Designation: F 2038 – 00 (Reapproved 2005)

Standard Guide for

Silicone Elastomers, Gels, and Foams Used in Medical
Applications Part I—Formulations and Uncured Materials1
This standard is issued under the fixed designation F 2038; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.

1. Scope 2. Referenced Documents

1.1 This guide is intended to educate potential users of 2.1 ASTM Standards: 2
silicone elastomers, gels, and foams relative to their formula- D 1566 Terminology Relating to Rubber
tion and use. It does not provide information relative to silicone F 813 Practice for Direct Contact Cell Culture Evaluation of
powders, fluids, and other silicones. The information provided Materials for Medical Devices
is offered to guide users in the selection of appropriate 2.2 Sterility Standards:3
materials, after consideration of the chemical, physical, and ANSI/AAMI ST41 Good Hospital Practice: Ethylene Oxide
toxicological properties of individual ingredients or by- Sterilization and Sterility Assurance
products. This guide offers general information about silicone ANSI/AAMI ST50 Dry Heat (Heated Air) Sterilizers
materials typically used for medical applications. Detail on the ANSI/AAMI ST29 Recommended Practice for Determin-
crosslinking and fabrication of silicone materials is found in ing Ethylene Oxide in Medical Devices
Part II of this guide. ANSI/AAM1 ST30 Determining Residual Ethylene Chlo-
1.2 Fabrication and properties of elastomers is covered in rohydrin and Ethylene Glycol in Medical Devices
the companion document, F 604, Part II. This monograph AAMI 13409-251 Sterilization of Health Care Products—
addresses only components of uncured elastomers, gels, and Radiation Sterilization—Substantiation of 25kGy as a
foams. Sterilization Dose for Small or Infrequent Production
1.3 Silicone biocompatibility issues can be addressed at Batches
several levels, but ultimately the device manufacturer must AAMI TIRS-251 Microbiological Methods for Gamma Ir-
assess biological suitability relative to intended use. radiation Sterilization of Medical Devices
1.4 Biological and physical properties tend to be more 2.3 Quality Standards4:
reproducible when materials are manufactured in accordance ANSI/ASQC Q9001 Quality Systems—Model for Quality
with accepted quality standards such as ANSI ISO 9001 and Assurance in Design, Development Production, Installa-
current FDA Quality System Regulations/Good Manufacturing tion, and Servicing
Practice Regulations. 21 CFR 820 Quality System Regulation (current revision)
1.5 This standard does not purport to address all of the 21 CFR 210 Current Good Manufacturing Practice in
safety concerns, if any, associated with its use. It is the Manufacturing, Processing, Packing or Holding of Drugs;
responsibility of the user of this standard to establish appro- General (current revision)
priate safety and health practices and determine the applica- 21 CFR 211 Current Good Manufacturing Practice for
bility of regulatory limitations prior to use. Users are also Finished Pharmaceuticals (current revision)
advised to refer to Material Safety Data Sheets provided with
uncured silicone components. 2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
1
This specification is under the jurisdiction of ASTM Committee F04 on the ASTM website.
3
Medical and Surgical Materials and Devices and is the direct responsibility of Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
Subcommittee F04.11 on Polymeric Materials. 4th Floor, New York, NY 10036.
4
Current edition approved Mar. 1, 2005. Published March 2005. Originally Available from U.S. Government Printing Office Superintendent of Documents,
published in 2000. Last previous edition approved in 2000 as F 2038 – 00e1. 732 N. Capitol St., NW, Mail Stop: SDE, Washington, DC 20401.

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.

