Biomacromolecules 2004, 5, 197-201
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Preparation of Ultrafine Oxidized Cellulose Mats via Electrospinning Won Keun Son,§ Ji Ho Youk,† and Won Ho Park*,‡ Research Institute of Advanced Materials, Chungnam National University, Daejeon 305-764, South Korea, Department of Textile Engineering, College of Engineering, Inha University, Incheon, 402-751, South Korea, and Department of Textile Engineering, College of Engineering, Chungnam National University, Daejeon 305-764, South Korea Received August 21, 2003; Revised Manuscript Received September 25, 2003
Ultrafine oxidized cellulose (OC) mats were prepared by oxidation of ultrafine cellulose mats produced by electrospinning and subsequent deacetylation of cellulose acetate for potential applications in nonwoven adhesion barriers. When ultrafine cellulose mats were oxidized with a mixture of HNO3/H3PO4-NaNO2 (2/1/1.4 v/v/wt %), their ultrafine mat structure remained unchanged. The yield and carboxyl content of OC mats were 86.7% and 16.8%, respectively. OC showed lower crystallinity than cellulose because the oxidation of cellulose proceeded via disruption of hydrogen bonds between cellulose chains. The swelling behaviors of ultrafine OC mats were dependent on the type of swelling solution. In a physiological salt solution, their degree of swelling was ∼230%. Introduction Oxidized cellulose (OC) is completely bioresorbable and readily degrades under physiological conditions. It is commonly used in medical and related applications such as hemostats, bodying agents in cosmetic and pharmaceutical preparations, fibrin formation-accelerating agents, and adhesion barriers.1-3 OC adhesion barriers in a fabric, liquid gellike solution, or membrane form have been used to reduce the formation of postsurgical adhesions especially after pelvic or abdominal surgery. These barriers also assist the healing process and are readily absorbed within a short period of time. For example, Interceed TC7 (Johnson and Johnson Medical Inc.), a fabric adhesion barrier made from oxidized regenerated cellulose, becomes a gel to form a protective coating within 8 h and is completely absorbed within 28 days.4-6 On the other hand, in electrospinning, polymer solutions or melts are ejected from a millimeter size capillary and deposited as nonwoven fibrous mats on a template serving as the ground for the electric charges by applying a strong electrostatic force.7-10 These polymer mats exhibit high surface area-to-volume ratio, which is essential for applications such as separation membranes, scaffolds for tissue engineering, wound dressing materials, artificial blood vessels, and sensors.11-20 It is expected that ultrafine OC mats produced by electrospinning would be more flexible and easier to handle than commercial adhesion barriers and extend their applications in immobilizing matrixes for various drugs, enzymes, and proteins.2,21,22 However, it is impossible * Corresponding author. Tel: +82-42-821-6613. Fax: +82-42-823-3736. E-mail address:
[email protected]. § Research Institute of Advanced Materials, Chungnam National University. † Inha University. ‡ College of Engineering, Chungnam National University.
to directly prepare ultrafine OC mats via electrospinning of OC because of its poor solubility to organic solvents. Therefore, as an alternative method, they can be prepared by oxidizing ultrafine cellulose mats. In our previous study,23 ultrafine cellulose mats were successfully prepared via electrospinning and subsequent deacetylation of cellulose acetate (CA). These cellulose mats can be oxidized with various oxidants such as gaseous oxygen, hydrogen peroxide, peracetic acid, nitrogen oxide (dinitrogen tetraoxide), and hypochlorous acid.24 Recently, Kumar and Yang21 prepared OC having various carboxyl contents in high yield by treating cellulose with a mixture of HNO3/H3PO4-NaNO2 at room temperature. In this study, ultrafine OC mats were prepared by oxidizing ultrafine cellulose mats produced by electrospinning and subsequent deacetylation of CA with a mixture of HNO3/ H3PO4-NaNO2, and their physical properties were investigated for potential applications in nonwoven adhesion barriers. Experimental Section Materials. CA (39.8%, acetyl content, MW ) 30 000) was purchased from Aldrich Co. and used without further purification. Acetone, KOH, NaNO2, H3PO4 (85%, w/v), HNO3 (69.7%, w/v), and H2SO4 (95%, w/v) were purchased from Duksan Chemical Co. HCl and NaOH aqueous solutions were 0.1 N standard solutions. Electrospinning. Figure 1 shows a schematic diagram of the electrospinning setup used in this study.10 It consisted of a syringe and needle (i.d. ) 0.84 mm), a ground electrode (d ) 21.5 cm, stainless steel sheet on a drum the rotation speed of which can be varied), and a high-voltage supply (Chungpa EMT, CPS-40K03). The needle was connected to the high-voltage supply. The distance between the needle
10.1021/bm034312g CCC: $27.50 © 2004 American Chemical Society Published on Web 11/11/2003
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the hydroxylamine hydrochloride solution was then transferred into the flask containing ultrafine OC mats by tilting the assembly. The reaction mixture was then heated at 50 °C for 2 h. After cooling to room temperature, a 25 mL aliquot of the reaction supernatant was removed and titrated with a 0.1 N HCl solution to pH 3.2. A blank titration using 25 mL of the hydroxylamine hydrochloride solution was conducted with the same method. The carbonyl contents in OCs were calculated according to the following equation: carbonyl groups (% w/w) )
Figure 1. Schematic diagram of electrospinning setup.
