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JOURNAL OF MATERIALS SCIENCE: MATERIALS IN MEDICI NE, 7 (1996) 355 357

Hyd roxyapatite c oat ing by dipp ing m ethod,and bo ne bon ding strengthT U A N T U A N L I* , J U N H E E LEE ~ , T A K A Y U K I K O B A Y A S H I *, H I D EK I A O K I **Division o f Inorganic Mater ials, Inst i tute for Me dical and Dental Engineer ing, TokyoMe dical and D ental University, To kyo 101, Japan~Department o f Metal lurgica l Engineer ing, Dong-A University, Pu san South Korea~Animal Clinic Kobayashi 715-3 Sa kia Fukaya-city, Saitama, JapanHydrox yapa t i t e (HA) was coa ted on to t i t an ium rods by a d ip coa t ing me tho d us ing HA so l.The HA so ls we re prepared by d ispers ing HA crys ta ls less than 100 nm length in d is t i l ledwa te r o r ph ys io log i ca l sa l t so lu t i on us ing an u l t rason ic homo gen ize r . The su r face o f t he HAcoa t ing was hom ogen eous as de te rm ined by scann ing e lec t ron m ic roscop y (SEM). A f te rimp lan ta t ion o f uncoated and HA d ip coated t i tan ium rods in dog fem urs , new bone fo rma t ionwas observed on ly around the coated ma ter ia l . The bone bon d ing s t rength to HA coated rodswa s 1.0, 1.5, 2.0 and 2.5 M pa af ter 1,2,3 and 4 wee ks im pla ntat io n, respect ively, as determ inedby pu l l - ou t t es t i ng . These va lues were ove r tw ice tha t o f t he uncoa ted t i t an ium rodsa t 1 4 weeks a f te r imp lan ta t i on . The d ip coa ted t i t an ium exh ib i t ed supe r io r b iocom pa t ib i l i t yto t he uncoa ted imp lan t and may be o f g rea t va lue fo r bone repacemen t app l i ca t i ons .

1 . I n t r o d u c t i o nT h e r e a r e a n u m b e r o f t e ch n i q u e s fo r c o a ti n g H A o nm e t a l s , in c l u d i n g p l a s m a s p r a y i n g , f l a m e s p r a y i n g , i o ns p u t t e r i n g , c h e m i c a l v a p o u r d e p o s i t i o n a n d t h e r m a ld e c o m p o s i t i o n t e c h n i q u e s . A m o n g t h e s e t e c h n i q u e s ,t h e p l a s m a s p r a y i n g t e c h n i q u e i s p r e s e n t l y b e i n g u s e dwo r ldw ide (1 8 ). Ho wev e r , t he se a re a ll h igh - t emp er -a tu re t e chn iques . The re fo re , i t i s ve ry d i f f i cu l t t o coa tH A o n t o m e t a ls h o m o g e n e o u s l y w i t h a t h ic k n e s s o fl ess t ha n 10 t sm , w i tho u t e i t he r deco mp os i t i o n o r r eac -t i o n w i t h t h e m e t a l s u b s t r a t e . I n t h i s s t u d y , H A w a sc o a t e d o n t i t a n i u m m e t a l b y d i p p i n g i t i n t o H A m i c r o -c r y s t a l s u s p e n s i o n , i . e . H A - s o l a s d e v e l o p e d b y A o k ie t a l . ( 9, 1 0) . T h e H A - c o a t e d t i t a n i u m r o d s w e r e i m -p l a n t e d i n t o d o g f e m u r s , a n d t h e b i o c o m p a t i b i l i t y a n db o n e b o n d i n g s t r e n g t h w e r e e v a l u a t e d .

2 . M a t e r ia l s a n d m e t h o d s2 . 1 . M a t e r i a l sH A c r y s t a l s (F i g . 1) w e r e p r e p a r e d b y t h e n e u t r a l i z a -t i o n r e a c t i o n o f c a l c i u m h y d r o x i d e s u s p e n s i o n a n dp h o s p h o r i c a c i d s o l u t i o n u s i n g a n u l t r a s o n i c h o m o -gen ize r (F ig . 2 ). The HA c rys t a l s we re d i spe r sed i n tod i s ti l le d w a t e r o r p h y s i o l o g i c a l s a l t s o l u t i o n i n o r d e rt o p r e p a r e H A s o l s . P u r e t i t a n i u m r o d s w e r e d i p p e do n c e a n d s e v e n ti m e s i n t o H A s o l s o f c o n c e n t r a t i o n sb e t w e e n 1 . 5 a n d 1 0 w t % . T h e c o a t i n g l a y e r w a s c h a r -a c t e r iz e d b y X - r a y d i f f r a c t io n ( X R D ) a n d i n f r a r e ds p e c t r o s c o p y ( [R S ). T h e t h i c k n e s s o f H A c o a t i n g w a s

m e a s u r e d a n d t h e s u r f a c e w a s e x a m i n e d b y s c a n n i n ge l e c t r o n m i c r o s c o p y ( S E M ) .

