Bioinspired Surface Functionalization of Titanium for Enhanced Lubrication and Sustained Drug Release

Article abstract of DOI:10.1021/acs.langmuir.9b00338

Titanium and its alloys have long been used as implantable biomaterials in orthopedics; however, to the best of our knowledge, few studies were reported to investigate surface functionalization of titanium for enhanced lubrication and sustained drug relea

Full text of DOI:10.1021/acs.langmuir.9b00338

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Interface-Rich Materials and Assemblies
Bioinspired Surface Functionalization of Titanium for
Enhanced Lubrication and Sustained Drug Release
Ying Han, Sizhe Liu, Yulong Sun, Yanhong Gu, and Hongyu Zhang
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00338 • Publication Date (Web): 29 Apr 2019
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Bioinspired Surface Functionalization of Titanium for Enhanced Lubrication
and Sustained Drug Release
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Ying Han1, Sizhe Liu1, Yulong Sun1, Yanhong Gu2, Hongyu Zhang1,*
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1State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua
University, Beijing 100084, China
2Beijing Key Laboratory of Pipeline Critical Technology and Equipment for Deepwater
Oil & Gas Development, Beijing Institute of Petrochemical Technology, Beijing
102617, China
*Correspondence to Prof. Hongyu Zhang
State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua
University, Beijing 100084, China
Email: zhanghyu@tsinghua.edu.cn
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ABSTRACT
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Titanium and its alloys have long been used as implantable biomaterials in orthopedics,
however, to the best of our knowledge, few studies are reported to investigate surface
functionalization of titanium for enhanced lubrication and sustained drug release. In the
present study, titania nanotube arrays (TNTs) were prepared by anodization as effective
drug nanocarriers, using titanium as the substrate. Meanwhile, motivated by articular
cartilage-inspired superlubricity and mussel-inspired adhesion, a copolymer containing
both dopamine methacrylamide and 2-methacryloyloxyethyl phosphorylcholine was
synthesized (DMA-MPC) and spontaneously grafted onto the TNTs surface, which was
validated by the characterizations of scanning electron microscopy, water contact angle,
and X-ray photoelectron spectroscopy. Additionally, the lubrication test showed that
the copolymer-grafted TNTs remarkably reduced friction coefficient compared with the
bare TNTs. Furthermore, the drug release test demonstrated that the copolymer-grafted
TNTs inhibited burst drug release and achieved sustained drug release in comparison
with the bare TNTs. In conclusion, the bioinspired surface functionalization strategy
developed here, namely DMA-MPC copolymer-grafted TNTs, can be applied to
modify orthopedic biomaterials (such as titanium) for enhanced lubrication and
sustained drug release.
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Keywords: TNTs; articular cartilage; mussel; hydration lubrication; drug delivery
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INTRODUCTION
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Osteoarthritis is a degenerative joint disease caused by wear of the articular cartilage.1,2
Artificial joint replacement is the most common surgery for clinical treatment of end
stage osteoarthritis.3 Titanium and its alloys have been widely used as implantable
biomaterials due to their outstanding properties, including excellent biocompatibility,
good mechanical stability and corrosion resistance.4-6 However, titanium and its alloys
experience wear in vivo, and the wear debris-induced inflammation response occurs
following orthopedic surgery, which can result in serious complications and greatly
affect the lifetime of implants.7 Basically, it is difficult for the anti-inflammatory drugs
taken via oral administration to reach the joints, partly due to the restrictions including
poor targeting, poor distribution and uncontrolled pharmacokinetics.8,9 Consequently,
local drug delivery may represent a proper solution to solve this problem because this
technique, with lower dosage and extended release time, can deliver drugs directly to
the required location.10
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Titania nanotube arrays (TNTs) prepared with titanium as the substrate are reported to
be promising nanocarriers for drug delivery systems, which have attracted considerable
attention due to their excellent biocompatibility, high surface area, thermal stability,
and the potential for further surface modification.11-13 TNTs are tubular and self-
organized nanostructures that enable them to be loaded with therapeutic agents such as
antibiotics, anti-inflammatory agent, and growth factors using physical adsorption and
deposition method.14-17 Meanwhile, TNTs can be simply prepared in large quantities
via various techniques such as hydrothermal treatment of sol-gel template and anodic
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oxidation method. Generally, anodization is an effective method that can synthesize
TNTs with controlled nanostructure and morphology, being aligned perpendicularly to
the titanium substrate.18,19 Nevertheless, TNTs have relatively lower wear and abrasion
resistance, and as a consequence TNTs are easy to be removed away under shear force.
