ステンレス鋼の表面改質による生体適合性向上
Surface tailoring of stainless steels for biocompatibility
Nagoya University, IMaSS, Professor Masazumi Okido
1．Introduction
Biomaterials can be roughly classified into ceramics, polymers, and metals dependingon their property, and used depending on the application 1-3). Metals have been widelyused in dental restorations, prostheses and fracture fixation. However, metallic
materials in contact with living tissue may be toxic to the human body if they are eluted
as metal ions due to corrosion or become wear powder due to frictional wear.
Biomaterials are required to be composed by non-toxic metal elements and not to cause
inflammation even when dissolved as ions 4). Therefore, austenitic stainless steels, CoCr alloys, pure Ti, Ti alloys, etc. have been used as excellent high corrosion resistance.
The corrosion resistance of these metals is maintained by the passive film 5). Austenitic
stainless steels are used as bone screws and bone nails in orthopedics because of their
good mechanical properties and easy processing 6-8). At present, stainless steel is used inJapan as a short-term implant material from the viewpoint of the risk of metal ion
elution, and rarely used for long-term implantation. On the other hand, in Southeast
Asia, it is used for long-term implants because it is inexpensive. There is little doubtthat austenitic stainless steel is a promising implant material, but at present thesurface modification is hardly performed for enhancing its corrosion resistance.
Apart from corrosion resistance, osteoconductivity is another vital index for theproperty evaluation of the implanted materials 9). The relationship between surface
properties and bone conductivity of metal implants have been reported by Yamamoto etal., where the hydrophilization of smooth TiO2 film (Ra/μm < 0.1) prepared on Ti surface
improved bone conductivity 10,11). According to Schwartz et al.12), bone conduction
proceeds in the sequence of protein adsorption to the implant, cell adhesion andproliferation, and then bone remodeling. For example, osteoblast is considered to havefavorable adhesion to fibronectin-adsorbed implant surfaces and the bone formation isprone to happen 13). Hydrophilization is thought to facilitate adsorption of cell adhesion
proteins and promote osteoblast adhesion. However, these are based on the results
derived from the evaluation for Ti alloys and so on for biomaterials, and it has not been
judged whether it can be applied to stainless steel. There have been few studies on therelationship between the surface properties of stainless steel and osteoconductivity
when used as implant.

－ 65 －

As a promising and inexpensive material for implant application, stainless steelneeds to be studied in a more intensive way, particularly in its osteoconductivity andanti-corrosion properties. In other words, it is integral to improve osteoconductivity
along with guaranteeing its anti-corrosion property. In a corrosive environment
containing chloride ions in a living body, passive film formed in air may not be able toensure sufficient corrosion resistance 14,15). Metal ions may dissolve chemically orelectrochemically in body 14). Therefore, in order to improve osteoconductivity andcorrosion resistance in stainless steel, it is considered to be necessary to create a stablepassive layer on which cell adhesible proteins and osteoblasts are positively adsorbed
as well.
In this paper, we tried to prepare a protective film that is highly hydrophilic andsuppresses the elution of metal ions by surface modification, and aimed to improve bone
conductivity and reduce inflammation. The samples used were 304 stainless steel which
is used for short-term implantation in vivo, and 316L stainless steel with high corrosion
resistance by adding Mo for long-term implantation. As the surface modification method,
the protective film was prepared by electrochemical anodizing treatment, immersion
treatment in an acid with oxidizer activity, and hydrothermal treatment immersed in
distilled water at high temperature and high pressure 15,16). The chemical composition,roughness, hydrophilicity, corrosion resistance, protein adsorption and so on werecharacterized for surface film formed by surface treatments. In addition, in vivo tests
were employed to evaluate osteoconductivity and inflammation due to dissolved metal
ions .
2．Materials and methods
2.1 Specimen
Two types of cold-rolled austenitic stainless steels with standard specifications wereused as specimens. Table 1 shows the chemical composition of the 304 and 316 L steels.

