Use of plasma polymerization for preparing silicone-coated membranes for possible use in blood oxygenators

Polylysine-immobilized chitosan beads as adsorbents for bilirubin

Hyperbilirubinemia generally relates to an elevated bilirubin level in the blood and is usually an indication of a disease of the blood, liver, or biliary tract. Hemoperfusion using synthetic resins as sorbents has been one of the ways to reduce bilirubin. In this study, chitosan, a natural polysaccharide having structural characteristics similar to glycosaminoglycans and which is nontoxic and biocompatible, has been used for bilirubin binding. Several layers of poly-L-lysine have been coated covalently onto chitosan beads, using N2 plasma and carbodiimide treatments. Such surface-modified chitosan beads exhibited high binding affinities for bilirubin (1.13 +/- 0.18 mg/g beads) in aqueous phosphate buffer solutions at 4 degrees C in relation to activated charcoal (0.74 +/- 0.2 mg/g). The polylysine-coated resins have been reported to have an improved binding affinity for bilirubin over cholestyramine. It seems that the surface-immobilized polylysine has an increased bilirubin binding affinity and is highly stable. The binding capacity is proportional to the amount of polylysine bonded to the chitosan beads. The hemolytic potential of all modified beads is compatible with polystyrene control tubes. Studies were also performed against albumin as proof of specificity toward bilirubin binding. The albumin-coated beads have shown the highest blood compatibility and selectivity over the other modified beads. However, it appears that polylysine-modified chitosan may be an excellent sorbent system for hemoperfusion due to its high binding affinity, capacity, and blood compatibility. Further studies are needed to determine its behavior under clinical conditions.

Ex vivo and in vitro platelet adhesion on RFGD deposited polymers

Clinical applications of small-diameter synthetic vascular grafts are hindered by their highly thrombogenic surfaces. To develop vascular grafts that resist thrombotic occlusion, a radio frequency glow discharge (RFGD) process was employed to modify the surface of existing graft materials. Ultrathin coatings of RFGD polymers of ethylene (E), tetrafluoroethylene (TFE), and hexamethyldisiloxane (HMDS) were deposited on the lumen of Dacron grafts. Surfaces were characterized by electron spectroscopy for chemical analysis (ESCA). The effect of glow discharge treatments on platelet-graft interactions was evaluated in an ex vivo baboon shunt model. Following placement of an untreated or RFGD-treated graft in the shunt, deposition of 111Indium-labeled platelets was monitored for 60 min by gamma camera imaging. Untreated Dacron rapidly accumulated large numbers of platelets, reaching a plateau in 60 min. HMDS- and TFE-treated Dacron had significantly lower levels of platelet deposition compared to the untreated control. In contrast, the ethylene treatment of Dacron augmented platelet deposition, making it the most platelet-adherent surface studied. In vitro studies were also performed using untreated and RFGD-treated poly (ethylene terephthalate) (PET) coverslips. ESCA verified that the surface composition of the untreated and RFGD-treated coverslips were virtually identical to their untreated and treated Dacron graft counterparts. Samples were incubated in washed baboon platelet suspensions for 2 h at 37 degrees C. Platelet adhesion on the untreated PET was relatively high, and many of the platelets had a completely spread morphology. The HMDS and TFE treatment of PET reduced the number of adherent platelets and prevented platelet spreading on the surface. Platelet adhesion and spreading on the ethylene-treated surface was the highest among the four studied. There is a remarkable linear correlation of the ex vivo and in vitro platelet adhesion data.

Effect of plasma glow, glutaraldehyde and carbodiimide treatments on the enzymic degradation of poly (L-lactic acid) and poly (gamma-benzyl-L-glutamate) films

The hydrolytic and enzymic degradation of poly(L-lactic acid) (PLA) and poly(gamma-benzyl L-glutamate) (PBGA) films, together with a series of surface treatments, were studied, as a function of exposure time. The degradation of these polymers was monitored by weight loss, contact angle, pH changes and tensile strength studies. Glutaraldehyde treatment retained the maximum strength of PLA in buffer, followed by carbodiimide, compared with control films. On the other hand, plasma glow reversed the effect. The ability of alpha-chymotrypsin, carboxypeptidase, ficin, esterase, bromelain and leucine aminopeptidase to modulate the degradation of PLA and PBGA was also investigated. Addition of these enzymes to the polymer-buffer system reduced the tensile strength of these polymers variably. Among the six enzymes studied, leucine aminopeptidase showed the highest enzymic effect on the degradation of the glutaraldehyde-treated and bare PLA or bare PBGA films. However, glutaraldehyde-cross-linked PLA demonstrated maximum stability in buffers or in all other enzyme systems studied compared with bare PLA. It is conceivable that surface treatments on these polymers might have altered their physical and chemical configuration and the subsequent degradation properties. Surface modifications may provide new ways of controlling the biodegradation of polymers for a variety of biomedical applications.

Immobilization of enzymes on polypropylene bead surfaces by anhydrous ammonia gaseous plasma technique

Anhydrous ammonia gaseous plasma technique was used for the surface modification of polypropylene beads. Amino groups were added onto the surfaces of beads by exposing them to ammonia plasma. Through these amino groups covalent immobilization of glucose oxidase and peroxidase were carried out. The total amounts of immobilized glucose oxidase and immobilized peroxidase were found to be 52 and 43 micrograms/cm2, respectively. To assess the stability of enzyme-polypropylene linkage, beads with covalently immobilized glucose oxidase and peroxidase were washed with phosphate buffer. It was found that after the removal of the adsorbed enzymes, the concentration of covalently immobilized enzymes tended to reach a steady state. After additional washing with buffer for 5 to 6 h, 40-55% of the immobilized enzymes were found to be in the active form.