Therefore, a suitable surface modification is desirable to enhance the lubrication
property of TNTs, that is to say, TNTs should be modified with effective lubricating
materials to achieve better lubrication.
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In nature, articular cartilage can form excellent superlubricity under high loading based
on hydration lubrication mechanism, which is attributed to the supramolecular synergy
of the brush-like biomacromolecules.20-22 Specifically, the charged zwitterionic groups
–
(N+(CH3)3 and PO4 ) in the phosphocholine lipids attract a tenacious hydration layer
surrounding them, and the hydration layer responds in a fluid-like manner under shear,
significantly enhancing lubrication property between two sliding surfaces. Inspired by
this, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brush, with the same
zwitterionic charges as phosphocholine lipids, has been paid much attention recently.
Typically, Klein et al. modified mica with PMPC brush and obtained an extremely low
friction coefficient (COF, less than 0.001 as measured using surface force balance).23
However, the grafting-from strategy (such as atom transfer radical polymerization) to
modify the substrate with PMPC brush is complicated and often needs harsh reaction
conditions. Therefore, a universal and facile surface modification technique that can
functionalize the substrate with PMPC brush is required.
Mussels can attach to almost all kinds of substrates with high binding strength, and this
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has been attributed to the 3,4-dihydroxy-L-phenylalanine and lysine-enriched proteins
secreted by mussels byssus at the plaque-substrate interface. Dopamine, as an important
derivative of dihydroxy-phenylalanine with similar adhesion function to mussels, has
received increasing attention since it was first reported by Lee et al. for multifunctional
coatings.24-26 Regarding the mechanism of spontaneous attachment, it is well accepted
that the catechol groups are susceptible to oxidation to quinone under neutral or alkaline
conditions,27,28 and a strong bond is formed between dopamine and the substrate during
this process.
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In the present study, TNTs with a series of length to diameter ratios were prepared by
anodization, and simultaneously, motivated by cartilage-inspired superlubricity and
mussel-inspired adhesion, a novel copolymer (DMA-MPC) containing PMPC brush
and dopamine was synthesized, which could spontaneously attach onto the TNTs. As
shown in Figure 1, the copolymer can, on the one hand, enhance lubrication of TNTs
and thus reduce wear of the titanium implant, and on the other hand, enable sustained
drug release for anti-inflammatory purpose. It is hypothesized that, as a universal and
facile technique for enhanced lubrication and sustained drug release, the copolymer-
grafted TNTs developed here can be used for surface functionalization of titanium to
promote the lifetime of orthopedic implants.
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Figure 1. Schematic illustration showing the preparation of the bioinspired copolymer-grafted TNTs for
enhanced lubrication and sustained drug release.
MATERIALS AND METHODS
Preparation of TNTs. High purity titanium sheet (99 %, Goodfellow Cambridge
Ltd, UK) with a thickness of 1 mm was used as the substrate. Initially, the sheet was
polished and cleaned by sonication in deionized water for 5 min. The two-electrode
system was used in the electrochemical anodization process, with graphite rod as the
cathode and titanium sheet as the anode.29 The electrolyte contained 0.5 wt.%
ammonium fluoride, 3 vt.% deionized water, and 97 vt.% ethylene glycol. The
anodization was performed at room temperature using a direct current power supply,
with the anodization voltage of 5 V, 10 V and 15 V sequentially for 12 h. After
anodization, the titanium sheets were cleaned by sonication in deionized water and
ethanol for 2 min, and dried in vacuum.
Synthesis of Bioinspired Copolymers. Dopamine methacrylamide (DMA) was
initially synthesized according to the method previously reported.30-32 Briefly, sodium
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tetraborate (20 g, 52.4 mmol) and sodium bicarbonate (8 g, 95.2 mmol) were dissolved
in 200 mL of deionized water under nitrogen atmosphere, followed by rapidly adding
dopamine hydrochloride (10 g, 53 mmol, purchased from J&K Scientific Ltd., Beijing,
China). Meanwhile, 9.4 mL (63.4 mmol) of methacrylic anhydride was dissolved in 50
mL of tetrahydrofuran and added dropwise to the above solution (pH=8). The mixed
solution was stirred overnight under nitrogen atmosphere. Subsequently, the pH value
of the solution was adjusted to 2 by hydrochloric acid (0.2 M), followed by extracting
with ethyl acetate and filtering with magnesium sulfate. Finally, the filtered solution
was recrystallization with petroleum ether to produce a white solid power.
