Hydrogels for Medical and Related Applications

21 Radiation-Induced Co-Graft Polymerization of 2-Hydroxyethyl Methacrylate and Ethyl Methacrylate

Downloaded by UNIV ILLINOIS URBANA on June 7, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0031.ch021

onto Silicone Rubber Films TAKASHI SASAKI,* BUDDY D. RATNER, and ALLAN S. HOFFMAN Department of Chemical Engineering and Center for Bioengineering, University of Washington, Seattle, Wash. 98195

A number of hydrogel systems have demonstrated good tissue compatibility and low thrombogenicity in in vivo and in vitro tests (1). The poor mechanical properties of hydrogels have encouraged the development of techniques for reinforcing hydro gels to make them suitable for use in biomedical devices. By radiation grafting monomers such as 2-hydroxyethyl methacrylate (HEMA), N-vinyl-2-pyrrolidone, and acrylamide onto strong, inert polymeric supports, materials have been produced which combine the desirable biocompatibility properties of the hydrogel systems with the good mechanical properties of the substrate polymer (2-8). A clear correlation between the surface hydrophilicity of radiation grafted hydrogels and their biocompatibility has not yet been firmly established. Many useful biomaterials demon strate a balance between hydrophilic and hydrophobic sites which might be important for biocompatibility. Some of these materials are summarized in Table I. In order to systematically investigate the interrelationship between the hydrophilic-hydrophobic composition of a polymeric material and biological interactions with that material, a series of well characterized radiation graft copolymers of poly(HEMA) and poly(ethylmethacrylate) (EMA) have been prepared. A prelim inary report on the preparation and characterization of these graft copolymers is presented here. Experimental Silicone rubber films, 1.9 cm. χ 3.8 cm. χ 10 mils thick (Silastic, type 500-3, Dow Corning Corp.) were cleaned with five minute sonications in 0.1% Ivory soap solution and then in distilled water (3 times). After washing they were equilibrated at *0n a leave of absence from the Takasaki Radiation Chemistry Research Establishment, J.A.E.R.I., Takasaki, Japan. 283

In Hydrogels for Medical and Related Applications; Andrade, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

284

HYDROGELS FOR MEDICAL AND RELATED APPLICATIONS

Table I Thromboresistant Surfaces Which May Demonstrate a Balance Between H y d r o p h i l i c and Hydrophobic S i t e s .

Hydrophobic S i t e

Material


blood v e s s e l w a l l

lipid

Hydrophilic Site

Reference

protein, polysaccharide

(9)

polyether blocks

(10)

copolyetherpolyurethanes

polyurethane blocks

perfluorobutyryl ethylcellulose

perfluorobutyryl groups

cellulose rings

S t e l l i t e 21

tallow

steel

Poly(HEMA)

backbone methyl groups

h y d r o x y l groups

(11)

(?) (12)

carboxylic acid (8) v i n y l acetate poly(vinyl groups groups acetate-crotonic acid) 52% R.H. The i n i t i a l weight of the s i l i c o n e rubber (W ) was measured a t 52% R.H. H i g h l y p u r i f i e d HEMA was generously s u p p l i e d by Hydron L a b o r a t o r i e s , Inc. and was used as r e c e i v e d . EMA monomer was used a f t e r d i s t i l l i n g a t a reduced pressure. Reagent grade s o l v e n t s and chemicals were used as r e c e i v e d . The f i l m s were immersed i n monomer s o l u t i o n s and the system was deoxygenated by bubbling n i t r o g e n gas through the s o l u t i o n for 30 minutes. I r r a d i a t i o n was performed i n a ca. 20,000 C i Co-60 source. The i r r a d i a t i o n dose used was 0.25 Mrad unless otherwise s p e c i f i e d . A f t e r the i r r a d i a t i o n , the f i l m s were washed twice (30 min, and 2 hrs.) w i t h acetone^methanol 111 m i x t u r e , and then w i t h d i s t i l l e d water f o r 24 hours changing the water twice d u r i n g t h i s p e r i o d . The wet weight (W ) of the g r a f t e d f i l m s was measured by b l o t t i n g between two sheets of f i l t e r paper (Whatman #1) f o r t e n seconds under a constant p r e s s s u r e and then weighing. F i l m s were then d r i e d i n an evacuated d e s i c c a t o r c o n t a i n i n g magnesium p e r c h l o r a t e f o r more than 12 h r s . a t which p o i n t the dry weight (W^) was measured. The degree of g r a f t and the water content i n the g r a f t were defined as: S


21.

