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Structural and surface properties of polyurethane membranes of different porosities
Polymer Testing 14 (1995) 115-120 0 1995 Elsevier Science Limited Printed in Malta. All rights reserved 0142-9418/95/$930 ELSEVIER
Structural and Surface Properties of Polyurethane Membranes of Different Porosities Mehlika Gazi Universitesi, Fen-Edebiyat
Pulat & Cemil Senvar Fakiiltesi Kimya Biiliimii, Teknikokullar Ankara, Turkey
06500,
(Received 7 April 1994; accepted 17 May 1994)
ABSTRACT The structural and surface properties of polyurethane (PU) membranes prepared by a classical solvent-casting method were tested. A number of casting solutions of Pellethane@, in dimethylformamide, dioxan, tetrahydrofuran and their mixtures, were used to obtain a series of PU membranes of different porosities. Structural properties of the membranes were investigated by scanning electron microscopy and surface free energies were calculated from
contact-angle measurements. By changing
the composition and type of casting solution, different physical, chemical and surface characteristics were obtained.
INTRODUCTION Polymeric materials are used in a number of clinically important bloodcontacting implants and devices. Biocompatibility of polymeric materials implanted in the human body is the ultimate goal in the application of artificial organs. It is known that the biocompatibility of polymeric materials is related to the structure and surface properties.lW3 Segmented polyurethanes (PU) are widely used in both commercial and experimental blood-contacting applications, such as catheters, heart assist pumps and chambers for artificial hearts, because of their physiological acceptability, relatively good blood tolerability, excellent stability over long implant periods and excellent physical and mechanical properties.4-7 These properties are related to the surface chemistry and physics of the PU implants and vary as a result of variation in the composition of the prepolymer and also the casting procedure. 115
116
Mehlika
Pulac, Cemil Senaar
In this study the structural and surface properties of PU membranes prepared by a classical solvent-casting method were determined. Porosities of the membranes were investigated by using the scanning electron microscope (SEM). Surface free energies (SFE) were calculated from the measurements of contact angles. In addition, equilibrium water contents (EWC) and water permeabilities (WP) were examined and the results are discussed. The interactions of these membranes with fibroblastic and blood cells has been presented in our previous papers.8,9 EXPERIMENTAL Preparation of PU membranes
PU membranes used in this study were prepared by the solvent-casting procedure performed with polyether urethane (Pellethane@ 2363-80A, Upjohn, The Netherland) as given in the literature.* In order to obtain a series of PU membranes which have different structural properties, a number of casting solutions of Pellethane@ were prepared by dissolving it in different solvent systems. These systems included dimethylformamide (DMF) (Merck, Germany), dioxan (BDH, UK), tetrahydrofuran (THF) (Merck) and their mixtures; 8% (w/v) solutions of Pellethane@ were cast in a casting apparatus and then the precipitation process was applied. Average thicknesses of the membranes were 50 ( f 5) pm; 20% glycerol-80% water solution was used as the precipitation medium. They were extensively washed with distilled water and dried at 40°C under vacuum for about a week. Determination
of EWC, WP and contact angles of PU membranes
Swelling experiments at 37°C were used to determine the void volume (VV) and the EWC of PU membranes.* EWC (%) was defined as:” [(wet weight-dry
weight)/dry weight] x 100
Water permeabilities through .the membrane were measured under controlled conditions at 37 )0*5”C and 60% relative humidity. The water permeabilities were calculated from weight losses of beakers filled with water and sealed with the tested membranes.” Contact-angle measurements based on the captive bubble method12 were realized by placing both air and octane bubbles on the membrane surface submerged in a container filled with bi-distilled water. The contact
Structural and surface properties of polyurethane membranes
117
angles were calculated from the photographs of the bubblesI as given in the literature.8 The porosities of the membranes were investigated from the SEM photographs. Membrane samples were prepared by coating with gold at 3OOA thickness. The photographs were used to determine the average pore densities and the average pore radius, as described in the literature.14 Void volumes of the membranes were found using the same reference.
