Biocompatible Polymer Composites Based on Ultrahigh Molecular Weight

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Biocompatible polymer composites based on ultrahigh molecular weight

polyethylene perspective for cartilage defects replacement

F.S. Senatov a, M.V. Gorshenkov a,⇑, S.D. Kaloshkin a, V.V. Tcherdyntsev a, N.Yu. Anisimova b,A.N. Kopylov b, M.V. Kiselevsky b

a National University of Science and Technology ‘‘MISIS’’, Moscow, Russian Federationb N.N. Blokhin Russian Cancer Research Center of RAMS, Moscow, Russian Federation

a r t i c l e i n f o

 Article history:

Available online 22 October 2012

Keywords:

UHMWPE

Composite

Biomaterial

Wear

Mechanical activation

a b s t r a c t

The purpose of this study was to evaluate mechanical properties, microstructure and biocompatibility of 

nano-compounds obtained by mechanical activation of UHMWPE powder and several types of particulate

fillers in a planetary ball mill. The effect of particle shape and concentration of Al 2O3 on properties of the

composite was investigated. The effectiveness of reinforcing a polyethylene matrix by mechanoactivated

alumina in the form of nanopowder or microspheres was shown. Reinforcing of the polymer by nanopar-

ticles significantly improves wear-resistance. No signs of blood serum fibrin deposition on the surface of 

the samples were observed. Also no signs of pathological changes in physiology or anatomy were noticed

on the animals with implanted samples. UHMWPE-based composites obtained by mechanical activation

andhot pressing and filled with ceramic particles can be used as a biocompatible material for thereplace-

ment of cartilage.

 2012 Elsevier B.V. All rights reserved.

1. Introduction

The development of synthetic implantates to replace cartilage

defects, for example, the replacement of joints, is an actual prob-

lem of modern medicine. Modern biocompatible materials, used

for joint implantates, must have high impact strength, wear-resis-

tance and corrosion resistance. Ultrahigh molecular weight poly-

ethylene (UHMWPE) is used in joint replacement prostheses

because of its excellent properties of bio-compatibility, chemical

stability, and low friction coefficient. To improve the functional

characteristics, such as strength, polymers may be reinforced with

hard inorganic fillers [1]. Ultrafine fillers, including nanosized par-

ticles, are of particular interest. It was shown  [2]  that the use of 

nanosized powders as fillers for polymer matrices can significantly

enhance the mechanical, tribological and other functional charac-teristics of polymer composites.

Currently, for obtaining composite polymeric materials

mechanical activation of polymer and filler is widely used for

pre-processing of raw materials. Mechanical activation increases

the adhesion of the filler to the matrix [3,4]. Mechanical activation

also helps to reduce the particle size of polymers and in some

cases, may be accompanied by rupture of polymer chains, the

transformation of the crystalline and amorphous structures, the

appearance of oriented amorphous regions   [5–9]. As noted in

[10], fibrous form of UHMWPE particles increases the contact areaof the particles during compaction and gets a denser structure than

in the case of spherical particles. Therefore in this paper, we pro-

pose to make a preliminary mechanical treatment of UHMWPE

powder and several types of particulate fillers in a planetary ball

mill and evaluate microstructure and biocompatibility of obtained

nano-compounds.

2. Materials and experiment

UHMWPE powder ‘‘Polinit-2’’ (OJSC ‘‘Kazanorgsintez’’, Russia) with molecular

weight 2.106 g/mol was used as the polymer matrix, Al2O3 was used as a strength-

ening phase in the form of nanopowder with 50 nm particles (JSC ‘‘Siberian Chem-

ical Combine’’, obtained by plasma method) or microspheres with size 1000 nm

(National University of Science and Technology ‘‘MISIS’’, obtained by pyrolysis of 

ultrasonic aerosols). The first stage of the process of polymer composite obtaining

was mechanical activation of UHMWPE powder and alumina with various ratios

in a planetaryball mill Fritsch Pulverisette5 (Germany), equipped with agate grind-

ing bowl (500 ml) and alumina grinding balls (10 mm). The grinding bowl filling

with grinding bodies was 45 vol.%. Based on changes of morphology of the particles

of UHMWPE during mechanical activation the optimal duration was chosen. Hot

pressing of the mechanoactivated mixture was performedusing a hydraulic thermo

press MEGA KSC-10A (Spain) on the following mode: compression load 76 MPa

with heating for 1.5h from 20 C to 160 C, holding at this temperature for

30 min, followed by hot pressing with a load of 80 MPa and subsequent cooling un-

der pressure.

Study of the shape and size of the powder particles after the mechanical activa-

tions was performed with a scanning electron microscope JEOL JSM-6610LV (accel-

erating voltage 20 kV). For studying of non-conductive polymeric samples, the

surface of polymer composite samples was covered with a layer of platinum (10–

20 nm) by Auto Fine Coater JFC-1600 (Jeol, USA).

0925-8388/$ - see front matter    2012 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2012.10.014

⇑ Corresponding author. Tel.: +7 9262810334.

