In vitro degradation of polydimethylsiloxanes in breast implant applications
Abstract
Background
The durability of breast implant material is associated with failure probability, increasing with time from implantation. The current study avoided the bias introduced by biological factors, to systematically investigate the degradation over time of shell materials. The same fundamental physical and chemical conditions were maintained (temperature and pH) throughout the study, to decouple biological aspects from the degradation process.
Methods
Six virgin implants of 2 brands were submitted to the in vitro degradation process, mechanical testing of shell materials, surface change analysis (via scanning electron microscopy [SEM]) and chemical composition analysis by Fourier transform infrared (FTIR) spectroscopy.
Results
FTIR results showed that the principal chemical bonds of the material remained intact after 12 weeks of degradation. Apparently the implants’ shell structures remained unchanged. Despite this observation, there were statistically significant differences between strain at failure at different time points for the shells of both brands, translated into a stiffening of the material over time.
Conclusions
Material stiffening is reported as an indicator of material degradation. This altered mechanical behavior, added to the mechanical friction from tissue–tissue and tissue–implant contact and to the external mechanical loading (physical activity), may alter the material performance in women’s bodies. Ultimately these changes may affect the implants’ durability. Further work is needed to understand the biological aspects of the degradation process and their impact on implant durability.
J Appl Biomater Funct Mater 2017; 15(4): e369 - e375
Article Type: ORIGINAL RESEARCH ARTICLE
DOI:10.5301/jabfm.5000354
Authors
Nilza G. Ramião, Pedro S. Martins, Maria L. Barroso, Diana C. Santos, António A. Fernandes
Article History
• Accepted on 24/03/2017
• Available online on 24/05/2017
• Published online on 10/11/2017
Disclosures
Financial support: The authors gratefully acknowledge funding from the Ministério da Ciência, Inovação e do Ensino Superior, FCT - Fundação para a Ciência e a Tecnologia, Portugal, under grants SFRH/BD/85090/2012 and SFRH/BPD/111846/2015 and projects: LAETA - UID/EMS/50022/2013; UROSPHINX - Project 16842, cofinanced by Programa Operacional Competitividade e Internacionalização (COMPETE2020), through the Fundo Europeu de Desenvolvimento Regional (FEDER) and by National Funds through FCT; NORTE-01-0145-FEDER-000022 – SciTech – Science and Technology for Competitive and Sustainable Industries (NORTE2020).
Conflict of interest: None of the authors has any financial interest related to this study to disclose.
The polydimethylsiloxanes (PDMS) are the basis of both the gel and shell of breast implants. The shell is produced from liquid components and an amorphous “fumed” silica (SiO2) filler. SiO2 is added to make high-performance silicone rubber for many purposes, including for the enhancement of mechanical properties (1-2-3-4-5). The gel is a weakly cross-linked material which forms a 3-dimensional polymer network. Cross-linking occurs due to the reaction of vinyl groups present in the copolymer chains (dimethyl- and methylvinyl-siloxane) (3-4-5). Increasing the degree of PDMS cross-linking can lead to stronger and stiffer shell and gel materials (4). The aging of implant material, is associated to slow degradation and involves multiple physical and/or chemical processes.
A major risk for breast implant failure is material degradation during implantation time (1-2-3, 6-7-8-9). Several causes for degradation of the breast implant have been identified in the literature:
additional cross-linking that embrittles the silicone (2);
degradation or weakening of the Si-O-Si cross-links in the shells, to Si-OH (9-10-11);
degradation leading to the production of dimethyl siloxanes (e.g., octamethylcyclotetrasiloxane [D4]) (2);
swelling of the silicone elastomer shell by silicone fluid from the gel (2, 7, 12);
silicone degradation produced by prolonged contact with lipids (3, 8, 13, 14);
degradation of the base polymer to a lower average molecular weight material (2).
Although biodegradation effects have been identified, in vitro biodurability testing of PDMS breast implants is not well-documented in the open literature. Most of the work done by breast implant producers for implant development is confidential.
The purpose of this present research was to characterize breast implant degradation under the physical and chemical conditions of the human body (temperature and pH): a “normal” physiology (pH 7.4) and an inflammatory process (pH 4.0). This study comprised the following stages: (i) an in vitro degradation process, (ii) analysis of the mechanical performance of the shell, (iii) analysis of the surface (via scanning electron microscopy [SEM]) and (iv) analysis of the chemical composition by Fourier transform infrared (FTIR) spectroscopy.
