Journal of Biomaterials and Nanobiotechnology, 2011, 2, 361-368
doi:10.4236/jbnb.2011.24045 Published Online October 2011 (http://www.SciRP.org/journal/jbnb)
361
The Influence of Different Sterilization Techniques
on the Time-Dependent Behavior of Polyamides
Galina Kubyshkina1, Barbara Zupančič2*, Marina Štukelj3, Dušan Grošelj4, Ljubo Marion4,
Igor Emri2
1Elektromaterial Lendava d.d., Lendava, Slovenia; 2Center for Experimental Mechanics, Faculty of Mechanical Engineering, Univer-
sity of Ljubljana, Ljubljana, Slovenia; 3Veterinary Faculty, University of Ljubljana, Ljubljana, Slovenia; 4Faculty of Medicine, Uni-
versity of Ljubljana, Ljubljana, Slovenia.
Received July 18th, 2011; revised August 22nd, 2011; accepted September 27th, 2011.
ABSTRACT
For this investigation conventional polyamide 6 with monomodal molecular mass distribution, and the newly developed
bimodal one were used. Conventional polyamide 6 was used as a reference material in order to emphasize prospects of us-
ing bimodal material for medical applications from the point of view of sterilization resistance and improved creep behavior.
Time-dependent mechanical properties of testing samples were characterized by torsional creep measurements in non-ster-
ilized state and after sterilization with three different techniques: with autoclave, ethylene oxide, and hydrogen peroxide
plasma. Results show that the two materials exhibit pronounced difference in morphology and consequently, mechanical
properties. Both of them were not significantly affected by any of used sterilization techniques. However, bimodal material,
originally being noticeably more time-stable in comparison to monomodal one, retains these preferences also post steriliza-
tion.
Keywords: Sterilization, Polyamide, Creep, Time Stability
1. Introduction
Polymers are often the materials of choice for medical
devices, medical packaging, and food packaging, replac-
ing traditional materials such as stainless steel and glass
[1]. Polymers are beside non-dental applications exten-
sively used in dentistry as composite (resin-ceramic) res-
torative materials, implants, dental cements, and denture
based teeth.
Theoretically, they can be modified for maximum bio-
compatibility and acceptable mechanical properties. Al-
though all three groups of polymers (thermoplasts, ther-
mosets, elastomers) are used for medical applications,
the thermoplastics are most widely used [2].
They offer manufacturing cost savings, lighter weight,
and performance characteristics that meet and exceed the
demand in many highend applications. However, the pro-
cess of sterilization can strongly affect the properties of
the polymers [1,3].
Sterilization is the complete elimination of microbial
viability, including the vegetative forms of bacteria and
spores. Sterilization of surgical equipment, implants, linens,
and attire is one aspect of a series of highly regimented
steps constituting aseptic technique. The methods of ster-
ilization can be divided into two general groups: physi
cal and chemical. Although sterility can be achieved with
certain chemicals, physical methods are generally more
reliable. Heat, filtration, and radiation are the most com-
monly used physical methods of sterilizing medical and
surgical materials. Chemical sterilization is usually ac-
complished with ethylene oxide or hydrogen peroxide,
although formaldehyde and β-propiolactone are also used
occasionally. The most commonly used techniques in ve-
terinary medicine are steam, ethylene oxide, and hydro-
gen peroxide gas plasma sterilization [4].
Sterilization has increasingly been a topic of discus-
sion when comes to the implantology. There are several
issues to be considered when sterilization methods are
being selected for implantable medical devices [5].
The number of agents capable of sterilizing product or
material without adversely or deleteriously affecting pro-
duct quality or material integrity is few. There is no sin-
gular sterilization method that is compatible with all
healthcare products including drugs, polymers, devices,
and materials, because of the severity of a process to
meet the sterilization criteria and definition [6].