Table 1 Chemical compositions of stainless steel (mass %)
304
316L

Cr
Ni
18.00 ｰ– 20.00 8.00 ｰ– 10.50
16.00 – 18.00 10.00 – 14.00

MoMn–< 2.002.00 – 3.00 < 2.00

Si
< 1.00< 1.00P< 0.045< 0.045

S< 0.030< 0.030C< 0.08< 0.08

Plates with surface area of approximately 1 cm2 were used for in vitro testing and rods
were used for in vivo testing, i.e., 2 mm (φ) × 5 mm (L) for bone conductivity evaluationand 1 mm (φ) × 5 mm (L) for inflammatory test. All specimen surfaces were wet-polished－ 66 －

to #2000 with SiC abrasive paper, and buff-polished with 0.05 μm alumina powder togive surface roughness Ra/μm <0.1. After degreasing with acetone and washing with
distilled water, water droplets on the surface were removed using an air blower.
2.2 Surface modifications
Anodizing treatment, acid immersion treatment, and hydrothermal treatment wereused as surface treatments to improve the corrosion resistance. The anodic oxidation
was performed by increasing the potential to 2 V (vs Ag / AgCl sat. KCl) at a rate of 0.01
Vs -1 in a 0.1 M aqueous solution of NaOH at room temperature. The acid immersion
was performed by immersing the specimen in concentrated sulfuric acid (18 M) at room
temperature for 10 s and then washing with water. After acid immersion, it was placedin a Teflon container containing distilled water (40 mL) for hydrothermal treatment,
sealed in a stainless steel capsule, and kept at 230 °C or 180 °C for 3 h.
2.3 Surface characterization
The surface morphology was observed with a scanning electron microscope (SEM).
Chemical compositions of the austenitic stainless steel were analyzed with an X-ray
photoelectron spectrometer (XPS, Thermo Fisher Scientific Escalab 250). It is known
that surface roughness at μm level affects bone conductivity 10). The surface roughness(analytical range 150 μm × 112 μm) was measured with an ultra-deep profile measuring
microscope and evaluated by the arithmetic mean roughness Ra that is less susceptible
to local flaws.
Hydrophilicity and hydrophobicity were evaluated using a water contact angle, WCA,
by static droplet method (sessile drop method), on a drop shape instrument (FTA1000,
First Ten Angstroms). Immediately after preparation of the film, 2 μL of distilled water
was deposited on the specimen, and WCA after 10 s was measured by the curve fitting
method of elliptic approximation.
2.4 Protein adsorption test
As well as the previous protein adsorption test of Ti alloys 17), stainless steels were
immersed for 60 min in 0.5 mg mL-1 fibronectin aqueous solution or 25 mg mL- 1 albumin
solution at 37 °C. The amount of adsorbed protein on the surface was calculated basedon reflection ATR method of Fourier transform infrared spectrophotometer.
2.5 Corrosion test
The amount of metal ions dissolved from the surface-modified specimens was－ 67 －