SASAKI ET AL.

Radiation Induced Co-Graft Polymerization

285

wt. % g r a f t = [(W, - W )/W,] χ 100 α s α water content (%) = [(W - W,)/(W - W ) ] χ 100 w d w s


Results A p r e l i m i n a r y experiment on the r a d i a t i o n induced bulk c o p o l y m e r i z a t i o n of HEMA and EMA showed that the p o l y m e r i z a t i o n r a t e decreases a b r u p t l y as the f r a c t i o n of EMA i n a HEMA-EMA monomer mixture i n c r e a s e s . S i m i l a r behavior was noted i n a cog r a f t p o l y m e r i z a t i o n of HEMA and EMA onto s i l i c o n e f i l m s i n acetone o r acetone-methanol (1:1 m i x t u r e ) , as i s shown i n F i g . 1. When the HEMA p o r t i o n i n the monomer mixture i s more than 90 v o l . %, p r e c i p i t a t i o n of homopolymer i n acetone was observed.

EMA

(vol 7·) IN MONOMER

MIXTURE

Figure 1. Co-graft polymerization of HEMA-EMA at various monomer compositions. Total monomer, 20 vol %.

I n surveying a p p r o p r i a t e s o l v e n t systems f o r the c o - g r a f t p o l y m e r i z a t i o n an a c c e l e r a t i v e e f f e c t by water on the g r a f t p o l y m e r i z a t i o n , e s p e c i a l l y i n a l c o h o l i c systems, was noted. F i g . 2 shows the e f f e c t of water on the g r a f t i n g r a t e of 1:1 (by volume) mixture of HEMA and EMA i n methanol (MeOH), ethanol (EtOH) and acetone. I t can be seen from the f i g u r e that the a d d i t i o n of water t o any o f these s o l v e n t s i n c r e a s e s the g r a f t i n g r a t e . Pure EtOH appears t o be somewhat s u p e r i o r t o pure MeOH f o r o b t a i n i n g higher l e v e l s of g r a f t , but the a c c e l e r a t i v e e f f e c t o f