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 F 2038 – 00 (2005)
3. Terminology 3.2.10.1 high consistency rubbers (HCRS)—are materials
3.1 Additional pertinent definitions can be found in Termi- which cannot be pumped by conventional pumping equipment.
nology D 1566. They normally must be processed using high shear equipment
such as a two-roll mill and parts are typically fabricated using
3.2 Definitions:
compression or transfer molding techniques.
3.2.1 silicone polymer—polymer chains having a backbone
3.2.10.2 low consistency rubbers or liquid silicone rubbers
consisting of repeating silicon-oxygen atoms where each
(LSRS)—are normally flowable materials which can be readily
silicon atom bears two organic groups. The organic groups are
pumped. They can be mixed by pumping through static mixers
typically methyl, but can be vinyl, phenyl, fluorine, or other
and parts can be fabricated using injection molding techniques.
organic groups.
3.2.10.3 RTVs (room temperature vulcanization)— are one-
3.2.2 cyclics and linears—low molecular weight volatile
part elastomers which cure in the presence of atmospheric
cyclic siloxane species are referred to using the “D” nomen-
moisture. Little, if any, acceleration of cure rate is realized by
clature which designates the number of Si-O linkages in the
increasing temperature. Because cure is dependent upon diffu-
material (usually D4-D20); species from D7 to D 40(or more)
sion of water into the elastomer, cure in depths greater than
may be called “macrocyclics”. Linears are straight chain
0.25 in. (0.635 cm) is not recommended.
oligomers that may be volatile or of higher molecular weight,
depending on chain length; they are designated by “M” and 3.2.10.4 gels—are lightly crosslinked materials having no
“D” combinations, where “M” is R3Si-O, and D is as explained or relatively low levels of reinforcement beyond that provided
above; “R” is usually methyl. (For example, MDM is (CH3) by the crosslinked polymer. They are usually two-part formu-
3SiOSiOSi(CH3)3). Low molecular weight species are present
lations utilizing a platinum catalyzed addition cure system. The
in silicone components to varying degrees depending on hardness of the gel can be adjusted within wide limits. The
process and storage. The levels of macrocyclics that can be material is not usually designed to bear heavy loads but rather
removed from silicone polymers by vacuum, high temperature to conform to an irregular surface providing intimate contact.
stripping, or oven post-cure is dependent on the conditions As a result, loads are distributed over a wider area. These
used. materials may also be used to provide protection from envi-
ronmental contaminants.
3.2.3 catalyst—a component of a silicone elastomer formu-
lation that initiates the crosslinking reaction when the material 3.2.10.5 foams—are crosslinked materials which have a
is vulcanized. component added to them that generates a volatile gas as the
material is being vulcanized. This results in a material with a
3.2.4 crosslinker or crosslinking agent—a component of a
very low density. These are usually two-part formulations
silicone elastomer that is a reactant in the crosslinking reaction
utilizing a platinum catalyzed addition cure system. They
that occurs when an elastomer is vulcanized.
conform to an irregular surface as they expand to provide
3.2.5 inhibitor—a component of a silicone elastomer added intimate contact and protection from the environment but are
to moderate the rate of the crosslinking reaction. more rigid and provide more strength than gels. Since foams
3.2.6 filler—a finely divided solid that is intimately mixed are expanded elastomers, on a weight basis they are highly
with silicone polymers during manufacture to achieve specific crosslinked relative to gels. Most cure conditions will result in
properties. The fillers used in silicone elastomers are one of a closed cell foam.
two types: 3.2.11 lot or batch—a quantity of material made with a
3.2.6.1 reinforcing fillers—usually have high surface areas fixed, specified formulation in a single, manufacturing run
and are amorphous in nature such as fumed or precipitated carried out under specific processing techniques and condi-
silica. Such fillers impart high strength and elastomeric physi- tions.
cal properties to the elastomer. 3.2.12 vulcanization—an irreversible process in which co-
3.2.6.2 extending fillers—typically have lower surface area valent chemical bonds are formed between silicone polymer
and lower cost than reinforcing fillers. They include crystalline chains. During vulcanization, the material changes from a
forms of silica and diatomaceous earths. While they provide flowable or moldable compound to an elastomeric material
some reinforcement, because they are relatively inexpensive, which cannot be reshaped except by its physical destruction.
they are used primarily to extend the bulk of the silicone. 3.2.13 types of cure—based upon the cure chemistry em-
3.2.7 additives—a component of a silicone elastomer used ployed, silicone elastomers used in medical applications fall
in relatively small amounts to perform functions such as into one of three categories: condensation cure, peroxide cure,
marking, coloring, or providing opacity to the elastomer. and addition cure.
3.2.8 silicone base—a uniformly blended mixture of sili- 3.2.13.1 condensation cure—these materials liberate an or-
cone polymers, fillers, and additives which does not contain ganic leaving group during curing and are normally catalyzed
crosslinkers or catalyst. by an organometallic compound.
3.2.9 uncured elastomer—a silicone base which contains one-part—material supplied ready to use in an air tight
crosslinker and/or catalyst but has not been vulcanized. container which cures upon exposure to atmospheric moisture.
3.2.10 silicone elastomer—an uncured elastomer that has The material cures from the surface down and cure depths of
been subjected to conditions which cause it to become greater than about 0.25 inches (0.635 cm) are not practical.
crosslinked. Elastomers may be either high consistency rub- two-part—material supplied in two separate containers
bers, low consistency rubbers, or RTVs (see below). which must be intimately mixed in the prescribed proportions

