論文の公開元へSynergistic effect of sulfonation followed by precipitation of amorphous calcium phosphate on the bone-bonding strength of carbon fiber reinforced polyetheretherketone
概要
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Synergistic effect of sulfonation
followed by precipitation
of amorphous calcium phosphate
on the bone‑bonding strength
of carbon fiber reinforced
polyetheretherketone
Yusuke Takaoka 1*, Shunsuke Fujibayashi 1, Takeshi Yabutsuka 2, Yuya Yamane 2,
Chihiro Ishizaki 2, Koji Goto 1, Bungo Otsuki 1, Toshiyuki Kawai 1, Takayoshi Shimizu 1,
Yaichiro Okuzu 1, Kazutaka Masamoto 1, Yu Shimizu 1, Makoto Hayashi 1, Norimasa Ikeda 1 &
Shuichi Matsuda 1
Sulfonation and applications of amorphous calcium phosphate are known to make
polyetheretherketone (PEEK) bioactive. Sulfonation followed by precipitation of amorphous calcium
phosphate (AN-treatment) may provide PEEK with further bone-bonding strength. Herein, we
prepared a carbon-fiber-reinforced PEEK (CPEEK) with similar tensile strength to cortical bone and
a CPEEK subjected to AN-treatment (CPEEK-AN). The effect of AN-treatment on the bone-bonding
strength generated at the interface between the rabbit’s tibia and a base material was investigated
using a detaching test at two time-points (4 and 8 weeks). At 4 weeks, the strength of CPEEK-AN was
significantly higher than that of CPEEK due to the direct bonding between the interfaces. Between 4
and 8 weeks, the different bone forming processes showed that, with CPEEK-AN, bone consolidation
was achieved, thus improving bone-bonding strength. In contrast, with CPEEK, a new bone was
absorbed mainly on the interface, leading to poor strength. These observations were supported by
an in vitro study, which showed that pre-osteoblast on CPEEK-AN caused earlier maturation and
mineralization of the extracellular matrix than on CPEEK. Consequently, AN-treatment, comprising
a combination of two efficient treatments, generated a synergetic effect on the bonding strength of
CPEEK.
Polyetheretherketone (PEEK) is a high-performance engineering plastic with exceptional resistance to chemicals,
wear, and fatigue. In addition, because of its mechanical properties and biocompatibility, it is used as an orthopedic implant1–3. Radiolucency, one of its most attractive characteristics, enables clear interpretation of postoperative medical images2,3. Furthermore, according to the circumstances, PEEK can change its tensile strength to the
desired strength by carbon-fiber reinforcement, and the tensile strength of 50% carbon-fiber-reinforced PEEK
(CPEEK) is about 120 MPa and close to that of cortical bones4–6. Therefore, it potentially resolves the problem
caused by gaps in tensile strength between implants and bones, for example, stress shielding7–9, (often seen when
metallic implants are used), and the weakness of pure PEEK material.
Because of its chemical inertness, PEEK is not bioactive. Therefore, various treatments have been used to
achieve osseointegration with P
EEK9–12. Among them, s ulfonation9,11,13–16 and applications of amorphous calcium
phosphate (ACP)17–19 are popular treatments for gaining bone-bonding strength in PEEK implants. Sulfonation,
acid-etching by immersion in sulfuric acid, changes the surface topography and gives proton conductivity to the
benzene rings of PEEK13,20 for chemical bonding with other compounds, including ACP17. Despite the advantage
1
Department of Orthopedic Surgery, Kyoto University Graduate School of Medicine, Kyoto, Japan. 2Department
of Fundamental Energy Science, Graduate School of Energy Science, Kyoto University, Kyoto, Japan. *email:
ytakaoka@kuhp.kyoto-u.ac.jp
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of sulfuric acid immersion in changing the surface topography, including pore size, it may be a disadvantage for
this treatment. If the immersion time is too long, it harms the human body because of the residual sulfuric acid
on the s ubstrates13,14,21. Additionally, it damages PEEK itself and weakens its material strength14,21.
Alternatively, ACP is used as coatings and cement for orthopedic and dental a pplications17. ACP is an intermediate phase that precipitates from a highly supersaturated calcium phosphate solution and converts easily to a
stable crystalline phase. ACP is already known to precipitate on the surface of materials in solution with C
a+ and
17,22
3−
PO4 by increasing the pH and temperature of the s olution . It has better conductivity and biodegradability
than hydroxyapatite and tricalcium p
hosphate18. In this way, ACP is expected to not only overcome the major
drawbacks of the excellent method, sulfuric acid treatment, but also extend its advantages. In this study, we
focused on both treatments and tried to combine them for a synergetic effect. AN-treatment involves short-time
sulfonation and glow discharging followed by precipitation of ACP produced by immersion in modified simulated
body solution (SBF), dubbed “apatite nuclei”, developed for better bone-bonding strength23–25. Modified SBF
identified in previous studies to search for the ideal SBF for better apatite deposition on C
PEEK23.