The Influence of Different Sterilization Techniques on the Time-Dependent Behavior of Polyamides 362
In our investigation we applied the three most com-
monly used sterilization techniques: autoclave, Ethylene
Oxide (EtO), and H2O2 plasma. In continuation, the brief
overviews of the three sterilization methods effects on
polymers’ properties are presented.
Steam sterilization by autoclaving has traditionally
been the most widely used method for medical instru-
ments [7]. The steam method is convenient, fast, and
widely available. It is inexpensive and can quickly ster-
ilize porous materials and accessible surfaces and spaces
[8]. However, this method has some disadvantages. It is
well known that heat and steam can drastically alter the
mechanical and chemical properties of many synthetic
biomaterials. The main disadvantage of steam steriliza-
tion is the hydrophilicity of some polymers, which ren-
ders the implant mechanically weaker [9]. Lastly, the
steam itself may carry contaminants to implant surfaces
and directly lead to a lesser degree of in vivo biocom-
patibility [5].
Ethylene oxide-EtO (CH2-O-CH2) and gamma sterili-
zation are the most common sterilization methods for
thermoplastics used in the disposable medical device
industry [10]. The gas vapor sterilant most commonly
used is ethylene oxide; it is capable of killing very effi-
ciently all the species and forms of micro-organisms in a
short time, but in pure form is extremely toxic, irritating,
water and common organic solvents soluble, and explo-
sive. EtO is bactericidal, virucidal, and sporicidal. Its
germicidal activity results from its action on the proteins,
DNA and RNA.
The flammability hazard of EtO has been reduced by
mixing it with an inert gas, usually Freon chlorofluoro-
carbon, and after year 1995, an alternate gas, hydro-
chlorofluorocarbon is being used as a carrier for EtO
sterilization [10]. A non-flammable preparation is also
obtained when EtO is mixed with carbon dioxide in a
ratio 1:9.
Marois et al. reported on investigation of sterilization
effect on poly beta-hydroxy octanoate (PHO). The phy-
sical and structural properties of PHO were shown to be
well preserved following EtO (ethylene oxide) steriliza-
tion, whereas gamma radiation caused random chain
scission and physical cross-linking, a frequent phenome-
non observed with organic polymers [11].
Lucas et al. reports that EtO residues in medical de-
vices and polymers depend on the type and size of the
material. Some materials retain little EtO residues, such
as polyurethane PU 80A and hollow catheters, whether
others retain much larger amounts, such as polyurethane
PU 75D and nylon 66. Presented work illustrates that
EtO levels and effects are highly dependent on the type
of material used. Residual EtO, the amount of EtO that is
bioavailable, and the effect of EtO in vitro must be con-
sidered for each type of material or device [12].
Buben et al. report on rough generalizing conclusions:
acrylate polymers and copolymers frequently used as
hard components of medical devices release absorbed
EtO very slowly. This applies to the following materials:
ABS, MBS, MMABS, SAN/ABS. In addition to acrylate
polymers EtO is released also by other hard polymers e.g.
parts of polycarbonate medical devices, hardened PVC
or cross-linked polypropylene [13].
The study of Brown et al. investigated the effects of a
range of methods on the tensile strength of latex rubber,
silicone elastomer, 2 different formulations of polyure-
thane, nylon, and high-density polyethylene (HDPE) spe-
cimens. The methods of disinfection and sterilization
used were hypochlorite bleach, peracetic acid + hydro-
gen peroxide, formaldehyde gas, low-temperature per-
acetic acid and gas plasma, and low-temperature hydro-
gen peroxide gas plasma. The results showed that sili-
cone elastomer was minimally affected, whereas the
strengths of nylon, polyethylene, and latex were reduced
by some of the methods [14].
Plasma-based methods a priori offer the required effi-
cacy near ambient temperature to sterilize polymeric de-
vices, but they inevitably also modify the exposed poly-
mer surfaces. The hydrogen peroxide plasma sterilization
causes surface oxidation of many types of polymers [15].
The study has shown that two commercial plasma-based
sterilizers, Sterrad® and PlazlyteTM, can substantially
alter the surface characteristics of all the polymers inves-
tigated in this paper, namely their surface composition
and morphology, and their wettability, even after a single
sterilization cycle [15].
Regarding sterilization with EtO, they have generally
found a lesser extent of surface modification than follow-
ing plasma-based techniques, but this advantage must be
weighted against the possible hazards of retained EtO or
its toxic by-products in treated polymers [15].
It has already been noted that nominally identical
polymer materials, for example the PVC film and tubing,
or the two polyurethane catheter materials, could reveal
quite different surface compositions, oxidation behav-
iours, wettabilities, and surface topographies. This under-
lines the important fact that the generic composition of a
polymer is insufficient for judging whether a particular
device can be safely resterilized; clearly, the nature and
concentration of additives in the polymer formulation,
the manufacturing process, and numerous other variables
