is made of

two PZTs mounted on the ends of a titanium rod. The gen-erated ultrasonic waves propagate perpendicularly to the res-onating surface. The waves interact with liquid media to gen-erate cavitation implosion.

Cavitations. When a solution is subjected to the rapidoscillation from the high frequency sound waves, minute vac-uum bubbles are generated. These bubbles grow to a certaincritical size based on number of variables and then implode.This phenomenon is known as cavitations.

When using ultrasonics, the cleaning action relies on cavi-tations or microimplosions. The micromechanical scrubbingaction of ultrasonic cavitations is directionless and can reachminute crevices, blind holes, intricate or irregular surfaces orinternal passages, and other hard to reach areas. When cavi-tations occur in the vicinity of contaminated surface, mechan-ically held contamination is released from the surface, solublematerials rapidly dissolve, and oils and other similar contam-inants are easily displaced with the cleaning chemicals. Thereleased energies can reach areas that are inaccessible toother cleaning methods such as high velocity spray or contactbrush (Figure 2).5

Frequencies. At a frequency of 68 kHz, the total time(in microseconds) from nucleation incipient to implosion isestimated to be about one third of that at 25 kHz. The ener-gy released from an implosion in close vicinity to the surfacecollides with and fragments, or disintegrates, the contami-

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20-40 kHz Properties at 20 kHz

Diameter: 50-200 µm

Impact: 35-70 k Pascals

Streaming Velocity: ≈ 400 km/hr.

60-70 kHz 120-140 kHz

FIGURE 1.

nants, allowing the detergent or the cleaning solvent to dis-place it at a fast rate. The implosion also produces dynamicpressure waves, which carry the fragments away from thesurface being cleaned. High-speed microstreaming currentsof the liquid molecules accompany the implosion.

The abundance of cavities generated in a solutionincreases with frequency but the energy released by indi-

vidual cavities decreases and becomes milder, thus idealfor small particle removal.6

At low frequencies (20-30 kHz), for example, a relative-ly small number of high energy powerful cavitations aregenerated (Figure 3). Low frequencies are appropriatefor cleaning heavy and large-size components, while mid-range frequency ultrasonics (40-80 kHz) is recommendedfor cleaning a variety of surfaces.

At higher frequencies (68-200 kHz), more cavitationswith moderate or lower energies are generated, which aregood for cleaning delicate components such as stents. Thisfeature also makes it effective in the rinse step. For exam-ple, at 68 kHz and 132 kHz the cavitation abundance arehigh enough and mild enough to completely remove deter-gent films and small particles, without inflicting surfaceerosion. Selecting the proper frequency for a particularapplication is an extremely important step and must becarefully investigated.

Ultrasonic Cleaning EquipmentUltrasonic aqueous batch cleaning equipment typically con-sists of four components: an ultrasonic wash unit, a minimumof two ultrasonic separate (or reverse cascading) water rinsetanks, and a heated re-circulated clean air unit for drying.The drying step is not included if the post cleaning operationincludes an aqueous bath, such as in electro- or electroless

plating.Ultrasonic transducers are bonded to

the outside bottom surface or outside ofthe sidewalls or provided as immersiblesinside the tanks. Two types of immer-sibles are commercially available in vari-ous sizes and frequencies. The traditionalsealed metal box contains a multi-trans-ducer system. The cylindrical immersibleis powered by two main transducers,which are located at both ends and areknown as push-pull immersibles.

An effective cleaning process must firstbe developed and then the number andthe size of the stations are subsequentlydetermined based on the required clean-liness level, yield, total process time, andspace limitation.

Automation of the ultrasonic cleaningsystem is a well-established feature, whichincludes a computerized transport sys-tem, able to run different processes forvarious parts simultaneously, and data ac-quisition and monitoring capabilities.Advantages of automation are numerous,including consistency, achieving desiredthroughputs, and full control on all pro-cess parameters.

Typical industrial tank sizes range from 10 to 2500 L;selection is based on the size of the parts, productionthroughput, and required drying time. The entire machinecan be enclosed to provide a cleanroom environment meet-ing Class 10,000 down to Class 100 cleanroom specifications.Process control and monitoring equipment consist of flow-controls, chemical feed-pumps, and in-line particle countersas well as equipment for TOC measurement, pH, turbidity,conductivity, and refractive index. Tanks are typically con-structed from corrosion resistant stainless steel or electropo-lished stainless steel. Titanium nitride or similar coating isused to extend the lifetime of the radiating surface in thetanks or the immersible transducers.

Cleaning and ChemistryThe use of ultrasonics does not eliminate the need for propercleaning chemicals and implementing and maintaining prop-er process parameters.

