Elsevier

Applied Surface Science

Volume 329, 28 February 2015, Pages 32-39
Applied Surface Science

Modification of polylactic acid surface using RF plasma discharge with sputter deposition of a hydroxyapatite target for increased biocompatibility

https://doi.org/10.1016/j.apsusc.2014.12.127Get rights and content

Highlights

  • The treatment by plasma of radio-frequency magnetron discharge with hydroxyapatite target sputtering improves the biocompatibility of PLLA surface.

  • The treatment significantly increases the roughness of PLLA surface.

  • The formation of rough highly porous surface is due to the etching and crystallization processes on PLLA surface during treatment.

  • Maximum concentration of the ions from the sputtered target is achieved at 60 s of the plasma treatment.

Abstract

Surface modification of polylactic acid (PLLA) by plasma of radio-frequency magnetron discharge with hydroxyapatite target sputtering was investigated. Increased biocompatibility was demonstrated using studies with bone marrow multipotent mesenchymal stromal cells. Atomic force microscopy demonstrates that the plasma treatment modifies the surface morphology of PLLA to produce rougher surface. Infrared spectroscopy and X-ray diffraction revealed that changes in the surface morphology are caused by the processes of PLLA crystallization. Fluorescent X-ray spectroscopy showed that the plasma treatment also changes the chemical composition of PLLA, enriching it with ions of the sputtered target: calcium, phosphorus and oxygen. It is hypothesized that these surface modifications increase biocompatibility of PLLA without increasing toxicity.

Introduction

A biomaterial is a material intended to interface with biological systems to treat, enhance or replace any tissue, organ or function of the body [1]. Currently polylactic acid (PLLA) is one of the most widely used biomaterials [2].

PLLA is a polymer with the degree of crystallinity dependent on the molecular weight and polymer treatment parameters. PLLA is biocompatible and degrades into non-toxic components with a well-described degradation rate in vivo and has been used as degradable surgical sutures for a long time. It has gained US Food and Drug Administration approval for clinical use [3].

The surface properties of polymer materials play a crucial role in determining the overall biocompatibility, since the surface of materials comes first in contact with biological environment [4]. Surface morphology and its physiochemical properties have a major influence on the attachment of cells, they determine cell topology, spatial orientation of cell's cytoskeleton components and many other important parameters [5], [6], [7].

PLLA is chemically inert and has no reactive side-chain groups, which makes it challenging to modify its surface and bulk. PLLA is comparatively hydrophobic, with a static water contact angle of about 80°. This leads to low cell affinity and can provoke an inflammatory response from the living host upon direct contact with biological fluids [8], [9].

Non-thermal plasma treatments (plasma corona discharge, dielectric barrier discharge, etc.) are often used for inserting chemically reactive functional groups on polymeric substrates to increase the biocompatibility [10]. While non-thermal plasma surface treatments are preferred for simplicity, modification of PLLA under thermal plasma conditions remain less popular owing to inherent difficulties associated with identifying appropriate plasma conditions and complimenting target material(s) for a specific (bio)-material surface treatment [11].

Radio-frequency magnetron sputtering (RFMS) of a solid dielectric target is a common way to create coatings with high biocompatibility on metal surfaces and biostable polymeric materials such as polyethylene, silicone and polytetrafluoroethylene [12], [13], [14], [15], [16], [17], [18]. The RFMS method is based on the sputtering of material in vacuum due to the bombardment of the target surface with the working gas ions which are formed in the abnormal glow discharge plasma when a magnetic field is applied. Thus, the application of the RFMS method allows modifying the plasma composition in a wide range not only by changing the atmosphere in the vacuum chamber, but also by changing the chemical composition of the sputtered target [19], [20], [21], [22], [23], which opens up new possibilities for the modification of PLLA surface.

Until now only few papers investigated the PLLA surface modification by using RFMS method [24], [25]. Such situation limits the application of the RFMS method as a universal technique for modifying various types of polymeric materials and narrows the range of possible methods of surface modification of biodegradable polymers for biomedical applications.

We have previously shown that the application of this method for modifying PLLA surface allows increasing the free energy and the biocompatibility of the surface [26]. In the present paper we continue the study of the effect of the RFMS modification on biocompatibility and chemical composition of the treated films. The mechanisms of the formation of highly rough surface during the plasma treatment were further investigated.

Section snippets

Materials and methods

Polymer films were prepared from a 4% solution of the polymeric material (Poly (l-lactide) PURASORB® PL 38, Purac) in dichloromethane (CH2Cl2, Panreac Química S.L.U.). The polymer solution of 12 (±1) g was placed in a specially designed glass bath with a polished bottom and left at room temperature. After 24 h when most of the solvent had vaporized, the polymer films were removed from the bath using milli-Q water. The formed polymer films were then placed into a vacuum chamber with initial

Results and discussion

Fig. 2 shows high resolution AFM images of the surface of the investigated samples for different plasma treatment times. The AFM study of the surface shows the change in morphology and the increase in PLLA surface roughness with increasing the plasma treatment time. Specifically, Fig. 2(a)–(b) shows that non-modified PLLA surface has no significant cavities and protrusions. Fig. 2(c)–(d) shows that after the interaction with plasma for 30 s a significant number of round formations with an

Conclusions

It was shown that the RFMS plasma treatment with the solid hydroxyapatite target sputtering increases the biocompatibility of PLLA surface by stimulating processes of attachment and differentiation of MMSC pool. The FCM method revealed that the plasma treatment does not cause an adverse cellular response (apoptosis, necrosis). This surface modification can therefore enhance PLLA usability as a biomaterial. It is hypothesized that the increased surface roughness of PLLA, which was demonstrated

Acknowledgments

The authors would like to thank V. Novikov (Tomsk State University) for conducting AFM studies. This work was financially supported by the Federal Target Program (agreement # 14.578.21.0031, unique identifier RFMEFI57714X0036); Russian Foundation for Basic Research (projects #13-08-98052 r_sibir_a).

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