must also be taken into account [15].
Polymer degradation caused by sterilization is more
often associated with oxidative processes rather than
with hydrolysis. As known, there are some sterilization
technologies, such hydrogen peroxide plasma steriliza-
tion systems, which use hydrogen peroxide (H2O2)-pla-
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The Influence of Different Sterilization Techniques on the Time-Dependent Behavior of Polyamides363
sma and peracetic acid, respectively, as an oxidative ster-
ilizing agent [16].
In our study we compared the effects of steam under
pressure as physical sterilant and of ethylene oxide and
plasma gas as gas vapour method of sterilization on the
time-dependent mechanical properties of two types of
polyamide PA6 (Nylon).
Nylon is biostable polymer [10]. Polyamide and co-
polymers utilized for healthcare products such as cathe-
ters used in cardiovascular procedures, sutures, epidural
catheters, laparoscopy devices, blood sets, joints, kidney
dialysis, composite materials and films, and special pack-
ages might be submitted to the following sterilisation
processes: steam, limited dry heat depending on steriliza-
tion temperature up to 130˚C [6], EtO, limited radiation,
limited H2O2. Polyamides are sensitive to crosslinking
from radiation, but are suitable for a single dose of radia-
tion. PA6 may be at least radiation resistant [6].
The sterilization stability of nylon and filled nylon
with EtO is very good but there exist some susceptibility
of material to oxidizing agents. The stability of nylon to
autoclave sterilization is very good but components may
swell slightly due to water absorption [17].
Sicard et al. have shown that sterilization by steam or
EtO had no significant effect on the strength of the nylon
fishing material. Ethylene oxide was the preferred me-
thod of sterilization to preserve strength and minimize
elongation of the fishing material [18].
Kirby et al. reported that there was no difference be-
tween untreated and EtO sterilized bands with respect to
testing to failure using a splint circular jaw mounted on a
tensile testing machine, whereas bands subjected to auto-
claving failed at less force [19].
Bochkova et al. investigated the uniaxial stretching of
microfiltration membranes made of polyamides. Changes
were found in the mechanical properties of the mem-
branes after autoclaving, which had been induced by
spon-taneous polymer arrangement on heating; the ar-
rangement was expressed as either a great self-elongation
of the membrane or its shrinkage [20].
As analyzed by Navarrete & Hermanson (1998), after
exposure to HCFC 124, HCFC 124/22 and pure EtO, the
tensile and Izod properties of Nylon appear relatively
unaffected. The physical properties of Nylon, and Acry-
lic were not significantly affected by any of the three
sterilant gases [10].
Gatineau et al. compared the effects of hydrogen per-
oxide gas plasma (HPGP) and ethylene oxide (EtO) ster-
ilizations on the mechanical properties of nylon lines
used for stabilization of the canine stifle. Sterilization
with HPGP seems a good alternative to EtO for nylon
lines [21].
For the purpose of our investigation two different
types of polyamide material PA6 were used, conven-
tional one with monomodal molecular distribution, and
novel material with bimodal one. The objective of this
paper was to analyze how three of the commonly used
sterilization methods will affect novel bimodal material
in comparison to monomodal one with respect to their
time-dependent (long-term) mechanical properties. Cha-
racterization of the investigated materials was performed
by means of torsional creep tests.
2. Experimental Part
2.1. Materials
We have tested the effect of sterilization on the newly
developed I-PA6 (intelligent polyamide) described by
Emri et al. [22], which was compared to commercially
available PA6 material. Conventional PA6 has monomo-
dal molecular mass distribution, whereas the new I-PA6
has bimodal one. Multimodality of the material was
achieved by proper modification of its molecular mass
distribution to put it more precisely, by varying the av-
erage chain length of the PA6 polymer chains during
polymerization process starting from the same PA6 mo-
nomer units. The average molecular weight of both ma-
terials is roughly the same.
Structure of both materials, observed by means of op-
tical microscopy, is shown in the Figure 1. By compari-
son one may infer significant difference in morphology
of non-sterilized standard material and modified one,
prepared by the same technology. In the case of mono-
modal PA6 obtained image indicates formation of coarse
spherulites that can be observed as Maltese cross patterns
(Figure 1(a)), while bimodal I-PA6 forms homogeneous
fine grain structure when exposed to the same processing
conditions (Figure 1(b)).
From both types of polyamide materials samples were
prepared to test their time-dependent mechanical proper-
ties after exposure to three different sterilization tech-
niques: autoclave sterilization, EtO sterilization, and H2O2
plasma sterilization. Standard monomodal PA6 was used
as a control material in this study.
2.2. Sample Preparation
In order to perform creep measurements PA cylindrical
Figure 1. (a) Morphology of non-sterilized monomodal PA6,
and (b) bimodal I-PA6; Axioskop 2 MAT (Carl Zeiss), po-
larized transmitted light, 200×.

