J. BIOMED. MATER. RES.
VOL. 9, PP. 561-568 (1975)
Relative Thrombogenicity of Polydimethylsiloxane and Silicone Rubber Constituents PAUL K. WEATHERSBY,* Radiation Biology Department, Armed Forces Radiobiology Research Institute, Bethesda, and THEODOR KOLOBOW and EDWARD W. STOOL, Laboratory of Technical Development, National Heart and Lung Institute, National Institutes of Health, Bethesda, Maryland SOOl4
Summary Medical grade silicone rubber (MGSR) is composed of polydimethylsiloxane (PDMS) as well as silica filler, oxidation products from the curing process, and other components. In a test that excludes air-blood interfaces, PDMS radiation cured under nitrogen has a whole blood clotting time 22% longer than MGSR. Curing the PDMS under oxidizing conditions maintains a 10% prolongation, but addition of silica filler to the PDMS returns the clotting time to that of MGSR. Extracting MGSR with solvents other than water appreciably lowers the clotting time. These results indicate that “pure” PDMS has an intrinsically high thromboresistance. Thrombogenicity is increased by the use of silica filler and oxidizing cure, as in MGSR.
INTRODUCTION Progress in artificial internal organs has been slowed by the lack of a material compatible with blood. Part of the problem arises from a scanty data base and inadequate theoretical understanding of thrombogenesis. Further difficulties arise from the testing of poorly categorized biomaterials. Medical grade silicone rubber is one of the most widely used biomaterials. Typical formulations of this material include the base polymer polydimethylsiloxane (PDMS), a filler such as silica, resi*Present address: Department of Chemical Engineering, University of Washington, Seattle Washington 98195 561 @ 1975 by John Wiley & Sons, Inc.
562
WEATHERSBY, KOLOBOW, AND STOOL
dues of the crosslinking agent (c.g., an organic peroxide), and various processing aids.2* 3 Additional contaminants can be introduced by fabrication, cleaning, handling, and ~terilization.~J This study was designed to determine the thromboresistancc of the base polymer PMDS and the effects of curing, SiOzfiller, and solvent extraction. Comparison was made with commercially available “Medical Grade” Silicone rubber.
MATERIALS AND METHODS Materials The PDMS used was a commercial linear polymer (SE-30 and SE76, General Electric Co., Waterford, N.Y .) of viscosity-average molecular weight 6.4 to 8.0 x lo5. The only additional purification was solution centrifugation t o remove particulate contaminants. Infrared spectroscopy detected only those bands attributable to linear PDMS.6 The filled rubber stock was a commercial preparation of approximately 18% SiO, in PDMS (SE-406, General Electric Co., Waterford, N.Y.) This fumed silica filler (Cab-O-Sil, M-S series) is similar t o that used in the medical grade product, but represents a somewhat lower loading.
Sample Preparation Chemical crosslinking was provided by 2,4-dichlorobenzoyl peroxide (50% 2,4-dichlorobenzoyl peroxide in silicone oil, Luperco CST, Pennwalt Corp., Buffalo, N.Y.), added as 4 wt% to the polymer and cured 1 hr a t 195°C under nitrogen. Infrared spectra of samples cured in this manner showed a new peak a t 1710 cm-l, presumably due to carbonyl groups. Ionizing radiation was employed to cure the PDMS without the addition of another chemical such as the peroxide. A dose of 5 x lo6 rad was delivered either by a linear accelerator (20 MeV electrons) or a 6OCo source. Activation analysis showed t h a t less than 1 carbon atom in 1013was activated by the electron dose. As expected from the radiation chemistry of PDMS,’ infrared spectra of irradiated samples showed no change when the radiation was done under nitrogen gas, but the peak a t 1710 cm-’ appeared when done in air.
THROMBOGENICITY OF PDMS AND MGSR
563
Swelling and extraction data were obtained for a commercial medical grade silicone rubber tubing (.250 in. i.d.-.375 in. o.d., DowCorning Corp., Midland, Mich.) in water, ethanol, l-butanol, 2butanone, ethyl acetate and toluene. The tubing samples were immersed in several changes of stagnant solvent. Solvents were reagent grade used as received. Swelling is reported as the percent increase in outside diameter after 2 days immersion in the given solvent. Extraction is reported as the percent weight loss of the original sample after 7 days immersion in the solvent and room temperature drying to constant weight. Each value is the average of a t least three determinations. Coating of silicone rubber tubing was accomplished by filling the tubing with a solution of 10 to 15 g/dl PDMS in cyclohexane or toluene, standing for 2 min, then gravity draining, solvent evaporating and curing. These conditions gave a rather uniform surface layer of 50 p to 100 p . The surface coatings tested were PDMS cured by radiation under nitrogen, by radiation in air, by peroxide decomposition, and SiOz filled PDMS cured by radiation under nitrogen.
