Osteoblast Biocompatibility and Antibacterial Effects Using 2-Methacryloyloxyethyl Phosphocholine-Grafted Stainless-Steel Composite for Implant Applications

Poor osteogenesis and bacterial infections lead to an implant failure, so the enhanced osteogenic and antimicrobial activity of the implantable device is of great importance in orthopedic applications. In this study, 2-methacryloyloxyethyl phosphocholine (MPC) was grafted onto 316L stainless steel (SS) using a facile photo-induced radical graft polymerization method via a benzophenone (BP) photo initiator. Atomic force microscopy (AFM) was employed to determine the nanoscale morphological changes on the surface. The grafted BP-MPC layer was estimated to be tens of nanometers thick. The SS-BP-MPC composite was more hydrophilic and smoother than the untreated and BP-treated SS samples. Staphylococcus aureus (S. aureus) bacteria binding onto the SS-BP-MPC composite film surface was significantly reduced compared with the pristine SS and SS-BP samples. Mouse pre-osteoblast (MC3T3-E1) cells showed good adhesion on the MPC-modified samples and better proliferation and metabolic activity (73% higher) than the pristine SS sample. Biological studies revealed that grafting MPC onto the SS substrate enhanced the antibacterial efficiency and also retained osteoblast biocompatibility. This proposed procedure is promising for use with other implant materials.

Swelling of Thin Poly(ethylene glycol)-Containing Hydrogel Films in Water Vapor-A Neutron Reflectivity Study

Hydrogels are widely used in biomedicine and for bioanalytical purposes, normally under wet conditions. For certain applications, processing steps, or process monitoring, hydrogel films are used or treated under ambient conditions, and because they are hygroscopic, it is of interest to investigate how they respond to changes in atmospheric humidity. We have used neutron reflectometry to follow the swelling of thin UV-polymerized hydrogel films in air under different relative humidities (RHs). These polymers were prepared to similar thicknesses on silica and gold substrates, and the chemical similarity between them was verified by infrared spectroscopy. The swelling in response to variations in RH was different for the layers on the two substrate types, reflecting structural changes induced by differences in the UV exposure required to achieve a given polymer thickness, as demonstrated also by differences in the Flory-Huggins interaction parameter, obtained by fitting a Flory-Huggins-type sorption model to the swelling data. Wetting studies show small changes in contact angles with surrounding humidity variations, indicating that structural reorganization at the interface in response to humidity changes is limited.

Peptide-functionalized poly[oligo(ethylene glycol) methacrylate] brushes on dopamine-coated stainless steel for controlled cell adhesion

The modification of the surface of surgical implants with cell adhesion ligands has emerged as a promising approach to improve biomaterial-host interactions. However, these approaches are limited by the non-specific adsorption of biomolecules and uncontrolled presentation of desired bioactive ligands on implant surfaces. This leads to sub-optimal integration with host tissue and delayed healing. Here we present a strategy to grow non-fouling polymer brushes of oligo(ethylene glycol) methacrylate by atom transfer radical polymerization from dopamine-functionalized clinical grade 316 stainless steel. These brushes prevent non-specific adsorption of proteins and attachment of cells. Subsequently, the brushes can be modified with covalently tethered adhesive peptides that provide controlled cell adhesion. This approach may therefore have broad application to promote bone growth and improvements in osseointegration.

STATEMENT OF SIGNIFICANCE

Stainless steel (SS) implants are widely used clinically for orthopaedic, spinal, dental and cardiovascular applications. However, non-specific adsorption of biomolecules onto implant surfaces results in sub-optimal integration with host tissue. To allow controlled cell-SS interactions, we have developed a strategy to grow non-fouling polymer brushes that prevent protein adsorption and cell adhesion and can be subsequently functionalized with adhesive peptides to direct cell adhesion and signaling. This approach has broad application to improve osseointegration onto stainless steel implants in bone repair.

Icephobic Surfaces Induced by Interfacial Nonfrozen Water

It is known that smooth, hydrophobic solid surfaces exhibit low ice adhesion values, which have been shown to approach a lower ice adhesion strength limit (∼150 kPa) defined by the water receding contact angle. To overcome this limit, we have designed self-lubricating icephobic coatings by blending polydimethylsiloxane (PDMS)-poly(ethylene glycol) (PEG) amphiphilic copolymers into a polymer matrix. Such coatings provide low ice adhesion strength values (∼50 kPa) that can substantially reduce the lower bound of the ice adhesion strength achieved previously on smooth, hydrophobic solid surfaces. Different molecular mechanisms are responsible for the low ice adhesion strength attained by these two approaches. For the smooth hydrophobic surfaces, an increased water depletion layer thickness at the interface weakens the van der Waals' interactions between the ice and the polymeric substrate. For the self-lubricating icephobic coatings, the PEG component of the amphiphilic copolymer is capable of strongly hydrogen bonding with water molecules. The surface hydrogen-bonded water molecules do not freeze, even at substantial levels of subcooling, and therefore serve as a self-lubricating interfacial liquid-like layer that helps to reduce the adhesion strength of ice to the surface. The existence of nonfrozen water molecules at the ice-solid interface is confirmed by solid-state nuclear magnetic resonance (NMR) spectroscopy.