2 . 2 . A n i m a l t e s t i n gT h e e x p e r i m e n t a l a n i m a l u s e d w a s o n e m a l e b e a g l ed o g , o v e r 1 y e a r s o f a g e a n d w i t h a w e i g h t o f 12 k g .A n e s t h e s i a w a s i n d u c e d w i t h a n i n t r a v e n o u s i n j e c t i o no f G O I a n d w i t h n i t ro u s o x i d e i n h a l a t io n a c c o r d i n g t oa c c e p t e d s t a n d a r d s . T w e l v e H A - c o a t e d t i t a n i u m r o d sw e r e i m p l a n t e d i n t o f e m u r s o f d o g . T h e b o n e t is s u er e a c t i o n o f t h e m a t e r ia l s w a s e x a m i n e d h i s t o l o g i c a ll ya n d t h e b o n e b o n d i n g s t r e n g t h w a s m e a s u r e d b y p u l l -ou t t e s t i ng a t a speed o f 0 .5 r am / ra in a f t e r imp lan t a -t i on fo r I t o 4 weeks .

3 . R e s u l t s3 .1 . D i p c o a t i n g m e t h o dT h e h y d r o x y a p a t i t e c r y s t a l s w e r e le ss t h a n 1 0 0 n m i nl e n g t h a n d h a d a p l a t e - l i k e m o r p h o l o g y . T h e c o n c e n -t r a t i o n o f H A s o l r a n g e d f r o m 1 .5 t o 1 0 w t % . T h ec o a t i n g t h i c k n e s s r a n g e d f r o m 1 0 t a m t o 2 0 0 t a m a n dd e p e n d e d o n t h e d i p p in g t i m e a n d t h e c o n c e n t r a t i o no f t h e H A s o l. T h e c o a t i n g t h i c k n e s s in c r e a s e d n o t a b l yw i t h t h e n u m b e r o f d i p s a n d i n s o l s o f c o n c e n t r a t i o n o f7 w t % o r m o r e . T i t a n i u m p r e t r e a t e d b y s a n d b l a s t i n gp r o v i d e d a m u c h b e t t e r c o a t i n g s u b s t r a t e t h a n t h esmooth un t rea t ed su r face (F ig . 3 ) . Th in coa t i ngs l e sstha n 501, ti n i n t h i ckness w e re ob t a ine d i n d i l u t e HA

This paper was accepted for publicat ion after the 1995 Conference of the European Society of Biomaterials, Oporto , Portugal, 10 13September.0957 4530 ,c 1996 Chapman & ttall 3 5 5

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sol less than 5.34 wt %. The HA w as deposi ted hom o-geneously.

3.2. HistologyFig. 4a is a mic rogra ph i l lustrat ing new bone grow thin close contact wi th an HA d ip coated implan t af t er2 weeks in vivo. Fig. 4b sho ws the h is tology of thecont ro l t i t an ium implan t . Fibrous connect ive t i s suewas observed after 2 weeks.

Figure 1 Transmission electron micrograph of HA crystals.

StirreUItrasonichomogenizer

'iH3P04solut ion

Ca(OH)2suspension

Figure 2 Schema of preparation of HA crystals by ultrasonic ir-radiation.

Figure 4 Undecalcified histological section of femo ra 4 weeks afterimpl antatio n (H. E. stain, 200) T: titanum, C: co ated layer, B: bone.(a) HA-sol coated implant; (b) titanium control implant.

Figure 3 SEM micrographs of HA coating layer after dipping titanium with (a} smooth surface and (b) sandblasted surface once in5.34 wt % HA sol.356

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F i g u r e 5 C h a n g e s in p u l l -o u t s t r e n g th o f u n c o a t e d ( x ) a n d H A -c o a t e d ( 0 ) t i t a n i u m i m p l a n t s a s a f u n c t i o n o f i m p l a n t a t i o n p e r io d .

3 . 3 . B o n e b o n d i n g s t r e n g t hF i g. 5 s h ow s t h e c h a n g e s i n b o n e b o n d i n g s t r e n g th o fH A d i p c o a t e d o n t i t a n i u m . T h e b o n d i n g s t r e n g t h o fH A c o a t i n g w a s o v e r tw i c e t h a t o f t i t a n i u m r o d s a t2 4 w e e k s a f t e r i m p l a n t a t i o n .