Inhibition of platelet adhesion to glow discharge modified surfaces

Plasma glow technique has created much interest in the field of surface modification of polymers due to its versatility of generating active polar groups on the surface without affecting the bulk properties. Here an attempt is made to inter-relate the surface properties and platelet adhesion on various polymeric substrates due to plasma treatments. Initially, a critical review of the process and development of thrombosis upon contact of an artificial surface with blood, has been provided, which has been extended with the need for surface modifications to improve their blood compatibility and the versatility of plasma treatments for such modifications have been emphasized. Phospholipids like phosphoryl choline, phosphatidyl choline and phosphoryl ethanolamine were attached to Angioflex surface by plasma glow. The role of such modified substrates to interact with platelets were investigated using Tyrode washed calf platelets. It seems, glow discharge modified phosphoryl choline bilayers dramatically inhibited the platelet-surface binding, which may be due to their biochemical resemblance with thromboresistant surfaces of human blood cells. Further, the behaviour of all phospholipids towards bloodpolymer interaction is not similar and may change depending on the nature of their functional groups, net charge of the phospholipid adsorbed surface and their interaction with platelets and its activation. It is possible to chemically immobilize lipid bilayers on standard polymers, using plasma glow, to improve their biological performance; by suitably selecting the phospholipid combinations.

Characterization of plasma polymerized polypropylene coatings

Propylene was polymerized at low pressure in a radio frequency plasma reactor. The plasma polymerized propylene (PPP) films were insoluble in toluene. Thermal analysis of PPP showed no phase transition took place up to 255 degrees C. From these results, it was concluded that PPP is a highly crosslinked polymer. To further characterize the deposited PPP, Fourier Transform infrared (FTIR) spectroscopy in the attenuated total reflection (ATR) mode was used. The deposited material was shown to be polymerized propylene as a spectrum of the material showed absorption bands characteristic of polypropylene.

Characterization of plasma polymerized silicone coatings useful as biomaterials

Plasma polymerization techniques were used to deposit a layer of filler-free silicone rubber on a variety of substrate materials. The thickness of the deposited film was 0.5-0.8 micron. As it is the surface of the biomaterial that comes in direct contact with the body fluids, the surface of the biomaterial is of paramount importance. In this study, the plasma polymerized biomaterials were characterized. Thus, the scanning electron microscope (SEM) showed the surfaces to be smooth. To study the surface layer of the deposited polymer, Fourier-transform infrared spectrometry in the attenuated total reflection (ATR) mode was used. The deposited material was indeed silicone polymer with adsorption bands at 1262, 1020, and 802 cm-1 for the Si-CH3 bending, Si-0-Si stretching, and Si-CH3 bending, respectively. To find the bonding nature of the polymer, electron spectrometry for chemical analysis (ESCA) was used. The silicone polymer was shown to be highly cross-linked. To find the molecular weight between cross-links, swelling studies were done. Thus the results of the study show that the plasma polymerization could produce a filler-free silicone layer on a variety of substrate materials.

Biomedical applications of plasma polymerization and plasma treatment of polymer surfaces

Thin polymer films obtained by plasma polymerization usually show good biocompatibility when compared to classical biomaterials such as Silastic. The thickness of these films (from several hundreds of A to several micrometers) make them suitable for the purpose of changing the surface properties of the substrate without altering its bulk properties. Both the above features together indicate the possible biomedical use of plasma polymerization as well as plasma treatment of polymer surfaces processes. The purpose of this review is to present the most significant efforts to develop such applications with the specification of particular fields where these efforts are directed.

Albuminated polymer surfaces for biomedical application

Amino groups were added on to the surfaces of Celgard-2400 membranes by exposing them to an ammonia plasma. The presence of amino groups on the surfaces was detected by an attenuated total reflection Fourier Transform infrared spectrometer and by the Auger electron spectrometer. Through these amino groups, albumin was attached to the membranes. In some experiments, the attached albumin was further stabilized by cross-linking with glutaraldehyde. The effect of washing the albuminated membranes with saline and with plasma was investigated. It was observed that after the initial wash-out of albumin, the concentration of attached albumin tends to level off. The amount of albumin retained on the membranes varied between 275 to 357 micrograms/cm2.

Evaluation of plasma polymerized hexamethylcyclotrisiloxane biomaterials towards adhesion of canine platelets and leucocytes

Silicone coated Celgard-2400 and Silastic membranes were prepared by plasma polymerization of hexamethylcyclotrisiloxane. The adhesion of canine platelets and leucocytes was tested by passing whole blood from the anaesthetized mongrel dog in an ex-vivo shunt system. The silicone control Celgard and silicone coated Silastic membranes had fewer platelets and fewer leucocytes compared to those on the control Silastic membranes. Furthermore, these blood cells underwent fewer morphological changes on the silicone coated Celgard and Silastic compared to those on the control Silastic. From these observations the silicone coated biomaterials were judged to be better than the Silastic as far as the adhesion of platelets and leucocytes are concerned.