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 F 2038 – 00 (2005)
shortly before use. Because they do not rely upon dispersion of cally contain from 0.5 to 2.0 weight percent organotin com-
atmospheric moisture into the silicone, the cure depth is not pound. The ligands attached to tin will be some combination of
limited. alkyl groups, alkoxy groups, or the anions of a carboxylic acid.
3.2.13.2 peroxide cure—one-part formulations vulcanized 5.1.3 Crosslinker or crosslinking agent:
by free radicals generated by the decomposition of an organic 5.1.3.1 Two-part, addition cure formulation—the
peroxide. crosslinker is a polymer of the structure shown in Fig. 2 where
3.2.13.3 addition cure—two-part elastomers which must R is generally a methyl or a hydrogen group such as to provide
first be mixed together and then cure by addition of a at least 2.0 SiH groups per chain and x and y are integers
silylhydride to a vinyl silane in the presence of a platinum greater than or equal to zero. In order to avoid chain extension,
catalyst. the functionality of either the vinyl-containing polymer or the
3.2.14 dispersion—an uncured silicone elastomer dispersed SiH-containing crosslinker must be at least 3.0.
in a suitable solvent to allow application of a thin layer of Because of the limitless possibilities for the structure of both
elastomer to a substrate by either dipping or spraying. the crosslinker and the functional (vinyl containing) polymer, it
would be meaningless to define a weight range for the level of
4. Significance and Use crosslinker in a formulation. However, the amount of
4.1 This guide is intended to provide guidance for the crosslinker will typically be sufficient to provide a stoichio-
specification and selection of silicone materials for medical metric excess of SiH groups over the amount of unsaturated
device applications. alkyl groups when the 2 components (parts) of the addition
4.2 Silicone manufacturers supplying materials to the medi- cure silicone elastomer are mixed together in the manufactur-
cal device industry should readily provide information regard- er’s recommended ratio.
ing non-proprietary product formulation to their customers 5.1.3.2 One-part RTVs and two-part addition cure
either directly, or through the US FDA master file program. formulations—the crosslinker may be an organosilane mono-
mer of the general formula:
5. Formulation
RxSi~OR’!42x (1)
5.1 Elastomers, gels, and foams shall be manufactured using
formulations containing combinations of the following raw where:
materials. R = organic group excluding phenyl
5.1.1 silicone polymer—any polymer of medium or high OR’ = hydrolyzable group such as alkoxy, acetoxy, ke-
molecular weight of the structure shown in Fig. 1 where R is a toximo, etc.
methyl, an unsaturated alkyl group or a hydroxy group, R is 5.1.3.3 Peroxide vulcanized elastomers—organic peroxides
generally a methyl or an unsaturated alkyl group but may also comprise a third type of crosslinking agent which participates
be a phenyl, trifuoropropyl, or other hydrocarbon radical, and in the crosslinking reaction that does not become directly
x and y are integers greater than or equal to zero. At least 2.0 incorporated into the crosslinked network. Peroxide levels
alkenyl groups must exist per chain if R is not a hydroxy group. range from less than a percent to as high as a couple of weight
5.1.2 catalyst—an organometallic complex of platinum or percent in the total formulation. These peroxides decompose, at
tin bonded to ligands made of any suitable combination of a rate which is dependent upon the temperature, to form
elements such as carbon, hydrogen, oxygen, fluorine and radicals which then abstract hydrogen atoms from some of the
silicon. alkyl groups attached to the silicone backbone. Recombination
5.1.2.1 platinum—this catalyst may be dispersed in a sili- of these radicals results in the formation of a crosslinked
cone polymer of the structure shown in Fig. 1 having a silicone network. One commonly used peroxide is 2,4,-
viscosity low enough that the resulting dispersion is easily dichlorobenzoyl peroxide. Decomposition of this peroxide
pourable. Platinum catalysts can be used in the range of 5 to 20 results in the formation of small amounts of polychlorinated
ppm of active platinum but typically are present at about 7.5 biphenyls and other catalyst decomposition by-products which
ppm. must be, and are, removed from the cured elastomer during
5.1.2.2 tin—one-part condensation cure formulations will post-curing.
typically contain from 0.1 to 0.5 wt percent of an organotin 5.1.4 Filler—a high purity amorphous silica commercially
compound. Two-part condensation cure formulations will typi- known as fumed or precipitated silica. This silica can be treated
with a silane of the formula Me3SiX or Me2SiX 2 where X is
a hydrolyzable group or treated with one or more polysiloxane
oligomers of the formula HOMe2SiO(SiMe2O)x (SiMeRO)