RESULTS AND DISCUSSION Surface free energies of the membranes were calculated from the harmonic mean equation1 5 using contact-angle values. Surface free energies (y) and air contact angles (O,,,) of the PU membranes, prepared in this study by changing the composition of the solvent system, are given in Table 1. It is observed from Table 1 that Oairdecreases as SFE increases. The relationship between EWC and SFE is illustrated in Fig. 1. As can be seen from the figure, EWC is directly proportional to SFE. TABLE 1 Oairand y Values of PU Membranes Sample no.
Composition of solvent system
eair (")*
7 (erslcm2)
PU-1 PU-2 PU-3 PU-4 PU-5 PU-6
THF 10% DMF-90% THF 50% THF-50% dioxan 10% DMF-45% dioxan-45% THF 10% DMF-90% dioxan dioxan
76.11 78.70 59.20 64.85 73.94 6684
32.54 30.75 43.95 40.01 33.86 39.61
PU-3 l’
PU-4 . /
01 27
0’ 30
, 33
I
36
I
39
I
42
I 45
SFE (erg/cm*) Fig. 1. The variation of EWC vs SFE.
118
Mehlika Pulat, Cemil Senvar
The VV is that fraction of the membrane volume which is not occupied by the polymer substrate and can be calculated from the difference between the wet and dry weights of the membrane. The VV can be connected to the SFE and wettability. In Table 2, it is seen that the VV is directly proportional to the EWC. The porosity values of the membranes which are given in Table 3 were directly determined from SEM photographs as given in the literature.* SEM photographs are presented in Fig. 2. Water permeability (WP) of the PU membranes increases as the porosity increases (Table 3) and can be compared to the water permeabilities of some commercial membranes” used in wound healing (Table 4). It is clearly seen from Table 3 that DMF affects the porosites inversely. The adding of DMF to the solvent system increases the porosities in the THF-PU system, which has less pores, and decreases it in the dioxan-PU system, which has more pores. TABLE 2 Variation of VV Values with EWC Sample no.
Void volume x I O3(g/ml )
EWCs
PU-3 PU-4 PU-6 PU-5 PU-1 PU-2
11.5 9.7 6.9 3.4 1.3 0.5
23.71 16.58 1000 7QO 2.60 0,75
TABLE 3 Surface Properties and WP of the Membranes Sample no.
WP( g/m2 24 h)
PU-1” PU-2” PU-3b PU-4 PU-5” PU-6
1205 1562 1677 1417 1641 1682
“In Ref. 16. bin Ref. 9.
Average pore density (no. of pores/cm2) 2.2 x 6.7 x 5.6 x 3.2 x 4.1 x 4.7 x
105 IO* lo6 lo5 106 106
Average pore radius (pm) 1.0 2.5 1.3 1.4 3.9 4.5
Structural and surface properties of polyurethane membranes
Fig. 2. SEM photographs
of PU membranes: (a) PU-1; (b) PU-2; (c) PU-3; (d) PU-4; (4 PI-J-5; (f) PU-6.
TABLE 4
WP of Some Commercial Membranes’ ’ Membrane (commercial name)
Omiderm Biobrane Op site
119
WP (g/in’ 24 h)
5ooo 1400 500
120
Mehlika
Pulat, Cemil Senvar
CONCLUSION As a result
1. Piskin, E. & Chang, T. M. S., The Past, Present and Future of Artijicial Organs, 1988, 32. 2. Williams, D. F. (eds), Fundamental Aspects ofBiocompatibility, Vols I-II, CRC Press, Boca Raton, FL, 1981. 3. Wachem, van P. B. et al., Interaction of cultured human endothelial cells with polymeric surfaces of different wettabilities. Biomaterials, 6 (1985) 403. 4. Boretos, J. W. & Pierce, W. S., Segmented polyurethane: a polyether polymer. J. Biomed. Mater. Res., 2 (1968) 121. 5. Farrar, D. J. et al., In uiuo evaluations of a new thromboresistant polyurethane for artificial heart blood pumps. J. Thorac. Cardiovasc. Surg., 95 (1988) 191. 6. How, T. V. & Annis, D., Viscoelastic behavior of polyurethane vascular prostheses. J. Biomed. Mater. Res., 21 (1987) 1093. 7. Ratner, B. D., Gladhill, K. W. & Horbett, T. A. Analysis of in oitro enzymatic and oxidative degradation of polyurethanes. J. Biomed. Mater. Res., 22 (1988) 509. 8. Kiremitci, M., Pulat, M., Senvar, C., Serbetci, A. 1. & Piskin, E., Structural and cellular characterization of solvent casted polyurethane membranes. Clin. Mater. 6 (1990) 227. 9. Kiremitci, M. Pesmen, A., Pulat, M. & Giirhan, I., Relationship of surface characteristics to cellular attachment in PU and PHEMA. J. Biomater. Appl., 7(3) (1993) 250.