evaluated by the static corrosion test (JIS T0304). After coating with insulating tape to
expose the modified surface, the specimen was immersed into 5 mL test solution at 37± 0.5 °C and left to stand for 7 d. In addition, in order to calculate the change in metal
ion amount over time, the specimen was immersed in the 3 mL test solution, and wasremoved from the test solution every 1, 4 and 7 d, washed with distilled water, and thenimmersed in the new test solution. Test solutions was 3 mass% NaCl for accelerated
corrosion test and Phosphate buffered salts (PBS(-)) adjusted according to the chloride
concentration in the living body fluid. PBS(-) consisted of 137 mM NaCl, 2.7 mM KCl,
10 mM Na2HPO4, 1.76 mM KH2PO4 at pH 7.4. Metal ions corroded into the solutions
were analyzed by an ICP spectrophotometer.
On the other hand, the influence of protein pre-adsorption on corrosion behavior of
the specimens was investigated. 200 μL aqueous solution of 25 mg mL-1 albumin was
dropped on the hydrophobic surface and 100 μL dropped on the hydrophilic surface, andthese droplets were kept for 60 min at 37 °C, to realize the saturated adsorption ofalbumin to the entire surface of the specimen. Then, the specimens were immersed in
PBS (-) of 3 mL in volume for 7 d in a thermostatic chamber set at 37 ± 0.5 °C, and thedissolved ions were detected by ICP.
2.6 Evaluation of osteoconductivity
The in vivo test was carried out using the specimen of a round bar (Φ2 mm × 5 mmL) polished and then surface-modified. In order to control the hydrophilicity, the surfacemodified specimen was stored in PBS(-) for 1 d, and the as-polished material was stored
in a sterile bottle before implantation. The specimen was implanted for 2 weeks into
tibial bones of rats that were 8 weeks age, SD (Sprague Dawley), IGS (International
Genetic Standard), male SPF (Specific Pathogen Free), using normal rats acclimatizedand quarantined for 4 d. The rats were reared at 24 ± 3 °C, humidity of 50 ± 20 %,ventilation frequency of about 10 to 15 times/h, and lighting time of 12 h/d.
After 2 weeks of implantation, the animals were bred and observed, and then thespecimens were removed together with the tibia and fixed with 10 % neutral formalinsolution. The specimens were polished to a thickness of 20 μm and stained withtoluidine blue, and then the cortical bone area was observed using a microscope (MegaLight 100, Shot Japan Co., Ltd.). RB-I was used as an index of osteoconductivity. The RBI value is defined as the percentage (%) of direct adhesion of hard tissue but not soft

tissue at the interface between the embedded stainless steel sample and bone. All
animal experiments were conducted at the facilities of AAALAC International(International Association for Accreditation of Experimental Animals).

－ 68 －

2.7 Anti-inflammatory evaluation
To prepare a specimen for anti-inflammatory evaluation, a round bar (Φ 1 mm × 5mm L) was polished and then surface-modified. All specimens were stored in PBS(-) to
exclude initial dissolution of metal ions and to maintain hydrophilicity. The specimens
were implanted to the back of rat subcutaneously with 6 to 8 specimens per animal
using a needle and observed for 2 weeks. Other conditions were the same as proposedin osteoconductivity evaluation. The rats were reared at temperature of 24 ± 3 °C,
humidity of 50 ± 20 %, ventilation frequency of about 10-15 times/h, and lighting timeof 12 h/d. After 2 weeks of implantation, rearing and observation, the whole skin wasremoved and fixed in 10% neutral formalin solution.
The anti-inflammatory property was, firstly, evaluated from presence or absence of
suppuration or hemorrhage around the implanting specimen, and neovascularizationor hemorrhage from the blood vessel, in the naked eye findings. Secondly, it wasevaluated from the presence of inflammatory cells such as lymphocytes and neutrophils,
and the thickness of fibrous capsule in tissue observation using light microscopy (MegaLight 100, shot Japan)
3．Results and discussion
3.1 Change in film composition by surface treatment
For the steels treated by the usual
surface modification methods, mostof them maintained smooth surfaces
with Ra/μm < 0.1. Only the smooth
specimens with Ra/μm < 0.1 wasused because the surface roughness
affected osteoconductivity according
to the previous researches. Figure 1
shows XPS profiles of as-polished
316L steel used in the research. The
ratio

of

calculated

thecomponents

based

on

the

was
de-

convolution of Fe 2p3/2, Cr 2p, Ni 2p
and Mo 3d spectras 18,19). Figure 2

Fig.1 XPS profiles and peak analysis for 316L
stainless steel as-polishedshows the relative ratio of eachelement in oxidized form present in the oxide film for the specimens as-polished,hydrothermally treated in distilled water at 180 °C or 230 °C for 3 h, hydrothermally－ 69 －

treated at 230 °C for 3 hours after
immersing in concentrated sulfuric

316L
Fe2+

acid at 25 °C, and anodized in 0.1 M

MoO3

NaOH solution at 25 °C. The film

Ni(OH)2
Ni2O3

formed by the surface modification

Cr(OH)3

was different in composition from

Cr2O3

the natural oxide film by polishing.