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Scrubbing Forces:

IMPLOSION

Contaminants/Particles/Detergent Film

UULLTTRRAASSOONNIICC CCLLEEAANNIINNGG

Cavitational Wave ShockMicrostreamingMiniature EddiesShear Forces

FIGURE 2.

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The chemical composition of the cleaning medium (foraqueous or solvent cleaning) is a critical factor in achievingthe complete removal of various contaminants without inflict-ing any damage to the components.

Ultrasonic cleaning using only plain water is workable, butonly for a very short time. Redeposition of contaminants willoccur shortly after a buildup of these contaminants takesplace. Evidently, some reactive contaminants must be cat-alytically hydrolyzed to break the chemical bonds with somemetallic surfaces. Furthermore, cleaning is more complex innature than simply extracting the contaminants from the sur-face. Soil loading and encapsulation/dispersion of contami-nants are important determining factors for the effective life-time of the cleaning medium and the cleaning results.

Reproducibility and consistency of the cleaning results areessential requirements for all successful cleaning processes.Cleaning chemistry, as part of the overall cleaning process, isan essential element in achieving such consistency. The fol-lowing factors must be addressed when deciding on theappropriate chemistry:

◗the ability to cavitate well with ultrasonics ◗its compatibility with components to be cleaned◗its wettability, stability, soil loading, oil separation,

effectiveness, dispersion, or encapsulation of solid residues◗its ability to rinse freely ◗the ability to be disposable at low cost

Both aqueous and solvent cleaning have advantages anddisadvantages. Aqueous cleaning is universal and achievesbetter cleaning results. Solvents perform well when removingorganic contaminants, but fall short on removing inorganicsalts.7 With aqueous cleaning, for example, the drying andprotection of steel components are valid concerns. The cur-rent available aqueous cleaning technologies, however, offereffective ways to alleviate these concerns.

The chemistry’s role is a multi-tasked one in order to dis-

place oils, solubilize or emulsify organiccontaminants, encapsulate particles, anddisperse and prevent re-deposition of con-taminants after cleaning. Special additivesin the cleaning chemistries are used toassist in the process of breaking chemicalbonding, removing oxides, preventing cor-rosion, or enhancing the physical proper-ties of the surfactants or the surface finish.Ultrasonic rinsing with water for injection(WFI), deionized (DI) water, or reverseosmosis (RO) water is important in orderto achieve spot-free surfaces. A minimumof two rinse steps is recommended.

Parts Handling and OrientationTo maximize the use of ultrasonic cavita-

tions in cleaning, parts must be racked on a fixture orarranged in one layer and placed on an open mesh (prefer-ably wire screen) basket and immersed in the ultrasonic tank.Stacking the components in layers is not recommended sincethe lower layers partially mask the ultrasonic waves. For opti-mum results, parts must be placed 11⁄2-2 in. away from theultrasonic radiating surface. To achieve cleaning with con-stant rotation, if needed, special requirements must be con-sidered such as surface characters and basket loading.

For best results, it is recommend that parts to be posi-tioned so that all surfaces receive equal exposure of ultrasonicenergy. Parts should be oriented to maximize drainage as well.

Vertical oscillation of parts may be essential to have in thewash and the rinse steps in certain applications.

Examples of Ultrasonics ApplicationsSuccessful ultrasonic cleaning processes developed for medicaland pharmaceutical applications include hip and knee metalreplacements, metal stents, pacemakers, polymer intraocularlenses, CR-39, and polycarbonate lenses. Scopes used insurgery such as endoscopes and laproscopes have also beensuccessfully cleaned using ultrasonics. Additional ultrasonicsapplications include the cleaning of various components fordiagnostic X-ray and CAT scan machines, microcatheters,hearing aids, regular and electro-surgical blades, cannulas, dis-posable syringes and needles, tools, and implants. Cleanlinesslevels were measured and verified with fourier transforminfrared spectroscopy (FTIR) analysis, optical microscopy, anda liquid particle counter (LPC). The following are severalexamples with more details on ultrasonic processes.

Cleaning Tablet Punches and DiesThese devices are frequently cleaned, particularly with everynew batch or on switching from making one type of medicineto another. A solvent vapor degreaser was used with mixedresults. Some components of certain drugs are not solvent sol-

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HIGH POWER GENERATORLow frequency current 50-60 Hz

high and ultra-high frequency 20 kHz-x MHz

TRANSDUCER ASSEMBLY (PZT)HF or UHF current and transducer

ultrasonic or megasonic waves

Sonic waves and liquid mediumcavitations and/or microstreaming

+

+

+

FIGURE 3. Ultrasonic and megasonics generation.

