ISO 13781:2017

Implants for surgery — Homopolymers, copolymers and blends on poly(lactide) — In vitro degradation testing

ISO 13781:2017 describes methods for the determination of chemical and mechanical changes in poly(lactide)-based homopolymers, copolymers and/or blends induced under in vitro degradation testing conditions. This document covers polymers based on L-lactide, D-lactide, and/or D, L-lactide monomeric units. The purpose of this document is to compare and/or evaluate materials or processing conditions. This document also describes the fundamental physical and mechanical evaluations needed for an in vitro degradation characterization of an absorbable poly(lactide) or other hydrolysable material or device. ISO 13781:2017 is applicable to poly(lactide)-based homopolymers, copolymers and/or blends in bulk or processed forms and used for the manufacture of surgical implants, including finished products (packaged and sterilized implants). The test methods specified in this document are also intended to determine the in vitro degradation rate and related changes in material properties of polylactide-based copolymers and/or blends with various other comonomers, such as glycolid, trimethylene, carbonate and/or ε-caprolactone. Unless otherwise validated for a specific device, these in vitro methods cannot be used to definitively predict device behaviour under in vivo conditions.

Implants chirurgicaux — Homopolymères, copolymères et mélanges sur poly(lactide) — Essais de dégradation in vitro

General Information

• Status: Published
• Publication Date: 19-Jul-2017
• ICS: 11.040.40 - Implants for surgery, prosthetics and orthotics
• Technical Committee: ISO/TC 150/SC 1 - Materials
• Drafting Committee: ISO/TC 150/SC 1/WG 5 - Plastics
• Current Stage: 9093 - International Standard confirmed
• Completion Date: 12-Jan-2023

Relations

• Revises: ISO 15814:1999 - Implants for surgery — Copolymers and blends based on polylactide — In vitro degradation testing
• Effective Date: 20-Jul-2013
• Revises: ISO 13781:1997 - Poly(L-lactide) resins and fabricated forms for surgical implants — In vitro degradation testing
• Effective Date: 20-Jul-2013

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 13781
Second edition
2017-07
Implants for surgery —
Homopolymers, copolymers and
blends on poly(lactide) — In vitro
degradation testing
Implants chirurgicaux — Homopolymères, copolymères et mélanges
sur poly(lactide) — Essais de dégradation in vitro
Reference number
ISO 13781:2017(E)
©
ISO 2017

---------------------- Page: 1 ----------------------
ISO 13781:2017(E)

COPYRIGHT PROTECTED DOCUMENT
© ISO 2017, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. [phone]
Fax [phone]
[e-mail]
www.iso.org
ii © ISO 2017 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 13781:2017(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Degradation evaluation . 3
4.1 General . 3
4.2 Apparatus and reagents . 4
4.3 Real-time degradation — Sample conditioning procedure . 5
4.3.1 Sample loading and placement. 5
4.3.2 Control of temperature . 5
4.3.3 Control of buffer solution . 5
4.3.4 Sample retrieval. 5
5 Physical, chemical and mechanical tests . 6
5.1 General . 6
5.2 Loss of sample mass . 6
5.2.1 Apparatus . 6
5.2.2 Number of test samples . 6
5.2.3 Procedure . 7
5.2.4 Reusability of test specimens. 7
5.3 Evaluation of molar mass . 8
5.3.1 Via inherent viscosity . 8
5.3.2 Via gel permeation chromatography/size exclusion chromatography . 8
5.4 Mechanical tests . 8
5.4.1 General. 8
5.4.2 Conditioning of test samples . 8
5.4.3 Test methods . 9
5.5 Additional evaluation methods for consideration . 9
6 Test termination . 9
7 Test report .10
Annex A (informative) Nomenclature of absorb, degrade and related terms .12
Annex B (informative) Additional analytic methods for consideration .13
Bibliography .14
© ISO 2017 – All rights reserved iii

---------------------- Page: 3 ----------------------
ISO 13781:2017(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: w w w . i s o .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 150, Implants for surgery, Subcommittee
SC 1, Materials.
This second edition cancels and replaces the first edition (ISO 13781:1997) and ISO 15814, which have
been technically revised.
The main change compared to the previous edition is as follows:
— the principle contents of ISO 15814 are incorporated into this document.
iv © ISO 2017 – All rights reserved