On the hydration of subnanometric antifouling organosilane adlayers: a molecular dynamics simulation

The connection between antifouling and surface hydration is a fascinating but daunting question to answer. Herein, we use molecular dynamics (MD) computer simulations to gain further insight into the role of surface functionalities in the molecular-level structuration of water (surface kosmotropicity)--within and atop subnanometric organosilane adlayers that were shown in previous experimental work to display varied antifouling behavior. Our simulations support the hypothesized intimate link between surface hydration and antifouling, in particular the importance of both internal and interfacial hydrophilicity and kosmotropicity. The antifouling mechanism is also discussed in terms of surface dehydration energy and water dynamicity (lability and mobility), notably the crucial requirement for clustered water molecules to remain tightly bound for extensive periods of time--i.e. exhibit slow exchange dynamics. A substrate effect on surface hydration, which would also participate in endowing antifouling adlayers with hydrogel-like characteristics, is also proposed. In contrast, the role of adlayer flexibility, if any, is assigned a secondary role in these ultrathin structures made of short building blocks. The conclusions from this work are well in line with those previously drawn in the literature.

Methacrylate polymer layers bearing poly(ethylene oxide) and phosphorylcholine side chains as non-fouling surfaces: in vitro interactions with plasma proteins and platelets

Two methacrylate monomers, oligo(ethylene glycol) methyl ether methacrylate (OEGMA; MW=300 g mol(-1), poly(ethylene glycol) (PEG) side chains of average length n=4.5) and 2-methacryloyloxyethyl phosphorylcholine (MPC; MW=295 g mol(-1)), were grafted from silicon wafer surfaces via surface-initiated atom transfer radical polymerization. The grafted surfaces were used as model PEG and phosphorylcholine surface systems to allow comparison of the effectiveness of these two motifs in the prevention of plasma protein adsorption and platelet adhesion. It was found that at high graft density fibrinogen adsorption from plasma on the poly(MPC) and poly(OEGMA) surfaces for a given graft chain length was comparable and extremely low. At low graft density, poly(OEGMA) was slightly more effective than poly(MPC) in resisting fibrinogen adsorption from plasma. Flowing whole blood experiments showed that at low graft density the poly(OEGMA) surfaces were more resistant to fibrinogen adsorption and platelet adhesion than the poly(MPC) surfaces. At high graft density, both the poly(MPC) and poly(OEGMA) surfaces were highly resistant to fibrinogen and platelets. Immunoblots of proteins eluted from the surfaces after contact with human plasma were probed with antibodies against a range of proteins, including the contact phase clotting factors, fibrinogen, albumin, complement C3, IgG, vitronectin and apolipoprotein A-I. The blot responses were weak on the poly(MPC) and poly(OEGMA) surfaces at low graft density and zero at high graft density, again indicating strongly protein resistant properties for these surfaces. Since the side chains of the poly(OEGMA) are about 50% greater in size than those of poly(MPC), the difference in protein resistance between the poly(MPC) and poly(OEGMA) surfaces at low graft density may be due to the difference in surface coverage of the two graft types.

Synthesis of biocompatible sterically-stabilized poly(2-(methacryloyloxy)ethyl phosphorylcholine) latexes via dispersion polymerization in alcohol/water mixtures

Poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) is soluble in either 2-propanol or water but becomes insoluble in certain alcohol-rich 2-propanol/water mixtures. We have exploited this unusual cononsolvency behavior in order to prepare new biocompatible sterically stabilized PMPC latexes via nonaqueous dispersion polymerization in 2-propanol/water mixtures. All polymerizations were conducted in the presence of monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) as a reactive stabilizer, with some formulations including ethylene glycol dimethacrylate (EGDMA) as a cross-linker. Under optimized conditions, unimodal size distributions could be obtained with a mean latex diameter of approximately 1 microm, as judged by laser diffraction and DLS. The mean latex diameter depended on both the PEGMA and initiator concentration but was almost independent of the cross-linking density. Smaller PMPC latexes were obtained by increasing the alcohol content of the dispersion medium. On dilution with water, these latexes acquired microgel character. The microgel solution viscosity was insensitive to added salt due to the so-called "antipolyelectrolyte" effect, which is characteristic of polyzwitterions. Finally, copolymerization of MPC with a fluorescein-based methacrylic comonomer produced fluorescently labeled PMPC latexes, which may have potential biomedical applications.