Test Procedures Lee-White Clotting Test.-Lee-White whole blood clotting times8 were measured in test tubes of glass, siliconized (Siliclad, Clay-Adams, Inc., New York, N.Y.) glass, and PDMS-coated glass. The PDMS glass test tubes were filled with a 10% solution of PDMS in toluene, decanted, dried and irradiated under nitrogen. Closed Tubing Clotting Test.-A new clotting test, similar to the approach of Wallin and K r i ~ i kwas , ~ devised t o avoid the air-blood interface present in the test tube method. Thin-wall medical grade silicone rubber tubing (.132 in. i.d.-.183 in. o.d., Dow-Corning Corp., Midland, Mich.), coated with the surface to be tested, was cut into lengths of approximately 15 in. The tubing was filled with sterile isotonic saline (Abbott Laboratories, North Chicago, Ill.) which displaced all air. Fresh flowing blood from a venipunrture site in the same fasting male volunteer was directed through a 19 gauge needle, $way stopcock and siliconized stainless steel connector into one end of the tubing. The flowing blood displaced the saline and gradually filled the tubing without ever encountering an air interface. Both
564
WEATHERSBY, KOLOBOW, AND STOOL
ends of the tubing were clamped with hemostats and the tubing was placed in a 37°C water bath. At various time intervals, a 1 to 1.5 in. length segment from each end of the tubing was clamped, cut off, opened, and examined for clots. I n some samples, small (5 mm or Iess) clots were discovered early in incubation, usually at the point where the tubing had been clamped. These clots seldom progressed to a longer size. However, in examining adjacent segments, a time interval of only a few rnin could be defined during which a clot formed that grew to fill SO~O-lOO~O of the tubing segment volume. The end point was taken as the first appearance of a clot greater than one-half of the available volume. Thfl end point for one end of a tubing sample occurred within about 15 rnin of the other end, and the averagc of the two end points is given as the clotting time of the sample. At least 2 each of the solvent extracted tubing samples and a t least 6 each of the tubing samples with the 4 types of coated surface were tested. No two identical samples were tested on the same day. Every experiment included one piece of untreated tubing from the same commercial batch to serve as a n internal standard.
RESULTS Lee-White Clotting Test.-Several sets of Lee-White clotting tests were performed. Glass test tubes had firm clots within 10 min and Siliclad-coated tubes within 15 min. The test tubes coated with PDMS and irradiated did not clot within 45 min. By this time, a firm film had formed at the blood surface which produced an “end point.’’ If the film was removed, however, the bulk of the blood beneath could be transferred t o a clean glass test tube where it formed a normal clot 3 to 10 min later. This test was subsequently abandoned because of this consistent artifact. Closed Tubing Clotting Test.-The closed tubing clotting test proved more useful. I n 14 experiments the untreated commercial medical grade silicone rubber tubing had a clotting time of 82.7 f 9.5 rnin (S.D.). Under these conditions, a glass capillary of the same dimensions clotted before 15 min. The three-tube Lee-White clotting time for the same blood samples was 7.3 =t1.3 min. Commercial silicone rubber tubing was tested after extraction for 1 week in various solvents. Both oily and crystalline materials were
THROMBOGENICITY OF PDMS AND MGSR
565
recovered from the extracting solvents. No further analysis was performed on the extracts. As the solvent affinity for PDMS (i.e., swelling) increased, the amount extracted became constant (Table I). The ratios of the clotting time of an extracted sample to the clotting for an unexposed commercial sample are also presented in Table I. TABLE I Swelling, Extraction, and Clotting Ratio (clotting time of sample over clotting time of untreated commercial medical grade silicone rubber) of Tubing Samples Exposed to the Given Solvent for 7 days8 Swelling (yo)
Solvent
0.6 2.7 5.5 22.7 31.7 43.3
Water Ethanol 1-Butanol 2-Butanone Ethyl acetate Toluene a
Extraction (yo)
Clotting Ratio
0.3 2.9 3.0 3.0 3.1 3.1
0.95, 0.99, 1.03 0.53,0.66 0.63,0.63 0.80,0.83 0.81, 0.90 0.40,0.40
See text for prodecure.