---------------------- Page: 4 ----------------------
ISO 13781:2017(E)

Introduction
With the development of absorbable polymers for use in implantable devices, there is a need to
define standard test methods to evaluate the behaviour of bulk material or devices under simulated
physiological environments. On the other hand, the behaviour of absorbable materials and devices in
situ depends on the conditions in which the material is implanted. These conditions differ, so that the
site-specific behaviour of the material or device can differ. The interpretation of in vitro test results
therefore needs to be considered carefully, taking into account any correlation of test results under
in vitro and in vivo conditions. Only functional in vivo tests with the final product can answer actual
degradation behaviour in situ.
© ISO 2017 – All rights reserved v

---------------------- Page: 5 ----------------------
INTERNATIONAL STANDARD ISO 13781:2017(E)
Implants for surgery — Homopolymers, copolymers and
blends on poly(lactide) — In vitro degradation testing
1 Scope
This document describes methods for the determination of chemical and mechanical changes in
poly(lactide)-based homopolymers, copolymers and/or blends induced under in vitro degradation
testing conditions. This document covers polymers based on L-lactide, D-lactide, and/or D, L-lactide
monomeric units.
The purpose of this document is to compare and/or evaluate materials or processing conditions. This
document also describes the fundamental physical and mechanical evaluations needed for an in vitro
degradation characterization of an absorbable poly(lactide) or other hydrolysable material or device.
This document is applicable to poly(lactide)-based homopolymers, copolymers and/or blends in bulk
or processed forms and used for the manufacture of surgical implants, including finished products
(packaged and sterilized implants).
The test methods specified in this document are also intended to determine the in vitro degradation
rate and related changes in material properties of polylactide-based copolymers and/or blends with
various other comonomers, such as glycolid, trimethylene, carbonate and/or ε-caprolactone. Unless
otherwise validated for a specific device, these in vitro methods cannot be used to definitively predict
device behaviour under in vivo conditions.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 178, Plastics — Determination of flexural properties
ISO 180, Plastics — Determination of Izod impact strength
ISO 527-1, Plastics — Determination of tensile properties — Part 1: General principles
ISO 527-2, Plastics — Determination of tensile properties — Part 2: Test conditions for moulding and
extrusion plastics
ISO 527-3, Plastics — Determination of tensile properties — Part 3: Test conditions for films and sheets
ISO 604,Plastics — Determination of compressive properties
ISO 1628-1, Plastics — Determination of the viscosity of polymers in dilute solution using capillary
viscometers — Part 1: General principles
ISO 1805, Fishing nets — Determination of breaking force and knot breaking force of netting yarns
ISO 2062, Textiles — Yarns from packages — Determination of single-end breaking force and elongation at
break using constant rate of extension (CRE) tester
ISO 6721-2, Plastics — Determination of dynamic mechanical properties — Part 2: Torsion-pendulum method
ISO 13934-1, Textiles — Tensile properties of fabrics — Part 1: Determination of maximum force and
elongation at maximum force using the strip method
© ISO 2017 – All rights reserved 1

---------------------- Page: 6 ----------------------
ISO 13781:2017(E)

ISO 14130, Fibre-reinforced plastic composites — Determination of apparent interlaminar shear strength
by short-beam method
ISO 16014-1, Plastics — Determination of average molecular mass and molecular mass distribution of
polymers using size-exclusion chromatography — Part 1: General principles
ISO 16014-2, Plastics — Determination of average molecular mass and molecular mass distribution of
polymers using size-exclusion chromatography — Part 2: Universal calibration method
ISO 16014-3, Plastics — Determination of average molecular mass and molecular mass distribution of
polymers using size-exclusion chromatography — Part 3: Low-temperature method
ISO 16014-4, Plastics — Determination of average molecular mass and molecular mass distribution of
polymers using size-exclusion chromatography — Part 4: High-temperature method
ISO 16014-5, Plastics — Determination of average molecular mass and molecular mass distribution of
polymers using size-exclusion chromatography — Part 5: Method using light-scattering detection
ASTM D2990,Standard test methods for tensile, compressive, and flexural creep and creep-rupture of
plastics
ASTM D5296, Test method for molecular weight averages and molecular weight distribution of polystyrene
by high performance size-exclusion chromatography
ASTM F1635-16, Standard test method for in vitro degradation testing of hydrolytically degradable
polymer resins and fabricated forms for surgical implants
ASTM F2902-16, Standard guide for assessment of absorbable polymeric implants
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
NOTE A discussion regarding the basis for some of the terms and definitions in this clause is available in
Annex A.
3.1
absorbable polymer
non-endogenous (foreign) polymeric material that is capable of passing through or being assimilated by
cells and/or tissue over time
3.2
absorption
act of a non-endogenous (foreign) material or substance passing through or being assimilated by cells
and/or tissue over time
3.3
blend
physical mixture of two or more different thermoplastic polymers and/or copolymers (3.4)
3.4
copolymer
polymer synthesized by polymerizing two or more monomer units together
2 © ISO 2017 – All rights reserved

