Chemical Modification of Poly (Vinyl Chloride) - MedIND

Trends Biomater. Artif. Organs, Vol 18 (2), January 2005

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Chemical Modification of Poly (Vinyl Chloride) using Poly (Ethylene Glycol) to Improve Blood Compatibility Biji Balakrishnan and A. Jayakrishnan Polymer Chemistry Division, Biomedical Technology Wing Sree Chitra Tirunal Institute for Medical Sciences and Technology Satelmond Palace Campus, Trivandrum 695 012, India Email: [email protected]

Abstract: Poly(vinyl chloride) (PVC) was aminated by treating the resin with a concentrated aqueous solution of ethylenediamine. The aminated PVC was then reacted with hexamethylene diisocyanate to incorporate the isocyanate group onto the polymer backbone. The isocyanated PVC was further reacted with poly(ethylene glycol) (PEG) of molecular weight 600 Da. The modified polymer was characterized using infrared spectroscopy and thermal analysis. Infrared spectra showed the incorporation of PEG onto PVC. The thermal stability of the modified polymer was found to be lowered by the incorporation of PEG. Contact angle measurements on the surface of polymer films cast from a tetrahydrofuran solution of the polymer demonstrated that the modified polymer gave rise to a significantly hydrophilic surface compared to unmodified PVC. The solid/water interfacial free energy of the modified surface was 3.9 ergs/cm2 as opposed to 19.4 ergs/cm2 for bare PVC surface. Static platelet adhesion studies using platelet rich plasma showed significantly reduced platelet adhesion on the surface of the modified polymer compared to control PVC. The study showed that bulk modification of PVC using PEG using appropriate chemistry can give rise to a polymer that possesses the anti-fouling property of PEG and such bulk modifications are less cumbersome compared to surface modifications on the finished product to impart anti-fouling properties to the PVC surface. Keywords: Poly(vinyl chloride); poly(ethylene glycol); Polymer Modification; Surface Energy; Platelet Adhesion; Biocompatibility. Running Head: Poly(ethylene glycol) modified PVC.

Introduction: Poly(vinyl chloride) (PVC) finds extensive application in the medical field (1). Bags for the storage of blood and its components, tubings for extracorporeal circulation and endotracheal intubation and intravenous catheters are some of the medical devices wherein plasticized PVC is employed. PVC is not a blood-compatible polymer per se and

additives such as plasticizers added to the polymer during processing to impart flexible character to an otherwise rigid PVC also contribute to many adverse effects when used in contact with tissue or blood (2). Many attempts to improve the biocompatibility of PVC have been reported in the literature. This includes polymer surface modification with endpoint attachment of heparin (3 – 4), 230

Chemical Modification of Poly (Vinyl Chloride) using Poly (Ethylene Glycol) to Improve Blood Compatibility

70 was from Sriram Fibres Ltd., Kota, India. PEG of average molecular weight 600 Da was from Central Drug House Ltd., Mumbai, India. Hexamethylene diisocyanate (HMDI) was from Merck-Shuchardt, Germany. Ethylenediamine, ethanol, benzene, tetrahydrofuran (THF) and hexane were from S.D. Fine Chemicals Ltd., Mumbai, India. THF was refluxed over sodium in the presence of benzophenone and distilled prior to use. Hexane was dried over sodium and distilled. Methods Amination of PVC PVC was aminated by treating with a large excess of an 80% aqueous solution of ethylenediamine. Thus, 1 g resin was added to 10 ml of an 80% solution of the amine in distilled water and stirred magnetically in an oil bath maintained at 80°C for 1 h. After the reaction, the resin was filtered and washed with copious amounts of tap water followed by distilled water to remove ethylenediamine and dried in an air oven. Grafting of PEG onto aminated PVC Aminated PVC, 1 g was dissolved in 25 ml dry THF in a 100 ml round-bottomed flask, 2 ml of HMDI was added, stoppered and the mixture was stirred magnetically for 1 h at room temperature. The resultant product was precipitated in dry hexane and washed with the same solvent over a fluted filter paper many times to remove the excess HMDI. It was then redissolved in THF and treated with 2.0 g of PEG for 15 min under magnetic stirring at room temperature. After the reaction, the product was precipitated in ethanol and extensively washed with the same solvent to remove the unreacted PEG and dried in vacuum. Films of modified PVC were cast from a 4% solution in THF in glass petri dishes. Solvent evaporation was allowed to take place slowly at room temperature by