The copolymer (DMA-MPC) was synthesized by free radical copolymerization, with
the feeding mole ratio of DMA/MPC at 1/4 and azodiisobutyronitrile as the initiator.
Briefly, DMA (0.2 g), MPC (0.8 g, purchased from Joy-Nature Institute of Technology,
Nanjing, China), and azodiisobutyronitrile (3 mg) were dissolved in 50 mL of N, N-
dimethylformamide under nitrogen atmosphere, and then the reaction was proceeded at
65 °C for 24 h. Subsequently, the solution was dialyzed against deionized water and
freeze-dried to obtain a flocculent product.
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Preparation of Copolymer-Grafted TNTs. The DMA-MPC copolymer was
grafted onto TNTs using the dripping addition method. Briefly, DMA-MPC was
dissolved in phosphate buffered saline (PBS), and 60 μL of the copolymer solution (1
mg/mL, 2 mg/mL, and 5 mg/mL) was dropped onto the TNTs (bare TNTs or drug-
loaded TNTs) twice. The copolymer-grafted TNTs were washed with deionized water
and dried in vacuum.
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Lubrication Test. The lubrication test was performed using a universal materials
tester (UMT-3, Bruker Nano Inc., Germany). The experiments were completed in a
reciprocating mode (oscillation amplitude: 4 mm; sliding frequency: 1 Hz; normal load:
1~3 N) at 25 °C, each for a duration of 10 min. Three kinds of TNTs (bare TNTs or
copolymer-grafted TNTs) with different length to diameter ratios (i.e. prepared at the
anodization voltage of 5 V, 10 V, and 15 V) were used as the lower specimen, which
contacted directly with a polytetrafluoroethylene (PTFE) sphere pin (diameter: 8 mm)
as the upper specimen. The lubricant was deionized water for all the experiments to
ensure the PMPC brush was well stretched in the medium. Under each test condition,
at least three different locations on the TNTs surface were selected to calculate the
average COF value to ensure repeatability.
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Drug Loading and Release. Honokiol is a natural lignin that has been proved for its
anti-inflammatory effect in previous study.33 In the present study, honokiol was
selected to be loaded to TNTs by physical adsorption and deposition method. Briefly,
TNTs were immersed in 2 mg/mL of honokiol solution (dissolved in dimethyl
sulphoxide, DMSO), and the dynamic adsorption was allowed by shaking at 37 °C for
24 h. After loading, the TNTs were rinsed thoroughly by PBS to remove the drug
remaining on the surface of TNTs.
Subsequently, two groups of samples including drug-loaded bare TNTs and drug-
loaded copolymer-grafted TNTs (six specimens in parallel) were immersed in 5 mL of
PBS, and then placed in the shaker for drug release at 37 °C.34,35 At predetermined time
intervals, 1 mL of the medium was taken out and mixed with 1 mL of DMSO. The
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absorbance of the solution was measured by a UV-vis spectrophotometer (UV-8000,
Metash Instruments, China) at a wavelength of 296 nm. Meanwhile, 1 mL of fresh PBS
was added to the medium for further drug release test. The standard curve of honokiol
in DMSO/PBS (volume ratio at 1/1) solution was obtained, and the drug release
percentage (wt.%) was calculated based on the following equation (1).
amount of released drug
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Drug release percentage 
100
(1)
total amount of loaded drug
Characterizations. The surface and cross-section morphologies of the TNTs were
observed by a scanning electron microscope (SEM, SU8220, Hitachi, Japan) associated
with an energy dispersive spectrometer (EDS) for surface composition detection. The
surface wettability of the TNTs was examined by water contact angle employing a
1
goniometer (OCA25, Dataphysics Instruments, Germany). The H NMR spectra of
DMA and DMA-MPC were measured using a nuclear magnetic resonance (NMR)
spectrometer (JNM-ECS400, JEOL, Japan), with DMSO and D2O as the solvent. The
element analysis of the TNTs was performed by an X-ray photoelectron spectroscopy
(XPS, PHI Quantera II, Ulvac-Phi Inc., Japan). The Raman spectra of the TNTs were
recorded employing an XploRA Plus Raman spectrometer (Horiba, France) under the
following conditions: excitation wavelength of 638 nm, laser power of 0.5 mW, and
accumulation time of 50 s.