286


water i s more marked i n the methanolic system. Co-graft p o l y m e r i z a t i o n w i t h v a r i o u s monomer compositions was c a r r i e d out i n a MeOH-H 0 o r an EtOH-I^O system. F i g . 3 and 4 show the r e l a t i o n s h i p between the degree of g r a f t and monomer r a t i o i n these systems. In F i g . 3, i t should be noted that the wt.% g r a f t i s almost constant i n the range of 25 t o 75 v o l . % of EMA i n the monomer mixture a t two monomer c o n c e n t r a t i o n s . This tendency i s q u i t e d i f f e r e n t from that shown i n F i g . 1. I n the s o l u t i o n s c o n s i s t ing of only EMA, some p r e c i p i t a t i o n of homopolymer occurred though the v i s c o s i t y of the s o l u t i o n s themselves remained r a t h e r low. F i g . 4 shows another aspect of the c o p o l y m e r i z a t i o n behavior of t h i s system. The c o g r a f t i n g i n pure EtOH has a somewhat s i m i l a r tendency to that seen i n acetone-MeOH ( F i g . 1 ) . That i s , the wt. % g r a f t decreases w i t h an i n c r e a s i n g p r o p o r t i o n of EMA i n the monomer. The a d d i t i o n of water t o the monomer s o l u t i o n , however, changes the s i t u a t i o n e n t i r e l y . Thus, w i t h 7.5 v o l . % of water i n the s o l u t i o n , the wt. % g r a f t i s n e a r l y constant i n the range of 25 t o 100 v o l . % EMA i n the monomer mixture. A d d i t i o n of more water leads t o p r a c t i c a l l y the reverse trend from that seen i n pure EtOH. The e f f e c t of water on the g r a f t i n g r a t e o f EMA o r HEMA i n MeOH-H 0 and EtOH-H 0 i s shown i n F i g u r e 5 which i s a c r o s s p l o t of some of the data i n F i g u r e 4. The degree of g r a f t of pure EMA markedly i n c r e a s e s w i t h an i n c r e a s e of water i n the s o l u t i o n . P r e c i p i t a t i o n of pure EMA homopolymer occurred when the f r a c t i o n of water was more than 5% i n the MeOH-^O system o r 15% i n the EtOH-H 0 system. I n the e t h a n o l i c system, the a d d i t i o n of water to the HEMA s o l u t i o n shows a r e t a r d i n g e f f e c t . However, very l i t t l e e f f e c t on HEMA g r a f t l e v e l i s noted upon the a d d i t i o n o f water to the methanolic system i n the c o n c e n t r a t i o n range studied. F i g . 6 shows the g r a f t water content f o r those samples which were prepared w i t h v a r i o u s HEMA-EMA mixtures i n an EtOH-H 0 s o l vent system (7.5% of water i n the s o l u t i o n ) . The wt. % g r a f t f o r these samples are almost constant (~22%). I t can be seen from t h i s f i g u r e that the water content decreases w i t h i n c r e a s i n g EMA p o r t i o n s i n the monomer m i x t u r e . Upon dehydration and then r e h y d r a t i o n of g r a f t s contains l a r g e r p o r t i o n s of EMA, water contents are found t o drop even f u r t h e r . A f t e r t h i s treatment these f i l m s gave constant water c o n t e n t s , ca. 2-3% f o r 1/3 HEMAEMA g r a f t e d f i l m s and > 1% f o r pure EMA g r a f t e d f i l m s , r e g a r d l e s s of the g r a f t l e v e l . The water content of the g r a f t i s u s u a l l y independent of g r a f t l e v e l a t a g i v e n monomer r a t i o i f more than 50% HEMA i s present i n the monomer m i x t u r e . However, t h i s i s no longer observed f o r l a r g e r EMA p o r t i o n s . I n F i g . 7 the water contents of a number of HEMA-EMA g r a f t e d polymers a r e p l o t t e d a g a i n s t the wt. % g r a f t . I n cases where more than 50% o f the monomer


2

2

2

2

2



21.

SASAKI

E T AL.

Radiation

Induced

Co-Graft

Polymerization

WATER (vol.%)

Figure 2. Effect of water on the grafting of a 1:1 mixture of HEMA and EMA in various solvents. Dose, 0.25 mrad; total monomer, 25 vol %.

Ο

25 vol.% MONOMER

•

20 vol.% MONOMER

25

50

EMA IN

75

100

(vol.%)

MONOMER

MIXTURE

Figure 3. Co-graft polymerization of HEMA-EMA in MeOH-H Q (5 vol % H Q) 2

2


287



288



21.

SASAKI ET AL.

Radiation

Induced

25

Co-Graft

50

Polymerization

75

100

EMA (vol.%) IN MONOMER MIXTURE

Figure 6. Effect of monomer composition on the graft water content. Samples were prepared in EtOH—H Ù. Monomer, 25 vol %. H 0 = 7.5%; dose, 0.25 Mrad or 0.16 Mrad (HEMA). 2

2

SOLVENT MeOH - H 0

HEMA/EMA Vol. RATIO 4/0

3/1 2/2

1/3 0 / 4

φ

V

•

Δ

·

EtOH-H 0

Ο

V

•

Δ

Ο

Acetone-H 0

Φ

2

2

2

Π

Φ

—ι

1

1

1

ΙΟ

20

30

40

γ50

Wt. % GRAFT

Figure 7. Plot of water content vs. wt % graft. Total monomer, 25 vol %.