immobilization of albumin (5), amination of PVC followed by complexation with heparin (6 – 7), grafting hydrophilic polymers onto PVC surface (8) and plasma modification (9). In a recent study, it was shown that grafting poly(ethylene glycol) (PEG) onto PVC surface by the well known Williamson reaction, taking advantage of the labile nature of chlorine atoms on PVC can generate a protein and platelet repelling surface (10). Although surface modification is the key to altering the surface properties of the polymer without changing its physical and mechanical properties, the method has inherent disadvantages in that the modification has to be performed on the finished product. This is often undesirable and modification of the polymer prior to processing would be more advantageous if the desirable surface properties are retained to a significant extent in the finished product. This is especially the case when the intended devices are disposable after single use. Most of the PVC-based devices used for medical applications fall into this category. Introduction of sophisticated chemical structures on the backbone of the polymer chain prior to processing is resorted to in many cases to produce the final product with the required physical, chemical and biological properties (11 – 12). This work was undertaken in order to examine whether chemical modification of PVC resin using PEG can give rise to a more blood-compatible surface. This communication reports a procedure to chemically graft PEG onto PVC resin and examines the thermal behavior of the graft polymer and the surface properties of polymer films prepared by solution casting and its blood compatibility. Materials and Methods: Materials Medical grade PVC resin having a k value of 231

Biji Balakrishnan and A. Jayakrishnan

covering the dishes partially with a watch glass to obtain clear, transparent films slight yellow in colour. Residual solvent if any in the films was removed in vacuum in a desiccator. Films were stored in a desiccator until use. Contact angle measurements: Measurements were carried out on films using, captive air-in-water and octane-in-water methods using a goniometer (Rame’-Hart, NJ, USA). Films were equilibrated overnight in distilled water before measurements. Values reported are average of 8 to 10 measurements on different parts of the film. Surface energy parameters were calculated according to the method of Andrade et al (13). Infrared spectra: Infrared spectra of unmodified and modified PVC were recorded using thin films cast from a 4% THF solution of the polymers in a Fourier transform infrared spectrophotometer (Nicolet, Model Impact 410, Madison, WI, USA). Platelet adhesion studies: For platelet adhesion studies, films of 1.5 x 1.5 cm size were incubated for 1 h phosphate buffered saline (0.1 M, pH 7.4). Fresh human blood anticoagulated with ACD was centrifuged at 400 g for 5 min to obtain platelet rich plasma (PRP). Platelet count was adjusted to 2-2.5 x 108/mL using platelet poor plasma obtained by centrifugation of anticoagulated blood at 1500 g for 15 min. The films were laid flat in small glass petri dishes and were submerged with PRP and left at 37°C for 1 h in an incubator. After washing gently with buffer many times to remove non-adhering platelets, fixing was done with 2.5% buffered glutaraldehyde solution overnight in the refrigerator at 4°C. Specimens were then washed with distilled water, stained with Leishman stain and examined in an optical microscope (Leica, DMR, Germany). For

electron microscopy, films after glutaraldehyde fixation and washing with distilled water were frozen in distilled water, lyophilized, coated with gold and examined in a scanning electron microscope (Hitachi, S-2400, Japan). Results and Discussions: PEG has attracted considerable attention as a biomaterial in recent years because of its protein and cell repelling characteristics and there are numerous reports in the literature on the anti-fouling properties of PEG-rich surfaces (14 – 15). The amination of chlorine containing polymers using alkyl amines has been reported in the literature. Dragen et al (16 – 17) reported the formation of soluble and crosslinked polymers by reacting chloromethylated polystyrene with tris(2-hydroxyethyl) amine. Ferruti et al (6 – 7) aminated PVC using a concentrated aqueous solution of bis-(2-aminoethyl) amine. The aminated PVC was then coupled onto acrylamido end-capped poly(amido-amine) which was used to complex heparin to generate an antithrombogenic surface on PVC. Although reaction of alkyl halides with ammonia or primary amines can result in the formation of quaternary salts, in the presence of a large excess of amine, the reaction can yield primary and secondary amines. The polymeric species will be less reactive towards the amine as is the case of PVC. The use of a bifunctional amine such as ethylenediamine can yield a crosslinked product on prolonged reaction with the polymer. Indeed, such crosslinking was observed when PVC was reacted with ethylenediamine for prolonged periods. The products thus obtained were found to be partially or completely insoluble in good solvents for PVC. Reaction for a period of 1 h yielded the aminated resin which was found to be completely soluble in THF. 232