RESULTS AND DISCUSSION
Characterization of TNTs. Figure 2a-h shows the SEM micrographs of the surface
and cross-section morphologies for titanium substrate and ordered TNTs prepared by
oxidation at different voltages. The average diameter of the TNTs oxidized at 5 V, 10
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V, and 15 V is 25 nm, 30 nm, and 40 nm respectively, and the length is 0.6 μm, 1.0 μm,
and 1.0 μm respectively, resulting in the length to diameter ratio to be 24, 33, and 25
for the three kind of TNTs. This indicates that increasing the anodization voltage within
a certain range can increase the diameter of the TNTs as a result of a faster anodization
process. The EDS analysis of the TNTs is shown in Figure 2e, and clearly the O element
is detected on the surface. The formation of TNTs by anodization is attributed to the
equilibrium between the electrochemical process (oxidation reaction at the metal/oxide
interface, equation (2)) and the chemical process (etching reaction at the oxide/solution
interface, equation (3)).36,37 Briefly, the titanium substrate reacts with water molecules
under the action of electric field to generate an oxidation TiO2 layer. Subsequently, the
TiO2 layer was chemically attacked by fluoride ions to form the [TiF6]2- complexes,
which are water-soluble, and therefore a series of nanopores are produced and the
oxidation film becomes thinner. As a consequence, in these areas the ion migration rate
will be greater and the etching of the nanopores will be further accelerated to finally
generate TNTs.
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Ti + 2H2O → TiO2 + 4H+ + 4e-
(2)
TiO2 + 6HF → [TiF6]2- + 2H2O + 2H+
(3)
Figure 2i-l shows the water contact angle of the titanium substrate and ordered TNTs
prepared by oxidation at different voltages, which is measured after the water droplets
stay on the sample surface for 5 sec. The TNTs surface (contact angle: 2.6~3.5°) is
more hydrophilic than the titanium substrate (contact angle: 84.5°). The higher
hydrophilicity of TNTs can be attributed to two aspects. One is that the special tubular
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structure itself allows for seeping of the water droplet into the TNTs because of
capillary action.38 The other is that the Ti-OH group remains on the surface of TNTs
(formed owing to the cathode process consumes H+ to produce H2 during anodization),
thus increasing the hydrophilicity of TNTs.
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Figure 2. Characterization of TNTs: (a-d) SEM micrographs of the surface morphology for (a) titanium
substrate and TNTs anodized at (b) 5 V, (c) 10 V, and (d) 15 V; (e) typical EDS analysis of TNTs with
the detection of O element; (f-h) SEM micrographs of the cross-section morphology for TNTs anodized
at (f) 5 V, (g) 10 V, and (h) 15 V; (i-l) water contact angle of the (i) titanium substrate and TNTs anodized
at (j) 5 V, (k) 10 V, and (l) 15 V.
1
Characterization of DMA-MPC Copolymer. The H NMR spectra of DMA and
DMA-MPC are displayed in Figure 3. As shown in Figure 3a, the signal at 7.93 ppm is
assigned to the -NHCO-group of DMA, which indicates the success of the amidation
reaction between dopamine hydrochloride and methacrylic anhydride. Additionally, the
signals at 8.74 ppm and 8.62 ppm are attributed to the catechol group of DMA, which
maintains similar adhesion property to dopamine. As shown in Figure 3b, the signals at
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6.83 ppm and 6.71 ppm are assigned to the 3, 4, 6-trihydrophenyl groups of DMA in
the copolymer. The signals at 4.23 ppm, 4.03 ppm, 3.60 ppm, and 3.16 ppm belong to
the groups of MPC in the copolymer. It is calculated via integration that the content of
DMA in the DMA-MPC copolymer is ~20.3 wt.%, which conforms to the feeding mole
ratio of DMA/MPC (1/4) in the copolymerization reaction.
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Figure 3. 1H NMR spectra of (a) DMA monomer and (b) DMA-MPC copolymer.
Surface Grafting of TNTs with DMA-MPC Copolymer. The SEM micrographs
of DMA-MPC copolymer-grafted TNTs (TNTs@DMA-MPC) are presented in Figure
4a-c. The tubular structure of TNTs disappears, and it is covered by a dense copolymer
layer after surface grafting. The water contact angle of TNTs@DMA-MPC is shown in
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Figure 4d-f (in the range of 8.9~10.4°), indicating that the DMA-MPC copolymer is
superhydrophilic. Moreover, the presence of the copolymer layer grafted onto the TNTs
is evaluated by XPS. As shown in Figure 4g, the constituents of O 1s, Ti 2p, C 1s, and
Ti 3p with corresponding binding energies at 531 eV, 459 eV, 285 eV, and 40 eV are
observed in the titanium substrate and the TNTs@DMA-MPC. Compared with that of
the titanium substrate, the spectra of TNTs@DMA-MPC demonstrate new peaks of N
1s (402 eV, corresponding to -N+(CH3)3 and –NHCO-), P 2s, and P 2p (190 eV and 132
eV, corresponding to -OPOCH2).
