Elsevier

Materials Letters

Volume 97, 15 April 2013, Pages 81-85
Materials Letters

Electrohydrodynamic atomization technique for applying phospholipid coatings to titanium implant materials

https://doi.org/10.1016/j.matlet.2013.01.091Get rights and content

Abstract

Phospholipid coatings on titanium implants have been shown to enhance osteoblast activity, promote mineralization, and facilitate implant osseointegration in vivo. To date, dip and drip coating techniques have been used to apply these coatings. These coating techniques are easy to perform, but present difficulties in controlling the characteristics of the coating on the final surface, resulting in inconsistent, non-uniform, non-conformal coatings that are too thick for in vivo use.

Electrohydrodynamic atomization (electro-spraying or e-spraying) is a versatile and easy method of creating thin, uniform and consistent coatings by atomizing a liquid with electrical forces. e-Spraying provides the advantage of being able to create coatings with relatively high efficiencies, while providing good control of coating morphology, especially on rough and intricately shaped surfaces, which is an important consideration for cell attachment and growth. Other advantages of this technique are low cost and easy setup. The purpose of this study was to develop an e-spraying technique for applying phospholipid coatings on commercially pure titanium implant materials.

Highlights

► We showed that thin phospholipid coatings can be electro-sprayed on titanium. ► Key e-spray parameters provide predictable control of coating characteristics. ► e-Spraying is highly repeatable on flat and 3D titanium shapes. ► e-Spraying is easy to perform, low cost and flexible.

Introduction

More than 4.4 million people have at least one metallic orthopedic implant, and the number of revisions is growing substantially [1]. Roughly a third of total joint replacements fail due to mechanical loosening from various causes [2]. Bone cements are often used to address these issues; however, cemented implants are prone to cement failure, leading many surgeons to prefer cementless implants, particularly for younger, more active patients.

Poor mineralization at an implant/bone interface leads to the development of a thin fibrous layer between the implant and bone, which frequently correlates with implant failure. With our increased understanding of the complex biological processes that underlie implant failure in vivo, there has been a shift in focus from bone cements and mechanical fixation techniques to the design of novel mechanical/biomimetic strategies that promote natural physiological integration.

Calcium phosphate-based bio-ceramics, primarily in the form of hydroxyapatite, have become popular cementless implant coatings [3]. However, long-term success of these coatings on load-bearing implants has been hindered by sub-optimal mechanical properties [4], delamination of the coating from the substrate [4] and poor mineralization and integration of newly forming bone with the implant surface [5]. As a result, considerable research effort has focused on development of new coating materials and techniques for popular implant materials.

Phospholipids are one such candidate material. Phospholipids are broadly implicated in the development of new bone. The phospholipid bilayer of osteoblast vesicles is believed to be the primary nucleation and mineralization site of new bone [5], [6]. Recent studies indicate that titanium surfaces coated with phospholipid alone (i.e. not in the presence of the additional constituents found in the matrix vesicle) enhance osteoblast activity, promote mineralization, and facilitate implant osseointegration in vivo [7], [8], [9]. These studies found that synthetic phosphatidylserine (PS) achieved the best in vitro biomineralization and in vitro osseointegration.

PS coatings also present challenges: newly grown bone did not make intimate contact with the implant surface. Numerous techniques have been proposed for applying bioactive coatings to implant materials, including but not limited to plasma spraying [4] and sputtering [3]. High temperature processes cannot be used with phospholipids, and prior dip [10] and drip [7], [11] coating processes resulted in relatively thick (approximately 100 μm) layers of soft PS that tended to form 3-dimensional gels in simulated body fluid. Such gels were not well adhered to the titanium substrate, resulting in mechanical instability [9]. These issues led to recommendations for thinner coatings to ensure mechanical stability of the implant [7], thus motivating improved coating processes.

Electro-hydrodynamic atomization (electrostatic, electro-spraying or e-spraying) is a versatile method of creating thin, adherent coatings by atomizing a liquid by means of electrical forces [12]. Electro-spraying provides the advantage of being able to create coatings with relatively high efficiencies [13] (i.e. most or all of the material sprayed becomes part of the coating on the target) because the charged liquid source material is carried by the electrical field rather than being mechanically atomized or carried on another liquid, as in typical pressure-based (e.g. aerosol) spraying techniques [14]. This is especially advantageous for more costly coating materials, and it enables good control of coating uniformity and morphology, especially on rough and intricately shaped surfaces. Other advantages of electro-spraying are low cost and easy setup. Detailed technical explanations and reviews are found elsewhere [12].

The goal of this study was to develop a technique to electro-spray thin, conformal, and consistent PS coatings on flat titanium implant materials and titanium surgical screws.

Section snippets

Materials and methods

1,2-Dioleoyl-sn-glycero-3-phospho-l-serine (“DOPS”) (Avanti Polar Lipids, Alabaster, AL) was dissolved in chloroform to a 20 mM concentration and drawn into a glass syringe (10 cc, Hamilton) which was then mounted on a syringe pump (Kent Scientific) (see Fig. 1). A double hub syringe tube (12 in., 20 gauge, Hamilton) connected the syringe to a blunt needle (22 gauge, Hamilton). Target titanium samples (flat, commercially pure Ti, 0.016 in. thickness, 0.75 cm×0.75 cm), were cleaned by successive 30 min

Results

Consistent with prior studies [14], small variations in e-spray parameters led to dramatically different coating morphologies. Fig. 2 presents SEM images of representative DOPS coatings e-sprayed at parameters shown in Table 1. Higher DOPS concentration and pump rate (Fig. 2A) produced a relatively smooth, uniform coating surface devoid of pores, particles or distinct surface features. Reducing the amount of material in the electric field (e.g. by decreasing DOPS concentration and pump rate),

Discussion

Kumbar and others [14], [15] found that holding certain parameters constant while varying others produced PLG and PEG coatings with widely varying morphologies and thicknesses. Higher electric field strength increases field current [12], which increases mass flow rate from the needle to the target, all other parameters held constant (to the limit of the amount of material available in the electric field at a given pump rate). A higher mass flow rate increases the number of charged particles in

Conclusions

The worldwide need for thin, conformal, osseointegrative coatings on orthopedic implants is well documented. e-Spraying is a suitable method for creating DOPS coatings on titanium. Compared to commonly used drip and dip techniques, e-sprayed DOPS coatings are much thinner, which could lead to enhanced mechanical stability for un-cemented implants [7]. And, e-sprayed coatings are more consistent and uniform, enabling more dependable and predictable device coverage.

Manipulation of the key

Acknowledgments

This paper was supported by the Colorado State University Cancer Supercluster Translational Research Grant Program and Colorado Bioscience Discovery Evaluation Grant Program.

Neither funding source had any role in study design, collection, analysis or interpretation of data; in the writing of the report; or in the decision to submit this article for publication.

References (17)

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