Effects of cloud-point grafting, chain length, and density of PEG layers on competitive adsorption of ocular proteins
Introduction
Non-specific bioadhesion (protein adsorption, bacterial adhesion, platelet deposition) continues to hinder biomaterials utilization because of the failure of existing and emerging biomedical device materials to prevent such uncontrolled accumulation of biological material at the surface [1]. Attempts at combating protein adsorption and cellular adhesion by surface modification include the use of polysaccharides [2], [3], [4], phospholipids [5], [6], and many others [7], [8]. We have recently reviewed some of the most recent attempts at preventing bioadhesion [8]; surface modification with polyethylene glycol (PEG) or polyethylene oxide (PEO) continues to provide the most promising results in terms of reducing undesirable protein adsorption and cell adhesion [8], [9].
The key properties that determine the protein-repellent effects of PEG coatings have been the subject of much discussion. Reported experimental results are at times inconsistent; one challenge is to verify complete PEG coverage and thereby exclude artefactual protein adsorption at coating defects. Molecular models have been proposed which predict the necessary interfacial properties of PEG layers required to prevent protein adsorption, such as the effects of chain length and graft density of end-attached PEGs [10], [11], or the requirement of a combined strong interaction between the PEG chains and the substrate and high chain density [12], [13]. An essential finding of these theoretical models is that the PEG layer (whether in a brush or mushroom conformation) should provide an interfacial barrier to prevent the protein from interacting with the underlying substrate. Recent experimental evidence is supportive of this notion [14], [15], [16], [17], [18], [19], with the interfacial graft density proving to be the most crucial property of a PEG layer for minimizing protein adsorption. However, comparative studies of ethylene oxide (EO) terminated self-assembled monolayers (SAMs) on Au and Ag suggest that adsorption of fibrinogen can occur when the graft density becomes too high: when EO groups are too tightly packed on the surface they lose their helical character, a condition for maintaining a stable hydration shell, and adsorb protein [20].
Many studies of PEG coating have used carefully constructed model systems such as the SAMs or grafting onto surface-modified silicon wafers; the transferability of these approaches to biomedical devices is, however, not assured. In order to apply PEG coatings onto contact lenses and similar devices, we have chosen an approach based on thin interfacial bonding layers deposited from radio-frequency glow discharges (r.f.g.d.'s) established in vapours of organic molecules. This r.f.g.d. polymerization method enables for instance the deposition of thin polymeric coatings with surface amine groups [21] which can subsequently be used for the covalent immobilization of bioactive molecules or suitably functionalized PEGs. By appropriate choice of the ‘monomer’ vapour and r.f.g.d. deposition conditions, it is possible to produce thin uniform coatings with a range of densities of surface amine groups [22], [23], [24]. Another advantage of r.f.g.d. polymerization is the strong adhesion of r.f.g.d. deposited polymeric coatings on a variety of substrates. Furthermore, the deposition method is usually readily transferable from model substrates, suited to coating characterizations, to real-life devices such as contact lenses, and thereby allows application of coatings optimized in vitro to devices for use in vivo. Thin amine r.f.g.d. interlayers have previously been used for the covalent immobilization of polysaccharides onto highly oxygen permeable contact lenses [25].
In this study, we have built onto previous findings that the graft density of PEG chains and their length (molecular weight) together are important for achieving high protein repellency. Utilizing the ability of r.f.g.d. interlayers to provide surfaces with varying densities of amine pinning sites for covalent PEG anchoring, we extend previous work by studying how the density and the PEG chain length interrelate with the density of surface pinning groups. Using varying grafting conditions we also compared grafting under conditions ranging from good solubility of PEG chains to marginal solvation (‘cloud point’, CP). The latter condition results in reduced chain repulsion during the grafting reaction and hence greater chain packing density at the interface. We conjectured that grafting under marginal solvation conditions therefore may require a higher density of surface amine sites. By using surfaces with different densities of pinning groups, we aimed to study whether the density of pinning sites is important, and if so, how the effects of pinning site density, PEG chain length, and chain solvation would be interrelated. We conjectured that surfaces with less dense pinning sites would require longer ‘brush’ or well-hydrated ‘mushroom’ graft chains to yield a coating that shields the underlying r.f.g.d. surface from irreversible protein adsorption. Thus, we studied protein adsorption onto coatings containing various combinations of surface pinning densities, PEG chain lengths, and chain densities (modulated by solvation conditions during grafting and possibly also by the surface pinning density). For protein adsorption we used a multicomponent solution containing four proteins that are used as constituents in artificial tear fluid [26]. Elucidation of which of the proteins, if any, would succeed in competitive adsorption might yield information on the mechanisms of protein adsorption onto the coatings.
Section snippets
Plasma polymer surfaces
Perfluorinated poly(ethylene-co-propylene) tape (Teflon FEP 100 Type A; DuPont) and poly(ethylene terephthalate) (PET, DuPont) sheets were coated with thin amine polymer layers deposited using a r.f.g.d. struck in ‘monomer’ vapour of either n-heptylamine (HA; Aldrich, 99%) or allylamine (AlA; Fluka, 99%) in a custom-built r.f.g.d. apparatus [27], as previously described [20], [21]. X-ray photoelectron spectroscopy (XPS) was used to assess the N content of the coatings and high-resolution N 1s
Amine pinning layers
Previous reports from this laboratory [21], [22] have detailed the chemical composition and properties of the n-heptylamine (HA) and allylamine (AlA) r.f.g.d. polymer coatings produced on the r.f.g.d. reactor used. On freshly deposited coatings, the XPS N 1s BE of 399.1–399.2 eV indicates that the N is predominantly in the form of amine groups, as expected. However, when the r.f.g.d. coatings are exposed to air for periods of days to weeks, there is substantial oxygen uptake, and a shift in the
Discussion
In the present study, we have varied both the density of surface pinning groups and the length of PEG chains, and we have also used varying grafting conditions to study which factors and structures of PEG coating matter most.
By using a methoxylated PEG and amine surfaces of different densities of pinning sites, we fabricated coatings with a constant length of the PEG chain but different surface coverages due both to varying grafting conditions and different densities of reactive surface pinning
Conclusions
XPS and Surface-MALDI-MS analyses of PEG coatings exposed to a multicomponent protein solution consisting of lysozyme, lactoferrin, albumin, and IgG have shown that linear PEG coatings can prevent protein adsorption through specific tailoring of the coating structure. PEG coatings are protein resistant (within the sensitivity of our techniques; a few ng/cm2 of protein coverage) only if fabricated under suitable conditions, requiring as a minimum the use of marginal solvation as obtained by CP
Acknowledgments
We acknowledge partial support for this research by the Australian Government under the Generic Technology component of the Industry Research and Development Act 1986 (GIRD Project ‘Polymer Surface Engineering’).
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