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Furthermore, the Raman spectra of the bare TNTs and TNTs@DMA-MPC are
shown in Figure 5. It is clear that a few new peaks (such as P-O, P=O, and C=O) are
observed for the TNTs@DMA-MPC compared with the bare TNTs, indicating the
presence of the DMA-MPC copolymer on the TNTs surface. More importantly, the Ti-
O bond splits sharply for the TNTs@DMA-MPC, forming a chemical bond at 650 cm-1
between the Ti atoms (in TNTs) and the hydroxyl of catechol groups (in DMA-MPC
copolymer). This confirms that the DMA-MPC copolymer has been successfully
grafted onto the TNTs surface via an ion-dipole interaction.39
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Figure 4. Characterization of copolymer-grafted TNTs: (a-c) SEM micrographs and (d-f) water contact
angles of the TNTs@DMA-MPC anodized at (a, d) 5 V, (b, e) 10 V, and (c, f) 15 V; (g) XPS of the
titanium substrate and TNTs@DMA-MPC.
Figure 5. Raman spectra of (a) the bare and (b-d) DMA-MPC copolymer-grafted TNTs anodized at (b)
5 V, (c) 10 V, and (d) 15 V, respectively.
Lubrication Performance. The lubrication test is performed to characterize the
lubrication performance of the DMA-MPC copolymer-grafted TNTs. Figure 6a shows
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the schematic experimental setup of the lubrication test, and Figure 6b-d displays the
COF-loading plots for the TNTs prepared by oxidation at different voltages. Obviously,
all the COF values of the TNTs@DMA-MPC are lower than those of bare TNTs. For
instance, the COF value decreases evidently from 0.16 for the bare TNTs to 0.10 for
the TNTs@DMA-MPC (~37 %) prepared at 10 V under 3 N (Figure 6c). Additionally,
the COF value of the TNTs@DMA-MPC slightly increases under 3 N than that under
1 N and 2 N. It is considered that the PMPC brush in the DMA-MPC copolymer (grafted
on the TNTs) is well stretched in aqueous medium under low load, but the brush can be
crushed under relatively higher load, resulting in the increase of COF value. However,
on the other hand, the crush of the brush may be beneficial for accelerated release of
the anti-inflammatory drug, when the increased friction causes generation of more wear
debris at the sliding interface and correspondingly inflammation response of the joint.
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Figure 6. Lubrication performance of copolymer-grafted TNTs: (a) schematic diagram showing the
experimental setup of the lubrication test; (b-d) COF-loading plots for the bare TNTs and TNTs@DMA-
MPC anodized at (b) 5 V, (c) 10 V, and (d) 15 V.
Drug Loading and Release Property. Figure 7a demonstrates the standard curve of
honokiol in the DMSO/PBS mixed solution, and Figure 7b-d shows the drug release
behavior of the bare and copolymer-grafted TNTs prepared by oxidation at different
voltages. Clearly, all the curves have two stages including the initial burst release stage
followed by a relative plateau stage. The drug release of bare TNTs is remarkably
higher than that of TNTs@DMA-MPC (especially at the higher concentration),
indicating that the DMA-MPC copolymer layer can effectively block the drug from
burst release and prolong the drug release duration. For instance, the drug release
reduced from 60 % (at 11.5 h) for the bare TNTs to less than 40 % (at 50 h) for the
TNTs@DMA-MPC (5 mg/mL) prepared at 15 V (Figure 7d). Moreover, it is noted that
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the burst drug release and the total drug release are the highest for the bare TNTs
anodized at 15 V, which is caused due to the largest diameter of the tubular structure.
Additionally, the total drug release cannot reach 100 % during the test period owing to
the slow diffusion rate of the drug molecules entrapped inside the TNTs. These results
indicate that the TNTs@DMA-MPC can achieve a sustained release of the loaded drug,
which is desirable for the implanted orthopedic biomaterials.
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Figure 7. Drug loading and release property of copolymer-grafted TNTs: (a) standard curve of honokiol
in the DMSO/PBS mixed solution; (b-d) drug release curves of the bare TNTs and TNTs@DMA-MPC
anodized at (b) 5 V, (c) 10 V, and (d) 15 V.
