Cleanroom Garment System Application and Use

By C. W. Berndt, E. S. Burnett, Ph.D. and Robert Spector

Abstract: This presentation will take and analytical approach to informed selection and use of cleanroom garment systems used in a variety of industries i.e., from semiconductor wafer fabrication to parenteral drug aseptic fill. It will review human sourced contamination along with appropriate counter measures for control and will present garment system selection guidelines for various industrial applications. Other relevant factors reviewed include recommended maintenance and processing practices, relevant testing and cost factors.

Introduction

In many cases, there is a lack of understanding in the contamination control community regarding the optimum garment system to be used to control particulate, chemical and biological contamination from human operators. Frequently, personnel not completely conversant with the latest developments in contamination control are asked to establish cleanroom garment programs, in many instances without adequate knowledge of the relevant performance parameters and limitations of competing fabrics and garment systems.

This paper introduces those factors which must be addressed in selecting cleanroom garment systems for many different applications. Recognizing that the requirements for cleanroom garment systems are increasingly demanding and volatile as technology changes and progresses. This presentation is confined to providing broad guidelines for garment system selection based on currently available fabrics, garment configurations, cleaning and sterilization technology. These guidelines are based on perceived user needs and cost-effectiveness for selected industrial applications.

Contaminants and Contamination Control

The human operator has been characterized as A broad-spectrum particle generator enclosed by inefficient mechanical filters which may also be sources of particulates. He or she is also capable of generating and releasing chemical and biological aerosols to the surrounding environment together with potentially destructive electric charges (ESD).

While primary attention in cleanroom technology and fabric/garment system design has always focused on the control of particulate contamination and, to a lesser extent, the control of ESD, the control of chemical and biological contamination assumed greater prominence following a pre-launch onboard computer failure on the Space Shuttle program. Subsequently, other semiconductor failures attributable to human chemical and biological contaminants were also reported by other investigators.

Street clothes and cosmetics are potential sources of particulate, fiber and chemical contamination. The concentration of particles per application of specific cosmetics is shown in Table I.

The data shown is presented to emphasize the need to exclude cosmetics other than moisturizing creams from the cleanroom. Conversely, it has been shown that use of cosmetics and /or lanolin-based moisturizers (containing little or no sodium, potassium, other metals or silicone) significantly reduces the release of skin flakes as particulates especially from the facial region of both male and female operators. This use of cosmetics and moisturizers reduces a prolific source of particulates viz., skin flakes and eyelashes from the upper facial region.

Specific Contaminants

Lacking a properly designed, installed and controlled cleanroom garment system, any or all of the contaminants listed in Table II may be released from a human operator.

By far, the largest number of particles released from the operator are skin flakes, varying in size from a few micrometers to between 40 and 50 µm with a median size of 20 µm. In moderate motion, between 500,000 and 1,000,000 skin flakes may be released per minute, with subsequent disintegrations in the air stream rapidly doubling or trebling this number. Bacteria-carrying skin cells typically released from males number 1,000 per minute, with less dispersion from females. Little or no quantitative data has been published on bacterial or other contaminants released from an ungowned operator.

Contamination Control Requirements

The overwhelming functional priority for a cleanroom garment system (including undergarments) is containment of particles/fibers released from the human operator. Secondary requirements include containment of chemical or biological releases from the operator and, in some cases, control of electrostatic discharge (ESD). Implicit in these requirements is the need to minimize, or ideally, to eliminate any secondary release of particles/fibers, ESD, chemical or biological residues from the garment system. For aseptic fill operations, garment system components must be capable of being sterilized prior to use.

In almost all ultraclean (FS 209 Class M1.5/1 to M2.5/10), aseptic (FS 209 Class M3.5/100 to M5.5/10,000) and many clean (FS 209 Class M4.5/1,000 to M6.5/100,000) applications, containment translates directly to full body coverage. In the limiting case, where absolute protection of product overrides all other considerations (including cost) and no alternate mitigation is feasible, full containment may demand the use of the Space Helmet head covering with HEPA-filtered self-contained breathing apparatus (SCUBA).