All extracted samples had a decreased clotting time, but there was no clear relationship between clotting time and either swelling or extraction. The results of the tubing clotting test for the coated tubing surfaces are given in Table 11. The Si02-filled coating clotted just as rapidly as the commercial tubing even though it was crosslinked free of peroxides. The two PDMS coatings which were free of silica filler but cured under oxidizing conditions (peroxide or radiation in air) had a 10% prolongation in clotting time ( p < 0.2). A coating TABLE I1 Ratio of Clotting Times of Coated Tubing to the Clotting Time of Uncoated Commercial Medical Grade Silicone Rubber Tubing
-_
Coating (Cure)
Clotting Ratio -
PDMS (radiation, Nz) PDMS (radiation, air) PDMS (peroxide, Nz) SiOz (radiation, Nz) PDMS
+
1.22 f 0.16 (1 S.D.) 1.10 f 0.14 1.10 f 0.12 0.98 f 0.11
566
WEATHERSBY, KOLOBOW, AND STOOL
of PDMS with a “clean” cure had a 22% prolongation of clotting time over the uncoated tubing ( p < 0.02), and was significantly longer than the PDMS peroxide cure or the PDMS radiation cure in air (both p < 0.2).
DISCUSSION Silicone rubber has been used in blood contacting applications for many years. Recently Nyilas published an extensive study4 on the blood compatibility of artificial heart bladders related to manufacture and fabrication additives of silicone rubber. This study did not consider the blood compatibility of PDMS alone. I n view of the inherently poor mechanical strength of PDMS rubber (approximately 50 psi tensile), special techniques are needed to produce a useful device having a PDRlS surface. Testing for blood compatibility is not a well-defined procedure. Except for the Lee-White clotting time, different investigators use different methods to assess the material influence in in vitro coagulation. We have seen that the exclusion of a n air-blood interface is necessary for a reliable, extended (i.e., over 30 min) clotting test. The procedure outlined here is reproducible: a coefficient of variation of about 12% compared to 18% for the Lee-White time in glass. It has proven to be a useful tool in discovering silicone treatments which change the clotting time by as little as 10%. The exposure of silicone tubing to any solvent except water, appreciably lowers the clotting time. Possible explanations include one or more extractable substances found in medical grade silicone rubber that act to delay coagulation, or a deleterious change in the rubber structure produced by the cycle of solvent swelling and subsequent evaporation. Coating experiments prove that PDMS by itself is less thrombogenic than commercial medical grade silicone rubber. It is further seen that the SiOz filler decreases the whole blood clotting time by about 20% while the various oxidation products known to result from peroxides or radiation in 0 2 decrease it by about 10%. The relationship between in vitro testing and clinical extraeorporeal use remains unproven. Evidence exists, however, that the silica and oxidation products of silicone rubber result in appreciable decreases in ex vivo performance. It has been shown that silicone
THROMBOGENICITY O F PDMS AND MGSIt
,567
polymers with deliberately oxidized side groups (-COOH) do poorer as venu cuvu implants than do fully methylated chain^.^ The blood compatibility of silicone rubber heart bladders was improved by the complete removal of the decomposition product of the peroxide c a t a l y ~ t . ~We have recently shown that entire membrane oxygenator perfusion systems with blood contacting surfaces of filler-free PDMS (cured under oxidizing conditions) consistently maintain a constant platelet count in the animal and steady perfusion pressure.10 The animal work, preliminary t o this study, has shown that small membrane envelopes coated with PDMS (radiation cured under nitrogen) and tested ex vivo remained patent for over 2 hr wher econtrol envelopes showed severe clotting within 10 min. l1 The mechanical properties of PDMS alone are not sufficient for most extracorporeal applications. I n our study i t was necessary t o produce a surface coat of the polymer on a substrate more mechanically suitable for a given test. The polymer can be radiation grafted onto a variety of surfaces or i t can be cast as a discrete layer in a membrane.12 Thus a polymer generally considered too weak may become a useful biomaterial when used on a stronger, though less biologically acceptable material. Medical grade silicone rubber is considered an acceptable material in many blood contacting applications. Polydimethylsiloxane alone has been shown to be less thrombogenic than the commercially produced oxidized polymer plus filler, and should be used where maximum blood compatibility is desired. The original impetus for this study came from Prof. E. W. Merrill of MIT and took the form of an M.S. thesis of one of the authors (PKW). The authors are also grateful to E. Sokoloski for the infrared data, and J. Smarsh of AFRRI and P. Riesz of NIH for use of the radiation sources.