---------------------- Page: 7 ----------------------
ISO 13781:2017(E)

3.5
in vitro degradation
loss of mechanical properties and/or chemical integrity through chemical changes induced by a
simulated physiological environment
3.6
poly(D,L-lactide)
polymer synthesized with approximately equimolar concentrations of D-lactide and L-lactide
monomeric units
3.7
poly(L-lactide)
polymer synthesized exclusively from L-lactide monomeric units
3.8
sample
finite subset, object or individual of a group, class or population whose properties or characteristics are
studied to gain information regarding the whole
3.9
specimen
specific portion or quantity of a sample utilized to obtain a single measurement of a property or
characteristic
4 Degradation evaluation
4.1 General
Appropriate in vitro characterization of the degradation of an absorbable polymeric material monitors
the progressive loss of both sample mass and molar mass (i.e. molecular weight) from ongoing exposure
to a physiologically relevant environment. Such an environment provides fluid and thermal conditions
that generate an in vitro degradation rate that approximates the rate observed in vivo. Such an in vitro
degradation characterization also monitors the loss of device-relevant mechanical properties over
time. Thus, unless justified otherwise, such a material or device degradation characterization shall
include a systematic monitoring of the loss of sample mass (see 5.2), molar mass (via inherent viscosity
and/or GPC/SEC, see 5.3) and at least one mechanical property that can be considered relevant to the
characteristics and intended use of the device (see 5.4).
The degradation rate of an absorbable material or device can be affected by sample dimensions and
other manufacturing/processing parameters (e.g. fibre draw ratio). As the shape and the structure of
the test sample can have a strong influence on the degradation kinetics, where applicable, the initial
test sample should be comparable to the intended product in shape and structure (i.e. fibre, film, plate,
rod, bulk material or other relevant form, as appropriate). The test sample may be a finished product or
a test coupon fabricated to emulate the relevant physical and/or mechanical properties of the intended
product.
Evaluation of samples representative of the finished product shall be conducted following terminal
sterilization at a level that meets or exceeds anticipated commercial exposure.
While the described general degradation evaluations and related tests can be useful toward
characterizing a finished product, they are not necessarily adequate to address all device-specific
issues. For example, a device that undergoes in vivo loading can degrade significantly faster than an
identical device implanted in an unloaded application. Thus, a device that is intended to undergo in
vivo loading should be additionally evaluated in vitro under real-time degradation conditions that
approximate the anticipated mechanical loading, including, if applicable, cyclic loading, shear, creep
loading, with further guidance available in ASTM F2902-16, 6.2.2.2.
The initial values for all tests shall be determined directly before starting the degradation test (time
zero). All subsequent tests shall be carried out on degraded samples at each test interval. The presence
© ISO 2017 – All rights reserved 3

