Surface functionalization of an osteoconductive filler by plasma polymerization of poly(ε-caprolactone) and poly(acrylic acid) films
Introduction
Apatite is one of nearly 5000 naturally occurring compounds on Earth [1] and can vary widely in physical, chemical, and crystallographic properties [2]. The general formula of apatite is M10(ZO4)6X2 where chemical substitution (M = Ca, Sr, Pb, Na…, Z = P, As, Si, V…, and X = F, OH, Cl…) [3], originates the apatite group containing the Ca phosphate apatites, Ca10(PO4)6(F, OH, Cl)2.
Hydroxyapatite (Ca10(PO4)6(OH)2—HAp), is the major component of animal bones [4], [5], attracting increasing interest for use in bone grafting [6] or scaffolding in bone tissue engineering [7], [8].
Synthetic structures fabricated [9], [10], [11] to temporarily replace and promote bone healing [12], [13], [14] generally consist of a biodegradable polymer [10], [15] and natural [4], [5], [6] or synthetic HA [7], [16], [17]. Since the forces that determine the mode of failure in reinforced composites are concentrated in a nanometric interphase [18], maximum reinforcement will only be achieved if the reinforcing particles are well bonded to the matrix phase and well dispersed in the host polymer matrix [19], [20].
Current state of development of surface modification techniques, such as plasma polymerization technology, allows surface modification of particles by creating a film with functional groups at nanoscale thickness [21] keeping unaltered their bulk properties [22]. The interest in plasma polymerized films lies in their unique coating properties which include good adherence and conformal coating of substrates [23]. Plasma has been successfully used for the surface modification [24], [25] of low surface energy polymers such as PE [26], PP [27] and PTFE [28], [29], metals such as Al [30], Cu [31] and Ti [32] and other engineering materials such as borosilicate glass [33], diamond-like carbon films [34] or carbon nanofibers [35].
The high controllability and reproducibility of plasma processes allow the physico-chemical design of surfaces suited to specific applications [36], [37]. Thus, the enhancement of antibacterial performance [38], or the improvement of the adhesion of collagen type I [39] evidence the vibrant potential of plasma technology in biomedical applications.
Cold plasma treatment is one of the best methods for creating tailored surface structures. The cold-plasma state is created and sustained by energetic electrons generated under the influence of electric and electromagnetic fields. These electrons have sufficiently high kinetic energies to induce, in low pressure gas environments, ionization, excitation and molecular fragmentation processes. The resulting charged particles and some of the neutral species (e.g. free radicals) interact also with electric and electromagnetic fields and with each other, resulting in recombination, ionization, neutralization and fragmentation mechanisms, defining the plasma state under certain experimental conditions [40].
The use of non-polymerisable atoms or molecules such as oxygen, nitrogen, carbon dioxide, ammonia and argon in order to create the plasma activated species [41] has been successfully used for multiple purposes such as nanotexturing [42], activation [43], etching [44] or sterilization [45], [46], [47]. In contrast, plasma polymerisation includes the deposition of material from the plasma phase to which the material is exposed [48]. Plasma polymerization is a specific type of plasma chemistry, which involves reactions between plasma species, between plasma and surface species, and between surface species. In consequence, this technique allows the polymerization of unconventional starting materials such as saturated alkanes or benzene and fabricating thin polymer films from almost any organic vapour [49]. Amongst these, acrylic acid has been intensely employed for the incorporation of carboxylic groups [50], [51], [52], [53], [54], [55], [56], [57] and to improve the dispersion of particles [58].
Conversely, catalyst-free routes for the polymerization of poly(ε-caprolactone) films continue to be an active field of research [59], [60] and moreover by plasma polymerisation [61]. Polycaprolactone (PCL) polymer obtained from traditional routes, is a semi-crystalline polymer [62] with good thermal plasticity that allows easy shaping and compounding [63], [64]. However, its intrinsic hydrophobicity [65] restricts its applications as cell colonizing material [66]. Therefore, thorough efforts have been employed to introduce hydrophilic compounds onto the polymeric surfaces as a strategy to increase its wettability in order to promote cell attachment [67], [68], [69], [70].
The experimental work presented here analyses the functionalization of HA particles by plasma polymerization starting from two different monomers, namely acrylic acid and ε-caprolactone. Subsequently, coated particles will be used for the fabrication of biocompatible polymer matrix composites [71] with tailored interfacial properties between reinforcement and matrix materials [72], and, subsequently characterized, whose results constitute the object of a forthcoming publication. Surface analytical techniques such as XPS and FTIR have been used for the characterization of the thin-film polymer coatings. Additionally, AFM imaging has allowed the quantification of the thickness of deposited layers and evaluation of the surface roughness of HA substrates as a function of preconditioning and plasma polymerization processes. Further, the hydrophobic/hydrophilic character of the plasma deposited films has been evaluated by means of their water contact angles. As will be shown in the present study, the chemical structure of the thin-film coatings obtained by the plasma polymerization of acrylic acid and ε-caprolactone over hydroxyapatite particles show high monomer functionality retention upon the plasma polymerization conditions used in this work.
Section snippets
Materials
Hydroxyapatite (Plasma Biotal Ltd, UK), was used as received without any further purification.
The monomers used for plasma treatments were ε-caprolactone (99%, Alfa Aesar, Germany) and acrylic acid (anhydrous, 99%) from Sigma-Aldrich Chemie GmbH, Germany. Oxygen (99.99% purity) was kindly supplied by Air Liquide Spain S. A.
Plasma treatment procedure
The surface modification of hydroxyapatite by plasma deposition was performed in a Pico LF40 (Diener electronic GmbH&Co., Germany). In order to guarantee the total exposure
Results and discussion
The thermal degradation behaviour of the organic layers deposited by plasma polymerization was measured through the evolution of the weight loss as depicted in Fig. 1. Detail of weight losses is summarized in Table 1. As can be seen, plasma treated particles show a noticeable weight loss, in contrast with the quasi-horizontal trace of the unmodified HA particles, which only show a marginal weight loss of ∼0.5% at 100 °C, attributable to the loss of humidity. In addition, acrylic acid plasma
Conclusions
According to the results presented in this work, plasma treatments are a valuable route to deposit polymeric layers based on acrylic acid and ε-caprolactone monomers upon the surface of HA particles. Under optimized conditions the structure of the organic monomers can be preserved. FTIR spectra of the plasma polymerized films closely resemble the conventional counterparts, yet the minor differences observed are attributable to the different polymerization paths.
Modification of the HA surface
Acknowledgments
The authors thank for technical and human support provided by SGIker of UPV/EHU and European funding (ERDF and ESF) and for financial support received from the Basque Government (Ref: SA-2011/00075 and GIC10/152-IT-334-10).
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