Gopinath Mani

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.

Drug delivery from gold and titanium surfaces using self-assembled monolayers.

Currently available drug-eluting stents (DES) use polymers for coating and releasing drugs. Increasing evidence suggests that inflammatory and hypersensitive reactions are caused by such polymer coatings. This study focused on developing new techniques for delivering drugs directly from metal implant surfaces. Hydroxyl-terminated self-assembled monolayers (SAMs) were coated on Au and Ti surfaces. Therapeutic self-assembled monolayers (TSAMs) were prepared by chemically attaching the model drug, flufenamic acid, to SAM coated metal surfaces. Three different methods of esterification (acid chloride esterification, dry heat esterification, and direct esterification) were explored to attach flufenamic acid to SAMs. TSAMs were characterized using X-ray photoelectron spectroscopy, fluorescence microscopy, atomic force microscopy, and contact angle goniometry. These techniques collectively confirmed the attachment of drug onto SAM coated metal surfaces. In vitro drug release was investigated by immersing TSAM coated metal specimens in tris-buffered saline (TBS) at 37 degrees C for 28 days. TBS was analyzed at 1, 3, 7, 14, 21, and 28 days for the amount of drug eluted using high performance liquid chromatography. Large data scatter was observed for the release profiles of TSAMs prepared by acid chloride esterification. TSAMs prepared by dry heat and direct esterification methods showed an initial burst release of the drug followed by a sustained slow release for up to 2 weeks. Thus, this study suggests the potential for using self-assembled monolayers as an alternate system for delivering drugs from coronary stents and other metal implants.

Delivery of paclitaxel from cobalt-chromium alloy surfaces without polymeric carriers

Polymer-based carriers are commonly used to deliver drugs from stents. However, adverse responses to polymer coatings have raised serious concerns. This research is focused on delivering drugs from stents without using polymers or any carriers. Paclitaxel (PAT), an anti-restenotic drug, has strong adhesion towards a variety of material surfaces. In this study, we have utilized such natural adhesion property of PAT to attach these molecules directly to cobalt-chromium (Co-Cr) alloy, an ultra-thin stent strut material. Four different groups of drug coated specimens were prepared by directly adding PAT to Co-Cr alloy surfaces: Group-A (PAT coated, unheated, and ethanol cleaned); Group-B (PAT coated, heat treated, and ethanol cleaned); Group-C (PAT coated, unheated, and not ethanol cleaned); and Group-D (PAT coated, heat treated and not ethanol cleaned). In vitro drug release of these specimens was investigated using high performance liquid chromatography. Groups A and B showed sustained PAT release for up to 56 days. A simple ethanol cleaning procedure after PAT deposition can remove the loosely bound drug crystals from the alloy surfaces and thereby allowing the remaining strongly bound drug molecules to be released at a sustained rate. The heat treatment after PAT coating further improved the stability of PAT on Co-Cr alloy and allowed the drug to be delivered at a much slower rate, especially during the initial 7 days. The specimens which were not cleaned in ethanol, Groups C and D, showed burst release. PAT coated Co-Cr alloy specimens were thoroughly characterized using scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. These techniques were collectively useful in studying the morphology, distribution, and attachment of PAT molecules on Co-Cr alloy surfaces. Thus, this study suggests the potential for delivering paclitaxel from Co-Cr alloy surfaces without using any carriers.

Interaction of Endothelial Cells with Self-Assembled Monolayers for Potential Use in Drug-Eluting Coronary Stents

Drug-eluting stents (DES) are implanted in patients to treat in-stent restenosis. Commercially available DES use polymers for coating and releasing drugs. Several studies have showed that polymer coatings cause adverse reactions. Delayed endothelialization of polymer-coated DES leads to late stent thrombosis. Recently, the potential for using self-assembled monolayers (self-assembled monolayers (SAMs)-organic constructs composed of (a) chemical groups which attach to metal surfaces, (b) long hydrocarbon chains, and (c) terminal functional groups) as an alternate drug delivery system for coronary stents has been demonstrated. In this study, the interaction of human aortic endothelial cells (HAECs) with SAMs and therapeutic SAMs (therapeutic self-assembled monolayers (TSAMs)-SAMs derivatized with the drug, flufenamic acid) was investigated. HAECs were cultured on plain glass, control, SAMs-, and TSAMs-coated titanium (Ti) and gold (Au) specimens. The viability and proliferation of HAECs were investigated using MTT colorimetric assay. The adhesion of HAECs on SAMs and TSAMs was equivalent to that of control metal surfaces and superior to that of plain glass surfaces. The cells continued to proliferate on both SAMs and TSAMs even though the rate of proliferation was slower than plain glass or control-Ti. The spreading of HAECs on TSAMs with typical polygonal shape indicated that these surfaces are conducive to endothelialization. The expression of surface adhesion protein, platelet endothelial cell adhesion molecule-1, on TSAMs indicated that the endothelial cells preserved their phenotype on these surfaces. Thus, this study demonstrated that SAMs and TSAMs do not elicit an adverse response from endothelial cells in in vitro conditions.