289


290


mixture i s EMA the water content increases w i t h i n c r e a s i n g g r a f t level. Figure 8 shows the r e s u l t s of a 2 hour vena cava r i n g t e s t f o r a s e r i e s of HEMA-EMA c o - g r a f t e d r i n g s . The g r a f t l e v e l s of these samples are d i f f e r e n t and the water contents are l a r g e r than those f o r the f i l m s i n v e s t i g a t e d above, probably due t o d i f f e r e n c e s i n the f o r m u l a t i o n of s i l i c o n e rubbers. The r e s u l t s , however, c l e a r l y show that HEMA-EMA co-grafted r i n g s as w e l l as pure HEMA g r a f t e d r i n g s have much lower thrombogenicity than pure EMA g r a f t e d o r ungrafted s i l i c o n e rubber r i n g s . Discussion The pronounced e f f e c t of s m a l l amounts o f water on the g r a f t i n g of HEMA-EMA mixtures to s i l i c o n e rubber can be mainly a t t r i b u t e d to the Trommsdorff e f f e c t . That i s , the a d d i t i o n of the non-solvent water to an a l c o h o l i c s o l u t i o n of EMA w i l l cause the growing chains to adopt a l e s s expanded conformation which hinders the chain t e r m i n a t i o n step to produce h i g h e r g r a f t i n g r a t e s . The s i t u a t i o n i s the r e v e r s e f o r pure HEMA systems as EtOH o r MeOH mixtures w i t h water are good s o l v e n t s f o r HEMA p o l y mer. T h i s e x p l a n a t i o n could account f o r the c o p o l y m e r i z a t i o n behavior shown i n F i g . 3 and 4. Changes i n the p a r t i t i o n i n g of the monomer between the s i l i c o n e rubber and the g r a f t i n g s o l u t i o n may a l s o i n f l u e n c e the g r a f t i n g behavior demonstrated by t h i s system upon the a d d i t i o n of water. The v a r i a t i o n i n water content w i t h degree of g r a f t f o r a given monomer r a t i o only a t f r a c t i o n s of EMA g r e a t e r than 50% i n the monomer mixture may be due t o a l t e r a t i o n s i n the p o r o s i t y of these g r a f t e d f i l m s . The water i n the solvent system may cause p r e c i p i t a t i o n o f EMA polymer as i t i s formed. This procès occurs simultaneously w i t h the s u r f a c e g r a f t i n g . Such p r e c i p i t a t e d polymers o f t e n have an open c e l l u l a r , porous s t r u c t u r e . Thus, the water content i n the g e l should be r e l a t e d t o the p o r o s i t y o f the g r a f t as w e l l as to the HEMA-EMA r a t i o and should t h e r e f o r e be h i g h l y dependent on the s o l v e n t c o n d i t i o n s a t the time of g r a f t i n g . Upon d r y i n g , such a porous s t r u c t u r e may c o l l a p s e and, due to hydrophobic i n t e r a c t i o n s between the poly(EMA) chains, be r e s i s t a n t t o r e h y d r a t i o n . T h i s s i t u a t i o n would account f o r the behavior seen a t h i g h EMA r a t i o s i n Figure 6 i n which the water contents of g r a f t e d f i l m s which have been d r i e d and then r e hydrated are found t o be lower than they were before d r y i n g . The i n v i v o e v a l u a t i o n of the thrombogenicity of s i l i c o n e rubber r i n g s by the vena cava r i n g t e s t ( f i g u r e 8) i s c o n s i s t e n t w i t h the idea that a balance o f hydrophobic and h y d r o p h i l i c s i t e s on s u r f a c e might be important f o r the blood c o m p a t i b i l i t y of m a t e r i a l s . Two other c o n c l u s i o n s can a l s o be drawn from t h i s experiment. F i r s t , low apparent thrombogenicity (as evaluated by the vena cava r i n g t e s t ) can be achieved u s i n g hydrogel systems which have low water contents ( i . e . dow to ~ 12% H 0 ) . 9


21.

SASAKI ET AL.

Radiation

Induced

Co-Graft

291

Polymerization


EMA/HEMA COPOLYMER GRAFTS

MONOMER RATIO

GRAFT LEVEL (mg/cm2)

WATER IN GRAFT %

TWO HOUR IVC RING TESTS (Dr. J.D. Whiff en. U. Wise.) Θ

Η

Ι

Ι

Φ

Ι

Ι

Ι

I

I

I

(SILASTIC)

Ο

negl.

