Poly(N-vinyl-2-pyrrolidone) hydrogel production by ultraviolet radiation: new methodologies to accelerate crosslinking

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1 Polymer 45 (2004) Poly(N-vinyl-2-pyrrolidone) hydrogel production by ultraviolet radiation: new methodologies to accelerate crosslinking G.J.M. Fechine*, J.A.G. Barros, L.H. Catalani Institute of Chemistry, University of São Paulo, CP 26077, , São Paulo, Brazil Received 16 February 2004; received in revised form 30 April 2004; accepted 3 May 2004 Abstract Poly(N-vinyl-2-pyrrolidone) hydrogels produced by high-energy radiation relies on water radiolysis as a primary process leading to crosslinks. Conversely, ultraviolet direct irradiation into PVP leads to crosslinking trough pyrrolidinone moiety photolysis. However, this process showed to be rather inefficient. This work describes the crosslinking of poly(n-vinyl-2-pyrrolidone) based on hydrogen peroxide photolysis, therefore mimicking water radiolysis, using UV-C (e.g. low pressure Hg lamp) or UV-A radiation sources. The process efficiency and the properties of the hydrogel formed are discussed and compared with other methods of hydrogel production. q 2004 Elsevier Ltd. All rights reserved. Keywords: Poly(N-vinyl-2-pyrrolidone); Hydrogel; Photocrosslinking 1. Introduction Polymers are today the largest group of materials used for biomedical purposes. They are used separately and/or in combination with other materials. The main areas of today s applications of hydrogel materials include wound dressing, drug delivery systems, transdermal systems, dental applications, injectable polymers, implants and ophthalmic applications. Since the pioneering work of Wichterle and Lim in 1960 on crosslinked HEMA hydrogels [1], and due to of their hydrophilic character and potential compatibility, hydrogels have been of great interest to biomaterial scientists in the last years [2 7]. The processes of hydrogel formation are essentially due to monomer polymerization or crosslinking of preformed polymers. A great variety of methods to promote crosslinking have indeed been used to prepare hydrogels including chemical and physical methods. Among these, High-energy radiation, in particular gamma rays and electron beam, can be used to polymerize unsaturated compounds. This means that water-soluble polymers can be converted into hydrogels using high-energy * Corresponding author. Tel.: þ ; fax: þ addresses: guilherminof@yahoo.com.br (G.J.M. Fechine), catalani@iq.usp.br (L.H. Catalani). radiation. Rosiak and coworkers [8 14] have presented a successful methodology of hydrogel dressing production based on high-energy radiation. These hydrogels are prepared through irradiation of poly(n-vinyl-2-pyrrolidone) (PVP) aqueous solutions. Hydroxyl radicals formed during water radiolysis react with PVP generating macroradicals. One of the resulting reactions from these radicals is recombination leading to crosslinks. The commercial diffusion of this hydrogels is inhibited by the necessity of high-energy radiation sources. Typically, these sources are very expensive and not easily available. Ultraviolet radiation may represent an alternative to highenergy radiation. Lopergolo et al. [15] presented a study about PVP crosslinking using UV-C radiation (l ¼254 nm) from arc mercury lamps. However, the necessity of the high doses precludes the use of this process, since photodegradation competes with crosslinking. In this work, we present new methodologies based on the use of hydrogen peroxide and photo-fenton reactions to accelerate the crosslinking process of the PVP aqueous solution through UV radiation. 2. Experimental The PVP used in this study, known as Luviskol w K-90 ðm w ¼ 1: Þ was supplied by BASF. Irradiations were carried out using a PCQ-X1 reactor from Ultra Violet /$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi: /j.polymer