Mechanism for Enhanced Lubrication and Sustained Drug Release. Figure 8
schematically demonstrates the mechanism for enhanced lubrication and sustained drug
release of the copolymer-grafted TNTs, which is attributed to the zwitterionic charges
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(N+(CH3)3 and PO4 ) contained in the PMPC brush and the catechol groups contained
in the adhesive DMA. Water molecule is overall neutral, but it has a large electric dipole
due to the residual charges on the H and O atoms. Therefore, the zwitterionic charges
in the PMPC brush can attract water molecules to form a tenacious hydration shell due
to the charge-dipole interaction. The water molecules in the hydration shell can support
high pressures without being squeezed out, and exchange rapidly with the nearby “free”
water molecules, resulting in a fluidlike response when being sheared and consequently
an enhanced lubrication at the interface. This mechanism, proposed by Klein et al., is
called hydration lubrication.23 Additionally, the adhesive copolymer layer can reduce
the diffusion rate of the drug previously encapsulated into the TNTs, thus inhibiting
burst drug release into the solution and achieving sustained drug release property.
–
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Figure 8. Schematic illustration showing the mechanism for enhanced lubrication and sustained drug
release of copolymer-grafted TNTs, which can be attributed to hydration lubrication of the
phosphorylcholine groups in PMPC brush and spontaneous adhesion of the catechol groups in DMA.
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CONCLUSIONS
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In the present study, bioinspired by superlubricity of articular cartilage and spontaneous
adhesion of mussel, a novel DMA-MPC copolymer was synthesized and used to modify
the TNTs for enhanced lubrication and sustained drug release. The characterizations of
surface morphology, surface wettability, and surface composition all confirmed that the
copolymer was successfully grafted onto the TNTs. Additionally, the lubrication test
showed that the COF value of the copolymer-grafted TNTs was lower than that of the
bare TNTs, which was attributed to the hydration lubrication mechanism of the PMPC
brush with zwitterionic charges. The drug release test demonstrated that the copolymer
layer grafted on the surface of TNTs effectively inhibited burst drug release and thus
achieved sustained drug release property. The copolymer-grafted TNTs, as a promising
lubricated drug nanocarrier, can be used to functionalize orthopedic biomaterials (such
as titanium) for enhanced lubrication and sustained drug release.
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ASSOCIATED CONTENT
The Supporting Information of the deconvolution study of XPS spectra (DMA-MPC
and TNTs@DMA-MPC) for C 1s, O 1s, N 1s and P 2p, and the drug release test of
honokiol from the DMA-MPC copolymer.
ACKNOWLEDGEMENTS
This study was financially supported by National Natural Science Foundation of China
(grant no. 51675296), Ng Teng Fong Charitable Foundation (grant no. 202-276-132-
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13), and Research Fund of State Key Laboratory of Tribology, Tsinghua University,
China (grant no. SKLT2018B08). The authors thank Dr. Yilin Lu for his help in Raman
spectrum characterization.
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REFERENCES
(1) Knoop, J.; Dekker, J.; Klein, J.; Leeden M. V. D. Biomechanical factors and
physical examination findings in osteoarthritis of the knee: associations with tissue
abnormalities assessed by conventional radiography and high-resolution 3.0 Tesla
magnetic resonance imaging. Arthritis. Res. Ther. 2012, 14, R212.
(2) Lawrence, R. C.; Helmick, C. G.; Arnett, F. C.; Deyo, R. A.; Felson, D. T.; Giannini,
E. H.; Heyse, S. P.; Hirsch, R.; Hochberg, M. C.; Hunder, G. G.; Liang, M. H.; Pillemer,
S. R.; Steen, V. D.; Wolfe, F. Estimates of the prevalence of arthritis and selected
musculoskeletal disorders in the United States. Arthritis. Rheum. 1998, 41, 778-799.
(3) Zhang, H.; Blunt, L.; Jiang, X.; Brown, L.; Barrans, S. The significance of the
micropores at the stem-cement interface in total hip replacement. J. Biomater. Sci.
Polym. Ed. 2011, 22, 845-856.
(4) Roy, P.; Berger, S.; Schmuki, P. TiO2 nanotubes: synthesis and applications. Angew.
Chem. Int. Ed. 2011, 50, 2904-2939.
(5) Alhoshan, M. S.; Baqais, A. A.; Al-Hazza, M. I.; Al-Mayouf, A. M. Heat treatment
and electrochemical activation of titanium oxide nanotubes: the effect of hydrogen
doping on electrochemical behavior. Electrochimica 2012, 62, 390-395.