References 1. E. W. Salzman, Blood. 38, 509 (1971). 2. W. Noll, Chemistry and Technology of Silicones, Academic Press, New York, 1968. 3. M. C. Musolf, V. D. Hulce, D. R. Bennett, and M. Ramos, Trans. Amer. SOC.Artif. Int. Organs, 15, 18 (1969). 4. E. Nyilas, E. L. Kupski, P. Burnett, and R. M. Haag, J . Biomed. Mater. Res., 4, 369 (1970). 5. S. M. Bruck, J . Biomed. Mater. Res., 5 , 139 (1971).
568
WEATHERSBY, KOLOBOW, AND STOOL
6. I). C. Smith, J. M. French, and J. J. O’Neill, U.S. Naval Research Laboratory Report No. P-2746, 1946. 7. A. A. Miller, J. Amer. Chem. Soc., 83, 31, (1961). 8. M. M. Wintrobe, Clinical Hematology, 5th ed., Lea and Febiger, Philadelphia, 1961. 9. R. F. Wallin and M. Krisik, J. Biomed. Muter. Res., 6, 49 (1972). 10. T. Kolobow, E. W. Stool, P. K. Weathersby, J. Pierce, F. Hayano, and J. Suadeau, Trans. Amer. Soc. Artif. Int. OTgans, 20, 269 (1974). 11. P. K. Weathersby, T. Kolobow, and E. W. Stool, AFRRI Technical Note TN74-5, Armed Forces Radiobiology Research Institute, Bethesda, Maryland, 1974. 12. T. Kolobow, F. Hayano, and P. K. Weathersby, Med. Instrum., 9,124 (1975).
Received October 28, 1974 Revised January 28, 1975
VOL. 9, PP. 561-568 (1975)
Relative Thrombogenicity of Polydimethylsiloxane and Silicone Rubber Constituents PAUL K. WEATHERSBY,* Radiation Biology Department, Armed Forces Radiobiology Research Institute, Bethesda, and THEODOR KOLOBOW and EDWARD W. STOOL, Laboratory of Technical Development, National Heart and Lung Institute, National Institutes of Health, Bethesda, Maryland SOOl4
Summary Medical grade silicone rubber (MGSR) is composed of polydimethylsiloxane (PDMS) as well as silica filler, oxidation products from the curing process, and other components. In a test that excludes air-blood interfaces, PDMS radiation cured under nitrogen has a whole blood clotting time 22% longer than MGSR. Curing the PDMS under oxidizing conditions maintains a 10% prolongation, but addition of silica filler to the PDMS returns the clotting time to that of MGSR. Extracting MGSR with solvents other than water appreciably lowers the clotting time. These results indicate that “pure” PDMS has an intrinsically high thromboresistance. Thrombogenicity is increased by the use of silica filler and oxidizing cure, as in MGSR.
INTRODUCTION Progress in artificial internal organs has been slowed by the lack of a material compatible with blood. Part of the problem arises from a scanty data base and inadequate theoretical understanding of thrombogenesis. Further difficulties arise from the testing of poorly categorized biomaterials. Medical grade silicone rubber is one of the most widely used biomaterials. Typical formulations of this material include the base polymer polydimethylsiloxane (PDMS), a filler such as silica, resi*Present address: Department of Chemical Engineering, University of Washington, Seattle Washington 98195 561 @ 1975 by John Wiley & Sons, Inc.
562
WEATHERSBY, KOLOBOW, AND STOOL
dues of the crosslinking agent (c.g., an organic peroxide), and various processing aids.2* 3 Additional contaminants can be introduced by fabrication, cleaning, handling, and ~terilization.~J This study was designed to determine the thromboresistancc of the base polymer PMDS and the effects of curing, SiOzfiller, and solvent extraction. Comparison was made with commercially available “Medical Grade” Silicone rubber.