The aminated PVC was reacted with HMDI to introduce the highly reactive isocyanate groups onto the polymer. Prolonged reaction with HMDI was also found to yield a product which was insoluble in THF indicating crosslinking. The optimum period for the reaction was 1 h giving rise to the modified resin which was soluble in THF. Reaction of the isocyanate modified PVC with PEG resulted in grafting. The reaction was carried out for 15 min duration to obtain a product soluble in THF. Prolonged reaction times at this stage resulted in a crosslinked product insoluble in THF and other good solvents for PVC such as cyclohexanone. The reactions are outlined in scheme 1.

Fig 1: FTIR spectrum of unmodified PVC (a) and PEG-modified PVC (b) films cast from THF.

The air-in-water and octane-in-water contact angles on bare and modified PVC surface are given Table 1 along with the surface energy parameters calculated. The solid/water surface free energy of the modified PVC surface is nearly one fourth of the unmodified surface indicating considerable hydrophilicity on the modified surface. When PEG was grafted onto the surface of plasticized PVC by the Williamson reaction, we found that the solid/ water free energy of the modified surface was virtually zero (10). In surface modification, there was uniform coverage of the PEG on the polymer surface. It should be noted here that the measurements reported in the present study are on films of the modified polymer prepared by solvent casting. All the PEG incorporated into the polymer by grafting will not be exposed onto the surface on contact with water. Even then the decrease in contact angle and solid/water free energy of the surface is striking.

Figure 1 a and b show the infrared spectra of unmodified and PEG-grafted PVC films cast from a 4% solution in THF respectively. The peak at 3300 cm-1 is due to the hydroxyl groups of PEG and the peak at 1600 cm-1 is due to the carbonyl group both of which are absent in PVC. Thus, the infrared spectra unambiguously confirm the incorporation of PEG into the PVC backbone.

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Biji Balakrishnan and A. Jayakrishnan Table 1: Contact angles and surface energy parameters of unmodified and PEG-modified PVC films.

Figure 2 shows the optical photograph of platelet adhesion on PVC and PEG-modified PVC films. As can be seen from the photomicrographs, the adhesion is significantly less on the film prepared from the modified polymer compared to control. It is well known that PEG repels proteins and the absence of proteins having affinity for the platelets is responsible for the reduced cell adhesion that is seen PEG-rich surfaces. Although the modified polymer is not expected to be covered uniformly by PEG unlike surface grafting, the reduced platelet adhesion seen

on the modified polymer surface is a pointer to the enhanced blood compatibility of the surface. The results obtained in this study shows that although surface modification of PVC using PEG results in high grafting density on the surface, even bulk modification by appropriate chemistry will be able to change the surface properties of the polymer to a significant extent. Chemical modification of PVC using appropriate chemistry is a feasible proposition due to the labile nature of the chlorine atoms present on the polymer (21). It would also be interesting look at the possibility of radiation grafting of PEG-like molecules onto the resin and examine the surface properties of films prepared from such modified resin. Bulk modification of polymers such as PVC to prepare more blood and tissue-compatible devices will be more cost effective than surface modification of finished products especially when such devices are disposable after single use. Acknowledgement: The author thanks S. Lakshmi, and R. Latha, for technical assistance and the Director, SCTIMST for permission to publish this manuscript.

(a)

(b)

Fig 2: Optical photographs of platelet adhesion onto (a) unmodified PVC & (b) PEG-modified PVC films. 234


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