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uble and the instrument had to be manually re-cleaned after-ward. Switching from using solvent to aqueous ultrasoniccleaning was not an easy task. The punches are made of specialhard steel that is susceptible to flash rusting.

Aqueous cleaning processes for cleaning ferrous and non-ferrous metals together in the same aqueous cleaner, usingproperly formulated chemistries, was met with complete suc-cess. For example, aluminum, copper, brass, steel, and stain-less steel can all be cleaned in the same cleaning chemistrywithout interactions.

An aqueous ultrasonic process was specially designed utiliz-ing 25 kHz in the wash and 40 kHz in the DI water rinses. Aspecial proprietary inhibitor was developed and used in therinses to protect the parts in the rinse and drying steps. Theanalysis of the cleaned parts performed by the user had clear-

ly shown thoroughly cleaned surfaces with no sign of flash rust.

Cleaning Stents Stents are important medical cylindrical metal devicesdesigned to support the interiors of blood vessels and urinarytracts to keep them from collapsing after catheterization.

These devices are made of special fine mesh, are cylindri-cal in shape, and have different lengths and diameters. Anultrasonic cleaning process was developed to remove thelubricants and metal fines residues. Following a comparison ofthe FTIR analysis performed on the ultrasonically cleanedstents with the results from their manual cleaning, the processclearly showed success in achieving very clean parts.

The process utilized the proper cleaning chemistry, 68 kHzultrasonic frequency in the wash and in the pure water rinsesteps, and drying under HEPA filtered high velocity hot air.To avoid any stress fatigue damage to the product’s delicatestructures, the ultrasonic frequency and its total duration in

the process were selected after an extensive study by the man-ufacturer to determine the resonant frequency of the mesh.

Cleaning of Bone Replacement ImplantsHip and knee replacement components are manufacturedfrom titanium and other special alloys. They have differentdesigns and surface textures to enhance tissue growth into thedevice. These parts are manufactured in a multi-step customoperation. The process includes utilizing lubricants, specialpolymers, and complex buffing compounds, which must all becompletely removed in the cleaning step prior to passivationand sterilization.

Without ultrasonics, it takes a labor-intensive operation toclean such surfaces. An additional problem with the manualcleaning operation was an in-consistency in cleaning results.

The fully automated ultrasonic clean-ing process for this application success-fully utilized 40 kHz ultrasonics, two dif-ferent cleaning chemistries, and includedan acid passivation step in the machine.

ConclusionUltrasonic technology offers a uniquemeans to achieve ultra clean surfaces invarious fields, particularly for the med-ical and pharmaceutical industries(Figure 4). Aqueous or solvent ultra-sonics cleaning is a value-added optionthat offers several advantages overmechanical or non-ultrasonic methods.Selecting the right ultrasonic frequency,the appropriate cleaning chemistry, andoptimizing the cleaning process para-meters are necessary elements toachieve consistency and reproducibilityof the desired clean surfaces and achiev-

ing better overall yields.

References�� Sami B. Awad, “Ultrasonic Cavitations and Precision Cleaning,” Preci-

sion Cleaning, Nov. 1996, p. 12. (Figure 1 and Figure 2)�� Sami B. Awad, “Ultrasonic Aqueous Cleaning and Particle Removal of

Disk Drive Components,” Datatech, 1999, p. 59.�� Jon Harmon, “Ultrasonic Applications in the Life Sciences,” A2C2,

March 1999, p. 7.�� Patent assigned to Martin Walter Co., Germany/Crest Ultrasonics Corp.�� Andrew Shoh, Ultrasound, Its Chemical, Physical and Biological Effects,

K.S. Suslick, Editor, VCH Publishers, Inc., 1988, p. 97.�� Ahmed A. Busnaina, Glenn W. Gale, and Ismail I. Kashkoush, “Ultra-

sonic and Megasonic Theory and Experimentation,” Precision Cleaning,April 1994, p. 13.

�� Barbara Kanegsburg, “Aqueous Cleaning for High-Value Processes,”A2C2, September 1999, p. 25.

Contact Sami B. Awad, Ph.D., Vice President of Technology, CrestUltrasonics Corp., at Scotch Rd., Trenton, NJ 08628; 609-883-4000,fax 609-406-7045; sawad@crest-ultrasonics.com

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FIGURE 4.Advantages of ultrasonic cleaning.

◗ Efficient cleaning in recessed areas and blind holes.

◗ Capability of cleaning assemblies or devices.

◗ Removal of micro- and sub-microcontaminants.

◗ Various scrubbing intensities with different ultrasonic frequencies.

◗ With proper chemistry, ultrapure, consistent cleaning.

◗ Shorter process time.

◗ Full automation and control, batch and continuousprocesses.