10. Hirabayashi, Y., Satoh, M., Ogawa, H. & Ohtsuka, Y., Amphiphilic graft copolymer latex membranes. VI. Poly(viny1 alcohol)-acrylonitrile-N-hydroxyethyl acrylamide graft copolymer latex membranes. J. Appl. Polym. Sci., 28 (1983) 109. 11. Behar, D., et al., Omiderm, a new synthetic wound covering: physical properties and drug permeability studies. J. Biomed. Mater. Res., 20 (1986) 731. 12. Bikermann, J. J., Physical Surfaces, Academic Press, New York, 1970. 13. Guidelines for Physicochemical Characterization of Biomaterials. US Department of Health and Human Services, 1980. 14. Kesting, R. E., Synthetic Polymeric Membranes, McGraw-Hill, New York, 1971. 15. Andrade, J. D., King, R. & Gregonis, D. E., Contact angles at the solid-water interface, J. Polym. Sci. Symp. C, Sci., 72 (1979) 488. 16. Abbasoglu, U. & Pulat, M., Investigation of water and antimicrobial agents permeation of polyurethane membranes in relation to their surface properties. FABAD, 19(l) (1994) 1.
Structural and Surface Properties of Polyurethane Membranes of Different Porosities Mehlika Gazi Universitesi, Fen-Edebiyat
Pulat & Cemil Senvar Fakiiltesi Kimya Biiliimii, Teknikokullar Ankara, Turkey
06500,
(Received 7 April 1994; accepted 17 May 1994)
ABSTRACT The structural and surface properties of polyurethane (PU) membranes prepared by a classical solvent-casting method were tested. A number of casting solutions of Pellethane@, in dimethylformamide, dioxan, tetrahydrofuran and their mixtures, were used to obtain a series of PU membranes of different porosities. Structural properties of the membranes were investigated by scanning electron microscopy and surface free energies were calculated from
contact-angle measurements. By changing
the composition and type of casting solution, different physical, chemical and surface characteristics were obtained.