Full body coverage immediately raises issues related to operator comfort, acceptance and productivity. There are many trade-offs related to filtration efficiency, fabric density, garment closures, air porosity and moisture vapor transmission rate that impact on operator comfort and acceptance. In the interest of operator safety, in the event of an emergency evacuation, the garment system must be designed for easy removal while maintaining contamination control integrity in normal use.

Realistically, many factors mitigate against total containment of human body debris. These include incomplete facial coverage, billowing of the body covering, incomplete filtration by many woven fabrics and improper installation of the individual components of the garment system. The latter problem can often be traced to lack of discipline and/or training of the individual.

Implementation

Recognizing the need for near total containment and sterility, many years ago the pharmaceutical industry adopted a simple style of body covering consisting of a coverall with elasticized arm and leg cuff closures, and open-faced hood, a surgical mask with suitable gloves and boots. The coverall and hood were fabricated from Tyvek, a limited use spunbonded polyolefin fabric with excellent barrier properties, lowdensity and having a low but marginally acceptable moisture vapor transmission rate. The basic Tyvek coverall and hood remains the workhorse of the pharmaceutical and related industries where barrier properties and sterility override considerations of operator comfort and acceptance. Only very recently have new, light weight, high filtration efficiency polyester fabrics been developed for use in the healthcare and pharmaceutical industries. These fabrics are currently being evaluated in the field as potentially cost effective and ecologically sound replacements for the limited use fabrics currently in use.

Modelled after the basic full body coverings described above, there have been many developments in fabric and garment system design, aimed primarily at the ultraclean (Class M1.5/1 to M2.5/10) and non-aseptic (Class M3.5/100) applications for the semiconductor, disk drive and electronics industries. The need to minimize particle release from the garment system while providing good chemical resistance and biological growth inhibition effectively limits the choice of fabrics to 100 percent woven polyester or the significantly more expensive laminated fabrics pioneered by W. L. Gore Associates under the trade name of Gore-Tex. This fabric is used in the leading edge cleanrooms in the USA. It must be emphasized that garment systems manufactured from the laminated fabrics are significantly more expensive than woven polyester garments.

Where cost is a prime consideration and the operating environment is non-critical, it may be possible to dispense with full body coveralls (Bunny Suits) in favor of smocks or frocks. The trade-offs involved in establishing a working station layout or micro-environment that permits the confinement of operator particulate contamination control to the upper torso while guaranteeing the product integrity should be carefully considered.

For full-body coverage and particulate control to 0.3 µm, the cleanroom garment system is required to act as a high-efficiency membrane filter. Furthermore, garment closures must be designed in conjunction with other components of the garment system (boots, gloves, hood and face-mask) to minimize or eliminate particle release at the interfaces and the main garment closure, normally a Delrin zipper with plackets. However, the same filter assembly must permit transfer of body moisture through the garment to establish an acceptable level of comfort for the operator.

Perceived operator comfort is dependent on the following factors: The temperature and relative humidity in the cleanroom Activity level of the operator Fabric density Air permeability Moisture Vapor Transmission Rate (MVTR)

Ideally, the cleanroom should be operating at  relative humidity. In the interest of contamination control and operator comfort, excessive and rapid motions should be minimized through improved process design. This leaves fabric density, air permeability and MVTR as the controlling variables in the trade-offs of particulate control vs. operator comfort.

Absolute values of the most critical parameter affecting operator comfort i.e. MVTR, differ widely depending on the test method used. The problem of determining MVTR is further compounded by the transition from diffusive to capillary moisture transfer, which takes place as equivalent pore size and air permeability decrease. For this reason, it is advisable to conduct Body Box testing and in-situ evaluation of a garment system to determine whether an acceptable comfort level can be realized within acceptable particulate control parameters. Under appropriate conditions of use, both lightweight Tyvek with very low air permeability and low MVTR and the denser membrane garment with higher air permeability and MVTR, both of which are excellent particle barriers, are acceptable from the stand point of comfort. However, the limited lifetime of Tyvek garments before they begin to particulate make them unsuitable for use with most critical particulate control environments, while the high cost of the membrane garments limits their use to the critical FS Class 1 to 10 environments. The newer 100% polyester static-dissipative woven garments with relatively low density, good particle filtration efficiency and acceptable MVTR represent the most cost-effective compromise in many applications, particularly where ESD control is important.