The Influence of Different Sterilization Techniques on the Time-Dependent Behavior of Polyamides 364
specimens with diameter D = 3.0 mm and length L =
60.0 mm were made from both materials by gravimetric
casting procedure, which is briefly described below. This
method was developed to avoid orientation of molecules
caused by melt flow when using injection molding or
extrusion in sample preparation. After casting all sam-
ples are annealed to minimize the effect of physical age-
ing to the level that is negligible. The annealing proce-
dure was thoroughly studied in our laboratory in the past.
Based on these studies we determined the temperature
profile which brings sample sufficiently close to ther-
modynamic equilibrium.
The specimen preparation equipment is illustrated in
Figures 2(a) and 2(b). The casting procedure starts with
filling up a glass tube with an inner diameter of 3 mm
with PA pellets (Figure 2(a)). Following this, the heater
is placed on the bottom of the tube to start the melting
process, and a pressure of about 1MPa is applied from
above to overcome friction forces between the material
and the tube inner surface. The heater temperature is
maintained at 270˚C and it is moved upwards the glass
tube at slow rate (2.25 mm/min), giving the pellets
enough time to melt (Figure 2(b)). The applied pressure
pushes upper non-molten pellets into the melt, thus re-
moving any possible trapped air. This procedure is fol-
lowed until the spiral heater slides over the whole surface
of the tube. Then the heater is removed from the glass
tube and the specimen is left to cool down slowly to the
room temperature. Finally, received cylindrical rod is
extracted from the glass tube, cut down to the length of
60mm, and then polished and glued to the special grips
as shown in Figure 3.
2.3. Methods of Sterilization
In this investigation we have applied three commonly
Figure 2. Schematic representation of cylindrical rods pre-
paration for creep measurements.
Figure 3. Cylindrical rod sample glued to the special grips.
used sterilization techniques: a) autoclave sterilization, b)
EtO sterilization, and c) H2O2 plasma sterilization. Con-
ditions of each method are briefly explained below:
2.3.1. Autoclave or Steam Sterilization
Investigated samples were sterilized by the autoclave
type Euroklav 23 V-S, manufactured by Melag, Germany.
Before sterilization was applied, thin slices were placed
inside the sterilization bags and sealed. The cylindrical
rod specimens were put and sealed directly into steriliza-
tion bags. Steam sterilization was applied for 75 min at
121˚C and 0.2 MPa. After the procedure the specimens
were allowed to dry for 2 hours.
2.3.2. Ethylene Oxide
In our study sterilizer De Lama S. Martino Sicc. (Pavia
It.) of Blood Transfusion Centre of Slovenia has been
utilized. The characteristics of the gas sterilizer are: EtO
pressure 0.2 MPa, EtO/CO2 mixing ratio of 10%/90%,
sterilization temperature 50˚C, relative humidity 60%,
exposure time 8 hours. After the sterilization cycle 10
repeated air flushing cycles per hour are applied to re-
move the EtO retained gas. The aeration procedure is
validated with regard to sterilization cycle and nature of
the material designed to be fumigated. For sterilization of
our polyamide samples the chamber DLOG, manufac-
tured by De-Lama was used. The samples were sterilized
for 1 hour, and then they were aerated for 2 hours inside
the chamber. Then, the sterilized samples in the sealed
bags were taken out of the chamber and allowed to aerate
for 10 days before tests were performed.
2.3.3. Hydrogen Peroxide Plasma
Sterilization of our samples was done by using the Ster-
rad® 100S system manufactured by Johnson & Johnson,
USA. Both materials were sterilized at 50˚C for 1 hour.
2.4. Methods of Characterization
Since properties of polymers may considerably change
with time, which means that functionality and stability of
polymeric products may be affected over time of their
exploitation [23,24], we investigated how the steriliza-
tion affects mono- and bi-modal polyamide time-depen-
dent (long-term) mechanical properties. In order to char-
acterize the effect of different sterilization techniques on
materials’ long-term behavior, shear creep compliance
measurements were performed according to the procedure
described below.
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The Influence of Different Sterilization Techniques on the Time-Dependent Behavior of Polyamides365
Shear creep measurements of investigated PA materi-
als were performed with the Apparatus for Torsional
Creep Measurements that has been designed and manu-
factured at the Centre for Experimental Mechanics at the
University of Ljubljana, Slovenia [25,26].
Initial part of the creep measuring procedure started
with the annealing phase at higher temperature (110˚C)
that should erase mechanical stress-strain history of the
material. The thermal history to which specimens were
exposed during the creep experiment is schematically
shown in Figure 4.
After the annealing phase the torsional creep meas-
urements were performed in segmental form at 8 differ-
ent elevated temperatures, nominally at 30˚C, 40˚C, 50˚C,
60˚C, 70˚C, 80˚C, 90˚C, and 100˚C in the time interval
of 3 hours, under loading in shear by torque, t
M
, of the
constant magnitude ranging from 0.007 - 0.07 MPa, de-
pending on the temperature at which the measurements
were performed. In all cases the applied torque resulted
in shear strain, not exceeding 0.05%. The response of the
material was measured in terms of rotational angle of the
specimen, ()t