E-mail address: mvg@misis.ru (M.V. Gorshenkov).

 Journal of Alloys and Compounds 586 (2014) S544–S547

Contents lists available at  SciVerse ScienceDirect

 Journal of Alloys and Compounds

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j a l c o m

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To study the effect of particle shape and concentration of Al2O3 on properties of 

the composite microindentation, the determination of tensile strength and impact

strength were used. Microhardness measurements were performed using microind-

enter CSM Micro Indentation (CSM Instruments SA, Switzerland) with a load of 

100 mN (the maximum depth of 200 microns, the depth resolution 0.3 nm). Mea-

surement of tensile strength, yield strength, elongation and Young’s modulus of 

the specimen in tension was carried out according to Russian measurement stan-

dards (GOST11262-80) on Instron 150LX (USA) with the strain-rate 10 mm/min

using a rectangular sample measuring 60 10 2 mm. Toughness was determined

on samples of size 80 10 4 mm with a V-notch Charpy method in accordancewith Russian measurement standards (GOST 4647-80) on POE Instron 2000 (USA)

with a reserve of the pendulum energy 50 J.

Tribological tests were carried out in the regime of friction in distilled water by

pin-on-disk method on CETR-UMT-3 (Bruker AXS, Switzerland). Form of the test

sample: pin with a diameter of 6.3 mm and a length of 10 mm; counter-body: steel

45; sliding velocity200 rpm, normalload5 MPa, the test time4 h, the length of fric-

tion 4 km, temperature 27 C. These parameters were similar to standard parame-

ters of cartilage friction tests in some other works [11–13].

Biocompatibility of the samples (Al2O3/UHMWPE-0.50.5) was researched

invitro and invivo. Samples weresterilizedin the 96% ethanol during24 h and then

– were dried in sterile conditions. We made our research on mouse bone-marrow

derived MSC(mesenchymal stem cells) andhuman fibroblasts. We took C57/BlackB

mouse bone marrow as a source of MSC. Cell concentration was 440 105 cell/ml.

MSC were placed on the surface of a sample, and cultivated in cultural medium

(RPMI-1640 (Sigma, USA) containing 10% heat-inactivated (56 C, 30 min) fetal bo-

vine serum (Hyclone Laboratories, Logan, UK), 2 mM   L -glutamine and antibiotics

(100lg/ml penicillin sodium salt and 100lg/ml streptomycin sulfate (Sigma,

USA)) in 37 C,5%CO2 during30 days fixed by May– Grunwald giemsaand staining

by hematoxylin–eosin [14].

For evaluation of fibrin dropping we took citrated healthy donor blood serum

andextracted plasmaon the surface of the sample. After moistening of the samples,

we incubated them in blood serum during five days, in 37 C and 5% CO2.

In vivo we researched sample toxicity in six mice C57/Black, they were narco-

tized by ether, skin on their back was dissected and 0.6 0.6 samples were im-

planted subcutaneously into 1 1 area. After 60 days mice were euthanized and

samples were taken out. Also connective tissue surrounding the sample was taken

out too. Samples were fixed and stained.

Biocompatibility features were evaluated with light reflecting microscopy, with

AxioScop 40 microscope (Zeiss, Germany).

3. Results and discussion

It was shown [15] that due to the high melt viscosity UHMWPE

particles can retain the shape of the initial particles after thermo

pressing. Owing to this fact the microstructure of UHMWPE can

be regarded as a polycrystalline structure in metals, where grains

are the particles of UHMWPE. An example of the UHMWPE struc-

ture is shown at the Fig. 1. Mechanical activation in certain modes

can contribute to obtaining a more dense structure of the compos-

ite by changing the shape of the original UHMWPE particles. The

hard reinforcing particles at a rate much smaller than the size of 

the original polymer particles are distributed on the surface of 

UHMWPE only. Therefore, the hard particles reinforce only the

‘‘grains’’ boundary and do not reinforce the body of the UHMWPE

particle as it shown at Figs. 2 and 3.

The homogeneous tension composite without necking associ-

ated with high molecular weight of UHMWPE, while stretching

the compacted samples containing different amounts of micro-

spheres and ultrafine alumina powder, is observed, and it confirms

the results of   [16]. Both strength and Young’s modulus increase

was determined after addition of a filler in UHMWPE. According

to [17], reinforcing of the material by mechanically activated dis-

persed fillers due to the retention of oxide particles in a polymer

matrix by van der Waals forces and hydrogen bonds with themolecular chains of UHMWPE.