304

Fe2+

(a)

(b)

(c)

(d)

(e)

(a)

(b)

(c)

(d)

(e)

Compared with the as-polishedFig.2 Metal ratio in XPS surface analysis for

specimen, the percentage of Cr (III)stainlessin surface increased and Fe (sum ofhydrothermal treated for 3 h in distilled water

Fe(II) and Fe(III)) decreased by theat (b) 180 °C and (c) 230 °C, (d) hydrothermalhydrothermal treatment. Above all,treated at 230 °C after immersing for 10 s in 18

when the hydrothermal treatment

M H2SO4 at 25 °C, and (e) anodized in 0.1 M

was

NaOH at 25 °C

performed

atahighersteelsof(a)as-polished,temperature of 230 °C, the Cr (III)ratio in the film increased. Since film containing Cr (III) has high passive corrosion
resistance, it is expected to suppress the corrosion in vivo. In the case of SUS316L,
Molybdenum was present in the oxide film as hexavalent MoO3 under all surface
modification conditions. On the other hand, in the anodized specimens, the proportionof Cr (III) decreased and oxidized-state Ni and Fe tended to increase.
3.2 Corrosion behavior
Figure 3 shows the results of dissolution amount of metal ions for 7 d in 3 mass%
NaCl and PBS(-) solutions at 37 °C under static dissolution test. The 316L steel had
much less dissolution amount of total metal ions compared with that of 304 steel, in the
NaCl aqueous solution, indicating 316L steel had higher corrosion resistance than 304
steel. Since the chloride ion concentration was low in PBS solution, the corrosion
resistance was improved and the difference between 304 and 316 L became small.
Corrosion resistance was most highly improved by hydrothermal treatment after
sulfuric acid immersion. On the contrary, the corrosion resistance of the anodized
specimen deteriorated and the dissolved amount of Fe and Cr ions increased greatly.
Thus, metal ion dissolution was affected by the protective film formed by surface
modification.

－ 70 －

The static corrosion test with 3
mass% NaCl solution was different

400

from the corrosive environment inevaluate the metal ion amountdissolved from the stainless steels
for biomaterial in the biologicalsimulated environment. Surface
morphology

after

the

staticdissolution test was not changed incomparison with that before thetest, and no dissolution mark due topitting corrosion was found. It is

316L

300

Dissolution amount, M / nmol cm-2

the living body, so it is necessary to

304

3 mass%NaCl

200
100
0
30
20

Mo
Cr
Fe

PBS

10
0

considered that the dissolved metal(a) (b) (c) (d) (e) (a) (b) (c) (d) (e)ions were not from the substrate
but from the dissolution of theFig.3 Dissolved ion for 7 d in 3 mass% NaCl

surface oxide film.

and PBS solutions at 37 °C for 304 and 316L

Next, the time dependence of thestainless steels stainless steels of (a) as-

dissolved metal ion amount waspolished, hydrothermal treated for 3 h inmeasuredbiologicaldistilled water at (b) 180 °C and (c) 230 °C, (d)environment. From the results ashydrothermal treated after immersing for 10 s

