ASTM F3268-18a

Standard Guide for <emph type="bdit">in vitro</emph> Degradation Testing of Absorbable Metals

SIGNIFICANCE AND USE
5.1 This standard provides an itemization of potential in vitro test methods to evaluate the degradation of absorbable metals. The provided approach defers to the user of this standard to pick most appropriate method(s) based on the specific requirements of the intended application. However, a minimum of at least two different corrosion evaluation methods is considered necessary for basic profiling of the material or device, with additional methods potentially needed for an adequate characterization. However, in some instances there may be only one method that correlates to in vivo degradation results.  
5.2 It is recognized that not all test methods will be meaningful for every situation. In addition, some methods carry different potential than others regarding their relative approximation to the in vivo conditions within which actual use is to occur. As a result, some discussion and ranking of the relevance of the described methods is provided by this guidance.  
5.3 It should be noted that degradation of absorbable metals is not linear. Thus, precautions should be taken that evaluations of the degradation profile of a metal or metal device are appropriately adapted to reflect the varying stages and rates of degradation. Relevant factors can include the amount or percentage (%) of tissue coverage of the implanted device and the metabolic rate of surrounding tissue, which is not necessarily accompanied by a high perfusion rate.  
5.4 It is recognized that in vivo environments will impart specialized considerations that can directly affect the corrosion rate, even when compared with other in vivo locations. Thus, a basic understanding of the biochemistry and physiology of the specific targeted implant location (e.g. hard tissue; soft tissue; high, low or zero perfusion areas/tissue; high, low or zero loading environments) is needed to optimize in vitro and in vivo evaluations.  
5.5 Within the evaluation of absorbable metals, rate uniformity is considered to be ...
SCOPE
1.1 The purpose of this standard is to outline appropriate experimental approaches for conducting an initial evaluation of the in vitro degradation properties of a device or test sample fabricated from an absorbable metal or alloy.  
1.2 The described experimental approaches are intended to control the corrosion test environment through standardization of conditions and utilization of physiologically relevant electrolyte fluids. Evaluation of a standardized degradation control material is also incorporated to facilitate comparison and normalization of results across laboratories.  
1.3 The obtained test results may be used to screen materials and/or constructs prior to evaluation of a more refined fabricated device. The described tests may also be utilized to define a device’s performance threshold prior to more extensive in vitro performance evaluations (e.g. fatigue testing) or in vivo evaluations.  
1.4 This standard is considered to be applicable to all absorbable metals, including magnesium, iron, and zinc-based metals and alloys.  
1.5 The described tests are not considered to be representative of in vivo conditions and could potentially provide a more rapid or slower degradation rate than an absorbable metal’s actual in vivo corrosion rate. The herein described test methods are to be used for material comparison purposes only and are not to act as either a predictor or substitute for evaluation of the in vivo degradation properties of a device.  
1.6 This standard only provides guidance regarding the in vitro degradation of absorbable metals and does not address any aspect regarding either in vivo or biocompatibility evaluations.  
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limit...

General Information

• Status: Published
• Publication Date: 30-Sep-2018
• ICS: 77.040.30 - Chemical analysis of metals
• ICS: 77.060 - Corrosion of metals
• Technical Committee: F04 - Medical and Surgical Materials and Devices
• Drafting Committee: F04.15 - Material Test Methods

Relations

• Refers: ASTM F2739-19 - Standard Guide for Quantifying Cell Viability and Related Attributes within Biomaterial Scaffolds
• Effective Date: 01-Sep-2019
• Refers: ASTM G3-14(2019) - Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing
• Effective Date: 01-May-2019
• Refers: ASTM G16-13(2019) - Standard Guide for Applying Statistics to Analysis of Corrosion Data
• Effective Date: 15-Feb-2019
• Refers: ASTM F2129-19a - Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices
• Effective Date: 15-Jan-2019
• Refers: ASTM F2129-19 - Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices
• Effective Date: 01-Jan-2019
• Refers: ASTM F2129-17b - Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices
• Effective Date: 01-Dec-2017
• Refers: ASTM F2129-17a - Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices
• Effective Date: 15-Nov-2017
• Refers: ASTM G215-17 - Standard Guide for Electrode Potential Measurement
• Effective Date: 01-Jan-2017
• Refers: ASTM F2129-17 - Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices
• Effective Date: 01-Jan-2017
• Refers: ASTM F2739-16 - Standard Guide for Quantifying Cell Viability within Biomaterial Scaffolds
• Effective Date: 01-Oct-2016
• Refers: ASTM G215-16 - Standard Guide for Electrode Potential Measurement
• Effective Date: 01-May-2016
• Refers: ASTM F1854-15 - Standard Test Method for Stereological Evaluation of Porous Coatings on Medical Implants (Withdrawn 2024)
• Effective Date: 15-Mar-2015
• Refers: ASTM F2129-15 - Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices
• Effective Date: 01-Mar-2015
• Refers: ASTM G3-14 - Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing
• Effective Date: 15-Dec-2014
• Refers: ASTM G16-13 - Standard Guide for Applying Statistics to Analysis of Corrosion Data
• Effective Date: 01-Dec-2013

Overview

ASTM F3268-18a - Standard Guide for in vitro Degradation Testing of Absorbable Metals outlines recommended methods for the controlled laboratory evaluation of corrosion and degradation processes in absorbable metallic materials, such as magnesium, iron, and zinc-based alloys. This guide serves as a foundational document for researchers and manufacturers involved in the development, characterization, and comparison of absorbable metals intended for medical devices and implants. ASTM F3268-18a emphasizes the significance of simulating physiologically relevant conditions, adopting appropriate corrosion testing methodologies, and ensuring reproducibility and comparability of results across different laboratories and applications.

Key Topics

• Selection of Test Methods:

  • The standard provides a list of possible in vitro corrosion evaluation techniques, allowing users to select methods best suited to their application. At least two distinct corrosion evaluation techniques are recommended for a comprehensive material profile.
  • Not all methods are equally suitable for every situation; relevance to the real-life application and correlation to in vivo behavior are important considerations.

• Control of Experimental Conditions:

  • Physiologically relevant electrolyte fluids and controlled environments (including pH, temperature, and fluid flow) are crucial to ensure meaningful and repeatable results.
  • Electrolyte composition, fluid movement, and atmospheric conditions are significant factors that can influence degradation rates.

• Degradation Control Material:

  • Using a standardized degradation control material helps normalize experimental setups and results, enabling better comparison across laboratories.

• Test Result Applications:

  • Results from in vitro degradation tests can be used to screen materials, evaluate constructs prior to advanced performance testing, or set initial device performance thresholds.
  • The standard stresses that in vitro tests are not precise predictors of in vivo corrosion rates, but are valuable for comparative material selection and preliminary development stages.

• Comprehensive Material Characterization:

  • Full compositional and morphological characterization of the test material is necessary prior to corrosion testing.
  • Consideration of device porosity and microstructure is crucial, as these factors impact degradation.

Applications

The practical value of ASTM F3268-18a lies in its wide applicability within biomedical engineering and medical device manufacturing:

• Medical Device Development:

  • Supports the development of absorbable implants by enabling controlled laboratory screening of metals for coronary stents, bone screws, pins, orthopedic implants, and similar applications.
  • Facilitates early-stage material selection and optimization before extensive in vivo testing.

• Regulatory Submissions & Quality Assurance:

  • Provides a standardized test framework for generating reproducible data necessary for regulatory documentation and quality management.
  • Helps ensure that test results are comparable, facilitating communication with regulatory bodies.

• Research and Material Comparison:

  • Assists researchers in benchmarking new absorbable metal alloys against existing materials under standardized in vitro conditions.
  • Promotes exchange and normalization of results between laboratories and across international borders, advancing the science of medical biomaterials.