When implants are embedded in a living body, fibrous tissues form on its surface, interfering with the direct
bonding with the b
one9,12. This encapsulation, sometimes seen as a radiolucent line in the post-operative X-ray,
results in the failure of implantation caused by aseptic loosening26. Furthermore, thick layers of fibrous tissues
are unsuitable for weight translation26. Additionally, this encapsulation site can become the site of inflammatory responses to fine particles caused by the wear of the implant, for example, the bearing surface of artificial
joints27. Ideally, the integration without encapsulation between the base material and new bone will improve
bone-bonding strength and resolve these problems.
Bone-bonding strength is essential for osseointegration in vivo to get rigid fixation during functional loading.
This is achieved by ossification, based on the delicate balance between bone formation and resorption, which
changes with t ime28 and requires two time-points evaluations. Therefore, this study aimed to measure the interfacial bone-bonding strength at two time-points (4 and 8 weeks after implantation), with CPEEK and CPEEKAN, to evaluate the effect of AN-treatment on the PEEK interface. Furthermore, we investigated the factors in
an in vitro study that may influence the differences in bone-bonding strength, with and without the treatment.
Results
Substrates. Surface characteristics. There was no obvious difference between CPEEK and CPEEK-AN
substrates; however, that of CPEEK-AN looked slightly white (Fig. 1). A sequential procedure, including immersion in sulfuric acid twice for 4 s, exposing glow disposing, and immersion in modified SBF (Table 1) for 24 h
changed the surface (Fig. 2). From the scanning electron microscopy (SEM) observation (Fig. 3a), the CPEEK
surface was changed to a complicated porous structure by sulfonation. This porous structure possessed various
shapes and diameters to 800 nm, as shown in Fig. 3. The surface after AN-treatment was almost covered with the
precipitate of apatite nuclei (Fig. 3a); this was confirmed by the X-ray photoelectron spectroscopy (XPS) profile,
For in vivo
CPEEK-AN
CPEEK
For in vitro
Figure 1. Photographs of the substrates (CPEEK, CPEEK-AN) for in vivo and in vitro study. White bar
indicates 15 mm.
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Dissolved amount or volume in
1 dm3
Reagent
SBF
Modified SBF
NaCl
137 mmol·dm−3
–
NaHCO3
4.17 mmol·dm−3
–
KCl
3.00 mmol·dm−3
–
K2HPO4·3H2O
1.31 mmol·dm−3
1.31 mmol·dm−3
MgCl2·6H2O
1.72 mmol·dm−3
1.72 mmol·dm−3
1 mol·dm−3 HCl
35 cm3·dm−3
35 cm3·dm−3
CaCl2
2.50 mmol·dm−3
2.50 mmol·dm−3
Na2SO4
0.50 mmol·dm−3
–
Table 1. Amounts of dissolved reagent in the preparation of 1 dm3 SBF and modified-SBF.
Figure 2. Schematic diagram of the fabrication process for CPEEK-AN, and its magnification, as well as the
reaction equation for sulfonation. Emphasis of H in –SO3H expresses the proton conductivity.
as shown in Fig. 4a. Although CPEEK-AN showed peaks intensity in Ca2p and P2p derived from calcium (Ca)
and phosphorus (P), respectively, no peaks were observed for CPEEK other than C1s derived from O=C–O.
Additionally, the peak derived from S–O, which expresses the presence of sulfuric acid, observed in sulfonated
CPEEK was weakened in CPEEK-AN. The water contact angle showed that this treatment improved wettability
(Fig. 4b). A detailed evaluation of the properties of these materials has been reported in previous studies.
Evaluation of the apatite‑forming ability. After soaking in S BF29 (Table 1) for 1 day, apatite formation was confirmed on the surface of CPEEK-AN, whereas almost no deposition was observed on other substrates (Fig. S1).
This study was performed before the in vivo and in vitro experiments; we compared CPEEK and CPEEK-AN
based on the results. The sulfonated CPEEK was not included in subsequent experiments.
In vivo study. Bone‑bonding strength evaluation by the detaching test. The extracted blocks were obtained
from the femur of rabbits to evaluate the interfacial bone-bonding strength between bones and substrates with
the apparatus (Fig. 5a–c). As shown in Fig. 5d the average failure loads obtained from each group (CPEEK in
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(a)
3µm
3µm
3µm
CPEEK
Sulfonated CPEEK
CPEEK-AN
Occupancy
(b)
Pore size
Figure 3. (a) SEM photographs showing the surface of CPEEK, sulfonated PEEK, and CPEEK-AN. (b)
Characterization of pores formed on the sulfonated PEEK.
week 4, CPEEK-AN in week 4, CPEEK in week 8, and CPEEK-AN in week 8) were 3.25 ± 2.3N, 12.5 ± 6.1 N,
3.13 ± 2.0 N, and 28.7 ± 7.3 N, respectively. ...
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