---------------------- Page: 8 ----------------------
ISO 13781:2017(E)

or absence of a need for load testing should be based on a developed understanding of the device and
any related safety concerns at the time of loading. Such an assessment should incorporate any needed
considerations for extreme limits and/or creep aspects of the loading.
When load testing, attention should be directed toward mitigating any potential effects that fixturing
may have on the degradation rate of the device. For example, constriction of a fixtured device with
a moisture barrier on one or more sides carries potential to inhibit outward diffusion of acidic
degradation products that could thereby affect the device’s interstitial pH and accelerate degradation.
Physical constriction also carries potential to inhibit normal device swelling, which, as a result, carries
potential to induce artifacts in the mechanical response being measured.
4.2 Apparatus and reagents
4.2.1 Soaking solution
4.2.1.1 General
For the in vitro degradation study, the test sample shall be immersed in a Sörensen or other suitable
pH 7,4 buffer solution. However, if the normal pH of the intended in vivo application is not pH 7,4
(e.g. synovial fluid, portions of the alimentary canal), other appropriate pH and buffer solutions may
be used. For pH 7,4 applications, suitable solutions typically contain potassium dihydrogenphosphate
and disodium hydrogenphosphate in analytical water (e.g. Grade 2 in accordance with ISO 3696) and
include phosphate buffered saline (140 mM NaCl, 10 mM phosphate buffer and 3 mM KCl), commonly
known as “PBS”. The salts used for the preparation of the buffer solution shall be of analytical grade
and dried to constant mass. Any utilized buffer solution should have sufficient buffer capacity to allow
maintenance of the appropriate targeted pH to within ±0,2 pH units throughout the degradation
process. The adequacy of buffer systems that encounter excursions outside these limits can assessed
by determining the time weighted average, more details of which may be found in ASTM F1635-16, 6.1
and X1.3.1.
4.2.1.2 Preparation of Sörensen buffer solution
If a Sörensen solution is used, it shall be prepared by mixing 18,2 % (volume fraction) from solution A
and 81,8 % (volume fraction) from solution B.
— Solution A: 1/15 mol/L KH PO , prepared by dissolving 9,08 g KH PO in 1 l water.
2 4 2 4
— Solution B: 1/15 mol/L Na HPO , prepared by dissolving 11,9 g Na HPO • 2H O in 1 l water.
2 4 2 4 2
When prepared in this manner, the pH value of this buffer solution will be 7,4 ± 0,2.
4.2.2 Sample container
The container (e.g. bottle, jar, vial) shall be either inert plastic or glass and capable of holding the test
sample for each material and time interval and the required volume of soaking solution. Each container
shall be sealable against loss of solution by evaporation and to prevent microbial contamination.
4.2.3 Constant-temperature bath or oven
The constant-temperature bath or oven (for example, circulating air dryer) shall be capable of
maintaining all the sample containers at the specified degradation temperature ±1 °C for the specified
test duration.
4.2.4 pH-meter
A pH-meter with a precision of 0,02 or better shall be used to monitor for pH change. A pH-meter with a
calibrated accuracy of 0,02 or better shall be used to determine pH.
4 © ISO 2017 – All rights reserved

---------------------- Page: 9 ----------------------
ISO 13781:2017(E)

4.2.5 Balance
A calibrated balance shall be used to monitor mass loss in the samples. The precision of the balance
shall be sufficient to measure mass changes equal to or less than 0,1 % of the original dry mass of the
samples.
4.3 Real-time degradation — Sample conditioning procedure
4.3.1 Sample loading and placement
Before beginning real-time degradation, determine initial mass in accordance with 5.2.3.1.
Place the test sample into a container (4.2.3) with soaking solution (4.2.1) and seal the container. The
test sample shall be fully immersed in the soaking solution.
The minimum volume of the buffer solution used shall be 10 ml. To provide adequate buffer capacity, the
ratio of volume of the buffer solution, in millilitres, to the test sample mass, in grams, is recommended
to be greater than 30:1. However, the actual buffer capacity shall be equal or greater than the maximum
calculated post-hydrolysis acid concentration.
4.3.2 Control of temperature
Using the constant-temperature bath or oven (4.2.4), maintain the test sample containers at
physiological temperature of (37 ± 1) °C.
4.3.3 Control of buffer solution
4.3.3.1 Changes in pH value
The pH of the buffer solution shall be measured in at least two different containers at each test interval
or every six weeks, whichever is shorter. However, unless prior experience indicates otherwise,
monitoring should preferably be undertaken weekly (or more frequently, if needed).
If in one container the pH value has shifted beyond the limits, measure the value in all containers and
adjust, as needed, to the appropriate targeted pH ± 0,2. If encountered, assess the acceptability of any
excursion beyond pH ± 0,2 in accordance with the provisions of ASTM F1635-16, 6.1 and X1.3.1.
For samples designated solely for mechanical evaluation per 5.4, the buffer solution can be replaced
instead of adjusted, providing such fluid replacement does not compromise the mechanical integrity of
the degraded sample.
4.3.3.2 Clouding of buffer solution
Clouding of the buffer solution may indicate the contamination with microorganisms. Discard the test
sample if any clouding is visible which cannot be related directly to the material itself or its degradation
products.
It is recommended that the containers and soaking solutions be sterilized in order to avoid
contamination with microorganisms.
4.3.4 Sample retrieval
Remove the samples from the soaking solution and test according to the selected Clause 5 tests at
predetermined intervals.
Test sample retrieval intervals shall be selected to both maintain clinical relevance and profile the
degradation properties of the device. Mechanical properties shall be profiled from an initial clinically
relevant condition until a point where measurement of the mechanical attribute becomes impractical.
Additionally, mechanical properties of the device at the point of full hydration should also be
© ISO 2017 – All rights reserved 5