Related Standards

ASTM F3268-18a references and complements several other recognized standards for corrosion testing, material characterization, and medical device evaluation, including:

• ASTM G31: Laboratory Immersion Corrosion Testing of Metals
• ASTM G1: Preparing, Cleaning, and Evaluating Corrosion Test Specimens
• ASTM F2129: Conducting Cyclic Potentiodynamic Polarization Measurements
• ASTM F3160: Metallurgical Characterization of Absorbable Metallic Materials for Medical Implants
• ASTM G59/G102/G106: Electrochemical measurement procedures for corrosion analysis
• ISO 10993-15: Biological evaluation of medical devices - Identification and quantification of degradation products from metals and alloys
• ISO 13485: Medical devices - Quality management systems

By following ASTM F3268-18a in conjunction with these related standards, organizations can ensure robust, reliable, and internationally recognized in vitro testing of absorbable metals, thereby advancing the safe and effective use of bioabsorbable metallic implants.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: F3268 − 18a
Standard Guide for
in vitro Degradation Testing of Absorbable Metals
This standard is issued under the fixed designation F3268; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.8 This international standard was developed in accor-
dance with internationally recognized principles on standard-
1.1 The purpose of this standard is to outline appropriate
ization established in the Decision on Principles for the
experimentalapproachesforconductinganinitialevaluationof
Development of International Standards, Guides and Recom-
the in vitro degradation properties of a device or test sample
mendations issued by the World Trade Organization Technical
fabricated from an absorbable metal or alloy.
Barriers to Trade (TBT) Committee.
1.2 The described experimental approaches are intended to
control the corrosion test environment through standardization
2. Referenced Documents
of conditions and utilization of physiologically relevant elec-
2.1 ASTM Standards:
trolyte fluids. Evaluation of a standardized degradation control
B943 Specification for Zinc and Tin Alloy Wire Used in
material is also incorporated to facilitate comparison and
Thermal Spraying for Electronic Applications
normalization of results across laboratories.
B954 Test Method for Analysis of Magnesium and Magne-
1.3 Theobtainedtestresultsmaybeusedtoscreenmaterials
sium Alloys by Atomic Emission Spectrometry
and/or constructs prior to evaluation of a more refined fabri-
E2375 Practice for Ultrasonic Testing of Wrought Products
cated device. The described tests may also be utilized to define
F1854 Test Method for Stereological Evaluation of Porous
a device’s performance threshold prior to more extensive in
Coatings on Medical Implants
vitro performance evaluations (e.g. fatigue testing) or in vivo
F2129 Test Method for Conducting Cyclic Potentiodynamic
evaluations.
Polarization Measurements to Determine the Corrosion
Susceptibility of Small Implant Devices
1.4 This standard is considered to be applicable to all
F2739 Guide for Quantifying Cell Viability and Related
absorbable metals, including magnesium, iron, and zinc-based
Attributes within Biomaterial Scaffolds
metals and alloys.
F3160 Guide for Metallurgical Characterization of Absorb-
1.5 The described tests are not considered to be representa-
able Metallic Materials for Medical Implants
tive of in vivo conditions and could potentially provide a more
G1 Practice for Preparing, Cleaning, and Evaluating Corro-
rapid or slower degradation rate than an absorbable metal’s
sion Test Specimens
actual in vivo corrosion rate.The herein described test methods
G3 Practice for Conventions Applicable to Electrochemical
are to be used for material comparison purposes only and are
Measurements in Corrosion Testing
nottoactaseitherapredictororsubstituteforevaluationofthe
G4 Guide for Conducting Corrosion Tests in Field Applica-
in vivo degradation properties of a device.
tions
1.6 This standard only provides guidance regarding the in
G16 Guide for Applying Statistics to Analysis of Corrosion
vitro degradation of absorbable metals and does not address
Data
any aspect regarding either in vivo or biocompatibility evalu-
G31 Guide for Laboratory Immersion Corrosion Testing of
ations.
Metals
G46 Guide for Examination and Evaluation of Pitting Cor-
1.7 This standard does not purport to address all of the
rosion
safety concerns, if any, associated with its use. It is the
G59 Test Method for Conducting Potentiodynamic Polariza-
responsibility of the user of this standard to establish appro-
tion Resistance Measurements
priate safety, health, and environmental practices and deter-
G102 Practice for Calculation of Corrosion Rates and Re-
mine the applicability of regulatory limitations prior to use.
lated Information from Electrochemical Measurements
This guide is under the jurisdiction of ASTM Committee F04 on Medical and
Surgical Materials and Devices and is the direct responsibility of Subcommittee
F04.15 on Material Test Methods. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Oct. 1, 2018. Published November 2018. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 2018. Last previous edition approved in 2018 as F3268–18. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/F3268-18A. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F3268 − 18a
G106 Practice for Verification of Algorithm and Equipment carbonate buffer or if oxygen in the electrolyte is consumed
for Electrochemical Impedance Measurements during the corrosion process, as with iron-based alloys. Mea-
G215 Guide for Electrode Potential Measurement surement and control of the atmospheric composition may be
2.2 DIN Standards: important, depending on the specific circumstances of the
experiment.
DIN 50918 Elektrochemische Korrosionsuntersuchungen.
Deutsche Normen. Berlin: Beuth Verlag; 1978. p. 1-6 4.1.4 Measurements of corrosion may include weight loss
of the sample, accumulation of corrosion products in the
2.3 ISO Standards:
experiment, generation of H gas, and changes to physical and
ISO 10993-15 Biological evaluation of medical devices Part
mechanical properties.
15: Identification and quantification of degradation prod-
ucts from metals and alloys
4.2 Electrochemical methods, Polarization Resistance, and
ISO 13485 Medical devices – Quality management systems
Electrochemical Impedance Spectroscopy also can be used to
– Requirements for regulatory purposes
measurerelativecorrosionratesandgenerateadditionalinsight
into the corrosion process. The electrolyte used in these
3. Terminology
methods may not be relevant to in vivo conditions and may not
3.1 Definitions:
mimic the process in vivo. It is important to fully document
3.1.1 absorbable, adj—in the body, referring to an initially
relevant experimental conditions (e.g. electrolyte composition,
distinct foreign material or substance that either directly or
current, current density and atmosphere), so that their impact
through intended degradation can be excreted, metabolized or
on the test results can be understood.
assimilated by cells and/or tissue.
4.3 Use of a degradation control material to monitor the
3.1.2 surface roughness, R ,n—the arithmetic average de-
A consistency of the experimental system is recommended, but
viation of the surface profile from the centerline, normally
not mandatory. See Annex A1 for details.
reported in micrometers.
5. Significance and Use
3.2 Definitions of Terms Specific to This Standard:
5.1 This standard provides an itemization of potential in
3.2.1 degradation, n—the breakdown of a metallic test
material or metallic device principally due to corrosion in an vitro test methods to evaluate the degradation of absorbable
metals. The provided approach defers to the user of this
electrolyte solution relevant to physiologic conditions.
standard to pick most appropriate method(s) based on the
3.2.2 degradation control material, n—multiplebatchesofa
specific requirements of the intended application. However, a
defined metallic compositon with sufficiently uniform corro-
minimum of at least two different corrosion evaluation meth-
sion properties to verify an experimental setup and to compare
ods is considered necessary for basic profiling of the material
relative intra-laboratory and/or inter-laboratory corrosion rates.
or device, with additional methods potentially needed for an
adequate characterization. However, in some instances there
4. Summary of Guide
may be only one method that correlates to in vivo degradation
4.1 Guidance is given on in vitro evaluation of the
results.
corrosion/degradation properties of absorbable metal materials
and devices fabricated from absorbable metals. Considerations 5.2 It is recognized that not all test methods will be
meaningful for every situation. In addition, some methods
specific to the application of corrosion testing methods to
absorbablemetalmaterialsareoutlinedforbothimmersionand carry different potential than others regarding their relative
electrochemical methods. approximationtothe in vivoconditionswithinwhichactualuse
4.1.1 Electrolytecompositionisacriticalfactorincorrosion is to occur. As a result, some discussion and ranking of the