Materials and methods
Six virgin implants of 2 different brands (see Fig. 1) were tested. All implants were round shaped, with textured surface and low profile. The degradation study was carried out taking into account
Breast implants considered: round-shaped, textured and low profile: brand 1 (A) and brand 2 (B).
interbrand comparison: different brands were compared at the same time point;
intrabrand comparison: for each brand, the same implant lot was used for all time points.
Degradation test
The degradation process was conducted according with the ISO 10993 standard: Biological Evaluation of Medical Devices. To study in vitro degradation processes of the materials, 2 buffer solutions were used: phosphate-buffered saline with pH 7.4 (Ref. P4417; Sigma-Aldrich), and potassium hydrogen phthalate buffer solution (Ref. 82560; Sigma-Aldrich) with pH 4.0. The degradation process was carried out for 12 weeks at a controlled temperature of 37°C. Every week, a batch of samples was removed from the thermal bath, and the samples’ weights were measured. Percentage weight loss was calculated using Equation [1]:
%massloss=pf−pipi*100 Eq. [1]
where pi is the initial sample’s weight and pf its weight after different degradation stages (0-12 weeks).
Mechanical test
The mechanical properties were evaluated at the end of each stage of degradation. These properties were obtained from uniaxial tensile testing data, through a prototype developed at INEGI Biomechanics Laboratory (Porto, Portugal). The experimental protocol followed the ISO standards for shell integrity (ISO 14607:2007) and determination of tensile stress-strain properties (ISO 37:2005). The equipment used has alloy aluminium arms connected to actuators and 2 load cells with 50-N capacity. Before the tensile test, samples were subjected to a 0.25 preload to guarantee a pretesting controlled initial geometry. The samples were tested until failure, at a constant displacement rate of 20 mm/min.
Morphological characterization
SEM analyses were carried out in a JEOL JSM 6301F/Oxford INCA Energy 350/Gatan Alto 2500 microscope (Tokyo, Japan) at CEMUP (University of Porto, Portugal). This technique was used to analyze the morphology evolution due to degradation. Samples were analyzed at 4 time points: 0, 4, 8 and 12 weeks.
Samples were coated with an Au/Pd thin film, by sputtering, using SPI Module Sputter Coater equipment, for 120 seconds at 10.00 kV voltage | 15 mA current.
Surface characterization by FTIR spectroscopy
The chemical composition of the surfaces were analyzed using a Cary 630 FTIR Spectrometer (Agilent Technologies, USA) equipped with a diamond attenuated total reflectance (ATR) accessory. The tests were carried out at INEGI’s tribology laboratory (CETRIB). Each spectrum was acquired over 20 scans with wavelengths ranging from 600 to 4,000 cm-1, with a resolution of 4 cm-1. A background scan was performed before each sample measurement.
Statistical analysis
The statistical analysis was performed using IBM SPSS software version 20.0, with the significance level set at a p value <0.05. The normal distribution of the data was verified with Kolmogorov-Smirnov and Shapiro-Wilk tests. The statistical differences in the mechanical properties among groups were assessed using independent-samples t-test and Manny-Whitney U-test. The groups considered were brand 1 and lot 1.1, brand 1 and lot 1.2, and brand 2. Each group was controlled at 2 time points (0 and 12 weeks), while subjected to in vitro aging using different buffer solutions: pH 7.4 and pH 4.0.
Results
Mass loss during in vitro aging
This study included 4 implants from brand 1: 2 of the implant from 1 lot (ref. lot 1.1) and 2 more implants from another lot (ref. lot 1.2). Lot 1.1, after 12 weeks of degradation, lost 0.10% of its mass soaked in pH 7.4 solution and 0.11% in pH 4.0 solution (Fig. 2A); lot 1.2 lost 0.52% mass, soaked in pH 7.4 solution and 0.75% in pH 4.0 solution (Fig. 2B). The initial pH of the buffer solutions did not change during the degradation period.
Experimental results of loss of mass for brand 1 (A, B), and brand 2 (C) during the degradation period in different buffer solutions.
Brand 2 aged in a pH 7.4 solution from week 11 onwards displayed a mass loss rate of -0.34% which appeared as an asymptotic convergence (Fig. 2C). Implants soaked in pH 7.4 solution lost 0.34% in mass, and 0.58% in pH 4.0 solution.