shown in Fig.3, we targeted three

in 18 M H2SO4 at 25 °C, and (e) anodized in 0.1

specimens, i.e., specimen employed

M NaOH at 25 °C

in

the

for hydrothermal treatment at
230 °C after immersion in 18 M H2SO4 which had the highest corrosion resistance,
specimen anodized in a 0.1 M NaOH aqueous solution which was worst in corrosion
resistance, and the as-polished. Pre-adsorption of albumin was also employed in the
corrosion behavior investigations. Since in vivo there is protein, it is integral to analyze
the effect of protein on corrosion resistance. Figure 4 shows the integration values ofdissolved metal ions for 7 d. The initial amount after 1 d immersion was noticeable in
304 and 316L steels and the cumulative values increased gradually thereafter.
The Cr dissolution from the anodized material was confirmed only up to 1 d after
immersion, and thereafter it was below the detection limit. From this fact, it isconsidered that Cr ions contained in the anodized film was corroded in the initial stageof immersion. The amount of dissolved Fe ions tended to increase in the albumin preadsorbed specimen compared with the specimen having no albumin. The promotion of－ 71 －

property

wasrealized through the formation of
the oxygen concentration cell in
the albumin adsorption part andthe non-adsorption part or theformation of the oxygen deficientpart. In addition, albumin could bepolarized when it is adsorbed on anegatively charged surface such as

Dissolution amount, M / nmol cm-2

anti-corrosion

20

(c)
(c)

304

15

Cr

(c)

10

(a)

(a)
(a)

5
0
20
15

(c) Cr

(c)

316L

(c)

5

(a)

Mo

(a)
(a)

10

Fe

(b)

(b)

(b)

(b)

(b)

4

7

(b)

Fe

0

1

Immersing period, t / dhydroxyl group and it is thoughtthat the specimen surface becomesFig.4 Change in dissolved ions by ICP analysis

acidic by electrically attracting

immersed in PBS solutions for 304 and 316L

surrounding protons 20-22). Thesestainlesscould be the reasons for thehydrothermal treated for 3 h in distilled water

specimenbyat 230 °C after immersing for 10 s in 18 M H2SO4hydrothermal treatment after acidat 25 °C, (c) anodized in 0.1 M NaOH at 25 °C.

immersion

Left bar : albumin free, right bar : adsorbed

modifiedhavingthelowestdissolution amount of metal ions.

steelsof(a)as-polished,(b)

albumin before immersion

3.3 Protein adsorption and osteoconductivity
The protein is firstly adsorbed on implant surface and then osteocyte is attracted tothe protein layer, the osteocyte is attached to the protein layer, and finally the osteocyte
is stretched out along the protein layer. So favorable protein adsorption was theprerequisite for good osteoconductivity. It is known that osteoconductivity of Ti alloy
implants is related to hydrophilicity. This is because the more hydrophilic orhydrophobic the surface was, the more adsorption of proteins was promoted, according
to our previous researches 10,11,17). Therefore, the relationship between hydrophilicity
and protein adsorption along with osteoconductivity were also investigated for stainlesssteel. A peak around 1650 cm-1 due to C = O stretching of the peptide bond contained in
the protein was observed in the FT-IR results of the specimen immersed in albumin or
fibronectin solution, as shown in Figure 5. The protein adsorption amount wascalculated based on comparison of the peak area with that of specimen with known
adsorption value. The relationship with the water droplet contact angle, WCA, is shownin Figure 6 where the past results of Ti alloy are also shown as small marks. The Vshaped curves were obtained for Ti alloys with inflection points at 48 deg. for fibronectin
and 65 deg. for albumin. The amount of protein adsorption increased as more

－ 72 －

hydrophilic or hydrophobic for Ti alloy.
Similar correlations were observed formore corrosive compared to Ti alloys. It
was found that WCA was greatly affectedby the surface treatment method ofstainlesssteel.

hydrothermalInparticular,

treatment(a)Transmittancestainless steels in which metal ions were

(c)

the

showedsuperhydrophilicity with WCA <10 deg.

1800

and the protein adsorption capacity was
the highest.