experiments. Several electrolytes are commonly used to mimic relevance of the described methods is provided by this guid-
ance.
in vivo conditions. Electrolyte selection may also take into
consideration the alloy being tested.
5.3 It should be noted that degradation of absorbable metals
4.1.2 Control of the experimental conditions (i.e.,
isnotlinear.Thus,precautionsshouldbetakenthatevaluations
temperature, pH and fluid movement around the test piece(s))
of the degradation profile of a metal or metal device are
can markedly affect the corrosion rates and experimental
appropriately adapted to reflect the varying stages and rates of
outcomes. Controlling and documenting these factors are
degradation. Relevant factors can include the amount or
important with regard to generating consistent, reproducible
percentage (%) of tissue coverage of the implanted device and
results. Experimental conditions may be altered, depending on
the metabolic rate of surrounding tissue, which is not neces-
the intent of the experiment.
sarily accompanied by a high perfusion rate.
4.1.3 The surrounding atmosphere may interact with the
5.4 It is recognized that in vivo environments will impart
electrolyte solution (liquid-gas interface), depending on elec-
specialized considerations that can directly affect the corrosion
trolyte composition, particularly if the electrolyte contains a
rate, even when compared with other in vivo locations. Thus, a
basic understanding of the biochemistry and physiology of the
Available from Deutsches Institut für Normung e.V.(DIN), Am DIN-Platz,
specific targeted implant location (e.g. hard tissue; soft tissue;
Burggrafenstrasse 6, 10787 Berlin, Germany, http://www.din.de.
high, low or zero perfusion areas/tissue; high, low or zero
Available from International Organization for Standardization (ISO), ISO
loading environments) is needed to optimize in vitro and in
Central Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier,
Geneva, Switzerland, http://www.iso.org. vivo evaluations.
F3268 − 18a
5.5 Within the evaluation of absorbable metals, rate unifor- lipids [see references (1-7)]. As a result, solutions with a
mity is considered to be the principle concern and design goal. physiologically relevant combination of electrolytes should be
The recognized primary value for the herein described in vitro used.
testing under static (i.e. not dynamic) conditions is to monitor
NOTE 1—If the intention of the experiment is to provide an in vitro
and screen materials and/or devices for their corrosion consis-
approximation to an in vivo system, the use of a well-controlled, simpler
tency. Such an evaluation may provide a practical understand-
electrolyte system that has been correlated to in vivo data may be
preferable to a more complex, less stable system.
ing of the uniformity of the device prior to any subsequent in
vivo testing - where device consistency is considered to be
7.2.2 Numerous formulations exist for simulated body flu-
critical for optimizing the quality of the obtained observations.
ids(SBFs)orbufferingsolutionsthatareintendedtomimicthe
in vivo condition. Hank’s Solution, which is phosphate-based
5.6 Once a suitable level of device corrosion consistency
and designed to buffer in a normal atmosphere, provides an
has been established (either directly or historically), static
approximation of the electrolyte composition found in the
and/or dynamic fatigue testing can then be undertaken, if
body. However, while it does provide a reasonable approxima-
needed, to further enhance the understanding of the corrosion
tion of inorganic moieties, it does NOT provide the body’s
processwithinthecontextofthedevice’soveralldesignandits
buffer capacity (as enhanced through carbonate equilibria)or
intended application/use.
the presence of a myriad of organic molecules – many of
5.7 Depending on the intended application, appropriate
which, particularly proteins, can be expected to adsorb to the
levels of implant loading may range from minimal to severe.
implant surface and further affect the degradation rate. Table 1
Thus, this standard does NOT directly address the appropriate
defines the main ions in several common SBF solutions.
level of loading of absorbable metallic devices, guidance for
7.2.3 Additional factors to consider in electrolyte solutions
which may be found in documents specific to the intended
are the levels of dissolved O and CO , which, depending on a
2 2
implant application and the design requirements for the prod-
particular metal’s composition and corrosion mechanism, can
uct.
significantly affect the degradation rate. The reduced or zero
5.8 This standard does NOT directly address dynamic
level of bicarbonate within Hank’s Solution limits its ability to
fatigue testing of absorbable metallic devices.
interact with a CO atmosphere, resulting in a limitation to its
bufferingcapacity.Conversely,biologicalsaltsolutionsthatare
6. Material/Metallurgical Characterization
bicarbonate-based and thereby interact with a CO atmosphere
6.1 A full understanding of the compositional and morpho-
are Earles and possibly Krebs Ringers. However, these bal-
logical features of the material or device to be tested is needed
anced salts and culture media are designed to buffer at either 5
priortoconductinganyinvitrodegradationevaluation.Lackof
or 10% CO , depending on the amount of bicarbonate in-
control of critical material features (e.g. elemental
cluded. This concern is also true for complete medium (e.g.
composition, contamination, grain size, etc.) may lead to
Dulbecco’s Modified Eagle’s Medium (DMEM), Minimum
inconsistent results both in vitro and/or in vivo. Characteriza-
Essential Media (MEM), Roswell Park Memorial Institute
tion of the test material should be undertaken in accordance
Medium 1640 (RPMI 1640), etc.) that introduce additional
with ASTM F3160. constituents, such as amino acids, chemicals, and proteins,
which can impact corrosion rates. Thus it can be surmised that
6.2 Depending on the goals of the experiment, selected
such buffered electrolyte solutions can only approximate the
mechanical tests may be repeated at various intervals during
actual corrosion occurring in vivo and may need adjustment
the corrosion experiment. In most cases, it would be appropri-
during the degradation process to meet the requirements of the
ate to retire mechanically tested samples.
intended evaluation. Buffering capacity can affect results.
7. General Testing Conditions
NOTE 2—The actual in vivo concentration of O and CO may not be
2 2
7.1 The intention of the following listing of general consid- known. However, maintaining control of O and CO in the in vitro
2 2
erations is to provide a fundamental overview of the critical
factors involved with generating consistent in vitro corrosion
characterization results. Hank’s Balanced Salt Solution can be purchased at various concentrations of
calcium,magnesiumandbicarbonate.Bicarbonatebufferingistypically4.2mmol/L
7.2 Fluid Composition:
which is lower than other Salt Solutions.
7.2.1 For all in vitro test systems, fluid composition is a
A
critical factor that requires both control and disclosure.
TABLE 1 Composition of SBF Test Solutions
Additionally, pH (which can be influenced by degradation
Extra-
Solution
Ion
product composition and generation rate), fluid flow, and Cellular
mmol/L
Tyrode’s Hanks’ Ringer’s Isotonic
Fluid
solution buffer capacity are significant variables that can affect
+
Na 149 141 147 154 142
an absorbable metal’s corrosion rate. While it is desirable to
+
K 2.7 5.4 4—5
2+
maintain an in vitro pH at a level that is representative of the
Ca 1.8 1.3 3 — 2.5
2+
Mg 1.0 0.74 0 — 1.5
in vivocondition,itisimportanttonotethatthecompositionof
-
Cl 145 145 157 154 103+
a buffer’s anions can significantly affect the corrosion rate.
A
Table obtained from: Corrosion Mechanisms of Metallic Biomaterials, Barbosa,
Critical electrolytes and biomolecules that are known to
M. (Table 3, p. 239); in Biomaterials Degradation – Fundamental Aspects and
directly affect the corrosion rate of Mg alloys include
Related Clinical Phenomena, Ed. Mario Barbosa, pp. 227-239+ (1991).
phosphate, carbonate, chloride, calcium, serum proteins, and
F3268 − 18a
experiment enhances experimental reproducibility.
ambient concentration and exchange rate of dissolved gasses
can be expected to differ between lung, arterial and intramus-
7.2.4 In summary, the composition of electrolytes and
cular implantation applications.
atmospheres should be tailored to approximate the expected
conditionsintheintendedclinicalapplication,totheextentthat
7.5 Temperature:
they are known.As a result, full composition of the electrolyte
7.5.1 Solution temperature is a critical part of any corrosion
solution and overlying atmosphere shall be included in the
environment (even when used in screening tests), in that
reportalongwithrelevantdetailsregardingtheirreplenishment
elevated temperatures will typically increase reaction rates as
rate.
well as introduce the potential for other chemical reactions.As
7.2.5 For an overview discussion regarding environmental
aresult,(37 61°C)shallbeusedforallprinciplecorrosiontest
factors for absorbable metals, see Section 1.3.2 of Y.F. Zheng