In accordance with previous study   [18], the optimal polymer

filling was 3 wt.% of ceramic particles. Increasing of ultimate stress

limit by 38% and Young’s modulus in 2.85 times was achieved by

filling UHMWPE with alumina particles (Table 1). There is less

yield strength in the case of filling the polymer matrix by micro-

spheres. This fact can be explained by a smaller surface area

(12 m2/g) and spherical shape of particles, which can leave the ma-

trix under the load. Nanoparticles also have a greater surface area

(55 m2/g) and irregular shape, which increases the contact area

with the polymer matrix and improves the adhesion. Such behav-

ior of mechanical properties of polymeric composites is conven-

tional, reinforcing of polymer matrix with hard particles

(including nanosized powders) results in an increase of elasticitymodules and yield stress   [19]. Very often this gain in elasticity

Fig. 1.   SEM micrograph of quasibrittle fracture surface of UHMWPE-basedcomposite.

Fig. 2.   ‘‘Grain’’ boundary of UHMWPE-based composite reinforced by Al2O3

microspheres.

Fig. 3.   ‘‘Grain’’ boundary of UHMWPE-based composite reinforced by Al2O3

nanoparticles.

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modulus and yield stress is desirable, because in most cases these

properties define the stress limit of material under loading and it

does not undergo plastic deformation.

The destruction of UHMWPE, as other amorphous–crystalline

polymers, carried out by spreading of microcracks. Hard ceramic

particles inhibit the spreading of cracks that leads to an increase

of elongation. As a consequence, there was a significant increase

in the impact strength of the composite. Positive effect on the im-

pact strength of the composite states in the case of filling by micro-spheres is higher than in the case of filling by nanopowders,

because microspheres behave like closed pores.

The ratio of the elastic modulus of the nanocomposite to the

elastic modulus of bone tissue is 1:13 and 1:14 versus 1:21 in

the case of unfilled UHMWPE, which is especially important be-

cause a large difference of the elastic modulus can lead to micro-

strain on the border of the prosthesis-bone and lead to

degradation of the implantate.

The friction coefficient has decreased in 2.5 times after 4 h of 

friction in case of composite with microspheres in comparison

with pure UHMWPE, but increased in 1.4 times in the case of com-

posite with nanoparticles as it is shown in Fig. 4. But wear-resis-

tance has decreased in case of filling polymer matrix with

microspheres (Fig. 5). This can be attributed to the larger size of microparticles compared with the nanoparticles and, most impor-

tantly, their spherical shape. Consequently, the ceramic particles

more active crumble from the polymer matrix during the loading,

leading to intensive wear of the composite body and counter-body.

Based on the tribological properties composites with nanoparticles

are preferred, because of higher wear-resistance. This fact is very

important for using this material in joint replacement operations.

We did not observed signs of blood serum fibrin deposition on

the surface of the samples. That is a good feature of material for

cartilage defects replacement, because fibrin dropping with follow-

ing calcification can cause decreasing of the smoothness of the

 joint surface, pain and loss of function. Studying the surface of 

samples showed absence of MSC monolayer on the surface, we no-

ticed only single adhered cells (0–4 cells/sm2

) on the surface of thetested samples after cocultivation with human fibroblasts (Fig. 6).

No signs of pathological changes in physiology or anatomy werenoticed on the animals with implanted samples. Studying the

microstructure showed absence of signs of colonization by recipi-

ent cells on the surface. The implantate for replacement cartilage

defects must not adhere cells of recipient tissues on its surface be-

cause this will result in decreasing its quality and characteristics.

So that characterizes Al2O3/UHMWPE composite as a very perspec-

tive for using as a implantate.

4. Conclusions

As a result of the investigation it was shown that UHMWPE-

based composites filled by ceramic particles with better

physical–mechanical properties can be obtained by mechanical

activation and hot pressing, and can be used as a biocompatiblematerial for the replacement of cartilage.

 Table 1

Comparison of the values of various parameters for the pure UHMWPE and the composite material with 3 wt.% Al 2O3 of different types.

Ultimate

strength (MPa)

Young’s

modulus (MPa)

E material/E bone   Yield strength

(MPa)

Elongation

(%)

Hardness

(MPa)

Impact elasticity

(kJ/m2)

UHMWPE 31.9 0.97 1:21 22.0 120 50 50

Composite with nanoparticles 40.0 1.53 1:13 26.5 160 109 63

Composite with microspheres 36.0 1.4 1:14 24.5 150 100 73

Fig. 4.  Friction coefficients of UHMWPE and UHMWPE-based composites.

Fig. 6.   UHMWPE-based composite surface with single adhered cells (0–4 cells/sm2)

after cocultivation with human fibroblasts.

Fig. 5.  Wear of UHMWPE and UHMWPE-based composites.

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According to the study of the complex mechanical properties of 

composite materials based on UHMWPE, the effectiveness of rein-

forcing a polyethylene matrix by mechanoactivated alumina in the

form of nanopowder or microspheres was shown. Effect of micro-

spheres on the mechanical properties of the composite is compara-

ble to the effect of nanopowder, but the reinforcing of the polymer

matrix occurs to a much lesser extent, due to the smaller surface

area because of the spherical shape and smaller surface area. Be-

cause of this fact wear of samples with microspheres is much more

intensive. Therefore, the composites with nanoparticles are prefer-

able to use as implantates for replacement cartilage defects.

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