Chain conformation of a new class of PEG-based thermoresponsive polymer brushes grafted on silicon as determined by neutron reflectometry

The thermoresponsive PEG-based copolymer poly[2-(2-methoxyethoxy)ethyl methacrylate-co-oligo(ethylene glycol) methacrylate) (P(MEO(2)MA-co-OEGMA)] was grafted onto a silicon wafer, and its chain conformation in aqueous solution was studied by neutron reflectometry. The effects of temperature and salt concentration on the polymer's conformation were evaluated. With increasing temperature, it was found that the polymer brushes underwent a transition from an extended state to a compressed state, and eventually a collapsed state above the lower critical solution temperature. The presence of salt significantly affected the well-extended brushes but had little effect on compressed and collapsed brushes. This PEG-based thermoresponsive surface exhibited good protein adsorption resistance. Interestingly, extended and collapsed brushes showed the same level of protein repulsion, something that was not expected.

End terminal, poly(ethylene oxide) graft layers: surface forces and protein adsorption

Covalently grafted poly(ethylene oxide) coatings have been widely studied for use in biomedical applications, particularly for the reduction of protein and other biomolecule adsorption. However, many of these studies have not characterized the hydrated structure of the coatings. This new study uses a combination of silica colloid probe interaction force measurements using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) in order to determine the grafting density and hydrated layer structure of monomethoxy poly(ethylene oxide) aldehyde layers, covalently grafted onto amine plasma polymer surfaces, and their interactions with silica surfaces. For high grafting densities, purely repulsive interactions were measured as expected for densely grafted polymer brushes. These interactions could be described by theoretical expectations for compression of one polymer brush layer. However, at lower grafting densities, attractive interactions were observed at larger separation distances, originating from bridging interactions due to adsorption of the PEO chains on the surface of the silica colloid probe. This is a new finding indicating that the coupled PEO molecules have sufficient conformational freedom to interact strongly with an adjacent surface or, for example, protein molecules for which there is an affinity. The attractive interactions could be removed by grafting an additional PEO layer onto the silica colloid probe. Protein adsorption measurements confirmed that at high grafting densities, the amount of adsorbed protein on the PEO grafted surfaces was greatly reduced, to the order of the detection limit for the XPS technique.

Oligo(ethylene oxide) self-assembled monolayers with self-limiting packing densities for the inhibition of nonspecific protein adsorption

We have created a molecule that forms self-assembled monolayers (SAMs) on Au, possessing the characteristics for inhibition of nonspecific protein adsorption, i.e., uniformly distributed, loosely packed, conformationally mobile, hydrated ethylene oxide (EO) chains of near optimal packing densities. SAMs of the bipodal molecule CH(3)O(CH(2)CH(2)O)(5)CH(2)CON(CH(2)CH(2)CH(2)SCOCH(3))(2) [N,N-(bis-3'-thioacetylpropyl)-3,6,9,12,15,18-hexaoxanonadecanamide (BTHA)] on polycrystalline Au are described. Spectroscopic ellipsometry (SE) and reflection-absorption infrared spectroscopy data indicate that BTHA SAM thickness and EO chain disorder closely match that of partially formed monothio-(EO)(5-6)CH(3) SAMs when they exhibit maximum inhibition of protein adsorption. However, in contrast to the monothio-(EO)(5-6)CH(3) SAMs, the BTHA SAM thickness and EO chain disorder remain constant in the presence of unbound molecules because of the structurally imposed upper limit of one EO chain per two Au occupancy sites. SE data indicate high resistance to protein adsorption for bovine serum albumin, fibrinogen, and a mixture of the two, suggesting uniform EO surface coverage on a length scale at least equal to the smallest dimension of these proteins.

Lubrication at physiological pressures by polyzwitterionic brushes

The very low sliding friction at natural synovial joints, which have friction coefficients of mu < 0.002 at pressures up to 5 megapascals or more, has to date not been attained in any human-made joints or between model surfaces in aqueous environments. We found that surfaces in water bearing polyzwitterionic brushes that were polymerized directly from the surface can have mu values as low as 0.0004 at pressures as high as 7.5 megapascals. This extreme lubrication is attributed primarily to the strong hydration of the phosphorylcholine-like monomers that make up the robustly attached brushes, and may have relevance to a wide range of human-made aqueous lubrication situations.