conditions, where achievable, regardless of any
et al. Materials Science and Engineering R 77 (2014) 1-34 (8).
recommendations/requirements described within a cited refer-
ence method. If 6 1 °C is not achievable in a particular
7.3 Fluid Flow:
experimental setup, the temperature shall be maintained within
7.3.1 An additional factor for consideration is the level of
the minimum practical limits to maintain experimental consis-
fluid flow, which can affect both the rate of chemical exchange
tency and reproducibility. Such a universally recognized in
and the adhesion of degradation products. It has been recog-
vivo temperature is considered to broadly represent the physi-
nizedthatflow-inducedshearstresscansignificantlyaccelerate
ological condition and thereby provide the most broadly
degradation of Mg (see reference (8)). Thus, fluid flow should
applicable scientific value. However, testing at other tempera-
be considered as a potentially critical factor when assessing a
tures may also be included to determine differences in reaction
material’s or device’s degradation rate. However, due to the
mechanisms and rates. An additional alternate evaluation
presence of both tissue and proteins, it should be noted that the
temperature may be especially useful if the temperature at the
impact of fluid flow under in vivo conditions may not be as
intended implant application and/or the temperature in a
pronounced as is found in in vitro models (see articles by
particular animal model differs significantly from (37 6 1 °C).
Wittchow (11) and Bowen (12)].
7.3.2 While an active flow environment may not be needed
7.6 Utilization of a Degradation Control Material:
when preliminarily screening materials, a physiologically rel-
7.6.1 In any corrosion evaluation, the experimental rate
evant fluid flow that resembles the targeted application is
observed for a degradation control material may be included,
highly recommended when attempting an approximation of in
both in the test and in the generated report. Such an evaluation
vivo performance. For example, the flow-induced shear stress
of the corrosion rate of a degradation control material allows
on a newly deployed coronary stent may be very different from
for continued monitoring of the consistency of a specific
that of an absorbable screw placed into tibial bone.
corrosion test system. Once included and tested, the experi-
Additionally, an initial perfusion rate may change over time
mentally obtained results can be utilized to demonstrate the
(e.g. from 10 to 2 relative units), which could then potentially
repeatability of the specific test system while simultaneously
alter the related rate of degradation. Also, the additional
providing a comparative control that allows for a normalized
influence of normal cell and protein coverage and any drugs
assessment of results across different test laboratories.
(e.g. anti-proliferatives) or other biological factors that may
7.6.2 In some cases, it may be desirable to use a material
affect cell coverage and/or perfusion at the corrosion interface
already in use for the same or similar product as a control
also requires consideration.
material, particularly if relative differences are of interest. A
7.3.3 Flow induced through use of a shaker table may (or
material for which the experimenter(s) have corrosion perfor-
may not) be adequate to be representative of the in vivo
mance data may be advantageous in some experimental situa-
condition.
tions.
7.4 Atmospheric Composition:
NOTE 3—Caution: Some alloys may not be sufficiently consistent from
7.4.1 Since atmospheric composition can significantly influ-
heat to heat to provide reproducible degradation results. It is up to the user
to verify the suitability of a material as a degradation control material.
ence the electrolyte solution (see prior discussion under fluid
composition), a definable and, if needed, controlled (can be
7.6.3 Guidance and further discussion regarding the appro-
ambient)atmosphereshouldbemaintainedforalltestmethods,
priate specification and manufacturing of a degradation control
including during materials screening.Any selected atmosphere
material may be found in Annex A1.
should be reflective of the intent of the experiment and the
7.6.4 Guidance regarding the appropriate utilization of a
material’s specific degradation considerations. An atmosphere
degradation control material within an experimental environ-
for maintenance of pH through CO / bicarbonate buffering
ment may be found in Annex A2.
may be chosen. For experiments meant to approximate in vivo
conditions, appropriate considerations would result in an at- 7.7 In summary, flow conditions along with electrolytes,
mosphere that is compositionally relevant to both the implant’s atmospheres, temperature and any other conditions relevant to
intended application as well as to the particular metal’s the experiment should be tailored to approximate the expected
projected corrosion chemistry once it is placed in vivo. Se- conditions in the intended clinical application.As a result, full
lected atmospheric compositions may also be reflective of the composition of the electrolyte solution and overlying atmo-
projectedgaseousexchangeratewithintheinvivoenvironment sphere shall be reported along with relevant details regarding
of the intended clinical application. For example, both the flow.
F3268 − 18a
8. Initial Sample Characterization and Preparation pH fluctuations during the course of an experiment. The closer
the initial pH is set to the target value, (i.e., within plus or
8.1 Sample dimensions will be dictated primarily by the
minus 0.1 pH units), the longer the corrosion experiment may
limitations of the selected test method. However, the utilized
stay within predefined limits. If there is justification for a
size and description shall be reported in sufficient detail,
different pH specification for a particular experimental
including surface area and/or other features that may affect the
application, it should take precedence.
results, so that its dimensions and features can be both easily
understood and readily reproduced by others wishing to repeat NOTE 4—Caution: The use of 2-(4-(2-hydroxyethyl)-1-pipeazinyl)
ethanesulfonic acid (HEPES) buffer may negatively affect the corrosion
the evaluation. Thus, information that will allow ready deter-
rates of magnesium alloys (8).
minationofasample’sexternaldimensionsandoverallvolume
shall be reported. Additionally, if the evaluated structure is 9.3.3 Corrosion rates for iron-based alloys are affected by
porous, a description of the porosity and the relative size and the oxygen content of the electrolyte. It may be necessary to
interconnectivity of the pores also should be reported so as to control oxygen saturation to achieve consistent results [ASTM
allow a full understanding of the material’s or device’s G31], (8, 9).
electrolyte-accessible surface-to-volume ratio. ASTM F1854
NOTE 5—Bioreactors are well suited to adequately control corrosion
contains methods useful for evaluating surface porosity.
conditions, with the potential to simultaneously expose the corroding
8.1.1 Sufficient information about the sample shall be re- implant to hard and/or soft tissues SeeASTM F2739 for additional details
on bioreactors and their potential use.
ported such that all pertinent references may be followed, e.g.
ASTM G31 recommends a minimum electrolyte volume to
9.4 Mass Loss—Loss of sample mass can be used as a basic
sample surface area, for which the sample surface should be
measure of corrosion.
known within the limits specified in the reference.
9.4.1 Corrosion products should be removed from the
sample as completely as possible prior to weighing a post-
9. Immersion Corrosion Evaluation corrosion test sample. ASTM G1 and ASTM G31 give guid-
ance on removing corrosion products from test articles prior to
9.1 The intent of an immersion corrosion evaluation is to
weighing.
understand the rate of degradation when an implant or material
is exposed (at rest) to a corrosive environment. As described
NOTE 6—Chromic acid is carcinogenic and, in some jurisdictions, may
be restricted by local regulation.
previously, an approximation of the in vivo flow environment
should be provided if it carries potential for clinical relevance.
9.4.2 Weighing accuracy of the post-corrosion sample
should be equivalent to the accuracy of the initial sample
9.2 Specific methods that guide the observation of passive
weight and sufficiently accurate to assess weight loss.
corrosion in absorbable metal implants are:
9.4.3 ASTM G1 and ASTM G31 provide formulas for
9.2.1 NACE TM0169/ASTM G31 – Laboratory Immersion
converting mass loss to corrosion rate for flat prismatic
Corrosion Testing of Metals.
geometries.
9.2.2 Also worthy of consideration are evaluation ap-
proaches contained withinASTM G1,ASTM G4,ASTM G46, 9.5 Measurement of Corrosion Products—The quantitative
andASTMG102.ISO10993-15addressesmethodsforpassive
evaluation of corrosion products -m may be used as a measure
and polarization corrosion testing. of corrosion if the reactions are well understood.
9.2.3 Guide G16 provides statistical methods appropriate to 9.5.1 Hydrogen Gas Evolution—For magnesium metals and