Stability of antibacterial self-assembled monolayers on hydroxyapatite

Open fractures are common in battlefields, motor vehicle accidents, gunshot wounds, sports injuries, and high-energy falls. Such fractures are treated using hydroxyapatite (HA)-based bone graft substitutes. However, open fracture wounds are highly susceptible to bacterial infections. Hence, this study was focused on incorporating antibacterial properties to HA using silver (Ag) carrying self-assembled monolayers (SAMs). Also, the stability of Ag carrying SAMs on HA was investigated under sterilization and physiological conditions. Initially, the -COOH terminated phosphonic acid SAMs of two different chain lengths (11 carbon atoms - shorter chain and 16 carbon atoms - longer chain) were deposited on HA. Antibacterial SAMs (ASAMs) were prepared by chemically attaching Ag to shorter and longer chain SAMs coated HA. X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle goniometry collectively confirmed the attachment of Ag onto SAMs coated HA. The bacterial adhesion study showed that the adherence of Staphylococcus aureus was significantly reduced on ASAMs coated HA when compared to control-HA. The stability studies showed that gas plasma, dry heat and autoclave degraded most of the ASAMs on HA. UV irradiation did not damage the shorter chain ASAMs as vigorously as other treatments, while it degraded the longer chain ASAMs completely. Ethylene oxide treatment did not degrade the longer chain ASAMs unlike all other treatments but it severely damaged the shorter chain ASAMs. Both shorter and longer chain ASAMs significantly desorbed from the HA surfaces under physiological conditions although longer chain ASAMs exhibited better stability than shorter chain ASAMs. This study demonstrated the potential for using ASAMs to provide antibacterial properties to HA and the need for developing techniques to improve stability of SAMs under sterilization and physiological conditions.

Paclitaxel delivery from cobalt-chromium alloy surfaces using self-assembled monolayers.

Polymer-based platforms in drug-eluting stents (DESs) can cause adverse reactions in patients. Hence, the development of a polymer-free drug delivery platform may reduce adverse reactions to DES. In this study, the use of a polymer-free platform, self-assembled monolayers (SAMs), is explored for delivering an antiproliferative drug [paclitaxel (PAT)] from a stent material [cobalt-chromium ((Co-Cr) alloy]. Initially, carboxylic acid terminated phosphonic acid SAMs were coated on Co-Cr alloy. Two different doses (25 and 100 μg/cm²) of PAT were coated on SAM coated Co-Cr surfaces using a microdrop deposition method. Also, control experiments were carried out to coat PAT directly on Co-Cr surfaces with no SAM modification. The PAT coated specimens were characterized using the Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and atomic force microscopy (AFM). FTIR spectra showed the successful deposition of PAT on SAM coated and control-Co-Cr surfaces. SEM images showed islands of high density PAT crystals on SAM coated surfaces, while low density PAT crystals were observed on control-Co-Cr alloy. AFM images showed molecular distribution of PAT on SAM coated as well as control-Co-Cr alloy surfaces. In vitro drug release studies showed that PAT was released from SAM coated Co-Cr surfaces in a biphasic manner (an initial burst release in first 7 days was followed by a slow release for up to 35 days), while the PAT was burst released from control-Co-Cr surfaces within 1-3 days. Thus, this study demonstrated the use of SAMs for delivering PAT from Co-Cr alloy surfaces for potential use in drug-eluting stents.

Endothelial cell behaviour on gas-plasma-treated PLA surfaces: the roles of surface chemistry and roughness

Glow‐discharge gas‐plasma (GP) treatment has been shown to induce surface modifications such that cell adhesion and growth are enhanced. However, it is not known which gas used in GP treatment is optimal for endothelial cell function. Polylactic acid (PLA) films treated oxygen, argon, or nitrogen GP were characterized using contact angles, scanning electron microscopy, atomic force microscopy, optical profilometry, and x‐ray photoelectron spectroscopy. All three GP treatments decreased the carbon atomic concentration and surface roughness and increased the oxygen atomic concentration. Human umbilical vein endothelial cells were cultured on the PLA films for up to 7 days. Based on proliferation and live/dead assays, surface chemistry was shown to have the greatest effect on the attachment, proliferation, and viability of these cells, while roughness did not have a significant influence. Of the different gases, endothelial cell viability, attachment and proliferation were most significantly increased on PLA surfaces treated with oxygen and argon gas plasma. Copyright

Microrough Cobalt–Chromium Alloy Surfaces for Paclitaxel Delivery: Preparation, Characterization, and In Vitro Drug Release Studies