INTRODUCTION Polymeric materials are used in a number of clinically important bloodcontacting implants and devices. Biocompatibility of polymeric materials implanted in the human body is the ultimate goal in the application of artificial organs. It is known that the biocompatibility of polymeric materials is related to the structure and surface properties.lW3 Segmented polyurethanes (PU) are widely used in both commercial and experimental blood-contacting applications, such as catheters, heart assist pumps and chambers for artificial hearts, because of their physiological acceptability, relatively good blood tolerability, excellent stability over long implant periods and excellent physical and mechanical properties.4-7 These properties are related to the surface chemistry and physics of the PU implants and vary as a result of variation in the composition of the prepolymer and also the casting procedure. 115
116
Mehlika
Pulac, Cemil Senaar
In this study the structural and surface properties of PU membranes prepared by a classical solvent-casting method were determined. Porosities of the membranes were investigated by using the scanning electron microscope (SEM). Surface free energies (SFE) were calculated from the measurements of contact angles. In addition, equilibrium water contents (EWC) and water permeabilities (WP) were examined and the results are discussed. The interactions of these membranes with fibroblastic and blood cells has been presented in our previous papers.8,9 EXPERIMENTAL Preparation of PU membranes
PU membranes used in this study were prepared by the solvent-casting procedure performed with polyether urethane (Pellethane@ 2363-80A, Upjohn, The Netherland) as given in the literature.* In order to obtain a series of PU membranes which have different structural properties, a number of casting solutions of Pellethane@ were prepared by dissolving it in different solvent systems. These systems included dimethylformamide (DMF) (Merck, Germany), dioxan (BDH, UK), tetrahydrofuran (THF) (Merck) and their mixtures; 8% (w/v) solutions of Pellethane@ were cast in a casting apparatus and then the precipitation process was applied. Average thicknesses of the membranes were 50 ( f 5) pm; 20% glycerol-80% water solution was used as the precipitation medium. They were extensively washed with distilled water and dried at 40°C under vacuum for about a week. Determination
of EWC, WP and contact angles of PU membranes
Swelling experiments at 37°C were used to determine the void volume (VV) and the EWC of PU membranes.* EWC (%) was defined as:” [(wet weight-dry
weight)/dry weight] x 100
Water permeabilities through .the membrane were measured under controlled conditions at 37 )0*5”C and 60% relative humidity. The water permeabilities were calculated from weight losses of beakers filled with water and sealed with the tested membranes.” Contact-angle measurements based on the captive bubble method12 were realized by placing both air and octane bubbles on the membrane surface submerged in a container filled with bi-distilled water. The contact
Structural and surface properties of polyurethane membranes
117
angles were calculated from the photographs of the bubblesI as given in the literature.8 The porosities of the membranes were investigated from the SEM photographs. Membrane samples were prepared by coating with gold at 3OOA thickness. The photographs were used to determine the average pore densities and the average pore radius, as described in the literature.14 Void volumes of the membranes were found using the same reference.
RESULTS AND DISCUSSION Surface free energies of the membranes were calculated from the harmonic mean equation1 5 using contact-angle values. Surface free energies (y) and air contact angles (O,,,) of the PU membranes, prepared in this study by changing the composition of the solvent system, are given in Table 1. It is observed from Table 1 that Oairdecreases as SFE increases. The relationship between EWC and SFE is illustrated in Fig. 1. As can be seen from the figure, EWC is directly proportional to SFE. TABLE 1 Oairand y Values of PU Membranes Sample no.
Composition of solvent system
eair (")*
7 (erslcm2)
PU-1 PU-2 PU-3 PU-4 PU-5 PU-6
THF 10% DMF-90% THF 50% THF-50% dioxan 10% DMF-45% dioxan-45% THF 10% DMF-90% dioxan dioxan
76.11 78.70 59.20 64.85 73.94 6684
32.54 30.75 43.95 40.01 33.86 39.61
PU-3 l’
PU-4 . /
01 27
0’ 30
, 33
I
36
I
39
I
42
I 45
SFE (erg/cm*) Fig. 1. The variation of EWC vs SFE.
118
Mehlika Pulat, Cemil Senvar
The VV is that fraction of the membrane volume which is not occupied by the polymer substrate and can be calculated from the difference between the wet and dry weights of the membrane. The VV can be connected to the SFE and wettability. In Table 2, it is seen that the VV is directly proportional to the EWC. The porosity values of the membranes which are given in Table 3 were directly determined from SEM photographs as given in the literature.* SEM photographs are presented in Fig. 2. Water permeability (WP) of the PU membranes increases as the porosity increases (Table 3) and can be compared to the water permeabilities of some commercial membranes” used in wound healing (Table 4). It is clearly seen from Table 3 that DMF affects the porosites inversely. The adding of DMF to the solvent system increases the porosities in the THF-PU system, which has less pores, and decreases it in the dioxan-PU system, which has more pores. TABLE 2 Variation of VV Values with EWC Sample no.