corrosiondata,includingregressionmethodsfortheanalysisof alloys with relatively short longevities, hydrogen (H ) gas
longitudinal data. The methods may be applied to corrosion evolution can also be considered as a means for determining
rates and/or changes in mechanical or other properties. the corrosion rate or be used to confirm other measures of
corrosion rate.
9.3 Test Solution:
9.5.1.1 This method/approach is applicable when the major-
9.3.1 Preparation and documentation of electrolyte solu-
ity of the alloy evolves hydrogen during the corrosion process
tions is necessary so that it can be accurately reproduced over
(e.g. magnesium and group I and II metals) (16).
multiple test setups, allowing for comparable results across
9.5.1.2 The collected hydrogen volume can be converted to
testingregimesandlaboratories.ASTMG31givesguidanceon
moles after correcting for non-standard temperature, pressure,
electrolyte compositions and documentation.
and the partial (vapor) pressure of electrolyte solution (16).
9.3.2 Small changes in electrolyte composition during the
9.5.1.3 For experiments not taken to complete dissolution,
testperiod,byeitherdepletionofelectrolytecomponentsorthe
corrosionratemaybecalculatedusingthefollowingequation:
addition of corrosion products to the electrolyte during the test
P 5 2.279V (1)
period, may affect corrosion rates, giving skewed or erroneous
H H
results [ASTM G31]. The ratio of electrolyte solution to
where:
surfaceareaofthetestpiececanimpactthedegreetowhichthe
V = volume (ml) / surface area (cm ) / days, and
H
electrolyte solution may change during the test. ASTM G31
P = mm/year.
H
gives guidance for the ratio of solution volumes to test piece
9.5.1.4 Slow or low-volume evolution may require special
surface area which should be considered during experimental
sensors or may not be practical or possible to detect.
design. Corrosion rates for magnesium-based alloys and other
group I and group II metals are affected by changes in
electrolyte pH (8). Starting pH is more easily controlled than Shi, Atrens. Corrosion Science. 53 (2011) 226-246.
F3268 − 18a
9.5.2 Metal Ion Release—Monitoring of the accumulation (log i) from polarization from the open-circuit potential in the
of metal ions within the electrolyte solution provides an anodic and cathodic directions. Many specific corrosion details
indication of the corrosion release. can be derived from these plots, the scope of which can be
9.5.3 Measurement of corrosion products provides addi- found in the following methods, which also include means for
tional fundamental information about the degradation/ the polarization resistance determination of E :
corr
corrosion process (16).
10.3.1 ASTM F2129 – Standard Test Method for Conduct-
9.5.3.1 Precipitates produced during the corrosion process
ing Cyclic Potentiodynamic Polarization Measurements to
must be completely isolated for a quantitative evaluation of the
Determine the Corrosion Susceptibility of Small Implant
corrosion process. Precipitates may often be filtered for com-
Devices
plete removal from the electrolyte.
10.3.2 ASTM G59 – Standard Test Method for Conducting
9.5.3.2 Precipitates produced during the evaluation may be
Potentiodynamic Polarization Resistance Measurements
completely dissolved by adding strong acid (HCl or H SO )
2 4 10.3.3 DIN 50918 – Elektrochemische Korrosionsuntersuc-
prior to analyzing the solution content (16). Some components
hungen. Deutsche Normen. Berlin: Beuth Verlag; 1978. p. 1-6.
of a sample (e.g. marker materials) may not completely
10.3.4 ISO 10993-15 – Biological evaluation of medical
dissolve. The extent of dissolution shall be verified.
devices Part 15: Identification and quantification of degrada-
9.5.3.3 The use of acid-washed glass containers may pre-
tion products from metals and alloys
vent corrosion products from adhering to the glass surfaces of
10.3.5 The oxygen-poor environment utilized with electro-
the vessel. Alternatively, they may be dissolved with strong
chemical evaluations may yield results not reflective of results
acids (16).
achieved in an in vivo environment, particularly with iron-
9.5.3.4 The addition of corrosion products to the electrolyte
based alloys.
may affect ongoing corrosion rates [ASTM G31], (16).An
10.3.6 Corrosionratescalculatedformagnesiumalloysmay
understanding of the effect of corrosion products on the
be underestimated because of the Negative Difference Effect
corrosion rates may improve the experimental design.
(NDE) which causes hydrogen evolution to increase at poten-
9.5.3.5 The electrolyte may need to be monitored for both
tials more positive than Ecorr instead of decreasing and thus
addition of corrosion product and depletion of essential elec-
diverging from Tafel kinetics (15).
trolyte components [ASTM G31].
10.4 Electrochemical Impedance Spectroscopy (also known
9.5.3.6 Achange of more than 10% in electrolyte composi-
as Dielectric Spectroscopy or Impedance Spectroscopy)—This
tion may be adjusted by adding additional electrolyte [ASTM
approach measures the dielectric properties of a medium as a
G31].
function of frequency. It is based on the interaction of an
10. Electrochemical Corrosion Evaluation
external field with the electric dipole moment of the sample,
often expressed by permittivity. The technique measures the
10.1 Corrosion, in general, is an electrochemical proc
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: F3268 − 18 F3268 − 18a
Standard Guide for
in vitro Degradation Testing of Absorbable Metals
This standard is issued under the fixed designation F3268; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 The purpose of this standard is to outline appropriate experimental approaches for conducting an initial evaluation of the
in vitro degradation properties of a device or test sample fabricated from an absorbable metal or alloy.
1.2 The described experimental approaches are intended to control the corrosion test environment through standardization of
conditions and utilization of physiologically relevant electrolyte fluids. Evaluation of a standardized degradation control material
is also incorporated to facilitate comparison and normalization of results across laboratories.
1.3 The obtained test results may be used to screen materials and/or constructs prior to evaluation of a more refined fabricated
device. The described tests may also be utilized to define a device’s performance threshold prior to more extensive in vitro
performance evaluations (e.g. fatigue testing) or in vivo evaluations.
1.4 This standard is considered to be applicable to all absorbable metals, including magnesium, iron, and zinc-based metals and
alloys.
1.5 The described tests are not considered to be representative of in vivo conditions and could potentially provide a more rapid
or slower degradation rate than an absorbable metal’s actual in vivo corrosion rate. The herein described test methods are to be
used for material comparison purposes only and are not to act as either a predictor or substitute for evaluation of the in vivo
degradation properties of a device.
1.6 This standard only provides guidance regarding the in vitro degradation of absorbable metals and does not address any
aspect regarding either in vivo or biocompatibility evaluations.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.8 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
B943 Specification for Zinc and Tin Alloy Wire Used in Thermal Spraying for Electronic Applications
B954 Test Method for Analysis of Magnesium and Magnesium Alloys by Atomic Emission Spectrometry
E2375 Practice for Ultrasonic Testing of Wrought Products
F1854 Test Method for Stereological Evaluation of Porous Coatings on Medical Implants
F2129 Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Suscepti-
bility of Small Implant Devices
F2739 Guide for Quantifying Cell Viability within Biomaterial Scaffolds
F3160 Guide for Metallurgical Characterization of Absorbable Metallic Materials for Medical Implants
G1 Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens
G3 Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing
This guide is under the jurisdiction of ASTM Committee F04 on Medical and Surgical Materials and Devices and is the direct responsibility of Subcommittee F04.15
on Material Test Methods.
Current edition approved April 1, 2018Oct. 1, 2018. Published May 2018November 2018. Originally approved in 2018. Last previous edition approved in 2018 as
F3268–18. DOI: 10.1520/F3268-18.vb h10.1520/F3268-18A.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F3268 − 18a
G4 Guide for Conducting Corrosion Tests in Field Applications
G16 Guide for Applying Statistics to Analysis of Corrosion Data
G31 Guide for Laboratory Immersion Corrosion Testing of Metals
G46 Guide for Examination and Evaluation of Pitting Corrosion
G59 Test Method for Conducting Potentiodynamic Polarization Resistance Measurements
G102 Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements
G106 Practice for Verification of Algorithm and Equipment for Electrochemical Impedance Measurements
G215 Guide for Electrode Potential Measurement
2.2 DIN Standards:
DIN 50918 Elektrochemische Korrosionsuntersuchungen. Deutsche Normen. Berlin: Beuth Verlag; 1978. p. 1-6
