The adhesion of glow--discharge polymers, Silastic and Parylene to implantable platinum electrodes: results of tensile pull tests after exposure to isotonic sodium chloride.

Thin organic coatings commonly are used for insulating microelectrodes and electronic packages designed for implant applications. The adherence of these coatings to the underlying substrates is a key parameter in their selection for various devices. Instron pull tests were performed on glow-discharge polymerized monomers, Parylene-N, medical-grade Silastic and various epoxies. The application of a thin coating of glow-discharge polymerized methane under a thicker Parylene-N coating improved the adhesion of the latter to the underlying substrate in isotonic sodium chloride solution and during accelerated testing conditions done by boiling.

Fouling and protein adsorption. Effect of low-temperature plasma treatment of membrane surfaces.

Adsorption of proteins and the effect of the chemical nature of membrane surfaces on protein adsorption were investigated using 14C-tagged albumin and several microporous membranes (polyvinilydene fluoride, PVDF; nylon; polypropylene, PP; and polycarbonate, PC). The membrane surfaces were modified by exposing them to low-temperature plasma of several different monomers (n-butane, oxygen, nitrogen alone or as mixtures) in a radiofrequency plasma reactor. Transients in the permeability of albumin solutions through the membranes and changes in flux of distilled water through the membranes before and after adsorption of albumin were used to investigate the role of protein adsorption on membrane fouling. The results show that the extent of adsorption of albumin on hydrophobic membranes was considerably more than that on hydrophilic membranes. The hydrophilic membranes were susceptible to electrostatic interactions and less prone to fouling. A pore-blocking model was successfully used to correlate the loss of water flux through pores of defined geometry.

ESR study of MMA polymerization by a peroxide/amine system: bone cement formation.

Electron spin resonance (ESR) spectroscopy was used to gain insight at the molecular level into the curing of bone cement. Methyl methacrylate was polymerized using a N,N-dimethyl-p-toluidine (TD)/benzoyl peroxide (BPO) redox system in the presence of polymethyl methacrylate (PMMA) powder. The conventional nine-line ESR spectrum for the growing polymer radical was detected at the gel stage of polymerization. While the optimum free radical concentration was observed near the equimolar amine/BPO concentration, excess amine led to a change in the chemical structure of the trapped radical and inhibited the polymerization process. At a high amine/BPO ratio the nine-line signal disappeared and a three-line nitroxide-based radical appeared. The appearance of this nitroxide signal seems to depend on the amine/BPO molar ratio and on the presence of PMMA. An excess amount of amine with respect to BPO was found to inhibit the polymerization process. When BPO was removed, the system still polymerized but with a longer gelation time and a lower radical concentration. These results demonstrate that trapped free radicals in the bulk polymerization of MMA convert to polymeric peroxides that act as initiators in bone cement. When the accelerator 4-dimethylamino phenethyl alcohol (TDOH) was used, a higher radical concentration was observed in the polymerizing system. TDOH shows potential for being a more effective accelerator than TD for bone cement curing.

Improvement of fatigue properties of poly(methyl methacrylate) bone cement by means of plasma surface treatment of fillers.

The fatigue properties of poly(methyl methacrylate) (PMMA) bone cement were significantly improved through 13.56-MHz radio frequency plasma treatments on the X-ray opaque powder and on the reinforcing fibers. For the plasma treatments of particle surfaces, a specially designed plasma reactor was used to modify the surfaces of the ZrO2 powder (X-ray opaque filler) and milled carbon fibers (reinforcing filler). The reactor chamber was designed to be rotated continuously to mix the particles during the plasma treatments to obtain uniformly treated particle surfaces. The surface peroxides resulting from the plasma treatments seemed to have a significant effect on the improvement of fatigue properties. The peroxides on the particles may yield free radicals by the reaction with the reducing agent (N,N-dimethyl-p-toluidine) in the bone cement mixture, which can initiate methyl methacrylate (MMA) polymerization. Through this graft polymerization process, the interfacial bond strength between the filler particles and the MMA matrix may be enhanced, resulting in efficient stress transfer from the matrix to the fillers. The best results of the fatigue tests were seen in the reinforced bone cement, which contained surface modified fillers, with hexamethyldisiloxane plasma, and with O2 plasma posttreatment.