2 4706 G.J.M. Fechine et al. / Polymer 45 (2004) Products (Cambridge, UK) with three low-pressure doughnut-type Hg lamps with maximum emission l max ¼254 nm (17.2 W/lamp). Alternatively, for photo-fenton experiments a similar set of phosphorous-coated lamps were used, but with maximum emission in 360 nm. The solution was placed in quartz tube of 1 cm diameter positioned in the center of the lamp set. This setting produces 5.85 MW/cm 2 and 2.3 MW/cm 2 of radiant flux at its center for 254 and 360 nm maximum emission lamps, respectively. The absorbed doses were calculated taking into account the sample surface area of 30 cm 2. The gel content, g(%), crosslinking density, r x ; the number-average molecular weight between crosslinks, M c ; swelling ratio, ðqþ; mesh size, j; and p 0 =q 0 (degradation and crosslinking ratio) were calculated as described elsewhere [15]. The cytotoxicity of the membranes was tested by an in vitro cell viability method according to Ciapetti et al. [16]. The test was carried out with dilution of the extracts of hydrogel membranes, in contact with mouse subconjuntive tissue cell culture from NCTC Clone 929 line (ATCC- CCL1). Phenol solution (0.02%) and poly(vinyl chloride) (PVC) extracts were used as positive and negative controls, respectively. Scheme 1. Production of hydroxyl radical from the photo-homolysis of peroxide. 3. Results and discussion 3.1. Irradiation of PVP aqueous solution containing hydrogen peroxide Fig. 1(A) and (B) show the dependence of gel content, g(%), with increasing radiation dose, when 20 mm H 2 O 2 or no peroxide is used. In both cases the gel content increases to a maximum, when it levels up. As a distinctive advantage over direct photolysis of PVP aqueous solution, the radiation doses necessary to form hydrogels when H 2 O 2 is used were typically one order of magnitude lower. Fig. 1. Effect of 254 nm radiation dose on the gel content of a PVP solution alone (A) or with 20 mm H 2 O 2. (B) [PVP] ¼ 80 g/l ( X ); 90 g/l ( B ); 100 g/l ( O ). Fig. 2. Sol/gel data treatment according to Charlesby-Pinner relation. [PVP] ¼ 90 g/l pure (A) and (B) with 20 mm H 2 O 2.

3 G.J.M. Fechine et al. / Polymer 45 (2004) Table 1 Properties of the hydrogel produced by UV radiation PVP concentration (g/l) Dose Gel content (%) Q (%) M c (10 3 g/mol) r x (10 26 mol/cm 3 ) j (nm) J 81.8 ^ ^ ^ ^ ^ þ 10 mm H 2 O J 80.9 ^ ^ ^ ^ ^ þ 20 mm H 2 O J 83.9 ^ ^ ^ ^ ^ a 20 kgy 78.8 ^ a Prepared with 8 and 30 g/l of agar and poly(ethylene glycol), respectively [17]. The peroxide addition accelerates the crosslinking kinetics through the hydroxyl radical (zoh) generation during its photo-homolysis (see Scheme 1). Hydroxyl radical reacts with polymer macromolecules giving rise to macroradicals. If macroradicals located on different polymer chains are favorably positioned, they may undergo recombination leading to crosslinking. Hydroxyl radicals have been shown to be the main species responsible for crosslinking reactions during highenergy irradiation of polymer in aqueous solutions, since water radiolysis is the primary process involved [11 14]. However, the energy doses required are 20 kgy or 400 J [17] (for a sample weighting of the 20 g) to obtain hydrogel with 80% of the gel content. In contrast, hydrogels with similar gel content produced from peroxide photo-homolysis need lower doses, about 200 J (see Fig. 1(B)), than those produced by high-energy. More importantly, UV radiation is safer, more portable and less expensive than high-energy radiation. Charlesby-Pinner treatment provides a mean to evaluate the degradation ability of the crosslinking method by calculating p 0 =q 0 ; the ratio of degradation vs. crosslinking density [9,14,18]. p This value is shown as the linear coefficient of a ðs þ ffi sþ (where s is the sol fraction) against the reciprocal of the applied dose. An updated value of p 0 =q 0 equal to zero was calculated for direct UV crosslinking of 8, 9 and 10% PVP aqueous solutions, using doses between 600 and 2400 J, showing that UV crosslinking occurs basically with no chain scission. Fig. 2(A) shows this analysis for the irradiation of 9% PVP aqueous solution. The linear regression gave a p 0 =q 0 value of around zero. However, this value goes to,0.17 when 20 mm H 2 O 2 is present, using doses between 100 and 600 J, as shows the Fig. 2(B). The p 0 =q 0 values for PVP aqueous solution (8, 9 and 10 mg/ ml) with 10 and 20 mm H 2 O 2 were between 0.10 and This strongly indicates that the hydroxyl radicals produced from the photo-homolysis of peroxide are responsible for degradation reactions in addition to crosslinking reactions. Contrarily, very little hydroxyl radical is expected to be formed during UV direct photolysis of PVP, where it is produced mostly by hydroperoxide decomposition, formed via autoxidative processes. To support this argument, the p 0 =q 0 values observed on the photolysis of PVP in presence of H 2 O 2 are similar to those observed using high energy methods, where water radiolysis producing hydroxyl radicals is the primary process involved. Olejniczak et al. [18] reported p 0 =q 0 values for g-irradiated PVP aqueous solution of 0.22 at normally aerated solution and zero from an argon purged solution. The gel dose, D g ; which is the dose necessary to produce the first insoluble fraction, was calculated from the same treatment as, 520 J for systems without peroxide. However, for systems containing peroxide the D g decreases to values lower than 100 J, as an explicit demonstration of the advantage in using this activator. Table 1 compares the hydrogel properties obtained from aqueous solution of PVP 8% with and without peroxide. For comparison, a literature example of hydrogel obtained from high-energy radiation was included [17]. In all cases, hydrogels of approximately 80% gel content were selected. The values of gel content, g(%), swelling ratio, Q; crosslinking density, r x ; the number-average molecular weight between crosslinks, M c ; mesh size, j; presented significant differences, demonstrating the possibility of control over these parameters. These data also suggest that hydrogels produced in the presence of H 2 O 2 show a more compact network than those produced without peroxide or from high-energy radiation. Mesh size is ultimately the parameter to be controlled when outlining hydrogel individual characteristics, especially when these hydrogels are proposed as drug Table 2 Number-average molecular weight between crosslinks ðm c Þ and pore size ðjþ of the hydrogels prepared by 254 nm irradiation in presence of hydrogen peroxide Dose (J) [PVP] (g/l) M c (10 3 g/mol) j (nm) ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ 0.46 [H 2 O 2 ] ¼ 20 mm.