tip and the ground electrode was 8 cm, and the mass flow rate was 10 mL/h. A positive voltage was set at 12 kV. The electrospinning of CA mats was carried out with a 17 wt % CA solution in acetone/water (85/15 v/v) at room temperature. Deacetylation of Ultrafine CA Mats. Electrospun CA mats (0.25 g) were swollen in 25 mL of an acetone/water mixture (1/1 v/v) at room temperature for 24 h and then deacetylated with 12.5 mL of 0.5 N KOH in ethanol for 1 h. Oxidation of Ultrafine Cellulose Mats. Ultrafine cellulose mats (5.0 g) were completely soaked in 70 mL of HNO3/H3PO4 (2/1 v/v) or HNO3/H2SO4 (2/1 v/v) solution, and then 1 g of NaNO2 was added [HNO3/H3PO4-NaNO2 (2/1/1.43 v/v/wt %)]. The reaction mixture was allowed to react at room temperature for 48 h. The reaction was terminated by slowly adding an excess of water. The diluted reaction mixture was filtered, washed with water until the filtrate showed pH of about 4, washed with acetone, and then air-dried at room temperature. Determination of Carboxyl Contents. The determination of carboxyl and carbonyl contents in OCs was performed according to the methods employed by Kumar and Yang;21 hence, only main points were described here. About 0.5 g of ultrafine OC mats was accurately weighed and soaked in 50 mL of a 2% (w/v) calcium acetate solution for 30 min. The mixture was titrated with a 0.1 N NaOH solution using phenolphthalein as an indicator. The carboxyl contents in OCs were calculated as follows: carboxyl groups (% w/w) )
NVMWCOOH weight of sample (mg)
× 100
where N is the normality of NaOH, and V is the volume (milliliters) of NaOH consumed in titration after correcting for the blank. Determination of Carbonyl Contents. Accurately weighed ∼0.5 g of ultrafine OC mats and 50 mL of hydroxylamine hydrochloride were separately placed in 250 mL round flasks. One of the flasks had a sidearm with a stopcock for evacuation. The flasks were connected by a manifold and evacuated with a water pump. The stopcock was closed, and
MWCO(B - S) 10(weight of sample)
× 100
where B and S are volumes (milliliters) of a 0.1 N HCl solution consumed for titrations of blank and sample solutions, respectively. Swelling Behavior of Ultrafine OC Mats. Ultrafine OC mats were cut into 2 cm × 2 cm squares and dried in a vacuum oven at 80 °C for 24 h. The mats were accurately weighted and then immersed into deionized water, a 0.1 N HCl solution, a 0.1 N NaOH solution, or NaCl solutions at 25 °C. The swollen ultrafine OC mats were withdrawn at intervals from the solution and weighed after removal of excess surface water. The degree of swelling (DS) was determined as follows: degree of swelling (%) )
W t - W0 × 100 W0
where Wt is the weight of swollen ultrafine OC mats at time t and W0 is the weight of dried ultrafine OC mats. Measurement and Characterization. The morphologies of ultrafine mats were observed on a scanning electron microscope (SEM, Hidachi S-2350) after gold coating. Fourier transform infrared (FT-IR) spectra of celluloses were obtained with a Nicolet Magna-IR 560 spectrophotometer. The crystalline structures and thermal stabilities of celluloses were analyzed on a wide-angle X-ray diffractometer (model Rigaku D/max-IIB, Regaku Co.) and on a thermogravimetric analyzer (Perkin-Elmer TGA-7) at a heating rate of 10 °C/ min under nitrogen atmosphere, respectively. The images of swollen OC mats were obtained by using an image analyzer (KH-2200 MD3, HI-SCOBE Co. Japan) with a magnification of 1000×. Results and Discussion Ultrafine CA mats were electrospun from a 17 wt % CA solution in acetone/water (85/15 v/v) and deacetylated with KOH in ethanol after swelling in 