A simple multi-specimen apparatus for fixed stress fatigue testing.

To cope with the time-consuming characteristics of fatigue tests, a multi-specimen fatigue testing apparatus, which could test 10 specimens at a time, was designed, constructed, and tested. The specimens are fixed around a rotating axis, and the required stresses are applied by weights attached on the other end of each specimen. The test mode can be categorized as a stress-controlled flexural fatigue test. Its performance was tested by comparing it with a commercial three-point bending fatigue testing apparatus. The stress versus number of cycles to failure curves of poly(methylmethacrylate) (PMMA), which were obtained from both fatigue testing equipment, showed results that were similar to each other. The fatigue test results of acrylic bone cement in a fixed-stress mode also showed good agreement between the data obtained from the new apparatus and the commercial apparatus. The test results seem quite reliable and show feasibility of significantly reducing the overall test periods. It may be valuable, especially for the fatigue tests, which must be done with a low frequency and a low applied stress level such as a fatigue test of bone cements.

Evaluation of plasma polymer-coated contact lenses by electrochemical impedance spectroscopy.

The influence of ultra-thin (i.e., 5-50 nm) plasma polymer coatings on siloxane-based hydrogel contact lenses was investigated by electrochemical impedance spectroscopy (EIS). Impedance measurements as a function of frequency (Bode plots) were taken at regular intervals until steady impedance was obtained, indicating that the lenses were saturated. Appropriate equivalent circuit models were constructed to describe the salt intrusion characteristics of the plasma polymer-coated contact lenses. This provided information pertaining to the resistance and capacitance of interfacial and bulk layers in the plasma polymer-coated lenses. Resistance relates to ion permeability and capacitance for water uptake. This investigation showed that some of an ultra-thin layer of plasma polymer applied onto a dry hydrogel remains a contiguous film after the substrate hydrogel swells upon hydration. In some cases, however, the overall impedance of the coated lens is so low that the state of the plasma polymer layer after the hydration of the substrate hydrogel could not be judged by EIS.

Nonporous silicone polymer coating of expanded polytetrafluoroethylene grafts reduces graft neointimal hyperplasia in dog and baboon models.

PURPOSE: Neointimal hyperplasia frequently develops after placement of prosthetic vascular grafts and is a major cause of graft failure. This study was an attempt to prevent vascular lesion formation by coating the graft luminal surface with a thin layer of nonporous silicone polymer, and subsequently with an ultrathin layer of vapor phase (plasma gas)-deposited fluoropolymer, thereby providing a smooth and chemically uniform surface that was postulated to limit pannus tissue ingrowth across the graft anastomoses. METHODS: Bilateral femoral arteriovenous (AV) conduits were constructed in four dogs using expanded polytetrafluoroethylene graft materials (ePTFE; 6-mm inside diameter, 2.5-cm long). In each animal, one femoral AV shunt was constructed from a graft whose luminal surface was entirely coated with polymer. On the contralateral side, an uncoated graft served as a control. Bilateral aortoiliac grafts were placed in three baboons using 5-cm segments of ePTFE (4-mm inside diameter). One end (1 cm) of each graft had been coated with polymer. In each animal, the coated end of one graft was placed proximally and the coated end of the second graft was placed distally in the contralateral vessels. RESULTS: All grafts were patent at 30 days. In the dog model, there was a significant reduction in graft neointimal area at the venous anastomoses for the coated grafts compared with the uncoated grafts (0.03 +/- 0.02 mm2 and 1.11 +/- 0.54 mm2, respectively; p < 0.05). In the baboon model, the silicone coating significantly reduced the graft neointimal thickness (0.003 +/- 0.003 mm vs 0.21 +/- 0.05 mm; p < 0.05) and neointimal area (0.05 +/- 0.08 mm2 vs 0.82 +/- 0.58 mm2; p < 0.05). CONCLUSIONS: These data demonstrate that healing of ePTFE grafts can be effectively modified by altering the physical properties of the graft surface. Neointimal hyperplasia within ePTFE grafts is significantly reduced by the local application of a fluorocarbon-coated, silicone-based polymer. The resulting graft flow surface effectively prevents tissue ingrowth from the adjacent native vessel, thereby preserving the anastomosis luminal area. This approach could represent a new strategy for limiting graft surface anastomotic neointimal hyperplasia.