MATERIALS AND METHODS Materials The PDMS used was a commercial linear polymer (SE-30 and SE76, General Electric Co., Waterford, N.Y .) of viscosity-average molecular weight 6.4 to 8.0 x lo5. The only additional purification was solution centrifugation t o remove particulate contaminants. Infrared spectroscopy detected only those bands attributable to linear PDMS.6 The filled rubber stock was a commercial preparation of approximately 18% SiO, in PDMS (SE-406, General Electric Co., Waterford, N.Y.) This fumed silica filler (Cab-O-Sil, M-S series) is similar t o that used in the medical grade product, but represents a somewhat lower loading.
Sample Preparation Chemical crosslinking was provided by 2,4-dichlorobenzoyl peroxide (50% 2,4-dichlorobenzoyl peroxide in silicone oil, Luperco CST, Pennwalt Corp., Buffalo, N.Y.), added as 4 wt% to the polymer and cured 1 hr a t 195°C under nitrogen. Infrared spectra of samples cured in this manner showed a new peak a t 1710 cm-l, presumably due to carbonyl groups. Ionizing radiation was employed to cure the PDMS without the addition of another chemical such as the peroxide. A dose of 5 x lo6 rad was delivered either by a linear accelerator (20 MeV electrons) or a 6OCo source. Activation analysis showed t h a t less than 1 carbon atom in 1013was activated by the electron dose. As expected from the radiation chemistry of PDMS,’ infrared spectra of irradiated samples showed no change when the radiation was done under nitrogen gas, but the peak a t 1710 cm-’ appeared when done in air.
THROMBOGENICITY OF PDMS AND MGSR
563
Swelling and extraction data were obtained for a commercial medical grade silicone rubber tubing (.250 in. i.d.-.375 in. o.d., DowCorning Corp., Midland, Mich.) in water, ethanol, l-butanol, 2butanone, ethyl acetate and toluene. The tubing samples were immersed in several changes of stagnant solvent. Solvents were reagent grade used as received. Swelling is reported as the percent increase in outside diameter after 2 days immersion in the given solvent. Extraction is reported as the percent weight loss of the original sample after 7 days immersion in the solvent and room temperature drying to constant weight. Each value is the average of a t least three determinations. Coating of silicone rubber tubing was accomplished by filling the tubing with a solution of 10 to 15 g/dl PDMS in cyclohexane or toluene, standing for 2 min, then gravity draining, solvent evaporating and curing. These conditions gave a rather uniform surface layer of 50 p to 100 p . The surface coatings tested were PDMS cured by radiation under nitrogen, by radiation in air, by peroxide decomposition, and SiOz filled PDMS cured by radiation under nitrogen.
Test Procedures Lee-White Clotting Test.-Lee-White whole blood clotting times8 were measured in test tubes of glass, siliconized (Siliclad, Clay-Adams, Inc., New York, N.Y.) glass, and PDMS-coated glass. The PDMS glass test tubes were filled with a 10% solution of PDMS in toluene, decanted, dried and irradiated under nitrogen. Closed Tubing Clotting Test.-A new clotting test, similar to the approach of Wallin and K r i ~ i kwas , ~ devised t o avoid the air-blood interface present in the test tube method. Thin-wall medical grade silicone rubber tubing (.132 in. i.d.-.183 in. o.d., Dow-Corning Corp., Midland, Mich.), coated with the surface to be tested, was cut into lengths of approximately 15 in. The tubing was filled with sterile isotonic saline (Abbott Laboratories, North Chicago, Ill.) which displaced all air. Fresh flowing blood from a venipunrture site in the same fasting male volunteer was directed through a 19 gauge needle, $way stopcock and siliconized stainless steel connector into one end of the tubing. The flowing blood displaced the saline and gradually filled the tubing without ever encountering an air interface. Both
564
WEATHERSBY, KOLOBOW, AND STOOL
ends of the tubing were clamped with hemostats and the tubing was placed in a 37°C water bath. At various time intervals, a 1 to 1.5 in. length segment from each end of the tubing was clamped, cut off, opened, and examined for clots. I n some samples, small (5 mm or Iess) clots were discovered early in incubation, usually at the point where the tubing had been clamped. These clots seldom progressed to a longer size. However, in examining adjacent segments, a time interval of only a few rnin could be defined during which a clot formed that grew to fill SO~O-lOO~O of the tubing segment volume. The end point was taken as the first appearance of a clot greater than one-half of the available volume. Thfl end point for one end of a tubing sample occurred within about 15 rnin of the other end, and the averagc of the two end points is given as the clotting time of the sample. At least 2 each of the solvent extracted tubing samples and a t least 6 each of the tubing samples with the 4 types of coated surface were tested. No two identical samples were tested on the same day. Every experiment included one piece of untreated tubing from the same commercial batch to serve as a n internal standard.