(b)

1600

1400

1200

Wave Number (cm-1)

Fig.5 FT-IR spectrums for 304 stainlesssteel after immersing for 60 min at 298
K in distilled water (a), and in PBS
solutions with 50 mgcm-3 albumin (b)and with 1 mg.cm-3 fibronectin (c)Fig.6Amount of protein adsorbed on 304 and 316L stainless steels of (a) as-

polished, (b) hydrothermal treated for 3 h in distilled water at 230 °C after
immersing for 10 s in 18 M H2SO4 at 25 °C, and (c) anodized in 0.1 M NaOH at
25 °C. Small marks were results of Ti alloys in previous research
A typical picture of the interface between stainless steel embedded in a rat andcortical bone is shown in Figure 7. The interface was divided into parts in contact withthe hard tissue or fibrous tissue of the new bone, and the proportion was affected by thesurface treatment of the specimen to be embedded. Figure 8 shows the relationship－ 73 －

betweentheindicating

RB-I

value

osteoconductivity

and the WCA measured before
implantation. The past resultsof other Ti alloys are also
shown.

For

both

304

and

SUS316L, the RB-I value was
improved in the sample whose
WCA

was

hydrothermal

loweredbytreatment

at

230 °C after immersion in 18 M
H2SO4 compared with the aspolished stainless steel. It wasalso found that plots of stainlessFig.7

steels was well located at the V-

implanted for 14 d in cortical bone of the tibiae ofshaped curve obtained from the

8-week-old male rats

Ti alloy results. Therefore, it(a) as-polished specimen, (b) hydrothermallycan be deduced that RB-I istreated for 3 h in distilled water at 230 °C after

closely correlated with WCA

immersing for 10 s in 18 M H2SO4 at 25 °C

Photos of interface of 304 stainless steels

Fig.8 Osteoconductivity of 304 (a)(c) and 316L (b)(d) stainless steels of (c)(d)as-polished, (a)(b) hydrothermal treated for 3 h in distilled water at 230 °C
after immersing for 10 s in 18 M H2SO4 at 25 °C, Small gray marks were
results of Ti alloys in previous researchvalue, regardless of the type of metal, i.e., a certain WCA value would lead to almost the－ 74 －

same RB-I. Stainless steel is more likely to corrode as metal ions than titanium, but noeffect on bone conductivity was observed. The reason is considered to be that the protein
adsorption ability is increased by hydrophilization and henceforth the bone conductivity
is improved. From the above, protein adsorption capacity and osteoconductivity forstainless steel tended to be the same as other Ti alloys, indicating the surfacehydrophilicity property of the implanted metals was the dominant factor to affect theosteoconductivity. Furthermore, the effect of metal ion elution was small compared withthe effect of hydrophilicity property of implanted metals. So adjusting WCA value of
implanted metals could be the primary consideration to improve bone conductivity.
The in vitro protein adsorption test had high consistency to the in vivo bone
conductivity results. So it again proved that the protein adsorption on metal surface isthe prerequisite for bone conductivity. The super-hydrophilicity could greatly increase
the protein adsorption and enhance bone conductivity. Henceforth, t yhe surface
modification by hydrothermal treatment after acid immersion was superior to aspolished specimen as control group, and the anodized specimen.
3.4 Inflammation
In gross observation, no inflammatory
reaction such as suppuration or hemorrhage
was observed around the stainless implant.
Furthermore, no histological findings ofhemorrhage and remarkable blood vessels
were observed in the tissue observation of thespecimens, and no cell necrosis was observed.
Therefore, severe acute inflammation andchronic inflammation are considered not to
occur. Therefore, we estimated the thicknessof the fibrous capsule membrane formed
around the implant as an indicator of
inflammatory property as well as previous
work 23). When biomaterials are implanted in
the body, acute inflammation occurs, which

