Synthesis and Characterization of Biocompatible Polymer Interlayers on Titanium Implant Materials

Influence of quaternization of ammonium on antibacterial activity and cytocompatibility of thin copolymer layers on titanium

Antimicrobial coatings are able to improve the osseointegration of dental implants. Copolymers are promising materials for such applications due to their combined properties of two different monomers. To investigate the influence of different monomer mixtures, we have been synthesized copolymers of dimethyl (methacryloxyethyl) phosphonate (DMMEP) and dipicolyl aminoethyl methacrylate in different compositions and have them characterized to obtain the r-parameters. Some of the copolymers with different compositions have also been alkylated with 1-bromohexane, resulting in quaternized ammonium groups. The copolymers have been deposited onto titanium surfaces resulting in ultrathin, covalently bound layers. These layers have been characterized by water contact angle measurements and ellipsometry. The influence of quaternary ammonium groups on antibacterial properties and cytocompatibility was studied: Activity against bacteria was tested with a gram positive Staphylococcus aureus strain. Cytocompatibility was tested with a modified LDH assay after 24 and 72 h to investigate adhesion and proliferation of human fibroblast cells on modified surfaces. The copolymer with the highest content of DMMEP showed a good reduction of S. aureus and in the alkylated version a very good reduction of about 95%. On the other hand, poor cytocompatibility is observed. However, our results show that this trend cannot be generalized for this copolymer system.

Effects of interfacial layer wettability and thickness on the coating morphology and sirolimus release for drug-eluting stent

Drug-eluting stents (DESs) have been used to treat coronary artery diseases by placing in the arteries. However, current DESs still suffer from polymer coating defects such as delamination and peeling-off that follows stent deployment. Such coating defects could increase the roughness of DES and might act as a source of late or very late thrombosis and might increase the incident of restenosis. In this regard, we modified the cobalt-chromium (Co-Cr) alloy surface with hydrophilic poly(2-hydroxyethyl methacrylate) (PHEMA) or hydrophobic poly(2-hydroxyethyl methacrylate)-grafted-poly(caprolactone) (PHEMA-g-PCL) brushes. The resulting surfaces were biocompatible and biodegradable, which could act as anchoring layer for the drug-in-polymer matrix coating. The two modifications were characterized by ATR-FTIR, XPS, water contact angle measurements, SEM and AFM. On the control and modified Co-Cr samples, a sirolimus (SRL)-containing poly(D,L-lactide) (PDLLA) were ultrasonically spray-coated, and the drug release was examined for 8weeks under physiological conditions. The results demonstrated that PHEMA as a primer coating improved the coating stability and degradation morphology, and drug release profile for short-term as compared to control Co-Cr, but fails after 7weeks in physiological buffer. On the other hand, the hydrophobic PHEMA-g-PCL brushes not only enhanced the stability and degradation morphology of the PDLLA coating layer, but also sustained SRL release for long-term. At 8-week of release test, the surface morphologies and release profiles of coated PDLLA layers verified the beneficial effect of hydrophobic PCL brushes as well as their thickness on coating stability. Our study concludes that 200nm thickness of PHEMA-g-PCL as interfacial layer affects the stability and degradation morphology of the biodegradable coating intensively to be applied for various biodegradable-based DESs.

Surface modification of titanium substrate with a novel covalently-bound copolymer thin film for improving its platelet compatibility

Despite of its widely uses in various clinical applications, the titanium-based material still faces different challenges, such as hemocompatibility and anti-biofouling characteristics required in various situations. The objective of this investigation was to develop a novel surface modification strategy for titanium-based material to improve the platelet compatibility that is important in rigorous blood-contacting cardiovascular applications. In this work, a series of copolymers, which composed of novel 6-acryloyloxy hexyl phosphonic acid (AcrHPA) and sulfobetaine methacrylate (SBMA) was synthesized. The phosphonic acid group in these copolymers can impart covalent binding to the titanium substrate while the zwitterionic sulfobetaine functionality is considered being able to reduce the platelet adhesion and activation on the modified titanium substrate. NMR analyses suggested that copolymerization reaction is likely not an ideal statistical reaction but to add the monomers in a random order. Studies have shown that the composition of the monomers affected the surface characteristics and platelet compatibility of these covalent-bound AcrHPA-SBMA copolymers on titanium substrate. Contact angle analysis has shown the addition of SBMA can increase surface hydrophilicity of the spun-coated copolymers. In addition, AFM analyses have revealed that the surface roughness of the spun-coated copolymer layer were varied with the ratio of AcrHPA and SBMA. The most platelet compatible surface was noted on the one modified by the highest amount of SBMA added (i.e. 70 mol%) in copolymerization. In summary, the surface modification scheme presented here would be of potential as well as manufacturing process applicable for future development in blood-contacting titanium-based biomedical devices.