Mechanical properties analysis
To analyze the material behavior during different degradation stages (weeks), a total of 384 samples were tested under uniaxial loading. For comparison purposes, a subsample corresponding to 0 (beginning) and 12 (end) weeks was considered after the degradation process. The mechanical properties of each tested implant and the statistical results are presented in Table I.
Mechanical properties for breast implant samples at different stages of degradation (0 and 12 weeks)
Implant
Weeks
No.
Tensile strength (MPa)
p value*
Strain at failure
p value†
Bold type indicates significant differences (p<0.05).
* Analysis by independent-samples t-test.
† Analysis by Mann-Whitney U-test.
Brand 1
Lot 1.1
pH 7.4
0
6
10.34 ± 1.39
0.496
3 .06 ± 0.25
0.004
12
6
11.22 ± 1.87
2.50 ± 0.07
pH 4.0
0
6
11.68 ± 1.56
0.217
2.98 ± 0.01
0.575
12
6
10.07 ± 1.5
2.76 ± 0.26
Lot 2.1
pH 7.4
0
9
12.28 ± 1.56
0.245
2.87 ± 0.34
0.171
12
9
13.32 ± 2.08
2.85 ± 0.14
pH 4.0
0
9
12.43 ± 1.89
0.811
2.91 ± 0.38
0.031
12
9
12.66 ± 2.04
2.77 ± 0.10
Brand 2
Lot 2
pH 7.4
0
18
15.01 ± 1.15
0.164
3.08 ± 0.24
0.003
12
18
15.58 ± 1.34
2.96 ± 0.04
pH 4.0
0
18
14.45 ± 0.91
0.211
3.11 ± 0.31
0.030
12
18
14.99 ± 1.52
2.88 ± 0.10
There were no statistically significant differences in the tensile strength for any of the groups analyzed (Fig. 3). However, the strain at failure results for brand 1 in lot 1.1 under pH 7.4 solution and for lot 1.2 under pH 4.0 solution were significantly different (p<0.05). The strain at failure of brand 2 samples showed statistically significant differences (p<0.05) under both solutions. It is worth noting that strain at failure decreased between 0 and 12 weeks (Tab. I and Fig. 3).
Tensile test results during degradation in 2 buffer solutions: brand 1 and lot 1.2 (A, B); and brand 2 (C, D). Blue and red lines represent the stiffening of the shell.
SEM analysis
SEM was used to analyze material surfaces. Before degradation (0 weeks), implants from the same lot showed similar surface morphology.
Brand 1 and brand 2 showed similar surface morphology at different degradation stages for pH 7.4 and pH 4.0 buffer solutions (Fig. 4). The textured outer surface of brand 1 showed a different morphology (Fig. 4A) from that of brand 2 (Fig. 4B). Brand 1 revealed larger surface structures extending over several hundred micrometers, whereas brand 2 structures were squarer and had larger gaps between them.
Scanning electron microscopy images of the outer surface of brand 1 implants (×75 for lot 1.1 and ×200 for lot 1.2) (A) and brand 2 implants (×75 and ×200) (B), over 2 time points (0 and 12 weeks) in pH 7.4 and pH 4.0 solutions.
ATR-: FTIR analysis
Figure 5 show the FTIR spectra for all implants over different stages and in degradation solutions of pH 7.4 and pH 4.0. All FTIR spectra showed that samples were of the same type of material, with similar spectra to a PDMS spectrum found in the literature (8). There was no evidence of any significant chemical structure modifications.
Fourier transform infrared (FTIR) spectra of brand 1 (A) and brand 2 (B) soaked in different solutions and at different degradation stages.
The wave numbers (cm-1) showed significant peaks (for material identification) at 786 cm-1, corresponding to Si–C bond vibrations (800-760 cm-1). The peaks at 1,005 cm-1 and 1,004 cm-1 correspond to the stretching vibrations of Si–O–Si bonds, with a broad band in the region 1,100-1,000 cm-1 (polymer backbone). The peak at 1,258 cm-1 is associated with Si–CH3 bonds (1, 280-1, 250 cm-1), and the weak band at 1,412 cm-1 is correlated with asymmetric deformation vibration. A peak at 2,964 cm-1 corresponds to vibrations of the Si–OH bonds. The variation in the absorbance seen in Figure 5 is likely due to the differences in elasticity and thickness of samples.