RESULTS Lee-White Clotting Test.-Several sets of Lee-White clotting tests were performed. Glass test tubes had firm clots within 10 min and Siliclad-coated tubes within 15 min. The test tubes coated with PDMS and irradiated did not clot within 45 min. By this time, a firm film had formed at the blood surface which produced an “end point.’’ If the film was removed, however, the bulk of the blood beneath could be transferred t o a clean glass test tube where it formed a normal clot 3 to 10 min later. This test was subsequently abandoned because of this consistent artifact. Closed Tubing Clotting Test.-The closed tubing clotting test proved more useful. I n 14 experiments the untreated commercial medical grade silicone rubber tubing had a clotting time of 82.7 f 9.5 rnin (S.D.). Under these conditions, a glass capillary of the same dimensions clotted before 15 min. The three-tube Lee-White clotting time for the same blood samples was 7.3 =t1.3 min. Commercial silicone rubber tubing was tested after extraction for 1 week in various solvents. Both oily and crystalline materials were
THROMBOGENICITY OF PDMS AND MGSR
565
recovered from the extracting solvents. No further analysis was performed on the extracts. As the solvent affinity for PDMS (i.e., swelling) increased, the amount extracted became constant (Table I). The ratios of the clotting time of an extracted sample to the clotting for an unexposed commercial sample are also presented in Table I. TABLE I Swelling, Extraction, and Clotting Ratio (clotting time of sample over clotting time of untreated commercial medical grade silicone rubber) of Tubing Samples Exposed to the Given Solvent for 7 days8 Swelling (yo)
Solvent
0.6 2.7 5.5 22.7 31.7 43.3
Water Ethanol 1-Butanol 2-Butanone Ethyl acetate Toluene a
Extraction (yo)
Clotting Ratio
0.3 2.9 3.0 3.0 3.1 3.1
0.95, 0.99, 1.03 0.53,0.66 0.63,0.63 0.80,0.83 0.81, 0.90 0.40,0.40
See text for prodecure.
All extracted samples had a decreased clotting time, but there was no clear relationship between clotting time and either swelling or extraction. The results of the tubing clotting test for the coated tubing surfaces are given in Table 11. The Si02-filled coating clotted just as rapidly as the commercial tubing even though it was crosslinked free of peroxides. The two PDMS coatings which were free of silica filler but cured under oxidizing conditions (peroxide or radiation in air) had a 10% prolongation in clotting time ( p < 0.2). A coating TABLE I1 Ratio of Clotting Times of Coated Tubing to the Clotting Time of Uncoated Commercial Medical Grade Silicone Rubber Tubing
-_
Coating (Cure)
Clotting Ratio -
PDMS (radiation, Nz) PDMS (radiation, air) PDMS (peroxide, Nz) SiOz (radiation, Nz) PDMS
+
1.22 f 0.16 (1 S.D.) 1.10 f 0.14 1.10 f 0.12 0.98 f 0.11
566
WEATHERSBY, KOLOBOW, AND STOOL
of PDMS with a “clean” cure had a 22% prolongation of clotting time over the uncoated tubing ( p < 0.02), and was significantly longer than the PDMS peroxide cure or the PDMS radiation cure in air (both p < 0.2).