25 mL of an acetone/water mixture (v/v 1/1) for 24 h.23 The resulting cellulose mats were oxidized by using a mixture of HNO3/H3PO4-NaNO2 or HNO3/H2SO4-NaNO2 as an oxidant [HNO3/H3PO4NaNO2 (2/1/1.43 v/v/wt %)]. The reactions are schematically represented in Figure 2. The morphologies of ultrafine mats were observed on SEM. Figure 3a,b shows SEM images of an electrospun CA mat and the resulting cellulose mat after deacetylation, respectively. The morphology of the CA mat was slightly changed after deacetylation, but its nonwoven mat structure was maintained. Figure 3c,d shows SEM
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Figure 2. Synthetic scheme for ultafine OC fiber mat.
Figure 4. FT-IR spectra of ultafine CA, cellulose, and OC fiber mats.
Figure 3. SEM images of (a) electrospun CA, (b) deacetylated CA, (c) OC prepared with a mixture of HNO3/H3PO4-NaNO2, and (d) OC prepared with a mixture of HNO3/H2SO4-NaNO2.
images of ultrafine OC mats obtained after oxidation of ultrafine cellulose mats with HNO3/H3PO4-NaNO2 and HNO3/H2SO4-NaNO2, respectively. When oxidized with the former, the ultrafine mat structure remained unchanged, and the average diameter of ultrafine fibers was slightly changed from 3.94 to 3.67 µm. In the case of oxidation with the latter, small fibers and aggregated particles were observed because of the severe degradation of cellulose mats. The yields of OC obtained from the former and the latter were 86.7% and 37.5%, respectively. H2SO4, a strong acid, rapidly hydrolyzed cellulose and produced OC in low yield. The chemical reactions were traced by using FT-IR spectroscopy (Figure 4). The characteristic adsorption peaks attributed to the vibrations of acetate group at 1745 (υCdO), 1375 (υC-CH3), and 1235 cm-1 (υC-O-C) disappeared after deacetylation [Figure 4b]. An adsorption peak at 1745 cm-1 (υCdO) was detected again after oxidation [Figure 4c], and an absorption peak at 3500 cm-1 (υO-H) was observed for both cellulose and OC. Usually, OC contains different amounts of carboxyl, aldehyde, and ketone groups, depending on the nature of
oxidants and reaction conditions used. The carboxyl contents of OCs obtained with mixtures of HNO3/H3PO4-NaNO2 and HNO3/H2SO4-NaNO2 were 16.8% and 19.2%, respectively. OC with 16-24% carboxyl content is commonly available in a powder or a knitted fabric form for hemostats. Although the carboxyl content of ultrafine OC mats prepared with a HNO3/H3PO4-NaNO2 mixture was a little bit lower than that reported by Kumar and Yang,21 they had high enough carboxyl content to exhibit bioresorbability. The amount of ketone groups present in OC was determined by subtracting the aldehyde content from the carbonyl content. The carbonyl contents in OCs prepared with mixtures of HNO3/H3PO4-NaNO2 and HNO3/H2SO4NaNO2 were 4.2% and 4.6%, respectively, whereas their carboxyl contents after treatment with sodium chlorite (reagent for the oxidation of aldehyde groups to carboxylic acids) were 16.9% and 19.2%, respectively. This result indicates that they contained no aldehyde groups, and the carbonyl contents present were wholly due to ketone groups. The carbonyl content and yield of ultrafine OC mats were dependent on the amount of NaNO2 added. As shown in Figure 5, the yield of OC was decreased with increasing the amount of NaNO2 added, whereas the carboxyl content was increased. They were leveled off at a NaNO2 concentration of 1.5% [NaNO2/(HNO3/H3PO4), w/v]. It was found that
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Figure 5. Changes in carboxyl content and yield of ultafine OC fiber mats with the amount of NaNO2 added for the oxidation of cellulose.