Antimicrobial surface coatings for a permanent percutaneous passage in the concept of osseointegrated extremity prosthesis

The clinical implementation of percutaneous implants is still limited owing to infections at the side of the stoma. In our concept, this issue is addressed by designing copolymer surface coatings possessing biocompatibility and antimicrobial activity to improve the maintenance of a physiological skin seal at the skin-implant interface. Different copolymers with surface-active phosphonate and antimicrobial cationic groups were designed. Thus, coated titanium samples were cultured with bacterial strains or fibroblasts, respectively. Antimicrobial impact was evaluated by imaging the reduction of bacterial adherence. Biocompatibility was displayed by fibroblast proliferation and morphology. A variety of copolymers of 4-vinylpyridine with vinylbenzylphosphonate or dimethyl(2-methacryloyloxy-ethyl) phosphonate were prepared by free radical polymerization. The optimized polymer coating (copolymer D) showed a reduction of adherent bacteria up to 95%, with only a slight reduction in the adherence of human fibroblasts compared with blank titanium controls. In this study, we demonstrate in vitro that polymer surface coatings can be simultaneously antimicrobial and biocompatible. We consider this to be a promising technology for the realization of a permanent aseptic percutaneous passage as needed for the advancement of osseointegrated limb prosthesis.

Effect of oligonucleotide mediated immobilization of bone morphogenic proteins on titanium surfaces

The aim of the present study was to test the hypothesis that oligonucleotides can be used for anchorage and slow release of osteogenic growth factors such as BMP to enhance the osteogenic activity of a titanium implant surface. Strands of 60-mer non-coding DNA oligonucleotides (ODN) were bound to an acid-etched sandblasted cp Ti-surface by nanomechanical fixation using anodic polarization. RhBMP2 that had been conjugated to complementary strands of DNA oligonucleotides was then bound to the anchored ODN strands by hybridization. Binding studies showed a higher binding capacity compared to non-conjugated BMP2. Long term release experiments demonstrated a continuous release from all surfaces that was lowest for the conjugated BMP2 bound to the ODN anchor strands. Proliferation of human bone marrow stroma cells (hBMSC) was significantly increased on these surfaces. Immunofluorescence showed that hBMSC grown on surfaces coated with specifically bound conjugated BMP2 developed significantly higher numbers of focal adhesion points and exhibited significantly higher levels of transcription of osteogenic markers alkaline phosphatase and osteopontin at early intervals. Biological activity (induction of alkaline phosphatase) of conjugated BMP2 released from the surface was comparable to released non-conjugated BMP2, indicating that conjugation did not negatively affect the activity of the released molecules. In conclusion the present study has shown that BMP2 conjugated to ODN strands and hybridized to complementary ODN strands anchored to a titanium surface has led to slow growth factor release and can enhance the osteogenic activity of the titanium surface.

Self-assembled antimicrobial and biocompatible copolymer films on titanium

Copolymers of 4-vinyl-N-hexylpyridinium bromide and dimethyl(2-methacryloyloxyethyl) phosphonate self-assemble to form ultrathin layers on titanium surfaces that show antimicrobial activity, and biocompatibility. The copolymer layers are characterized by contact angle measurements, ellipsometry and XPS. Antibacterial activity is assessed by investigation of adherence of S. mutans. Biocompatibility is rated based on human gingival fibroblast adhesion and proliferation. By balancing the opposing effects of the chemical composition on biocompatibility and antimicrobial activity, copolymer coatings are fabricated that are able to inhibit the growth of S. mutans on the surface but still show attachment of gingival fibroblasts, and therefore might prevent biofilm formation on implants.