as a function of time and then recalcu-
lated to the material creep compliance:



0
J
ttrl

  (1)
where denotes radius of the circular cross section of
the sample and -length of the sample.0
r
l

is the magni-
tude of constant shear stress loading, calculated as:
0tp
M
rI

 (2)
with polar momentum
p
I
for circular cross section
geometry of cylindrical samples. The magnitudes of ap-
plied shear stress loading were sufficiently low to ensure
that the entire measurement procedure remained within
the scope of the theory of linear viscoelasticity.
Following the time-temperature superposition princi-
ple, measured segments were shifted along the time-scale
in relation to the segment, measured at the nominal ref-
erence temperature, Tref = 50˚C. Shifting was executed
3715 182139
24 27 30 333642 45 485154 5760
Time, t (h)
Temperature, T (°C)
110
100
90
80
70
60
50
40
30
3715 182139
24 27 30 333642 45 485154 5760
Time, t (h)
Temperature, T (°C)
110
100
90
80
70
60
50
40
30
Figure 4. Schematic representation of the temperature-
loading profile of the shear creep measurements on PA
samples.
by using the recently developed closed-form shifting
(CFS) procedure [27]. CFS shifting methodology brings
in error that is more than 10-times smaller as the under-
lying experimental error of data being shifted.
Obtained shift factors were then modeled by the WLF
equation in order to obtain material constants, 1 and
2, which are presented in Table 1. By knowing these
constants we calculated shift factors that corresponded to
the shifting of master curves in order to represent the
creep behavior at the reference temperature of 37˚C,
which is a relevant temperature for medical applications.
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C
3. Results and Discussion
3.1. Non-Sterilized Monomodal and Bimodal
Materials Comparison
Performing creep tests according to the procedure de-
scribed above we firstly investigated the difference in
time-dependent mechanical behavior of non-sterilized
monomodal and bimodal PA6. In the concerned case it is
appropriate to present the long-term creep behavior at the
reference temperature close to the temperature of a hu-
man body with respect to the proposed conditions of use.
Figure 5 shows comparison of time-dependent creep
compliances for both non-sterilized PA6 materials for the
reference temperature 37˚C.
Comparison of creep compliances may be interpreted in
terms of long-time stability for the products made of
such materials. In the diagram it is clearly seen that creep
compliance of both materials increase with time, but,
nevertheless, creep compliance of bimodal material is
lower than such of monomodal along the complete ex-
amined time scale. From the point of view of desired
life-span of dental implants, which is more than 20 years
[28], it seems to be reasonable to analyze mechanical
properties and their change during this period of time. In
our research the time period of 30 years was considered.
In 30 years ≈ 109 s, it can be observed that creep com-
pliance of bimodal material is approximately 25% lower
Table 1. Constants C1 and C2, calculated from WLF mod-
eling of shift factors for non-sterilized and sterilized mono-
and bimodal PA6.
Type of materialType of sterilization C1 (/) C2 (˚C)
Non-sterilized 17.88 69.25
Ethylene Oxide (EtO) 10.41 31.40
Steam (autoclave) 13.84 58.92
Monomodal PA6
H2O2 plasma 10.96 36.52
Non-sterilized 17.50 81.72
Ethylene Oxide (EtO) 7.86 26.48
Steam (autoclave) 13.95 63.97
Bimodal PA6
H2O2 plasma 17.98 84.44