Figure 6. X-ray diffraction patterns of ultrafine CA, cellulose, and OC fiber mats.
1.43% of NaNO2 used in this study was in the optimum concentration range for the oxidation of ultrafine cellulose mats. Figure 6 shows X-ray diffraction patterns of ultrafine CA, cellulose, and OC mats. After deacetylation, a peak (2θ ) 5°-15°) attributed to the crystal structure of CA disappeared, and the intensity of a peak (2θ ) 22°) ascribed to the crystal structure of cellulose was increased. The X-ray diffraction pattern of OC was very similar to that of cellulose, but its peak intensity was greatly weakened, indicating that the oxidation of cellulose proceeded by disrupting hydrogen bonds between cellulose chains. Figure 7 shows thermogravimetric analysis (TGA) curves of ultrafine CA, cellulose, and OC mats. The initial degradation temperatures of CA, cellulose, and OC were 252, 256, and 168 °C, respectively. As expected, carboxyl groups in OC catalyzed the thermal degradation, and that greatly lowered its initial degradation temperature.25 Kumar and Yang21 reported that degradation
Son et al.
Figure 7. TGA curves of CA, cellulose, and ultrafine OC fiber mats.
Figure 8. Changes in degree of swelling of ultrafine OC fiber mats with time in various aqueous solutions at 25 °C.
temperature of OC was decreased with increasing the content of carboxyl groups in OC. The swelling behaviors of ultrafine OC mats were investigated with deionized water, a 0.1 N HCl solution, a 0.1 N NaOH solution, or NaCl solutions at 25 °C. Figure 8 shows changes in DS of ultrafine OC mats in various solutions with time. In the case of a 0.1 N NaOH solution, they were completely swollen within 10 min. However, in cases of deionized water, a 0.1 N HCl solution, and a 0.5 M NaCl solution, they were rapidly swelled within 30 min and gradually reached an equilibrium state. These swelling behaviors were dependent on the rate and degree of dissociation of carboxyl groups according to the type of solution. The dissociation of carboxyl groups occurred rapidly and completely in a 0.1 N NaOH solution, whereas it was suppressed in a 0.1 N HCl solution. Figure 9 shows photographs of ultrafine OC mats before and after swelling in a 0.5 M NaCl aqueous solution for 24 h. The ultrafine
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Conclusions Ultrafine OC mats were prepared via oxidization of ultrafine cellulose mats produced by electrospinning and subsequent deacetylation of CA. When ultrafine cellulose mats were oxidized with a mixture of HNO3/H3PO4-NaNO2 (2/1/1.4 v/v/wt %), the ultrafine mat structure was maintained, but when they were oxidized with a mixture of HNO3/ H2SO4-NaNO2 (2/1/1.4 v/v/wt %), small fibers and aggregated particles were obtained because of the severe degradation. The yield and carboxyl content of ultrafine OC mats obtained from a mixture of HNO3/H3PO4-NaNO2 were 86.7% and 16.8%, respectively. They did not contain aldehyde groups. The yield of OC was decreased with increasing amount of NaNO2 added, whereas the carboxyl content was increased. They were leveled off at a NaNO2 concentration of 1.5%. OC showed lower crystallinity than cellulose, indicating that oxidation of cellulose proceeded via disruption of hydrogen bonds between cellulose chains. Carboxyl groups in OC catalyzed the thermal degradation, and that greatly lowered its initial degradation temperature. The swelling behaviors of ultrafine OC mats were dependent on the rate and degree of dissociation of their carboxyl groups according to the type of solution. In a physiological salt solution, DS of ultrafine OC mat was ∼230%. Figure 9. Images of (a) ultrafine OC fiber mat and (b) swollen ultrafine OC fiber mat in a 0.5 M NaCl aqueous solution for 24 h.