4 4708 G.J.M. Fechine et al. / Polymer 45 (2004) Table 3 Gel content and number-average molecular weight between crosslinks ðm c Þ of the hydrogels prepared by photo-fenton reactions Dose (J) [PVP] (g/l) Gel content (%) M c (10 3 g/mol) ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ 3.37 [Fe 3þ ] ¼ 5 mm; [H 2 O 2 ] ¼ 50 mm. delivery and related systems. Table 2 compares j (nm) values of hydrogels prepared with 20 mm H 2 O 2 and different concentrations of PVP. Increasing values of j are obtained from increasing concentration of PVP at the same radiation doses. However, when the PVP concentration is maintained and radiation dose is increased, theses values decrease. The amount and character of the imbibed water in hydrogels determines absorption and diffusion of solutes through the hydrogel. In particular, the average pore size and the pore size distribution are important factors in solute permeation into and out of hydrogel. These factors are, in turn, most influenced by composition and crosslink density of the network. The control of this crosslink density can be easily achieved by choosing the correct conditions. However, for gels with reasonable mechanical properties, a minimum of 70% gel content is needed Photo-Fenton reactions The critical condition to produce hydrogels via hydrogen peroxide photo-homolysis is the radiation wavelength dependence, which necessarily takes place within the UV- C or deep-uv region (l,260 nm). Hence, feasible sources are restricted to low pressure mercury lamps, which are devices almost monochromatic emitting at 254 nm. Photo-Fenton reactions produce hydroxyl radicals through of the absorption the UVA visible radiation ( nm) using simple components like ferric aqueous solution in presence of hydrogen peroxide. (see Scheme 2). In this case, a range of alternative inexpensive sources can be used like medium pressure Hg vapor lamps or phosphorous coated low pressure Hg vapor lamps. Tables 3 and 4 show the correlation of some gel properties with increasing radiation dose and PVP concentration, when 5 mm Fe 3þ and 50 mm H 2 O 2 are used. Similar to the case described above, this correlation is also seen for gel content, although the necessary dose for an equivalent material is three times higher. The hydrogels produced for photo-fenton reactions have similar properties of those produced by direct photolysis of PVP or by photo-homolysis of H 2 O 2. Examples for comparison are hydrogels showing, 80% gel content prepared from the aqueous solution with 80 mg/ml of the PVP produced by (i) UV radiation at 254 nm without peroxide (ii) idem with 20 mm H 2 O 2, and (iii) UV radiation at 360 nm with 5 mm Fe 3þ and 50 mm H 2 O 2, where similar values of the M c ; r x and mesh size were obtained. Despite the overall similarities, Charlesby-Pinner treatment showed a p 0 =q 0 value of,0.50 for 8, 9 and 10% PVP aqueous solutions with doses between and J. The elevated decomposition is, however, expected if the same argument is used: hydroxyl radicals are equally produced, the amount of H 2 O 2 necessary is higher as well as the exposure time (dose) Cytotoxicity tests The conclusion from the cytotoxicity tests is that that hydrogels prepared by these methods do not impose any toxic effect to living organisms. These hydrogels were also submitted to dermal inflammation test in albino rabbits, where dressing made of hydrogel were maintained in direct contact with the rabbit skin for 72 h. All samples presented inflammation indexes within a satisfactory range, i.e. as a non-irritating material. 4. Conclusions Scheme 2. Production of hydroxyl radical from the photo-fenton reactions. The use of high-energy radiation to produce hydrogel shows some advantages like readiness and sterility of the product upon radiation exposure of the polymer solution. Alternatively, PVP hydrogel production can be achieved by direct irradiation using a low-pressure Hg lamp eliciting 254 nm light.