Fig.9

Photos of interface of 304

stainless steels implanted for 14 d incortical bone of the tibiae of 8-weekold male rats

has a rapid and premature termination. At
this time, the material with stronger recognition as a foreign substance is covered witha thick fibrous soft tissue for a long time. This fibrous soft tissue is a capsule membrane.
Figure 9 shows an example of a photograph of the capsule membrane-sample interface－ 75 －

around the stainless steel specimen. The thickness of the capsule membrane was used
as an index of inflammation.
Corrosion of implanted materials is considered to have an influence on inflammation.
It is, however, difficult to evaluate the dissolution amount and metal ions actuallyinserted in vivo. Therefore, the amount of metal ions corroded in vivo was substituted
by in vitro tests, i.e., the amount of metal ions from the specimen with adsorbed albuminin static corrosion test was used as an index indicating the case of corrosion in vivo. The
specimen for evaluation of inflammation was surface-modified and stored in PBS (-) justbefore implantation to maintain hydrophilicity, so it is considered that initial corrosion
within 1 d is finished before implantation in rats. Therefore, the amounts of Fe and Mo
during immersion period 2 d to 4 d was used as an index. Figure 10 shows therelationship between the thickness of fibrous capsule and the amount of dissolved ions.
It was found that the capsule membrane becomes thicker and the inflammation
becomes stronger as the metal ions were easily dissolved. Considering the influence ofion species on inflammation,twostraitlines,

noproportional

relation,areobtained corresponding to 304
and 316L steels respectively,in the case of Fe ions as shownin Fig.10 lower graph. The
proportional relation between
thethicknessofcapsulemembrane and the dissolved
molar amount of Fe and Mo
can be obtained as uppergraph.

From

the

above

results, corroded metal ionsincreases inflammation. The
amount of eluted metal ionsFig.10was positively related to themembrane in in vivo test and amount of dissolved

inflammation.

ions from stainless steels adsorbed albumin

Henceforth,

Relations between thickness of capsule

this in vivo tests support that(a) as-polished, (b) hydrothermal treated for 3 h inwhen evaluating the amountdistilled water at 230 °C after immersing for 10 s inof metal corrosion in vivo, it is

18 M H2SO4 at 25 °C , (c) anodized in 0.1 M NaOH atnecessary to carry out in an

25 °C－ 76 －

environment where proteins closer to the in vivo environment exist. The 304 steel withrelatively higher corrosion resistance showed the more suppressed inflammatory
property. As mentioned in the corrosion test, the hydrothermal treatment for 3 h in
distilled water at 230 °C after immersing for 10 s in 18 M H2SO4 at 25 °C had the lowest
metal ion elution amount, suggesting the highest anti-corrosion property, which also
meet well with the situation in vivo test that this specimen exhibited the lowest
inflammation. So the inflammation test in vivo further proved the advantages of thisparticular surface modification to stainless steel. The protective film with low metal ion
elution was prepared by surface modification of hydrothermal treatment after acidimmersion, and the reduction of inflammation was realized by suppressing metal ion
supply.
4．Conclusions
We aimed to improve osteoconductivity and reduce inflammation by surface
treatment for two kinds of stainless steel 24). Preparation of protective film by 230 °C
hydrothermal treatment in distilled water after immersion in 18 M H2SO4 at 25 °C wasa process effective in improving osteoconductivity and corrosion resistance. The
percentage of Cr in the surface passivation coating increased and the percentage of Fe
decreased by hydrothermal treatment. On the contrary, in the anodizing treatment, theproportion of Cr decreased and the proportion of Fe increased. As a result, the former
showed higher osteoconductivity and higher corrosion resistance than the latter.
Moreover, super-hydrophllic surface with water droplet contact angle, WCA, less than
10 deg. can be obtained by performing hydrothermal treatment. The relationshipbetween the WCA and the osteoconductivity RB-I was similar to that of the titanium
alloy etc. in our previous research. Effective surface treatment makes the surface superhydrophilic, which results in an increase in protein adsorption, leading to an
improvement in osteoconductivity. This study provided a close relationship between invitro tests and in vivo tests.
Acknowledgements
This work was financially supported by the JFE21st Century Foundation.
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