Figure 10. The effect of NaCl concentration on the degree of swelling of ultrafine OC fiber mats.
fibrous structure could not be observed after swelling (DS ) ∼270%) as shown in Figure 9b, whereas individual ultrafine fibers were separately observed before swelling. Figure 10 shows the effect of NaCl concentration on the swelling of ultrafine OC mats for 24 h without pH adjustment. DS was increased with increasing the NaCl concentration up to 0.4 M. DS of ultrafine OC mats was ∼230% in the physiological salt solution (0.225 M NaCl solution).
Acknowledgment. This work was supported by the Korea Research Foundation (Grant KRF-2001-005-E00037). References and Notes (1) Banker, G. S.; Kumar, V. U.S. Patent 5,405,953, 1995. (2) Wiseman, D. M.; Saferstein, L.; Wolf, S. U.S. Patent 6,500,777 B1, 2002. (3) Galgut, P. N. Biomaterials 1990, 11, 561-564. (4) Franklin, R. R. Obstet. Gynecol. 1995, 86, 335-340. (5) Reid, R. L.; Tulandi, T.; Hahn, P. M.; Yuzpe, A. A.; Spence, J. E. H.; Wiseman, D. M. Fertil. Steril. 1997, 67, 23-29. (6) Johns, D. B.; Keyport, G. M.; Hoehler, F.; diZerega, G. S. Fertil. Steril. 2001, 76, 595-604. (7) Doshi, J.; Reneker, D. H. J. Electrost. 1995, 35, 151-160. (8) Reneker, D. H.; Chun, I. S. Nanotechnology 1996, 7, 216-223. (9) Gibson, P.; Schreuder-Gibson, H.; Rivin, D. Colloids Surf., A 2001, 187-188, 469-481. (10) Reneker, D. H.; Yarin, A. L.; Fong, H.; Koombhongse, S. J. Appl. Phys. 2000, 87, 4531-4547. (11) Kim, J. S.; Reneker, D. H. Polym. Eng. Sci. 1999, 39, 849-854. (12) Norris, I. D.; Shaker, M. M.; Ko, F. K.; MacDiarmid, A. G. Synth. Met. 2000, 114, 109-114. (13) Kim, J. S.; Lee, D. S. Polym. J. 2000, 32, 616-618. (14) Shin, Y. M.; Hohman, M. M.; Brenner, M. P.; Rutledge, G. C. Polymer 2001, 42, 9955-9967. (15) Deitzel, J. M.; Kosik, W.; McKnight, S. H.; Beck Tan, N. C.; DeSimone, J. M.; Crette, S. Polymer 2002, 43, 1025-1029. (16) Matthews, J. A.; Wnek, G. E.; Simpson, D. G.; Bowlin, G. L. Biomacromolecules 2002, 3, 232-238. (17) Luu, Y. K.; Kim, K.; Hsiao, B. S.; Chu, B.; Hadjiargyrou, M. J. Controlled Release 2003, 89, 341-353. (18) Yoshimoto, H.; Shin, Y. M.; Terai, H.; Vacanti, J. P. Biomaterials 2003, 24, 2077-2082. (19) Wang, X.; Drew, C.; Lee, S. H.; Senecal, K. J.; Kumar, J.; Samuelson, L. A. Nano Lett. 2002, 2, 1373-1375. (20) Wnek, G. E.; Carr, M. E.; Simpson, D. G.; Bowlin, G. L. Nano Lett. 2003, 3, 213-216. (21) Kumar, V.; Yang, T. Carbohydr. Polym. 2002, 48, 403-412. (22) Zhu, L.; Kumar, V.; Banker, G. S. Int. J. Pharm. 2001, 223, 35-47. (23) Son, W. K.; Youk, J. H.; Lee, T. S.; Park, W. H. J. Polym. Sci., Polym. Phys. Ed., in press. (24) Banker, G. S.; Kumar, V. U.S. Patent 5,780,618, 1998. (25) Varma, A. J.; Chavan, V. B. Polym. Degrad. Stab. 1995, 49, 245-250.
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