Void volume x I O3(g/ml )
EWCs
PU-3 PU-4 PU-6 PU-5 PU-1 PU-2
11.5 9.7 6.9 3.4 1.3 0.5
23.71 16.58 1000 7QO 2.60 0,75
TABLE 3 Surface Properties and WP of the Membranes Sample no.
WP( g/m2 24 h)
PU-1” PU-2” PU-3b PU-4 PU-5” PU-6
1205 1562 1677 1417 1641 1682
“In Ref. 16. bin Ref. 9.
Average pore density (no. of pores/cm2) 2.2 x 6.7 x 5.6 x 3.2 x 4.1 x 4.7 x
105 IO* lo6 lo5 106 106
Average pore radius (pm) 1.0 2.5 1.3 1.4 3.9 4.5
Structural and surface properties of polyurethane membranes
Fig. 2. SEM photographs
of PU membranes: (a) PU-1; (b) PU-2; (c) PU-3; (d) PU-4; (4 PI-J-5; (f) PU-6.
TABLE 4
WP of Some Commercial Membranes’ ’ Membrane (commercial name)
Omiderm Biobrane Op site
119
WP (g/in’ 24 h)
5ooo 1400 500
120
Mehlika
Pulat, Cemil Senvar
CONCLUSION As a result
1. Piskin, E. & Chang, T. M. S., The Past, Present and Future of Artijicial Organs, 1988, 32. 2. Williams, D. F. (eds), Fundamental Aspects ofBiocompatibility, Vols I-II, CRC Press, Boca Raton, FL, 1981. 3. Wachem, van P. B. et al., Interaction of cultured human endothelial cells with polymeric surfaces of different wettabilities. Biomaterials, 6 (1985) 403. 4. Boretos, J. W. & Pierce, W. S., Segmented polyurethane: a polyether polymer. J. Biomed. Mater. Res., 2 (1968) 121. 5. Farrar, D. J. et al., In uiuo evaluations of a new thromboresistant polyurethane for artificial heart blood pumps. J. Thorac. Cardiovasc. Surg., 95 (1988) 191. 6. How, T. V. & Annis, D., Viscoelastic behavior of polyurethane vascular prostheses. J. Biomed. Mater. Res., 21 (1987) 1093. 7. Ratner, B. D., Gladhill, K. W. & Horbett, T. A. Analysis of in oitro enzymatic and oxidative degradation of polyurethanes. J. Biomed. Mater. Res., 22 (1988) 509. 8. Kiremitci, M., Pulat, M., Senvar, C., Serbetci, A. 1. & Piskin, E., Structural and cellular characterization of solvent casted polyurethane membranes. Clin. Mater. 6 (1990) 227. 9. Kiremitci, M. Pesmen, A., Pulat, M. & Giirhan, I., Relationship of surface characteristics to cellular attachment in PU and PHEMA. J. Biomater. Appl., 7(3) (1993) 250.
10. Hirabayashi, Y., Satoh, M., Ogawa, H. & Ohtsuka, Y., Amphiphilic graft copolymer latex membranes. VI. Poly(viny1 alcohol)-acrylonitrile-N-hydroxyethyl acrylamide graft copolymer latex membranes. J. Appl. Polym. Sci., 28 (1983) 109. 11. Behar, D., et al., Omiderm, a new synthetic wound covering: physical properties and drug permeability studies. J. Biomed. Mater. Res., 20 (1986) 731. 12. Bikermann, J. J., Physical Surfaces, Academic Press, New York, 1970. 13. Guidelines for Physicochemical Characterization of Biomaterials. US Department of Health and Human Services, 1980. 14. Kesting, R. E., Synthetic Polymeric Membranes, McGraw-Hill, New York, 1971. 15. Andrade, J. D., King, R. & Gregonis, D. E., Contact angles at the solid-water interface, J. Polym. Sci. Symp. C, Sci., 72 (1979) 488. 16. Abbasoglu, U. & Pulat, M., Investigation of water and antimicrobial agents permeation of polyurethane membranes in relation to their surface properties. FABAD, 19(l) (1994) 1.