Discussion
The durability and useful life of a breast implant continues to be a subject of intense interest and debate among both patients and the plastic surgery community. To evaluate the potential impact of in vivo degradation on the mechanical properties of implant shells, an experimental protocol including uniaxial tension testing and in vitro degradation was carried out with 2 implant brands. The observation that implant shells are sensitive to degradation in the body has been demonstrated for a long time in several studies (2, 6, 9, 12, 15-16-17). Several authors showed a negative correlation between implant duration and mechanical resistance (1, 2, 6, 7, 9, 16, 17). Furthermore, Yildirimer et al (9) using FTIR, found evidence of degradation of the Si-O-Si cross-links to Si-OH on silicone shells, which may be related to inflammation.
However, Brandon et al (12, 18, 19), Wolf et al (20) and Swarts et al (21) showed that there was no time-dependent degradation in the shell tensile strength over years of implantation. These authors considered implant failure to be highly correlated with the implantation and explantation procedures, trauma of the breast or manufacturing defects.
This assessment of the problem does not take into consideration the baseline implant shell properties, since all previous studies were conducted on explanted implants (in vivo) (1, 2, 6, 7, 9, 12, 15-16-17-18-19-20-21). The current study avoided the bias introduced by biological factors, to systematically investigate the degradation over time of shell materials. The fundamental physical and chemical conditions were maintained (temperature and pH) in an effort to decouple biological aspects from the whole degradation process. The overall material degradation differs from person to person, from tissue to tissue and over time for the same person (22). The present study found evidence of mass loss over the degradation period (Fig. 2); however, the tensile strength of the shell material was not significantly affected, as can be seen in Table I and Figure 3.
There was a statistically significant strain-at-failure reduction for the shells of both brands after a degradation period of 12 weeks (Tab. I). This is translated into a stiffening of the shell, as illustrated by the blue and red lines in Figure 3D. Material stiffening is reported as an indicative factor of material degradation (23). Considering brand 2 (Fig. 3D) for a 2.5 level of strain, the stress after 12 weeks of degradation was higher than the initial stress.
SEM results revealed that the surface morphology did not show any differences after 12 weeks of degradation. Apparently the implants’ shell structures remained unchanged.
FTIR results (Fig. 5) indicated the presence of the same type of PDMS for implant material in all samples, which agrees with the literature (8, 9, 24-25-26). No spectral deviations were observed during the degradation period, which suggests a lack of chemical degradation.
Conclusion
In this study, we attempted to decouple the biological aspects from the whole degradation process, by maintaining the human body physical and chemical conditions – i.e., temperature and pH.
FTIR results showed that the principal chemical bonds of the material remained intact after 12 weeks of degradation. Despite this observation, there were statistically significant differences between the strain at different time points, which translated into a stiffening of the material over time. This change may alter the mechanical friction, tissue–tissue and/or tissue–implant, affecting significantly the performance of the implant material in a woman’s body when subjected to external mechanical loading such as physical activity etc. Ultimately these changes may affect the implant durability.
Further work is needed to understand the biological aspects of the degradation process especially over longer testing periods. In particular, aspects such as oxidation (due to oxidants produced by tissues) and enzymatic degradation should be analyzed. Longer degradation periods may also shed some light on fundamental degradation mechanisms such as weight loss, which was not fully understood with the current study. Another factor that possibly contributes to implant failure is the loading frequency imposed on the breast. In this context, a dynamic-mechanical analysis could expose changes of mechanical behavior induced by degradation, not revealed by a static analysis.
Disclosures
Financial support: The authors gratefully acknowledge funding from the Ministério da Ciência, Inovação e do Ensino Superior, FCT - Fundação para a Ciência e a Tecnologia, Portugal, under grants SFRH/BD/85090/2012 and SFRH/BPD/111846/2015 and projects: LAETA - UID/EMS/50022/2013; UROSPHINX - Project 16842, cofinanced by Programa Operacional Competitividade e Internacionalização (COMPETE2020), through the Fundo Europeu de Desenvolvimento Regional (FEDER) and by National Funds through FCT; NORTE-01-0145-FEDER-000022 – SciTech – Science and Technology for Competitive and Sustainable Industries (NORTE2020).
Conflict of interest: None of the authors has any financial interest related to this study to disclose.
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INEGI, Associated Laboratory for Energy, Transports and Aeronautics (LAETA), Faculty of Engineering, University of Porto, Porto - Portugal
Department of Plastic Surgery, Gaia Hospital Center, Vila Nova de Gaia - Portugal
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