EMA

2.2

-10

5.1

-12

0.8

^25

Ο •

Ο •

Ο •

Ο

0.8

- 4 6

ΟΠ3

Ο Π

Ο •

O

I5EMA/ 5EMA/

5 H E M A

| 5 H E M A

HEMA

d

O

I

D

I

ODODOÛQU

Figure 8. Results of 2-hr vena cava ring tests for silastic and various grafted copolymers on silastic

• Q

HEMA/EMA



292


Second, the vena cava r i n g t e s t does not have the s e n s i t i v i t y to d i s t i n g u i s h among most of the g r a f t e d polymers used i n t h i s experiment. As the s e r i e s of g r a f t e d polymers d i f f e r d r a m a t i c a l l y i n water contents and s u r f a c e p r o p e r t i e s , some d i f f e r e n c e i n performanace between g r a f t e d HEMA and HEMA-EMA copolymers would be expected. The a d d i t i o n of 25% HEMA to the monomer mixture was found s u f f i c i e n t t o produce a s u r f a c e which, although possessing a low water content, i s s i g n i f i c a n t l y l e s s thrombogenic than the hydrophobic S i l a s t i c o r P o l y ( E M A ) - g - S i l a s t i c s u r f a c e s . The r e l a t i v e l y l i n e a r decrease i n water content w i t h i n c r e a s i n g EMA i n the monomer mixture (Figure 6) i s taken t o i n d i c a t e that both HEMA and EMA enter i n t o the g r a f t copolymer a t s i m i l a r r a t e s and that the g r a f t copolymer composition i s not d i f f e r e n t by a l a r g e degree from the monomer s o l u t i o n composition. However, t h i s experiment does r a i s e a number of questions which e f f e c t the i n t e r p r e t a t i o n of the r e s u l t s . The degree t o which the HEMA and EMA are d i s t r i b u t e d between the s i l i c o n e rubber surface and the b u l k o f the s i l i c o n e rubber i s not p r e s e n t l y known. The p r e c i s e f r a c t i o n s o f HEMA and EMA i n the copolymer, and perhaps more important, the d i s t r i b u t i o n of HEMA and EMA groups a t the s u r f a c e of the g r a f t copolymer i n t e r f a c i n g w i t h the b l o o d , i s not known. F i n a l l y , the importance of the l e v e l of g r a f t on the apparent s u r f a c e thrombogenicity of the g r a f t polymer cannot be determined from the p r e l i m i n a r y vena cava r i n g i m p l a n t a t i o n experiment. These problems are c u r r e n t l y under study i n an e f f o r t to c l a r i f y the i n t e r p r e t a t i o n of these r e s u l t s and to determine the importance of a h y d r o p h i l i c - h y d r o p h o b i c balance on the thrombogenicity of m a t e r i a l s . I t should be emphasized that the vena cava r i n g t e s t does not d i s t i n g u i s h between non-thrombogenic m a t e r i a l s and those which are only non-thromboadherent. Thus, the t r u e thrombog e n i c i t y of the m a t e r i a l s remains t o be evaluated. Tests a r e underway which should a l l o w a more r e a l i s t i c and q u a n t i f i a b l e assessment o f the thrombogenicity of HEMA-EMA g r a f t e d m a t e r i a l s . An e v a l u a t i o n o f the b i o l o g i c a l response t o these g r a f t e d copolymers implanted i n s o f t t i s s u e i s a l s o underway. Conclusions A survey has been made o f v a r i o u s s o l v e n t systems f o r use i n the p r e p a r a t i o n of HEMA-EMA co-grafted s i l i c o n e rubber f i l m s by the r a d i a t i o n g r a f t i n g technique. The a d d i t i o n of water t o the s o l v e n t s i n v e s t i g a t e d has a marked a c c e l e r a t i v e e f f e c t on the g r a f t i n g r a t e of EMA. By the a d d i t i o n of a p p r o p r i a t e amounts of water, i t i s p o s s i b l e t o prepare co-grafted f i l m s w i t h the same l e v e l s of g r a f t throughout a wide range of HEMA/EMA r a t i o s . The e f f e c t of water on the g r a f t i n g process f o r t h i s system has been discussed and i t i s suggested that the changes i n g r a f t l e v e l s noted might be due t o the Trommsdorff e f e c t o r d i f f e r e n c e s i n the



21.