Biofunctionalization of a "clickable" organic layer photochemically grafted on titanium substrates

We have developed a general method combining photochemical grafting and copper-catalyzed click chemistry for biofunctionalization of titanium substrates. The UV-activated grafting of an α,ω-alkenyne onto TiO(2)/Ti substrates provided a "clickable" thin film platform. The selective attachment of the vinyl end of the molecule to the surface was achieved by masking the alkynyl end with a trimethylgermanyl (TMG) protecting group. Subsequently, various oligo(ethylene glycol) (OEG) derivatives terminated with an azido group were attached to the TMG-alkynyl modified titanium surface via a one-pot deprotection/click reaction. The films were characterized by X-ray photoelectron spectroscopy (XPS), contact angle goniometry, ellipsometry, and atomic force microscopy (AFM). We showed that the titanium surface presenting click-immobilized OEG substantially suppressed the nonspecific attachment of protein and cells as compared to the unmodified titanium substrate. Furthermore, glycine-arginine-glycine-aspartate (GRGD), a cell adhesion peptide, was coimmobilized with OEG on the platform. We demonstrated that the resultant GRGD-presenting thin film on Ti substrates can promote the specific adhesion and spreading of AsPC-1 cells.

Coating of titanium implant materials with thin polymeric films for binding the signaling protein BMP2

A fast and simple approach for immobilization using copolymers as interlayers is reported. The synthesized copolymers form stable self-assembled layers on implant materials like, e.g., titanium in a simple coating/drying/washing sequence and have functional groups which can bind proteins from an aqueous solution. The copolymer films have been characterized via ellipsometry and contact angle measurements and were tested for biocompatibility. An immunoassay was used to determine the amount of BMP2 and demonstrated an approximately 10-fold increase as compared to previously used self-assembled monolayers. A BMP2-responsive cell line with luciferase detection was used to determine the biological activity of the bound signaling protein.

A "clickable" titanium surface platform

A straightforward functionalization of a titanium surface using "click" chemistry is reported. A "clickable" titanium surface platform was prepared by the immobilization of an azide-functionalized electroactive catechol anchor and was subsequently derivatized with an electroactive or fluorinated probe via the CuAAC (copper-catalyzed azide-alkyne cycloaddition) reaction. The course of the reaction was investigated by contact angle, XPS, and electrochemical measurements.

Biological nano-functionalization of titanium-based biomaterial surfaces: a flexible toolbox

Surface functionalization with bioactive molecules (BAMs) on a nanometre scale is a main field in current biomaterial research. The immobilization of a vast number of substances and molecules, ranging from inorganic calcium phosphate phases up to peptides and proteins, has been investigated throughout recent decades. However, in vitro and in vivo results are heterogeneous. This may be at least partially attributed to the limits of the applied immobilization methods. Therefore, this paper highlights, in the first part, advantages and limits of the currently applied methods for the biological nano-functionalization of titanium-based biomaterial surfaces. The second part describes a new immobilization system recently developed in our groups. It uses the nanomechanical fixation of at least partially single-stranded nucleic acids (NAs) into an anodic titanium oxide layer as an immobilization principle and their hybridization ability for the functionalization of the surface with BAMs conjugated to the respective complementary NA strands.

The Janus-SAM approach for the flexible functionalization of gold and titanium oxide surfaces

A novel approach is developed to address the requirement of multiple stamps and inks for microcontact printing (microCP) onto different substrate surfaces. This approach relies on microCP one divalent molecule, which is able to form Janus self-assembled monolayers (JSAMs) with a labile cleavable centre, thus providing a facile method for the chemical derivatization of different substrate surfaces. This study presents an answer to the challenges presented within a highly versatile application, microCP. N-(3-diethylphosphatoxy)propyl-11-mercaptoundecanamide is used for the first time as an ink for microCP onto both gold and titanium oxide surfaces, utilizing the same polydimethylsiloxane stamp. Following printing, the JSAMs are enzymatically treated on these two different substrates to reveal different functional groups. The newly formed surfaces are subjected to additional surface reactions and used for the chemisorption of bovine serum albumin. At each stage, these JSAMs are characterized by X-ray photoelectron spectroscopy and dynamic water-contact-angle measurements. Confocal laser scanning microscopy is used for the characterization of the adsorbed proteins.