Sterilization

Since sterile cleanrooms are supplied with microbial-free air and are also positively pressurized against the admission of contaminated air from adjacent areas, airborne contamination can only come from within the room. Other than some unusual set of circumstances, the source of the microbial contamination can only be the people in the room (microbe laden skin cells, hair, internal and other external effluvia). As such, the people working in these cleanrooms must be garmented so as to completely cover the body. This is done using appropriate barrier-type fabrics with acceptable garment configurations. These garments must be cleanroom laundered to control particulates and extraneous contaminants and, must also be sterilized.

Three methods have been identified for sterilization of cleanroom garments that are typically used in sterile cleanrooms i.e., woven polyesters and nonwoven spun-bonded polyolefins or Tyvek. These are, Steam Autoclave Ethylene Oxide (EtO) Ionizing Radiation (gamma or electron beam) The oldest form of sterilization is steam autoclave. This method utilizes high temperatures and pressures (approximately 270 (F  at 15 psi. for 15 minutes) and is most commonly used for cleansing or sterilizing reusable instruments and durable or heat-resistant materials. Unfortunately, the use of steam autoclave on cleanroom garments is highly deleterious and not recommended. Undesirable effects include shrinkage of garments by at least one size, puckering of seams (thereby allowing gaps which will compromise the ability of the garment to contain microbial and particulate/fiber contamination), premature fiber degradation and alteration of intended filtration ability/characteristics of the fabric.

EtO and, more recently, ionizing radiation are more commonly used for sterilizing cleanroom garments. EtO is a colorless gas that is used industrially as both a sterilant and a fumigant and has shown to be effective for sterilizing bulk loads of cleanroom garments. This form of sterilization requires validation employing guidelines as set for by the Association for the Advancement of Medical Instrumentation (AAMI). As defined by AAMI, validation is a term that describes the overall program used to show that a specified product can be reliably sterilized by the designed process.

Once garments have been sterilized with EtO, it is very important to provide for efficient outgas or aeration of the packaged and sterilized garments so that residual EtO, ethylene chlorohydrin (EtCH) and ethylene glycol (EtG) are kept to a minimum. Although cleanroom garments are not regulated as medical devices, good guidelines to follow would be those for maximum residues for devices contacting the skin (Proposed 21 CFR Part 821, Section 821.100). These maximum limits are, EtO ----- 250 ppm EtCH --- 250 ppm EtG ----- 5000 ppm In terms of meeting residual criteria for Et0 and, after proper outgas or aeration, cleanroom garments made of Tyvek show the least EtO residuals (usually far less than 20 ppm, three days post-sterilization). Polyester, on the other hand, shows residuals for EtO at approximately 650 ppm to more than 1500 ppm, seven days post-sterilization. The reason for this is that polyester is dielectric and tends to tenaciously hold on to the EtO molecule. As a consequence, sterilization via ionizing radiation is recommended for polyester.

The release of EtO sterilized cleanroom garments as sterile articles requires sterility testing of biological indicators (spore strips of Bacillus subtilis at a population of 10 million ) as described in the United States Pharmacopoeia (USP). As a result, a quarantine period (usually a minimum of 5 days) is mandated prior to allowing release and certification for sterility.

Two forms of ionizing radiation that are commonly used for sterilizing cleanroom garments are gamma rays and high-energy electron beams (E-beam). Gamma rays are produced by the decay of Cobalt-60 (Co) while electron beams are produced by accelerating systems such as linear accelerators. As with EtO, these forms of sterilization also require validation as outlined by AAMI. Elements of the validation cycle include materials compatibility, dose setting, product loading pattern, dose distribution mapping, cycle timer setting and cycle validation. For E-beam, on-line control of voltage, current, conveyor speed and electron beam scan dimension must be validated. In addition, penetration depths and related dose distributions must be considered.