4 . D i s c u s s i o nT h e d i p c o a t i n g m e t h o d c o a t e d H A h o m o g e n e o u s l yo n t i t a n i u m w i t h a 1 0 2 0 0 p m t h i c k n e ss a t r o o mt e m p e r a t u r e a n d t h e r e f o r e n o r e a c t i o n o c c u r r e d b e -t w e e n H A c r y s t a l s a n d t i t a n i u m m e t a l . T h e c o a t i n gl a y e r w a s c o m p o s e d o f n a n o s c a l e H A c r y s t a l s a n d t h eb o n d i n g s t r e n g t h b e t w e e n H A a n d t i t a n i u m w a s v e r yw e a k . T h e m i c r o c r y s t a l s q u i c k l y i n d u c e n e w b o n ef o r m a t i o n b y b o n d i n g w i t h s o m e b o n e f o r m a t i o nc o m p o n e n t s i n b l o o d a n d t h e n t h e y w i l l b e a b s o r b e di n t o c e ll s f o r a s h o r t t i m e . T h e r e f o r e , H A c o a t e d b y t h ed i p c o a t i n g m e t h o d i s o n l y u s e fu l fo r q u ic k b o n e

f o r m a t i o n , a s t h e H A c o a t i n g l a y e r w i l l d i s a p p e a rr e l a t i v e ly q u i c k l y .

5 . C o n c l u s i o n sH A - s o l w a s p r e p a r e d b y d i s p e rs i n g H A m i c r o c r y s ta l si n t o d i s t i ll e d w a t e r o r p h y s i o l o g i c a l s a lt s o l u t i o n u s i n ga n u l t r a s o n i c h o m o g e n i z e r . H A c o a t i n g o f t i t a n i u mw a s p r e p a r e d b y d i p p i n g t i t a n i u m i n t o H A s o l a tr o o m t e m p e r a t u r e . T h e c o a t i n g t h i c k n e s s w a s v e r yt h i n , r a n g i n g f r o m 1 0 t am t o 2 0 0 t, m .

B o n d i n g s t r e n g t h b e t w e e n h y d r o x y a p a t i t e a n d t i t a -n i u m w a s v e r y w e a k , h o w e v e r , b o n d i n g s t r e n g t h b e -t w e e n b o n e a n d H A - c o a t e d t i t a n i u m w a s s t r o n g e r ,o v e r t w ic e t h a t o f u n c o a t e d t i t a n i u m . T h e d i p c o a t e dH A w a s q u i c k l y a b s o r b e d i n t o c e l l s . I t w a s c o n c l u d e dt h a t t h e d i p p i n g m e t h o d i s a n e f f e c ti v e t e c h n i q u e f o rp r e p a r i n g H A c o a t e d t i t a n i u m m a t e r i a l s w i t h c o m -p l i c a t e d s h a p e s . T h e m a t e r i a l h a d g o o d b i o c o m p a t i -b i l it y a n d m a y b e e f f e c ti v e f o r u s e a s p r o s t h e s e s .

R e f e r e n c e s1 . H . A O K I , i n " ' M e d i c a l a p p l i c a t i o n s o f h y d r o x y a p a t i t e "

( I s h i y a k u E u r o A m e r i c a , T o k y o , S t . L o u i s , 19 94 ).2 . l d em . , i n " S c ie n c e a n d m e d i c a l a p p l i c a t i o n s o f h y d r o x y a p a t i t e "

( J p n . A s s o c . A p a t i t e S c i ., 1 9 91 ).3 . H . A O K I a n d K . K A T O , J. Dent . En ,qml . 18 (1977) 86 .4 . L . S E N N E R B Y a n d P. T H O M S E N , B i o m a t e r i a l s 14 (1993)

4 1 3 .5 . A . P I A T T E L L I a n d P . T R I S I , i b i d . 1 4 ( 1 9 93 ) 2 17 .6 . Z . Z Y M A N , Y . C A O a n d X . Z H A N G , i b i d . 1 4 ( 1 99 3 ) I I 40 .7 . H . W . D E N I S S E N , i n " H y d r o x y a p t i t e i m p l a n t s " (P i c ci n ,

P a d u a , I t a l y , 1 9 8 5 ) p . 3 9 .8. P . D U C H E Y N E , S . R A D I N . M . H E U G H E B A E R T a nd

J. C. H E U G H E B A E R T , B i o m a t e r i a l s 11 (1990) 244 .9 . H . A O K I , S . O H G A K I a n d S. K A N O , R ep . I n s t . M ed D en t .

E n g . 27 (1993) 39 .1 0. H . A O K I , M . A K A O a n d K . K A N O ibid. 26 (1992) 15 .I 1 . J . L E E and H. A O K I , B i o m ed . M a t e r . E n ,q n9. 5 (1995) in p res s .

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