FIG. 1 Typical Polymeric Silicone Dispersing Agent FIG. 2 Typical Polymeric Crosslinker Agent

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 F 2038 – 00 (2005)
ySiMe2OH where R is a methyl or an unsaturated alkyl group. 7.2 Each type of filler has its own associated hazards.
Filler will be added at an appropriate level to provide the Potential respiratory effects are typically of no concern, since
desired physical property profile. This level will be dependent fillers cannot easily become airborne from compounded mate-
upon the type of treatment, level of treatment, and surface area rials.
of the fumed silica. Alternatively the filler may be a crystalline 7.3 Catalysts may have some toxicological effects, depend-
quartz compound such as sand which has been ground to a fine ing on concentration, form and type. Inhibitors are usually
powder. Diatomaceous earths which contain approximately volatilized or incorporated into the elastomeric network during
88% silica are also sometimes used. cure, or are at levels generally considered to have negligible
5.1.4.1 plasticizers/filler treating agents—materials added toxicological effects.
during compounding of an elastomer formulation to pacify the 7.4 Suitability for intended use (that is, biocompatibility)
surface of fumed silica. These materials stabilize elastomer must be determined on final devices tested in their intended
properties and allow higher silica loadings, and therefore better applications. Information about the biocompatibility of cured
reinforcement, to be attained. Filler treating agents are typi- silicone elastomers is most relevant to the selection of device
cally silanol ended polydiorganosiloxanes or siloxanes which constituents, and is specific to preparation, sterilization, and
can be hydrolyzed to form silanols as described in 5.1.4. These testing conditions.
materials are used to treat, or pacify, the highly reactive surface 7.5 Where uncured silicone elastomers are applied to the
body directly and cured in situ, consideration must be given to
of fumed silica particles. Treating occurs by either hydrogen
formulation components and cure by-products. Some of these
bonding or chemical reaction with the reactive silanol groups
elastomers utilize cure systems, which generate organic com-
on the surface of the silica to form covalent bonds. By
pounds, which can cause local tissue irritation.
preferentially reacting filler treating agents with the silica
7.6 Toxicological test data on uncured materials applies
surface, similar interactions with the higher molecular weight
only when all formulating and manufacturing starts with
base polymer are avoided, thereby minimizing creping of the
specified ingredients, and is accomplished in accordance with
uncured elastomer and changes in hardness of the cured
accepted quality standards such as ISO 9001 and current
elastomer that would otherwise occur. Quality System Regulations/Good Manufacturing Practices
5.1.5 Additives—any of a class of materials usually com- (GMP) regulations promulgated by the FDA.
prised of salts or oxides of metals such as barium, titanium, or
calcium. 8. Sterilization
5.1.6 Inhibitor—a substance or mixture of substances ca- 8.1 Manufacturers of uncured silicone elastomers may sup-
pable of reducing the rate of crosslinking. It typically contains ply such materials sterile or may want to advise fabricators on
an unsaturated alkyl group, capable of reacting with the sterilization methods. These methods should be validated
crosslinker via a hydrosilylation (addition) reaction. It is before use.
composed of any suitable combination of elements such as 8.2 Ethylene oxide is highly soluble in silicone. Those users
carbon, hydrogen, oxygen, and silicon. sterilizing with ethylene oxide must do testing to ensure
5.1.7 Solvent—a liquid used to form a dispersion by mixing acceptable levels of residues if such sterilized material is used
with an uncured elastomer. It typically is toluene or xylene but as is (see references). Cell culture tests, such as Practice F 813,
may be composed of any suitable combination of the following may be used to show absence of sterilant residues.
elements: carbon, hydrogen, oxygen, and possibly silicon. 8.3 Autoclave sterilization of uncured elastomers, gels, and
foams is not typically used and is not recommended because it
may result in fabrication difficulties. Specific details relating to
6. Packaging, Labeling, and Storage
autoclaving will not be discussed here because it is more
6.1 Uncured silicone elastomer components for use in typically performed on devices made from fabricated elas-
medical applications shall be supplied in proper packaging to tomers.
prevent their contamination during typical conditions of ship- 8.4 Radiation sterilization of uncured elastomers, gels and
ment and storage, as well as their adulteration from the foams is not typically used and is not recommended. Addi-
package itself. tional information about radiation sterilization is available in
6.2 All packages shall be labeled so as to identify the Part II.
manufacturer, specific product name, and lot or batch number.
9. Quality Control Provisions
6.3 The material supplier shall provide information regard-
9.1 Silicone elastomers should be designed and manufac-
ing recommended material storage conditions and product
tured utilizing quality control programs such as that discussed
warranties.
in ANSI/ASQC CI (Specification of General Requirements for
a Quality Program), preferably in accordance with ISO 9001
7. Health and Safety
and ISO 9002 standards. Batch-to-batch consistency/
7.1 Because this guide applies only to uncured silicone acceptability can also be monitored by matching product
elastomers, and most silicone elastomers are cured at the time performance to lot acceptance requirements, providing these
of end use, most health and safety concerns related to the use are specific and reasonably narrow.
of these materials are those encountered during handling; 9.2 Manufacturers of uncured silicone elastomers will in-
namely eye, skin, and respiratory exposure. form customers of changes in formulation, test methods,