When employing a validated sterilizing tool such as ionization radiation, Process Specifications and General Operating Guidelines must be defined. These include,

• Processing Specifications            
• General Operating Guidelines
• Pre-irradiation product handling      
• Good Manufacturing Practices Product irradiation                                       
• Product handling Post-irradiation product handling    
• Calibration Processing Records                       
• Source activity
• Sterilization lot                                                                     
• Product sterility release
• Process interruption (routine and preventative)

Finally, sterility dose auditing to detect changes in bioburden should be performed every three months unless bioburden data indicate otherwise. This is done to detect changes in the bioburden that could require an increase in the established sterilization dose i.e., continued validity of a sterilizing dose.

Both forms of ionization radiation sterilization (Co and E-Beam) allow for dosimetric release. According to AAMI, this is defined as the release of product lots as sterile upon confirmation that all aspects of the manufacturing process are within specifications and that the specified minimum dose has been delivered to the product. This presents an obvious advantage over EtO in that no quarantine period is required. Confirmation of sterility without USP testing reduces cleanroom garment turnaround time which suggests a lower assigned garment inventory thusly reducing cost to the user.

Cleanroom garments made of Tyvek are not compatible with repeated ionizing radiation sterilization techniques i.e., more than 50 kGy accumulated dose. The loss of physical properties is too great i.e., unacceptable garment degradation (particulation) as well as literal deterioration (garment tendency to fall apart when donned). Single ionizing radiation sterilization at no greater than 25 kGy for disposable (one-time use) Tyvek garments has been found to be acceptable.

Cleanroom garments made of 100 percent polyester are very compatible with repeated ionizing radiation sterilization techniques nevertheless, the lower the sterilizing dose, the longer the garment will last. Accumulated doses of 800 kGy or more for 100 percent polyester cleanroom garments have shown no degradation other than a very slight yellowing. It should also be noted that overdosing garments with gamma radiation may result in producing an unacceptable pungent odor (much like regurgitative material). This will dissipate fairly rapidly once the garments have been removed from their polyethylene packaging and donned. The odour is very unpleasant but harmless under conditions of use nevertheless, operator acceptance may be negatively impacted if garments are consistently overdosed.

Selection Criteria

Contamination Control Needs by Industry A generalized approach to identifying general contaminants by industry is presented in Table III. The particle size column was arbitrarily divided into >1.0 and <0.5 (m categories based on identifiable industry interest.

Fabric Recommendations

Table IV presents overall fabric-type use and preference by the same industry categories as shown in Table III. Fabric use information as presented is not all inclusive or exclusive and is generally reflective of performance (specifications) based on type of contamination control needed. In all cases fabric garment and garment program selection should be based on the well understood needs of each individual user and choices made accordingly.

Definitions

Herr

Herringbone, a broken twill weave giving a zigzag effect produced by alternating the direction of the twill (same as chevron weave; structural design resembles the backbone of a herring). A true herringbone should have the same number of yarns in each direction, right and left, and be evenly balanced. Thus, all herringbones are broken twills but all broken twills are not true herringbones.

Taff

Taffeta, a fine plain-weave fabric, smooth on both sides, usually with a sheen on its surface.

Twill

A fundamental weave characterized by diagonal lines produced by a series of floats staggered in the warp direction. The floats are normally formed by filling i.e., filling-faced twill. A warp-face twill is a weave in which the warp yarns produce the diagonal effect.

High Density

A term that is used to describe a material with heavier than normal weight per unit volume (usually a taffeta).

Spun-bonded polyolefin

Fabrics formed by filaments which have been extruded, drawn, then laid on a continuous belt. Bonding is accomplished by several methods such as by hot-roll calendering or by passing the web through a saturated-steam chamber at an elevated pressure. In this case, an olefin fiber-forming substance is any long-chain synthetic polymer composed of at least 85% by weight of ethylene, propylene or other olefin units.

PTFE

Polytetrafluoroethylene or Teflon laminate bonded on a polyester knit fabric employing multiple layers. Urethane - A polyurethane film (or hydrophilic thermoplastic membrane) laminate bonded on a polyester knit fabric.

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