2.3 ISO Standards:
ISO 10993-15 Biological evaluation of medical devices Part 15: Identification and quantification of degradation products from
metals and alloys
ISO 13485 Medical devices – Quality management systems – Requirements for regulatory purposes
3. Terminology
3.1 Definitions:
3.1.1 absorbable, adj—in the body, referring to an initially distinct foreign material or substance that either directly or through
intended degradation can be excreted, metabolized or assimilated by cells and/or tissue.
3.1.2 surface roughness, R , n—the arithmetic average deviation of the surface profile from the centerline, normally reported
A
in micrometers.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 degradation, n—the breakdown of a metallic test material or metallic device principally due to corrosion in an electrolyte
solution relevant to physiologic conditions.
3.2.2 degradation control material, n—multiple batches of a defined metallic compositon with sufficiently uniform corrosion
properties to verify an experimental setup and to compare relative intra-laboratory and/or inter-laboratory corrosion rates.
4. Summary of Guide
4.1 Guidance is given on in vitro evaluation of the corrosion/degradation properties of absorbable metal materials and devices
fabricated from absorbable metals. Considerations specific to the application of corrosion testing methods to absorbable metal
materials are outlined for both immersion and electrochemical methods.
4.1.1 Electrolyte composition is a critical factor in corrosion experiments. Several electrolytes are commonly used to mimic in
vivo conditions. Electrolyte selection may also take into consideration the alloy being tested.
4.1.2 Control of the experimental conditions (i.e., temperature, pH and fluid movement around the test piece(s)) can markedly
affect the corrosion rates and experimental outcomes. Controlling and documenting these factors are important with regard to
generating consistent, reproducible results. Experimental conditions may be altered, depending on the intent of the experiment.
4.1.3 The surrounding atmosphere may interact with the electrolyte solution (liquid-gas interface), depending on electrolyte
composition, particularly if the electrolyte contains a carbonate buffer or if oxygen in the electrolyte is consumed during the
corrosion process, as with iron-based alloys. Measurement and control of the atmospheric composition may be important,
depending on the specific circumstances of the experiment.
4.1.4 Measurements of corrosion may include weight loss of the sample, accumulation of corrosion products in the experiment,
generation of H gas, and changes to physical and mechanical properties.
4.2 Electrochemical methods, Polarization Resistance, and Electrochemical Impedance Spectroscopy also can be used to
measure relative corrosion rates and generate additional insight into the corrosion process. The electrolyte used in these methods
may not be relevant to in vivo conditions and may not mimic the process in vivo. It is important to fully document relevant
experimental conditions (e.g. electrolyte composition, current, current density and atmosphere), so that their impact on the test
results can be understood.
4.3 Use of a degradation control material to monitor the consistency of the experimental system is recommended, but not
mandatory. See Annex A1 for details.
5. Significance and Use
5.1 This standard provides an itemization of potential in vitro test methods to evaluate the degradation of absorbable metals.
The provided approach defers to the user of this standard to pick most appropriate method(s) based on the specific requirements
Available from Deutsches Institut für Normung e.V.(DIN), Am DIN-Platz, Burggrafenstrasse 6, 10787 Berlin, Germany, http://www.din.de.
Available from International Organization for Standardization (ISO), ISO Central Secretariat, BIBC II, Chemin de Blandonnet 8, CP 401, 1214 Vernier, Geneva,
Switzerland, http://www.iso.org.
F3268 − 18a
of the intended application. However, a minimum of at least two different corrosion evaluation methods is considered necessary
for basic profiling of the material or device, with additional methods potentially needed for an adequate characterization. However,
in some instances there may be only one method that correlates to in vivo degradation results.
5.2 It is recognized that not all test methods will be meaningful for every situation. In addition, some methods carry different
potential than others regarding their relative approximation to the in vivo conditions within which actual use is to occur. As a result,
some discussion and ranking of the relevance of the described methods is provided by this guidance.
5.3 It should be noted that degradation of absorbable metals is not linear. Thus, precautions should be taken that evaluations
of the degradation profile of a metal or metal device are appropriately adapted to reflect the varying stages and rates of degradation.
Relevant factors can include the amount or percentage (%) of tissue coverage of the implanted device and the metabolic rate of
surrounding tissue, which is not necessarily accompanied by a high perfusion rate.
5.4 It is recognized that in vivo environments will impart specialized considerations that can directly affect the corrosion rate,
even when compared with other in vivo locations. Thus, a basic understanding of the biochemistry and physiology of the specific
targeted implant location (e.g. hard tissue; soft tissue; high, low or zero perfusion areas/tissue; high, low or zero loading
environments) is needed to optimize in vitro and in vivo evaluations.
5.5 Within the evaluation of absorbable metals, rate uniformity is considered to be the principle concern and design goal. The
recognized primary value for the herein described in vitro testing under static (i.e. not dynamic) conditions is to monitor and screen
materials and/or devices for their corrosion consistency. Such an evaluation may provide a practical understanding of the
uniformity of the device prior to any subsequent in vivo testing - where device consistency is considered to be critical for
optimizing the quality of the obtained observations.
5.6 Once a suitable level of device corrosion consistency has been established (either directly or historically), static and/or
dynamic fatigue testing can then be undertaken, if needed, to further enhance the understanding of the corrosion process within
the context of the device’s overall design and its intended application/use.
5.7 Depending on the intended application, appropriate levels of implant loading may range from minimal to severe. Thus, this
standard does NOT directly address the appropriate level of loading of absorbable metallic devices, guidance for which may be
found in documents specific to the intended implant application and the design requirements for the product.
5.8 This standard does NOT directly address dynamic fatigue testing of absorbable metallic devices.
6. Material/Metallurgical Characterization
6.1 A full understanding of the compositional and morphological features of the material or device to be tested is needed prior
to conducting any in vitro degradation evaluation. Lack of control of critical material features (e.g. elemental composition,
contamination, grain size, etc.) may lead to inconsistent results both in vitro and/or in vivo. Characterization of the test material
should be undertaken in accordance with ASTM F3160.
6.2 Depending on the goals of the experiment, selected mechanical tests may be repeated at various intervals during the
corrosion experiment. In most cases, it would be appropriate to retire mechanically tested samples.
7. General Testing Conditions
7.1 The intention of the following listing of general considerations is to provide a fundamental overview of the critical factors
involved with generating consistent in vitro corrosion characterization results.
7.2 Fluid Composition:
7.2.1 For all in vitro test systems, fluid composition is a critical factor that requires both control and disclosure. Additionally,
pH (which can be influenced by degradation product composition and generation rate), fluid flow, and solution buffer capacity are
significant variables that can affect an absorbable metal’s corrosion rate. While it is desirable to maintain an in vitro pH at a level
that is representative of the in vivo condition, it is important to note that the composition of a buffer’s anions can significantly affect
the corrosion rate. Critical electrolytes and biomolecules that are known to directly affect the corrosion rate of Mg alloys include
phosphate, carbonate, chloride, calcium, serum proteins, and lipids [see references (1-7)]. As a result, solutions with a
physiologically relevant combination of electrolytes should be used.
NOTE 1—If the intention of the experiment is to provide an in vitro approximation to an in vivo system, the use of a well-controlled, simpler electrolyte
system that has been correlated to in vivo data may be preferable to a more complex, less stable system.
7.2.2 Numerous formulations exist for simulated body fluids (SBFs) or buffering solutions that are intended to mimic the in vivo