DISCUSSION Silicone rubber has been used in blood contacting applications for many years. Recently Nyilas published an extensive study4 on the blood compatibility of artificial heart bladders related to manufacture and fabrication additives of silicone rubber. This study did not consider the blood compatibility of PDMS alone. I n view of the inherently poor mechanical strength of PDMS rubber (approximately 50 psi tensile), special techniques are needed to produce a useful device having a PDRlS surface. Testing for blood compatibility is not a well-defined procedure. Except for the Lee-White clotting time, different investigators use different methods to assess the material influence in in vitro coagulation. We have seen that the exclusion of a n air-blood interface is necessary for a reliable, extended (i.e., over 30 min) clotting test. The procedure outlined here is reproducible: a coefficient of variation of about 12% compared to 18% for the Lee-White time in glass. It has proven to be a useful tool in discovering silicone treatments which change the clotting time by as little as 10%. The exposure of silicone tubing to any solvent except water, appreciably lowers the clotting time. Possible explanations include one or more extractable substances found in medical grade silicone rubber that act to delay coagulation, or a deleterious change in the rubber structure produced by the cycle of solvent swelling and subsequent evaporation. Coating experiments prove that PDMS by itself is less thrombogenic than commercial medical grade silicone rubber. It is further seen that the SiOz filler decreases the whole blood clotting time by about 20% while the various oxidation products known to result from peroxides or radiation in 0 2 decrease it by about 10%. The relationship between in vitro testing and clinical extraeorporeal use remains unproven. Evidence exists, however, that the silica and oxidation products of silicone rubber result in appreciable decreases in ex vivo performance. It has been shown that silicone
THROMBOGENICITY O F PDMS AND MGSIt
,567
polymers with deliberately oxidized side groups (-COOH) do poorer as venu cuvu implants than do fully methylated chain^.^ The blood compatibility of silicone rubber heart bladders was improved by the complete removal of the decomposition product of the peroxide c a t a l y ~ t . ~We have recently shown that entire membrane oxygenator perfusion systems with blood contacting surfaces of filler-free PDMS (cured under oxidizing conditions) consistently maintain a constant platelet count in the animal and steady perfusion pressure.10 The animal work, preliminary t o this study, has shown that small membrane envelopes coated with PDMS (radiation cured under nitrogen) and tested ex vivo remained patent for over 2 hr wher econtrol envelopes showed severe clotting within 10 min. l1 The mechanical properties of PDMS alone are not sufficient for most extracorporeal applications. I n our study i t was necessary t o produce a surface coat of the polymer on a substrate more mechanically suitable for a given test. The polymer can be radiation grafted onto a variety of surfaces or i t can be cast as a discrete layer in a membrane.12 Thus a polymer generally considered too weak may become a useful biomaterial when used on a stronger, though less biologically acceptable material. Medical grade silicone rubber is considered an acceptable material in many blood contacting applications. Polydimethylsiloxane alone has been shown to be less thrombogenic than the commercially produced oxidized polymer plus filler, and should be used where maximum blood compatibility is desired. The original impetus for this study came from Prof. E. W. Merrill of MIT and took the form of an M.S. thesis of one of the authors (PKW). The authors are also grateful to E. Sokoloski for the infrared data, and J. Smarsh of AFRRI and P. Riesz of NIH for use of the radiation sources.
References 1. E. W. Salzman, Blood. 38, 509 (1971). 2. W. Noll, Chemistry and Technology of Silicones, Academic Press, New York, 1968. 3. M. C. Musolf, V. D. Hulce, D. R. Bennett, and M. Ramos, Trans. Amer. SOC.Artif. Int. Organs, 15, 18 (1969). 4. E. Nyilas, E. L. Kupski, P. Burnett, and R. M. Haag, J . Biomed. Mater. Res., 4, 369 (1970). 5. S. M. Bruck, J . Biomed. Mater. Res., 5 , 139 (1971).
568
WEATHERSBY, KOLOBOW, AND STOOL
6. I). C. Smith, J. M. French, and J. J. O’Neill, U.S. Naval Research Laboratory Report No. P-2746, 1946. 7. A. A. Miller, J. Amer. Chem. Soc., 83, 31, (1961). 8. M. M. Wintrobe, Clinical Hematology, 5th ed., Lea and Febiger, Philadelphia, 1961. 9. R. F. Wallin and M. Krisik, J. Biomed. Muter. Res., 6, 49 (1972). 10. T. Kolobow, E. W. Stool, P. K. Weathersby, J. Pierce, F. Hayano, and J. Suadeau, Trans. Amer. Soc. Artif. Int. OTgans, 20, 269 (1974). 11. P. K. Weathersby, T. Kolobow, and E. W. Stool, AFRRI Technical Note TN74-5, Armed Forces Radiobiology Research Institute, Bethesda, Maryland, 1974. 12. T. Kolobow, F. Hayano, and P. K. Weathersby, Med. Instrum., 9,124 (1975).
Received October 28, 1974 Revised January 28, 1975