(6) Al-Mobarak, N. A.; Al-Swayih, A. A. Development of titanium surgery implants
20
ACS Paragon Plus Environment

Page 21 of 25
Langmuir
1
2
3
4
5
6
7
for improving osseointegration through formation of a titanium nanotube layer. Int. J.
Electrochem. Sci. 2014, 9, 32-45.
8
9
(7) Mandal, S. S.; Jose, D.; Bhattacharyya, A. J. Role of surface chemistry in
modulating drug release kinetics in titania nanotubes. Mater. Chem. Phys. 2014, 147,
247-253.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(8) Aw, M. S.; Gulati, K.; Losic, D. Controlling drug release from titania nanotube
arrays using polymer nanocarriers and biopolymer coating. J. Biomater.
Nanobiotechnol. 2011, 2, 477-484.
(9) SinnáAw, M. A multi-drug delivery system with sequential release using titania
nanotube arrays. Chem. Commun. 2012, 48, 3348-3350.
(10) Wu, P.; Grainger, D. W. Drug device combinations for local drug therapies and
infection prophylaxis. Biomaterials 2006, 27, 2450-2467.
(11) Sobieszczyk, S.; Klotzke, R. Nanotubular titanium oxide layers for enhancement
of bone-implant bonding and bioactivity. Adv. Mater. Sci. 2011, 11, 17-26.
(12) Wang, W. Y.; Chen, B. R. Characterization and photocatalytic activity of TiO2
nanotube films prepared by anodization. Int. J. Photoenergy 2013, 2013, 348171.
(13) Tan, A.; Pingguan-Murphy, B.; Ahmad, R.; Akbar, S. Review of titania nanotubes:
fabrication and cellular response. Ceram. Int. 2012, 38, 4421-4435.
(14) Park, J.; Bauer, S.; Pittrof, A.; Killian, M. S.; Schmuki, P.; Von Der Mark, K.
Synergistic control of mesenchymal stem cell differentiation by nanoscale surface
geometry and immobilized growth factors on TiO2 nanotubes. Small 2011, 8, 98-107.
(15) Ma, M.; Kazemzadeh-Narbat, M.; Hui, Y.; Lu, S.; Ding, C.; Chen, D. D. Y.;
21
ACS Paragon Plus Environment

Langmuir
Page 22 of 25
1
2
3
4
5
6
7
8
9
Hancock, R. E. W.; Wang, R. Local delivery of antimicrobial peptides using self-
organized TiO2 nanotube arrays for peri-implant infections. J. Biomed. Mater. Res.
2012, 2, 278-285.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(16) Moseke, C.; Hage, F.; Vorndran, E.; Gbureck, U. TiO2 nanotube arrays deposited
on Ti substrate by anodic oxidation and their potential as a long-term drug delivery
system for antimicrobial agents. Appl. Surf. Sci. 2012, 258, 5399-5404.
(17) Aninwene, G. E.; Yao, C.; Webster, T. J. Enhanced osteoblast adhesion to drug-
coated anodized nanotubular titanium surface. Int. J. Nanomedicine 2008, 3, 257-264.
(18) Kulkarni, M.; Mazare, A.; Gongadze, E.; Perutkova1, Š.; Kralj-Iglič, V.; Milošev,
I.; Schmuki, P.; Iglič, A.; Mozetič, M. Titanium nanostructures for biomedical
applications. Nanotechnology 2015, 26, 062002.
(19) Oliveira, W. F.; Arruda, I. R. S.; Silva, G. M. M.; Machado, G.; Coelho, L. C. B.
B.; Correia, M. T. S. Functionalization of titanium dioxide nanotubes with
biomolecules for biomedical applications. Mater. Sci. Eng. C 2017, 81, 597-606.
(20) Jay, G. D.; Torres, J. R.; Warman, M. L.; Laderer, M. C.; Breuer, K. S. The role
of lubricin in the mechanical behavior of synovial fluid. PNAS 2007, 104, 6194-6199.
(21) Seror, J.; Zhu, L.; Goldberg, R.; Day, A. J.; Klein, J. Supramolecular synergy in
the boundary lubrication of synovial joints. Nat. Commun. 2015, 6, 6497-6503.
(22) Jahn, S.; Klein, J. Lubrication of articular cartilage. Annu. Rev. Biomed. Eng. 2016,
18, 235-258.
(23) Klein, J. Hydration lubrication. Friction 2013, 1, 1-23.