condition. Hank’s Solution, which is phosphate-based and designed to buffer in a normal atmosphere, provides an approximation
of the electrolyte composition found in the body. However, while it does provide a reasonable approximation of inorganic moieties,
F3268 − 18a
it does NOT provide the body’s buffer capacity (as enhanced through carbonate equilibria ) or the presence of a myriad of organic
molecules – many of which, particularly proteins, can be expected to adsorb to the implant surface and further affect the
degradation rate. Table 1 defines the main ions in several common SBF solutions.
7.2.3 Additional factors to consider in electrolyte solutions are the levels of dissolved O and CO , which, depending on a
2 2
particular metal’s composition and corrosion mechanism, can significantly affect the degradation rate. The reduced or zero level
of bicarbonate within Hank’s Solution limits its ability to interact with a CO atmosphere, resulting in a limitation to its buffering
capacity. Conversely, biological salt solutions that are bicarbonate-based and thereby interact with a CO atmosphere are Earles
and possibly Krebs Ringers. However, these balanced salts and culture media are designed to buffer at either 5 or 10% CO ,
depending on the amount of bicarbonate included. This concern is also true for complete medium (e.g. Dulbecco’s Modified
Eagle’s Medium (DMEM), Minimum Essential Media (MEM), Roswell Park Memorial Institute Medium 1640 (RPMI 1640), etc.)
that introduce additional constituents, such as amino acids, chemicals, and proteins, which can impact corrosion rates. Thus it can
be surmised that such buffered electrolyte solutions can only approximate the actual corrosion occurring in vivo and may need
adjustment during the degradation process to meet the requirements of the intended evaluation. Buffering capacity can affect
results.
NOTE 2—The actual in vivo concentration of O and CO may not be known. However, maintaining control of O and CO in the in vitro experiment
2 2 2 2
enhances experimental reproducibility.
7.2.4 In summary, the composition of electrolytes and atmospheres should be tailored to approximate the expected conditions
in the intended clinical application, to the extent that they are known. As a result, full composition of the electrolyte solution and
overlying atmosphere shall be included in the report along with relevant details regarding their replenishment rate.
7.2.5 For an overview discussion regarding environmental factors for absorbable metals, see Section 1.3.2 of Y.F. Zheng et al.
Materials Science and Engineering R 77 (2014) 1-34 (8).
7.3 Fluid Flow:
7.3.1 An additional factor for consideration is the level of fluid flow, which can affect both the rate of chemical exchange and
the adhesion of degradation products. It has been recognized that flow-induced shear stress can significantly accelerate degradation
of Mg (see reference (8)). Thus, fluid flow should be considered as a potentially critical factor when assessing a material’s or
device’s degradation rate. However, due to the presence of both tissue and proteins, it should be noted that the impact of fluid flow
under in vivo conditions may not be as pronounced as is found in in vitro models (see articles by Wittchow (11) and Bowen (12)].
7.3.2 While an active flow environment may not be needed when preliminarily screening materials, a physiologically relevant
fluid flow that resembles the targeted application is highly recommended when attempting an approximation of in vivo
performance. For example, the flow-induced shear stress on a newly deployed coronary stent may be very different from that of
an absorbable screw placed into tibial bone. Additionally, an initial perfusion rate may change over time (e.g. from 10 to 2 relative
units), which could then potentially alter the related rate of degradation. Also, the additional influence of normal cell and protein
coverage and any drugs (e.g. anti-proliferatives) or other biological factors that may affect cell coverage and/or perfusion at the
corrosion interface also requires consideration.
7.3.3 Flow induced through use of a shaker table may (or may not) be adequate to be representative of the in vivo condition.
7.4 Atmospheric Composition:
7.4.1 Since atmospheric composition can significantly influence the electrolyte solution (see prior discussion under fluid
composition), a definable and, if needed, controlled (can be ambient) atmosphere should be maintained for all test methods,
including during materials screening. Any selected atmosphere should be reflective of the intent of the experiment and the
material’s specific degradation considerations. An atmosphere for maintenance of pH through CO / bicarbonate buffering may be
chosen. For experiments meant to approximate in vivo conditions, appropriate considerations would result in an atmosphere that
is compositionally relevant to both the implant’s intended application as well as to the particular metal’s projected corrosion
A
TABLE 1 Composition of SBF Test Solutions
Extra-
Solution
Ion
Cellular
mmol/L
Tyrode’s Hanks’ Ringer’s Isotonic
Fluid
+
Na 149 141 147 154 142
+
K 2.7 5.4 4 — 5
2+
Ca 1.8 1.3 3 — 2.5
2+
Mg 1.0 0.74 0 — 1.5
-
Cl 145 145 157 154 103+
A
Table obtained from: Corrosion Mechanisms of Metallic Biomaterials, Barbosa,
M. (Table 3, p. 239); in Biomaterials Degradation – Fundamental Aspects and
Related Clinical Phenomena, Ed. Mario Barbosa, pp. 227-239+ (1991).
Hank’s Balanced Salt Solution can be purchased at various concentrations of calcium, magnesium and bicarbonate. Bicarbonate buffering is typically 4.2 mmol/L which
is lower than other Salt Solutions.
F3268 − 18a
chemistry once it is placed in vivo. Selected atmospheric compositions may also be reflective of the projected gaseous exchange
rate within the in vivo environment of the intended clinical application. For example, both the ambient concentration and exchange
rate of dissolved gasses can be expected to differ between lung, arterial and intramuscular implantation applications.
7.5 Temperature:
7.5.1 Solution temperature is a critical part of any corrosion environment (even when used in screening tests), in that elevated
temperatures will typically increase reaction rates as well as introduce the potential for other chemical reactions. As a result, (37
61 °C) shall be used for all principle corrosion test conditions, where achievable, regardless of any recommendations/requirements
described within a cited reference method. If 6 1 °C is not achievable in a particular experimental setup, the temperature shall
be maintained within the minimum practical limits to maintain experimental consistency and reproducibility. Such a universally
recognized in vivo temperature is considered to broadly represent the physiological condition and thereby provide the most broadly
applicable scientific value. However, testing at other temperatures may also be included to determine differences in reaction
mechanisms and rates. An additional alternate evaluation temperature may be especially useful if the temperature at the intended
implant application and/or the temperature in a particular animal model differs significantly from (37 6 1 °C).
7.6 Utilization of a Degradation Control Material:
7.6.1 In any corrosion evaluation, the experimental rate observed for a degradation control material may be included, both in
the test and in the generated report. Such an evaluation of the corrosion rate of a degradation control material allows for continued
monitoring of the consistency of a specific corrosion test system. Once included and tested, the experimentally obtained results
can be utilized to demonstrate the repeatability of the specific test system while simultaneously providing a comparative control
that allows for a normalized assessment of results across different test laboratories.
7.6.2 In some cases, it may be desirable to use a material already in use for the same or similar product as a control material,
particularly if relative differences are of interest. A material for which the experimenter(s) have corrosion performance data may
be advantageous in some experimental situations.
NOTE 3—Caution: Some alloys may not be sufficiently consistent from heat to heat to provide reproducible degradation results. It is up to the user
to verify the suitability of a material as a degradation control material.
7.6.3 Guidance and further discussion regarding the appropriate specification and manufacturing of a degradation control
material may be found in Annex A1.
7.6.4 Guidance regarding the appropriate utilization of a degradation control material within an experimental environment may
be found in Annex A2.
7.7 In summary, flow conditions along with electrolytes, atmospheres, temperature and any other conditions relevant to the
experiment should be tailored to approximate the expected conditions in the intended clinical application. As a result, full
composition of the electrolyte solution and overlying atmosphere shall be reported along with relevant details regarding flow.
8. Initial Sample Characterization and Preparation
8.1 Sample dimensions will be dictated primarily by the limitations of the selected test method. However, the utilized size and