---------------------- Page: 10 ----------------------
ISO 13781:2017(E)

understood. Thus, dependent on the intended clinical application, potential retrieval durations could
range from less than a minute (i.e. minimal soaking solution exposure) to extended time intervals
for the generation of samples with marginal mechanical strength. In addition, when determining the
physical or molar mass of degraded samples, retrieval durations may be extended beyond the threshold
of mechanical integrity to allow evaluation of fragmented particles. As a result and dependent on the
specific attribute being measured, retrieval intervals may vary for the same device. Thus, to adequately
characterize each measured attribute, a minimum of an initial sample and retrieved samples from at
least five relatively evenly spaced degradation intervals shall be evaluated, with additional data points
potentially necessary for adequate characterization of complex profiles.
NOTE As degradation proceeds, physically independent manipulation and mechanical measurement of the
sample will eventually become impossible. Thus, before evaluating a large quantity of samples, it is suggested to
conduct a pilot evaluation to better ascertain appropriate sample retrieval timing and quantities.
Both fluid flow and mechanical loading carry potential to significantly influence the rate of sample
degradation. If the intended use of the device is subject to either fluid flow or loading conditions, the
user of this document should consult the guidance available in ASTM F1635-16 and ASTM F2902-16.
If such conditions are likely to significantly affect test results, it is suggested that the degradation
evaluation be undertaken in a manner that approximates the device’s in vivo service condition. However,
the nature and extent of such service conditions will be both device and application specific and are
therefore beyond the scope of this document.
5 Physical, chemical and mechanical tests
5.1 General
The testing described in each of the following subclauses is considered essential for an appropriate
characterization of an absorbable implant. Both the evaluation of mass loss in accordance with 5.2 and
the evaluation of molar mass in accordance with 5.3 are required. Additionally required is a mechanical
evaluation in accordance with 5.4 utilizing at least one test method, with additional methods potentially
necessary dependent on the mechanical performance requirements of the material and/or device.
A listing of potentially useful evaluations of a device’s mechanical, degradation and performance
properties can be found in ASTM F2902-16, Table 2.
5.2 Loss of sample mass
5.2.1 Apparatus
5.2.1.1 Balance, a calibrated weighing device capable of measuring sample mass to the requisite
precision.
5.2.1.2 Desiccator, a sealable container that retains a desiccant to absorb moisture for the purpose of
drying the test samples. For example, silica gel beads containing an indicator can be used.
5.2.1.3 Vacuum pump, a pump capable of producing a vacuum with a pressure of at most 5 kPa
(50 mbar) in the desiccator.
5.2.1.4 Filter system, an appropriate apparatus for the separation of the debris produced during the
degradation study. This may involve an inert filter and rinse system, a temperature-controlled centrifuge,
or a combination thereof. The apparatus shall be described and defined in the test report.
5.2.2 Number of test samples
At least three test samples shall be evaluated at each retrieval interval. Since mass loss is sample
specific, a separate container shall be used for each mass loss sample.
6 © ISO 2017 – All rights reserved

---------------------- Page: 11 ----------------------
ISO 13781:2017(E)

5.2.3 Procedure
5.2.3.1 Measurement of initial mass
Dry each un-degraded test
...