(24) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-inspired
22
ACS Paragon Plus Environment

Page 23 of 25
Langmuir
1
2
3
4
5
6
7
8
9
surface chemistry for multifunctional coatings. Science 2007, 318, 426-430.
(25) Pan, G.; Sun, S.; Zhang, W.; Zhao, R.; Cui, W.; He, F.; Huang, L.; Lee, S. H.;
Shea, K. J.; Shi, Q.; Yang, H. Biomimetic design of mussel-derived bioactive peptides
for dual-functionalization of titanium-based biomaterials. J. Am. Chem. Soc. 2016, 138,
15078-15086.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(26) Liu, S.; Pan, J.; Liu, J.; Ma, Y.; Qiu, F.; Mei, L.; Zeng, X.; Pan, G. Dynamically
PEGylated and borate-coordination-polymer-coated polydopamine nanoparticles for
synergetic tumor-targeted, chemo-photothermal combination therapy. Small. 2018, 14,
1703968.
(27) Liu, Y. L.; Ai, K. L.; Lu, L. H. Polydopamine and its derivative materials: synthesis
and promising applications in energy, environmental, and biomedical fields. Chem. Rev.
2014, 114, 5057-5115.
(28) Wang, W.; Ji, X.; Na, H. B.; Safi, M.; Smith, A.; Palui, G.; Perez, J. M.; Mattoussi,
H. Design of a multi-dopamine-modified polymer ligand optimally suited for
interfacing magnetic nanoparticles with biological systems. Langmuir 2014, 30, 6197-
6208.
(29) Shahrzad, S.; Fariborz, T.; Nahid, H. N.; Abbas, F. Drug delivery behavior of
titania nanotube arrays coated with chitosan polymer. Mater. Discov. 2017, 8, 9-17.
(30) Lee, H.; Lee, Y.; Statz, A. R.; Rho, J.; Park, T. G.; Messersmith, P. B. Substrate-
independent layer-by-layer assembly by using mussel adhesive-inspired polymers. Adv.
Mater. 2010, 20, 1619-1623.
(31) Jiao, Y.; Liu, S.; Sun, Y.; Yue, W.; Zhang, H. Bioinspired surface functionalization
23
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Page 24 of 25
1
2
3
4
5
6
7
8
9
of nanodiamonds for enhanced lubrication. Langmuir 2018, 34, 12436-12444.
(32) Wei, Q.; Wang, X.; Zhou, F. A versatile macro-initiator with dual functional
anchoring groups for surface-initiated atom transfer radical polymerization on various
substrates. Polym. Chem. 2012, 3, 2129-2137.
10
11
12
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55
56
57
58
59
60
(33) Lee, J.; Jung, E.; Park, J.; Jung, K.; Lee, S.; Hong, S.; Park, E.; Kim, J.; Park, S.;
Park, D. Anti-inflammatory effects of magnolol and honokiol are mediated through
inhibition of the downstream pathway of MEKK-1 in NF-κB activation signaling.
Planta Medica 2005, 71, 338-343.
(34) Sun, Y.; Yang, B.; Zhang, X.; Yang, Y. Cucurbit[7]uril pseudorotaxanes-based
photoresponsive supramolecular nanovalve. Chem. Eur. J. 2012, 18, 9212-9216.
(35) Li, Q.; Sun, Y.; Sun, Y.; Wen, J.; Zhou, Y.; Bing, Q.; Isaacs, L.; Jin, Y.; Gao, H.;
Yang, Y. Mesoporous silica nanoparticles coated by layer-by-layer self-assembly using
Cucurbit[7]uril for in vitro and in vivo anticancer drug release. Chem. Mater. 2014, 26,
6418-6431.
(36) Lee, W. J.; Smyrl, W. H. Oxide nanotube arrays fabricated by anodizing processes
for advanced material application. Curr. Appl. Phys. 2008, 8, 818-821.
(37) Hao, X.; Qian, Z.; Chun, L. Application of ultrasonic wave to clean the surface of
the TiO2 nanotubes prepared by the electrochemical anodization. Appl. Sur. Sci. 2011,
257, 8478-8480.
(38) Ran, C.; Ding, G. D.; Liu, W. C.; Deng, Y.; Hou, W. T. Wetting on nanoporous
alumina surface: transition between Wenzel and Cssie states controlled by surface
structure. Langmuir 2008, 24, 9952-9955.
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(39) Moser, J.; Punchihewa, S.; Infelta, P. P.; Gratzel, M. Surface complexation of
colloidal semiconductors strongly enhances interfacial electron-transfer rates.
Langmuir 1991, 7, 3012-3018.
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