The Influence of Different Sterilization Techniques on the Time-Dependent Behavior of Polyamides 366
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
1234567891011121
Log t (s)
J
ref
(1/MPa)
3
mono-modal PA 6, Non-sterilized
bi-modal PA 6, Non-sterilizedT
ref
= 37°C
t ~ 30 years
J
t
bimodal
J
t
monomodal
Figure 5. Comparison of creep compliances of non-steri-
lized monomodal PA6 and bimodal PA6 at reference tem-
perature Tref = 37˚C.
relative to the creep compliance of monomodal PA6
(Figure 5) which indicates better creep resistance (long-
term stability) properties of bimodal PA6. As it was ob-
served from the optical microscopy pictures above (Fig-
ure 1), bimodal PA6 has finer structure in comparison to
monomodal PA6, which can consequently lead to better
creep resistance properties of bimodal material. It is evi-
dent that both materials become less stiff with time, but
for monomodal material this process is much faster than
that of bimodal one.
The absolute change in creep compliance,


,
ref ref
J
tT,
which represents the magnitude of total deformation
when the implant is exposed to constant load over certain
period of time, for both materials may be estimated from
the same diagram as well. For the monomodal material
this change is bigger than for the bimodal one. Thus, one
may summarize that bimodal PA6 creeps much less in
the same time period in comparison with monomodal
PA6 both in absolute and relative numbers.
3.2. Sterilization Techniques Influence on
Materials Long-Term Mechanical Properties
After exposure of both materials to three different ster-
ilization techniques it was investigated how much the
process of sterilization affected their mechanical proper-
ties. Figure 6 presents the comparison of creep compli-
ances of non-sterilized and sterilized monomodal PA6
materials at reference temperature 37˚C, whereas Figure
7 shows the same comparison for bimodal PA6.
From Figure 6 one may observe all sterilization tech-
niques have noticeable effect on long-term stability of
monomodal material and all exhibit a tendency to dete-
riorate it at that. From the diagram it is clearly seen that
EtO has the most severe effect on monomodal material
time stability within whole time scale.
The diagram in Figure 7 shows that in case of bi-
modal PA6 sterilization effect is not as significant: the
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
012345678910111
Log t (s)
J
ref
(1/MPa)
2
mono-modal PA 6, Non-sterilized
mono-modal PA 6, Ethylene Oxide (EtO)
mono-modal PA 6, Steam (autoclave)
mono-modal PA 6, H2O2 plasma
T
ref
= 37°C
t ~ 30 years
non-sterilized
material
Figure 6. Creep behavior of sterilized and non-sterilized
monomodal PA6 at Tref = 37˚C.
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
012345678910111
Log t (s)
J
ref
(1/MPa)
2
bi-modal PA 6, Non-sterilize d
bi-modal PA 6, Ethylene Oxide (EtO)
bi-modal PA 6, Steam (autoclave)
bi-modal PA 6, H2O2 plasma
T
ref
= 37°C
t ~ 30 years
non-sterilized
material
Figure 7. Creep behavior of sterilized and non-sterilized
bimodal PA6 at Tref = 37˚C.
creep compliance curves of sterilized material stand much
less apart from the one of non-sterilized than in case of
monomodal material. At the same time it is also observed
that EtO has the strongest effect on both materials.
In Table 2 the magnitudes of creep compliance,