5 G.J.M. Fechine et al. / Polymer 45 (2004) Table 4 Crosslinking density ðr x Þ and pore size ðjþ of the hydrogels prepared by photo-fenton reactions Dose (J) [PVP] (g/l) r x (10 26 mol/cm 3 ) j (nm) ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ 0.33 [Fe 3þ ] ¼ 5 mm; [H 2 O 2 ] ¼ 50 mm. If UV radiation is used, the time of exposure is typically much higher, even using high-intensity lamps. However, the results in this work show that, by using low concentration of H 2 O 2, the exposure time to a low pressure Hg lamp can be shortened by one order of magnitude, maintaining the same level of crosslinking. This process mimics that of highenergy radiation, since the photolysis of H 2 O 2 generates hydroxyl radicals, the same species responsible for PVP crosslinking in the water radiolysis process. The resulting material formed showed to be well suited to all PVP hydrogel applications like wound dressings and drug delivery devices. These results have special significance where high-energy radiation is not available. Acknowledgements The authors would like to thank the Fundação de Amparo a Pesquisa do Estado de São Paulo, FAPESP, São Paulo, Brazil, for financial support. References [2] Peppas NA, Keys KB, Torres-Lugo M, Lowman AM. J Controlled Release 1999;62:81 7. [3] Lee KY, Mooney DJ. Chem Rev 2001;101(7): [4] Ichijo H, Hirasa O, Kishi R, Oowada M, Sahara K, Kokufuta E, Kohno S. Radiat Phys Chem 1995;46(2): [5] Jabbari E, Nozari S. Eur Polym J 2000;36: [6] Darwis D, Hilmy N, Hardiningsih L, Erlinda T. Radiat Phys Chem 1993;42(4-6): [7] Hoffman AS. Adv Drug Deliv Rev 2002;43:3 12. [8] Rosiak J, ska-rybus AR, Pekala W. Method of manufacturing hydrogel dressing US PATENT. 4,871, ; [9] Rosiak J, Olejniczak J, Charlesby A. Radiat Phys Chem 1988;32(5): [10] Rosiak JM, Ulanski P, Pajewski LA, Yoshii F, Makuuchi K. Radiat Phys Chem 1995;46(2): [11] Rosiak JM. J Controlled Release 1994;31:9 19. [12] Rosiak JM, Ulanski P. Radiat Phys Chem 1999;55: [13] Rosiak JM, Yoshii F. Nucl Instrum Methods Phys Res, B 1999;151: [14] Rosiak JM, Olejniczak J. Radiat Phys Chem 1993;42(4-6): [15] Lopergolo LC, Lugão AB, Catalani LH. Polymer 2003;44: [16] Ciapetti G, Granchi D, Verri E, Savarino L, Cavedagna D, Pizzoferrato A. Biomaterials 1996;17: [17] Miranda LF, Lugão AB, Machado LDB, Ramanathan LV. Radiat Phys Chem 1999;55: [18] Olejniczak J, Rosiak J, Charlesby A. Radiat Phys Chem 1991;38(1): [1] Wichterle O, Lim D. Nature 1960;185:117 8.

Poly(N-vinyl-2-pyrrolidone) hydrogels produced by Fenton reaction

Polymer 47 (2006) 8414e8419 www.elsevier.com/locate/polymer Poly(N-vinyl-2-pyrrolidone) hydrogels produced by Fenton reaction J.A.G. Barros, G.J.M. Fechine, M.R. Alcantara, L.H. Catalani Instituto de Química