4
 F 2038 – 00 (2005)
specifications or packaging. Details of the changes and a means AAMI 13409-251 Sterilization of Health Care Products—Radiation Sterilization—
to identify when each change occurred shall be provided. Substantiation of 25kGy as a Sterilization Dose for Small or Infrequent
Production Batches
9.3 Sterilization will be performed using quality standards AAMI TIR8-251 Microbiological Methods for Gamma Irradiation Sterilization of
such as: Medical Devices
ANSI/AAMI ST46 Good Hospital Practice: Steam Sterilization and Sterility
Assurance 10. Keywords
ANSI/AAMI ST41 Good Hospital Practice: Ethylene Oxide Sterilization and
Sterility Assurance 10.1 elastomer; foam; gel; HCR; high consistency rubber;
ANSI/AAMI ST50 Dry Heat (Heated Air) Sterilizers
ANSI/AAMI ST29 Recommended Practice for Determining Ethylene Oxide in liquid silicone rubber; LSR; moisture cure; medical device
Medical Devices material; peroxide cure; platinum cure; RTV
ANSI/AAMI ST30 Determining Residual Ethylene Chlorohydrin and Ethylene
Glycol in Medical Devices

APPENDIX

(Nonmandatory Information)

X1. RATIONALE

X1.1 Medical devices made from silicone elastomer are tion. The codes previously used in this guide were not widely
widely used in the care of public health and have a history of accepted by manufacturers, and therefore the monograph had
biocompatibility in many applications. This guide educates the minimal utility. The information provided here (and some
user as to the formulation of such elastomers, the first level at suggestions for further investigation) at least provide a starting
which biocompatibility of the ultimate device is affected. Also point from which the user can seek guidance on the biological
impacting the biocompatibility of the finished device is the impact of silicone formulations. Manufacturers’ responsibili-
fabrication of the silicone elastomer; fabrication is addressed in ties as defined here are now or are expected to be practiced in
the companion standard. the industry; manufacturers can be differentiated on the basis of
X1.2 The previous version of this guide has now been split the information they provide on the topics introduced herein.
into two parts, one addressing formulation, and one, fabrica-

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