Plasma modification of polylactic acid in a medium pressure DBD

Abstract

In this paper, a dielectric barrier discharge (DBD) operating in different atmospheres (air, nitrogen, helium and argon) and at medium pressure is employed to modify the surface properties of polylactic acid (PLA). Chemical and physical changes on the plasma-treated surfaces are examined using contact angle, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements. Results show that the discharge gas can have a significant influence on the chemical composition of the PLA surfaces: air and argon plasmas introduce oxygen-containing groups, while nitrogen discharges add nitrogen groups to the PLA surface. Quite surprisingly, also helium plasmas incorporate a small amount of nitrogen-containing functionalities: this observation can however be explained by the fact that the helium discharge operates in the glow mode. In the near future, it will be examined whether the performed plasma treatments can enhance PLA cell attachment and proliferation, which might open the door to many interesting biomedical applications.

Introduction

Non-thermal plasmas are commonly used to modify the surface properties of polymers [1], [2], [3]. Due to excellent properties such as a high strength to weight ratio, thermal stability, transparency, etc. polymers are frequently used as structural materials, for packaging and sealing applications and as protective coatings [4], [5]. Despite these excellent properties, polymers are not employable for many industrial uses due to their low surface energies and their normally poor chemical reactivity [4], [6], [7]. Thus, surface pre-treatment is usually required to achieve satisfactory surface wetting and adhesion, while retaining the advantageous bulk properties. In the past decades, plasma modification of polymers has been actively studied, however, this research has mainly focussed on well-known polymers, such as polypropylene [8], [9], [10], polyethylene [3], [11], [12], polyethylene terephthalate [5], [8], [13], polyamide [14], [15], etc. Although plasma treatments have been successfully applied to these popular polymers, only more recently they have been used to modify the surface properties of uncommon biodegradable polymers, such as for example polylactic acid (PLA) [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. PLA is a well-known biodegradable aliphatic polymer and has been previously used for several biomedical applications such as bone fixation devices (plates, pins, screws, etc.) and as tissue engineering scaffolds [16], [20]. Although PLA is known to be biocompatible and widely used in the field of medicine, its low wettability and surface energy affect cell attachment and proliferation and remain an important issue [16]. Therefore, surface properties of PLA need to be altered to create additional functional groups, which can in turn be used to link peptide sequences (for example the tripeptide sequence arginine–glycine–aspartate) to the PLA surface [26]. These short peptide sequences are recognized by cell membrane receptors leading to the promotion of cell attachment and spreading [26].

A few authors [20], [21], [22], [23], [24], [25] have studied the effect of non-thermal plasmas on PLA samples, however, their studies are mostly limited to vacuum technologies. Although vacuum treatments afford good control over gas chemistry and provide the possibility to use high energetic species (in the range of several eV to hundreds of eV) in surface modification processes, atmospheric pressure technologies offer attractive perspectives in today's industrial processes due to the elimination of expensive vacuum equipment, easier handling of the samples and scalability for industrial in-line processing [27], [28]. Therefore, in recent years, a lot of effort has been put into the development of non-thermal plasma reactors working at (or near) atmospheric pressure [28], [29]. In between vacuum and atmospheric pressure plasma technology lays a wide – almost unexplored – pressure range (so-called medium pressure) which might combine the advantages of both vacuum and atmospheric pressure technologies. At medium pressure, a large plasma volume can be easily obtained, which can result in a higher overall productivity [10]. Recently, it has been shown that at low energy densities, plasma treatment at medium pressure is more energy-efficient than at atmospheric pressure [2]. Furthermore, different (toxic) chemicals and gases can be used, since medium pressure technology works in a closed system. In addition, at medium pressure, the pumping equipment is relatively inexpensive [10]. Recognizing the above, a medium pressure non-thermal plasma will be employed in this paper to alter the surface properties of PLA. Among various non-thermal plasmas, DBDs are very convenient for the activation or modification of polymer surfaces due to the easy formation of a stable regime and due to their scalability [27]. DBDs are characterized by the presence of one or more insulating layers (so-called dielectric barriers) with a discharge gap in the current path between the metal electrodes. The presence of this dielectric barrier limits the current flowing through the gas, which prevents the discharge from developing into an electrical arc [30]. Usually, a DBD operates in the filamentary mode: the breakdown starts at many points, followed by the development of independent current filaments, named microdischarges [31], [32], [33], [34]. These microdischarges are of nanosecond duration and are randomly distributed over the dielectric surface. Due to this lack of uniformity, extensive efforts to homogenize DBDs have been made and it was found that under special, quite restrictive conditions homogeneous (or so-called glow) DBDs can be obtained [35], [36]. It is generally believed that in the case of a filamentary DBD, a large number of spike-like current pulses with nanosecond duration are appearing during certain sections of every half cycle of the applied voltage [33], [34], [37]. In contrast, a glow discharge is characterized by a single current pulse with durations up to a few µs [38], [39]. Besides these 2 well-known modes, some authors also report on what they call the “pseudoglow” discharge [40], [41], [42]. This regime is characterized by the presence of successive short current pulses (a few µs) and is also called the “multi-glow” mode, since the current–voltage characteristic for each current pulse is similar to the one obtained in the glow mode [42].

The present study investigates the potential of medium pressure DBDs for surface modification of PLA. After the description of the experimental set-up, DBD surface treatments in 4 different atmospheres (air, argon, nitrogen and helium) will be explored. The differences in surface modifications among the four discharges will be investigated by contact angle measurements, XPS and AFM. Contact angle measurements will be used to evaluate the wettability of the untreated and plasma-treated PLA films, while the chemical composition of the films can be studied using XPS. Finally, AFM will be employed to visualize and compare the topography of untreated and plasma-treated PLA surfaces.