Cobalt-chromium (Co-Cr) alloys have extensive biomedical applications including drug-eluting stents (DES). This study investigates the use of eight different microrough Co-Cr alloy surfaces for delivering paclitaxel (PAT) for potential use in DES. The eight different surfaces include four bare microrough and four self-assembled monolayer (SAM) coated microrough surfaces. The bare microrough surfaces were prepared by grit blasting Co-Cr with glass beads (50 and 100 μm in size) and Al(2)O(3) (50 and 110 μm). The SAM coated surfaces were prepared by depositing a -COOH terminated phosphonic acid monolayer on the different microrough surfaces. PAT was then deposited on all the bare and SAM coated microrough surfaces. The surfaces were characterized using scanning electron microscopy (SEM), 3D optical profilometry, and Fourier transform infrared spectroscopy (FTIR). SEM showed the different morphologies of microrough surfaces without and with PAT coating. An optical profiler showed the 3D topography of the different surfaces and the changes in surface roughness and surface area after SAM and PAT deposition. FTIR showed ordered SAMs were formed on glass bead grit blasted surfaces, while the molecules were disordered on Al(2)O(3) grit blasted surfaces. Also, FTIR showed the successful deposition of PAT on these surfaces. The PAT release was investigated for up to two weeks using high performance liquid chromatography. Al(2)O(3) grit blasted bare microrough surfaces showed sustained release profiles, while the glass bead grit blasted surfaces showed burst release profiles. All SAM coated surfaces showed biphasic drug release profiles, which is an initial burst release followed by a slow and sustained release. SAM coated Al(2)O(3) grit blasted surfaces prolonged the sustained release of PAT in a significant amount during the second week of drug elution studies, while this behavior was not observed for any other surfaces used in this study. Thus, this study demonstrates the use of different microrough Co-Cr alloy surfaces for delivering PAT for potential applications in DES and other medical devices.

Long-term stability of self-assembled monolayers on 316L stainless steel.

316L stainless steel (316L SS) has been extensively used for making orthopedic, dental and cardiovascular implants. The use of phosphonic acid self-assembled monolayers (SAMs) on 316L SS has been previously explored for potential biomedical applications. In this study, we have investigated the long-term stability of methyl (-CH(3)) and carboxylic acid (-COOH)-terminated phosphonic acid SAMs on 316L under physiological conditions. The stability of SAMs on mechanically polished and electropolished 316L SS was also investigated as a part of this study. Well-ordered and uniform -CH(3)- and -COOH-terminated SAMs were coated on mechanically polished and electropolished 316L SS surfaces. The long-term stability of SAMs on 316L SS was investigated for up to 28 days in Tris-buffered saline (TBS) at 37 degrees C using x-ray photoelectron spectroscopy, atomic force microscopy and contact angle goniometry. A significant amount of phosphonic acid molecules was desorbed from the 316L SS surfaces within 1 to 7 days of TBS immersion followed by a slow desorption of molecules over the remaining days. The -COOH-terminated SAM was found to be more stable than the -CH(3)-terminated SAM on both mechanically and electropolished surfaces. No significant differences in the desorption behavior of SAMs were observed between mechanically and electropolished 316L SS surfaces.

Long-term stability of self-assembled monolayers on electropolished L605 cobalt chromium alloy for stent applications

Commercially available drug-eluting stents have the potential to induce inflammatory and hypersensitive adverse reactions due to their polymer coating. The use of self assembled monolayers (SAMs) as an alternate drug delivery platform for stents has recently been demonstrated. In this study, the formation and stability of phosphonic acid SAMs were investigated using the material and surface preparation commonly used to make ultra-thin stent struts-electropolished L605 Cobalt Chromium (CoCr) alloy. Methyl (⁻CH₃) and carboxylic acid (⁻OOH) terminated phosphonic acid SAMs were coated on electropolished CoCr alloy using a combination of solution immersion and dip-evaporation cycle deposition methods. SAMs-coated CoCr alloy specimens were thoroughly characterized using contact angle goniometry, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). These characterizations suggested that uniform and well-ordered monolayers were coated on the electropolished CoCr alloy. The long-term physiological stability of monolayers was investigated in tris-buffered saline (TBS) at 37°C for up to 28 days. Contact angles, FTIR, XPS, and AFM suggested that both ⁻CH₃ and ⁻COOH terminated phosphonic acid SAMs desorb from electropolished CoCr alloy surfaces in a biphasic manner. A significant desorption of ⁻CH₃ and ⁻COOH terminated SAMs occurs within 1-3 days followed by a slower desorption for up to 28 days. Thus, there is a need to develop techniques that can improve the long-term stability of SAMs on electropolished CoCr alloy for stent and other biomedical applications.