SASAKI ET AL.

Radiation

Induced

Co-Graft

Polymerization

293

i n the p a r t i t i o n i n g of the monomers between the s i l i c o n e rubber and the monomer s o l u t i o n . The water content of g r a f t e d f i l m s a t a given monomer composition i s independent of g r a f t l e v e l a t higher HEMA r a t i o s , but increases w i t h i n c r e a s e s i n the g r a f t l e v e l a t higher EMA r a t i o s . This was concluded t o be due t o p o r o s i t y a f f e c t s i n the graft. HEMA-EMA g r a f t e d copolymers on s i l i c o n e rubber and pure HEMA g r a f t s on S i l i c o n e rubber show low apparent thrombogenicity by the vena cava r i n g t e s t . Pure g r a f t e d EMA m a t e r i a l s were found t o be thrombogenic by t h i s t e s t . An a n a l y s i s of the exact copolymer composition of these f i l m s i s planned f o r the near f u t u r e . A l s o a wide range of a d d i t i o n a l s'tudies on the i n t e r a c t i o n of these m a t e r i a l s w i t h b i o l o g i c a l systems are underway. Acknowledgement One of the authors (T.S.) would l i k e t o g r a t e f u l l y acknowledge the support of the Japanese Atomic Energy I n s t i t u t e , Takasaki, Japan. We would a l s o l i k e t o thank the U.S. Atomic Energy Commission, D i v i s i o n of B i o m e d i c a l and Environmental Research (Contract AT (45-1)-2225) f o r t h e i r generous support f o r these s t u d i e s , and Dr. James D. Whiffen of the U n i v e r s i t y of Wisconsin f o r the vena cava r i n g t e s t s .

Literature Cited 1. Bruck, S. D., J. Biomed. Mater. Res., (1973),7,387. 2. Hoffman, A. S. and Kraft, W. G., ACS Polymer Preprints, (1972), 13(2), 723. 3. Hoffman, A. S. and Harris, C., ACS Polymer Preprints, (1972), 13(2), 740. 4. Lee, H. B., Shim, H. S. and Andrade, J. D., ACS Polymer Preprints, (1972), 13(2), 729. 5. Ratner, B. D. and Hoffman, A. S., Preprints - ACS Division of Organic Coatings and Plastics Chemistry, (1973), 33(2), 386. 6. Kearney, J. J., Amara, I. and McDevitt, M. B., Preprints ACS Division of Organic Coatings and Plastics Chemistry, (1973), 33(2), 346. 7. Ratner, B. D. and Hoffman, A. S., J. Appl. Polymer Sci., (1974), 18, 3183. 8. Kwiatkowski, G. T., Byck, J. S., Camp, R. L., Creasy, W. S. and Stewart, D. D., "Blood Compatible Polyelectrolytes for Use in Medical Devices", Contract No. N01-HL3-2950T, National Heart and Lung Institute, National Institutes of Health, Bethesda, Maryland, Annual Report, July 1, 1972 June 30, 1973, PB 225-636.



294


9. Baier, R. E., Dutton, R. C. and Gott, V. L., "Surface Chemical Features of Blood Vessel Walls and Synthetic Materials Exhibiting Thromboresistance" in "Surface Chemistry of Bio logical Systems", M. Black, ed., pp. 235-260, Plenum Press, N.Y., 1970. 10. Brash, J. L., Fritzinger, Β. K. and Bruck, S. D., J. Biomed. Mater. Res., (1973), 7, 313. 11. Peterson, R. J. and Rozelle, L. T., "Ultrathin Membranes for Blood Oxygenators", Contract No. N01-HB-1-2364, National Heart and Lung Institute, National Institutes of Health, Bethesda, Maryland, Annual Report, December 1974, PB 231 324/5WM. 12. Ratner, B. D. and Miller, I. F., J. Polymer Sci., (1972), A-1, 10, 2425.


Hydrogels for Medical and Related Applications

Recommend Documents