description shall be reported in sufficient detail, including surface area and/or other features that may affect the results, so that its
dimensions and features can be both easily understood and readily reproduced by others wishing to repeat the evaluation. Thus,
information that will allow ready determination of a sample’s external dimensions and overall volume shall be reported.
Additionally, if the evaluated structure is porous, a description of the porosity and the relative size and interconnectivity of the
pores also should be reported so as to allow a full understanding of the material’s or device’s electrolyte-accessible
surface-to-volume ratio. ASTM F1854 contains methods useful for evaluating surface porosity.
8.1.1 Sufficient information about the sample shall be reported such that all pertinent references may be followed, e.g. ASTM
G31 recommends a minimum electrolyte volume to sample surface area, for which the sample surface should be known within
the limits specified in the reference.
9. Immersion Corrosion Evaluation
9.1 The intent of an immersion corrosion evaluation is to understand the rate of degradation when an implant or material is
exposed (at rest) to a corrosive environment. As described previously, an approximation of the in vivo flow environment should
be provided if it carries potential for clinical relevance.
9.2 Specific methods that guide the observation of passive corrosion in absorbable metal implants are:
9.2.1 NACE TM0169/ASTM G31 – Laboratory Immersion Corrosion Testing of Metals.
9.2.2 Also worthy of consideration are evaluation approaches contained within ASTM G1, ASTM G4, ASTM G46, and ASTM
G102. ISO 10993-15 addresses methods for passive and polarization corrosion testing.
9.2.3 Guide G16 provides statistical methods appropriate to corrosion data, including regression methods for the analysis of
longitudinal data. The methods may be applied to corrosion rates and/or changes in mechanical or other properties.
9.3 Test Solution:
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9.3.1 Preparation and documentation of electrolyte solutions is necessary so that it can be accurately reproduced over multiple
test setups, allowing for comparable results across testing regimes and laboratories. ASTM G31 gives guidance on electrolyte
compositions and documentation.
9.3.2 Small changes in electrolyte composition during the test period, by either depletion of electrolyte components or the
addition of corrosion products to the electrolyte during the test period, may affect corrosion rates, giving skewed or erroneous
results [ASTM G31]. The ratio of electrolyte solution to surface area of the test piece can impact the degree to which the electrolyte
solution may change during the test. ASTM G31 gives guidance for the ratio of solution volumes to test piece surface area which
should be considered during experimental design. Corrosion rates for magnesium-based alloys and other group I and group II
metals are affected by changes in electrolyte pH (8). Starting pH is more easily controlled than pH fluctuations during the course
of an experiment. The closer the initial pH is set to the target value, (i.e., within plus or minus 0.1 pH units), the longer the
corrosion experiment may stay within predefined limits. If there is justification for a different pH specification for a particular
experimental application, it should take precedence.
NOTE 4—Caution: The use of 2-(4-(2-hydroxyethyl)-1-pipeazinyl) ethanesulfonic acid (HEPES) buffer may negatively affect the corrosion rates of
magnesium alloys (8).
9.3.3 Corrosion rates for iron-based alloys are affected by the oxygen content of the electrolyte. It may be necessary to control
oxygen saturation to achieve consistent results [ASTM G31], (8, 9).
NOTE 5—Bioreactors are well suited to adequately control corrosion conditions, with the potential to simultaneously expose the corroding implant to
hard and/or soft tissues See ASTM F2739 for additional details on bioreactors and their potential use.
9.4 Mass Loss—Loss of sample mass can be used as a basic measure of corrosion.
9.4.1 Corrosion products should be removed from the sample as completely as possible prior to weighing a post-corrosion test
sample. ASTM G1 and ASTM G31 give guidance on removing corrosion products from test articles prior to weighing.
NOTE 6—Chromic acid is carcinogenic and, in some jurisdictions, may be restricted by local regulation.
9.4.2 Weighing accuracy of the post-corrosion sample should be equivalent to the accuracy of the initial sample weight and
sufficiently accurate to assess weight loss.
9.4.3 ASTM G1 and ASTM G31 provide formulas for converting mass loss to corrosion rate for flat prismatic geometries.
9.5 Measurement of Corrosion Products—The quantitative evaluation of corrosion products -m may be used as a measure of
corrosion if the reactions are well understood.
9.5.1 Hydrogen Gas Evolution—For magnesium metals and alloys with relatively short longevities, hydrogen (H ) gas evolution
can also be considered as a means for determining the corrosion rate or be used to confirm other measures of corrosion rate.
9.5.1.1 This method/approach is applicable when the majority of the alloy evolves hydrogen during the corrosion process (e.g.
magnesium and group I and II metals) (16).
9.5.1.2 The collected hydrogen volume can be converted to moles after correcting for non-standard temperature, pressure, and
the partial (vapor) pressure of electrolyte solution (16).
9.5.1.3 For experiments not taken to complete dissolution, corrosion rate may be calculated using the following equation:
P 5 2.279V (1)
H H
where:
V = volume (ml) / surface area (cm ) / days, and
H
P = mm/year.
H
9.5.1.4 Slow or low-volume evolution may require special sensors or may not be practical or possible to detect.
9.5.2 Metal Ion Release—Monitoring of the accumulation of metal ions within the electrolyte solution provides an indication
of the corrosion release.
9.5.3 Measurement of corrosion products provides additional fundamental information about the degradation/corrosion process
(16).
9.5.3.1 Precipitates produced during the corrosion process must be completely isolated for a quantitative evaluation of the
corrosion process. Precipitates may often be filtered for complete removal from the electrolyte.
9.5.3.2 Precipitates produced during the evaluation may be completely dissolved by adding strong acid (HCl or H SO ) prior
2 4
to analyzing the solution content (16). Some components of a sample (e.g. marker materials) may not completely dissolve. The
extent of dissolution shall be verified.
9.5.3.3 The use of acid-washed glass containers may prevent corrosion products from adhering to the glass surfaces of the
vessel. Alternatively, they may be dissolved with strong acids (16).
9.5.3.4 The addition of corrosion products to the electrolyte may affect ongoing corrosion rates [ASTM G31], (16). An
understanding of the effect of corrosion products on the corrosion rates may improve the experimental design.
Shi, Atrens. Corrosion Science. 53 (2011) 226-246.
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9.5.3.5 The electrolyte may need to be monitored for both addition of corrosion product and depletion of essential electrolyte
components [ASTM G31].
9.5.3.6 A change of more than 10% in electrolyte composition may be adjusted by adding additional electrolyte [ASTM G31].
10. Electrochemical Corrosion Evaluation
10.1 Corrosion, in general, is an electrochemical process that requires both an anode and a cathode, as well as an ionic path
through the electrolyte. Corrosion is also highly pH-dependent, so its monitoring is an essential component of any electrochemical
evaluation. Typical electrochemical methods applicable to absorbable metals include polarization resistance, Tafel extrapolation,
and electrochemical impedance spectroscopy (EIS).
10.2 Polarization Resistance—Electrochemical corrosion evaluations typically begin with this test, which is conducted by
monitoring the corrosion potential Ecorr (also known as electrochemical corrosion potential, free corrosion potential, and
open-circuit potential) of an electrolyte-immersed corroding sample versus a standard calomel electrode for a specified period of
time. The sample is typically polarized at 6 10 mV each side of the Open Circuit potential, recording the induced current between
the working and counter electrodes. The resistance to corrosion is measured as the slope of the potential-versus-current curve.
10.3 Potentiodynamic Polarization (also known as DC Polarization)—This test refers to a technique wherein the potential of
an electrode with respect to a reference electrode is varied at a selected rate by application of a current through the electrolyte.
The test is typically applied after a definable rest potential has been achieved. The result are anodic and cathodic polarization plots,
which depicts the relationship between the change in potential (E) and the logarithm of the current density (log i) from polarization
from the open-circuit potential in the anodic and cathodic directions. Many specific corrosion details can be derived from these
plots, the scope of which can be found in
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