,
ref ref
Jt T
9
10
ref
t s
, are presented at the reference time
, and reference temperature, Tref =
37˚C. Besides this, the information in the columns
named
30years


T
10
,
ster
Jt

ref
t
non
rel

refref in Table 2 shows the rela-
tive difference in the magnitude of creep compliance at
the reference time, , and reference
temperature, Tref = 37˚C for sterilized samples in relation
to the creep compliance magnitude of non-sterilized
sample.
930s years
Taking into account spread of experimental data, we
may say that sterilization affects both materials. It should
be taken into account that due to the accuracy of the
measuring technique (which is within 10%) the calcu-
lated relative difference, , below 10%
could not be recognized as the difference.

,
non ster
relref ref
JtT 


There were found rather discrepant data on polyamide
sterilization resistance in the literature. Most likely it can
be explained with variety of properties that are taken into
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The Influence of Different Sterilization Techniques on the Time-Dependent Behavior of Polyamides 367
Table 2. Creep compliance, J(tref, Tref), and relative difference in creep compliance, (, )


non ster
relref ref
J
tT , at the reference time tref
= 109 s ≈ 30 years, and reference temperature Tref = 37˚C for monomodal and bimodal PA6.
Monomodal PA6 Bimodal PA6
Type of sterilization J(tref, Tref), (1/MPa) (, )
non ster
relref ref
J
tT 
, (%) J(tref, Tref), (1/MPa) (, ) non ster
relref ref
J
tT 
, (%)
Non-sterilized 0.0049 / 0.0038 /
Ethylene Oxide (EtO) 0.0061 23.4 0.0032 15.5
Steam (autoclave) 0.0052 5.2 0.0042 10.8
H2O2 plasma 0.0060 21.4 0.0038 1.6
account when sterilization effect is analyzed as how dif-
ferent sterilization techniques affect different properties.
Generally, it is stated that PA materials are affected by
high humidity, temperature and oxidants. This makes
sense, since polyamides are very sensitive on moisture,
which has analogous effect as increase of temperature
[29], and susceptible oxidative and thermal degradation.
Regarding steam sterilization, in a number of sources
it is recommended not to sterilize polyamides in auto-
clave since they are susceptible to water vapor and heat
sensitive [8]. From another hand, some publications [1,6,
18] claim that nylon exhibits very good resistance to
steam sterilization. According to the experimental results
we observed, time stability of both mono- and bi-modal
materials sterilized with autoclave was not significantly
affected, which is in agreement with the second state-
ment.
As reported by Sicard et al. [18] sterilization stability
of nylon and filled nylon with EtO is very good but there
exist some susceptibility of material to oxidizing agents.
Generally in literature it is stated that PA appear rela-
tively unaffected after exposure to EtO [1,19]. In practice,
PA sutures are mostly EtO sterilized. But, regarding ob-
servations in our study, results of performed experiments
go in contradiction with information that PA exhibits
very good resistance to EtO, since it was observed that
sterilization with ethylene oxide had the most significant
effect on time-dependent mechanical properties of both
investigated materials. Deteriorated time stability of
monomodal PA6, but improved both characteristics of
bimodal PA6. This observation merits further attention.
As to H2O2 plasma sterilization it is mentioned in lit-
erature that it causes oxidation in several types of poly-
mers [15,16]. Thus, the use of this technique is limited.
In the performed measurements there was observed no
significant effect on monomodal PA and modified I-PA
time-dependent behavior after H2O2 plasma sterilization.
4. Conclusions
From the performed analysis of mechanical properties
for both, monomodal and bimodal PA6 materials in
sterilization techniques, the following observations may
be summarized:
1) Non-sterilized
non-sterilized state and after exposure to three different
bimodal material yields more com-
pl
) Characterization of time-dependent behavior in
te
ore signifi-
ca
the time stability at
37
knowledgements
he support provided by
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