Protection of sensors for biological applications by photoinitiated chemical vapor deposition of hydrogel thin films

We report photoinitiated chemical vapor deposition (piCVD), a gentle synthetic method for the preparation of ultrathin films (approximately 100 nm) of the hydrogel poly(hydroxyethyl methacrylate) (pHEMA). piCVD occurs near room temperature and requires only mild vacuum conditions. The deposited films swell rapidly and reversibly in buffer solution, and the swelling properties can be controlled via the deposition conditions. Analysis of the swelling data indicates that the mesh size of the hydrogel creates a selectively permeable coating. The mesh is large enough to allow small molecule analytes to permeate the film but small enough to prevent the transport of large biomolecules such as proteins. X-ray photoelectron spectroscopy (XPS) shows that the films decrease nonspecific adhesion of the protein albumin by nearly 8-fold over bare silicon. A dry process, piCVD is suitable for coating particles with diameters as small as 5 microm. The absence of solvents and plasmas in piCVD allows films to be directly synthesized on optode sensors without degradation of sensitivity or response time.

Stability of self-assembled monolayers on titanium and gold

Methyl- and hydroxyl-terminated phosphonic acid self-assembled monolayers (SAMs) were coated on Ti from aqueous solution. Dodecyl phosphate and dodecyltrichlorosilane SAMs were also coated on Ti using solution-phase deposition. The stability of SAMs on Ti was investigated in Tris-buffered saline (TBS) at 37 degrees C using X-ray photoelectron spectroscopy, contact angle goniometry, and atomic force microscopy. For comparison purposes, a hydroxyl-terminated thiol SAM was coated on Au, and its stability was also investigated under similar conditions. In TBS, a significant proportion of phosphonic acid or phosphate molecules were desorbed from the Ti surface within 1 day, while the trichlorosilane SAM on Ti or thiol SAM on Au was stable for up to 7 days under similar conditions. The stability of hydroxyl-terminated phosphonic acid SAM coated Ti and thiol SAM coated Au was investigated in ambient air and ultraviolet (UV) light. In ambient air, the phosphonic acid SAM on Ti was stable for up to 14 days, while the thiol SAM on Au was not stable for 1 day. Under UV-radiation exposure, the alkyl chains of the phosphonic acid SAM were decomposed, leaving only the phosphonate groups on the Ti surface after 12 h. Under similar conditions, decomposition of alkyl chains of the thiol SAM was observed on the Au surface accompanied by oxidation of thiolates.

Surface modification of plastic, glass and titanium by photoimmobilization of polyethylene glycol for antibiofouling

Photoreactive poly(ethylene glycol) (PEG) was prepared and the polymer was photoimmobilized on organic, inorganic and metal surfaces to reduce their interaction with proteins and cells. The photoreactive PEG was synthesized by co-polymerization of methacrylate-PEG and acryloyl 4-azidobenzene. Surface modification was carried in the presence and the absence of a micropatterned photomask. It was then straightforward to confirm the immobilization using the micropatterning. Using the micropatterning method, immobilization of the photoreactive PEG on plastic (Thermanox), glass and titanium was confirmed by time-of-flight secondary ion mass spectroscopy and atomic force microscopy observations. The contact angle on an unpatterned surface was measured. Although the original surfaces have different contact angles, the contact angle on PEG-immobilized surfaces was the same on all surfaces. This result demonstrated that the surface was completely covered with PEG by the photoimmobilization. To assess non-specific protein adsorption on the micropatterned surface, horseradish peroxidase (HRP)-conjugated proteins were adsorbed. Reduced protein adsorption was confirmed by vanishingly small staining of HRP substrates on the immobilized regions. COS-7 cells were cultured on the micropatterned surface. The cells did not adhere to the PEG-coated regions. In conclusion, photoreactive PEG was immobilized on various surfaces and tended to reduce interactions with proteins and cells.